IS-IS Prefix Attributes for Extended IPv4 and IPv6 Reachability
Three new IS-IS sub-TLVs that tag a route with extra flags and name the router that first announced it.
Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4 and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794, March 2016, <https://www.rfc-editor.org/info/rfc7794>.
@misc{rfc7794,
series = {Request for Comments},
number = 7794,
howpublished = {RFC 7794},
publisher = {RFC Editor},
doi = {10.17487/RFC7794},
url = {https://www.rfc-editor.org/info/rfc7794},
author = {L. Ginsberg and B. Decraene and S. Previdi and X. Xu and U. Chunduri},
title = {{IS-IS Prefix Attributes for Extended IPv4 and IPv6 Reachability}},
pagetotal = 9,
year = 2016,
month = mar,
} RFC text
Contents
Internet Engineering Task Force (IETF) L. Ginsberg, Ed.
Request for Comments: 7794 Cisco Systems
Category: Standards Track B. Decraene
ISSN: 2070-1721 Orange
S. Previdi
Cisco Systems
X. Xu
Huawei
U. Chunduri
Ericsson
March 2016
IS-IS Prefix Attributes for Extended IPv4 and IPv6 Reachability
Abstract
This document introduces new sub-TLVs to support advertisement of
IPv4 and IPv6 prefix attribute flags and the source router ID of the
router that originated a prefix advertisement.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7794.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. New Sub-TLVs for Extended Reachability TLVs . . . . . . . . . 3
2.1. IPv4/IPv6 Extended Reachability Attribute Flags . . . . . 4
2.2. IPv4/IPv6 Source Router ID . . . . . . . . . . . . . . . 5
2.3. Advertising Router IDs . . . . . . . . . . . . . . . . . 6
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Normative References . . . . . . . . . . . . . . . . . . 7
5.2. Informative References . . . . . . . . . . . . . . . . . 8
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
IS-IS is a link-state routing protocol defined in [ISO10589] and
[RFC1195]. Extensions in support of advertising new forms of
IPv4/IPv6 prefix reachability are defined in [RFC5305], [RFC5308],
and [RFC5120].
There are existing use cases in which knowing additional attributes
of a prefix is useful.
It is useful to know whether or not an advertised prefix is directly
connected to the advertising router. In the case of Segment Routing
as described in [SR], knowing whether or not a prefix is directly
connected determines what action should be taken as regards
processing of labels associated with an incoming packet.
It is useful to know what addresses can be used as addresses of the
node in support of services (e.g., Remote Loop Free Alternate (RLFA)
endpoint).
Current formats of the Extended Reachability TLVs for both IPv4 and
IPv6 are fixed and do not allow the introduction of additional flags
without backwards compatibility issues. Therefore, this document
defines a new sub-TLV that supports the advertisement of attribute
flags associated with prefix advertisements.
In cases where multiple node addresses are advertised by a given
router, it is also useful to be able to associate all of these
addresses with a single Router ID even when prefixes are advertised
outside of the area in which they originated. Therefore, a new sub-
TLV is introduced to advertise the Router ID of the originator of a
prefix advertisement.
1.1. Requirements Language
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. New Sub-TLVs for Extended Reachability TLVs
The following new sub-TLVs are introduced:
o Prefix Attribute Flags
o IPv4 Source Router ID
o IPv6 Source Router ID
All sub-TLVs are applicable to TLVs 135, 235, 236, and 237.
2.1. IPv4/IPv6 Extended Reachability Attribute Flags
This sub-TLV supports the advertisement of additional flags
associated with a given prefix advertisement. The behavior of each
flag when a prefix advertisement is leaked from one level to another
(upwards or downwards) is explicitly defined below.
All flags are applicable to TLVs 135, 235, 236, and 237, unless
otherwise stated.
Prefix Attribute Flags
Type: 4
Length: Number of octets of the Value field.
Value:
(Length * 8) bits.
0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|X|R|N| ...
+-+-+-+-+-+-+-+-+...
Bits are defined/sent starting with Bit 0 defined below. Additional
bit definitions that may be defined in the future SHOULD be assigned
in ascending bit order so as to minimize the number of bits that will
need to be transmitted.
Undefined bits MUST be transmitted as 0 and MUST be ignored on
receipt.
Bits that are NOT transmitted MUST be treated as if they are set to 0
on receipt.
X-Flag: External Prefix Flag (Bit 0)
Set if the prefix has been redistributed from another protocol.
This includes the case where multiple virtual routers are
supported and the source of the redistributed prefix is another
IS-IS instance.
The flag MUST be preserved when leaked between levels.
In TLVs 236 and 237, this flag SHOULD always be sent as 0 and MUST
be ignored on receipt. This is because there is an existing X
flag defined in the fixed format of these TLVs as specified in
[RFC5308] and [RFC5120].
R-Flag: Re-advertisement Flag (Bit 1)
Set when the prefix has been leaked from one level to another
(upwards or downwards).
N-flag: Node Flag (Bit 2)
Set when the prefix identifies the advertising router, i.e., the
prefix is a host prefix advertising a globally reachable address
typically associated with a loopback address.
The advertising router MAY choose to NOT set this flag even when
the above conditions are met.
If the flag is set and the prefix length is NOT a host prefix (/32
for IPV4, /128 for IPv6), then the flag MUST be ignored. The flag
MUST be preserved when leaked between levels.
2.2. IPv4/IPv6 Source Router ID
When a reachability advertisement is leaked from one level to
another, the source of the original advertisement is unknown. In
cases where the advertisement is an identifier for the advertising
router (e.g., with the N-flag set in the Prefix Attribute Flags sub-
TLV as described in Section 2.1), it may be useful for other routers
to know the source of the advertisement. The sub-TLVs defined below
provide that information.
Note that the Router ID advertised is always the Router ID of the
IS-IS instance that originated the advertisement. This would be true
even if the prefix had been learned from another protocol (i.e., with
the X-flag set as defined in Section 2.1).
IPv4 Source Router ID
Type: 11
Length: 4
Value: IPv4 Router ID of the source of the advertisement
Inclusion of this TLV is optional and MAY occur in TLVs 135, 235,
236, or 237. When included, the value MUST be identical to the value
advertised in the Traffic Engineering router ID (TLV 134) defined in
[RFC5305].
If present the sub-TLV MUST be included when the prefix advertisement
is leaked to another level.
IPv6 Source Router ID
Type: 12
Length: 16
Value: IPv6 Router ID of the source of the advertisement
Inclusion of this TLV is optional and MAY occur in TLVs 135, 235,
236, or 237. When included, the value MUST be identical to the value
advertised in the IPv6 TE Router ID (TLV 140) defined in [RFC6119].
If present, the sub-TLV MUST be included when the prefix
advertisement is leaked to another level.
2.3. Advertising Router IDs
[RFC5305] and [RFC6119] define the advertisement of router IDs for
IPv4 and IPv6, respectively. Although both documents discuss the use
of router ID in the context of Traffic Engineering (TE), the
advertisement of router IDs is explicitly allowed for purposes other
than TE. The use of router IDs to identify the source of a prefix
advertisement as defined in Section 2.2 is one such use case.
Therefore, whenever an IPv4 or IPv6 Source Router ID sub-TLV (as
defined in Section 2.2) is used, the originating router SHOULD also
advertise the corresponding address-family-specific router ID TLV.
3. IANA Considerations
This document adds the following new sub-TLVs to the registry of sub-
TLVs for TLVs 135, 235, 236, and 237.
Value: 4
Name: Prefix Attribute Flags
Value: 11
Name: IPv4 Source Router ID
Value: 12
Name: IPv6 Source Router ID
This document also introduces a new registry for bit values in the
Prefix Attribute Flags sub-TLV. The registration policy is Expert
Review as defined in [RFC5226]. This registry is part of the "IS-IS
TLV Codepoints" registry. The name of the registry is "Bit Values
for Prefix Attribute Flags Sub-TLV". The defined values are:
Bit # Name
----- ------------------------------
0 External Prefix Flag (X-flag)
1 Re-advertisement Flag (R-flag)
2 Node Flag (N-flag)
4. Security Considerations
Security concerns for IS-IS are addressed in [RFC5304] and [RFC5310].
Advertisement of the additional information defined in this document
introduces no new security concerns.
5. References
5.1. Normative References
[ISO10589] International Organization for Standardization,
"Intermediate system to Intermediate system intra-domain
routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473)",
ISO/IEC 10589:2002, Second Edition, Nov. 2002.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC1195,
December 1990, <http://www.rfc-editor.org/info/rfc1195>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<http://www.rfc-editor.org/info/rfc5120>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <http://www.rfc-editor.org/info/rfc5304>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <http://www.rfc-editor.org/info/rfc5305>.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
<http://www.rfc-editor.org/info/rfc5308>.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310, February
2009, <http://www.rfc-editor.org/info/rfc5310>.
[RFC6119] Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
Engineering in IS-IS", RFC 6119, DOI 10.17487/RFC6119,
February 2011, <http://www.rfc-editor.org/info/rfc6119>.
5.2. Informative References
[SR] Previdi, S., Ed., Filsfils, C., Bashandy, A., Gredler, H.,
Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS
Extensions for Segment Routing", Work in Progress,
draft-ietf-isis-segment-routing-extensions-06, December
2015.
Contributors
The following people gave a substantial contribution to the content
of this document:
Clarence Filsfils
Cisco Systems
Email: cf@cisco.com
Stephane Litkowski
Orange Business Service
Email: stephane.litkowski@orange.com
Authors' Addresses
Les Ginsberg (editor)
Cisco Systems
510 McCarthy Blvd.
Milpitas, CA 95035
United States
Email: ginsberg@cisco.com
Bruno Decraene
Orange
38 rue du General Leclerc
Issy Moulineaux cedex 9 92794
France
Email: bruno.decraene@orange.com
Stefano Previdi
Cisco Systems
Via Del Serafico 200
Rome 0144
Italy
Email: sprevidi@cisco.com
Xiaohu Xu
Huawei
Email: xuxiaohu@huawei.com
Uma Chunduri
Ericsson
Email: uma.chunduri@ericsson.com
AI ELI5
On this page
§1Why Prefixes Need Attributes
IS-IS is a link-state routing protocol. Earlier RFCs define extended ways to advertise IPv4 and IPv6 prefix reachability, but their formats are fixed, so new flags cannot be added without backwards compatibility problems.
The RFC gives two motivating cases. Knowing whether a prefix is directly connected to the advertising router matters for Segment Routing, because it decides how labels on an incoming packet are processed. Knowing which addresses identify a node is useful for services such as a Remote Loop Free Alternate (RLFA) endpoint.
A second problem is that when a router advertises several node addresses and they are leaked out of the area where they started, it helps to link them all to one Router ID. A new sub-TLV carries that Router ID.
The old route announcements were like a form with no blank spaces left. This RFC adds a sticky note area so routers can say more about each route.
§2The New Sub-TLVs
Three sub-TLVs are introduced: Prefix Attribute Flags, IPv4 Source Router ID and IPv6 Source Router ID. All of them apply to the extended reachability TLVs numbered 135, 235, 236 and 237.
The Prefix Attribute Flags sub-TLV has Type 4. Its Value is a variable number of bits (Length times 8), sent starting with bit 0. Future bits SHOULD be assigned in ascending order so that as few bits as possible need to be sent.
There are three new kinds of sticker: one holding a row of yes-or-no switches, and two that name the router that started the announcement.
§2.1The Attribute Flags
Three flags are defined. Undefined bits MUST be sent as 0 and MUST be ignored on receipt, and bits that are not transmitted MUST be treated as 0.
- X-flag (External Prefix, bit 0): set if the prefix was redistributed from another protocol, including from another IS-IS instance. It MUST be preserved when leaked between levels. In TLVs 236 and 237 it SHOULD always be sent as 0 and MUST be ignored on receipt, because those TLVs already have an X flag in their fixed format.
- R-flag (Re-advertisement, bit 1): set when the prefix has been leaked from one level to another, upwards or downwards.
- N-flag (Node, bit 2): set when the prefix identifies the advertising router, meaning a host prefix for a globally reachable address, typically a loopback. A router MAY choose not to set it. If it is set on a prefix that is not a host prefix (/32 for IPv4, /128 for IPv6) it MUST be ignored, and it MUST be preserved when leaked between levels.
The three switches say: 'I came from outside', 'I was passed along from another level', and 'this address belongs to the router itself'.
- 0X-flag — External prefix: redistributed from another protocol.
- 1R-flag — Re-advertisement: leaked from one level to another.
- 2N-flag — Node: host prefix identifying the advertising router.
§2.2Source Router ID
When a route is leaked from one level to another, the source of the original advertisement becomes unknown. The Source Router ID sub-TLVs fix that. The IPv4 version has Type 11 and Length 4. The IPv6 version has Type 12 and Length 16.
Including either one is optional and it MAY appear in TLVs 135, 235, 236 or 237. The value MUST be identical to the router ID in the Traffic Engineering router ID TLV 134 (IPv4) or the IPv6 TE Router ID TLV 140. If the sub-TLV is present, it MUST be included when the advertisement is leaked to another level.
The Router ID is always that of the IS-IS instance that originated the advertisement, even when the prefix was learned from another protocol and the X-flag is set.
It is a name tag that stays on the announcement even when it is passed to a different neighbourhood, so everyone knows which router first said it.
§2.3Advertising Router IDs
The earlier RFCs 5305 and 6119 define router ID advertisement for IPv4 and IPv6. Although they discuss it in the Traffic Engineering context, they explicitly allow router IDs to be advertised for other purposes. Identifying the source of a prefix is one such purpose.
Whenever a Source Router ID sub-TLV is used, the originating router SHOULD also advertise the matching address-family-specific router ID TLV.
If you put a name tag on your announcement, you should also have your name posted on the board where people can look it up.
§3IANA and Security
IANA adds the three sub-TLVs (values 4, 11 and 12) to the registry of sub-TLVs for TLVs 135, 235, 236 and 237. It also creates a new registry, "Bit Values for Prefix Attribute Flags Sub-TLV", under the "IS-IS TLV Codepoints" registry, with Expert Review as the registration policy. Bits 0, 1 and 2 are the X, R and N flags.
For security, the RFC points to the existing IS-IS authentication RFCs (RFC 5304 and RFC 5310) and states that advertising this additional information introduces no new security concerns.
A public list keeps track of which sticker means what, and the authors say the stickers do not make routing any less safe.