Definition of an IS-IS Link Attribute Sub-TLV
A small optional flag field in IS-IS that tells other routers whether a link has local protection.
Vasseur, JP. and S. Previdi, "Definition of an IS-IS Link Attribute Sub-TLV", RFC 5029, DOI 10.17487/RFC5029, September 2007, <https://www.rfc-editor.org/info/rfc5029>.
@misc{rfc5029,
series = {Request for Comments},
number = 5029,
howpublished = {RFC 5029},
publisher = {RFC Editor},
doi = {10.17487/RFC5029},
url = {https://www.rfc-editor.org/info/rfc5029},
author = {JP. Vasseur and S. Previdi},
title = {{Definition of an IS-IS Link Attribute Sub-TLV}},
pagetotal = 6,
year = 2007,
month = sep,
} RFC text
Contents
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
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.
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.
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.
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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On this page
§1Background: IS-IS, TLVs and Sub-TLVs
IS-IS is the routing protocol specified in ISO 10589, with IPv4 support added in RFC 1195. A router advertises one or several Link State Protocol data units, which are built from variable-length tuples called TLVs (Type-Length-Value).
RFC 3784 defined new TLVs to add information about link characteristics, widen the range of IS-IS metrics and optimize the encoding of prefixes. This document adds one new sub-TLV, named Link-attributes, to the extended IS reachability TLV (type 22) from that RFC.
A router tells its neighbors about its roads using little labelled boxes of information. This RFC adds a new kind of box that can go inside one of the existing boxes.
§2The Link-Attributes Sub-TLV Format
The sub-TLV is carried within TLV 22 and uses the same format as the sub-TLVs of the traffic engineering extensions in RFC 3784: 1 octet of sub-type, 1 octet giving the length of the value, then the value. Here the value is a 16-bit flags field. The sub-type is 19 and the length is 2 octets.
The 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, an implementation SHOULD consider only the first one it finds.
The sticker has a label saying what kind it is, a note of how big it is, and two on/off switches inside.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
SUB-TYPE | Sub-type | 1 byte | 19 | Identifies the sub-TLV. |
LENGTH | Length | 1 byte | 2 | Length of the value field in octets. |
FLAGS | Flags | 2 bytes | Bit flags: 0x01 Local Protection Available, 0x02 Link excluded from local protection path. |
§3The Defined Flag Bits
Two bits are defined in the flags field.
- Local Protection Available (0x01): when set, the link is protected by some local protection mechanism, for example the fast reroute extensions of RFC 4090.
- 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.
What causes a router to set the second bit is out of the scope of this document.
One switch says 'this road has a backup road ready'. The other says 'please do not send detour traffic down this road'.
§4Routers That Do Not Support It
A router that does not support the link-attribute sub-TLV just silently ignores it. Since the sub-TLV is also optional, routers that do and do not understand it can sit in the same network without changing how they otherwise behave.
If a router does not know what the sticker means, it just does not look at it, and everything else keeps working.
§5IANA Code Points
IANA assigned codepoint 19 for the link-attribute sub-TLV carried within TLV 22. IANA also created a registry for the bit values inside the sub-TLV, with initial contents 0x1 (Local Protection Available) and 0x2 (Link Excluded from Local Protection).
Further values are to be allocated by the Standards Action process defined in RFC 2434, with Early Allocation (defined in RFC 4020) permitted.
There is an official list of what each switch means, and adding a new switch needs a proper standards process.
§6Security Considerations
This document raises no new issues beyond the opportunity for LSPs to be snooped and modified, which it does not make easier or harder. Because LSPs may now carry additional information about router capabilities, that information also becomes available to an attacker.
Specifications based on this mechanism need to describe the security considerations around disclosure and modification of their information. An integrity mechanism such as the one in RFC 3567 should be applied if modifying capability information carries a high risk.
Anyone who can peek at the router's messages could read the stickers too, so if that matters, the messages should be sealed.