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RFC 7926


Problem Statement and Architecture for Information Exchange between Interconnected Traffic-Engineered Networks

Part 3 of 3, p. 44 to 67
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prevText      Top      ToC       Page 44 
7.  Application of the Architecture to the User-Network Interface

   The User-Network Interface (UNI) is an important architectural
   concept in many implementations and deployments of client-server
   networks, especially those where the client and server network have
   different technologies.  The UNI is described in [G.8080], and the
   GMPLS approach to the UNI is documented in [RFC4208].  Other
   GMPLS-related documents describe the application of GMPLS to specific
   UNI scenarios: for example, [RFC6005] describes how GMPLS can support
   a UNI that provides access to Ethernet services.

   Figure 1 of [RFC6005] is reproduced here as Figure 22.  It shows the
   Ethernet UNI reference model, and that figure can serve as an example
   for all similar UNIs.  In this case, the UNI is an interface between
   client network edge nodes and the server network.  It should be noted
   that neither the client network nor the server network need be an
   Ethernet switching network.

   There are three network layers in this model: the client network, the
   "Ethernet service network", and the server network.  The so-called
   Ethernet service network consists of links comprising the UNI links
   and the tunnels across the server network, and nodes comprising the
   client network edge nodes and various server network nodes.  That is,
   the Ethernet service network is equivalent to the abstraction layer
   network, with the UNI links being the physical links between the
   client and server networks, the client edge nodes taking the role of
   UNI Client-side (UNI-C) nodes, and the server edge nodes acting as
   the UNI Network-side (UNI-N) nodes.

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        Client                                            Client
        Network       +----------+    +-----------+       Network
   -------------+     |          |    |           |     +-------------
         +----+ |     |  +-----+ |    |  +-----+  |     | +----+
   ------+    | |     |  |     | |    |  |     |  |     | |    +------
   ------+ EN +-+-----+--+ CN  +-+----+--+  CN +--+-----+-+ EN +------
         |    | |  +--+--|     +-+-+  |  |     +--+-----+-+    |
         +----+ |  |  |  +--+--+ | |  |  +--+--+  |     | +----+
                |  |  |     |    | |  |     |     |     |
   -------------+  |  |     |    | |  |     |     |     +-------------
                   |  |     |    | |  |     |     |
   -------------+  |  |     |    | |  |     |     |     +-------------
                |  |  |  +--+--+ | |  |  +--+--+  |     |
         +----+ |  |  |  |     | | +--+--+     |  |     | +----+
   ------+    +-+--+  |  | CN  +-+----+--+ CN  |  |     | |    +------
   ------+ EN +-+-----+--+     | |    |  |     +--+-----+-+ EN +------
         |    | |     |  +-----+ |    |  +-----+  |     | |    |
         +----+ |     |          |    |           |     | +----+
                |     +----------+    |-----------+     |
   -------------+           Server Networks             +-------------
        Client    UNI                               UNI   Client
        Network <----->                           <-----> Network
                          Scope of This Document

                        Legend:   EN  -  Client Network Edge Node
                                  CN  -  Server Network (Core) Node

                  Figure 22: Ethernet UNI Reference Model

   An issue that is often raised relates to how a dual-homed client
   network edge node (such as that shown at the bottom left-hand corner
   of Figure 22) can make determinations about how they connect across
   the UNI.  This can be particularly important when reachability across
   the server network is limited or when two diverse paths are desired
   (for example, to provide protection).  However, in the model
   described in this network, the edge node (the UNI-C node) is part of
   the abstraction layer network and can see sufficient topology
   information to make these decisions.  If the approach introduced in
   this document is used to model the UNI as described in this section,
   there is no need to enhance the signaling protocols at the GMPLS UNI
   nor to add routing exchanges at the UNI.

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8.  Application of the Architecture to L3VPN Multi-AS Environments

   Serving Layer 3 VPNs (L3VPNs) across a multi-AS or multi-operator
   environment currently provides a significant planning challenge.
   Figure 6 shows the general case of the problem that needs to be
   solved.  This section shows how the abstraction layer network can
   address this problem.

   In the VPN architecture, the CE nodes are the client network edge
   nodes, and the PE nodes are the server network edge nodes.  The
   abstraction layer network is made up of the CE nodes, the CE-PE
   links, the PE nodes, and PE-PE tunnels that are the abstract links.

   In the multi-AS or multi-operator case, the abstraction layer network
   also includes the PEs (maybe Autonomous System Border Routers
   (ASBRs)) at the edges of the multiple server networks, and the PE-PE
   (maybe inter-AS) links.  This gives rise to the architecture shown in
   Figure 23.

   The policy for adding abstract links to the abstraction layer network
   will be driven substantially by the needs of the VPN.  Thus, when a
   new VPN site is added and the existing abstraction layer network
   cannot support the required connectivity, a new abstract link will be
   created out of the underlying network.

       ...........                                     .............
        VPN Site :                                     : VPN Site
        --   --  :                                     :  --   --
       |C1|-|CE| :                                     : |CE|-|C2|
        --  |  | :                                     : |  |  --
            |  | :                                     : |  |
            |  | :                                     : |  |
            |  | :                                     : |  |
            |  | :   --           --     --       --   : |  |
            |  |----|PE|=========|PE|---|PE|=====|PE|----|  |
             --  :  |  |         |  |   |  |     |  |  :  --
       ...........  |  |         |  |   |  |     |  |  ............
                    |  |         |  |   |  |     |  |
                    |  |         |  |   |  |     |  |
                    |  |         |  |   |  |     |  |
                    |  |  -   -  |  |   |  |  -  |  |
                    |  |-|P|-|P|-|  |   |  |-|P|-|  |
                     --   -   -   --     --   -   --

        Figure 23: The Abstraction Layer Network for a Multi-AS VPN

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   It is important to note that each VPN instance can have a separate
   abstraction layer network.  This means that the server network
   resources can be partitioned and that traffic can be kept separate.

   This can be achieved even when VPN sites from different VPNs connect
   at the same PE.  Alternatively, multiple VPNs can share the same
   abstraction layer network if that is operationally preferable.

   Lastly, just as for the UNI discussed in Section 7, the issue of
   dual-homing of VPN sites is a function of the abstraction layer
   network and so is just a normal routing problem in that network.

9.  Scoping Future Work

   This section is provided to help guide the work on this problem.  The
   overarching view is that it is important to limit and focus the work
   on those things that are core and necessary to achieve the main
   function, and to not attempt to add unnecessary features or to
   over-complicate the architecture or the solution by attempting to
   address marginal use cases or corner cases.  This guidance is
   non-normative for this architecture description.

9.1.  Limiting Scope to Only Part of the Internet

   The scope of the use cases and problem statement in this document is
   limited to "some small set of interconnected domains."  In
   particular, it is not the objective of this work to turn the whole
   Internet into one large, interconnected TE network.

9.2.  Working with "Related" Domains

   Starting with this subsection, the intention of this work is to solve
   the TE interconnectivity for only "related" domains.  Such domains
   may be under common administrative operation (such as IGP areas
   within a single AS, or ASes belonging to a single operator) or may
   have a direct commercial arrangement for the sharing of TE
   information to provide specific services.  Thus, in both cases, there
   is a strong opportunity for the application of policy.

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9.3.  Not Finding Optimal Paths in All Situations

   As has been well described in this document, abstraction necessarily
   involves compromises and removal of information.  That means that it
   is not possible to guarantee that an end-to-end path over
   interconnected TE domains follows the absolute optimal (by any
   measure of optimality) path.  This is taken as understood, and future
   work should not attempt to achieve such paths, which can only be
   found by a full examination of all network information across all
   connected networks.

9.4.  Sanity and Scaling

   All of the above points play into a final observation.  This work is
   intended to "bite off" a small problem for some relatively simple use
   cases as described in Section 2.  It is not intended that this work
   will be immediately (or even soon) extended to cover many large
   interconnected domains.  Obviously, the solution should, as far as
   possible, be designed to be extensible and scalable; however, it is
   also reasonable to make trade-offs in favor of utility and

10.  Manageability Considerations

   Manageability should not be a significant additional burden.  Each
   layer in the network model can, and should, be managed independently.

   That is, each client network will run its own management systems and
   tools to manage the nodes and links in the client network: each
   client network link that uses an abstract link will still be
   available for management in the client network as any other link.

   Similarly, each server network will run its own management systems
   and tools to manage the nodes and links in that network just as

   Three issues remain for consideration:

   -  How is the abstraction layer network managed?

   -  How is the interface between the client network and the
      abstraction layer network managed?

   -  How is the interface between the abstraction layer network and the
      server network managed?

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10.1.  Managing the Abstraction Layer Network

   Management of the abstraction layer network differs from the client
   and server networks because not all of the links that are visible in
   the TED are real links.  That is, it is not possible to run
   Operations, Administration, and Maintenance (OAM) on the links that
   constitute the potential of a link.

   Other than that, however, the management of the abstraction layer
   network should be essentially the same.  Routing and signaling
   protocols can be run in the abstraction layer (using out-of-band
   channels for links that have not yet been established), and a
   centralized TED can be constructed and used to examine the
   availability and status of the links and nodes in the network.

   Note that different deployment models will place the "ownership" of
   the abstraction layer network differently.  In some cases, the
   abstraction layer network will be constructed by the operator of the
   server network and run by that operator as a service for one or more
   client networks.  In other cases, one or more server networks will
   present the potential of links to an abstraction layer network run by
   the operator of the client network.  And it is feasible that a
   business model could be built where a third-party operator manages
   the abstraction layer network, constructing it from the connectivity
   available in multiple server networks and facilitating connectivity
   for multiple client networks.

10.2.  Managing Interactions of Abstraction Layer and Client Networks

   The interaction between the client network and the abstraction layer
   network is a management task.  It might be automated (software
   driven), or it might require manual intervention.

   This is a two-way interaction:

   -  The client network can express the need for additional
      connectivity.  For example, the client network may try, and fail,
      to find a path across the client network and may request
      additional, specific connectivity (this is similar to the
      situation with the Virtual Network Topology Manager (VNTM)
      [RFC5623]).  Alternatively, a more proactive client network
      management system may monitor traffic demands (current and
      predicted), network usage, and network "hot spots" and may request
      changes in connectivity by both releasing unused links and
      requesting new links.

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   -  The abstraction layer network can make links available to the
      client network or can withdraw them.  These actions can be in
      response to requests from the client network or can be driven by
      processes within the abstraction layer (perhaps reorganizing the
      use of server network resources).  In any case, the presentation
      of new links to the client network is heavily subject to policy,
      since this is both operationally key to the success of this
      architecture and the central plank of the commercial model
      described in this document.  Such policies belong to the operator
      of the abstraction layer network and are expected to be fully

      Once the abstraction layer network has decided to make a link
      available to the client network, it will install it at the link
      end points (which are nodes in the client network) such that it
      appears and can be advertised as a link in the client network.

   In all cases, it is important that the operators of both networks are
   able to track the requests and responses, and the operator of the
   client network should be able to see which links in that network are
   "real" physical links and which links are presented by the
   abstraction layer network.

10.3.  Managing Interactions of Abstraction Layer and Server Networks

   The interactions between the abstraction layer network and the server
   network are similar to those described in Section 10.2, but there is
   a difference in that the server network is more likely to offer up
   connectivity and the abstraction layer network is less likely to ask
   for it.

   That is, the server network will, according to policy that may
   include commercial relationships, offer the abstraction layer network
   a "set" of potential connectivity that the abstraction layer network
   can treat as links.  This server network policy will include:

   -  how much connectivity to offer

   -  what level of server network redundancy to include

   -  how to support the use of the abstract links

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   This process of offering links from the server network may include a
   mechanism to indicate which links have been pre-established in the
   server network and can include other properties, such as:

   -  link-level protection [RFC4202]

   -  SRLGs and MSRLGs (see Appendix B.1)

   -  mutual exclusivity (see Appendix B.2)

   The abstraction layer network needs a mechanism to tell the server
   network which links it is using.  This mechanism could also include
   the ability to request additional connectivity from the server
   network, although it seems most likely that the server network will
   already have presented as much connectivity as it is physically
   capable of, subject to the constraints of policy.

   Finally, the server network will need to confirm the establishment of
   connectivity, withdraw links if they are no longer feasible, and
   report failures.

   Again, it is important that the operators of both networks are able
   to track the requests and responses, and the operator of the server
   network should be able to see which links are in use.

11.  Security Considerations

   Security of signaling and routing protocols is usually administered
   and achieved within the boundaries of a domain.  Thus, and for
   example, a domain with a GMPLS control plane [RFC3945] would apply
   the security mechanisms and considerations that are appropriate to
   GMPLS [RFC5920].  Furthermore, domain-based security relies strongly
   on ensuring that control-plane messages are not allowed to enter the
   domain from outside.

   In this context, additional security considerations arising from this
   document relate to the exchange of control-plane information between
   domains.  Messages are passed between domains using control-plane
   protocols operating between peers that have predictable relationships
   (for example, UNI-C to UNI-N, between BGP-LS speakers, or between
   peer domains).  Thus, the security that needs to be given additional
   attention for inter-domain TE concentrates on authentication of
   peers; assertion that messages have not been tampered with; and, to a
   lesser extent, protecting the content of the messages from
   inspection, since that might give away sensitive information about
   the networks.  The protocols described in Appendix A, which are
   likely to provide the foundation for solutions to this architecture,

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   already include such protection and also can be run over protected
   transports such as IPsec [RFC6071], Transport Layer Security (TLS)
   [RFC5246], and the TCP Authentication Option (TCP-AO) [RFC5925].

   It is worth noting that the control plane of the abstraction layer
   network is likely to be out of band.  That is, control-plane messages
   will be exchanged over network links that are not the links to which
   they apply.  This models the facilities of GMPLS (but not of
   MPLS-TE), and the security mechanisms can be applied to the protocols
   operating in the out-of-band network.

12.  Informative References

   [G.8080]   International Telecommunication Union, "Architecture for
              the automatically switched optical network", ITU-T
              Recommendation G.8080/Y.1304, February 2012,

              Bryskin, I., Ed., Doonan, W., Beeram, V., Ed., Drake, J.,
              Ed., Grammel, G., Paul, M., Kunze, R., Armbruster, F.,
              Margaria, C., Gonzalez de Dios, O., and D. Ceccarelli,
              "Generalized Multiprotocol Label Switching (GMPLS)
              External Network Network Interface (E-NNI): Virtual Link
              Enhancements for the Overlay Model", Work in Progress,
              draft-beeram-ccamp-gmpls-enni-03, September 2013.

   [RFC2702]  Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
              McManus, "Requirements for Traffic Engineering Over MPLS",
              RFC 2702, DOI 10.17487/RFC2702, September 1999,

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,

   [RFC3473]  Berger, L., Ed., "Generalized Multi-Protocol Label
              Switching (GMPLS) Signaling Resource ReserVation
              Protocol-Traffic Engineering (RSVP-TE) Extensions",
              RFC 3473, DOI 10.17487/RFC3473, January 2003,

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,

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   [RFC3945]  Mannie, E., Ed., "Generalized Multi-Protocol Label
              Switching (GMPLS) Architecture", RFC 3945,
              DOI 10.17487/RFC3945, October 2004,

   [RFC4105]  Le Roux, J.-L., Ed., Vasseur, J.-P., Ed., and J. Boyle,
              Ed., "Requirements for Inter-Area MPLS Traffic
              Engineering", RFC 4105, DOI 10.17487/RFC4105, June 2005,

   [RFC4202]  Kompella, K., Ed., and Y. Rekhter, Ed., "Routing
              Extensions in Support of Generalized Multi-Protocol Label
              Switching (GMPLS)", RFC 4202, DOI 10.17487/RFC4202,
              October 2005, <>.

   [RFC4206]  Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
              Hierarchy with Generalized Multi-Protocol Label Switching
              (GMPLS) Traffic Engineering (TE)", RFC 4206,
              DOI 10.17487/RFC4206, 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, DOI 10.17487/RFC4208,
              October 2005, <>.

   [RFC4216]  Zhang, R., Ed., and J.-P. Vasseur, Ed., "MPLS
              Inter-Autonomous System (AS) Traffic Engineering (TE)
              Requirements", RFC 4216, DOI 10.17487/RFC4216,
              November 2005, <>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364,
              February 2006, <>.

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,

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   [RFC4726]  Farrel, A., Vasseur, J.-P., and A. Ayyangar, "A Framework
              for Inter-Domain Multiprotocol Label Switching Traffic
              Engineering", RFC 4726, DOI 10.17487/RFC4726,
              November 2006, <>.

   [RFC4847]  Takeda, T., Ed., "Framework and Requirements for Layer 1
              Virtual Private Networks", RFC 4847, DOI 10.17487/RFC4847,
              April 2007, <>.

   [RFC4874]  Lee, CY., Farrel, A., and S. De Cnodder, "Exclude Routes -
              Extension to Resource ReserVation Protocol-Traffic
              Engineering (RSVP-TE)", RFC 4874, DOI 10.17487/RFC4874,
              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, DOI 10.17487/RFC4920, 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, DOI 10.17487/RFC5150,
              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, DOI 10.17487/RFC5152, February 2008,

   [RFC5195]  Ould-Brahim, H., Fedyk, D., and Y. Rekhter, "BGP-Based
              Auto-Discovery for Layer-1 VPNs", RFC 5195,
              DOI 10.17487/RFC5195, June 2008,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC5251]  Fedyk, D., Ed., Rekhter, Y., Ed., Papadimitriou, D.,
              Rabbat, R., and L. Berger, "Layer 1 VPN Basic Mode",
              RFC 5251, DOI 10.17487/RFC5251, July 2008,

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   [RFC5252]  Bryskin, I. and L. Berger, "OSPF-Based Layer 1 VPN
              Auto-Discovery", RFC 5252, DOI 10.17487/RFC5252,
              July 2008, <>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305,
              October 2008, <>.

   [RFC5440]  Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,

   [RFC5441]  Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
              "A Backward-Recursive PCE-Based Computation (BRPC)
              Procedure to Compute Shortest Constrained Inter-Domain
              Traffic Engineering Label Switched Paths", RFC 5441,
              DOI 10.17487/RFC5441, April 2009,

   [RFC5523]  Berger, L., "OSPFv3-Based Layer 1 VPN Auto-Discovery",
              RFC 5523, DOI 10.17487/RFC5523, April 2009,

   [RFC5553]  Farrel, A., Ed., Bradford, R., and JP. Vasseur, "Resource
              Reservation Protocol (RSVP) Extensions for Path Key
              Support", RFC 5553, DOI 10.17487/RFC5553, May 2009,

   [RFC5623]  Oki, E., Takeda, T., Le Roux, JL., and A. Farrel,
              "Framework for PCE-Based Inter-Layer MPLS and GMPLS
              Traffic Engineering", RFC 5623, DOI 10.17487/RFC5623,
              September 2009, <>.

   [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <>.

   [RFC6005]  Berger, L. and D. Fedyk, "Generalized MPLS (GMPLS) Support
              for Metro Ethernet Forum and G.8011 User Network Interface
              (UNI)", RFC 6005, DOI 10.17487/RFC6005, October 2010,

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   [RFC6071]  Frankel, S. and S. Krishnan, "IP Security (IPsec) and
              Internet Key Exchange (IKE) Document Roadmap", RFC 6071,
              DOI 10.17487/RFC6071, February 2011,

   [RFC6107]  Shiomoto, K., Ed., and A. Farrel, Ed., "Procedures for
              Dynamically Signaled Hierarchical Label Switched Paths",
              RFC 6107, DOI 10.17487/RFC6107, February 2011,

   [RFC6805]  King, D., Ed., and A. Farrel, Ed., "The Application of the
              Path Computation Element Architecture to the Determination
              of a Sequence of Domains in MPLS and GMPLS", RFC 6805,
              DOI 10.17487/RFC6805, November 2012,

   [RFC6827]  Malis, A., Ed., Lindem, A., Ed., and D. Papadimitriou,
              Ed., "Automatically Switched Optical Network (ASON)
              Routing for OSPFv2 Protocols", RFC 6827,
              DOI 10.17487/RFC6827, January 2013,

   [RFC6996]  Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", BCP 6, RFC 6996, DOI 10.17487/RFC6996,
              July 2013, <>.

   [RFC7399]  Farrel, A. and D. King, "Unanswered Questions in the Path
              Computation Element Architecture", RFC 7399,
              DOI 10.17487/RFC7399, October 2014,

   [RFC7579]  Bernstein, G., Ed., Lee, Y., Ed., Li, D., Imajuku, W., and
              J. Han, "General Network Element Constraint Encoding for
              GMPLS-Controlled Networks", RFC 7579,
              DOI 10.17487/RFC7579, June 2015,

   [RFC7580]  Zhang, F., Lee, Y., Han, J., Bernstein, G., and Y. Xu,
              "OSPF-TE Extensions for General Network Element
              Constraints", RFC 7580, DOI 10.17487/RFC7580, June 2015,

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,

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              Ali, Z., Ed., Swallow, G., Ed., Zhang, F., Ed., and D.
              Beller, Ed., "Resource ReserVation Protocol-Traffic
              Engineering (RSVP-TE) Path Diversity using Exclude Route",
              Work in Progress, draft-ietf-teas-lsp-diversity-05,
              June 2016.

              Zhang, F., Ed., Gonzalez de Dios, O., Ed., Hartley, M.,
              Ali, Z., and C. Margaria, "RSVP-TE Extensions for
              Collecting SRLG Information", Work in Progress,
              draft-ietf-teas-rsvp-te-srlg-collect-06, May 2016.

              Ali, Z., Swallow, G., Filsfils, C., Hartley, M., Kumaki,
              K., and R. Kunze, "Resource ReserVation Protocol-Traffic
              Engineering (RSVP-TE) extension for recording TE Metric of
              a Label Switched Path", Work in Progress,
              draft-ietf-teas-te-metric-recording-04, March 2016.

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Appendix A.  Existing Work

   This appendix briefly summarizes relevant existing work that is used
   to route TE paths across multiple domains.  It is non-normative.

A.1.  Per-Domain Path Computation

   The mechanism for per-domain path establishment is described in
   [RFC5152], and its applicability is discussed in [RFC4726].  In
   summary, this mechanism assumes that each domain entry point is
   responsible for computing the path across the domain but that details
   regarding the path in the next domain are left to the next domain
   entry point.  The computation may be performed directly by the entry
   point or may be delegated to a computation server.

   This basic mode of operation can run into many of the issues
   described alongside the use cases in Section 2.  However, in practice
   it can be used effectively, with a little operational guidance.

   For example, RSVP-TE [RFC3209] includes the concept of a "loose hop"
   in the explicit path that is signaled.  This allows the original
   request for an LSP to list the domains or even domain entry points to
   include on the path.  Thus, in the example in Figure 1, the source
   can be told to use interconnection x2.  Then, the source computes the
   path from itself to x2 and initiates the signaling.  When the
   signaling message reaches Domain Z, the entry point to the domain
   computes the remaining path to the destination and continues the

   Another alternative suggested in [RFC5152] is to make TE routing
   attempt to follow inter-domain IP routing.  Thus, in the example
   shown in Figure 2, the source would examine the BGP routing
   information to determine the correct interconnection point for
   forwarding IP packets and would use that to compute and then signal a
   path for Domain A.  Each domain in turn would apply the same approach
   so that the path is progressively computed and signaled domain by

   Although the per-domain approach has many issues and drawbacks in
   terms of achieving optimal (or, indeed, any) paths, it has been the
   mainstay of inter-domain LSP setup to date.

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A.2.  Crankback

   Crankback addresses one of the main issues with per-domain path
   computation: What happens when an initial path is selected that
   cannot be completed toward the destination?  For example, what
   happens if, in Figure 2, the source attempts to route the path
   through interconnection x2 but Domain C does not have the right TE
   resources or connectivity to route the path further?

   Crankback for MPLS-TE and GMPLS networks is described in [RFC4920]
   and is based on a concept similar to the Acceptable Label Set
   mechanism described for GMPLS signaling in [RFC3473].  When a node
   (i.e., a domain entry point) is unable to compute a path further
   across the domain, it returns an error message in the signaling
   protocol that states where the blockage occurred (link identifier,
   node identifier, domain identifier, etc.) and gives some clues about
   what caused the blockage (bad choice of label, insufficient bandwidth
   available, etc.).  This information allows a previous computation
   point to select an alternative path, or to aggregate crankback
   information and return it upstream to a previous computation point.

   Crankback is a very powerful mechanism and can be used to find an
   end-to-end path in a multi-domain network if one exists.

   On the other hand, crankback can be quite resource-intensive, as
   signaling messages and path setup attempts may "wander around" in the
   network, attempting to find the correct path for a long time.  Since
   (1) RSVP-TE signaling ties up network resources for partially
   established LSPs, (2) network conditions may be in flux, and (3) most
   particularly, LSP setup within well-known time limits is highly
   desirable, crankback is not a popular mechanism.

   Furthermore, even if crankback can always find an end-to-end path, it
   does not guarantee that the optimal path will be found.  (Note that
   there have been some academic proposals to use signaling-like
   techniques to explore the whole network in order to find optimal
   paths, but these tend to place even greater burdens on network

A.3.  Path Computation Element

   The Path Computation Element (PCE) is introduced in [RFC4655].  It is
   an abstract functional entity that computes paths.  Thus, in the
   example of per-domain path computation (see Appendix A.1), both the
   source node and each domain entry point are PCEs.  On the other hand,
   the PCE can also be realized as a separate network element (a server)
   to which computation requests can be sent using the Path Computation
   Element Communication Protocol (PCEP) [RFC5440].

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   Each PCE is responsible for computations within a domain and has
   visibility of the attributes within that domain.  This immediately
   enables per-domain path computation with the opportunity to offload
   complex, CPU-intensive, or memory-intensive computation functions
   from routers in the network.  But the use of PCEs in this way
   does not solve any of the problems articulated in Appendices A.1
   and A.2.

   Two significant mechanisms for cooperation between PCEs have been
   described.  These mechanisms are intended to specifically address the
   problems of computing optimal end-to-end paths in multi-domain

   -  The Backward-Recursive PCE-Based Computation (BRPC) mechanism
      [RFC5441] involves cooperation between the set of PCEs along the
      inter-domain path.  Each one computes the possible paths from the
      domain entry point (or source node) to the domain exit point (or
      destination node) and shares the information with its upstream
      neighbor PCE, which is able to build a tree of possible paths
      rooted at the destination.  The PCE in the source domain can
      select the optimal path.

      BRPC is sometimes described as "crankback at computation time".
      It is capable of determining the optimal path in a multi-domain
      network but depends on knowing the domain that contains the
      destination node.  Furthermore, the mechanism can become quite
      complicated and can involve a lot of data in a mesh of
      interconnected domains.  Thus, BRPC is most often proposed for a
      simple mesh of domains and specifically for a path that will cross
      a known sequence of domains, but where there may be a choice of
      domain interconnections.  In this way, BRPC would only be applied
      to Figure 2 if a decision had been made (externally) to traverse
      Domain C rather than Domain D (notwithstanding that it could
      functionally be used to make that choice itself), but BRPC could
      be used very effectively to select between interconnections x1 and
      x2 in Figure 1.

   -  The Hierarchical PCE (H-PCE) [RFC6805] mechanism offers a parent
      PCE that is responsible for navigating a path across the domain
      mesh and for coordinating intra-domain computations by the child
      PCEs responsible for each domain.  This approach makes computing
      an end-to-end path across a mesh of domains far more tractable.
      However, it still leaves unanswered the issue of determining the
      location of the destination (i.e., discovering the destination
      domain) as described in Section 2.1.  Furthermore, it raises the
      question of who operates the parent PCE, especially in networks
      where the domains are under different administrative and
      commercial control.

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   It should also be noted that [RFC5623] discusses how PCEs are used in
   a multi-layer network with coordination between PCEs operating at
   each network layer.  Further issues and considerations regarding the
   use of PCEs can be found in [RFC7399].

A.4.  GMPLS UNI and Overlay Networks

   [RFC4208] defines the GMPLS User-Network Interface (UNI) to present a
   routing boundary between an overlay (client) network and the server
   network, i.e., the client-server interface.  In the client network,
   the nodes connected directly to the server network are known as edge
   nodes, while the nodes in the server network are called core nodes.

   In the overlay model defined by [RFC4208], the core nodes act as a
   closed system and the edge nodes do not participate in the routing
   protocol instance that runs among the core nodes.  Thus, the UNI
   allows access to, and limited control of, the core nodes by edge
   nodes that are unaware of the topology of the core nodes.  This
   respects the operational and layer boundaries while scaling the

   [RFC4208] does not define any routing protocol extension for the
   interaction between core and edge nodes but allows for the exchange
   of reachability information between them.  In terms of a VPN, the
   client network can be considered as the customer network comprised of
   a number of disjoint sites, and the edge nodes match the VPN CE
   nodes.  Similarly, the provider network in the VPN model is
   equivalent to the server network.

   [RFC4208] is, therefore, a signaling-only solution that allows edge
   nodes to request connectivity across the server network and leaves
   the server network to select the paths for the LSPs as they traverse
   the core nodes (setting up hierarchical LSPs if necessitated by the
   technology).  This solution is supplemented by a number of signaling
   extensions, such as [RFC4874], [RFC5553], [RSVP-TE-EXCL],
   [RSVP-TE-EXT], and [RSVP-TE-METRIC], to give the edge node more
   control over the path within the server network and by allowing the
   edge nodes to supply additional constraints on the path used in the
   server network.  Nevertheless, in this UNI/overlay model, the edge
   node has limited information regarding precisely what LSPs could be
   set up across the server network and what TE services (diverse routes
   for end-to-end protection, end-to-end bandwidth, etc.) can be

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A.5.  Layer 1 VPN

   A Layer 1 VPN (L1VPN) is a service offered by a Layer 1 server
   network to provide Layer 1 connectivity (Time-Division Multiplexing
   (TDM), Lambda Switch Capable (LSC)) between two or more customer
   networks in an overlay service model [RFC4847].

   As in the UNI case, the customer edge has some control over the
   establishment and type of connectivity.  In the L1VPN context, three
   different service models have been defined, classified by the
   semantics of information exchanged over the customer interface: the
   management-based model, the signaling-based (a.k.a. basic) service
   model, and the signaling and routing (a.k.a. enhanced) service model.

   In the management-based model, all edge-to-edge connections are
   set up using configuration and management tools.  This is not a
   dynamic control-plane solution and need not concern us here.

   In the signaling-based (basic) service model [RFC5251], the CE-PE
   interface allows only for signaling message exchange, and the
   provider network does not export any routing information about the
   server network.  VPN membership is known a priori (presumably through
   configuration) or is discovered using a routing protocol [RFC5195]
   [RFC5252] [RFC5523], as is the relationship between CE nodes and
   ports on the PE.  This service model is much in line with GMPLS UNI
   as defined in [RFC4208].

   In the signaling and routing (enhanced) service model, there is an
   additional limited exchange of routing information over the CE-PE
   interface between the provider network and the customer network.  The
   enhanced model considers four different types of service models,
   namely the overlay extension, virtual node, virtual link, and per-VPN
   service models.  All of these represent particular cases of the TE
   information aggregation and representation.

A.6.  Policy and Link Advertisement

   Inter-domain networking relies on policy and management input to
   coordinate the allocation of resources under different administrative
   control.  [RFC5623] introduces a functional component called the VNTM
   for this purpose.

   An important companion to this function is determining how
   connectivity across the abstraction layer network is made available
   as a TE link in the client network.  Obviously, if the connectivity
   is established using management intervention, the consequent client
   network TE link can also be configured manually.  However, if
   connectivity from client edge to client edge is achieved using

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   dynamic signaling, then there is need for the end points to exchange
   the link properties that they should advertise within the client
   network, and in the case of support for more than one client network,
   it will be necessary to indicate which client network or networks can
   use the link.  This capability it provided in [RFC6107].

Appendix B.  Additional Features

   This appendix describes additional features that may be desirable and
   that can be achieved within this architecture.  It is non-normative.

B.1.  Macro Shared Risk Link Groups

   Network links often share fate with one or more other links.  That
   is, a scenario that may cause a link to fail could cause one or more
   other links to fail.  This may occur, for example, if the links are
   supported by the same fiber bundle, or if some links are routed down
   the same duct or in a common piece of infrastructure such as a
   bridge.  A common way to identify the links that may share fate is to
   label them as belonging to a Shared Risk Link Group (SRLG) [RFC4202].

   TE links created from LSPs in lower layers may also share fate, and
   it can be hard for a client network to know about this problem
   because it does not know the topology of the server network or the
   path of the server network LSPs that are used to create the links in
   the client network.

   For example, looking at the example used in Section 4.2.3 and
   considering the two abstract links S1-S3 and S1-S9, there is no way
   for the client network to know whether links C2-C0 and C2-C3 share
   fate.  Clearly, if the client layer uses these links to provide a
   link-diverse end-to-end protection scheme, it needs to know that the
   links actually share a piece of network infrastructure (the server
   network link S1-S2).

   Per [RFC4202], an SRLG represents a shared physical network resource
   upon which the normal functioning of a link depends.  Multiple SRLGs
   can be identified and advertised for every TE link in a network.
   However, this can produce a scalability problem in a multi-layer
   network that equates to advertising in the client network the server
   network route of each TE link.

   Macro SRLGs (MSRLGs) address this scaling problem and are a form of
   abstraction performed at the same time that the abstract links are
   derived.  In this way, links that actually share resources in the
   server network are advertised as having the same MSRLG, rather than
   advertising each SRLG for each resource on each path in the server

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   network.  This saving is possible because the abstract links are
   formulated on behalf of the server network by a central management
   agency that is aware of all of the link abstractions being offered.

   It may be noted that a less optimal alternative path for the abstract
   link S1-S9 exists in the server network (S1-S4-S7-S8-S9).  It would
   be possible for the client network request for C2-C0 connectivity to
   also ask that the path be maximally disjoint from path C2-C3.
   Although nothing can be done about the shared link C2-S1, the
   abstraction layer could make a request to use link S1-S9 in a way
   that is diverse from the use of link S1-S3, and this request could be
   honored if the server network policy allows it.

   Note that SRLGs and MSRLGs may be very hard to describe in the case
   of multiple server networks because the abstraction points will not
   know whether the resources in the various server layers share
   physical locations.

B.2.  Mutual Exclusivity

   As noted in the discussion of Figure 13, it is possible that some
   abstraction layer links cannot be used at the same time.  This arises
   when the potentiality of the links is indicated by the server
   network, but the use of the links would actually compete for server
   network resources.  Referring to Figure 13, this situation would
   arise when both link S1-S3 and link S7-S9 are used to carry LSPs: in
   that case, link S1-S9 could no longer be used.

   Such a situation need not be an issue when client-edge-to-client-edge
   LSPs are set up one by one, because the use of one abstraction layer
   link and the corresponding use of server network resources will cause
   the server network to withdraw the availability of the other
   abstraction layer links, and these will become unavailable for
   further abstraction layer path computations.

   Furthermore, in deployments where abstraction layer links are only
   presented as available after server network LSPs have been
   established to support them, the problem is unlikely to exist.

   However, when the server network is constrained but chooses to
   advertise the potential of multiple abstraction layer links even
   though they compete for resources, and when multiple client-edge-to-
   client-edge LSPs are computed simultaneously (perhaps to provide
   protection services), there may be contention for server network
   resources.  In the case where protected abstraction layer LSPs are
   being established, this situation would be avoided through the use of
   SRLGs and/or MSRLGs, since the two abstraction layer links that
   compete for server network resources must also fate-share across

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   those resources.  But in the case where the multiple client-edge-to-
   client-edge LSPs do not care about fate sharing, it may be necessary
   to flag the mutually exclusive links in the abstraction layer TED so
   that path computation can avoid accidentally attempting to utilize
   two of a set of such links at the same time.


   Thanks to Igor Bryskin for useful discussions in the early stages of
   this work and to Gert Grammel for discussions on the extent of
   aggregation in abstract nodes and links.

   Thanks to Deborah Brungard, Dieter Beller, Dhruv Dhody, Vallinayakam
   Somasundaram, Hannes Gredler, Stewart Bryant, Brian Carpenter, and
   Hilarie Orman for review and input.

   Particular thanks to Vishnu Pavan Beeram for detailed discussions and
   white-board scribbling that made many of the ideas in this document
   come to life.

   Text in Section 4.2.3 is freely adapted from the work of Igor
   Bryskin, Wes Doonan, Vishnu Pavan Beeram, John Drake, Gert Grammel,
   Manuel Paul, Ruediger Kunze, Friedrich Armbruster, Cyril Margaria,
   Oscar Gonzalez de Dios, and Daniele Ceccarelli in [GMPLS-ENNI], for
   which the authors of this document express their thanks.

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   Gert Grammel
   Juniper Networks

   Vishnu Pavan Beeram
   Juniper Networks

   Oscar Gonzalez de Dios

   Fatai Zhang

   Zafar Ali

   Rajan Rao

   Sergio Belotti

   Diego Caviglia

   Jeff Tantsura

   Khuzema Pithewan

   Cyril Margaria

   Victor Lopez

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Authors' Addresses

   Adrian Farrel (editor)
   Juniper Networks


   John Drake
   Juniper Networks


   Nabil Bitar


   George Swallow
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719


   Daniele Ceccarelli
   Via A. Negrone 1/A
   Genova - Sestri Ponente


   Xian Zhang
   Huawei Technologies