Internet Engineering Task Force (IETF) L. Ginsberg
Request for Comments: 6823 S. Previdi
Category: Standards Track M. Shand
ISSN: 2070-1721 Cisco Systems
Advertising Generic Information in IS-IS
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
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.
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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
[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
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
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
Length: Number of octets in the value field (3 to 255)
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)
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
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
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
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-
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
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
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
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
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
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
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 Allowed in Instance zero (Y/N)
o Reference Specification
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,
[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.
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