GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1716

Network Working Group P. Almquist, Author Request for Comments: 1716 Consultant Category: Informational F. Kastenholz, Editor

                                                    FTP Software, Inc.
                                                         November 1994
                Towards Requirements for IP Routers

Status of this Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.

Almquist & Kastenholz [Page i] RFC 1716 Towards Requirements for IP Routers November 1994

Table of Contents

0. PREFACE ………………………………………………. 1 1. INTRODUCTION ………………………………………….. 2 1.1 Reading this Document …………………………………. 4 1.1.1 Organization ……………………………………….. 4 1.1.2 Requirements ……………………………………….. 5 1.1.3 Compliance …………………………………………. 6 1.2 Relationships to Other Standards ……………………….. 7 1.3 General Considerations ………………………………… 8 1.3.1 Continuing Internet Evolution ………………………… 8 1.3.2 Robustness Principle ………………………………… 9 1.3.3 Error Logging ………………………………………. 9 1.3.4 Configuration ………………………………………. 10 1.4 Algorithms …………………………………………… 11 2. INTERNET ARCHITECTURE ………………………………….. 13 2.1 Introduction …………………………………………. 13 2.2 Elements of the Architecture …………………………… 14 2.2.1 Protocol Layering …………………………………… 14 2.2.2 Networks …………………………………………… 16 2.2.3 Routers ……………………………………………. 17 2.2.4 Autonomous Systems ………………………………….. 18 2.2.5 Addresses and Subnets ……………………………….. 18 2.2.6 IP Multicasting …………………………………….. 20 2.2.7 Unnumbered Lines and Networks and Subnets ……………… 20 2.2.8 Notable Oddities ……………………………………. 22 2.2.8.1 Embedded Routers ………………………………….. 22 2.2.8.2 Transparent Routers ……………………………….. 23 2.3 Router Characteristics ………………………………… 24 2.4 Architectural Assumptions ……………………………… 27 3. LINK LAYER ……………………………………………. 29 3.1 INTRODUCTION …………………………………………. 29 3.2 LINK/INTERNET LAYER INTERFACE ………………………….. 29 3.3 SPECIFIC ISSUES ………………………………………. 30 3.3.1 Trailer Encapsulation ……………………………….. 30 3.3.2 Address Resolution Protocol - ARP …………………….. 31 3.3.3 Ethernet and 802.3 Coexistence ……………………….. 31 3.3.4 Maximum Transmission Unit - MTU ………………………. 31 3.3.5 Point-to-Point Protocol - PPP ………………………… 32 3.3.5.1 Introduction ……………………………………… 32 3.3.5.2 Link Control Protocol (LCP) Options …………………. 33 3.3.5.3 IP Control Protocol (ICP) Options …………………… 34 3.3.6 Interface Testing …………………………………… 35 4. INTERNET LAYER - PROTOCOLS ……………………………… 36 4.1 INTRODUCTION …………………………………………. 36 4.2 INTERNET PROTOCOL - IP ………………………………… 36

Almquist & Kastenholz [Page ii] RFC 1716 Towards Requirements for IP Routers November 1994

4.2.1 INTRODUCTION ……………………………………….. 36 4.2.2 PROTOCOL WALK-THROUGH ……………………………….. 37 4.2.2.1 Options: RFC-791 Section 3.2 ……………………….. 37 4.2.2.2 Addresses in Options: RFC-791 Section 3.1 ……………. 40 4.2.2.3 Unused IP Header Bits: RFC-791 Section 3.1 …………… 40 4.2.2.4 Type of Service: RFC-791 Section 3.1 ………………… 41 4.2.2.5 Header Checksum: RFC-791 Section 3.1 ………………… 41 4.2.2.6 Unrecognized Header Options: RFC-791 Section 3.1 ……… 41 4.2.2.7 Fragmentation: RFC-791 Section 3.2 ………………….. 42 4.2.2.8 Reassembly: RFC-791 Section 3.2 …………………….. 43 4.2.2.9 Time to Live: RFC-791 Section 3.2 …………………… 43 4.2.2.10 Multi-subnet Broadcasts: RFC-922 …………………… 43 4.2.2.11 Addressing: RFC-791 Section 3.2 ……………………. 43 4.2.3 SPECIFIC ISSUES …………………………………….. 47 4.2.3.1 IP Broadcast Addresses …………………………….. 47 4.2.3.2 IP Multicasting …………………………………… 48 4.2.3.3 Path MTU Discovery ………………………………… 48 4.2.3.4 Subnetting ……………………………………….. 49 4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP ………………… 50 4.3.1 INTRODUCTION ……………………………………….. 50 4.3.2 GENERAL ISSUES ……………………………………… 50 4.3.2.1 Unknown Message Types ……………………………… 50 4.3.2.2 ICMP Message TTL ………………………………….. 51 4.3.2.3 Original Message Header ……………………………. 51 4.3.2.4 ICMP Message Source Address ………………………… 51 4.3.2.5 TOS and Precedence ………………………………… 51 4.3.2.6 Source Route ……………………………………… 52 4.3.2.7 When Not to Send ICMP Errors ……………………….. 53 4.3.2.8 Rate Limiting …………………………………….. 54 4.3.3 SPECIFIC ISSUES …………………………………….. 55 4.3.3.1 Destination Unreachable ……………………………. 55 4.3.3.2 Redirect …………………………………………. 55 4.3.3.3 Source Quench …………………………………….. 56 4.3.3.4 Time Exceeded …………………………………….. 56 4.3.3.5 Parameter Problem …………………………………. 57 4.3.3.6 Echo Request/Reply ………………………………… 57 4.3.3.7 Information Request/Reply ………………………….. 58 4.3.3.8 Timestamp and Timestamp Reply ………………………. 58 4.3.3.9 Address Mask Request/Reply …………………………. 59 4.3.3.10 Router Advertisement and Solicitations ……………… 61 4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ……………….. 61 5. INTERNET LAYER - FORWARDING …………………………….. 62 5.1 INTRODUCTION …………………………………………. 62 5.2 FORWARDING WALK-THROUGH ……………………………….. 62 5.2.1 Forwarding Algorithm ………………………………… 62 5.2.1.1 General ………………………………………….. 63 5.2.1.2 Unicast ………………………………………….. 64

Almquist & Kastenholz [Page iii] RFC 1716 Towards Requirements for IP Routers November 1994

5.2.1.3 Multicast ………………………………………… 65 5.2.2 IP Header Validation ………………………………… 66 5.2.3 Local Delivery Decision ……………………………… 68 5.2.4 Determining the Next Hop Address ……………………… 70 5.2.4.1 Immediate Destination Address ………………………. 71 5.2.4.2 Local/Remote Decision ……………………………… 71 5.2.4.3 Next Hop Address ………………………………….. 72 5.2.4.4 Administrative Preference ………………………….. 77 5.2.4.6 Load Splitting ……………………………………. 78 5.2.5 Unused IP Header Bits: RFC-791 Section 3.1 …………….. 79 5.2.6 Fragmentation and Reassembly: RFC-791 Section 3.2 ………. 79 5.2.7 Internet Control Message Protocol - ICMP ………………. 80 5.2.7.1 Destination Unreachable ……………………………. 80 5.2.7.2 Redirect …………………………………………. 82 5.2.7.3 Time Exceeded …………………………………….. 84 5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ……………… 84 5.3 SPECIFIC ISSUES ………………………………………. 84 5.3.1 Time to Live (TTL) ………………………………….. 84 5.3.2 Type of Service (TOS) ……………………………….. 85 5.3.3 IP Precedence ………………………………………. 87 5.3.3.1 Precedence-Ordered Queue Service ……………………. 88 5.3.3.2 Lower Layer Precedence Mappings …………………….. 88 5.3.3.3 Precedence Handling For All Routers …………………. 89 5.3.4 Forwarding of Link Layer Broadcasts …………………… 92 5.3.5 Forwarding of Internet Layer Broadcasts ……………….. 92 5.3.5.1 Limited Broadcasts ………………………………… 94 5.3.5.2 Net-directed Broadcasts ……………………………. 94 5.3.5.3 All-subnets-directed Broadcasts …………………….. 95 5.3.5.4 Subnet-directed Broadcasts …………………………. 97 5.3.6 Congestion Control ………………………………….. 97 5.3.7 Martian Address Filtering ……………………………. 99 5.3.8 Source Address Validation ……………………………. 99 5.3.9 Packet Filtering and Access Lists …………………….. 100 5.3.10 Multicast Routing ………………………………….. 101 5.3.11 Controls on Forwarding ……………………………… 101 5.3.12 State Changes ……………………………………… 101 5.3.12.1 When a Router Ceases Forwarding ……………………. 102 5.3.12.2 When a Router Starts Forwarding ……………………. 102 5.3.12.3 When an Interface Fails or is Disabled ……………… 103 5.3.12.4 When an Interface is Enabled ………………………. 103 5.3.13 IP Options ………………………………………… 103 5.3.13.1 Unrecognized Options ……………………………… 103 5.3.13.2 Security Option ………………………………….. 104 5.3.13.3 Stream Identifier Option ………………………….. 104 5.3.13.4 Source Route Options ……………………………… 104 5.3.13.5 Record Route Option ………………………………. 104 5.3.13.6 Timestamp Option …………………………………. 105

Almquist & Kastenholz [Page iv] RFC 1716 Towards Requirements for IP Routers November 1994

6. TRANSPORT LAYER ……………………………………….. 106 6.1 USER DATAGRAM PROTOCOL - UDP …………………………… 106 6.2 TRANSMISSION CONTROL PROTOCOL - TCP …………………….. 106 7. APPLICATION LAYER - ROUTING PROTOCOLS ……………………. 109 7.1 INTRODUCTION …………………………………………. 109 7.1.1 Routing Security Considerations ………………………. 109 7.1.2 Precedence …………………………………………. 110 7.2 INTERIOR GATEWAY PROTOCOLS …………………………….. 110 7.2.1 INTRODUCTION ……………………………………….. 110 7.2.2 OPEN SHORTEST PATH FIRST - OSPF ………………………. 111 7.2.2.1 Introduction ……………………………………… 111 7.2.2.2 Specific Issues …………………………………… 111 7.2.2.3 New Version of OSPF ……………………………….. 112 7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM - DUAL IS-IS

   ..............................................................  112

7.2.4 ROUTING INFORMATION PROTOCOL - RIP ……………………. 113 7.2.4.1 Introduction ……………………………………… 113 7.2.4.2 Protocol Walk-Through ……………………………… 113 7.2.4.3 Specific Issues …………………………………… 118 7.2.5 GATEWAY TO GATEWAY PROTOCOL - GGP …………………….. 119 7.3 EXTERIOR GATEWAY PROTOCOLS …………………………….. 119 7.3.1 INTRODUCTION ……………………………………….. 119 7.3.2 BORDER GATEWAY PROTOCOL - BGP ………………………… 120 7.3.2.1 Introduction ……………………………………… 120 7.3.2.2 Protocol Walk-through ……………………………… 120 7.3.3 EXTERIOR GATEWAY PROTOCOL - EGP ………………………. 121 7.3.3.1 Introduction ……………………………………… 121 7.3.3.2 Protocol Walk-through ……………………………… 122 7.3.4 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL ………….. 124 7.4 STATIC ROUTING ……………………………………….. 125 7.5 FILTERING OF ROUTING INFORMATION ……………………….. 127 7.5.1 Route Validation ……………………………………. 127 7.5.2 Basic Route Filtering ……………………………….. 127 7.5.3 Advanced Route Filtering …………………………….. 128 7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE ……………… 129 8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS ………….. 131 8.1 The Simple Network Management Protocol - SNMP ……………. 131 8.1.1 SNMP Protocol Elements ………………………………. 131 8.2 Community Table ………………………………………. 132 8.3 Standard MIBS ………………………………………… 133 8.4 Vendor Specific MIBS ………………………………….. 134 8.5 Saving Changes ……………………………………….. 135 9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS ………………. 137 9.1 BOOTP ……………………………………………….. 137 9.1.1 Introduction ……………………………………….. 137 9.1.2 BOOTP Relay Agents ………………………………….. 137 10. OPERATIONS AND MAINTENANCE …………………………….. 139

Almquist & Kastenholz [Page v] RFC 1716 Towards Requirements for IP Routers November 1994

10.1 Introduction ………………………………………… 139 10.2 Router Initialization ………………………………… 140 10.2.1 Minimum Router Configuration ………………………… 140 10.2.2 Address and Address Mask Initialization ………………. 141 10.2.3 Network Booting using BOOTP and TFTP …………………. 142 10.3 Operation and Maintenance …………………………….. 143 10.3.1 Introduction ………………………………………. 143 10.3.2 Out Of Band Access …………………………………. 144 10.3.2 Router O&M Functions ……………………………….. 144 10.3.2.1 Maintenance - Hardware Diagnosis …………………… 144 10.3.2.2 Control - Dumping and Rebooting ……………………. 145 10.3.2.3 Control - Configuring the Router …………………… 145 10.3.2.4 Netbooting of System Software ……………………… 146 10.3.2.5 Detecting and responding to misconfiguration ………… 146 10.3.2.6 Minimizing Disruption …………………………….. 147 10.3.2.7 Control - Troubleshooting Problems …………………. 148 10.4 Security Considerations ………………………………. 149 10.4.1 Auditing and Audit Trails …………………………… 149 10.4.2 Configuration Control ………………………………. 150 11. REFERENCES …………………………………………… 152 APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ……………. 162 APPENDIX B. GLOSSARY ……………………………………… 164 APPENDIX C. FUTURE DIRECTIONS ……………………………… 169 APPENDIX D. Multicast Routing Protocols …………………….. 172 D.1 Introduction …………………………………………. 172 D.2 Distance Vector Multicast Routing Protocol - DVMRP ……….. 172 D.3 Multicast Extensions to OSPF - MOSPF ……………………. 173 APPENDIX E Additional Next-Hop Selection Algorithms ………….. 174 E.1. Some Historical Perspective ……………………………. 174 E.2. Additional Pruning Rules ………………………………. 176 E.3 Some Route Lookup Algorithms …………………………… 177 E.3.1 The Revised Classic Algorithm …………………………. 178 E.3.2 The Variant Router Requirements Algorithm ………………. 179 E.3.3 The OSPF Algorithm …………………………………… 179 E.3.4 The Integrated IS-IS Algorithm ………………………… 180 Security Considerations ……………………………………. 182 Acknowledgments …………………………………………… 183 Editor's Address ………………………………………….. 186

Almquist & Kastenholz [Page vi] RFC 1716 Towards Requirements for IP Routers November 1994

0. PREFACE

This document is a snapshot of the work of the Router Requirements working group as of November 1991. At that time, the working group had essentially finished its task. There were some final technical matters to be nailed down, and a great deal of editing needed to be done in order to get the document ready for publication. Unfortunately, these tasks were never completed.

At the request of the Internet Area Director, the current editor took the last draft of the document and, after consulting the mailing list archives, meeting minutes, notes, and other members of the working group, edited the document to its current form. This effort included the following tasks: 1) Deleting all the parenthetical material (such as editor's comments). Useful information was turned into DISCUSSION sections, the rest was deleted. 2) Completing the tasks listed in the last draft's To be Done sections. As a part of this task, a new "to be done" list was developed and included as an appendix to the current document. 3) Rolling Philip Almquist's "Ruminations on the Next Hop" and "Ruminations on Route Leaking" into this document. These represent significant work and should be kept. 4) Fulfilling the last intents of the working group as determined from the archival material. The intent of this effort was to get the document into a form suitable for publication as an Historical RFC so that the significant work which went into the creation of this document would be preserved.

The content and form of this document are due, in large part, to the working group's chair, and document's original editor and author: Philip Almquist. Without his efforts, this document would not exist.

Almquist & Kastenholz [Page 1] RFC 1716 Towards Requirements for IP Routers November 1994

1. INTRODUCTION

The goal of this work is to replace RFC-1009, Requirements for Internet Gateways ([INTRO:1]) with a new document.

This memo is an intermediate step toward that goal. It defines and discusses requirements for devices which perform the network layer forwarding function of the Internet protocol suite. The Internet community usually refers to such devices as IP routers or simply routers; The OSI community refers to such devices as intermediate systems. Many older Internet documents refer to these devices as gateways, a name which more recently has largely passed out of favor to avoid confusion with application gateways.

An IP router can be distinguished from other sorts of packet switching devices in that a router examines the IP protocol header as part of the switching process. It generally has to modify the IP header and to strip off and replace the Link Layer framing.

The authors of this memo recognize, as should its readers, that many routers support multiple protocol suites, and that support for multiple protocol suites will be required in increasingly large parts of the Internet in the future. This memo, however, does not attempt to specify Internet requirements for protocol suites other than TCP/IP.

This document enumerates standard protocols that a router connected to the Internet must use, and it incorporates by reference the RFCs and other documents describing the current specifications for these protocols. It corrects errors in the referenced documents and adds additional discussion and guidance for an implementor.

For each protocol, this final version of this memo also contains an explicit set of requirements, recommendations, and options. The reader must understand that the list of requirements in this memo is incomplete by itself; the complete set of requirements for an Internet protocol router is primarily defined in the standard protocol specification documents, with the corrections, amendments, and supplements contained in this memo.

This memo should be read in conjunction with the Requirements for Internet Hosts RFCs ([INTRO:2] and [INTRO:3]). Internet hosts and routers must both be capable of originating IP datagrams and receiving IP datagrams destined for them. The major distinction between Internet hosts and routers is that routers are required to implement forwarding algorithms and Internet hosts do not require forwarding capabilities. Any Internet host acting as a router must adhere to the requirements contained in the final version of this memo.

Almquist & Kastenholz [Page 2] RFC 1716 Towards Requirements for IP Routers November 1994

The goal of open system interconnection dictates that routers must function correctly as Internet hosts when necessary. To achieve this, this memo provides guidelines for such instances. For simplification and ease of document updates, this memo tries to avoid overlapping discussions of host requirements with [INTRO:2] and [INTRO:3] and incorporates the relevant requirements of those documents by reference. In some cases the requirements stated in [INTRO:2] and [INTRO:3] are superseded by the final version of this document.

A good-faith implementation of the protocols produced after careful reading of the RFCs, with some interaction with the Internet technical community, and that follows good communications software engineering practices, should differ from the requirements of this memo in only minor ways. Thus, in many cases, the requirements in this document are already stated or implied in the standard protocol documents, so that their inclusion here is, in a sense, redundant. However, they were included because some past implementation has made the wrong choice, causing problems of interoperability, performance, and/or robustness.

This memo includes discussion and explanation of many of the requirements and recommendations. A simple list of requirements would be dangerous, because:

o Some required features are more important than others, and some

 features are optional.

o Some features are critical in some applications of routers but

 irrelevant in others.

o There may be valid reasons why particular vendor products that are

 designed for restricted contexts might choose to use different
 specifications.

However, the specifications of this memo must be followed to meet the general goal of arbitrary router interoperation across the diversity and complexity of the Internet. Although most current implementations fail to meet these requirements in various ways, some minor and some major, this specification is the ideal towards which we need to move.

These requirements are based on the current level of Internet architecture. This memo will be updated as required to provide additional clarifications or to include additional information in those areas in which specifications are still evolving.

Almquist & Kastenholz [Page 3] RFC 1716 Towards Requirements for IP Routers November 1994

1.1 Reading this Document

1.1.1 Organization

    This memo emulates the layered organization used by [INTRO:2] and
    [INTRO:3].  Thus, Chapter 2 describes the layers found in the
    Internet architecture.  Chapter 3 covers the Link Layer.  Chapters
    4 and 5 are concerned with the Internet Layer protocols and
    forwarding algorithms.  Chapter 6 covers the Transport Layer.
    Upper layer protocols are divided between Chapter 7, which
    discusses the protocols which routers use to exchange routing
    information with each other, Chapter 8, which discusses network
    management, and Chapter 9, which discusses other upper layer
    protocols.  The final chapter covers operations and maintenance
    features.  This organization was chosen for simplicity, clarity,
    and consistency with the Host Requirements RFCs.  Appendices to
    this memo include a bibliography, a glossary, and some conjectures
    about future directions of router standards.
    In describing the requirements, we assume that an implementation
    strictly mirrors the layering of the protocols.  However, strict
    layering is an imperfect model, both for the protocol suite and
    for recommended implementation approaches.  Protocols in different
    layers interact in complex and sometimes subtle ways, and
    particular functions often involve multiple layers.  There are
    many design choices in an implementation, many of which involve
    creative breaking of strict layering.  Every implementor is urged
    to read [INTRO:4] and [INTRO:5].
    In general, each major section of this memo is organized into the
    following subsections:
    (1)  Introduction
    (2)  Protocol Walk-Through - considers the protocol specification
         documents section-by-section, correcting errors, stating
         requirements that may be ambiguous or ill-defined, and
         providing further clarification or explanation.
    (3)  Specific Issues - discusses protocol design and
         implementation issues that were not included in the walk-
         through.
    Under many of the individual topics in this memo, there is
    parenthetical material labeled DISCUSSION or IMPLEMENTATION. This
    material is intended to give a justification, clarification or

Almquist & Kastenholz [Page 4] RFC 1716 Towards Requirements for IP Routers November 1994

    explanation to the preceding requirements text.  The
    implementation material contains suggested approaches that an
    implementor may want to consider.  The DISCUSSION and
    IMPLEMENTATION sections are not part of the standard.

1.1.2 Requirements

    In this memo, the words that are used to define the significance
    of each particular requirement are capitalized.  These words are:
    o  MUST
       This word means that the item is an absolute requirement of the
       specification.
    o  MUST IMPLEMENT
       This phrase means that this specification requires that the
       item be implemented, but does not require that it be enabled by
       default.
    o  MUST NOT
       This phrase means that the item is an absolute prohibition of
       the specification.
    o  SHOULD
       This word means that there may exist valid reasons in
       particular circumstances to ignore this item, but the full
       implications should be understood and the case carefully
       weighed before choosing a different course.
    o  SHOULD IMPLEMENT
       This phrase is similar in meaning to SHOULD, but is used when
       we recommend that a particular feature be provided but does not
       necessarily recommend that it be enabled by default.
    o  SHOULD NOT
       This phrase means that there may exist valid reasons in
       particular circumstances when the described behavior is
       acceptable or even useful, but the full implications should be
       understood and the case carefully weighed before implementing
       any behavior described with this label.
    o  MAY
       This word means that this item is truly optional.  One vendor
       may choose to include the item because a particular marketplace
       requires it or because it enhances the product, for example;
       another vendor may omit the same item.

Almquist & Kastenholz [Page 5] RFC 1716 Towards Requirements for IP Routers November 1994

1.1.3 Compliance

    Some requirements are applicable to all routers.  Other
    requirements are applicable only to those which implement
    particular features or protocols.  In the following paragraphs,
    Relevant refers to the union of the requirements applicable to all
    routers and the set of requirements applicable to a particular
    router because of the set of features and protocols it has
    implemented.
    Note that not all Relevant requirements are stated directly in
    this memo.  Various parts of this memo incorporate by reference
    sections of the Host Requirements specification, [INTRO:2] and
    [INTRO:3].  For purposes of determining compliance with this memo,
    it does not matter whether a Relevant requirement is stated
    directly in this memo or merely incorporated by reference from one
    of those documents.
    An implementation is said to be conditionally compliant if it
    satisfies all of the Relevant MUST, MUST IMPLEMENT, and MUST NOT
    requirements.  An implementation is said to be unconditionally
    compliant if it is conditionally compliant and also satisfies all
    of the Relevant SHOULD, SHOULD IMPLEMENT, and SHOULD NOT
    requirements.  An implementation is not compliant if it is not
    conditionally compliant (i.e., it fails to satisfy one or more of
    the Relevant MUST, MUST IMPLEMENT, or MUST NOT requirements).
    For any of the SHOULD and SHOULD NOT requirements, a router may
    provide a configuration option that will cause the router to act
    other than as specified by the requirement.  Having such a
    configuration option does not void a router's claim to
    unconditional compliance as long as the option has a default
    setting, and that leaving the option at its default setting causes
    the router to operate in a manner which conforms to the
    requirement.
    Likewise, routers may provide, except where explicitly prohibited
    by this memo, options which cause them to violate MUST or MUST NOT
    requirements.  A router which provides such options is compliant
    (either fully or conditionally) if and only if each such option
    has a default setting which causes the router to conform to the
    requirements of this memo.  Please note that the authors of this
    memo, although aware of market realities, strongly recommend
    against provision of such options.  Requirements are labeled MUST
    or MUST NOT because experts in the field have judged them to be
    particularly important to interoperability or proper functioning
    in the Internet.  Vendors should weigh carefully the customer

Almquist & Kastenholz [Page 6] RFC 1716 Towards Requirements for IP Routers November 1994

    support costs of providing options which violate those rules.
    Of course, this memo is not a complete specification of an IP
    router, but rather is closer to what in the OSI world is called a
    profile.  For example, this memo requires that a number of
    protocols be implemented.  Although most of the contents of their
    protocol specifications are not repeated in this memo,
    implementors are nonetheless required to implement the protocols
    according to those specifications.

1.2 Relationships to Other Standards

 There are several reference documents of interest in checking the
 current status of protocol specifications and standardization:
   o  INTERNET OFFICIAL PROTOCOL STANDARDS
      This document describes the Internet standards process and lists
      the standards status of the protocols.  As of this writing, the
      current version of this document is STD 1, RFC 1610, [ARCH:7].
      This document is periodically re-issued.  You should always
      consult an RFC repository and use the latest version of this
      document.
   o  Assigned Numbers
      This document lists the assigned values of the parameters used
      in the various protocols.  For example, IP protocol codes, TCP
      port numbers, Telnet Option Codes, ARP hardware types, and
      Terminal Type names.  As of this writing, the current version of
      this document is STD 2, RFC 1700, [INTRO:7].  This document is
      periodically re-issued.  You should always consult an RFC
      repository and use the latest version of this document.
   o  Host Requirements
      This pair of documents reviews the specifications that apply to
      hosts and supplies guidance and clarification for any
      ambiguities.  Note that these requirements also apply to
      routers, except where otherwise specified in this memo.  As of
      this writing (December, 1993) the current versions of these
      documents are RFC 1122 and RFC 1123, (STD 3) [INTRO:2], and
      [INTRO:3] respectively.
   o  Router Requirements (formerly Gateway Requirements)
      This memo.
   Note that these documents are revised and updated at different
   times; in case of differences between these documents, the most
   recent must prevail.

Almquist & Kastenholz [Page 7] RFC 1716 Towards Requirements for IP Routers November 1994

   These and other Internet protocol documents may be obtained from
   the:
   The InterNIC
   DS.INTERNIC.NET
   InterNIC Directory and Database Service
   +1 (800) 444-4345 or +1 (619) 445-4600
   info@internic.net

1.3 General Considerations

 There are several important lessons that vendors of Internet software
 have learned and which a new vendor should consider seriously.

1.3.1 Continuing Internet Evolution

    The enormous growth of the Internet has revealed problems of
    management and scaling in a large datagram-based packet
    communication system.  These problems are being addressed, and as
    a result there will be continuing evolution of the specifications
    described in this memo.  New routing protocols, algorithms, and
    architectures are constantly being developed.  New and additional
    internet-layer protocols are also constantly being devised.
    Because routers play such a crucial role in the Internet, and
    because the number of routers deployed in the Internet is much
    smaller than the number of hosts, vendors should expect that
    router standards will continue to evolve much more quickly than
    host standards.  These changes will be carefully planned and
    controlled since there is extensive participation in this planning
    by the vendors and by the organizations responsible for operation
    of the networks.
    Development, evolution, and revision are characteristic of
    computer network protocols today, and this situation will persist
    for some years.  A vendor who develops computer communications
    software for the Internet protocol suite (or any other protocol
    suite!) and then fails to maintain and update that software for
    changing specifications is going to leave a trail of unhappy
    customers.  The Internet is a large communication network, and the
    users are in constant contact through it.  Experience has shown
    that knowledge of deficiencies in vendor software propagates
    quickly through the Internet technical community.

Almquist & Kastenholz [Page 8] RFC 1716 Towards Requirements for IP Routers November 1994

1.3.2 Robustness Principle

    At every layer of the protocols, there is a general rule (from
    [TRANS:2] by Jon Postel) whose application can lead to enormous
    benefits in robustness and interoperability:
                     Be conservative in what you do,
                be liberal in what you accept from others.
    Software should be written to deal with every conceivable error,
    no matter how unlikely; sooner or later a packet will come in with
    that particular combination of errors and attributes, and unless
    the software is prepared, chaos can ensue.  In general, it is best
    to assume that the network is filled with malevolent entities that
    will send packets designed to have the worst possible effect.
    This assumption will lead to suitably protective design.  The most
    serious problems in the Internet have been caused by unforeseen
    mechanisms triggered by low probability events; mere human malice
    would never have taken so devious a course!
    Adaptability to change must be designed into all levels of router
    software.  As a simple example, consider a protocol specification
    that contains an enumeration of values for a particular header
    field - e.g., a type field, a port number, or an error code; this
    enumeration must be assumed to be incomplete.  If the protocol
    specification defines four possible error codes, the software must
    not break when a fifth code shows up.  An undefined code might be
    logged, but it must not cause a failure.
    The second part of the principle is almost as important: software
    on hosts or other routers may contain deficiencies that make it
    unwise to exploit legal but obscure protocol features.  It is
    unwise to stray far from the obvious and simple, lest untoward
    effects result elsewhere.  A corollary of this is watch out for
    misbehaving hosts; router software should be prepared to survive
    in the presence of misbehaving hosts.  An important function of
    routers in the Internet is to limit the amount of disruption such
    hosts can inflict on the shared communication facility.

1.3.3 Error Logging

    The Internet includes a great variety of systems, each
    implementing many protocols and protocol layers, and some of these
    contain bugs and misfeatures in their Internet protocol software.
    As a result of complexity, diversity, and distribution of
    function, the diagnosis of problems is often very difficult.

Almquist & Kastenholz [Page 9] RFC 1716 Towards Requirements for IP Routers November 1994

    Problem diagnosis will be aided if routers include a carefully
    designed facility for logging erroneous or strange events.  It is
    important to include as much diagnostic information as possible
    when an error is logged.  In particular, it is often useful to
    record the header(s) of a packet that caused an error.  However,
    care must be taken to ensure that error logging does not consume
    prohibitive amounts of resources or otherwise interfere with the
    operation of the router.
    There is a tendency for abnormal but harmless protocol events to
    overflow error logging files; this can be avoided by using a
    circular log, or by enabling logging only while diagnosing a known
    failure.  It may be useful to filter and count duplicate
    successive messages.  One strategy that seems to work well is to
    both:
    o  Always count abnormalities and make such counts accessible
       through the management protocol (see Chapter 8); and
    o  Allow the logging of a great variety of events to be
       selectively enabled.  For example, it might useful to be able
       to log everything or to log everything for host X.
    This topic is further discussed in [MGT:5].

1.3.4 Configuration

    In an ideal world, routers would be easy to configure, and perhaps
    even entirely self-configuring.  However, practical experience in
    the real world suggests that this is an impossible goal, and that
    in fact many attempts by vendors to make configuration easy
    actually cause customers more grief than they prevent.  As an
    extreme example, a router designed to come up and start routing
    packets without requiring any configuration information at all
    would almost certainly choose some incorrect parameter, possibly
    causing serious problems on any networks unfortunate enough to be
    connected to it.
    Often this memo requires that a parameter be a configurable
    option.  There are several reasons for this.  In a few cases there
    currently is some uncertainty or disagreement about the best value
    and it may be necessary to update the recommended value in the
    future.  In other cases, the value really depends on external
    factors - e.g., the distribution of its communication load, or the
    speeds and topology of nearby networks - and self-tuning
    algorithms are unavailable and may be insufficient.  In some
    cases, configurability is needed because of administrative
    requirements.

Almquist & Kastenholz [Page 10] RFC 1716 Towards Requirements for IP Routers November 1994

    Finally, some configuration options are required to communicate
    with obsolete or incorrect implementations of the protocols,
    distributed without sources, that persist in many parts of the
    Internet.  To make correct systems coexist with these faulty
    systems, administrators must occasionally misconfigure the correct
    systems.  This problem will correct itself gradually as the faulty
    systems are retired, but cannot be ignored by vendors.
    When we say that a parameter must be configurable, we do not
    intend to require that its value be explicitly read from a
    configuration file at every boot time.  For many parameters, there
    is one value that is appropriate for all but the most unusual
    situations.  In such cases, it is quite reasonable that the
    parameter default to that value if not explicitly set.
    This memo requires a particular value for such defaults in some
    cases.  The choice of default is a sensitive issue when the
    configuration item controls accommodation of existing, faulty,
    systems.  If the Internet is to converge successfully to complete
    interoperability, the default values built into implementations
    must implement the official protocol, not misconfigurations to
    accommodate faulty implementations.  Although marketing
    considerations have led some vendors to choose misconfiguration
    defaults, we urge vendors to choose defaults that will conform to
    the standard.
    Finally, we note that a vendor needs to provide adequate
    documentation on all configuration parameters, their limits and
    effects.

1.4 Algorithms

 In several places in this memo, specific algorithms that a router
 ought to follow are specified.  These algorithms are not, per se,
 required of the router.  A router need not implement each algorithm
 as it is written in this document.  Rather, an implementation must
 present a behavior to the external world that is the same as a
 strict, literal, implementation of the specified algorithm.
 Algorithms are described in a manner that differs from the way a good
 implementor would implement them.  For expository purposes, a style
 that emphasizes conciseness, clarity, and independence from
 implementation details has been chosen.  A good implementor will
 choose algorithms and implementation methods which produce the same
 results as these algorithms, but may be more efficient or less
 general.

Almquist & Kastenholz [Page 11] RFC 1716 Towards Requirements for IP Routers November 1994

 We note that the art of efficient router implementation is outside of
 the scope of this memo.

Almquist & Kastenholz [Page 12] RFC 1716 Towards Requirements for IP Routers November 1994

2. INTERNET ARCHITECTURE

This chapter does not contain any requirements. However, it does contain useful background information on the general architecture of the Internet and of routers.

General background and discussion on the Internet architecture and supporting protocol suite can be found in the DDN Protocol Handbook [ARCH:1]; for background see for example [ARCH:2], [ARCH:3], and [ARCH:4]. The Internet architecture and protocols are also covered in an ever-growing number of textbooks, such as [ARCH:5] and [ARCH:6].

2.1 Introduction

 The Internet system consists of a number of interconnected packet
 networks supporting communication among host computers using the
 Internet protocols.  These protocols include the Internet Protocol
 (IP), the Internet Control Message Protocol (ICMP), the Internet
 Group Management Protocol (IGMP), and a variety transport and
 application protocols that depend upon them.  As was described in
 Section [1.2], the Internet Engineering Steering Group periodically
 releases an Official Protocols memo listing all of the Internet
 protocols.
 All Internet protocols use IP as the basic data transport mechanism.
 IP is a datagram, or connectionless, internetwork service and
 includes provision for addressing, type-of-service specification,
 fragmentation and reassembly, and security.  ICMP and IGMP are
 considered integral parts of IP, although they are architecturally
 layered upon IP.  ICMP provides error reporting, flow control,
 first-hop router redirection, and other maintenance and control
 functions.  IGMP provides the mechanisms by which hosts and routers
 can join and leave IP multicast groups.
 Reliable data delivery is provided in the Internet protocol suite by
 Transport Layer protocols such as the Transmission Control Protocol
 (TCP), which provides end-end retransmission, resequencing and
 connection control.  Transport Layer connectionless service is
 provided by the User Datagram Protocol (UDP).

Almquist & Kastenholz [Page 13] RFC 1716 Towards Requirements for IP Routers November 1994

2.2 Elements of the Architecture

2.2.1 Protocol Layering

    To communicate using the Internet system, a host must implement
    the layered set of protocols comprising the Internet protocol
    suite.  A host typically must implement at least one protocol from
    each layer.
    The protocol layers used in the Internet architecture are as
    follows [ARCH:7]:
    o  Application Layer
       The Application Layer is the top layer of the Internet protocol
       suite.  The Internet suite does not further subdivide the
       Application Layer, although some application layer protocols do
       contain some internal sub-layering.  The application layer of
       the Internet suite essentially combines the functions of the
       top two layers - Presentation and Application - of the OSI
       Reference Model [ARCH:8].  The Application Layer in the
       Internet protocol suite also includes some of the function
       relegated to the Session Layer in the OSI Reference Model.
       We distinguish two categories of application layer protocols:
       user protocols that provide service directly to users, and
       support protocols that provide common system functions.  The
       most common Internet user protocols are:
       - Telnet (remote login)
       - FTP (file transfer)
       - SMTP (electronic mail delivery)
       There are a number of other standardized user protocols and
       many private user protocols.
       Support protocols, used for host name mapping, booting, and
       management, include SNMP, BOOTP, TFTP, the Domain Name System
       (DNS) protocol, and a variety of routing protocols.
       Application Layer protocols relevant to routers are discussed
       in chapters 7, 8, and 9 of this memo.
    o  Transport Layer
       The Transport Layer provides end-to-end communication services.
       This layer is roughly equivalent to the Transport Layer in the
       OSI Reference Model, except that it also incorporates some of
       OSI's Session Layer establishment and destruction functions.

Almquist & Kastenholz [Page 14] RFC 1716 Towards Requirements for IP Routers November 1994

       There are two primary Transport Layer protocols at present:
       - Transmission Control Protocol (TCP)
       - User Datagram Protocol (UDP)
       TCP is a reliable connection-oriented transport service that
       provides end-to-end reliability, resequencing, and flow
       control.  UDP is a connectionless (datagram) transport service.
       Other transport protocols have been developed by the research
       community, and the set of official Internet transport protocols
       may be expanded in the future.
       Transport Layer protocols relevant to routers are discussed in
       Chapter 6.
    o  Internet Layer
       All Internet transport protocols use the Internet Protocol (IP)
       to carry data from source host to destination host.  IP is a
       connectionless or datagram internetwork service, providing no
       end-to-end delivery guarantees. IP datagrams may arrive at the
       destination host damaged, duplicated, out of order, or not at
       all.  The layers above IP are responsible for reliable delivery
       service when it is required.  The IP protocol includes
       provision for addressing, type-of-service specification,
       fragmentation and reassembly, and security.
       The datagram or connectionless nature of IP is a fundamental
       and characteristic feature of the Internet architecture.
       The Internet Control Message Protocol (ICMP) is a control
       protocol that is considered to be an integral part of IP,
       although it is architecturally layered upon IP, i.e., it uses
       IP to carry its data end-to-end.  ICMP provides error
       reporting, congestion reporting, and first-hop router
       redirection.
       The Internet Group Management Protocol (IGMP) is an Internet
       layer protocol used for establishing dynamic host groups for IP
       multicasting.
       The Internet layer protocols IP, ICMP, and IGMP are discussed
       in chapter 4.
    o  Link Layer
       To communicate on its directly-connected network, a host must
       implement the communication protocol used to interface to that
       network.  We call this a Link Layer layer protocol.

Almquist & Kastenholz [Page 15] RFC 1716 Towards Requirements for IP Routers November 1994

       Some older Internet documents refer to this layer as the
       Network Layer, but it is not the same as the Network Layer in
       the OSI Reference Model.
       This layer contains everything below the Internet Layer.
       Protocols in this Layer are generally outside the scope of
       Internet standardization; the Internet (intentionally) uses
       existing standards whenever possible.  Thus, Internet Link
       Layer standards usually address only address resolution and
       rules for transmitting IP packets over specific Link Layer
       protocols.  Internet Link Layer standards are discussed in
       chapter 3.

2.2.2 Networks

    The constituent networks of the Internet system are required to
    provide only packet (connectionless) transport.  According to the
    IP service specification, datagrams can be delivered out of order,
    be lost or duplicated, and/or contain errors.
    For reasonable performance of the protocols that use IP (e.g.,
    TCP), the loss rate of the network should be very low.  In
    networks providing connection-oriented service, the extra
    reliability provided by virtual circuits enhances the end-end
    robustness of the system, but is not necessary for Internet
    operation.
    Constituent networks may generally be divided into two classes:
      o  Local-Area Networks (LANs)
         LANs may have a variety of designs.  In general, a LAN will
         cover a small geographical area (e.g., a single building or
         plant site) and provide high bandwidth with low delays.  LANs
         may be passive (similar to Ethernet) or they may be active
         (such as ATM).
      o  Wide-Area Networks (WANs)
         Geographically-dispersed hosts and LANs are interconnected by
         wide-area networks, also called long-haul networks.  These
         networks may have a complex internal structure of lines and
         packet-switches, or they may be as simple as point-to-point
         lines.

Almquist & Kastenholz [Page 16] RFC 1716 Towards Requirements for IP Routers November 1994

2.2.3 Routers

    In the Internet model, constituent networks are connected together
    by IP datagram forwarders which are called routers or IP routers.
    In this document, every use of the term router is equivalent to IP
    router.  Many older Internet documents refer to routers as
    gateways.
    Historically, routers have been realized with packet-switching
    software executing on a general-purpose CPU.  However, as custom
    hardware development becomes cheaper and as higher throughput is
    required, but special-purpose hardware is becoming increasingly
    common.  This specification applies to routers regardless of how
    they are implemented.
    A router is connected to two or more networks, appearing to each
    of these networks as a connected host.  Thus, it has (at least)
    one physical interface and (at least) one IP address on each of
    the connected networks (this ignores the concept of un-numbered
    links, which is discussed in section [2.2.7]).  Forwarding an IP
    datagram generally requires the router to choose the address of
    the next-hop router or (for the final hop) the destination host.
    This choice, called routing, depends upon a routing database
    within the router.  The routing database is also sometimes known
    as a routing table or forwarding table.
    The routing database should be maintained dynamically to reflect
    the current topology of the Internet system.  A router normally
    accomplishes this by participating in distributed routing and
    reachability algorithms with other routers.
    Routers provide datagram transport only, and they seek to minimize
    the state information necessary to sustain this service in the
    interest of routing flexibility and robustness.
    Packet switching devices may also operate at the Link Layer; such
    devices are usually called bridges. Network segments which are
    connected by bridges share the same IP network number, i.e., they
    logically form a single IP network.  These other devices are
    outside of the scope of this document.
    Another variation on the simple model of networks connected with
    routers sometimes occurs: a set of routers may be interconnected
    with only serial lines, to form a network in which the packet
    switching is performed at the Internetwork (IP) Layer rather than
    the Link Layer.

Almquist & Kastenholz [Page 17] RFC 1716 Towards Requirements for IP Routers November 1994

2.2.4 Autonomous Systems

    For technical, managerial, and sometimes political reasons, the
    routers of the Internet system are grouped into collections called
    autonomous systems.  The routers included in a single autonomous
    system (AS) are expected to:
    o  Be under the control of a single operations and maintenance
       (O&M) organization;
    o  Employ common routing protocols among themselves, to
       dynamically maintain their routing databases.
    A number of different dynamic routing protocols have been
    developed (see Section [7.2]); the routing protocol within a
    single AS is generically called an interior gateway protocol or
    IGP.
    An IP datagram may have to traverse the routers of two or more ASs
    to reach its destination, and the ASs must provide each other with
    topology information to allow such forwarding.  An exterior
    gateway protocol (generally BGP or EGP) is used for this purpose.

2.2.5 Addresses and Subnets

    An IP datagram carries 32-bit source and destination addresses,
    each of which is partitioned into two parts - a constituent
    network number and a host number on that network.  Symbolically:
       IP-address  ::=  { <Network-number>, <Host-number> }
    To finally deliver the datagram, the last router in its path must
    map the Host-number (or rest) part of an IP address into the
    physical address of a host connection to the constituent network.
    This simple notion has been extended by the concept of subnets,
    which were introduced in order to allow arbitrary complexity of
    interconnected LAN structures within an organization, while
    insulating the Internet system against explosive growth in network
    numbers and routing complexity.  Subnets essentially provide a
    multi-level hierarchical routing structure for the Internet
    system.  The subnet extension, described in [INTERNET:2], is now a
    required part of the Internet architecture.  The basic idea is to
    partition the <Host-number> field into two parts: a subnet number,
    and a true host number on that subnet:
       IP-address  ::=

Almquist & Kastenholz [Page 18] RFC 1716 Towards Requirements for IP Routers November 1994

         { <Network-number>, <Subnet-number>, <Host-number> }
    The interconnected physical networks within an organization will
    be given the same network number but different subnet numbers.
    The distinction between the subnets of such a subnetted network is
    normally not visible outside of that network.  Thus, routing in
    the rest of the Internet will be based only upon the <Network-
    number> part of the IP destination address; routers outside the
    network will combine <Subnet-number> and <Host-number> together to
    form an uninterpreted rest part of the 32-bit IP address.  Within
    the subnetted network, the routers must route on the basis of an
    extended network number:
       { <Network-number>, <Subnet-number> }
    Under certain circumstances, it may be desirable to support
    subnets of a particular network being interconnected only via a
    path which is not part of the subnetted network.  Even though many
    IGP's and no EGP's currently support this configuration
    effectively, routers need to be able to support this configuration
    of subnetting (see Section [4.2.3.4]).  In general, routers should
    not make assumptions about what are subnets and what are not, but
    simply ignore the concept of Class in networks, and treat each
    route as a { network, mask }-tuple.
    DISCUSSION:
       It is becoming clear that as the Internet grows larger and
       larger, the traditional uses of Class A, B, and C networks will
       be modified in order to achieve better use of IP's 32-bit
       address space.  Classless Interdomain Routing (CIDR)
       [INTERNET:15] is a method currently being deployed in the
       Internet backbones to achieve this added efficiency.  CIDR
       depends on the ability of assigning and routing to networks
       that are not based on Class A, B, or C networks.  Thus, routers
       should always treat a route as a network with a mask.
    Furthermore, for similar reasons, a subnetted network need not
    have a consistent subnet mask through all parts of the network.
    For example, one subnet may use an 8 bit subnet mask, another 10
    bit, and another 6 bit.  Routers need to be able to support this
    type of configuration (see Section [4.2.3.4]).
    The bit positions containing this extended network number are
    indicated by a 32-bit mask called the subnet mask; it is
    recommended but not required that the <Subnet-number> bits be
    contiguous and fall between the <Network-number> and the <Host-
    number> fields.  No subnet should be assigned the value zero or -1

Almquist & Kastenholz [Page 19] RFC 1716 Towards Requirements for IP Routers November 1994

    (all one bits).
    Although the inventors of the subnet mechanism probably expected
    that each piece of an organization's network would have only a
    single subnet number, in practice it has often proven necessary or
    useful to have several subnets share a single physical cable.
    There are special considerations for the router when a connected
    network provides a broadcast or multicast capability; these will
    be discussed later.

2.2.6 IP Multicasting

    IP multicasting is an extension of Link Layer multicast to IP
    internets.  Using IP multicasts, a single datagram can be
    addressed to multiple hosts. This collection of hosts is called a
    multicast group.  Each multicast group is represented as a Class D
    IP address.  An IP datagram sent to the group is to be delivered
    to each group member with the same best-effort delivery as that
    provided for unicast IP traffic.  The sender of the datagram does
    not itself need to be a member of the destination group.
    The semantics of IP multicast group membership are defined in
    [INTERNET:4].  That document describes how hosts and routers join
    and leave multicast groups.  It also defines a protocol, the
    Internet Group Management Protocol (IGMP), that monitors IP
    multicast group membership.
    Forwarding of IP multicast datagrams is accomplished either
    through static routing information or via a multicast routing
    protocol.  Devices that forward IP multicast datagrams are called
    multicast routers. They may or may not also forward IP unicasts.
    In general, multicast datagrams are forwarded on the basis of both
    their source and destination addresses.  Forwarding of IP
    multicast packets is described in more detail in Section [5.2.1].
    Appendix D discusses multicast routing protocols.

2.2.7 Unnumbered Lines and Networks and Subnets

    Traditionally, each network interface on an IP host or router has
    its own IP address.  Over the years, people have observed that
    this can cause inefficient use of the scarce IP address space,
    since it forces allocation of an IP network number, or at least a
    subnet number, to every point-to-point link.
    To solve this problem, a number of people have proposed and
    implemented the concept of unnumbered serial lines.  An unnumbered

Almquist & Kastenholz [Page 20] RFC 1716 Towards Requirements for IP Routers November 1994

    serial line does not have any IP network or subnet number
    associated with it.  As a consequence, the network interfaces
    connected to an unnumbered serial line do not have IP addresses.
    Because the IP architecture has traditionally assumed that all
    interfaces had IP addresses, these unnumbered interfaces cause
    some interesting dilemmas.  For example, some IP options (e.g.
    Record Route) specify that a router must insert the interface
    address into the option, but an unnumbered interface has no IP
    address.  Even more fundamental (as we shall see in chapter 5) is
    that routes contain the IP address of the next hop router.  A
    router expects that that IP address will be on an IP (sub)net that
    the router is connected to.  That assumption is of course violated
    if the only connection is an unnumbered serial line.
    To get around these difficulties, two schemes have been invented.
    The first scheme says that two routers connected by an unnumbered
    serial line aren't really two routers at all, but rather two
    half-routers which together make up a single (virtual) router.
    The unnumbered serial line is essentially considered to be an
    internal bus in the virtual router.  The two halves of the virtual
    router must coordinate their activities in such a way that they
    act exactly like a single router.
    This scheme fits in well with the IP architecture, but suffers
    from two important drawbacks.  The first is that, although it
    handles the common case of a single unnumbered serial line, it is
    not readily extensible to handle the case of a mesh of routers and
    unnumbered serial lines.  The second drawback is that the
    interactions between the half routers are necessarily complex and
    are not standardized, effectively precluding the connection of
    equipment from different vendors using unnumbered serial lines.
    Because of these drawbacks, this memo has adopted an alternative
    scheme, which has been invented multiple times but which is
    probably originally attributable to Phil Karn.  In this scheme, a
    router which has unnumbered serial lines also has a special IP
    address, called a router-id in this memo.  The router-id is one of
    the router's IP addresses (a router is required to have at least
    one IP address).  This router-id is used as if it is the IP
    address of all unnumbered interfaces.

Almquist & Kastenholz [Page 21] RFC 1716 Towards Requirements for IP Routers November 1994

2.2.8 Notable Oddities

2.2.8.1 Embedded Routers

       A router may be a stand-alone computer system, dedicated to its
       IP router functions.  Alternatively, it is possible to embed
       router functions within a host operating system which supports
       connections to two or more networks.  The best-known example of
       an operating system with embedded router code is the Berkeley
       BSD system.  The embedded router feature seems to make
       internetting easy, but it has a number of hidden pitfalls:
       (1)  If a host has only a single constituent-network interface,
            it should not act as a router.
            For example, hosts with embedded router code that
            gratuitously forward broadcast packets or datagrams on the
            same net often cause packet avalanches.
       (2)  If a (multihomed) host acts as a router, it must implement
            ALL the relevant router requirements contained in this
            document.
            For example, the routing protocol issues and the router
            control and monitoring problems are as hard and important
            for embedded routers as for stand-alone routers.
            Since Internet router requirements and specifications may
            change independently of operating system changes, an
            administration that operates an embedded router in the
            Internet is strongly advised to have the ability to
            maintain and update the router code (e.g., this might
            require router code source).
       (3)  Once a host runs embedded router code, it becomes part of
            the Internet system.  Thus, errors in software or
            configuration can hinder communication between other
            hosts.  As a consequence, the host administrator must lose
            some autonomy.
            In many circumstances, a host administrator will need to
            disable router code embedded in the operating system, and
            any embedded router code must be organized so that it can
            be easily disabled.
       (4)  If a host running embedded router code is concurrently

Almquist & Kastenholz [Page 22] RFC 1716 Towards Requirements for IP Routers November 1994

            used for other services, the O&M (Operation and
            Maintenance) requirements for the two modes of use may be
            in serious conflict.
            For example, router O&M will in many cases be performed
            remotely by an operations center; this may require
            privileged system access which the host administrator
            would not normally want to distribute.

2.2.8.2 Transparent Routers

       There are two basic models for interconnecting local-area
       networks and wide-area (or long-haul) networks in the Internet.
       In the first, the local-area network is assigned a network
       number and all routers in the Internet must know how to route
       to that network.  In the second, the local-area network shares
       (a small part of) the address space of the wide-area network.
       Routers that support this second model are called address
       sharing routers or transparent routers.  The focus of this memo
       is on routers that support the first model, but this is not
       intended to exclude the use of transparent routers.
       The basic idea of a transparent router is that the hosts on the
       local-area network behind such a router share the address space
       of the wide-area network in front of the router.  In certain
       situations this is a very useful approach and the limitations
       do not present significant drawbacks.
       The words in front and behind indicate one of the limitations
       of this approach: this model of interconnection is suitable
       only for a geographically (and topologically) limited stub
       environment.  It requires that there be some form of logical
       addressing in the network level addressing of the wide-area
       network.  All of the IP addresses in the local environment map
       to a few (usually one) physical address in the wide-area
       network.  This mapping occurs in a way consistent with the { IP
       address <-> network address } mapping used throughout the
       wide-area network.
       Multihoming is possible on one wide-area network, but may
       present routing problems if the interfaces are geographically
       or topologically separated.  Multihoming on two (or more)
       wide-area networks is a problem due to the confusion of
       addresses.
       The behavior that hosts see from other hosts in what is
       apparently the same network may differ if the transparent

Almquist & Kastenholz [Page 23] RFC 1716 Towards Requirements for IP Routers November 1994

       router cannot fully emulate the normal wide-area network
       service.  For example, the ARPANET used a Link Layer protocol
       that provided a Destination Dead indication in response to an
       attempt to send to a host which was powered off.  However, if
       there were a transparent router between the ARPANET and an
       Ethernet, a host on the ARPANET would not receive a Destination
       Dead indication if it sent a datagram to a host that was
       powered off and was connected to the ARPANET via the
       transparent router instead of directly.

2.3 Router Characteristics

 An Internet router performs the following functions:
 (1)  Conforms to specific Internet protocols specified in this
      document, including the Internet Protocol (IP), Internet Control
      Message Protocol (ICMP), and others as necessary.
 (2)  Interfaces to two or more packet networks.  For each connected
      network the router must implement the functions required by that
      network.  These functions typically include:
      o  Encapsulating and decapsulating the IP datagrams with the
         connected network framing (e.g., an Ethernet header and
         checksum),
      o  Sending and receiving IP datagrams up to the maximum size
         supported by that network, this size is the network's Maximum
         Transmission Unit or MTU,
      o  Translating the IP destination address into an appropriate
         network-level address for the connected network (e.g., an
         Ethernet hardware address), if needed, and
      o  Responding to the network flow control and error indication,
         if any.
      See chapter 3 (Link Layer).
 (3)  Receives and forwards Internet datagrams.  Important issues in
      this process are buffer management, congestion control, and
      fairness.
      o  Recognizes various error conditions and generates ICMP error
         and information messages as required.
      o  Drops datagrams whose time-to-live fields have reached zero.

Almquist & Kastenholz [Page 24] RFC 1716 Towards Requirements for IP Routers November 1994

      o  Fragments datagrams when necessary to fit into the MTU of the
         next network.
      See chapter 4 (Internet Layer - Protocols) and chapter 5
      (Internet Layer - Forwarding) for more information.
 (4)  Chooses a next-hop destination for each IP datagram, based on
      the information in its routing database.  See chapter 5
      (Internet Layer - Forwarding) for more information.
 (5)  (Usually) supports an interior gateway protocol (IGP) to carry
      out distributed routing and reachability algorithms with the
      other routers in the same autonomous system.  In addition, some
      routers will need to support an exterior gateway protocol (EGP)
      to exchange topological information with other autonomous
      systems.  See chapter 7 (Application Layer - Routing Protocols)
      for more information.
 (6)  Provides network management and system support facilities,
      including loading, debugging, status reporting, exception
      reporting and control.  See chapter 8 (Application Layer -
      Network Management Protocols) and chapter 10 (Operation and
      Maintenance) for more information.
 A router vendor will have many choices on power, complexity, and
 features for a particular router product.  It may be helpful to
 observe that the Internet system is neither homogeneous nor fully-
 connected.  For reasons of technology and geography it is growing
 into a global interconnect system plus a fringe of LANs around the
 edge. More and more these fringe LANs are becoming richly
 interconnected, thus making them less out on the fringe and more
 demanding on router requirements.
 o  The global interconnect system is comprised of a number of wide-
    area networks to which are attached routers of several Autonomous
    Systems (AS); there are relatively few hosts connected directly to
    the system.
 o  Most hosts are connected to LANs.  Many organizations have
    clusters of LANs interconnected by local routers.  Each such
    cluster is connected by routers at one or more points into the
    global interconnect system.  If it is connected at only one point,
    a LAN is known as a stub network.
 Routers in the global interconnect system generally require:
 o  Advanced Routing and Forwarding Algorithms

Almquist & Kastenholz [Page 25] RFC 1716 Towards Requirements for IP Routers November 1994

    These routers need routing algorithms which are highly dynamic and
    also offer type-of-service routing.  Congestion is still not a
    completely resolved issue (see Section [5.3.6]).  Improvements in
    these areas are expected, as the research community is actively
    working on these issues.
 o  High Availability
    These routers need to be highly reliable, providing 24 hours a
    day, 7 days a week service.  Equipment and software faults can
    have a wide-spread (sometimes global) effect.  In case of failure,
    they must recover quickly.  In any environment, a router must be
    highly robust and able to operate, possibly in a degraded state,
    under conditions of extreme congestion or failure of network
    resources.
 o  Advanced O&M Features
    Internet routers normally operate in an unattended mode.  They
    will typically be operated remotely from a centralized monitoring
    center.  They need to provide sophisticated means for monitoring
    and measuring traffic and other events and for diagnosing faults.
 o  High Performance
    Long-haul lines in the Internet today are most frequently 56 Kbps,
    DS1 (1.4Mbps), and DS3 (45Mbps) speeds.  LANs are typically
    Ethernet (10Mbps) and, to a lesser degree, FDDI (100Mbps).
    However, network media technology is constantly advancing and even
    higher speeds are likely in the future.  Full-duplex operation is
    provided at all of these speeds.
 The requirements for routers used in the LAN fringe (e.g., campus
 networks) depend greatly on the demands of the local networks.  These
 may be high or medium-performance devices, probably competitively
 procured from several different vendors and operated by an internal
 organization (e.g., a campus computing center).  The design of these
 routers should emphasize low average latency and good burst
 performance, together with delay and type-of-service sensitive
 resource management. In this environment there may be less formal O&M
 but it will not be less important.  The need for the routing
 mechanism to be highly dynamic will become more important as networks
 become more complex and interconnected.  Users will demand more out
 of their local connections because of the speed of the global
 interconnects.
 As networks have grown, and as more networks have become old enough

Almquist & Kastenholz [Page 26] RFC 1716 Towards Requirements for IP Routers November 1994

 that they are phasing out older equipment, it has become increasingly
 imperative that routers interoperate with routers from other vendors.
 Even though the Internet system is not fully interconnected, many
 parts of the system need to have redundant connectivity.  Rich
 connectivity allows reliable service despite failures of
 communication lines and routers, and it can also improve service by
 shortening Internet paths and by providing additional capacity.
 Unfortunately, this richer topology can make it much more difficult
 to choose the best path to a particular destination.

2.4 Architectural Assumptions

 The current Internet architecture is based on a set of assumptions
 about the communication system.  The assumptions most relevant to
 routers are as follows:
 o  The Internet is a network of networks.
    Each host is directly connected to some particular network(s); its
    connection to the Internet is only conceptual.  Two hosts on the
    same network communicate with each other using the same set of
    protocols that they would use to communicate with hosts on distant
    networks.
 o  Routers don't keep connection state information.
    To improve the robustness of the communication system, routers are
    designed to be stateless, forwarding each IP packet independently
    of other packets.  As a result, redundant paths can be exploited
    to provide robust service in spite of failures of intervening
    routers and networks.
    All state information required for end-to-end flow control and
    reliability is implemented in the hosts, in the transport layer or
    in application programs.  All connection control information is
    thus co-located with the end points of the communication, so it
    will be lost only if an end point fails.  Routers effect flow
    control only indirectly, by dropping packets or increasing network
    delay.
    Note that future protocol developments may well end up putting
    some more state into routers.  This is especially likely for
    resource reservation and flows.

Almquist & Kastenholz [Page 27] RFC 1716 Towards Requirements for IP Routers November 1994

 o  Routing complexity should be in the routers.
    Routing is a complex and difficult problem, and ought to be
    performed by the routers, not the hosts.  An important objective
    is to insulate host software from changes caused by the inevitable
    evolution of the Internet routing architecture.
 o  The system must tolerate wide network variation.
    A basic objective of the Internet design is to tolerate a wide
    range of network characteristics - e.g., bandwidth, delay, packet
    loss, packet reordering, and maximum packet size.  Another
    objective is robustness against failure of individual networks,
    routers, and hosts, using whatever bandwidth is still available.
    Finally, the goal is full open system interconnection: an Internet
    router must be able to interoperate robustly and effectively with
    any other router or Internet host, across diverse Internet paths.
    Sometimes implementors have designed for less ambitious goals.
    For example, the LAN environment is typically much more benign
    than the Internet as a whole; LANs have low packet loss and delay
    and do not reorder packets.  Some vendors have fielded
    implementations that are adequate for a simple LAN environment,
    but work badly for general interoperation.  The vendor justifies
    such a product as being economical within the restricted LAN
    market.  However, isolated LANs seldom stay isolated for long;
    they are soon connected to each other, to organization-wide
    internets, and eventually to the global Internet system.  In the
    end, neither the customer nor the vendor is served by incomplete
    or substandard routers.
    The requirements spelled out in this document are designed for a
    full-function router.  It is intended that fully compliant routers
    will be usable in almost any part of the Internet.

Almquist & Kastenholz [Page 28] RFC 1716 Towards Requirements for IP Routers November 1994

3. LINK LAYER

Although [INTRO:1] covers Link Layer standards (IP over foo, ARP, etc.), this document anticipates that Link-Layer material will be covered in a separate Link Layer Requirements document. A Link-Layer requirements document would be applicable to both hosts and routers. Thus, this document will not obsolete the parts of [INTRO:1] that deal with link-layer issues.

3.1 INTRODUCTION

 Routers have essentially the same Link Layer protocol requirements as
 other sorts of Internet systems.  These requirements are given in
 chapter 3 of Requirements for Internet Gateways [INTRO:1].  A router
 MUST comply with its requirements and SHOULD comply with its
 recommendations.  Since some of the material in that document has
 become somewhat dated, some additional requirements and explanations
 are included below.
 DISCUSSION:
    It is expected that the Internet community will produce a
    Requirements for Internet Link Layer standard which will supersede
    both this chapter and chapter 3 of [INTRO:1].

3.2 LINK/INTERNET LAYER INTERFACE

 Although this document does not attempt to specify the interface
 between the Link Layer and the upper layers, it is worth noting here
 that other parts of this document, particularly chapter 5, require
 various sorts of information to be passed across this layer boundary.
 This section uses the following definitions:
 o  Source physical address
    The source physical address is the Link Layer address of the host
    or router from which the packet was received.
 o  Destination physical address
    The destination physical address is the Link Layer address to
    which the packet was sent.
 The information that must pass from the Link Layer to the
 Internetwork Layer for each received packet is:

Almquist & Kastenholz [Page 29] RFC 1716 Towards Requirements for IP Routers November 1994

 (1)  The IP packet [5.2.2],
 (2)  The length of the data portion (i.e., not including the Link-
      Layer framing) of the Link Layer frame [5.2.2],
 (3)  The identity of the physical interface from which the IP packet
      was received [5.2.3], and
 (4)  The classification of the packet's destination physical address
      as a Link Layer unicast, broadcast, or multicast [4.3.2],
      [5.3.4].
 In addition, the Link Layer also should provide:
 (5)  The source physical address.
 The information that must pass from the Internetwork Layer to the
 Link Layer for each transmitted packet is:
 (1)  The IP packet [5.2.1]
 (2)  The length of the IP packet [5.2.1]
 (3)  The destination physical interface [5.2.1]
 (4)  The next hop IP address [5.2.1]
 In addition, the Internetwork Layer also should provide:
 (5)  The Link Layer priority value [5.3.3.2]
 The Link Layer must also notify the Internetwork Layer if the packet
 to be transmitted causes a Link Layer precedence-related error
 [5.3.3.3].

3.3 SPECIFIC ISSUES

3.3.1 Trailer Encapsulation

    Routers which can connect to 10Mb Ethernets MAY be able to receive
    and forward Ethernet packets encapsulated using the trailer
    encapsulation described in [LINK:1].  However, a router SHOULD NOT
    originate trailer encapsulated packets.  A router MUST NOT
    originate trailer encapsulated packets without first verifying,
    using the mechanism described in section 2.3.1 of [INTRO:2], that
    the immediate destination of the packet is willing and able to

Almquist & Kastenholz [Page 30] RFC 1716 Towards Requirements for IP Routers November 1994

    accept trailer-encapsulated packets.  A router SHOULD NOT agree
    (using these same mechanisms) to accept trailer-encapsulated
    packets.

3.3.2 Address Resolution Protocol - ARP

    Routers which implement ARP MUST be compliant and SHOULD be
    unconditionally compliant with the requirements in section 2.3.2
    of [INTRO:2].
    The link layer MUST NOT report a Destination Unreachable error to
    IP solely because there is no ARP cache entry for a destination.
    A router MUST not believe any ARP reply which claims that the Link
    Layer address of another host or router is a broadcast or
    multicast address.

3.3.3 Ethernet and 802.3 Coexistence

    Routers which can connect to 10Mb Ethernets MUST be compliant and
    SHOULD be unconditionally compliant with the requirements of
    Section [2.3.3] of [INTRO:2].

3.3.4 Maximum Transmission Unit - MTU

    The MTU of each logical interface MUST be configurable.
    Many Link Layer protocols define a maximum frame size that may be
    sent.  In such cases, a router MUST NOT allow an MTU to be set
    which would allow sending of frames larger than those allowed by
    the Link Layer protocol.  However, a router SHOULD be willing to
    receive a packet as large as the maximum frame size even if that
    is larger than the MTU.
    DISCUSSION:
       Note that this is a stricter requirement than imposed on hosts
       by [INTRO:2], which requires that the MTU of each physical
       interface be configurable.
       If a network is using an MTU smaller than the maximum frame
       size for the Link Layer, a router may receive packets larger
       than the MTU from hosts which are in the process of
       initializing themselves, or which have been misconfigured.
       In general, the Robustness Principle indicates that these
       packets should be successfully received, if at all possible.

Almquist & Kastenholz [Page 31] RFC 1716 Towards Requirements for IP Routers November 1994

3.3.5 Point-to-Point Protocol - PPP

    Contrary to [INTRO:1], the Internet does have a standard serial
    line protocol: the Point-to-Point Protocol (PPP), defined in
    [LINK:2], [LINK:3], [LINK:4], and [LINK:5].
    A serial line interface is any interface which is designed to send
    data over a telephone, leased, dedicated or direct line (either 2
    or 4 wire) using a standardized modem or bit serial interface
    (such as RS-232, RS-449 or V.35), using either synchronous or
    asynchronous clocking.
    A general purpose serial interface is a serial line interface
    which is not solely for use as an access line to a network for
    which an alternative IP link layer specification exists (such as
    X.25 or Frame Relay).
    Routers which contain such general purpose serial interfaces MUST
    implement PPP.
    PPP MUST be supported on all general purpose serial interfaces on
    a router.  The router MAY allow the line to be configured to use
    serial line protocols other than PPP, all general purpose serial
    interfaces MUST default to using PPP.

3.3.5.1 Introduction

       This section provides guidelines to router implementors so that
       they can ensure interoperability with other routers using PPP
       over either synchronous or asynchronous links.
       It is critical that an implementor understand the semantics of
       the option negotiation mechanism.  Options are a means for a
       local device to indicate to a remote peer what the local device
       will *accept* from the remote peer, not what it wishes to send.
       It is up to the remote peer to decide what is most convenient
       to send within the confines of the set of options that the
       local device has stated that it can accept.  Therefore it is
       perfectly acceptable and normal for a remote peer to ACK all
       the options indicated in an LCP Configuration Request (CR) even
       if the remote peer does not support any of those options.
       Again, the options are simply a mechanism for either device to
       indicate to its peer what it will accept, not necessarily what
       it will send.

Almquist & Kastenholz [Page 32] RFC 1716 Towards Requirements for IP Routers November 1994

3.3.5.2 Link Control Protocol (LCP) Options

       The PPP Link Control Protocol (LCP) offers a number of options
       that may be negotiated.  These options include (among others)
       address and control field compression, protocol field
       compression, asynchronous character map, Maximum Receive Unit
       (MRU), Link Quality Monitoring (LQM), magic number (for
       loopback detection), Password Authentication Protocol (PAP),
       Challenge Handshake Authentication Protocol (CHAP), and the
       32-bit Frame Check Sequence (FCS).
       A router MAY do address/control field compression on either
       synchronous or asynchronous links.  A router MAY do protocol
       field compression on either synchronous or asynchronous links.
       A router MAY indicate that it can accept these compressions,
       but MUST be able to accept uncompressed PPP header information
       even if it has indicated a willingness to receive compressed
       PPP headers.
       DISCUSSION:
          These options control the appearance of the PPP header.
          Normally the PPP header consists of the address field (one
          byte containing the value 0xff), the control field (one byte
          containing the value 0x03), and the two-byte protocol field
          that identifies the contents of the data area of the frame.
          If a system negotiates address and control field compression
          it indicates to its peer that it will accept PPP frames that
          have or do not have these fields at the front of the header.
          It does not indicate that it will be sending frames with
          these fields removed.  The protocol field may also be
          compressed from two to one byte in most cases.
       IMPLEMENTATION:
          Some hardware does not deal well with variable length header
          information.  In those cases it makes most sense for the
          remote peer to send the full PPP header.  Implementations
          may ensure this by not sending the address/control field and
          protocol field compression options to the remote peer.  Even
          if the remote peer has indicated an ability to receive
          compressed headers there is no requirement for the local
          router to send compressed headers.
       A router MUST negotiate the Async Control Character Map (ACCM)
       for asynchronous PPP links, but SHOULD NOT negotiate the ACCM
       for synchronous links.  If a router receives an attempt to
       negotiate the ACCM over a synchronous link, it MUST ACKnowledge

Almquist & Kastenholz [Page 33] RFC 1716 Towards Requirements for IP Routers November 1994

       the option and then ignore it.
       DISCUSSION:
          There are implementations that offer both sync and async
          modes of operation and may use the same code to implement
          the option negotiation.  In this situation it is possible
          that one end or the other may send the ACCM option on a
          synchronous link.
       A router SHOULD properly negotiate the maximum receive unit
       (MRU).  Even if a system negotiates an MRU smaller than 1,500
       bytes, it MUST be able to receive a 1,500 byte frame.
       A router SHOULD negotiate and enable the link quality
       monitoring (LQM) option.
       DISCUSSION:
          This memo does not specify a policy for deciding whether the
          link's quality is adequate.  However, it is important (see
          Section [3.3.6]) that a router disable failed links.
       A router SHOULD implement and negotiate the magic number option
       for loopback detection.
       A router MAY support the authentication options (PAP - password
       authentication protocol, and/or CHAP - challenge handshake
       authentication protocol).
       A router MUST support 16-bit CRC frame check sequence (FCS) and
       MAY support the 32-bit CRC.

3.3.5.3 IP Control Protocol (ICP) Options

       A router MAY offer to perform IP address negotiation.  A router
       MUST accept a refusal (REJect) to perform IP address
       negotiation from the peer.
       A router SHOULD NOT perform Van Jacobson header compression of
       TCP/IP packets if the link speed is in excess of 64 Kbps.
       Below that speed the router MAY perform Van Jacobson (VJ)
       header compression.  At link speeds of 19,200 bps or less the
       router SHOULD perform VJ header compression.

Almquist & Kastenholz [Page 34] RFC 1716 Towards Requirements for IP Routers November 1994

3.3.6 Interface Testing

    A router MUST have a mechanism to allow routing software to
    determine whether a physical interface is available to send
    packets or not.  A router SHOULD have a mechanism to allow routing
    software to judge the quality of a physical interface.  A router
    MUST have a mechanism for informing the routing software when a
    physical interface becomes available or unavailable to send
    packets because of administrative action.  A router MUST have a
    mechanism for informing the routing software when it detects a
    Link level interface has become available or unavailable, for any
    reason.
    DISCUSSION:
       It is crucial that routers have workable mechanisms for
       determining that their network connections are functioning
       properly, since failure to do so (or failure to take the proper
       actions when a problem is detected) can lead to black holes.
       The mechanisms available for detecting problems with network
       connections vary considerably, depending on the Link Layer
       protocols in use and also in some cases on the interface
       hardware chosen by the router manufacturer.  The intent is to
       maximize the capability to detect failures within the Link-
       Layer constraints.

Almquist & Kastenholz [Page 35] RFC 1716 Towards Requirements for IP Routers November 1994

4. INTERNET LAYER - PROTOCOLS

4.1 INTRODUCTION

 This chapter and chapter 5 discuss the protocols used at the Internet
 Layer: IP, ICMP, and IGMP.  Since forwarding is obviously a crucial
 topic in a document discussing routers, chapter 5 limits itself to
 the aspects of the protocols which directly relate to forwarding.
 The current chapter contains the remainder of the discussion of the
 Internet Layer protocols.

4.2 INTERNET PROTOCOL - IP

4.2.1 INTRODUCTION

    Routers MUST implement the IP protocol, as defined by
    [INTERNET:1].  They MUST also implement its mandatory extensions:
    subnets (defined in [INTERNET:2]), and IP broadcast (defined in
    [INTERNET:3]).
    A router  MUST be compliant, and SHOULD be unconditionally
    compliant, with the requirements of sections 3.2.1 and 3.3 of
    [INTRO:2], except that:
    o  Section 3.2.1.1 may be ignored, since it duplicates
       requirements found in this memo.
    o  Section 3.2.1.2 may be ignored, since it duplicates
       requirements found in this memo.
    o  Section 3.2.1.3 should be ignored, since it is superseded by
       Section [4.2.2.11] of this memo.
    o  Section 3.2.1.4 may be ignored, since it duplicates
       requirements found in this memo.
    o  Section 3.2.1.6 should be ignored, since it is superseded by
       Section [4.2.2.4] of this memo.
    o  Section 3.2.1.8 should be ignored, since it is superseded by
       Section [4.2.2.1] of this memo.
    In the following, the action specified in certain cases is to
    silently discard a received datagram.  This means that the
    datagram will be discarded without further processing and that the

Almquist & Kastenholz [Page 36] RFC 1716 Towards Requirements for IP Routers November 1994

    router will not send any ICMP error message (see Section [4.3]) as
    a result.  However, for diagnosis of problems a router SHOULD
    provide the capability of logging the error (see Section [1.3.3]),
    including the contents of the silently-discarded datagram, and
    SHOULD record the event in a statistics counter.

4.2.2 PROTOCOL WALK-THROUGH

    RFC 791 is [INTERNET:1], the specification for the Internet
    Protocol.

4.2.2.1 Options: RFC-791 Section 3.2

       In datagrams received by the router itself, the IP layer MUST
       interpret those IP options that it understands and preserve the
       rest unchanged for use by higher layer protocols.
       Higher layer protocols may require the ability to set IP
       options in datagrams they send or examine IP options in
       datagrams they receive.  Later sections of this document
       discuss specific IP option support required by higher layer
       protocols.
       DISCUSSION:
          Neither this memo nor [INTRO:2] define the order in which a
          receiver must process multiple options in the same IP
          header.  Hosts and routers originating datagrams containing
          multiple options must be aware that this introduces an
          ambiguity in the meaning of certain options when combined
          with a source-route option.
       Here are the requirements for specific IP options:
       (a)  Security Option
            Some environments require the Security option in every
            packet originated or received.  Routers SHOULD IMPLEMENT
            the revised security option described in [INTERNET:5].
            DISCUSSION:
               Note that the security options described in
               [INTERNET:1] and RFC 1038 ([INTERNET:16]) are obsolete.
       (b)  Stream Identifier Option
            This option is obsolete; routers SHOULD NOT place this
            option in a datagram that the router originates.  This

Almquist & Kastenholz [Page 37] RFC 1716 Towards Requirements for IP Routers November 1994

            option MUST be ignored in datagrams received by the
            router.
       (c)  Source Route Options
            A router MUST be able to act as the final destination of a
            source route.  If a router receives a packet containing a
            completed source route (i.e., the pointer points beyond
            the last field and the destination address in the IP
            header addresses the router), the packet has reached its
            final destination; the option as received (the recorded
            route) MUST be passed up to the transport layer (or to
            ICMP message processing).
            In order to respond correctly to source-routed datagrams
            it receives, a router MUST provide a means whereby
            transport protocols and applications can reverse the
            source route in a received datagram and insert the
            reversed source route into datagrams they originate (see
            Section 4 of [INTRO:2] for details).
            Some applications in the router MAY require that the user
            be able to enter a source route.
            A router MUST NOT originate a datagram containing multiple
            source route options.  What a router should do if asked to
            forward a packet containing multiple source route options
            is described in Section [5.2.4.1].
            When a source route option is created, it MUST be
            correctly formed even if it is being created by reversing
            a recorded route that erroneously includes the source host
            (see case (B) in the discussion below).
            DISCUSSION:
               Suppose a source routed datagram is to be routed from
               source S to destination D via routers G1, G2, ... Gn.
               Source S constructs a datagram with G1's IP address as
               its destination address, and a source route option to
               get the datagram the rest of the way to its
               destination.  However, there is an ambiguity in the
               specification over whether the source route option in a
               datagram sent out by S should be (A) or (B):
               (A):  {>>G2, G3, ... Gn, D}     <--- CORRECT
               (B):  {S, >>G2, G3, ... Gn, D}  <---- WRONG

Almquist & Kastenholz [Page 38] RFC 1716 Towards Requirements for IP Routers November 1994

               (where >> represents the pointer).  If (A) is sent, the
               datagram received at D will contain the option: {G1,
               G2, ... Gn >>}, with S and D as the IP source and
               destination addresses.  If (B) were sent, the datagram
               received at D would again contain S and D as the same
               IP source and destination addresses, but the option
               would be: {S, G1, ...Gn >>}; i.e., the originating host
               would be the first hop in the route.
       (d)  Record Route Option
            Routers MAY support the Record Route option in datagrams
            originated by the router.
       (e)  Timestamp Option
            Routers MAY support the timestamp option in datagrams
            originated by the router.  The following rules apply:
            o  When originating a datagram containing a Timestamp
               Option, a router MUST record a timestamp in the option
               if
  1. Its Internet address fields are not pre-specified or
  2. Its first pre-specified address is the IP address of

the logical interface over which the datagram is

                  being sent (or the router's router-id if the
                  datagram is being sent over an unnumbered
                  interface).
            o  If the router itself receives a datagram containing a
               Timestamp Option, the router MUST insert the current
               timestamp into the Timestamp Option (if there is space
               in the option to do so) before passing the option to
               the transport layer or to ICMP for processing.
            o  A timestamp value MUST follow the rules given in
               Section [3.2.2.8] of [INTRO:2].
            IMPLEMENTATION:
               To maximize the utility of the timestamps contained in
               the timestamp option, it is suggested that the
               timestamp inserted be, as nearly as practical, the time
               at which the packet arrived at the router.  For
               datagrams originated by the router, the timestamp
               inserted should be, as nearly as practical, the time at
               which the datagram was passed to the Link Layer for

Almquist & Kastenholz [Page 39] RFC 1716 Towards Requirements for IP Routers November 1994

               transmission.

4.2.2.2 Addresses in Options: RFC-791 Section 3.1

       When a router inserts its address into a Record Route, Strict
       Source and Record Route, Loose Source and Record Route, or
       Timestamp, it MUST use the IP address of the logical interface
       on which the packet is being sent.  Where this rule cannot be
       obeyed because the output interface has no IP address (i.e., is
       an unnumbered interface), the router MUST instead insert its
       router-id.  The router's router-id is one of the router's IP
       addresses.  Which of the router's addresses is used as the
       router-id MUST NOT change (even across reboots) unless changed
       by the network manager or unless the configuration of the
       router is changed such that the IP address used as the router-
       id ceases to be one of the router's IP addresses.  Routers with
       multiple unnumbered interfaces MAY have multiple router-id's.
       Each unnumbered interface MUST be associated with a particular
       router-id.  This association MUST NOT change (even across
       reboots) without reconfiguration of the router.
       DISCUSSION:
          This specification does not allow for routers which do not
          have at least one IP address.  We do not view this as a
          serious limitation, since a router needs an IP address to
          meet the manageability requirements of Chapter [8] even if
          the router is connected only to point-to-point links.
       IMPLEMENTATION:
          One possible method of choosing the router-id that fulfills
          this requirement is to use the numerically smallest (or
          greatest) IP address (treating the address as a 32-bit
          integer) that is assigned to the router.

4.2.2.3 Unused IP Header Bits: RFC-791 Section 3.1

       The IP header contains two reserved bits: one in the Type of
       Service byte and the other in the Flags field.  A router MUST
       NOT set either of these bits to one in datagrams originated by
       the router.  A router MUST NOT drop (refuse to receive or
       forward) a packet merely because one or more of these reserved
       bits has a non-zero value.

Almquist & Kastenholz [Page 40] RFC 1716 Towards Requirements for IP Routers November 1994

       DISCUSSION:
          Future revisions to the IP protocol may make use of these
          unused bits.  These rules are intended to ensure that these
          revisions can be deployed without having to simultaneously
          upgrade all routers in the Internet.

4.2.2.4 Type of Service: RFC-791 Section 3.1

       The Type-of-Service byte in the IP header is divided into three
       sections:  the Precedence field (high-order 3 bits), a field
       that is customarily called Type of Service or TOS (next 4
       bits), and a reserved bit (the low order bit).
       Rules governing the reserved bit were described in Section
       [4.2.2.3].
       A more extensive discussion of the TOS field and its use can be
       found in [ROUTE:11].
       The description of the IP Precedence field is superseded by
       Section [5.3.3].  RFC-795, Service Mappings, is obsolete and
       SHOULD NOT be implemented.

4.2.2.5 Header Checksum: RFC-791 Section 3.1

       As stated in Section [5.2.2], a router MUST verify the IP
       checksum of any packet which is received.  The router MUST NOT
       provide a means to disable this checksum verification.
       IMPLEMENTATION:
          A more extensive description of the IP checksum, including
          extensive implementation hints, can be found in [INTERNET:6]
          and [INTERNET:7].

4.2.2.6 Unrecognized Header Options: RFC-791 Section 3.1

       A router MUST ignore IP options which it does not recognize.  A
       corollary of this requirement is that a router MUST implement
       the End of Option List option and the No Operation option,
       since neither contains an explicit length.

Almquist & Kastenholz [Page 41] RFC 1716 Towards Requirements for IP Routers November 1994

       DISCUSSION:
          All future IP options will include an explicit length.

4.2.2.7 Fragmentation: RFC-791 Section 3.2

       Fragmentation, as described in [INTERNET:1], MUST be supported
       by a router.
       When a router fragments an IP datagram, it SHOULD minimize the
       number of fragments.  When a router fragments an IP datagram,
       it MUST send the fragments in order.  A fragmentation method
       which may generate one IP fragment which is significantly
       smaller than the other MAY cause the first IP fragment to be
       the smaller one.
       DISCUSSION:
          There are several fragmentation techniques in common use in
          the Internet.  One involves splitting the IP datagram into
          IP fragments with the first being MTU sized, and the others
          being approximately the same size, smaller than the MTU.
          The reason for this is twofold.  The first IP fragment in
          the sequence will be the effective MTU of the current path
          between the hosts, and the following IP fragments are sized
          to hopefully minimize the further fragmentation of the IP
          datagram.  Another technique is to split the IP datagram
          into MTU sized IP fragments, with the last fragment being
          the only one smaller, as per page 26 of [INTERNET:1].
          A common trick used by some implementations of TCP/IP is to
          fragment an IP datagram into IP fragments that are no larger
          than 576 bytes when the IP datagram is to travel through a
          router.  In general, this allows the resulting IP fragments
          to pass the rest of the path without further fragmentation.
          This would, though, create more of a load on the destination
          host, since it would have a larger number of IP fragments to
          reassemble into one IP datagram.  It would also not be
          efficient on networks where the MTU only changes once, and
          stays much larger than 576 bytes (such as an 802.5 network
          with a MTU of 2048 or an Ethernet network with an MTU of
          1536).
          One other fragmentation technique discussed was splitting
          the IP datagram into approximately equal sized IP fragments,
          with the size being smaller than the next hop network's MTU.
          This is intended to minimize the number of fragments that
          would result from additional fragmentation further down the

Almquist & Kastenholz [Page 42] RFC 1716 Towards Requirements for IP Routers November 1994

          path.
          In most cases, routers should try and create situations that
          will generate the lowest number of IP fragments possible.
          Work with slow machines leads us to believe that if it is
          necessary to send small packets in a fragmentation scheme,
          sending the small IP fragment first maximizes the chance of
          a host with a slow interface of receiving all the fragments.

4.2.2.8 Reassembly: RFC-791 Section 3.2

       As specified in Section 3.3.2 of [INTRO:2], a router MUST
       support reassembly of datagrams which it delivers to itself.

4.2.2.9 Time to Live: RFC-791 Section 3.2

       Time to Live (TTL) handling for packets originated or received
       by the router is governed by [INTRO:2].  Note in particular
       that a router MUST NOT check the TTL of a packet except when
       forwarding it.

4.2.2.10 Multi-subnet Broadcasts: RFC-922

       All-subnets broadcasts (called multi-subnet broadcasts in
       [INTERNET:3]) have been deprecated.  See Section [5.3.5.3].

4.2.2.11 Addressing: RFC-791 Section 3.2

       There are now five classes of IP addresses: Class A through
       Class E.  Class D addresses are used for IP multicasting
       [INTERNET:4], while Class E addresses are reserved for
       experimental use.
       A multicast (Class D) address is a 28-bit logical address that
       stands for a group of hosts, and may be either permanent or
       transient.  Permanent multicast addresses are allocated by the
       Internet Assigned Number Authority [INTRO:7], while transient
       addresses may be allocated dynamically to transient groups.
       Group membership is determined dynamically using IGMP
       [INTERNET:4].
       We now summarize the important special cases for Unicast (that
       is class A, B, and C) IP addresses, using the following
       notation for an IP address:

Almquist & Kastenholz [Page 43] RFC 1716 Towards Requirements for IP Routers November 1994

          { <Network-number>, <Host-number> }
       or
          { <Network-number>, <Subnet-number>, <Host-number> }
       and the notation -1 for a field that contains all 1 bits and
       the notation 0 for a field that contains all 0 bits.  This
       notation is not intended to imply that the 1-bits in a subnet
       mask need be contiguous.
       (a)  { 0, 0 }
            This host on this network.  It MUST NOT be used as a
            source address by routers, except the router MAY use this
            as a source address as part of an initialization procedure
            (e.g., if the router is using BOOTP to load its
            configuration information).
            Incoming datagrams with a source address of { 0, 0 } which
            are received for local delivery (see Section [5.2.3]),
            MUST be accepted if the router implements the associated
            protocol and that protocol clearly defines appropriate
            action to be taken.  Otherwise, a router MUST silently
            discard any locally-delivered datagram whose source
            address is { 0, 0 }.
            DISCUSSION:
               Some protocols define specific actions to take in
               response to a received datagram whose source address is
               { 0, 0 }.  Two examples are BOOTP and ICMP Mask
               Request.  The proper operation of these protocols often
               depends on the ability to receive datagrams whose
               source address is { 0, 0 }.  For most protocols,
               however, it is best to ignore datagrams having a source
               address of { 0, 0 } since they were probably generated
               by a misconfigured host or router.  Thus, if a router
               knows how to deal with a given datagram having a { 0, 0
               } source address, the router MUST accept it.
               Otherwise, the router MUST discard it.
            See also Section [4.2.3.1] for a non-standard use of { 0,
            0 }.
       (b)  { 0, <Host-number> }
            Specified host on this network.  It MUST NOT be sent by

Almquist & Kastenholz [Page 44] RFC 1716 Towards Requirements for IP Routers November 1994

            routers except that the router MAY uses this as a source
            address as part of an initialization procedure by which
            the it learns its own IP address.
       (c)  { -1, -1 }
            Limited broadcast.  It MUST NOT be used as a source
            address.
            A datagram with this destination address will be received
            by every host and router on the connected physical
            network, but will not be forwarded outside that network.
       (d)  { <Network-number>, -1 }
            Network Directed Broadcast - a broadcast directed to the
            specified network.  It MUST NOT be used as a source
            address.  A router MAY originate Network Directed
            Broadcast packets.  A router MUST receive Network Directed
            Broadcast packets; however a router MAY have a
            configuration option to prevent reception of these
            packets.  Such an option MUST default to allowing
            reception.
       (e)  { <Network-number>, <Subnet-number>, -1 }
            Subnetwork Directed Broadcast - a broadcast sent to the
            specified subnet.  It MUST NOT be used as a source
            address.  A router MAY originate Network Directed
            Broadcast packets.  A router MUST receive Network Directed
            Broadcast packets; however a router MAY have a
            configuration option to prevent reception of these
            packets.  Such an option MUST default to allowing
            reception.
       (f)  { <Network-number>, -1, -1 }
            All Subnets Directed Broadcast - a broadcast sent to all
            subnets of the specified subnetted network.  It MUST NOT
            be used as a source address.  A router MAY originate
            Network Directed Broadcast packets.  A router MUST receive
            Network Directed Broadcast packets; however a router MAY
            have a configuration option to prevent reception of these
            packets.  Such an option MUST default to allowing
            reception.

Almquist & Kastenholz [Page 45] RFC 1716 Towards Requirements for IP Routers November 1994

       (g)  { 127, <any> }
            Internal host loopback address.  Addresses of this form
            MUST NOT appear outside a host.
       The <Network-number> is administratively assigned so that its
       value will be unique in the entire world.
       IP addresses are not permitted to have the value 0 or -1 for
       any of the <Host-number>, <Network-number>, or <Subnet-number>
       fields (except in the special cases listed above).  This
       implies that each of these fields will be at least two bits
       long.
       For further discussion of broadcast addresses, see Section
       [4.2.3.1].
       Since (as described in Section [4.2.1]) a router must support
       the subnet extensions to IP, there will be a subnet mask of the
       form: { -1, -1, 0 } associated with each of the host's local IP
       addresses; see Sections [4.3.3.9], [5.2.4.2], and [10.2.2].
       When a router originates any datagram, the IP source address
       MUST be one of its own IP addresses (but not a broadcast or
       multicast address).  The only exception is during
       initialization.
       For most purposes, a datagram addressed to a broadcast or
       multicast destination is processed as if it had been addressed
       to one of the router's IP addresses; that is to say:
       o  A router MUST receive and process normally any packets with
          a broadcast destination address.
       o  A router MUST receive and process normally any packets sent
          to a multicast destination address which the router is
          interested in.
       The term specific-destination address means the equivalent
       local IP address of the host.  The specific-destination address
       is defined to be the destination address in the IP header
       unless the header contains a broadcast or multicast address, in
       which case the specific-destination is an IP address assigned
       to the physical interface on which the datagram arrived.
       A router MUST silently discard any received datagram containing
       an IP source address that is invalid by the rules of this

Almquist & Kastenholz [Page 46] RFC 1716 Towards Requirements for IP Routers November 1994

       section.  This validation could be done either by the IP layer
       or by each protocol in the transport layer.
       DISCUSSION:
          A misaddressed datagram might be caused by a Link Layer
          broadcast of a unicast datagram or by another router or host
          that is confused or misconfigured.

4.2.3 SPECIFIC ISSUES

4.2.3.1 IP Broadcast Addresses

       For historical reasons, there are a number of IP addresses
       (some standard and some not) which are used to indicate that an
       IP packet is an IP broadcast.  A router
       (1)  MUST treat as IP broadcasts packets addressed to
            255.255.255.255, { <Network-number>, -1 }, { <Network-
            number>, <Subnet-number>, -1 }, and { <Network-number>,
            -1, -1 }.
       (2)  SHOULD silently discard on receipt (i.e., don't even
            deliver to applications in the router) any packet
            addressed to 0.0.0.0, { <Network-number>, 0 }, {
            <Network-number>, <Subnet-number>, 0 }, or { <Network-
            number>, 0, 0 }; if these packets are not silently
            discarded, they MUST be treated as IP broadcasts (see
            Section [5.3.5]).  There MAY be a configuration option to
            allow receipt of these packets.  This option SHOULD
            default to discarding them.
       (3)  SHOULD (by default) use the limited broadcast address
            (255.255.255.255) when originating an IP broadcast
            destined for a connected network or subnet (except when
            sending an ICMP Address Mask Reply, as discussed in
            Section [4.3.3.9]).  A router MUST receive limited
            broadcasts.
       (4)  SHOULD NOT originate datagrams addressed to 0.0.0.0, {
            <Network-number>, 0 }, { <Network-number>, <Subnet-
            number>, 0 }, or { <Network-number>, 0, 0 }.  There MAY be
            a configuration option to allow generation of these
            packets (instead of using the relevant 1s format
            broadcast).  This option SHOULD default to not generating
            them.

Almquist & Kastenholz [Page 47] RFC 1716 Towards Requirements for IP Routers November 1994

       DISCUSSION:
          In the second bullet, the router obviously cannot recognize
          addresses of the form { <Network-number>, <Subnet-number>, 0
          } if the router does not know how the particular network is
          subnetted.  In that case, the rules of the second bullet do
          not apply because, from the point of view of the router, the
          packet is not an IP broadcast packet.

4.2.3.2 IP Multicasting

       An IP router SHOULD satisfy the Host Requirements with respect
       to IP multicasting, as specified in Section 3.3.7 of [INTRO:2].
       An IP router SHOULD support local IP multicasting on all
       connected networks for which a mapping from Class D IP
       addresses to link-layer addresses has been specified (see the
       various IP-over-xxx specifications), and on all connected
       point-to-point links.  Support for local IP multicasting
       includes originating multicast datagrams, joining multicast
       groups and receiving multicast datagrams, and leaving multicast
       groups.  This implies support for all of [INTERNET:4] including
       IGMP (see Section [4.4]).
       DISCUSSION:
          Although [INTERNET:4] is entitled Host Extensions for IP
          Multicasting, it applies to all IP systems, both hosts and
          routers.  In particular, since routers may join multicast
          groups, it is correct for them to perform the host part of
          IGMP, reporting their group memberships to any multicast
          routers that may be present on their attached networks
          (whether or not they themselves are multicast routers).
          Some router protocols may specifically require support for
          IP multicasting (e.g., OSPF [ROUTE:1]), or may recommend it
          (e.g., ICMP Router Discovery [INTERNET:13]).

4.2.3.3 Path MTU Discovery

       In order to eliminate fragmentation or minimize it, it is
       desirable to know what is the path MTU along the path from the
       source to destination.  The path MTU is the minimum of the MTUs
       of each hop in the path.  [INTERNET:14] describes a technique
       for dynamically discovering the maximum transmission unit (MTU)
       of an arbitrary internet path.  For a path that passes through
       a router that does not support [INTERNET:14], this technique
       might not discover the correct Path MTU, but it will always

Almquist & Kastenholz [Page 48] RFC 1716 Towards Requirements for IP Routers November 1994

       choose a Path MTU as accurate as, and in many cases more
       accurate than, the Path MTU that would be chosen by older
       techniques or the current practice.
       When a router is originating an IP datagram, it SHOULD use the
       scheme described in [INTERNET:14] to limit the datagram's size.
       If the router's route to the datagram's destination was learned
       from a routing protocol that provides Path MTU information, the
       scheme described in [INTERNET:14] is still used, but the Path
       MTU information from the routing protocol SHOULD be used as the
       initial guess as to the Path MTU and also as an upper bound on
       the Path MTU.

4.2.3.4 Subnetting

       Under certain circumstances, it may be desirable to support
       subnets of a particular network being interconnected only via a
       path which is not part of the subnetted network.  This is known
       as discontiguous subnetwork support.
       Routers MUST support discontiguous subnetworks.
       IMPLEMENTATION:
          In general, a router should not make assumptions about what
          are subnets and what are not, but simply ignore the concept
          of Class in networks, and treat each route as a { network,
          mask }-tuple.
       DISCUSSION:
          The Internet has been growing at a tremendous rate of late.
          This has been placing severe strains on the IP addressing
          technology.  A major factor in this strain is the strict IP
          Address class boundaries.  These make it difficult to
          efficiently size network numbers to their networks and
          aggregate several network numbers into a single route
          advertisement.  By eliminating the strict class boundaries
          of the IP address and treating each route as a {network
          number, mask}-tuple these strains may be greatly reduced.
          The technology for currently doing this is Classless
          Interdomain Routing (CIDR) [INTERNET:15].
       Furthermore, for similar reasons, a subnetted network need not
       have a consistent subnet mask through all parts of the network.
       For example, one subnet may use an 8 bit subnet mask, another
       10 bit, and another 6 bit.  This is known as variable subnet-

Almquist & Kastenholz [Page 49] RFC 1716 Towards Requirements for IP Routers November 1994

       masks.
       Routers MUST support variable subnet-masks.

4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP

4.3.1 INTRODUCTION

    ICMP is an auxiliary protocol, which provides routing, diagnostic
    and and error functionality for IP. It is described in
    [INTERNET:8].  A router MUST support ICMP.
    ICMP messages are grouped in two classes which are discussed in
    the following sections:
    ICMP error messages:
    Destination Unreachable     Section 4.3.3.1
    Redirect                    Section 4.3.3.2
    Source Quench               Section 4.3.3.3
    Time Exceeded               Section 4.3.3.4
    Parameter Problem           Section 4.3.3.5
    ICMP query messages:
    Echo                        Section 4.3.3.6
    Information                 Section 4.3.3.7
    Timestamp                   Section 4.3.3.8
    Address Mask                Section 4.3.3.9
    Router Discovery            Section 4.3.3.10
    General ICMP requirements and discussion are in the next section.

4.3.2 GENERAL ISSUES

4.3.2.1 Unknown Message Types

       If an ICMP message of unknown type is received, it MUST be
       passed to the ICMP user interface (if the router has one) or
       silently discarded (if the router doesn't have one).

Almquist & Kastenholz [Page 50] RFC 1716 Towards Requirements for IP Routers November 1994

4.3.2.2 ICMP Message TTL

       When originating an ICMP message, the router MUST initialize
       the TTL.  The TTL for ICMP responses must not be taken from the
       packet which triggered the response.

4.3.2.3 Original Message Header

       Every ICMP error message includes the Internet header and at
       least the first 8 data bytes of the datagram that triggered the
       error.  More than 8 bytes MAY be sent, but the resulting ICMP
       datagram SHOULD have a length of less than or equal to 576
       bytes.  The returned IP header (and user data) MUST be
       identical to that which was received, except that the router is
       not required to undo any modifications to the IP header that
       are normally performed in forwarding that were performed before
       the error was detected (e.g., decrementing the TTL, updating
       options).  Note that the requirements of Section [4.3.3.5]
       supersede this requirement in some cases (i.e., for a Parameter
       Problem message, if the problem  is in a modified field, the
       router must undo the modification).  See Section [4.3.3.5])

4.3.2.4 ICMP Message Source Address

       Except where this document specifies otherwise, the IP source
       address in an ICMP message originated by the router MUST be one
       of the IP addresses associated with the physical interface over
       which the ICMP message is transmitted.  If the interface has no
       IP addresses associated with it, the router's router-id (see
       Section [5.2.5]) is used instead.

4.3.2.5 TOS and Precedence

       ICMP error messages SHOULD have their TOS bits set to the same
       value as the TOS bits in the packet which provoked the sending
       of the ICMP error message, unless setting them to that value
       would cause the ICMP error message to be immediately discarded
       because it could not be routed to its destination.  Otherwise,
       ICMP error messages MUST be sent with a normal (i.e. zero) TOS.
       An ICMP reply message SHOULD have its TOS bits set to the same
       value as the TOS bits in the ICMP request that provoked the
       reply.
       EDITOR'S COMMENTS:
          The following paragraph originally read:
             ICMP error messages MUST have their IP Precedence field

Almquist & Kastenholz [Page 51] RFC 1716 Towards Requirements for IP Routers November 1994

             set to the same value as the IP Precedence field in the
             packet which provoked the sending of the ICMP error
             message, except that the precedence value MUST be 6
             (INTERNETWORK CONTROL) or 7 (NETWORK CONTROL), SHOULD be
             7, and MAY be settable for the following types of ICMP
             error messages: Unreachable, Redirect, Time Exceeded, and
             Parameter Problem.
          I believe that the following paragraph is equivalent and
          easier for humans to parse (Source Quench is the only other
          ICMP Error message).  Other interpretations of the original
          are sought.
       ICMP Source Quench error messages MUST have their IP Precedence
       field set to the same value as the IP Precedence field in the
       packet which provoked the sending of the ICMP Source Quench
       message.  All other ICMP error messages (Destination
       Unreachable, Redirect, Time Exceeded, and Parameter Problem)
       MUST have their precedence value set to 6 (INTERNETWORK
       CONTROL) or 7 (NETWORK CONTROL), SHOULD be 7.  The IP
       Precedence value for these error messages MAY be settable.
       An ICMP reply message MUST have its IP Precedence field set to
       the same value as the IP Precedence field in the ICMP request
       that provoked the reply.

4.3.2.6 Source Route

       If the packet which provokes the sending of an ICMP error
       message contains a source route option, the ICMP error message
       SHOULD also contain a source route option of the same type
       (strict or loose), created by reversing the portion before the
       pointer of the route recorded in the source route option of the
       original packet UNLESS the ICMP error message is an ICMP
       Parameter Problem complaining about a source route option in
       the original packet.
       DISCUSSION:
          In environments which use the U.S. Department of Defense
          security option (defined in [INTERNET:5]), ICMP messages may
          need to include a security option.  Detailed information on
          this topic should be available from the Defense
          Communications Agency.

Almquist & Kastenholz [Page 52] RFC 1716 Towards Requirements for IP Routers November 1994

4.3.2.7 When Not to Send ICMP Errors

       An ICMP error message MUST NOT be sent as the result of
       receiving:
       o  An ICMP error message, or
       o  A packet which fails the IP header validation tests
          described in Section [5.2.2] (except where that section
          specifically permits the sending of an ICMP error message),
          or
       o  A packet destined to an IP broadcast or IP multicast
          address, or
       o  A packet sent as a Link Layer broadcast or multicast, or
       o  A packet whose source address has a network number of zero
          or is an invalid source address (as defined in Section
          [5.3.7]), or
       o  Any fragment of a datagram other then the first fragment
          (i.e., a packet for which the fragment offset in the IP
          header is nonzero).
       Furthermore, an ICMP error message MUST NOT be sent in any case
       where this memo states that a packet is to be silently
       discarded.
       NOTE:  THESE RESTRICTIONS TAKE PRECEDENCE OVER ANY REQUIREMENT
       ELSEWHERE IN THIS DOCUMENT FOR SENDING ICMP ERROR MESSAGES.
       DISCUSSION:
          These rules aim to prevent the broadcast storms that have
          resulted from routers or hosts returning ICMP error messages
          in response to broadcast packets.  For example, a broadcast
          UDP packet to a non-existent port could trigger a flood of
          ICMP Destination Unreachable datagrams from all devices that
          do not have a client for that destination port.  On a large
          Ethernet, the resulting collisions can render the network
          useless for a second or more.
          Every packet that is broadcast on the connected network
          should have a valid IP broadcast address as its IP
          destination (see Section [5.3.4] and [INTRO:2]).  However,
          some devices violate this rule.  To be certain to detect
          broadcast packets, therefore, routers are required to check

Almquist & Kastenholz [Page 53] RFC 1716 Towards Requirements for IP Routers November 1994

          for a link-layer broadcast as well as an IP-layer address.
       IMPLEMENTATION:
          This requires that the link layer inform the IP layer when a
          link-layer broadcast packet has been received; see Section
          [3.1].

4.3.2.8 Rate Limiting

       A router which sends ICMP Source Quench messages MUST be able
       to limit the rate at which the messages can be generated.  A
       router SHOULD also be able to limit the rate at which it sends
       other sorts of ICMP error messages (Destination Unreachable,
       Redirect, Time Exceeded, Parameter Problem).  The rate limit
       parameters SHOULD be settable as part of the configuration of
       the router.  How the limits are applied (e.g., per router or
       per interface) is left to the implementor's discretion.
       DISCUSSION:
          Two problems for a router sending ICMP error message are:
          (1)  The consumption of bandwidth on the reverse path, and
          (2)  The use of router resources (e.g., memory, CPU time)
          To help solve these problems a router can limit the
          frequency with which it generates ICMP error messages.  For
          similar reasons, a router may limit the frequency at which
          some other sorts of messages, such as ICMP Echo Replies, are
          generated.
       IMPLEMENTATION:
          Various mechanisms have been used or proposed for limiting
          the rate at which ICMP messages are sent:
          (1)  Count-based - for example, send an ICMP error message
               for every N dropped packets overall or per given source
               host.  This mechanism might be appropriate for ICMP
               Source Quench, but probably not for other types of ICMP
               messages.
          (2)  Timer-based - for example, send an ICMP error message
               to a given source host or overall at most once per T
               milliseconds.
          (3)  Bandwidth-based - for example, limit the rate at which

Almquist & Kastenholz [Page 54] RFC 1716 Towards Requirements for IP Routers November 1994

               ICMP messages are sent over a particular interface to
               some fraction of the attached network's bandwidth.

4.3.3 SPECIFIC ISSUES

4.3.3.1 Destination Unreachable

       If a route can not forward a packet because it has no routes at
       all to the destination network specified in the packet then the
       router MUST generate a Destination Unreachable, Code 0 (Network
       Unreachable) ICMP message.  If the router does have routes to
       the destination network specified in the packet but the TOS
       specified for the routes is neither the default TOS (0000) nor
       the TOS of the packet that the router is attempting to route,
       then the router MUST generate a Destination Unreachable, Code
       11 (Network Unreachable for TOS) ICMP message.
       If a packet is to be forwarded to a host on a network that is
       directly connected to the router (i.e., the router is the
       last-hop router) and the router has ascertained that there is
       no path to the destination host then the router MUST generate a
       Destination Unreachable, Code 1 (Host Unreachable) ICMP
       message.  If a packet is to be forwarded to a host that is on a
       network that is directly connected to the router and the router
       cannot forward the packet because because no route to the
       destination has a TOS that is either equal to the TOS requested
       in the packet or is the default TOS (0000) then the router MUST
       generate a Destination Unreachable, Code 12 (Host Unreachable
       for TOS) ICMP message.
       DISCUSSION:
          The intent is that a router generates the "generic"
          host/network unreachable if it has no path at all (including
          default routes) to the destination.  If the router has one
          or more paths to the destination, but none of those paths
          have an acceptable TOS, then the router generates the
          "unreachable for TOS" message.

4.3.3.2 Redirect

       The ICMP Redirect message is generated to inform a host on the
       same subnet that the router used by the host to route certain
       packets should be changed.

Almquist & Kastenholz [Page 55] RFC 1716 Towards Requirements for IP Routers November 1994

       Contrary to section 3.2.2.2 of [INTRO:2], a router MAY ignore
       ICMP Redirects when choosing a path for a packet originated by
       the router if the router is running a routing protocol or if
       forwarding is enabled on the router and on the interface over
       which the packet is being sent.

4.3.3.3 Source Quench

       A router SHOULD NOT originate ICMP Source Quench messages.  As
       specified in Section [4.3.2], a router which does originate
       Source Quench messages MUST be able to limit the rate at which
       they are generated.
       DISCUSSION:
          Research seems to suggest that Source Quench consumes
          network bandwidth but is an ineffective (and unfair)
          antidote to congestion.  See, for example, [INTERNET:9] and
          [INTERNET:10].  Section [5.3.6] discusses the current
          thinking on how routers ought to deal with overload and
          network congestion.
       A router MAY ignore any ICMP Source Quench messages it
       receives.
       DISCUSSION:
          A router itself may receive a Source Quench as the result of
          originating a packet sent to another router or host.  Such
          datagrams might be, e.g., an EGP update sent to another
          router, or a telnet stream sent to a host.  A mechanism has
          been proposed ([INTERNET:11], [INTERNET:12]) to make the IP
          layer respond directly to Source Quench by controlling the
          rate at which packets are sent, however, this proposal is
          currently experimental and not currently recommended.

4.3.3.4 Time Exceeded

       When a router is forwarding a packet and the TTL field of the
       packet is reduced to 0, the requirements of section [5.2.3.8]
       apply.
       When the router is reassembling a packet that is destined for
       the router, it MUST fulfill requirements of [INTRO:2], section
       [3.3.2] apply.
       When the router receives (i.e., is destined for the router) a
       Time Exceeded message, it MUST comply with section 3.2.2.4 of

Almquist & Kastenholz [Page 56] RFC 1716 Towards Requirements for IP Routers November 1994

       [INTRO:2].

4.3.3.5 Parameter Problem

       A router MUST generate a Parameter Problem message for any
       error not specifically covered by another ICMP message.  The IP
       header field or IP option including the byte indicated by the
       pointer field MUST be included unchanged in the IP header
       returned with this ICMP message.  Section [4.3.2] defines an
       exception to this requirement.
       A new variant of the Parameter Problem message was defined in
       [INTRO:2]:
            Code 1 = required option is missing.
       DISCUSSION:
          This variant is currently in use in the military community
          for a missing security option.

4.3.3.6 Echo Request/Reply

       A router MUST implement an ICMP Echo server function that
       receives Echo Requests and sends corresponding Echo Replies.  A
       router MUST be prepared to receive, reassemble and echo an ICMP
       Echo Request datagram at least as large as the maximum of 576
       and the MTUs of all the connected networks.
       The Echo server function MAY choose not to respond to ICMP echo
       requests addressed to IP broadcast or IP multicast addresses.
       A router SHOULD have a configuration option which, if enabled,
       causes the router to silently ignore all ICMP echo requests; if
       provided, this option MUST default to allowing responses.
       DISCUSSION:
          The neutral provision about responding to broadcast and
          multicast Echo Requests results from the conclusions reached
          in section [3.2.2.6] of [INTRO:2].
       As stated in Section [10.3.3], a router MUST also implement an
       user/application-layer interface for sending an Echo Request
       and receiving an Echo Reply, for diagnostic purposes.  All ICMP
       Echo Reply messages MUST be passed to this interface.
       The IP source address in an ICMP Echo Reply MUST be the same as
       the specific-destination address of the corresponding ICMP Echo

Almquist & Kastenholz [Page 57] RFC 1716 Towards Requirements for IP Routers November 1994

       Request message.
       Data received in an ICMP Echo Request MUST be entirely included
       in the resulting Echo Reply.
       If a Record Route and/or Timestamp option is received in an
       ICMP Echo Request, this option (these options) SHOULD be
       updated to include the current router and included in the IP
       header of the Echo Reply message, without truncation.  Thus,
       the recorded route will be for the entire round trip.
       If a Source Route option is received in an ICMP Echo Request,
       the return route MUST be reversed and used as a Source Route
       option for the Echo Reply message.

4.3.3.7 Information Request/Reply

       A router SHOULD NOT originate or respond to these messages.
       DISCUSSION:
          The Information Request/Reply pair was intended to support
          self-configuring systems such as diskless workstations, to
          allow them to discover their IP network numbers at boot
          time.  However, these messages are now obsolete.  The RARP
          and BOOTP protocols provide better mechanisms for a host to
          discover its own IP address.

4.3.3.8 Timestamp and Timestamp Reply

       A router MAY implement Timestamp and Timestamp Reply.  If they
       are implemented then:
       o  The ICMP Timestamp server function MUST return a Timestamp
          Reply to every Timestamp message that is received.  It
          SHOULD be designed for minimum variability in delay.
       o  An ICMP Timestamp Request message to an IP broadcast or IP
          multicast address MAY be silently discarded.
       o  The IP source address in an ICMP Timestamp Reply MUST be the
          same as the specific-destination address of the
          corresponding Timestamp Request message.
       o  If a Source Route option is received in an ICMP Timestamp
          Request, the return route MUST be reversed and used as a
          Source Route option for the Timestamp Reply message.

Almquist & Kastenholz [Page 58] RFC 1716 Towards Requirements for IP Routers November 1994

       o  If a Record Route and/or Timestamp option is received in a
          Timestamp Request, this (these) option(s) SHOULD be updated
          to include the current router and included in the IP header
          of the Timestamp Reply message.
       o  If the router provides an application-layer interface for
          sending Timestamp Request messages then incoming Timestamp
          Reply messages MUST be passed up to the ICMP user interface.
       The preferred form for a timestamp value (the standard value)
       is milliseconds since midnight, Universal Time.  However, it
       may be difficult to provide this value with millisecond
       resolution. For example, many systems use clocks that update
       only at line frequency, 50 or 60 times per second.  Therefore,
       some latitude is allowed in a standard value:
       (a)  A standard value MUST be updated at least 16 times per
            second (i.e., at most the six low-order bits of the value
            may be undefined).
       (b)  The accuracy of a standard value MUST approximate that of
            operator-set CPU clocks, i.e., correct within a few
            minutes.
       IMPLEMENTATION:
          To meet the second condition, a router may need to query
          some time server when the router is booted or restarted. It
          is recommended that the UDP Time Server Protocol be used for
          this purpose. A more advanced implementation would use the
          Network Time Protocol (NTP) to achieve nearly millisecond
          clock synchronization; however, this is not required.

4.3.3.9 Address Mask Request/Reply

       A router MUST implement support for receiving ICMP Address Mask
       Request messages and responding with ICMP Address Mask Reply
       messages.  These messages are defined in [INTERNET:2].
       A router SHOULD have a configuration option for each logical
       interface specifying whether the router is allowed to answer
       Address Mask Requests for that interface; this option MUST
       default to allowing responses.  A router MUST NOT respond to an
       Address Mask Request before the router knows the correct subnet
       mask.
       A router MUST NOT respond to an Address Mask Request which has

Almquist & Kastenholz [Page 59] RFC 1716 Towards Requirements for IP Routers November 1994

       a source address of 0.0.0.0 and which arrives on a physical
       interface which has associated with it multiple logical
       interfaces and the subnet masks for those interfaces are not
       all the same.
       A router SHOULD examine all ICMP Address Mask Replies which it
       receives to determine whether the information it contains
       matches the router's knowledge of the subnet mask.  If the ICMP
       Address Mask Reply appears to be in error, the router SHOULD
       log the subnet mask and the sender's IP address.  A router MUST
       NOT use the contents of an ICMP Address Mask Reply to determine
       the correct subnet mask.
       Because hosts may not be able to learn the subnet mask if a
       router is down when the host boots up, a router MAY broadcast a
       gratuitous ICMP Address Mask Reply on each of its logical
       interfaces after it has configured its own subnet masks.
       However, this feature can be dangerous in environments which
       use variable length subnet masks.  Therefore, if this feature
       is implemented, gratuitous Address Mask Replies MUST NOT be
       broadcast over any logical interface(s) which either:
       o  Are not configured to send gratuitous Address Mask Replies.
          Each logical interface MUST have a configuration parameter
          controlling this, and that parameter MUST default to not
          sending the gratuitous Address Mask Replies.
       o  Share the same IP network number and physical interface but
          have different subnet masks.
       The { <Network-number>, -1, -1 } form (on subnetted networks)
       or the { <Network-number>, -1 } form (on non-subnetted
       networks) of the IP broadcast address MUST be used for
       broadcast Address Mask Replies.
       DISCUSSION:
          The ability to disable sending Address Mask Replies by
          routers is required at a few sites which intentionally lie
          to their hosts about the subnet mask.  The need for this is
          expected to go away as more and more hosts become compliant
          with the Host Requirements standards.
          The reason for both the second bullet above and the
          requirement about which IP broadcast address to use is to
          prevent problems when multiple IP networks or subnets are in
          use on the same physical network.

Almquist & Kastenholz [Page 60] RFC 1716 Towards Requirements for IP Routers November 1994

4.3.3.10 Router Advertisement and Solicitations

       An IP router MUST support the router part of the ICMP Router
       Discovery Protocol [INTERNET:13] on all connected networks on
       which the router supports either IP multicast or IP broadcast
       addressing.  The implementation MUST include all of the
       configuration variables specified for routers, with the
       specified defaults.
       DISCUSSION:
          Routers are not required to implement the host part of the
          ICMP Router Discovery Protocol, but might find it useful for
          operation while IP forwarding is disabled (i.e., when
          operating as a host).
       DISCUSSION:
          We note that it is quite common for hosts to use RIP as the
          router discovery protocol.  Such hosts listen to RIP traffic
          and use and use information extracted from that traffic to
          discover routers and to make decisions as to which router to
          use as a first-hop router for a given destination.  While
          this behavior is discouraged, it is still common and
          implementors should be aware of it.

4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP

 IGMP [INTERNET:4] is a protocol used between hosts and multicast
 routers on a single physical network to establish hosts' membership
 in particular multicast groups.  Multicast routers use this
 information, in conjunction with a multicast routing protocol, to
 support IP multicast forwarding across the Internet.
 A router SHOULD implement the host part of IGMP.

Almquist & Kastenholz [Page 61] RFC 1716 Towards Requirements for IP Routers November 1994

5. INTERNET LAYER - FORWARDING

5.1 INTRODUCTION

 This section describes the process of forwarding packets.

5.2 FORWARDING WALK-THROUGH

 There is no separate specification of the forwarding function in IP.
 Instead, forwarding is covered by the protocol specifications for the
 internet layer protocols ([INTERNET:1], [INTERNET:2], [INTERNET:3],
 [INTERNET:8], and [ROUTE:11]).

5.2.1 Forwarding Algorithm

    Since none of the primary protocol documents describe the
    forwarding algorithm in any detail, we present it here.  This is
    just a general outline, and omits important details, such as
    handling of congestion, that are dealt with in later sections.
    It is not required that an implementation follow exactly the
    algorithms given in sections [5.2.1.1], [5.2.1.2], and [5.2.1.3].
    Much of the challenge of writing router software is to maximize
    the rate at which the router can forward packets while still
    achieving the same effect of the algorithm.  Details of how to do
    that are beyond the scope of this document, in part because they
    are heavily dependent on the architecture of the router.  Instead,
    we merely point out the order dependencies among the steps:
    (1)  A router MUST verify the IP header, as described in section
         [5.2.2], before performing any actions based on the contents
         of the header.  This allows the router to detect and discard
         bad packets before the expenditure of other resources.
    (2)  Processing of certain IP options requires that the router
         insert its IP address into the option.  As noted in Section
         [5.2.4], the address inserted MUST be the address of the
         logical interface on which the packet is sent or the router's
         router-id if the packet is sent over an unnumbered interface.
         Thus, processing of these options cannot be completed until
         after the output interface is chosen.
    (3)  The router cannot check and decrement the TTL before checking
         whether the packet should be delivered to the router itself,
         for reasons mentioned in Section [4.2.2.9].

Almquist & Kastenholz [Page 62] RFC 1716 Towards Requirements for IP Routers November 1994

    (4)  More generally, when a packet is delivered locally to the
         router, its IP header MUST NOT be modified in any way (except
         that a router may be required to insert a timestamp into any
         Timestamp options in the IP header).  Thus, before the router
         determines whether the packet is to be delivered locally to
         the router, it cannot update the IP header in any way that it
         is not prepared to undo.

5.2.1.1 General

       This section covers the general forwarding algorithm.  This
       algorithm applies to all forms of packets to be forwarded:
       unicast, multicast, and broadcast.
       (1)  The router receives the IP packet (plus additional
            information about it, as described in Section [3.1]) from
            the Link Layer.
       (2)  The router validates the IP header, as described in
            Section [5.2.2].  Note that IP reassembly is not done,
            except on IP fragments to be queued for local delivery in
            step (4).
       (3)  The router performs most of the processing of any IP
            options.  As described in Section [5.2.4], some IP options
            require additional processing after the routing decision
            has been made.
       (4)  The router examines the destination IP address of the IP
            datagram, as described in Section [5.2.3], to determine
            how it should continue to process the IP datagram.  There
            are three possibilities:
            o  The IP datagram is destined for the router, and should
               be queued for local delivery, doing reassembly if
               needed.
            o  The IP datagram is not destined for the router, and
               should be queued for forwarding.
            o  The IP datagram should be queued for forwarding, but (a
               copy) must also be queued for local delivery.

Almquist & Kastenholz [Page 63] RFC 1716 Towards Requirements for IP Routers November 1994

5.2.1.2 Unicast

       Since the local delivery case is well-covered by [INTRO:2], the
       following assumes that the IP datagram was queued for
       forwarding.  If the destination is an IP unicast address:
       (5)  The forwarder determines the next hop IP address for the
            packet, usually by looking up the packet's destination in
            the router's routing table.  This procedure is described
            in more detail in Section [5.2.4].  This procedure also
            decides which network interface should be used to send the
            packet.
       (6)  The forwarder verifies that forwarding the packet is
            permitted.  The source and destination addresses should be
            valid, as described in Section [5.3.7] and Section [5.3.4]
            If the router supports administrative constraints on
            forwarding, such as those described in Section [5.3.9],
            those constraints must be satisfied.
       (7)  The forwarder decrements (by at least one) and checks the
            packet's TTL, as described in Section [5.3.1].
       (8)  The forwarder performs any IP option processing that could
            not be completed in step 3.
       (9)  The forwarder performs any necessary IP fragmentation, as
            described in Section [4.2.2.7].  Since this step occurs
            after outbound interface selection (step 5), all fragments
            of the same datagram will be transmitted out the same
            interface.
       (10) The forwarder determines the Link Layer address of the
            packet's next hop.  The mechanisms for doing this are Link
            Layer-dependent (see chapter 3).
       (11) The forwarder encapsulates the IP datagram (or each of the
            fragments thereof) in an appropriate Link Layer frame and
            queues it for output on the interface selected in step 5.
       (12) The forwarder sends an ICMP redirect if necessary, as
            described in Section [4.3.3.2].

Almquist & Kastenholz [Page 64] RFC 1716 Towards Requirements for IP Routers November 1994

5.2.1.3 Multicast

       If the destination is an IP multicast, the following steps are
       taken.
       Note that the main differences between the forwarding of IP
       unicasts and the forwarding of IP multicasts are
       o  IP multicasts are usually forwarded based on both the
          datagram's source and destination IP addresses,
       o  IP multicast uses an expanding ring search,
       o  IP multicasts are forwarded as Link Level multicasts, and
       o  ICMP errors are never sent in response to IP multicast
          datagrams.
       Note that the forwarding of IP multicasts is still somewhat
       experimental. As a result, the algorithm presented below is not
       mandatory, and is provided as an example only.
       (5a) Based on the IP source and destination addresses found in
            the datagram header, the router determines whether the
            datagram has been received on the proper interface for
            forwarding. If not, the datagram is dropped silently.  The
            method for determining the proper receiving interface
            depends on the multicast routing algorithm(s) in use. In
            one of the simplest algorithms, reverse path forwarding
            (RPF), the proper interface is the one that would be used
            to forward unicasts back to the datagram source.
       (6a) Based on the IP source and destination addresses found in
            the datagram header, the router determines the datagram's
            outgoing interfaces. In order to implement IP multicast's
            expanding ring search (see [INTERNET:4]) a minimum TTL
            value is specified for each outgoing interface. A copy of
            the multicast datagram is forwarded out each outgoing
            interface whose minimum TTL value is less than or equal to
            the TTL value in the datagram header, by separately
            applying the remaining steps on each such interface.
       (7a) The router decrements the packet's TTL by one.
       (8a) The forwarder performs any IP option processing that could
            not be completed in step (3).

Almquist & Kastenholz [Page 65] RFC 1716 Towards Requirements for IP Routers November 1994

       (9a) The forwarder performs any necessary IP fragmentation, as
            described in Section [4.2.2.7].
       (10a) The forwarder determines the Link Layer address to use in
            the Link Level encapsulation. The mechanisms for doing
            this are Link Layer-dependent. On LANs a Link Level
            multicast or broadcast is selected, as an algorithmic
            translation of the datagrams' class D destination address.
            See the various IP-over-xxx specifications for more
            details.
       (11a) The forwarder encapsulates the packet (or each of the
            fragments thereof) in an appropriate Link Layer frame and
            queues it for output on the appropriate interface.

5.2.2 IP Header Validation

    Before a router can process any IP packet, it MUST perform a the
    following basic validity checks on the packet's IP header to
    ensure that the header is meaningful.  If the packet fails any of
    the following tests, it MUST be silently discarded, and the error
    SHOULD be logged.
    (1)  The packet length reported by the Link Layer must be large
         enough to hold the minimum length legal IP datagram (20
         bytes).
    (2)  The IP checksum must be correct.
    (3)  The IP version number must be 4.  If the version number is
         not 4 then the packet may well be another version of IP, such
         as ST-II.
    (4)  The IP header length field must be at least 5.
    (5)  The IP total length field must be at least 4 * IP header
         length field.
    A router MUST NOT have a configuration option which allows
    disabling any of these tests.
    If the packet passes the second and third tests, the IP header
    length field is at least 4, and both the IP total length field and
    the packet length reported by the Link Layer are at least 16 then,
    despite the above rule, the router MAY respond with an ICMP
    Parameter Problem message, whose pointer points at the IP header
    length field (if it failed the fourth test) or the IP total length

Almquist & Kastenholz [Page 66] RFC 1716 Towards Requirements for IP Routers November 1994

    field (if it failed the fifth test).  However, it still MUST
    discard the packet and still SHOULD log the error.
    These rules (and this entire document) apply only to version 4 of
    the Internet Protocol.  These rules should not be construed as
    prohibiting routers from supporting other versions of IP.
    Furthermore, if a router can truly classify a packet as being some
    other version of IP then it ought not treat that packet as an
    error packet within the context of this memo.
    IMPLEMENTATION:
       It is desirable for purposes of error reporting, though not
       always entirely possible, to determine why a header was
       invalid.  There are four possible reasons:
       o  The Link Layer truncated the IP header
       o  The datagram is using a version of IP other than the
          standard one (version 4).
       o  The IP header has been corrupted in transit.
       o  The sender generated an illegal IP header.
       It is probably desirable to perform the checks in the order
       listed, since we believe that this ordering is most likely to
       correctly categorize the cause of the error.  For purposes of
       error reporting, it may also be desirable to check if a packet
       which fails these tests has an IP version number equal to 6.
       If it does, the packet is probably an ST-II datagram and should
       be treated as such.  ST-II is described in [FORWARD:1].
    Additionally, the router SHOULD verify that the packet length
    reported by the Link Layer is at least as large as the IP total
    length recorded in the packet's IP header.  If it appears that the
    packet has been truncated, the packet MUST be discarded, the error
    SHOULD be logged, and the router SHOULD respond with an ICMP
    Parameter Problem message whose pointer points at the IP total
    length field.
    DISCUSSION:
       Because any higher layer protocol which concerns itself with
       data corruption will detect truncation of the packet data when
       it reaches its final destination, it is not absolutely
       necessary for routers to perform the check suggested above in
       order to maintain protocol correctness.  However, by making
       this check a router can simplify considerably the task of

Almquist & Kastenholz [Page 67] RFC 1716 Towards Requirements for IP Routers November 1994

       determining which hop in the path is truncating the packets.
       It will also reduce the expenditure of resources down-stream
       from the router in that down-stream systems will not need to
       deal with the packet.
    Finally, if the destination address in the IP header is not one of
    the addresses of the router, the router SHOULD verify that the
    packet does not contain a Strict Source and Record Route option.
    If a packet fails this test, the router SHOULD log the error and
    SHOULD respond with an ICMP Parameter Problem error with the
    pointer pointing at the offending packet's IP destination address.
    DISCUSSION:
       Some people might suggest that the router should respond with a
       Bad Source Route message instead of a Parameter Problem
       message.  However, when a packet fails this test, it usually
       indicates a protocol error by the previous hop router, whereas
       Bad Source Route would suggest that the source host had
       requested a nonexistent or broken path through the network.

5.2.3 Local Delivery Decision

    When a router receives an IP packet, it must decide whether the
    packet is addressed to the router (and should be delivered
    locally) or the packet is addressed to another system (and should
    be handled by the forwarder).  There is also a hybrid case, where
    certain IP broadcasts and IP multicasts are both delivered locally
    and forwarded.  A router MUST determine which of the these three
    cases applies using the following rules:
    o  An unexpired source route option is one whose pointer value
       does not point past the last entry in the source route.  If the
       packet contains an unexpired source route option, the pointer
       in the option is advanced until either the pointer does point
       past the last address in the option or else the next address is
       not one of the router's own addresses.  In the latter (normal)
       case, the  packet is forwarded (and not delivered locally)
       regardless of the rules below.
    o  The packet is delivered locally and not considered for
       forwarding in the following cases:
  1. The packet's destination address exactly matches one of the

router's IP addresses,

  1. The packet's destination address is a limited broadcast

Almquist & Kastenholz [Page 68] RFC 1716 Towards Requirements for IP Routers November 1994

          address ({-1, -1}), and
  1. The packet's destination is an IP multicast address which is

limited to a single subnet (such as 224.0.0.1 or 224.0.0.2)

          and (at least) one of the logical interfaces associated with
          the physical interface on which the packet arrived is a
          member of the destination multicast group.
    o  The packet is passed to the forwarder AND delivered locally in
       the following cases:
  1. The packet's destination address is an IP broadcast address

that addresses at least one of the router's logical

          interfaces but does not address any of the logical
          interfaces associated with the physical interface on which
          the packet arrived
  1. The packet's destination is an IP multicast address which is

not limited to a single subnetwork (such as 224.0.0.1 and

          224.0.0.2 are) and (at least) one of the logical interfaces
          associated with the physical interface on which the packet
          arrived is a member of the destination multicast group.
    o  The packet is delivered locally if the packet's destination
       address is an IP broadcast address (other than a limited
       broadcast address) that addresses at least one of the logical
       interfaces associated with the physical interface on which the
       packet arrived.  The packet is ALSO passed to the forwarder
       unless the link on which the packet arrived uses an IP
       encapsulation that does not encapsulate broadcasts differently
       than unicasts (e.g. by using different Link Layer destination
       addresses).
    o  The packet is passed to the forwarder in all other cases.
    DISCUSSION:
       The purpose of the requirement in the last sentence of the
       fourth bullet is to deal with a directed broadcast to another
       net or subnet on the same physical cable.  Normally, this works
       as expected: the sender sends the broadcast to the router as a
       Link Layer unicast.  The router notes that it arrived as a
       unicast, and therefore must be destined for a different logical
       net (or subnet) than the sender sent it on.  Therefore, the
       router can safely send it as a Link Layer broadcast out the
       same (physical) interface over which it arrived.  However, if
       the router can't tell whether the packet was received as a Link
       Layer unicast, the sentence ensures that the router does the

Almquist & Kastenholz [Page 69] RFC 1716 Towards Requirements for IP Routers November 1994

       safe but wrong thing rather than the unsafe but right thing.
    IMPLEMENTATION:
       As described in Section [5.3.4], packets received as Link Layer
       broadcasts are generally not forwarded.  It may be advantageous
       to avoid passing to the forwarder packets it would later
       discard because of the rules in that section.
       Some Link Layers (either because of the hardware or because of
       special code in the drivers) can deliver to the router copies
       of all Link Layer broadcasts and multicasts it transmits.  Use
       of this feature can simplify the implementation of cases where
       a packet has to both be passed to the forwarder and delivered
       locally, since forwarding the packet will automatically cause
       the router to receive a copy of the packet that it can then
       deliver locally.  One must use care in these circumstances in
       order to prevent treating a received loop-back packet as a
       normal packet that was received (and then being subject to the
       rules of forwarding, etc etc).
       Even in the absence of such a Link Layer, it is of course
       hardly necessary to make a copy of an entire packet in order to
       queue it both for forwarding and for local delivery, though
       care must be taken with fragments, since reassembly is
       performed on locally delivered packets but not on forwarded
       packets.  One simple scheme is to associate a flag with each
       packet on the router's output queue which indicates whether it
       should be queued for local delivery after it has been sent.

5.2.4 Determining the Next Hop Address

    When a router is going to forward a packet, it must determine
    whether it can send it directly to its destination, or whether it
    needs to pass it through another router.  If the latter, it needs
    to determine which router to use.  This section explains how these
    determinations are made.
    This section makes use of the following definitions:
    o  LSRR - IP Loose Source and Record Route option
    o  SSRR - IP Strict Source and Record Route option
    o  Source Route Option - an LSRR or an SSRR
    o  Ultimate Destination Address - where the packet is being sent

Almquist & Kastenholz [Page 70] RFC 1716 Towards Requirements for IP Routers November 1994

       to: the last address in the source route of a source-routed
       packet, or the destination address in the IP header of a non-
       source-routed packet
    o  Adjacent - reachable without going through any IP routers
    o  Next Hop Address - the IP address of the adjacent host or
       router to which the packet should be sent next
    o  Immediate Destination Address - the ultimate destination
       address, except in source routed packets, where it is the next
       address specified in the source route
    o  Immediate Destination - the node, system, router, end-system,
       or whatever that is addressed by the Immediate Destination
       Address.

5.2.4.1 Immediate Destination Address

       If the destination address in the IP header is one of the
       addresses of the router and the packet contains a Source Route
       Option, the Immediate Destination Address is the address
       pointed at by the pointer in that option if the pointer does
       not point past the end of the option.  Otherwise, the Immediate
       Destination Address is the same as the IP destination address
       in the IP header.
       A router MUST use the Immediate Destination Address, not the
       Ultimate Destination Address, when determining how to handle a
       packet.
       It is an error for more than one source route option to appear
       in a datagram.  If it receives one, it SHOULD discard the
       packet and reply with an ICMP Parameter Problem message whose
       pointer points at the beginning of the second source route
       option.

5.2.4.2 Local/Remote Decision

       After it has been determined that the IP packet needs to be
       forwarded in accordance with the rules specified in Section
       [5.2.3], the following algorithm MUST be used to determine if
       the Immediate Destination is directly accessible (see
       [INTERNET:2]):
       (1)  For each network interface that has not been assigned any
            IP address (the unnumbered lines as described in Section

Almquist & Kastenholz [Page 71] RFC 1716 Towards Requirements for IP Routers November 1994

            [2.2.7]), compare the router-id of the other end of the
            line to the Immediate Destination Address.  If they are
            exactly equal, the packet can be transmitted through this
            interface.
            DISCUSSION:
               In other words, the router or host at the remote end of
               the line is the destination of the packet or is the
               next step in the source route of a source routed
               packet.
       (2)  If no network interface has been selected in the first
            step, for each IP address assigned to the router:
            (a)  Apply the subnet mask associated with the address to
                 this IP address.
                 IMPLEMENTATION:
                    The result of this operation will usually have
                    been computed and saved during initialization.
            (b)  Apply the same subnet mask to the Immediate
                 Destination Address of the packet.
            (c)  Compare the resulting values. If they are equal to
                 each other, the packet can be transmitted through the
                 corresponding network interface.
       (3)  If an interface has still not been selected, the Immediate
            Destination is accessible only through some other router.
            The selection of the router and the next hop IP address is
            described in Section [5.2.4.3].

5.2.4.3 Next Hop Address

       EDITOR'S COMMENTS:
          Note that this section has been extensively rewritten.  The
          original document indicated that Phil Almquist wished to
          revise this section to conform to his "Ruminations on the
          Next Hop" document.  I am under the assumption that the
          working group generally agreed with this goal; there was an
          editor's note from Phil that remained in this document to
          that effect, and the RoNH document contains a "mandatory
          RRWG algorithm".
          So, I have taken said algorithm from RoNH and moved it into
          here.

Almquist & Kastenholz [Page 72] RFC 1716 Towards Requirements for IP Routers November 1994

          Additional useful or interesting information from RoNH has
          been extracted and placed into an appendix to this note.
       The router applies the algorithm in the previous section to
       determine if the Immediate Destination Address is adjacent.  If
       so, the next hop address is the same as the Immediate
       Destination Address.  Otherwise, the packet must be forwarded
       through another router to reach its Immediate Destination.  The
       selection of this router is the topic of this section.
       If the packet contains an SSRR, the router MUST discard the
       packet and reply with an ICMP Bad Source Route error.
       Otherwise, the router looks up the Immediate Destination
       Address in its routing table to determine an appropriate next
       hop address.
       DISCUSSION:
          Per the IP specification, a Strict Source Route must specify
          a sequence of nodes through which the packet must traverse;
          the packet must go from one node of the source route to the
          next, traversing intermediate networks only.  Thus, if the
          router is not adjacent to the next step of the source route,
          the source route can not be fulfilled.  Therefore, the ICMP
          Bad Source Route error.
       The goal of the next-hop selection process is to examine the
       entries in the router's Forwarding Information Base (FIB) and
       select the best route (if there is one) for the packet from
       those available in the FIB.
       Conceptually, any route lookup algorithm starts out with a set
       of candidate routes which consists of the entire contents of
       the FIB.  The algorithm consists of a series of steps which
       discard routes from the set.  These steps are referred to as
       Pruning Rules.  Normally, when the algorithm terminates there
       is exactly one route remaining in the set.  If the set ever
       becomes empty, the packet is discarded because the destination
       is unreachable.  It is also possible for the algorithm to
       terminate when more than one route remains in the set.  In this
       case, the router may arbitrarily discard all but one of them,
       or may perform "load-splitting" by choosing whichever of the
       routes has been least recently used.
       With the exception of rule 3 (Weak TOS), a router MUST use the
       following Pruning Rules when selecting a next hop for a packet.
       If a router does consider TOS when making next-hop decisions,
       the Rule 3 must be applied in the order indicated below.  These

Almquist & Kastenholz [Page 73] RFC 1716 Towards Requirements for IP Routers November 1994

       rules MUST be (conceptually) applied to the FIB in the order
       that they are presented.  (For some historical perspective,
       additional pruning rules, and other common algorithms in use,
       see Appendix E).
       DISCUSSION:
          Rule 3 is optional in that Section [5.3.2] says that a
          router only SHOULD consider TOS when making forwarding
          decisions.
       (1)  Basic Match
            This rule discards any routes to destinations other than
            the Immediate Destination Address of the packet.  For
            example, if a packet's Immediate Destination Address is
            36.144.2.5, this step would discard a route to net
            128.12.0.0 but would retain any routes to net 36.0.0.0,
            any routes to subnet 36.144.0.0, and any default routes.
            More precisely, we assume that each route has a
            destination attribute, called route.dest, and a
            corresponding mask, called route.mask, to specify which
            bits of route.dest are significant.  The Immediate
            Destination Address of the packet being forwarded is
            ip.dest.  This rule discards all routes from the set of
            candidate routes except those for which (route.dest &
            route.mask) = (ip.dest & route.mask).
       (2)  Longest Match
            Longest Match is a refinement of Basic Match, described
            above.  After Basic Match pruning is performed, the
            remaining routes are examined to determine the maximum
            number of bits set in any of their route.mask attributes.
            The step then discards from the set of candidate routes
            any routes which have fewer than that maximum number of
            bits set in their route.mask attributes.
            For example, if a packet's Immediate Destination Address
            is 36.144.2.5 and there are  {route.dest, route.mask}
            pairs of {36.144.2.0, 255.255.255.0}, {36.144.0.5,
            255.255.0.255}, {36.144.0.0, 255.255.0.0}, and {36.0.0.0,
            255.0.0.0}, then this rule would keep only the first two
            pairs; {36.144.2.0, 255.255.255.0} and {36.144.0.5,
            255.255.0.255}.

Almquist & Kastenholz [Page 74] RFC 1716 Towards Requirements for IP Routers November 1994

       (3)  Weak TOS
            Each route has a type of service attribute, called
            route.tos, whose possible values are assumed to be
            identical to those used in the TOS field of the IP header.
            Routing protocols which distribute TOS information fill in
            route.tos appropriately in routes they add to the FIB;
            routes from other routing protocols are treated as if they
            have the default TOS (0000).  The TOS field in the IP
            header of the packet being routed is called ip.tos.
            The set of candidate routes is examined to determine if it
            contains any routes for which route.tos = ip.tos.  If so,
            all routes except those for which route.tos = ip.tos are
            discarded.  If not, all routes except those for which
            route.tos = 0000 are discarded from the set of candidate
            routes.
            Additional discussion of routing based on Weak TOS may be
            found in [ROUTE:11].
            DISCUSSION:
               The effect of this rule is to select only those routes
               which have a TOS that matches the TOS requested in the
               packet.  If no such routes exist then routes with the
               default TOS are considered.  Routes with a non-default
               TOS that is not the TOS requested in the packet are
               never used, even if such routes are the only available
               routes that go to the packet's destination.
       (4)  Best Metric
            Each route has a metric attribute, called route.metric,
            and a routing domain identifier, called route.domain.
            Each member of the set of candidate routes is compared
            with each other member of the set.  If route.domain is
            equal for the two routes and route.metric is strictly
            inferior for one when compared with the other, then the
            one with the inferior metric is discarded from the set.
            The determination of inferior is usually by a simple
            arithmetic comparison, though some protocols may have
            structured metrics requiring more complex comparisons.
       (5)  Vendor Policy
            Vendor Policy is sort of a catch-all to make up for the
            fact that the previously listed rules are often inadequate
            to chose from among the possible routes.  Vendor Policy
            pruning rules are extremely vendor-specific.  See section
            [5.2.4.4].

Almquist & Kastenholz [Page 75] RFC 1716 Towards Requirements for IP Routers November 1994

       This algorithm has two distinct disadvantages.  Presumably, a
       router implementor might develop techniques to deal with these
       disadvantages and make them a part of the Vendor Policy pruning
       rule.
       (1)  IS-IS and OSPF route classes are not directly handled.
       (2)  Path properties other than type of service (e.g. MTU) are
            ignored.
       It is also worth noting a deficiency in the way that TOS is
       supported: routing protocols which support TOS are implicitly
       preferred when forwarding packets which have non-zero TOS
       values.
       The Basic Match and Longest Match pruning rules generalize the
       treatment of a number of particular types of routes.  These
       routes are selected in the following, decreasing, order of
       preference:
       (1)  Host Route: This is a route to a specific end system.
       (2)  Subnetwork Route: This is a route to a particular subnet
            of a network.
       (3)  Default Subnetwork Route: This is a route to all subnets
            of a particular net for which there are not (explicit)
            subnet routes.
       (4)  Network Route: This is a route to a particular network.
       (5)  Default Network Route (also known as the default route):
            This is a route to all networks for which there are no
            explicit routes to the net or any of its subnets.
       If, after application of the pruning rules, the set of routes
       is empty (i.e., no routes were found), the packet MUST be
       discarded and an appropriate ICMP error generated (ICMP Bad
       Source Route if the Immediate Destination Address came from a
       source route option; otherwise, whichever of ICMP Destination
       Host Unreachable or Destination Network Unreachable is
       appropriate, as described in Section [4.3.3.1]).

Almquist & Kastenholz [Page 76] RFC 1716 Towards Requirements for IP Routers November 1994

5.2.4.4 Administrative Preference

       One suggested mechanism for the Vendor Policy Pruning Rule is
       to use administrative preference.
       Each route has associated with it a preference value, based on
       various attributes of the route (specific mechanisms for
       assignment of preference values are suggested below).  This
       preference value is an integer in the range [0..255], with zero
       being the most preferred and 254 being the least preferred.
       255 is a special value that means that the route should never
       be used.  The first step in the Vendor Policy pruning rule
       discards all but the most preferable routes (and always
       discards routes whose preference value is 255).
       This policy is not safe in that it can easily be misused to
       create routing loops.  Since no protocol ensures that the
       preferences configured for a router are consistent with the
       preferences configured in its neighbors, network managers must
       exercise care in configuring preferences.
       o  Address Match
          It is useful to be able to assign a single preference value
          to all routes (learned from the same routing domain) to any
          of a specified set of destinations, where the set of
          destinations is all destinations that match a specified
          address/mask pair.
       o  Route Class
          For routing protocols which maintain the distinction, it is
          useful to be able to assign a single preference value to all
          routes (learned from the same routing domain) which have a
          particular route class (intra-area, inter-area, external
          with internal metrics, or external with external metrics).
       o  Interface
          It is useful to be able to assign a single preference value
          to all routes (learned from a particular routing domain)
          that would cause packets to be routed out a particular
          logical interface on the router (logical interfaces
          generally map one-to-one onto the router's network
          interfaces, except that any network interface which has
          multiple IP addresses will have multiple logical interfaces
          associated with it).
       o  Source router
          It is useful to be able to assign a single preference value

Almquist & Kastenholz [Page 77] RFC 1716 Towards Requirements for IP Routers November 1994

          to all routes (learned from the same routing domain) which
          were learned from any of a set of routers, where the set of
          routers are those whose updates have a source address which
          match a specified address/mask pair.
       o  Originating AS
          For routing protocols which provide the information, it is
          useful to be able to assign a single preference value to all
          routes (learned from a particular routing domain) which
          originated in another particular routing domain.  For BGP
          routes, the originating AS is the first AS listed in the
          route's AS_PATH attribute.  For OSPF external routes, the
          originating AS may be considered to be the low order 16 bits
          of the route's external route tag if the tag's Automatic bit
          is set and the tag's PathLength is not equal to 3.
       o  External route tag
          It is useful to be able to assign a single preference value
          to all OSPF external routes (learned from the same routing
          domain) whose external route tags match any of a list of
          specified values.  Because the external route tag may
          contain a structured value, it may be useful to provide the
          ability to match particular subfields of the tag.
       o  AS path
          It may be useful to be able to assign a single preference
          value to all BGP routes (learned from the same routing
          domain) whose AS path "matches" any of a set of specified
          values.  It is not yet clear exactly what kinds of matches
          are most useful.  A simple option would be to allow matching
          of all routes for which a particular AS number appears (or
          alternatively, does not appear) anywhere in the route's
          AS_PATH attribute.  A more general but somewhat more
          difficult alternative would be to allow matching all routes
          for which the AS path matches a specified regular
          expression.

5.2.4.6 Load Splitting

       At the end of the Next-hop selection process, multiple routes
       may still remain.  A router has several options when this
       occurs.  It may arbitrarily discard some of the routes.  It may
       reduce the number of candidate routes by comparing metrics of
       routes from routing domains which are not considered
       equivalent.  It may retain more than one route and employ a
       load-splitting mechanism to divide traffic among them.  Perhaps
       the only thing that can be said about the relative merits of

Almquist & Kastenholz [Page 78] RFC 1716 Towards Requirements for IP Routers November 1994

       the options is that load-splitting is useful in some situations
       but not in others, so a wise implementor who implements load-
       splitting will also provide a way for the network manager to
       disable it.

5.2.5 Unused IP Header Bits: RFC-791 Section 3.1

    The IP header contains several reserved bits, in the Type of
    Service field and in the Flags field.  Routers MUST NOT drop
    packets merely because one or more of these reserved bits has a
    non-zero value.
    Routers MUST ignore and MUST pass through unchanged the values of
    these reserved bits.  If a router fragments a packet, it MUST copy
    these bits into each fragment.
    DISCUSSION:
       Future revisions to the IP protocol may make use of these
       unused bits.  These rules are intended to ensure that these
       revisions can be deployed without having to simultaneously
       upgrade all routers in the Internet.

5.2.6 Fragmentation and Reassembly: RFC-791 Section 3.2

    As was discussed in Section [4.2.2.7], a router MUST support IP
    fragmentation.
    A router MUST NOT reassemble any datagram before forwarding it.
    DISCUSSION:
       A few people have suggested that there might be some topologies
       where reassembly of transit datagrams by routers might improve
       performance.  In general, however, the fact that fragments may
       take different paths to the destination precludes safe use of
       such a feature.
       Nothing in this section should be construed to control or limit
       fragmentation or reassembly performed as a link layer function
       by the router.

Almquist & Kastenholz [Page 79] RFC 1716 Towards Requirements for IP Routers November 1994

5.2.7 Internet Control Message Protocol - ICMP

    General requirements for ICMP were discussed in Section [4.3].
    This section discusses ICMP messages which are sent only by
    routers.

5.2.7.1 Destination Unreachable

       The ICMP Destination Unreachable message is sent by a router in
       response to a packet which it cannot forward because the
       destination (or next hop) is unreachable or a service is
       unavailable
       A router MUST be able to generate ICMP Destination Unreachable
       messages and SHOULD choose a response code that most closely
       matches the reason why the message is being generated.
       The following codes are defined in [INTERNET:8] and [INTRO:2]:
       0 =  Network Unreachable - generated by a router if a
            forwarding path (route) to the destination network is not
            available;
       1 =  Host Unreachable - generated by a router if a forwarding
            path (route) to the destination host on a directly
            connected network is not available;
       2 =  Protocol Unreachable - generated if the transport protocol
            designated in a datagram is not supported in the transport
            layer of the final destination;
       3 =  Port Unreachable -  generated if the designated transport
            protocol (e.g. UDP) is unable to demultiplex the datagram
            in the transport layer of the final destination but has no
            protocol mechanism to inform the sender;
       4 =  Fragmentation Needed and DF Set - generated if a router
            needs to fragment a datagram but cannot since the DF flag
            is set;
       5 =  Source Route Failed - generated if a router cannot forward
            a packet to the next hop in a source route option;
       6 =  Destination Network Unknown - This code SHOULD NOT be
            generated since it would imply on the part of the router
            that the destination network does not exist (net
            unreachable code 0 SHOULD be used in place of code 6);

Almquist & Kastenholz [Page 80] RFC 1716 Towards Requirements for IP Routers November 1994

       7 =  Destination Host Unknown - generated only when a router
            can determine (from link layer advice) that the
            destination host does not exist;
       11 = Network Unreachable For Type Of Service - generated by a
            router if a forwarding path (route) to the destination
            network with the requested or default TOS is not
            available;
       12 = Host Unreachable For Type Of Service - generated if a
            router cannot forward a packet because its route(s) to the
            destination do not match either the TOS requested in the
            datagram or the default TOS (0).
       The following additional codes are hereby defined:
       13 = Communication Administratively Prohibited - generated if a
            router cannot forward a packet due to administrative
            filtering;
       14 = Host Precedence Violation.  Sent by the first hop router
            to a host to indicate that a requested precedence is not
            permitted for the particular combination of
            source/destination host or network, upper layer protocol,
            and source/destination port;
       15 = Precedence cutoff in effect.  The network operators have
            imposed a minimum level of precedence required for
            operation, the datagram was sent with a precedence below
            this level;
       NOTE: [INTRO:2] defined Code 8 for source host isolated.
       Routers SHOULD NOT generate Code 8; whichever of Codes 0
       (Network Unreachable) and 1 (Host Unreachable) is appropriate
       SHOULD be used instead.  [INTRO:2] also defined Code 9 for
       communication with destination network administratively
       prohibited and Code 10 for communication with destination host
       administratively prohibited.  These codes were intended for use
       by end-to-end encryption devices used by U.S military agencies.
       Routers SHOULD use the newly defined Code 13 (Communication
       Administratively Prohibited) if they administratively filter
       packets.
       Routers MAY have a configuration option that causes Code 13
       (Communication Administratively Prohibited) messages not to be
       generated.  When this option is enabled, no ICMP error message
       is sent in response to a packet which is dropped because its

Almquist & Kastenholz [Page 81] RFC 1716 Towards Requirements for IP Routers November 1994

       forwarding is administratively prohibited.
       Similarly, routers MAY have a configuration option that causes
       Code 14 (Host Precedence Violation) and Code 15 (Precedence
       Cutoff in Effect) messages not to be generated.  When this
       option is enabled, no ICMP error message is sent in response to
       a packet which is dropped  because of a precedence violation.
       Routers MUST use Host Unreachable or Destination Host Unknown
       codes whenever other hosts on the same destination network
       might be reachable; otherwise, the source host may erroneously
       conclude that all hosts on the network are unreachable, and
       that may not be the case.
       [INTERNET:14] describes a slight modification the form of
       Destination Unreachable messages containing Code 4
       (Fragmentation needed and DF set).  A router MUST use this
       modified form when originating Code 4 Destination Unreachable
       messages.

5.2.7.2 Redirect

       The ICMP Redirect message is generated to inform a host on the
       same subnet that the router used by the host to route certain
       packets should be changed.
       Routers MUST NOT generate the Redirect for Network or Redirect
       for Network and Type of Service messages (Codes 0 and 2)
       specified in [INTERNET:8].  Routers MUST be able to generate
       the Redirect for Host message (Code 1) and SHOULD be able to
       generate the Redirect for Type of Service and Host message
       (Code 3) specified in [INTERNET:8].
       DISCUSSION:
          If the directly-connected network is not subnetted, a router
          can normally generate a network Redirect which applies to
          all hosts on a specified remote network.  Using a network
          rather than a host Redirect may economize slightly on
          network traffic and on host routing table storage.  However,
          the savings are not significant, and subnets create an
          ambiguity about the subnet mask to be used to interpret a
          network Redirect.  In a general subnet environment, it is
          difficult to specify precisely the cases in which network
          Redirects can be used.  Therefore, routers must send only
          host (or host and type of service) Redirects.
       A Code 3 (Redirect for Host and Type of Service) message is

Almquist & Kastenholz [Page 82] RFC 1716 Towards Requirements for IP Routers November 1994

       generated when the packet provoking the redirect has a
       destination for which the path chosen by the router would
       depend (in part) on the TOS requested.
       Routers which can generate Code 3 redirects (Host and Type of
       Service) MUST have a configuration option (which defaults to
       on) to enable Code 1 (Host) redirects to be substituted for
       Code 3 redirects.  A router MUST send a Code 1 Redirect in
       place of a Code 3 Redirect if it has been configured to do so.
       If a router is not able to generate Code 3 Redirects then it
       MUST generate Code 1 Redirects in situations where a Code 3
       Redirect is called for.
       Routers MUST NOT generate a Redirect Message unless all of the
       following conditions are met:
       o  The packet is being forwarded out the same physical
          interface that it was received from,
       o  The IP source address in the packet is on the same Logical
          IP (sub)network as the next-hop IP address, and
       o  The packet does not contain an IP source route option.
       The source address used in the ICMP Redirect MUST belong to the
       same logical (sub)net as the destination address.
       A router using a routing protocol (other than static routes)
       MUST NOT consider paths learned from ICMP Redirects when
       forwarding a packet.  If a router is not using a routing
       protocol, a router MAY have a configuration which, if set,
       allows the router to consider routes learned via ICMP Redirects
       when forwarding packets.
       DISCUSSION:
          ICMP Redirect is a mechanism for routers to convey routing
          information to hosts.  Routers use other mechanisms to learn
          routing information, and therefore have no reason to obey
          redirects.  Believing a redirect which contradicted the
          router's other information would likely create routing
          loops.
          On the other hand, when a router is not acting as a router,
          it MUST comply with the behavior required of a host.

Almquist & Kastenholz [Page 83] RFC 1716 Towards Requirements for IP Routers November 1994

5.2.7.3 Time Exceeded

       A router MUST generate a Time Exceeded message Code 0 (In
       Transit) when it discards a packet due to an expired TTL field.
       A router MAY have a per-interface option to disable origination
       of these messages on that interface, but that option MUST
       default to allowing the messages to be originated.

5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP

    IGMP [INTERNET:4] is a protocol used between hosts and multicast
    routers on a single physical network to establish hosts'
    membership in particular multicast groups.  Multicast routers use
    this information, in conjunction with a multicast routing
    protocol, to support IP multicast forwarding across the Internet.
    A router SHOULD implement the multicast router part of IGMP.

5.3 SPECIFIC ISSUES

5.3.1 Time to Live (TTL)

    The Time-to-Live (TTL) field of the IP header is defined to be a
    timer limiting the lifetime of a datagram.  It is an 8-bit field
    and the units are seconds.  Each router (or other module) that
    handles a packet MUST decrement the TTL by at least one, even if
    the elapsed time was much less than a second.  Since this is very
    often the case, the TTL is effectively a hop count limit on how
    far a datagram can propagate through the Internet.
    When a router forwards a packet, it MUST reduce the TTL by at
    least one.  If it holds a packet for more than one second, it MAY
    decrement the TTL by one for each second.
    If the TTL is reduced to zero (or less), the packet MUST be
    discarded, and if the destination is not a multicast address the
    router MUST send an ICMP Time Exceeded message, Code 0 (TTL
    Exceeded in Transit) message to the source.  Note that a router
    MUST NOT discard an IP unicast or broadcast packet with a non-zero
    TTL merely because it can predict that another router on the path
    to the packet's final destination will decrement the TTL to zero.
    However, a router MAY do so for IP multicasts, in order to more
    efficiently implement IP multicast's expanding ring search
    algorithm (see [INTERNET:4]).

Almquist & Kastenholz [Page 84] RFC 1716 Towards Requirements for IP Routers November 1994

    DISCUSSION:
       The IP TTL is used, somewhat schizophrenically, as both a hop
       count limit and a time limit.  Its hop count function is
       critical to ensuring that routing problems can't melt down the
       network by causing packets to loop infinitely in the network.
       The time limit function is used by transport protocols such as
       TCP to ensure reliable data transfer.  Many current
       implementations treat TTL as a pure hop count, and in parts of
       the Internet community there is a strong sentiment that the
       time limit function should instead be performed by the
       transport protocols that need it.
       In this specification, we have reluctantly decided to follow
       the strong belief among the router vendors that the time limit
       function should be optional.  They argued that implementation
       of the time limit function is difficult enough that it is
       currently not generally done.  They further pointed to the lack
       of documented cases where this shortcut has caused TCP to
       corrupt data (of course, we would expect the problems created
       to be rare and difficult to reproduce, so the lack of
       documented cases provides little reassurance that there haven't
       been a number of undocumented cases).
       IP multicast notions such as the expanding ring search may not
       work as expected unless the TTL is treated as a pure hop count.
       The same thing is somewhat true of traceroute.
       ICMP Time Exceeded messages are required because the traceroute
       diagnostic tool depends on them.
       Thus, the tradeoff is between severely crippling, if not
       eliminating, two very useful tools vs. a very rare and
       transient data transport problem (which may not occur at all).

5.3.2 Type of Service (TOS)

    The Type-of-Service byte in the IP header is divided into three
    sections:  the Precedence field (high-order 3 bits), a field that
    is customarily called Type of Service or "TOS (next 4 bits), and a
    reserved bit (the low order bit).  Rules governing the reserved
    bit were described in Section [4.2.2.3].  The Precedence field
    will be discussed in Section [5.3.3].  A more extensive discussion
    of the TOS field and its use can be found in [ROUTE:11].
    A router SHOULD consider the TOS field in a packet's IP header
    when deciding how to forward it.  The remainder of this section

Almquist & Kastenholz [Page 85] RFC 1716 Towards Requirements for IP Routers November 1994

    describes the rules that apply to routers that conform to this
    requirement.
    A router MUST maintain a TOS value for each route in its routing
    table.  Routes learned via a routing protocol which does not
    support TOS MUST be assigned a TOS of zero (the default TOS).
    To choose a route to a destination, a router MUST use an algorithm
    equivalent to the following:
    (1)  The router locates in its routing table all available routes
         to the destination (see Section [5.2.4]).
    (2)  If there are none, the router drops the packet because the
         destination is unreachable.  See section [5.2.4].
    (3)  If one or more of those routes have a TOS that exactly
         matches the TOS specified in the packet, the router chooses
         the route with the best metric.
    (4)  Otherwise, the router repeats the above step, except looking
         at routes whose TOS is zero.
    (5)  If no route was chosen above, the router drops the packet
         because the destination is unreachable.  The router returns
         an ICMP Destination Unreachable error specifying the
         appropriate code: either Network Unreachable with Type of
         Service (code 11) or Host Unreachable with Type of Service
         (code 12).
    DISCUSSION:
       Although TOS has been little used in the past, its use by hosts
       is now mandated by the Requirements for Internet Hosts RFCs
       ([INTRO:2] and [INTRO:3]).  Support for TOS in routers may
       become a MUST in the future, but is a SHOULD for now until we
       get more experience with it and can better judge both its
       benefits and its costs.
       Various people have proposed that TOS should affect other
       aspects of the forwarding function.  For example:
       (1)  A router could place packets which have the Low Delay bit
            set ahead of other packets in its output queues.
       (2)  a router is forced to discard packets, it could try to
            avoid discarding those which have the High Reliability bit
            set.

Almquist & Kastenholz [Page 86] RFC 1716 Towards Requirements for IP Routers November 1994

       These ideas have been explored in more detail in [INTERNET:17]
       but we don't yet have enough experience with such schemes to
       make requirements in this area.

5.3.3 IP Precedence

    This section specifies requirements and guidelines for appropriate
    processing of the IP Precedence field in routers.  Precedence is a
    scheme for allocating resources in the network based on the
    relative importance of different traffic flows.  The IP
    specification defines specific values to be used in this field for
    various types of traffic.
    The basic mechanisms for precedence processing in a router are
    preferential resource allocation, including both precedence-
    ordered queue service and precedence-based congestion control, and
    selection of Link Layer priority features.  The router also
    selects the IP precedence for routing, management and control
    traffic it originates.  For a more extensive discussion of IP
    Precedence and its implementation see [FORWARD:6].
    Precedence-ordered queue service, as discussed in this section,
    includes but is not limited to the queue for the forwarding
    process and queues for outgoing links.  It is intended that a
    router supporting precedence should also use the precedence
    indication at whatever points in its processing are concerned with
    allocation of finite resources, such as packet buffers or Link
    Layer connections.  The set of such points is implementation-
    dependent.
    DISCUSSION:
       Although the Precedence field was originally provided for use
       in DOD systems where large traffic surges or major damage to
       the network are viewed as inherent threats, it has useful
       applications for many non-military IP networks.  Although the
       traffic handling capacity of networks has grown greatly in
       recent years, the traffic generating ability of the users has
       also grown, and network overload conditions still occur at
       times.  Since IP-based routing and management protocols have
       become more critical to the successful operation of the
       Internet, overloads present two additional risks to the
       network:
       (1)  High delays may result in routing protocol packets being
            lost.  This may cause the routing protocol to falsely
            deduce a topology change and propagate this false

Almquist & Kastenholz [Page 87] RFC 1716 Towards Requirements for IP Routers November 1994

            information to other routers.  Not only can this cause
            routes to oscillate, but an extra processing burden may be
            placed on other routers.
       (2)  High delays may interfere with the use of network
            management tools to analyze and perhaps correct or relieve
            the problem in the network that caused the overload
            condition to occur.
       Implementation and appropriate use of the Precedence mechanism
       alleviates both of these problems.

5.3.3.1 Precedence-Ordered Queue Service

       Routers SHOULD implement precedence-ordered queue service.
       Precedence-ordered queue service means that when a packet is
       selected for output on a (logical) link, the packet of highest
       precedence that has been queued for that link is sent.  Routers
       that implement precedence-ordered queue service MUST also have
       a configuration option to suppress precedence-ordered queue
       service in the Internet Layer.
       Any router MAY implement other policy-based throughput
       management procedures that result in other than strict
       precedence ordering, but it MUST be configurable to suppress
       them (i.e., use strict ordering).
       As detailed in Section [5.3.6], routers that implement
       precedence-ordered queue service discard low precedence packets
       before discarding high precedence packets for congestion
       control purposes.
       Preemption (interruption of processing or transmission of a
       packet) is not envisioned as a function of the Internet Layer.
       Some protocols at other layers may provide preemption features.

5.3.3.2 Lower Layer Precedence Mappings

       Routers that implement precedence-ordered queueing MUST
       IMPLEMENT, and other routers SHOULD IMPLEMENT, Lower Layer
       Precedence Mapping.
       A router which implements Lower Layer Precedence Mapping:
       o  MUST be able to map IP Precedence to Link Layer priority
          mechanisms for link layers that have such a feature defined.

Almquist & Kastenholz [Page 88] RFC 1716 Towards Requirements for IP Routers November 1994

       o  MUST have a configuration option to select the Link Layer's
          default priority treatment for all IP traffic
       o  SHOULD be able to configure specific nonstandard mappings of
          IP precedence values to Link Layer priority values for each
          interface.
       DISCUSSION:
          Some research questions the workability of the priority
          features of some Link Layer protocols, and some networks may
          have faulty implementations of the link layer priority
          mechanism.  It seems prudent to provide an escape mechanism
          in case such problems show up in a network.
          On the other hand, there are proposals to use novel queueing
          strategies to implement special services such as low-delay
          service.  Special services and queueing strategies to
          support them need further research and experimentation
          before they are put into widespread use in the Internet.
          Since these requirements are intended to encourage (but not
          force) the use of precedence features in the hope of
          providing better Internet service to all users, routers
          supporting precedence-ordered queue service should default
          to maintaining strict precedence ordering regardless of the
          type of service requested.
          Implementors may wish to consider that correct link layer
          mapping of IP precedence is required by DOD policy for
          TCP/IP systems used on DOD networks.

5.3.3.3 Precedence Handling For All Routers

       A router (whether or not it employs precedence-ordered queue
       service):
       (1)  MUST accept and process incoming traffic of all precedence
            levels normally, unless it has been administratively
            configured to do otherwise.
       (2)  MAY implement a validation filter to administratively
            restrict the use of precedence levels by particular
            traffic sources.  If provided, this filter MUST NOT filter
            out or cut off the following sorts of ICMP error messages:
            Destination Unreachable, Redirect, Time Exceeded, and
            Parameter Problem.  If this filter is provided, the
            procedures required for packet filtering by addresses are

Almquist & Kastenholz [Page 89] RFC 1716 Towards Requirements for IP Routers November 1994

            required for this filter also.
            DISCUSSION:
               Precedence filtering should be applicable to specific
               source/destination IP Address pairs, specific
               protocols, specific ports, and so on.
            An ICMP Destination Unreachable message with code 14
            SHOULD be sent when a packet is dropped by the validation
            filter, unless this has been suppressed by configuration
            choice.
       (3)  MAY implement a cutoff function which allows the router to
            be set to refuse or drop traffic with precedence below a
            specified level.  This function may be activated by
            management actions or by some implementation dependent
            heuristics, but there MUST be a configuration option to
            disable any heuristic mechanism that operates without
            human intervention.  An ICMP Destination Unreachable
            message with code 15 SHOULD be sent when a packet is
            dropped by the cutoff function, unless this has been
            suppressed by configuration choice.
            A router MUST NOT refuse to forward datagrams with IP
            precedence of 6 (Internetwork Control) or 7 (Network
            Control) solely due to precedence cutoff.  However, other
            criteria may be used in conjunction with precedence cutoff
            to filter high precedence traffic.
            DISCUSSION:
               Unrestricted precedence cutoff could result in an
               unintentional cutoff of routing and control traffic.
               In general, host traffic should be restricted to a
               value of 5 (CRITIC/ECP) or below although this is not a
               requirement and may not be valid in certain systems.
       (4)  MUST NOT change precedence settings on packets it did not
            originate.
       (5)  SHOULD be able to configure distinct precedence values to
            be used for each routing or management protocol supported
            (except for those protocols, such as OSPF, which specify
            which precedence value must be used).
       (6)  MAY be able to configure routing or management traffic
            precedence values independently for each peer address.

Almquist & Kastenholz [Page 90] RFC 1716 Towards Requirements for IP Routers November 1994

       (7)  MUST respond appropriately to Link Layer precedence-
            related error indications where provided.  An ICMP
            Destination Unreachable message with code 15 SHOULD be
            sent when a packet is dropped because a link cannot accept
            it due to a precedence-related condition, unless this has
            been suppressed by configuration choice.
            DISCUSSION:
               The precedence cutoff mechanism described in (3) is
               somewhat controversial.  Depending on the topological
               location of the area affected by the cutoff, transit
               traffic may be directed by routing protocols into the
               area of the cutoff, where it will be dropped.  This is
               only a problem if another path which is unaffected by
               the cutoff exists between the communicating points.
               Proposed ways of avoiding this problem include
               providing some minimum bandwidth to all precedence
               levels even under overload conditions, or propagating
               cutoff information in routing protocols.  In the
               absence of a widely accepted (and implemented) solution
               to this problem, great caution is recommended in
               activating cutoff mechanisms in transit networks.
               A transport layer relay could legitimately provide the
               function prohibited by (4) above.  Changing precedence
               levels may cause subtle interactions with TCP and
               perhaps other protocols; a correct design is a non-
               trivial task.
               The intent of (5) and (6) (and the discussion of IP
               Precedence in ICMP messages in Section [4.3.2]) is that
               the IP precedence bits should be appropriately set,
               whether or not this router acts upon those bits in any
               other way.  We expect that in the future specifications
               for routing protocols and network management protocols
               will specify how the IP Precedence should be set for
               messages sent by those protocols.
               The appropriate response for (7) depends on the link
               layer protocol in use.  Typically, the router should
               stop trying to send offensive traffic to that
               destination for some period of time, and should return
               an ICMP Destination Unreachable message with code 15
               (service not available for precedence requested) to the
               traffic source.  It also should not try to reestablish
               a preempted Link Layer connection for some period of
               time.

Almquist & Kastenholz [Page 91] RFC 1716 Towards Requirements for IP Routers November 1994

5.3.4 Forwarding of Link Layer Broadcasts

    The encapsulation of IP packets in most Link Layer protocols
    (except PPP) allows a receiver to distinguish broadcasts and
    multicasts from unicasts simply by examining the Link Layer
    protocol headers (most commonly, the Link Layer destination
    address).  The rules in this section which refer to Link Layer
    broadcasts apply only to Link Layer protocols which allow
    broadcasts to be distinguished; likewise, the rules which refer to
    Link Layer multicasts apply only to Link Layer protocols which
    allow multicasts to be distinguished.
    A router MUST NOT forward any packet which the router received as
    a Link Layer broadcast (even if the IP destination address is also
    some form of broadcast address) unless the packet is an all-
    subnets-directed broadcast being forwarded as specified in
    [INTERNET:3].
    DISCUSSION:
       As noted in Section [5.3.5.3], forwarding of all-subnets-
       directed broadcasts in accordance with [INTERNET:3] is optional
       and is not something that routers do by default.
    A router MUST NOT forward any packet which the router received as
    a Link Layer multicast unless the packet's destination address is
    an IP multicast address.
    A router SHOULD silently discard a packet that is received via a
    Link Layer broadcast but does not specify an IP multicast or IP
    broadcast destination address.
    When a router sends a packet as a Link Layer broadcast, the IP
    destination address MUST be a legal IP broadcast or IP multicast
    address.

5.3.5 Forwarding of Internet Layer Broadcasts

    There are two major types of IP broadcast addresses; limited
    broadcast and directed broadcast.  In addition, there are three
    subtypes of directed broadcast; a broadcast directed to a
    specified network, a broadcast directed to a specified subnetwork,
    and a broadcast directed to all subnets of a specified network.
    Classification by a router of a broadcast into one of these
    categories depends on the broadcast address and on the router's
    understanding (if any) of the subnet structure of the destination
    network.  The same broadcast will be classified differently by
    different routers.

Almquist & Kastenholz [Page 92] RFC 1716 Towards Requirements for IP Routers November 1994

    A limited IP broadcast address is defined to be all-ones: { -1, -1
    } or 255.255.255.255.
    A net-directed broadcast is composed of the network portion of the
    IP address with a local part of all-ones, { <Network-number>, -1
    }.  For example, a Class A net broadcast address is
    net.255.255.255, a Class B net broadcast address is
    net.net.255.255 and a Class C net broadcast address is
    net.net.net.255 where net is a byte of the network address.
    An all-subnets-directed broadcast is composed of the network part
    of the IP address with a subnet and a host part of all-ones, {
    <Network-number>, -1, -1 }.  For example, an all-subnets broadcast
    on a subnetted class B network is net.net.255.255.  A network must
    be known to be subnetted and the subnet part must be all-ones
    before a broadcast can be classified as all-subnets-directed.
    A subnet-directed broadcast address is composed of the network and
    subnet part of the IP address with a host part of all-ones, {
    <Network-number>, <Subnet-number>, -1 }.  For example, a subnet-
    directed broadcast to subnet 2 of a class B network might be
    net.net.2.255 (if the subnet mask was 255.255.255.0) or
    net.net.1.127 (if the subnet mask was 255.255.255.128).  A network
    must be known to be subnetted and the net and subnet part must not
    be all-ones before an IP broadcast can be classified as subnet-
    directed.
    As was described in Section [4.2.3.1], a router may encounter
    certain non-standard IP broadcast addresses:
    o  0.0.0.0 is an obsolete form of the limited broadcast address
    o  { broadcast address.
    o  { broadcast address.
    o  { form of a subnet-directed broadcast address.
    As was described in that section, packets addressed to any of
    these addresses SHOULD be silently discarded, but if they are not,
    they MUST be treated in accordance with the same rules that apply
    to packets addressed to the non-obsolete forms of the broadcast
    addresses described above.  These rules are described in the next
    few sections.

Almquist & Kastenholz [Page 93] RFC 1716 Towards Requirements for IP Routers November 1994

5.3.5.1 Limited Broadcasts

       Limited broadcasts MUST NOT be forwarded.  Limited broadcasts
       MUST NOT be discarded.  Limited broadcasts MAY be sent and
       SHOULD be sent instead of directed broadcasts where limited
       broadcasts will suffice.
       DISCUSSION:
          Some routers contain UDP servers which function by resending
          the requests (as unicasts or directed broadcasts) to other
          servers.  This requirement should not be interpreted as
          prohibiting such servers.  Note, however, that such servers
          can easily cause packet looping if misconfigured.  Thus,
          providers of such servers would probably be well-advised to
          document their setup carefully and to consider carefully the
          TTL on packets which are sent.

5.3.5.2 Net-directed Broadcasts

       A router MUST classify as net-directed broadcasts all valid,
       directed broadcasts destined for a remote network or an
       attached nonsubnetted network.  A router MUST forward net-
       directed broadcasts.  Net-directed broadcasts MAY be sent.
       A router MAY have an option to disable receiving net-directed
       broadcasts on an interface and MUST have an option to disable
       forwarding net-directed broadcasts.  These options MUST default
       to permit receiving and forwarding net-directed broadcasts.
       DISCUSSION:
          There has been some debate about forwarding or not
          forwarding directed broadcasts.  In this memo we have made
          the forwarding decision depend on the router's knowledge of
          the subnet mask for the destination network.  Forwarding
          decisions for subnetted networks should be made by routers
          with an understanding of the subnet structure.  Therefore,
          in general, routers must forward directed broadcasts for
          networks they are not attached to and for which they do not
          understand the subnet structure.  One router may interpret
          and handle the same IP broadcast packet differently than
          another, depending on its own understanding of the structure
          of the destination (sub)network.

Almquist & Kastenholz [Page 94] RFC 1716 Towards Requirements for IP Routers November 1994

5.3.5.3 All-subnets-directed Broadcasts

       A router MUST classify as all-subnets-directed broadcasts all
       valid directed broadcasts destined for a directly attached
       subnetted network which have all-ones in the subnet part of the
       address.  If the destination network is not subnetted, the
       broadcast MUST be treated as a net-directed broadcast.
       A router MUST forward an all-subnets-directed broadcast as a
       link level broadcast out all physical interfaces connected to
       the IP network addressed by the broadcast, except that:
       o  A router MUST NOT forward an all-subnet-directed broadcast
          that was received by the router as a Link Layer broadcast,
          unless the router is forwarding the broadcast in accordance
          with [INTERNET:3] (see below).
       o  If a router receives an all-subnets-directed broadcast over
          a network which does not indicate via Link Layer framing
          whether the frame is a broadcast or a unicast, the packet
          MUST NOT be forwarded to any network which likewise does not
          indicate whether a frame is a broadcast.
       o  A router MUST NOT forward an all-subnets-directed broadcast
          if the router is configured not to forward such broadcasts.
          A router MUST have a configuration option to deny forwarding
          of all-subnets-directed broadcasts.  The configuration
          option MUST default to permit forwarding of all-subnets-
          directed broadcasts.
       EDITOR'S COMMENTS:
          The algorithm presented here is broken.  The working group
          explicitly desired this algorithm, knowing its failures.
          The second bullet, above, prevents All Subnets Directed
          Broadcasts from traversing more than one PPP (or other
          serial) link in a row.  Such a topology is easily conceived.
          Suppose that some corporation builds its corporate backbone
          out of PPP links, connecting routers at geographically
          dispersed locations.  Suppose that this corporation has 3
          sites (S1, S2, and S3) and there is a router at each site
          (R1, R2, and R3).  At each site there are also several LANs
          connected to the local router.  Let there be a PPP link
          connecting S1 to S2 and one connecting S2 to S3 (i.e. the
          links are R1-R2 and R2-R3).  So, if a host on a LAN at S1
          sends a All Subnets Directed Broadcast, R1 will forward the
          broadcast over the R1-R2 link to R2.  R2 will forward the

Almquist & Kastenholz [Page 95] RFC 1716 Towards Requirements for IP Routers November 1994

          broadcast to the LAN(s) connected to R2.  Since the PPP does
          not differentiate broadcast from non-broadcast frames, R2
          will NOT forward the broadcast onto the R2-R3 link.
          Therefore, the broadcast will not reach S3.
       [INTERNET:3] describes an alternative set of rules for
       forwarding of all-subnets-directed broadcasts (called multi-
       subnet-broadcasts in that document).  A router MAY IMPLEMENT
       that alternative set of rules, but MUST use the set of rules
       described above unless explicitly configured to use the
       [INTERNET:3] rules.  If routers will do [INTERNET:3]-style
       forwarding, then the router MUST have a configuration option
       which MUST default to doing the rules presented in this
       document.
       DISCUSSION:
          As far as we know, the rules for multi-subnet broadcasts
          described in [INTERNET:3] have never been implemented,
          suggesting that either they are too complex or the utility
          of multi-subnet broadcasts is low.  The rules described in
          this section match current practice.  In the future, we
          expect that IP multicast (see [INTERNET:4]) will be used to
          better solve the sorts of problems that multi-subnets
          broadcasts were intended to address.
          We were also concerned that hosts whose system managers
          neglected to configure with a subnet mask could
          unintentionally send multi-subnet broadcasts.
       A router SHOULD NOT originate all-subnets broadcasts, except as
       required by Section [4.3.3.9] when sending ICMP Address Mask
       Replies on subnetted networks.
       DISCUSSION:
          The current intention is to decree that (like 0-filled IP
          broadcasts) the notion of the all-subnets broadcast is
          obsolete.  It should be treated as a directed broadcast to
          the first subnet of the net in question that it appears on.
          Routers may implement a switch (default off) which if turned
          on enables the [INTERNET:3] behavior for all-subnets
          broadcasts.
          If a router has a configuration option to allow for
          forwarding all-subnet broadcasts, it should use a spanning
          tree, RPF, or other multicast forwarding algorithm (which
          may be computed for other purposes such as bridging or OSPF)

Almquist & Kastenholz [Page 96] RFC 1716 Towards Requirements for IP Routers November 1994

          to distribute the all-subnets broadcast efficiently.  In
          general, it is better to use an IP multicast address rather
          than an all-subnets broadcast.

5.3.5.4 Subnet-directed Broadcasts

       A router MUST classify as subnet-directed broadcasts all valid
       directed broadcasts destined for a directly attached subnetted
       network in which the subnet part is not all-ones.  If the
       destination network is not subnetted, the broadcast MUST be
       treated as a net-directed broadcast.
       A router MUST forward subnet-directed broadcasts.
       A router MUST have a configuration option to prohibit
       forwarding of subnet-directed broadcasts.  Its default setting
       MUST permit forwarding of subnet-directed broadcasts.
       A router MAY have a configuration option to prohibit forwarding
       of subnet-directed broadcasts from a source on a network on
       which the router has an interface.  If such an option is
       provided, its default setting MUST permit forwarding of
       subnet-directed broadcasts.

5.3.6 Congestion Control

    Congestion in a network is loosely defined as a condition where
    demand for resources (usually bandwidth or CPU time) exceeds
    capacity.  Congestion avoidance tries to prevent demand from
    exceeding capacity, while congestion recovery tries to restore an
    operative state.  It is possible for a router to contribute to
    both of these mechanisms.  A great deal of effort has been spent
    studying the problem.  The reader is encouraged to read
    [FORWARD:2] for a survey of the work.  Important papers on the
    subject include [FORWARD:3], [FORWARD:4], [FORWARD:5], and
    [INTERNET:10], among others.
    The amount of storage that router should have available to handle
    peak instantaneous demand when hosts use reasonable congestion
    policies, such as described in [FORWARD:5], is a function of the
    product of the bandwidth of the link times the path delay of the
    flows using the link, and therefore storage should increase as
    this Bandwidth*Delay product increases.  The exact function
    relating storage capacity to probability of discard is not known.
    When a router receives a packet beyond its storage capacity it

Almquist & Kastenholz [Page 97] RFC 1716 Towards Requirements for IP Routers November 1994

    must (by definition, not by decree) discard it or some other
    packet or packets.  Which packet to discard is the subject of much
    study but, unfortunately, little agreement so far.
    A router MAY discard the packet it has just received; this is the
    simplest but not the best policy.  It is considered better policy
    to randomly pick some transit packet on the queue and discard it
    (see [FORWARD:2]).  A router MAY use this Random Drop algorithm to
    determine which packet to discard.
    If a router implements a discard policy (such as Random Drop)
    under which it chooses a packet to discard from among a pool of
    eligible packets:
    o  If precedence-ordered queue service (described in Section
       [5.3.3.1]) is implemented and enabled, the router MUST NOT
       discard a packet whose IP precedence is higher than that of a
       packet which is not discarded.
    o  A router MAY protect packets whose IP headers request the
       maximize reliability TOS, except where doing so would be in
       violation of the previous rule.
    o  A router MAY protect fragmented IP packets, on the theory that
       dropping a fragment of a datagram may increase congestion by
       causing all fragments of the datagram to be retransmitted by
       the source.
    o  To help prevent routing perturbations or disruption of
       management functions, the router MAY protect packets used for
       routing control, link control, or network management from being
       discarded.  Dedicated routers (i.e.. routers which are not also
       general purpose hosts, terminal servers, etc.) can achieve an
       approximation of this rule by protecting packets whose source
       or destination is the router itself.
    Advanced methods of congestion control include a notion of
    fairness, so that the 'user' that is penalized by losing a packet
    is the one that contributed the most to the congestion.  No matter
    what mechanism is implemented to deal with bandwidth congestion
    control, it is important that the CPU effort expended be
    sufficiently small that the router is not driven into CPU
    congestion also.
    As described in Section [4.3.3.3], this document recommends that a
    router should not send a Source Quench to the sender of the packet
    that it is discarding.  ICMP Source Quench is a very weak

Almquist & Kastenholz [Page 98] RFC 1716 Towards Requirements for IP Routers November 1994

    mechanism, so it is not necessary for a router to send it, and
    host software should not use it exclusively as an indicator of
    congestion.

5.3.7 Martian Address Filtering

    An IP source address is invalid if it is an IP broadcast address
    or is not a class A, B, or C address.
    An IP destination address is invalid if it is not a class A, B, C,
    or D address.
    A router SHOULD NOT forward any packet which has an invalid IP
    source address or a source address on network 0.  A router SHOULD
    NOT forward, except over a loopback interface, any packet which
    has a source address on network 127.  A router MAY have a switch
    which allows the network manager to disable these checks.  If such
    a switch is provided, it MUST default to performing the checks.
    A router SHOULD NOT forward any packet which has an invalid IP
    destination address or a destination address on network 0.  A
    router SHOULD NOT forward, except over a loopback interface, any
    packet which has a destination address on network 127.  A router
    MAY have a switch which allows the network manager to disable
    these checks.  If such a switch is provided, it MUST default to
    performing the checks.
    If a router discards a packet because of these rules, it SHOULD
    log at least the IP source address, the IP destination address,
    and, if the problem was with the source address, the physical
    interface on which the packet was received and the Link Layer
    address of the host or router from which the packet was received.

5.3.8 Source Address Validation

    A router SHOULD IMPLEMENT the ability to filter traffic based on a
    comparison of the source address of a packet and the forwarding
    table for a logical interface on which the packet was received.
    If this filtering is enabled, the router MUST silently discard a
    packet if the interface on which the packet was received is not
    the interface on which a packet would be forwarded to reach the
    address contained in the source address.  In simpler terms, if a
    router wouldn't route a packet containing this address through a
    particular interface, it shouldn't believe the address if it
    appears as a source address in a packet read from this interface.
    If this feature is implemented, it MUST be disabled by default.

Almquist & Kastenholz [Page 99] RFC 1716 Towards Requirements for IP Routers November 1994

    DISCUSSION:
       This feature can provide useful security improvements in some
       situations, but can erroneously discard valid packets in
       situations where paths are asymmetric.

5.3.9 Packet Filtering and Access Lists

    As a means of providing security and/or limiting traffic through
    portions of a network a router SHOULD provide the ability to
    selectively forward (or filter) packets.  If this capability is
    provided, filtering of packets MUST be configurable either to
    forward all packets or to selectively forward them based upon the
    source and destination addresses.  Each source and destination
    address SHOULD allow specification of an arbitrary mask.
    If supported, a router MUST be configurable to allow one of an
    o  Include list -  specification of a list of address pairs to be
       forwarded, or an
    o  Exclude list -  specification of a list of address pairs NOT to
       be forwarded.
    A router MAY provide a configuration switch which allows a choice
    between specifying an include or an exclude list.
    A value matching any address (e.g. a keyword any or an address
    with a mask of all 0's) MUST be allowed as a source and/or
    destination address.
    In addition to address pairs, the router MAY allow any combination
    of transport and/or application protocol and source and
    destination ports to be specified.
    The router MUST allow packets to be silently discarded (i.e..
    discarded without an ICMP error message being sent).
    The router SHOULD allow an appropriate ICMP unreachable message to
    be sent when a packet is discarded. The ICMP message SHOULD
    specify Communication Administratively Prohibited (code 13) as the
    reason for the destination being unreachable.
    The router SHOULD allow the sending of ICMP destination
    unreachable messages (code 13) to be configured for each
    combination of address pairs, protocol types, and ports it allows
    to be specified.

Almquist & Kastenholz [Page 100] RFC 1716 Towards Requirements for IP Routers November 1994

    The router SHOULD count and SHOULD allow selective logging of
    packets not forwarded.

5.3.10 Multicast Routing

    An IP router SHOULD support forwarding of IP multicast packets,
    based either on static multicast routes or on routes dynamically
    determined by a multicast routing protocol (e.g., DVMRP
    [ROUTE:9]).  A router that forwards IP multicast packets is called
    a multicast router.

5.3.11 Controls on Forwarding

    For each physical interface, a router SHOULD have a configuration
    option which specifies whether forwarding is enabled on that
    interface.  When forwarding on an interface is disabled, the
    router:
    o  MUST silently discard any packets which are received on that
       interface but are not addressed to the router
    o  MUST NOT send packets out that interface, except for datagrams
       originated by the router
    o  MUST NOT announce via any routing protocols the availability of
       paths through the interface
    DISCUSSION:
       This feature allows the network manager to essentially turn off
       an interface but leaves it accessible for network management.
       Ideally, this control would apply to logical rather than
       physical interfaces, but cannot because there is no known way
       for a router to determine which logical interface a packet
       arrived on when there is not a one-to-one correspondence
       between logical and physical interfaces.

5.3.12 State Changes

    During the course of router operation, interfaces may fail or be
    manually disabled, or may become available for use by the router.
    Similarly, forwarding may be disabled for a particular interface
    or for the entire router or may be (re)enabled.  While such
    transitions are (usually) uncommon, it is important that routers
    handle them correctly.

Almquist & Kastenholz [Page 101] RFC 1716 Towards Requirements for IP Routers November 1994

5.3.12.1 When a Router Ceases Forwarding

       When a router ceases forwarding it MUST stop advertising all
       routes, except for third party routes.  It MAY continue to
       receive and use routes from other routers in its routing
       domains.  If the forwarding database is retained, the router
       MUST NOT cease timing the routes in the forwarding database.
       If routes that have been received from other routers are
       remembered, the router MUST NOT cease timing the routes which
       it has remembered.  It MUST discard any routes whose timers
       expire while forwarding is disabled, just as it would do if
       forwarding were enabled.
       DISCUSSION:
          When a router ceases forwarding, it essentially ceases being
          a router.  It is still a host, and must follow all of the
          requirements of Host Requirements [INTRO: 2].  The router
          may still be a passive member of one or more routing
          domains, however.  As such, it is allowed to maintain its
          forwarding database by listening to other routers in its
          routing domain.  It may not, however, advertise any of the
          routes in its forwarding database, since it itself is doing
          no forwarding.  The only exception to this rule is when the
          router is advertising a route which uses only some other
          router, but which this router has been asked to advertise.
       A router MAY send ICMP destination unreachable (host
       unreachable) messages to the senders of packets that it is
       unable to forward. It SHOULD NOT send ICMP redirect messages.
       DISCUSSION:
          Note that sending an ICMP destination unreachable (host
          unreachable) is a router action.  This message should not be
          sent by hosts.   This exception to the rules for hosts is
          allowed so that packets may be rerouted in the shortest
          possible time, and so that black holes are avoided.

5.3.12.2 When a Router Starts Forwarding

       When a router begins forwarding, it SHOULD expedite the sending
       of new routing information to all routers with which it
       normally exchanges routing information.

Almquist & Kastenholz [Page 102] RFC 1716 Towards Requirements for IP Routers November 1994

5.3.12.3 When an Interface Fails or is Disabled

       If an interface fails or is disabled a router MUST remove and
       stop advertising all routes in its forwarding database which
       make use of that interface.  It MUST disable all static routes
       which make use of that interface.  If other routes to the same
       destination and TOS are learned or remembered by the router,
       the router MUST choose the best alternate, and add it to its
       forwarding database.  The router SHOULD send ICMP destination
       unreachable or ICMP redirect messages, as appropriate, in reply
       to all packets which it is unable to forward due to the
       interface being unavailable.

5.3.12.4 When an Interface is Enabled

       If an interface which had not been available becomes available,
       a router MUST reenable any static routes which use that
       interface.  If routes which would use that interface are
       learned by the router,  then these routes MUST be evaluated
       along with all of the other learned routes, and the router MUST
       make a decision as to which routes should be placed in the
       forwarding database.  The implementor is referred to Chapter
       [7], Application Layer - Routing Protocols for further
       information on how this decision is made.
       A router SHOULD expedite the sending of new routing information
       to all routers with which it normally exchanges routing
       information.

5.3.13 IP Options

    Several options, such as Record Route and Timestamp, contain slots
    into which a router inserts its address when forwarding the
    packet.  However, each such option has a finite number of slots,
    and therefore a router may find that there is not free slot into
    which it can insert its address.  No requirement listed below
    should be construed as requiring a router to insert its address
    into an option that has no remaining slot to insert it into.
    Section [5.2.5] discusses how a router must choose which of its
    addresses to insert into an option.

5.3.13.1 Unrecognized Options

       Unrecognized IP options in forwarded packets MUST be passed
       through unchanged.

Almquist & Kastenholz [Page 103] RFC 1716 Towards Requirements for IP Routers November 1994

5.3.13.2 Security Option

       Some environments require the Security option in every packet;
       such a requirement is outside the scope of this document and
       the IP standard specification.  Note, however, that the
       security options described in [INTERNET:1] and [INTERNET:16]
       are obsolete.  Routers SHOULD IMPLEMENT the revised security
       option described in [INTERNET:5].

5.3.13.3 Stream Identifier Option

       This option is obsolete.  If the Stream Identifier option is
       present in a packet forwarded by the router, the option MUST be
       ignored and passed through unchanged.

5.3.13.4 Source Route Options

       A router MUST implement support for source route options in
       forwarded packets.  A router MAY implement a configuration
       option which, when enabled, causes all source-routed packets to
       be discarded.  However, such an option MUST NOT be enabled by
       default.
       DISCUSSION:
          The ability to source route datagrams through the Internet
          is important to various network diagnostic tools.  However,
          in a few rare cases, source routing may be used to bypass
          administrative and security controls within a network.
          Specifically, those cases where manipulation of routing
          tables is used to provide administrative separation in lieu
          of other methods such as packet filtering may be vulnerable
          through source routed packets.

5.3.13.5 Record Route Option

       Routers MUST support the Record Route option in forwarded
       packets.
       A router MAY provide a configuration option which, if enabled,
       will cause the router to ignore (i.e. pass through unchanged)
       Record Route options in forwarded packets.  If provided, such
       an option MUST default to enabling the record-route.  This
       option does not affect the processing of Record Route options
       in datagrams received by the router itself (in particular,
       Record Route options in ICMP echo requests will still be
       processed in accordance with Section [4.3.3.6]).

Almquist & Kastenholz [Page 104] RFC 1716 Towards Requirements for IP Routers November 1994

       DISCUSSION:
          There are some people who believe that Record Route is a
          security problem because it discloses information about the
          topology of the network.  Thus, this document allows it to
          be disabled.

5.3.13.6 Timestamp Option

       Routers MUST support the timestamp option in forwarded packets.
       A timestamp value MUST follow the rules given in Section
       [3.2.2.8] of [INTRO:2].
       If the flags field = 3 (timestamp and prespecified address),
       the router MUST add its timestamp if the next prespecified
       address matches any of the router's IP addresses.  It is not
       necessary that the prespecified address be either the address
       of the interface on which the packet arrived or the address of
       the interface over which it will be sent.
       IMPLEMENTATION:
          To maximize the utility of the timestamps contained in the
          timestamp option, it is suggested that the timestamp
          inserted be, as nearly as practical, the time at which the
          packet arrived at the router.  For datagrams originated by
          the router, the timestamp inserted should be, as nearly as
          practical, the time at which the datagram was passed to the
          network layer for transmission.
       A router MAY provide a configuration option which, if enabled,
       will cause the router to ignore (i.e. pass through unchanged)
       Timestamp options in forwarded datagrams when the flag word is
       set to zero (timestamps only) or one (timestamp and registering
       IP address).  If provided, such an option MUST default to off
       (that is, the router does not ignore the timestamp).  This
       option does not affect the processing of Timestamp options in
       datagrams received by the router itself (in particular, a
       router will insert timestamps into Timestamp options in
       datagrams received by the router, and Timestamp options in ICMP
       echo requests will still be processed in accordance with
       Section [4.3.3.6]).
       DISCUSSION:
          Like the Record Route option, the Timestamp option can
          reveal information about a network's topology.  Some people
          consider this to be a security concern.

Almquist & Kastenholz [Page 105] RFC 1716 Towards Requirements for IP Routers November 1994

6. TRANSPORT LAYER

A router is not required to implement any Transport Layer protocols except those required to support Application Layer protocols supported by the router. In practice, this means that most routers implement both the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

6.1 USER DATAGRAM PROTOCOL - UDP

 The User Datagram Protocol (UDP) is specified in [TRANS:1].
 A router which implements UDP MUST be compliant, and SHOULD be
 unconditionally compliant, with the requirements of section 4.1.3 of
 [INTRO:2], except that:
 o  This specification does not specify the interfaces between the
    various protocol layers.  Thus, a router need not comply with
    sections 4.1.3.2, 4.1.3.3, and 4.1.3.5 of [INTRO:2] (except of
    course where compliance is required for proper functioning of
    Application Layer protocols supported by the router).
 o  Contrary to section 4.1.3.4 of [INTRO:2], an application MUST NOT
    be able to disable to generation of UDP checksums.
 DISCUSSION:
    Although a particular application protocol may require that UDP
    datagrams it receives must contain a UDP checksum, there is no
    general requirement that received UDP datagrams contain UDP
    checksums.  Of course, if a UDP checksum is present in a received
    datagram, the checksum must be verified and the datagram discarded
    if the checksum is incorrect.

6.2 TRANSMISSION CONTROL PROTOCOL - TCP

 The Transmission Control Protocol (TCP) is specified in [TRANS:2].
 A router which implements TCP MUST be compliant, and SHOULD be
 unconditionally compliant, with the requirements of section 4.2 of
 [INTRO:2], except that:
 o  This specification does not specify the interfaces between the
    various protocol layers.  Thus, a router need not comply with the
    following requirements of [INTRO:2] (except of course where
    compliance is required for proper functioning of Application Layer

Almquist & Kastenholz [Page 106] RFC 1716 Towards Requirements for IP Routers November 1994

    protocols supported by the router):
    Section 4.2.2.2:
         Passing a received PSH flag to the application layer is now
         OPTIONAL.
    Section 4.2.2.4:
         A TCP MUST inform the application layer asynchronously
         whenever it receives an Urgent pointer and there was
         previously no pending urgent data, or whenever the Urgent
         pointer advances in the data stream.  There MUST be a way for
         the application to learn how much urgent data remains to be
         read from the connection, or at least to determine whether or
         not more urgent data remains to be read.
    Section 4.2.3.5:
         An application MUST be able to set the value for R2 for a
         particular connection.  For example, an interactive
         application might set R2 to ``infinity,'' giving the user
         control over when to disconnect.
    Section 4.2.3.7:
         If an application on a multihomed host does not specify the
         local IP address when actively opening a TCP connection, then
         the TCP MUST ask the IP layer to select a local IP address
         before sending the (first) SYN.  See the function
         GET_SRCADDR() in Section 3.4.
    Section 4.2.3.8:
         An application MUST be able to specify a source route when it
         actively opens a TCP connection, and this MUST take
         precedence over a source route received in a datagram.
 o  For similar reasons, a router need not comply with any of the
    requirements of section 4.2.4 of [INTRO:2].
 o  The requirements of section 4.2.2.6 of [INTRO:2] are amended as
    follows: a router which implements the host portion of MTU
    discovery (discussed in Section [4.2.3.3] of this memo) uses 536
    as the default value of SendMSS only if the path MTU is unknown;
    if the path MTU is known, the default value for SendMSS is the
    path MTU - 40.
 o  The requirements of section 4.2.2.6 of [INTRO:2] are amended as
    follows: ICMP Destination Unreachable codes 11 and 12 are
    additional soft error conditions.  Therefore, these message MUST
    NOT cause TCP to abort a connection.

Almquist & Kastenholz [Page 107] RFC 1716 Towards Requirements for IP Routers November 1994

 DISCUSSION:
    It should particularly be noted that a TCP implementation in a
    router must conform to the following requirements of [INTRO:2]:
    o  Providing a configurable TTL. [4.2.2.1]
    o  Providing an interface to configure keep-alive behavior, if
       keep-alives are used at all. [4.2.3.6]
    o  Providing an error reporting mechanism, and the ability to
       manage it.  [4.2.4.1]
    o  Specifying type of service. [4.2.4.2]
    The general paradigm applied is that if a particular interface is
    visible outside the router, then all requirements for the
    interface must be followed.  For example, if a router provides a
    telnet function, then it will be generating traffic, likely to be
    routed in the external networks.  Therefore, it must be able to
    set the type of service correctly or else the telnet traffic may
    not get through.

Almquist & Kastenholz [Page 108] RFC 1716 Towards Requirements for IP Routers November 1994

7. APPLICATION LAYER - ROUTING PROTOCOLS

7.1 INTRODUCTION

 An Autonomous System (AS) is defined as a set of routers all
 belonging under the same authority and all subject to a consistent
 set of routing policies.  Interior gateway protocols (IGPs) are used
 to distribute routing information inside of an AS (i.e.  intra-AS
 routing). Exterior gateway protocols are used to exchange routing
 information between ASs (i.e. inter-AS routing).

7.1.1 Routing Security Considerations

    Routing is one of the few places where the Robustness Principle
    (be liberal in what you accept) does not apply.  Routers should be
    relatively suspicious in accepting routing data from other routing
    systems.
    A router SHOULD provide the ability to rank routing information
    sources from most trustworthy to least trustworthy and to accept
    routing information about any particular destination from the most
    trustworthy sources first.  This was implicit in the original
    core/stub autonomous system routing model using EGP and various
    interior routing protocols.  It is even more important with the
    demise of a central, trusted core.
    A router SHOULD provide a mechanism to filter out obviously
    invalid routes (such as those for net 127).
    Routers MUST NOT by default redistribute routing data they do not
    themselves use, trust or otherwise consider invalid.  In rare
    cases, it may be necessary to redistribute suspicious information,
    but this should only happen under direct intercession by some
    human agency.
    In general, routers must be at least a little paranoid about
    accepting routing data from anyone, and must be especially careful
    when they distribute routing information provided to them by
    another party.  See below for specific guidelines.
    Routers SHOULD IMPLEMENT peer-to-peer authentication for those
    routing protocols that support them.

Almquist & Kastenholz [Page 109] RFC 1716 Towards Requirements for IP Routers November 1994

7.1.2 Precedence

    Except where the specification for a particular routing protocol
    specifies otherwise, a router SHOULD set the IP Precedence value
    for IP datagrams carrying routing traffic it originates to 6
    (INTERNETWORK CONTROL).
    DISCUSSION:
       Routing traffic with VERY FEW exceptions should be the highest
       precedence traffic on any network.  If a system's routing
       traffic can't get through, chances are nothing else will.

7.2 INTERIOR GATEWAY PROTOCOLS

7.2.1 INTRODUCTION

    An Interior Gateway Protocol (IGP) is used to distribute routing
    information between the various routers in a particular AS.
    Independent of the algorithm used to implement a particular IGP,
    it should perform the following functions:
    (1)  Respond quickly to changes in the internal topology of an AS
    (2)  Provide a mechanism such that circuit flapping does not cause
         continuous routing updates
    (3)  Provide quick convergence to loop-free routing
    (4)  Utilize minimal bandwidth
    (5)  Provide equal cost routes to enable load-splitting
    (6)  Provide a means for authentication of routing updates
    Current IGPs used in the internet today are characterized as
    either being being based on a distance-vector or a link-state
    algorithm.
    Several IGPs are detailed in this section, including those most
    commonly used and some recently developed protocols which may be
    widely used in the future.  Numerous other protocols intended for
    use in intra-AS routing exist in the Internet community.
    A router which implements any routing protocol (other than static
    routes) MUST IMPLEMENT OSPF (see Section [7.2.2]) and MUST

Almquist & Kastenholz [Page 110] RFC 1716 Towards Requirements for IP Routers November 1994

    IMPLEMENT RIP (see Section [7.2.4]).  A router MAY implement
    additional IGPs.

7.2.2 OPEN SHORTEST PATH FIRST - OSPF

7.2.2.1 Introduction

       Shortest Path First (SPF) based routing protocols are a class
       of link-state algorithms which are based on the shortest-path
       algorithm of Dijkstra.  Although SPF based algorithms have been
       around since the inception of the ARPANet, it is only recently
       that they have achieved popularity both inside both the IP and
       the OSI communities.  In an SPF based system, each router
       obtains an exact replica of the entire topology database via a
       process known as flooding.  Flooding insures a reliable
       transfer of the information. Each individual router then runs
       the SPF algorithm on its database to build the IP routing
       table.  The OSPF routing protocol is an implementation of an
       SPF algorithm.  The current version, OSPF version 2, is
       specified in [ROUTE:1].  Note that RFC-1131, which describes
       OSPF version 1, is obsolete.
       Note that to comply with Section [8.3] of this memo, a router
       which implements OSPF MUST implement the OSPF MIB [MGT:14].

7.2.2.2 Specific Issues

       Virtual Links
            There is a minor error in the specification that can cause
            routing loops when all of the following conditions are
            simultaneously true:
            (1)  A virtual link is configured through a transit area,
            (2)  Two separate paths exist, each having the same
                 endpoints, but one utilizing only non-virtual
                 backbone links, and the other using links in the
                 transit area, and
            (3)  The latter path is part of the (underlying physical
                 representation of the) configured virtual link,
                 routing loops may occur.
            To prevent this, an implementation of OSPF SHOULD invoke

Almquist & Kastenholz [Page 111] RFC 1716 Towards Requirements for IP Routers November 1994

            the calculation in Section 16.3 of [ROUTE:1] whenever any
            part of the path to the destination is a virtual link (the
            specification only says this is necessary when the first
            hop is a virtual link).

7.2.2.3 New Version of OSPF

       As of this writing (4/4/94) there is a new version of the OSPF
       specification that is winding its way through the Internet
       standardization process.  A prudent implementor will be aware
       of this and develop an implementation accordingly.
       The new version fixes several errors in the current
       specification [ROUTE:1].  For this reason, implementors and
       vendors ought to expect to upgrade to the new version
       relatively soon.  In particular, the following problems exist
       in [ROUTE:1] that the new version fixes:
       o  In [ROUTE:1], certain configurations of virtual links can
          lead to incorrect routing and/or routing loops. A fix for
          this is specified in the new specification.
       o  In [ROUTE:1], OSPF external routes to For example, a router
          cannot import into an OSPF domain external routes both for
          192.2.0.0, 255.255.0.0 and 192.2.0.0, 255.255.255.0.  Routes
          such as these may become common with the deployment of CIDR
          [INTERNET:15].  This has been addressed in the new OSPF
          specification.
       o  In [ROUTE:1], OSPF Network-LSAs originated before a router
          changes its OSPF Router ID can confuse the Dijkstra
          calculation if the router again becomes Designated Router
          for the network. This has been fixed.

7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM - DUAL IS-IS

    The American National Standards Institute (ANSI) X3S3.3 committee
    has defined an intra-domain routing protocol.  This protocol is
    titled Intermediate System to Intermediate System Routeing
    Exchange Protocol.
    Its application to an IP network has been defined in [ROUTE:2],
    and is referred to as Dual IS-IS (or sometimes as Integrated IS-
    IS).  IS-IS is based on a link-state (SPF) routing algorithm and
    shares all the advantages for this class of protocols.

Almquist & Kastenholz [Page 112] RFC 1716 Towards Requirements for IP Routers November 1994

7.2.4 ROUTING INFORMATION PROTOCOL - RIP

7.2.4.1 Introduction

       RIP is specified in [ROUTE:3].  Although RIP is still quite
       important in the Internet, it is being replaced in
       sophisticated applications by more modern IGPs such as the ones
       described above.
       Another common use for RIP is as a router discovery protocol.
       Section [4.3.3.10] briefly touches upon this subject.

7.2.4.2 Protocol Walk-Through

       Dealing with changes in topology: [ROUTE:3], pp. 11
            An implementation of RIP MUST provide a means for timing
            out routes.  Since messages are occasionally lost,
            implementations MUST NOT invalidate a route based on a
            single missed update.
            Implementations MUST by default wait six times the update
            interval before invalidating a route.  A router MAY have
            configuration options to alter this value.
            DISCUSSION:
               It is important to routing stability that all routers
               in a RIP autonomous system use similar timeout value
               for invalidating routes, and therefore it is important
               that an implementation default to the timeout value
               specified in the RIP specification.  However, that
               timeout value is overly conservative in environments
               where packet loss is reasonably rare.  In such an
               environment, a network manager may wish to be able to
               decrease the timeout period in order to promote faster
               recovery from failures.
            IMPLEMENTATION:
               There is a very simple mechanism which a router may use
               to meet the requirement to invalidate routes promptly
               after they time out.  Whenever the router scans the
               routing table to see if any routes have timed out, it
               also notes the age of the least recently updated route
               which has not yet timed out.  Subtracting this age from

Almquist & Kastenholz [Page 113] RFC 1716 Towards Requirements for IP Routers November 1994

               the timeout period gives the amount of time until the
               router again needs to scan the table for timed out
               routes.
       Split Horizon: [ROUTE:3], pp. 14-15
            An implementation of RIP MUST implement split horizon, a
            scheme used for avoiding problems caused by including
            routes in updates sent to the router from which they were
            learned.
            An implementation of RIP SHOULD implement Split horizon
            with poisoned reverse, a variant of split horizon which
            includes routes learned from a router sent to that router,
            but sets their metric to infinity.  Because of the routing
            overhead which may be incurred by implementing split
            horizon with poisoned reverse, implementations MAY include
            an option to select whether poisoned reverse is in effect.
            An implementation SHOULD limit the period of time in which
            it sends reverse routes at an infinite metric.
            IMPLEMENTATION:
               Each of the following algorithms can be used to limit
               the period of time for which poisoned reverse is
               applied to a route.  The first algorithm is more
               complex but does a more complete job of limiting
               poisoned reverse to only those cases where it is
               necessary.
               The goal of both algorithms is to ensure that poison
               reverse is done for any destination whose route has
               changed in the last Route Lifetime (typically 180
               seconds), unless it can be sure that the previous route
               used the same output interface.  The Route Lifetime is
               used because that is the amount of time RIP will keep
               around an old route before declaring it stale.
               The time intervals (and derived variables) used in the
               following algorithms are as follows:
               Tu   The Update Timer; the number of seconds between
                    RIP updates.  This typically defaults to 30
                    seconds.
               Rl   The Route Lifetime, in seconds.  This is the
                    amount of time that a route is presumed to be

Almquist & Kastenholz [Page 114] RFC 1716 Towards Requirements for IP Routers November 1994

                    good, without requiring an update.  This typically
                    defaults to 180 seconds.
               Ul   The Update Loss; the number of consecutive updates
                    that have to be lost or fail to mention a route
                    before RIP deletes the route.  Ul is calculated to
                    be (Rl/Tu)+1.  The +1 is to account for the fact
                    that the first time the ifcounter is decremented
                    will be less than Tu seconds after it is
                    initialized.  Typically, Ul will be 7: (180/30)+1.
               In   The value to set ifcounter to when a destination
                    is newly learned.  This value is Ul-4, where the 4
                    is RIP's garbage collection timer/30
               The first algorithm is:
  1. Associated with each destination is a counter, called

the ifcounter below. Poison reverse is done for any

                  route whose destination's ifcounter is greater than
                  zero.
  1. After a regular (not triggered or in response to a

request) update is sent, all of the non-zero

                  ifcounters are decremented by one.
  1. When a route to a destination is created, its

ifcounter is set as follows:

  1. If the new route is superseding a valid route, and

the old route used a different (logical) output

                     interface, then the ifcounter is set to Ul.
  1. If the new route is superseding a stale route, and

the old route used a different (logical) output

                     interface, then the ifcounter is set to MAX(0, Ul
                     - INT(seconds that the route has been stale/Ut).
  1. If there was no previous route to the destination,

the ifcounter is set to In.

  1. Otherwise, the ifcounter is set to zero
  1. RIP also maintains a timer, called the resettimer

below. Poison reverse is done on all routes

                  whenever resettimer has not expired (regardless of

Almquist & Kastenholz [Page 115] RFC 1716 Towards Requirements for IP Routers November 1994

                  the ifcounter values).
  1. When RIP is started, restarted, reset, or otherwise

has its routing table cleared, it sets the

                  resettimer to go off in Rl seconds.
               The second algorithm is identical to the first except
               that:
  1. The rules which set the ifcounter to non-zero values

are changed to always set it to Rl/Tu, and

  1. The resettimer is eliminated.
          Triggered updates: [ROUTE:3], pp. 15-16; pp. 29
               Triggered updates (also called flash updates) are a
               mechanism for immediately notifying a router's
               neighbors when the router adds or deletes routes or
               changes their metrics.  A router MUST send a triggered
               update when routes are deleted or their metrics are
               increased.  A router MAY send a triggered update when
               routes are added or their metrics decreased.
               Since triggered updates can cause excessive routing
               overhead, implementations MUST use the following
               mechanism to limit the frequency of triggered updates:
               (1)  When a router sends a triggered update, it sets a
                    timer to a random time between one and five
                    seconds in the future.  The router must not
                    generate additional triggered updates before this
                    timer expires.
               (2)  If the router would generate a triggered update
                    during this interval it sets a flag indicating
                    that a triggered update is desired.  The router
                    also logs the desired triggered update.
               (3)  When the triggered update timer expires, the
                    router checks the triggered update flag. If the
                    flag is set then the router sends a single
                    triggered update which includes all of the changes
                    that were logged.  The router then clears the flag
                    and, since a triggered update was sent, restarts
                    this algorithm.

Almquist & Kastenholz [Page 116] RFC 1716 Towards Requirements for IP Routers November 1994

               (4)  The flag is also cleared whenever a regular update
                    is sent.
               Triggered updates SHOULD include all routes that have
               changed since the most recent regular (non-triggered)
               update.  Triggered updates MUST NOT include routes that
               have not changed since the most recent regular update.
               DISCUSSION:
                  Sending all routes, whether they have changed
                  recently or not, is unacceptable in triggered
                  updates because the tremendous size of many Internet
                  routing tables could otherwise result in
                  considerable bandwidth being wasted on triggered
                  updates.
          Use of UDP: [ROUTE:3], pp. 18-19.
               RIP packets sent to an IP broadcast address SHOULD have
               their initial TTL set to one.
               Note that to comply with Section [6.1] of this memo, a
               router MUST use UDP checksums in RIP packets which it
               originates, MUST discard RIP packets received with
               invalid UDP checksums, but MUST not discard received
               RIP packets simply because they do not contain UDP
               checksums.
          Addressing Considerations: [ROUTE:3], pp. 22
               A RIP implementation SHOULD support host routes.  If it
               does not, it MUST (as described on page 27 of
               [ROUTE:3]) ignore host routes in received updates.  A
               router MAY log ignored hosts routes.
               The special address 0.0.0.0 is used to describe a
               default route. A default route is used as the route of
               last resort (i.e. when a route to the specific net does
               not exist in the routing table). The router MUST be
               able to create a RIP entry for the address 0.0.0.0.
          Input Processing - Response: [ROUTE:3], pp. 26
               When processing an update, the following validity
               checks MUST be performed:
               o  The response MUST be from UDP port 520.

Almquist & Kastenholz [Page 117] RFC 1716 Towards Requirements for IP Routers November 1994

               o  The source address MUST be on a directly connected
                  subnet (or on a directly connected, non-subnetted
                  network) to be considered valid.
               o  The source address MUST NOT be one of the router's
                  addresses.
                  DISCUSSION:
                     Some networks, media, and interfaces allow a
                     sending node to receive packets that it
                     broadcasts.  A router must not accept its own
                     packets as valid routing updates and process
                     them.  The last requirement prevents a router
                     from accepting its own routing updates and
                     processing them (on the assumption that they were
                     sent by some other router on the network).
               An implementation MUST NOT replace an existing route if
               the metric received is equal to the existing metric
               except in accordance with the following heuristic.
               An implementation MAY choose to implement the following
               heuristic to deal with the above situation. Normally,
               it is useless to change the route to a network from one
               router to another if both are advertised at the same
               metric. However, the route being advertised by one of
               the routers may be in the process of timing out.
               Instead of waiting for the route to timeout, the new
               route can be used after a specified amount of time has
               elapsed. If this heuristic is implemented, it MUST wait
               at least halfway to the expiration point before the new
               route is installed.

7.2.4.3 Specific Issues

       RIP Shutdown
            An implementation of RIP SHOULD provide for a graceful
            shutdown using the following steps:
            (1)  Input processing is terminated,
            (2)  Four updates are generated at random intervals of
                 between two and four seconds, These updates contain
                 all routes that were previously announced, but with
                 some metric changes.  Routes that were being

Almquist & Kastenholz [Page 118] RFC 1716 Towards Requirements for IP Routers November 1994

                 announced at a metric of infinity should continue to
                 use this metric.  Routes that had been announced with
                 a non-infinite metric should be announced with a
                 metric of 15 (infinity - 1).
                 DISCUSSION:
                    The metric used for the above really ought to be
                    16 (infinity); setting it to 15 is a kludge to
                    avoid breaking certain old hosts which wiretap the
                    RIP protocol.  Such a host will (erroneously)
                    abort a TCP connection if it tries to send a
                    datagram on the connection while the host has no
                    route to the destination (even if the period when
                    the host has no route lasts only a few seconds
                    while RIP chooses an alternate path to the
                    destination).
       RIP Split Horizon and Static Routes
            Split horizon SHOULD be applied to static routes by
            default.  An implementation SHOULD provide a way to
            specify, per static route, that split horizon should not
            be applied to this route.

7.2.5 GATEWAY TO GATEWAY PROTOCOL - GGP

    The Gateway to Gateway protocol is considered obsolete and SHOULD
    NOT be implemented.

7.3 EXTERIOR GATEWAY PROTOCOLS

7.3.1 INTRODUCTION

    Exterior Gateway Protocols are utilized for inter-Autonomous
    System routing to exchange reachability information for a set of
    networks internal to a particular autonomous system to a
    neighboring autonomous system.
    The area of inter-AS routing is a current topic of research inside
    the Internet Engineering Task Force.  The Exterior Gateway
    Protocol (EGP) described in Section [7.3.3] has traditionally been
    the inter-AS protocol of choice.  The Border Gateway Protocol
    (BGP) eliminates many of the restrictions and limitations of EGP,
    and is therefore growing rapidly in popularity.  A router is not
    required to implement any inter-AS routing protocol.  However, if
    a router does implement EGP it also MUST IMPLEMENT BGP.

Almquist & Kastenholz [Page 119] RFC 1716 Towards Requirements for IP Routers November 1994

    Although it was not designed as an exterior gateway protocol, RIP
    (described in Section [7.2.4]) is sometimes used for inter-AS
    routing.

7.3.2 BORDER GATEWAY PROTOCOL - BGP

7.3.2.1 Introduction

       The Border Gateway Protocol (BGP) is an inter-AS routing
       protocol which exchanges network reachability information with
       other BGP speakers. The information for a network includes the
       complete list of ASs that traffic must transit to reach that
       network. This information can then be used to insure loop-free
       paths.  This information is sufficient to construct a graph of
       AS connectivity from which routing loops may be pruned and some
       policy decisions at the AS level may be enforced.
       BGP is defined by [ROUTE:4].  [ROUTE:5] specifies the proper
       usage of BGP in the Internet, and provides some useful
       implementation hints and guidelines.  [ROUTE:12] and [ROUTE:13]
       provide additional useful information.
       To comply with Section [8.3] of this memo, a router which
       implements BGP MUST also implement the BGP MIB [MGT:15].
       To characterize the set of policy decisions that can be
       enforced using BGP, one must focus on the rule that an AS
       advertises to its neighbor ASs only those routes that it itself
       uses.  This rule reflects the hop-by-hop routing paradigm
       generally used throughout the current Internet.  Note that some
       policies cannot be supported by the hop-by-hop routing paradigm
       and thus require techniques such as source routing to enforce.
       For example, BGP does not enable one AS to send traffic to a
       neighbor AS intending that that traffic take a different route
       from that taken by traffic originating in the neighbor AS.  On
       the other hand, BGP can support any policy conforming to the
       hop-by-hop routing paradigm.
       Implementors of BGP are strongly encouraged to follow the
       recommendations outlined in Section 6 of [ROUTE:5].

7.3.2.2 Protocol Walk-through

       While BGP provides support for quite complex routing policies
       (as an example see Section 4.2 in [ROUTE:5]), it is not
       required for all BGP implementors to support such policies.  At

Almquist & Kastenholz [Page 120] RFC 1716 Towards Requirements for IP Routers November 1994

       a minimum, however, a BGP implementation:
       (1)  SHOULD allow an AS to control announcements of the BGP
            learned routes to adjacent AS's. Implementations SHOULD
            support such control with at least the granularity of a
            single network. Implementations SHOULD also support such
            control with the granularity of an autonomous system,
            where the autonomous system may be either the autonomous
            system that originated the route, or the autonomous system
            that advertised the route to the local system (adjacent
            autonomous system).
       (2)  SHOULD allow an AS to prefer a particular path to a
            destination (when more than one path is available).  Such
            function SHOULD be implemented by allowing system
            administrator to assign weights to Autonomous Systems, and
            making route selection process to select a route with the
            lowest weight (where weight of a route is defined as a sum
            of weights of all AS's in the AS_PATH path attribute
            associated with that route).
       (3)  SHOULD allow an AS to ignore routes with certain AS's in
            the AS_PATH path attribute. Such function can be
            implemented by using technique outlined in (2), and by
            assigning infinity as weights for such AS's. The route
            selection process must ignore routes that have weight
            equal to infinity.

7.3.3 EXTERIOR GATEWAY PROTOCOL - EGP

7.3.3.1 Introduction

       The Exterior Gateway Protocol (EGP) specifies an EGP which is
       used to exchange reachability information between routers of
       the same or differing autonomous systems. EGP is not considered
       a routing protocol since there is no standard interpretation
       (i.e. metric) for the distance fields in the EGP update
       message, so distances are comparable only among routers of the
       same AS.  It is however designed to provide high-quality
       reachability information, both about neighbor routers and about
       routes to non-neighbor routers.
       EGP is defined by [ROUTE:6].  An implementor almost certainly
       wants to read [ROUTE:7] and [ROUTE:8] as well, for they contain
       useful explanations and background material.

Almquist & Kastenholz [Page 121] RFC 1716 Towards Requirements for IP Routers November 1994

       DISCUSSION:
          The present EGP specification has serious limitations, most
          importantly a restriction which limits routers to
          advertising only those networks which are reachable from
          within the router's autonomous system.  This restriction
          against propagating third party EGP information is to
          prevent long-lived routing loops.  This effectively limits
          EGP to a two-level hierarchy.
          RFC-975 is not a part of the EGP specification, and should
          be ignored.

7.3.3.2 Protocol Walk-through

       Indirect Neighbors: RFC-888, pp. 26
          An implementation of EGP MUST include indirect neighbor
          support.
       Polling Intervals: RFC-904, pp. 10
          The interval between Hello command retransmissions and the
          interval between Poll retransmissions SHOULD be configurable
          but there MUST be a minimum value defined.
          The interval at which an implementation will respond to
          Hello commands and Poll commands SHOULD be configurable but
          there MUST be a minimum value defined.
       Network Reachability: RFC-904, pp. 15
          An implementation MUST default to not providing the external
          list of routers in other autonomous systems; only the
          internal list of routers together with the nets which are
          reachable via those routers should be included in an Update
          Response/Indication packet.  However, an implementation MAY
          elect to provide a configuration option enabling the
          external list to be provided.  An implementation MUST NOT
          include in the external list routers which were learned via
          the external list provided by a router in another autonomous
          system. An implementation MUST NOT send a network back to
          the autonomous system from which it is learned, i.e. it MUST
          do split-horizon on an autonomous system level.
          If more than 255 internal or 255 external routers need to be

Almquist & Kastenholz [Page 122] RFC 1716 Towards Requirements for IP Routers November 1994

          specified in a Network Reachability update, the networks
          reachable from routers that can not be listed MUST be merged
          into the list for one of the listed routers.  Which of the
          listed routers is chosen for this purpose SHOULD be user
          configurable, but SHOULD default to the source address of
          the EGP update being generated.
          An EGP update contains a series of blocks of network
          numbers, where each block contains a list of network numbers
          reachable at a particular distance via a particular router.
          If more than 255 networks are reachable at a particular
          distance via a particular router, they are split into
          multiple blocks (all of which have the same distance).
          Similarly, if more than 255 blocks are required to list the
          networks reachable via a particular router, the router's
          address is listed as many times as necessary to include all
          of the blocks in the update.
       Unsolicited Updates: RFC-904, pp. 16
          If a network is shared with the peer, an implementation MUST
          send an unsolicited update upon entry to the Up state
          assuming that the source network is the shared network.
       Neighbor Reachability: RFC-904, pp. 6, 13-15
          The table on page 6 which describes the values of j and k
          (the neighbor up and down thresholds) is incorrect.  It is
          reproduced correctly here:
             Name    Active  Passive Description
             -----------------------------------------------
              j         3       1    neighbor-up threshold
              k         1       0    neighbor-down threshold
          The value for k in passive mode also specified incorrectly
          in RFC-904, pp. 14 The values in parenthesis should read:
             (j = 1, k = 0, and T3/T1 = 4)
          As an optimization, an implementation can refrain from
          sending a Hello command when a Poll is due.  If an
          implementation does so, it SHOULD provide a user
          configurable option to disable this optimization.
       Abort timer: RFC-904, pp. 6, 12, 13

Almquist & Kastenholz [Page 123] RFC 1716 Towards Requirements for IP Routers November 1994

          An EGP implementation MUST include support for the abort
          timer (as documented in section 4.1.4 of RFC-904).  An
          implementation SHOULD use the abort timer in the Idle state
          to automatically issue a Start event to restart the protocol
          machine.  Recommended values are P4 for a critical error
          (Administratively prohibited, Protocol Violation and
          Parameter Problem) and P5 for all others.  The abort timer
          SHOULD NOT be started when a Stop event was manually
          initiated (such as via a network management protocol).
       Cease command received in Idle state: RFC-904, pp. 13
          When the EGP state machine is in the Idle state, it MUST
          reply to Cease commands with a Cease-ack response.
       Hello Polling Mode: RFC-904, pp. 11
          An EGP implementation MUST include support for both active
          and passive polling modes.
       Neighbor Acquisition Messages: RFC-904, pp. 18
          As noted the Hello and Poll Intervals should only be present
          in Request and Confirm messages.  Therefore the length of an
          EGP Neighbor Acquisition Message is 14 bytes for a Request
          or Confirm message and 10 bytes for a Refuse, Cease or
          Cease-ack message.  Implementations MUST NOT send 14 bytes
          for Refuse, Cease or Cease-ack messages but MUST allow for
          implementations that send 14 bytes for these messages.
       Sequence Numbers: RFC-904, pp. 10
          Response or indication packets received with a sequence
          number not equal to S MUST be discarded.  The send sequence
          number S MUST be incremented just before the time a Poll
          command is sent and at no other times.

7.3.4 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL

    It is possible to exchange routing information between two
    autonomous systems or routing domains without using a standard
    exterior routing protocol between two separate, standard interior
    routing protocols.  The most common way of doing this is to run
    both interior protocols independently in one of the border routers
    with an exchange of route information between the two processes.
    As with the exchange of information from an EGP to an IGP, without

Almquist & Kastenholz [Page 124] RFC 1716 Towards Requirements for IP Routers November 1994

    appropriate controls these exchanges of routing information
    between two IGPs in a single router are subject to creation of
    routing loops.

7.4 STATIC ROUTING

 Static routing provides a means of explicitly defining the next hop
 from a router for a particular destination.  A router SHOULD provide
 a means for defining a static route to a destination, where the
 destination is defined by an address and an address mask.  The
 mechanism SHOULD also allow for a metric to be specified for each
 static route.
 A router which supports a dynamic routing protocol MUST allow static
 routes to be defined with any metric valid for the routing protocol
 used.  The router MUST provide the ability for the user to specify a
 list of static routes which may or may not be propagated via the
 routing protocol.  In addition, a router SHOULD support the following
 additional information if it supports a routing protocol that could
 make use of the information. They are:
 o  TOS,
 o  Subnet mask, or
 o  A metric specific to a given routing protocol that can import the
    route.
 DISCUSSION:
    We intend that one needs to support only the things useful to the
    given routing protocol.  The need for TOS should not require the
    vendor to implement the other parts if they are not used.
 Whether a router prefers a static route over a dynamic route (or vice
 versa) or whether the associated metrics are used to choose between
 conflicting static and dynamic routes SHOULD be configurable for each
 static route.
 A router MUST allow a metric to be assigned to a static route for
 each routing domain that it supports.  Each such metric MUST be
 explicitly assigned to a specific routing domain.  For example:
      route 36.0.0.0 255.0.0.0 via 192.19.200.3 rip metric 3
      route 36.21.0.0 255.255.0.0 via 192.19.200.4 ospf inter-area
      metric 27

Almquist & Kastenholz [Page 125] RFC 1716 Towards Requirements for IP Routers November 1994

      route 36.22.0.0 255.255.0.0 via 192.19.200.5 egp 123 metric 99
      route 36.23.0.0 255.255.0.0 via 192.19.200.6 igrp 47 metric 1 2
      3 4 5
 DISCUSSION:
    It has been suggested that, ideally, static routes should have
    preference values rather than metrics (since metrics can only be
    compared with metrics of other routes in the same routing domain,
    the metric of a static route could only be compared with metrics
    of other static routes).  This is contrary to some current
    implementations, where static routes really do have metrics, and
    those metrics are used to determine whether a particular dynamic
    route overrides the static route to the same destination.  Thus,
    this document uses the term metric rather than preference.
    This technique essentially makes the static route into a RIP
    route, or an OSPF route (or whatever, depending on the domain of
    the metric).  Thus, the route lookup algorithm of that domain
    applies.  However, this is NOT route leaking, in that coercing a
    static route into a dynamic routing domain does not authorize the
    router to redistribute the route into the dynamic routing domain.
    For static routes not put into a specific routing domain, the
    route lookup algorithm is:
    (1)  Basic match
    (2)  Longest match
    (3)  Weak TOS (if TOS supported)
    (4)  Best metric (where metric are implementation-defined)
    The last step may not be necessary, but it's useful in the case
    where you want to have a primary static route over one interface
    and a secondary static route over an alternate interface, with
    failover to the alternate path if the interface for the primary
    route fails.

Almquist & Kastenholz [Page 126] RFC 1716 Towards Requirements for IP Routers November 1994

7.5 FILTERING OF ROUTING INFORMATION

 Each router within a network makes forwarding decisions based upon
 information contained within its forwarding database.  In a simple
 network the contents of the database may be statically configured.
 As the network grows more complex, the need for dynamic updating of
 the forwarding database becomes critical to the efficient operation
 of the network.
 If the data flow through a network is to be as efficient as possible,
 it is necessary to provide a mechanism for controlling the
 propagation of the information a router uses to build its forwarding
 database.  This control takes the form of choosing which sources of
 routing information should be trusted and selecting which pieces of
 the information to believe.  The resulting forwarding database is a
 filtered version of the available routing information.
 In addition to efficiency, controlling the propagation of routing
 information can reduce instability by preventing the spread of
 incorrect or bad routing information.
 In some cases local policy may require that complete routing
 information not be widely propagated.
 These filtering requirements apply only to non-SPF-based protocols
 (and therefore not at all to routers which don't implement any
 distance vector protocols).

7.5.1 Route Validation

    A router SHOULD log as an error any routing update advertising a
    route to network zero, subnet zero, or subnet -1, unless the
    routing protocol from which the update was received uses those
    values to encode special routes (such as default routes).

7.5.2 Basic Route Filtering

    Filtering of routing information allows control of paths used by a
    router to forward packets it receives.  A router should be
    selective in which sources of routing information it listens to
    and what routes it believes.  Therefore, a router MUST provide the
    ability to specify:
    o  On which logical interfaces routing information will be
       accepted and which routes will be accepted from each logical
       interface.

Almquist & Kastenholz [Page 127] RFC 1716 Towards Requirements for IP Routers November 1994

    o  Whether all routes or only a default route is advertised on a
       logical interface.
    Some routing protocols do not recognize logical interfaces as a
    source of routing information.  In such cases the router MUST
    provide the ability to specify
    o  from which other routers routing information will be accepted.
    For example, assume a router connecting one or more leaf networks
    to the main portion or backbone of a larger network.  Since each
    of the leaf networks has only one path in and out, the router can
    simply send a default route to them.  It advertises the leaf
    networks to the main network.

7.5.3 Advanced Route Filtering

    As the topology of a network grows more complex, the need for more
    complex route filtering arises.  Therefore, a router SHOULD
    provide the ability to specify independently for each routing
    protocol:
    o  Which logical interfaces or routers routing information
       (routes) will be accepted from and which routes will be
       believed from each other router or logical interface,
    o  Which routes will be sent via which logical interface(s), and
    o  Which routers routing information will be sent to, if this is
       supported by the routing protocol in use.
    In many situations it is desirable to assign a reliability
    ordering to routing information received from another router
    instead of the simple believe or don't believe choice listed in
    the first bullet above.  A router MAY provide the ability to
    specify:
    o  A reliability or preference to be assigned to each route
       received.  A route with higher reliability will be chosen over
       one with lower reliability regardless of the routing metric
       associated with each route.
    If a router supports assignment of preferences, the router MUST
    NOT propagate any routes it does not prefer as first party
    information.  If the routing protocol being used to propagate the
    routes does not support distinguishing between first and third
    party information, the router MUST NOT propagate any routes it

Almquist & Kastenholz [Page 128] RFC 1716 Towards Requirements for IP Routers November 1994

    does not prefer.
    DISCUSSION:
       For example, assume a router receives a route to network C from
       router R and a route to the same network from router S.  If
       router R is considered more reliable than router S traffic
       destined for network C will be forwarded to router R regardless
       of the route received from router S.
    Routing information for routes which the router does not use
    (router S in the above example) MUST NOT be passed to any other
    router.

7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE

 Routers MUST be able to exchange routing information between separate
 IP interior routing protocols, if independent IP routing processes
 can run in the same router.  Routers MUST provide some mechanism for
 avoiding routing loops when routers are configured for bi-directional
 exchange of routing information between two separate interior routing
 processes.  Routers MUST provide some priority mechanism for choosing
 routes from among independent routing processes.  Routers SHOULD
 provide administrative control of IGP-IGP exchange when used across
 administrative boundaries.
 Routers SHOULD provide some mechanism for translating or transforming
 metrics on a per network basis.  Routers (or routing protocols) MAY
 allow for global preference of exterior routes imported into an IGP.
 DISCUSSION:
    Different IGPs use different metrics, requiring some translation
    technique when introducing information from one protocol into
    another protocol with a different form of metric.  Some IGPs can
    run multiple instances within the same router or set of routers.
    In this case metric information can be preserved exactly or
    translated.
    There are at least two techniques for translation between
    different routing processes.  The static (or reachability)
    approach uses the existence of a route advertisement in one IGP to
    generate a route advertisement in the other IGP with a given
    metric.  The translation or tabular approach uses the metric in
    one IGP to create a metric in the other IGP through use of either
    a function (such as adding a constant) or a table lookup.
    Bi-directional exchange of routing information is dangerous
    without control mechanisms to limit feedback.  This is the same

Almquist & Kastenholz [Page 129] RFC 1716 Towards Requirements for IP Routers November 1994

    problem that distance vector routing protocols must address with
    the split horizon technique and that EGP addresses with the
    third-party rule.  Routing loops can be avoided explicitly through
    use of tables or lists of permitted/denied routes or implicitly
    through use of a split horizon rule, a no-third-party rule, or a
    route tagging mechanism.  Vendors are encouraged to use implicit
    techniques where possible to make administration easier for
    network operators.

Almquist & Kastenholz [Page 130] RFC 1716 Towards Requirements for IP Routers November 1994

8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS

Note that this chapter supersedes any requirements stated in section 6.3 of [INTRO:3].

8.1 The Simple Network Management Protocol - SNMP

8.1.1 SNMP Protocol Elements

    Routers MUST be manageable by SNMP [MGT:3].  The SNMP MUST operate
    using UDP/IP as its transport and network protocols.  Others MAY
    be supported (e.g., see [MGT:25, MGT:26, MGT:27, and MGT:28]).
    SNMP management operations MUST operate as if the SNMP was
    implemented on the router itself. Specifically, management
    operations MUST be effected by sending SNMP management requests to
    any of the IP addresses assigned to any of the router's
    interfaces. The actual management operation may be performed
    either by the router or by a proxy for the router.
    DISCUSSION:
       This wording is intended to allow management either by proxy,
       where the proxy device responds to SNMP packets which have one
       of the router's IP addresses in the packets destination address
       field, or the SNMP is implemented directly in the router itself
       and receives packets and responds to them in the proper manner.
       It is important that management operations can be sent to one
       of the router's IP Addresses.  In diagnosing network problems
       the only thing identifying the router that is available may be
       one of the router's IP address; obtained perhaps by looking
       through another router's routing table.
    All SNMP operations (get, get-next, get-response, set, and trap)
    MUST be implemented.
    Routers MUST provide a mechanism for rate-limiting the generation
    of SNMP trap messages.  Routers MAY provide this mechanism via the
    algorithms for asynchronous alert management described in [MGT:5].
    DISCUSSION:
       Although there is general agreement about the need to rate-
       limit traps, there is not yet consensus on how this is best
       achieved.  The reference cited is considered experimental.

Almquist & Kastenholz [Page 131] RFC 1716 Towards Requirements for IP Routers November 1994

8.2 Community Table

 For the purposes of this specification, we assume that there is an
 abstract `community table' in the router.  This table contains
 several entries, each entry for a specific community and containing
 the parameters necessary to completely define the attributes of that
 community.  The actual implementation method of the abstract
 community table is, of course, implementation specific.
 A router's community table MUST allow for at least one entry and
 SHOULD allow for at least two entries.
 DISCUSSION:
    A community table with zero capacity is useless.  It means that
    the router will not recognize any communities and, therefore, all
    SNMP operations will be rejected.
    Therefore, one entry is the minimal useful size of the table.
    Having two entries allows one entry to be limited to read-only
    access while the other would have write capabilities.
 Routers MUST allow the user to manually (i.e., without using SNMP)
 examine, add, delete and change entries in the SNMP community table.
 The user MUST be able to set the community name.  The user MUST be
 able to configure communities as read-only (i.e., they do not allow
 SETs) or read-write (i.e., they do allow SETs).
 The user MUST be able to define at least one IP address to which
 traps are sent for each community.  These addresses MUST be definable
 on a per-community basis.  Traps MUST be enablable or disablable on a
 per-community basis.
 A router SHOULD provide the ability to specify a list of valid
 network managers for any particular community.  If enabled, a router
 MUST validate the source address of the SNMP datagram against the
 list and MUST discard the datagram if its address does not appear.
 If the datagram is discarded the router MUST take all actions
 appropriate to an SNMP authentication failure.
 DISCUSSION:
    This is a rather limited authentication system, but coupled with
    various forms of packet filtering may provide some small measure
    of increased security.
 The community table MUST be saved in non-volatile storage.
 The initial state of the community table SHOULD contain one entry,

Almquist & Kastenholz [Page 132] RFC 1716 Towards Requirements for IP Routers November 1994

 with the community name string public and read-only access.  The
 default state of this entry MUST NOT send traps.  If it is
 implemented, then this entry MUST remain in the community table until
 the administrator changes it or deletes it.
 DISCUSSION:
    By default, traps are not sent to this community.  Trap PDUs are
    sent to unicast IP addresses. This address must be configured into
    the router in some manner. Before the configuration occurs, there
    is no such address, so to whom should the trap be sent? Therefore
    trap sending to the public community defaults to be disabled. This
    can, of course, be changed by an administrative operation once the
    router is operational.

8.3 Standard MIBS

 All MIBS relevant to a router's configuration are to be implemented.
 To wit:
 o  The System, Interface, IP, ICMP, and UDP groups of MIB-II [MGT:2]
    MUST be implemented.
 o  The Interface Extensions MIB [MGT:18] MUST be implemented.
 o  The IP Forwarding Table MIB [MGT:20] MUST be implemented.
 o  If the router implements TCP (e.g. for Telnet) then the TCP group
    of MIB-II [MGT:2] MUST be implemented.
 o  If the router implements EGP then the EGP group of MIB-II [MGT:2]
    MUST be implemented.
 o  If the router supports OSPF then the OSPF MIB [MGT:14] MUST be
    implemented.
 o  If the router supports BGP then the BGP MIB [MGT:15] MUST be
    implemented.
 o  If the router has Ethernet, 802.3, or StarLan interfaces then the
    Ethernet-Like MIB [MGT:6] MUST be implemented.
 o  If the router has 802.4 interfaces then the 802.4 MIB [MGT:7] MAY
    be implemented.
 o  If the router has 802.5 interfaces then the 802.5 MIB [MGT:8] MUST
    be implemented.

Almquist & Kastenholz [Page 133] RFC 1716 Towards Requirements for IP Routers November 1994

 o  If the router has FDDI interfaces that implement ANSI SMT 7.3 then
    the FDDI MIB [MGT:9] MUST be implemented.
 o  If the router has FDDI interfaces that implement ANSI SMT 6.2 then
    the FDDI MIB [MGT:29] MUST be implemented.
 o  If the router has RS-232 interfaces then the RS-232 [MGT:10] MIB
    MUST be implemented.
 o  If the router has T1/DS1 interfaces then the T1/DS1 MIB [MGT:16]
    MUST be implemented.
 o  If the router has T3/DS3 interfaces then the T3/DS3 MIB [MGT:17]
    MUST be implemented.
 o  If the router has SMDS interfaces then the SMDS Interface Protocol
    MIB [MGT:19] MUST be implemented.
 o  If the router supports PPP over any of its interfaces then the PPP
    MIBs [MGT:11], [MGT:12], and [MGT:13] MUST be implemented.
 o  If the router supports RIP Version 2 then the RIP Version 2 MIB
    [MGT:21] MUST be implemented.
 o  If the router supports X.25 over any of its interfaces then the
    X.25 MIBs [MGT:22, MGT:23 and MGT:24] MUST be implemented.

8.4 Vendor Specific MIBS

 The Internet Standard and Experimental MIBs do not cover the entire
 range of statistical, state, configuration and control information
 that may be available in a network element. This information is,
 never the less, extremely useful. Vendors of routers (and other
 network devices) generally have developed MIB extensions that cover
 this information. These MIB extensions are called Vendor Specific
 MIBs.
 The Vendor Specific MIB for the router MUST provide access to all
 statistical, state, configuration, and control information that is
 not available through the Standard and Experimental MIBs that have
 been implemented.  This information MUST be available for both
 monitoring and control operations.

Almquist & Kastenholz [Page 134] RFC 1716 Towards Requirements for IP Routers November 1994

 DISCUSSION:
    The intent of this requirement is to provide the ability to do
    anything on the router via SNMP that can be done via a console.  A
    certain minimal amount of configuration is necessary before SNMP
    can operate (e.g., the router must have an IP address). This
    initial configuration can not be done via SNMP. However, once the
    initial configuration is done, full capabilities ought to be
    available via network management.
 The vendor SHOULD make available the specifications for all Vendor
 Specific MIB variables. These specifications MUST conform to the SMI
 [MGT:1] and the descriptions MUST be in the form specified in
 [MGT:4].
 DISCUSSION:
    Making the Vendor Specific MIB available to the user is necessary.
    Without this information the users would not be able to configure
    their network management systems to be able to access the Vendor
    Specific parameters.  These parameters would then be useless.
    The format of the MIB specification is also specified.  Parsers
    which read MIB specifications and generate the needed tables for
    the network management station are available.  These parsers
    generally understand only the standard MIB specification format.

8.5 Saving Changes

 Parameters altered by SNMP MAY be saved to non-volatile storage.
 DISCUSSION:
    Reasons why this requirement is a MAY:
    o  The exact physical nature of non-volatile storage is not
       specified in this document.  Hence, parameters may be saved in
       NVRAM/EEPROM, local floppy or hard disk, or in some TFTP file
       server or BOOTP server, etc. Suppose that that this information
       is in a file that is retrieved via TFTP. In that case, a change
       made to a configuration parameter on the router would need to
       be propagated back to the file server holding the configuration
       file.  Alternatively, the SNMP operation would need to be
       directed to the file server, and then the change somehow
       propagated to the router.  The answer to this problem does not
       seem obvious.
       This also places more requirements on the host holding the
       configuration information than just having an available tftp

Almquist & Kastenholz [Page 135] RFC 1716 Towards Requirements for IP Routers November 1994

       server, so much more that its probably unsafe for a vendor to
       assume that any potential customer will have a suitable host
       available.
    o  The timing of committing changed parameters to non-volatile
       storage is still an issue for debate. Some prefer to commit all
       changes immediately. Others prefer to commit changes to non-
       volatile storage only upon an explicit command.

Almquist & Kastenholz [Page 136] RFC 1716 Towards Requirements for IP Routers November 1994

9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS

For all additional application protocols that a router implements, the router MUST be compliant and SHOULD be unconditionally compliant with the relevant requirements of [INTRO:3].

9.1 BOOTP

9.1.1 Introduction

    The Bootstrap Protocol (BOOTP) is a UDP/IP-based protocol which
    allows a booting host to configure itself dynamically and without
    user supervision.  BOOTP provides a means to notify a host of its
    assigned IP address, the IP address of a boot server host, and the
    name of a file to be loaded into memory and executed ([APPL:1]).
    Other configuration information such as the local subnet mask, the
    local time offset, the addresses of default routers, and the
    addresses of various Internet servers can also be communicated to
    a host using BOOTP ([APPL:2]).

9.1.2 BOOTP Relay Agents

    In many cases, BOOTP clients and their associated BOOTP server(s)
    do not reside on the same IP network or subnet.  In such cases, a
    third-party agent is required to transfer BOOTP messages between
    clients and servers.  Such an agent was originally referred to as
    a BOOTP forwarding agent.  However, in order to avoid confusion
    with the IP forwarding function of a router, the name BOOTP relay
    agent has been adopted instead.
    DISCUSSION:
       A BOOTP relay agent performs a task which is distinct from a
       router's normal IP forwarding function.  While a router
       normally switches IP datagrams between networks more-or-less
       transparently, a BOOTP relay agent may more properly be thought
       to receive BOOTP messages as a final destination and then
       generate new BOOTP messages as a result.  One should resist the
       notion of simply forwarding a BOOTP message straight through
       like a regular packet.
    This relay-agent functionality is most conveniently located in the
    routers which interconnect the clients and servers (although it
    may alternatively be located in a host which is directly connected
    to the client subnet).
    A router MAY provide BOOTP relay-agent capability.  If it does, it

Almquist & Kastenholz [Page 137] RFC 1716 Towards Requirements for IP Routers November 1994

    MUST conform to the specifications in [APPL:3].
    Section [5.2.3] discussed the circumstances under which a packet
    is delivered locally (to the router).  All locally delivered UDP
    messages whose UDP destination port number is BOOTPS (67) are
    considered for special processing by the router's logical BOOTP
    relay agent.
    Sections [4.2.2.11] and [5.3.7] discussed invalid IP source
    addresses.  According to these rules, a router must not forward
    any received datagram whose IP source address is 0.0.0.0.
    However, routers which support a BOOTP relay agent MUST accept for
    local delivery to the relay agent BOOTREQUEST messages whose IP
    source address is 0.0.0.0.

Almquist & Kastenholz [Page 138] RFC 1716 Towards Requirements for IP Routers November 1994

10. OPERATIONS AND MAINTENANCE

This chapter supersedes any requirements stated in section 6.2 of [INTRO:3].

Facilities to support operation and maintenance (O&M) activities form an essential part of any router implementation. Although these functions do not seem to relate directly to interoperability, they are essential to the network manager who must make the router interoperate and must track down problems when it doesn't. This chapter also includes some discussion of router initialization and of facilities to assist network managers in securing and accounting for their networks.

10.1 Introduction

 The following kinds of activities are included under router O&M:
 o  Diagnosing hardware problems in the router's processor, in its
    network interfaces, or in its connected networks, modems, or
    communication lines.
 o  Installing new hardware
 o  Installing new software.
 o  Restarting or rebooting the router after a crash.
 o  Configuring (or reconfiguring) the router.
 o  Detecting and diagnosing Internet problems such as congestion,
    routing loops, bad IP addresses, black holes, packet avalanches,
    and misbehaved hosts.
 o  Changing network topology, either temporarily (e.g., to bypass a
    communication line problem) or permanently.
 o  Monitoring the status and performance of the routers and the
    connected networks.
 o  Collecting traffic statistics for use in (Inter-)network planning.
 o  Coordinating the above activities with appropriate vendors and
    telecommunications specialists.
 Routers and their connected communication lines are often operated as
 a system by a centralized O&M organization.  This organization may
 maintain a (Inter-)network operation center, or NOC, to carry out its

Almquist & Kastenholz [Page 139] RFC 1716 Towards Requirements for IP Routers November 1994

 O&M functions.  It is essential that routers support remote control
 and monitoring from such a NOC through an Internet path, since
 routers might not be connected to the same network as their NOC.
 Since a network failure may temporarily preclude network access, many
 NOCs insist that routers be accessible for network management via an
 alternative means, often dialup modems attached to console ports on
 the routers.
 Since an IP packet traversing an internet will often use routers
 under the control of more than one NOC, Internet problem diagnosis
 will often involve cooperation of personnel of more than one NOC.  In
 some cases, the same router may need to be monitored by more than one
 NOC, but only if necessary, because excessive monitoring could impact
 a router's performance.
 The tools available for monitoring at a NOC may cover a wide range of
 sophistication. Current implementations include multi-window, dynamic
 displays of the entire router system. The use of AI techniques for
 automatic problem diagnosis is proposed for the future.
 Router O&M facilities discussed here are only a part of the large and
 difficult problem of Internet management.  These problems encompass
 not only multiple management organizations, but also multiple
 protocol layers.  For example, at the current stage of evolution of
 the Internet architecture, there is a strong coupling between host
 TCP implementations and eventual IP-level congestion in the router
 system [OPER:1].  Therefore, diagnosis of congestion problems will
 sometimes require the monitoring of TCP statistics in hosts.  There
 are currently a number of R&D efforts in progress in the area of
 Internet management and more specifically router O&M. These R&D
 efforts have already produced standards for router O&M. This is also
 an area in which vendor creativity can make a significant
 contribution.

10.2 Router Initialization

10.2.1 Minimum Router Configuration

    There exists a minimum set of conditions that must be satisfied
    before a router may forward packets.  A router MUST NOT enable
    forwarding on any physical interface unless either:
    (1)  The router knows the IP address and associated subnet mask of
         at least one logical interface associated with that physical
         interface, or

Almquist & Kastenholz [Page 140] RFC 1716 Towards Requirements for IP Routers November 1994

    (2)  The router knows that the interface is an unnumbered
         interface and also knows its router-id.
    These parameters MUST be explicitly configured:
    o  A router MUST NOT use factory-configured default values for its
       IP addresses, subnet masks, or router-id, and
    o  A router MUST NOT assume that an unconfigured interface is an
       unnumbered interface.
    DISCUSSION:
       There have been instances in which routers have been shipped
       with vendor-installed default addresses for interfaces.  In a
       few cases, this has resulted in routers advertising these
       default addresses into active networks.

10.2.2 Address and Address Mask Initialization

    A router MUST allow its IP addresses and their subnet masks to be
    statically configured and saved in permanent storage.
    A router MAY obtain its IP addresses and their corresponding
    subnet masks dynamically as a side effect of the system
    initialization process (see Section 10.2.3]);
    If the dynamic method is provided, the choice of method to be used
    in a particular router MUST be configurable.
    As was described in Section [4.2.2.11], IP addresses are not
    permitted to have the value 0 or -1 for any of the <Host-number>,
    <Network-number>, or <Subnet-number> fields.  Therefore, a router
    SHOULD NOT allow an IP address or subnet mask to be set to a value
    which would make any of the the three fields above have the value
    zero or -1.
    DISCUSSION:
       It is possible using variable length subnet masks to create
       situations in which routing is ambiguous (i.e., two routes with
       different but equally-specific subnet masks match a particular
       destination address).  We suspect that a router could, when
       setting a subnet mask, check whether the mask would cause
       routing to be ambiguous, and that implementors might be able to
       decrease their customer support costs by having routers
       prohibit or log such erroneous configurations.  However, at
       this time we do not require routers to make such checks because

Almquist & Kastenholz [Page 141] RFC 1716 Towards Requirements for IP Routers November 1994

       we know of no published method for accurately making this
       check.
    A router SHOULD make the following checks on any subnet mask it
    installs:
    o  The mask is not all 1-bits.
    o  The bits which correspond to the network number part of the
       address are all set to 1.
    DISCUSSION:
       The masks associated with routes are also sometimes called
       subnet masks, this test should not be applied to them.

10.2.3 Network Booting using BOOTP and TFTP

    There has been a lot of discussion on how routers can and should
    be booted from the network.  In general, these discussions have
    centered around BOOTP and TFTP.  Currently, there are routers that
    boot with TFTP from the network.  There is no reason that BOOTP
    could not be used for locating the server that the boot image
    should be loaded from.
    In general, BOOTP is a protocol used to boot end systems, and
    requires some stretching to accommodate its use with routers.  If
    a router is using BOOTP to locate the current boot host, it should
    send a BOOTP Request with its hardware address for its first
    interface, or, if it has been previously configured otherwise,
    with either another interface's hardware address, or another
    number to put in the hardware address field of the BOOTP packet.
    This is to allow routers without hardware addresses (like sync
    line only routers) to use BOOTP for bootload discovery.  TFTP can
    then be used to retrieve the image found in the BOOTP Reply.  If
    there are no configured interfaces or numbers to use, a router MAY
    cycle through the interface hardware addresses it has until a
    match is found by the BOOTP server.
    A router SHOULD IMPLEMENT the ability to store parameters learned
    via BOOTP into local stable storage.  A router MAY implement the
    ability to store a system image loaded over the network into local
    stable storage.
    A router MAY have a facility to allow a remote user to request
    that the router get a new boot image.  Differentiation should be

Almquist & Kastenholz [Page 142] RFC 1716 Towards Requirements for IP Routers November 1994

    made between getting the new boot image from one of three
    locations: the one included in the request, from the last boot
    image server, and using BOOTP to locate a server.

10.3 Operation and Maintenance

10.3.1 Introduction

    There is a range of possible models for performing O&M functions
    on a router.  At one extreme is the local-only model, under which
    the O&M functions can only be executed locally (e.g., from a
    terminal plugged into the router machine).  At the other extreme,
    the fully-remote model allows only an absolute minimum of
    functions to be performed locally (e.g., forcing a boot), with
    most O&M being done remotely from the NOC.  There are intermediate
    models, such as one in which NOC personnel can log into the router
    as a host, using the Telnet protocol, to perform functions which
    can also be invoked locally.  The local-only model may be adequate
    in a few router installations, but in general remote operation
    from a NOC will be required, and therefore remote O&M provisions
    are required for most routers.
    Remote O&M functions may be exercised through a control agent
    (program).  In the direct approach, the router would support
    remote O&M functions directly from the NOC using standard Internet
    protocols (e.g., SNMP, UDP or TCP); in the indirect approach, the
    control agent would support these protocols and control the router
    itself using proprietary protocols.  The direct approach is
    preferred, although either approach is acceptable.  The use of
    specialized host hardware and/or software requiring significant
    additional investment is discouraged; nevertheless, some vendors
    may elect to provide the control agent as an integrated part of
    the network in which the routers are a part.  If this is the case,
    it is required that a means be available to operate the control
    agent from a remote site using Internet protocols and paths and
    with equivalent functionality with respect to a local agent
    terminal.
    It is desirable that a control agent and any other NOC software
    tools which a vendor provides operate as user programs in a
    standard operating system.  The use of the standard Internet
    protocols UDP and TCP for communicating with the routers should
    facilitate this.
    Remote router monitoring and (especially) remote router control
    present important access control problems which must be addressed.

Almquist & Kastenholz [Page 143] RFC 1716 Towards Requirements for IP Routers November 1994

    Care must also be taken to ensure control of the use of router
    resources for these functions.  It is not desirable to let router
    monitoring take more than some limited fraction of the router CPU
    time, for example.  On the other hand, O&M functions must receive
    priority so they can be exercised when the router is congested,
    since often that is when O&M is most needed.

10.3.2 Out Of Band Access

    Routers MUST support Out-Of-Band (OOB) access.  OOB access SHOULD
    provide the same functionality as in-band access.
    DISCUSSION:
       This Out-Of-Band access will allow the NOC a way to access
       isolated routers during times when network access is not
       available.
       Out-Of-Band access is an important management tool for the
       network administrator.  It allows the access of equipment
       independent of the network connections.  There are many ways to
       achieve this access.  Whichever one is used it is important
       that the access is independent of the network connections.  An
       example of Out-Of-Band access would be a serial port connected
       to a modem that provides dial up access to the router.
       It is important that the OOB access provides the same
       functionality as in-band access.  In-band access, or accessing
       equipment through the existing network connection, is limiting,
       because most of the time, administrators need to reach
       equipment to figure out why it is unreachable.  In band access
       is still very important for configuring a router, and for
       troubleshooting more subtle problems.

10.3.2 Router O&M Functions

10.3.2.1 Maintenance - Hardware Diagnosis

       Each router SHOULD operate as a stand-alone device for the
       purposes of local hardware maintenance.  Means SHOULD be
       available to run diagnostic programs at the router site using
       only on-site tools.  A router SHOULD be able to run diagnostics
       in case of a fault.  For suggested hardware and software
       diagnostics see Section [10.3.3].

Almquist & Kastenholz [Page 144] RFC 1716 Towards Requirements for IP Routers November 1994

10.3.2.2 Control - Dumping and Rebooting

       A router MUST include both in-band and out-of-band mechanisms
       to allow the network manager to reload, stop, and restart the
       router.  A router SHOULD also contain a mechanism (such as a
       watchdog timer) which will reboot the router automatically if
       it hangs due to a software or hardware fault.
       A router SHOULD IMPLEMENT a mechanism for dumping the contents
       of a router's memory (and/or other state useful for vendor
       debugging after a crash), and either saving them on a stable
       storage device local to the router or saving them on another
       host via an up-line dump mechanism such as TFTP (see [OPER:2],
       [INTRO:3]).

10.3.2.3 Control - Configuring the Router

       Every router has configuration parameters which may need to be
       set.  It SHOULD be possible to update the parameters without
       rebooting the router; at worst, a restart MAY be required.
       There may be cases when it is not possible to change parameters
       without rebooting the router (for instance, changing the IP
       address of an interface).  In these cases, care should be taken
       to minimize disruption to the router and the surrounding
       network.
       There SHOULD be a way to configure the router over the network
       either manually or automatically.  A router SHOULD be able to
       upload or download its parameters from a host or another
       router, and these parameters SHOULD be convertible into some
       sort of text format for making changes and then back to the
       form the router can read.  A router SHOULD have some sort of
       stable storage for its configuration. A router SHOULD NOT
       believe protocols such as RARP, ICMP Address Mask Reply, and
       MAY not believe BOOTP.
       DISCUSSION:
          It is necessary to note here that in the future RARP, ICMP
          Address Mask Reply, BOOTP and other mechanisms may be needed
          to allow a router to auto-configure.  Although routers may
          in the future be able to configure automatically, the intent
          here is to discourage this practice in a production
          environment until such time as auto-configuration has been
          tested more thoroughly. The intent is NOT to discourage
          auto-configuration all together.  In cases where a router is
          expected to get its configuration automatically it may be
          wise to allow the router to believe these things as it comes

Almquist & Kastenholz [Page 145] RFC 1716 Towards Requirements for IP Routers November 1994

          up and then ignore them after it has gotten its
          configuration.

10.3.2.4 Netbooting of System Software

       A router SHOULD keep its system image in local non-volatile
       storage such as PROM, NVRAM, or disk. It MAY also be able to
       load its system software over the network from a host or
       another router.
       A router which can keep its system image in local non-volatile
       storage MAY be configurable to boot its system image over the
       network.  A router which offers this option SHOULD be
       configurable to boot the system image in its non-volatile local
       storage if it is unable to boot its system image over the
       network.
       DISCUSSION:
          It is important that the router be able to come up and run
          on its own.  NVRAM may be a particular solution for routers
          used in large networks, since changing PROMs can be quite
          time consuming for a network manager responsible for
          numerous or geographically dispersed routers.  It is
          important to be able to netboot the system image because
          there should be an easy way for a router to get a bug fix or
          new feature more quickly than getting PROMS installed.  Also
          if the router has NVRAM instead of PROMs, it will netboot
          the image and then put it in NVRAM.
       A router MAY also be able to distinguish between different
       configurations based on which software it is running. If
       configuration commands change from one software version to
       another, it would be helpful if the router could use the
       configuration that was compatible with the software.

10.3.2.5 Detecting and responding to misconfiguration

       There MUST be mechanisms for detecting and responding to
       misconfigurations.  If a command is executed incorrectly, the
       router SHOULD give an error message.  The router SHOULD NOT
       accept a poorly formed command as if it were correct.

Almquist & Kastenholz [Page 146] RFC 1716 Towards Requirements for IP Routers November 1994

       DISCUSSION:
          There are cases where it is not possible to detect errors:
          the command is correctly formed, but incorrect with respect
          to the network.  This may be detected by the router, but may
          not be possible.
       Another form of misconfiguration is misconfiguration of the
       network to which the router is attached.  A router MAY detect
       misconfigurations in the network.  The router MAY log these
       findings to a file, either on the router or a host, so that the
       network manager will see that there are possible problems on
       the network.
       DISCUSSION:
          Examples of such misconfigurations might be another router
          with the same address as the one in question or a router
          with the wrong subnet mask.  If a router detects such
          problems it is probably not the best idea for the router to
          try to fix the situation.  That could cause more harm than
          good.

10.3.2.6 Minimizing Disruption

       Changing the configuration of a router SHOULD have minimal
       affect on the network.   Routing tables SHOULD NOT be
       unnecessarily flushed when a simple change is made to the
       router.  If a router is running several routing protocols,
       stopping one routing protocol SHOULD NOT disrupt other routing
       protocols, except in the case where one network is learned by
       more than one routing protocol.
       DISCUSSION:
          It is the goal of a network manager to run a network so that
          users of the network get the best connectivity possible.
          Reloading a router for simple configuration changes can
          cause disruptions in routing and ultimately cause
          disruptions to the network and its users.  If routing tables
          are unnecessarily flushed, for instance, the default route
          will be lost as well as specific routes to sites within the
          network.  This sort of disruption will cause significant
          downtime for the users. It is the purpose of this section to
          point out that whenever possible, these disruptions should
          be avoided.

Almquist & Kastenholz [Page 147] RFC 1716 Towards Requirements for IP Routers November 1994

10.3.2.7 Control - Troubleshooting Problems

       (1)  A router MUST provide in-band network access, but (except
            as required by Section [8.2]) for security considerations
            this access SHOULD be disabled by default.  Vendors MUST
            document the default state of any in-band access.
            DISCUSSION:
               In-band access primarily refers to access via the
               normal network protocols which may or may not affect
               the permanent operational state of the router.  This
               includes, but is not limited to Telnet/RLOGIN console
               access and SNMP operations.
               This was a point of contention between the operational
               out of the box and secure out of the box contingents.
               Any automagic access to the router may introduce
               insecurities, but it may be more important for the
               customer to have a router which is accessible over the
               network as soon as it is plugged in.  At least one
               vendor supplies routers without any external console
               access and depends on being able to access the router
               via the network to complete its configuration.
               Basically, it is the vendors call whether or not in-
               band access is enabled by default; but it is also the
               vendors responsibility to make its customers aware of
               possible insecurities.
       (2)  A router MUST provide the ability to initiate an ICMP
            echo.  The following options SHOULD be implemented:
            o  Choice of data patterns
            o  Choice of packet size
            o  Record route
            and the following additional options MAY be implemented:
            o  Loose source route
            o  Strict source route
            o  Timestamps

Almquist & Kastenholz [Page 148] RFC 1716 Towards Requirements for IP Routers November 1994

       (3)  A router SHOULD provide the ability to initiate a
            traceroute.  If traceroute is provided, then the 3rd party
            traceroute SHOULD be implemented.
       Each of the above three facilities (if implemented) SHOULD have
       access restrictions placed on it to prevent its abuse by
       unauthorized persons.

10.4 Security Considerations

10.4.1 Auditing and Audit Trails

    Auditing and billing are the bane of the network operator, but are
    the two features most requested by those in charge of network
    security and those who are responsible for paying the bills.  In
    the context of security, auditing is desirable if it helps you
    keep your network working and protects your resources from abuse,
    without costing you more than those resources are worth.
    (1)  Configuration Changes
         Router SHOULD provide a method for auditing a configuration
         change of a router, even if it's something as simple as
         recording the operator's initials and time of change.
         DISCUSSION:
            Having the ability to track who made changes and when is
            highly desirable, especially if your packets suddenly
            start getting routed through Alaska on their way across
            town.
    (2)  Packet Accounting
         Vendors should strongly consider providing a system for
         tracking traffic levels between pairs of hosts or networks.
         A mechanism for limiting the collection of this information
         to specific pairs of hosts or networks is also strongly
         encouraged.
         DISCUSSION:
            A host traffic matrix as described above can give the
            network operator a glimpse of traffic trends not apparent
            from other statistics.  It can also identify hosts or
            networks which are probing the structure of the attached
            networks - e.g., a single external host which tries to
            send packets to every IP address in the network address

Almquist & Kastenholz [Page 149] RFC 1716 Towards Requirements for IP Routers November 1994

            range for a connected network.
    (3)  Security Auditing
         Routers MUST provide a method for auditing security related
         failures or violations to include:
         o  Authorization Failures:  bad passwords, invalid SNMP
            communities, invalid authorization tokens,
         o  Violations of Policy Controls:  Prohibited Source Routes,
            Filtered Destinations, and
         o  Authorization Approvals:  good passwords - Telnet in-band
            access, console access.
         Routers MUST provide a method of limiting or disabling such
         auditing but auditing SHOULD be on by default.  Possible
         methods for auditing include listing violations to a console
         if present, logging or counting them internally, or logging
         them to a remote security server via the SNMP trap mechanism
         or the Unix logging mechanism as appropriate.  A router MUST
         implement at least one of these reporting mechanisms - it MAY
         implement more than one.

10.4.2 Configuration Control

    A vendor has a responsibility to use good configuration control
    practices in the creation of the software/firmware loads for their
    routers.  In particular, if a vendor makes updates and loads
    available for retrieval over the Internet, the vendor should also
    provide a way for the customer to confirm the load is a valid one,
    perhaps by the verification of a checksum over the load.
    DISCUSSION:
       Many vendors currently provide short notice updates of their
       software products via the Internet.  This a good trend and
       should be encouraged, but provides a point of vulnerability in
       the configuration control process.
    If a vendor provides the ability for the customer to change the
    configuration parameters of a router remotely, for example via a
    Telnet session, the ability to do so SHOULD be configurable and
    SHOULD default to off.  The router SHOULD require a password or
    other valid authentication before permitting remote
    reconfiguration.

Almquist & Kastenholz [Page 150] RFC 1716 Towards Requirements for IP Routers November 1994

    DISCUSSION:
       Allowing your properly identified network operator to twiddle
       with your routers is necessary; allowing anyone else to do so
       is foolhardy.
    A router MUST NOT have undocumented back door access and master
    passwords.  A vendor MUST ensure any such access added for
    purposes of debugging or product development are deleted before
    the product is distributed to its customers.
    DISCUSSION:
       A vendor has a responsibility to its customers to ensure they
       are aware of the vulnerabilities present in its code by
       intention - e.g.  in-band access.  Trap doors, back doors and
       master passwords intentional or unintentional can turn a
       relatively secure router into a major problem on an operational
       network.  The supposed operational benefits are not matched by
       the potential problems.

Almquist & Kastenholz [Page 151] RFC 1716 Towards Requirements for IP Routers November 1994

11. REFERENCES

Implementors should be aware that Internet protocol standards are occasionally updated. These references are current as of this writing, but a cautious implementor will always check a recent version of the RFC index to ensure that an RFC has not been updated or superseded by another, more recent RFC. Reference [INTRO:6] explains various ways to obtain a current RFC index.

APPL:1.

   B. Croft and J. Gilmore, Bootstrap Protocol (BOOTP), Request For
   Comments (RFC) 951, Stanford and SUN Microsystems, September 1985.

APPL:2.

   S. Alexander and R. Droms, DHCP Options and BOOTP Vendor
   Extensions, Request For Comments (RFC) 1533, Lachman Technology,
   Inc., Bucknell University, October 1993.

APPL:3.

   W. Wimer, Clarifications and Extensions for the Bootstrap Protocol,
   Request For Comments (RFC) 1542, Carnegie Mellon University,
   October 1993.

ARCH:1.

   DDN Protocol Handbook, NIC-50004, NIC-50005, NIC-50006 (three
   volumes), DDN Network Information Center, SRI International, Menlo
   Park, California, USA, December 1985.

ARCH:2.

   V. Cerf and R. Kahn, A Protocol for Packet Network
   Intercommunication," IEEE Transactions on Communication, May 1974.
   Also included in [ARCH:1].

ARCH:3.

   J. Postel, C. Sunshine, and D. Cohen, The ARPA Internet Protocol,"
   Computer Networks, vol. 5, no. 4, July 1981.  Also included in
   [ARCH:1].

ARCH:4.

   B. Leiner, J. Postel, R. Cole, and D. Mills, The DARPA Internet
   Protocol Suite, Proceedings of INFOCOM '85, IEEE, Washington, DC,
   March 1985.  Also in: IEEE Communications Magazine, March 1985.
   Also available from the Information Sciences Institute, University
   of Southern California as Technical Report ISI-RS-85-153.

Almquist & Kastenholz [Page 152] RFC 1716 Towards Requirements for IP Routers November 1994

ARCH:5.

   D. Comer, Internetworking With TCP/IP Volume 1: Principles,
   Protocols, and Architecture, Prentice Hall, Englewood Cliffs, NJ,
   1991.

ARCH:6.

   W. Stallings, Handbook of Computer-Communications Standards Volume
   3: The TCP/IP Protocol Suite, Macmillan, New York, NY, 1990.

ARCH:7.

   J. Postel, Internet Official Protocol Standards, Request For
   Comments (RFC) 1610, STD 1, USC/Information Sciences Institute,
   July 1994.

ARCH:8.

   Information processing systems - Open Systems Interconnection -
   Basic Reference Model, ISO 7489, International Standards
   Organization, 1984.

FORWARD:1.

   IETF CIP Working Group (C. Topolcic, Editor), Experimental Internet
   Stream Protocol, Version 2 (ST-II), Request For Comments (RFC)
   1190, CIP Working Group, October 1990.

FORWARD:2.

   A. Mankin and K. Ramakrishnan, Editors, Gateway Congestion Control
   Survey, Request For Comments (RFC) 1254, MITRE, Digital Equipment
   Corporation, August 1991.

FORWARD:3.

   J. Nagle, On Packet Switches with Infinite Storage, IEEE
   Transactions on Communications, vol. COM-35, no. 4, April 1987.

FORWARD:4.

   R. Jain, K. Ramakrishnan, and D. Chiu, Congestion Avoidance in
   Computer Networks With a Connectionless Network Layer, Technical
   Report DEC-TR-506, Digital Equipment Corporation.

FORWARD:5.

   V. Jacobson, Congestion Avoidance and Control, Proceedings of
   SIGCOMM '88, Association for Computing Machinery, August 1988.

FORWARD:6.

   W. Barns, Precedence and Priority Access Implementation for
   Department of Defense Data Networks, Technical Report MTR-91W00029,
   The Mitre Corporation, McLean, Virginia, USA, July 1991.

Almquist & Kastenholz [Page 153] RFC 1716 Towards Requirements for IP Routers November 1994

INTERNET:1.

   J. Postel, Internet Protocol, Request For Comments (RFC) 791, STD
   5, USC/Information Sciences Institute, September 1981.

INTERNET:2.

   J. Mogul and J. Postel, Internet Standard Subnetting Procedure,
   Request For Comments (RFC) 950, STD 5, USC/Information Sciences
   Institute, August 1985.

INTERNET:3.

   J. Mogul, Broadcasting Internet Datagrams in the Presence of
   Subnets, Request For Comments (RFC) 922, STD 5, Stanford, October
   1984.

INTERNET:4.

   S. Deering, Host Extensions for IP Multicasting, Request For
   Comments (RFC) 1112, STD 5, Stanford University, August 1989.

INTERNET:5.

   S. Kent, U.S. Department of Defense Security Options for the
   Internet Protocol, Request for Comments (RFC) 1108, BBN
   Communications, November 1991.

INTERNET:6.

   R. Braden, D. Borman, and C. Partridge, Computing the Internet
   Checksum, Request For Comments (RFC) 1071, USC/Information Sciences
   Institute, Cray Researc, BBN, September 1988.

INTERNET:7.

   T. Mallory and A. Kullberg, Incremental Updating of the Internet
   Checksum, Request For Comments (RFC) 1141, BBN, January 1990.

INTERNET:8.

   J. Postel, Internet Control Message Protocol, Request For Comments
   (RFC) 792, STD 5, USC/Information Sciences Institute, September
   1981.

INTERNET:9.

   A. Mankin, G. Hollingsworth, G. Reichlen, K. Thompson, R.  Wilder,
   and R. Zahavi, Evaluation of Internet Performance - FY89, Technical
   Report MTR-89W00216, MITRE Corporation, February, 1990.

INTERNET:10.

   G. Finn, A Connectionless Congestion Control Algorithm, Computer
   Communications Review, vol. 19, no. 5, Association for Computing
   Machinery, October 1989.

Almquist & Kastenholz [Page 154] RFC 1716 Towards Requirements for IP Routers November 1994

INTERNET:11.

   W. Prue, J. Postel, The Source Quench Introduced Delay (SQuID),
   Request For Comments (RFC) 1016, USC/Information Sciences
   Institute, August 1987.

INTERNET:12.

   A. McKenzie, Some comments on SQuID, Request For Comments (RFC)
   1018, BBN, August 1987.

INTERNET:13.

   S. Deering, ICMP Router Discovery Messages, Request For Comments
   (RFC) 1256, Xerox PARC, September 1991.

INTERNET:14.

   J. Mogul and S. Deering, Path MTU Discovery, Request For Comments
   (RFC) 1191, DECWRL, Stanford University, November 1990.

INTERNET:15

   V. Fuller, T. Li, J. Yi, and K. Varadhan, Classless Inter-Domain
   Routing (CIDR): an Address Assignment and Aggregation Strategy
   Request For Comments (RFC) 1519, BARRNet, cisco, Merit, OARnet,
   September 1993.

INTERNET:16

   M. St. Johns, Draft Revised IP Security Option, Request for
   Comments 1038, IETF, January 1988.

INTERNET:17

   W. Prue and J. Postel, Queuing Algorithm to Provide Type-of-service
   For IP Links, Request for Comments 1046, USC/Information Sciences
   Institute, February 1988.

INTRO:1.

   R. Braden and J. Postel, Requirements for Internet Gateways,
   Request For Comments (RFC) 1009, STD 4, USC/Information Sciences
   Institute, June 1987.

INTRO:2.

   Internet Engineering Task Force (R. Braden, Editor), Requirements
   for Internet Hosts - Communication Layers, Request For Comments
   (RFC) 1122, STD 3, USC/Information Sciences Institute, October
   1989.

Almquist & Kastenholz [Page 155] RFC 1716 Towards Requirements for IP Routers November 1994

INTRO:3.

   Internet Engineering Task Force (R. Braden, Editor), Requirements
   for Internet Hosts - Application and Support, Request For Comments
   (RFC) 1123, STD 3, USC/Information Sciences Institute, October
   1989.

INTRO:4.

   D. Clark, Modularity and Efficiency in Protocol Implementations,
   Request For Comments (RFC) 817, MIT, July 1982.

INTRO:5.

   D. Clark, The Structuring of Systems Using Upcalls, Proceedings of
   10th ACM SOSP, December 1985.

INTRO:6.

   O. Jacobsen and J. Postel, Protocol Document Order Information,
   Request For Comments (RFC) 980, SRI, USC/Information Sciences
   Institute, March 1986.

INTRO:7.

   J. Reynolds and J. Postel, Assigned Numbers, Request For Comments
   (RFC) 1700, STD 2, USC/Information Sciences Institute, October
   1994.  This document is periodically updated and reissued with a
   new number.  It is wise to verify occasionally that the version you
   have is still current.

INTRO:8.

   DoD Trusted Computer System Evaluation Criteria, DoD publication
   5200.28-STD, U.S. Department of Defense, December 1985.

INTRO:9

   G. Malkin and T. LaQuey Parker, Internet Users' Glossary, Request
   for Comments (RFC) 1392 (also FYI 0018), Xylogics, Inc., UTexas,
   January 1993.

LINK:1.

   S. Leffler and M. Karels, Trailer Encapsulations, Request For
   Comments (RFC) 893, U. C. Berkeley, April 1984.

LINK:2

   W. Simpson, The Point-to-Point Protocol (PPP) for the Transmission
   of Multi-protocol Datagrams over Point-to-Point Links, Daydreamer,
   Request For Comments (RFC) 1331, May 1992.

Almquist & Kastenholz [Page 156] RFC 1716 Towards Requirements for IP Routers November 1994

LINK:3

   G. McGregor, The PPP Internet Protocol Control Protocol (IPCP),
   Request For Comments (RFC) 1332, Merit, May 1992.

LINK:4

   B. Lloyd, W. Simpson, PPP Authentication Protocols, Request For
   Comments (RFC) 1334, Daydreamer, May 1992.

LINK:5

   W. Simpson, PPP Link Quality Monitoring, Daydreamer, Request For
   Comments (RFC) 1333, May 1992.

MGT:1.

   M. Rose and K. McCloghrie, Structure and Identification of
   Management Information of TCP/IP-based Internets, Request For
   Comments (RFC) 1155, STD 16, Performance Systems International,
   Hughes LAN Systems, May 1990.

MGT:2.

   K. McCloghrie and M. Rose (Editors), Management Information Base of
   TCP/IP-Based Internets: MIB-II, Request For Comments (RFC) 1213,
   STD 16, Hughes LAN Systems, Performance Systems International,
   March 1991.

MGT:3.

   J. Case, M. Fedor, M. Schoffstall, and J. Davin, Simple Network
   Management Protocol, Request For Comments (RFC) 1157, STD 15, SNMP
   Research, Performance Systems International, MIT Laboratory for
   Computer Science, May 1990.

MGT:4.

   M. Rose and K. McCloghrie (Editors), Towards Concise MIB
   Definitions, Request For Comments (RFC) 1212, STD 16, Performance
   Systems International, Hughes LAN Systems, March 1991.

MGT:5.

   L. Steinberg, Techniques for Managing Asynchronously Generated
   Alerts, Request for Comments (RFC) 1224, IBM, May 1991.

MGT:6.

   F. Kastenholz, Definitions of Managed Objects for the Ethernet-like
   Interface Types, Request for Comments (RFC) 1398, FTP Software
   January 1993.

Almquist & Kastenholz [Page 157] RFC 1716 Towards Requirements for IP Routers November 1994

MGT:7.

   R. Fox and K. McCloghrie, IEEE 802.4 Token Bus MIB, Request for
   Comments (RFC) 1230, Hughes LAN Systems, Synoptics, Inc., May 1991.

MGT:8.

   K. McCloghrie, R. Fox and E. Decker, IEEE 802.5 Token Ring MIB,
   Request for Comments (RFC) 1231, Hughes LAN Systems, Synoptics,
   Inc., cisco Systems, Inc., February 1993.

MGT:9.

   J. Case and A. Rijsinghani, FDDI Management Information Base,
   Request for Comments (RFC) 1512, SNMP Research, Digital Equipment
   Corporation, September 1993.

MGT:10.

   B. Stewart, Definitions of Managed Objects for RS-232-like Hardware
   Devices, Request for Comments (RFC) 1317, Xyplex, Inc., April 1992.

MGT:11.

   F. Kastenholz, Definitions of Managed Objects for the Link Control
   Protocol of the Point-to-Point Protocol, Request For Comments (RFC)
   1471, FTP Software, June 1992.

MGT:12.

   F. Kastenholz, The Definitions of Managed Objects for the Security
   Protocols of the Point-to-Point Protocol, Request For Comments
   (RFC) 1472, FTP Software, June 1992.

MGT:13.

   F. Kastenholz, The Definitions of Managed Objects for the IP
   Network Control Protocol of the Point-to-Point Protocol, Request
   For Comments (RFC) 1473, FTP Software, June 1992.

MGT:14.

   F. Baker and R. Coltun, OSPF Version 2 Management Information Base,
   Request For Comments (RFC) 1253, ACC, Computer Science Center,
   August 1991.

MGT:15.

   S. Willis and J. Burruss, Definitions of Managed Objects for the
   Border Gateway Protocol (Version 3), Request For Comments (RFC)
   1269, Wellfleet Communications Inc., October 1991.

MGT:16.

   F. Baker, J. Watt, Definitions of Managed Objects for the DS1 and
   E1 Interface Types, Request For Comments (RFC) 1406, Advanced
   Computer Communications, Newbridge Networks Corporation, January

Almquist & Kastenholz [Page 158] RFC 1716 Towards Requirements for IP Routers November 1994

   1993.

MGT:17.

   T. Cox and K. Tesink, Definitions of Managed Objects for the DS3/E3
   Interface Types, Request For Comments (RFC) 1407, Bell
   Communications Research, January 1993.

MGT:18.

   K. McCloghrie, Extensions to the Generic-Interface MIB, Request For
   Comments (RFC) 1229,  Hughes LAN Systems, August 1992.

MGT:19.

   T. Cox and K. Tesink, Definitions of Managed Objects for the SIP
   Interface Type, Request For Comments (RFC) 1304, Bell
   Communications Research, February 1992.

MGT:20

   F. Baker, IP Forwarding Table MIB, Request For Comments (RFC) 1354,
   ACC, July 1992.

MGT:21.

   G. Malkin and F. Baker, RIP Version 2 MIB Extension, Request For
   Comments (RFC) 1389, Xylogics, Inc., Advanced Computer
   Communications, January 1993.

MGT:22.

   D. Throop, SNMP MIB Extension for the X.25 Packet Layer, Request
   For Comments (RFC) 1382, Data General Corporation, November 1992.

MGT:23.

   D. Throop and F. Baker, SNMP MIB Extension for X.25 LAPB, Request
   For Comments (RFC) 1381, Data General Corporation, Advanced
   Computer Communications, November 1992.

MGT:24.

   D. Throop and F. Baker, SNMP MIB Extension for MultiProtocol
   Interconnect over X.25, Request For Comments (RFC) 1461, Data
   General Corporation, May 1993.

MGT:25.

   M. Rose, SNMP over OSI, Request For Comments (RFC) 1418, Dover
   Beach Consulting, Inc., March 1993.

MGT:26.

   G. Minshall and M. Ritter, SNMP over AppleTalk, Request For
   Comments (RFC) 1419, Novell, Inc., Apple Computer, Inc., March
   1993.

Almquist & Kastenholz [Page 159] RFC 1716 Towards Requirements for IP Routers November 1994

MGT:27.

   S. Bostock, SNMP over IPX, Request For Comments (RFC) 1420, Novell,
   Inc., March 1993.

MGT:28.

   M. Schoffstall, C. Davin, M. Fedor, J. Case, SNMP over Ethernet,
   Request For Comments (RFC) 1089, Rensselaer Polytechnic Institute,
   MIT Laboratory for Computer Science, NYSERNet, Inc., University of
   Tennessee at Knoxville, February 1989.

MGT:29.

   J. Case, FDDI Management Information Base, Request For Comments
   (RFC) 1285, SNMP Research, Incorporated, January 1992.

OPER:1.

   J. Nagle, Congestion Control in IP/TCP Internetworks, Request For
   Comments (RFC) 896, FACC, January 1984.

OPER:2.

   K.R. Sollins, TFTP Protocol (revision 2), Request For Comments
   (RFC) 1350, MIT, July 1992.

ROUTE:1.

   J. Moy, OSPF Version 2, Request For Comments (RFC) 1247, Proteon,
   July 1991.

ROUTE:2.

   R. Callon, Use of OSI IS-IS for Routing in TCP/IP and Dual
   Environments, Request For Comments (RFC) 1195, DEC, December 1990.

ROUTE:3.

   C. L. Hedrick, Routing Information Protocol, Request For Comments
   (RFC) 1058, Rutgers University, June 1988.

ROUTE:4.

   K. Lougheed and Y. Rekhter, A Border Gateway Protocol 3 (BGP-3),
   Request For Comments (RFC) 1267, cisco, T.J. Watson Research
   Center, IBM Corp., October 1991.

ROUTE:5.

   Y. Rekhter and P. Gross Application of the Border Gateway Protocol
   in the Internet, Request For Comments (RFC) 1268, T.J. Watson
   Research Center, IBM Corp., ANS, October 1991.

Almquist & Kastenholz [Page 160] RFC 1716 Towards Requirements for IP Routers November 1994

ROUTE:6.

   D. Mills, Exterior Gateway Protocol Formal Specification, Request
   For Comments (RFC) 904, UDEL, April 1984.

ROUTE:7.

   E. Rosen, Exterior Gateway Protocol (EGP), Request For Comments
   (RFC) 827, BBN, October 1982.

ROUTE:8.

   L. Seamonson and E. Rosen, "STUB" Exterior Gateway Protocol,
   Request For Comments (RFC) 888, BBN, January 1984.

ROUTE:9.

   D. Waitzman, C. Partridge, and S. Deering, Distance Vector
   Multicast Routing Protocol, Request For Comments (RFC) 1075, BBN,
   Stanford, November 1988.

ROUTE:10.

   S. Deering, Multicast Routing in Internetworks and Extended LANs,
   Proceedings of SIGCOMM '88, Association for Computing Machinery,
   August 1988.

ROUTE:11.

   P. Almquist, Type of Service in the Internet Protocol Suite,
   Request for Comments (RFC) 1349, Consultant, July 1992.

ROUTE:12.

   Y. Rekhter, Experience with the BGP Protocol, Request For Comments
   (RFC) 1266, T.J. Watson Research Center, IBM Corp., October 1991.

ROUTE:13.

   Y. Rekhter, BGP Protocol Analysis, Request For Comments (RFC) 1265,
   T.J. Watson Research Center, IBM Corp., October 1991.

TRANS:1.

   J. Postel, User Datagram Protocol, Request For Comments (RFC) 768,
   STD 6, USC/Information Sciences Institute, August 1980.

TRANS:2.

   J. Postel, Transmission Control Protocol, Request For Comments
   (RFC) 793, STD 7, T.J. Watson Research Center, IBM Corp., September
   1981.

Almquist & Kastenholz [Page 161] RFC 1716 Towards Requirements for IP Routers November 1994

APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS

Subject to restrictions given below, a host MAY be able to act as an intermediate hop in a source route, forwarding a source-routed datagram to the next specified hop.

However, in performing this router-like function, the host MUST obey all the relevant rules for a router forwarding source-routed datagrams [INTRO:2]. This includes the following specific provisions:

(A) TTL

   The TTL field MUST be decremented and the datagram perhaps
   discarded as specified for a router in [INTRO:2].

(B) ICMP Destination Unreachable

   A host MUST be able to generate Destination Unreachable messages
   with the following codes:
   4 (Fragmentation Required but DF Set) when a source-routed datagram
     cannot be fragmented to fit into the target network;
   5 (Source Route Failed) when a source-routed datagram cannot be
     forwarded, e.g., because of a routing problem or because the next
     hop of a strict source route is not on a connected network.

(C) IP Source Address

   A source-routed datagram being forwarded MAY (and normally will)
   have a source address that is not one of the IP addresses of the
   forwarding host.

(D) Record Route Option

   A host that is forwarding a source-routed datagram containing a
   Record Route option MUST update that option, if it has room.

(E) Timestamp Option

   A host that is forwarding a source-routed datagram containing a
   Timestamp Option MUST add the current timestamp to that option,
   according to the rules for this option.

To define the rules restricting host forwarding of source-routed datagrams, we use the term local source-routing if the next hop will be through the same physical interface through which the datagram arrived; otherwise, it is non-local source-routing.

A host is permitted to perform local source-routing without restriction.

A host that supports non-local source-routing MUST have a configurable switch to disable forwarding, and this switch MUST default to disabled.

Almquist & Kastenholz [Page 162] RFC 1716 Towards Requirements for IP Routers November 1994

The host MUST satisfy all router requirements for configurable policy filters [INTRO:2] restricting non-local forwarding.

If a host receives a datagram with an incomplete source route but does not forward it for some reason, the host SHOULD return an ICMP Destination Unreachable (code 5, Source Route Failed) message, unless the datagram was itself an ICMP error message.

Almquist & Kastenholz [Page 163] RFC 1716 Towards Requirements for IP Routers November 1994

APPENDIX B. GLOSSARY

This Appendix defines specific terms used in this memo. It also defines some general purpose terms that may be of interest. See also [INTRO:9] for a more general set of definitions.

AS

   Autonomous System A collection of routers under a single
   administrative authority using a common Interior Gateway Protocol
   for routing packets.

Connected Network

   A network to which a router is interfaced is often known as the
   local network or the subnetwork relative to that router. However,
   these terms can cause confusion, and therefore we use the term
   Connected Network in this memo.

Connected (Sub)Network

   A Connected (Sub)Network is an IP subnetwork to which a router is
   interfaced, or a connected network if the connected network is not
   subnetted.  See also Connected Network.

Datagram

   The unit transmitted between a pair of internet modules.  data,
   called datagrams, from sources to destinations.  The Internet
   Protocol does not provide a reliable communication facility.  There
   are no acknowledgments either end-to-end or hop-by-hop.  There is
   no error no retransmissions.  There is no flow control.  See IP.

Default Route

   A routing table entry which is used to direct any data addressed to
   any network numbers not explicitly listed in the routing table.

EGP

   Exterior Gateway Protocol A protocol which distributes routing
   information to the gateways (routers) which connect autonomous
   systems.  See IGP.

EGP-2

   Exterior Gateway Protocol version 2 This is an EGP routing protocol
   developed to handle traffic between AS's in the Internet.

Forwarder

   The logical entity within a router that is responsible for
   switching packets among the router's interfaces.  The Forwarder
   also makes the decisions to queue a packet for local delivery, to

Almquist & Kastenholz [Page 164] RFC 1716 Towards Requirements for IP Routers November 1994

   queue a packet for transmission out another interface, or both.

Forwarding

   Forwarding is the process a router goes through for each packet
   received by the router.  The packet may be consumed by the router,
   it may be output on one or more interfaces of the router, or both.
   Forwarding includes the process of deciding what to do with the
   packet as well as queuing it up for (possible) output or internal
   consumption.

Fragment

   An IP datagram which represents a portion of a higher layer's
   packet which was too large to be sent in its entirety over the
   output network.

IGP

   Interior Gateway Protocol A protocol which distributes routing
   information with an Autonomous System (AS).  See EGP.

Interface IP Address

   The IP Address and subnet mask that is assigned to a specific
   interface of a router.

Internet Address

   An assigned number which identifies a host in an internet.  It has
   two or three parts: network number, optional subnet number, and
   host number.

IP

   Internet Protocol The network layer protocol for the Internet.  It
   is a packet switching, datagram protocol defined in RFC 791.  IP
   does not provide a reliable communications facility; that is, there
   are no end-to-end of hop-by-hop acknowledgments.

IP Datagram

   An IP Datagram is the unit of end-to-end transmission in the
   Internet Protocol.  An IP Datagram consists of an IP header
   followed by all of higher-layer data (such as TCP, UDP, ICMP, and
   the like).  An IP Datagram is an IP header followed by a message.
   An IP Datagram is a complete IP end-to-end transmission unit.  An
   IP Datagram is composed of one or more IP Fragments.
   In this memo, the unqualified term Datagram should be understood to
   refer to an IP Datagram.

Almquist & Kastenholz [Page 165] RFC 1716 Towards Requirements for IP Routers November 1994

IP Fragment

   An IP Fragment is a component of an IP Datagram.  An IP Fragment
   consists of an IP header followed by all or part of the higher-
   layer of the original IP Datagram.
   One or more IP Fragments comprises a single IP Datagram.
   In this memo, the unqualified term Fragment should be understood to
   refer to an IP Fragment.

IP Packet

   An IP Datagram or an IP Fragment.
   In this memo, the unqualified term Packet should generally be
   understood to refer to an IP Packet.

Logical [network] interface

   We define a logical [network] interface to be a logical path,
   distinguished by a unique IP address, to a connected network.

Martian Filtering

   A packet which contains an invalid source or destination address is
   considered to be martian and discarded.

MTU (Maximum Transmission Unit)

   The size of the largest packet that can be transmitted or received
   through a logical interface.  This size includes the IP header but
   does not include the size of any Link Layer headers or framing.

Multicast

   A packet which is destined for multiple hosts.  See broadcast.

Multicast Address

   A special type of address which is recognized by multiple hosts.
   A Multicast Address is sometimes known as a Functional Address or a
   Group Address.

Originate

   Packets can be transmitted by a router for one of two reasons: 1)
   the packet was received and is being forwarded or 2) the router
   itself created the packet for transmission (such as route
   advertisements).  Packets that the router creates for transmission
   are said to originate at the router.

Packet

   A packet is the unit of data passed across the interface between

Almquist & Kastenholz [Page 166] RFC 1716 Towards Requirements for IP Routers November 1994

   the Internet Layer and the Link Layer.  It includes an IP header
   and data.  A packet may be a complete IP datagram or a fragment of
   an IP datagram.

Path

   The sequence of routers and (sub-)networks which a packet traverses
   from a particular router to a particular destination host.  Note
   that a path is uni-directional; it is not unusual to have different
   paths in the two directions between a given host pair.

Physical Network

   A Physical Network is a network (or a piece of an internet) which
   is contiguous at the Link Layer.  Its internal structure (if any)
   is transparent to the Internet Layer.
   In this memo, several media components that are connected together
   via devices such as bridges or repeaters are considered to be a
   single Physical Network since such devices are transparent to the
   IP.

Physical Network Interface

   This is a physical interface to a Connected Network and has a
   (possibly unique) Link-Layer address.  Multiple Physical Network
   Interfaces on a single router may share the same Link-Layer
   address, but the address must be unique for different routers on
   the same Physical Network.

router

   A special-purpose dedicated computer that attaches several networks
   together.  Routers switch packets between these networks in a
   process known as forwarding.  This process may be repeated several
   times on a single packet by multiple routers until the packet can
   be delivered to the final destination - switching the packet from
   router to router to router... until the packet gets to its
   destination.

RPF

   Reverse Path Forwarding A method used to deduce the next hops for
   broadcast and multicast packets.

serial line

   A physical medium which we cannot define, but we recognize one when
   we see one.  See the U.S. Supreme Court's definitions on
   pornography.

Silently Discard

   This memo specifies several cases where a router is to Silently

Almquist & Kastenholz [Page 167] RFC 1716 Towards Requirements for IP Routers November 1994

   Discard a received packet (or datagram).  This means that the
   router should discard the packet without further processing, and
   that the router will not send any ICMP error message (see Section
   [4.3.2]) as a result.  However, for diagnosis of problems, the
   router should provide the capability of logging the error (see
   Section [1.3.3]), including the contents of the silently-discarded
   packet, and should record the event in a statistics counter.

Silently Ignore

   A router is said to Silently Ignore an error or condition if it
   takes no action other than possibly generating an error report in
   an error log or via some network management protocol, and
   discarding, or ignoring, the source of the error.  In particular,
   the router does NOT generate an ICMP error message.

Specific-destination address

   This is defined to be the destination address in the IP header
   unless the header contains an IP broadcast or IP multicast address,
   in which case the specific-destination is an IP address assigned to
   the physical interface on which the packet arrived.

subnet

   A portion of a network, which may be a physically independent
   network, which shares a network address with other portions of the
   network and is distinguished by a subnet number.  A subnet is to a
   network what a network is to an internet.

subnet number

   A part of the internet address which designates a subnet.  It is
   ignored for the purposes internet routing, but is used for intranet
   routing.

TOS

   Type Of Service A field in the IP header which represents the
   degree of reliability expected from the network layer by the
   transport layer or application.

TTL

   Time To Live A field in the IP header which represents how long a
   packet is considered valid.  It is a combination hop count and
   timer value.

Almquist & Kastenholz [Page 168] RFC 1716 Towards Requirements for IP Routers November 1994

APPENDIX C. FUTURE DIRECTIONS

This appendix lists work that future revisions of this document may wish to address.

In the preparation of Router Requirements, we stumbled across several other architectural issues. Each of these is dealt with somewhat in the document, but still ought to be classified as an open issue in the IP architecture.

Most of the he topics presented here generally indicate areas where the technology is still relatively new and it is not appropriate to develop specific requirements since the community is still gaining operational experience.

Other topics represent areas of ongoing research and indicate areas that the prudent developer would closely monitor.

(1) SNMP Version 2

(2) Additional SNMP MIBs

(3) IDPR

(4) CIPSO

(5) IP Next Generation research

(6) More detailed requirements for next-hop selection

(7) More detailed requirements for leaking routes between routing

   protocols

(8) Router system security

(9) Routing protocol security

(10) Internetwork Protocol layer security. There has been extensive

   work refining the security of IP since the original work writing
   this document.  This security work should be included in here.

(11) Route caching

(12) Load Splitting

(13) Sending fragments along different paths

Almquist & Kastenholz [Page 169] RFC 1716 Towards Requirements for IP Routers November 1994

(14) Variable width subnet masks (i.e., not all subnets of a particular

   net use the same subnet mask).  Routers are required (MUST) support
   them, but are not required to detect ambiguous configurations.

(15) Multiple logical (sub)nets on the same wire. Router Requirements

   does not require support for this.  We made some attempt to
   identify pieces of the architecture (e.g. forwarding of directed
   broadcasts and issuing of Redirects) where the wording of the rules
   has to be done carefully to make the right thing happen, and tried
   to clearly distinguish logical interfaces from physical interfaces.
   However, we did not study this issue in detail, and we are not at
   all confident that all of the rules in the document are correct in
   the presence of multiple logical (sub)nets on the same wire.

(15) Congestion control and resource management. On the advice of the

   IETF's experts (Mankin and Ramakrishnan) we deprecated (SHOULD NOT)
   Source Quench and said little else concrete (Section 5.3.6).

(16) Developing a Link-Layer requirements document that would be common

   for both routers and hosts.

(17) Developing a common PPP LQM algorithm.

(18) Investigate of other information (above and beyond section [3.2])

   that passes between the layers, such as physical network MTU,
   mappings of IP precedence to Link Layer priority values, etc.

(19) Should the Link Layer notify IP if address resolution failed (just

   like it notifies IP when there is a Link Layer priority value
   problem)?

(20) Should all routers be required to implement a DNS resolver?

(21) Should a human user be able to use a host name anywhere you can use

   an IP address when configuring the router? Even in ping and
   traceroute?

(22) Almquist's draft ruminations on the next hop and ruminations on

   route leaking need to be reviewed, brought up to date, and
   published.

(23) Investigation is needed to determine if a redirect message for

   precedence is needed or not. If not, are the type-of-service
   redirects acceptable?

(24) RIPv2 and RIP+CIDR and variable length subnet masks.

Almquist & Kastenholz [Page 170] RFC 1716 Towards Requirements for IP Routers November 1994

(25) BGP-4 CIDR is going to be important, and everyone is betting on

   BGP-4. We can't avoid mentioning it.  Probably need to describe the
   differences between BGP-3 and BGP-4, and explore upgrade issues...

(26) Loose Source Route Mobile IP and some multicasting may require

   this.  Perhaps it should be elevated to a SHOULD (per Fred Baker's
   Suggestion).

Almquist & Kastenholz [Page 171] RFC 1716 Towards Requirements for IP Routers November 1994

APPENDIX D. Multicast Routing Protocols

Multicasting is a relatively new technology within the Internet Protocol family. It is not widely deployed or commonly in use yet. Its importance, however, is expected to grow over the coming years.

This Appendix describes some of the technologies being investigated for routing multicasts through the Internet.

A diligent implementor will keep abreast of developments in this area in order to properly develop multicast facilities.

This Appendix does not specify any standards or requirements.

D.1 Introduction

 Multicast routing protocols enable the forwarding of IP multicast
 datagrams throughout a TCP/IP internet. Generally these algorithms
 forward the datagram based on its source and destination addresses.
 Additionally, the datagram may need to be forwarded to several
 multicast group members, at times requiring the datagram to be
 replicated and sent out multiple interfaces.
 The state of multicast routing protocols is less developed than the
 protocols available for the forwarding of IP unicasts.  Two multicast
 routing protocols have been documented for TCP/IP; both are currently
 considered to be experimental.  Both also use the IGMP protocol
 (discussed in Section [4.4]) to monitor multicast group membership.

D.2 Distance Vector Multicast Routing Protocol - DVMRP

 DVMRP, documented in [ROUTE:9], is based on Distance Vector or
 Bellman-Ford technology. It routes multicast datagrams only, and does
 so within a single Autonomous System. DVMRP is an implementation of
 the Truncated Reverse Path Broadcasting algorithm described in
 [ROUTE:10].  In addition, it specifies the tunneling of IP multicasts
 through non-multicast-routing-capable IP domains.

Almquist & Kastenholz [Page 172] RFC 1716 Towards Requirements for IP Routers November 1994

D.3 Multicast Extensions to OSPF - MOSPF

 MOSPF, currently under development, is a backward-compatible addition
 to OSPF that allows the forwarding of both IP multicasts and unicasts
 within an Autonomous System. MOSPF routers can be mixed with OSPF
 routers within a routing domain, and they will interoperate in the
 forwarding of unicasts. OSPF is a link-state or SPF-based protocol.
 By adding link state advertisements that pinpoint group membership,
 MOSPF routers can calculate the path of a multicast datagram as a
 tree rooted at the datagram source. Those branches that do not
 contain group members can then be discarded, eliminating unnecessary
 datagram forwarding hops.

Almquist & Kastenholz [Page 173] RFC 1716 Towards Requirements for IP Routers November 1994

APPENDIX E Additional Next-Hop Selection Algorithms

Section [5.2.4.3] specifies an algorithm that routers ought to use when selecting a next-hop for a packet.

This appendix provides historical perspective for the next-hop selection problem. It also presents several additional pruning rules and next-hop selection algorithms that might be found in the Internet.

This appendix presents material drawn from an earlier, unpublished, work by Philip Almquist; Ruminations on the Next Hop.

This Appendix does not specify any standards or requirements.

E.1. Some Historical Perspective

 It is useful to briefly review the history of the topic, beginning
 with what is sometimes called the "classic model" of how a router
 makes routing decisions.  This model predates IP.  In this model, a
 router speaks some single routing protocol such as RIP.  The protocol
 completely determines the contents of the router's FIB.  The route
 lookup algorithm is trivial: the router looks in the FIB for a route
 whose destination attribute exactly matches the network number
 portion of the destination address in the packet.  If one is found,
 it is used; if none is found, the destination is unreachable.
 Because the routing protocol keeps at most one route to each
 destination, the problem of what to do when there are multiple routes
 which match the same destination cannot arise.
 Over the years, this classic model has been augmented in small ways.
 With the advent of default routes, subnets, and host routes, it
 became possible to have more than one routing table entry which in
 some sense matched the destination.  This was easily resolved by a
 consensus that there was a hierarchy of routes: host routes should be
 preferred over subnet routes, subnet routes over net routes, and net
 routes over default routes.
 With the advent of variable length subnet masks, the general approach
 remained the same although its description became a little more
 complicated. We now say that each route has a bit mask associated
 with it.  If a particular bit in a route's bit mask is set, the
 corresponding bit in the route's destination attribute is
 significant. A route cannot be used to route a packet unless each
 significant bit in the route's destination attribute matches the
 corresponding bit in the packet's destination address, and routes
 with more bits set in their masks are preferred over routes which
 have fewer bits set in their masks. This is simply a generalization

Almquist & Kastenholz [Page 174] RFC 1716 Towards Requirements for IP Routers November 1994

 of the hierarchy of routes described above, and will be referred to
 for the rest of this memo as choosing a route by preferring longest
 match.
 Another way the classic model has been augmented is through a small
 amount of relaxation of the notion that a routing protocol has
 complete control over the contents of the routing table.  First,
 static routes were introduced.  For the first time, it was possible
 to simultaneously have two routes (one dynamic and one static) to the
 same destination.  When this happened, a router had to have a policy
 (in some cases configurable, and in other cases chosen by the author
 of the router's software) which determined whether the static route
 or the dynamic route was preferred. However, this policy was only
 used as a tie-breaker when longest match didn't uniquely determine
 which route to use. Thus, for example, a static default route would
 never be preferred over a dynamic net route even if the policy
 preferred static routes over dynamic routes.
 The classic model had to be further augmented when inter-domain
 routing protocols were invented. Traditional routing protocols came
 to be called "interior gateway protocols" (IGPs), and at each
 Internet site there was a strange new beast called an "exterior
 gateway", a router which spoke EGP to several "BBN Core Gateways"
 (the routers which made up the Internet backbone at the time) at the
 same time as it spoke its IGP to the other routers at its site. Both
 protocols wanted to determine the contents of the router's routing
 table. Theoretically, this could result in a router having three
 routes (EGP, IGP, and static) to the same destination.  Because of
 the Internet topology at the time, it was resolved with little debate
 that routers would be best served by a policy of preferring IGP
 routes over EGP routes.  However, the sanctity of longest match
 remained unquestioned: a default route learned from the IGP would
 never be preferred over a net route from learned EGP.
 Although the Internet topology, and consequently routing in the
 Internet, have evolved considerably since then, this slightly
 augmented version of the classic model has survived pretty much
 intact to this day in the Internet (except that BGP has replaced
 EGP).  Conceptually (and often in implementation) each router has a
 routing table and one or more routing protocol processes.  Each of
 these processes can add any entry that it pleases, and can delete or
 modify any entry that it has created. When routing a packet, the
 router picks the best route using longest match, augmented with a
 policy mechanism to break ties. Although this augmented classic model
 has served us well, it has a number of shortcomings:
 o  It ignores (although it could be augmented to consider) path

Almquist & Kastenholz [Page 175] RFC 1716 Towards Requirements for IP Routers November 1994

    characteristics such as quality of service and MTU.
 o  It doesn't support routing protocols (such as OSPF and Integrated
    IS-IS) that require route lookup algorithms different than pure
    longest match.
 o  There has not been a firm consensus on what the tie-breaking
    mechanism ought to be. Tie-breaking mechanisms have often been
    found to be difficult if not impossible to configure in such a way
    that the router will always pick what the network manger considers
    to be the "correct" route.

E.2. Additional Pruning Rules

 Section [5.2.4.3] defined several pruning rules to use to select
 routes from the FIB.  There are other rules that could also be used.
 o  OSPF Route Class
    Routing protocols which have areas or make a distinction between
    internal and external routes divide their routes into classes,
    where classes are rank-ordered in terms of preference. A route is
    always chosen from the most preferred class unless none is
    available, in which case one is chosen from the second most
    preferred class, and so on. In OSPF, the classes (in order from
    most preferred to least preferred) are intra-area, inter-area,
    type 1 external (external routes with internal metrics), and type
    2 external. As an additional wrinkle, a router is configured to
    know what addresses ought to be accessible via intra-area routes,
    and will not use inter- area or external routes to reach these
    destinations even when no intra-area route is available.
    More precisely, we assume that each route has a class attribute,
    called route.class, which is assigned by the routing protocol.
    The set of candidate routes is examined to determine if it
    contains any for which route.class = intra-area.  If so, all
    routes except those for which route.class = intra-area are
    discarded.  Otherwise, router checks whether the packet's
    destination falls within the address ranges configured for the
    local area.  If so, the entire set of candidate routes is deleted.
    Otherwise, the set of candidate routes is examined to determine if
    it contains any for which route.class = inter-area.  If so, all
    routes except those for which route.class = inter-area are
    discarded.  Otherwise, the set of candidate routes is examined to
    determine if it contains any for which route.class = type 1
    external.  If so, all routes except those for which route.class =
    type 1 external are discarded.

Almquist & Kastenholz [Page 176] RFC 1716 Towards Requirements for IP Routers November 1994

 o  IS-IS Route Class
    IS-IS route classes work identically to OSPF's. However, the set
    of classes defined by Integrated IS-IS is different, such that
    there isn't a one-to-one mapping between IS-IS route classes and
    OSPF route classes. The route classes used by Integrated IS-IS are
    (in order from most preferred to least preferred) intra-area,
    inter-area, and external.
    The Integrated IS-IS internal class is equivalent to the OSPF
    internal class. Likewise, the Integrated IS-IS external class is
    equivalent to OSPF's type 2 external class. However, Integrated
    IS-IS does not make a distinction between inter-area routes and
    external routes with internal metrics - both are considered to be
    inter-area routes. Thus, OSPF prefers true inter-area routes over
    external routes with internal metrics, whereas Integrated IS-IS
    gives the two types of routes equal preference.
 o  IDPR Policy
    A specific case of Policy. The IETF's Inter-domain Policy Routing
    Working Group is devising a routing protocol called Inter-Domain
    Policy Routing (IDPR) to support true policy-based routing in the
    Internet. Packets with certain combinations of header attributes
    (such as specific combinations of source and destination addresses
    or special IDPR source route options) are required to use routes
    provided by the IDPR protocol. Thus, unlike other Policy pruning
    rules, IDPR Policy would have to be applied before any other
    pruning rules except Basic Match.
    Specifically, IDPR Policy examines the packet being forwarded to
    ascertain if its attributes require that it be forwarded using
    policy-based routes. If so, IDPR Policy deletes all routes not
    provided by the IDPR protocol.

E.3 Some Route Lookup Algorithms

 This section examines several route lookup algorithms that are in use
 or have been proposed.  Each is described by giving the sequence of
 pruning rules it uses.  The strengths and weaknesses of each
 algorithm are presented

Almquist & Kastenholz [Page 177] RFC 1716 Towards Requirements for IP Routers November 1994

E.3.1 The Revised Classic Algorithm

    The Revised Classic Algorithm is the form of the traditional
    algorithm which was discussed in Section [E.1].  The steps of this
    algorithm are:
    1.  Basic match
    2.  Longest match
    3.  Best metric
    4.  Policy
    Some implementations omit the Policy step, since it is needed only
    when routes may have metrics that are not comparable (because they
    were learned from different routing domains).
    The advantages of this algorithm are:
    (1)  It is widely implemented.
    (2)  Except for the Policy step (which an implementor can choose
         to make arbitrarily complex) the algorithm is simple both to
         understand and to implement.
    Its disadvantages are:
    (1)  It does not handle IS-IS or OSPF route classes, and therefore
         cannot be used for Integrated IS-IS or OSPF.
    (2)  It does not handle TOS or other path attributes.
    (3)  The policy mechanisms are not standardized in any way, and
         are therefore are often implementation-specific.  This causes
         extra work for implementors (who must invent appropriate
         policy mechanisms) and for users (who must learn how to use
         the mechanisms.  This lack of a standardized mechanism also
         makes it difficult to build consistent configurations for
         routers from different vendors.  This presents a significant
         practical deterrent to multi-vendor interoperability.
    (4)  The proprietary policy mechanisms currently provided by
         vendors are often inadequate in complex parts of the
         Internet.
    (5)  The algorithm has not been written down in any generally
         available document or standard.  It is, in effect, a part of
         the Internet Folklore.

Almquist & Kastenholz [Page 178] RFC 1716 Towards Requirements for IP Routers November 1994

E.3.2 The Variant Router Requirements Algorithm

    Some Router Requirements Working Group members have proposed a
    slight variant of the algorithm described in the Section
    [5.2.4.3].  In this variant, matching the type of service
    requested is considered to be more important, rather than less
    important, than matching as much of the destination address as
    possible.  For example, this algorithm would prefer a default
    route which had the correct type of service over a network route
    which had the default type of service, whereas the algorithm in
    [5.2.4.3] would make the opposite choice.
    The steps of the algorithm are:
    1.  Basic match
    2.  Weak TOS
    3.  Longest match
    4.  Best metric
    5.  Policy
    Debate between the proponents of this algorithm and the regular
    Router Requirements Algorithm suggests that each side can show
    cases where its algorithm leads to simpler, more intuitive routing
    than the other's algorithm does.  In general, this variant has the
    same set of advantages and disadvantages that the algorithm
    specified in [5.2.4.3] does, except that pruning on Weak TOS
    before pruning on Longest Match makes this algorithm less
    compatible with OSPF and Integrated IS-IS than the standard Router
    Requirements Algorithm.

E.3.3 The OSPF Algorithm

    OSPF uses an algorithm which is virtually identical to the Router
    Requirements Algorithm except for one crucial difference: OSPF
    considers OSPF route classes.
    The algorithm is:
    1.  Basic match
    2.  OSPF route class
    3.  Longest match
    4.  Weak TOS
    5.  Best metric
    6.  Policy
    Type of service support is not always present.  If it is not
    present then, of course, the fourth step would be omitted
    This algorithm has some advantages over the Revised Classic

Almquist & Kastenholz [Page 179] RFC 1716 Towards Requirements for IP Routers November 1994

    Algorithm:
    (1)  It supports type of service routing.
    (2)  Its rules are written down, rather than merely being a part
         of the Internet folklore.
    (3)  It (obviously) works with OSPF.
    However, this algorithm also retains some of the disadvantages of
    the Revised Classic Algorithm:
    (1)  Path properties other than type of service (e.g. MTU) are
         ignored.
    (2)  As in the Revised Classic Algorithm, the details (or even the
         existence) of the Policy step are left to the discretion of
         the implementor.
    The OSPF Algorithm also has a further disadvantage (which is not
    shared by the Revised Classic Algorithm).  OSPF internal (intra-
    area or inter-area) routes are always considered to be superior to
    routes learned from other routing protocols, even in cases where
    the OSPF route matches fewer bits of the destination address.
    This is a policy decision that is inappropriate in some networks.
    Finally, it is worth noting that the OSPF Algorithm's TOS support
    suffers from a deficiency in that routing protocols which support
    TOS are implicitly preferred when forwarding packets which have
    non-zero TOS values.  This may not be appropriate in some cases.

E.3.4 The Integrated IS-IS Algorithm

    Integrated IS-IS uses an algorithm which is similar to but not
    quite identical to the OSPF Algorithm.  Integrated IS-IS uses a
    different set of route classes, and also differs slightly in its
    handling of type of service.  The algorithm is:
    1. Basic Match
    2. IS-IS Route Classes
    3. Longest Match
    4. Weak TOS
    5. Best Metric
    6. Policy
    Although Integrated IS-IS uses Weak TOS, the protocol is only
    capable of carrying routes for a small specific subset of the
    possible values for the TOS field in the IP header.  Packets

Almquist & Kastenholz [Page 180] RFC 1716 Towards Requirements for IP Routers November 1994

    containing other values in the TOS field are routed using the
    default TOS.
    Type of service support is optional; if disabled, the fourth step
    would be omitted.  As in OSPF, the specification does not include
    the Policy step.
    This algorithm has some advantages over the Revised Classic
    Algorithm:
    (1)  It supports type of service routing.
    (2)  Its rules are written down, rather than merely being a part
         of the Internet folklore.
    (3)  It (obviously) works with Integrated IS-IS.
    However, this algorithm also retains some of the disadvantages of
    the Revised Classic Algorithm:
    (1)  Path properties other than type of service (e.g. MTU) are
         ignored.
    (2)  As in the Revised Classic Algorithm, the details (or even the
         existence) of the Policy step are left to the discretion of
         the implementor.
    (3)  It doesn't work with OSPF because of the differences between
         IS-IS route classes and OSPF route classes.  Also, because
         IS-IS supports only a subset of the possible TOS values, some
         obvious implementations of the Integrated IS-IS algorithm
         would not support OSPF's interpretation of TOS.
    The Integrated IS-IS Algorithm also has a further disadvantage
    (which is not shared by the Revised Classic Algorithm): IS-IS
    internal (intra-area or inter-area) routes are always considered
    to be superior to routes learned from other routing protocols,
    even in cases where the IS-IS route matches fewer bits of the
    destination address and doesn't provide the requested type of
    service.  This is a policy decision that may not be appropriate in
    all cases.
    Finally, it is worth noting that the Integrated IS-IS Algorithm's
    TOS support suffers from the same deficiency noted for the OSPF
    Algorithm.

Almquist & Kastenholz [Page 181] RFC 1716 Towards Requirements for IP Routers November 1994

Security Considerations

Although the focus of this document is interoperability rather than security, there are obviously many sections of this document which have some ramifications on network security.

Security means different things to different people. Security from a router's point of view is anything that helps to keep its own networks operational and in addition helps to keep the Internet as a whole healthy. For the purposes of this document, the security services we are concerned with are denial of service, integrity, and authentication as it applies to the first two. Privacy as a security service is important, but only peripherally a concern of a router - at least as of the date of this document.

In several places in this document there are sections entitled … Security Considerations. These sections discuss specific considerations that apply to the general topic under discussion.

Rarely does this document say do this and your router/network will be secure. More likely, it says this is a good idea and if you do it, it *may* improve the security of the Internet and your local system in general.

Unfortunately, this is the state-of-the-art AT THIS TIME. Few if any of the network protocols a router is concerned with have reasonable, built-in security features. Industry and the protocol designers have been and are continuing to struggle with these issues. There is progress, but only small baby steps such as the peer-to-peer authentication available in the BGP and OSPF routing protocols.

In particular, this document notes the current research into developing and enhancing network security. Specific areas of research, development, and engineering that are underway as of this writing (December 1993) are in IP Security, SNMP Security, and common authentication technologies.

Notwithstanding all of the above, there are things both vendors and users can do to improve the security of their router. Vendors should get a copy of Trusted Computer System Interpretation [INTRO:8]. Even if a vendor decides not to submit their device for formal verification under these guidelines, the publication provides excellent guidance on general security design and practices for computing devices.

Almquist & Kastenholz [Page 182] RFC 1716 Towards Requirements for IP Routers November 1994

Acknowledgments

O that we now had here But one ten thousand of those men in England That do no work to-day!

What's he that wishes so? My cousin Westmoreland? No, my fair cousin: If we are mark'd to die, we are enow To do our country loss; and if to live, The fewer men, the greater share of honour. God's will! I pray thee, wish not one man more. By Jove, I am not covetous for gold, Nor care I who doth feed upon my cost; It yearns me not if men my garments wear; Such outward things dwell not in my desires: But if it be a sin to covet honour, I am the most offending soul alive. No, faith, my coz, wish not a man from England: God's peace! I would not lose so great an honour As one man more, methinks, would share from me For the best hope I have. O, do not wish one more! Rather proclaim it, Westmoreland, through my host, That he which hath no stomach to this fight, Let him depart; his passport shall be made And crowns for convoy put into his purse: We would not die in that man's company That fears his fellowship to die with us. This day is called the feast of Crispian: He that outlives this day, and comes safe home, Will stand a tip-toe when the day is named, And rouse him at the name of Crispian. He that shall live this day, and see old age, Will yearly on the vigil feast his neighbours, And say 'To-morrow is Saint Crispian:' Then will he strip his sleeve and show his scars. And say 'These wounds I had on Crispin's day.' Old men forget: yet all shall be forgot, But he'll remember with advantages What feats he did that day: then shall our names. Familiar in his mouth as household words Harry the king, Bedford and Exeter, Warwick and Talbot, Salisbury and Gloucester, Be in their flowing cups freshly remember'd. This story shall the good man teach his son; And Crispin Crispian shall ne'er go by,

Almquist & Kastenholz [Page 183] RFC 1716 Towards Requirements for IP Routers November 1994

From this day to the ending of the world, But we in it shall be remember'd; We few, we happy few, we band of brothers; For he to-day that sheds his blood with me Shall be my brother; be he ne'er so vile, This day shall gentle his condition: And gentlemen in England now a-bed Shall think themselves accursed they were not here, And hold their manhoods cheap whiles any speaks That fought with us upon Saint Crispin's day.

This memo is a product of the IETF's Router Requirements Working Group. A memo such as this one is of necessity the work of many more people than could be listed here. A wide variety of vendors, network managers, and other experts from the Internet community graciously contributed their time and wisdom to improve the quality of this memo. The editor wishes to extend sincere thanks to all of them.

The current editor also wishes to single out and extend his heartfelt gratitude and appreciation to the original editor of this document; Philip Almquist. Without Philip's work, both as the original editor and as the Chair of the working group, this document would not have been produced.

Philip Almquist, Jeffrey Burgan, Frank Kastenholz, and Cathy Wittbrodt each wrote major chapters of this memo. Others who made major contributions to the document included Bill Barns, Steve Deering, Kent England, Jim Forster, Martin Gross, Jeff Honig, Steve Knowles, Yoni Malachi, Michael Reilly, and Walt Wimer.

Additional text came from Art Berggreen, John Cavanaugh, Ross Callon, John Lekashman, Brian Lloyd, Gary Malkin, Milo Medin, John Moy, Craig Partridge, Stephanie Price, Yakov Rekhter, Steve Senum, Richard Smith, Frank Solensky, Rich Woundy, and others who have been inadvertently overlooked.

Some of the text in this memo has been (shamelessly) plagiarized from earlier documents, most notably RFC-1122 by Bob Braden and the Host Requirements Working Group, and RFC-1009 by Bob Braden and Jon Postel. The work of these earlier authors is gratefully acknowledged.

Jim Forster was a co-chair of the Router Requirements Working Group during its early meetings, and was instrumental in getting the group off to a good start. Jon Postel, Bob Braden, and Walt Prue also contributed to the success by providing a wealth of good advice prior to the group's first meeting. Later on, Phill Gross, Vint Cerf, and Noel Chiappa all provided valuable advice and support.

Almquist & Kastenholz [Page 184] RFC 1716 Towards Requirements for IP Routers November 1994

Mike St. Johns coordinated the Working Group's interactions with the security community, and Frank Kastenholz coordinated the Working Group's interactions with the network management area. Allison Mankin and K.K. Ramakrishnan provided expertise on the issues of congestion control and resource allocation.

Many more people than could possibly be listed or credited here participated in the deliberations of the Router Requirements Working Group, either through electronic mail or by attending meetings. However, the efforts of Ross Callon and Vince Fuller in sorting out the difficult issues of route choice and route leaking are especially acknowledged.

The previous editor, Philip Almquist, wishes to extend his thanks and appreciation to his former employers, Stanford University and BARRNet, for allowing him to spend a large fraction (probably far more than they ever imagined when he started on this) of his time working on this project.

The current editor wishes to thank his employer, FTP Software, for allowing him to spend the time necessary to finish this document.

Almquist & Kastenholz [Page 185] RFC 1716 Towards Requirements for IP Routers November 1994

Editor's Address

The address of the current editor of this document is

 Frank J. Kastenholz
 FTP Software
 2 High Street
 North Andover, MA, 01845-2620
 USA
 Phone: +1 508-685-4000
 EMail: kasten@ftp.com

Almquist & Kastenholz [Page 186]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1716.txt · Last modified: 1994/11/03 21:12 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki