Network work group                                          Mach Chen
Internet Draft                                            Renhai Zhang
Expires: October 2007                      Huawei Technologies Co.,Ltd
Category: Standards Track                                April 2, 2007

    OSPF Traffic Engineering (OSPF-TE) Extensions in Support of Inter-AS
     Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS)
                            Traffic Engineering

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   This document describes extensions to the OSPF Traffic Engineering
   (OSPF-TE) mechanisms to support Multiprotocol Label Switching (MPLS)
   and Generalized MPLS (GMPLS) Traffic Engineering (TE) for multiple
   Autonomous Systems (ASes). It defines OSPF-TE extensions for the
   flooding of TE information about inter-AS links which can be used to
   perform inter-AS TE path computation.

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Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [RFC2119].

Table of Contents

   1. Introduction.................................................2
   2. Problem statement............................................3
      2.1. A Note on Non-Objectives................................3
      2.2. Per-Domain Path Determination...........................4
      2.3. Backward Recursive Path Computation.....................5
   3. Extensions to OSPF-TE........................................6
      3.1. Remote AS Number Sub-TLV................................6
      3.2. Inter-AS Link Type......................................7
      3.3. Link ID.................................................7
   4. Procedure for Inter-AS TE Links..............................7
   5. Security Considerations......................................8
   6. IANA Considerations..........................................8
      6.1. OSPF LSA Sub-TLVs type..................................8
      6.2. OSPF TE Link Type.......................................9
   7. Acknowledgments..............................................9
   8. References...................................................9
      8.1. Normative References....................................9
      8.2. Informative References.................................10
   Author's Addresses.............................................10
   Intellectual Property Statement................................11
   Disclaimer of Validity.........................................11
   Copyright Statement............................................11

1. Introduction

   [OSPF-TE] defines extensions to the OSPF protocol [OSPF] to support
   intra-area Traffic Engineering (TE). The extensions provide a way of
   encoding the TE information for TE-enabled links within the network
   (TE links) and flooding this information within an area. Type 10
   opaque LSAs [RFC2370] are used to carry such TE information. Two top-
   level TLVs are defined in [OSPF-TE]: Router Address TLV and Link TLV.
   The Link TLV has several nested sub-TLVs which describe the TE
   attributes for a TE link.

   [OSPF-TE-V3] defines similar extensions to OSPFv3 [OSPFV3].

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   Requirements for establishing Multiprotocol Label Switching (MPLS) TE
   Label Switched Paths (LSPs) that cross multiple Autonomous Systems
   (ASes) are described in [INTER-AS-TE-REQ]. As described in [INTER-AS-
   TE-REQ], a method SHOULD provide the ability to compute a path
   spanning multiple ASes. So a path computation entity that may be the
   head-end Label Switching Router (LSR), an AS Border Router (ASBR), or
   a Path Computation Element (PCE [PCE]) needs to know the TE
   information not only of the links within an AS, but also of the links
   that connect to other ASes.

   In this document, some extensions to OSPF-TE are defined in support
   of carrying inter-AS TE link information for inter-AS Traffic
   Engineering. A new sub-TLV is added to the Link TLV and a new link
   type is introduced. The extensions are equally applicable to OSPFv2
   and OSPFv3 as identical extensions to [OSPF-TE] and [OSPF-TE-V3]. The
   detailed definitions and procedures are discussed in the following

2. Problem statement

   As described in [INTER-AS-TE-REQ], in the case of establishing an
   inter-AS TE LSP traversing multiple ASes, the Path message [RFC3209]
   may include the following elements in the Explicit Route Object (ERO)
   in order to describe the path of the LSP:

     - a set of AS numbers as loose hops; and/or

     - a set of LSRs including ASBRs as loose hops.

   Two methods for determining inter-AS paths are currently discussed.
   The per-domain method [PD-PATH] determines the path one domain at a
   time. The backward recursive method [BRPC] uses cooperation between
   PCEs to determine an optimum inter-domain path. The sections that
   follow examine how inter-AS TE link information could be useful in
   both cases.

2.1. A Note on Non-Objectives

   It is important to note that this document does not make any change
   to the confidentiality and scaling assumptions surrounding the use of
   ASes in the Internet. In particular, this document is conformant to
   the requirements set out in [INTER-AS-TE-REQ].

   The following lists of features are explicit exclusions.

   o There is no attempt to distribute TE information from within one
      AS to another AS.

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   o There is no mechanism proposed to distribute any form of TE
      reachability information for destinations outside the AS.

   o There is no proposed change to the PCE architecture or usage.

   o TE aggregation is not supported or recommended.

   o There is no exchange of private information between ASes.

   Note further that the extensions proposed in this document are
   limited to use for information about inter-AS TE links. L1VPN Auto-
   Discovery [L1VPN-OSPF-AD] defines how TE information about links
   between Customer Edge (CE) equipment and Provider Edge (PE) equipment
   can be advertised in OSPF alongside the auto-discovery information
   for the CE-PE links. That is separate functionality and does not
   overlap with the function defined in this document.

2.2. Per-Domain Path Determination

   In the per-domain method of determining an inter-AS path for an MPLS-
   TE LSP, when an LSR that is an entry-point to an AS receives a PATH
   message from an upstream AS with an ERO containing a next hop that is
   an AS number, it needs to find which LSRs within the local AS are
   connected to the downstream AS so that it can compute a TE LSP
   segment across the AS to that LSR and forward the PATH message to the
   LSR and hence into the next AS. See the figure below for an example:

                        |     | \    |      / |
                        |     |  \   |  ----  |
                        |     |   \  | /      |
                R2------R4----R6   --R8------R10----R12
                           :              :
                <-- AS1 -->:<---- AS2 --->:<--- AS3 --->

                  Figure 1: Inter-AS Reference Model

   The figure shows three ASes (AS1, AS2, and AS3) and twelve LSRs (R1
   through R12). R3 and R4 are ASBRs in AS1. R5, R6, R7, and R8 are
   ASBRs in AS2. R9 and R10 are ASBRs in AS3.

   If an inter-AS TE LSP is planned to be established from R1 to R12,
   the AS sequence is limited as: AS1, AS2, AS3.

   Suppose that the Path message enters AS2 from R3. The next hop in the
   ERO shows AS3, and R5 must determine a path segment across AS2 to
   reach AS3. It has a choice of three exit points from AS2 (R6, R7, and

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   R8) and it needs to know which of these provide TE connectivity to
   AS3, and whether the TE connectivity (for example, available
   bandwidth) is adequate for the requested LSP.

   Alternatively, if the next hop in the ERO is the entry ASBR for AS3
   (say R9), R5 needs to know which of its exit ASBRs has a TE link that
   connects to R9. Since there may be multiple exist ASBRs that are
   connected to R9 (both R7 and R8 in this example), R5 also needs to
   know the TE properties of the inter-AS TE links so that it can select
   the correct exit ASBR.

   Once the path message reaches the exit ASBR, any choice of inter-AS
   TE link can be made by the ASBR if not already made by entry ASBR
   that computed the segment.

   More details can be found in the Section 4.0 of [PD-PATH], which
   clearly points out why advertising of inter-AS links is desired.

   To enable R5 to make the correct choice of exit ASBR the following
   information is needed:

o List of all inter-AS TE links for the local AS.

o TE properties of each inter-AS TE link.

o AS number of the neighboring AS connected to by each inter-AS TE link.

o Identity (TE Router ID) of the neighboring ASBR connected to by each
   inter-AS TE link.

   In GMPLS networks further information may also be required to select
   the correct TE links as defined in [GMPLS-TE].

   The example above shows how this information is needed at the entry
   point ASBRs for each AS (or the PCEs that provide computation
   services for the ASBRs), but this information is also needed
   throughout the local AS if path computation function is fully
   distributed among LSRs in the local AS, for example to support LSPs
   that have start points (ingress nodes) within the AS.

2.3. Backward Recursive Path Computation

   Another scenario using PCE techniques has the same problem. [BRPC]
   defines a PCE-based TE LSP computation method (called Backward
   Recursive Path Computation) to compute optimal inter-domain
   constrained MPLS-TE or GMPLS LSPs. In this path computation method, a
   specific set of traversed domains are assumed to be selected before

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   computation starts. Each downstream PCE in domain(i) returns a
   multipoint-to-point tree of potential paths to its upstream neighbor
   PCE in domain(i-1). Each tree consists of the set of paths from all
   Boundary Nodes located in domain(i) to the destination where each
   path satisfies the set of required constraints for the TE LSP
   (bandwidth, affinities, etc.).

   So a PCE needs to select Boundary Nodes (that is, ASBRs) that provide
   connectivity from the upstream AS. In order that the tree of paths
   provided by one PCE to its neighbor can be correlated, the identities
   of the ASBRs for each path need to be referenced, so the PCE must
   know the identities of the ASBRs in the remote AS reached by any
   inter-AS TE link, and, in order that it provides only suitable paths
   in the tree, the PCE must know the TE properties of the inter-AS TE

   Thus, to support Backward Recursive Path Computation the same
   information as listed in Section 2.2 is required.

3. Extensions to OSPF-TE

   Note that this document does not define mechanisms for distribution
   of TE information from one AS to another, does not distribute any
   form of TE reachability information for destinations outside the AS,
   does not change the PCE architecture or usage, does not suggest or
   recommend any form of TE aggregation, and does not feed private
   information between ASes. See section 2.1.

   The extensions defined in this document allow an inter-AS TE link
   advertisement to be easily identified as such by the use of a new
   link type. A new sub-TLV to the Link TLV is defined to carry the
   information about the neighboring AS. The extensions are equally
   applicable to TE distribution using OSPFv2 and OSPFv3.

3.1. Remote AS Number Sub-TLV

   As described in [OSPF-TE], the Link TLV describes a single link and
   consists of a set of sub-TLVs. A new sub-TLV, the Remote AS Number
   sub-TLV is added to the Link TLV when advertising inter-AS links. The
   Remote AS Number sub-TLV specifies the AS number of the neighboring
   AS to which the advertised link connects.

   The Remote AS number sub-TLV is TLV type 21 (which needs to be
   confirmed by IANA), and is four octets in length. The format is as

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |              Type             |             Length            |
   |                       Remote AS Number                        |

   The Remote AS number field has 4 octets. When two octets are used for
   the AS number, as in current deployments, the left (high-order) two
   octets MUST be set to zero.

3.2. Inter-AS Link Type

   To identify a link as an inter-AS link and allow easy identification
   of these new advertisements, a new Link Type value is defined for use
   in the Link Type sub-TLV. The value of the Link Type for an inter-AS
   point-to-point link is 3 (which needs to be confirmed by IANA). The
   use of multi-access inter-AS TE links is for future study.

3.3. Link ID

   For an inter-AS link, the Link ID carried in the Link ID sub-TLV is
   the TE Router ID of the remote ASBR reached through this inter-AS

4. Procedure for Inter-AS TE Links

   When TE is enabled on an inter-AS link and the link is up, the ASBR
   SHOULD advertise this link using the normal procedures for OSPF-TE
   [OSPF-TE]. When either the link is down or TE is disabled on the link,
   the ASBR SHOULD withdraw the advertisement. When there are changes to
   the TE parameters for the link (for example, when the available
   bandwidth changes) the ASBR SHOULD re-advertise the link, but the
   ASBR MUST take precautions against excessive re-advertisements as
   described in [OSPF-TE].

   The information advertised comes from the ASBR's knowledge of the TE
   capabilities of the link, the ASBR's knowledge of the current status
   and usage of the link, and configuration at the ASBR of the remote AS
   number and remote ASBR TE Router ID.

   The TE link advertisement SHOULD be carried in a Type 10 Opaque LSA
   if the flooding scope is to be limited to within the single IGP area
   to which the ASBR belongs, or MAY be carried in a Type 11 Opaque LSA
   if the information should reach all routers (including area border
   routers, ASBRs, and PCEs) in the AS.

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   Legacy routers receiving an advertisement for an inter-AS TE link are
   able to ignore it because the Link Type carries an unknown value.
   They will continue to flood the LSA, but will not attempt to use the
   information received as if the link were an intra-AS TE link.

   Routers or PCEs that are capable of processing advertisements of
   inter-AS TE links SHOULD NOT use such links to compute paths that
   exit an AS to a remote ASBR and then immediately re-enter the AS
   through another TE link. Such paths would constitute extremely rare
   occurrences and SHOULD NOT be allowed except as the result of
   specific policy configurations at the router or PCE computing the

5. Security Considerations

   The protocol extensions defined in this document are relatively minor
   and can be secured within the AS in which they are used by the
   existing OSPF security mechanisms.

   It should be noted, however, that some of the information included in
   these new advertisements(the remote AS number and the remote ASBR ID)
   are obtained from a neighboring administration and cannot be verified
   in anyway. Since the means of delivery of this information is likely
   to be part of a commercial relationship, the source of the
   information should be carefully checked before it is entered as
   configuration information at the ASBR responsible for advertising the
   inter-AS TE links.

6. IANA Considerations

   IANA is requested to make the following allocations from registries
   under its control.

6.1. OSPF LSA Sub-TLVs type

   IANA maintains the "Open Shortest Path First (OSPF) Traffic
   Engineering TLVs" registry with sub-registry "Types for sub-TLVs in a
   TE Link TLV". IANA is requested to assign a new sub-TLV as follows.
   The number 21 is suggested.

   Value     Meaning

   21        Remote AS Number sub-TLV.

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6.2. OSPF TE Link Type

   IANA is requested to create a new sub-registry "TE Link Types" of the
   registry "Open Shortest Path First (OSPF) Traffic Engineering TLVs"
   to track TE Link Types.

   The sub-registry should read as follows:

   [OSPF-TE] defines the Link Type sub-TLV of the Link TLV. The
   following values are defined.

   Value     Meaning                 Reference

   1         Point-to-point link     [OSPF-TE]

   2         Multi-access link       [OSPF-TE]

   3         Inter-AS link           [this document]

    New allocations from this registry are by IETF Standards Action.

7. Acknowledgments

   The authors would like to thank Adrian Farrel, Acee Lindem, JP
   Vasseur, and Dean Cheng for their review and comments to this

8. References

8.1. Normative References

   [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.

   [RFC2370]  R. Coltun, "The OSPF Opaque LSA Option", RFC2370, July

   [OSPF]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

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

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

8.2. Informative References

   [INTER-AS-TE-REQ] Zhang and Vasseur, "MPLS Inter-AS Traffic
             Engineering Requirements", RFC4216, November 2005.

   [PD-PATH] Ayyangar, A., Vasseur, JP., and Zhang, R., "A Per-domain
             path computation method for establishing Inter-domain",
             draft-ietf-ccamp-inter-domain-pd-path-comp, (work in

   [BRPC] JP. Vasseur, Ed., R. Zhang, N. Bitar, JL. Le Roux, "A Backward
             Recursive PCE-based Computation (BRPC) procedure to compute
             shortest inter-domain Traffic Engineering Label Switched
             Paths ", draft-ietf-pce-brpc, (work in progress)

   [PCE] Farrel, A., Vasseur, JP., and Ash, J., "A Path Computation
             Element (PCE)-Based Architecture", RFC4655, August 2006.

   [OSPF-TE-V3] Ishiguro K., Manral V., Davey A., and Lindem A. "Traffic
             Engineering Extensions to OSPF version 3", draft-ietf-ospf-
             ospfv3-traffic, {work in progress}.

   [OSPFV3]   Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6", RFC
             2740, April 1998.

   [L1VPN-OSPF-AD] Bryskin, I., and Berger, L., "OSPF Based L1VPN Auto-
             Discovery", draft-ietf-l1vpn-ospf-auto-discovery-02.txt,
             (work in progress).

Author's Addresses

   Mach Chen
   Huawei Technologies Co.,Ltd
   KuiKe Building, No.9 Xinxi Rd.,
   Hai-Dian District
   Beijing, 100085
   P.R. China


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   Renhai Zhang
   Huawei Technologies Co.,Ltd
   KuiKe Building, No.9 Xinxi Rd.,
   Hai-Dian District
   Beijing, 100085
   P.R. China


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