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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                                       Eiji Oki
   Internet Draft                                                   NTT
   Category: Informational                           Jean-Louis Le Roux
   Expires: January 2008                                 France Telecom
                                                          Adrian Farrel
                                                     Old Dog Consulting
                                                              July 2007

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

                  draft-ietf-pce-inter-layer-frwk-04.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.

   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).








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   Table of Contents

  1. Introduction....................................................2
  1.1.  Terminology..................................................3
  2. Inter-Layer Path Computation....................................3
  3. Inter-layer Path Computation Models.............................5
  3.1.  Single PCE Inter-Layer Path Computation......................5
  3.2.  Multiple PCE Inter-Layer Path Computation....................5
  3.3.  General Observations.........................................6
  4. Inter-Layer Path Control........................................7
  4.1.  VNT Management...............................................7
  4.2.  Inter-Layer Path Control Models..............................7
  4.2.1.  Cooperation Model Between PCE and VNTM.....................7
  4.2.2.  Higher-Layer Signaling Trigger Model.......................9
  4.2.3.  Examples of Multi-Layer ERO...............................11
  5. Choosing Between Inter-Layer Path Control Models...............11
  5.1.  VNTM Functions:.............................................11
  5.2.  Border LSR Functions:.......................................12
  5.3.  Complete Inter-Layer LSP Setup Time:........................12
  5.4.  Network Complexity..........................................12
  5.5.  Separation of Layer Management..............................13
  6. Security Considerations........................................13
  7. Acknowledgment.................................................14
  8. References.....................................................14
  8.1.  Normative Reference.........................................14
  8.2.  Informative Reference.......................................14
  9. Authors' Addresses.............................................14
  10.  Intellectual Property Statement..............................15

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) to the higher layer 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.


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   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, and called the Virtual Network Topology
   Manager (VNTM).

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.

   Consider, for instance, a two-layer network where the higher-layer
   network is a packet-based IP/MPLS or GMPLS network, and the lower-
   layer network is a GMPLS optical network. An ingress LSR in the
   higher-layer network tries to set up an LSP to an egress LSR 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.
   Suppose also that the ingress LSR does not have topology visibility
   into the lower layer. If a single-layer path computation is applied
   for 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 and a suggestion that a
   lower-layer LSP be set up between the border LSRs.



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   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
   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 from 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




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   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 1, 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.


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

     Figure 1 : Multi-Layer PCE - A single PCE with multi-layer
   visibility

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.

   In Figure 2, 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




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

   Figure 2 : Cooperating Mono-Layer PCEs - Multiple PCEs with single-
   layer visibility


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
   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



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   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 inter-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 network 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.
         Cooperation Model Between PCE and VNTM

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

   Figure 3: Cooperation Model Between PCE and VNTM

   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 3. Consider that H1 requests 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 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



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   information about the higher-layer demand (from H2 to H3). 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. It could then compute the complete end-to-end path for the
   higher-layer LSP and 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 operates a timeout. An example of such a cooperative
   procedure between PCE and VNTM is as follows using the exmaple
   network in Figure 3.

   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.







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 4.2.2.
         Higher-Layer Signaling Trigger Model


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

   Figure 4: Higher-layer Signaling Trigger Model

   Figure 4 shows the higher-layer signaling trigger 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 visiblity 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.

   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.





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   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.

   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.







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 4.2.3.
        Examples of Multi-Layer ERO

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

   Figure 5: Example of a Multi-Layer Network

   This section describes 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)}

   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.

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:

   In the cooperation model, VNTM functions are required. In this model,
   communications are required between PCE and VNTM, and between VNTM
   and a border LSR. VNTM-LSR communication can rely on existing GMPLS-
   TE MIB modules. PCE-VNTM communication will be detailed in further
   revisions of this document.

   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.





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   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 cooperation 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, and VNTM and LSR. In the cooperation 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
   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



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   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 [CRANK] 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
   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. Security Considerations

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

   In the cooperation model between PCE and VNTM, when the PCE judges a
   new lower-layer LSP, communications between PCE and VNTM and between
   VNTM and a border LSR are needed. In this case, there are some
   security concerns that need to be addressed for these communications.
   These communications should have some security mechanisms to ensure
   authenticity, privacy and integrity. In particular, it is important
   to protect against false triggers for LSP setup in the lower-layer
   network.



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   In the higher-layer signaling trigger model, there are several
   security concerns. First, PCE may inform PCC, which is located in
   the higher-layer network, of multi-layer path information that
   includes an ERO in the lower-layer network, while the PCC may not
   have TE topology visibility into the lower-layer network. This
   raises a security concern, where lower-layer hop information is
   known to transit LSRs supporting a higher-layer LSP. Some security
   mechanisms to ensure authenticity, privacy and integrity may be used.

   Security issues may also exist when a single PCE is granted full
   visibility of TE information that applies to multiple layers.

7. Acknowledgment

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

8. References

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

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

8.2.  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).

   [CRANK] A. Farrel et al., "Crankback Signaling Extensions for MPLS
   and GMPLS RSVP-TE", draft-ietf-ccamp-crankback (work in progress).

9. Authors' Addresses

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



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