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RFC 4971 - Intermediate System to Intermediate System (IS-IS) Ex

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Network Working Group                                   JP. Vasseur, Ed.
Request for Comments: 4971                                  N. Shen, Ed.
Category: Standards Track                            Cisco Systems, Inc.
                                                        R. Aggarwal, Ed.
                                                        Juniper Networks
                                                               July 2007

     Intermediate System to Intermediate System (IS-IS) Extensions
                  for Advertising Router Information

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document defines a new optional Intermediate System to
   Intermediate System (IS-IS) TLV named CAPABILITY, formed of multiple
   sub-TLVs, which allows a router to announce its capabilities within
   an IS-IS level or the entire routing domain.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................2
   2. IS-IS Router CAPABILITY TLV .....................................3
   3. Elements of Procedure ...........................................4
   4. Interoperability with Routers Not Supporting the
      Capability TLV ..................................................5
   5. Security Considerations .........................................6
   6. IANA Considerations .............................................6
   7. Acknowledgment ..................................................6
   8. References ......................................................6
      8.1. Normative References .......................................6
      8.2. Informative References .....................................8

1.  Introduction

   There are several situations where it is useful for the IS-IS [IS-IS]
   [IS-IS-IP] routers to learn the capabilities of the other routers of
   their IS-IS level, area, or routing domain.  For the sake of
   illustration, three examples related to MPLS Traffic Engineering (TE)
   are described here:

   1. Mesh-group: the setting up of a mesh of TE Label Switched Paths
      (LSPs) [IS-IS-TE] requires some significant configuration effort.
      [AUTOMESH] proposes an auto-discovery mechanism whereby every
      Label Switching Router (LSR) of a mesh advertises its mesh-group
      membership by means of IS-IS extensions.

   2. Point to Multipoint TE LSP (P2MP LSP).  A specific sub-TLV
      ([TE-NODE-CAP]) allows an LSR to advertise its Point To Multipoint
      capabilities ([P2MP] and [P2MP-REQS]).

   3. Inter-area traffic engineering: Advertisement of the IPv4 and/or
      the IPv6 Traffic Engineering Router IDs.

   The use of IS-IS for Path Computation Element (PCE) discovery may
   also be considered and will be discussed in the PCE WG.

   The capabilities mentioned above require the specification of new
   sub-TLVs carried within the CAPABILITY TLV defined in this document.

   Note that the examples above are provided for the sake of
   illustration.  This document proposes a generic capability
   advertising mechanism that is not limited to MPLS Traffic

   This document defines a new optional IS-IS TLV named CAPABILITY,
   formed of multiple sub-TLVs, which allows a router to announce its
   capabilities within an IS-IS level or the entire routing domain.  The
   applications mentioned above require the specification of new sub-
   TLVs carried within the CAPABILITY TLV defined in this document.

   Definition of these sub-TLVs is outside the scope of this document.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC-2119].


   The IS-IS Router CAPABILITY TLV is composed of 1 octet for the type,
   1 octet that specifies the number of bytes in the value field, and a
   variable length value field that starts with 4 octets of Router ID,
   indicating the source of the TLV, and followed by 1 octet of flags.

   A set of optional sub-TLVs may follow the flag field.  Sub-TLVs are
   formatted as described in RFC 3784 [IS-IS-TE].

   TYPE: 242
   LENGTH: from 5 to 255
     Router ID (4 octets)
     Flags (1 octet)
     Set of optional sub-TLVs (0-250 octets)


             0 1 2 3 4 5 6 7
             | Reserved  |D|S|

   Currently two bit flags are defined.

   S bit (0x01): If the S bit is set(1), the IS-IS Router CAPABILITY TLV
   MUST be flooded across the entire routing domain.  If the S bit is
   not set(0), the TLV MUST NOT be leaked between levels.  This bit MUST
   NOT be altered during the TLV leaking.

   D bit (0x02): When the IS-IS Router CAPABILITY TLV is leaked from
   level-2 to level-1, the D bit MUST be set.  Otherwise, this bit MUST
   be clear.  IS-IS Router capability TLVs with the D bit set MUST NOT
   be leaked from level-1 to level-2.  This is to prevent TLV looping.

   The Router CAPABILITY TLV is OPTIONAL.  As specified in Section 3,
   more than one Router CAPABILITY TLV from the same source MAY be

   This document does not specify how an application may use the Router
   Capability TLV and such specification is outside the scope of this

3.  Elements of Procedure

   A router that generates a CAPABILITY TLV MUST have a Router ID that
   is a 32-bit number.  The ID MUST be unique within the IS-IS area.  If
   the router generates any capability TLVs with domain flooding scope,
   then the ID MUST also be unique within the IS-IS routing domain.

   When advertising capabilities with different flooding scopes, a
   router MUST originate a minimum of two Router CAPABILITY TLVs, each
   TLV carrying the set of sub-TLVs with the same flooding scope.  For
   instance, if a router advertises two sets of capabilities, C1 and C2,
   with an area/level scope and routing domain scope respectively, C1
   and C2 being specified by their respective sub-TLV(s), the router
   will originate two Router CAPABILITY TLVs:

   -  One Router CAPABILITY TLV with the S flag cleared, carrying the
      sub-TLV(s) relative to C1.  This Router CAPABILITY TLV will not be
      leaked into another level.

   -  One Router CAPABILITY TLV with the S flag set, carrying the sub-
      TLV(s) relative to C2.  This Router CAPABILITY TLV will be leaked
      into other IS-IS levels.  When the TLV is leaked from level-2 to
      level-1, the D bit will be set in the level-1 LSP advertisement.

   In order to prevent the use of stale capabilities, a system MUST NOT
   use a Capability TLV present in an LSP of a system that is not
   currently reachable via Level-x paths, where "x" is the level (1 or
   2) in which the sending system advertised the TLV.  This requirement
   applies regardless of whether or not the sending system is the
   originator of the Capabilities TLV.  Note that leaking a Capabilities
   TLV is one of the uses that is prohibited under these conditions.

      Example: If Level-1 router A generates a Capability TLV and floods
      it to two L1/L2 routers, S and T, they will flood it into the
      Level-2 domain.  Now suppose the Level-1 area partitions, such
      that A and S are in one partition and T is in another.  IP routing
      will still continue to work, but if A now issues a revised version
      of the CAP TLV, or decides to stop advertising it, S will follow
      suit, but T will continue to advertise the old version until the
      LSP times out.

   Routers in other areas have to choose whether to trust T's copy of
   A's capabilities or S's copy of A's information and, they have no
   reliable way to choose.  By making sure that T stops leaking A's
   information, this removes the possibility that other routers will use
   stale information from A.

   In IS-IS, the atomic unit of the update process is a TLV -- or more
   precisely, in the case of TLVs that allow multiple entries to appear
   in the value field (e.g., IS-neighbors), the atomic unit is an entry
   in the value field of a TLV.  If an update to an entry in a TLV is
   advertised in an LSP fragment different from the LSP fragment
   associated with the old advertisement, the possibility exists that
   other systems can temporarily have either 0 copies of a particular
   advertisement or 2 copies of a particular advertisement, depending on
   the order in which new copies of the LSP fragment that had the old
   advertisement and the fragment that has the new advertisement arrive
   at other systems.

   Wherever possible, an implementation SHOULD advertise the update to a
   capabilities TLV in the same LSP fragment as the advertisement that
   it replaces.  Where this is not possible, the two affected LSP
   fragments should be flooded as an atomic action.

   Systems that receive an update to an existing capability TLV can
   minimize the potential disruption associated with the update by
   employing a holddown time prior to processing the update so as to
   allow for the receipt of multiple LSP fragments associated with the
   same update prior to beginning processing.

   Where a receiving system has two copies of a capabilities TLV from
   the same system that have different settings for a given attribute,
   the procedure used to choose which copy shall be used is undefined.

4.  Interoperability with Routers Not Supporting the Capability TLV

   Routers that do not support the Router CAPABILITY TLV MUST silently
   ignore the TLV(s) and continue processing other TLVs in the same LSP.
   Routers that do not support specific sub-TLVs carried within a Router
   CAPABILITY TLV MUST silently ignore the unsupported sub-TLVs and
   continue processing those sub-TLVs that are supported in the Router
   CAPABILITY TLV.  How partial support may impact the operation of the
   capabilities advertised within the Router CAPABILITY TLV is outside
   the scope of this document.

   In order for Router CAPABILITY TLVs with domain-wide scope originated
   by L1 Routers to be flooded across the entire domain, at least one
   L1/L2 Router in every area of the domain MUST support the Router

   If leaking of the CAPABILITY TLV is required, the entire CAPABILITY
   TLV MUST be leaked into another level even though it may contain some
   of the unsupported sub-TLVs.

5.  Security Considerations

   Any new security issues raised by the procedures in this document
   depend upon the opportunity for LSPs to be snooped and modified, the
   ease/difficulty of which has not been altered.  As the LSPs may now
   contain additional information regarding router capabilities, this
   new information would also become available to an attacker.
   Specifications based on this mechanism need to describe the security
   considerations around the disclosure and modification of their
   information.  Note that an integrity mechanism, such as the one
   defined in [RFC-3567] or [IS-IS-HMAC], should be applied if there is
   high risk resulting from modification of capability information.

6.  IANA Considerations

   IANA assigned a new IS-IS TLV code-point for the newly defined IS-IS
   TLV type named the IS-IS Router CAPABILITY TLV and defined in this
   document.  The assigned value is 242.

7.  Acknowledgment

   The authors would like to thank Jean-Louis Le Roux, Paul Mabey,
   Andrew Partan, and Adrian Farrel for their useful comments.

8.  References

8.1.  Normative References

   [RFC-2119]    Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [IS-IS]       "Intermediate System to Intermediate System Intra-
                 Domain Routeing Exchange Protocol for use in
                 Conjunction with the Protocol for Providing the
                 Connectionless-mode Network Service (ISO 8473)", ISO

   [RFC-3567]    Li, T. and R. Atkinson, "Intermediate System to
                 Intermediate System (IS-IS) Cryptographic
                 Authentication", RFC 3567, July 2003.

   [IS-IS-IP]    Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
                 dual environments", RFC 1195, December 1990.

   [IS-IS-TE]    Smit, H. and T. Li, "Intermediate System to
                 Intermediate System (IS-IS) Extensions for Traffic
                 Engineering (TE)", RFC 3784, June 2004.

8.2.  Informative References

   [AUTOMESH]    Vasseur, JP., Ed., Le Roux, JL., Ed., Yasukawa, S.,
                 Previdi, S., Psenak, P., and P. Mabbey, "Routing
                 extensions for Discovery of Multiprotocol (MPLS) Label
                 Switch Router (LSR) Traffic Engineering (TE) Mesh
                 Membership", RFC 4972, July 2007.

   [TE-NODE-CAP] Vasseur, JP., Ed., and J.L. Le Roux, "Routing
                 Extensions for Discovery of Traffic Engineering Node
                 Capabilities", Work in Progress, April 2007.

   [P2MP]        Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
                 Yasukawa, Ed., "Extensions to Resource Reservation
                 Protocol - Traffic Engineering (RSVP-TE) for Point-to-
                 Multipoint TE Label Switched Paths (LSPs)", RFC 4875,
                 May 2007.

   [P2MP-REQS]   Yasukawa, S., Ed., "Signaling Requirements for Point-
                 to-Multipoint Traffic-Engineered MPLS Label Switched
                 Paths (LSPs)", RFC 4461, April 2006.

   [IS-IS-HMAC]  Bhatia, M., Ed. and V. Manral, Ed., "IS-IS Generic
                 Cryptographic Authentication", Work in Progress, May

Authors' Addresses

   Jean-Philippe Vasseur
   CISCO Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719
   EMail: jpv@cisco.com

   Stefano Previdi
   CISCO Systems, Inc.
   Via Del Serafico 200
   00142 - Roma
   EMail: sprevidi@cisco.com

   Mike Shand
   Cisco Systems
   250 Longwater Avenue,
   RG2 6GB
   EMail: mshand@cisco.com

   Les Ginsberg
   Cisco Systems
   510 McCarthy Blvd.
   Milpitas, Ca. 95035 USA
   EMail: ginsberg@cisco.com

   Acee Lindem
   Redback Networks
   102 Carric Bend Court
   Cary, NC 27519
   EMail: acee@redback.com

   Naiming Shen
   Cisco Systems
   225 West Tasman Drive
   San Jose, CA 95134
   EMail: naiming@cisco.com

   Rahul Aggarwal
   Juniper Networks
   1194 N. Mathilda Avenue
   San Jose, CA 94089
   EMail: rahul@juniper.net

   Scott Shaffer
   EMail: sshaffer@bridgeport-networks.com

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