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.


   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",
   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

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   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.

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   The adjacency three-way state can be one of the following types:

      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.

      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.

      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.

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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

   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

   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

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   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.

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   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 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, "Three-Way Handshake", immediately prior to
   section "IIH PDU Processing" (section 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 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.

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      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 are followed with
      following changes:

      a) In section 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 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 c and d.

      d) If the new action is "Initialize", "Up" or "Accept", follow
         section e.

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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

   Rajesh Saluja
   Tenet Technologies
   30/31, 100 Feet Road, Madiwala
   Bangalore - 560 068  INDIA

   Phone: +91 80 552 2215

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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
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   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

   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


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

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