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Versions: 00 01 02 draft-ietf-pce-path-key

   Networking Working Group                        Rich Bradford (Ed)
   IETF Internet Draft                                    JP Vasseur
                                                  Cisco Systems, Inc.
                                                        Adrian Farrel
                                                   Old Dog Consulting



   Proposed Status: Standard
   Expires: April 2007
                                                       October 2006



                  draft-bradford-pce-path-key-01.txt


   Preserving Topology Confidentiality in Inter-Domain Path
   Computation using a key based mechanism

Status of this Memo

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   Abstract

   Multiprotocol Label Switching (MPLS) Traffic Engineering (TE)
   Label Switched Paths (LSPs) may be computed by Path Computation
   Elements (PCEs). Where the TE LSP crosses multiple domains, such
   as Autonomous Systems (ASs), the path may be computed by multiple
   PCEs that cooperate, with each responsible for computing a segment
   of the path. However, in some cases (e.g.  when ASs are
   administered by separate Service Providers), it would break
   confidentiality rules for a PCE to supply a path segment to a PCE
   in another domain, thus disclosing internal topology information.
   This issue may be circumvented by returning a loose hop and by
   invoking a new path computation from the domain boundary LSR
   during TE LSP setup as the LSP enters the second domain, but this
   technique has several issues including the problem of maintaining
   path diversity.

   This document defines a mechanism to hide the contents of a
   segment of a path, called the Confidential Path Segment (CPS). The
   CPS may be replaced by a path-key that can be conveyed in the PCE
   Communication Protocol (PCEP) and signaled within in a Resource
   Reservation Protocol (RSVP) explicit route object.



   Table of contents
   To be Added

   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 [RFC2119].

1.  Note

   This document proposes PCEP protocol extensions necessary to
   support use of Path Keys. Such Path Keys are then used upon
   signaling by being carried out using RSVP-TE protocol extensions
   called out in [RSVP-PKS].


2.  Terminology

   ASBR Routers: border routers used to connect to another AS of a
   different or the same Service Provider via one or more links
   inter-connecting between ASs.


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   CPS: Confidential Path Segment. A segment of a path that contains
   nodes and links that the AS policy requires to not be disclosed
   outside the AS.

   Inter-AS TE LSP: A TE LSP that crosses an AS boundary.

   LSR: Label Switching Router.

   LSP: Label Switched Path.

   PCC: Path Computation Client: any client application requesting a
   path computation to be performed by a Path Computation Element.

   PCE: Path Computation Element: an entity (component, application
   or network node) that is capable of computing a network path or
   route based on a network graph and applying computational
   constraints.

   TE LSP: Traffic Engineering Label Switched Path


3.  Introduction

   Path computation techniques using the Path Computation Element
   (PCE) have been described in [PCE-ARCH] and allow for path
   computation of inter-domain Multiprotocol Label Switching (MPLS)
   traffic engineering (TE) Label Switched Paths (LSPs).

   An important element of inter-domain TE is that TE information is
   not usually shared between domains for scalability and
   confidentiality reasons ([RFC4105] and [RFC4216]). Therefore, a
   single PCE is unlikely to be able to compute a full inter-domain
   path.

   Two path computation scenarios can be used for inter-domain TE
   LSPs: one using per-domain path computation (defined in [PD-PATH-
   COMP]), and the other using a PCE-based path computation technique
   with cooperation between PCEs (as described in [PCE-ARCH]). In
   this second case, paths for inter-domain LSPs can be computed by
   cooperation between PCEs each of which computes a segment of the
   path across one domain. Such a path computation procedure is
   described in [BRPC].

   If confidentiality is required between domains (such as would very
   likely be the case between ASs belonging to different Service
   Providers) then cooperating PCEs cannot exchange path segments or
   else the receiving PCE and the Path Computation Client (PCC) will
   be able to see the individual hops through another domain thus non
   conforming to the confidentiality requirement stated in [RFC4105]

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   and [RFC4216]. We define the part of the path which we wish to
   keep confidential as the Confidential Path Segment (CPS).

   One mechanism for preserving the confidentiality of the CPS is for
   the PCE to return a path containing a loose hop for the segment
   internal to a domain that must be kept confidential. The concept
   of loose hops for the route of a TE LSP is described in [RFC3209].
   The Path Computation Element Communication Protocol (PCEP) defined
   in [PCEP] supports the use of paths with loose hops, and it is a
   local policy decision at a PCE whether it returns a full explicit
   path or uses loose hops. Note that a Path computation Request may
   request a loose or explicit path as detailed in [PCEP].

   One option may consist of returning loose hop without further
   extensions: if loose hops are used, the TE LSPs are signaled as
   normal ([RFC3209]), and when a loose hop is encountered in the
   explicit route it is resolved by performing a secondary path
   computation to reach the next loose hop. Given the nature of the
   cooperation between PCEs in computing the original path, this
   secondary computation occurs at a Label Switching Router (LSR) at
   a domain boundary (i.e. an ABR or ASBR) and the path is expanded
   as described in [PD-PATH-COMP].The PCE-based computation model is
   particularly useful for determining mutually disjoint inter-domain
   paths such as might be required for service protection. A single
   path computation request is used. However, if loose hops are
   returned, the path of each TE LSP must be recomputed at the domain
   boundaries as the TE LSPs are signaled, and since the TE LSP
   signaling proceeds independently for each LSP, disjoint paths
   cannot be guaranteed since the LSRs in charge of expanding the
   EROs are not synchronized. Therefore, using the loose hop
   technique without further extensions, path segment confidentiality
   and path diversity are mutually incompatible requirements.

   This document defines the notion of a Path Key that is a token
   that replaces a path segment in an explicit route. The Path Key is
   encoded as a Path Key Sub-object (PKS) returned in the PCEP Path
   Computation Reply message (PCReq) ([PCEP]). Upon receiving the
   computed path, the PKS sub-object will be carried out in an RSVP-
   TE Path message (RSVP-TE [RFC3209]) during signaling. The PKS may
   also, optionally, be used in recorded routes in RSVP-TE.



4.  Path-Key Solution

   The Path-Key solution may be applied in the PCE-based path
   computation context as follows. A PCE computes a path segment
   related to a particular domain and replaces it in the path


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   reported to the requesting PCC (or another PCE) by one or more
   sub-objects referred to as the PKS. The entry and boundary LSR of
   each CPS SHOULD be specified as hops in the returned path
   immediately preceding the PKS, but where two PKSs are supplied in
   sequence the entry node to the second MAY be encoded within the
   first. The exit node of a CPS MAY be present as a strict hop
   immediately following the PKS, but MAY also be hidden as part of
   the PKS.

4.1.   Mode of Operation

   During path computation, when local policy dictates that
   confidentiality must be preserved for all or part of the path
   segment being computed or if explicitly requested by the Path
   Computation Request, the PCE associates a path-key with the
   computed path for the CPS, places its own identifier (its PCE-ID)
   along with the path-key in a PKS, and inserts the PKS object in
   the path returned to the requesting PCC or PCE immediately after
   the IPv4 sub-object defined in [RFC3209]sub-object that identifies
   the LSR that will expand the PKS into a explicit path hops. This
   will usually be the LSR that is the start point of the CPS. The
   PCE that generates a PKS MUST store the computed path segment and
   the path-key for later retrieval. A local policy SHOULD be used to
   determine for how long to retain such stored information, and
   whether to discard the information after it has been queried using
   the procedures described below. TBD: Need to define the scope of
   the PKS and spell out the restrictions on Path Key re-use.

   A head-end LSR that is a PCC converts the path returned by a PCE
   into an explicit route object (ERO) that it includes in the
   Resource Reservation Protocol (RSVP) Path message. If the path
   returned by the PCE contains PKSs these are included in the ERO.
   Like any other sub-objects, the PKS is passed transparently from
   hop to hop, until it becomes the first sub-object in the ERO. This
   will occur at the start of the CPS which will usually be the
   domain boundary. The PKS MUST be preceded by an ERO sub-object
   that identifies the LSR that must expand the PKS, so the PKS will
   not be encountered in ERO processing until the LSR that can
   process it.

   An LSR that encounters a PKS when trying to identify the next-hop
   retrieves the PCE-ID from the PKS and sends the path-key to the
   PCE in a PCEP query the details of which will be described in a
   further revision of this document.

   Upon receiving the path query, the PCE identifies the computed
   path segment using the supplied path-key, and returns the
   previously computed path segment in the form of explicit hops to

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   the requesting node. The requesting node inserts the explicit hops
   into the ERO and continues to process the LSP setup as per
   [RFC3209].

   Note that if a PCE fails to expand a PKS a new PCEP error message
   will be returned. The details of this error message will be
   detailed in a further revision of this document. Upon receipt of
   this error message the requesting LSR MUST fail the LSP setup and
   SHOULD use the procedures associated with loose hop expansion
   failure [RFC3209].


5.  PCEP/RSVP-TE Path Key Sub-object


   The Path Key sub-object (PKS) may be carried in the Explicit Route
   Object (ERO) of a PCEP PCRep message [PCEP] or an RSVP-TE Path
   message [RFC3209].

      The contents of this sub-object are identical in encoding to
   the contents of the PKS as defined in [RSVP-PKS]

      The PKS Sub-Object-Type is to be assigned by IANA (recommended
   value is identical to the value assigned for the PKS for [RSVP-
   PKS])


6.  Security Considerations

   This document proposes tunneling secure topology information
   across an untrusted AS, so the security considerations are many
   and apply to PCEP and RSVP-TE.
   Issues include:
  - Security of the CPS (can other network elements probe for
     expansion of path-keys, possibly at random?).
  - Authenticity of the path-key (resilience to alteration by
     intermediaries, resilience to fake expansion of path-keys).
  - Resilience from DNS attacks (insertion of spurious path-keys;
     flooding of bogus path-key expansion requests).

     Most of the interactions required by this extension are point to
     point, can be authenticated and made secure. These interactions
     include the:
       - PCC->PCE request
       - PCE->PCE request(s)
       - PCE->PCE response(s)
       - PCE->PCC response


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       - LSR->LSR request and response (Note that a rogue LSR could
          modify the ERO and insert or modify Path Keys. This would
          result in an LSR (which is downstream in the ERO) sending
          decode requests to a PCE. This is actually a larger problem
          with RSVP.  The rogue LSR is an existing issue with RSVP and
          will not be addressed here.
       - LSR->PCE request. Note that the PCE can check that the LSR
          requesting the decode is the LSR at the head of the Path Key.
          This largely contains the previous problem to DoS rather than
          a security issue. A rogue LSR can issue random decode
          requests, but these will amount only to DoS.
       - PCE->LSR response.

     Thus, the major security issues can be dealt with using standard
     techniques for securing and authenticating pt-pt links. In
     addition, it is recommended that the PCE providing a decode
     response should check that the LSR that issued the decode request
     is the head end of the decoded ERO segment.



7.  Manageability Considerations

   To be detailed in a further revision of this document.

8.  IANA considerations

   The IANA section will be detailed in further revision of this
   document.


   For PCEP, it will include code point requests for the three new
   computed path sub-objects.


9.  Intellectual Property Considerations

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed
   to pertain to the implementation or use of the technology
   described in this document or the extent to which any license
   under such rights might or might not be available; nor does it
   represent that it has made any independent effort to identify any
   such rights. Information on the procedures with respect to rights
   in RFC documents can be found in BCP 78 and BCP 79.


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   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use
   of such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository
   at http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention
   any copyrights, patents or patent applications, or other
   proprietary rights that may cover technology that may be required
   to implement this standard. Please address the information to the
   IETF at ietf-ipr@ietf.org.

10.  References

10.1.  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.

   [PCEP] Vasseur, J.P., Le Roux, J.L., Ayyangar, A., Oki, E.,
   Ikejiri, A., Atlas, A., Dolganow, A., "Path Computation Element
   (PCE) communication Protocol (PCEP)", draft-vasseur-pce-pcep,
   work in progress.

   [RSVP-PKS] Bradford, R., Vasseur, J.P., Farrel, A., "RSVP
   Extensions for Path Key Support", draft-bradford-ccamp-path-key-
   ero, work in progress.


10.2.  Informational References

   [PCE-ARCH] Farrel, A., Vasseur, J.P., Ash, J., "Path Computation
   Element (PCE) Architecture", RFC4655, September 2006.

   [PD-PATH-COMP] Vasseur, J., et al "A Per-domain path computation
   method for establishing Inter-domain Traffic  Engineering (TE)
   Label
   Switched Paths (LSPs)", draft-ietf-ccamp-inter-domain-pd-path-
   comp, work in progress.

   [BRPC] Vasseur, J., et al "A Backward Recursive PCE-based
   Computation



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   (BRPC) procedure to compute shortest inter-domain Traffic
   Engineering Label Switched Path", draft-ietf-pce-brpc, work in
   progress.

   [RFC4105]    Le Roux, J., Vasseur, JP, Boyle, J., "Requirements
   for Support of Inter-Area and Inter-AS MPLS Traffic Engineering",
   RFC 4105, June 2005.

   [RFC4216]    Zhang, R., Vasseur, JP., et. al., "MPLS Inter-AS
   Traffic Engineering requirements", RFC 4216, November 2005.



11.                          Authors' Addresses:

   Rich Bradford (Editor)
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough , MA - 01719
   USA
   Email: rbradfor@cisco.com

   J.-P Vasseur
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough , MA - 01719
   USA
   Email: jpv@cisco.com

   Adrian Farrel
   Old Dog Consulting
   EMail:  adrian@olddog.co.uk


















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