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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: April 2007                                   France Telecom
                                                            Adrian Farrel
                                                       Old Dog Consulting
                                                             October 2006
  
         Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic
                                 Engineering
  
                   draft-ietf-pce-inter-layer-frwk-02.txt
  
  
  
     Status of this Memo
  
     By submitting this Internet-Draft, each author represents that any
     applicable patent or other IPR claims of which he or she is aware
     have been or will be disclosed, and any of which he or she becomes
     aware will be disclosed, in accordance with Section 6 of BCP 79.
  
     Internet-Drafts are working documents of the Internet Engineering
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     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.txt.
  
     The list of Internet-Draft Shadow Directories can be accessed at
     http://www.ietf.org/shadow.html.
  
     Abstract
  
     A network may comprise of multiple layers. It is important to
     globally optimize network resources 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 path
     computation architecture to inter-layer MPLS and 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,
  
  
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        draft-ietf-pce-inter-layer-frwk-02.txt   October 2006
  
     and the relationship between PCE and a functional component called
     the Virtual Network Topology Manager (VNTM).
  
     Table of Contents
  
    1. Terminology.....................................................2
    2. Introduction....................................................2
    3. Inter-Layer Path Computation....................................3
    4. Inter-layer Path Computation Models.............................5
    4.1.  Single PCE Inter-Layer Path Computation......................5
    4.2.  Multiple PCE Inter-Layer Path Computation....................6
    4.3.  General observations.........................................7
    5. Inter-Layer Path Control........................................7
    5.1.  VNT Management...............................................8
    5.2.  Inter-Layer Path Control Models..............................8
    5.2.1.  Cooperation model between PCE and VNTM.....................8
    5.2.2.  Higher-Layer Signaling Trigger Model......................10
    5.2.3.  Examples of multi-layer ERO...............................12
    6. Choosing between inter-layer path control models...............12
    6.1.  VNTM functions:.............................................13
    6.2.  Border LSR functions:.......................................13
    6.3.  Complete inter-layer LSP setup time:........................13
    6.4.  Network complexity..........................................14
    7. Security Considerations........................................14
    8. Acknowledgment.................................................15
    9. References.....................................................15
    9.1.  Normative Reference.........................................15
    9.2.  Informative Reference.......................................15
    10.  Authors' Addresses...........................................16
    11.  Intellectual Property Statement..............................16
  
  
  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. Introduction
  
     A network may comprise of multiple layers. These layers may
     represent separations of technologies (e.g., packet switch capable
     (PSC), time division multiplex (TDM) lambda switch capable (LSC))
     [RFC3945], separation of data plane switching granularity levels
     (e.g. PSC-1, PSC-2, VC4, 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
  
  
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          draft-ietf-pce-inter-layer-frwk-02.txt   October 2006
  
     topology formed by lower-layer LSPs and advertised to the higher
     layer is called a 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 VNT by setting up and releasing LSPs in the lower layers
     [MLN-REQ].
  
     Inter-layer traffic engineering is included in the scope of the
     PCE-based path computation 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 forth in [PCE-INTER-LAYER-REQ].
  
     This document describes a framework for applying the PCE-based path
     computation 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).
  
  3. 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, whose
     explicit route 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 lower-layer topology, particularly in an overlay
     or augmented model, and hence may not be able to compute an end-to-
     end path across layers.
  
     PCE-based inter-layer path computation, consists of relying on one
     or more PCEs to compute an end-to-end path across layers. This
     could be achieved by rely on 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 have information about the topology of
  
  
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     one or more layers, but not all layers, and 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 network 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 Traffic Engineering (TE) link between border LSRs,
     which are located on the boundary between the higher-layer and
     lower-layer networks, and that the ingress LSR does not have
     topology visibility in the lower layer. If a single-layer path
     computation is applied for the higher-layer, the path computation
     fails. 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 setup between border LSRs, considering both layers' TE
     topologies.
  
     Lower-layer LSPs form a Virtual Network Topology (VNT), which can
     be used for routing higher-layer LSPs or to carry IP traffic.
     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. 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
     between 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 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 computes and returns a path to the PCC that the PCC can use
     to build an MPLS or GMPLS LSP once 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 expected lower-layer LSPs
     to be established: that is the resulting ERO includes sub-object(s)
     corresponding to lower-layer hierarchical LSPs expressed as the TE
     link identifiers, which can be numbered or unnumbered ones, of the
  
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     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 regular TE link, this does not trigger new
     lower-layer LSP setup, but the utilization of existing lower-layer
     LSPs. If it is a virtual TE link, this triggers a new lower-layer
     LSP setup (provided that there are available resources in the lower
     layer). A transit LSR corresponding to the entry point of the
     virtual TE link is expected to trigger the new lower-layer LSP
     setup. Note that the path of a virtual TE link is not necessarily
     known in advance, and this may require path computation either on
     the entry point or on a PCE. The second case is that the PCE
     computes a path that includes loose hop(s). The higher layer path
     would select which lower layer paths to use and would select the
     entry and exit points from those layers, but would not select the
     path across the layers. A transit LSR corresponding to the entry
     point 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.
  
  4. Inter-layer Path Computation Models
  
     As stated in Section 3, two PCE modes defined in the PCE
     architecture can be used to perform inter-layer path computation.
     They are discussed below.
  
  4.1.  Single PCE Inter-Layer Path Computation
  
     In this model Inter-layer path computation is performed by a single
     PCE that has topology visibility in all layers. Such a PCE is
     called a multi-layer PCE.
  
     In Figure 1, the network is comprised of two layers. LSR H1, H2, H3
     and H4 belong to the higher layer, and LSRs 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 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 established or not.
  
  
  
  
  
  
  
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                             -----
                            | 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
  
  4.2.  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 other 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 of one layer (TED size
     reduction).
  
      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
     Lo is responsible for path computation in the lower layer. A simple
     example of cooperation between the PCEs could be: PCE Hi requests a
     path H2-H3 from PCE Lo. Of course more complex cooperation may be
     required if an end-to-end optimal path is desired.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
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                                  -----
                                 | 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
  
  
  4.3.  General observations
  
     - Depending on implementation details, inter-layer path computation
     time 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 layered 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 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.
  
  5. Inter-Layer Path Control
  
  
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  5.1.  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 are provisioned across the lower-
     layer network. Further, 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 Management" are distinct
     functions that may or may not be co-located. To describe each
     function clearly, VNTM is considered as a functional element in
     this draft.
  
  5.2.  Inter-Layer Path Control Models
  
   5.2.1. 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.
  
     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) should be established to
     support future LSP requests. 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
  
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     establish a lower-layer LSP. VNTM or the ingress LSR (H2) may use a
     PCE with visibility into the lower layer to compute the path of
     this new LSP.
  
     If the PCE cannot compute a path for the higher-layer LSP without
     the establishment of a further lower-layer LSP, the PCE may notify
     VNTM and wait for the lower-layer LSP to be set up and advertised
     as a TE link. It can 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 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, the PCE operates a timeout, or the PCE will be
     notified by VNTM that no new LSP will become available. An example
     of such a cooperative procedure between PCE and VNTM is as follows.
  
     Step 1: H1 (PCC) requests PCE to compute a path between H1 and H4.
     In the request, it 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 suggests to VNTM that a new lower-layer LSP should be
     established if necessary and if acceptable within VNTM's policy
     constraints. The inter-layer path route computed by PCE may include
     one or more virtual TE links. If PCE knows the inclusion of the
     virtual TE link(s) in the inter-layer route, PCE may suggest VNTM
     that the corresponding new lower-layer LSP(s) should be established.
     Otherwise, new lower-layer LSP(s) may be setup according to the
     higher-layer signaling trigger model.
  
     In the above description, it is assumed that a higher layer LSP is
     supported by a single lower layer LSP. However, in case of VCAT,
     several lower layer LSPs may be used to transport a single higher
     layer LSP.
  
     Step 4: VNTM requests an ingress LSR (e.g. H2) to establish a
     lower-layer LSP. The request message may include a pre-computed
     lower-layer LSP route obtained from the PCE responsible for the
     lower-layer network.
  
     Step 5: The ingress LSR starts signaling to establish a lower-layer
     LSP.
  
     Step 6: If the lower-layer LSP setup is completed, the ingress LSR
     notifies VNTM that the LSP is complete and supplies the tunnel
     information.
  
     Step 7: VNTM replies to PCE to inform it that the lower-layer LSP
     is now established, and includes the lower-layer tunnel information.
     Alternatively, PCE may get to know about the existence of the
     lower-layer LSP when a new TE link in the higher-layer
  
  
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     corresponding to the lower-layer LSP is advertised to PCE through
     the IGP.
  
     Step 8: 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. The higher-layer
     route is specified as 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.
  
   5.2.2. 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 5.2.1, 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.
  
     If PCE judges that a lower-layer LSP needs to be established based
     on the inter-layer path computation result, a lower-layer LSP is
     established during the higher-layer signaling procedure. After PCE
     completes inter-layer path computation, PCE sends a reply message
     including explicit route to the ingress LSR (PCC). There are two
     ways to express the higher-layer LSP route, which are a multi-layer
     path and a mono-layer path that includes loose hop(s).
  
     In the higher-layer signaling trigger model with a multi-layer path,
     a high-layer LSP route includes a route for a lower-layer LSP that
     is not yet established. An LSR that is located at the boundary
     between the higher-layer and lower-layer networks, called a border
     LSR, receives a higher-layer signaling message and then may start
     to setup the lower-layer LSP. Note that it depends on a policy at
     the border LSR whether the higher-layer signaling triggers a lower-
     layer LSP setup. An example procedure of the signaling trigger
     model with a multi-layer path is as follows.
  
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     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 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 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 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.
  
     On the other hand, in the signaling trigger model with a mono-layer
     path, a higher-layer LSP route includes a loose or strict hop to
     traverse the lower-layer network between the two border LSRs. In
     the strict hop case, a virtual TE link may be advertised, but a
     lower-layer LSP is not setup. 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 a lower-layer LSP. Again, it is
     possible that a suitable lower-layer LSP has been established (or
     become available) between the time that the higher-layer
     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. 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.
  
   5.2.3. Examples of multi-layer ERO
  
     PCE
      ^
      :
      :
      V
     H1--H2             H3--H4
          \                  /
           L1==L2==L3--L4--L5
                    |
                    |
                   L6--L7
                         \
                          H5--H6
  
     Figure 5 Example of 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, 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.
  
  6. 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 is chosen, taking into all these considerations.
  
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     6.1. VNTM functions:
  
     In the cooperation model, VNTM functions are required. In this
     model, additional overhead communications between PCE and VNTM and
     between VNTM and a border LSR are required.
  
     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 not available in the trigger model.
  
     6.2. 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. The triggering signaling is also
     required in the cooperation case when the VNTM support virtual TE
     links. Note that, if only the cooperation model is applied, it is
     required that a PCE knows whether a link is a regular TE link or
     virtual TE link.
  
     If the ERO in the higher-layer Path message uses a mono-layer path
     or specifies loose hop, a border LSR receiving the Path message
     MUST obtain a lower-layer route either by consulting PCE or by
     using its own computation engine. If the ERO in the higher-layer
     Path message uses multi-layer path, the border LSR MUST judge
     whether lower-layer signaling is needed.
  
     In the cooperation model, no additional function for triggered
     signaling in border LSRs is required except when virtual TE links
     are used. Therefore, if these additional functions are not
     supported in border LSRs, the cooperation model, where a border LSR
     is controlled by VNTM to set up a lower-layer LSP, has to be chosen.
  
     6.3. Complete inter-layer LSP setup time:
  
     Complete inter-layer LSP setup time includes inter-layer path
     computation, signaling, and 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
  
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     of the cooperation model except when any 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.
  
     An appropriate model is chosen, taking into all of the above
     considerations.
  
     6.4. 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 an issue for a VNT Manager component.
  
     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 each
     layer, are used with a 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 to
     compute a path by an ingress LSR expects a border LSR to setup a
     lower-layer path triggered by high-layer signaling when there is no
     TE link between border LSRs.
  
  7. Security Considerations
  
     Inter-layer traffic engineering with PCE may raise new security
     issues in both inter-layer path control models.
  
     In the cooperation model between PCE and VNTM, when 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
  
  
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     communications. These communications should have some security
     mechanisms to ensure authenticity, privacy and integrity.
  
     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.
  
  8. Acknowledgment
  
    We would like to thank Kohei Shiomoto, Ichiro Inoue, Julien Meuric,
    Jean-Francois Peltier, Young Lee, and Ina Minei for their useful
    comments.
  
  9. References
  
  9.1.  Normative Reference
  
     [RFC2119] Bradner, S., "Key words for use in RFCs to indicate
     requirements levels", RFC 2119, March 1997.
  
     [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.
  
     [RFC4208] G. Swallow et al., "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.
  
  9.2.  Informative Reference
  
     [RFC4657] J. Ash, J.L Le Roux et al., " Path Communication Element
     (PCE) Communication Protocol Generic Requirements", RFC 4657,
     September 2006.
  
     [RFC4674] JL Le Roux et al., "Requirements for Path Computation
     Element (PCE) Discovery", RFC 4674, September 2006.
  
  
  
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     [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).
  
     [PCEP] JP. Vasseur et al, "Path Computation Element (PCE)
     communication Protocol (PCEP) - Version 1 -" draft-ietf-pce-pcep
     (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).
  
  10.     Authors' Addresses
  
     Eiji Oki
     NTT
     3-9-11 Midori-cho,
     Musashino-shi, Tokyo 180-8585, Japan
     Email: oki.eiji@lab.ntt.co.jp
  
     Jean-Louis Le Roux
     France Telecom R&D,
     Av Pierre Marzin,
     22300 Lannion, France
     Email: jeanlouis.leroux@orange-ft.com
  
     Adrian Farrel
     Old Dog Consulting
     Email: adrian@olddog.co.uk
  
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