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Versions: (draft-oki-pce-inter-layer-frwk) 00 01 02 03 04 05 06 07 08 09 10 RFC 5623

   Network Working Group                                         E. Oki
   Internet Draft                                                   NTT
   Category: Informational                              Tomonori Takeda
   Expires: December 2008                                           NTT
                                                            J-L Le Roux
                                                         France Telecom
                                                              A. Farrel
                                                     Old Dog Consulting
                                                              June 2008

        Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic
                               Engineering

                  draft-ietf-pce-inter-layer-frwk-07.txt



Status of this Memo

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Abstract

   A network may comprise multiple layers. It is important to globally
   optimize network resource utilization, taking into account all
   layers, rather than optimizing resource utilization at each layer
   independently. This allows better network efficiency to be achieved
   through a process that we call inter-layer traffic engineering. The
   Path Computation Element (PCE) can be a powerful tool to achieve
   inter-layer traffic engineering.




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   This document describes a framework for applying the PCE-based
   architecture to inter-layer Multiprotocol Label Switching (MPLS) and
   Generalized MPLS (GMPLS) traffic engineering. It provides
   suggestions for the deployment of PCE in support of multi-layer
   networks. This document also describes network models where PCE
   performs inter-layer traffic engineering, and the relationship
   between PCE and a functional component called the Virtual Network
   Topology Manager (VNTM).


Table of Contents

  1. Introduction....................................................3
  1.1.  Terminology..................................................4
  2. Inter-Layer Path Computation....................................4
  3. Inter-Layer Path Computation Models.............................6
  3.1.  Single PCE Inter-Layer Path Computation......................6
  3.2.  Multiple PCE Inter-Layer Path Computation....................7
  3.3.  General Observations.........................................8
  4. Inter-Layer Path Control........................................9
  4.1.  VNT Management...............................................9
  4.2.  Inter-Layer Path Control Models..............................9
  4.2.1.  PCE-VNTM Cooperation Model.................................9
  4.2.2.  Higher-Layer Signaling Trigger Model......................11
  4.2.3.  NMS-VNTM Cooperation Model................................15
  4.2.4.  Possible Combinations of Inter-Layer Path Computation and
  Inter-Layer Path Control Models...................................17
  5. Choosing Between Inter-Layer Path Control Models...............18
  5.1.  VNTM Functions..............................................18
  5.2.  Border LSR Functions........................................19
  5.3.  Complete Inter-Layer LSP Setup Time.........................19
  5.4.  Network Complexity..........................................20
  5.5.  Separation of Layer Management..............................20
  6. Stability Considerations.......................................21
  7. Manageability Considerations...................................22
  7.1.  Control of Function and Policy..............................22
  7.1.1.  Control of Inter-Layer Computation Function...............22
  7.1.2.  Control of Per-Layer Policy...............................22
  7.1.3.  Control of Inter-Layer Policy.............................23
  7.2.  Information and Data Models.................................23
  7.3.  Liveness Detection and Monitoring...........................24
  7.4.  Verifying Correct Operation.................................24
  7.5.  Requirements on Other Protocols and Functional Components...25
  7.6.  Impact on Network Operation.................................25
  8. Security Considerations........................................25


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  9. Acknowledgments................................................27
  10.  References...................................................27
  10.1. Normative Reference.........................................27
  10.2. Informative Reference.......................................27
  11.  Authors's Addresses..........................................28
  12.  Intellectual Property Statement..............................29

1. Introduction

   A network may comprise multiple layers. These layers may represent
   separations of technologies (e.g., packet switch capable (PSC), time
   division multiplex (TDM), or lambda switch capable (LSC)) [RFC3945],
   separation of data plane switching granularity levels (e.g., PSC-1,
   PSC-2, VC4, or VC12) [MLN-REQ], or a distinction between client and
   server networking roles. In this multi-layer network, Label Switched
   Paths (LSPs) in a lower layer are used to carry higher-layer LSPs
   across the lower-layer network. The network topology formed by
   lower-layer LSPs and advertised as traffic engineering links (TE
   links) in the higher layer network is called the Virtual Network
   Topology (VNT) [MLN-REQ].

   It may be effective to optimize network resource utilization
   globally, i.e., taking into account all layers, rather than
   optimizing resource utilization at each layer independently. This
   allows better network efficiency to be achieved and is what we call
   inter-layer traffic engineering. This includes mechanisms allowing
   the computation of end-to-end paths across layers (known as inter-
   layer path computation), and mechanisms for control and management
   of the Virtual Network Topology (VNT) by setting up and releasing
   LSPs in the lower layers [MLN-REQ].

   Inter-layer traffic engineering is included in the scope of the Path
   Computation Element (PCE)-based architecture [RFC4655], and PCE can
   provide a suitable mechanism for resolving inter-layer path
   computation issues.

   PCE Communication Protocol requirements for inter-layer traffic
   engineering are set out in [PCE-INTER-LAYER-REQ].

   This document describes a framework for applying the PCE-based
   architecture to inter-layer traffic engineering. It provides
   suggestions for the deployment of PCE in support of multi-layer
   networks. This document also describes network models where PCE
   performs inter-layer traffic engineering, and the relationship
   between PCE and a functional component in charge of the control and
   management of the VNT, called the Virtual Network Topology Manager
   (VNTM).




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1.1. Terminology

   This document uses terminology from the PCE-based path computation
   architecture [RFC4655] and also common terminology from Multi
   Protocol Label Switching (MPLS) [RFC3031], Generalized MPLS (GMPLS)
   [RFC3945], and Multi-Layer Networks [MLN-REQ].

2. Inter-Layer Path Computation

   This section describes key topics of inter-layer path computation in
   MPLS and GMPLS networks.

   [RFC4206] defines a way to signal a higher-layer LSP, which has an
   explicit route that includes hops traversed by LSPs in lower layers.
   The computation of end-to-end paths across layers is called Inter-
   Layer Path Computation.

   A Label Switching Router (LSR) in the higher-layer might not have
   information on the topology of the lower-layer, particularly in an
   overlay or augmented model deployment, and hence may not be able to
   compute an end-to-end path across layers.

   PCE-based Inter-Layer Path Computation consists of using one or more
   PCEs to compute an end-to-end path across layers. This could be
   achieved by a single PCE path computation where the PCE has topology
   information about multiple layers and can directly compute an end-
   to-end path across layers considering the topology of all of the
   layers. Alternatively, the inter-layer path computation could be
   performed as a multiple PCE computation where each member of a set
   of PCEs has information about the topology of one or more layers
   (but not all layers), and the PCEs collaborate to compute an end-to-
   end path.

       -----    -----                  -----    -----
      | LSR |--| LSR |................| LSR |--| LSR |
      | H1  |  | H2  |                | H3  |  | H4  |
       -----    -----\                /-----    -----
                      \-----    -----/
                      | LSR |--| LSR |
                      | L1  |  | L2  |
                       -----    -----

   Figure 1 - A Simple Example of a Multi-Layer Network.

   Consider, for instance, the two-layer network shown in Figure 1,
   where the higher-layer network is a packet-based IP/MPLS or GMPLS
   network  (LSRs H1, H2, H3, and H4), and the lower-layer network
   (LSRs, H2, L1, L2, and H3) is a GMPLS optical network. An ingress
   LSR in the higher-layer network (H1) tries to set up an LSP to an



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   egress LSR (H4) also in the higher-layer network across the lower-
   layer network, and needs a path in the higher-layer network. However,
   suppose that there is no TE link in the higher-layer network between
   the border LSRs located on the boundary between the higher-layer and
   lower-layer networks (H2 and H3). Suppose also that the ingress LSR
   does not have topology visibility into the lower layer. If a single-
   layer path computation is applied in the higher-layer, the path
   computation fails because of the missing TE link. On the other hand,
   inter-layer path computation is able to provide a route in the
   higher-layer (H1-H2-H3-H4) and a suggestion that a lower-layer LSP
   be set up between the border LSRs (H2-L1-L2-H3).

   Lower-layer LSPs that are advertised as TE links into the higher-
   layer network form a Virtual Network Topology (VNT) that can be used
   for routing higher-layer LSPs. Inter-layer path computation for end-
   to-end LSPs in the higher-layer network that span the lower-layer
   network may utilize the VNT, and PCE is a candidate for computing
   the paths of such higher-layer LSPs within the higher-layer network.
   Alternatively, the PCE-based path computation model can:

   - Perform a single computation on behalf of the ingress LSR using
   information gathered from more than one layer. This mode is referred
   to as Single PCE Computation in [RFC4655].

   - Compute a path on behalf of the ingress LSR through cooperation
   with PCEs responsible for each layer. This mode is referred to as
   Multiple PCE Computation with inter-PCE communication in [RFC4655].

   - Perform separate path computations on behalf of the TE-LSP head-
   end and each transit border LSR that is the entry point to a new
   layer. This mode is referred to as Multiple PCE Computation (without
   inter-PCE communication) in [RFC4655]. This option utilizes per-
   layer path computation performed independently by successive PCEs.

   The PCE invoked by the head-end LSR computes a path that the LSR can
   use to signal an MPLS-TE or GMPLS LSP once the path information has
   been converted to an Explicit Route Object (ERO) for use in RSVP-TE
   signaling. There are two options.

   - Option 1: Mono-layer path.
   The PCE computes a "mono-layer" path, i.e., a path that includes
   only TE links from the same layer. There are two cases for this
   option. In the first case the PCE computes a path that includes
   already established lower-layer LSPs or lower-layer LSPs to be
   established on demand. That is, the resulting ERO includes sub-
   object(s) corresponding to lower-layer hierarchical LSPs expressed
   as the TE link identifiers of the hierarchical LSPs when advertised
   as TE links in the higher-layer network. The TE link may be a
   regular TE link that is actually established, or a virtual TE link



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   that is not established yet (see [MLN-REQ]). If it is a virtual TE
   link, this triggers a setup attempt for a new lower-layer LSP when
   signaling reaches the head-end of the lower-layer LSP. Note that the
   path of a virtual TE link is not necessarily known in advance, and
   this may require a further (lower-layer) path computation.

   The second case is that the PCE computes a path that includes a
   loose hop that spans the lower-layer network. The higher layer path
   computation selects which lower layer network to use, and selects
   the entry and exit points of that lower-layer network, but does not
   select the path across the lower-layer network. A transit LSR that
   is the entry point to the lower-layer network is expected to expand
   the loose hop (either itself or relying on the services of a PCE).
   The path expansion process on the border LSR may result either in
   the selection of an existing lower-layer LSP, or in the computation
   and setup of a new lower-layer LSP.

   - Option 2: Multi-layer path.
   The PCE computes a "multi-layer" path, i.e., a path that includes TE
   links from distinct layers [RFC4206]. Such a path can include the
   complete path of one or more lower-layer LSPs that already exist or
   are not yet established. In the latter case, the signaling of the
   higher-layer LSP will trigger the establishment of the lower-layer
   LSPs.

3. Inter-Layer Path Computation Models

   As stated in Section 2, two PCE modes defined in the PCE
   architecture can be used to perform inter-layer path computation.
   They are discussed in the sections that follow.

3.1. Single PCE Inter-Layer Path Computation

   In this model inter-layer path computation is performed by a single
   PCE that has topology visibility into all layers. Such a PCE is
   called a multi-layer PCE.

   In Figure 2, the network is comprised of two layers. LSRs H1, H2, H3,
   and H4 belong to the higher layer, and LSRs H2, H3, L1, and L2
   belong to the lower layer. The PCE is a multi-layer PCE that has
   visibility into both layers. It can perform end-to-end path
   computation across layers (single PCE path computation). For
   instance, it can compute an optimal path H1-H2-L1-L2-H3-H4, for a
   higher layer LSP from H1 to H4. This path includes the path of a
   lower layer LSP from H2 to H3, already in existence or not yet
   established.


                           -----



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                          | PCE |
                           -----
       -----    -----                  -----    -----
      | LSR |--| LSR |................| LSR |--| LSR |
      | H1  |  | H2  |                | H3  |  | H4  |
       -----    -----\                /-----    -----
                      \-----    -----/
                      | LSR |--| LSR |
                      | L1  |  | L2  |
                       -----    -----

     Figure 2: Single PCE Inter-Layer Path Computation

3.2. Multiple PCE Inter-Layer Path Computation

   In this model there is at least one PCE per layer, and each PCE has
   topology visibility restricted to its own layer. Some providers may
   want to keep the layer boundaries due to factors such as
   organizational and/or service management issues. The choice for
   multiple PCE computation instead of single PCE computation may also
   be driven by scalability considerations, as in this mode a PCE only
   needs to maintain topology information for one layer (resulting in a
   size reduction for the Traffic Engineering Database (TED)).

   These PCEs are called mono-layer PCEs. Mono-layer PCEs collaborate
   to compute an end-to-end optimal path across layers.

   Figure 3 shows multiple PCE inter-layer computation with inter-PCE
   communication. There is one PCE in each layer. The PCEs from each
   layer collaborate to compute an end-to-end path across layers. PCE
   Hi is responsible for computations in the higher layer and may
   "consult" with PCE Lo to compute paths across the lower layer. PCE
   Lo is responsible for path computation in the lower layer. A simple
   example of cooperation between the PCEs could be as follows:
   - LSR H1 sends a request for a path H1-H4 to PCE Hi
   - PCE Hi selects H2 as the entry point to the lower layer, and H3 as
   the exit point.
   - PCE Hi requests a path H2-H3 from PCE Lo.
   - PCE Lo returns H2-L1-L2-H3 to PCE Hi.
   - PEC Hi is now able to compute the full path (H1-H2-L1-L2-H3-H4)
   and return it to H1.

   Of course, more complex cooperation may be required if an optimal
   end-to-end path is desired.


                                -----
                               | PCE |
                               | Hi  |



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                                --+--
                                  |
       -----    -----             |            -----    -----
      | LSR |--| LSR |............|...........| LSR |--| LSR |
      | H1  |  | H2  |            |           | H3  |  | H4  |
       -----    -----\          --+--         /-----    -----
                      \        | PCE |       /
                       \       | Lo  |      /
                        \       -----      /
                         \                /
                          \-----    -----/
                          | LSR |--| LSR |
                          | L1  |  | L2  |
                           -----    -----

   Figure 3: Multiple PCE Inter-Layer Path Computation with Inter-PCE
   Communication

   Figure 4 shows multiple PCE inter-layer path computation without
   inter-PCE communication. As described in Section 2, separate path
   computations are performed on behalf of the TE-LSP head-end and each
   transit border LSR that is the entry point to a new layer.

                                -----
                               | PCE |
                               | Hi  |
                                -----

       -----    -----                          -----    -----
      | LSR |--| LSR |........................| LSR |--| LSR |
      | H1  |  | H2  |                        | H3  |  | H4  |
       -----    -----\          -----         /-----    -----
                      \        | PCE |       /
                       \       | Lo  |      /
                        \       -----      /
                         \                /
                          \-----    -----/
                          | LSR |--| LSR |
                          | L1  |  | L2  |
                           -----    -----

   Figure 4: Multiple PCE Inter-layer Path Computation Without Inter-
   PCE Communication

3.3. General Observations

   - Depending on implementation details, the time to perform inter-
   layer path computation in the single PCE inter-layer path
   computation model may be less than that of the multiple PCE model



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   with cooperating mono-layer PCEs, because there is no requirement to
   exchange messages between cooperating PCEs.

   - When TE topology for all layer networks is visible within one
   routing domain, the single PCE inter-layer path computation model
   may be adopted because a PCE is able to collect all layers' TE
   topologies by participating in only one routing domain.

   - As the single PCE inter-layer path computation model uses more TE
   topology information in one computation than is used by PCEs in the
   multiple PCE path computation model, it requires more computation
   power and memory.

   When there are multiple candidate layer border nodes (we may say
   that the higher layer is multi-homed), optimal path computation
   requires that all the possible paths transiting different layer
   border nodes or links be examined. This is relatively simple in the
   single PCE inter-layer path computation model because the PCE has
   full visibility - the computation is similar to the computation
   within a single domain of a single layer. In the multiple PCE inter-
   layer path computation model, backward recursive techniques
   described in [BRPC] could be used, by considering layers as separate
   domains.

4. Inter-Layer Path Control

4.1. VNT Management

   As a result of mono-layer path computation, a PCE may determine that
   there is insufficient bandwidth available in the higher-layer
   network to support this or future higher-layer LSPs. The problem
   might be resolved if new LSPs were provisioned across the lower-
   layer network. Furthermore, the modification, re-organization and
   new provisioning of lower-layer LSPs may enable better utilization
   of lower-layer network resources given the demands of the higher-
   layer network. In other words, the VNT needs to be controlled or
   managed in cooperation with inter-layer path computation.

   A VNT Manager (VNTM) is defined as a functional element that manages
   and controls the VNT. PCE and VNT Manager are distinct functional
   elements that may or may not be co-located.

4.2. Inter-Layer Path Control Models

4.2.1. PCE-VNTM Cooperation Model

      -----      ------
     | PCE |--->| VNTM |
      -----      ------



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        ^           :
        :           :
        :           :
        v           V
       -----      -----                  -----      -----
      | LSR |----| LSR |................| LSR |----| LSR |
      | H1  |    | H2  |                | H3  |    | H4  |
       -----      -----\                /-----      -----
                        \-----    -----/
                        | LSR |--| LSR |
                        | L1  |  | L2  |
                         -----    -----

   Figure 5: PCE-VNTM Cooperation Model

   A multi-layer network consists of higher-layer and lower-layer
   networks. LSRs H1, H2, H3, and H4 belong to the higher-layer network,
   LSRs H2, L1, L2, and H3 belong to the lower-layer network, as shown
   in Figure 5. The case of single PCE inter-layer path computation is
   considered here to explain the cooperation model between PCE and
   VNTM, but multiple PCE path computation with or without inter-PCE
   communication can also be applied to this model.

   Consider that H1 requests the PCE to compute an inter-layer path
   between H1 and H4. There is no TE link in the higher-layer between
   H2 and H3 before the path computation request, so the request fails.
   But the PCE may provide information to the VNT Manager responsible
   for the lower layer network that may help resolve the situation for
   future higher-layer LSP setup.

   The roles of PCE and VNTM are as follows. PCE performs inter-layer
   path computation and is unable to supply a path because there is no
   TE link between H2 and H3. The computation fails, but PCE suggests
   to VNTM that a lower-layer LSP (H2-H3) could be established to
   support future LSP requests. Messages from PCE to VNTM contain
   information about the higher-layer demand (from H2 to H3), and may
   include a suggested path in the lower layer (if the PCE has
   visibility into the lower layer network). VNTM uses local policy and
   possibly management/configuration input to determine how to process
   the suggestion from PCE, and may request an ingress LSR (e.g. H2) to
   establish a lower-layer LSP. VNTM or the ingress LSR (H2) may
   themselves use a PCE with visibility into the lower layer to compute
   the path of this new LSP.

   When the higher-layer PCE fails to compute a path and notifies VNTM,
   it may wait for the lower-layer LSP to be set up and advertised as a
   TE link. PCE may have a timer. After TED is updated within a
   specified duration, PCE will know a new TE link. It could then
   compute the complete end-to-end path for the higher-layer LSP and



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   return the result to the PCC. In this case, the PCC may be kept
   waiting for some time, and it is important that the PCC understands
   this. It is also important that the PCE and VNTM have an agreement
   that the lower-layer LSP will be set up in a timely manner, or that
   the PCE will be notified by VNTM that no new LSP will become
   available. In any case, if the PCE decides to wait, it must operate
   a timeout. An example of such a cooperative procedure between PCE
   and VNTM is as follows using the example network in Figure 4.

   Step 1: H1 (PCC) requests PCE to compute a path between H1 and H4.

   Step 2: The path computation fails because there is no TE link
   across the lower-layer network.

   Step 3: PCE suggests to VNTM that a new TE link connecting H2 and H3
   would be useful. The PCE notifies VNTM that it will be waiting for
   the TE link to be created. VNTM considers whether lower-layer LSPs
   should be established if necessary and if acceptable within VNTM's
   policy constraints.

   Step 4: VNTM requests an ingress LSR in the lower-layer network
   (e.g., H2) to establish a lower-layer LSP. The request message may
   include a lower-layer LSP route obtained from the PCE responsible
   for the lower-layer network.

   Step 5: The ingress LSR signals to establish the lower-layer LSP.

   Step 6: If the lower-layer LSP setup is successful, the ingress LSR
   notifies VNTM that the LSP is complete and supplies the tunnel
   information.

   Step 7: The ingress LSR (H2) advertises the new LSP as a TE link in
   the higher-layer network routing instance.

   Step 8: PCE notices the new TE link advertisement and recomputes the
   requested path.

   Step 9: PCE replies to H1 (PCC) with a computed higher-layer LSP
   route. The computed path is categorized as a mono-layer path that
   includes the already-established lower layer-LSP as a single hop in
   the higher layer. The higher-layer route is specified as H1-H2-H3-H4,
   where all hops are strict.

   Step 9: H1 initiates signaling with the computed path H2-H3-H4 to
   establish the higher-layer LSP.

4.2.2. Higher-Layer Signaling Trigger Model

      -----



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     | PCE |
      -----
        ^
        :
        :
        v
       -----      -----                  -----    -----
      | LSR |----| LSR |................| LSR |--| LSR |
      | H1  |    | H2  |                | H3  |  | H4  |
       -----      -----\                /-----    -----
                        \-----    -----/
                        | LSR |--| LSR |
                        | L1  |  | L2  |
                         -----    -----

   Figure 6: Higher-layer Signaling Trigger Model

   Figure 6 shows the higher-layer signaling trigger model. The case of
   single PCE path computation is considered to explain the higher-
   layer signaling trigger model here, but multiple PCE path
   computation with/without inter-PCE communication can also be applied
   to this model.

   As in the case described in Section 4.2.1, consider that H1 requests
   PCE to compute a path between H1 and H4. There is no TE link in the
   higher-layer between H2 and H3 before the path computation request.

   PCE is unable to compute a mono-layer path, but may judge that the
   establishment of a lower-layer LSP between H2 and H3 would provide
   adequate connectivity. If the PCE has inter-layer visibility it may
   return a path that includes hops in the lower layer (H1-H2-L1-L2-H3-
   H4), but if it has no visibility into the lower layer, it may return
   a path with a loose hop from H2 to H3 (H1-H2-H3(loose)-H4). The
   former is a multi-layer path, and the latter a mono-layer path that
   includes loose hops.

   In the higher-layer signaling trigger model with a multi-layer path,
   the LSP route supplied by the PCE includes the route of a lower-
   layer LSP that is not yet established. A border LSR that is located
   at the boundary between the higher-layer and lower-layer networks
   (H2 in this example) receives a higher-layer signaling message,
   notices that the next hop is in the lower-layer network, starts to
   setup the lower-layer LSP as described in [RFC4206]. Note that these
   actions depends on a policy being applied at the border LSR. An
   example procedure of the signaling trigger model with a multi-layer
   path is as follows.

   Step 1: H1 (PCC) requests PCE to compute a path between H1 and H4.
   The request indicates that inter-layer path computation is allowed.



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   Step 2: As a result of the inter-layer path computation, PCE judges
   that a new lower-layer LSP needs to be established.

   Step 3: PCE replies to H1 (PCC) with a computed multi-layer route
   including higher-layer and lower-layer LSP routes. The route may be
   specified as H1-H2-L1-L2-H3-H4, where all hops are strict.

   Step 4: H1 initiates higher-layer signaling using the computed
   explicit router of H2-L1-L2-H3-H4.

   Step 5: The border LSR (H2) that receives the higher-layer signaling
   message starts lower-layer signaling to establish a lower-layer LSP
   along the specified lower-layer route of H2-L1-L2-H3. That is, the
   border LSR recognizes the hops within the explicit route that apply
   to the lower-layer network, verifies with local policy that a new
   LSP is acceptable, and establishes the required lower-layer LSP.
   Note that it is possible that a suitable lower-layer LSP has already
   been established (or become available) between the time that the
   computation was performed and the moment when the higher-layer
   signaling message reached the border LSR. In this case, the border
   LSR may select such a lower-layer LSP without the need to signal a
   new LSP provided that the lower-layer LSP satisfies the explicit
   route in the higher-layer signaling request.

   Step 6: After the lower-layer LSP is established, the higher-layer
   signaling continues along the specified higher-layer route of H2-H3-
   H4 using hierarchical signaling [RFC4206].

   On the other hand, in the signaling trigger model with a mono-layer
   path, a higher-layer LSP route includes a loose hop to traverse the
   lower-layer network between the two border LSRs. A border LSR that
   receives a higher-layer signaling message needs to determine a path
   for a new lower-layer LSP. It applies local policy to verify that a
   new LSP is acceptable and then either consults a PCE with
   responsibility for the lower-layer network or computes the path by
   itself, and initiates signaling to establish the lower-layer LSP.
   Again, it is possible that a suitable lower-layer LSP has already
   been established (or become available). In this case, the border LSR
   may select such a lower-layer LSP without the need to signal a new
   LSP provided that the existing lower-layer LSP satisfies the
   explicit route in the higher-layer signaling request. Since the
   higher-layer signaling request used a loose hop without specifying
   any specifics of the path within the lower-layer network, the border
   LSR has greater freedom to choose a lower-layer LSP than in the
   previous example.






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   The difference between procedures of the signaling trigger model
   with a multi-layer path and a mono-layer path is Step 5. Step 5 of
   the signaling trigger model with a mono-layer path is as follows:

   Step 5': The border LSR (H2) that receives the higher-layer
   signaling message applies local policy to verify that a new LSP is
   acceptable and then initiates establishment of a lower-layer LSP. It
   either consults a PCE with responsibility for the lower-layer
   network or computes the route by itself to expand the loose hop
   route in the higher-layer path.

   Finally, note that a virtual TE link may have been advertised into
   the higher-layer network. This causes the PCE to return a path H1-
   H2-H3-H4 where all the hops are strict. But when the higher-layer
   signaling message reaches the layer border node H2 (that was
   responsible for advertising the virtual TE link) it realizes that
   the TE link does not exist yet, and signals the necessary LSP across
   the lower-layer network using its own path determination (just as
   for a loose hop in the higher layer) before continuing with the
   higher-layer signaling.

   PCE
    ^
    :
    :
    V
   H1--H2                  H3--H4
        \                  /
         L1==L2==L3--L4--L5
                  |
                  |
                 L6--L7
                       \
                        H5--H6

   Figure 7: Example of a Multi-Layer Network

   Examples of multi-layer EROs are explained using Figure 7. It is
   described how lower-layer LSP setup is performed in the higher-layer
   signaling trigger model using an ERO that can include subobjects in
   both the higher and lower layers. It gives rise to several options
   for the ERO when it reaches the last LSR in the higher layer network
   (H2).
   1. The next subobject is a loose hop to H3 (mono layer ERO).
   2. The next subobject is a strict hop to L1 followed by a loose hop
   to H3.
   3. The next subobjects are a series of hops (strict or loose) in the
   lower-layer network followed by H3. For example, {L1(strict),
   L3(loose), L5(loose), H3(strict)}



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   In the first example, the lower layer can utilize any LSP tunnel
   that will deliver the end-to-end LSP to H3. In the third case, the
   lower layer must select an LSP tunnel that traverses L3 and L5.
   However, this does not mean that the lower layer can or should use
   an LSP from L1 to L3 and another from L3 to L5.

4.2.3. NMS-VNTM Cooperation Model

       -----
      | NMS |
      |     |   -----
       -----   | PCE |
       ^   ^   | Hi  |
       :   :    -----
       :   :    ^
       :   :    :
       :   :    :
       :   v    v
       :   ------    -----                          -----    ------
       :  | LSR  |--| LSR |........................| LSR |--| LSR  |
       :  | H1   |  | H2  |                        | H3  |  | H4   |
       :   ------    -----\                        /-----    ------
       :             ^     \                      /
       :             :      \                    /
       :     --------        \                  /
       v    :                 \                /
       ------      -----       \-----    -----/
      | VNTM |<-->| PCE |      | LSR |--| LSR |
      |      |    | Lo  |      | L1  |  | L2  |
       ------      -----        -----    -----

   Figure 8: NMS-VNTM Cooperation Model

   Figure 8 show the Network Management System (NMS) in the NMS-VNTM
   cooperation model. The NMS manages the higher layer. The case of
   multiple PCE computation without inter-PCE communication is used to
   explain the NMS-VNTM cooperation model here, but single PCE path
   computation could also be applied to this model. Note that multiple
   PCE path computation with inter-PCE communication does not fit in
   with this model.

   The NMS requests a head-end LSR (H1 in this example) to set up a
   higher-layer LSP between head-end and tail-end LSRs without
   specifying any route. The head-end LSR, which is a PCC, requests the
   higher-layer PCE to compute a path between head-end and tail-end
   LSRs. There is no TE link in the higher-layer between border LSRs
   (H2 and H3 in this example). When the PCE fails to compute a path,
   it informs the PCC (i.e., head-end LSR) that notifies the NMS. The



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   notification may include the information that there is no TE link
   between the border LSRs.

   Note that it is equally valid for the higher-layer PCE to be
   consulted by the NMS rather than by the head-end LSR. In this case,
   the result is the same - the NMS discovers that an end-to-end LSP
   cannot be provisioned owing to the lack of a TE link between H2 and
   H3.

   The NMS may now suggest (or request) to the VNTM that a lower-layer
   LSP between the border LSRs could be established and could be
   advertised as a TE link in the higher layer to support future
   higher-layer LSP requests. The communication between the NMS and the
   VNTM may be performed in an automatic manner or in a manual manner,
   and is a key interaction between layers that may also be separate
   administrative domains. Thus, this communication is potentially a
   point of application of administrative, billing, and security policy.
   The NMS may wait for the lower-layer LSP to be set up and advertised
   as a TE link, or may reject the operator's request for the service
   that requires the higher-layer LSP with a suggestion that the
   operator tries again later.

   The VNTM requests the lower-layer PCE to compute a path, and then
   requests H2 to establish a lower-layer LSP. Alternatively, the VNTM
   may make a direct request to H2 for the LSP, and H2 may consult the
   lower-layer PCE. After the NMS is informed or notices that the
   lower-layer LSP has been established, it can request the head-end
   LSR (H1) to set up the higher-layer end-to-end LSP between H1 and H4.

   Thus, cooperation between the high layer and lower layer is
   performed though communication between NMS and VNTM. An example of
   such a procedure of the NSM-VNTM cooperation model is as follows
   using the example network in Figure 6.

   Step 1: NMS requests a head-end LSR (H1) to set up a higher-layer
   LSP between H1 and H4 without specifying any route.

   Step 2: H1 (PCC) requests PCE to compute a path between H2 and H3.

   Step 3: The path computation fails because there is no TE link
   across the lower-layer network.

   Step 4: H1 (PCC) notifies NMS. The notification may include an
   indication that there is no TE link between H2 and H4.

   Step 5: NMS suggests (or requests) to VNTM that a new TE link
   connecting H2 and H3 would be useful. The NMS notifies VNTM that it
   will be waiting for the TE link to be created. VNTM considers




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   whether lower-layer LSPs should be established if necessary and if
   acceptable within VNTM's policy constraints.

   Step 6: VNTM requests the lower-layer PCE for path computation.

   Step 7: VNTM requests the ingress LSR in the lower-layer network
   (H2) to establish a lower-layer LSP. The request message includes a
   lower-layer LSP route obtained from the lower-layer PCE responsible
   for the lower-layer network.

   Step 5: H2 signals the lower-layer LSP.

   Step 6: If the lower-layer LSP setup is successful, H2 notifies VNTM
   that the LSP is complete and supplies the tunnel information.

   Step 7: H2 advertises the new LSP as a TE link in the higher-layer
   network routing instance.

   Step 8: VNTM notifies NMS that the underlying lower-layer LSP has
   been set up, and NMS notices the new TE link advertisement.

   Step 9: NMS again requests H1 to set up a higher-layer LSP between
   H1 and H4.

   Step 10: H1 requests the higher-layer PCE to compute a path and
   obtains a successful result that includes the higher-layer route
   that is specified as H1-H2-H3-H4, where all hops are strict.

   Step 11: H1 initiates signaling with the computed path H2-H3-H4 to
   establish the higher-layer LSP.

4.2.4. Possible Combinations of Inter-Layer Path Computation and Inter-
      Layer Path Control Models

   Table 1 summarizes the possible combinations of inter-layer path
   computation and inter-layer path control models. There are three
   inter-layer path computation models: the single PCE path computation
   model; the multiple PCE path computation with inter-PCE
   communication model; and the multiple PCE path computation without
   inter-PCE communication model. There are also three inter-layer path
   control models:  the PCE-VNTM cooperation model; the higher-layer
   signaling trigger model; and the NMS-VNTM cooperation model. All the
   combinations between inter-layer path computation and path control
   models, except for the combination of the multiple PCE path
   computation with inter-layer PCE communication model and the NMS-
   VNTM cooperation model are possible.

   Table 1: Possible Combinations of Inter-Layer Path Computation and
   Inter-Layer Path Control Models.


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    ----------------------------------------------------
   | Path computation  | Single | Multiple  | Multiple  |
   |      \            | PCE    | PCE with  | PCE w/o   |
   | Path control      |        | inter-PCE | inter-PCE |
   |----------------------------------------------------|
   | PCE-VNTM          |  Yes   | Yes       | Yes       |
   | cooperation       |        |           |           |
   |----------------------------+-----------+-----------|
   | Higher-layer      |  Yes   | Yes       | Yes       |
   | signaling trigger |        |           |           |
   |----------------------------------------------------|
   | NMS-VNTM          |  No*   | No        | Yes       |
   | cooperation       |        |           |           |
    -------------------+--------+-----------+-----------

   *Note that, in case of NSM-VNTM cooperation and single PCE inter-
   layer path computation, the PCE function used by NMS and VNTM may be
   collocated, but it will operate on separate TEDs.

5. Choosing Between Inter-Layer Path Control Models

   This section compares the cooperation model between PCE and VNTM,
   and the higher-layer signaling trigger model, in terms of VNTM
   functions, border LSR functions, higher-layer signaling time, and
   complexity (in terms of number of states and messages). An
   appropriate model may be chosen by a network operator in different
   deployment scenarios taking all these considerations into account.

5.1. VNTM Functions

   VNTM functions are required in both the PCE-VNTM cooperation model
   and the NMS-VNTM model. In the PCE-VNTM cooperation model,
   communications are required between PCE and VNTM, and between VNTM
   and a border LSR. Communications between a higher-layer PCE and the
   VNTM are event notifications and may use SNMP notifications from the
   PCE MIB modules [PCE-MIB]. Note that communications from the PCE to
   the VNTM do not have any acknowledgements.

   VNTM-LSR communication can use existing GMPLS-TE MIB modules
   [RFC4802]. In the NMS-VNTM cooperation model, communications are
   required between NMS and VNTM, between VNTM and a lower-layer PCE,
   and between VNTM and a border LSR. NMS-VNTM communications, which
   are out of scope of this document, may use proprietary or standard
   interfaces, some of which, for example, are standardized in TM Forum.
   Communications between VNTM and a lower-layer PCE use PCEP [PCEP].
   VNTM-LSR communications are the same as in the PCE-VNTM cooperation
   model.




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   In the higher-layer signaling trigger model, no VNTM functions are
   required, and no such communications are required.

   If VNTM functions are not supported in a multi-layer network, the
   higher-layer signaling trigger model has to be chosen.

   The inclusion of VNTM functionality allows better coordination of
   cross-network LSP tunnels and application of network-wide policy
   that is far harder to apply in the trigger model since it requires
   the coordination of policy between multiple border LSRs.

5.2. Border LSR Functions

   In the higher-layer signaling trigger model, a border LSR must have
   some additional functions. It needs to trigger lower-layer signaling
   when a higher-layer path message suggests that lower-layer LSP setup
   is necessary. Note that, if virtual TE links are used, the border
   LSRs must be capable of triggered signaling.

   If the ERO in the higher-layer Path message uses a mono-layer path
   or specifies a loose hop, the border LSR receiving the Path message
   must obtain a lower-layer route either by consulting a PCE or by
   using its own computation engine. If the ERO in the higher-layer
   Path message uses a multi-layer path, the border LSR must judge
   whether lower-layer signaling is needed.

   In the PCE-VNTM cooperation model and the NMS-VNTM model, no
   additional function for triggered signaling is required in border
   LSRs except when virtual TE links are used. Therefore, if these
   additional functions are not supported in border LSRs, where a
   border LSR is controlled by VNTM to set up a lower-layer LSP, the
   cooperation model has to be chosen.

5.3. Complete Inter-Layer LSP Setup Time

   The complete inter-layer LSP setup time includes inter-layer path
   computation, signaling, and the communication time between PCC and
   PCE, PCE and VNTM, NSM and VNTM, and VNTM and LSR. In the PCE-VNTM
   cooperation model and the NMS-VNTM model, the additional
   communication steps are required compared with the higher-layer
   signaling trigger model. On the other hand, the cooperation model
   provides better control at the cost of a longer service setup time.

   Note that, in terms of higher-layer signaling time, in the higher-
   layer signaling trigger model, the required time from when higher-
   layer signaling starts to when it is completed, is more than that of
   the cooperation model except when a virtual TE link is included.
   This is because the former model requires lower-layer signaling to
   take place during the higher-layer signaling. A higher-layer ingress



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   LSR has to wait for more time until the higher-layer signaling is
   completed. A higher-layer ingress LSR is required to be tolerant of
   longer path setup times.

5.4. Network Complexity

   If the higher and lower layer networks have multiple interconnects
   then optimal path computation for end-to-end LSPs that cross the
   layer boundaries is non-trivial. The higher layer LSP must be routed
   to the correct layer border nodes to achieve optimality in both
   layers.

   Where the lower layer LSPs are advertised into the higher layer
   network as TE links, the computation can be resolved in the higher
   layer network. Care needs to be taken in the allocation of TE
   metrics (i.e., costs) to the lower layer LSPs as they are advertised
   as TE links into the higher layer network, and this might be a
   function for a VNT Manager component. Similarly, attention should be
   given to the fact that the LSPs crossing the lower-layer network
   might share points of common failure (e.g., they might traverse the
   same link in the lower-layer network) and the shared risk link
   groups (SRLGs) for the TE links advertised in the higher-layer must
   be set accordingly.

   In the single PCE model an end-to-end path can be found in a single
   computation because there is full visibility into both layers and
   all possible paths through all layer interconnects can be considered.

   Where PCEs cooperate to determine a path, an iterative computation
   model such as [BRPC] can be used to select an optimal path across
   layers.

   When non-cooperating mono-layer PCEs, each of which is in a separate
   layer, are used with the triggered LSP model, it is not possible to
   determine the best border LSRs, and connectivity cannot even be
   guaranteed. In this case, signaling crankback techniques [RFC4920]
   can be used to eventually achieve connectivity, but optimality is
   far harder to achieve. In this model, a PCE that is requested by an
   ingress LSR to compute a path expects a border LSR to setup a lower-
   layer path triggered by high-layer signaling when there is no TE
   link between border LSRs.

5.5. Separation of Layer Management

   Many network operators may want to provide a clear separation
   between the management of the different layer networks. In some
   cases, the lower layer network may come from a separate commercial
   arm of an organization or from a different corporate body entirely.
   In these cases, the policy applied to the establishment of LSPs in



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   the lower-layer network and to the advertisement of these LSPs as TE
   links in the higher-layer network will reflect commercial agreements
   and security concerns (see next section). Since the capacity of the
   LSPs in the lower-layer network are likely to be significantly
   larger than those in the client higher-layer network (multiplex-
   server model), the administrator of the lower-layer network may want
   to exercise caution before allowing a single small demand in the
   higher layer to tie up valuable resources in the lower layer.

   The necessary policy points for this separation of administration
   and management are more easily achieved through the VNTM approach
   than by using triggered signaling. In effect, the VNTM is the
   coordination point for all lower layer LSPs and can be closely tied
   to a human operator as well as to policy and billing. Such a model
   can also be achieved using triggered signaling.

6. Stability Considerations

   Inter-layer traffic engineering needs to be managed and operated
   correctly to avoid introducing instability problems.

   Lower-layer LSPs are likely, by the nature of the technologies used
   in layered networks, to be of considerably higher capacity than the
   higher-layer LSPs. This has the benefit of allowing multiple higher-
   layer LSPs to be carried across the lower-layer network in a single
   lower-layer LSP. However, when a new lower-layer LSP is set up to
   support a request for a higher-layer LSP because there is no
   suitable route in the higher-layer network, it may be the case that
   a very large LSP is established in support of a very small traffic
   demand. Further, if the higher-layer LSP is short-lived, the
   requirement for the lower-layer LSP will go away leaving it either
   in-place but unused, or requiring it to be torn down. This may cause
   excessive tie-up of unused lower-layer network resources, or may
   introduce instability into the lower-layer network. It is important
   that appropriate policy controls or configuration features are
   available so that demand-led establishment of lower-layer LSPs (the
   so-called "bandwidth on demand") is filtered according to the
   requirements of the lower-layer network.

   When a higher-layer LSP is requested to be set up, a new lower-layer
   LSP may be established if there is no route with the requested
   bandwidth for the higher-layer LSP. After the lower-layer LSP is
   established, existing high-layer LSPs could be re-routed to use the
   newly established lower-layer LSP if using the lower-layer LSP
   provides a better route than that taken by the existing LSPs. This
   re-routing may result in lower utilization of other lower-layer LSPs
   that used to carry the existing higher-layer LSPs. When the
   utilization of a lower-layer LSP drops below a threshold (or drops
   to zero), the LSP is deleted according to lower-layer network policy.



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   But consider that some other new higher-layer LSP may be requested
   at once requiring the establishment or re-establishment of a lower-
   layer LSP. This, in turn, may cause higher-layer re-routing making
   other lower-layer LSPs under-utilized, in a cyclic manner. This
   behavior makes the higher-layer network unstable.

   Inter-layer traffic engineering needs to avoid network instability
   problems. To solve the problem, network operators may have some
   constraints achieved through configuration or policy, where inter-
   layer path control actions such as re-routing and deletion of lower-
   layer LSPs are not easily allowed. For example, threshold parameters
   for the actions are determined so that hysteresis control behavior
   can be performed.

7. Manageability Considerations

   Inter-layer MPLS or GMPLS traffic engineering must be considered in
   the light of administrative and management boundaries that are
   likely to coincide with the technology layer boundaries. That is,
   each layer network may possibly be under separate management control
   with different policies applied to the networks, and specific policy
   rules applied at the boundaries between the layers.

   Management mechanisms are required to make sure that inter-layer
   traffic engineering can be applied without violating the policy and
   administrative operational procedures used by the network operators.

7.1. Control of Function and Policy

7.1.1. Control of Inter-Layer Computation Function

   PCE implementations that are capable of supporting inter-layer
   computations should provide a configuration switch to allow support
   of inter-layer path computations to be enabled or disabled.

   When a PCE is capable of, and configured for, inter-layer path
   computation, it should advertise this capability as described in
   [PCE-INTER-LAYER-REQ], but this advertisement may be suppressed
   through a secondary configuration option.

7.1.2. Control of Per-Layer Policy

   Where each layer is operated as a separate network, the operators
   must have control over the policies applicable to each network, and
   that control should be independent of the control of policies for
   other networks.

   Where multiple layers are operated as part of the same network, the
   operator may have a single point of control for an integrated policy


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   across all layers, or may have control of separate policies for each
   layer.

7.1.3. Control of Inter-Layer Policy

   Probably the most important issue for inter-layer traffic
   engineering is inter-layer policy. This may cover issues such as
   under what circumstances a lower layer LSP may be established to
   provide connectivity in the higher layer network. Inter-layer policy
   may exist to protect the lower layer (high capacity) network from
   very dynamic changes in micro-demand in the higher layer network
   (see Section 6). It may also be used to ensure appropriate billing
   for the lower layer LSPs.

   Inter-layer policy should include the definition of the points of
   connectivity between the network layers, the inter-layer TE model to
   be applied (for example, the selection between the models described
   in this document), and the rules for path computation and LSP setup.
   Where inter-layer policy is defined, it must be used consistently
   throughout the network, and should be made available to the PCEs
   that perform inter-layer computation so that appropriate paths are
   computed. Mechanisms for providing policy information to PCEs are
   discussed in [PCE-POLICY].

   VNTM may provide a suitable functional component for the
   implementation of inter-layer policy. Use of VNTM allows the
   administrator of the lower layer network to apply inter-layer policy
   without making that policy public to the operator of the higher
   layer network. Similarly, a cooperative PCE model (with or without
   inter-PCE communication) allows separate application of policy
   during the selection of paths.

7.2. Information and Data Models

   Any protocol extensions to support inter-layer computations must be
   accompanied by the definition of MIB objects for the control and
   monitoring of the protocol extensions. These MIB object definitions
   will conventionally be placed in a separate document from that which
   defines the protocol extensions. The MIB objects may be provided in
   the same MIB module as used for the management of the base protocol
   that is being extended.

   Note that inter-layer PCE functions should, themselves, be
   manageable through MIB modules. In general, this means that the MIB
   modules for managing PCEs should include objects that can be used to
   select and report on the inter-layer behavior of each PCE. It may
   also be appropriate to provide statistical information that reports
   on the inter-layer PCE interactions.




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   Where there are communications between a PCE and VNTM, additional
   MIB modules may be necessary to manage and model these
   communications. On the other hand, if these communications are
   provided through MIB notifications, then those notifications must
   form part of a MIB module definition.

   Policy Information Base (PIB) modules may also be appropriate to
   meet the requirements as described in Section 6.1 and [PCE-POLICY].

7.3. Liveness Detection and Monitoring

   Liveness detection and monitoring is required between PCEs and PCCs,
   and between cooperating PCEs as described in [RFC4657]. Inter-layer
   traffic engineering does not change this requirement.

   Where there are communications between a PCE and VNTM, additional
   liveness detection and monitoring may be required to allow the PCE
   to know whether the VNTM has received its information about failed
   path computations and desired TE links.

   When a lower layer LSP fails (perhaps because of the failure of a
   lower layer network resource) or is torn down as a result of lower
   layer network policy, the consequent change should be reported to
   the higher layer as a change in the VNT, although inter-layer policy
   may dictate that such a change is hidden from the higher layer. The
   higher layer network may additionally operate data plane failure
   techniques over the virtual TE links in the VNT in order to monitor
   the liveness of the connections, but it should be noted that if the
   virtual TE link is advertised but not yet established as an LSP in
   the lower layer, such higher layer OAM techniques will report a
   failure.

7.4. Verifying Correct Operation

   The correct operation of the PCE computations and interactions are
   described in [RFC4657], [PCEP], etc., and does not need further
   discussion here.

   The correct operation of inter-layer traffic engineering may be
   measured in several ways. First, the failure rate of higher layer
   path computations owing to an absence of connectivity across the
   lower layer may be observed as a measure of the effectiveness of the
   VNT and may be reported as part of the data model described in
   Section 6.2. Second, the rate of change of the VNT (i.e., the rate
   of establishment and removal of higher layer TE links based on lower
   layer LSPs) may be seen as a measure of the correct planning of the
   VNT and may also form part of the data model described in Section
   6.2. Third, network resource utilization in the lower layer (both in
   terms of resource congestion, and in consideration of under



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   utilization of LSPs set up to support virtual TE links) can indicate
   whether effective inter-layer traffic engineering is being applied.

   Management tools in the higher layer network should provide a view
   of which TE links are provided using planned lower layer capacity
   (that is, physical connectivity or permanent connections) and which
   TE links are dynamic and achieved through inter-layer traffic
   engineering. Management tools in the lower layer should provide a
   view of the use to which lower layer LSPs are put including whether
   they have been set up to support TE links in a VNT, and if so for
   which client network.

7.5. Requirements on Other Protocols and Functional Components

   There are no protocols or protocol extensions defined in this
   document and so it is not appropriate to consider specific
   interactions with other protocols. It should be noted, however, that
   the objective of this document is to enable inter-layer traffic
   engineering for MPLS-TE and GMPLS networks and so it is assumed that
   the necessary features for inter-layer operation of routing and
   signaling protocols are in existence or will be developed.

   This document introduces roles for various network components (PCE,
   LSR, NMS, and VNTM). Those components are all required to play their
   part in order that inter-layer TE can be effective. That is, an
   inter-layer TE model that assumes the presence and operation of any
   of these functional components obviously depends on those components
   to fulfill their roles as described in this document.

7.6. Impact on Network Operation

   The use of a PCE to compute inter-layer paths is expected to have a
   significant and beneficial impact on network operations. Inter-layer
   traffic engineering of itself may provide additional flexibility to
   the higher layer network while allowing the lower layer network to
   support more and varied client networks in a more efficient way.
   Traffic engineering across network layers allows optimal use to be
   made of network resources in all layers.

   The use of PCE as described in this document may also have a
   beneficial effect on the loading of PCEs responsible for performing
   inter-layer path computation while facilitating a more independent
   operation model for the network layers.

8. Security Considerations

   Inter-layer traffic engineering with PCE raises new security issues
   in all three inter-layer path control models.




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   In the cooperation model between PCE and VNTM, when the PCE
   determines that a new lower-layer LSP is desirable, communications
   are needed between the PCE and VNTM and between VNTM and a border
   LSR. In this case, these communications should have security
   mechanisms to ensure authenticity, privacy and integrity of the
   information exchanged. In particular, it is important to protect
   against false triggers for LSP setup in the lower-layer network
   since such falsification could tie up lower-layer network resources
   (achieving a denial of service attack on the lower-layer network and
   on the higher layer network that is attempting to use it) and could
   result in incorrect billing for services provided by the lower-layer
   network. Where the PCE MIB modules are used to provide the
   notification exchanges between the higher-layer PCE and the VNTM,
   SNMP v3 should be used to ensure adequate security. Additionally,
   the VNTM should provide configurable or dynamic policy functions so
   that the VNTM behavior upon receiving notification from a higher-
   layer PCE can be controlled.

   The main security concern in the higher-layer signaling trigger
   model is related to confidentiality. The PCE may inform a higher-
   layer PCC about a multi-layer path that includes an ERO in the
   lower-layer network, but the PCC may not have TE topology visibility
   into the lower-layer network and might not be trusted with this
   information. A loose hop across the lower-layer network could be
   used, but this decreases the benefit of multi-layer traffic
   engineering. A better alternative may be to mask the lower-layer
   path using a path key [PATH-KEY] that can be expanded within the
   lower-layer network. Consideration must also be given to filtering
   the recorded path information from the lower-layer - see [RFC4208],
   for example.

   Additionally, in the higher-layer signaling trigger model,
   consideration must be given to the security of signaling at the
   inter-layer interface since the layers may belong to different
   administrative or trust domains.

   The NMS-VNTM cooperation model introduces communication between the
   NMS and the VNTM. Both of these components belong to the management
   plane and the communication is out of scope for this PCE document.
   Note that the NMS-VNTM cooperation model may be considered to
   address many security and policy concerns because the control and
   decision-making is placed within the sphere of influence of the
   operator in contrast to the more dynamic mechanisms of the other
   models. However, the security issues have simply moved, and will
   require authentication of operators and of policy.

   Security issues may also exist when a single PCE is granted full
   visibility of TE information that applies to multiple layers. Any
   access to the single PCE will immediately gain access to the



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   topology information for all network layers - effectively, a single
   security breach can expose information that requires multiple
   breaches in other models.

   Note that, as described in Section 6, inter-layer TE can cause
   network stability issues, and this could be leveraged to attack
   either the higher or lower layer network. Precautionary measures,
   such as those described in Section 7.1.3, can be applied through
   policy or configuration to dampen any network oscillations.

9. Acknowledgments

  We would like to thank Kohei Shiomoto, Ichiro Inoue, Julien Meuric,
  Jean-Francois Peltier, Young Lee, Ina Minei, and Jean-Philippe
  Vasseur for their useful comments.

10. References

10.1. Normative Reference

   [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
   Label Switching Architecture", RFC 3031, January 2001.

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

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

10.2. Informative Reference

   [MLN-REQ] K. Shiomoto et al., "Requirements for GMPLS-based multi-
   region networks (MRN)", draft-ietf-ccamp-gmpls-mln-reqs (work in
   progress).

   [PCE-INTER-LAYER-REQ] E. Oki et al., "PCC-PCE Communication
   Requirements for Inter-Layer Traffic Engineering", draft-ietf-pce-
   inter-layer-req (work in progress).

   [BRPC] JP. Vasseur et al., "A Backward Recursive PCE-based
   Computation (BRPC) procedure to compute shortest inter-domain
   Traffic Engineering Label Switched Paths", draft-ietf-pce-brpc (work
   in progress).

   [RFC4920] A. Farrel et al., "Crankback Signaling Extensions for MPLS
   and GMPLS RSVP-TE", RFC 4920, July 2007.





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   [PCE-MIB] E. Stephan, "Definitions of Textual Conventions for Path
   Computation Element", draft-ietf-pce-tc-mib.txt (work in progress).

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

   [PATH-KEY] Bradford, R., Vasseur, JP., and Farrel, A., "Preserving
   Topology Confidentiality in Inter-Domain Path Computation Using a
   Key Based Mechanism", draft-ietf-pce-path-key, work in progress.

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

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

   [RFC4657] J. Ash and J.L. Le Roux (Ed.), "Path Computation Element
   (PCE) Communication Protocol Generic Requirements", RFC 4657,
   September 2006.

   [PCE-POLICY] Bryskin, I., Papadimitriou, P., and Berger, L.,
   "Policy-Enabled Path Computation Framework", draft-ietf-pce-policy-
   enabled-path-comp, (work in progress).

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


11. Authors' Addresses

   Eiji Oki
   NTT
   3-9-11 Midori-cho,
   Musashino-shi, Tokyo 180-8585, Japan
   Email: oki.eiji@lab.ntt.co.jp

   Tomonori Takeda
   NTT
   3-9-11 Midori-cho,
   Musashino-shi, Tokyo 180-8585, Japan
   Email: takeda.tomonori@lab.ntt.co.jp

   Jean-Louis Le Roux
   France Telecom R&D,
   Av Pierre Marzin,



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   22300 Lannion, France
   Email: jeanlouis.leroux@orange-ftgroup.com

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

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   This document and the information contained herein are provided on
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   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
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