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Versions: (draft-leroux-pce-pcecp-interarea-reqs) 00 01 02 03 04 05 06 RFC 5376

Network Working Group                Nabil Bitar
Internet Draft                       (Editor)
Intended Status: Informational       Verizon
Expires: September 2008              Raymond Zhang
                                     (Editor)
                                     BT
                                     Kenji Kumaki
                                     (Editor)
                                     KDDI Corporation
                                     February 2008


        Inter-AS Requirements for the Path Computation Element
                  Communication Protocol (PCECP)

               draft-ietf-pce-interas-pcecp-reqs-04.txt

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Abstract

   Multiprotocol Label Switching Traffic Engineered (MPLS-TE) Label
   Switched Paths (LSPs) may be established wholly within an Autonomous
   System (AS) or may cross AS boundaries.

   The Path Computation Element (PCE) is a component that is capable of
   computing paths for MPLS-TE LSPs. The PCE Communication
   Protocol(PCECP) is defined to allow communication between Path
   Computation Clients (PCCs) and PCEs, and between PCEs. The PCECP is
   used to request paths and to supply computed paths in response.
   Generic requirements for the PCECP are set out in "Path Computation
   Element(PCE) Communication Protocol Generic Requirements", RFC 4657.
   This document extends those requirements to cover the use of PCECP
   in support of inter-AS MPLS-TE.

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.

   Table of Contents

   1. Introduction....................................................3
   2. Definitions.....................................................4
   3. Reference Model.................................................4
   3.1. Scope of Deployment Model.....................................5
   4. Detailed PCECP Requirements for Inter-AS Computation............6
   4.1. PCE Communication Protocol Requirements.......................6
   4.1.1. Requirements for path computation requests..................6
   4.1.2. Requirements for path computation responses.................7
   4.2. Scalability and Performance Considerations....................8
   4.3. Management Considerations.....................................8
   4.4. Confidentiality...............................................9
   4.5. Policy Controls Affecting inter-AS PCECP.....................10
   4.5.1. Inter-AS PCE Peering Policy Controls.......................10
   4.5.2. Inter-AS PCE Re-interpretation Policies....................11
   5. Security Considerations........................................11
   5.1. Use and Distribution of Keys.................................11
   5.2. Application of Policy........................................12
   5.3. Confidentiality..............................................12
   5.4. Falsification of Information.................................13
   6. IANA Considerations............................................13
   7. Acknowledgments................................................13
   8. Authors' Addresses.............................................13
   9. Normative References...........................................14

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   10. Informative References........................................14

1. Introduction

   [RFC4216] defines the scenarios motivating the deployment of inter-
   AS Multiprotocol Label Switching Traffic Engineering (MPLS-TE) and
   specifies the requirements for inter-AS MPLS-TE when the ASes are
   under the administration of one Service Provider (SP) or the
   administration of different SPs.

   Three signaling options are defined for setting up an inter-AS TE
   LSP:
       1) contiguous TE LSP as documented in [INTERD-TESIG];
       2) stitched inter-AS TE LSP discussed in [LSP-STITCHING];
       3) nested TE LSP as in [RFC4206].

   [INTERD-TE-PDPC] defines mechanisms for the computation of inter-
   domain TE LSPs using network elements along the signaling paths to
   compute per-domain path segments. The mechanisms in [INTERD-TE-PDPC]
   do not guarantee an optimum path across multiple ASes where an
   optimum path for an LSP is one that has the smallest cost, according
   to a normalized TE metric (based upon a TE-metric or IGP metric
   adopted in each transit AS) among all possible paths that satisfy
   the LSP TE-constraints.

   The Path Computation Element (PCE) [RFC4655] is a component that is
   capable of computing paths for MPLS-TE LSPs. The requirements for a
   PCE have come from Service Provider (SP) demands to compute optimum
   paths across multiple areas and/or domains, and to be able to
   separate the path computation elements from the forwarding elements.

   The PCE Communication Protocol (PCECP) is defined to allow
   communication between Path Computation Clients (PCCs) and PCEs, and
   between PCEs. The PCECP is used to request paths and to supply
   computed paths in response. Generic requirements for the PCECP are
   discussed in [RFC4657]. This document provides a set of PCECP
   requirements that are specific to inter-AS (G)MPLS-TE path
   computation.






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2. Definitions

   This document adopts the definitions and acronyms defined in Section
   3 of [RFC4216] and Section 2 of [RFC4655]. In addition, we use the
   following terminology:

   PCECP: PCE Communication Protocol

   Inter-AS (G)MPLS-TE: MPLS or Generalized MPLS Traffic Engineering

   Inter-AS (G)MPLS-TE path: An MPLS-TE or Generalized MPLS (GMPLS)
   path that traverses two or more ASes.

   Intra-AS (G)MPLS-TE path: An MPLS-TE or GMPLS path that is confined
   to a single AS. It may traverse one or more IGP areas.

   Intra-AS PCE: A PCE responsible for computing MPLS-TE or GMPLS paths
   remaining within a single AS.

   Inter-AS PCE: A PCE responsible for computing inter-AS MPLS-TE or
   GMPLS paths or path segments, possibly by cooperating with intra-AS
   PCEs.

3. Reference Model

   Figure 1 depicts the reference model for PCEs in an inter-AS
   application. We refer to two types of PCE functions in this
   document: inter-AS PCEs and intra-AS PCEs. Inter-AS PCEs perform the
   procedures needed for inter-AS MPLS-TE or GMPLS path computation
   while intra-AS PCEs perform the functions needed for intra-AS MPLS-
   TE or GMPLS path computation.

   Let's follow a scenario that illustrates the interaction among PCCs,
   inter-AS PCEs and intra-AS PCEs as shown Figure 1. R1 in AS1 wants
   to setup a MPLS-TE or a GMPLS path, call it LSP1, with certain
   constraints to R7 in AS3. R1 determines, using mechanisms out of the
   scope of this document, that R7 is an inter-AS route and that it
   needs to contact its Inter-AS PCE1 to compute the path. R1, as a
   PCC, sends a PCECP path request to PCE1. PCE1 determines that R7 is
   reachable via AS2 and that PCE2 is the PCE to ask for path
   computation across AS2. PCE1 sends a PCECP path request to PCE2.
   Inter-AS PCE2, in turn, sends a PCECP path request to Intra-AS PCE
   R4 to compute a path within AS2 (in certain cases, the same router
   such as R3 can assume both inter-AS and intra-AS path computation
   functions). R4 returns a PCECP path response to PCE2 with ASBR3 as

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   the entry point to AS2 from AS1 and ASBR7 as the exit point to AS3.
   PCE2 then sends a PCECP path request to PCE3 to compute the path
   segment across AS3, starting at ASBR7 and terminating at R7. PCE3
   returns a PCECP path response to PCE2 with the path segment ASBR7-
   R7. PCE2 then return path ASBR3-ASBR5-ASBR7-R7 to PCE1 which, in
   turn, returns path ASBR1-ASBR3-ASBR5-ASBR7-R7 to PCC R1.

   As described in the above scenario, in general, a PCC may contact an
   inter-AS PCE to request an inter-AS path, and that PCE may supply
   the path itself, or may solicit the services of other PCEs which
   may, themselves be inter-AS PCEs, or may be intra-AS PCEs with the
   responsibility for computing path segments within just one AS.

   This document describes the PCE Communication Protocol requirements
   for inter-AS path computation. That is, for PCCs to communicate path
   requests for inter-AS paths to a PCE, and for the PCE to respond. It
   also includes the requirements for PCEs to communicate inter-AS path
   requests and responses.

             Inter-AS        Inter-AS              Inter-AS
        PCC <->PCE1<--------->PCE2<--------------->PCE3
         ::     ::             ::                   ::
         R1---ASBR1====ASBR3---R3---ASBR5====ASBR7---R5---R7
         |      |        |            |        |           |
         |      |        |            |        |           |
         R2---ASBR2====ASBR4---R4---ASBR6====ASBR8---R6---R8
                               ::
                             Intra-AS
                               PCE
         <==AS1=>        <====AS2======>       <=====AS3===>

      Figure 1 Inter and Intra-AS PCE Reference Model

3.1. Scope of Deployment Model

   All attempts to predict future deployment scopes within the Internet
   have proven fruitless. Nevertheless, it may be helpful to provide
   some discussion of the scope of the inter-AS deployment model as
   envisioned at the time of writing.

   It is expected that most, if not all, inter-AS PCECP-based
   communications will be between PCEs operating in the cooperative PCE
   model described in [RFC4655]. Clearly, in this model, the requesting
   PCE acts as a PCC for the purpose of issuing a path computation


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   request, but nevertheless, the requesting node fills the wider role
   of a PCE in its own AS. It is currently considered unlikely that a
   PCC (for example, a normal Label Switching Router) will make a path
   computation request to a PCE outside its own AS. This means that the
   PCECP relationships between ASes are limited to at most n-squared
   where n is the number of peering PCEs in the various ASes
   (considered to be no greater than 100 in [RFC4657]). In practice,
   however, it is likely that only a few PCEs in one AS will be
   designated for PCECP communications with a PCE in an adjacent AS,
   and each of these will only have a few PCEs in the adjacent AS to
   choose from. A deployment model might place the PCEs as co-resident
   with the ASBRs, resulting in a manageable scaling of the PCE-PCE
   relationships. Scaling considerations (Section 4.2), manageability
   considerations (Section 4.3), and security considerations (Section
   5) should be examined in the light of these deployment expectations.



4. Detailed PCECP Requirements for Inter-AS Computation

   This section discusses detailed PCECP requirements for inter-AS
   MPLS-TE and GMPLS LSPs. Depending on the deployment environment,
   some or all of the requirements described here may be utilized.
   Specifically, some requirements are more applicable to inter-
   provider inter-AS MPLS-TE and GMPLS operations than to intra-
   provider operations.

4.1. PCE Communication Protocol Requirements

   Requirements specific to inter-AS PCECP path computation requests
   and responses are discussed in the following sections.

4.1.1. Requirements for path computation requests

   The following are inter-AS specific requirements for PCECP requests
   for path computation:

   1. [RFC4657] states the requirement for a priority level to be
   associated with each path computation request. This document does
   not change that requirement, but, in addition, it MUST be possible
   for an inter-AS PCE to apply local policy to vary the priority of
   path computation requests received across AS borders. PCECP MAY
   include a mechanism to inform the requesting inter-AS PCE of the
   change in priority that was applied.


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   2. A path computation request by an inter-AS PCE or a PCC to another
   inter-AS PCE MUST be able to specify the sequence of ASes and/or
   ASBRs across the network by providing ASBRs and/or ASes as hops in
   the desired path of the LSP to the destination. For instance, an
   inter-AS PCE MUST be able to specify to the inter-AS PCE serving the
   neighboring AS a preferred ASBR for exiting to that AS and reach the
   destination. That is, where multiple ASBRs exist, the requester MUST
   be able to indicate a preference for one of them. The PCE must be
   able to indicate whether the specified ASBR or AS as mandatory or
   non-mandatory to be on the (G)MPLS-TE path.

   3. PCECP MUST allow a requester to provide a list of ASes and/or
   ASBRs to be excluded from the computed path.

   4. A PCECP path request from one inter-AS PCE to another MUST
   include the previous AS number in the path of the LSP to enable the
   correct application of local policy at the second inter-AS PCE.

   5. A path computation request from a PCC to an inter-AS PCE or an
   inter-AS PCE to another MUST be able to specify the need for
   protection against node, link, or SRLG failure using 1:1 detours or
   facility backup. It MUST be possible to request protection across
   all ASes or across specific ASes.

   6. PCECP MUST support the disjoint path requirements specified in
   [RFC4657] and MUST further allow the specification of AS-diversity
   for the computation of a set of two or more paths.

   7. A PCECP path computation request message MUST be able to identify
   the scope of diversified path computation to be end-to-end (i.e.,
   between the endpoints of the (G)MPLS-TE tunnel) or to be limited to
   a specific AS.

4.1.2. Requirements for path computation responses

The following are inter-AS specific requirements for PCECP responses
for path computation:

   1. A PCECP path computation response from one inter-AS PCE to
another MUST be able to include both ASBRs and ASes in the computed
path while preserving path segment and topology confidentiality.

   2. A PCECP path computation response from one inter-AS PCE to the
requesting inter-AS PCE MUST be able to carry an identifier for a path
segment it computes to preserve path segment and topology
confidentiality. The objective of the identifier is to be included in

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the LSP signaling, whose mechanism is out of scope of this document, to
be used for path expansion during LSP signaling.

   3. If a constraint for a desired ASBR (see Section 4.1.1,
requirement 2) cannot be satisfied by a PCE, PCECP SHOULD allow the PCE
to notify the requester of that fact as an error in a path computation
response.

   4. A PCECP path computation from an inter-AS PCE to a requesting
inter-AS PCE or a PCC MUST be able to carry a cumulative inter-AS path
cost. Path cost normalization across ASes is out of scope of this
document.

   5. A PCECP path computation response from an inter-AS PCE to a PCC
SHOULD be able to carry the intra-AS cost of the path segment within
the PCC AS.

   6. A PCECP path computation response MUST be able to identify
diversified paths for the same (G)MPLS-TE LSP. End-to-end (i.e.,
between the two endpoints of the (G)MPLS-TE tunnel) disjoint paths are
paths that do not share nodes, links or SRLGs except for the LSP head-
end and tail-end. In cases where diversified path segments are desired
within one or more ASes, the disjoint path segments may share only the
ASBRs of the first AS and the ASBR of the last AS across these ASes.


4.2. Scalability and Performance Considerations

PCECP design for use in the inter-AS case SHOULD consider the following
criteria:

    - PCE message processing load.
    - Scalability as a function of the following parameters:
      - number of PCCs within the scope of an inter-AS PCE
- number of intra-AS PCEs within the scope of an inter-AS PCE
      - number of peering inter-AS PCEs per inter-AS PCE
      - Added complexity caused by inter-AS features.

4.3. Management Considerations

[RFC4657] specifies generic requirements for PCECP management. This
document addresses new requirements that apply to inter-AS operations.


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   The PCECP MIB module MUST provide objects to control the behavior of
   PCECP in inter-AS applications.  They include the ASes within the
   scope of an inter-AS PCE, Inter-AS PCEs in neighboring ASes to which
   the requesting PCE will or will not communicate, confidentiality and
   policies, etc..

   The built-in diagnostic tools MUST enable failure detection and
   status checking of PCC/PCE-PCE PCECP. Diagnostic tools include
   statistics collection on the historical behavior of PCECP as
   specified in [RFC4657], but additionally it MUST be possible to
   analyze this statistics on a neighboring AS basis (i.e., across the
   inter-AS PCEs that belong to a neighboring AS).

   The MIB module MUST support trap functions when thresholds are
   crossed or when important events occur as stated in [RFC4657]. These
   thresholds SHOULD be specifiable per neighbor AS as well as per peer
   inter-AS PCE, and traps should be accordingly generated.

   Basic liveliness detection for PCC/PCE-PCE PCECP is described in
   [RFC4657]. The  PCECP MIB module SHOULD allow control of liveliness
   check behavior by providing a liveliness message frequency MIB
   object and this frequency object SHOULD be specified per inter-AS
   PCE peer. In addition, there SHOULD be a MIB object that specifies
   the dead-interval as a multiplier of the liveliness message
   frequency so that if no liveliness message is received within that
   time from an inter-AS PCE, the inter-AS PCE is declared unreachable.

4.4. Confidentiality

Confidentiality mainly applies to inter-provider (inter-AS) PCE
communication. It is about protecting the information exchanged between
PCEs and about protecting the topology information within a provider's
network. Confidentiality rules may also apply among ASes under a single
provider. Each SP will in most cases designate some PCEs for inter-AS
MPLS-TE or GMPLS path computation within its own administrative domain
and some other PCEs for inter-provider inter-AS MPLS-TE or GMPLS path
computation. Among the inter-provider-scoped inter-AS PCEs in each SP
domain, there may also be a subset of the PCEs specifically enabled for
path computation across a specific set of ASes of different peer SPs.




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   PCECP SHOULD allow an SP to hide from other SPs the set of hops
   within its own ASes that are traversed by an inter-AS inter-provider
   LSP (c.f., Section 5.2.1 of [RFC4216]). In a multi-SP administrative
   domain environment, SPs may want to hide their network topologies
   for security or commercial reasons. Thus, for each inter-AS LSP path
   segment an inter-AS PCE computes, it may return to the requesting
   inter-AS PCE an inter-AS TE LSP path segment from its own ASes
   without detailing the explicit intra-AS hops. As stated earlier,
   PCECP responses SHOULD be able to carry path-segment identifiers
   that replace the details of that path segment. The potential use of
   that identifier for path expansion, for instance, during LSP
   signaling is out of scope of this document.

4.5. Policy Controls Affecting inter-AS PCECP

Section 5.2.2 of [RFC4216] discusses the policy control requirements
for inter-AS RSVP-TE signaling at the AS boundaries for the enforcement
of interconnect agreements, attribute/parameter translation and
security hardening.

   This section discusses those policy control requirements that are
   similar to what are discussed in section 5.2.2 of [RFC4216] for
   PCECP. Please note that SPs may still require policy controls during
   signaling of LSPs to enforce their bilateral or multi-lateral
   agreements at AS boundaries, but signaling is out of scope for this
   document.

4.5.1. Inter-AS PCE Peering Policy Controls

An inter-AS PCE sends path computation requests to its neighboring
inter-AS PCEs, and an inter-AS PCE that receives such a request
enforces policies applicable to the sender of the request. These
policies may include rewriting some of the parameters, or rejecting
requests based on parameter values. Such policies may be applied for
PCEs belonging to different SPs or to PCEs responsible for ASes within
a single SP administrative domain. Parameters that might be subject to
policy include bandwidth, setup/holding priority, Fast Reroute request,
Differentiated Services Traffic Engineering (DS-TE) Class Type (CT),
and others as specified in section 5.2.2.1 of [RFC4216].


   For path computation requests that are not compliant with locally
   configured policies, PCECP SHOULD enable a PCE to send an error

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   message to the requesting PCC or PCE indicating that the request has
   been rejected because a specific parameter did not satisfy the local
   policy.

4.5.2. Inter-AS PCE Re-interpretation Policies

Each SP may have different definitions in its use of, for example, DS-
TE TE classes. An inter-AS PCE receiving a path computation request
needs to interpret the parameters and constraints and adapt them to the
local environment. Specifically, a request constructed by a PCC or PCE
in one AS may have parameters and constraints that should be
interpreted differently or translated by the receiving PCE that is in a
different AS. A list of signaling parameters subject to policy re-
interpretation at AS borders can be found in section 5.2.2.2 of
[RFC4216], and the list for path computation request parameters and
constraints is the same. In addition, the transit SPs along the inter-
AS TE path may be GMPLS transport providers which may require re-
interpretation of MPLS specific PCECP path request objects to enable
path computation over a GMPLS network or vice versa.


5. Security Considerations

  The PCECP is a communications protocol for use between potentially
  remote entities (PCCs and PCEs) over an IP network. Security
  concerns arise in order to protect the PCC and PCE, and the
  information they exchange. [RFC4758] specifies requirements on the
  PCECP to protect against spoofing, snooping, and DoS attacks. That
  document is concerned with general protocol requirements applicable
  to the basic use of the PCECP. This document is specific to the
  application of the PCE architecture in an inter-AS environment, and
  so it is appropriate to highlight the security considerations that
  apply in that environment.

  Security requirements that exist within a single administrative
  domain become critical in the multi-AS case when the control of IP
  traffic and access to the network may leave the authority of a
  single administration.

5.1. Use and Distribution of Keys

How the participants in a PCECP session discover each other and the
need for the session is out of scope of this document. It may be


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through configuration or automatic discovery. However, when a PCECP
session is established, the PCECP speakers MUST have mechanisms to
authenticate each other's identities and validate the data the
exchange.  They also SHOULD have mechanisms protect the data that they
exchange via encryption. Such mechanisms usually require the use of
keys, and so the PCECP MUST describe techniques for the exchange and
use of security keys. Where inter-AS PCE discovery is used, and PCECP
security is required, automated key distribution mechanisms MUST also
be used. Since such key exchange must (necessarily) operate over an AS
boundary, proper consideration needs to be given to how inter-
administration key exchanges may be carried out and how the key
exchange, itself, may be secured. Key distribution mechanisms MUST be
defined with consideration of [RFC4107]. Where a PCECP session is
configured between a pair of inter-AS PCEs, a security key may be
manually set for that session.

5.2. Application of Policy

Policy forms an important part of the operation of PCEs in an inter-AS
environment as described in Section 4.5, especially when ASes are
administrated by different Service Providers. A wider discussion of the
application of policy to the PCE architecture can be found in [PCE-
POLICY].

Policy may also form part of the security model for the PCECP and may
be particularly applicable to inter-AS path computation requests. A
fundamental element of the application of policy at a PCE is the
identity of the requesting PCC/PCE. This makes the use of
authentication described in Section 5.1 particularly important.  Where
policy information is exchanged as part of the computation request
and/or response, the policy object is transparent to the PCECP being
delivered un-inspected and unmodified to the policy component of a PCE
or PCC. Therefore, the policy components are responsible for protecting
(for example, encrypting) the policy information and using additional
identification and authentication if a higher level of validation is
required than is provided by the base protocol elements of the PCECP.

5.3. Confidentiality

The PCECP SHOULD also provide mechanism to preserve the confidentiality
of path segments computed by a PCE in one AS and provided to a
computation response in another AS. Not only is it necessary for such
mechanisms to be provided in PCECP responses, but signaling messages

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MUST also provide mechanisms such that an ASBR receiving an incoming
signaling request can apply policy to reject signaling messages that do
not contain the computation responses produced by the local PCE.

Furthermore, a PCE SHOULD be provided with a mechanism to mask its
identity such that its presence in the path computation chain in a
cooperative PCE model (such as described in [BRPC]) cannot be derived
from the computed path. This will help to protect the PCE from targeted
attacks. Clearly, such confidentiality does not extend to the PCECP
peer (i.e., a PCC or another PCE) that invokes the PCE with a path
computation request.

5.4. Falsification of Information

In the PCE architecture, when PCEs cooperate, one PCE may return a path
computation result that is composed of multiple path segments each
computed by a different PCE. In the inter-AS case, each PCE may belong
to a different administrative domain, and the source PCC might not know
about the downstream PCEs, nor fully trust them. Although it is
possible and RECOMMENDED to establish a chain of trust between PCEs,
this might not always be possible. In this case, it becomes necessary
to guard against a PCE changing the information provided by another
downstream PCE. Some mechanism MUST be available in the PCECP, and
echoed in the corresponding signaling, that allows an AS to verify that
the signaled path conforms to the path segment computed by the local
PCE and returned on the path computation request.


6. IANA Considerations

   This document makes no requests for IANA action.

7. Acknowledgments

We would like to thank Adrian Farrel, Jean-Philippe Vasseur, and Jean
Louis Le Roux for their useful comments and suggestions. Pasi Eronen
and Sandy Murphy provided valuable early Security Directorate reviews.
Adrian Farrel re-wrote the Security Considerations section.

8. Authors' Addresses

   Nabil Bitar
   Verizon
   40 Sylvan Road

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   Waltham, MA 02451
   Email: nabil.n.bitar@verizon.com

   Kenji Kumaki
   KDDI Corporation
   Garden Air Tower
   Iidabashi, Chiyoda-ku,
   Tokyo 102-8460, JAPAN
   Phone: +81-3-6678-3103
   Email: ke-kumaki@kddi.com

   Raymond Zhang
   BT
   2160 E. Grand Ave.
   El Segundo, CA 90245 USA
   Email: Raymond_zhang@bt.com

9. Normative References

   [RFC4216] Zhang and Vasseur, "MPLS Inter-AS Traffic    Engineering
   Requirements", RFC 4216, November 2005.

   [RFC4655] Farrel, Vasseur & Ash, "A Path Computation Element (PCE)-
   Based Architecture", RFC 4755, August 2006.

   [RFC4657] J. Ash, J.L Le Roux et al., "PCE Communication Protocol
   Generic Requirements", RFC 4657, September 2006.

   [RFC4107] Bellovin, S. and Housley, R., "Guidelines for Cryptographic
   Key Management", BCP 107, RFC 4107, June 2005.

   [RFC4758] Nystroem, "Cryptographic Token Key Initialization
   Protocol (CT-KIP)", November 2006

10. Informative References

   [INTERD-TESIG] Ayyangar and Vasseur, "Inter domain GMPLS Traffic
   Engineering - RSVP-TE extensions", draft-ietf-ccamp-inter-domain-
   rsvp-te-07.txt, September 2007 (Work in Progress)

   [LSP-STITCHING] Ayyangar A., Vasseur JP., "LSP Stitching with
   Generalized MPLS TE", draft-ietf-ccamp-lsp-stitching-06.txt,
   April 2007, (work in progress).

   [RFC4206] Kompella K., Rekhter Y., "Label switched Paths(LSP)
   Hierarchy with Generalized MPLS TE", RFC4206, October 2005.

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   [INTERD-TE-PDPC] Vasseur, Ayyangar and Zhang, "A Per-domain path
   computation method for computing Inter-domain Traffic Engineering
   (TE) Label Switched Path (LSP)", draft-ietf-ccamp-inter-domain-pd-
   path-comp-06.txt, November 2007, (Work in Progress).

   [PCE-POLICY]  Bryskin, I., Berger, L. and Ash, J., "Policy-Enabled
   Path Computation Framework", draft-ietf-pce-policy-enabled-path-
   comp-03, October 2007, work in progress.

   [BRPC] Vasseur,etc. "A Backward Recursive PCE-based Computation
   (BRPC) procedure to compute shortest inter-domain Traffic
   Engineering Label Switched Paths", draft-ietf-pce-brpc-07.txt,
   February 2008 (Work in Progress)


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