





Network Working Group                                     T. Takeda, Ed.
Request for Comments: 5298                                           NTT
Category: Informational                                   A. Farrel, Ed.
                                                      Old Dog Consulting
                                                              Y. Ikejiri
                                                      NTT Communications
                                                             JP. Vasseur
                                                     Cisco Systems, Inc.
                                                             August 2008


      Analysis of Inter-Domain Label Switched Path (LSP) Recovery

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   Protection and recovery are important features of service offerings
   in Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS)
   networks.  Increasingly, MPLS and GMPLS networks are being extended
   from single domain scope to multi-domain environments.

   Various schemes and processes have been developed to establish Label
   Switched Paths (LSPs) in multi-domain environments.  These are
   discussed in RFC 4726: "A Framework for Inter-Domain Multiprotocol
   Label Switching Traffic Engineering".

   This document analyzes the application of these techniques to
   protection and recovery in multi-domain networks.  The main focus for
   this document is on establishing end-to-end diverse Traffic
   Engineering (TE) LSPs in multi-domain networks.
















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Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................3
      1.2. Domain .....................................................4
      1.3. Document Scope .............................................5
      1.4. Note on Other Recovery Techniques ..........................6
      1.5. Signaling Options ..........................................6
   2. Diversity in Multi-Domain Networks ..............................6
      2.1. Multi-Domain Network Topology ..............................7
      2.2. Note on Domain Diversity ...................................8
   3. Factors to Consider .............................................9
      3.1. Scalability versus Optimality ..............................9
      3.2. Key Concepts ..............................................10
   4. Diverse LSP Setup Schemes without Confidentiality ..............12
      4.1. Management Configuration ..................................12
      4.2. Head-End Path Computation (with Multi-Domain Visibility) ..12
      4.3. Per-Domain Path Computation ...............................12
           4.3.1. Sequential Path Computation ........................13
           4.3.2. Simultaneous Path Computation ......................14
      4.4. Inter-Domain Collaborative Path Computation ...............15
           4.4.1. Sequential Path Computation ........................15
           4.4.2. Simultaneous Path Computation ......................15
   5. Diverse LSP Setup Schemes with Confidentiality .................16
      5.1. Management Configuration ..................................17
      5.2. Head-End Path Computation (with Multi-Domain Visibility) ..17
      5.3. Per-Domain Path Computation ...............................17
           5.3.1. Sequential Path Computation ........................18
           5.3.2. Simultaneous Path Computation ......................19
      5.4. Inter-Domain Collaborative Path Computation ...............20
           5.4.1. Sequential Path Computation ........................20
           5.4.2. Simultaneous Path Computation ......................20
   6. Network Topology Specific Considerations .......................20
   7. Addressing Considerations ......................................21
   8. Note on SRLG Diversity .........................................21
   9. Security Considerations ........................................21
   10. References ....................................................22
      10.1. Normative References .....................................22
      10.2. Informative References ...................................22
   11. Acknowledgements ..............................................25











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1.  Introduction

   Protection and recovery in Multiprotocol Label Switching (MPLS) and
   Generalized MPLS (GMPLS) networks are described in [RFC4428].  These
   are important features for service delivery in MPLS and GMPLS
   networks.

   MPLS and GMPLS networks were originally limited to single domain
   environments.  Increasingly, multi-domain MPLS and GMPLS networks are
   being considered, where 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
   Interior Gateway Protocol (IGP) areas and Autonomous Systems (ASes).

   [RFC4726] provides a framework for inter-domain MPLS and GMPLS
   traffic engineering.  It introduces and discusses the various schemes
   and processes to establish Label Switched Paths (LSPs) in multi-
   domain environments.

   However, protection and recovery introduce additional complexities to
   LSP establishment.  Protection LSPs are generally required to be path
   diverse from the working LSPs that they protect.  Achieving this is
   particularly challenging in multi-domain environments because no
   single path computation or planning point is capable of determining
   path diversity for both paths from one end to the other.

   This document analyzes various schemes to realize MPLS and GMPLS LSP
   recovery in multi-domain networks.  The main focus for this document
   is on establishing end-to-end diverse Traffic Engineering (TE) LSPs
   in multi-domain networks.

1.1.  Terminology

   The reader is assumed to be familiar with the terminology for LSP
   recovery set out in [RFC4427], and with the terms introduced in
   [RFC4726] that provides a framework for inter-domain Label Switched
   Path (LSP) setup.  Key terminology may also be found in [RFC4216]
   that sets out requirements for inter-AS MPLS traffic engineering.

   The following key terms from those sources are used within this
   document.

   - Domain: See [RFC4726].  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.  Section 1.2 provides additional
     details.




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   - 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 also
     carry user traffic even when the working LSP is operating normally
     and transporting the user traffic (e.g., 1+1 protection).  The
     recovery LSP is sometimes referred to as a protecting LSP.

   - Diversity: See [RFC4726].  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)).
     Diversity is also referred to as disjointness.

     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.  The two diverse LSPs are
     referred to as the working LSP and recovery LSP.

   - Confidentiality: See [RFC4216].  Confidentiality refers to the
     protection of information about the topology and resources of one
     domain from visibility by people or applications outside that
     domain.

1.2.  Domain

   In order to fully understand the issues addressed in this document,
   it is necessary to carefully define and scope the term "domain".

   As defined in [RFC4726], 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.  Networks accessed over the GMPLS
   User-to-Network Interface (UNI) [RFC4208], and Layer One Virtual
   Private Networks (L1VPNs) [RFC4847] are special cases of multi-domain
   networks.

   Example motivations for using more than one domain include
   administrative separation, scalability, and the construction of
   domains for the purpose of providing protection.  These latter
   "protection domains" offer edge-to-edge protection facilities for
   spans or segments of end-to-end LSPs.







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   As described in [RFC4726], there could be TE parameters (such as
   color and priority) whose meanings are specific to each domain.  In
   such scenarios, in order to set up inter-domain LSPs, mapping
   functions may be needed to transform the TE parameters based on
   policy agreements between domain administrators.

1.3.  Document Scope

   This document analyzes various schemes to realize MPLS and GMPLS LSP
   recovery in multi-domain networks.  It is based on the existing
   framework for multi-domain LSP setup [RFC4726].  Note that this
   document does not prevent the development of additional techniques
   where appropriate (i.e., additional to the ones described in this
   document).  In other words, this document shows how the existing
   techniques can be applied.

   There are various recovery techniques for LSPs.  For TE LSPs, the
   major techniques are end-to-end recovery [RFC4872], local protection
   such as Fast Reroute (FRR) [RFC4090] (in packet switching
   environments), and segment recovery [RFC4873] (in GMPLS).

   The main focus of this document is the analysis of diverse TE LSP
   setup schemes that can be used in the context of end-to-end recovery.
   The methodology is to show different combinations of functional
   elements such as path computation and signaling techniques.

   [RFC4105] and [RFC4216] describe requirements for diverse LSPs.
   There are various types of diversity, and this document focuses on
   node, link, and shared risk link group (SRLG) diversity.

   Recovery LSPs may be used for 1+1 protection, 1:1 protection, or
   shared mesh restoration.  However, the requirements for path
   diversity, the ways to compute diverse paths, and the signaling of
   these TE LSPs are common across all uses.  These topics are the main
   scope of this document.

   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 node failure.  This document does not preclude use of diverse
   LSP setup schemes for other purposes.

   The following are beyond the scope of this document.

   - Analysis of recovery techniques other than the use of link, node,
     or SRLG diverse LSPs (see Section 1.4).





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   - Details of maintenance of diverse LSPs, such as re-optimization and
     Operations and Maintenance (OAM).

   - Comparative evaluation of LSP setup schemes.

1.4.  Note on Other Recovery Techniques

   Fast Reroute and segment recovery in multi-domain networks are
   described in Section 5.4 of [RFC4726], and a more detailed analysis
   is provided in Section 5 of [RFC5151].  This document does not cover
   any additional analysis for Fast Reroute and segment recovery in
   multi-domain networks.

   The recovery type of an LSP or service may change at a domain
   boundary.  That is, the recovery type could remain the same within
   one domain, but might be different in the next domain or on the
   connections between domains.  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 links, but where protection
   within each domain is achieved through segment recovery.  This
   mixture of protection techniques is beyond the scope of this
   document.

   Domain diversity (that is, the selection of paths that have only the
   ingress and egress domains in common) may be considered as one form
   of diversity in multi-domain networks, but this is beyond the scope
   of this document (see Section 2.2).

1.5.  Signaling Options

   There are three signaling options for establishing inter-domain TE
   LSPs: nesting, contiguous LSPs, and stitching [RFC4726].  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 considerations for Fast Reroute and
   segment recovery are described in [RFC5151].

2.  Diversity in Multi-Domain Networks

   This section describes some assumptions about achieving path
   diversity in multi-domain networks.








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2.1.  Multi-Domain Network Topology

   Figures 1 and 2 show examples of 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 Domain Connectivity


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

   Figure 2: Meshed Domain Connectivity





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   In Figure 2, domains are connected in a mesh topology.  There are
   multiple paths between nodes A and D, and these paths do not cross
   the same domains.  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 used in IP networks.

   In this document, we assume that domain-level routing is given via
   configuration, policy, or some external mechanism, and that this is
   not part of the path computation process described later in this
   document.

   Domain-level routing may also allow "domain re-entry" where a path
   re-enters a domain that it has previously exited (e.g., domain#X-
   domain#Y-domain#X).  This requires specific considerations when
   confidentiality (described in Section 3.2) is required, and is beyond
   the scope of this document.

   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
   document, we assume that the working LSP and recovery LSP follow the
   same set of domains in the same order (via configuration, policy or
   some external mechanism).  That is, we assume that the domain mesh
   topology is reduced to a linear domain topology for each pair of
   working and recovery LSPs.

   In summary,

   - There is no assumption about the multi-domain network topology.
     For example, there could be more than two domain boundary nodes or
     inter-domain links (links connecting a pair of domain boundary
     nodes belonging to different domains).

   - It is assumed that in a multi-domain topology, the sequence of
     domains that the working LSP and the recovery LSP follow must be
     the same and is pre-configured.

   - Domain re-entry is out of scope and is not considered further.

2.2.  Note on Domain Diversity

   As described in Section 1.4, 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.






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   In Section 2.1, 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).

   Further details of the operation of domain diversity are beyond the
   scope of this document.

3.  Factors to Consider

3.1.  Scalability versus Optimality

   As described in [RFC4726], 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:

   - Minimizing the sum of their cost while maintaining diversity.

   - Restricting the difference of their costs (for example, so as to
     minimize the 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 might not be possible to compute diverse paths, even if they
   exist.  However, if we assume domain-level routing is given (as
   discussed in Section 2), it would be possible to compute diverse
   paths using specific computation schemes, if such paths exist.  This
   is discussed further in Section 4.













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3.2.  Key Concepts

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

   o With or without confidentiality

     - Without confidentiality

       It is possible to specify a path across multiple domains in
       signaling (by means of the Resource Reservation Protocol-TE
       (RSVP-TE) Explicit Route Object (ERO)), and to obtain record of
       the inter-domain path used (by means of the RSVP-TE Record Route
       Object (RRO)).  In this case, it is clear that one domain has
       control over the path followed in another domain, and that the
       path actually used in one domain is visible from within another
       domain.

       Examples of this configuration are multi-area networks, and some
       forms of multi-AS networks (especially within the same provider).
       In these cases, there is no requirement for confidentiality.

     - With confidentiality

       Where confidentiality of domain topology and operational policy
       is required, it is not possible to specify or obtain a full path
       across other domains.  Partial paths may be specified and
       reported using domain identifiers or the addresses of domain
       border routers in the EROs and RROs.

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

   o Multi-domain path computation, per-domain path computation, and
     inter-domain collaborative path computation

     - Multi-domain path computation

       If a single network element can see the topology of all domains
       along the path, it is able to compute a full end-to-end path.
       Clearly, this is only possible where confidentiality is not
       required.

       Such a network element might be the head-end Label Switching
       Router (LSR), a Path Computation Element (PCE) [RFC4655], or a
       Network Management System (NMS).  This mode of path computation
       is discussed in Sections 4 and 5.



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     - Per-domain path computation

       The path of an LSP may be computed domain-by-domain as LSP
       signaling progresses through the network.  This scheme requires
       ERO expansion in each domain to construct the next segment of the
       path toward the destination.  The establishment of unprotected
       LSPs in this way is covered in [RFC5152].

     - Inter-domain collaborative path computation

       In this scheme, path computation is typically done before
       signaling and uses communication between cooperating PCEs.  An
       example of such a scheme is Backward Recursive Path Computation
       (BRPC) [BRPC].

       It is possible to combine multiple path computation techniques
       (including using a different technique for the working and
       recovery LSPs), but details are beyond the scope of this
       document.

   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.
       This scheme is applicable when the recovery LSP is added later as
       a change to the service grade, but may also be used when both the
       working and recovery LSPs are established from the start.

       Using this approach, it may not be possible to find diverse paths
       for the LSPs in "trap" topologies.  Furthermore, such sequential
       path computation approaches reduce the likelihood of selecting 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.  In this scheme, it is possible to avoid
       trap conditions and it may be more possible to achieve an optimal
       result.

   Note that LSP setup, with or without confidentiality, depends on per-
   domain configuration.  The choice of per-domain path computation or
   inter-domain collaborative path computation, and the choice between
   sequential path computation or simultaneous path computation can be
   determined for each individual pair of working/recovery LSPs.




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   The analysis of various diverse LSP setup schemes is described in
   Sections 4 and 5, based on the concepts set out above.

   Some other considerations, such as network topology-specific
   considerations, addressing considerations, and SRLG diversity are
   described in Sections 6, 7, and 8.

4.  Diverse LSP Setup Schemes without Confidentiality

   This section examines schemes for diverse LSP setup based on
   different path computation techniques assuming that there is no
   requirement for domain confidentiality.  Section 5 makes a similar
   examination of schemes where domain confidentiality is required.

4.1.  Management Configuration

   [RFC4726] describes this path computation technique where 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 considerations are needed.

4.2.  Head-End Path Computation (with Multi-Domain Visibility)

   Section 3.2.1 [RFC4726] 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
   considerations are needed.

4.3.  Per-Domain Path Computation

   Sections 3.2.2, 3.2.3, and 3.3 of [RFC4726] describe this path
   computation technique.  More detailed procedures are described in
   [RFC5152].

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

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

     - The LSP destination.

   Thus, in order to navigate each domain, the path must be expanded at
   each domain boundary, i.e., per-domain.  This path computation is
   performed by the boundary LSR or by a PCE on its behalf.



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   There are two schemes for establishing diverse LSPs using per-domain
   computation.  These are described Sections 4.3.1 and 4.3.2.

4.3.1.  Sequential Path Computation

   As previously noted, the main issue with sequential path computation
   is that diverse paths cannot be guaranteed.  Where a per-domain path
   computation scheme is applied, the computation of second path needs
   to be aware of the path used by the first path in order that path
   diversity can be attempted.

   The RSVP-TE EXCLUDE_ROUTE Object (XRO) [RFC4874] can be used when the
   second path is signaled to report the details of the first path.  It
   should be noted that the PRIMARY_PATH_ROUTE Object defined in
   [RFC4872] for end-to-end protection is not intended as a path
   exclusion mechanism and should not be used for this purpose.

   The process for sequential path computation is as follows:

     - The working LSP is established using per-domain path computation
       as described in [RFC5152].  The path of the working LSP is
       available at the head-end through the RSVP-TE RRO since domain
       confidentiality is not required.

     - The path of the recovery LSP across the first domain is computed
       excluding the resources used by the working LSP within that
       domain.  If a PCE is used, the resources to be avoided can be
       passed to the PCE using the Exclude Route Object (XRO) extensions
       to the PCE Protocol [PCEP-XRO], [PCEP].

     - The recovery LSP is now signaled across the first domain as
       usual, but the path of the working LSP is also conveyed in an
       RSVP-TE XRO.  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.

     - At each subsequent domain boundary, a segment of the path of the
       recovery LSP can be computed across the new domain excluding the
       resources indicated in the RSVP-TE XRO.

   This scheme cannot guarantee to establish diverse LSPs (even if they
   could exist) because the first (working) LSP is established without
   consideration of the need for a diverse recovery LSP.  It is possible
   to modify the path of the working LSP using the crankback techniques
   [RFC4920] if the setup of the recovery LSP is blocked or if some
   resources are shared.





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   Note that, even if a solution is found, the degree of optimality of
   the solution (i.e., of the set of diverse TE LSPs) might not be
   maximal.

4.3.2.  Simultaneous Path Computation

   Simultaneous path computation gives a better likelihood of finding a
   pair of diverse paths as the diversity requirement forms part of the
   computation process.  In per-domain path computation mechanisms,
   there are several aspects to consider.

   Simultaneous path computation means that the paths of the working and
   recovery LSPs are computed at the same time.  Since we are
   considering per-domain path computation, these two paths must be
   computed at the boundary of each domain.

   The process for simultaneous path computation is as follows:

     - The ingress LSR (or a PCE) computes a pair of diverse paths
       across the first domain.  If a PCE is used, PCEP offers the
       ability to request disjoint paths.

     - The working LSP is signaled across the first domain as usual, but
       must carry with it the requirement for a disjoint recovery LSP
       and the information about the path already computed for the
       recovery LSP across the first domain.  In particular, the domain
       boundary node used by the recovery LSP must be reported.

     - Each domain boundary router, in turn, computes a pair of disjoint
       paths across the next domain.  The working LSP is signaled as
       usual, and the computed path of the recovery LSP is collected in
       the signaling messages.

     - When the working LSP has been set up, the full path of the
       recovery LSP is returned to the head-end LSR in the signaling
       messages for the working LSP.  This allows the head-end LSR to
       signal the recovery LSP using a full path without the need for
       further path computation.

   Note that the signaling protocol mechanisms do not currently exist to
   collect the path of the recovery LSP during the signaling of the
   working LSP.  Definition of protocol mechanisms are beyond the scope
   of this document, but it is believed that such mechanisms would be
   simple to define and implement.

   Note also that the mechanism described is still not able to guarantee
   the selection of diverse paths even where they exist since, when
   domains are multiply interconnected, the determination of diverse



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   end-to-end paths may depend on the correct selection of inter-domain
   links.  Crankback [RFC4920] 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 (i.e., set of diverse TE LSPs) might not be maximal.

4.4.  Inter-Domain Collaborative Path Computation

   Collaborative path computation requires the cooperation between PCEs
   that are responsible for different domains.  This approach is
   described in Section 3.4 of [RFC4726].  Backward recursive path
   computation (BRPC) [BRPC] provides a collaborative path computation
   technique where the paths of LSPs are fully determined by
   communication between PCEs before the LSPs are established.  Two ways
   to use BRPC for diverse LSPs are described in the following sections.

4.4.1.  Sequential Path Computation

   In sequential path computation, the path of the working LSP is
   computed using BRPC as described in [BRPC].  The path of the recovery
   LSP is then computed also using BRPC with the addition that the path
   of the working LSP is explicitly excluded using the XRO route
   exclusion techniques described in [PCEP-XRO].

   In this case, the working LSP could be set up before or after the
   path of the recovery LSP is computed.  In the latter case, the actual
   path of the working LSP as reported in the RSVP-TE RRO should be used
   when computing the path of the recovery LSP.

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

4.4.2.  Simultaneous Path Computation

   In simultaneous path computation, the PCEs collaborate to compute the
   paths of both the working and the recovery LSPs at the same time.
   Since both LSPs are computed in a single pass, the LSPs can be
   signaled simultaneously of sequentially according to the preference
   of the head-end LSR.









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   Collaborative simultaneous path computation is achieved using the
   Synchronization Vector (SVEC) object in the PCE Protocol [PCEP].
   This object allows two computation requests to be associated and made
   dependent.  The coordination of multiple computation requests within
   the BRPC mechanism is not described in [BRPC].  It is believed that
   it is possible to specify procedures for such coordination, but the
   development of new procedures is outside the scope of this document.

   This scheme can guarantee to establish diverse LSPs where they are
   possible, assuming that domain-level routing is pre-determined as
   described in Section 2.  Furthermore, the computed set of TE LSPs can
   be guaranteed to be optimal with respect to some objective functions.

5.  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, and the advertisements of TE information.  The
   confidentiality does not apply to the protection of user traffic.

   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: A path key is used in place of a segment of the path of
       an LSP when the LSP is signaled, when the path of the LSP is
       reported by signaling, or when the LSP's path is generated by a
       PCE.  This allows the exact path of the LSP to remain
       confidential through the substitution of "confidential path
       segments" (CPSs) by these path keys.

       [PCE-PATH-KEY] describes how path keys can be used by PCEs to
       preserve path confidentiality, and [RSVP-PATH-KEY] explains how
       path keys are used in signaling.  Note that if path keys are
       signaled in RSVP-TE EROs they must be expanded so that the
       signaling can proceed.  This expansion normally takes place when
       the first node in the CPS is reached.  The expansion of the path
       key would normally be carried out by the PCE that generated the
       key, and for that reason, the path key contains an identifier of
       the PCE (the PCE-ID).

     - LSP segment: LSP segments can be pre-established across domains
       according to some management policy.  The LSP segments can be
       used to support end-to-end LSPs as hierarchical LSPs [RFC4206] or
       as LSP stitching segments [RFC5150].




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       The end-to-end LSPs are signaled indicating just the series of
       domains or domain border routers.  Upon entry to each domain, an
       existing trans-domain LSP is selected and used to carry the end-
       to-end LSP across the domain.

       Note that although the LSP segments are described as being pre-
       established, they could be set up on demand on receipt of the
       request for the end-to-end LSP at the domain border.

       It is also worth noting that in schemes that result in a single
       contiguous end-to-end LSP (without LSP tunneling or stitching),
       the same concept of LSP segments may apply provided that ERO
       expansion is applied at domain boundaries and that the path of
       the LSP is not reported in the RSVP-TE RRO.

   These techniques may be applied directly or may require protocol
   extensions depending on the 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.

5.1.  Management Configuration

   Although management systems may exist that can determine end-to-end
   paths even in the presence of domain confidentiality, the full paths
   cannot be provided to the head-end LSR for LSP signaling as this
   would break the confidentiality requirements.

   Thus, for LSPs that are configured through management applications,
   the end-to-end path must either be constructed using LSP segments
   that cross the domains, or communicated to the head-end LSR with the
   use of path keys.

5.2.  Head-End Path Computation (with Multi-Domain Visibility)

   It is not possible for the head-end LSR to compute the full end-to-
   end path of an inter-domain LSP when domain confidentiality is in use
   because the LSR will not have any TE information about the other
   domains.

5.3.  Per-Domain Path Computation

   Per-domain path computation for working and recovery LSPs is
   practical with domain confidentiality.  As when there are no
   confidentiality restrictions, we can separate the cases of sequential
   and simultaneous path computation.





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5.3.1.  Sequential Path Computation

   In sequential path computation, we can assume that the working LSP
   has its path computed and is set up using the normal per-domain
   technique as described in [RFC5152].  However, because of
   confidentiality issues, the full path of the working LSP is not
   returned in the signaling messages and is not available to the head-
   end LSR.

   To compute a disjoint path for the recovery LSP, each domain border
   node needs to know the path of the working LSP within the domain to
   which it provides entry.  This is easy for the ingress LSR as it has
   access to the RSVP-TE RRO within first domain.  In subsequent
   domains, the process requires that the path of the working LSP is
   somehow made available to the domain border router as the recovery
   LSP is signaled.  Note that the working and recovery LSPs do not use
   the same border routers if the LSPs are node or SRLG diverse.

   There are several possible mechanisms to achieve this.

     - Path keys could be used in the RRO for the working LSP.  As the
       signaling messages are propagated back towards the head-end LSR,
       each domain border router substitutes a path key for the segment
       of the working LSP's path within its domain.  Thus, the RRO
       received at the head-end LSR consists of the path within the
       initial domain followed by a series of path keys.

       When the recovery LSP is signaled, the path keys can be included
       in the ERO as exclusions.  Each domain border router in turn can
       expand the path key for its domain and know which resources must
       be avoided.  PCEP provides a protocol that can be used to request
       the expansion of the path key from the domain border router used
       by the working LSP, or from some third party such as a PCE.

     - Instead of using path keys, each confidential path segment in the
       RRO of the working LSP could be encrypted by the domain border
       routers.  These encrypted segments would appear as exclusions in
       the ERO for the recovery LSP and could be decrypted by the domain
       border routers.

       No mechanism currently exists in RSVP-TE for this function, which
       would probably assume a domain-wide encryption key.

     - The identity of the working LSP could be included in the XRO of
       the recovery LSP to indicate that a disjoint path must be found.

       This option could require a simple extension to the current XRO
       specification [RFC4874] to allow LSP identifiers to be included.



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       It could also use the Association Object [RFC4872] to identify
       the working LSP.

       This scheme would also need a way for a domain border router to
       find the path of an LSP within its domain.  An efficient way to
       achieve this would be to also include the domain border router
       used by the working LSP in the signaling for the recovery LSP,
       but other schemes based on management applications or stateful
       PCEs might also be developed.

       Clearly, the details of this alternative have not been specified.

5.3.2.  Simultaneous Path Computation

   In per-domain simultaneous path computation the path of the recovery
   LSP is computed at the same time as the working LSP (i.e., as the
   working LSP is signaled).  The computed path of the recovery LSP is
   propagated to the head-end LSR as part of the signaling process for
   the working LSP, but confidentiality must be maintained, so the full
   path cannot be returned.  There are two options as follows.

     - LSP segment: As the signaling of the working LSP progresses and
       the path of the recovery LSP is computed domain-by-domain,
       trans-domain LSPs can be set up for use by the recovery LSP.
       When the recovery LSP is signaled, it will pick up these LSP
       segments and use them for tunneling or stitching.

       This mechanism needs coordination through the management plane
       between domain border routers so that a router on the working
       path can request the establishment of an LSP segment for use by
       the protection path.  This could be achieved through the TE MIB
       modules [RFC3812], [RFC4802].

       Furthermore, when the recovery LSP is signaled it needs to be
       sure to pick up the correct LSP segment.  Therefore, some form of
       LSP segment identifier needs to be reported in the signaling of
       the working LSP and propagated in the signaling of the recovery
       LSP.  Mechanisms for this do not currently exist, but would be
       relatively simple to construct.

       Alternatively, the LSP segments could be marked as providing
       protection for the working LSP.  In this case, the recovery LSP
       can be signaled with the identifier of the working LSP using the
       Association Object [RFC4872] enabling the correct LSP segments to
       be selected.






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     - Path key: The path of the recovery LSP can be determined and
       returned to the head-end LSR just described in Section 4.4.2, but
       each CPS is replaced by a path key.  As the recovery path is
       signaled each path key is expanded, domain-by-domain to achieve
       the correct path.  This requires that the entity that computes
       the path of the recovery LSP (domain border LSR or PCE) is
       stateful.

5.4 Inter-Domain Collaborative Path Computation

   Cooperative collaboration between PCEs is also applicable when domain
   confidentiality is required.

5.4.1.  Sequential Path Computation

   In sequential cooperative path computation, the path of the working
   LSP is determined first using a mechanism such as BRPC.  Since domain
   confidentiality is in use, the path returned may contain path keys.

   When the path of the recovery LSP is computed (which may be before or
   after the working LSP is signaled) the path exclusions supplied to
   the PCE and exchanged between PCEs must use path keys as described in
   [PCEP-XRO].

5.4.2.  Simultaneous Path Computation

   As described in Section 4.4.2, diverse path computation can be
   requested using the PCEP SVEC Object [PCEP], and BRPC could be
   extended for inter-domain diverse path computation.  However, to
   conform to domain confidentiality requirements, path keys must be
   used in the paths returned by the PCEs and signaled by RSVP-TE.

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

6.  Network Topology Specific Considerations

   In some specific network topologies the schemes for setting up
   diverse LSPs could be significantly simplified.

   For example, consider the L1VPN or GMPLS UNI case.  This may be
   viewed as a linear sequence of three domains where the first and last
   domains contain only a single node each.  In such a scenario, no BRPC
   procedures are needed, because there is no need for path computation
   in the first and last domains even if the source and destination
   nodes are multi-homed.





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7.  Addressing Considerations

   All of the schemes described in this document are applicable when a
   single address space is used across all domains.

   There may also be cases where private address spaces are used within
   some of the domains.  This problem is similar to the use of domain
   confidentiality since the ERO and RRO are meaningless outside a
   domain even if they are available, and the problem can be solved
   using the same techniques.

8.  Note on SRLG Diversity

   The schemes described in this document are applicable when the nodes
   and links in different domains belong to different SRLGs, which is
   normally the case.

   However, it is possible that nodes or links in different domains
   belong to the same SRLG.  That is, an SRLG may span domain
   boundaries.  In such cases, in order to establish SRLG diverse LSPs,
   several considerations are needed:

     - 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.

   In this case, there is a conflict between any requirement for domain
   confidentiality, and the requirement for SRLG diversity.  One of the
   requirements must be compromised.

   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 1.2.

9.  Security Considerations

   The core protocols used to achieve the procedures described in this
   document are RSVP-TE and PCEP.  These protocols include policy and
   authentication capabilities as described in [RFC3209] and [PCEP].
   Furthermore, these protocols may be operated using more advanced
   security features such as IPsec [RFC4301] and TLS [RFC4346].

   Security may be regarded as particularly important in inter-domain
   deployments and serious consideration should be given to applying the
   available security techniques, as described in the documents
   referenced above and as set out in [RFC4726].



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   Additional discussion of security considerations for MPLG/GMPLS
   networks can be found in [SECURITY-FW].

   This document does not of itself require additional security measures
   and does not modify the trust model implicit in the protocols used.
   Note, however, that domain confidentiality (that is the
   confidentiality of the topology and path information from within any
   one domain) is an important consideration in this document, and a
   significant number of the mechanisms described in this document are
   designed to preserve domain confidentiality.

10.  References

10.1.  Normative References

   [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.

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

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

   [RFC4726]        Farrel, A., Vasseur, J.-P., and A. Ayyangar, "A
                    Framework for Inter-Domain Multiprotocol Label
                    Switching Traffic Engineering", RFC 4726, November
                    2006.

10.2.  Informative References

   [RFC3812]        Srinivasan, C., Viswanathan, A., and T. Nadeau,
                    "Multiprotocol Label Switching (MPLS) Traffic
                    Engineering (TE) Management Information Base (MIB)",
                    RFC 3812, June 2004.

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

   [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.




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   [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.

   [RFC4208]        Swallow, G., Drake, J., Ishimatsu, H., and Y.
                    Rekhter, "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.

   [RFC4301]        Kent, S. and K. Seo, "Security Architecture for the
                    Internet Protocol", RFC 4301, December 2005.

   [RFC4346]        Dierks, T. and E. Rescorla, "The Transport Layer
                    Security (TLS) Protocol Version 1.1", RFC 4346,
                    April 2006.

   [RFC4428]        Papadimitriou, D., Ed., and E. Mannie, Ed.,
                    "Analysis of Generalized Multi-Protocol Label
                    Switching (GMPLS)-based Recovery Mechanisms
                    (including Protection and Restoration)", RFC 4428,
                    March 2006.

   [RFC4655]        Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
                    Computation Element (PCE)-Based Architecture", RFC
                    4655, August 2006.

   [RFC4802]        Nadeau, T., Ed., and A. Farrel, Ed., "Generalized
                    Multiprotocol Label Switching (GMPLS) Traffic
                    Engineering Management Information Base", RFC 4802,
                    February 2007.

   [RFC4847]        Takeda, T., Ed., "Framework and Requirements for
                    Layer 1 Virtual Private Networks", RFC 4847, April
                    2007.

   [RFC4872]        Lang, J., Ed., Rekhter, Y., Ed., and D.
                    Papadimitriou, Ed., "RSVP-TE Extensions in Support
                    of End-to-End Generalized Multi-Protocol Label
                    Switching (GMPLS) Recovery", RFC 4872, May 2007.

   [RFC4873]        Berger, L., Bryskin, I., Papadimitriou, D., and A.
                    Farrel, "GMPLS Segment Recovery", RFC 4873, May
                    2007.





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   [RFC4874]        Lee, CY., Farrel, A., and S. De Cnodder, "Exclude
                    Routes - Extension to Resource ReserVation
                    Protocol-Traffic Engineering (RSVP-TE)", RFC 4874,
                    April 2007.

   [RFC4920]        Farrel, A., Ed., Satyanarayana, A., Iwata, A.,
                    Fujita, N., and G. Ash, "Crankback Signaling
                    Extensions for MPLS and GMPLS RSVP-TE", RFC 4920,
                    July 2007.

   [RFC5150]        Ayyangar, A., Kompella, K., Vasseur, JP., and A.
                    Farrel, "Label Switched Path Stitching with
                    Generalized Multiprotocol Label Switching Traffic
                    Engineering (GMPLS TE)", RFC 5150, February 2008.

   [RFC5151]        Farrel, A., Ed., Ayyangar, A., and JP. Vasseur,
                    "Inter-Domain MPLS and GMPLS Traffic Engineering --
                    Resource Reservation Protocol-Traffic Engineering
                    (RSVP-TE) Extensions", RFC 5151, February 2008.

   [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.

   [BRPC]           Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le
                    Roux, "A Backward Recursive PCE-Based Computation
                    (BRPC) Procedure to Compute Shortest Inter-Domain
                    Traffic Engineering Label Switched Paths", Work in
                    Progress, April 2008.

   [PCE-PATH-KEY]   Bradford, R., Vasseur, JP., and A. Farrel,
                    "Preserving Topology Confidentiality in Inter-Domain
                    Path Computation Using a Key-Based Mechanism", Work
                    in Progress, May 2008.

   [PCEP]           Vasseur, JP., Ed., and  JL. Le Roux, Ed., "Path
                    Computation Element (PCE) Communication Protocol
                    (PCEP)", Work in Progress, March 2008.

   [PCEP-XRO]       Oki, E., Takeda, T., and A. Farrel, "Extensions to
                    the Path Computation Element Communication Protocol
                    (PCEP) for Route Exclusions", Work in Progress, July
                    2008.






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   [RSVP-PATH-KEY]  Bradford, R., Vasseur, JP., and A. Farrel, "RSVP
                    Extensions for Path Key Support", Work in Progress,
                    May 2008.

   [SECURITY-FW]    Fang, L., Ed., " Security Framework for MPLS and
                    GMPLS Networks", Work in Progress, July 2008.

11.  Acknowledgments

   The authors would like to thank Eiji Oki, Ichiro Inoue, Kazuhiro
   Fujihara, Dimitri Papadimitriou, and Meral Shirazipour for valuable
   comments. Deborah Brungard provided useful advice about the text.

Authors' 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














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Full Copyright Statement

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