    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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    RFC 5029                  IS-IS Link Attribute            September 2007

    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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    RFC 5029                  IS-IS Link Attribute            September 2007

    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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    RFC 5029                  IS-IS Link Attribute            September 2007

    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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    RFC 5029                  IS-IS Link Attribute            September 2007

    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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    RFC 5029                  IS-IS Link Attribute            September 2007

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