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rfc:rfc5520

Network Working Group R. Bradford, Ed. Request for Comments: 5520 JP. Vasseur Category: Standards Track Cisco Systems, Inc.

                                                             A. Farrel
                                                    Old Dog Consulting
                                                            April 2009
              Preserving Topology Confidentiality in
   Inter-Domain Path Computation Using a Path-Key-Based Mechanism

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 in effect on the date of
 publication of this document (http://trustee.ietf.org/license-info).
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.

Abstract

 Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS)
 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 (ASes), 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
 ASes 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 AS-internal topology information.
 This issue may be circumvented by returning a loose hop and by
 invoking a new path computation from the domain boundary Label
 Switching Router (LSR) during TE LSP setup as the signaling message
 enters the second domain, but this technique has several issues
 including the problem of maintaining path diversity.

Bradford, et al. Standards Track [Page 1] RFC 5520 Preserving Topology Confidentiality April 2009

 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 TE (RSVP-TE) explicit route object.

Table of contents

 1. Introduction ....................................................3
    1.1. Terminology ................................................4
 2. Path-Key Solution ...............................................5
    2.1. Mode of Operation ..........................................5
    2.2. Example ....................................................6
 3. PCEP Protocol Extensions ........................................7
    3.1. Path-Keys in PCRep Messages ................................7
         3.1.1. PKS with 32-Bit PCE ID ..............................8
         3.1.2. PKS with 128-Bit PCE ID .............................9
    3.2. Unlocking Path-Keys .......................................10
         3.2.1. Path-Key Bit .......................................10
         3.2.2. PATH-KEY Object ....................................10
         3.2.3. Path Computation Request (PCReq) Message
                with Path-Key ......................................11
 4. PCEP Mode of Operation for Path-Key Expansion ..................12
 5. Security Considerations ........................................12
 6. Manageability Considerations ...................................13
    6.1. Control of Function through Configuration and Policy ......13
    6.2. Information and Data Models ...............................14
    6.3. Liveness Detection and Monitoring .........................15
    6.4. Verifying Correct Operation ...............................15
    6.5. Requirements on Other Protocols and Functional
         Components ................................................15
    6.6. Impact on Network Operation ...............................16
 7. IANA Considerations ............................................16
    7.1. New Subobjects for the ERO Object .........................16
    7.2. New PCEP Object ...........................................17
    7.3. New RP Object Bit Flag ....................................17
    7.4. New NO-PATH-VECTOR TLV Bit Flag ...........................17
 8. References .....................................................17
    8.1. Normative References ......................................17
    8.2. Informative References ....................................18
 Acknowledgements ..................................................19

Bradford, et al. Standards Track [Page 2] RFC 5520 Preserving Topology Confidentiality April 2009

1. Introduction

 Path computation techniques using the Path Computation Element (PCE)
 are described in [RFC4655] and allow for path computation of inter-
 domain Multiprotocol Label Switching (MPLS) Traffic Engineering (TE)
 and Generalized MPLS (GMPLS) Label Switched Paths (LSPs).
 An important element of inter-domain TE is that TE information is not
 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 [RFC5152]), and the
 other using a PCE-based path computation technique with cooperation
 between PCEs (as described in [RFC4655]).  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 [RFC5441].
 If confidentiality is required between domains (such as would very
 likely be the case between Autonomous Systems (ASes) 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 breaking the confidentiality requirement stated in
 [RFC4105] and [RFC4216].  We define the part of the path that 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 in place of the
 segment that must be kept confidential.  The concept of loose and
 strict hops for the route of a TE LSP is described in [RFC3209].  The
 Path Computation Element Communication Protocol (PCEP) defined in
 [RFC5440] 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 with strict hops or uses loose hops.  Note that a path
 computation request may request an explicit path with strict hops or
 may allow loose hops as detailed in [RFC5440].
 The option of returning a loose hop in place of the CPS can be
 achieved without further extensions to PCEP or the signaling
 protocol.  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 resource or set of resources identified by the loose hop.
 Given the nature of the cooperation between PCEs in computing the
 original path, this secondary computation occurs at or on behalf of a

Bradford, et al. Standards Track [Page 3] RFC 5520 Preserving Topology Confidentiality April 2009

 Label Switching Router (LSR) at a domain boundary (i.e., an Area
 Border Router (ABR) or an AS Border Router (ASBR)) and the path is
 expanded as described in [RFC5152].
 The PCE-based computation model is particularly useful for
 determining mutually disjoint inter-domain paths such as might be
 required for service protection [RFC5298].  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 TE LSP, disjoint paths cannot be guaranteed
 since the LSRs in charge of expanding the explicit route objects
 (EROs) are not synchronized.  Therefore, if the loose hop technique
 is used 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 Subobject (PKS) returned in the PCEP Path
 Computation Reply message (PCRep) ([RFC5440]).  Upon receiving the
 computed path, the PKS will be carried in an RSVP-TE Path message
 (RSVP-TE [RFC3209] and [RSVP-PKS]) during signaling.
 The BNF in this document follows the format described in [RBNF].
 Please note that the term "path-key" used in this document refers to
 an identifier allocated by a PCE to represent a segment of a computed
 path.  This term has no relation to the term "cryptographic key" used
 in some documents that describe security mechanisms.

1.1. Terminology

 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].
 This document makes use of the following terminology and acronyms.
 AS: Autonomous System.
 ASBR: Autonomous System Border Routers used to connect to another AS
 of a different or the same Service Provider via one or more links
 inter-connecting between ASes.
 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.

Bradford, et al. Standards Track [Page 4] RFC 5520 Preserving Topology Confidentiality April 2009

 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.

2. 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 any CPS in the path
 reported to the requesting PCC (or another PCE) by one or more
 subobjects referred to as PKSes.  The entry boundary LSR of each CPS
 SHOULD be specified using its TE Router Id as a hop in the returned
 path immediately preceding the CPS, and other subobjects MAY be
 included in the path immediately before the hop identifying the
 boundary LSR to indicate link and label choices.  Where two PKSes are
 supplied in sequence with no intervening nodes, the entry node to the
 second CPS MAY be part of the first CPS and does not need to be
 explicitly present in the returned path.  The exit node of a CPS MAY
 be present as a strict hop immediately following the PKS.

2.1. 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 as defined in Section 3.1)
 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
 subobject that identifies (using the TE Router Id) the LSR that will
 expand the PKS into explicit path hops.  This will usually be the LSR
 that is the starting point of the CPS.  The PCE that generates a PKS
 SHOULD 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

Bradford, et al. Standards Track [Page 5] RFC 5520 Preserving Topology Confidentiality April 2009

 information, and whether to discard the information after it has been
 queried using the procedures described below.  It is RECOMMENDED for
 a PCE to store the PKS for a period of 10 minutes.
 A path-key value is scoped to the PCE that computed it as identified
 by the PCE-ID carried in the PKS.  A PCE MUST NOT re-use a path-key
 value to represent a new CPS for at least 30 minutes after discarding
 the previous use of the same path-key.  A PCE that is unable to
 retain information about previously used path-key values over a
 restart SHOULD use some other mechanism to guarantee uniqueness of
 path-key values such as embedding a timestamp or version number in
 the path-key.
 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 a PKS, this is included in the ERO.  Like any other
 subobjects, the PKS is passed transparently from hop to hop, until it
 becomes the first subobject 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 subobject that identifies the LSR that must
 expand the PKS.  This means that (following the rules for ERO
 processing set out in [RFC3209]) the PKS will not be encountered in
 ERO processing until the ERO is being processed by the LSR that is
 capable of correctly handling the PKS.
 An LSR that encounters a PKS when trying to identify the next hop
 retrieves the PCE-ID from the PKS and sends a Path Computation
 Request (PCReq) message as defined in [RFC5440] to the PCE identified
 by the PCE-ID that contains the path-key object .
 Upon receiving the PCReq message, 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 using an ERO
 object contained in the Path Computation Reply (PCRep) to the
 requesting node as defined in [RFC5440].  The requesting node inserts
 the explicit hops into the ERO and continues to process the TE LSP
 setup as per [RFC3209].

2.2. Example

 Figure 1 shows a simple two-AS topology with a PCE responsible for
 the path computations in each AS.  An LSP is requested from the
 ingress LSR in one AS to the egress LSR in the other AS.  The
 ingress, acting as the PCC, sends a path computation request to
 PCE-1.  PCE-1 is unable to compute an end-to-end path and invokes
 PCE-2 (possibly using the techniques described in [RFC5441]).  PCE-2
 computes a path segment from ASBR-2 to the egress as {ASBR-2, C, D,

Bradford, et al. Standards Track [Page 6] RFC 5520 Preserving Topology Confidentiality April 2009

 Egress}.  It could pass this path segment back to PCE-1 in full, or
 it could send back the path {ASBR-2, Egress} where the second hop is
 a loose hop.
 However, in order to protect the confidentiality of the topology in
 the second AS while still specifying the path in full, PCE-2 may send
 PCE-1 a path segment expressed as {ASBR-2, PKS, Egress} where the PKS
 is a Path-Key Subobject as defined in this document.  In this case,
 PCE-2 has identified the segment {ASBR-2, C, D, Egress} as a
 Confidential Path Segment (CPS).  PCE-1 will compute the path segment
 that it is responsible for, and will supply the full path to the PCC
 as {Ingress, A, B, ASBR-1, ASBR-2, PKS, Egress}.
 Signaling proceeds in the first AS as normal, but when the Path
 message reaches ASBR-2, the next hop is the PKS, and this must be
 expanded before signaling can progress further.  ASBR-2 uses the
 information in the PKS to request PCE-2 for a path segment, and PCE-2
 will return the segment {ASBR-2, C, D, Egress} allowing signaling to
 continue to set up the LSP.
  1. —————————- —————————-

| ——- | | ——- |

    |    | PCE-1 |<---------------+--+-->| PCE-2 |                |
    |     -------                 |  |    -------                 |
    |      ^                      |  |    ^                       |
    |      |                      |  |    |                       |
    |      v                      |  |    v                       |
    |  -------              ----  |  |  ----                      |
    | |  PCC  |   -    -   |ASBR| |  | |ASBR|   -    -    ------  |
    | |Ingress|--|A|--|B|--|  1 |-+--+-|  2 |--|C|--|D|--|Egress| |
    |  -------    -    -    ----- |  |  ----    -    -    ------  |
    |                             |  |                            |
     -----------------------------    ----------------------------
      Figure 1 : A Simple network to demonstrate the use of the PKS

3. PCEP Protocol Extensions

3.1. Path-Keys in PCRep Messages

 Path-Keys are carried in PCReq and PCRep messages as part of the
 various objects that carry path definitions.  In particular, a Path-
 Key is carried in the Explicit Route Object (ERO) on PCRep messages.
 In all cases, the Path-Key is carried in a Path-Key Subobject (PKS).

Bradford, et al. Standards Track [Page 7] RFC 5520 Preserving Topology Confidentiality April 2009

 The PKS is a fixed-length subobject containing a Path-Key and a
 PCE-ID.  The Path-Key is an identifier, or token used to represent
 the CPS within the context of the PCE identified by the PCE-ID.  The
 PCE-ID identifies the PCE that can decode the Path-Key using an
 identifier that is unique within the domain that the PCE serves.  The
 PCE-ID has to be mapped to a reachable IPv4 or IPv6 address of the
 PCE by the first node of the CPS (usually a domain border router) and
 a PCE MAY use one of its reachable IP addresses as its PCE-ID.
 Alternatively and to provide greater security (see Section 5) or
 increased confidentiality, according to domain-local policy, the PCE
 MAY use some other identifier that is scoped only within the domain.
 To allow IPv4 and IPv6 addresses to be carried, two subobjects are
 defined in the following subsections.
 The Path-Key Subobject may be present in the PCEP ERO or the PCEP
 PATH-KEY object (see Section 3.2).

3.1.1. PKS with 32-Bit PCE ID

 The Subobject Type for the PKS with 32-bit PCE ID is 64.  The format
 of this subobject is as follows:
   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |L|    Type     |     Length    |           Path-Key            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         PCE ID (4 bytes)                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    L
       The L bit SHOULD NOT be set, so that the subobject represents a
       strict hop in the explicit route.
    Type
       Subobject Type for a Path-Key with 32-bit PCE ID (64).
    Length
       The Length contains the total length of the subobject in bytes,
       including the Type and Length fields.  The Length is always 8.

Bradford, et al. Standards Track [Page 8] RFC 5520 Preserving Topology Confidentiality April 2009

    PCE ID
       A 32-bit identifier of the PCE that can decode this path-key.
       The identifier MUST be unique within the scope of the domain
       that the CPS crosses, and MUST be understood by the LSR that
       will act as PCC for the expansion of the PKS.  The
       interpretation of the PCE-ID is subject to domain-local policy.
       It MAY be an IPv4 address of the PCE that is always reachable,
       and MAY be an address that is restricted to the domain in which
       the LSR that is called upon to expand the CPS lies.  Other
       values that have no meaning outside the domain (for example,
       the Router ID of the PCE) MAY be used to increase security or
       confidentiality (see Section 5).

3.1.2. PKS with 128-Bit PCE ID

 The Subobject Type for the PKS with 128-bit PCE ID is 65.  The format
 of the subobject is as follows.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |L|    Type     |     Length    |           Path-Key            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        PCE ID (16 bytes)                      |
 |                                                               |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    L
       As above.
    Type
       Subobject Type for a Path-Key with 128-bit PCE ID (65).
    Length
       The Length contains the total length of the subobject in bytes,
       including the Type and Length fields.  The Length is always 20.
    PCE ID
       A 128-bit identifier of the PCE that can decode this path-key.
       The identifier MUST be unique within the scope of the domain
       that the CPS crosses, and MUST be understood by the LSR that

Bradford, et al. Standards Track [Page 9] RFC 5520 Preserving Topology Confidentiality April 2009

       will act as PCC for the expansion of the PKS.  The
       interpretation of the PCE-ID is subject to domain-local policy.
       It MAY be an IPv6 address of the PCE that is always reachable,
       but MAY be an address that is restricted to the domain in which
       the LSR that is called upon to expand the CPS lies.  Other
       values that have no meaning outside the domain (for example,
       the IPv6 TE Router ID) MAY be used to increase security (see
       Section 5).

3.2. Unlocking Path-Keys

 When a network node needs to decode a Path-Key so that it can
 continue signaling for an LSP, it must send a PCReq to the designated
 PCE.  The PCReq defined in [RFC5440] needs to be modified to support
 this usage, which differs from the normal path computation request.
 To that end, a new flag is defined to show that the PCReq relates to
 the expansion of a PKS, and a new object is defined to carry the PKS
 in the PCReq.  These result in an update to the BNF for the message.
 The BNF used in this document is as described in [RBNF].

3.2.1. Path-Key Bit

 [RFC5440] defines the Request Parameters (RP) object that is used to
 specify various characteristics of the Path Computation Request
 (PCReq).
 In this document, we define a new bit named the Path-Key bit as
 follows.  See Section 7.3 for the IANA assignment of the appropriate
 bit number.
 Path-Key bit: When set, the requesting PCC requires the retrieval of
 a Confidential Path Segment that corresponds to the PKS carried in a
 PATH-KEY object in the path computation request.  The Path-Key bit
 MUST be cleared when the path computation request is not related to a
 CPS retrieval.

3.2.2. PATH-KEY Object

 When a PCC needs to expand a path-key in order to expand a CPS, it
 issues a Path Computation Request (PCReq) to the PCE identified in
 the PKS in the RSVP-TE ERO that it is processing.  The PCC supplies
 the PKS to be expanded in a PATH-KEY Object in the PCReq message.

Bradford, et al. Standards Track [Page 10] RFC 5520 Preserving Topology Confidentiality April 2009

 The PATH-KEY Object is defined as follows:
 PATH-KEY Object-Class is 16.
 Path-Key Object-Type is 1.
 The PATH-KEY Object MUST contain at least one Path-Key Subobject (see
 Section 3.1).  The first PKS MUST be processed by the PCE.
 Subsequent subobjects SHOULD be ignored.

3.2.3. Path Computation Request (PCReq) Message with Path-Key

 The format of a PCReq message including a PATH-KEY object is
 unchanged as follows:
     <PCReq Message>::= <Common Header>
                        [<SVEC-list>]
                        <request-list>
     where:
        <svec-list>::=<SVEC>[<svec-list>]
        <request-list>::=<request>[<request-list>]
 To support the use of the message to expand a PKS, the definition of
 <request> is modified as follows :
     <request>::= <RP>
                  <segment-computation> | <path-key-expansion>
     where:
        <segment-computation> ::= <END-POINTS>
                                  [<LSPA>]
                                  [<BANDWIDTH>]
                                  [<BANDWIDTH>]
                                  [<metric-list>]
                                  [<RRO>]
                                  [<IRO>]
                                  [<LOAD-BALANCING>]
        <path-key-expansion> ::= <PATH-KEY>
 Thus, the format of the message for use in normal path computation is
 unmodified.

Bradford, et al. Standards Track [Page 11] RFC 5520 Preserving Topology Confidentiality April 2009

4. PCEP Mode of Operation for Path-Key Expansion

 The retrieval of the explicit path (the CPS) associated with a PKS by
 a PCC is no different than any other path computation request with
 the exception that the PCReq message MUST contain a PATH-KEY object
 and the Path-Key bit of the RP object MUST be set.  On receipt of a
 PCRep containing a CPS, the requesting PCC SHOULD insert the CPS into
 the ERO that it will signal, in accordance with local policy.
 If the receiving PCE does not recognize itself as identified by the
 PCE ID carried in the PKS, it MAY forward the PCReq message to
 another PCE according to local policy.  If the PCE does not forward
 such a PCReq, it MUST respond with a PCRep message containing a
 NO-PATH object.
 If the receiving PCE recognizes itself, but cannot find the related
 CPS, or if the retrieval of the CPS is not allowed by policy, the PCE
 MUST send a PCRep message that contains a NO-PATH object.  The
 NO-PATH-VECTOR TLV SHOULD be used as described in [RFC5440] and a new
 bit number (see Section 7.4) is assigned to indicate "Cannot expand
 PKS".
 Upon receipt of a negative reply, the requesting LSR MUST fail the
 LSP setup and SHOULD use the procedures associated with loose hop
 expansion failure [RFC3209].

5. Security Considerations

 This document describes tunneling confidential path information
 across an untrusted domain (such as an AS).  There are many security
 considerations that apply to PCEP and RSVP-TE.
 Issues include:
  1. Confidentiality of the CPS (can other network elements probe for

expansion of path-keys, possibly at random?).

  1. Authenticity of the path-key (resilience to alteration by

intermediaries, resilience to fake expansion of path-keys).

  1. Resilience from Denial-of-Service (DoS) 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 as described in [RFC5440]
 and [RFC3209].  These interactions include the:

Bradford, et al. Standards Track [Page 12] RFC 5520 Preserving Topology Confidentiality April 2009

  1. PCC→PCE request
  1. PCE→PCE request(s)
  1. PCE→PCE response(s)
  1. PCE→PCC response
  1. 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.
  1. 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 of DoS rather than a
       security issue.  A rogue LSR can issue random decode requests,
       but these will amount only to DoS.
  1. PCE→LSR response
 Thus, the major security issues can be dealt with using standard
 techniques for securing and authenticating point-to-point
 communications.  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.
 Further protection can be provided by using a PCE ID to identify the
 decoding PCE that is only meaningful within the domain that contains
 the LSR at the head of the CPS.  This may be an IP address that is
 only reachable from within the domain, or some not-address value.
 The former requires configuration of policy on the PCEs, the latter
 requires domain-wide policy.

6. Manageability Considerations

6.1. Control of Function through Configuration and Policy

 The treatment of a path segment as a CPS, and its substitution in a
 PCRep ERO with a PKS, is a function that MUST be under operator and
 policy control where a PCE supports the function.  The operator MUST
 be given the ability to specify which path segments are to be
 replaced and under what circumstances.  For example, an operator
 might set a policy that states that every path segment for the
 operator's domain will be replaced by a PKS when the PCReq has been
 issued from outside the domain.

Bradford, et al. Standards Track [Page 13] RFC 5520 Preserving Topology Confidentiality April 2009

 The operation of the PKS extensions require that path-keys are
 retained by the issuing PCE to be available for retrieval by an LSR
 (acting as a PCC) at a later date.  But it is possible that the
 retrieval request will never be made, so good housekeeping requires
 that a timer is run to discard unwanted path-keys.  A default value
 for this timer is suggested in Section 2.1.  Implementations SHOULD
 provide the ability for this value to be overridden through operator
 configuration or policy.
 After a PKS has been expanded in response to a retrieval request, it
 may be valuable to retain the path-key and CPS for debugging
 purposes.  Such retention SHOULD NOT be the default behavior of an
 implementation, but MAY be available in response to operator request.
 Once a path-key has been discarded, the path-key value SHOULD NOT be
 immediately available for re-use for a new CPS since this might lead
 to accidental misuse.  A default timer value is suggested in Section
 2.1.  Implementations SHOULD provide the ability for this value to be
 overridden through operator configuration or policy.
 A PCE must set a PCE-ID value in each PKS it creates so that PCCs can
 correctly identify it and send PCReq messages to expand the PKS to a
 path segment.  A PCE implementation SHOULD allow operator or policy
 control of the value to be used as the PCE-ID.  If the PCE allows
 PCE-ID values that are not routable addresses to be used, the PCCs
 MUST be configurable (by the operator or through policy) to allow the
 PCCs to map from the PCE-ID to a routable address of the PCE.  Such
 mapping may be algorithmic, procedural (for example, mapping a PCE-ID
 equal to the IGP Router ID into a routable address), or configured
 through a local or remote mapping table.

6.2. Information and Data Models

 A MIB module for PCEP is already defined in [PCEP-MIB].  The
 configurable items listed in Section 6.1 MUST be added as readable
 objects in the module and SHOULD be added as writable objects.
 A new MIB module MUST be created to allow inspection of path-keys.
 For a given PCE, this MIB module MUST provide a mapping from path-
 key to path segment (that is, a list of hops), and MUST supply other
 information including:
  1. The identity of the PCC that issued the original request that led

to the creation of the path-key.

  1. The request ID of the original PCReq.

Bradford, et al. Standards Track [Page 14] RFC 5520 Preserving Topology Confidentiality April 2009

  1. Whether the path-key has been retrieved yet, and if so, by which

PCC.

  1. How long until the path segment associated with the path-key will

be discarded.

  1. How long until the path-key will be available for re-use.

6.3. Liveness Detection and Monitoring

 The procedures in this document extend PCEP, but do not introduce new
 interactions between network entities.  Thus, no new liveness
 detection or monitoring is required.
 It is possible that a head-end LSR that has be given a path including
 PKSs replacing specific CPSs will want to know whether the path-keys
 are still valid (or have timed out).  However, rather than introduce
 a mechanism to poll the PCE that is responsible for the PKS, it is
 considered pragmatic to simply signal the associated LSP.

6.4. Verifying Correct Operation

 The procedures in this document extend PCEP, but do not introduce new
 interactions between network entities.  Thus, no new tools for
 verifying correct operation are required.
 A PCE SHOULD maintain counters and logs of the following events that
 might indicate incorrect operation (or might indicate security
 issues).
  1. Attempts to expand an unknown path-key.
  1. Attempts to expand an expired path-key.
  1. Duplicate attempts to expand the same path-key.
  1. Expiry of path-key without attempt to expand it.

6.5. Requirements on Other Protocols and Functional Components

 The procedures described in this document require that the LSRs
 signal PKSs as defined in [RSVP-PKS].  Note that the only changes to
 LSRs are at the PCCs.  Specifically, changes are only needed at the
 head-end LSRs that build RSVP-TE Path messages containing Path-Key
 Subobjects in their EROs, and the LSRs that discover such subobjects
 as next hops and must expand them.  Other LSRs in the network, even
 if they are on the path of the LSP, will not be called upon to
 process the PKS.

Bradford, et al. Standards Track [Page 15] RFC 5520 Preserving Topology Confidentiality April 2009

6.6. Impact on Network Operation

 As well as the security and confidentiality aspects addressed by the
 use of the PKS, there may be some scaling benefits associated with
 the procedures described in this document.  For example, a single PKS
 in an explicit route may substitute for many subobjects and can
 reduce the overall message size correspondingly.  In some
 circumstances, such as when the explicit route contains multiple
 subobjects for each hop (including node IDs, TE link IDs, component
 link IDs for each direction of a bidirectional LSP, and label IDs for
 each direction of a bidirectional LSP) or when the LSP is a point-
 to-multipoint LSP, this scaling improvement may be very significant.
 Note that a PCE will not supply a PKS unless it knows that the LSR
 that will receive the PKS through signaling will be able to handle
 it.  Furthermore, as noted in Section 6.5, only those LSRs
 specifically called upon to expand the PKS will be required to
 process the subobjects during signaling.  Thus, the only backward
 compatibility issues associated with the procedures introduced in
 this document arise when a head-end LSR receives a PCRep with an ERO
 containing a PKS, and it does not know how to encode this into
 signaling.
 Since the PCE that inserted the PKS is required to keep the CPS
 confidential, the legacy head-end LSR cannot be protected.  It must
 either fail the LSP setup, or request a new path computation avoiding
 the domain that has supplied it with unknown subobjects.

7. IANA Considerations

 IANA assigns values to PCEP parameters in registries defined in
 [RFC5440].  IANA has made the following additional assignments.

7.1. New Subobjects for the ERO Object

 IANA has previously assigned an Object-Class and Object-Type to the
 ERO carried in PCEP messages [RFC5440].  IANA also maintains a list
 of subobject types valid for inclusion in the ERO.
 IANA assigned two new subobject types for inclusion in the ERO as
 follows:
 Subobject Type                                         Reference
           64   Path-Key with 32-bit PCE ID             [RFC5520]
           65   Path-Key with 128-bit PCE ID            [RFC5520]

Bradford, et al. Standards Track [Page 16] RFC 5520 Preserving Topology Confidentiality April 2009

7.2. New PCEP Object

 IANA assigned a new object class in the registry of PCEP Objects as
 follows.
 Object  Name          Object  Name                     Reference
 Class                 Type
 16    PATH-KEY        1       Path-Key                 [RFC5520]
     Subobjects
        This object may carry the following subobjects as defined
        for the ERO object.
                64   Path-Key with 32-bit PCE ID        [RFC5520]
                65   Path-Key with 128-bit PCE ID       [RFC5520]

7.3. New RP Object Bit Flag

 IANA maintains a registry of bit flags carried in the PCEP RP object
 as defined in [RFC5440].  IANA assigned a new bit flag as follows:
 Bit Number  Hex       Name                             Reference
 23          0x000017  Path-Key (P-bit)                 [RFC5520]

7.4. New NO-PATH-VECTOR TLV Bit Flag

 IANA maintains a registry of bit flags carried in the PCEP NO-PATH-
 VECTOR TLV in the PCEP NO-PATH object as defined in [RFC5440].  IANA
 assigned a new bit flag as follows:
 Bit Number      Name Flag                    Reference
 27              PKS expansion failure        [RFC5520]

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.
 [RFC5440]  Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path Computation
            Element (PCE) Communication Protocol (PCEP)", RFC 5440,
            March 2009.

Bradford, et al. Standards Track [Page 17] RFC 5520 Preserving Topology Confidentiality April 2009

8.2. Informative References

 [PCEP-MIB] Koushik, K., and E. Stephan, "PCE Communication Protocol
            (PCEP) Management Information Base", Work in Progress,
            November 2008.
 [RBNF]     Farrel, A., "Reduced Backus-Naur Form (RBNF) A Syntax Used
            in Various Protocol Specifications", Work in Progress,
            November 2008.
 [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.
 [RFC4105]  Le Roux, J.-L., Ed., Vasseur, J.-P., Ed., and J. Boyle,
            Ed., "Requirements for Inter-Area MPLS Traffic
            Engineering", RFC 4105, June 2005.
 [RFC4216]  Zhang, R., Ed., and J.-P. Vasseur, Ed., "MPLS Inter-
            Autonomous System (AS) Traffic Engineering (TE)
            Requirements", RFC 4216, November 2005.
 [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
            Computation Element (PCE)-Based Architecture", RFC 4655,
            August 2006.
 [RFC5152]  Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
            Per-Domain Path Computation Method for Establishing
            Inter-Domain Traffic Engineering (TE) Label Switched Paths
            (LSPs)", RFC 5152, February 2008.
 [RFC5298]  Takeda, T., Ed., Farrel, A., Ed., Ikejiri, Y., and JP.
            Vasseur, "Analysis of Inter-Domain Label Switched Path
            (LSP) Recovery", RFC 5298, August 2008.
 [RFC5441]  Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
            "A Backward-Recursive PCE-Based Computation (BRPC)
            Procedure to Compute Shortest Constrained Inter-Domain
            Traffic Engineering Label Switched Paths", RFC 5441, April
            2009.
 [RSVP-PKS] Bradford, R., Vasseur, JP., and A. Farrel, "RSVP
            Extensions for Path Key Support", Work in Progress,
            February 2008.

Bradford, et al. Standards Track [Page 18] RFC 5520 Preserving Topology Confidentiality April 2009

Acknowledgements

 The authors would like to thank Eiji Oki, Ben Campbell, and Ross
 Callon for their comments on this document.

Authors' Addresses

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

Bradford, et al. Standards Track [Page 19]

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