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Versions: 00 01 02 03 04 05 RFC 5311

INTERNET DRAFT    Simplified Extension of LSP Space          Jun 2006



Network Working Group                                       L. Ginsberg
Internet Draft                                               S. Previdi
Expiration Date: Dec 2006                                      M. Shand
                                                          Cisco Systems
                                                              June 2006





              Simplified Extension of LSP Space for IS-IS
                    draft-ietf-isis-wg-extlsp-00.txt


Status of this Memo


   By submitting this Internet-Draft, each author represents that any
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Abstract


   This draft describes a simplified method for extending the LSP space
   beyond the 256 Link State PDU (LSP) limit defined in ISO 10589. This
   method is intended as a preferred replacement for the method defined
   in RFC 3786.

Table of Contents


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   1. Conventions used in this document..............................2
   2. Overview.......................................................2
   3. Definitions of Commonly Used Terms.............................3
   4. Utilizing Additional System IDs................................4
    4.1 Additional Information in Extended LSPs......................4
    4.2 Extended LSP Restrictions....................................4
     4.2.1 TLVs Which MUST NOT Appear................................4
     4.2.2 Leaf Advertisements in Extended LSPs......................5
     4.2.3 IS Neighbor Advertisement Restrictions....................5
     4.2.4 Area Addresses............................................6
     4.2.5 Overload, Attached, and Partition Repair Bits.............6
    4.3 Originating LSP Requirements.................................6
    4.4 IS Alias ID TLV (IS-Alias)...................................6
    4.5 New TLVs in support of IS Neighbor Attributes................7
   5. Comparison with the RFC 3786 Solution..........................8
   6. Deployment Considerations......................................8
    6.1 Advertising New TLVs in Extended LSPs........................8
    6.2 Reachability and non-SPF TLV Staleness.......................9
    6.3 Normal LSP OL State and the Use of Extended LSPs.............9
    6.4 Moving Neighbor Attribute Information to Extended LSPs.......9
    6.5 Advertising Leaf Information in Extended LSPs...............10
   7. Security Considerations.......................................10
   8. IANA Considerations...........................................10
   9. Normative References..........................................10
   10. Acknowledgments..............................................11
   11. Authors' Addresses...........................................11
   12. Full Copyright Statement.....................................12

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 [BCP14].

2.    Overview

   [IS-IS] defines the set of LSP fragments which may be originated by
   a system at each level. This set is limited to 256 fragments. [IS-
   IS] also defines a maximum value for an LSP fragment
   (originatingLxLSPBufferSize) as 1492 bytes. The carrying capacity of
   an LSP set, while bounded, has thus far been sufficient for
   advertisements associated with an area/domain in existing deployment
   scenarios. However, the definition of additional information to be
   included in LSPs (e.g. multitopology support, traffic engineering


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   information, router capabilities, etc.) has the potential to exceed
   the carrying capacity of an LSP set.

   This issue first drew interest when traffic engineering extensions
   were introduced. This interest resulted in the solution defined in
   RFC 3786. However, that solution suffers from restrictions required
   to maintain interoperability with systems which do not support the
   extensions.

   This document defines extensions which allow a system to exceed the
   256 fragment limit and do so in a way which has no interoperability
   issues with systems which do not support the extension. It is seen
   as a simpler and therefore preferred solution to the problem.

3.    Definitions of Commonly Used Terms

   This section provides definitions for terms that are used throughout
   the text. The terminology is consistent with that used in RFC 3786.

   Originating System: A physical IS running the IS-IS protocol.  As
     this document describes a method which allows a single physical IS
     to originate LSPs on behalf of multiple virtual ISs, the
     Originating System represents the single physical IS.

   Normal system-id: The system-id of an Originating System as defined
     by [IS-IS].

   Additional system-id: A system-id other than the "Normal system-id"
     that is assigned by the network administrator to an Originating
     System in order to allow the generation of extended LSP fragments.
     The Additional system-id, like the Normal system-id, must be
     unique throughout the routing area (Level-1) or domain (Level-2).

   Original LSP: An LSP using the Normal system-id in its LSP ID.

   Extended LSP: An LSP using an Additional system-id in its LSP ID.

   LSP set: Logical LSP. A group of LSP fragments (for a given level)
     which have the same LSPID. This term is used to resolve the
     ambiguity between a logical LSP and an LSP fragment, both of which
     are sometimes termed "LSP".

   Extended LSP set: An LSP set consisting of LSP fragments using an
     Additional system-id.

   Extension-capable IS: An IS implementing the mechanisms described in
     this document.





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4.    Utilizing Additional System IDs

   This extension allows an Originating System to be assigned
   additional system-ids which may be used to generate additional LSP
   sets. The additional system-ids are subject to the same restrictions
   as normal system-ids i.e. when used at Level-1 the additional
   system-id MUST be unique within the Level-1 area. When used at
   Level-2 the additional system-id MUST be unique within the domain.

   Extended LSPs are treated by the IS-IS Update Process in the same
   manner as normal LSPs i.e. the same rules as to generation,
   flooding, purging, etc. apply. In particular, if the Extended LSP
   with LSP Number zero and remaining lifetime > 0 is not present for a
   particular additional system-id then none of the extended LSPs in
   that Extended LSP set shall be processed.

4.1     Additional Information in Extended LSPs

   Fragment 0 of an Extended LSP Set MUST include the new IS alias ID
   TLV defined in Section 4.4. This allows the Extended LSP set to be
   associated with the Originating System which generated the LSP(s).

4.2     Extended LSP Restrictions

   The following restrictions on the information which may appear in an
   Extended LSP are defined in order to avoid interoperability issues
   with systems which do not support the extensions defined in this
   document. All TLV references are based on the current definitions in
   the IANA IS-IS TLV Codepoints Registry.

4.2.1       TLVs Which MUST NOT Appear

   The following TLVs MUST NOT appear in an Extended LSP:

             TLV Name (#)
             -----------
             ES Neighbors (3)
             Part. DIS (4)
             Prefix Neighbors (5)


   If any of the TLVs listed above appear in an Extended LSP, an
   Extension Capable IS MUST ignore those TLVs on receipt and SHOULD
   report an error. Other TLVs in that extended LSP set MUST be
   processed normally.






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4.2.2       Leaf Advertisements in Extended LSPs

   Advertisement of leaf information in Extended LSPs is allowed.
   Inclusion of such information requires the advertisement of a
   neighbor between the Originating System and the Virtual IS
   associated with the extended LSP set in which the leaf
   advertisements appear. See section 4.2.3.

   When leaf advertisements for multiple topologies (see [M-IS-IS]) are
   included in an Extended LSP set, the multi-topology TLV (229) MUST
   include all topologies for which a leaf advertisement is included.

   The following TLVs fall into this category:

             TLV Name (#)
             -----------
             IP Int. Reach (128)
             IP Ext. Address (130)
             The extended IP reachability TLV (135)
             MT IP Reach (235)
             IPv6 IP Reach (236)
             MT IPv6 IP Reach (237)


4.2.3       IS Neighbor Advertisement Restrictions

   Advertisement of IS Neighbor Reachability in an Extended LSP is
   restricted to advertisement of neighbor reachability to the
   Originating System. A neighbor to the Originating System MUST be
   advertised in Extended LSPs. If multi-topology capability [M-IS-IS]
   is supported, an MT IS Neighbor advertisement to the Originating
   System IS MUST be included for every topology advertised in the
   Extended LSP set. Neighbor advertisement(s) to the Originating
   System in an Extended LSP MUST use a non-zero metric and SHOULD use
   a metric of MaxLinkMetric-1.

   The restrictions defined here apply to all TLVs used to advertise
   neighbor reachability. These include the following TLVs:

             TLV Name (#)
             -----------
             IS Neighbors (2)
             The extended IS reachability TLV (22)
             MT-ISN (222)






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4.2.4       Area Addresses

   Fragment #0 of an Extended LSP set MUST include an Area Address TLV.
   The set of area addresses advertised MUST be a subset of the set of
   Area Addresses advertised in the normal LSP fragment #0 at the
   corresponding level. Preferably the advertisement SHOULD be
   syntactically identical to that included in the normal LSP fragment
   #0 at the corresponding level.

4.2.5       Overload, Attached, and Partition Repair Bits

   The Overload (OL), Attached (ATT), and Partition Repair (P) bits
   MUST be set to 0 in all Extended LSP fragments.

   Note that ISs NOT supporting these extensions will interpret these
   bits normally in Extended LSPs they receive. If the ATT bit were set
   in an Extended LSP this could indicate that the Virtual IS is
   attached to other areas when the Originating System is not. This
   might cause legacy systems to use the Virtual IS as a default exit
   point from the area.

4.3     Originating LSP Requirements

   The Original LSP set MUST include a neighbor to the Virtual IS
   associated with each Extended LSP set generated. If multi-topology
   capability [M-IS-IS] is supported, an MT IS Neighbor advertisement
   to the Virtual IS MUST be included for every topology advertised in
   the Extended LSP set. The neighbor advertisement(s) in the Original
   LSP MUST specify a metric of zero. This guarantees that the two way
   connectivity check between Originating System and Virtual IS will
   succeed and that the cost of reaching the Virtual IS is the same as
   the cost to reach the Originating System.

4.4     IS Alias ID TLV (IS-Alias)

   The IS-Alias TLV allows extension-capable ISs to recognize the
   Originating System of an Extended LSP set. It identifies the Normal
   system-id of the Originating System.














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         Type   24
         Length # of octets in the value field (7 to 255)
         Value

                                            No. of octets
             +-----------------------+
             | Normal System-id      |     6
             +-----------------------+
             | Sub-TLV length        |     1
             +-----------------------+
             | Sub-TLVs (optional)   |     0 to 248
             +-----------------------+


           Normal system-id

                 The Normal system-id of the Originating System

           Sub-TLVs length

                 Total length of all sub-TLVs.

           Sub-TLVs

   No subTLVs are defined in this document. Should future extensions
   define subTLVs, the subTLVs MUST be formatted as described in RFC
   3784.

4.5     New TLVs in support of IS Neighbor Attributes

   One of the major sources of additional information in LSPs is the
   subTLV information associated with the extended IS reachability TLV
   (22) and MT IS Neighbor TLV (222). This includes (but is not limited
   to) information required in support of Traffic Engineering (TE) as
   defined in RFC 3784 and RFC 4205. The restrictions defined in this
   document prohibit the presence of TLV 22 and/or TLV 222 in Extended
   LSPs except to advertise the neighbor relationship to the
   Originating System. In the event that there is a need to advertise
   in Extended LSPs such information associated with neighbors of the
   Originating System, it is necessary to define new TLVs to carry the
   subTLV information.

   Two new TLVs are therefore defined.

   1)IS Neighbor Attribute TLV (23). It is identical in format to the
   Extended IS Reachability TLV (22).





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   2)MT IS Neighbor Attribute TLV (223). It is identical in format to
   the MT IS Neighbor TLV (222).

   These new TLVs MAY be included in Original LSPs or Extended LSPs.
   Regardless of the type of LSP in which the TLVs appear, the
   information pertains to the neighbor relationship between the
   Originating System and the IS identified in the TLV.

   These TLVs MUST NOT be used to infer that a neighbor relationship
   exists in the absence of TLV 22 or TLV 222 (whichever applies) in
   the Originating LSP set for the specified neighbor. This restriction
   is necessary in order to maintain compatibility with systems which
   do not support these extensions.


5.    Comparison with the RFC 3786 Solution

   This document utilizes the same basic mechanism (additional system-
   ids) as RFC 3786 to allow an originating system to generate more
   than 256 LSP fragments. It differs from RFC 3786 in that it
   restricts the content of Extended LSPs to information which does
   NOT impact the building of a Shortest Path Tree (SPT).

   Legacy IS-IS implementations which do not support the extensions
   defined in this document see the extended LSPs as information
   associated with a system which is reachable only via the Originating
   System. As no other systems are reachable via the Virtual ISs, the
   SPF calculation in legacy ISs is therefore consistent with that
   performed by extension capable ISs. There is therefore no need for
   the two different operating modes defined in RFC 3786.

   There is also no need for the special handling of the original LSP
   set and the extended LSP set(s) as a single Logical LSP during the
   SPF as specified in Section 5 of RFC 3786.

6.    Deployment Considerations

   There are a number of deployment considerations which limit the
   usefulness of extended LSPs unless all systems are extension-capable
   ISs.

6.1     Advertising New TLVs in Extended LSPs

   As extended LSPs MAY be utilized to advertise TLVs associated with
   other protocol extensions (definition of which is outside the scope
   of this document) and/or the extensions defined in Section 4.5 of
   this document, it is obvious that the utilization of the information
   in extended LSPs by legacy IS-IS implementations will be limited.
   The implication of this is that as implementations are revised to
   support the protocol extensions which define new TLVs/subTLVs that


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   MAY be advertised in extended LSPs, the implementation SHOULD also
   be revised to support the extensions defined in this document so
   that it is capable of processing the new information whether it
   appears in normal or extended LSPs.

6.2     Reachability and non-SPF TLV Staleness

   In cases where non-SPF information is advertised in LSPs, it is
   necessary to determine whether the system which originated the
   advertisement is reachable in order to guarantee that a receiving IS
   does not use or leak stale information. So long as the OL bit is NOT
   set by the Originating System in normal LSPs, reachability to the
   Virtual IS will be consistent with reachability to the Originating
   System. Therefore, no special rules are required in this case.

6.3     Normal LSP OL State and the Use of Extended LSPs

   If the Originating System sets the OL bit in a normal LSP, legacy
   systems will see the Virtual ISs associated with that Originating
   System as unreachable and therefore will not use the information in
   the corresponding Extended LSPs. Under these circumstances,
   Extension capable ISs MUST also see the Virtual ISs as unreachable.
   This avoids potential routing loops in cases where leaf information
   is advertised in Extended LSPs.

6.4     Moving Neighbor Attribute Information to Extended LSPs

   Section 4.5 defines new TLVs which MAY be used to advertise neighbor
   attribute information in extended LSPs. In cases where neighbor
   attribute information associated with the same context (e.g. the
   same link) appears in both an Original LSP and in one or more
   Extended LSP Sets, the following rules apply for each attribute:

   o If the attribute information does not conflict, it MUST be
     considered additive

   o If the attribute information conflicts, then the information in
     the Original LSP, if present, MUST be used. If no information is
     in the Original LSP, then the information from the Extended LSP
     with the lowest system-id SHALL be preferred.

   Utilization of the new TLVs for neighbor attribute information would
   provide additional benefits which include:

   o Elimination of the need for redundant IS neighbor TLVs to be
     processed as part of the SPF.

   o Easier support for a set of TE information associated with a
     single link which exceeds the 255 byte TLV limit by allowing the



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     interpretation of multiple TLVs to be considered additive rather
     than mutually exclusive.

6.5     Advertising Leaf Information in Extended LSPs

   The need to advertise leaf information in Extended LSPs may arise
   because of extensive leaking of inter-level information or because
   of the support of multiple topologies as described in [M-IS-IS].
   When leaf information is advertised in Extended LSPs, these LSPs now
   contain information which MUST be processed in order to correctly
   update the forwarding plane of an IS. This may increase the
   frequency of SPF calculations by ISs in the network. It is therefore
   recommended that, when possible, leaf information be restricted to
   the normal LSP set.

7.    Security Considerations

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

8.    IANA Considerations

   This document defines the following new ISIS TLVs that need to be
   reflected in the ISIS TLV code-point registry:

    Type        Description                            IIH   LSP   SNP
    ----        -----------------------------------    ---   ---   ---
    23          IS Neighbor Attribute                   n     y     n
    24          IS Alias ID                             n     y     n
    223         MT IS Neighbor Attribute                n     y     n


9.    Normative References

   [IS-IS] ISO, "Intermediate system to Intermediate system 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.

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

   [RFC 3786] Hermelin, A., Previdi, S. and Shand, M., "Extending the
     Number of Intermediate to Intermediate (IS-IS) Link State PDU
     (LSP) Fragments Beyond the 256 Limit," RFC 3786, May 2004.

   [RFC 4205] Kompella, K. and Rehkter, Y., "Intermediate System to
     Intermediate System (IS-IS) Extensions in Support of Generalized
     Multi-Protocol Label Switching (GMPLS)", RFC 4205, October 2005.


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   [M-IS-IS] Pryzgienda, T., Shen, N., and Sheth, N., "Multi Topology
     (MT) Routing in IS-IS", draft-ietf-isis-wg-multi-topology-11.txt
     (work in progress), October 2005.

    [BCP9] Bradner, S., "The Internet Standards Process -- Revision
     3", BCP 9, RFC 2026, October 1996.

   [BCP14] Bradner, S., "Key words for use in RFCs to Indicate
     Requirement Levels", BCP 14, RFC 2119, March 1997

   [BCP26] Narten, T. and Alvestrand, H., "Guidelines for Writing an
     IANA Considerations Section in RFCs", BCP 26 , RFC 2434, October
     1998



   [BCP79] Bradner, S. Ed., "Intellectual Property Rights in IETF
     Technology ", BCP 79 , RFC 3979, March 2005

10.     Acknowledgments

11.     Authors' Addresses


   Les Ginsberg
   Cisco Systems
   510 McCarthy Blvd.
   Milpitas, Ca. 95035 USA
   Email: ginsberg@cisco.com


   Stefano Previdi
   CISCO Systems, Inc.
   Via Del Serafico 200
   00142 - Roma
   ITALY
   Email: sprevidi@cisco.com

   Mike Shand
   Cisco Systems
   250 Longwater Avenue,
   Reading,
   Berkshire,
   RG2 6GB
   UK
   Email: mshand@cisco.com






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12.     Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on
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