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rfc:rfc3373

Network Working Group D. Katz Request for Comments: 3373 Juniper Networks, Inc. Category: Informational R. Saluja

                                                    Tenet Technologies
                                                        September 2002
                      Three-Way Handshake for
        Intermediate System to Intermediate System (IS-IS)
                    Point-to-Point Adjacencies

Status of this Memo

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

Copyright Notice

 Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

 The IS-IS routing protocol (ISO 10589) requires reliable protocols at
 the link layer for point-to-point links.  As a result, it does not
 use a three-way handshake when establishing adjacencies on point-to-
 point media.  This paper defines a backward-compatible extension to
 the protocol that provides for a three-way handshake.  It is fully
 interoperable with systems that do not support the extension.
 Additionally, the extension allows the robust operation of more than
 256 point-to-point links on a single router.
 This extension has been implemented by multiple router vendors; this
 paper is provided as information to the Internet community in order
 to allow interoperable implementations to be built by other vendors.

1. Terms

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in BCP 14, RFC 2119.

2. Introduction

 The IS-IS protocol [1] assumes certain requirements stated in ISO
 10589 (section 6.7.2) for the operation of IS-IS over point-to-point
 links and hence provides only a two-way handshake when establishing

Katz & Saluja Informational [Page 1] RFC 3373 Three-Way Handshake for IS-IS September 2002

 adjacencies on point-to-point links.  The protocol does not operate
 correctly if these subnetwork requirements for point-to-point links
 are not met.  The basic mechanism defined in the standard is that
 each side declares the other side to be reachable if a Hello packet
 is heard from it.  Once this occurs, each side then sends a Complete
 Sequence Number PDU (CSNP) to trigger database synchronization.
 Three failure modes are known.  First, if the link goes down and then
 comes back up, or one of the systems restarts, and the CSNP packet is
 lost, and the network has a cut set of one through the link, the link
 state databases on either side of the link will not synchronize for a
 full LSP refresh period (up to eighteen hours).
 A second, more serious failure, is that if the link fails in only one
 direction, the failure will only be detected by one of the systems.
 Normally only one of the two systems will announce the adjacency in
 its link state packets, and the SPF algorithm will thus ignore the
 link.  However, if there are two parallel links between systems and
 one of them fails in one direction, SPF will still calculate paths
 between the two systems, and the system that does not notice the
 failure will attempt to pass traffic down the failed link (in the
 direction that does not work).
 The third issue is that on some physical layers, the
 interconnectivity between endpoints can change without causing a
 link-layer-down condition.  In this case, a system may receive
 packets that are actually destined for a different system (or a
 different link on the same system).  The receiving system may end up
 thinking that it has an adjacency with the remote system when in fact
 the remote system is adjacent with a third system.
 The solution proposed here ensures correct operation of the protocol
 over unreliable point-to-point links.  As part of the solution to the
 three-way handshaking issue,  a method is defined to remove the
 limitation of 255 point-to-point interfaces imposed by IS-IS [1].
 This method is more robust than the ad hoc methods currently in use.

3. Overview of Extensions

3.1 Handshaking

 The intent is to provide a three-way handshake for point-to-point
 adjacency establishment in a backward compatible fashion.  This is
 done by providing an optional mechanism that allows each system to
 report its adjacency three-way state; this allows a system to only
 declare an adjacency to be up if it knows that the other system is
 receiving its IS-IS Hello (IIH) packets.

Katz & Saluja Informational [Page 2] RFC 3373 Three-Way Handshake for IS-IS September 2002

 The adjacency three-way state can be one of the following types:
 Down
    This is the initial point-to-point adjacency three-way state.  The
    system has not received any IIH packet containing the three-way
    handshake option on this point-to-point circuit.
 Initializing
    The system has received IIH packet containing the three-way
    handshake option from a neighbor but does not know whether the
    neighbor is receiving its IIH packet.
 Up
    The system knows that the neighbor is receiving its IIH packets.
 The adjacency three-way state that is reported by this mechanism is
 not equal or equivalent to the adjacency state that is described in
 ISO 10589 [1].  If this mechanism is supported then an adjacency may
 have two states, its state as defined in ISO 10589 [1], and its
 three-way state.  For example according to ISO 10589 [1] receipt of
 an ISH will cause an adjacency to go to Initializing state; however
 receipt of an ISH will have no effect on the three-way state of an
 adjacency, which remains firmly Down until it receives an IIH from a
 neighbor that contains the three-way handshaking option.
 In addition, the neighbor's system ID and (newly-defined) extended
 circuit ID are reported in order to detect the case where the same
 stream is being received by multiple systems (only one of which can
 talk back).
 The mechanism is quite similar to the one defined in the Netware Link
 Services Protocol (NLSP) [2], a variant of IS-IS used for routing IPX
 traffic.  The difference between this mechanism and the one used in
 NLSP is the location where the information is carried (NLSP uses two
 of the reserved bits in the IIH header, whereas this solution adds a
 separate option to the IIH), and the presence of the neighbor's
 system ID and circuit ID.  In theory, using the reserved header bits
 should be backward compatible, since systems are supposed to ignore
 them.  However, it was felt that this was risky, as the use of
 untested mechanisms such as this have led to problems in the past in
 other protocols.  New option codes, on the other hand, have been
 demonstrated to work properly, as the deployment of Integrated IS-IS
 for IP [3] has done exactly this.
 The new mechanism only comes into play when the remote system
 includes the new option in its IIH packet; if the option is not
 present, it is assumed that the system does not support the new
 mechanism, and so the old procedures are used.

Katz & Saluja Informational [Page 3] RFC 3373 Three-Way Handshake for IS-IS September 2002

3.2 More Than 256 Interfaces

 The IS-IS specification has an implicit limit of 256 interfaces, as
 constrained by the eight bit Circuit ID field carried in various
 packets.  Moderately clever implementors have realized that the only
 true constraint is that of 256 LAN interfaces, and for that matter
 only 256 LAN interfaces for which a system is the Designated IS.
 This is because the only place that the circuit ID is advertised in
 LSPs is in the pseudonode LSP ID.
 Implementors have treated the point-to-point Circuit ID number space
 as being independent from that of the LAN interfaces, since these
 Circuit IDs appear only in IIH PDUs and are only used for detection
 of a change in identity at the other end of a link.  More than 256
 point-to-point interfaces have been supported by sending the same
 circuit ID on multiple interfaces.  This reduces the robustness of
 the ID change detection algorithm, since it would then be possible to
 switch links between interfaces on a system without detecting the
 change.
 Since the Circuit ID is an integral part of the new handshaking
 mechanism, a backward compatible mechanism for expanding the circuit
 ID number space is included in this specification.

4. Details

4.1 Syntax

 A new IS-IS Option type, "Point-to-Point Three-Way Adjacency", is
 defined:
 x Type - 0xF0 (decimal 240)
 x Length - total length of the value field (1 to 17 octets)
 x Value -
                                                     No. of Octets
               +-----------------------------------+
               | Adjacency Three-Way State         |      1
               +-----------------------------------+
               | Extended Local Circuit ID         |      4
               +-----------------------------------+
               | Neighbor System ID                |      ID Length
               +-----------------------------------+
               | Neighbor Extended Local Circuit ID|      4
               +-----------------------------------+

Katz & Saluja Informational [Page 4] RFC 3373 Three-Way Handshake for IS-IS September 2002

 Adjacency Three-Way State
    The adjacency three-way state of the point-to-point adjacency. The
    following values are defined:
       0  - Up
       1 -  Initializing
       2 -  Down
 Extended Local Circuit ID
    Unique ID assigned to this circuit when it is created by this
    Intermediate system.
 Neighbor System ID
    System ID of neighbor Intermediate system if known.  The length of
    this field is equal to "ID Length" of IIH PDU described in section
    "Point-to-point IS to IS hello PDU" (section 9.7 of [1]).
 Neighbor Extended Local Circuit ID
    Extended Local Circuit ID of the other end of the point-to-point
    adjacency if known.
 Any system that supports this mechanism SHALL include this option in
 its Point-to-Point IIH packets.
 Any system that does not understand this option SHALL ignore it, and
 (of course) SHALL NOT include it in its own IIH packets.
 Any system that supports this mechanism MUST include Adjacency
 Three-Way State field in this option.  The other fields in this
 option SHOULD be included as explained below in section 3.2.
 Any system that is able to process this option SHALL follow the
 procedures below.

4.2 Elements of Procedure

 The new handshake procedure is added to the IS-IS point-to-point IIH
 state machine after the PDU acceptance tests have been performed.
 Although the extended circuit ID is only used in the context of the
 three-way handshake, it is worth noting that it effectively protects
 against the unlikely event where a link is moved to another interface
 on a system that has the same local circuit ID, as the received PDUs
 will be ignored (via the checks defined below) and the existing
 adjacency will fail.

Katz & Saluja Informational [Page 5] RFC 3373 Three-Way Handshake for IS-IS September 2002

 Add a clause e) to the end of section "Receiving ISH PDUs by an
 intermediate system" (section 8.2.2 of [1]):
    Set the state to be reported in the Adjacency Three-Way State
    field of the Point-to-Point Three-Way Adjacency option to Down.
 Add a clause e) to the end of section "Sending point-to-point IIH
 PDUs" (section 8.2.3 of [1]):
    The IS SHALL include the Point-to-Point Three-Way Adjacency option
    in the transmitted Point-to-Point IIH PDU.  The current three-way
    state of the adjacency with its neighbor on the link (as defined
    in new section 8.2.4.1.1 introduced later in the document) SHALL
    be reported in the Adjacency Three-Way State field.  If no
    adjacency exists, the state SHALL be reported as Down.
    The Extended Local Circuit ID field SHALL contain a value assigned
    by this IS when the circuit is created.  This value SHALL be
    unique among all the circuits of this Intermediate System.  The
    value is not necessarily related to that carried in the Local
    Circuit ID field of the IIH PDU.
    If the system ID and Extended Local Circuit ID of the neighboring
    system are known (in adjacency three-way state Initializing or
    Up), the neighbor's system ID SHALL be reported in the Neighbor
    System ID field, and the neighbor's Extended Local Circuit ID
    SHALL be reported in the Neighbor Extended Local Circuit ID field.
 Add a section 8.2.4.1.1, "Three-Way Handshake", immediately prior to
 section "IIH PDU Processing" (section 8.2.4.2 of [1]):
    A received Point-to-Point IIH PDU may or may not contain the
    Point-to-Point Three-Way Adjacency option.  If it does not, the
    link is assumed to be functional in both directions, and the
    procedures described in section 8.2.4.2 are followed.
    If the option is present and contains invalid Adjacency Three-Way
    State, the PDU SHALL be discarded and no further action is taken.
    If the option with a valid Adjacency Three-Way State is present,
    the Neighbor System ID and Neighbor Extended Local Circuit ID
    fields, if present, SHALL be examined.  If they are present, and
    the Neighbor System ID contained therein does not match the local
    system's ID, or the Neighbor Extended Local Circuit ID does not
    match the local system's extended circuit ID, the PDU SHALL be
    discarded and no further action is taken.

Katz & Saluja Informational [Page 6] RFC 3373 Three-Way Handshake for IS-IS September 2002

    If the Neighbor System ID and Neighbor Extended Local Circuit ID
    fields match those of the local system, or are not present, the
    procedures described in section 8.2.4.2 are followed with
    following changes:
    a) In section 8.2.4.2 a and b, the action "Up" from state tables
       5, 6, 7 and 8 may create a new adjacency but the three-way
       state of the adjacency SHALL be Down.
    b) If the action taken from section 8.2.4.2 a or b  is "Up" or
       "Accept", the IS SHALL perform the action indicated by the
       new adjacency three-way state table below, based on the
       current adjacency three-way state and the received Adjacency
       Three-Way State value from the option.  (Note that the
       procedure works properly if neither field is ever included.
       This provides backward compatibility to an earlier version of
       this option.)
                        Received Adjacency Three-Way State
                          Down           Initializing          Up
                     -------------------------------------------------
       Down          |  Initialize            Up                Down
                     |
 adj   Initializing  |  Initialize            Up                Up
 three               |
 -way  Up            |  Initialize            Accept            Accept
 state               |
                     |
                   Adjacency Three-Way State Table
       If the new action is "Down", an adjacencyStateChange(Down)
       event is generated with the reason "Neighbor restarted" and the
       adjacency SHALL be deleted.
       If the new action is "Initialize", no event is generated and
       the adjacency three-way state SHALL be set to "Initializing".
       If the new action is "Up", an adjacencyStateChange(Up)
       event is generated.
    c) Skip section 8.2.4.2 c and d.
    d) If the new action is "Initialize", "Up" or "Accept", follow
       section 8.2.4.2 e.

Katz & Saluja Informational [Page 7] RFC 3373 Three-Way Handshake for IS-IS September 2002

5. Security Considerations

 This document raises no new security issues for IS-IS.

6. References

 [1] ISO, "Intermediate system to Intermediate system routeing
     information exchange protocol for use in conjunction with the
     Protocol for providing the Connectionless-mode Network Service
     (ISO 8473)", ISO/IEC 10589:1992.
 [2] "Netware Link Services Protocol Specification, Version 1.0",
     Novell, Inc., February 1994.
 [3] Callon, R., "OSI IS-IS for IP and Dual Environment", RFC 1195,
     December 1990.

7. Acknowledgements

 The authors would like to thank Tony Li, Henk Smit, Naiming Shen,
 Dave Ward, Jeff Learman, Les Ginsberg and Philip Christian for their
 contributions to this document.

8. Authors' Addresses

 Dave Katz
 Juniper Networks
 1194 N. Mathilda Ave.
 Sunnyvale, CA  94089
 Phone: (408) 745-2073
 EMail:  dkatz@juniper.net
 Rajesh Saluja
 Tenet Technologies
 30/31, 100 Feet Road, Madiwala
 Bangalore - 560 068  INDIA
 Phone: +91 80 552 2215
 EMail: rajesh.saluja@tenetindia.com

Katz & Saluja Informational [Page 8] RFC 3373 Three-Way Handshake for IS-IS September 2002

9. Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Katz & Saluja Informational [Page 9]

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