Network Working Group                                  D. McPherson, Ed.
Request for Comments: 5311                                Arbor Networks
Obsoletes: 3786                                              L. Ginsberg
                                                              S. Previdi
                                                                M. Shand
                                                           Cisco Systems
                                                           February 2009

Simplified Extension of Link State PDU (LSP) Space for IS-IS

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 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.

Abstract

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

   1. Overview ........................................................2
   2. Specification of Requirements ...................................3
   3. Definition 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 That 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, Partition Repair Bits ...........6
      4.3. Originating LSP Requirements ...............................6
      4.4. IS Alias ID TLV (IS Alias ID) ..............................7
      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 ......................9
      6.2. Reachability and Non-SPF TLV Staleness .....................9
      6.3. Normal LSP OL State and Use of Extended LSPs ...............9
      6.4. Moving Neighbor Attribute INFO LSPs ........................9
      6.5. Advertising Leaf INFO Extended LSPs .......................10
   7. Security Considerations ........................................10
   8. IANA Considerations ............................................10
   9. References .....................................................11
      9.1. Normative References ......................................11
      9.2. Informative References ....................................11

1. Overview

   [IS-IS] defines the set of LSPs that may be originated by a system at
   each level.  This set is limited to 256 LSPs.  [IS-IS] also defines a
   maximum value for an LSP (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., multi-topology support,
   traffic engineering 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
   [RFC3786].  However, that solution suffers from restrictions required
   to maintain interoperability with systems that do not support the
   extensions.

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

2. Specification of Requirements

   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].

3. Definition 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 that 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 LSPs.  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: All LSPs of a given level having the same system ID and
   Pseudonode ID.  (The LSPID field then only varies in the LSP number
   octet.) This constitutes the complete set of link state information
   at a given level originated using that system ID/Pseudonode ID.  This
   term is defined to resolve the ambiguity between a logical LSP and a
   single Link State PDU -- which is sometimes called an LSP fragment.
   The latter is the unit of information handled by the update process.

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

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

   Virtual IS: The system, identified by an Additional system-id,
   advertised as originating the Extended LSPs.  These LSPs specify the
   Additional system-id in their LSP IDs.

4. Utilizing Additional System IDs

   This extension allows an Originating System to be assigned additional
   system-ids that 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

   The LSP number zero 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 that generated the
   LSP(s).

4.2. Extended LSP Restrictions

   The following restrictions on the information that may appear in an
   Extended LSP are defined in order to avoid interoperability issues
   with systems that 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 That 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.

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 [RFC5120]) 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 [RFC5120]
   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 (#)
      -----------
      IIS Neighbors (2)
      The extended IS reachability TLV (22)
      MT-ISN (222)

4.2.4. Area Addresses

   LSP number zero 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 number zero at the
   corresponding level.  Preferably, the advertisement SHOULD be
   syntactically identical to that included in the normal LSP number
   zero at the corresponding level.

4.2.5. Overload, Attached, Partition Repair Bits

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

   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 [RFC5120] 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 ID)

   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.

      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 sub-TLVs are defined in this document.  Should future
      extensions define sub-TLVs, the sub-TLVs MUST be formatted as
      described in [RFC5305].

4.5. New TLVs in Support of IS Neighbor Attributes

   One of the major sources of additional information in LSPs is the
   sub-TLV information associated with the extended IS reachability TLV
   (22) and MT-ISN TLV (222).  This includes (but is not limited to)
   information required in support of Traffic Engineering (TE) as
   defined in [RFC5305] and [RFC5307].  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 sub-TLV
   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).

   2) MT IS Neighbor Attribute TLV (223).  It is identical in format to
      the MT-ISN 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 that 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 LSPs.  It differs from RFC 3786 in that it restricts the content
   of Extended LSPs to information that 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 that is reachable only via the Originating
   System.  As no other systems are reachable via the Virtual ISs, the
   Shortest Path First (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 that 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 4.4 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 that define new TLVs/sub-TLVs that
   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 that originated the
   advertisement is reachable in order to guarantee that a receiving IS
   does not use or leak stale information.  As 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 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 INFO LSPs

   Section 4.4 defines new TLVs that 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.

   o In cases where information about the same neighbor/link/attribute
     appears in both TLV 22 and TLV 23 (or TLV 222 and TLV 223 for the
     same MTID) then the information in TLV 22 (or TLV 222) MUST be used
     and the information in TLV 23 (or TLV 223) MUST be ignored.

   Utilization of the new TLVs for neighbor attribute information would
   provide additional benefits that 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 that exceeds the 255-byte TLV limit by allowing the
     interpretation of multiple TLVs to be considered additive rather
     than mutually exclusive.

6.5. Advertising Leaf INFO 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 [RFC5120].
     When leaf information is advertised in Extended LSPs, these LSPs
     now contain information that MUST be processed in order to
     correctly update the forwarding plane of an IS.  This may increase
     the frequency of events that trigger forwarding plane updates 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. For general
security considerations for IS-IS, see [RFC5304].

8. IANA Considerations

     This document defines the following new ISIS TLVs that are
     reflected in the ISIS TLV codepoint 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. References

9.1. 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.

   [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
             Engineering", RFC 5305, October 2008.

   [RFC5307] Kompella, K., Ed., and Y. Rekhter, Ed., "IS-IS Extensions
             in Support of Generalized Multi-Protocol Label Switching
             (GMPLS)", RFC 5307, October 2008.

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

   [RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
             Topology (MT) Routing in Intermediate System to
             Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

9.2. Informative References

   [RFC3786] Hermelin, A., Previdi, S., and M. Shand, "Extending the
             Number of Intermediate System to Intermediate System (IS-
             IS) Link State PDU (LSP) Fragments Beyond the 256 Limit",
             RFC 3786, May 2004.

   [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
             Authentication", RFC 5304, October 2008.

Authors' Addresses

   Danny McPherson (editor)
   Arbor Networks, Inc.
   EMail:  danny@arbor.net

   Les Ginsberg
   Cisco Systems
   EMail: ginsberg@cisco.com

   Stefano Previdi
   Cisco Systems
   EMail: sprevidi@cisco.com

   Mike Shand
   Cisco Systems
   EMail: mshand@cisco.com