Definition of an IS-IS Link Attribute Sub-TLV
A small sub-TLV that lets IS-IS routers advertise what their links can and cannot do.
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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ISO Plain Language
On this page
§1Where the Sub-TLV Fits
IS-IS, as specified in ISO 10589 with the IPv4 extensions in RFC 1195, has a router advertise one or several Link State Protocol data units. Those data units are composed of variable-length tuples called TLVs, of Type-Length-Value form.
RFC 3784 defines a set of new TLVs whose aims are to add more information about link characteristics, increase the range of IS-IS metrics, and optimise the encoding of IS-IS prefixes. This document defines a new sub-TLV named Link-attributes, carried within the extended IS reachability TLV, which is type 22.
IS-IS routers advertise their links in Link State Protocol data units, which are lists of TLVs. The new record travels inside the TLV that lists which neighbours are reachable.
§1.1Requirement Words
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. Those capitalised words set how strongly each rule applies.
The document uses the standard capitalised words for rules, so a reader can tell which parts are mandatory and which are advice.
§2The 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: 1 octet of sub-type, 1 octet of length of the value field of the sub-TLV, followed by the value field, which in this case is 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 neighbour. If a received Link State Packet contains more than one Link-Attribute Sub-TLV, an implementation SHOULD decide to consider only the first encountered instance.
Two bits are defined. Local Protection Available (0x01) means the link is protected by means of some local protection mechanism, such as the one in RFC 4090. Link excluded from local protection path (0x02) means 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 that bit are out of the scope of this document.
The record is four bytes: a type byte that says it is the link-attributes record, a length byte, and two bytes of flags. Two flags are defined, one saying the link has a fast protection path and one saying other routers should keep the link out of their repair paths.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
SUB-TYPE | Sub-TLV Type | 1 byte | 19 | Identifies the record. The value assigned by IANA for link-attributes is 19. |
LENGTH | Length | 1 byte | 2 | Length in octets of the value field that follows. |
FLAGS | Link-attribute Flags | 2 bytes | Bit field. Defined bits: Local Protection Available (0x01) and Link excluded from local protection path (0x02). |
- 0x1Local Protection Available — The link is protected by some local protection mechanism.
- 0x2Link Excluded from Local Protection — Other routers in the routing area should not include this link in a repair path computation.
§3Interoperability with Older Routers
A router that does not support the link-attribute sub-TLV will simply ignore it silently. No negotiation is needed and no failure results from an older router seeing the record.
An older router that does not know this record skips it. That is all the compatibility work the record needs.
§4IANA Registration
IANA has assigned codepoint 19 for the link-attribute sub-TLV defined in this document and carried within TLV 22. IANA has also created a registry for bit values inside the link-attributes sub-TLV. The initial contents of that registry are the value 0x1 for Local Protection Available and the value 0x2 for Link Excluded from Local Protection, both referencing this document.
Further values are to be allocated by the Standards Action process defined in RFC 2434, with early allocation, as defined in RFC 4020, permitted.
The record type and each of its flag bits get a number from IANA, so that different vendors agree on what each number means.
§5Security Considerations
Any new security issues raised by the procedures in this document depend on the opportunity for Link State Packets to be snooped and modified, and the ease or difficulty of that has not been altered. Because the packets may now contain 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 the disclosure and modification of their information. The document notes that an integrity mechanism, such as the one defined in RFC 3567, should be applied if there is a high risk resulting from the modification of capability information.
Link State Packets can already be read and changed on the network, and this record does not change that. It does reveal a little more about what the routers can do, so the RFC says an integrity check is worth adding where that matters.