





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: jpv@cisco.com


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

   EMail: sprevidi@cisco.com

































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