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Network Working Group                                          A. Farrel
Internet-Draft                                                  J. Drake
Intended status: Standards Track                        Juniper Networks
Expires: April 21, 2014
                                                                N. Bitar
                                                        Verizon Networks

                                                              G. Swallow
                                                     Cisco Systems, Inc.

                                                           D. Ceccarelli
                                                        October 21, 2013

     Problem Statement and Architecture for Information Exchange
         Between Interconnected Traffic Engineered Networks



   In Traffic Engineered (TE) systems, it is sometimes desirable to
   establish an end-to-end TE path with a set of constraints (such as
   bandwidth) across one or more network from a source to a destination.
   TE information is the data relating to nodes and TE links that is
   used in the process of selecting a TE path.  The availability of TE
   information is usually limited to within a network (such as an IGP
   area) often referred to as a domain.

   In order to determine the potential to establish a TE path through a
   series of connected networks, it is necessary to have available a
   certain amount of TE information about each network.  This need not
   be the full set of TE information available within each network, but
   does need to express the potential of providing TE connectivity. This
   subset of TE information is called TE reachability information.

   This document sets out the problem statement and architecture for the
   exchange of TE information between interconnected TE networks in
   support of end-to-end TE path establishment.  For reasons that are
   explained in the document, this work is limited to simple TE
   constraints and information that determine TE reachability.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute

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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction .................................................  5
   1.1.  What is TE Reachability? ...................................  6
   2.  Overview of Use Cases ........................................  6
   2.1.  Peer Networks ..............................................  6
   2.1.1.  Where is the Destination? ................................  7
   2.2.  Client-Server Networks .....................................  8
   2.3.  Dual-Homing ................................................ 10
   3.  Problem Statement ............................................ 11
   3.1.  Use of Existing Protocol Mechanisms ........................ 12
   3.2.  Policy and Filters ......................................... 12
   3.3.  Confidentiality ............................................ 13
   3.4.  Information Overload ....................................... 13
   3.5.  Issues of Information Churn ................................ 14
   3.6.  Issues of Aggregation ...................................... 15
   3.7.  Virtual Network Topology ................................... 15
   4.  Existing Work ................................................ 17
   4.1.  Per-Domain Path Computation ................................ 17
   4.2.  Crankback .................................................. 18
   4.3.  Path Computation Element ................................... 18
   4.4.  GMPLS UNI and Overlay Networks ............................. 20
   4.5.  Layer One VPN .............................................. 20
   4.6.  VNT Manager and Link Advertisement ......................... 21
   4.7.  What Else is Needed and Why? ............................... 22
   5.  Architectural Concepts ....................................... 22
   5.1.  Basic Components ........................................... 22
   5.1.1.  Peer Interconnection ..................................... 22
   5.1.2.  Client-Server Interconnection ............................ 23
   5.2.  TE Reachability ............................................ 24
   5.3.  Abstraction not Aggregation ................................ 25
   5.3.1.  Abstract Links ........................................... 25
   5.3.2.  The Abstraction Layer Network ............................ 26
   5.3.3.  Abstraction in Client-Server Networks..................... 27
   5.3.4.  Abstraction in Peer Networks ............................. 32
   5.4.  Considerations for Dynamic Abstraction ..................... 34
   5.5.  Requirements for Advertising Abstracted Links and Nodes .... 34
   6.  Building on Existing Protocols ............................... 34
   6.1.  BGP-LS ..................................................... 34
   6.2.  IGPs ....................................................... 34
   6.3.  RSVP-TE .................................................... 35
   7. Applicability to Optical Domains and Networks ................. 35
   8.  Abstraction in L3VPN Multi-AS Environments ................... 39
   9.  Scoping Future Work .......................................... 39
   9.1.  Not Solving the Internet ................................... 39
   9.2.  Working With "Related" Domains ............................. 39
   9.3.  Not Breaking Existing Protocols ............................ 39
   9.4.  Sanity and Scaling ......................................... 39

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   10.  Manageability Considerations ................................ 40
   11.  IANA Considerations ......................................... 40
   12.  Security Considerations ..................................... 40
   13.  Acknowledgements ............................................ 40
   14.  References .................................................. 40
   14.1.  Informative References .................................... 40
   Authors' Addresses ............................................... 44
   Appendix A.  Editor's Notes ...................................... 44

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1.  Introduction

   Traffic Engineered (TE) systems such as MPLS-TE [RFC2702] and GMPLS
   [RFC3945] offer a way to establish paths through a network in a
   controlled way that reserves network resources on specified links.
   TE paths are computed by examining the Traffic Engineering Database
   (TED) and selecting a sequence of links and nodes that are capable of
   meeting the requirements of the path to be established.  The TED is
   constructed from information distributed by the IGP running in the
   network, for example OSPF-TE [RFC3630] or ISIS-TE [RFC5305].

   It is sometimes desirable to establish an end-to-end TE path that
   crosses more than one network or administrative domain as described
   in [RFC4105] and [RFC4216].  In these cases, the availability of TE
   information is usually limited to within each network.  Such networks
   are often referred to as Domains [RFC4726] and we adopt that
   definition in this document: viz.

     For the purposes of this document, 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.

   In order to determine the potential to establish a TE path through a
   series of connected domains and to choose the appropriate domain
   connection points through which to route a path, it is necessary to
   have available a certain amount of TE information about each domain.
   This need not be the full set of TE information available within each
   domain, but does need to express the potential of providing TE
   connectivity.  This subset of TE information is called TE
   reachability information.  The TE reachability information can be
   exchanged between domains based on the information gathered from the
   local routing protocol, filtered by configured policy, or statically

   This document sets out the problem statement and architecture for the
   exchange of TE information between interconnected TE domains in
   support of end-to-end TE path establishment.  The scope of this
   document is limited to the simple TE constraints and information
   (TE metrics, hop count, bandwidth, delay, shared risk) necessary to
   determine TE reachability: discussion of multiple additional
   constraints that might qualify the reachability can significantly
   complicate aggregation of information and the stability of the
   mechanism used to present potential connectivity as is explained in
   the body of this document.

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1.1.  What is TE Reachability?

   In an IP network, reachability is the ability to deliver a packet to
   a specific address or prefix.  That is, the existence of an IP path
   to that address or prefix.  TE reachability is the ability to reach a
   specific address along a TE path.

   TE reachability may be unqualified (there is a TE path, but no
   information about available resources or other constraints is
   supplied) which is helpful especially in determining a path to a
   destination that lies in an unknown domain, or may be qualified by TE
   attributes such as TE metrics, hop count, available bandwidth, delay,
   shared risk, etc.

2.  Overview of Use Cases

2.1.  Peer Networks

   The peer network use case can be most simply illustrated by the
   example in Figure 1.  A TE path is required between the source (Src)
   and destination (Dst), that are located in different domains.  There
   are two points of interconnection between the domains, and selecting
   the wrong point of interconnection can lead to a sub-optimal path, or
   even fail to make a path available.

   For example, when Domain A attempts to select a path, it may
   determine that adequate bandwidth is available on from Src through
   both interconnection points x1 and x2.  It may pick the path through
   x1 for local policy reasons: perhaps the TE metric is smaller.
   However, if there is no connectivity in Domain Z from x1 to Dst, the
   path cannot be established.  Techniques such as crankback (see
   Section 4.2) may be used to allieviate this situation, but do not
   lead to rapid setup or guaranteed optimality.  Furthermore RSVP
   signalling creates state in the network that is immediately removed
   by the crankback procedure. Frequent events of such a kind impact
   scalability in a non-deterministic manner.

     --------------      --------------
    | Domain A     | x1 |     Domain Z |
    |              +----+              |
    |  -----       |    |       -----  |
    | | Src |      |    |      | Dst | |
    |  -----       +----+       -----  |
    |              | x2 |              |
     --------------      --------------

        Figure 1 : Peer Networks

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   There are countless more complicated examples of the problem of peer
   networks.  Figure 2 shows the case where there is a simple mesh of
   domains.  Clearly, to find a TE path from Src to Dst, Domain A must
   not select a path leaving through interconnect x1 since Domain B has
   no connectivity to Domain Z.  Furthermore, in deciding whether to
   select interconnection x2 (through Domain C) or interconnection x3
   though Domain D, Domain A must be sensitive to the TE connectivity
   available through each of Domains C and D, as well the TE
   connectivity from each of interconnections x4 and x5 to Dst within
   Domain Z.

                      |     Domain B |
                      |              |
                      |              |
    --------------/                       --------------
   | Domain A     |                      |     Domain Z |
   |              |    --------------    |              |
   |  -----       | x2|     Domain C | x4|       -----  |
   | | Src |      +---+              +---+      | Dst | |
   |  -----       |   |              |   |       -----  |
   |              |    --------------    |              |
    --------------\                      /--------------
                   \x3                  /
                    \                  /
                     \                /x5
                      |     Domain D |
                      |              |
                      |              |

        Figure 2 : Peer Networks in a Mesh

   Of course, many network interconnection scenarios are going to be a
   combination of the situations expressed in these two examples.  There
   may be a mesh of domains, and the domains may have multiple points of

2.1.1.  Where is the Destination?

   A variation of the problems expressed in Section 2.1 arises when the
   source domain (Domain A in both figures) does not know where the

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   destination is located.  That is, when the domain in which the
   destination node is located is not known to the source domain.

   This is most easily seen in consideration of Figure 2 where the
   decision about which interconnection to select needs to be based on
   building a path toward the destination domain.  Yet this can only be
   achieved if it is known in which domain the destination node lies, or
   at least if there is some indication in which direction the
   destination lies.  This function is obviously provided in IP networks
   by inter-domain routing [RFC4271].

2.2.  Client-Server Networks

   Two specific use cases relate to the client-server relationship
   between networks.  These use cases have sometimes been referred to as
   overlay networks.

   The first case, shown is Figure 3, occurs when domains belonging to
   one network are connected by a domain belonging to another network.
   In this scenario, once connections (or tunnels) are formed across the
   lower layer network, the domains of the upper layer network can be
   merged into a single domain by running IGP adjacencies over the
   tunnels, and treating the tunnels as links in the higher layer
   network.  The TE relationship between the domains (higher and lower
   layer) in this case is reduced to determining which tunnels to set
   up, how to trigger them, how to route them, and what capacity to
   assign them.  As the demands in the higher layer network vary, these
   tunnels may need to be modified.

    --------------                         --------------
   | Domain A     |                       |     Domain Z |
   |              |                       |              |
   |  -----       |                       |       -----  |
   | | Src |      |                       |      | Dst | |
   |  -----       |                       |       -----  |
   |              |                       |              |
    --------------\                       /--------------
                   \x1                 x2/
                    \                   /
                     \                 /
                      | Server Domain |
                      |               |
                      |               |

             Figure 3 : Client-Server Networks

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   The second use case relating to client-server networking is for
   Virtual Private Networks (VPNs).  In this case, as opposed to the
   former one, it is assumed that the client network has a different
   address space than that of the server layer where non-overlapping IP
   addresses between the client and the server networks cannot be
   guaranteed.  A simple example is shown in Figure 4.  The VPN sites
   comprise a set of domains that are interconnected over a core domain,
   the provider network.

    --------------                         --------------
   | Domain A     |                       |     Domain Z |
   | (VPN site)   |                       |   (VPN site) |
   |              |                       |              |
   |  -----       |                       |       -----  |
   | | Src |      |                       |      | Dst | |
   |  -----       |                       |       -----  |
   |              |                       |              |
    --------------\                       /--------------
                   \x1                 x2/
                    \                   /
                     \                 /
                      |  Core Domain  |
                      |               |
                      |               |
                     /                 \
                    /                   \
                   /x3                 x4\
    --------------/                       \--------------
   | Domain B     |                       |     Domain C |
   | (VPN site)   |                       |   (VPN site) |
   |              |                       |              |
   |              |                       |              |
    --------------                         --------------

        Figure 4 : A Virtual Private Network

   Note that in the use cases shown in Figures 3 and 4 the client layer
   domains may (and, in fact, probably do) operate as a single connected

   Both use cases in this section become "more interesting" when
   combined with the use case in Section 2.1.  That is, when the
   connectivity between higher layer domains or VPN sites is provided
   by a sequence or mesh of lower layer domains.  Figure 5 shows how
   this might look in the case of a VPN.

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    ------------                                   ------------
   | Domain A   |                                 |   Domain Z |
   | (VPN site) |                                 | (VPN site) |
   |  -----     |                                 |     -----  |
   | | Src |    |                                 |    | Dst | |
   |  -----     |                                 |     -----  |
   |            |                                 |            |
    ------------\                                 /------------
                 \x1                           x2/
                  \                             /
                   \                           /
                    \----------     ----------/
                    | Domain X |x5 | Domain Y |
                    | (core)   +---+ (core)   |
                    |          |   |          |
                    |          +---+          |
                    |          |x6 |          |
                    /----------     ----------\
                   /                           \
                  /                             \
                 /x3                           x4\
    ------------/                                 \------------
   | Domain B   |                                 |   Domain C |
   | (VPN site) |                                 | (VPN site) |
   |            |                                 |            |
    ------------                                   ------------

        Figure 5 : A VPN Supported Over Multiple Server Domains

2.3.  Dual-Homing

   A further complication may be added to the client-server relationship
   described in Section 2.2 by considering what happens when a client
   domain is attached to more than one server domain, or has two points
   of attachment to a server domain.  Figure 6 shows an example of this
   for a VPN.

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                           | Domain A   |
                           | (VPN site) |
    ------------           |  -----     |
   | Domain B   |          | | Src |    |
   | (VPN site) |          |  -----     |
   |            |          |            |
    ------------\           -+--------+-
                 \x1         |        |
                  \        x2|        |x3
                   \         |        |              ------------
                    \--------+-      -+--------     |   Domain Z |
                    | Domain X | x8 | Domain Y | x4 | (VPN site) |
                    | (core)   +----+ (core)   +----+     -----  |
                    |          |    |          |    |    | Dst | |
                    |          +----+          +----+     -----  |
                    |          | x9 |          | x5 |            |
                    /----------      ----------\     ------------
                   /                            \
                  /                              \
                 /x6                            x7\
    ------------/                                  \------------
   | Domain C   |                                  |   Domain D |
   | (VPN site) |                                  | (VPN site) |
   |            |                                  |            |
    ------------                                    ------------

        Figure 6 : Dual-Homing in a Virtual Private Network

3.  Problem Statement

   The problem statement presented in this section is as much about the
   issues that may arise in any solution (and so have to be avoided)
   and the features that are desirable within a solution, as it is about
   the actual problem to be solved.

   The problem can be stated very simply and with reference to the use
   cases presented in the previous section.

     A mechanism is required that allows TE-path computation in one
     domain to make informed choices about the TE-capabilities and exit
     point from the domain when signaling an end-to-end TE path that
     will extend across multiple domains.

   Thus, the problem is one of information collection and presentation,
   not about signaling.  Indeed, the existing signaling mechanisms for

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   TE LSP establishment are likely to prove adequate [RFC4726] with the
   possibility of minor extensions.

   An interesting annex to the problem is how the path is made available
   for use.  For example, in the case of a client-server network, the
   path established in the server network needs to be made available as
   a TE link to provide connectivity in the client network.

3.1.  Use of Existing Protocol Mechanisms

   TE information may currently be distributed in a domain by TE
   extensions to one of the two IGPs as described in OSPF-TE [RFC3630]
   and ISIS-TE [RFC5305].  TE information may be exported from a domain
   (for example, northbound) using link state extensions to BGP

   It is desirable that a solution to the problem described in this
   document does not require the implementation of a new, network-wide
   protocol.  Instead, it would be advantageous to make use of an
   existing protocol that is commonly implemented on routers and is
   currently deployed, or to use existing computational elements such as
   Path Computation Elements (PCEs).  This has many benefits in network
   stability, time to deployment, and operator training.

   It is recognized, however, that existing protocols are unlikely to be
   immediately suitable to this problem space without some protocol
   extensions.  Extending protocols must be done with care and with
   consideration for the stability of existing deployments. In extreme
   cases, a new protocol can be preferable to a messy hack of an
   existing protocol.

3.2.  Policy and Filters

   A solution must be amenable to the application of policy and filters.
   That is, the operator of a domain that is sharing information with
   another domain must be able to apply controls to what information is
   shared.  Furthermore, the operator of a domain that has information
   shared with it must be able to apply policies and filters to the
   received information.

   Additionally, the path computation within a domain must be able to
   weight the information received from other domains according to local
   policy such that the resultant computed path meets the local
   operator's needs and policies rather than those of the operators of
   other domains.

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3.3.  Confidentiality

   A feature of the policy described in Section 3.3 is that an operator
   of a domain may desire to keep confidential the details about its
   internal network topology and loading.  This information could be
   construed as commercially sensitive.

   Although it is possible that TE information exchange will take place
   only between parties that have significant trust, there are also use
   cases (such as the VPN supported over multiple server domains
   described in Section 2.4) where information will be shared between
   domains that have a commercial relationship, but a low level of

   Thus, it must be possible for a domain to limit the information share
   to just that which the computing domain needs to know with the
   understanding that less information that is made available the more
   likely it is that the result will be a less optimal path and/or more
   crankback events.

3.4.  Information Overload

   One reason that networks are partitioned into separate domains is to
   reduce the set of information that any one router has to handle.
   This also applies to the volume of information that routing protocols
   have to distribute.

   Over the years routers have become more sophisticated with greater
   processing capabilities and more storage, the control channels on
   which routing messages are exchanged have become higher capacity, and
   the routing protocols (and their implementations) have become more
   robust.  Thus, some of the arguments in favor of dividing a network
   into domains may have been reduced.  Conversely, however, the size of
   networks continues to grow dramatically with a consequent increase in
   the total amount of routing-related information available.
   Additionally, in this case, the problem space spans two or more

   Any solution to the problems voiced in this document must be aware of
   the issues of information overload.  If the solution was to simply
   share all TE information between all domains in the network, the
   effect from the point of view of the information load would be to
   create one single flat network domain.  Thus the solution must
   deliver enough information to make the computation practical (i.e.,
   to solve the problem), but not so much as to overload the receiving
   domain.  Furthermore, the solution cannot simply rely on the policies
   and filters described in Section 3.2 because such filters might not
   always be enabled.

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3.5.  Issues of Information Churn

   As LSPs are set up and torn down, the available TE resources on links
   in the network change.  In order to reliably compute a TE path
   through a network, the computation point must have an up-to-date view
   of the available TE resources.  However, collecting this information
   may result in considerable load on the distribution protocol and
   churn in the stored information.  In order to deal with this problem
   even in a single domain, updates are sent at periodic intervals or
   whenever there is a significant change in resources, whichever
   happens first.

   Consider, for example, that a TE LSP may traverse ten links in a
   network.  When the LSP is set up or torn down, the resources
   available on each link will change resulting in a new advertisement
   of the link's capabilities and capacity.  If the arrival rate of new
   LSPs is relatively fast, and the hold times relatively short, the
   network may be in a constant state of flux.  Note that the
   problem here is not limited to churn within a single domain, since
   the information shared between domains will also be changing.
   Furthermore, the information that one domain needs to share with
   another may change as the result of LSPs that are contained within or
   cross the first domain but which are of no direct relevance to the
   domain receiving the TE information.

   In packet networks, where the capacity of an LSP is often a small
   fraction of the resources available on any link, this issue is
   partially addressed by the advertising routers.  They can apply a
   threshold so that they do not bother to update the advertisement of
   available resources on a link if the change is less than a configured
   percentage of the total (or alternatively, the remaining) resources.
   The updated information in that case will be disseminated based on an
   update interval rather than a resource change event.

   In non-packet networks, where link resources are physical switching
   resources (such as timeslots or wavelengths) the capacity of an LSP
   may more frequently be a significant percentage of the available link
   resources.  Furthermore, in some switching environments, it is
   necessary to achieve end-to-end resource continuity (such as using
   the same wavelength on the whole length of an LSP), so it is far more
   desirable to keep the TE information held at the computation points
   up-to-date.  Fortunately, non-packet networks tend to be quite a bit
   smaller than packet networks, the arrival rates of non-packet LSPs
   are much lower, and the hold times considerably longer.  Thus the
   information churn may be sustainable.

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3.6.  Issues of Aggregation

   One possible solution to the issues raised in other sub-sections of
   this section is to aggregate the TE information shared between
   domains.  Two aggregation mechanisms are often considered:

   - Virtual node model.  In this view, the domain is aggregated as if
     it was a single node (or router / switch).  Its links to other
     domains are presented as real TE links, but the model assumes that
     any LSP entering the virtual node through a link can be routed to
     leave the virtual node through any other link.

   - Virtual link model.  In this model, the domain is reduced to a set
     of edge-to-edge TE links.  Thus, when computing a path for an LSP
     that crosses the domain, a computation point can see which domain
     entry points can be connected to which other and with what TE

   It is of the nature of aggregation that information is removed from
   the system.  This can cause inaccuracies and failed path computation.
   For example, in the virtual node model there might not actually be a
   TE path available between a pair of domain entry points, but the
   model lacks the sophistication to represent this "limited cross-
   connect capability" within the virtual node.  On the other hand, in
   the virtual link model it may prove very hard to aggregate multiple
   link characteristics: for example, there may be one path available
   with high bandwidth, and another with low delay, but this does not
   mean that the connectivity should be assumed or advertised as having
   both high bandwidth and low delay.

   The trick to this multidimensional problem, therefore, is to
   aggregate in a way that retains as much useful information as
   possible while removing the data that is not needed.  An important
   part of this trick is a clear understanding of what information is
   actually needed.

   It should also be noted in the context of Section 3.5 that changes in
   the information within a domain may have a bearing on what aggregated
   data is shared with another domain.  Thus, while the data shared in
   reduced, the aggregation algorithm (operating on the routers
   responsible for sharing information) may be heavily exercised.

3.7.  Virtual Network Topology

   The terms "virtual topology" and "virtual network topology" have
   become overloaded in a relatively short time.  We draw on [RFC5212]
   and [RFC5623] for inspiration to provide a definition for use in this
   document.  Our definition is based on the fact that a topology at the

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   client network layer is constructed of nodes and links.  Typically,
   the nodes are routers in the client layer, and the links are data
   links.  However, a layered network provides connectivity through the
   lower layer as LSPs, and these LSPs can provide links in the client
   layer.  Furthermore, those LSPs may have been established in advance,
   or might be LSPs that could be set up if required.  This leads to the

     A Virtual Network Topology (VNT) is made up of links in a network
     layer.  Those links may be realized as direct data links or as
     multi-hop connections (LSPs) in a lower network layer.  Those
     underlying LSPs may be established in advance or created on demand.

   The creation and management of a VNT requires interaction with
   management and policy.  Activity is needed in both the client and
   server layer:

   - In the server layer, LSPs need to be set up either in advance in
     response to management instructions or in answer to dynamic
     requests subject to policy considerations.

   - In the server layer, evaluation of available TE resources can lead
     to the announcement of potential connectivity (i.e., LSPs that
     could be set up on demand).

   - In the client layer, connectivity (lower layer LSPs or potential
     LSPs) needs to be announced in the IGP as a normal TE link.  Such
     links may or may not be made available to IP routing: but, they are
     never made available to IP until fully instantiated.

   - In the client layer, requests to establish lower layer LSPs need to
     be made either when links supported by potential LSPs are about to
     be used (i.e., when a higher layer LSP is signalled to cross the
     link, the setup of the lower layer LSP is triggered), or when the
     client layer determines it needs more connectivity or capacity.

   It is a fundamental of the use of a VNT that there is a policy point
   at the point of instantiation of a lower-layer LSP.  At the moment
   that the setup of a lower-layer LSP is triggered, whether from a
   client-layer management tool or from signaling in the client layer,
   the server layer must be able to apply policy to determine whether to
   actually set up the LSP.  Thus, fears that a micro-flow in the client
   layer might cause the activation of 100G optical resources in the
   server layer can be completely controlled by the policy of the server
   layer network's operator (and could even be subject to commercial

   These activities require an architecture and protocol elements as

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   well as management components and policy elements.

4.  Existing Work

   This section briefly summarizes relevant existing work that is used
   to route TE paths across multiple domains.

4.1.  Per-Domain Path Computation

   The per-domain mechanism of 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 of 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 the 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 domain.

   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 set-up to date.

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4.2.  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 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
   RSVP-TE signaling ties up networks resources for partially
   established LSPs, since network conditions may be in flux, and most
   particularly since 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 to find the optimal path. (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 processing.)

4.3.  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 (Section 4.1) the source node
   and each domain entry point is a PCE.  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 has responsibility 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 off-load
   complex, CPU-intensive, or memory-intensive computation functions
   from routers in the network.  But the use of PCE in this way does not
   solve any of the problems articulated in Sections 4.1 and 4.2.

   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
     domain entry point (or source node) to 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 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.

   - Hierarchical PCE (H-PCE) [RFC6805] 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 PCE.  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 2.1.1.  Furthermore, it raises the question of
     who operates the parent PCE especially in networks where the
     domains are under different administrative and commercial control.

   Further issues and considerations of the use of PCE  can be found in

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4.4.  GMPLS UNI and Overlay Networks

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

   [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 cross the core network, and leaves the
   core network to select the paths and set up the core LSPs.  This
   solution is supplemented by a number of signaling extensions such as
   [RFC5553], [I-D.ietf-ccamp-xro-lsp-subobject], and
   [I-D.ietf-ccamp-te-metric-recording] to give the edge node more
   control over the LSP that the core network will set up by exchanging
   information about core LSPs that have been established and by
   allowing the edge nodes to supply additional constraints on the core
   LSPs that are to be set up.

   Nevertheless, in this UNI/overlay model, the edge node has limited
   information of precisely what LSPs could be set up across the core,
   and what TE services (such as diverse routes for end-to-end
   protection, end-to-end bandwidth, etc.) can be supported.

4.5.  Layer One VPN

   A Layer One VPN (L1VPN) is a service offered by a core layer 1
   network to provide layer 1 connectivity (TDM, 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 the connectivity.  In the L1VPN context

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   three different service models have been defined classified by the
   semantics of information exchanged over the customer interface:
   Management Based, Signaling Based (a.k.a. basic), and Signaling and
   Routing service model (a.k.a. enhanced).

   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 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 core 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 enhanced 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: Overlay Extension, Virtual
   Node, Virtual Link and Per-VPN service models.  All of these
   represent particular cases of the TE information aggregation and

4.6.  VNT Manager and Link Advertisement

   As discussed in Section 3.7, operation of a VNT requires policy and
   management input.  In order to handle this, [RFC5623] introduces the
   concept of the Virtual Network Topology Manager.  This is a
   functional component that applies policy to requests from client
   networks (or agents of the client network, such as a PCE) for the
   establishment of LSPs in the server network to provide connectivity
   in the client network.

   The VNT Manager would, in fact, form part of the provisioning path
   for all server network LSPs whether they are set up ahead of client
   network demand or triggered by end-to-end client network LSP

   An important companion to this function is determining how the LSP
   set up across the server network is made available as a TE link in
   the client network.  Obviously, if the LSP is established using
   management intervention, the subsequent client network TE link can
   also be configured manually.  However, if the LSP is signaled
   dynamically 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 a server network that supports more than one client,

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   it will be necessary to indicate which client or clients can use the
   link.  This capability it provided in [RFC6107].

   Note that a potential server network LSP that is advertised as a TE
   link in the client network might to be determined dynamically by
   the edge nodes.  In this case there will need to be some effort to
   ensure that both ends of the link have the same view of the available
   TE resources, or else the advertised link will be asymmetrical.

4.7.  What Else is Needed and Why?

   As can be seen from Sections 4.1 through 4.6, a lot of effort has
   focused on client-server networks as described in Figure 3.  Far less
   consideration has been given to network peering or the combination of
   the two use cases.

   Various work has been suggested to extend the definition of the UNI
   such that routing information can be passed across the interface.
   However, this approach seems to break the architectural concept of
   network separation that the UNI facilitates.

   Other approaches are working toward a flattening of the network with
   complete visibility into the server networks being made available in
   the client network. These approaches, while functional, ignore the
   main reasons for introducing network separation in the first place.

   The remainder of this document introduces a new approach based on
   network abstraction that allows a server network to use its own
   knowledge of its resources and topology combined with its own
   policies to determine what edge-to-edge connectivity capabilities it
   will inform the client networks about.

5.  Architectural Concepts

5.1.  Basic Components

   This section revisits the use cases from Section 2 to present the
   basic architectural components that provide connectivity in the
   peer and client-server cases.  These component models can then be
   used in later sections to enable discussion of a solution

5.1.1.  Peer Interconnection

   Figure 7 shows the basic architectural concepts for connecting across
   peer networks.  Nodes from four networks are shown: A1 and A2 come
   from one network; B1, B2, and B3 from another network; etc.  The
   interfaces between the networks (sometimes known as External Network-

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   to-Network Interfaces - ENNIs) are A2-B1, B3-C1, and C3-D1.

   The objective is to be able to support an end-to-end connection A1-
   to-D2.  This connection is for TE connectivity.

   As shown in the figure, LSP tunnels that span the transit networks
   are used to achieve the required connectivity.  These transit LSPs
   form the key building blocks of the end-to-end connectivity.

   The transit tunnels can be used as hierarchical LSPs [RFC4206] to
   carry the end-to-end LSP, or can become stitching segments [RFC5150]
   of the end-to-end LSP.  The transit tunnels B1-B3 and C-C3 can be
   as an abstract link as discussed in Section 5.3.

               :                  :                  :
    Network A  :    Network B     :    Network C     :  Network D
               :                  :                  :
     --    --     --    --    --     --    --    --     --    --
     --    --    |  |   --   |  |   |  |   --   |  |    --    --
                 |  |========|  |   |  |========|  |
                  --          --     --          --

    --- Direct connection between two nodes
    === LSP tunnel across transit network

                Figure 7 : Architecture for Peering

5.1.2.  Client-Server Interconnection

   Figure 8 shows the basic architectural concepts for a client-server
   network.  The client network nodes are C1, C2, CE1, CE2, C3, and C4.
   The core network nodes are CN1, CN2, CN3, and CN4.  The interfaces
   CE1-CN1 and CE2-CN2 are the interfaces between the client and core

   The objective is to be able to support an end-to-end connection,
   C1-to-C4, in the client network.  This connection may support TE or
   normal IP forwarding.  To achieve this, CE1 is to be connected to CE2
   by a link in the client layer that is supported by a core network

   As shown in the figure, two LSPs are used to achieve the required
   connectivity.  One LSP is set up across the core from CN1 to CN2.
   This core LSP then supports a three-hop LSP from CE1 to CE2 with its
   middle hop being the core LSP.  It is this LSP that is presented as a
   link in the client network.

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   The practicalities of how the CE1-CE2 LSP is carried across the core
   LSP may depend on the switching and signaling options available in
   the core network.  The LSP may be tunneled down the core LSP using
   the mechanisms of a hierarchical LSP [RFC4206], or the LSP segments
   CE1-CN1 and CN2-CE2 may be stitched to the core LSP as described in

                     :                            :
     Client Network  :        Core Network        :  Client Network
                     :                            :
    --    --    ---                                  ---    --    --
    --    --   |   |    ---                  ---    |   |   --    --
               |   |---|CN1|================|CN4|---|   |
                ---    |   |   ---    ---   |   |    ---
                       |   |--|CN2|--|CN3|--|   |
                        ---    ---    ---    ---

    --- Direct connection between two nodes
    ... CE-to-CE LSP tunnel
    === LSP tunnel across the core

              Figure 8 : Architecture for Client-Server Network

5.2.  TE Reachability

   As described in Section 1.1, TE reachability is the ability to reach
   a specific address along a TE path.  The knowledge of TE reachability
   enables an end-to-end TE path to be computed.

   In a single network, TE reachability is derived from the Traffic
   Engineering Database (TED) that is the collection of all TE
   information about all TE links in the network.  The TED is usually
   built from the data exchanged by the IGP, although it can be
   supplemented by configuration and inventory details especially in
   transport networks.

   In multi-network scenarios, TE reachability information can be
   described as "You can get from node X to node Y with the following
   TE attributes."  For transit cases, nodes X and Y will be edge nodes
   of the transit network, but it is also important to consider the
   information about the TE connectivity between an edge node and a
   specific destination node.

   TE reachability may be unqualified (there is a TE path), or may be
   qualified by TE attributes such as TE metrics, hop count, available
   bandwidth, delay, shared risk, etc.

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   TE reachability information can be exchanged between networks so that
   nodes in one network can determine whether they can establish TE
   paths across or into another network.  Such exchanges are subject to
   a range of policies imposed by the advertiser (for security and
   administrative control) and by the receiver (for scalability and

5.3.  Abstraction not Aggregation

   Aggregation is the process of synthesizing from available
   information.  Thus, the virtual node and virtual link models
   described in Section 3.6 rely on processing the information available
   within a network to produce the aggregate representations of links
   and nodes that are presented to the consumer.  As described in
   Section 3, dynamic aggregation is subject to a number of pitfalls.

   In order to distinguish the architecture described in this document
   from the previous work on aggregation, we use the term "abstraction"
   in this document.  The process of abstraction is one of applying
   policy to the available TE information within a domain, to produce
   selective information that represents the potential ability to
   connect across the domain.

   Abstraction does not offer all possible connectivity options (refer
   to Section 3.6), but does present a general view of potential
   connectivity.  Abstraction may have a dynamic element, but is not
   intended to keep pace with the changes in TE attribute availability
   within the network.

   Thus, when relying on an abstraction to compute an end-to-end path,
   the process might not deliver a usable path.  That is, there is no
   actual guarantee that the abstractions are current or feasible.

   While abstraction uses available TE information, it is subject to
   policy and management choices.  Thus, not all potential connectivity
   will be advertised to each client.  The filters may depend on
   commercial relationships, the risk of disclosing confidential
   information, and concerns about what use is made of the connectivity
   that is offered.

5.3.1.  Abstract Links

   An abstract link is a measure of the potential to connect a pair of
   points with certain TE parameters.  An abstract link may be realized
   by an existing LSP, or may represent the possibility of setting up an

   When looking at a network such as that in Figure 8, the link from CN1

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   to CN4 may be an abstract link.  If the LSP has already been set up,
   it is easy to advertise it as a link with known TE attributes: policy
   will have been applied in the server network to decide what LSP to
   set up.  If the LSP has not yet been established, the potential for
   an LSP can be abstracted from the TE information in the core network
   subject to policy, and the resultant potential LSP can be advertised.

   Since the client nodes do not have visibility into the core network,
   they must rely on abstraction information delivered to them by the
   core network.  That is, the core network will report on the potential
   for connectivity.

5.3.2.  The Abstraction Layer Network

   Figure 9 introduces the Abstraction Layer Network.  This construct
   separates the client layer resources (nodes C1, C2, C3, and C4, and
   the corresponding links), and the server layer resources (nodes CN1,
   CN2, CN3, and CN4 and the corresponding links).  Additionally, the
   architecture introduces an intermediary layer called the Abstraction
   Layer.  The Abstraction Layer contains the client layer edge nodes
   (C2 and C3), the server layer edge nodes (CN1 and CN4), the client-
   server links (C2-CN1 and CN4-C3) and the abstract link CN1-CN4.

    --    --                                  --    --
   |C1|--|C2|                                |C3|--|C4|   Client Network
    --   |  |                                |  |   --
         |  |                                |  |  . . . . . . . . . . .
         |  |                                |  |
         |  |                                |  |
         |  |    ---                  ---    |  |          Abstraction
         |  |---|CN1|================|CN4|---|  |         Layer Network
          --    |   |                |   |    --
                |   |                |   |   . . . . . . . . . . . . . .
                |   |                |   |
                |   |                |   |
                |   |   ---    ---   |   |                Server Network
                |   |--|CN2|--|CN3|--|   |
                 ---    ---    ---    ---

    --- Direct connection between two nodes
    === Abstract link

      Figure 9 : Architecture for Abstraction Layer Network

   The client layer network is able to operate as normal.  Connectivity
   across the network can either be found or not found based on links

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   that appear in the client layer TED.  If connectivity cannot be
   found, end-to-end LSPs cannot be set up.  This failure may be
   reported but no dynamic action is taken by the client layer.

   The server network layer also operates as normal.  LSPs across the
   server layer are set up in response to management commands or in
   response to signaling requests.

   The Abstraction Layer consists of the physical links between the
   two networks, and also the abstract links.  The abstract links are
   created by the server network according to local policy and represent
   the potential connectivity that could be created across the server
   network and which the server network is willing to make available for
   use by the client network.  Thus, in this example, the diameter of
   the Abstraction Layer Network is only three hops, but an instance of
   an IGP could easily be run so that all nodes participating in the
   Abstraction Layer (and in particular the client network edge nodes)
   can see the TE connectivity in the layer.

   When the client layer needs additional connectivity it can make a
   request to the Abstraction Layer Network.  For example, the operator
   of the client network may want to create a link from C2 to C3.  The
   Abstraction Layer can see the potential path C2-CN1-CN4-C3, and asks
   the server layer to realise the abstract link CN1-CN4.  The server
   layer provisions the LSP CN1-CN2-CN3-CN4 and makes the LSP available
   as a hierarchical LSP to turn the abstract link into a link that can
   be used in the client network.  The Abstraction Layer can then set up
   an LSP C2-CN1-CN4-C3 using stitching or tunneling, and make the LSP
   available as a virtual link in the client network.

   Sections 5.3.3 and 5.3.4 show how this model is used to satisfy the
   requirements for connectivity in client-server networks and in peer

5.3.3.  Abstraction in Client-Server Networks

   Section 5.3.2 has already introduced the concept of the Abstraction
   Layer Network through an example of a simple layered network.  But it
   may be helpful to expand on the example using a slightly more complex

   Figure 10 shows a multi-layer network comprising client nodes
   (labeled as Cn for n= 0 to 9) and server nodes (labeled as Sn for
   n = 1 to 9).

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                                             --     --
                                            /--     --\
            --     --     --     --      --/           \--
           |C1|---|C2|---|S1|---|S2|----|S3|           |C5|
            --    /--     --\    --\     --\           /--
                 /           \--    \--     \--     --/    --
                /            |S4|   |S5|----|S6|---|C6|---|C7|
               /             /--     --\    /--    /--     --
            --/    --     --/    --     \--/    --/
            --     --     --     --      --     --

               Figure 10 : An example Multi-Layer Network

   If the network in Figure 10 is operated as separate client and server
   networks then the client layer topology will appear as shown in
   Figure 11.  As can be clearly seen, the network is partitioned and
   there is no way to set up an LSP from a node on the lefthand side
   (say C1) to a node on the righthand side (say C7).

                                 --     --
                                 --     --\
                 --     --                 \--
                |C1|---|C2|                |C5|
                 --    /--                 /--
                      /                 --/    --
                     /                 |C6|---|C7|
                    /                  /--     --
                 --/    --          --/
                |C8|---|C9|        |C0|
                 --     --          --

      Figure 11 : Client Layer Topology Showing Partitioned Network

   For reference, Figure 12 shows the corresponding server layer

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                       --     --     --
                       --\    --\     --\
                          \--    \--     \--
                          |S4|   |S5|----|S6|
                          /--     --\    /--
                       --/    --     \--/
                       --     --      --

                 Figure 12 : Server Layer Topology

   Operating on the TED for the server layer, a management entity or a
   software component may apply policy and consider what abstract links
   it might offer for use by the client layer.  To do this it obviously
   needs to be aware of the connections between the layers (there is no
   point in offering an abstract link S2-S8 since this could not be of
   any use in this example).

   In our example, after consideration of which LSPs could be set up in
   the server layer, four abstract links are offered: S1-S3, S3-S6,
   S1-S9, and S7-S9.  These abstract links are shown as double lines on
   the resulting topology of the Abstract Layer Network in Figure 13.

   The separate IGP instance running in the Abstract Layer Network mean
   that this topology is visible at the edge nodes (C2, C3, C6, C9, and
   C0) as well as at a PCE if one is present.

                   --     --            --/
                   --     --\\          --\\
                             \\            \\
                              \\            \\--     --
                               \\            |S6|---|C6|
                                \\            --     --
                   --     --     \\--     --
                   --     --       --     --

          Figure 13 : Abstraction Layer Network with Abstract Links

   Now the client layer is able to make requests to the Abstraction
   Layer Network to provide connectivity.  In our example, it requests
   that C2 is connected to C3 and that C2 is connected to C0.  This

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   results in several actions:

   1. The management component for the Abstraction Layer Network asks
      its PCE to compute the paths necessary to make the connections.
      This yields C2-S1-S3-C3 and C2-S1-S9-C0.

   2. The management component for the Abstraction Layer Network
      instructs C2 to start the signaling process for the new LSPs in
      the Abstraction Layer.

   3. C2 signals the LSPs for setup using the explicit routes
      C2-S1-S3-C3 and C2-S1-S9-C0.

   4. When the signaling messages reach S1 (in our example, both LSPs
      traverse S1) the Abstraction Layer Network may find that the
      necessary underlying LSPs (S1-S2-S3 and S1-S2-S5-S9) have not
      been established since it is not a requirement that an abstract
      link be backed up by a real LSP.  In this case, S1 computes the
      paths of the underlying LSPs and signals them.

   5. Once the serve layer LSPs have been established, S1 can continue
      to signal the Abstraction Layer LSPs either using the server layer
      LSPs as tunnels or as stitching segments.

   6. Finally, once the Abstraction Layer LSPs have been set up, the
      client layer can be informed and can start to advertise the
      new TE links C2-C3 and C2-C0.  The resulting client layer topology
      is shown in Figure 14.

                                   --   --
                                  /--   --\
                                 /         \--
                       --     --/          |C5|
                      |C1|---|C2|          /--
                       --    /--\       --/    --
                            /    \     |C6|---|C7|
                           /      \    /--     --
                          /        \--/
                       --/    --   |C0|
                      |C8|---|C9|   --
                       --     --

      Figure 14 : Connected Client Layer Network with Additional Links

   7. Now the client layer can compute an end-to-end path from C1 to C7.

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Internet-Draft  Information Exchange Between TE Networks    October 2013  Macro Shared Risk Link Groups

   Network links often share fate with one or more other links.  That
   is, a scenario that may cause a links 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 layer LSPs that are used to create the links in
   the client network.

   For example, looking at the example used in Section 5.3.3 and
   considering the two abstract links S1-S3 and S1-S9 the is no way for
   the client layer to know whether the 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
   layer 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 mutli-layer
   network that equates to advertising in the client layer the server
   layer 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, only the links tat are actually shared need to
   be advertised rather than every potentially shared link.  This saving
   is possible because the abstract links are formulated on behalf of
   the server layer 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 layer (S1-S4-S7-S8-S9).  It is would
   be possible for the client layer request for connectivity C2-C0 to
   request that the path be maximally disjoint from the path C2-C3.
   While nothing can be done about the shared link C2-S1, the
   Abstraction Layer could request that the server layer instantiate the
   link S1-S9 to be diverse from the link S1-S3, and this request could
   be honored if the server layer policy allows.

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Internet-Draft  Information Exchange Between TE Networks    October 2013  A Server with Multiple Clients

   A single server network may support multiple client networks.  This
   is not an uncommon state fo affairs for example when the server
   network provides connectivity for multiple customers.

   In this case, the abstraction provided by the server layer may vary
   considerably according to the policies and commercial relationships
   with each customer.  This variance would lead to a separate
   Abstraction Layer Network maintained to support each client network.

   On the other hand, it may be that multiple clients are subject to the
   same policies and the abstraction can be identical.  In this case, a
   single Abstraction Layer Network can support more than one client.

   The choices here are made as an operational issue by the server layer
   network.  A Client with Multiple Servers

   A single client network may be supported by multiple server networks.
   The server networks may provide connectivity between different parts
   of the client network or may provide parallel (redundant)
   connectivity for the client network.

   In this case the Abstraction Layer Network should contain the
   abstract links from all server networks so that it can make suitable
   computations and create the correct TE links in the client network.
   That is, the relationship between client network and Abstraction
   Layer Network should be one-to-one.

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

5.3.4.  Abstraction in Peer Networks

   Peer networks exist in many situations in the Internet.  Packet
   networks may peer as IGP areas (levels) or as ASes.  Transport
   networks (such as optical networks) may peer to provide
   concatenations of optical paths through single vendor environments
   (see Section 7).  Figure 15 shows a simple example of three peer
   networks (A, B, and C) each comprising a few nodes

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            Network A    :     Network B      :   Network C
                         :                    :
      --     --      --  :  --     --     --  :  --     --
      --     --\    /--  :  --    /--\    --  :  --     --
                \--/     :       /    \       :
                |A4|     :      /      \      :
                 --\     :     /        \     :
              --    \--  :  --/          \--  :  --     --
              --     --  :  --            --  :  --     --
                         :                    :
                         :                    :

       Figure 15 : A Network Comprising Three Peer Networks

   As discussed in Section 2, peered networks do not share visiblity of
   their topologies or TE capabilities for scaling and confidentiality
   reasons.  That means, in our example, that computing a path from A1
   to C4 can be impossible without the aid of cooperating PCEs or some
   form of crankback.

   But it is possible to produce abstract links for the reachability
   across transit peer networks and instantiate an Abstraction Layer
   Network.  That network can be enhanced with specific reachability
   information if a destination network is partitioned as is the case
   with Network C in Figure 15.

   Suppose Network B decides to offer three abstract links B1-B3, B4-B3,
   and B4-B6.  The Abstraction Layer Network could then be constructed
   to look like the network in Figure 16.

                     --     --      --      --
                     --     --    //--      --
                     --     --//     --     --
                     --     --       --     --

     Figure 15 : Abstraction Layer Network for the Peer Network Example

   Using a process similar to that described in Section 5.3.3, Network A
   can request connectivity to Network C and the abstract links can be
   instantiated as tunnels across the transit network, and edge-to-edge

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   LSPs can be set up to join the two networks.  Furthermore, if Network
   C is partitioned, reachability information can be exchanged to allow
   Network A to select the correct edge-to-edge LSP.

                    Network A       :      Network C
              --     --      --     :     --       --
              --     --\    /--     :     --       --
                        \--/        :
                        |A4|        :
                         --\        :
                      --    \--     :     --       --
                      --     --     :     --       --

     Figure 16 : Tunnel Connections to Network C with TE Reachability

   Peer networking cases can be made far more complex by dual homing
   between network peering nodes (for example, A3 might connect to B1
   and B4 in Figure 15) and by the networks themselves being arrange in
   a mesh (for example, A6 might connect to B4 and C1 in Figure 15).
   These additional complexities can be handled graceully by the
   Abstraction Layer Network model.

   Further examples of abstraction in peer networks can be found in
   Sections 7 and 8.

5.4.  Considerations for Dynamic Abstraction


5.5.  Requirements for Advertising Abstracted Links and Nodes


6.  Building on Existing Protocols

6.1.  BGP-LS


6.2.  IGPs


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6.3.  RSVP-TE


7. Applicability to Optical Domains and Networks

   Many optical networks are arranged a set of small domains. Each
   domain is a cluster of nodes, usually from the same equipment vendor
   and with the same properties.  The domain may be constructed as a
   mesh or a ring, or maybe as an interconnected set of rings.

   The network operator seeks to provide end-to-end connectivity across
   a network constructed from multiple domains, and so (of course) the
   domains are interconnected.  In a network under management control
   such as through an Operations Support System (OSS), each domain is
   under the operational control of a Network Management System (NMS).
   In this way, an end-to-end path may be commissioned by the OSS
   instructing each NMS, and the NMSes setting up the path fragments
   across the domains.

   However, in a system that uses a control plane, there is a need for
   integration between the domains.

   Consider a simple domain, D1, as shown in Figure 16.  In this case,
   the nodes A through F are arranged in a topological ring. Suppose
   that there is a control plane in use in this domain, and that OSPF is
   used as the TE routing protocol.

            |              D1 |
            |      B---C      |
            |     /     \     |
            |    /       \    |
            |   A         D   |
            |    \       /    |
            |     \     /     |
            |      F---E      |
            |                 |

       Figure 16 : A Simple Optical Domain

   Now consider that the operator's network is built from a mesh of such
   domains, D1 through D7, as shown in Figure 17.  It is possible that
   these domains share a single, common instance of OSPF in which case
   there is nothing further to say because that OSPF instance will

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   distribute sufficient information to build a single TED spanning the
   whole network, and an end-to-end path can be computed.  A a more
   likely scenario is that each domain is running its own OSPF instance.
   In this case, each is able to handle the peculiarities (or rather,
   advanced functions) of each vendor's equipment capabilities.

           ------     ------     ------     ------
          |      |   |      |   |      |   |      |
          |  D1  |---|  D2  |---|  D3  |---|  D4  |
          |      |   |      |   |      |   |      |
           ------\    ------\    ------\    ------
                  \    |     \     |    \     |
                   \------    \------    \------
                   |      |   |      |   |      |
                   |  D5  |---|  D6  |---|  D7  |
                   |      |   |      |   |      |
                    ------     ------     ------

       Figure 17 : A Simple Optical Domain

   The question now is how to combine the multiple sets of information
   distributed by the different OSPF instances.  Three possible models
   suggest themselves based on pre-existing routing practices.

   o In the first model (the Area-Based model) each domain is treated as
     a separate OSPF area.  The end-to-end path will be specified to
     traverse multiple areas, and each area will be left to determine
     the path across the nodes in the area.  The feasibility of an end-
     to-end path (and, thus, the selection of the sequence of areas and
     their interconnections) can be derived using hierarchical PCE.

     This approach, however, fits poorly with established use of the
     OSPF area: in this form of optical network, the interconnection
     points between domains are likely to be links; and the mesh of
     domains is far more interconnected and unstructured than we are
     used to seeing in the normal area-based routing paradigm.

     Furthermore, while hierarchical PCE may be able to solve this type
     of network, the effort involved may be considerable for more than a
     small collection of domains.

   o Another approach (the AS-Based model) treats each domain as a
     separate Autonomous System (AS).  The end-to-end path will be
     specified to traverse multiple ASes, and each AS will be left to
     determine the path across the AS.

     This model sits more comfortably with the established routing

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     paradigm, but causes a massive escalation of ASes in the global
     Internet.  It would, in practice, require that the operator used
     private AS numbers [RFC6996] of which there are plenty.

     Then, as suggested in the Area-Based model, hierarchical PCE
     could be used to determine the feasibility of an end-to-end path
     and to derive the sequence of domains and the points of
     interconnection to use.  But, just as in that other model, the
     scalability of the hierarchical PCE approach must be questioned.

     Furthermore, determining the mesh of domains (i.e., the inter-AS
     connections) conventionally requires the use of BGP as an inter-
     domain routing protocol.  However, not only is BGP not normally
     available on optical equipment, but this approach indicates that
     the TE properties of the inter-domain links would need to be
     distributed and updated using BGP: something for which it is not
     well suited.

   o The third approach (the ASON model) follows the architectual
     model set out by the ITU-T [G.8080] and uses the routing protocol
     extensions described in [RFC6827].  In this model the concept of
     "levels" is introduced to OSPF.  Referring back to Figure 17, each
     OSPF instance running in a domain would be construed as a "lower
     level" OSPF instance and would leak routes into a "higher level"
     instance of the protocol that runs across the whole network.

     This approach handles the awkwardness of representing the domains
     as areas or ASes by simply considering them as domains running
     distinct instances of OSPF.  Routing advertisements flow "upward"
     from the domains to the high level OSPF instance giving it a full
     view of the whole network and allowing end-to-end paths to be
     computed.  Routing advertisements may also flow "downward" from the
     network-wide OSPF instance to any one domain so that it has
     visibility of the connectivity of the whole network.

     While architecturally satisfying, this model suffers from having to
     handle the different characteristics of different equipment
     vendors.  The advertisements coming from each low level domain
     would be meaningless when distributed into the other domains, and
     the high level domain would need to be kept up-to-date with the
     semantics of each new release of each vendor's equipment.
     Additionally, the scaling issues associated with a well-meshed
     network of domains each with many entry and exit points and each
     with network resources that are continually being updated reduces
     to the same problem as noted in the virtual link model.
     Futhermore, in the event that the domains are under control of
     different administrations, the domains would not want to distribute
     the details of their topologies and TE resources.

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   Practically, this third model turns out to be very close to the
   methodology described in this document.  As noted in Section 7.1 of
   [RFC6827], there are policy rules that can be applied to define
   exactly what information is exported from or imported to a low level
   OSPF instance.  The document even notes that some forms of
   aggregation may be appropriate.  Thus, we can apply the following
   simplifications to the mechanisms defined in RFC 6827:

   - Zero information is imported to low level domains.

   - Low level domains export only abstracted links as defined in this
     document and according to local abstraction policy and with
     appropriate removal of vendor-specific information.

   - There is no need to formally define routing levels within OSPF.

   - Export of abstracted links from the domains to the network-wide
     routing instance (the abstraction routing layer) can take place
     through any mechanism including BGP-LS or direct interaction
     between OSPF implementations.

   With these simplifications, it can be seen that the framework defined
   in this document can be constructed from the architecture discussed
   in RFC 6827, but without needing any of the protocol extensions that
   that document defines.  Thus, using the terminology and concepts
   already established, the problem may solved as shown in Figure 18.
   The abstraction layer network is constructed from the inter-domain
   links, the domain border nodes, and the abstracted (cross-domain)

                                                       Abstraction Layer
      --             --    --             --    --             --
     |  |===========|  |--|  |===========|  |--|  |===========|  |
     |  |           |  |  |  |           |  |  |  |           |  |
   ..|  |...........|  |..|  |...........|  |..|  |...........|  |......
     |  |           |  |  |  |           |  |  |  |           |  |
     |  |  --   --  |  |  |  |  --   --  |  |  |  |  --   --  |  |
     |  |_|  |_|  |_|  |  |  |_|  |_|  |_|  |  |  |_|  |_|  |_|  |
     |  | |  | |  | |  |  |  | |  | |  | |  |  |  | |  | |  | |  |
      --   --   --   --    --   --   --   --    --   --   --   --
          Domain 1             Domain 2             Domain 3
     Key                                                   Optical Layer
       ...  Layer separation
       ---  Physical link
       ===  Abstract link

         Figure 18 : The Optical Network Implemented Through the
                       Abstraction Layer Network

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8.  Abstraction in L3VPN Multi-AS Environments

   Serving layer-3 VPNs (L3PVNs) across a multi-AS or multi-operator
   environment currently provides a significant planning challenge.
   This section shows how the Abstraction Layer Network can address this


9.  Scoping Future Work

   The section is provided to help guide the work on this problem and to
   ensure that oceans are not knowingly boiled.

9.1.  Not Solving 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

   Subsequent to Section 9.1, 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.

9.3.  Not Breaking Existing Protocols

   It is a clear objective of this work to not break existing protocols.
   The Internet relies on the stability of a few key routing protocols,
   and so it is critical that any new work must not make these protocols
   brittle or unstable.

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

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10.  Manageability Considerations


11.  IANA Considerations

   This document makes no requests for IANA action.

12.  Security Considerations


13.  Acknowledgements

   Thanks to Igor Bryskin for useful discussions in the early stages of
   this work.

   Thanks to Gert Grammel for discussions on the extent of aggregation
   in abstract nodes and links.

   Thanks to Deborah Brungard and Dieter Beller for review comments.

   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 5.3.3 is freely adapted from the work of Igo 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
   [I-D.beeram-ccamp-gmpls-enni] for which the authors of this document
   express their thanks.

14.  References

14.1.  Informative References

   [G.8080]  ITU-T, "Architecture for the automatically switched optical
             network (ASON)", Recommendation G.8080.

             Bryskin, I., Beeram, V. P.,  Drake, J. et al., "Generalized
             Multiprotocol Label Switching (GMPLS) External Network
             Network Interface (E-NNI): Virtual Link Enhancements for
             the Overlay Model", draft-beeram-ccamp-gmpls-enni, work in

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             Farrel, A., and D. King, "Unanswered Questions in the Path
             Computation Element Architecture", draft-farrkingel-pce-
             questions, work in progress.

             Z. Ali, et al., "Resource ReserVation Protocol-Traffic
             Engineering (RSVP-TE) LSP Route Diversity using Exclude
             Routes," draft-ali-ccamp-xro-lsp-subobject, work in

             Z. Ali, et al., "Resource ReserVation Protocol-Traffic
             Engineering (RSVP-TE) extension for recording TE Metric of
             a Label Switched Path," draft-ali-ccamp-te-metric-
             recording, work in progress.

             Gredler, H., Medved, J., Previdi, S., Farrel, A., and Ray,
             S., "North-Bound Distribution of Link-State and TE
             Information using BGP", draft-ietf-idr-ls-distribution,
             work in progress.

   [RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and
             McManus, J., "Requirements for Traffic Engineering Over
             MPLS", RFC 2702, 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, December 2001.

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

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

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

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

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Internet-Draft  Information Exchange Between TE Networks    October 2013

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

   [RFC4208] Swallow, G., Drake, J., Ishimatsu, H., and Y. Rekhter,
             "User-Network Interface (UNI): Resource ReserVation
             Protocol-Traffic Engineering (RSVP-TE) Support for the
             Overlay Model", RFC 4208, October 2005.

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

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

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

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

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

   [RFC4920] Farrel, A., Satyanarayana, A., Iwata, A., Fujita, N., and
             Ash, G., "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.

   [RFC5152] Vasseur, JP., Ayyangar, A., and Zhang, R., "A Per-Domain
             Path Computation Method for Establishing Inter-Domain
             Traffic Engineering (TE) Label Switched Paths (LSPs)",
             RFC 5152, February 2008.

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

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Internet-Draft  Information Exchange Between TE Networks    October 2013

   [RFC5212] Shiomoto, K., Papadimitriou, D., Le Roux, JL., Vigoureux,
             M., and D. Brungard, "Requirements for GMPLS-Based Multi-
             Region and Multi-Layer Networks (MRN/MLN)", RFC 5212, July

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

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

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

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

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

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

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

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

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

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

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Internet-Draft  Information Exchange Between TE Networks    October 2013

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

Authors' Addresses

   Adrian Farrel
   Juniper Networks
   EMail: adrian@olddog.co.uk

   John Drake
   Juniper Networks
   EMail: jdrake@juniper.net

   Nabil Bitar
   40 Sylvan Road
   Waltham, MA 02145
   EMail: nabil.bitar@verizon.com

   George Swallow
   Cisco Systems, Inc.
   1414 Massachusetts Ave
   Boxborough, MA 01719
   EMail: swallow@cisco.com

   Daniele Ceccarelli
   Via A. Negrone 1/A
   Genova - Sestri Ponente
   EMail: daniele.ceccarelli@ericsson.com

Appendix A.  Editor's Notes

[Editor Note:  Need to work up some text on addressing to cover the case
of each domain having a different (potentially overlapping) address
space and the need for inter-domain addressing. In fact, this should be
quite simple but needs discussion.]

[Editor Note: Need to explain how the IGP in the Abstraction Layer works
to distribute connectivity information when the Abstract Link is not yet
up. The answer will be that the DCN needs to exist regardless of the
state of the Abstract Link.]

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