IS-IS Capabilities: IS-IS Extensions for Advertising Router Information

Internet Engineering Task Force (IETF)                       L. Ginsberg
Request for Comments: 7981                                    S. Previdi
Obsoletes: 4971                                            Cisco Systems
Category: Standards Track                                        M. Chen
ISSN: 2070-1721                             Huawei Technologies Co., Ltd
                                                            October 2016


          IS-IS Extensions for Advertising Router Information

Abstract

   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.  This document obsoletes
   RFC 4971.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7981.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
      1.1. Requirements Language ......................................3
   2. IS-IS Router CAPABILITY TLV .....................................3
   3. Elements of Procedure ...........................................4
   4. Interoperability with Routers Not Supporting the IS-IS Router
      CAPABILITY TLV ..................................................6
   5. Security Considerations .........................................6
   6. IANA Considerations .............................................7
   7. References ......................................................7
      7.1. Normative References .......................................7
      7.2. Informative References .....................................8
   Appendix A.  Changes to RFC 4971 ...................................9
   Acknowledgements ..................................................10
   Authors' Addresses ................................................10

1.  Introduction

   There are several situations where it is useful for the IS-IS
   [ISO10589] [RFC1195] 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) [RFC5305] requires some significant configuration effort.
       [RFC4972] 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 (RFC4875): A specific sub-TLV
       [RFC5073] allows an LSR to advertise its Point-to-Multipoint
       capabilities ([RFC4875] and [RFC4461]).

   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 IS-IS Router 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
   Engineering.

   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 IS-IS Router CAPABILITY TLV defined in this
   document.

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

1.1.  Requirements Language

   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 RFC 2119 [RFC2119].

2.  IS-IS Router CAPABILITY TLV

   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, followed by 1 octet of flags.

   A set of optional sub-TLVs may follow the flag field.  Sub-TLVs are
   formatted as described in [RFC5305].

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

    Flags

                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 (L2) to Level 1 (L1), 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 IS-IS Router CAPABILITY TLV is OPTIONAL.  As specified in
   Section 3, more than one IS-IS Router CAPABILITY TLV from the same
   source MAY be present.

   This document does not specify how an application may use the IS-IS
   Router CAPABILITY TLV, and such specification is outside the scope of
   this document.

3.  Elements of Procedure

   The Router ID SHOULD be identical to the value advertised in the
   Traffic Engineering Router ID TLV [RFC5305].  If no Traffic
   Engineering Router ID is assigned, the Router ID SHOULD be identical
   to an IP Interface Address [RFC1195] advertised by the originating
   IS.  If the originating node does not support IPv4, then the reserved
   value 0.0.0.0 MUST be used in the Router ID field, and the IPv6 TE
   Router ID sub-TLV [RFC5316] MUST be present in the TLV.  IS-IS Router
   CAPABILITY TLVs that have a Router ID of 0.0.0.0 and do NOT have the
   IPv6 TE Router ID sub-TLV present MUST NOT be used.

   When advertising capabilities with different flooding scopes, a
   router MUST originate a minimum of two IS-IS 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 IS-IS Router CAPABILITY TLVs:

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

   o  One IS-IS Router CAPABILITY TLV with the S flag set, carrying the
      sub-TLV(s) relative to C2.  This IS-IS 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 IS-IS Router CAPABILITY TLVs, a
   system MUST NOT use an IS-IS Router 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 IS-IS Router CAPABILITY TLV.

   When an IS-IS Router CAPABILITY TLV is not used, either due to a lack
   of reachability to the originating router or due to an unusable
   Router ID, note that leaking the IS-IS Router CAPABILITY TLV is one
   of the uses that is prohibited under these conditions.

      Example: If Level 1 router A generates an IS-IS Router 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 without the above
      prohibition, 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 IS-IS Router CAPABILITY TLV or S's copy of A's IS-IS Router
      CAPABILITY TLV, and they have no reliable way to choose.  By
      making sure that T stops leaking A's information, the possibility
      that other routers will use stale information from A is
      eliminated.

   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
   an IS-IS Router CAPABILITY 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 IS-IS Router 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 an IS-IS Router CAPABILITY
   TLV from the same system that have conflicting information for a
   given sub-TLV, the procedure used to choose which copy shall be used
   is undefined.

4.  Interoperability with Routers Not Supporting the IS-IS Router
    CAPABILITY TLV

   Routers that do not support the IS-IS 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 an IS-IS Router CAPABILITY TLV MUST silently ignore the
   unsupported sub-TLVs and continue processing those sub-TLVs that are
   supported in the IS-IS Router CAPABILITY TLV.  How partial support
   may impact the operation of the capabilities advertised within the
   IS-IS Router CAPABILITY TLV is outside the scope of this document.

   In order for IS-IS 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 CAPABILITY TLV.

   If leaking of the IS-IS Router CAPABILITY TLV is required, the entire
   CAPABILITY TLV MUST be leaked into another level without change
   (except for changes to the TLV flags as noted in Section 2) even
   though it may contain some sub-TLVs that are unsupported by the
   router doing the leaking.

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 ones
   defined in [RFC5304] or [RFC5310], should be applied if there is high
   risk resulting from modification of capability information.

6.  IANA Considerations

   IANA originally assigned a TLV codepoint for the IS-IS Router
   CAPABILITY TLV (242) as described in RFC 4971.  IANA has updated this
   entry in the "TLV Codepoints Registry" to refer to this document.

7.  References

7.1.  Normative References

   [ISO10589] International Organization for Standardization,
              "Information technology -- Telecommunications and
              information exchange between systems -- Intermediate
              System to Intermediate System intra-domain routeing
              information exchange protocol for use in conjunction with
              the protocol for providing the connectionless-mode network
              service (ISO 8473)", ISO/IEC 10589:2002, Second Edition,
              November 2002.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, DOI 10.17487/RFC1195,
              December 1990, <http://www.rfc-editor.org/info/rfc1195>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5073]  Vasseur, J., Ed. and J. Le Roux, Ed., "IGP Routing
              Protocol Extensions for Discovery of Traffic Engineering
              Node Capabilities", RFC 5073, DOI 10.17487/RFC5073,
              December 2007, <http://www.rfc-editor.org/info/rfc5073>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <http://www.rfc-editor.org/info/rfc5304>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <http://www.rfc-editor.org/info/rfc5305>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <http://www.rfc-editor.org/info/rfc5310>.

   [RFC5316]  Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
              Support of Inter-Autonomous System (AS) MPLS and GMPLS
              Traffic Engineering", RFC 5316, DOI 10.17487/RFC5316,
              December 2008, <http://www.rfc-editor.org/info/rfc5316>.

7.2.  Informative References

   [RFC4461]  Yasukawa, S., Ed., "Signaling Requirements for Point-to-
              Multipoint Traffic-Engineered MPLS Label Switched Paths
              (LSPs)", RFC 4461, DOI 10.17487/RFC4461, April 2006,
              <http://www.rfc-editor.org/info/rfc4461>.

   [RFC4875]  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,
              DOI 10.17487/RFC4875, May 2007,
              <http://www.rfc-editor.org/info/rfc4875>.

   [RFC4972]  Vasseur, JP., Ed., Leroux, 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, DOI 10.17487/RFC4972, July 2007,
              <http://www.rfc-editor.org/info/rfc4972>.

Appendix A.  Changes to RFC 4971

   This document makes the following changes to RFC 4971.

   RFC 4971 only allowed a 32-bit Router ID in the fixed header of TLV
   242.  This is problematic in an IPv6-only deployment where an IPv4
   address may not be available.  This document specifies:

   1.  The Router ID SHOULD be identical to the value advertised in the
       Traffic Engineering Router ID TLV (134) if available.

   2.  If no Traffic Engineering Router ID is assigned, the Router ID
       SHOULD be identical to an IP Interface Address [RFC1195]
       advertised by the originating IS.

   3.  If the originating node does not support IPv4, then the reserved
       value 0.0.0.0 MUST be used in the Router ID field, and the IPv6
       TE Router ID sub-TLV [RFC5316] MUST be present in the TLV.

   In addition, some clarifying editorial changes have been made.

Acknowledgements

   The authors of RFC 4971 thanked Jean-Louis Le Roux, Paul Mabey,
   Andrew Partan, and Adrian Farrel for their useful comments.

   The authors of this document would like to thank Kris Michielsen for
   calling attention to the problem associated with an IPv6-only router.

Authors' Addresses

   Les Ginsberg
   Cisco Systems
   510 McCarthy Blvd.
   Milpitas, CA  95035
   United States of America

   Email: ginsberg@cisco.com


   Stefano Previdi
   Cisco Systems
   Via Del Serafico 200
   Rome  0144
   Italy

   Email: sprevidi@cisco.com


   Mach(Guoyi) Chen
   Huawei Technologies Co., Ltd
   KuiKe Building, No. 9 Xinxi Rd. Hai-Dian District
   Beijing  100085
   China

   Email: mach.chen@huawei.com