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rfc:rfc1009

Network Working Group R. Braden Request for Comments: 1009 J. Postel Obsoletes: 985 ISI

                                                             June 1987
                 Requirements for Internet Gateways

Status of this Memo

 This document is a formal statement of the requirements to be met by
 gateways used in the Internet system.  As such, it is an official
 specification for the Internet community.  Distribution of this memo
 is unlimited.
 This RFC summarizes the requirements for gateways to be used between
 networks supporting the Internet protocols.  While it was written
 specifically to support National Science Foundation research
 programs, the requirements are stated in a general context and are
 applicable throughout the Internet community.
 The purpose of this document is to present guidance for vendors
 offering gateway products that might be used or adapted for use in an
 Internet application.  It enumerates the protocols required and gives
 references to RFCs and other documents describing the current
 specifications.  In a number of cases the specifications are evolving
 and may contain ambiguous or incomplete information.  In these cases
 further discussion giving specific guidance is included in this
 document.  Specific policy issues relevant to the NSF scientific
 networking community are summarized in an Appendix.  As other
 specifications are updated this document will be revised.  Vendors
 are encouraged to maintain contact with the Internet research
 community.

1. Introduction

 The following material is intended as an introduction and background
 for those unfamiliar with the Internet architecture and the Internet
 gateway model.  General background and discussion on the Internet
 architecture and supporting protocol suite can be found in the DDN
 Protocol Handbook [25] and ARPANET Information Brochure [26], see
 also [19, 28, 30, 31].
 The Internet protocol architecture was originally developed under
 DARPA sponsorship to meet both military and civilian communication
 requirements [32].  The Internet system presently supports a variety
 of government and government-sponsored operational and research
 activities.  In particular, the National Science Foundation (NSF) is
 building a major extension to the Internet to provide user access to

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RFC 1009 - Requirements for Internet Gateways June 1987

 national supercomputer centers and other national scientific
 resources, and to provide a computer networking capability to a large
 number of universities and colleges.
 In this document there are many terms that may be obscure to one
 unfamiliar with the Internet protocols.  There is not much to be done
 about that but to learn, so dive in.  There are a few terms that are
 much abused in general discussion but are carefully and intentionally
 used in this document.  These few terms are defined here.
    Packet      A packet is the unit of transmission on a physical
                network.
    Datagram    A datagram is the unit of transmission in the IP
                protocol.  To cross a particular network a datagram is
                encapsulated inside a packet.
    Router      A router is a switch that receives data transmission
                units from input interfaces and, depending on the
                addresses in those units, routes them to the
                appropriate output interfaces.  There can be routers
                at different levels of protocol.  For example,
                Interface Message Processors (IMPs) are packet-level
                routers.
    Gateway     In the Internet documentation generally, and in this
                document specifically, a gateway is an IP-level
                router.  In the Internet community the term has a long
                history of this usage [32].
 1.1.  The DARPA Internet Architecture
    1.1.1.  Internet Protocols
       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 Transmission Control Protocol (TCP), and
       application protocols depending upon them [22].
       All Internet protocols use IP as the basic data transport
       mechanism.  IP [1,31] is a datagram, or connectionless,
       internetwork service and includes provision for addressing,
       type-of-service specification, fragmentation and reassembly,
       and security information.  ICMP [2] is considered an integral

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       part of IP, although it is architecturally layered upon IP.
       ICMP provides error reporting, flow control and first-hop
       gateway redirection.
       Reliable data delivery is provided in the Internet protocol
       suite by transport-level protocols such as the Transmission
       Control Protocol (TCP), which provides end-end retransmission,
       resequencing and connection control.  Transport-level
       connectionless service is provided by the User Datagram
       Protocol (UDP).
    1.1.2.  Networks and Gateways
       The constituent networks of the Internet system are required
       only to provide packet (connectionless) transport.  This
       requires only delivery of individual packets.  According to the
       IP service specification, datagrams can be delivered out of
       order, be lost or duplicated and/or contain errors.  Reasonable
       performance of the protocols that use IP (e.g., TCP) requires
       an IP datagram loss rate of less than 5%.  In those 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:
  • Local-Area Networks (LANs)
          LANs may have a variety of designs, typically based upon
          buss, ring, or star topologies.  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.
  • 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-routers (typified by ARPANET), or they may
          be as simple as point-to-point lines.
       In the Internet model, constituent networks are connected
       together by IP datagram forwarders which are called "gateways"
       or "IP routers".  In this document, every use of the term
       "gateway" is equivalent to "IP router".  In current practice,
       gateways are normally realized with packet-switching software

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       executing on a general-purpose CPU, but special-purpose
       hardware may also be used (and may be required for future
       higher-throughput gateways).
       A gateway is connected to two or more networks, appearing to
       each of these networks as a connected host.  Thus, it has a
       physical interface and an IP address on each of the connected
       networks.  Forwarding an IP datagram generally requires the
       gateway to choose the address of the next-hop gateway or (for
       the final hop) the destination host.  This choice, called
       "routing", depends upon a routing data-base within the gateway.
       This routing data-base should be maintained dynamically to
       reflect the current topology of the Internet system; a gateway
       normally accomplishes this by participating in distributed
       routing and reachability algorithms with other gateways.
       Gateways 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.
       Routing devices may also operate at the network level; in this
       memo we will call such devices MAC routers (informally called
       "level-2 routers", and also called "bridges").  The name
       derives from the fact that MAC routers base their routing
       decision on the addresses in the MAC headers; e.g., in IEEE
       802.3 networks, a MAC router bases its decision on the 48-bit
       addresses in the MAC header.  Network segments which are
       connected by MAC routers share the same IP network number,
       i.e., they logically form a single IP network.
       Another variation on the simple model of networks connected
       with gateways sometimes occurs: a set of gateways may be
       interconnected with only serial lines, to effectively form a
       network in which the routing is performed at the internetwork
       (IP) level rather than the network level.
    1.1.3.  Autonomous Systems
       For technical, managerial, and sometimes political reasons, the
       gateways of the Internet system are grouped into collections
       called "autonomous systems" [35].  The gateways included in a
       single autonomous system (AS) are expected to:
  • Be under the control of a single operations and

maintenance (O&M) organization;

  • Employ common routing protocols among themselves, to

maintain their routing data-bases dynamically.

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       A number of different dynamic routing protocols have been
       developed (see Section 4.1); the particular choice of routing
       protocol within a single AS is generically called an interior
       gateway protocol or IGP.
       An IP datagram may have to traverse the gateways of two or more
       ASs to reach its destination, and the ASs must provide each
       other with topology information to allow such forwarding.  The
       Exterior Gateway Protocol (EGP) is used for this purpose,
       between gateways of different autonomous systems.
    1.1.4.  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 gateway 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 two-level hierarchical routing
       structure for the Internet system.  The subnet extension,
       described in RFC-950 [21], 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 ::=
                  { <Network-number>, <Subnet-number>, <Host-number> }
       The interconnected LANs of an organization will be given the
       same network number but different subnet numbers.  The
       distinction between the subnets of such a subnetted network
       must not be visible outside that network.  Thus, wide-area
       routing in the rest of the Internet will be based only upon the
       <Network-number> part of the IP destination address; gateways
       outside the network will lump <Subnet-number> and <Host-number>

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       together to form an uninterpreted "rest" part of the 32-bit IP
       address.  Within the subnetted network, the local gateways must
       route on the basis of an extended network number:
          { <Network-number>, <Subnet-number> }.
       The bit positions containing this extended network number are
       indicated by a 32-bit mask called the "subnet mask" [21]; 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 (all one bits).
       Flexible use of the available address space will be
       increasingly important in coping with the anticipated growth of
       the Internet.  Thus, we allow a particular subnetted network to
       use more than one subnet mask.  Several campuses with very
       large LAN configurations are also creating nested hierarchies
       of subnets, sub-subnets, etc.
       There are special considerations for the gateway when a
       connected network provides a broadcast or multicast capability;
       these will be discussed later.
 1.2.  The Internet Gateway Model
    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
    gateways 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.  Gateways that support
    this second model are called "address sharing gateways" or
    "transparent gateways".  The focus of this memo is on gateways
    that support the first model, but this is not intended to exclude
    the use of transparent gateways.
    1.2.1.  Internet Gateways
       An Internet gateway is an IP-level router that 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.  See Section 2 (Protocols Required).
          2.  Interfaces to two or more packet networks.  For each

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              connected network the gateway must implement the
              functions required by that network.  These functions
              typically include:
             a.  encapsulating and decapsulating the IP datagrams with
                 the connected network framing (e.g., an Ethernet
                 header and checksum);
             b.  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";
             c.  translating the IP destination address into an
                 appropriate network-level address for the connected
                 network (e.g., an Ethernet hardware address);
             d.  responding to the network flow control and error
                 indication, if any.
             See Section 3 (Constituent Network Interface), for
             details on particular constituent network interfaces.
          3.  Receives and forwards Internet datagrams.  Important
              issues are buffer management, congestion control, and
              fairness.  See Section 4 (Gateway Algorithms).
             a.  Recognizes various error conditions and generates
                 ICMP error and information messages as required.
             b.  Drops datagrams whose time-to-live fields have
                 reached zero.
             c.  Fragments datagrams when necessary to fit into the
                 MTU of the next network.
          4.  Chooses a next-hop destination for each IP datagram,
              based on the information in its routing data-base.  See
              Section 4 (Gateway Algorithms).
          5.  Supports an interior gateway protocol (IGP) to carry out
              distributed routing and reachability algorithms with the
              other gateways in the same autonomous system.  In
              addition, some gateways will need to support the
              Exterior Gateway Protocol (EGP) to exchange topological
              information with other autonomous systems.  See
              Section 4 (Gateway Algorithms).

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          6.  Provides system support facilities, including loading,
              debugging, status reporting, exception reporting and
              control.  See Section 5 (Operation and Maintenance).
    1.2.2.  Embedded Gateways
       A gateway may be a stand-alone computer system, dedicated to
       its IP router functions.  Alternatively, it is possible to
       embed gateway functionality within a host operating system
       which supports connections to two or more networks.  The
       best-known example of an operating system with embedded gateway
       code is the Berkeley BSD system.  The embedded gateway 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 gateway.
              For example, hosts with embedded gateway code that
              gratuitously forward broadcast packets or datagrams on
              the same net often cause packet avalanches.
          2.  If a (multihomed) host acts as a gateway, it must
              implement ALL the relevant gateway requirements
              contained in this document.
              For example, the routing protocol issues (see Sections
              2.6 and 4.1) and the control and monitoring problems are
              as hard and important for embedded gateways as for
              stand-alone gateways.
                 Since Internet gateway requirements and
                 specifications may change independently of operating
                 system changes, an administration that operates an
                 embedded gateway in the Internet is strongly advised
                 to have an ability to maintain and update the gateway
                 code (e.g., this might require gateway code source).
          3.  Once a host runs embedded gateway code, it becomes part
              of the Internet system.  Thus, errors in software or
              configuration of such a host 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 gateway coded embedded in the operating system,
              and any embedded gateway code must be organized so it
              can be easily disabled.

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          4.  If a host running embedded gateway code is concurrently
              used for other services, the O&M (operation and
              maintenance) requirements for the two modes of use may
              be in serious conflict.
              For example, gateway 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.
    1.2.3.  Transparent Gateways
       The basic idea of a transparent gateway is that the hosts on
       the local-area network behind such a gateway share the address
       space of the wide-area network in front of the gateway.  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 (that is, all the IP addresses in the local
       environment map to a few (usually one) physical address in the
       wide-area network, 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
       gateway cannot fully emulate the normal wide-area network
       service.  For example, if there were a transparent gateway
       between the ARPANET and an Ethernet, a remote host would not
       receive a Destination Dead message [3] if it sent a datagram to
       an Ethernet host that was powered off.

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 1.3.  Gateway Characteristics
    Every Internet gateway must perform the functions listed above.
    However, a vendor will have many choices on power, complexity, and
    features for a particular gateway 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".
  • The global-interconnect system is comprised of a number of

wide-area networks to which are attached gateways of several

          ASs; there are relatively few hosts connected directly to
          it.  The global-interconnect system includes the ARPANET,
          the NSFNET "backbone", the various NSF regional and
          consortium networks, other ARPA sponsored networks such as
          the SATNET and the WBNET, and the DCA sponsored MILNET.  It
          is anticipated that additional networks sponsored by these
          and other agencies (such as NASA and DOE) will join the
          global-interconnect system.
  • Most hosts are connected to LANs, and many organizations

have clusters of LANs interconnected by local gateways.

          Each such cluster is connected by gateways 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.
    Gateways in the global-interconnect system generally require:
  • Advanced routing and forwarding algorithms
          These gateways need routing algorithms which are highly
          dynamic and also offer type-of-service routing.  Congestion
          is still not a completely resolved issue [24].  Improvements
          to the current situation will be implemented soon, as the
          research community is actively working on these issues.
  • High availability
          These gateways need to be highly reliable, providing 24 hour
          a day, 7 days a week service.  In case of failure, they must
          recover quickly.
  • Advanced O&M features
          These gateways will typically be operated remotely from a
          regional or national monitoring center.  In their

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          interconnect role, they will need to provide sophisticated
          means for monitoring and measuring traffic and other events
          and for diagnosing faults.
  • High performance
          Although long-haul lines in the Internet today are most
          frequently 56 Kbps, DS1 lines (1.5 Mbps) are of increasing
          importance, and even higher speeds are likely in the future.
          Full-duplex operation is provided at any of these speeds.
          The average size of Internet datagrams is rather small, of
          the order of 100 bytes.  At DS1 line speeds, the
          per-datagram processing capability of the gateways, rather
          than the line speed, is likely to be the bottleneck.  To
          fill a DS1 line with average-sized Internet datagrams, a
          gateway would need to pass -- receive, route, and send --
          2,000 datagrams per second per interface.  That is, a
          gateway which supported 3 DS1 lines and and Ethernet
          interface would need to be able to pass a dazzling 2,000
          datagrams per second in each direction on each of the
          interfaces, or a aggregate throughput of 8,000 datagrams per
          second, in order to fully utilize DS1 lines.  This is beyond
          the capability of current gateways.
             Note: some vendors count input and output operations
             separately in datagrams per second figures; for these
             vendors, the above example would imply 16,000 datagrams
             per second !
    Gateways used in the "LAN fringe" (e.g., campus networks) will
    generally have to meet less stringent requirements for
    performance, availability, and maintenance.  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 gateways
    should emphasize low average delay and good burst performance,
    together with delay and type-of-service sensitive resource
    management.  In this environment, there will be less formal O&M,
    more hand-crafted static configurations for special cases, and
    more need for inter-operation with gateways of other vendors.  The
    routing mechanism will need to be very flexible, but need not be
    so highly dynamic as in the global-interconnect system.
    It is important to realize that Internet gateways normally operate
    in an unattended mode, but that equipment and software faults can
    have a wide-spread (sometimes global) effect.  In any environment,

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    a gateway must be highly robust and able to operate, possibly in a
    degraded state, under conditions of extreme congestion or failure
    of network resources.
    Even though the Internet system is not fully-interconnected, many
    parts of the system do need to have redundant connectivity.  A
    rich connectivity allows reliable service despite failures of
    communication lines and gateways, and it can also improve service
    by shortening Internet paths and by providing additional capacity.
    The engineering tradeoff between cost and reliability must be made
    for each component of the Internet system.

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2. Protocols Required in Gateways

 The Internet architecture uses datagram gateways to interconnect
 constituent networks.  This section describes the various protocols
 which a gateway needs to implement.
 2.1.  Internet Protocol (IP)
    IP is the basic datagram protocol used in the Internet system [19,
    31].  It is described in RFC-791 [1] and also in MIL-STD-1777 [5]
    as clarified by RFC-963 [36] ([1] and [5] are intended to describe
    the same standard, but in quite different words).  The subnet
    extension is described in RFC-950 [21].
    With respect to current gateway requirements the following IP
    features can be ignored, although they may be required in the
    future:  Type of Service field, Security option, and Stream ID
    option.  However, if recognized, the interpretation of these
    quantities must conform to the standard specification.
    It is important for gateways to implement both the Loose and
    Strict Source Route options.  The Record Route and Timestamp
    options are useful diagnostic tools and must be supported in all
    gateways.
    The Internet model requires that a gateway be able to fragment
    datagrams as necessary to match the MTU of the network to which
    they are being forwarded, but reassembly of fragmented datagrams
    is generally left to the destination hosts.  Therefore, a gateway
    will not perform reassembly on datagrams it forwards.
    However, a gateway will generally receive some IP datagrams
    addressed to itself; for example, these may be ICMP Request/Reply
    messages, routing update messages (see Sections 2.3 and 2.6), or
    for monitoring and control (see Section 5).  For these datagrams,
    the gateway will be functioning as a destination host, so it must
    implement IP reassembly in case the datagrams have been fragmented
    by some transit gateway.  The destination gateway must have a
    reassembly buffer which is at least as large as the maximum of the
    MTU values for its network interfaces and 576.  Note also that it
    is possible for a particular protocol implemented by a host or
    gateway to require a lower bound on reassembly buffer size which
    is larger than 576.  Finally, a datagram which is addressed to a
    gateway may use any of that gateway's IP addresses as destination
    address, regardless of which interface the datagram enters.
    There are five classes of IP addresses:  Class A through
    Class E [23].  Of these, Class D and Class E addresses are

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    reserved for experimental use.  A gateway which is not
    participating in these experiments must ignore all datagrams with
    a Class D or Class E destination IP address.  ICMP Destination
    Unreachable or ICMP Redirect messages must not result from
    receiving such datagrams.
    There are certain special cases for IP addresses, defined in the
    latest Assigned Numbers document [23].  These special cases can be
    concisely summarized using the earlier notation for an IP address:
       IP-address ::=  { <Network-number>, <Host-number> }
          or
       IP-address ::=  { <Network-number>, <Subnet-number>,
                                                       <Host-number> }
    if we also use the notation "-1" to mean the field contains all 1
    bits.  Some common special cases are as follows:
       (a)   {0, 0}
          This host on this network.  Can only be used as a source
          address (see note later).
       (b)   {0, <Host-number>}
          Specified host on this network.  Can only be used as a
          source address.
       (c)   { -1, -1}
          Limited broadcast.  Can only be used as a destination
          address, and a datagram with this address must never be
          forwarded outside the (sub-)net of the source.
       (d)   {<Network-number>, -1}
          Directed broadcast to specified network.  Can only be used
          as a destination address.
       (e)   {<Network-number>, <Subnet-number>, -1}
          Directed broadcast to specified subnet.  Can only be used as
          a destination address.
       (f)   {<Network-number>, -1, -1}

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          Directed broadcast to all subnets of specified subnetted
          network.  Can only be used as a destination address.
       (g)   {127, <any>}
          Internal host loopback address.  Should never appear outside
          a host.
    The following two are conventional notation for network numbers,
    and do not really represent IP addresses.  They can never be used
    in an IP datagram header as an IP source or destination address.
       (h)   {<Network-number>, 0}
          Specified network (no host).
       (i)   {<Network-number>, <Subnet-number>, 0}
          Specified subnet (no host).
    Note also that the IP broadcast address, which has primary
    application to Ethernets and similar technologies that support an
    inherent broadcast function, has an all-ones value in the host
    field of the IP address.  Some early implementations chose the
    all-zeros value for this purpose, which is not in conformance with
    the specification [23, 49, 50].
 2.2.  Internet Control Message Protocol (ICMP)
    ICMP is an auxiliary protocol used to convey advice and error
    messages and is described in RFC-792 [2].
    We will discuss issues arising from gateway handling of particular
    ICMP messages.  The ICMP messages are grouped into two classes:
    error messages and information messages.  ICMP error messages are
    never sent about ICMP error messages, nor about broadcast or
    multicast datagrams.
       The ICMP error messages are: Destination Unreachable, Redirect,
       Source Quench, Time Exceeded, and Parameter Problem.
       The ICMP information messages are: Echo, Information,
       Timestamp, and Address Mask.

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    2.2.1.  Destination Unreachable
       The distinction between subnets of a subnetted network, which
       depends on the address mask described in RFC-950 [21], must not
       be visible outside that network.  This distinction is important
       in the case of the ICMP Destination Unreachable message.
       The ICMP Destination Unreachable message is sent by a gateway
       in response to a datagram which it cannot forward because the
       destination is unreachable or down.  The gateway chooses one of
       the following two types of Destination Unreachable messages to
       send:
  • Net Unreachable
  • Host Unreachable
       Net unreachable implies that an intermediate gateway was unable
       to forward a datagram, as its routing data-base gave no next
       hop for the datagram, or all paths were down.  Host Unreachable
       implies that the destination network was reachable, but that a
       gateway on that network was unable to reach the destination
       host.  This might occur if the particular destination network
       was able to determine that the desired host was unreachable or
       down.  It might also occur when the destination host was on a
       subnetted network and no path was available through the subnets
       of this network to the destination.  Gateways should send Host
       Unreachable messages 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.
    2.2.2.  Redirect
       The ICMP Redirect message is sent by a gateway to a host on the
       same network, in order to change the gateway used by the host
       for routing certain datagrams.  A choice of four types of
       Redirect messages is available to specify datagrams destined
       for a particular host or network, and possibly with a
       particular type-of-service.
       If the directly-connected network is not subnetted, a gateway
       can normally send 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 saving is not
       significant, and subnets create an ambiguity about the subnet

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       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, it is recommended that a gateway send only host (or
       host and type-of-service) Redirects.
    2.2.3.  Source Quench
       All gateways must contain code for sending ICMP Source Quench
       messages when they are forced to drop IP datagrams due to
       congestion.  Although the Source Quench mechanism is known to
       be an imperfect means for Internet congestion control, and
       research towards more effective means is in progress, Source
       Quench is considered to be too valuable to omit from production
       gateways.
       There is some argument that the Source Quench should be sent
       before the gateway is forced to drop datagrams [62].  For
       example, a parameter X could be established and set to have
       Source Quench sent when only X buffers remain.  Or, a parameter
       Y could be established and set to have Source Quench sent when
       only Y per cent of the buffers remain.
       Two problems for a gateway sending Source Quench are: (1) the
       consumption of bandwidth on the reverse path, and (2) the use
       of gateway CPU time.  To ameliorate these problems, a gateway
       must be prepared to limit the frequency with which it sends
       Source Quench messages.  This may be on the basis of a count
       (e.g., only send a Source Quench for every N dropped datagrams
       overall or per given source host), or on the basis of a time
       (e.g., send a Source Quench to a given source host or overall
       at most once per T millseconds).  The parameters (e.g., N or T)
       must be settable as part of the configuration of the gateway;
       furthermore, there should be some configuration setting which
       disables sending Source Quenches.  These configuration
       parameters, including disabling, should ideally be specifiable
       separately for each network interface.
       Note that a gateway itself may receive a Source Quench as the
       result of sending a datagram targeted to another gateway.  Such
       datagrams might be an EGP update, for example.
    2.2.4.  Time Exceeded
       The ICMP Time Exceeded message may be sent when a gateway
       discards a datagram due to the TTL being reduced to zero.  It

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       may also be sent by a gateway if the fragments of a datagram
       addressed to the gateway itself cannot be reassembled before
       the time limit.
    2.2.5.  Parameter Problem
       The ICMP Parameter Problem message may be sent to the source
       host for any problem not specifically covered by another ICMP
       message.
    2.2.6.  Address Mask
       Host and gateway implementations are expected to support the
       ICMP Address Mask messages described in RFC-950 [21].
    2.2.7.  Timestamp
       The ICMP Timestamp message has proven to be useful for
       diagnosing Internet problems.  The preferred form for a
       timestamp value, the "standard value", is in milliseconds since
       midnight GMT.  However, it may be difficult to provide this
       value with millisecond resolution.  For example, many systems
       use clocks which update only at line frequency, 50 or 60 times
       per second.  Therefore, some latitude is allowed in a
       "standard" value:
  • The value must be updated at a frequency of at least 30

times per second (i.e., at most five low-order bits of

             the value may be undefined).
  • The origin of the value must be within a few minutes of

midnight, i.e., the accuracy with which operators

             customarily set CPU clocks.
       To meet the second condition for a stand-alone gateway, it will
       be necessary to query some time server host when the gateway is
       booted or restarted.  It is recommended that the UDP Time
       Server Protocol [44] be used for this purpose.  A more advanced
       implementation would use NTP (Network Time Protocol) [45] to
       achieve nearly millisecond clock synchronization; however, this
       is not required.
       Even if a gateway is unable to establish its time origin, it
       ought to provide a "non-standard" timestamp value (i.e., with
       the non-standard bit set), as a time in milliseconds from
       system startup.

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       New gateways, especially those expecting to operate at T1 or
       higher speeds, are expected to have at least millisecond
       clocks.
    2.2.8.  Information Request/Reply
       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, the Reverse ARP (RARP) protocol [15] provides a better
       mechanism for a host to use to discover its own IP address, and
       RARP is recommended for this purpose.  Information
       Request/Reply need not be implemented in a gateway.
    2.2.9.  Echo Request/Reply
       A gateway must implement ICMP Echo, since it has proven to be
       an extremely useful diagnostic tool.  A gateway must be
       prepared to receive, reassemble, and echo an ICMP Echo Request
       datagram at least as large as the maximum of 576 and the MTU's
       of all of the connected networks.  See the discussion of IP
       reassembly in gateways, Section 2.1.
       The following rules resolve the question of the use of IP
       source routes in Echo Request and Reply datagrams.  Suppose a
       gateway D receives an ICMP Echo Request addressed to itself
       from host S.
          1.  If the Echo Request contained no source route, D should
              send an Echo Reply back to S using its normal routing
              rules.  As a result, the Echo Reply may take a different
              path than the Request; however, in any case, the pair
              will sample the complete round-trip path which any other
              higher-level protocol (e.g., TCP) would use for its data
              and ACK segments between S and D.
          2.  If the Echo Request did contain a source route, D should
              send an Echo Reply back to S using as a source route the
              return route built up in the source-routing option of
              the Echo Request.

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RFC 1009 - Requirements for Internet Gateways June 1987

 2.3.  Exterior Gateway Protocol (EGP)
    EGP is the protocol used to exchange reachability information
    between Autonomous Systems of gateways, and is defined in
    RFC-904 [11].  See also RFC-827 [51], RFC-888 [46], and
    RFC-975 [27] for background information.  The most widely used EGP
    implementation is described in RFC-911 [13].
    When a dynamic routing algorithm is operated in the gateways of an
    Autonomous System (AS), the routing data-base must be coupled to
    the EGP implementation.  This coupling should ensure that, when a
    net is determined to be unreachable by the routing algorithm, the
    net will not be declared reachable to other ASs via EGP.  This
    requirement is designed to minimize spurious traffic to "black
    holes" and to ensure fair utilization of the resources on other
    systems.
    The present EGP specification defines a model with serious
    limitations, most importantly a restriction against propagating
    "third party" EGP information in order to prevent long-lived
    routing loops [27].  This effectively limits EGP to a two-level
    hierarchy; the top level is formed by the "core" AS, while the
    lower level is composed of those ASs which are direct neighbor
    gateways to the core AS.  In practice, in the current Internet,
    nearly all of the "core gateways" are connected to the ARPANET,
    while the lower level is composed of those ASs which are directly
    gatewayed to the ARPANET or MILNET.
    RFC-975 [27] suggested one way to generalize EGP to lessen these
    topology restrictions;  it has not been adopted as an official
    specification, although its ideas are finding their way into the
    new EGP developments.  There are efforts underway in the research
    community to develop an EGP generalization which will remove these
    restrictions.
    In EGP, there is no standard interpretation (i.e., metric) for the
    distance fields in the update messages, so distances are
    comparable only among gateways of the same AS.  In using EGP data,
    a gateway should compare the distances among gateways of the same
    AS and prefer a route to that gateway which has the smallest
    distance value.
    The values to be announced in the distance fields for particular
    networks within the local AS should be a gateway configuration
    parameter; by suitable choice of these values, it will be possible
    to arrange primary and backup paths from other AS's.  There are
    other EGP parameters, such as polling intervals, which also need
    to be set in the gateway configuration.

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    When routing updates become large they must be transmitted in
    parts.  One strategy is to use IP fragmentation, another is to
    explicitly send the routing information in sections.  The Internet
    Engineering Task Force is currently preparing a recommendation on
    this and other EGP engineering issues.
 2.4.  Address Resolution Protocol (ARP)
    ARP is an auxiliary protocol used to perform dynamic address
    translation between LAN hardware addresses and Internet addresses,
    and is described in RFC-826 [4].
    ARP depends upon local network broadcast.  In normal ARP usage,
    the initiating host broadcasts an ARP Request carrying a target IP
    address; the corresponding target host, recognizing its own IP
    address, sends back an ARP Reply containing its own hardware
    interface address.
    A variation on this procedure, called "proxy ARP", has been used
    by gateways attached to broadcast LANs [14].  The gateway sends an
    ARP Reply specifying its interface address in response to an ARP
    Request for a target IP address which is not on the
    directly-connected network but for which the gateway offers an
    appropriate route.  By observing ARP and proxy ARP traffic, a
    gateway may accumulate a routing data-base [14].
    Proxy ARP (also known in some quarters as "promiscuous ARP" or
    "the ARP hack") is useful for routing datagrams from hosts which
    do not implement the standard Internet routing rules fully -- for
    example, host implementations which predate the introduction of
    subnetting.  Proxy ARP for subnetting is discussed in detail in
    RFC-925 [14].
    Reverse ARP (RARP) allows a host to map an Ethernet interface
    address into an IP address [15].  RARP is intended to allow a
    self-configuring host to learn its own IP address from a server at
    boot time.
 2.5.  Constituent Network Access Protocols
    See Section 3.

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 2.6.  Interior Gateway Protocols
    Distributed routing algorithms continue to be the subject of
    research and engineering, and it is likely that advances will be
    made over the next several years.  A good algorithm needs to
    respond rapidly to real changes in Internet connectivity, yet be
    stable and insensitive to transients.  It needs to synchronize the
    distributed data-base across gateways of its Autonomous System
    rapidly (to avoid routing loops), while consuming only a small
    fraction of the available bandwidth.
    Distributed routing algorithms are commonly broken down into the
    following three components:
       A.  An algorithm to assign a "length" to each Internet path.
          The "length" may be a simple count of hops (1, or infinity
          if the path is broken), or an administratively-assigned
          cost, or some dynamically-measured cost (usually an average
          delay).
          In order to determine a path length, each gateway must at
          least test whether each of its neighbors is reachable; for
          this purpose, there must be a "reachability" or "neighbor
          up/down" protocol.
       B.  An algorithm to compute the shortest path(s) to a given
           destination.
       C.  A gateway-gateway protocol used to exchange path length and
           routing information among gateways.
    The most commonly-used IGPs in Internet gateways are as follows.
    2.6.1.  Gateway-to-Gateway Protocol (GGP)
       GGP was designed and implemented by BBN for the first
       experimental Internet gateways [41].  It is still in use in the
       BBN LSI/11 gateways, but is regarded as having serious
       drawbacks [58].  GGP is based upon an algorithm used in the
       early ARPANET IMPs and later replaced by SPF (see below).
       GGP is a "min-hop" algorithm, i.e., its length measure is
       simply the number of network hops between gateway pairs.  It
       implements a distributed shortest-path algorithm, which
       requires global convergence of the routing tables after a
       change in topology or connectivity.  Each gateway sends a GGP

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       routing update only to its neighbors, but each update includes
       an entry for every known network, where each entry contains the
       hop count from the gateway sending the update.
    2.6.2.  Shortest-Path-First (SPF) Protocols
       SPF [40] is the name for a class of routing algorithms based on
       a shortest-path algorithm of Dijkstra.  The current ARPANET
       routing algorithm is SPF, and the BBN Butterfly gateways also
       use SPF.  Its characteristics are considered superior to
       GGP [58].
       Under SPF, the routing data-base is replicated rather than
       distributed.  Each gateway will have its own copy of the same
       data-base, containing the entire Internet topology and the
       lengths of every path.  Since each gateway has all the routing
       data and runs a shortest-path algorithm locally, there is no
       problem of global convergence of a distributed algorithm, as in
       GGP.  To build this replicated data-base, a gateway sends SPF
       routing updates to ALL other gateways; these updates only list
       the distances to each of the gateway's neighbors, making them
       much smaller than GGP updates.  The algorithm used to
       distribute SPF routing updates involves reliable flooding.
    2.6.3.  Routing Information (RIP)
       RIP is the name often used for a class of routing protocols
       based upon the Xerox PUP and XNS routing protocols.  These are
       relatively simple, and are widely available because they are
       incorporated in the embedded gateway code of Berkeley BSD
       systems.  Because of this simplicity, RIP protocols have come
       the closest of any to being an "Open IGP", i.e., a protocol
       which can be used between different vendors' gateways.
       Unfortunately, there is no standard, and in fact not even a
       good document, for RIP.
       As in GGP, gateways using RIP periodically broadcast their
       routing data-base to their neighbor gateways, and use a
       hop-count as the metric.
       A fixed value of the hop-count (normally 16) is defined to be
       "infinity", i.e., network unreachable.  A RIP implementation
       must include measures to avoid both the slow-convergence
       phenomen called "counting to infinity" and the formation of
       routing loops.  One such measure is a "hold-down" rule.  This
       rule establishes a period of time (typically 60 seconds) during
       which a gateway will ignore new routing information about a
       given network, once the gateway has learned that network is

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       unreachable (has hop-count "infinity").  The hold-down period
       must be settable in the gateway configuration; if gateways with
       different hold-down periods are using RIP in the same
       Autonomous System, routing loops are a distinct possibility.
       In general, the hold-down period is chosen large enough to
       allow time for unreachable status to propagate to all gateways
       in the AS.
    2.6.4.  Hello
       The "Fuzzball" software for an LSI/11 developed by Dave Mills
       incorporated an IGP called the "Hello" protocol [39].  This IGP
       is mentioned here because the Fuzzballs have been widely used
       in Internet experimentation, and because they have served as a
       testbed for many new routing ideas.
 2.7.  Monitoring Protocols
    See Section 5 of this document.
 2.8.  Internet Group Management Protocol (IGMP)
    An extension to the IP protocol has been defined to provide
    Internet-wide multicasting, i.e., delivery of copies of the same
    IP datagram to a set of Internet hosts [47, 48].  This delivery is
    to be performed by processes known as "multicasting agents", which
    reside either in a host on each net or (preferably) in the
    gateways.
    The set of hosts to which a datagram is delivered is called a
    "host group", and there is a host-agent protocol called IGMP,
    which a host uses to join, leave, or create a group.  Each host
    group is distinguished by a Class D IP address.
    This multicasting mechanism and its IGMP protocol are currently
    experimental; implementation in vendor gateways would be premature
    at this time.  A datagram containing a Class D IP address must be
    dropped, with no ICMP error message.

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RFC 1009 - Requirements for Internet Gateways June 1987

3. Constituent Network Interface

 This section discusses the rules used for transmission of IP
 datagrams on the most common types of constituent networks.  A
 gateway must be able to send and receive IP datagrams of any size up
 to the MTU of any constituent network to which it is connected.
 3.1.  Public data networks via X.25
    The formats specified for public data networks accessed via X.25
    are described in RFC-877 [8].  Datagrams are transmitted over
    standard level-3 virtual circuits as complete packet sequences.
    Virtual circuits are usually established dynamically as required
    and time-out after a period of no traffic.  Link-level
    retransmission, resequencing and flow control are performed by the
    network for each virtual circuit and by the LAPB link-level
    protocol.  Note that a single X.25 virtual circuit may be used to
    multiplex all IP traffic between a pair of hosts.  However,
    multiple parallel virtual circuits may be used in order to improve
    the utilization of the subscriber access line, in spite of small
    X.25 window sizes; this can result in random resequencing.
    The correspondence between Internet and X.121 addresses is usually
    established by table-lookup.  It is expected that this will be
    replaced by some sort of directory procedure in the future.  The
    table of the hosts on the Public Data Network is in the Assigned
    Numbers [23].
    The normal MTU is 576; however, the two DTE's (hosts or gateways)
    can use X.25 packet size negotiation to increase this value [8].
 3.2.  ARPANET via 1822 LH, DH, or HDH
    The formats specified for ARPANET networks using 1822 access are
    described in BBN Report 1822 [3], which includes the procedures
    for several subscriber access methods.  The Distant Host (DH)
    method is used when the host and IMP (the Defense Communication
    Agency calls it a Packet Switch Node or PSN) are separated by not
    more than about 2000 feet of cable, while the HDLC Distant Host
    (HDH) is used for greater distances where a modem is required.
    Under HDH, retransmission, resequencing and flow control are
    performed by the network and by the HDLC link-level protocol.
    The IP encapsulation format is simply to include the IP datagram
    as the data portion of an 1822 message.  In addition, the
    high-order 8 bits of the Message Id field (also known as the
    "link" field") should be set to 155 [23].  The MTU is 1007 octets.

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    While the ARPANET 1822 protocols are widely used at present, they
    are expected to be eventually overtaken by the DDN Standard X.25
    protocol (see Section 3.3).  The original IP address mapping
    (RFC-796 [38]) is in the process of being replaced by a new
    interface specification called AHIP-E; see RFC-1005 [61] for the
    proposal.
    Gateways connected to ARPANET or MILNET IMPs using 1822 access
    must incorporate features to avoid host-port blocking (i.e., RFNM
    counting) and to detect and report as ICMP Unreachable messages
    the failure of destination hosts or gateways (i.e., convert the
    1822 error messages to the appropriate ICMP messages).
    In the development of a network interface it will be useful to
    review the IMP end-to-end protocol described in RFC-979 [29].
 3.3.  ARPANET via DDN Standard X.25
    The formats specified for ARPANET networks via X.25 are described
    in the Defense Data Network X.25 Host Interface Specification [6],
    which describes two sets of procedures: the DDN Basic X.25, and
    the DDN Standard X.25.  Only DDN Standard X.25 provides the
    functionality required for interoperability assumptions of the
    Internet protocol.
    The DDN Standard X.25 procedures are similar to the public data
    network X.25 procedures, except in the address mappings.
    Retransmission, resequencing and flow control are performed by the
    network and by the LAPB link-level protocol.  Multiple parallel
    virtual circuits may be used in order to improve the utilization
    of the subscriber access line; this can result in random
    resequencing.
    Gateways connected to ARPANET or MILNET using Standard X.25 access
    must detect and report as ICMP Unreachable messages the failure of
    destination hosts or gateways (i.e., convert the X.25 diagnostic
    codes to the appropriate ICMP messages).
    To achieve compatibility with 1822 interfaces, the effective MTU
    for a Standard X.25 interface is 1007 octets.

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 3.4.  Ethernet and IEEE 802
    The formats specified for Ethernet networks are described in
    RFC-894 [10].  Datagrams are encapsulated as Ethernet packets with
    48-bit source and destination address fields and a 16-bit type
    field (the type field values are listed in the Assigned
    Numbers [23]).  Address translation between Ethernet addresses and
    Internet addresses is managed by the Address Resolution Protocol,
    which is required in all Ethernet implementations.  There is no
    explicit link-level retransmission, resequencing or flow control,
    although most hardware interfaces will retransmit automatically in
    case of collisions on the cable.
    The IEEE 802 networks use a Link Service Access Point (LSAP) field
    in much the same way the ARPANET uses the "link" field.  Further,
    there is an extension of the LSAP header called the Sub-Network
    Access Protocol (SNAP).
    The 802.2 encapsulation is used on 802.3, 802.4, and 802.5 network
    by using the SNAP with an organization code indicating that the
    following 16 bits specify the Ether-Type code [23].
    Headers:
       ...--------+--------+--------+
        MAC Header|      Length     |                  802.{3/4/5} MAC
       ...--------+--------+--------+
       +--------+--------+--------+
       | DSAP=K1| SSAP=K1| control|                          802.2 SAP
       +--------+--------+--------+
       +--------+--------+--------+--------+--------+
       |protocol id or org code=K2|    Ether-Type   |       802.2 SNAP
       +--------+--------+--------+--------+--------+
    The total length of the SAP Header and the SNAP header is
    8-octets, making the 802.2 protocol overhead come out on a 64-bit
    boundary.
    K1 is 170.  The IEEE likes to talk about things in bit
    transmission order and specifies this value as 01010101.  In
    big-endian order, as used in the Internet specifications, this
    becomes 10101010 binary, or AA hex, or 170 decimal.  K2 is 0
    (zero).
    The use of the IP LSAP (K1 = 6) is reserved for future
    development.

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    The assigned values for the Ether-Type field are the same for
    either this IEEE 802 encapsulation or the basic Ethernet
    encapsulation [10].
    In either Ethernets or IEEE 802 nets, the IP datagram is the data
    portion of the packet immediately following the Ether-Type.
    The MTU for an Ethernet or its IEEE-standard equivalent (802.3) is
    1500 octets.
 3.5.  Serial-Line Protocols
    In some configurations, gateways may be interconnected with each
    other by means of serial asynchronous or synchronous lines, with
    or without modems.  When justified by the expected error rate and
    other factors, a link-level protocol may be required on the serial
    line.  While there is no single Internet standard for this
    protocol, it is suggested that one of the following protocols be
    used.
  • X.25 LAPB (Synchronous Lines)
          This is the link-level protocol used for X.25 network
          access.  It includes HDLC "bit-stuffing" as well as
          rotating-window flow control and reliable delivery.
             A gateway must be configurable to play the role of either
             the DCE or the DTE.
  • HDLC Framing (Synchronous Lines)
          This is just the bit-stuffing and framing rules of LAPB.  It
          is the simplest choice, although it provides no flow control
          or reliable delivery; however, it does provide error
          detection.
  • Xerox Synchronous Point-to-Point (Synchronous Lines)
          This Xerox protocol is an elaboration upon HDLC framing that
          includes negotiation of maximum packet sizes, dial-up or
          dedicated circuits, and half- or full-duplex operation [12].
  • Serial Line Framing Protocol (Asynchronous Lines)
          This protocol is included in the MIT PC/IP package for an
          IBM PC and is defined in Appendix I to the manual for that
          system [20].

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    It will be important to make efficient use of the bandwidth
    available on a serial line between gateways.  For example, it is
    desirable to provide some form of data compression.  One possible
    standard compression algorithm, "Thinwire II", is described in
    RFC-914 [42].  This and similar algorithms are tuned to the
    particular types of redundancy which occur in IP and TCP headers;
    however, more work is necessary to define a standard serial-line
    compression protocol for Internet gateways.  Until a standard has
    been adopted, each vendor is free to choose a compression
    algorithm; of course, the result will only be useful on a serial
    line between two gateways using the same compression algorithm.
    Another way to ensure maximum use of the bandwidth is to avoid
    unnecessary retransmissions at the link level.  For some kinds of
    IP traffic, low delay is more important than reliable delivery.
    The serial line driver could distinguish such datagrams by their
    IP TOS field, and place them on a special high-priority,
    no-retransmission queue.
    A serial point-to-point line between two gateways may be
    considered to be a (particularly simple) network, a "null net".
    Considered in this way, a serial line requires no special
    considerations in the routing algorithms of the connected
    gateways, but does need an IP network number.  To avoid the
    wholesale consumption of Internet routing data-base space by null
    nets, we strongly recommend that subnetting be used for null net
    numbering, whenever possible.
       For example, assume that network 128.203 is to be constructed
       of gateways joined by null nets; these nets are given (sub-)net
       numbers 128.203.1, 128.203.2, etc., and the two interfaces on
       each end of null net 128.203.s might have IP addresses
       128.203.s.1 and 128.203.s.2.
    An alternative model of a serial line is that it is not a network,
    but rather an internal communication path joining two "half
    gateways".  It is possible to design an IGP and routing algorithm
    that treats a serial line in this manner [39, 52].

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4. Gateway Algorithms

 Gateways are general packet-switches that forward packets according
 to the IP address, i.e., they are IP routers.   While it is beyond
 the scope of this document to specify the details of the mechanisms
 used in any particular, perhaps proprietary, gateway architecture,
 there are a number of basic algorithms which must be provided by any
 acceptable design.
 4.1.  Routing Algorithm
    The routing mechanism is fundamental to Internet operation.  In
    all but trivial network topologies, robust Internet service
    requires some degree of routing dynamics, whether it be effected
    by manual or automatic means or by some combination of both.  In
    particular, if routing changes are made manually, it must be
    possible to make these routing changes from a remote Network
    Operation Center (NOC) without taking down the gateway for
    reconfiguration.  If static routes are used, there must be
    automatic fallback or rerouting features.
    Handling unpredictable changes in Internet connectivity must be
    considered the normal case, so that systems of gateways will
    normally be expected to have a routing algorithm with the
    capability of reacting to link and other gateway failures and
    changing the routing automatically.
    This document places no restriction on the type of routing
    algorithm, e.g., node-based, link-based or any other algorithm, or
    on the routing distance metric, e.g., delay or hop-count.
    However, the following features are considered necessary for a
    successful gateway routing algorithm:
       1.  The algorithm must sense the failure or restoration of a
           link or other gateway and switch to appropriate paths.  A
           design objective is to switch paths within an interval less
           than the typical TCP user time-out (one minute is a safe
           assumption).
       2.  The algorithm must suppress routing loops between neighbor
           gateways and must contain provisions to avoid or suppress
           routing loops that may form between non-neighbor gateways.
           A design objective is for no loop to persist for longer
           than an interval greater than the typical TCP user
           time-out.
       3.  The control traffic necessary to operate the routing
           algorithm must not significantly degrade or disrupt normal

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           network operation.  Changes in state which might
           momentarily disrupt normal operation in a local-area must
           not cause disruption in remote areas of the network.
       4.  As the size of the network increases, the demand on
           resources must be controlled in an efficient way.  Table
           lookups should be hashed, for example, and data-base
           updates handled piecemeal, with only incremental changes
           broadcast over a wide-area.
       5.  The size of the routing data-base must not be allowed to
           exceed a constant, independent of network topology, times
           the number of nodes times the mean connectivity (average
           number of incident links).  An advanced design might not
           require that the entire routing data-base be kept in any
           particular gateway, so that discovery and caching
           techniques would be necessary.
       6.  Reachability and delay metrics, if used, must not depend on
           direct connectivity to all other gateways or on the use of
           network-specific broadcast mechanisms.  Polling procedures
           (e.g., for consistency checking) must be used only
           sparingly and in no case introduce an overhead exceeding a
           constant, independent of network topology, times the
           longest non-looping path.
       7.  Default routes (generally intended as a means to reduce the
           size of the routing data-base) must be used with care,
           because of the many problems with multiple paths, loops,
           and mis-configurations which routing defaults have caused.
           The most common application of defaults is for routing
           within an Internet region which is connected in a strictly
           hierarchical fashion and is a stub from the rest of the
           Internet system.  In this case, the default is used for
           routing "up" the tree.  Unfortunately, such restricted
           topology seldom lasts very long, and defaults cease to
           work.
           More generally, defaults could be used for initial routing
           guesses, with final routes to be discovered and cached from
           external or internal data-bases via the routing algorithm
           or EGP.

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 4.2.  Subnets and Routing
    We will call a gateway "subnetted" if at least one of its
    interfaces is connected to a subnet; the set of gateways directly
    connected to subnets of the same network will be referred to as a
    "subnet cluster".  For example, in the following diagram, network
    2 is subnetted, with subnets 2.1 and 2.2, but network 1 is not;
    gateways 1, 2, and 3 are subnetted and are members of the same
    subnet cluster.
       (Net 1) === [Gwy 1] === (Net 2.1) === [Gwy 2] === (Net 2.2)
          |                                                   |
          |                                                   |
           =================== [Gwy 3] =======================
    Subnets have the following effects on gateway routing:
       A.  Non-subnetted gateways are not affected at all.
       B.  The routing data-base in a subnetted gateway must consider
           the address mask for subnet entries.
       C.  Routing updates among the gateways in the same subnet
           cluster must include entries for the various subnets.  The
           corresponding address mask(s) may be implicit, but for full
           generality the mask needs to be given explicitly for each
           entry.  Note that if the routing data-base included a full
           32-bit mask for every IP network, the gateway could deal
           with networks and subnets in a natural way.  This would
           also handle the case of multiple subnet masks for the same
           subnetted network.
       D.  Routing updates from a subnetted gateway to a gateway
           outside the cluster can contain nets, never subnets.
       E.  If a subnetted gateway (e.g., gateway 2 above) is unable to
           forward a datagram from one subnet to another subnet of the
           same network, then it must return a Host Unreachable, not a
           Net Unreachable, as discussed in Section 2.2.1.
    When considering the choice of routing protocol, a gateway builder
    must consider how that protocol generalizes for subnets.  For some
    routing protocols it will be possible to use the same procedures
    in a regular gateway and a subnetted gateway, with only a change
    of parameters (e.g., address masks).
    A different subnet address mask must be configurable for each

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    interface of a given gateway.  This will allow a subnetted gateway
    to connect to two different subnetted networks, or to connect two
    subnets of the same network with different masks.
 4.3   Resource Allocation
    In order to perform its basic datagram-forwarding functions, a
    gateway must allocate resources; its packet buffers and CPU time
    must be allocated to packets it receives from connected networks,
    while the bandwidth to each of the networks must also be allocated
    for sending packets.  The choice of allocation strategies will be
    critical when a particular resource is scarce.  The most obvious
    allocation strategy, first-come-first-served (FCFS), may not be
    appropriate under overload conditions, for reasons which we will
    now explore.
    A first example is buffer allocation.  It is important for a
    gateway to allocate buffers fairly among all of its connected
    networks, even if these networks have widely varying bandwidths.
    A high-speed interface must not be allowed to starve slower
    interfaces of buffers.  For example, consider a gateway with a
    10 Mbps Ethernet connection and two 56 Kbps serial lines.  A buggy
    host on the Ethernet may spray that gateway interface with packets
    at high speed.  Without careful algorithm design in the gateway,
    this could tie up all the gateway buffers in such a way that
    transit traffic between the serial lines would be completely
    stopped.
    Allocation of output bandwidth may also require non-FCFS
    strategies.  In an advanced gateway design, allocation of output
    bandwidth may depend upon Type-of-Service bits in the IP headers.
    A gateway may also want to give priority to datagrams for its own
    up/down and routing protocols.
    Finally, Nagle [24] has suggested that gateways implement "fair
    queueing", i.e., sharing output bandwidth equitably among the
    current traffic sources.  In his scheme, for each network
    interface there would be a dynamically-built set of output queues,
    one per IP source address; these queues would be serviced in a
    round-robin fashion to share the bandwidth.  If subsequent
    research shows fair queueing to be desirable, it will be added to
    a future version of this document as a universal requirement.

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 4.4.  Special Addresses and Filters
    Section 2.1 contained a list of the 32-bit IP addresses which have
    special meanings.  They do not in general represent unique IP
    addresses of Internet hosts, and there are restrictions on their
    use in IP headers.
    We can distinguish two classes of these special cases.  The first
    class (specifically, cases (a), (b), (c), (g), (h), and (i) in
    section 2.1) contains addresses which should never appear in the
    destination address field of any IP datagram, so a gateway should
    never be asked to route to one of these addresses.  However, in
    the real world of imperfect implementations and configuration
    errors, such bad destination addresses do occur.  It is the
    responsibility of a gateway to avoid propagating such erroneous
    addresses; this is especially important for gateways included in
    the global interconnect system.  In particular, a gateway which
    receives a datagram with one of these forbidden addresses should:
       1.  Avoid inserting that address into its routing database, and
           avoid including it in routing updates to any other gateway.
       2.  Avoid forwarding a datagram containing that address as a
           destination.
    To enforce these restrictions, it is suggested that a gateway
    include a configurable filter for datagrams and routing updates.
    A typical filter entry might consist of a 32-bit mask and value
    pair.  If the logical AND of the given address with the mask
    equals the value, a match has been found.  Since filtering will
    consume gateway resources, it is vital that the gateway
    configuration be able to control the degree of filtering in use.
    There is a second class of special case addresses (cases (d), (e),
    and (f) in section 2.1), the so-called "directed broadcasts".  A
    directed broadcast is a datagram to be forwarded normally to the
    specified destination (sub-)net and then broadcast on the final
    hop.  An Internet gateway is permitted, but not required, to
    filter out directed broadcasts destined for any of its
    locally-connected networks.  Hence, it should be possible to
    configure the filter to block the delivery of directed broadcasts.
    Finally, it will also be useful for Internet O&M to have a
    configurable filter on the IP source address.  This will allow a
    network manager to temporarily block traffic from a particular
    misbehaving host, for example.

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 4.5.  Redirects
    The ICMP Redirect message is specified only for use by a gateway
    to update the routing table of a host on the same connected net.
    However, the Redirect message is sometimes used between gateways,
    due to the following considerations:
       The routing function in a host is very much like that in a
       "dumb gateway" (i.e., a gateway having only static routes).  It
       is desirable to allow the routing tables of a dumb gateway to
       be changed under the control of a dynamic gateway (i.e., a
       gateway with full dynamic routing) on the same network.  By
       analogy, it is natural to let the dynamic gateway send ICMP
       Redirect messages to dumb gateway.
    The use of ICMP Redirect between gateways in this fashion may be
    considered to be part of the IGP (in fact, the totality of the
    IGP, as far as the dumb gateway is concerned!) in the particular
    Autonomous System.   Specification of an IGP is outside the scope
    of this document, so we only note the possibility of using
    Redirect in this fashion.  Gateways are not required to receive
    and act upon redirects, and in fact dynamic gateways must ignore
    them.  We also note that considerable experience shows that dumb
    gateways often create problems resulting in "black holes"; a full
    routing gateway is always preferable.
    Routing table entries established by redirect messages must be
    removed automatically, either by a time-out or when a use count
    goes to zero.
 4.6.  Broadcast and Multicast
    A host which is connected to a network (generally a LAN) with an
    intrinsic broadcast capability may want to use this capability to
    effect multidestination delivery of IP datagrams.  The basic
    Internet model assumes point-to-point messages, and we must take
    some care when we incorporate broadcasting.  It is important to
    note that broadcast addresses may occur at two protocol levels:
    the local network header and the IP header.
    Incorrect handling of broadcasting has often been the cause of
    packet avalanches (sometimes dubbed "meltdown") in LANs.  These
    avalanches are generally caused by gratuitous datagram-forwarding
    by hosts, or by hosts sending ICMP error messages when they
    discard broadcast datagrams.
    Gateways have a responsibility to prevent avalanches, or datagrams
    which can trigger avalanches, from escaping into another network.

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    In general, a gateway must not forward a datagram which arrives
    via local network broadcast, and must not send an ICMP error
    message when dropping the datagram.  A discussion of the rules
    will be found in Appendix A; see also [50].
    As noted in Section 4.4, a gateway is permitted to filter out
    directed broadcasts.  Hence, directed broadcasts will only be
    useful in limited Internet regions (e.g., the within the subnets
    of a particular campus) in which delivery is supported by the
    gateway administrators.  Host group multicasting (see Sections 2.8
    and 4.6) will soon provide a much more efficient mechanism than
    directed broadcasting.  Gateway algorithms for host group
    multicasting will be specified in future RFC's.
 4.7.  Reachability Procedures
    The architecture must provide a robust mechanism to establish the
    operational status of each link and node in the network, including
    the gateways, the links connecting them and, where appropriate,
    the hosts as well.  Ordinarily, this requires at least a
    link-level reachability protocol involving a periodic exchange of
    messages across each link.  This function might be intrinsic to
    the link-level protocols used (e.g., LAPB).  However, it is in
    general ill-advised to assume a host or gateway is operating
    correctly even if its link-level reachability protocol is
    operating correctly.  Additional confirmation is required in the
    form of an operating routing algorithm or peer-level reachability
    protocol (such as used in EGP).
    Failure and restoration of a link and/or gateway are considered
    network events and must be reported to the control center.  It is
    desirable, although not required, that reporting paths not require
    correct functioning of the routing algorithm itself.
 4.8.  Time-To-Live
    The Time-to-Live (TTL) field of the IP header is defined to be a
    timer limiting the lifetime of a datagram in the Internet.  It is
    an 8-bit field and the units are seconds.  This would imply that
    for a maximum TTL of 255 a datagram would time-out after about 4
    and a quarter minutes.  Another aspect of the definition requires
    each gateway (or other module) that handles a datagram to
    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 effectively becomes a hop count limit on how far a datagram
    can propagate through the Internet.

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    As the Internet grows, the number of hops needed to get from one
    edge to the opposite edge increases, i.e., the Internet diameter
    grows.
    If a gateway holds a datagram for more than one second, it must
    decrement the TTL by one for each second.
    If the TTL is reduced to zero, the datagram must be discarded, and
    the gateway may send an ICMP Time Exceeded message to the source.
    A datagram should never be received with a TTL of zero.
    When it originates a datagram, a gateway is acting in the role of
    a host and must supply a realistic initial value for the TTL.

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5. Operation and Maintenance

 5.1.  Introduction
    Facilities to support operation and maintenance (O&M) activities
    form an essential part of any gateway implementation.  The
    following kinds of activity are included under gateway O&M:
  • Diagnosing hardware problems in the gateway processor, in

its network interfaces, or in the connected networks,

          modems, or communication lines.
  • Installing a new version of the gateway software.
  • Restarting or rebooting a gateway after a crash.
  • Configuring (or reconfiguring) the gateway.
  • Detecting and diagnosing Internet problems such as

congestion, routing loops, bad IP addresses, black holes,

          packet avalanches, and misbehaved hosts.
  • Changing network topology, either temporarily (e.g., to

diagnose a communication line problem) or permanently.

  • Monitoring the status and performance of the gateways and

the connected networks.

  • Collecting traffic statistics for use in (Inter-)network

planning.

    Gateways, packet-switches, and their connected communication lines
    are often operated as a system by a centralized O&M organization.
    This organization will maintain a (Inter-)network operation
    center, or NOC, to carry out its O&M functions.  It is essential
    that gateways support remote control and monitoring from such a
    NOC, through an Internet path (since gateways might not be
    connected to the same network as their NOC).  Furthermore, an IP
    datagram traversing the Internet will often use gateways under the
    control of more than one NOC; therefore, Internet problem
    diagnosis will often involve cooperation of personnel of more than
    one NOC.  In some cases, the same gateway may need to be monitored
    by more than one NOC.
    The tools available for monitoring at a NOC may cover a wide range
    of sophistication.  Proposals have included multi-window, dynamic
    displays of the entire gateway system, and the use of AI
    techniques for automatic problem diagnosis.

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    Gateway 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 gateway system [9].  Therefore, diagnosis of congestion
    problems will sometimes require the monitoring of TCP statistics
    in hosts.  Gateway algorithms also interact with local network
    performance, especially through handling of broadcast packets and
    ARP, and again diagnosis will require access to hosts (e.g.,
    examining ARP caches).  However, consideration of host monitoring
    is beyond the scope of this RFC.
    There are currently a number of R&D efforts in progress in the
    area of Internet management and more specifically gateway O&M.  It
    is hoped that these will lead quickly to Internet standards for
    the gateway protocols and facilities required in this area.  This
    is also an area in which vendor creativity can make a significant
    contribution.
 5.2.   Gateway O&M Models
    There is a range of possible models for performing O&M functions
    on a gateway.  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 gateway 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 intermediate
    models, e.g., one in which NOC personnel can log into the gateway
    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 gateway installations, but in general remote operation
    from a NOC will be required, and therefore remote O&M provisions
    are required for most gateways.
    Remote O&M functions may be exercised through a control agent
    (program).  In the direct approach, the gateway would support
    remote O&M functions directly from the NOC using standard Internet
    protocols (e.g., UDP or TCP); in the indirect approach, the
    control agent would support these protocols and control the
    gateway 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 gateways are a part.  If this is the

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    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 gateways should
    facilitate this.
    Remote gateway monitoring and (especially) remote gateway control
    present important access control problems which must be addressed.
    Care must also be taken to ensure control of the use of gateway
    resources for these functions.  It is not desirable to let gateway
    monitoring take more than some limited fraction of the gateway CPU
    time, for example.  On the other hand, O&M functions must receive
    priority so they can be exercised when the gateway is congested,
    i.e., when O&M is most needed.
    There are no current Internet standards for the control and
    monitoring protocols, although work is in progress in this area.
    The Host Monitoring Protocol (HMP) [7] could be used as a model
    until a standard is developed; however, it is strongly recommended
    that gateway O&M protocol be built on top of one of the standard
    Internet end-to-end protocols UDP or TCP. An example of a very
    simple but effective approach to gateway monitoring is contained
    in RFC-996 [43].
 5.3.   Gateway O&M Functions
    The following O&M functions need to be performed in a gateway:
       A.  Maintenance -- Hardware Diagnosis
          Each gateway must operate as a stand-alone device for the
          purposes of local hardware maintenance.  Means must be
          available to run diagnostic programs at the gateway site
          using only on-site tools, which might be only a diskette or
          tape and local terminal.  It is desirable, although not
          required, to be able to run diagnostics or dump the gateway
          via the network in case of fault.  Means should be provided
          to allow remote control from the NOC of of modems attached
          to the gateway.  The most important modem control capability
          is entering and leaving loopback mode, to diagnose line
          problems.

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       B.  Control -- Dumping and Rebooting
          It must be possible to dump and reboot a stand-alone gateway
          upon command from the NOC.  In addition, a stand-alone
          gateway must include a watchdog timer that either initiates
          a reboot automatically or signals a remote control site if
          not reset periodically by the software.  It is desirable
          that the boot data involved reside at an Internet host
          (e.g., the NOC host) and be transmitted via the net;
          however, the use of local devices at the gateway site is
          acceptable.
       C.  Control -- Configuring the Gateway
          Every gateway will have a number of configuration parameters
          which must be set (see the next section for examples).  It
          must be possible to update the parameters without rebooting
          the gateway; at worst, a restart may be required.
       D.  Monitoring -- Status and Performance
          A mechanism must be provided for retrieving status and
          statistical information from a gateway.  A gateway must
          supply such information in response to a polling message
          from the NOC.  In addition, it may be desirable to configure
          a gateway to transmit status spontaneously and periodically
          to a NOC (or set of NOCs), for recording and display.
          Examples of interesting status information include: link
          status, queue lengths, buffer availability, CPU and memory
          utilization, the routing data-base, error counts, and packet
          counts.  Counts should be kept for dropped datagrams,
          separated by reason.  Counts of ICMP datagrams should be
          kept by type and categorized into those originating at the
          gateway, and those destined for the gateway.  It would be
          useful to maintain many of these statistics by network
          interface, by source/destination network pair, and/or by
          source/destination host pair.
          Note that a great deal of useful monitoring data is often to
          be found in the routing data-base.  It is therefore useful
          to be able to tap into this data-base from the NOC.
       E.  Monitoring -- Error Logging
          A gateway should be capable of asynchronously sending
          exception ("trap") reports to one or more specified Internet
          addresses, one of which will presumably be the NOC host.

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RFC 1009 - Requirements for Internet Gateways June 1987

          There must also be a mechanism to limit the frequency of
          such trap reports, and the parameters controlling this
          frequency must be settable in the gateway configuration.
          Examples of conditions which should result in traps include:
          datagrams discarded because of TTL expiration (an indicator
          of possible routing loops); resource shortages; or an
          interface changing its up/down status.
 5.4.   Gateway Configuration Parameters
    Every gateway will have a set of configuration parameters
    controlling its operation.  It must be possible to set these
    parameters remotely from the NOC or locally at any time, without
    taking the gateway down.
    The following is a partial but representative list of possible
    configuration parameters for a full-function gateway.  The items
    marked with "(i)" should be settable independently for each
    network interface.
  • (i) IP (sub-) network address
  • (i) Subnet address mask
  • (i) MTU of local network
  • (i) Hardware interface address
  • (i) Broadcast compatibility option (0s or 1s)
  • EGP parameters – neighbors, Autonomous System number,

and polling parameters

  • Static and/or default routes, if any
  • Enable/Disable Proxy ARP
  • Source Quench parameters
  • Address filter configuration
  • Boot-host address
  • IP address of time server host
  • IP address(es) of logging host(s)

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  • IP address(es) of hosts to receive traps
  • IP address(es) of hosts authorized to issue control

commands

  • Error level for logging
  • Maximum trap frequency
  • Hold-down period (if any)

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Appendix A. Technical Details

 This Appendix collects a number of technical details and rules
 concerning datagram forwarding by gateways and datagram handling by
 hosts, especially in the presence of broadcasting and subnets.
 A.1.  Rules for Broadcasting
    The following rules define how to handle broadcasts of packets and
    datagrams [50]:
       a.  Hosts (which do not contain embedded gateways) must NEVER
           forward any datagrams received from a connected network,
           broadcast or not.
           When a host receives an IP datagram, if the destination
           address identifies the host or is an IP broadcast address,
           the host passes the datagram to its appropriate
           higher-level protocol module (possibly sending ICMP
           protocol unreachable, but not if the IP address was a
           broadcast address).  Any other IP datagram must simply be
           discarded, without an ICMP error message.  Hosts never send
           redirects.
       b.  All packets containing IP datagrams which are sent to the
           local-network packet broadcast address must contain an IP
           broadcast address as the destination address in their IP
           header.  Expressed in another way, a gateway (or host) must
           not send in a local-network broadcast packet an IP datagram
           that has a specific IP host address as its destination
           field.
       c.  A gateway must never forward an IP datagram that arrives
           addressed to the IP limited broadcast address {-1,-1}.
           Furthermore, it must must not send an ICMP error message
           about discarding such a datagram.
       d.  A gateway must not forward an IP datagram addressed to
           network zero, i.e., {0, *}.
       e.  A gateway may forward a directed broadcast datagram, i.e.,
           a datagram with the IP destination address:
          { <Network-number>, -1}.
           However, it must not send such a directed broadcast out the
           same interface it came in, if this interface has
           <Network-number> as its network number.  If the code in the

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           gateway making this decision does not know what interface
           the directed-broadcast datagram arrived on, the gateway
           cannot support directed broadcast to this connected network
           at all.
       f.  A gateway is permitted to protect its connected networks by
           discarding directed broadcast datagrams.
    A gateway will broadcast an IP datagram on a connected network if
    it is a directed broadcast destined for that network.  Some
    gateway-gateway routing protocols (e.g., RIP) also require
    broadcasting routing updates on the connected networks.  In either
    case, the datagram must have an IP broadcast address as its
    destination.
       Note:  as observed earlier, some host implementations (those
       based on Berkeley 4.2BSD) use zero rather than -1 in the host
       field.  To provide compatibility during the period until these
       systems are fixed or retired, it may be useful for a gateway to
       be configurable to send either choice of IP broadcast address
       and accept both if received.
 A.2.  ICMP Redirects
    A gateway will generate an ICMP Redirect if and only if the
    destination IP address is reachable from the gateway (as
    determined by the routing algorithm) and the next-hop gateway is
    on the same (sub-)network as the source host.  Redirects must not
    be sent in response to an IP network or subnet broadcast address
    or in response to a Class D or Class E IP address.
    A host must discard an ICMP Redirect if the destination IP address
    is not its own IP address, or the new target address is not on the
    same (sub-)network.  An accepted Redirect updates the routing
    data-base for the old target address.  If there is no route
    associated with the old target address, the Redirect is ignored.
    If the old route is associated with a default gateway, a new route
    associated with the new target address is inserted in the
    data-base.

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Appendix B. NSFNET Specific Requirements

 The following sections discuss certain issues of special concern to
 the NSF scientific networking community.  These issues have primary
 relevance in the policy area, but also have ramifications in the
 technical area.
 B.1.  Proprietary and Extensibility Issues
    Although hosts, gateways and networks supporting Internet
    technology have been in continuous operation for several years,
    vendors users and operators must understand that not all
    networking issues are fully resolved.  As a result, when new needs
    or better solutions are developed for use in the NSF networking
    community, it may be necessary to field new protocols or augment
    existing ones.  Normally, these new protocols will be designed to
    interoperate in all practical respects with existing protocols;
    however, occasionally it may happen that existing systems must be
    upgraded to support these new or augmented protocols.
    NSF systems procurements may favor those vendors who undertake a
    commitment to remain aware of current Internet technology and be
    prepared to upgrade their products from time to time as
    appropriate.  As a result, vendors are strongly urged to consider
    extensibility and periodic upgrades as fundamental characteristics
    of their products.  One of the most productive and rewarding ways
    to do this on a long-term basis is to participate in ongoing
    Internet research and development programs in partnership with the
    academic community.
 B.2.  Interconnection Technology
    In order to ensure network-level interoperability of different
    vendor's gateways within the NSFNET context, we specify that a
    gateway must at a minimum support Ethernet connections and serial
    line protocol connections.
    Currently the most important common interconnection technology
    between Internet systems of different vendors is Ethernet.  Among
    the reasons for this are the following:
       1.  Ethernet specifications are well-understood and mature.
       2.  Ethernet technology is in almost all aspects vendor
           independent.
       3.  Ethernet-compatible systems are common and becoming more
           so.

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RFC 1009 - Requirements for Internet Gateways June 1987

    These advantages combined favor the use of Ethernet technology as
    the common point of demarcation between NSF network systems
    supplied by different vendors, regardless of technology.  It is a
    requirement of NSF gateways that, regardless of the possibly
    proprietary switching technology used to implement a given
    vendor-supplied network, its gateways must support an Ethernet
    attachment to gateways of other vendors.
    It is expected that future NSF gateway requirements will specify
    other interconnection technologies.  The most likely candidates
    are those based on X.25 or IEEE 802, but other technologies
    including broadband cable, optical fiber, or other media may also
    be considered.
 B.3.  Routing Interoperability
    The Internet does not currently have an "open IGP" standard, i.e.,
    a common IGP which would allow gateways from different vendors to
    form a single Autonomous System.  Several approaches to routing
    interoperability are currently in use among vendors and the NSF
    networking community.
  • Proprietary IGP
       At least one gateway vendor has implemented a proprietary IGP
       and uses EGP to interface to the rest of the Internet.
  • RIP
       Although RIP is undocumented and various implementations of it
       differ in subtle ways, it has been used successfully for
       interoperation among multiple vendors as an IGP.
  • Gateway Daemon
       The NSF networking community has built a "gateway daemon"
       program which can mediate among multiple routing protocols to
       create a mixed-IGP Autonomous System.  In particular, the
       prototype gateway daemon executes on a 4.3BSD machine acting as
       a gateway and exchanges routing information with other
       gateways, speaking both RIP and Hello protocols; in addition,
       it supports EGP to other Autonomous Systems.

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RFC 1009 - Requirements for Internet Gateways June 1987

 B.4.  Multi-Protocol Gateways
    The present NSF gateway requirements specify only the Internet
    protocol IP.  However, in a few years the Internet will begin a
    gradual transition to the functionally-equivalent subset of the
    ISO protocols [17].  In particular, an increasing percentage of
    the traffic will use the ISO Connectionless Mode Network Service
    (CLNS, but commonly called "ISO IP") [33] in place of IP.  It is
    expected that the ISO suite will eventually become the dominant
    one; however, it is also expected that requirements to support
    Internet IP will continue, perhaps indefinitely.
    To support the transition to ISO protocols and the coexistence
    stage, it is highly desirable that a gateway design provide for
    future extensions to support more than one protocol simultaneous,
    and in particular both IP and CLNS [18].
    Present NSF gateway requirements do not include protocols above
    the network layer, such as TCP, unless necessary for network
    monitoring or control.  Vendors should recognize that future
    requirements to interwork between Internet and ISO applications,
    for example, may result in an opportunity to market gateways
    supporting multiple protocols at all levels up through the
    application level [16].  It is expected that the network-level NSF
    gateway requirements summarized in this document will be
    incorporated in the requirements document for these
    application-level gateways.
    Internet gateways function as intermediate systems (IS) with
    respect to the ISO connectionless network model and incorporate
    defined packet formats, routing algorithms and related procedures
    [33, 34].  The ISO ES-IS [37] provides the functions of ARP and
    ICMP Redirect.
 B.5.  Access Control and Accounting
    There are no requirements for NSF gateways at this time to
    incorporate specific access-control and accounting mechanisms in
    the design;  however, these important issues are currently under
    study and will be incorporated into a subsequent edition of this
    document.  Vendors are encouraged to plan for the introduction of
    these mechanisms into their products.  While at this time no
    definitive common model for access control and accounting has
    emerged, it is possible to outline some general features such a
    model is likely to have, among them the following:

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RFC 1009 - Requirements for Internet Gateways June 1987

       1.  The primary access control and accounting mechanisms will
           be in the service hosts themselves, not the gateways,
           packet-switches or workstations.
       2.  Agents acting on behalf of access control and accounting
           mechanisms may be necessary in the gateways, to collect
           data, enforce password protection, or mitigate resource
           priority and fairness.  However, the architecture and
           protocols used by these agents may be a local matter and
           cannot be specified in advance.
       3.  NSF gateways may be required to incorporate access control
           and accounting mechanisms based on datagram
           source/destination address, as well as other fields in the
           IP header.
       4.  NSF gateways may be required to enforce policies on access
           to gateway and communication resources.  These policies may
           be based upon equity ("fairness") or upon inequity
           ("priority").

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RFC 1009 - Requirements for Internet Gateways June 1987

Acknowledgments

 An earlier version of this document (RFC-985) [60] was prepared by
 Dave Mills in behalf of the Gateway Requirements Subcommittee of the
 NSF Network Technical Advisory Group, in cooperation with the
 Internet Activities Board, Internet Architecture Task Force, and
 Internet Engineering Task Force.  This effort was chaired by Dave
 Mills, and contributed to by many people.
 The authors of current document have also received assistance from
 many people in the NSF and ARPA networking community.  We thank you,
 one and all.

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RFC 1009 - Requirements for Internet Gateways June 1987

References and Bibliography

 Many of these references are  available from the DDN Network
 Information Center, SRI International, 333 Ravenswood Avenue, Menlo
 Park, California 94025 (telephone: 800-235-3155).
 [1]   Postel, J., "Internet Protocol", RFC-791, USC Information
       Sciences Institute, September 1981.
 [2]   Postel, J., "Internet Control Message Protocol", RFC-792, USC
       Information Sciences Institute, September 1981.
 [3]   BBN, "Interface Message Processor - Specifications for the
       Interconnection of a Host and an IMP", Report 1822, Bolt
       Beranek and Newman, December 1981.
 [4]   Plummer, D., "An Ethernet Address Resolution Protocol",
       RFC-826, Symbolics, September 1982.
 [5]   DOD, "Military Standard Internet Protocol", Military Standard
       MIL-STD-1777, United States Department of Defense, August 1983.
 [6]   BBN, "Defense Data Network X.25 Host Interface Specification",
       Report 5476, Bolt Beranek and Newman, December 1983.
 [7]   Hinden, R., "A Host Monitoring Protocol", RFC-869, BBN
       Communications, December 1983.
 [8]   Korb, J.T., "A Standard for the Transmission of IP Datagrams
       over Public Data Networks", RFC-877, Purdue University,
       September 1983.
 [9]   Nagle, J., "Congestion Control in IP/TCP Internetworks",
       RFC-896, Ford Aerospace, January 1984.
 [10]  Hornig, C., "A Standard for the Transmission of IP Datagrams
       over Ethernet Networks", RFC-894, Symbolics, April 1984.
 [11]  Mills, D.L., "Exterior Gateway Formal Specification", RFC-904,
       M/A-COM Linkabit, April 1984.
 [12]  Xerox, "Xerox Synchronous Point-to-Point Protocol", Xerox
       System Integration Standard 158412, December 1984.
 [13]  Kirton, P., "EGP Gateway under Berkeley UNIX 4.2", RFC-911, USC
       Information Sciences Institute, August 1984.

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RFC 1009 - Requirements for Internet Gateways June 1987

 [14]  Postel, J., "Multi-LAN Address Resolution", RFC-925, USC
       Information Sciences Institute, October 1984.
 [15]  Finlayson, R., T. Mann, J. Mogul, and M. Theimer, "A Reverse
       Address Resolution Protocol", RFC-904, Stanford University,
       June 1984.
 [16]  NRC, "Transport Protocols for Department of Defense Data
       Networks", RFC-942, National Research Council, March 1985.
 [17]  Postel, J., "DOD Statement on NRC Report", RFC-945, USC
       Information Sciences Institute, April 1985.
 [18]  ISO, "Addendum to the Network Service Definition Covering
       Network Layer Addressing", RFC-941, International Standards
       Organization, April 1985.
 [19]  Leiner, B., J. Postel, R. Cole and D. Mills, "The DARPA
       Internet Protocol Suite", Proceedings INFOCOM 85, IEEE,
       Washington DC, March 1985.  Also in: IEEE Communications
       Magazine, March 1985.  Also available as ISI-RS-85-153.
 [20]  Romkey, J., "PC/IP Programmer's Manual", MIT Laboratory for
       Computer Science, pp. 57-59, April 1986.
 [21]  Mogul, J., and J. Postel, "Internet Standard Subnetting
       Procedure", RFC-950, Stanford University, August 1985.
 [22]  Reynolds, J., and J. Postel, "Official Internet Protocols",
       RFC-1011, USC Information Sciences Institute, May 1987.
 [23]  Reynolds, J., and J. Postel, "Assigned Numbers", RFC-1010, USC
       Information Sciences Institute, May 1987.
 [24]  Nagle, J., "On Packet Switches with Infinite Storage", RFC-970,
       Ford Aerospace, December 1985.
 [25]  SRI, "DDN Protocol Handbook", NIC-50004, NIC-50005, NIC-50006,
       (three volumes), SRI International, December 1985.
 [26]  SRI, "ARPANET Information Brochure", NIC-50003, SRI
       International, December 1985.
 [27]  Mills, D.L., "Autonomous Confederations", RFC-975, M/A-COM
       Linkabit, February 1986.
 [28]  Jacobsen, O., and J. Postel, "Protocol Document Order
       Information",  RFC-980, SRI International, March 1986.

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RFC 1009 - Requirements for Internet Gateways June 1987

 [29]  Malis, A.G., "PSN End-to-End Functional Specification",
       RFC-979, BBN Communications, March 1986.
 [30]  Postel, J, "Internetwork Applications using the DARPA Protocol
       Suite", Proceedings INFOCOM 85, IEEE, Washington DC,
       March 1985.  Also available as ISI-RS-85-151.
 [31]  Postel, J, C. Sunshine, and D. Cohen, "The ARPA Internet
       Protocol", Computer Networks, Vol. 5, No. 4, July 1981.
 [32]  Cerf, V., and R. Kahn, "A Protocol for Packet Network
       Intercommunication", IEEE Transactions on Communication,
       May 1974.
 [33]  ISO, "Protocol for Providing the Connectionless-mode Network
       Service", RFC-994, DIS-8473, International Standards
       Organization, March 1986.
 [34]  ANSI, "Draft Network Layer Routing Architecture", ANSI X3S3.3,
       86-215R, April 1987.
 [35]  Rosen, E., "Exterior Gateway Protocol (EGP)", RFC-827, Bolt
       Beranek and Newman, October 1982.
 [36]  Sidhu, D., "Some Problems with the Specification of the
       Military Standard Internet Protocol", RFC-963, Iowa State
       University, November 1985.
 [37]  ISO, "End System to Intermediate System Routing Exchange
       Protocol for use in conjunction with ISO 8473", RFC-995,
       April 1986.
 [38]  Postel, J., "Address Mappings", RFC-796, USC/Information
       Sciences Institute, September 1981.
 [39]  Mills, D., "DCN Local Network Protocols", RFC-891, M/A-COM
       Linkabit, December 1983.
 [40]  McQuillan, J. M., I. Richer, and E. C. Rosen, "The New Routing
       Algorithm for the ARPANET",  IEEE Transactions on
       Communications, May 1980.
 [41]  Hinden, R., and A. Sheltzer, "The DARPA Internet Gateway",
       RFC-823, Bolt Beranek and Newman, September 1982.
 [42]  Farber, D., G. Delp, and T. Conte, "A Thinwire Protocol for
       Connecting Personal Computers to the Internet", RFC-914,
       University of Delaware, September 1984.

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RFC 1009 - Requirements for Internet Gateways June 1987

 [43]  Mills, D., "Statistics Server", RFC-996, University Of
       Delaware, February 1987.
 [44]  Postel, J. and K. Harrenstien, "Time Protocol", RFC-868,
       May 1983.
 [45]  Mills, D., "Network Time Protocol (NTP)", RFC-958, M/A-Com
       Linkabit, September 1985.
 [46]  Seamonson, L., and E. Rosen, "Stub Exterior Gateway Protocol",
       RFC-888, Bolt Beranek And Newman, January 1984.
 [47]  Deering, S., and D. Cheriton, "Host Groups: A Multicast
       Extension to the Internet Protocol", RFC-966, Stanford
       University, December 1985.
 [48]  Deering, S., "Host Extensions for IP Multicasting", RFC-988,
       Stanford University, July 1986.
 [49]  Mogul, J., "Broadcasting Internet Datagrams", RFC-919, Stanford
       University, October 1984.
 [50]  Mogul, J., "Broadcasting Internet Datagrams in the Presence of
       Subnets", RFC-922, Stanford University, October 1984.
 [51]  Rosen, E., "Exterior Gateway Protocol", RFC-827, Bolt Beranek
       and Newman, October 1982.
 [52]  Rose, M., "Low Tech Connection into the ARPA Internet: The Raw
       Packet Split Gateway", Technical Report 216, Department of
       Information and Computer Science, University of California,
       Irvine, February 1984.
 [53]  Rosen, E., "Issues in Buffer Management", IEN-182, Bolt Beranek
       and Newman, May 1981.
 [54]  Rosen, E., "Logical Addressing", IEN-183, Bolt Beranek and
       Newman, May 1981.
 [55]  Rosen, E., "Issues in Internetting - Part 1: Modelling the
       Internet", IEN-184, Bolt Beranek and Newman, May 1981.
 [56]  Rosen, E., "Issues in Internetting - Part 2: Accessing the
       Internet", IEN-187, Bolt Beranek and Newman, June 1981.
 [57]  Rosen, E., "Issues in Internetting - Part 3: Addressing",
       IEN-188, Bolt Beranek and Newman, June 1981.

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RFC 1009 - Requirements for Internet Gateways June 1987

 [58]  Rosen, E., "Issues in Internetting - Part 4: Routing", IEN-189,
       Bolt Beranek and Newman, June 1981.
 [59]  Sunshine, C., "Comments on Rosen's Memos", IEN-191, USC
       Information Sciences Institute, July 1981.
 [60]  NTAG, "Requirements for Internet Gateways -- Draft", RFC-985,
       Network Technical Advisory Group, National Science Foundation,
       May 1986.
 [61]  Khanna, A., and Malis, A., "The ARPANET AHIP-E Host Access
       Protocol (Enhanced AHIP)", RFC-1005, BBN Communications,
       May 1987
 [62]  Nagle, J., "Congestion Control in IP/TCP Internetworks", ACM
       Computer Communications Review, Vol.14, no.4, October 1984.

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/data/webs/external/dokuwiki/data/pages/rfc/rfc1009.txt · Last modified: 1987/06/07 01:02 (external edit)