Advertising Generic Information in IS-IS
A single IS-IS TLV that carries any application's data without using up the protocol's limited type codes.
Ginsberg, L., Previdi, S., and M. Shand, "Advertising Generic Information in IS-IS", RFC 6823, DOI 10.17487/RFC6823, December 2012, <https://www.rfc-editor.org/info/rfc6823>.
@misc{rfc6823,
series = {Request for Comments},
number = 6823,
howpublished = {RFC 6823},
publisher = {RFC Editor},
doi = {10.17487/RFC6823},
url = {https://www.rfc-editor.org/info/rfc6823},
author = {L. Ginsberg and S. Previdi and M. Shand},
title = {{Advertising Generic Information in IS-IS}},
pagetotal = 11,
year = 2012,
month = dec,
} RFC text
Contents
Internet Engineering Task Force (IETF) L. Ginsberg
Request for Comments: 6823 S. Previdi
Category: Standards Track M. Shand
ISSN: 2070-1721 Cisco Systems
December 2010
Advertising Generic Information in IS-IS
Abstract
This document describes the manner in which generic application
information (i.e., information not directly related to the operation
of the Intermediate System to Intermediate System (IS-IS) protocol)
should be advertised in IS-IS Link State Protocol Data Units (LSPs)
and defines guidelines that should be used when flooding such
information.
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/rfc6823.
Copyright Notice
Copyright (c) 2010 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. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Encoding Format for GENINFO . . . . . . . . . . . . . . . . . 3
3.1. GENINFO TLV . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Use of Sub-TLVs in GENINFO TLV . . . . . . . . . . . . . . 5
4. GENINFO Flooding Procedures . . . . . . . . . . . . . . . . . 5
4.1. Leaking Procedures . . . . . . . . . . . . . . . . . . . . 6
4.2. Minimizing Update Confusion . . . . . . . . . . . . . . . 7
4.3. Interpreting Attribute Information . . . . . . . . . . . . 7
5. Use of a Separate Protocol Instance . . . . . . . . . . . . . 7
6. Applicability of GENINFO TLV . . . . . . . . . . . . . . . . . 8
7. Standardization Requirements . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
11. Normative References . . . . . . . . . . . . . . . . . . . . . 10
1. Overview
[ISO10589] defines the format of Type-Length-Values (TLVs) that may
be sent in IS-IS Protocol Data Units (PDUs). The first octet of a
TLV encodes the "type" or "codepoint" that provides a scope for the
information and information format that follows. The protocol is
therefore limited to 256 different codepoints that may be assigned.
This number has proved generous as regards the information required
for correct operation of the IS-IS protocol. However, the increasing
use of IS-IS Link State Protocol Data Units (LSPs) for advertisement
of generic information (GENINFO) not directly related to the
operation of the IS-IS protocol places additional demands on the TLV
encoding space that have the potential to consume a significant
number of TLV codepoints. This document therefore defines an
encoding format for GENINFO that minimizes the consumption of TLV
codepoints and also maximizes the flexibility of the formats that can
be used to represent GENINFO.
This document also discusses optimal behavior associated with the
advertisement and flooding of LSPs containing GENINFO in order to
avoid the advertisement of stale information and minimize the
presence of duplicate or conflicting information when advertisements
are updated.
The manner in which the information contained in GENINFO TLVs is
exchanged between an instance of the IS-IS protocol and the
application that generates or consumes the GENINFO is outside the
scope of this specification.
In order to minimize the impact that advertisement of GENINFO may
have on the operation of routing, such advertisements MUST occur in
the context of a non-zero instance of the IS-IS protocol as defined
in [RFC6822] except where the rules for the use of the zero instance
set out later in this document are followed.
2. 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 [RFC2119].
3. Encoding Format for GENINFO
The encoding format defined below has the following goals regarding
the advertisement of GENINFO in IS-IS LSPs:
o Minimize the number of IS-IS top level and sub-TLV codepoints
required
o Minimize the depth of sub-TLV levels required
In order to support these goals, a new IANA registry has been
created. This registry manages the assignment of IS-IS GENINFO
Application Identifiers. These numbers are unsigned 16-bit numbers
ranging in value from 1 to 65535. Application-specific sub-TLV
codepoints are unsigned 8-bit numbers ranging in value from 0 to 255.
The assignment of the sub-TLV codepoints is scoped by the Application
Identifier. Management of the application specific sub-TLV
codepoints is outside the scope of this document.
3.1. GENINFO TLV
The GENINFO TLV supports the advertisement of application-specific
information that is not directly related to the operation of the
IS-IS protocol.
Type: 251
Length: Number of octets in the value field (3 to 255)
Value:
No. of octets
+-----------------------+
| Flags | 1
+-----------------------+
| Application ID | 2
+-----------------------+
| Application |
| IP Address Info | 0 to 20
+-----------------------+
|Additional Application-| 0 to (252 -
| Specific Information | len of IP Address info)
+-----------------------+
Flags
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Rsvd |V|I|D|S|
+-+-+-+-+-+-+-+-+
The following bit flags are defined.
S bit (0x01): If the S bit is set (1), the GENINFO 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 GENINFO TLV is leaked from Level-2 to
Level-1, the D bit MUST be set. Otherwise, this bit MUST be
clear. GENINFO TLVs with the D bit set MUST NOT be leaked from
Level-1 to Level-2. This is to prevent TLV looping.
I bit (0x04): When the I bit is set, the 4-octet IPv4 address
associated with the application immediately follows the
Application ID.
V bit (0x08): When the V bit is set, the 16-octet IPv6 address
associated with the application immediately follows either the
Application ID (if I bit is clear) or the IPv4 address (if I bit
is set).
Application ID
An identifier assigned to this application via the IANA registry
defined later in this document.
Application IPv4 Address Info
The IPv4 address associated with the application. This is not
necessarily an address of a router running the IS-IS protocol.
Application IPv6 Address Info
The IPv6 address associated with the application. This is not
necessarily an address of a router running the IS-IS protocol.
Additional Application-Specific Information
Each application may define additional information to be encoded
in a GENINFO TLV following the fixed information. Definition of
such information is beyond the scope of this document.
3.2. Use of Sub-TLVs in GENINFO TLV
[RFC5305] introduced the definition and use of sub-TLVs. One of the
advantages of using sub-TLVs rather than fixed encoding of
information inside a TLV is to allow for the addition of new
information in a backwards compatible manner, i.e., just as with
TLVs, implementations are required to ignore sub-TLVs that they do
not understand.
GENINFO TLVs MAY include sub-TLVs in the application specific
information as deemed necessary and appropriate for each application.
The scope of the codepoints used in such sub-TLVs is defined by the
combination of the GENINFO TLV codepoint and the Application ID,
i.e., the sub-TLV codepoints are private to the application. Such
sub-TLVs are referred to as APPsub-TLVs.
Additional levels of APPsub-TLVs may be required when there is
variable information that is scoped by a specific APPsub-TLV. These
"nested" sub-TLVs MUST be encoded in the same manner as sub-TLVs,
i.e., with a one-octet Type field, a one-octet Length field, and zero
or more octets of Value.
4. GENINFO Flooding Procedures
This section describes procedures that apply to the propagation of
LSPs that contain GENINFO TLVs. These procedures have been
previously discussed in [RFC4971]. This section is intended to serve
as a reference specification for future documents that define the use
of GENINFO TLV(s) for a specific application -- eliminating the need
to repeat the definition of these procedures in the application-
specific documents.
Each GENINFO TLV contains information regarding exactly one
application instance as identified by the Application ID in the
GENINFO TLV. When it is necessary to advertise sets of information
with the same Application ID that have different flooding scopes, a
router MUST originate a minimum of one GENINFO TLV for each required
flooding scope. GENINFO TLVs that contain information having area/
level scope will have the S bit clear. These TLVs MUST NOT be leaked
into another level. GENINFO TLVs that contain information that has
domain scope will have the S bit set. These TLVs MUST be leaked into
other IS-IS levels. When a TLV is leaked from Level-2 to Level-1,
the D bit MUST be set in the Level-1 LSP advertisement.
4.1. Leaking Procedures
When leaking GENINFO TLVs downward from Level-2 into Level-1, if the
originator of the TLV is a Level-1 router in another area, it is
possible that multiple copies of the same TLV may be received from
multiple L2 routers in the originating area. A router performing
downward leaking MUST check for such duplication by comparing the
contents of the TLVs. The set of LSPs generated by a router for a
given level MUST NOT contain two or more copies of the same GENINFO
TLV.
In order to prevent the use of stale GENINFO information, a system
MUST NOT use a GENINFO 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) associated with the LSP in which the GENINFO TLV appears. Note
that leaking a GENINFO TLV is one of the uses that is prohibited
under these conditions. The following example illustrates what might
occur in the absence of this restriction.
Example: If Level-1 router A generates a GENINFO TLV and floods it to
two L1/L2 routers S and T, they will flood it into the Level-2 sub-
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
GENINFO 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 GENINFO TLV 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.
4.2. Minimizing Update Confusion
If an update to a TLV is advertised in an LSP with a different number
than the LSP 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 that had the
old advertisement and the LSP that has the new advertisement arrive
at other systems.
Whenever possible, an implementation SHOULD advertise the update to a
GENINFO TLV in the LSP with the same number as the advertisement that
it replaces. Where this is not possible, the two affected LSPs
SHOULD be flooded as an atomic action.
Systems that receive an update to an existing GENINFO TLV can
minimize the potential disruption associated with the update by
employing a hold-down time prior to processing the update so as to
allow for the receipt of multiple LSPs associated with the same
update prior to beginning processing.
4.3. Interpreting Attribute Information
Where a receiving system has two copies of a GENINFO TLV with the
same Application ID, attribute information in the two TLVs that does
not conflict MUST be considered additive. When information in the
two GENINFO TLVs conflicts, i.e., there are different settings for a
given attribute, the procedure used to choose which copy shall be
used is undefined.
5. Use of a Separate Protocol Instance
The use of the IS-IS flooding mechanism as a means of reliably and
efficiently propagating information is understandably attractive.
However, it is prudent to remember that the primary purpose of that
mechanism is to flood information necessary for the correct operation
of the IS-IS protocol. Flooding of information not directly related
to the use of the IS-IS protocol in support of routing degrades the
operation of the protocol. Degradation occurs because the frequency
of LSP updates is increased and because the processing of non-routing
information in each router consumes resources whose primary
responsibility is to efficiently respond to reachability changes in
the network.
Advertisement of GENINFO therefore MUST occur in the context of a
non-zero instance of the IS-IS protocol as defined in [RFC6822]
except when the use in the zero instance is defined in a Standards
Track RFC.
The use of a separate instance of the protocol allows both the
flooding and the processing of the non-routing information to be
decoupled from the information necessary to support correct routing
of data in the network. The flooding and processing of non-routing
information can then be prioritized appropriately.
Use of a separate protocol instance to advertise GENINFO does not
eliminate the need to use prudence in the frequency with which such
information is updated. One of the most egregious oversights is a
failure to appropriately dampen changes in the information to be
advertised; this can lead to flooding storms. Documents that specify
the use of the mechanisms defined here MUST define the expected rate
of change of the information to be advertised.
If desirable, independent control of the flooding scope for
information related to two different applications can be achieved by
utilizing separate non-zero protocol instances for each application
[RFC6822].
6. Applicability of GENINFO TLV
The GENINFO TLV supports the advertisement of application-specific
information in IS-IS LSPs that is not directly related to the
operation of the IS-IS protocol. Information advertised in the
GENINFO TLV MUST NOT alter basic IS-IS protocol operation including
(but not limited to) the establishment of adjacencies, the update
process, and the decision process.
7. Standardization Requirements
GENINFO is intended to advertise information on behalf of
applications whose operations have been defined in a public
specification as discussed in [RFC5226].
The public specification MUST include:
o a description of the sub-TLV allocation policy
o discussion of security issues
o discussion of the rate of change of the information being
advertised
o justification for the use of GENINFO
8. Security Considerations
The introduction and use of the new TLV codepoint for GENINFO in and
of itself raises no new security issues for IS-IS.
It is possible that information advertised in a GENINFO TLV by a
given application MAY introduce new security issues. The public
specification that defines the use of GENINFO by that application
MUST include a discussion of the security issues. Where appropriate,
it is recommended that either [RFC5304] or [RFC5310] be used.
9. IANA Considerations
Per this document, IANA has registered a new IS-IS TLV in the "IS-IS
TLV Codepoints" registry:
Type Description IIH LSP SNP Purge
---- ---------------------------------- --- --- --- -----
251 Generic Information n y n n
IANA has also created a new registry. The new registry manages the
assignment of Application Identifiers that may be used in the Generic
Information TLV. These identifiers are unsigned 16-bit numbers
ranging in value from 1 to 65535. The value 0 is reserved. The
registration procedure is "Expert Review" as defined in [RFC5226].
The expert MUST verify that the public specification that defines the
use of GENINFO for the application adequately discusses all points
mentioned in Section 7 of this document.
The following information MUST be specified in the registry:
o ID Value (1-65535)
o Description
o Allowed in Instance zero (Y/N)
o Reference Specification
10. Acknowledgements
The authors would like to thank JP. Vasseur and David Ward for
providing the need to produce this document and Tony Li for making
sure it was done with appropriate wisdom and prudence.
11. 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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4971] Vasseur, JP., Shen, N., and R. Aggarwal, "Intermediate
System to Intermediate System (IS-IS) Extensions for
Advertising Router Information", RFC 4971, July 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, October 2008.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, February 2009.
[RFC6822] Previdi, S., Ginsberg, L., Shand, M., Roy, A., and D.
Ward, "IS-IS Multi-Instance", RFC 6822, December 2012.
Authors' Addresses
Les Ginsberg
Cisco Systems
510 McCarthy Blvd.
Milpitas, CA 95035
USA
EMail: ginsberg@cisco.com
Stefano Previdi
Cisco Systems
Via Del Serafico 200
00142 - Roma
Italy
EMail: sprevidi@cisco.com
Mike Shand
Cisco Systems
250, Longwater Avenue.
Reading, Berks RG2 6GB
UK
EMail: imc.shand@gmail.com
AI ELI5
On this page
§1Why a Generic TLV
IS-IS sends information in TLVs (Type-Length-Values). The type is one octet, so only 256 codepoints exist. That has been generous for the protocol's own needs, but advertising generic information (GENINFO) that is not directly related to IS-IS operation could consume many of them.
RFC 6823 defines an encoding that uses few TLV codepoints and leaves the format of the information flexible. It also describes good behavior when advertising and flooding LSPs that contain GENINFO, so that stale information is avoided and duplicate or conflicting information is minimized. How the information passes between IS-IS and the application is out of scope.
There are only so many kinds of announcement cards. One flexible card for everyone's extra notes keeps the supply from running out.
§3.1The GENINFO TLV
The GENINFO TLV has type 251, and its length is the number of octets in the value (3 to 255). The value holds a 1-octet Flags field, a 2-octet Application ID, 0 to 20 octets of application IP address info, and then additional application-specific information.
The Flags are S (0x01), D (0x02), I (0x04) and V (0x08). If I is set, a 4-octet IPv4 address follows the Application ID. If V is set, a 16-octet IPv6 address follows the Application ID, or follows the IPv4 address when I is also set. These addresses are not necessarily those of a router running IS-IS.
Application IDs are unsigned 16-bit numbers from 1 to 65535, assigned through an IANA registry.
The card has a few check boxes, a number saying which group owns it, optional addresses, and then room for the group's own message.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
FLAGS | Flags | 1 byte | Reserved bits plus V, I, D and S flags. | |
APP-ID | Application ID | 2 bytes | Identifier assigned through the IANA registry. |
- 0x01S — Set: flood across the whole domain. Clear: do not leak between levels.
- 0x02D — Set when leaked from Level-2 to Level-1; prevents looping.
- 0x04I — A 4-octet IPv4 address follows the Application ID.
- 0x08V — A 16-octet IPv6 address follows the Application ID or IPv4 address.
§3.2Sub-TLVs for Applications
A GENINFO TLV MAY carry sub-TLVs in its application-specific part, as introduced by RFC 5305. Receivers are required to ignore sub-TLVs they do not understand, so new information can be added in a backwards compatible way.
Their codepoints are scoped by the GENINFO TLV codepoint together with the Application ID, so they are private to the application. They are called APPsub-TLVs, are unsigned 8-bit numbers (0 to 255), and their management is outside this document. Nested sub-TLVs use a one-octet Type, a one-octet Length and zero or more octets of Value.
Each group can put smaller labeled notes inside its card, and its labels mean something only for that group. Readers skip notes they do not know.
§4Flooding and Leaking Rules
Each GENINFO TLV describes exactly one application instance. If information with the same Application ID needs different flooding scopes, a router MUST originate at least one TLV per scope. With the S bit clear the TLV has area or level scope and MUST NOT be leaked into another level. With S set it has domain scope and MUST be leaked into other levels.
When a TLV is leaked from Level-2 to Level-1, the D bit MUST be set, and TLVs with D set MUST NOT be leaked from Level-1 to Level-2, which prevents looping. A router leaking downward MUST compare TLV contents so that its LSPs for a level never hold two copies of the same GENINFO TLV.
A system MUST NOT use, or leak, a GENINFO TLV from a system that is not currently reachable at that level, to avoid stale data after an area partition. Updates SHOULD go in the LSP with the same number as the one they replace, or the two LSPs SHOULD be flooded atomically. Receivers MAY use a hold-down time, and non-conflicting attributes in two TLVs with the same Application ID are additive. For conflicts, the choice is undefined.
The rules say how far a note may travel, stop notes from going in circles, and make sure nobody keeps passing along an old note after the sender is cut off.
§5Use a Separate Protocol Instance
Flooding non-routing data through IS-IS degrades the protocol, because LSP updates become more frequent and routers spend resources processing the data. So GENINFO advertisement MUST occur in a non-zero instance of IS-IS as defined in RFC 6822, unless use in the zero instance is defined in a Standards Track RFC.
A separate instance still calls for care: changes should be dampened to avoid flooding storms, and documents using GENINFO MUST define the expected rate of change. Separate non-zero instances can give independent flooding scope control to different applications. GENINFO MUST NOT alter basic IS-IS operation, including adjacencies, the update process and the decision process.
Routing news should not wait behind other notes, so the extra notes ride on their own separate announcer.
§7Rules for Applications
GENINFO is meant for applications whose operation is defined in a public specification. That specification MUST include a description of the sub-TLV allocation policy, a discussion of security issues, a discussion of the rate of change of the advertised information, and a justification for using GENINFO.
A group must write down its plan in public, including how its labels work and why it needs the card, before using it.
§9IANA Registrations and Security
IANA registered TLV type 251, Generic Information, valid in LSPs only. IANA also created a registry of Application Identifiers, 1 to 65535, with 0 reserved. The procedure is Expert Review, and the expert MUST verify that the public specification covers the points in Section 7. Each entry records the ID value, description, whether it is allowed in instance zero, and the reference specification.
The new TLV raises no new IS-IS security issues by itself, but an application's data might. The application's specification MUST discuss them, and use of RFC 5304 or RFC 5310 authentication is recommended where appropriate.
A keeper of the list hands out the group numbers and checks each group wrote its plan. Groups must also think about safety, and authentication is a good idea.