Network Working Group                                        JP. Vasseur
Request for Comments: 5029                                    S. Previdi
Category: Standards Track                             Cisco Systems, Inc
                                                         September 2007


            Definition of an IS-IS Link Attribute Sub-TLV

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.

Abstract

  This document defines a sub-TLV called "Link-attributes" carried
  within the TLV 22 and used to flood some link characteristics.

Table of Contents

  1. Introduction ....................................................2
     1.1. Terminology ................................................2
  2. Link-Attributes Sub-TLV Format ..................................2
  3. Interoperability with Routers Not Supporting This Capability ....3
  4. IANA Considerations .............................................3
  5. Security Considerations .........................................3
  6. Acknowledgements ................................................3
  7. References ......................................................4
     7.1. Normative References .......................................4
     7.2. Informative References .....................................4


















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1.  Introduction

  [IS-IS] specifies the IS-IS protocol (ISO 10589) with extensions to
  support IPv4 in [RFC1195].  A router advertises one or several Link
  State Protocol data units that are composed of variable length tuples
  called TLVs (Type-Length-Value).

  [RFC3784] defines a set of new TLVs whose aims are to add more
  information about links characteristics, increase the range of IS-IS
  metrics, and optimize the encoding of IS-IS prefixes.

  This document defines a new sub-TLV named "Link-attributes" carried
  within the extended IS reachability TLV (type 22) specified in
  [RFC3784].

1.1  Terminology

  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.  Link-Attributes Sub-TLV Format

  The link-attribute sub-TLV is carried within the TLV 22 and has a
  format identical to the sub-TLV format used by the Traffic
  Engineering Extensions for IS-IS ([RFC3784]): 1 octet of sub-type, 1
  octet of length of the value field of the sub-TLV followed by the
  value field -- in this case, a 16 bit flags field.

  The Link-attribute sub-type is 19 and the link-attribute has a length
  of 2 octets.

  This sub-TLV is OPTIONAL and MUST appear at most once for a single IS
  neighbor.  If a received Link State Packet (LSP) contains more than
  one Link-Attribute Sub-TLV, an implementation SHOULD decide to
  consider only the first encountered instance.

  The following bits are defined:

  Local Protection Available (0x01).  When set, this indicates that the
  link is protected by means of some local protection mechanism (e.g.,
  [RFC4090]).

  Link excluded from local protection path (0x02).  When set, this link
  SHOULD not be included in any computation of a repair path by any
  other router in the routing area.  The triggers for setting up this
  bit are out of the scope of this document.




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3.  Interoperability with Routers Not Supporting This Capability

  A router not supporting the link-attribute sub-TLV will just silently
  ignore this sub-TLV.

4.  IANA Considerations

  IANA has assigned codepoint 19 for the link-attribute sub-TLV defined
  in this document and carried within TLV 22.

  IANA has created a registry for bit values inside the link-attributes
  sub-TLV.  The initial contents of this registry are as follows

    Value   Name                                 Reference
    -----   ----                                 ---------
    0x1     Local Protection Available           [RFC5029]
    0x2     Link Excluded from Local Protection  [RFC5029]

  Further values are to be allocated by the Standards Action process
  defined in [RFC2434], with Early Allocation (defined in [RFC4020])
  permitted.

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 one defined
  in [RFC3567], should be applied if there is high risk resulting from
  the modification of capability information.

6.  Acknowledgements

  The authors would like to thank Mike Shand, Les Ginsberg, and Bill
  Fenner for their useful comments.












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7.  References

7.1.  Normative References

  [IS-IS]    "Intermediate System to Intermediate System Intra-Domain
             Routing Exchange Protocol for use in Conjunction with the
             Protocol for Providing the Connectionless-mode Network
             Service (ISO 8473)", ISO 10589.

  [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
             dual environments", RFC 1195, December 1990.

  [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

  [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 2434,
             October 1998.

  [RFC3784]  Smit, H. and T. Li, "Intermediate System to Intermediate
             System (IS-IS) Extensions for Traffic Engineering (TE)",
             RFC 3784, June 2004.

  [RFC4020]  Kompella, K. and A. Zinin, "Early IANA Allocation of
             Standards Track Code Points", BCP 100, RFC 4020, February
             2005.

7.2.  Informative References

  [RFC3567]  Li, T. and R. Atkinson, "Intermediate System to
             Intermediate System (IS-IS) Cryptographic Authentication",
             RFC 3567, July 2003.

  [RFC4090]  Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
             Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May
             2005.















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Authors' Addresses

  JP Vasseur
  Cisco Systems, Inc
  1414 Massachusetts Avenue
  Boxborough, MA  01719
  USA

  EMail: [email protected]


  Stefano Previdi
  Cisco Systems, Inc
  Via Del Serafico 200
  Roma  00142
  Italy

  EMail: [email protected]

































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