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Versions: 01 02 03 04 05 rfc3373                                        
Network Working Group                                          Dave Katz
Internet Draft                                    Juniper Networks, Inc.
Expiration Date: September 1999                               March 1999

        Three-Way Handshake for IS-IS Point-to-Point Adjacencies


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   The IS-IS routing protocol (ISO 10589) does not use a three-way
   handshake when establishing adjacencies on point-to-point media.
   This paper defines an 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.

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1.0  Introduction

   The IS-IS protocol [1]  provides only a two-way handshake when
   establishing adjacencies on point-to-point links.  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, this is not a serious issue;  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).

   In addition, SONET links bring a third issue, which is that when
   SONET protection is used, 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.

   As part of the solution to the three-way handshaking issue, a method
   is defined for supporting more than 256 point-to-point interfaces
   which is more robust than the ad hoc methods currently in use.

2.0  Overview of Extensions

2.1  Handshaking

   The intent is to provide a three-way handshake for point-to-point
   adjacency establishment in an backward compatible fashion.  This is
   done by providing an optional mechanism that allows each system to
   report its adjacency 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.  In addition, the neighbor's system ID and
   (newly-defined) extended circuit ID are reported in order to detect

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   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.  It is also worth noting that the
   encoding used in the NLSP solution does not lend itself to backward

   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.

2.2  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.

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3.0  Details

3.1  Syntax

   A new IS-IS Option type, "Point-to-Point Adjacency State", is

          Type = 0xF0 (decimal 240)
          Length = 5 to 17 octets
            Adjacency State (one octet):
              0 = Up
              1 = Initializing
              2 = Down
            Extended Local Circuit ID (four octets)
            Neighbor System ID if known (zero to eight octets)
            Neighbor Extended Local Circuit ID (four octets, if Neighbor
              System ID is present)

   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 will ignore it, and
   (of course) will not include it in its own IIH packets.

   Any system that is able to process this option shall follow the
   procedures below.

3.2 Elements of Procedure

   The new handshake procedure is added to the IS-IS point-to-point IIH
   state machine after the basic validity checks have been made, and
   before the existing state table is used.  The existing procedures are
   only executed if the neighbor is in the proper state for the
   adjacency to come up.

   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.

   Add a clause e) to the end of section 8.2.3:

     The IS shall include the Point-to-Point Adjacency State option in

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     the transmitted Point-to-Point IIH PDU.  The current state of the
     adjacency with its neighbor on the link (as defined in section shall be reported in the Adjacency 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 of the neighboring system is known (in state
     Initializing or Up), it 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

     A received Point-to-Point IIH PDU may or may not contain the Point-
     to-Point Adjacency State 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, the Neighbor System ID and Neighbor
     Extended Local Circuit ID fields, if present, shall be examined.
     If they are present, and the system ID contained therein does not
     match the local system's ID, or the extended circuit ID does not
     match the local system's extended circuit ID, the PDU shall be
     discarded and no further action is taken.

     If the Neighbor System ID and Neighbor Extended Local Circuit ID
     fields match those of the local system, or are not present, the
     state table below is used to determine the next state, based on the
     current adjacency state and the received 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.)

     If the new state is "Down", an adjacencyStateChange(Down) event is
     generated with the reason "Neighbor restarted" and the adjacency
     shall be deleted.

     If the new state is "Initializing", the adjacency state shall be
     set to "Initializing" and no further action is taken.

     If the new state is "Up", processing continues as described in

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                                             Received State
                               Down           Initializing          Up
           Down          |  Initializing      Initializing         Down
     Adj   Initializing  |  Initializing          Up                Up
     State               |
           Up            |  Initializing          Up                Up

4.0  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.


   The author would like to thank Tony Li, Henk Smit, Naiming Shen, and
   Dave Ward for their contributions to this document.

Author's Address

   Dave Katz
   Juniper Networks
   385 Ravendale Drive
   Mountain View, CA  94043 USA

   Phone:  +1 650 526 8073
   Email:  dkatz@juniper.net

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