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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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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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.
Vasseur & Previdi Standards Track [Page 4]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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