Advertising Generic Information in IS-IS
draft-ietf-isis-genapp-02.txt

Abstract

This draft describes the manner in which generic application information (i.e. information not directly related to the operation of the IS-IS protocol) SHOULD be advertised in IS-IS LSPs and defines guidelines which SHOULD be used when flooding such information.

Status of this Memo

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Table of Contents

1.  Conventions used in this Document
2.  Overview
3.  Encoding Format for GENINFO
    3.1.  GENINFO TLV
    3.2.  Use of subTLVs in GENINFO TLV
    3.3.  Standardization Requirements
4.  GENINFO Flooding Procedures
    4.1.  Leaking Procedures
    4.2.  Minimizing Update Confusion
    4.3.  Interpreting Attribute Information
5.  Use of a Separate Protocol Instance
6.  Applicability of GENINFO TLV
7.  Security Considerations
8.  IANA Considerations
9.  Acknowledgements
10.  References
    10.1.  Normative References
    10.2.  Informative References
§  Authors' Addresses

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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Overview

[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),” Nov 2002.) defines the format of TLVs which may be sent in IS-IS PDUs. The first octet of a TLV encodes the "type" or "codepoint" which provides a scope for the information and information format which follows. The protocol is therefore limited to 256 different codepoints which 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 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 which has the potential to consume a significant number of TLV codepoints. This document therefore defines an encoding format for GENINFO which minimizes the consumption of TLV codepoints and also maximizes the flexibility of the formats which 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 which generates/consumes the GENINFO is outside the scope of this specification.

In order to minimize the impact advertisement of GENINFO may have on the operation of routing, such advertisements SHOULD occur in the context of a non-zero instance of the IS-IS protocol as defined in [I‑D.ietf‑isis‑mi] (Previdi, S., Ginsberg, L., Shand, M., Ward, D., and A. Roy, “IS-IS Multi-Instance,” October 2009.).

3.  Encoding Format for GENINFO

The encoding format defined below has the following goals regarding the advertisement of GENINFO in IS-IS LSPs:

• Minimize the number of codepoints required
• Minimize the depth of subTLV levels required

In order to support these goals, a new IANA registry is required. This registry is required to manage the assignment of IS-IS GENINFO Application Identifiers. These numbers are unsigned 16 bit numbers ranging in value from 1 to 65535. The registry is also required to manage the assignment of application specific subTLV codepoints. These numbers are unsigned 8 bit numbers ranging in value from 0 to 255. The assignment of the subTLV codepoints is scoped by the Application Identifier.

3.1.  GENINFO TLV

The GENINFO TLV supports the advertisement of application specific information which is not directly related to the operation of the IS-IS protocol.

  Type   251
  Length # 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
                GENINFO-REG.

           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.

The Application ID in combination with the Application IPv4/IPv6 Address Information uniquely identifies the GENINFO Application Context (GENINFO-CTX).

3.2.  Use of subTLVs in GENINFO TLV

[RFC5305] (Li, T. and H. Smit, “IS-IS Extensions for Traffic Engineering,” October 2008.) introduced the definition and use of subTLVs. One of the advantages of using subTLVs 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 subTLVs which they do not understand.

GENINFO TLVs MAY include subTLVs in the application specific information as deemed necessary and appropriate for each application. The scope of the codepoints used in such subTLVs is defined by the GENINFO TLV codepoint AND the Application ID i.e. the subTLV codepoints are private to the application. Such subTLVs are referred to as APPSUBTLVs and MUST be assigned via the GENINFO-REG IANA registry.

Additional levels of APPSUBTLVs may be required when there is variable information which is scoped by a specific APPSUBTLV. These "nested" subTLVs MUST be encoded in the same manner as subTLVs i.e. with a one-octet Type field, a one-octet Length field, and zero or more octets of Value. These types MUST also be assigned via the GENINFO-REG IANA registry.

The use of additional levels of subTLVs is discouraged due to the inherent inefficiency in encoding introduced because the parent subTLV must encode the nested subTLV length. While this inefficiency is small (one additional octet), it may be sufficient to extend the total information about a single application object beyond the carrying capacity of a single GENINFO TLV. Given that each Application ID can utilize the full range of subTLV codepoints (0 to 255) without conflict with any other application, the need to be frugal in the use of APPSUBTLV codepoints is greatly reduced.

3.3.  Standardization Requirements

GENINFO is intended to advertise information on behalf of applications whose operations have been defined in public documents. Therefore the uses of GENINFO MUST be standardized.

GENINFO is NOT intended to be used for proprietary or experimental purposes.

4.  GENINFO Flooding Procedures

This section describes procedures which apply to the propagation of LSPs which contain GENINFO TLVs. These procedures have been previously discussed in [RFC4971] (Vasseur, JP., Shen, N., and R. Aggarwal, “Intermediate System to Intermediate System (IS-IS) Extensions for Advertising Router Information,” July 2007.). This section is intended to serve as a reference specification for future documents which 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 GENINFO-CTX. When it is necessary to advertise sets of information with the same GENINFO-CTX which have different flooding scopes, a router MUST originate a minimum of one GENINFO TLV for each required flooding scope. GENINFO TLVs which contain information having area/level scope will have the S bit clear. These TLVs MUST NOT be leaked into another level. GENINFO TLVs which contain information which 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 GENTLV.

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 which 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 which 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 which had the old advertisement and the LSP which 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 which it replaces. Where this is not possible, the two affected LSPs SHOULD be flooded as an atomic action.

Systems which receive an update to an existing GENINFO 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 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 GENINFO-CTX, attribute information in the two TLVs which 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 SHOULD occur in the context of a non-zero instance of the IS-IS protocol as defined in [I‑D.ietf‑isis‑mi] (Previdi, S., Ginsberg, L., Shand, M., Ward, D., and A. Roy, “IS-IS Multi-Instance,” October 2009.). 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, which can lead to flooding storms. Documents which 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.[I‑D.ietf‑isis‑mi] (Previdi, S., Ginsberg, L., Shand, M., Ward, D., and A. Roy, “IS-IS Multi-Instance,” October 2009.).

6.  Applicability of GENINFO TLV

The GENINFO TLV supports the advertisement of application specific information in IS-IS LSPs which is not directly related to the operation of the IS-IS protocol. Information which is not directly used by the IS-IS Decision process falls into this category. The Decision Process is defined by [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),” Nov 2002.) and extended by [RFC1195] (Callon, R., “Use of OSI IS-IS for routing in TCP/IP and dual environments,” December 1990.) and [RFC3906] (Shen, N. and H. Smit, “Calculating Interior Gateway Protocol (IGP) Routes Over Traffic Engineering Tunnels,” October 2004.).

The IS-IS WG of the IETF acts as the authority to determine whether information proposed to be advertised in IS-IS LSPs falls under this definition.

The applicability statement above is expected to cover some information currently being advertised by IS-IS in previously defined TLVs. It is expected and seen as desirable that an effort be made to migrate the advertisement of such information to utilize the procedures defined in this document.

7.  Security Considerations

This document raises no new security issues for IS-IS.

8.  IANA Considerations

This document defines a new ISIS TLV that needs to be reflected in the ISIS TLV code-point registry:

Type        Description                            IIH   LSP   SNP
----        -----------------------------------    ---   ---   ---
251         Generic Information                     n     y     n

This document also defines a new registry which needs to be created.

The new registry is required to manage two types of assigned numbers:

1)Application Identifiers which may be used in the Generic Information TLV. These identifiers are unsigned 16 bit numbers ranging in value from 1 to 65535.

2)Application specific subTLV codepoints which may be used in a GENINFO TLV when a specific Application Identifier is used. These numbers are unsigned 8 bit numbers ranging in value from 0 to 255.

9.  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.

10.  References

10.1. Normative References

10.2. Informative References

Authors' Addresses