Network Working Group JP. Vasseur, Ed.

Network Working Group JP. Vasseur, Ed.
Request for Comments: 4971 N. Shen, Ed.
Category: Standards Track Cisco Systems, Inc.
R. Aggarwal, Ed.
Juniper Networks
July 2007

Intermediate System to Intermediate System (IS-IS) Extensions
for Advertising Router Information

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.

Copyright Notice

Copyright (C) The IETF Trust (2007).

Abstract

This document defines a new optional Intermediate System to
Intermediate System (IS-IS) TLV named CAPABILITY, formed of multiple
sub-TLVs, which allows a router to announce its capabilities within
an IS-IS level or the entire routing domain.

Table of Contents

1. Introduction ....................................................2
1.1. Conventions Used in This Document ..........................2
2. IS-IS Router CAPABILITY TLV .....................................3
3. Elements of Procedure ...........................................4
4. Interoperability with Routers Not Supporting the
Capability TLV ..................................................5
5. Security Considerations .........................................6
6. IANA Considerations .............................................6
7. Acknowledgment ..................................................6
8. References ......................................................6
8.1. Normative References .......................................6
8.2. Informative References .....................................8

Vasseur, et al. Standards Track [Page 1]

RFC 4971 IS-IS Extensions for Advertising Router Info July 2007

1. Introduction

There are several situations where it is useful for the IS-IS [IS-IS]
[IS-IS-IP] routers to learn the capabilities of the other routers of
their IS-IS level, area, or routing domain. For the sake of
illustration, three examples related to MPLS Traffic Engineering (TE)
are described here:

1. Mesh-group: the setting up of a mesh of TE Label Switched Paths
(LSPs) [IS-IS-TE] requires some significant configuration effort.
[AUTOMESH] proposes an auto-discovery mechanism whereby every
Label Switching Router (LSR) of a mesh advertises its mesh-group
membership by means of IS-IS extensions.

2. Point to Multipoint TE LSP (P2MP LSP). A specific sub-TLV
([TE-NODE-CAP]) allows an LSR to advertise its Point To Multipoint
capabilities ([P2MP] and [P2MP-REQS]).

3. Inter-area traffic engineering: Advertisement of the IPv4 and/or
the IPv6 Traffic Engineering Router IDs.

The use of IS-IS for Path Computation Element (PCE) discovery may
also be considered and will be discussed in the PCE WG.

The capabilities mentioned above require the specification of new
sub-TLVs carried within the CAPABILITY TLV defined in this document.

Note that the examples above are provided for the sake of
illustration. This document proposes a generic capability
advertising mechanism that is not limited to MPLS Traffic
Engineering.

This document defines a new optional IS-IS TLV named CAPABILITY,
formed of multiple sub-TLVs, which allows a router to announce its
capabilities within an IS-IS level or the entire routing domain. The
applications mentioned above require the specification of new sub-
TLVs carried within the CAPABILITY TLV defined in this document.

Definition of these sub-TLVs is outside the scope of this document.

1.1. Conventions Used in This Document

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 [RFC-2119].

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RFC 4971 IS-IS Extensions for Advertising Router Info July 2007

2. IS-IS Router CAPABILITY TLV

The IS-IS Router CAPABILITY TLV is composed of 1 octet for the type,
1 octet that specifies the number of bytes in the value field, and a
variable length value field that starts with 4 octets of Router ID,
indicating the source of the TLV, and followed by 1 octet of flags.

A set of optional sub-TLVs may follow the flag field. Sub-TLVs are
formatted as described in RFC 3784 [IS-IS-TE].

TYPE: 242
LENGTH: from 5 to 255
VALUE:
Router ID (4 octets)
Flags (1 octet)
Set of optional sub-TLVs (0-250 octets)

Flags

0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Reserved |D|S|
+-+-+-+-+-+-+-+-+

Currently two bit flags are defined.

S bit (0x01): If the S bit is set(1), the IS-IS Router CAPABILITY TLV
MUST be flooded across the entire routing domain. If the S bit is
not set(0), the TLV MUST NOT be leaked between levels. This bit MUST
NOT be altered during the TLV leaking.

D bit (0x02): When the IS-IS Router CAPABILITY TLV is leaked from
level-2 to level-1, the D bit MUST be set. Otherwise, this bit MUST
be clear. IS-IS Router capability TLVs with the D bit set MUST NOT
be leaked from level-1 to level-2. This is to prevent TLV looping.

The Router CAPABILITY TLV is OPTIONAL. As specified in Section 3,
more than one Router CAPABILITY TLV from the same source MAY be
present.

This document does not specify how an application may use the Router
Capability TLV and such specification is outside the scope of this
document.

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RFC 4971 IS-IS Extensions for Advertising Router Info July 2007

3. Elements of Procedure

A router that generates a CAPABILITY TLV MUST have a Router ID that
is a 32-bit number. The ID MUST be unique within the IS-IS area. If
the router generates any capability TLVs with domain flooding scope,
then the ID MUST also be unique within the IS-IS routing domain.

When advertising capabilities with different flooding scopes, a
router MUST originate a minimum of two Router CAPABILITY TLVs, each
TLV carrying the set of sub-TLVs with the same flooding scope. For
instance, if a router advertises two sets of capabilities, C1 and C2,
with an area/level scope and routing domain scope respectively, C1
and C2 being specified by their respective sub-TLV(s), the router
will originate two Router CAPABILITY TLVs:

- One Router CAPABILITY TLV with the S flag cleared, carrying the
sub-TLV(s) relative to C1. This Router CAPABILITY TLV will not be
leaked into another level.

- One Router CAPABILITY TLV with the S flag set, carrying the sub-
TLV(s) relative to C2. This Router CAPABILITY TLV will be leaked
into other IS-IS levels. When the TLV is leaked from level-2 to
level-1, the D bit will be set in the level-1 LSP advertisement.

In order to prevent the use of stale capabilities, a system MUST NOT
use a Capability TLV present in an LSP of a system that is not
currently reachable via Level-x paths, where "x" is the level (1 or
2) in which the sending system advertised the TLV. This requirement
applies regardless of whether or not the sending system is the
originator of the Capabilities TLV. Note that leaking a Capabilities
TLV is one of the uses that is prohibited under these conditions.

Example: If Level-1 router A generates a Capability TLV and floods
it to two L1/L2 routers, S and T, they will flood it into the
Level-2 domain. Now suppose the Level-1 area partitions, such
that A and S are in one partition and T is in another. IP routing
will still continue to work, but if A now issues a revised version
of the CAP TLV, or decides to stop advertising it, S will follow
suit, but T will continue to advertise the old version until the
LSP times out.

Routers in other areas have to choose whether to trust T's copy of
A's capabilities or S's copy of A's information and, they have no
reliable way to choose. By making sure that T stops leaking A's
information, this removes the possibility that other routers will use
stale information from A.

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RFC 4971 IS-IS Extensions for Advertising Router Info July 2007

In IS-IS, the atomic unit of the update process is a TLV -- or more
precisely, in the case of TLVs that allow multiple entries to appear
in the value field (e.g., IS-neighbors), the atomic unit is an entry
in the value field of a TLV. If an update to an entry in a TLV is
advertised in an LSP fragment different from the LSP fragment
associated with the old advertisement, the possibility exists that
other systems can temporarily have either 0 copies of a particular
advertisement or 2 copies of a particular advertisement, depending on
the order in which new copies of the LSP fragment that had the old
advertisement and the fragment that has the new advertisement arrive
at other systems.

Wherever possible, an implementation SHOULD advertise the update to a
capabilities TLV in the same LSP fragment as the advertisement that
it replaces. Where this is not possible, the two affected LSP
fragments should be flooded as an atomic action.

Systems that receive an update to an existing capability TLV can
minimize the potential disruption associated with the update by
employing a holddown time prior to processing the update so as to
allow for the receipt of multiple LSP fragments associated with the
same update prior to beginning processing.

Where a receiving system has two copies of a capabilities TLV from
the same system that have different settings for a given attribute,
the procedure used to choose which copy shall be used is undefined.

4. Interoperability with Routers Not Supporting the Capability TLV

Routers that do not support the Router CAPABILITY TLV MUST silently
ignore the TLV(s) and continue processing other TLVs in the same LSP.
Routers that do not support specific sub-TLVs carried within a Router
CAPABILITY TLV MUST silently ignore the unsupported sub-TLVs and
continue processing those sub-TLVs that are supported in the Router
CAPABILITY TLV. How partial support may impact the operation of the
capabilities advertised within the Router CAPABILITY TLV is outside
the scope of this document.

In order for Router CAPABILITY TLVs with domain-wide scope originated
by L1 Routers to be flooded across the entire domain, at least one
L1/L2 Router in every area of the domain MUST support the Router
CAPABILITY TLV.

If leaking of the CAPABILITY TLV is required, the entire CAPABILITY
TLV MUST be leaked into another level even though it may contain some
of the unsupported sub-TLVs.

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RFC 4971 IS-IS Extensions for Advertising Router Info July 2007

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 the one
defined in [RFC-3567] or [IS-IS-HMAC], should be applied if there is
high risk resulting from modification of capability information.

6. IANA Considerations

IANA assigned a new IS-IS TLV code-point for the newly defined IS-IS
TLV type named the IS-IS Router CAPABILITY TLV and defined in this
document. The assigned value is 242.

7. Acknowledgment

The authors would like to thank Jean-Louis Le Roux, Paul Mabey,
Andrew Partan, and Adrian Farrel for their useful comments.

8. References

8.1. Normative References

[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[IS-IS] "Intermediate System to Intermediate System Intra-
Domain Routeing Exchange Protocol for use in
Conjunction with the Protocol for Providing the
Connectionless-mode Network Service (ISO 8473)", ISO
10589.

[RFC-3567] Li, T. and R. Atkinson, "Intermediate System to
Intermediate System (IS-IS) Cryptographic
Authentication", RFC 3567, July 2003.

[IS-IS-IP] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.

[IS-IS-TE] Smit, H. and T. Li, "Intermediate System to
Intermediate System (IS-IS) Extensions for Traffic
Engineering (TE)", RFC 3784, June 2004.

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RFC 4971 IS-IS Extensions for Advertising Router Info July 2007

8.2. Informative References

[AUTOMESH] Vasseur, JP., Ed., Le Roux, JL., Ed., Yasukawa, S.,
Previdi, S., Psenak, P., and P. Mabbey, "Routing
extensions for Discovery of Multiprotocol (MPLS) Label
Switch Router (LSR) Traffic Engineering (TE) Mesh
Membership", RFC 4972, July 2007.

[TE-NODE-CAP] Vasseur, JP., Ed., and J.L. Le Roux, "Routing
Extensions for Discovery of Traffic Engineering Node
Capabilities", Work in Progress, April 2007.

[P2MP] Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
Yasukawa, Ed., "Extensions to Resource Reservation
Protocol - Traffic Engineering (RSVP-TE) for Point-to-
Multipoint TE Label Switched Paths (LSPs)", RFC 4875,
May 2007.

[P2MP-REQS] Yasukawa, S., Ed., "Signaling Requirements for Point-
to-Multipoint Traffic-Engineered MPLS Label Switched
Paths (LSPs)", RFC 4461, April 2006.

[IS-IS-HMAC] Bhatia, M., Ed. and V. Manral, Ed., "IS-IS Generic
Cryptographic Authentication", Work in Progress, May
2007.

Authors' Addresses

Jean-Philippe Vasseur
CISCO Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
USA
EMail: [email protected]

Stefano Previdi
CISCO Systems, Inc.
Via Del Serafico 200
00142 - Roma
ITALY
EMail: [email protected]

Vasseur, et al. Standards Track [Page 7]

RFC 4971 IS-IS Extensions for Advertising Router Info July 2007

Mike Shand
Cisco Systems
250 Longwater Avenue,
Reading,
Berkshire,
RG2 6GB
UK
EMail: [email protected]

Les Ginsberg
Cisco Systems
510 McCarthy Blvd.
Milpitas, Ca. 95035 USA
EMail: [email protected]

Acee Lindem
Redback Networks
102 Carric Bend Court
Cary, NC 27519
USA
EMail: [email protected]

Naiming Shen
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
EMail: [email protected]

Rahul Aggarwal
Juniper Networks
1194 N. Mathilda Avenue
San Jose, CA 94089
USA
EMail: [email protected]

Scott Shaffer
EMail: [email protected]

Vasseur, et al. Standards Track [Page 8]

RFC 4971 IS-IS Extensions for Advertising Router Info July 2007

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