GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc5623

Network Working Group E. Oki Request for Comments: 5623 University of Electro-Communications Category: Informational T. Takeda

                                                                   NTT
                                                           JL. Le Roux
                                                        France Telecom
                                                             A. Farrel
                                                    Old Dog Consulting
                                                        September 2009

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

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).

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright and License Notice

 Copyright (c) 2009 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect

Oki, et al. Informational [Page 1] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the BSD License.

Table of Contents

 1. Introduction ....................................................3
    1.1. Terminology ................................................3
 2. Inter-Layer Path Computation ....................................4
 3. Inter-Layer Path Computation Models .............................7
    3.1. Single PCE Inter-Layer Path Computation ....................7
    3.2. Multiple PCE Inter-Layer Path Computation ..................7
    3.3. General Observations ......................................10
 4. Inter-Layer Path Control .......................................10
    4.1. VNT Management ............................................10
    4.2. Inter-Layer Path Control Models ...........................11
         4.2.1. PCE-VNTM Cooperation Model .........................11
         4.2.2. Higher-Layer Signaling Trigger Model ...............13
         4.2.3. NMS-VNTM Cooperation Model .........................16
         4.2.4. Possible Combinations of Inter-Layer Path
                Computation and Inter-Layer Path Control Models ....21
 5. Choosing between Inter-Layer Path Control Models ...............22
    5.1. VNTM Functions ............................................22
    5.2. Border LSR Functions ......................................23
    5.3. Complete Inter-Layer LSP Setup Time .......................24
    5.4. Network Complexity ........................................24
    5.5. Separation of Layer Management ............................25
 6. Stability Considerations .......................................25
 7. Manageability Considerations ...................................26
    7.1. Control of Function and Policy ............................27
         7.1.1. Control of Inter-Layer Computation Function ........27
         7.1.2. Control of Per-Layer Policy ........................27
         7.1.3. Control of Inter-Layer Policy ......................27
    7.2. Information and Data Models ...............................28
    7.3. Liveness Detection and Monitoring .........................28
    7.4. Verifying Correct Operation ...............................29
    7.5. Requirements on Other Protocols and Functional
         Components ................................................29
    7.6. Impact on Network Operation ...............................30
 8. Security Considerations ........................................30
 9. Acknowledgments ................................................31
 10. References ....................................................32
    10.1. Normative Reference ......................................32
    10.2. Informative Reference ....................................32

Oki, et al. Informational [Page 2] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

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) [RFC5212], 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) [RFC5212].
 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.  Inter-layer traffic engineering includes using
 mechanisms that allow the computation of end-to-end paths across
 layers (known as inter-layer path computation) and mechanisms that
 control and manage the Virtual Network Topology (VNT) by setting up
 and releasing LSPs in the lower layers [RFC5212].
 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 [PCC-PCE].
 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 as well as describing 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).

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 [RFC5212].

Oki, et al. Informational [Page 3] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

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 that has an
 explicit route and 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.
  1. —- —– —– —–

| 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 (LSRs H1, H2, H3, and H4) is a
 packet-based IP/MPLS or GMPLS network, 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
 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.

Oki, et al. Informational [Page 4] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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 to suggest 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:
  1. 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].
  1. 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].
  1. 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, which is performed
   independently by successive PCEs.
 Note that when a network consists of more than two layers (e.g., MPLS
 over SONET over Optical Transport Network (OTN)) and a path
 traversing more than two layers needs to be computed, it is possible
 to combine multiple PCE-based path computation models.  For example,
 the single PCE computation model could be used for computing a path
 across the SONET layer and the OTN layer, and the multiple PCE
 computation with inter-PCE communication model could be used for
 computing a path across the MPLS layer (computed by higher-layer PCE)
 and the SONET layer (computed by lower-layer PCE).
 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.

Oki, et al. Informational [Page 5] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

  1. 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
   subobject(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 [RFC5212]).  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 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.
   Note that even if a PCE computes a path with a loose hop expecting
   that the loose hop will be expanded across the lower-layer network,
   the LSR (that is an entry point to the lower-layer network) may
   simply expand the loose hop in the same layer.  If more strict
   control of how the LSR establishes the path is required, mechanisms
   such as Path Key [RFC5520] could be applied.
  1. 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 that are not yet established.  In the latter case, the
   signaling of the higher-layer LSP will trigger the establishment of
   the lower-layer LSPs.

Oki, et al. Informational [Page 6] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

3. Inter-Layer Path Computation Models

 In Section 2, three models are defined to perform PCE-based inter-
 layer path computation -- namely, single PCE computation, multiple
 PCE computation with inter-PCE communication, and multiple PCE
 computation without inter-PCE communication.  Single PCE computation
 is discussed in Section 3.1 below, and multiple PCE computation (with
 and without inter-PCE communication) is discussed in Section 3.2
 below.

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 that is already in existence or not yet
 established.
  1. —-

| PCE |

  1. —-
  2. —- —– —– —–

| 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

Oki, et al. Informational [Page 7] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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:
  1. LSR H1 sends a request to PCE Hi for a path H1-H4.
  1. PCE Hi selects H2 as the entry point to the lower layer and H3 as

the exit point.

  1. PCE Hi requests a path H2-H3 from PCE Lo.
  1. PCE Lo returns H2-L1-L2-H3 to PCE Hi.
  1. PCE 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.

Oki, et al. Informational [Page 8] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

  1. —-

| PCE |

                             | Hi  |
                              --+--
                                |
     -----    -----             |            -----    -----
    | 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.
  1. —-

| 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

Oki, et al. Informational [Page 9] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

3.3. General Observations

  1. 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.
  1. 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.
  1. 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
 [RFC5441] 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 are 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.  The PCE and VNT Manager are distinct
 functional elements that may or may not be collocated.

Oki, et al. Informational [Page 10] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

4.2. Inter-Layer Path Control Models

4.2.1. PCE-VNTM Cooperation Model

  1. —- ——

| PCE |—>| VNTM |

  1. —- ——

^ :

         :           :
         :           :
         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, and 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

Oki, et al. Informational [Page 11] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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
 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 the 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 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.

Oki, et al. Informational [Page 12] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

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

4.2.2. Higher-Layer Signaling Trigger Model

  1. —-

| PCE |

  1. —-

^

         :
         :
         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.

Oki, et al. Informational [Page 13] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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, and starts to set up
 the lower-layer LSP as described in [RFC4206].  Note that these
 actions depend 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.
   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].

Oki, et al. Informational [Page 14] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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.

Oki, et al. Informational [Page 15] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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, which
 shows 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.  The higher-layer signaling trigger
 model provides several options for the ERO when it reaches the last
 LSR in the higher layer 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.

4.2.3. NMS-VNTM Cooperation Model

 In this model, NMS and VNTM cooperate to establish a lower-layer LSP.
 There are two flavors in this model.  One is where interaction
 between layers in path computation is performed at the PCE level.
 This is called "integrated flavor".  The other is where interaction
 between layers in path computation is achieved through NMS and VNTM
 cooperation, which could be a point of application of administrative,
 billing, and security policy.  This is called "separated flavor".

Oki, et al. Informational [Page 16] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 o NMS-VNTM Cooperation Model (integrated flavor)
  1. —– —–

| NMS |←→| PCE |

   |      |     -----
   | ---- |
   ||VNTM||
   | ---- |
    ------
     :  :
     :   ---------
     :            :
     V            V
     -----      -----                  -----      -----
    | LSR |----| LSR |................| LSR |----| LSR |
    | H1  |    | H2  |                | H3  |    | H4  |
     -----      -----\                /-----      -----
                      \-----    -----/
                      | LSR |--| LSR |
                      | L1  |  | L2  |
                       -----    -----
       Figure 8: NMS-VNTM Cooperation Model (integrated flavor)
 Figure 8 shows the NMS-VNTM cooperation model (integrated flavor).
 The case of single PCE path computation is considered to explain the
 NMS-VNTM cooperation model (integrated flavor) here, but multiple PCE
 path computation with inter-PCE communication can also be applied to
 this model.  Note that multiple PCE path computation without inter-
 PCE communication does not fit in with this model.  For this model to
 have meaning, the VNTM and NMS are closely coupled.
 The NMS sends the path computation request to the PCE.  The PCE
 returns the inter-layer path computation result.  When the NMS
 receives the path computation result, the NMS works with the VNTM and
 sends the request to LSR H2 to set up the lower-layer LSP.  VNTM uses
 local policy and possibly management/configuration input to determine
 how to process the computation result from PCE.
 An example procedure of the NMS-VNTM cooperation model (integrated
 flavor) is as follows.
   Step 1:  NMS requests PCE to compute a path between H1 and H4.  The
            request indicates that inter-layer path computation is
            allowed.
   Step 2:  PCE computes a path.  The result (H1-H2-L1-L2-H3-H4) is
            sent back to the NMS.

Oki, et al. Informational [Page 17] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

   Step 3:  NMS discovers that a lower-layer LSP is needed.  NMS works
            with VNTM to determine whether the new TE LSP H2-L1-L2-H3
            is permitted according to policy, etc.
   Step 4:  VNTM requests the ingress LSR in the lower-layer network
            (H2) to establish a lower-layer LSP.  The request message
            includes the lower-layer LSP route obtained from PCE.
   Step 5:  H2 signals to establish 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 requests H1 to set up a higher-layer LSP between H1
            and H4 with the path computed in Step 2.  The lower-layer
            links are replaced by the corresponding higher-layer TE
            link.  Hence, the NMS sends the path H1-H2-H3-H4 to H1.
   Step 10: H1 initiates signaling with the path H2-H3-H4 to establish
            the higher-layer LSP.

Oki, et al. Informational [Page 18] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 o NMS-VNTM Cooperation Model (separate flavor)
  1. —-

| NMS |

    |     |   -----
     -----   | PCE |
     ^   ^   | Hi  |
     :   :    -----
     :   :    ^
     :   :    :
     :   :    :
     :   v    v
     :   ------    -----                          -----    ------
     :  | LSR  |--| LSR |........................| LSR |--| LSR  |
     :  | H1   |  | H2  |                        | H3  |  | H4   |
     :   ------    -----\                        /-----    ------
     :             ^     \                      /
     :             :      \                    /
     :     --------        \                  /
     v    :                 \                /
     ------      -----       \-----    -----/
    | VNTM |<-->| PCE |      | LSR |--| LSR |
    |      |    | Lo  |      | L1  |  | L2  |
     ------      -----        -----    -----
        Figure 9: NMS-VNTM Cooperation Model (separate flavor)
 Figure 9 shows the NMS-VNTM cooperation model (separate flavor).  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), which notifies the NMS.  The
 notification may include information about the reason for failure
 (such as that there is no TE link between the border LSRs or that
 computation constraints cannot be met).

Oki, et al. Informational [Page 19] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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 be established and 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 it
 may reject the operator's request for the service that requires the
 higher-layer LSP with a suggestion that the operator try 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 higher 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 whether lower-layer LSPs should
            be established, if necessary and acceptable within VNTM's
            policy constraints.

Oki, et al. Informational [Page 20] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

   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 8:  H2 signals the lower-layer LSP.
   Step 9:  If the lower-layer LSP setup is successful, H2 notifies
            VNTM that the LSP is complete and supplies the tunnel
            information.
   Step 10: H2 advertises the new LSP as a TE link in the higher-layer
            network routing instance.
   Step 11: VNTM notifies NMS that the underlying lower-layer LSP has
            been set up, and NMS notices the new TE link
            advertisement.
   Step 12: NMS again requests H1 to set up a higher-layer LSP between
            H1 and H4.
   Step 13: 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 14: 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 four inter-layer path control
 models:  the PCE-VNTM cooperation model, the higher-layer signaling
 trigger model, the NMS-VNTM cooperation model (integrated flavor),
 and the NMS-VNTM cooperation model (separate flavor).  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.

Oki, et al. Informational [Page 21] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

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

| 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            |  Yes   | Yes       | No        |
 | cooperation         |        |           |           |
 | (integrated flavor) |        |           |           |
 |---------------------+--------+-----------+-----------|
 | NMS-VNTM            |  No*   | No        | Yes       |
 | cooperation         |        |           |           |
 | (separate flavor)   |        |           |           |
  ---------------------+--------+-----------+-----------
  • Note that, in case of NSM-VNTM cooperation (separate flavor) 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 PCE-VNTM cooperation model, the higher-
 layer signaling trigger model, and the NMS-VNTM cooperation 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 Simple Network Management
 Protocol (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].

Oki, et al. Informational [Page 22] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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 the Path
 Computation Element Communication Protocol (PCEP) [RFC5440].  VNTM-
 LSR communications are the same as in the PCE-VNTM cooperation model.
 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.
 Also, VNTM functions could be applied to establish LSPs (or
 connections) in non-MPLS/GMPLS networks, which do not have signaling
 capabilities, by configuring each node along the path from the VNTM.

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 and NMS-VNTM cooperation models, 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.

Oki, et al. Informational [Page 23] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

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, NMS and VNTM, and VNTM and LSR.  In the PCE-VNTM
 and the NMS-VNTM cooperation models, 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
 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 [RFC5441] can be used to select an optimal path across
 layers.

Oki, et al. Informational [Page 24] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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, crankback signaling 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 set up 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 Section 8).  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, either leaving it in place but

Oki, et al. Informational [Page 25] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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.
 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.

Oki, et al. Informational [Page 26] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

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
 [PCC-PCE], 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
 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 [RFC5394].

Oki, et al. Informational [Page 27] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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.
 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 7.1 and [RFC5394].

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

Oki, et al. Informational [Page 28] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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 Operations, Administration, and
 Management (OAM) techniques will report a failure.

7.4. Verifying Correct Operation

 The correct operation of the PCE computations and interactions are
 described in [RFC4657], [RFC5440], 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 7.2.  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
 7.2.  Third, network resource utilization in the lower layer (both in
 terms of resource congestion and in consideration of under-
 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.

Oki, et al. Informational [Page 29] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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.
 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 the 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,
 SNMPv3 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

Oki, et al. Informational [Page 30] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 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
 [RFC5520] 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 such 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 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, Jean-Philippe Vasseur,
 and Franz Rambach for their useful comments.

Oki, et al. Informational [Page 31] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

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., Ed., "Generalized Multi-Protocol Label
            Switching (GMPLS) Architecture", RFC 3945, October 2004.
 [RFC4206]  Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
            Hierarchy with Generalized Multi-Protocol Label Switching
            (GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.

10.2. Informative Reference

 [PCE-MIB]  Stephan, E., "Definitions of Textual Conventions for Path
            Computation Element", Work in Progress, March 2009.
 [PCC-PCE]  Oki, E., Le Roux, JL., Kumaki, K., Farrel, A., and T.
            Takeda, "PCC-PCE Communication and PCE Discovery
            Requirements for Inter-Layer Traffic Engineering", Work in
            Progress, January 2009.
 [RFC4208]  Swallow, G., Drake, J., Ishimatsu, H., and Y. Rekhter,
            "Generalized Multiprotocol Label Switching (GMPLS) User-
            Network Interface (UNI): Resource ReserVation Protocol-
            Traffic Engineering (RSVP-TE) Support for the Overlay
            Model", RFC 4208, October 2005.
 [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
            Computation Element (PCE)-Based Architecture", RFC 4655,
            August 2006.
 [RFC4657]  Ash, J., Ed., and J. Le Roux, Ed., "Path Computation
            Element (PCE) Communication Protocol Generic
            Requirements", RFC 4657, September 2006.
 [RFC4802]  Nadeau, T., Ed., and A. Farrel, Ed., "Generalized
            Multiprotocol Label Switching (GMPLS) Traffic Engineering
            Management Information Base", RFC 4802, February 2007.
 [RFC4920]  Farrel, A., Ed., Satyanarayana, A., Iwata, A., Fujita, N.,
            and G. Ash, "Crankback Signaling Extensions for MPLS and
            GMPLS RSVP-TE", RFC 4920, July 2007.

Oki, et al. Informational [Page 32] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

 [RFC5212]  Shiomoto, K., Papadimitriou, D., Le Roux, JL., Vigoureux,
            M., and D. Brungard, "Requirements for GMPLS-Based Multi-
            Region and Multi-Layer Networks (MRN/MLN)", RFC 5212, July
            2008.
 [RFC5394]  Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
            "Policy-Enabled Path Computation Framework", RFC 5394,
            December 2008.
 [RFC5440]  Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path Computation
            Element (PCE) Communication Protocol (PCEP)", RFC 5440,
            March 2009.
 [RFC5441]  Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
            "A Backward-Recursive PCE-Based Computation (BRPC)
            Procedure to Compute Shortest Constrained Inter-Domain
            Traffic Engineering Label Switched Paths", RFC 5441, April
            2009.
 [RFC5520]  Bradford, R., Ed., Vasseur, JP., and A. Farrel,
            "Preserving Topology Confidentiality in Inter-Domain Path
            Computation Using a Path-Key-Based Mechanism", RFC 5520,
            April 2009.

Oki, et al. Informational [Page 33] RFC 5623 PCE-Based Inter-Layer MPLS and GMPLS TE September 2009

Authors' Addresses

 Eiji Oki
 University of Electro-Communications
 Tokyo
 Japan
 EMail: oki@ice.uec.ac.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,
 22300 Lannion, France
 EMail: jeanlouis.leroux@orange-ftgroup.com
 Adrian Farrel
 Old Dog Consulting
 EMail: adrian@olddog.co.uk

Oki, et al. Informational [Page 34]

/data/webs/external/dokuwiki/data/pages/rfc/rfc5623.txt · Last modified: 2009/09/17 22:25 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki