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RFC 4201 - Link Bundling in MPLS Traffic Engineering (TE)


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Network Working Group                                        K. Kompella
Request for Comments: 4201                                    Y. Rekhter
Updates: 3471, 3472, 3473                               Juniper Networks
Category: Standards Track                                      L. Berger
                                                          Movaz Networks
                                                            October 2005

             Link Bundling in MPLS Traffic Engineering (TE)

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) The Internet Society (2005).

Abstract

   For the purpose of Generalized Multi-Protocol Label Switching (GMPLS)
   signaling, in certain cases a combination of <link identifier, label>
   is not sufficient to unambiguously identify the appropriate resource
   used by a Label Switched Path (LSP).  Such cases are handled by using
   the link bundling construct, which is described in this document.
   This document updates the interface identification TLVs, which are
   defined in the GMPLS Signaling Functional Description.

Table of Contents

   1.  Introduction .................................................  2
       1.1.  Specification of Requirements ..........................  2
   2.  Link Bundling ................................................  3
       2.1.  Restrictions on Bundling ...............................  4
       2.2.  Routing Considerations .................................  4
       2.3.  Signaling Considerations ...............................  5
             2.3.1.  Interface Identification TLV Format ............  6
             2.3.2.  Errored Component Identification ...............  7
   3.  Traffic Engineering Parameters for Bundled Links .............  7
       3.1.  OSPF Link Type .........................................  7
       3.2.  OSPF Link ID ...........................................  7
       3.3.  Local and Remote Interface IP Address ..................  7
       3.4.  Local and Remote Identifiers ...........................  8
       3.5.  Traffic Engineering Metric .............................  8
       3.6.  Maximum Bandwidth ......................................  8
       3.7.  Maximum Reservable Bandwidth ...........................  8
       3.8.  Unreserved Bandwidth ...................................  8
       3.9.  Resource Classes (Administrative Groups) ...............  8
       3.10.  Maximum LSP Bandwidth .................................  8
   4.  Bandwidth Accounting .........................................  9
   5.  Security Considerations ......................................  9
   6.  IANA Considerations ..........................................  9
   7.  References ................................................... 10
       7.1.  Normative References ................................... 10
       7.2.  Informative References ................................. 11

1.  Introduction

   For the purpose of Generalized Multi-Protocol Label Switching (GMPLS)
   signaling, in certain cases a combination of <link identifier, label>
   is not sufficient to unambiguously identify the appropriate resource
   used by a Label Switched Path (LSP).  Such cases are handled by using
   the link bundling construct, which is described in this document.
   This document updates the interface identification TLVs, which are
   defined in the GMPLS Signaling Functional Description.

1.1.  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 [RFC2119].

2.  Link Bundling

   As defined in [GMPLS-ROUTING], a traffic engineering (TE) link is a
   logical construct that represents a way to group/map information
   about certain physical resources (and their properties) that
   interconnect LSRs with information that is used by Constrained SPF
   (for the purpose of path computation) and by GMPLS signaling.

   As stated in [GMPLS-ROUTING], depending on the nature of resources
   that form a particular TE link for the purpose of GMPLS signaling, in
   some cases a combination of <TE link identifier, label> is sufficient
   to unambiguously identify the appropriate resource used by an LSP.
   In other cases, a combination of <TE link identifier, label> is not
   sufficient.  Consider, for example, a TE link between a pair of
   SONET/SDH cross-connects, where this TE link is composed of several
   fibers.  In this case the label is a TDM time slot, and moreover,
   this time slot is significant only within a particular fiber.  Thus,
   when signaling an LSP over such a TE link, one needs to specify not
   just the identity of the link, but also the identity of a particular
   fiber within that TE link, as well as a particular label (time slot)
   within that fiber.  Such cases are handled by using the link bundling
   construct, which is described in this document.

   Consider a TE link such that, for the purpose of GMPLS signaling, a
   combination of <TE link identifier, label> is not sufficient to
   unambiguously identify the appropriate resources used by an LSP.  In
   this situation, the link bundling construct assumes that the set of
   resources that form the TE link could be partitioned into disjoint
   subsets, such that (a) the partition is minimal, and (b) within each
   subset, a label is sufficient to unambiguously identify the
   appropriate resources used by an LSP.  We refer to such subsets as
   "component links", and to the whole TE link as a "bundled link".
   Furthermore, we restrict the identifiers that can be used to identify
   component links such that they are unique for a given node.  On a
   bundled link, a combination of <component link identifier, label> is
   sufficient to unambiguously identify the appropriate resources used
   by an LSP.

   The partition of resources that form a bundled link into component
   links has to be done consistently at both ends of the bundled link.
   Both ends of the bundled link also have to understand the other end's
   component link identifiers.

   The purpose of link bundling is to improve routing scalability by
   reducing the amount of information that has to be handled by OSPF
   and/or IS-IS.  This reduction is accomplished by performing
   information aggregation/abstraction.  As with any other information
   aggregation/abstraction, this results in losing some of the

   information.  To limit the amount of losses, one needs to restrict
   the type of information that can be aggregated/abstracted.

2.1.  Restrictions on Bundling

   All component links in a bundle have the same Link Type (i.e.,
   point-to-point or multi-access), the same Traffic Engineering metric,
   the same set of resource classes at each end of the links, and must
   begin and end on the same pair of LSRs.

   A Forwarding Adjacency may be a component link; in fact, a bundle can
   consist of a mix of point-to-point links and FAs.

   If the component links are all multi-access links, the set of IS-IS
   or OSPF routers that are connected to each component link must be the
   same, and the Designated Router for each component link must be the
   same.  If these conditions cannot be enforced, multi-access links
   must not be bundled.

   Component link identifiers MUST be unique across both TE and
   component link identifiers on a particular node.  This means that
   unnumbered identifiers have a node-wide scope, and that numbered
   identifiers have the same scope as IP addresses.

2.2.  Routing Considerations

   A component link may be either numbered or unnumbered.  A bundled
   link may itself be numbered or unnumbered, independent of whether the
   component links of that bundled link are numbered.

   Handling identifiers for unnumbered component links, including the
   case in which a link is formed by a Forwarding Adjacency, follows the
   same rules as those for an unnumbered TE link (see Section "Link
   Identifiers" of [RFC3477]/[RFC3480]).  Furthermore, link local
   identifiers for all unnumbered links of a given LSR (whether
   component links, Forwarding Adjacencies, or bundled links) MUST be
   unique in the context of that LSR.

   The "liveness" of the bundled link is determined by the liveness of
   each of the component links within the bundled link; a bundled link
   is alive when at least one of its component links is determined to be
   alive.  The liveness of a component link can be determined by any of
   several means: IS-IS or OSPF hellos over the component link, RSVP
   Hello, LMP hellos (see [LMP]), or from layer 1 or layer 2
   indications.

   Once a bundled link is determined to be alive, it can be advertised
   as a TE link and the TE information can be flooded.  If IS-IS/OSPF
   hellos are run over the component links, IS-IS/OSPF flooding can be
   restricted to just one of the component links.  Procedures for doing
   this are outside the scope of this document.

   In the future, as new Traffic Engineering parameters are added to
   IS-IS and OSPF, they should be accompanied by descriptions as to how
   they can be bundled, and possible restrictions on bundling.

2.3.  Signaling Considerations

   Because information about the bundled link is flooded, but
   information about the component links is not, typically, an LSP's ERO
   will identify the bundled link to be used for the LSP, but not the
   component link.  While Discovery of component link identities to be
   used in an ERO is outside the scope of the document, it is envisioned
   that such information may be provided via configuration or via future
   RRO extensions.  When the bundled link is identified in an ERO or is
   dynamically identified, the choice of the component link for the LSP
   is a local matter between the two LSRs at each end of the bundled
   link.

   Signaling must identify both the component link and label to use.
   The choice of the component link to use is always made by the sender
   of the Path/REQUEST message.  If an LSP is bidirectional [RFC3471],
   the sender chooses a component link in each direction.  The handling
   of labels is not modified by this document.

   Component link identifiers are carried in RSVP messages, as described
   in section 8 of [RFC3473].  Component link identifiers are carried in
   CR-LDP messages, as described in section 8 of [RFC3473].  Additional
   processing related to unnumbered links is described in the
   "Processing the IF_ID RSVP_HOP object"/"Processing the IF_ID TLV",
   and "Unnumbered Forwarding Adjacencies" sections of
   [RFC3477]/[RFC3480].

   [RFC3471] defines the Interface Identification type-length-value
   (TLV) types.  This document specifies that the TLV types 1, 2, and 3
   SHOULD be used to indicate component links in IF_ID RSVP_HOP objects
   and IF_ID TLVs.

   Type 1 TLVs are used for IPv4 numbered component link identifiers.

   Type 2 TLVs are used for IPv6 numbered component link identifiers.

   Type 3 TLVs are used for unnumbered component link identifiers.

   The Component Interface TLVs, TLV types 4 and 5, SHOULD NOT be used.
   Note, in Path and REQUEST messages, link identifiers MUST be
   specified from the sender's perspective.

   Except in the special case noted below, for a unidirectional LSP,
   only a single TLV SHOULD be used in an IF_ID RSVP_HOP object or IF_ID
   TLV.  This TLV indicates the component link identifier of the
   downstream data channel on which label allocation must be done.

   Except in the special case noted below, for a bidirectional LSP, only
   one or two TLVs SHOULD be used in an IF_ID RSVP_HOP object or IF_ID
   TLV.  The first TLV always indicates the component link identifier of
   the downstream data channel on which label allocation must be done.
   When present, the second TLV always indicates the component link
   identifier of the upstream data channel on which label allocation
   must be done.  When only one TLV is present, it indicates the
   component link identifier for both downstream and upstream data
   channels.

   In the special case where the same label is to be valid across all
   component links, two TLVs SHOULD be used in an IF_ID RSVP_HOP object
   or IF_ID TLV.  The first TLV indicates the TE link identifier of the
   bundle on which label allocation must be done.  The second TLV
   indicates a bundle scope label.  For TLV types 1 and 2, this is done
   by using the special bit value of all ones (1) (e.g., 0xFFFFFFFF for
   a type 1 TLV).  Per [RFC3471], for TLV types 3, 4, and 5, this is
   done by setting the Interface ID field to the special value
   0xFFFFFFFF.  Note that this special case applies to both
   unidirectional and bidirectional LSPs.

   Although it SHOULD NOT be used, when used, the type 5 TLV MUST NOT be
   the first TLV in an IF_ID RSVP_HOP object or IF_ID TLV.

2.3.1.  Interface Identification TLV Format

   This section modifies section 9.1.1. of [RFC3471].  The definition of
   the IP Address field of the TLV types 3, 4, and 5 is clarified.

      For types 3, 4, and 5, the Value field has an identical format to
      the contents of the C-Type 1 LSP_TUNNEL_INTERFACE_ID object
      defined in [RFC3477].  Note that this results in the renaming of
      the IP Address field defined in [RFC3471].

2.3.2.  Errored Component Identification

   When Interface Identification TLVs are used, the TLVs are also used
   to indicate the specific components associated with an error.  For
   RSVP, this means that any received TLVs SHOULD be copied into the
   IF_ID ERROR_SPEC object (see Section 8.2 in [RFC3473]).  The Error
   Node Address field of the object SHOULD indicate the TE Link
   associated with the error.  For CR-LDP, this means that any received
   TLVs SHOULD be copied into the IF_ID Status TLV (see Section 8.2 in
   [RFC3472]).  The HOP Address field of the TLV SHOULD indicate the TE
   Link associated with the error.

3.  Traffic Engineering Parameters for Bundled Links

   In this section, we define the Traffic Engineering parameters to be
   advertised for a bundled link, based on the configuration of the
   component links and of the bundled link.  The definition of these
   parameters for component links was undertaken in [RFC3784] and
   [RFC3630]; we use the terminology from [RFC3630].

3.1.  OSPF Link Type

   The Link Type of a bundled link is the (unique) Link Type of the
   component links.  Note that this parameter is not present in IS-IS.

3.2.  OSPF Link ID

   For point-to-point links, the Link ID of a bundled link is the
   (unique) Router ID of the neighbor.  For multi-access links, this is
   the interface address of the (unique) Designated Router.  Note that
   this parameter is not present in IS-IS.

3.3.  Local and Remote Interface IP Address

   Note that in IS-IS, the Local Interface IP Address is known as the
   IPv4 Interface Address and the Remote Interface IP Address is known
   as the IPv4 Neighbor Address.

   If the bundled link is numbered, the Local Interface IP Address is
   the local address of the bundled link; similarly, the Remote
   Interface IP Address is the remote address of the bundled link.

3.4.  Local and Remote Identifiers

   If the bundled link is unnumbered, the link local identifier is set
   to the identifier chosen for the bundle by the advertising LSR.  The
   link remote identifier is set to the identifier chosen by the
   neighboring LSR for the reverse link corresponding to this bundle, if
   known; otherwise, this is set to 0.

3.5.  Traffic Engineering Metric

   The Traffic Engineering Metric for a bundled link is that of the
   component links.

3.6.  Maximum Bandwidth

   This parameter is not used.  The maximum LSP Bandwidth (as described
   below) replaces the Maximum Bandwidth for bundled links.

3.7.  Maximum Reservable Bandwidth

   For a given bundled link, we assume that either each of its component
   links is configured with the Maximum Reservable Bandwidth, or the
   bundled link is configured with the Maximum Reservable Bandwidth.  In
   the former case, the Maximum Reservable Bandwidth of the bundled link
   is set to the sum of the Maximum Reservable Bandwidths of all
   component links associated with the bundled link.

3.8.  Unreserved Bandwidth

   The unreserved bandwidth of a bundled link at priority p is the sum
   of the unreserved bandwidths at priority p of all the component links
   associated with the bundled link.

3.9.  Resource Classes (Administrative Groups)

   The Resource Classes for a bundled link are the same as those of the
   component links.

3.10.  Maximum LSP Bandwidth

   The Maximum LSP Bandwidth takes the place of the Maximum Bandwidth.
   For an unbundled link, the Maximum Bandwidth is defined in
   [GMPLS-ROUTING].  The Maximum LSP Bandwidth of a bundled link at
   priority p is defined to be the maximum of the Maximum LSP Bandwidth
   at priority p of all of its component links.

   The details of how Maximum LSP Bandwidth is carried in IS-IS is given
   in [GMPLS-ISIS].  The details of how Maximum LSP Bandwidth is carried
   in OSPF is given in [GMPLS-OSPF].

4.  Bandwidth Accounting

   The RSVP (or CR-LDP) Traffic Control module, or its equivalent, on an
   LSR with bundled links must apply admission control on a per-
   component link basis.  An LSP with a bandwidth requirement b and
   setup priority p fits in a bundled link if at least one component
   link has a maximum LSP bandwidth >= b at priority p.  If there are
   several such links, the implementation will choose which link to use
   for the LSP.

   In order to know the maximum LSP bandwidth (per priority) of each
   component link, the Traffic Control module must track the unreserved
   bandwidth (per priority) for each component link.

   A change in the unreserved bandwidth of a component link results in a
   change in the unreserved bandwidth of the bundled link.  It also
   potentially results in a change in the maximum LSP bandwidth of the
   bundle; thus, the maximum LSP bandwidth should be recomputed.

   If one of the component links goes down, the associated bundled link
   remains up and continues to be advertised, provided that at least one
   component link associated with the bundled link is up.  The
   unreserved bandwidth of the component link that is down is set to
   zero, and the unreserved bandwidth and maximum LSP bandwidth of the
   bundle must be recomputed.  If all the component links associated
   with a given bundled link are down, the bundled link MUST not be
   advertised into OSPF/IS-IS.

5.  Security Considerations

   This document defines ways of utilizing procedures defined in other
   documents, referenced herein.  Any security issues related to those
   procedures are addressed in the referenced documents.  Thus, this
   document raises no new security issues for RSVP-TE [RFC3209] or CR-
   LDP [RFC3212].

6.  IANA Considerations

   This document changes the recommended usage of two of the
   Interface_ID Types defined in [RFC3471].  For this reason, the IANA
   registry of GMPLS Signaling Parameters has been updated to read:

   4      12      COMPONENT_IF_DOWNSTREAM - DEPRECATED
   5      12      COMPONENT_IF_UPSTREAM   - DEPRECATED

7.  References

7.1.  Normative References

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

   [GMPLS-OSPF]    Kompella, K. Ed. and Y. Rekhter, Ed., "OSPF
                   Extensions in Support of Generalized Multi-Protocol
                   Label Switching (GMPLS)", RFC 4203, October 2005.

   [GMPLS-ROUTING] Kompella, K., Ed. and Y. Rekhter, Ed., "Routing
                   Extensions in Support of Generalized Multi-Protocol
                   Label Switching (GMPLS)", RFC 4202, October 2005.

   [RFC3471]       Berger, L., "Generalized Multi-Protocol Label
                   Switching (GMPLS) Signaling Functional Description",
                   RFC 3471, January 2003.

   [RFC3473]       Berger, L., "Generalized Multi-Protocol Label
                   Switching (GMPLS) Signaling Resource ReserVation
                   Protocol-Traffic Engineering (RSVP-TE) Extensions",
                   RFC 3473, January 2003.

   [RFC3472]       Ashwood-Smith, P. and L. Berger, "Generalized Multi-
                   Protocol Label Switching (GMPLS) Signaling
                   Constraint-based Routed Label Distribution Protocol
                   (CR-LDP) Extensions", RFC 3472, January 2003.

   [RFC3784]       Smit, H. and T. Li, "Intermediate System to
                   Intermediate System (IS-IS) Extensions for Traffic
                   Engineering (TE)", RFC 3784, June 2004.

   [RFC3630]       Katz, D., Kompella, K., and D. Yeung, "Traffic
                   Engineering (TE) Extensions to OSPF Version 2", RFC
                   3630, September 2003.

   [RFC3480]       Kompella, K., Rekhter, Y., and A. Kullberg,
                   "Signalling Unnumbered Links in CR-LDP (Constraint-
                   Routing Label Distribution Protocol)", RFC 3480,
                   February 2003.

   [RFC3477]       Kompella, K. and Y. Rekhter, "Signalling Unnumbered
                   Links in Resource ReSerVation Protocol - Traffic
                   Engineering (RSVP-TE)", RFC 3477, January 2003.

   [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3209]       Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
                   V., and G. Swallow, "RSVP-TE: Extensions to RSVP for
                   LSP Tunnels", RFC 3209, December 2001.

   [RFC3212]       Jamoussi, B., Andersson, L., Callon, R., Dantu, R.,
                   Wu, L., Doolan, P., Worster, T., Feldman, N.,
                   Fredette, A., Girish, M., Gray, E., Heinanen, J.,
                   Kilty, T., and A. Malis, "Constraint-Based LSP Setup
                   using LDP", RFC 3212, January 2002.

7.2.  Informative References

   [LMP]           Lang, J., Ed., "Link Management Protocol (LMP)", RFC
                   4204, October 2005.

Authors' Addresses

   Kireeti Kompella
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA 94089

   EMail: kireeti@juniper.net

   Yakov Rekhter
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA 94089

   EMail: yakov@juniper.net

   Lou Berger
   Movaz Networks, Inc.

   Phone: +1 703-847-1801
   EMail: lberger@movaz.com

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