Internet DRAFT - draft-ietf-pce-inter-layer-frwk

draft-ietf-pce-inter-layer-frwk





Network Working Group                                            E. Oki 
Internet Draft                     University of Electro-Communications 
Category: Informational                                 Tomonori Takeda 
Created: March, 2009                                                NTT 
Expires: November, 2009                                     J-L Le Roux 
                                                         France Telecom 
                                                              A. Farrel 
                                                     Old Dog Consulting 
                                      
 Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic Engineering 
    
                 draft-ietf-pce-inter-layer-frwk-10.txt 
 
Status of this Memo 
    
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Abstract 
    
   A network may comprise multiple layers. It is important to globally 
   optimize network resource utilization, taking into account all 
   layers, rather than optimizing resource utilization at each layer 
   independently. This allows better network efficiency to be achieved 
   through a process that we call inter-layer traffic engineering. The 
   Path Computation Element (PCE) can be a powerful tool to achieve 
   inter-layer traffic engineering. 
    
   This document describes a framework for applying the PCE-based 
   architecture to inter-layer Multiprotocol Label Switching (MPLS) and 
   Generalized MPLS (GMPLS) traffic engineering. It provides 
   suggestions for the deployment of PCE in support of multi-layer 
   networks. This document also describes network models where PCE 

 
 
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   performs inter-layer traffic engineering, and the relationship 
   between PCE and a functional component called the Virtual Network 
   Topology Manager (VNTM). 
    
Table of Contents 
    
   1. Introduction...................................................3 
   1.1. Terminology..................................................3 
   2. Inter-Layer Path Computation...................................4 
   3. Inter-Layer Path Computation Models............................6 
   3.1. Single PCE Inter-Layer Path Computation......................7 
   3.2. Multiple PCE Inter-Layer Path Computation....................7 
   3.3. General Observations.........................................9 
   4. Inter-Layer Path Control......................................10 
   4.1. VNT Management..............................................10 
   4.2. Inter-Layer Path Control Models.............................10 
   4.2.1. PCE-VNTM Cooperation Model................................10 
   4.2.2. Higher-Layer Signaling Trigger Model......................12 
   4.2.3. NMS-VNTM Cooperation Model................................15 
   4.2.4. Possible Combinations of Inter-Layer Path Computation and 
   Inter-Layer Path Control Models..................................20 
   5. Choosing Between Inter-Layer Path Control Models..............21 
   5.1. VNTM Functions..............................................21 
   5.2. Border LSR Functions........................................22 
   5.3. Complete Inter-Layer LSP Setup Time.........................22 
   5.4. Network Complexity..........................................23 
   5.5. Separation of Layer Management..............................24 
   6. Stability Considerations......................................24 
   7. IANA Considerations...........................................25 
   8. Manageability Considerations..................................25 
   8.1. Control of Function and Policy..............................25 
   8.1.1. Control of Inter-Layer Computation Function...............25 
   8.1.2. Control of Per-Layer Policy...............................26 
   8.1.3. Control of Inter-Layer Policy.............................26 
   8.2. Information and Data Models.................................27 
   8.3. Liveness Detection and Monitoring...........................27 
   8.4. Verifying Correct Operation.................................28 
   8.5. Requirements on Other Protocols and Functional Components...28 
   8.6. Impact on Network Operation.................................28 
   9. Security Considerations.......................................29 
   10. Acknowledgments..............................................30 
   11. References...................................................30 
   11.1. Normative Reference........................................30 
   11.2. Informative Reference......................................31 
   12. Authors' Addresses...........................................32 
   13. Intellectual Property Statement..............................32 
   14. Full Copyright Statement.....................................33 
    
 

 
 
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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. This includes mechanisms allowing 
   the computation of end-to-end paths across layers (known as inter- 
   layer path computation), and mechanisms for control and management 
   of the Virtual Network Topology (VNT) by setting up and releasing 
   LSPs in the lower layers [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 [PCE-INTER-LAYER-REQ]. 
    
   This document describes a framework for applying the PCE-based 
   architecture to inter-layer traffic engineering. It provides 
   suggestions for the deployment of PCE in support of multi-layer 
   networks. This document also describes network models where PCE 
   performs inter-layer traffic engineering, and the relationship 
   between PCE and a functional component in charge of the control and 
   management of the VNT, called the Virtual Network Topology Manager 
   (VNTM). 
    
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]. 
    

 
 
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2. Inter-Layer Path Computation 
    
   This section describes key topics of inter-layer path computation in 
   MPLS and GMPLS networks. 
    
   [RFC4206] defines a way to signal a higher-layer LSP, which has an 
   explicit route that includes hops traversed by LSPs in lower layers. 
   The computation of end-to-end paths across layers is called Inter- 
   Layer Path Computation. 
    
   A Label Switching Router (LSR) in the higher-layer might not have 
   information on the topology of the lower-layer, particularly in an 
   overlay or augmented model deployment, and hence may not be able to 
   compute an end-to-end path across layers. 
    
   PCE-based Inter-Layer Path Computation consists of using one or more 
   PCEs to compute an end-to-end path across layers. This could be 
   achieved by a single PCE path computation where the PCE has topology 
   information about multiple layers and can directly compute an end- 
   to-end path across layers considering the topology of all of the 
   layers. Alternatively, the inter-layer path computation could be 
   performed as a multiple PCE computation where each member of a set 
   of PCEs has information about the topology of one or more layers 
   (but not all layers), and the PCEs collaborate to compute an end-to- 
   end path. 
    
       -----    -----                  -----    ----- 
      | LSR |--| LSR |................| LSR |--| LSR | 
      | H1  |  | H2  |                | H3  |  | H4  | 
       -----    -----\                /-----    ----- 
                      \-----    -----/ 
                      | LSR |--| LSR | 
                      | L1  |  | L2  | 
                       -----    ----- 
    
   Figure 1 - A Simple Example of a Multi-Layer Network. 
    
   Consider, for instance, the two-layer network shown in Figure 1, 
   where the higher-layer network is a packet-based IP/MPLS or GMPLS 
   network  (LSRs H1, H2, H3, and H4), and the lower-layer network 
   (LSRs, H2, L1, L2, and H3) is a GMPLS optical network. An ingress 
   LSR in the higher-layer network (H1) tries to set up an LSP to an 
   egress LSR (H4) also in the higher-layer network across the lower- 
   layer network, and needs a path in the higher-layer network. However, 
   suppose that there is no TE link in the higher-layer network between 
   the border LSRs located on the boundary between the higher-layer and 
   lower-layer networks (H2 and H3). Suppose also that the ingress LSR 
   does not have topology visibility into the lower layer. If a single- 
   layer path computation is applied in the higher-layer, the path 

 
 
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   computation fails because of the missing TE link. On the other hand, 
   inter-layer path computation is able to provide a route in the 
   higher-layer (H1-H2-H3-H4) and a suggestion that a lower-layer LSP 
   be set up between the border LSRs (H2-L1-L2-H3). 
    
   Lower-layer LSPs that are advertised as TE links into the higher- 
   layer network form a Virtual Network Topology (VNT) that can be used 
   for routing higher-layer LSPs. Inter-layer path computation for end- 
   to-end LSPs in the higher-layer network that span the lower-layer 
   network may utilize the VNT, and PCE is a candidate for computing 
   the paths of such higher-layer LSPs within the higher-layer network. 
   Alternatively, the PCE-based path computation model can: 
    
   - Perform a single computation on behalf of the ingress LSR using 
     information gathered from more than one layer. This mode is 
     referred to as Single PCE Computation in [RFC4655]. 
    
   - Compute a path on behalf of the ingress LSR through cooperation 
     with PCEs responsible for each layer. This mode is referred to as 
     Multiple PCE Computation with inter-PCE communication in [RFC4655]. 
    
   - Perform separate path computations on behalf of the TE-LSP head- 
     end and each transit border LSR that is the entry point to a new 
     layer. This mode is referred to as Multiple PCE Computation 
     (without inter-PCE communication) in [RFC4655]. This option 
     utilizes per-layer path computation performed independently by 
     successive PCEs. 
    
   Note that when a network consists of more than two layers (e.g., MPLS 
   over SONET over 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. 
    
   - 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 

 
 
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     established on demand. That is, the resulting ERO includes sub- 
     object(s) corresponding to lower-layer hierarchical LSPs expressed 
     as the TE link identifiers of the hierarchical LSPs when advertised 
     as TE links in the higher-layer network. The TE link may be a 
     regular TE link that is actually established, or a virtual TE link 
     that is not established yet (see [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 selects 
     the entry and exit points of that lower-layer network, but does not 
     select the path across the lower-layer network. A transit LSR that 
     is the entry point to the lower-layer network is expected to expand 
     the loose hop (either itself or relying on the services of a PCE). 
     The path expansion process on the border LSR may result either in 
     the selection of an existing lower-layer LSP, or in the computation 
     and setup of a new lower-layer LSP. 
    
     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 [PATH-KEY] could be applied. 
    
   - Option 2: Multi-layer path. 
    
     The PCE computes a "multi-layer" path, i.e., a path that includes 
     TE links from distinct layers [RFC4206]. Such a path can include 
     the complete path of one or more lower-layer LSPs that already 
     exist or are not yet established. In the latter case, the signaling 
     of the higher-layer LSP will trigger the establishment of the 
     lower-layer LSPs. 
    
3. Inter-Layer Path Computation Models 
    
   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 
   or without inter-PCE communication) is discussed in Section 3.2  
   below. 
    


 
 
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3.1. Single PCE Inter-Layer Path Computation 
    
   In this model inter-layer path computation is performed by a single 
   PCE that has topology visibility into all layers. Such a PCE is 
   called a multi-layer PCE. 
    
   In Figure 2, the network is comprised of two layers. LSRs H1, H2, H3, 
   and H4 belong to the higher layer, and LSRs H2, H3, L1, and L2 
   belong to the lower layer. The PCE is a multi-layer PCE that has 
   visibility into both layers. It can perform end-to-end path 
   computation across layers (single PCE path computation). For 
   instance, it can compute an optimal path H1-H2-L1-L2-H3-H4, for a 
   higher layer LSP from H1 to H4. This path includes the path of a 
   lower layer LSP from H2 to H3, already in existence or not yet 
   established. 
    
                           ----- 
                          | PCE | 
                           ----- 
       -----    -----                  -----    ----- 
      | LSR |--| LSR |................| LSR |--| LSR | 
      | H1  |  | H2  |                | H3  |  | H4  | 
       -----    -----\                /-----    ----- 
                      \-----    -----/ 
                      | LSR |--| LSR | 
                      | L1  |  | L2  | 
                       -----    ----- 
    
     Figure 2: Single PCE Inter-Layer Path Computation 
    
3.2. Multiple PCE Inter-Layer Path Computation 
    
   In this model there is at least one PCE per layer, and each PCE has 
   topology visibility restricted to its own layer. Some providers may 
   want to keep the layer boundaries due to factors such as 
   organizational and/or service management issues. The choice for 
   multiple PCE computation instead of single PCE computation may also 
   be driven by scalability considerations, as in this mode a PCE only 
   needs to maintain topology information for one layer (resulting in a 
   size reduction for the Traffic Engineering Database (TED)). 
    
   These PCEs are called mono-layer PCEs. Mono-layer PCEs collaborate 
   to compute an end-to-end optimal path across layers. 
    
   Figure 3 shows multiple PCE inter-layer computation with inter-PCE 
   communication. There is one PCE in each layer. The PCEs from each 
   layer collaborate to compute an end-to-end path across layers. PCE 
   Hi is responsible for computations in the higher layer and may 
   "consult" with PCE Lo to compute paths across the lower layer. PCE 

 
 
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   Lo is responsible for path computation in the lower layer. A simple 
   example of cooperation between the PCEs could be as follows: 
    
   - LSR H1 sends a request for a path H1-H4 to PCE Hi 
   - PCE Hi selects H2 as the entry point to the lower layer, and H3 as 
     the exit point. 
   - PCE Hi requests a path H2-H3 from PCE Lo. 
   - PCE Lo returns H2-L1-L2-H3 to PCE Hi. 
   - PEC Hi is now able to compute the full path (H1-H2-L1-L2-H3-H4) 
     and return it to H1. 
    
   Of course, more complex cooperation may be required if an optimal 
   end-to-end path is desired. 
    
                                ----- 
                               | PCE | 
                               | Hi  | 
                                --+-- 
                                  | 
       -----    -----             |            -----    ----- 
      | LSR |--| LSR |............|...........| LSR |--| LSR | 
      | H1  |  | H2  |            |           | H3  |  | H4  | 
       -----    -----\          --+--         /-----    ----- 
                      \        | PCE |       / 
                       \       | Lo  |      / 
                        \       -----      / 
                         \                / 
                          \-----    -----/ 
                          | LSR |--| LSR | 
                          | L1  |  | L2  | 
                           -----    ----- 
    
   Figure 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. 
    
    
    
    
    
    
    
    
    
    

 
 
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                                ----- 
                               | PCE | 
                               | Hi  | 
                                ----- 
       -----    -----                          -----    ----- 
      | LSR |--| LSR |........................| LSR |--| LSR | 
      | H1  |  | H2  |                        | H3  |  | H4  | 
       -----    -----\          -----         /-----    ----- 
                      \        | PCE |       / 
                       \       | Lo  |      / 
                        \       -----      / 
                         \                / 
                          \-----    -----/ 
                          | LSR |--| LSR | 
                          | L1  |  | L2  | 
                           -----    ----- 
    
   Figure 4: Multiple PCE Inter-layer Path Computation Without Inter- 
   PCE Communication 
    
3.3. General Observations 
    
   - Depending on implementation details, the time to perform inter- 
     layer path computation in the single PCE inter-layer path 
     computation model may be less than that of the multiple PCE model 
     with cooperating mono-layer PCEs, because there is no requirement 
     to exchange messages between cooperating PCEs. 
    
   - When TE topology for all layer networks is visible within one 
     routing domain, the single PCE inter-layer path computation model 
     may be adopted because a PCE is able to collect all layers' TE 
     topologies by participating in only one routing domain. 
    
   - As the single PCE inter-layer path computation model uses more TE 
     topology information in one computation than is used by PCEs in the 
     multiple PCE path computation model, it requires more computation 
     power and memory. 
    
   When there are multiple candidate layer border nodes (we may say 
   that the higher layer is multi-homed), optimal path computation 
   requires that all the possible paths transiting different layer 
   border nodes or links be examined. This is relatively simple in the 
   single PCE inter-layer path computation model because the PCE has 
   full visibility - the computation is similar to the computation 
   within a single domain of a single layer. In the multiple PCE inter- 
   layer path computation model, backward recursive techniques 
   described in [BRPC] could be used, by considering layers as separate 
   domains. 
    

 
 
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4. Inter-Layer Path Control 
    
4.1. VNT Management 
    
   As a result of mono-layer path computation, a PCE may determine that 
   there is insufficient bandwidth available in the higher-layer 
   network to support this or future higher-layer LSPs. The problem 
   might be resolved if new LSPs were provisioned across the lower- 
   layer network. Furthermore, the modification, re-organization and 
   new provisioning of lower-layer LSPs may enable better utilization 
   of lower-layer network resources given the demands of the higher- 
   layer network. In other words, the VNT needs to be controlled or 
   managed in cooperation with inter-layer path computation. 
    
   A VNT Manager (VNTM) is defined as a functional element that manages 
   and controls the VNT. PCE and VNT Manager are distinct functional 
   elements that may or may not be co-located. 
    
4.2. Inter-Layer Path Control Models 
    
4.2.1. PCE-VNTM Cooperation Model 
    
      -----      ------ 
     | PCE |--->| VNTM | 
      -----      ------ 
        ^           : 
        :           : 
        :           : 
        v           V 
       -----      -----                  -----      ----- 
      | LSR |----| LSR |................| LSR |----| LSR | 
      | H1  |    | H2  |                | H3  |    | H4  | 
       -----      -----\                /-----      ----- 
                        \-----    -----/ 
                        | LSR |--| LSR | 
                        | L1  |  | L2  | 
                         -----    ----- 
    
   Figure 5: PCE-VNTM Cooperation Model 
    
   A multi-layer network consists of higher-layer and lower-layer 
   networks. LSRs H1, H2, H3, and H4 belong to the higher-layer network, 
   LSRs H2, L1, L2, and H3 belong to the lower-layer network, as shown 
   in Figure 5. The case of single PCE inter-layer path computation is 
   considered here to explain the cooperation model between PCE and 
   VNTM, but multiple PCE path computation with or without inter-PCE 
   communication can also be applied to this model. 
    
   Consider that H1 requests the PCE to compute an inter-layer path 

 
 
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   between H1 and H4. There is no TE link in the higher-layer between 
   H2 and H3 before the path computation request, so the request fails. 
   But the PCE may provide information to the VNT Manager responsible 
   for the lower layer network that may help resolve the situation for 
   future higher-layer LSP setup. 
    
   The roles of PCE and VNTM are as follows. PCE performs inter-layer 
   path computation and is unable to supply a path because there is no 
   TE link between H2 and H3. The computation fails, but PCE suggests 
   to VNTM that a lower-layer LSP (H2-H3) could be established to 
   support future LSP requests. Messages from PCE to VNTM contain 
   information about the higher-layer demand (from H2 to H3), and may 
   include a suggested path in the lower layer (if the PCE has 
   visibility into the lower layer network). VNTM uses local policy and 
   possibly management/configuration input to determine how to process 
   the suggestion from PCE, and may request an ingress LSR (e.g. H2) to 
   establish a lower-layer LSP. VNTM or the ingress LSR (H2) may 
   themselves use a PCE with visibility into the lower layer to compute 
   the path of this new LSP. 
    
   When the higher-layer PCE fails to compute a path and notifies VNTM, 
   it may wait for the lower-layer LSP to be set up and advertised as a 
   TE link. PCE may have a timer. After TED is updated within a 
   specified duration, PCE will know a new TE link. It could then 
   compute the complete end-to-end path for the higher-layer LSP and 
   return the result to the PCC. In this case, the PCC may be kept 
   waiting for some time, and it is important that the PCC understands 
   this. It is also important that the PCE and VNTM have an agreement 
   that the lower-layer LSP will be set up in a timely manner, or that 
   the PCE will be notified by VNTM that no new LSP will become 
   available. In any case, if the PCE decides to wait, it must operate 
   a timeout. An example of such a cooperative procedure between PCE 
   and VNTM is as follows using the example network in Figure 4. 
    
   Step 1: H1 (PCC) requests PCE to compute a path between H1 and H4. 
    
   Step 2: The path computation fails because there is no TE link 
   across the lower-layer network. 
    
   Step 3: PCE suggests to VNTM that a new TE link connecting H2 and H3 
   would be useful. The PCE notifies VNTM that it will be waiting for 
   the TE link to be created. VNTM considers whether lower-layer LSPs 
   should be established if necessary and if acceptable within VNTM's 
   policy constraints. 
    
   Step 4: VNTM requests an ingress LSR in the lower-layer network 
   (e.g., H2) to establish a lower-layer LSP. The request message may 
   include a lower-layer LSP route obtained from the PCE responsible 
   for the lower-layer network. 

 
 
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   Step 5: The ingress LSR signals to establish the lower-layer LSP. 
    
   Step 6: If the lower-layer LSP setup is successful, the ingress LSR 
   notifies VNTM that the LSP is complete and supplies the tunnel 
   information. 
    
   Step 7: The ingress LSR (H2) advertises the new LSP as a TE link in 
   the higher-layer network routing instance. 
    
   Step 8: PCE notices the new TE link advertisement and recomputes the 
   requested path. 
    
   Step 9: PCE replies to H1 (PCC) with a computed higher-layer LSP 
   route. The computed path is categorized as a mono-layer path that 
   includes the already-established lower layer-LSP as a single hop in 
   the higher layer. The higher-layer route is specified as H1-H2-H3-H4, 
   where all hops are strict. 
    
   Step 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 
    
      ----- 
     | PCE | 
      ----- 
        ^ 
        : 
        : 
        v 
       -----      -----                  -----    ----- 
      | LSR |----| LSR |................| LSR |--| LSR | 
      | H1  |    | H2  |                | H3  |  | H4  | 
       -----      -----\                /-----    ----- 
                        \-----    -----/ 
                        | LSR |--| LSR | 
                        | L1  |  | L2  | 
                         -----    ----- 
    
   Figure 6: Higher-layer Signaling Trigger Model 
    
   Figure 6 shows the higher-layer signaling trigger model. The case of 
   single PCE path computation is considered to explain the higher- 
   layer signaling trigger model here, but multiple PCE path 
   computation with/without inter-PCE communication can also be applied 
   to this model. 
    
   As in the case described in Section 4.2.1, consider that H1 requests 

 
 
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   PCE to compute a path between H1 and H4. There is no TE link in the 
   higher-layer between H2 and H3 before the path computation request. 
    
   PCE is unable to compute a mono-layer path, but may judge that the 
   establishment of a lower-layer LSP between H2 and H3 would provide 
   adequate connectivity. If the PCE has inter-layer visibility it may 
   return a path that includes hops in the lower layer (H1-H2-L1-L2-H3- 
   H4), but if it has no visibility into the lower layer, it may return 
   a path with a loose hop from H2 to H3 (H1-H2-H3(loose)-H4). The 
   former is a multi-layer path, and the latter a mono-layer path that 
   includes loose hops. 
    
   In the higher-layer signaling trigger model with a multi-layer path, 
   the LSP route supplied by the PCE includes the route of a lower- 
   layer LSP that is not yet established. A border LSR that is located 
   at the boundary between the higher-layer and lower-layer networks 
   (H2 in this example) receives a higher-layer signaling message, 
   notices that the next hop is in the lower-layer network, starts to 
   setup the lower-layer LSP as described in [RFC4206]. Note that these 
   actions 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. 

 
 
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   Step 6: After the lower-layer LSP is established, the higher-layer 
   signaling continues along the specified higher-layer route of H2-H3- 
   H4 using hierarchical signaling [RFC4206]. 
    
   On the other hand, in the signaling trigger model with a mono-layer 
   path, a higher-layer LSP route includes a loose hop to traverse the 
   lower-layer network between the two border LSRs. A border LSR that 
   receives a higher-layer signaling message needs to determine a path 
   for a new lower-layer LSP. It applies local policy to verify that a 
   new LSP is acceptable and then either consults a PCE with 
   responsibility for the lower-layer network or computes the path by 
   itself, and initiates signaling to establish the lower-layer LSP. 
   Again, it is possible that a suitable lower-layer LSP has already 
   been established (or become available). In this case, the border LSR 
   may select such a lower-layer LSP without the need to signal a new 
   LSP provided that the existing lower-layer LSP satisfies the 
   explicit route in the higher-layer signaling request. Since the 
   higher-layer signaling request used a loose hop without specifying 
   any specifics of the path within the lower-layer network, the border 
   LSR has greater freedom to choose a lower-layer LSP than in the 
   previous example. 
    
   The difference between procedures of the signaling trigger model 
   with a multi-layer path and a mono-layer path is Step 5. Step 5 of 
   the signaling trigger model with a mono-layer path is as follows: 
    
   Step 5': The border LSR (H2) that receives the higher-layer 
   signaling message applies local policy to verify that a new LSP is 
   acceptable and then initiates establishment of a lower-layer LSP. It 
   either consults a PCE with responsibility for the lower-layer 
   network or computes the route by itself to expand the loose hop 
   route in the higher-layer path. 
    
   Finally, note that a virtual TE link may have been advertised into 
   the higher-layer network. This causes the PCE to return a path H1- 
   H2-H3-H4 where all the hops are strict. But when the higher-layer 
   signaling message reaches the layer border node H2 (that was 
   responsible for advertising the virtual TE link) it realizes that 
   the TE link does not exist yet, and signals the necessary LSP across 
   the lower-layer network using its own path determination (just as 
   for a loose hop in the higher layer) before continuing with the 
   higher-layer signaling. 
    
    
    
    
    
    

 
 
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   PCE 
    ^ 
    : 
    : 
    V 
   H1--H2                  H3--H4 
        \                  / 
         L1==L2==L3--L4--L5 
                  | 
                  | 
                 L6--L7 
                       \ 
                        H5--H6 
    
   Figure 7: Example of a Multi-Layer Network 
    
   Examples of multi-layer EROs are explained using Figure 7. It is 
   described how lower-layer LSP setup is performed in the higher-layer 
   signaling trigger model using an ERO that can include subobjects in 
   both the higher and lower layers. It gives rise to several options 
   for the ERO when it reaches the last LSR in the higher layer network 
   (H2). 
   1. The next subobject is a loose hop to H3 (mono layer ERO). 
   2. The next subobject is a strict hop to L1 followed by a loose hop 
      to H3. 
   3. The next subobjects are a series of hops (strict or loose) in the 
      lower-layer network followed by H3. For example, {L1(strict), 
      L3(loose), L5(loose), H3(strict)} 
    
   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". 
    
   o NMS-VNTM Cooperation Model (integrated flavor) 
    
    
    

 
 
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      ------      ----- 
     | 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 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 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. 
    



 
 
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   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. 
    
   o NMS-VNTM Cooperation Model (separate flavor) 
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    

 
 
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       ----- 
      | 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) that 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). 
    
   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. 
    

 
 
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   The NMS may now suggest (or request) to the VNTM that a lower-layer 
   LSP between the border LSRs could be established and could be 
   advertised as a TE link in the higher layer to support future 
   higher-layer LSP requests. The communication between the NMS and the 
   VNTM may be performed in an automatic manner or in a manual manner, 
   and is a key interaction between layers that may also be separate 
   administrative domains. Thus, this communication is potentially a 
   point of application of administrative, billing, and security policy. 
   The NMS may wait for the lower-layer LSP to be set up and advertised 
   as a TE link, or may reject the operator's request for the service 
   that requires the higher-layer LSP with a suggestion that the 
   operator tries again later. 
    
   The VNTM requests the lower-layer PCE to compute a path, and then 
   requests H2 to establish a lower-layer LSP. Alternatively, the VNTM 
   may make a direct request to H2 for the LSP, and H2 may consult the 
   lower-layer PCE. After the NMS is informed or notices that the 
   lower-layer LSP has been established, it can request the head-end 
   LSR (H1) to set up the higher-layer end-to-end LSP between H1 and H4. 
    
   Thus, cooperation between the high layer and lower layer is 
   performed though communication between NMS and VNTM. An example of 
   such a procedure of the NSM-VNTM cooperation model is as follows 
   using the example network in Figure 6. 
    
   Step 1: NMS requests a head-end LSR (H1) to set up a higher-layer 
   LSP between H1 and H4 without specifying any route. 
    
   Step 2: H1 (PCC) requests PCE to compute a path between H2 and H3. 
    
   Step 3: The path computation fails because there is no TE link 
   across the lower-layer network. 
    
   Step 4: H1 (PCC) notifies NMS. The notification may include an 
   indication that there is no TE link between H2 and H4. 
    
   Step 5: NMS suggests (or requests) to VNTM that a new TE link 
   connecting H2 and H3 would be useful. The NMS notifies VNTM that it 
   will be waiting for the TE link to be created. VNTM considers 
   whether lower-layer LSPs should be established if necessary and if 
   acceptable within VNTM's policy constraints. 
    
   Step 6: VNTM requests the lower-layer PCE for path computation. 
    
   Step 7: VNTM requests the ingress LSR in the lower-layer network 
   (H2) to establish a lower-layer LSP. The request message includes a 
   lower-layer LSP route obtained from the lower-layer PCE responsible 
   for the lower-layer network. 
    

 
 
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   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); 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. 
    
    
    
    
    
    
    
    
    
    
    
    

 
 
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   Table 1: Possible Combinations of Inter-Layer Path Computation and 
   Inter-Layer Path Control Models. 
    
    ------------------------------------------------------ 
   | 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 cooperation model between PCE and VNTM, 
   the higher-layer signaling trigger model, and 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 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].  
    


 
 
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   In the NMS-VNTM cooperation model, communications are required 
   between NMS and VNTM, between VNTM and a lower-layer PCE, and between 
   VNTM and a border LSR. NMS-VNTM communications, which are out of 
   scope of this document, may use proprietary or standard interfaces, 
   some of which, for example, are standardized in TM Forum. 
   Communications between VNTM and a lower-layer PCE use PCEP [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 cooperation model and the NMS-VNTM model, no 
   additional function for triggered signaling is required in border 
   LSRs except when virtual TE links are used. Therefore, if these 
   additional functions are not supported in border LSRs, where a 
   border LSR is controlled by VNTM to set up a lower-layer LSP, the 
   cooperation model has to be chosen. 
    
5.3. Complete Inter-Layer LSP Setup Time 
    
   The complete inter-layer LSP setup time includes inter-layer path 

 
 
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   computation, signaling, and the communication time between PCC and 
   PCE, PCE and VNTM, NMS and VNTM, and VNTM and LSR. In the PCE-VNTM 
   cooperation model and the NMS-VNTM model, the additional 
   communication steps are required compared with the higher-layer 
   signaling trigger model. On the other hand, the cooperation model 
   provides better control at the cost of a longer service setup time. 
    
   Note that, in terms of higher-layer signaling time, in the higher- 
   layer signaling trigger model, the required time from when higher- 
   layer signaling starts to when it is completed, is more than that of 
   the cooperation model except when a virtual TE link is included. 
   This is because the former model requires lower-layer signaling to 
   take place during the higher-layer signaling. A higher-layer ingress 
   LSR has to wait for more time until the higher-layer signaling is 
   completed. A higher-layer ingress LSR is required to be tolerant of 
   longer path setup times. 
    
5.4. Network Complexity 
    
   If the higher and lower layer networks have multiple interconnects 
   then optimal path computation for end-to-end LSPs that cross the 
   layer boundaries is non-trivial. The higher layer LSP must be routed 
   to the correct layer border nodes to achieve optimality in both 
   layers. 
    
   Where the lower layer LSPs are advertised into the higher layer 
   network as TE links, the computation can be resolved in the higher 
   layer network. Care needs to be taken in the allocation of TE 
   metrics (i.e., costs) to the lower layer LSPs as they are advertised 
   as TE links into the higher layer network, and this might be a 
   function for a VNT Manager component. Similarly, attention should be 
   given to the fact that the LSPs crossing the lower-layer network 
   might share points of common failure (e.g., they might traverse the 
   same link in the lower-layer network) and the shared risk link 
   groups (SRLGs) for the TE links advertised in the higher-layer must 
   be set accordingly. 
    
   In the single PCE model an end-to-end path can be found in a single 
   computation because there is full visibility into both layers and 
   all possible paths through all layer interconnects can be considered. 
    
   Where PCEs cooperate to determine a path, an iterative computation 
   model such as [BRPC] can be used to select an optimal path across 
   layers. 
    
   When non-cooperating mono-layer PCEs, each of which is in a separate 
   layer, are used with the triggered LSP model, it is not possible to 
   determine the best border LSRs, and connectivity cannot even be 
   guaranteed. In this case, signaling crankback techniques [RFC4920] 

 
 
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   can be used to eventually achieve connectivity, but optimality is 
   far harder to achieve. In this model, a PCE that is requested by an 
   ingress LSR to compute a path expects a border LSR to setup a lower- 
   layer path triggered by high-layer signaling when there is no TE 
   link between border LSRs. 
    
5.5. Separation of Layer Management 
    
   Many network operators may want to provide a clear separation 
   between the management of the different layer networks. In some 
   cases, the lower layer network may come from a separate commercial 
   arm of an organization or from a different corporate body entirely. 
   In these cases, the policy applied to the establishment of LSPs in 
   the lower-layer network and to the advertisement of these LSPs as TE 
   links in the higher-layer network will reflect commercial agreements 
   and security concerns (see Section 9). Since the capacity of the 
   LSPs in the lower-layer network are likely to be significantly 
   larger than those in the client higher-layer network (multiplex- 
   server model), the administrator of the lower-layer network may want 
   to exercise caution before allowing a single small demand in the 
   higher layer to tie up valuable resources in the lower layer. 
    
   The necessary policy points for this separation of administration 
   and management are more easily achieved through the VNTM approach 
   than by using triggered signaling. In effect, the VNTM is the 
   coordination point for all lower layer LSPs and can be closely tied 
   to a human operator as well as to policy and billing. Such a model 
   can also be achieved using triggered signaling. 
    
6. Stability Considerations 
    
   Inter-layer traffic engineering needs to be managed and operated 
   correctly to avoid introducing instability problems. 
    
   Lower-layer LSPs are likely, by the nature of the technologies used 
   in layered networks, to be of considerably higher capacity than the 
   higher-layer LSPs. This has the benefit of allowing multiple higher- 
   layer LSPs to be carried across the lower-layer network in a single 
   lower-layer LSP. However, when a new lower-layer LSP is set up to 
   support a request for a higher-layer LSP because there is no 
   suitable route in the higher-layer network, it may be the case that 
   a very large LSP is established in support of a very small traffic 
   demand. Further, if the higher-layer LSP is short-lived, the 
   requirement for the lower-layer LSP will go away leaving it either 
   in-place but unused, or requiring it to be torn down. This may cause 
   excessive tie-up of unused lower-layer network resources, or may 
   introduce instability into the lower-layer network. It is important 
   that appropriate policy controls or configuration features are 
   available so that demand-led establishment of lower-layer LSPs (the 

 
 
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   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. IANA Considerations 
    
   This informational document makes no requests for IANA action. 
    
8. 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. 
    
8.1. Control of Function and Policy 
    
8.1.1. Control of Inter-Layer Computation Function 
    

 
 
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   PCE implementations that are capable of supporting inter-layer 
   computations should provide a configuration switch to allow support 
   of inter-layer path computations to be enabled or disabled. 
    
   When a PCE is capable of, and configured for, inter-layer path 
   computation, it should advertise this capability as described in 
   [PCE-INTER-LAYER-REQ], but this advertisement may be suppressed 
   through a secondary configuration option. 
    
8.1.2. 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. 
    
8.1.3. 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]. 
    
   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. 

 
 
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8.2. 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 6.1 and [RFC5394]. 
    
8.3. Liveness Detection and Monitoring 
    
   Liveness detection and monitoring is required between PCEs and PCCs, 
   and between cooperating PCEs as described in [RFC4657]. Inter-layer 
   traffic engineering does not change this requirement. 
    
   Where there are communications between a PCE and VNTM, additional 
   liveness detection and monitoring may be required to allow the PCE 
   to know whether the VNTM has received its information about failed 
   path computations and desired TE links. 
    
   When a lower layer LSP fails (perhaps because of the failure of a 
   lower layer network resource) or is torn down as a result of lower 
   layer network policy, the consequent change should be reported to 
   the higher layer as a change in the VNT, although inter-layer policy 
   may dictate that such a change is hidden from the higher layer. The 
   higher layer network may additionally operate data plane failure 
   techniques over the virtual TE links in the VNT in order to monitor 
   the liveness of the connections, but it should be noted that if the 
   virtual TE link is advertised but not yet established as an LSP in 
   the lower layer, such higher layer OAM techniques will report a 
   failure. 

 
 
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8.4. 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 6.2. Second, the rate of change of the VNT (i.e., the rate 
   of establishment and removal of higher layer TE links based on lower 
   layer LSPs) may be seen as a measure of the correct planning of the 
   VNT and may also form part of the data model described in Section 
   6.2. Third, network resource utilization in the lower layer (both in 
   terms of resource congestion, and in consideration of under 
   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. 
    
8.5. Requirements on Other Protocols and Functional Components 
    
   There are no protocols or protocol extensions defined in this 
   document and so it is not appropriate to consider specific 
   interactions with other protocols. It should be noted, however, that 
   the objective of this document is to enable inter-layer traffic 
   engineering for MPLS-TE and GMPLS networks and so it is assumed that 
   the necessary features for inter-layer operation of routing and 
   signaling protocols are in existence or will be developed. 
    
   This document introduces roles for various network components (PCE, 
   LSR, NMS, and VNTM). Those components are all required to play their 
   part in order that inter-layer TE can be effective. That is, an 
   inter-layer TE model that assumes the presence and operation of any 
   of these functional components obviously depends on those components 
   to fulfill their roles as described in this document. 
    
8.6. Impact on Network Operation 
    

 
 
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   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. 
    
9. 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 VNTM and a border 
   LSR. In this case, these communications should have security 
   mechanisms to ensure authenticity, privacy and integrity of the 
   information exchanged. In particular, it is important to protect 
   against false triggers for LSP setup in the lower-layer network 
   since such falsification could tie up lower-layer network resources 
   (achieving a denial of service attack on the lower-layer network and 
   on the higher layer network that is attempting to use it) and could 
   result in incorrect billing for services provided by the lower-layer 
   network. Where the PCE MIB modules are used to provide the 
   notification exchanges between the higher-layer PCE and the VNTM, 
   SNMP v3 should be used to ensure adequate security. Additionally, 
   the VNTM should provide configurable or dynamic policy functions so 
   that the VNTM behavior upon receiving notification from a higher- 
   layer PCE can be controlled. 
    
   The main security concern in the higher-layer signaling trigger 
   model is related to confidentiality. The PCE may inform a higher- 
   layer PCC about a multi-layer path that includes an ERO in the 
   lower-layer network, but the PCC may not have TE topology visibility 
   into the lower-layer network and might not be trusted with this 
   information. A loose hop across the lower-layer network could be 
   used, but this decreases the benefit of multi-layer traffic 
   engineering. A better alternative may be to mask the lower-layer 
   path using a path key [PATH-KEY] that can be expanded within the 
   lower-layer network. Consideration must also be given to filtering 
   the recorded path information from the lower-layer - see [RFC4208], 
   for example. 
    

 
 
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   Additionally, in the higher-layer signaling trigger model, 
   consideration must be given to the security of signaling at the 
   inter-layer interface since the layers may belong to different 
   administrative or trust domains. 
    
   The NMS-VNTM cooperation model introduces communication between the 
   NMS and the VNTM. Both of these components belong to the management 
   plane and the communication is out of scope for this PCE document. 
   Note that the NMS-VNTM cooperation model may be considered to 
   address many security and policy concerns because the control and 
   decision-making is placed within the sphere of influence of the 
   operator in contrast to the more dynamic mechanisms of the other 
   models. However, the security issues have simply moved, and will 
   require authentication of operators and of policy. 
    
   Security issues may also exist when a single PCE is granted full 
   visibility of TE information that applies to multiple layers. Any 
   access to the single PCE will immediately gain access to the 
   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 8.1.3, can be applied through 
   policy or configuration to dampen any network oscillations. 
    
10. Acknowledgments 
    
   We would like to thank Kohei Shiomoto, Ichiro Inoue, Julien Meuric, 
   Jean-Francois Peltier, Young Lee, Ina Minei, and Jean-Philippe 
   Vasseur, Franz Rambach for their useful comments. 
    
11. References 
    
11.1. Normative Reference 
    
   [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 
             Label Switching Architecture", RFC 3031, January 2001. 
    
   [RFC3945] Mannie, E., "Generalized Multi-Protocol Label Switching 
             Architecture", RFC 3945, October 2004. 
    
   [RFC4206] K. Kompella and Y. Rekhter, "Label Switched Paths (LSP) 
             Hierarchy with Generalized Multi-Protocol Label Switching 
             (GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005. 
    

 
 
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11.2. Informative Reference 
    
   [RFC5212] K. Shiomoto et al., "Requirements for GMPLS-Based Multi- 
             Region and Multi-Layer Networks (MRN/MLN)", RFC 5212, July 
             2008. 
    
   [PCE-INTER-LAYER-REQ] E. Oki et al., "PCC-PCE Communication 
             Requirements for Inter-Layer Traffic Engineering", draft- 
             ietf-pce-inter-layer-req work in progress. 
    
   [BRPC]    JP. Vasseur et al., "A Backward Recursive PCE-based 
             Computation (BRPC) procedure to compute shortest inter- 
             domain Traffic Engineering Label Switched Paths", draft- 
             ietf-pce-brpc, work in progress. 
    
   [RFC4920] A. Farrel et al., "Crankback Signaling Extensions for MPLS 
             and GMPLS RSVP-TE", RFC 4920, July 2007. 
    
   [PCE-MIB] E. Stephan, "Definitions of Textual Conventions for Path 
             Computation Element", draft-ietf-pce-tc-mib.txt, work in 
             progress. 
    
   [RFC4802] A. Farrel and T. Nadeau, "Generalized Multiprotocol Label 
             Switching (GMPLS) Traffic Engineering Management 
             Information Base", RFC 4802, February 2007. 
    
   [PATH-KEY] Bradford, R., Vasseur, JP., and Farrel, A., "Preserving 
             Topology Confidentiality in Inter-Domain Path Computation 
             Using a Key Based Mechanism", draft-ietf-pce-path-key, work 
             in progress. 
    
   [RFC4208] Swallow, G., Drake, J., Ishimatsu, H., and Rekhter, Y., 
             "Generalized Multiprotocol Label Switching (GMPLS) User- 
             Network Interface (UNI): Resource ReserVation Protocol- 
             Traffic Engineering (RSVP-TE) Support for the Overlay 
             Model", RFC 4208, October 2005. 
    
   [RFC4655] A. Farrel, JP. Vasseur and J. Ash, "A Path Computation 
             Element (PCE)-Based Architecture", RFC 4655, August 2006. 
    
   [RFC4657] J. Ash and J.L. Le Roux (Ed.), "Path Computation Element 
             (PCE) Communication Protocol Generic Requirements", RFC 
             4657, September 2006. 
    
   [RFC5394] Bryskin, I., Papadimitriou, P., Berger, L., and Ash J, 
             "Policy-Enabled Path Computation Framework", RFC 5394, 
             December 2008. 
    
   [RFC5440] JP. Vasseur et al, "Path Computation Element (PCE) 

 
 
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             communication Protocol (PCEP)" RFC 5440, March 2009. 
    
12. 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 
    
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