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RFC 6823 - Advertising Generic Information in IS-IS

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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


   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.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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",
   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.


   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)


                                          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
      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

   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
   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

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  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

   EMail: ginsberg@cisco.com

   Stefano Previdi
   Cisco Systems
   Via Del Serafico 200
   00142 - Roma

   EMail: sprevidi@cisco.com

   Mike Shand
   Cisco Systems
   250, Longwater Avenue.
   Reading, Berks  RG2 6GB

   EMail: imc.shand@gmail.com


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