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Versions: (draft-takeda-ccamp-inter-domain-recovery-analysis) 00 01 02 03 04 05 RFC 5298

Network Working Group                                   Tomonori Takeda
Internet Draft                                                      NTT
Proposed Status: Informational                           Yuichi Ikejiri
Expires: June 2007                                   NTT Communications
                                                          Adrian Farrel
                                                     Old Dog Consulting
                                                  Jean-Philippe Vasseur
                                                    Cisco Systems, Inc.

                                                          December 2006


        Analysis of Inter-domain Label Switched Path (LSP) Recovery
          draft-ietf-ccamp-inter-domain-recovery-analysis-00.txt


Status of this Memo

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Abstract

   This document analyzes various schemes to realize Multiprotocol Label
   Switching (MPLS) and Generalized MPLS (GMPLS) Label Switched Path
   (LSP) recovery in multi-domain networks based on the existing
   framework for multi-domain LSPs.

   The main focus for this document is on establishing end-to-end
   diverse Traffic Engineering (TE) LSPs in multi-domain networks. It
   presents various diverse LSP setup schemes based on existing
   functional elements.


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Table of Contents

   1. Terminology....................................................2
   2. Introduction...................................................3
   2.1 Domain........................................................3
   2.2 Document Scope................................................4
   2.3 Note on Other Recovery Techniques.............................5
   2.4 Signaling Options.............................................5
   3. Diversity in Multi-domain Networks.............................5
   3.1 Multi-domain Network Topology.................................6
   3.2 Note on Domain Diversity......................................7
   4. Factors to Consider............................................7
   4.1 Scalability versus Optimality.................................7
   4.2 Key Concepts..................................................8
   5. Diverse LSP Setup Schemes without Confidentiality.............10
   5.1 Management Configuration.....................................10
   5.2 Head-end Path Computation (with multi-domain visibility).....10
   5.3 Per-domain Path Computation..................................10
   5.3.1 Sequential Path Computation................................11
   5.3.2 Simultaneous Path Computation..............................11
   5.4 Inter-domain Collaborative Path Computation..................12
   5.4.1 Sequential Path Computation................................12
   5.4.2 Simultaneous Path Computation..............................13
   6. Diverse LSP Setup Schemes with Confidentiality................13
   6.1 Management Configuration.....................................14
   6.2 Head-end Path Computation (with multi-domain visibility).....14
   6.3 Per-Domain Path Computation..................................15
   6.3.1 Sequential Path Computation................................15
   6.3.2 Simultaneous Path Computation..............................15
   6.4 Inter-domain Collaborate Path Computation....................16
   6.4.1 Sequential Path Computation................................16
   6.4.2 Simultaneous Path Computation..............................16
   7. Network Topology Specific Considerations......................17
   8. Addressing Considerations.....................................17
   9. Note on SRLG Diversity........................................17
   10. Manageability Considerations.................................17
   11. Security Considerations......................................18
   12. References...................................................18
   12.1 Normative References........................................18
   12.2 Informative References......................................18
   13. Acknowledgments..............................................20
   14. Author's Addresses...........................................20
   15. Intellectual Property Consideration..........................20
   16. Full Copyright Statement.....................................21

1. Terminology




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   The reader is assumed to be familiar with the terminology in [inter-
   domain-fw] that provides a framework for inter-domain Label Switched
   Path (LSP) setup, and [RFC4427] that provides terminology for LSP
   recovery.

   The following are several key terminologies used within this
   document.

   - Domain: See [inter-domain-fw]. A domain is considered to be any
     collection of network elements within a common sphere of address
     management or path computational responsibility. Note that nested
     domains continue to be out of scope.

   - Working LSP: See [RFC4427]. The working LSP transports normal user
     traffic. Note that the term LSP and TE LSP will be used
     interchangeably.

   - Recovery LSP: See [RFC4427]. The recovery LSP transports normal
     user traffic when the working LSP fails. The recovery LSP may
     transport user traffic even when the working LSP is transporting
     normal user traffic (e.g., 1+1 protection). In such a scenario,
     the recovery LSP is sometimes referred to as a protecting LSP.

   - Diversity: See [inter-domain-fw]. Diversity means the relationship
     of multiple LSPs, where those LSPs do not share some specific type
     of resource (e.g., link, node, or shared risk link group (SRLG)).
     Diverse LSPs may be used for various purposes, such as load-
     balancing and recovery. In this document, the recovery aspect of
     diversity, specifically the end-to-end diversity of two traffic
     engineering (TE) LSPs, is the focus. Those two diverse LSPs are
     referred to as the working LSP and recovery LSP hereafter.
     Sometimes, diversity is referred to as disjointness.

   - Confidentiality: See [RFC4216]. The term confidentiality applies
     to the protection of information about the topology and resources
     present within one domain from visibility by people or
     applications outside that domain.

2. Introduction

2.1 Domain

   As defined in [inter-domain-fw], a domain is considered to be any
   collection of network elements within a common sphere of address
   management or path computational responsibility. Examples of such
   domains include IGP areas and Autonomous Systems. A network accessed
   over the Generalized Multiprotocol Label Switching (GMPLS) User-to-
   Network Interface (UNI) [RFC4208] and a Layer One Virtual Private



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   Network (L1VPNs) [L1VPN-FW] are special cases of multi-domain
   networks.

   Example objectives of domain usage include administrative separation,
   scalability, and forming protection domains.

   As described in [inter-domain-fw], there could be TE parameters (such
   as color and priority) whose meanings are specific to each domain. In
   such a scenarios, mapping functions could be performed based on
   policy agreements between domain administrators.

2.2 Document Scope

   This document analyzes various schemes to realize Multiprotocol Label
   Switching (MPLS) and Generalized MPLS (GMPLS) LSP recovery in multi-
   domain networks based on the existing framework for multi-domain LSP
   setup [inter-domain-fw].

   There are various recovery techniques for LSPs. For TE LSPs, major
   techniques are end-to-end recovery [e2e-recovery], local protection
   such as Fast Reroute (FRR) [RFC4090] (in packet switching
   environments), and segment recovery [seg-recovery] (in GMPLS).

   In this version of the document the main focus is the analysis of
   diverse TE LSP (hereafter LSP) setup schemes, which can
   advantageously used in the context of end-to-end recovery. This
   document presents various diverse LSP setup schemes by combining
   various functional elements. Analysis of other recovery techniques
   could be added in a later revision of this document if necessary.
   Furthermore, details of maintenance of diverse LSPs, such as re-
   optimization and OAM, are for further study.

   Note that the comparative evaluation of these various schemes is out
   of scope for this document, and should be described in separate
   applicability documents.

   [RFC4105] and [RFC4216] describe requirements for diverse LSPs. There
   could be various types of diversity, and this document focuses on
   node/link/SRLG diversity. Note that domain diversity (that is, the
   selection of paths that have only the ingress and egress domains in
   common) is discussed in section 3.2.

   Based on the service grade, both the working LSP and the recovery LSP
   may be established at the time of service establishment (e.g.,
   service requiring high resiliency). Alternatively, the recovery LSP
   may be added later due to a change in the grade of the service.

   Also, the recovery LSP may be used for 1+1 protection, 1:1
   protection, or shared mesh restoration. However, ways to compute


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   diverse paths, and the signaling of these TE LSPs are common across
   all uses, and these topics are the main scope of this document.

   Section 5 of [inter-domain-fw] describes some analysis of diverse
   LSPs in multi-domain networks, and this document provides more
   detailed analysis based on that content.

   Note that diverse LSPs may be used for various purposes, in addition
   to recovery. An example is for load-balancing, so as to limit the
   traffic disruption to a portion of the traffic flow in case of a
   single network element failure. This document does not preclude use
   of diverse LSP setup schemes for those purposes.

2.3 Note on Other Recovery Techniques

   Fast Reroute and segment recovery in multi-domain networks are
   described in section 5.4 of [inter-domain-fw], and a more detailed
   analysis is provided in section 5 of [inter-domain-rsvp]. Additional
   analysis may be added in a future revision of this document if
   necessary.

   Also, the recovery type of an LSP or service may change at a domain
   boundary. That is, the recovery type would remain the same within one
   domain, but might be different in the next domain. This may be due to
   the capabilities of each domain, administrative policies, or to
   topology limitations. An example is where protection at the domain
   boundary is provided by link protection on the inter-domain link(s),
   but where protection within each domain is achieved through segment
   recovery. This mixture of protection techniques is for further study.

2.4 Signaling Options

   There are three signaling options for establishing inter-domain TE
   LSPs: nesting, contiguous LSPs, and stitching [inter-domain-fw]. The
   description in this document of diverse LSP setup is agnostic in
   relation to the signaling option used, unless otherwise specified.

   Note that signaling option specific considerations for Fast Reroute
   and segment recovery are described in section 5 of [inter-domain-
   rsvp]. Also note that if the recovery type changes at the domain
   boundary, the nesting and stitching options may be more suitable.
   Details are for further study.

3. Diversity in Multi-domain Networks

   As described in section 2.2, analysis of diverse LSP setup schemes in
   multi-domain networks is the main focus of this document. This
   section describes some assumptions in this problem space made in this
   document.


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3.1 Multi-domain Network Topology

   Figures 1 and 2 show example multi-domain network topologies. In
   Figure 1, domains are connected in a linear topology. There are
   multiple paths between nodes A and L, but all of them cross domain#1-
   domain#2-domain#3 in that order.

           +--Domain#1--+   +--Domain#2--+   +--Domain#3--+
           |            |   |            |   |            |
           |     B------+---+---D-----E--+---+------J     |
           |    /       |   |    \   /   |   |       \    |
           |   /        |   |     \ /    |   |        \   |
           |  A         |   |      H     |   |         L  |
           |   \        |   |     / \    |   |        /   |
           |    \       |   |    /   \   |   |       /    |
           |     C------+---+---F-----G--+---+------K     |
           |            |   |            |   |            |
           +------------+   +------------+   +------------+

                    Figure 1: Linear Connectivity

                           +-----Domain#2-----+
                           |                  |
                           | E--------------F |
                           | |              | |
                           +-+--------------+-+
                             |              |
                  +-Domain#1-+-+  +---------+--+
                  |          | |  |         |  |
                  |  A-------B-+--+-C-------D  |
                  |  |         |  | |          |
                  +--+---------+  +-+-Domain#4-+
                     |              |
                   +-+--------------+-+
                   | |              | |
                   | G--------------H |
                   |                  |
                   +-----Domain#3-----+

                     Figure 2: Mesh Connectivity

   In Figure 2, domains are connected in a mesh topology. There are
   multiple paths between nodes A and D, and these paths do not
   necessarily follow the same set of domains.

   Indeed, if inter-domain connectivity forms a mesh, domain level
   routing is required (even for the unprotected case). This is tightly
   coupled with the next hop domain resolution/discovery mechanisms.


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   In this version of the document, we assume that domain level routing
   is given via configuration or policy, and this is not part of path
   computation process described later in this document. Details on more
   advanced domain level routing are for further study.

   In addition, domain level routing may perform "domain re-entry",
   where a path enters a domain after the path exits that domain (e.g.,
   domain#X-domain#Y-domain#X). This requires specific considerations
   when confidentiality is required (described in section 4.2), and is
   for further study.

   Furthermore, the working LSP and the recovery LSP may or may not be
   routed along the same set of domains in the same order. In this
   version of the document, we assume that the working LSP and recovery
   LSP follow the same set of domains in the same order (via
   configuration or policy). Details on other scenarios are for further
   study.

3.2 Note on Domain Diversity

   As described in section 2.2, domain diversity means the selection of
   paths that have only the ingress and egress domains in common. This
   may provide enhanced resilience against failures, and is a way to
   ensure path diversity for most of the path of the LSP.

   In section 3.1 we assumed that the working LSP and the recovery LSP
   follow the same set of domains in the same order. Under this
   assumption, domain diversity cannot be achieved. However, by relaxing
   this assumption, domain diversity could be achieved, e.g., by either
   of the following schemes.

   - Consider domain diversity as a special case of SRLG diversity
     (i.e., assign an SRLG ID to each domain)
   - Configure domain level routing to provide domain diverse paths
     (e.g., by means of AS_PATH in BGP)

   Details are for further study, should it been considered as a
   requirement.

4. Factors to Consider

4.1 Scalability versus Optimality

   As described in [inter-domain-fw], scalability and optimality are two
   conflicting objectives. Note that the meaning of optimality differs
   depending on each network operation. Some examples of optimality in
   the context of diverse LSPs are:



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   - Minimizing the sum of their cost while maintaining diversity
   - Restricting the difference of their cost (so as to minimize delay
     difference after switch-over) while maintaining diversity

   By restricting TE information distribution to only within each domain
   (and not across domain boundaries) as required by RFC4105 and
   RFC4216, it may not be possible to compute an optimal path. As such,
   it may not be possible to compute diverse paths, even if they exist.
   However, if we assume domain level routing is given (as discussed in
   section 3), it is possible to compute diverse paths in some schemes
   if such paths exist. This is discussed in section 5.

4.2 Key Concepts

   Three key concepts to classify various diverse LSP setup schemes are
   presented below.

   o With or without confidentiality

     - Without confidentiality

       Under this network configuration, it is possible to specify (by
       means of the Explicit Route Object - ERO comprising a list of
       strict hops) or obtain (by means of the recorded Route Object -
       RRO) a route across other domains.

       Examples of this configuration are multi-area networks, and some
       forms of multi-AS networks (especially within the same provider).

     - With confidentiality

       Under this network configuration, it is not possible to specify
       or obtain a route (by means of ERO/RRO) across other domains.
       Paths may be specified/obtained in the form of ERO/RRO containing
       loose hops. Therefore, it is not possible to specify or obtain a
       full route at the head-end of a multi-domain LSP.

       Examples of this configuration are some forms of multi-AS
       networks (especially inter-provider networks), GMPLS-UNI
       networks, and L1VPNs.

   o Per domain path computation or inter-domain collaborative path
     computation

     - Per domain path computation

       In this scheme, a path is computed domain by domain as LSP
       signaling progresses through the network. This scheme requires
       ERO expansion in each domain. The case for unprotected LSPs under


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       this scheme is covered in [inter-domain-pd].

     - Inter-domain collaborative path computation

       In this scheme, path computation is typically done before
       signaling. This scheme typically uses communication between
       cooperating path computation elements (PCEs) [PCE-arch]. The case
       for unprotected LSP under this scheme is covered in [brpc].

     Note that these are path computation techniques. It is also
     possible to obtain a path via management configuration, or head-end
     path computation (with multi-domain visibility). This is also
     discussed in sections 5 and 6.

     Note also that it is possible to combine multiple path computation
     techniques (including using a different technique for the working
     and recovery LSPs), but this is for further study and is likely
     to require sequential path computation (see below).

   o Sequential path computation or simultaneous path computation

     - Sequential path computation

       The path of the working LSP is computed (without considering the
       recovery LSP), and then the path of the recovery LSP is computed.
       Typically, this scheme is applicable when the recovery LSP is
       added later as change of the service grade. But this scheme can
       also be applicable when both of the working and recovery LSPs are
       established from the start. In this scheme, diverse LSPs may not
       be correctly computed (without some form of re-optimization or
       recomputation technique) in "trap" topologies. Furthermore, such
       sequential path computation approach may prevent from finding an
       "optimal" solution with regards to a specific objective function.

     - Simultaneous path computation

       The path of the working LSP and the path of the recovery LSP are
       computed simultaneously. Typically, this scheme is applicable
       when both the working LSP and the recovery LSP are established
       together. In this scheme, it is possible to avoid trap
       topologies. Furthermore it may allow for achieving more optimal
       results.

   Note that LSP setup with or without confidentiality is given as a
   per-domain configuration, while the choices of per-domain path
   computation or inter-domain collaborative path computation, and
   sequential path computation or simultaneous path computation may be a
   matter of choice for each individual pair of working/recovery LSPs.



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   The analysis of various diverse LSP setup schemes is described in
   sections 5 and 6, based on above criteria.

   Some other considerations, such as network topology specific
   considerations, addressing considerations, and SRLG diversity are
   described in sections 7, 8 and 9.

5. Diverse LSP Setup Schemes without Confidentiality

   In the following, various schemes for diverse LSP setup are presented
   based on different path computation techniques assuming that there is
   no requirement for confidentiality between domains. Section 6 makes a
   similar examination of schemes where inter-domain confidentiality is
   required.

5.1 Management Configuration

   Section 3.1 of [inter-domain-fw] describes this path computation
   technique. The full explicit paths for the working and recovery LSPs
   are specified by a management application at the head-end, and no
   further computation or signaling specific considerations are needed.

5.2 Head-end Path Computation (with multi-domain visibility)

   Section 3.2.1 of [inter-domain-fw] describes this path computation
   technique. The full explicit paths for the working and recovery LSPs
   are computed at the head-end either by the head-end itself or by a
   PCE. In either case the computing entity has full TE visibility
   across multiple domains and no further computation or signaling
   specific considerations are needed.

5.3 Per-domain Path Computation

   Sections 3.2.2, 3.2.3 and 3.3 of [inter-domain-fw] describe this path
   computation technique. More detailed procedures are described in
   [inter-domain-pd].

   In this scheme, the explicit paths of the working and recovery LSPs
   are specified as the complete strict path in the source domain
   followed by one of the following:

   - The complete list of boundary LSRs (or domain identifiers, e.g., AS
     numbers) along the path.

   - The boundary LSR for the source domain and the LSP destination.

   Thus, ERO expansion is needed at domain boundaries. Path computation
   is performed by, or by a PCE on behalf of, each domain boundary LSR.



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   For establishing diverse LSPs using per-domain computation, there are
   two specific schemes, which are described in sections 5.3.1 and 5.3.2
   respectively.

5.3.1 Sequential Path Computation

   The Exclude Route Object (XRO) [XRO] can be used. Details are as
   follows.

   Assume that the working LSP is established as described in [inter-
   domain-pd]. Also, assume that the route of the working LSP (full
   route) is available at the head-end through the RRO.

   o Path computation aspect

     When performing path computation (ERO expansion) for the recovery
     LSP as it crosses each domain boundary a path is selected that
     avoids the nodes/links/SRLGs used by the working path so that a
     diverse path is obtained.

   o Signaling aspect

     In order that the computation noted above can be performed, each
     computation point must be aware of the path of the working LSP.
     This information can be supplied in the XRO included in the Path
     message for recovery LSP. The XRO lists nodes, links and SRLGs that
     must be avoided by the LSP being signaled, and its contents are
     copied from the RRO of the working LSP.

   This scheme cannot guarantee to establish diverse LSPs (even if they
   could exist) because the first LSP is established without
   consideration of the need for a diverse recovery LSP. Crankback
   [crankback] may be used in combination with this scheme in order to
   improve the possibility of successful diverse LSP setup. Furthermore,
   re-optimization of the working LSP and the recovery LSP may be used
   to achieve fully diverse paths.

   Note that even if a solution is found, the degree of optimality of
   the solution (set of diverse TE LSP) might not be maximized.

5.3.2 Simultaneous Path Computation

   o Path computation aspect

     When signaling the working LSP, the path of not only the working
     LSP, but also the recovery LSP is computed. For example, in
     Figure 1, when node D receives a Path message for the working LSP
     between nodes A and L, node D expands the ERO to reach domain#3. At
     the same time, node D computes a path for the recovery LSP across


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     the same domain from node F to reach domain#3. The entry boundary
     node for the recovery LSP (node F) needs to be specified in the
     Path message for the working LSP. In this example the path for the
     working LSP may be computed by node D as D-E-domain#3, and the path
     for recovery LSP as F-G-domain#3.

   o Signaling aspect

     There must be a mechanism to force the recovery LSP to follow the
     route computed above. One way to realize this is to have a specific
     object (with the same format as the ERO) to collect the route of
     the recovery LSP in the Path message for the working LSP and to
     return is to the head-end in the Resv message. When signaling the
     recovery LSP, the content of the ERO is copied from this object.

   This scheme also cannot guarantee to establish diverse LSPs (even if
   they could exist) if there are more than two domain boundary nodes
   out of each domain. Crankback [crankback] may also be used in
   combination with this scheme to improve the chances of success.

   Note that even if a solution is found, the degree of optimality of
   the solution (set of diverse TE LSP) might not be maximized.

5.4 Inter-domain Collaborative Path Computation

   Section 3.4 of [inter-domain-fw] describes this approach. [brpc]
   provides some more detail. Path computation is performed for each of
   the working and recovery LSPs by the use of inter-PCE communication
   before each LSP is signaled.

   There are two specific schemes for establishing diverse LSPs using
   this scheme, which are described in sections 5.4.1 and 5.4.2.

5.4.1 Sequential Path Computation

   Route exclusion using the XRO [XRO] can be requested in the PCE
   communication protocol (PCEP) [PCEP] and this can be used to compute
   the path of a recovery LSP after the path of the working LSP has been
   determined. Details are as follows.

   The working LSP is computed and may be immediately established as
   described in [brpc]. Assume that the path of the working LSP (full
   route) is available from the RRO.

   o Path computation aspect

     When requesting path computation for the recovery LSP, an XRO is
     included in the PCEP path computation request message (see [PCEP]).
     The content of the XRO is copied from the RRO of the working LSP.


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     The computation request specifies that the full route must be
     returned. Per-domain PCEs may need to be invoked by the first PCE
     that is consulted in order to collaboratively determine the correct
     path for the recovery LSP (just as described in [PCE-arch] and
     [inter-domain-fw] for the computation of a single inter-domain LSP
     by collaborating PCEs), and these PCEs exchange the XRO information
     to ensure that the computed path is diverse from the working LSP.

   o Signaling aspect

     The recovery LSP is signaled by including an ERO whose content is
     copied from the result returned by the PCE.

   This scheme cannot guarantee to establish diverse LSPs (even if they
   exist) because the working LSP may be blocking. In such a scenario,
   re-optimization of the working and recovery LSPs may be used to
   achieve fully diverse paths.

   Note that PCEP [PCEP] does not currently include support for the XRO,
   but that this is planned to be added in a future version.

5.4.2 Simultaneous Path Computation

   o Path computation aspect

     The PCEP SVEC Object allows diverse path computation to be
     requested. It would be possible to extend [brpc] to compute diverse
     paths. Details are for further study.

   o Signaling aspect

     In this scheme the PCE returns the full routes for the working and
     recovery LSPs and they are signaled accordingly.

   This scheme can guarantee to establish diverse LSPs (if they exist),
   assuming domain level routing is given as described in section 3.

   Furthermore, the computed set of TE LSPs may be optimal with respect
   to some objective functions.

6. Diverse LSP Setup Schemes with Confidentiality

   In the context of this section, the term confidentiality applies to
   the protection of information about the topology and resources
   present within one domain from visibility by people or applications
   outside that domain. This includes, but is not limited to, recording
   of LSP routes, in addition to advertisements of TE information. The
   confidentiality does not apply to the protection of user traffic.



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   Diverse LSP setup schemes with confidentiality are similar to ones
   without confidentiality. However, several additional mechanisms are
   needed to preserve confidentiality. Examples of such mechanisms are:

   - Path key: Provide each per-domain segment of the path in advance to
     the domain boundary nodes or to the PCE that computed the path for
     a limited period of time (temporary caching) and identify it in the
     signaled ERO using a path key. When path computation is done by
     PCE, the identify of the PCE containing state for the path may be
     required as well (e.g., PCE I-D). The path key is provided by the
     PCE to the head-end for inclusion in the ERO [conf-segment].

   - LSP segment: Pre-establish each per-domain segments of the path
     using hierarchical LSPs [RFC4206] or LSP stitching segments
     [LSP-stitch] and signal the end-to-end LSP to use these per-domain
     LSPs. This scheme may need additional identifiers (such as LSP IDs)
     in the Path message so that the domain boundary node can identify
     which LSP segment or tunnel to use for the end-to-end LSP.
     Furthermore, this scheme may require communication to pre-establish
     the LSP segments.

   These techniques may be directly applied, or may be applied with
   extensions, depending on specific diverse LSP setup schemes described
   below.

   Note that in the schemes below, the paths of the working and recovery
   LSPs are not impacted by the confidentiality requirements.

6.1 Management Configuration

   It is not possible to obtain or specify the full explicit route for
   either LSP at the head-end due to confidentiality restrictions.
   Therefore, this information cannot be provided to signaling for LSP
   setup.

   Confidentially need not prevent the full computation of inter-domain
   paths and signaling of sufficient information to distinguish the
   paths. However the path information for each domain must be provided
   in a way that does not have meaning to other domains. Example
   mechanisms to preserve confidentiality as described above, include:

   - Path key
   - LSP segment

   Details are for further study.

6.2 Head-end Path Computation (with multi-domain visibility)




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   This scheme is not applicable since multi-domain visibility violates
   confidentiality.

6.3 Per-Domain Path Computation

6.3.1 Sequential Path Computation

   Assume the working LSP is established as described in [inter-domain-
   pd].

   It is not possible to obtain the route of the working LSP from the
   RRO at the head-end due to confidentiality. In order to provide the
   path of the working LSP through each domain to the computation point
   responsible for computing the path of the protection LSP through each
   domain additional mechanisms are needed. Examples of such mechanisms
   are:

   - Information identifying the working LSP is included in the Path
     message for the recovery LSP, and the domain boundary node consults
     the entity which computed the path of the working LSP (i.e., domain
     boundary node or PCE), so that the diverse path can be computed.
     When the entity which computed the path of the working LSP is the
     PCE, PCE needs to be temporarily stateful. An example of such
     information is the Association Object [e2e-recovery].

   Details are for further study.

6.3.2 Simultaneous Path Computation

   In this scheme the intention is to compute the path of the recovery
   LSP at the same time as the working LSP. In order to force the
   recovery LSP to follow the computed path as well as to preserve
   confidentiality, additional mechanisms are needed to communicate the
   computed recovery path from the path of the working LSP (where it was
   computed) to the recovery LSP. Examples of such mechanisms, that must
   continue to preserve confidentiality, are as follows.

   - LSP segment: As described before. The LSP segment for the recovery
     LSP is established domain-by-domain as the working LSP setup
     progresses.

   - Alternatively, information identifying the working LSP is included
     in the Path message for the recovery LSP, and the domain boundary
     node consults the entity which computed the path of the recovery
     LSP (i.e., domain boundary node or PCE), so as to obtain the path
     of the recovery LSP. This requires that the entity which computed
     the path of the recovery LSP is temporarily stateful. An example of
     such information is the Association Object [e2e-recovery].



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   Details are for further study.

6.4 Inter-domain Collaborate Path Computation

6.4.1 Sequential Path Computation

   Assume working LSP is established as described in [brpc].

   It is not possible to obtain RRO of working LSP (full route) at the
   head-end due to confidentiality.

   o Path computation aspect

     In order to obtain the path of the working LSP when computing the
     path of the recovery LSP, additional mechanisms are needed.
     Examples of such mechanisms are:

     - Information identifying the working LSP is included in the PCEP
       message when requesting path computation of the recovery LSP
       should the PCE stateful (temporarily). An example of such
       information is the Association Object [e2e-recovery].

   o Signaling aspect

     The full route for the recovery LSP can not be returned to the
     head-end by PCE because it cannot be collected from the other PCEs
     owing to confidentiality restrictions. In order to force the
     recovery LSP to follow the path computed above, additional
     mechanisms are needed. Examples of such mechanisms are as described
     before:

     - Path key
     - LSP segment

   Details are for further study.

6.4.2 Simultaneous Path Computation

   It is not possible for PCE to return the full route of the working
   LSP and recovery LSP to the head-end due to confidentiality. In order
   to force the working and recovery LSPs to follow the paths computed,
   additional mechanisms are needed. Examples of such mechanisms are as
   described before:

   - Path key: Use this for the working and recovery LSPs.

   Note that the LSP segment approach in section 6 may not be applicable
   here since a path cannot be determined until BRPC procedures are
   completed.


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   Details are for further study.

7. Network Topology Specific Considerations

   In some specific network topologies, diverse LSP setup schemes
   mentioned above could be drastically simplified.

   For example, assume there are only three domains connected linearly,
   and the first and the last domain contain only a single node. Assume
   that we need to establish diverse LSPs from the node in the first
   domain to the node in the last domain. In such a scenario, no BRPC
   procedures are needed, because there is no need for path computation
   in the first and last domains.

8. Addressing Considerations

   All of the above-mentioned schemes are applicable when a single
   address space is used across all domains.

   However, there could be several cases where private addresses are
   used within some of the domains. This case is similar to the one with
   confidentiality, since the ERO and RRO are meaningless even if they
   are available. Details are for further study.

9. Note on SRLG Diversity

   The above-mentioned schemes are applicable when the nodes and links
   in different domain belong to different SRLGs.

   However, there could be several cases where the nodes and links in
   different domain belong to the same SRLG. That is, where SRLGs span
   domain boundaries. In such cases, in order to establish SRLG diverse
   LSPs, several considerations are needed, such as:

   - Record of the SRLGs used by the working LSP
   - Indication of a set of SRLGs to exclude in the computation of the
     recovery LSP's path.

   Furthermore, SRLG IDs may be assigned independently in each domain,
   and might not have global meaning. In such a scenario, some mapping
   functions are necessary, similar to the mapping of other TE
   parameters mentioned in section 2.1.

   Details are for further study.

10. Manageability Considerations

   TBD


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11. Security Considerations

   TBD

12. References

12.1 Normative References

   [inter-domain-fw]   Farrel, A., et al., "A Framework for Inter-Domain
                       MPLS Traffic Engineering", draft-ietf-ccamp-
                       inter-domain-framework, work in progress.

   [RFC4427]           Mannie, E., Ed. and D. Papadimitriou, Ed.,
                       "Recovery (Protection and Restoration)
                       Terminology for Generalized Multi-Protocol Label
                       Switching (GMPLS)", RFC 4427, March 2006.

12.2 Informative References

   [RFC4208]           Swallow, G., et al., "Generalized Multiprotocol
                       Label Switching (GMPLS) User-Network Interface
                       (UNI): Resource ReserVation Protocol-Traffic
                       Engineering (RSVP-TE) Support for the Overlay
                       Model", RFC 4208, October 2005.

   [L1VPN-FW]          Takeda, T., Editor "Framework and Requirements
                       for Layer 1 Virtual Private Networks", draft-
                       ietf-l1vpn-framework, work in progress.

   [PCE-arch]          A. Farrel, JP. Vasseur and J. Ash, "Path
                       Computation Element (PCE) Architecture", draft-
                       ietf-pce-architecture, work in progress.

   [e2e-recovery]      Lang, J., Rekhter, Y., and Papadimitriou, D.
                       (Eds.), "RSVP-TE Extensions in support of End-to-
                       End Generalized Multi-Protocol Label Switching
                       (GMPLS)-based Recovery",
                       draft-ietf-ccamp-gmpls-recovery-e2e-signaling,
                       work in progress.

   [RFC4090]           Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
                       Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
                       May 2005.

   [seg-recovery]      Berger, L., Bryskin, I., Papadimitriou, D., and
                       Farrel, A., "GMPLS Based Segment Recovery",
                       draft-ietf-ccamp-gmpls-segment-recovery, work in
                       progress.


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

   [RFC4216]           Zhang, R., and Vasseur, J.-P., "MPLS Inter-
                       Autonomous System (AS) Traffic Engineering (TE)
                       Requirements", RFC 4216, November 2005

   [inter-domain-rsvp] Ayyangar, A., Vasseur, JP. "Inter-domain MPLS
                       Traffic Engineering - RSVP extensions", draft-
                       ietf-ccamp-inter-domain-rsvp-te, work in
                       progress.

   [XRO]               Lee et al., "Exclude Routes - Extension to
                       RSVP-TE", draft-ietf-ccamp-rsvp-te-exclude-route,
                       work in progress.

   [inter-domain-pd]   Vasseur JP., Ayyangar A., Zhang R., "A
                       per-domain path computation method for computing
                       Inter-domain Traffic Engineering Label Switched
                       Path", draft-ietf-ccamp-inter-domain-pd-path-
                       comp, work in progress.

   [brpc]              Vasseur, JP., Zhang, R., and Bitar, N., "A
                       Backward Recursive PCE-based Computation (BRPC)
                       procedure to compute shortest inter-domain
                       Traffic Engineering Label Switched Path", draft-
                       vasseur-ccamp-brpc, work in progress.

   [PCEP]              Vasseur, J., "Path Computation Element (PCE)
                       communication Protocol (PCEP) - Version 1 -",
                       draft-ietf-pce-pcep, work in progress.

   [conf-segment]      Bradford, R., Vasseur, JP., and Farrel, A.,
                       "Preserving Topology Confidentiality in Inter-
                       Domain Path Computation and Signaling", draft-
                       bradford-pce-path-key, work in progress.

   [crankback]         Farrel, A., et al., "Crankback Signaling
                       Extensions for MPLS Signaling", draft-ietf-
                       ccamp-crankback, work in progress.

   [RFC4206]           Kompella, K. and Y. Rekhter, "Label Switched
                       Paths (LSP) Hierarchy with Generalized Multi-
                       Protocol Label Switching (GMPLS) Traffic
                       Engineering (TE)", RFC 4206, October 2005.

   [LSP-stitch]        Ayyangar, A., and Vasseur, JP., "LSP Stitching


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                       with Generalized MPLS TE", draft-ietf-ccamp-lsp-
                       stitching, work in progress.

13. Acknowledgments

   Authors would like to thank Eiji Oki, Ichiro Inoue and Kazuhiro
   Fujihara for valuable comments.

14. Author's Addresses

   Tomonori Takeda
   NTT Network Service Systems Laboratories, NTT Corporation
   3-9-11, Midori-Cho
   Musashino-Shi, Tokyo 180-8585 Japan
   Email : takeda.tomonori@lab.ntt.co.jp

   Yuichi Ikejiri
   NTT Communications Corporation
   Tokyo Opera City Tower 3-20-2 Nishi Shinjuku, Shinjuku-ku
   Tokyo 163-1421, Japan
   Email: y.ikejiri@ntt.com

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

   Jean-Philippe Vasseur
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough , MA - 01719
   USA
   Email: jpv@cisco.com

15. Intellectual Property Consideration

   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.

   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


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   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.

16. Full Copyright Statement

   Copyright (C) The IETF Trust (2006).

   This document is subject to the rights, licenses and
   restrictions contained in BCP 78, and except as set
   forth therein, the authors retain all their rights.

   This document and the information contained herein are provided
   on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY,
   THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE
   DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT
   NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
   OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.



























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