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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Eiji Oki 3 Internet Draft NTT 4 Category: Informational Jean-Louis Le Roux 5 Expires: January 2008 France Telecom 6 Adrian Farrel 7 Old Dog Consulting 8 July 2007 10 Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic 11 Engineering 13 draft-ietf-pce-inter-layer-frwk-04.txt 15 Status of this Memo 17 By submitting this Internet-Draft, each author represents that any 18 applicable patent or other IPR claims of which he or she is aware 19 have been or will be disclosed, and any of which he or she becomes 20 aware will be disclosed, in accordance with Section 6 of BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF), its areas, and its working groups. Note that 24 other groups may also distribute working documents as Internet- 25 Drafts. 27 Internet-Drafts are draft documents valid for a maximum of six 28 months and may be updated, replaced, or obsoleted by other documents 29 at any time. It is inappropriate to use Internet- Drafts as 30 reference material or to cite them other than as "work in progress." 32 The list of current Internet-Drafts can be accessed at 33 http://www.ietf.org/ietf/1id-abstracts.txt. 35 The list of Internet-Draft Shadow Directories can be accessed at 36 http://www.ietf.org/shadow.html. 38 Abstract 40 A network may comprise multiple layers. It is important to globally 41 optimize network resource utilization, taking into account all 42 layers, rather than optimizing resource utilization at each layer 43 independently. This allows better network efficiency to be achieved 44 through a process that we call inter-layer traffic engineering. The 45 Path Computation Element (PCE) can be a powerful tool to achieve 46 inter-layer traffic engineering. 48 This document describes a framework for applying the PCE-based 49 architecture to inter-layer Multiprotocol Label Switching (MPLS) and 50 Generalized MPLS (GMPLS) traffic engineering. It provides 51 suggestions for the deployment of PCE in support of multi-layer 52 networks. This document also describes network models where PCE 53 performs inter-layer traffic engineering, and the relationship 54 between PCE and a functional component called the Virtual Network 55 Topology Manager (VNTM). 57 Table of Contents 59 1. Introduction....................................................2 60 1.1. Terminology..................................................3 61 2. Inter-Layer Path Computation....................................3 62 3. Inter-layer Path Computation Models.............................5 63 3.1. Single PCE Inter-Layer Path Computation......................5 64 3.2. Multiple PCE Inter-Layer Path Computation....................5 65 3.3. General Observations.........................................6 66 4. Inter-Layer Path Control........................................7 67 4.1. VNT Management...............................................7 68 4.2. Inter-Layer Path Control Models..............................7 69 4.2.1. Cooperation Model Between PCE and VNTM.....................7 70 4.2.2. Higher-Layer Signaling Trigger Model.......................9 71 4.2.3. Examples of Multi-Layer ERO...............................11 72 5. Choosing Between Inter-Layer Path Control Models...............11 73 5.1. VNTM Functions:.............................................11 74 5.2. Border LSR Functions:.......................................12 75 5.3. Complete Inter-Layer LSP Setup Time:........................12 76 5.4. Network Complexity..........................................12 77 5.5. Separation of Layer Management..............................13 78 6. Security Considerations........................................13 79 7. Acknowledgment.................................................14 80 8. References.....................................................14 81 8.1. Normative Reference.........................................14 82 8.2. Informative Reference.......................................14 83 9. Authors' Addresses.............................................14 84 10. Intellectual Property Statement..............................15 86 1. Introduction 88 A network may comprise multiple layers. These layers may represent 89 separations of technologies (e.g., packet switch capable (PSC), time 90 division multiplex (TDM), or lambda switch capable (LSC)) [RFC3945], 91 separation of data plane switching granularity levels (e.g., PSC-1, 92 PSC-2, VC4, or VC12) [MLN-REQ], or a distinction between client and 93 server networking roles. In this multi-layer network, Label Switched 94 Paths (LSPs) in a lower layer are used to carry higher-layer LSPs 95 across the lower-layer network. The network topology formed by 96 lower-layer LSPs and advertised as traffic engineering links (TE 97 links) to the higher layer is called the Virtual Network Topology 98 (VNT) [MLN-REQ]. 100 It may be effective to optimize network resource utilization 101 globally, i.e., taking into account all layers, rather than 102 optimizing resource utilization at each layer independently. This 103 allows better network efficiency to be achieved and is what we call 104 inter-layer traffic engineering. This includes mechanisms allowing 105 the computation of end-to-end paths across layers (known as inter- 106 layer path computation), and mechanisms for control and management 107 of the Virtual Network Topology (VNT) by setting up and releasing 108 LSPs in the lower layers [MLN-REQ]. 110 Inter-layer traffic engineering is included in the scope of the Path 111 Computation Element (PCE)-based architecture [RFC4655], and PCE can 112 provide a suitable mechanism for resolving inter-layer path 113 computation issues. 115 Oki et al Expires January 2008 2 116 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 118 PCE Communication Protocol requirements for inter-layer traffic 119 engineering are set out in [PCE-INTER-LAYER-REQ]. 121 This document describes a framework for applying the PCE-based 122 architecture to inter-layer traffic engineering. It provides 123 suggestions for the deployment of PCE in support of multi-layer 124 networks. This document also describes network models where PCE 125 performs inter-layer traffic engineering, and the relationship 126 between PCE and a functional component in charge of the control and 127 management of the VNT, and called the Virtual Network Topology 128 Manager (VNTM). 130 1.1. Terminology 132 This document uses terminology from the PCE-based path computation 133 architecture [RFC4655] and also common terminology from Multi 134 Protocol Label Switching (MPLS) [RFC3031], Generalized MPLS (GMPLS) 135 [RFC3945] and Multi-Layer Networks [MLN-REQ]. 137 2. Inter-Layer Path Computation 139 This section describes key topics of inter-layer path computation in 140 MPLS and GMPLS networks. 142 [RFC4206] defines a way to signal a higher-layer LSP, which has an 143 explicit route that includes hops traversed by LSPs in lower layers. 144 The computation of end-to-end paths across layers is called Inter- 145 Layer Path Computation. 147 A Label Switching Router (LSR) in the higher-layer might not have 148 information on the topology of the lower-layer, particularly in an 149 overlay or augmented model deployment, and hence may not be able to 150 compute an end-to-end path across layers. 152 PCE-based Inter-Layer Path Computation, consists of using one or 153 more PCEs to compute an end-to-end path across layers. This could be 154 achieved by a single PCE path computation where the PCE has topology 155 information about multiple layers and can directly compute an end- 156 to-end path across layers considering the topology of all of the 157 layers. Alternatively, the inter-layer path computation could be 158 performed as a multiple-PCE computation where each member of a set 159 of PCEs has information about the topology of one or more layers 160 (but not all layers), and the PCEs collaborate to compute an end-to- 161 end path. 163 Consider, for instance, a two-layer network where the higher-layer 164 network is a packet-based IP/MPLS or GMPLS network, and the lower- 165 layer network is a GMPLS optical network. An ingress LSR in the 166 higher-layer network tries to set up an LSP to an egress LSR also in 167 the higher-layer network across the lower-layer network, and needs a 168 path in the higher-layer network. However, suppose that there is no 169 TE link in the higher-layer network between the border LSRs located 170 on the boundary between the higher-layer and lower-layer networks. 171 Suppose also that the ingress LSR does not have topology visibility 172 into the lower layer. If a single-layer path computation is applied 173 for the higher-layer, the path computation fails because of the 174 missing TE link. On the other hand, inter-layer path computation is 175 able to provide a route in the higher-layer and a suggestion that a 176 lower-layer LSP be set up between the border LSRs. 178 Oki et al Expires January 2008 3 179 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 181 Lower-layer LSPs that are advertised as TE links into the higher- 182 layer network form a Virtual Network Topology (VNT) that can be used 183 for routing higher-layer LSPs. Inter-layer path computation for end- 184 to-end LSPs in the higher-layer network that span the lower-layer 185 network may utilize the VNT, and PCE is a candidate for computing 186 the paths of such higher-layer LSPs within the higher-layer network. 187 Alternatively, the PCE-based path computation model can: 189 - Perform a single computation on behalf of the ingress LSR using 190 information gathered from more than one layer. This mode is referred 191 to as Single PCE Computation in [RFC4655]. 193 - Compute a path on behalf of the ingress LSR through cooperation 194 with PCEs responsible for each layer. This mode is referred to as 195 Multiple PCE Computation with inter-PCE communication in [RFC4655]. 197 - Perform separate path computations on behalf of the TE-LSP head- 198 end and each transit border LSR that is the entry point to a new 199 layer. This mode is referred to as Multiple PCE Computation (without 200 inter-PCE communication) in [RFC4655]. This option utilizes per- 201 layer path computation performed independently by successive PCEs. 203 The PCE invoked by the head-end LSR computes a path that the LSR can 204 use to signal an MPLS-TE or GMPLS LSP once the path information has 205 been converted to an Explicit Route Object (ERO) for use in RSVP-TE 206 signaling. There are two options. 208 - Option 1: Mono-layer path. 209 The PCE computes a "mono-layer" path, i.e., a path that includes 210 only TE links from the same layer. There are two cases for this 211 option. In the first case the PCE computes a path that includes 212 already established lower-layer LSPs or lower-layer LSPs to be 213 established on demand. That is, the resulting ERO includes sub- 214 object(s) corresponding to lower-layer hierarchical LSPs expressed 215 as the TE link identifiers of the hierarchical LSPs when advertised 216 as TE links in the higher-layer network. The TE link may be a 217 regular TE link that is actually established, or a virtual TE link 218 that is not established yet (see [MLN-REQ]). If it is a virtual TE 219 link, this triggers a setup attempt for a new lower-layer LSP when 220 signaling reaches the head-end of the lower-layer LSP. Note that the 221 path of a virtual TE link is not necessarily known in advance, and 222 this may require a further (lower-layer) path computation. 224 The second case is that the PCE computes a path that includes a 225 loose hop that spans the lower-layer network. The higher layer path 226 computation selects which lower layer network to use, and selects 227 the entry and exit points from that lower-layer network, but does 228 not select the path across the lower-layer network. A transit LSR 229 that is the entry point to the lower-layer network is expected to 230 expand the loose hop (either itself or relying on the services of a 231 PCE). The path expansion process on the border LSR may result either 232 in the selection of an existing lower-layer LSP, or in the 233 computation and setup of a new lower-layer LSP. 235 - Option 2: Multi-layer path. The PCE computes a "multi-layer" path, 236 i.e., a path that includes TE links from distinct layers [RFC4206]. 237 Such a path can include the complete path of one or more lower-layer 238 LSPs that already exist or are not yet established. In the latter 240 Oki et al Expires January 2008 4 241 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 243 case, the signaling of the higher-layer LSP will trigger the 244 establishment of the lower-layer LSPs. 246 3. Inter-layer Path Computation Models 248 As stated in Section 2, two PCE modes defined in the PCE 249 architecture can be used to perform inter-layer path computation. 250 They are discussed in the sections that follow. 252 3.1. Single PCE Inter-Layer Path Computation 254 In this model Inter-layer path computation is performed by a single 255 PCE that has topology visibility into all layers. Such a PCE is 256 called a multi-layer PCE. 258 In Figure 1, the network is comprised of two layers. LSRs H1, H2, H3, 259 and H4 belong to the higher layer, and LSRs H2, H3, L1, and L2 260 belong to the lower layer. The PCE is a multi-layer PCE that has 261 visibility into both layers. It can perform end-to-end path 262 computation across layers (single PCE path computation). For 263 instance, it can compute an optimal path H1-H2-L1-L2-H3-H4, for a 264 higher layer LSP from H1 to H4. This path includes the path of a 265 lower layer LSP from H2 to H3, already in existence or not yet 266 established. 268 ----- 269 | PCE | 270 ----- 271 ----- ----- ----- ----- 272 | LSR |--| LSR |................| LSR |--| LSR | 273 | H1 | | H2 | | H3 | | H4 | 274 ----- -----\ /----- ----- 275 \----- -----/ 276 | LSR |--| LSR | 277 | L1 | | L2 | 278 ----- ----- 280 Figure 1 : Multi-Layer PCE - A single PCE with multi-layer 281 visibility 283 3.2. Multiple PCE Inter-Layer Path Computation 285 In this model there is at least one PCE per layer, and each PCE has 286 topology visibility restricted to its own layer. Some providers may 287 want to keep the layer boundaries due to factors such as 288 organizational and/or service management issues. The choice for 289 multiple PCE computation instead of single PCE computation may also 290 be driven by scalability considerations, as in this mode a PCE only 291 needs to maintain topology information for one layer (resulting in a 292 size reduction for the Traffic Engineering Database (TED)). 294 These PCEs are called mono-layer PCEs. Mono-layer PCEs collaborate 295 to compute an end-to-end optimal path across layers. 297 In Figure 2, there is one PCE in each layer. The PCEs from each 298 layer collaborate to compute an end-to-end path across layers. PCE 299 Hi is responsible for computations in the higher layer and may 300 consult with PCE Lo to compute paths across the lower layer. PCE 302 Oki et al Expires January 2008 5 303 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 305 Lo is responsible for path computation in the lower layer. A simple 306 example of cooperation between the PCEs could be as follows: 307 - LSR H1 sends a request for a path H1-H4 to PCE Hi 308 - PCE Hi selects H2 as the entry point to the lower layer, and H3 as 309 the exit point. 310 - PCE Hi requests a path H2-H3 from PCE Lo. 311 - PCE Lo returns H2-L1-L2-H3 to PCE Hi. 312 - PEC Hi is now able to compute the full path (H1-H2-L1-L2-H3-H4) 313 and return it to H1. 315 Of course more complex cooperation may be required if an optimal 316 end-to-end path is desired. 318 ----- 319 | PCE | 320 | Hi | 321 --+-- 322 | 323 ----- ----- | ----- ----- 324 | LSR |--| LSR |............|...........| LSR |--| LSR | 325 | H1 | | H2 | | | H3 | | H4 | 326 ----- -----\ --+-- /----- ----- 327 \ | PCE | / 328 \ | Lo | / 329 \ ----- / 330 \ / 331 \----- -----/ 332 | LSR |--| LSR | 333 | L1 | | L2 | 334 ----- ----- 336 Figure 2 : Cooperating Mono-Layer PCEs - Multiple PCEs with single- 337 layer visibility 339 3.3. General Observations 341 - Depending on implementation details, the time to perform inter- 342 layer path computation in the Single PCE inter-layer path 343 computation model may be less than that of the Multiple PCE model 344 with cooperating mono-layer PCEs, because there is no requirement to 345 exchange messages between cooperating PCEs. 347 - When TE topology for all layer networks is visible within one 348 routing domain, the single PCE inter-layer path computation model 349 may be adopted because a PCE is able to collect all layers' TE 350 topologies by participating in only one routing domain. 352 - As the single PCE inter-layer path computation model uses more TE 353 topology information in one computation than is used by PCEs in the 354 Multiple PCE path computation model, it requires more computation 355 power and memory. 357 When there are multiple candidate layer border nodes (we may say 358 that the higher layer is multi-homed), optimal path computation 359 requires that all the possible paths transiting different layer 360 border nodes or links be examined. This is relatively simple in the 361 single PCE inter-layer path computation model because the PCE has 362 full visibility - the computation is similar to the computation 364 Oki et al Expires January 2008 6 365 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 367 within a single domain of a single layer. In the multiple PCE inter- 368 layer path computation model, backward recursive techniques 369 described in [BRPC] could be used, by considering layers as separate 370 domains. 372 4. Inter-Layer Path Control 374 4.1. VNT Management 376 As a result of inter-layer path computation, a PCE may determine 377 that there is insufficient bandwidth available in the higher-layer 378 network to support this or future higher-layer LSPs. The problem 379 might be resolved if new LSPs were provisioned across the lower- 380 layer network. Furthermore, the modification, re-organization and 381 new provisioning of lower-layer LSPs may enable better utilization 382 of lower-layer network resources given the demands of the higher- 383 layer network. In other words, the VNT needs to be controlled or 384 managed in cooperation with inter-layer path computation. 386 A VNT Manager (VNTM) is defined as a network element that manages 387 and controls the VNT. PCE and VNT Manager are distinct functional 388 elements that may or may not be co-located. 390 4.2. Inter-Layer Path Control Models 392 4.2.1. 393 Cooperation Model Between PCE and VNTM 395 ----- ------ 396 | PCE |--->| VNTM | 397 ----- ------ 398 ^ : 399 : : 400 : : 401 v V 402 ----- ----- ----- ----- 403 | LSR |----| LSR |................| LSR |----| LSR | 404 | H1 | | H2 | | H3 | | H4 | 405 ----- -----\ /----- ----- 406 \----- -----/ 407 | LSR |--| LSR | 408 | L1 | | L2 | 409 ----- ----- 411 Figure 3: Cooperation Model Between PCE and VNTM 413 A multi-layer network consists of higher-layer and lower-layer 414 networks. LSRs H1, H2, H3, and H4 belong to the higher-layer network, 415 LSRs H2, L1, L2, and H3 belong to the lower-layer network, as shown 416 in Figure 3. Consider that H1 requests PCE to compute an inter-layer 417 path between H1 and H4. There is no TE link in the higher-layer 418 between H2 and H3 before the path computation request fails. But the 419 PCE may provide information to the VNT Manager responsible for the 420 lower layer network that may help resolve the situation for future 421 higher-layer LSP setup. 423 The roles of PCE and VNTM are as follows. PCE performs inter-layer 424 path computation and is unable to supply a path because there is no 425 TE link between H2 and H3. The computation fails, but PCE suggests 426 to VNTM that a lower-layer LSP (H2-H3) could be established to 427 support future LSP requests. Messages from PCE to VNTM contain 429 Oki et al Expires January 2008 7 430 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 432 information about the higher-layer demand (from H2 to H3). VNTM uses 433 local policy and possibly management/configuration input to 434 determine how to process the suggestion from PCE, and may request an 435 ingress LSR (e.g. H2) to establish a lower-layer LSP. VNTM or the 436 ingress LSR (H2) may themselves use a PCE with visibility into the 437 lower layer to compute the path of this new LSP. 439 When the higher-layer PCE fails to compute a path and notifies VNTM, 440 it may wait for the lower-layer LSP to be set up and advertised as a 441 TE link. It could then compute the complete end-to-end path for the 442 higher-layer LSP and return the result to the PCC. In this case, the 443 PCC may be kept waiting for some time, and it is important that the 444 PCC understands this. It is also important that the PCE and VNTM 445 have an agreement that the lower-layer LSP will be set up in a 446 timely manner, or that the PCE will be notified by VNTM that no new 447 LSP will become available. In any case, if the PCE decides to wait, 448 it must operates a timeout. An example of such a cooperative 449 procedure between PCE and VNTM is as follows using the exmaple 450 network in Figure 3. 452 Step 1: H1 (PCC) requests PCE to compute a path between H1 and H4. 454 Step 2: The path computation fails because there is no TE link 455 across the lower-layer network. 457 Step 3: PCE suggests to VNTM that a new TE link connecting H2 and H3 458 would be useful. The PCE notifies VNTM that it will be waiting for 459 the TE link to be created. VNTM considers whether lower-layer LSPs 460 should be established if necessary and if acceptable within VNTM's 461 policy constraints. 463 Step 4: VNTM requests an ingress LSR in the lower-layer network 464 (e.g., H2) to establish a lower-layer LSP. The request message may 465 include a lower-layer LSP route obtained from the PCE responsible 466 for the lower-layer network. 468 Step 5: The ingress LSR signals to establish the lower-layer LSP. 470 Step 6: If the lower-layer LSP setup is successful, the ingress LSR 471 notifies VNTM that the LSP is complete and supplies the tunnel 472 information. 474 Step 7: The ingress LSR (H2) advertises the new LSP as a TE link in 475 the higher-layer network routing instance. 477 Step 8: PCE notices the new TE link advertisement and recomputes the 478 requested path. 480 Step 9: PCE replies to H1 (PCC) with a computed higher-layer LSP 481 route. The computed path is categorized as a mono-layer path that 482 includes the already-established lower layer-LSP as a single hop in 483 the higher layer. The higher-layer route is specified as H1-H2-H3-H4, 484 where all hops are strict. 486 Step 9: H1 initiates signaling with the computed path H2-H3-H4 to 487 establish the higher-layer LSP. 489 Oki et al Expires January 2008 8 490 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 492 4.2.2. 493 Higher-Layer Signaling Trigger Model 495 ----- 496 | PCE | 497 ----- 498 ^ 499 : 500 : 501 v 502 ----- ----- ----- ----- 503 | LSR |----| LSR |................| LSR |--| LSR | 504 | H1 | | H2 | | H3 | | H4 | 505 ----- -----\ /----- ----- 506 \----- -----/ 507 | LSR |--| LSR | 508 | L1 | | L2 | 509 ----- ----- 511 Figure 4: Higher-layer Signaling Trigger Model 513 Figure 4 shows the higher-layer signaling trigger model. As in the 514 case described in Section 4.2.1, consider that H1 requests PCE to 515 compute a path between H1 and H4. There is no TE link in the higher- 516 layer between H2 and H3 before the path computation request. 518 PCE is unable to compute a mono-layer path, but may judge that the 519 establishment of a lower-layer LSP between H2 and H3 would provide 520 adequate connectivity. If the PCE has inter-layer visibility it may 521 return a path that includes hops in the lower layer (H1-H2-L1-L2-H3- 522 H4), but if it has no visiblity into the lower layer, it may return 523 a path with a loose hop from H2 to H3 (H1-H2-H3(loose)-H4). The 524 former is a multi-layer path, and the latter a mono-layer path that 525 includes loose hops. 527 In the higher-layer signaling trigger model with a multi-layer path, 528 the LSP route supplied by the PCE includes the route of a lower- 529 layer LSP that is not yet established. A border LSR that is located 530 at the boundary between the higher-layer and lower-layer networks 531 (H2 in this example) receives a higher-layer signaling message, 532 notices that the next hop is in the lower-layer network, starts to 533 setup the lower-layer LSP as described in [RFC4206]. Note that these 534 actions depends on a policy being applied at the border LSR. An 535 example procedure of the signaling trigger model with a multi-layer 536 path is as follows. 538 Step 1: H1 (PCC) requests PCE to compute a path between H1 and H4. 539 The request indicates that inter-layer path computation is allowed. 541 Step 2: As a result of the inter-layer path computation, PCE judges 542 that a new lower-layer LSP needs to be established. 544 Step 3: PCE replies to H1 (PCC) with a computed multi-layer route 545 including higher-layer and lower-layer LSP routes. The route may be 546 specified as H1-H2-L1-L2-H3-H4, where all hops are strict. 548 Step 4: H1 initiates higher-layer signaling using the computed 549 explicit router of H2-L1-L2-H3-H4. 551 Oki et al Expires January 2008 9 552 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 554 Step 5: The border LSR (H2) that receives the higher-layer signaling 555 message starts lower-layer signaling to establish a lower-layer LSP 556 along the specified lower-layer route of H2-L1-L2-H3. That is, the 557 border LSR recognizes the hops within the explicit route that apply 558 to the lower-layer network, verifies with local policy that a new 559 LSP is acceptable, and establishes the required lower-layer LSP. 560 Note that it is possible that a suitable lower-layer LSP has already 561 been established (or become available) between the time that the 562 computation was performed and the moment when the higher-layer 563 signaling message reached the border LSR. In this case, the border 564 LSR may select such a lower-layer LSP without the need to signal a 565 new LSP provided that the lower-layer LSP satisfies the explicit 566 route in the higher-layer signaling request. 568 Step 6: After the lower-layer LSP is established, the higher-layer 569 signaling continues along the specified higher-layer route of H2-H3- 570 H4 using hierarchical signaling [RFC4206]. 572 On the other hand, in the signaling trigger model with a mono-layer 573 path, a higher-layer LSP route includes a loose hop to traverse the 574 lower-layer network between the two border LSRs. A border LSR that 575 receives a higher-layer signaling message needs to determine a path 576 for a new lower-layer LSP. It applies local policy to verify that a 577 new LSP is acceptable and then either consults a PCE with 578 responsibility for the lower-layer network or computes the path by 579 itself, and initiates signaling to establish the lower-layer LSP. 580 Again, it is possible that a suitable lower-layer LSP has already 581 been established (or become available). In this case, the border LSR 582 may select such a lower-layer LSP without the need to signal a new 583 LSP provided that the existing lower-layer LSP satisfies the 584 explicit route in the higher-layer signaling request. Since the 585 higher-layer signaling request used a loose hop without specifying 586 any specifics of the path within the lower-layer network, the border 587 LSR has greater freedom to choose a lower-layer LSP than in the 588 previous example. 590 The difference between procedures of the signaling trigger model 591 with a multi-layer path and a mono-layer path is Step 5. Step 5 of 592 the signaling trigger model with a mono layer path is as follows: 594 Step 5': The border LSR (H2) that receives the higher-layer 595 signaling message applies local policy to verify that a new LSP is 596 acceptable and then initiates establishment of a lower-layer LSP. It 597 either consults a PCE with responsibility for the lower-layer 598 network or computes the route by itself to expand the loose hop 599 route in the higher-layer path. 601 Finally, note that a virtual TE link may have been advertised into 602 the higher-layer network. This causes the PCE to return a path H1- 603 H2-H3-H4 where all the hops are strict. But when the higher-layer 604 signaling message reaches the layer border node H2 (that was 605 responsible for advertising the virtual TE link) it realizes that 606 the TE link does not exist yet, and signals the necessary LSP across 607 the lower-layer network using its own path determination (just as 608 for a loose hop in the higher layer) before continuing with the 609 higher-layer signaling. 611 Oki et al Expires January 2008 10 612 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 614 4.2.3. 615 Examples of Multi-Layer ERO 617 PCE 618 ^ 619 : 620 : 621 V 622 H1--H2 H3--H4 623 \ / 624 L1==L2==L3--L4--L5 625 | 626 | 627 L6--L7 628 \ 629 H5--H6 631 Figure 5: Example of a Multi-Layer Network 633 This section describes how lower-layer LSP setup is performed in the 634 higher-layer signaling trigger model using an ERO that can include 635 subobjects in both the higher and lower layers. It gives rise to 636 several options for the ERO when it reaches the last LSR in the 637 higher layer network (H2). 638 1. The next subobject is a loose hop to H3 (mono layer ERO). 639 2. The next subobject is a strict hop to L1 followed by a loose hop 640 to H3. 641 3. The next subobjects are a series of hops (strict or loose) in the 642 lower-layer network followed by H3. For example, {L1(strict), 643 L3(loose), L5(loose), H3(strict)} 645 In the first example, the lower layer can utilize any LSP tunnel 646 that will deliver the end-to-end LSP to H3. In the third case, the 647 lower layer must select an LSP tunnel that traverses L3 and L5. 648 However, this does not mean that the lower layer can or should use 649 an LSP from L1 to L3 and another from L3 to L5. 651 5. Choosing Between Inter-Layer Path Control Models 653 This section compares the cooperation model between PCE and VNTM, 654 and the higher-layer signaling trigger model, in terms of VNTM 655 functions, border LSR functions, higher-layer signaling time, and 656 complexity (in terms of number of states and messages). An 657 appropriate model may be chosen by a network operator in different 658 deployment scenarios taking all these considerations into account. 660 5.1. VNTM Functions: 662 In the cooperation model, VNTM functions are required. In this model, 663 communications are required between PCE and VNTM, and between VNTM 664 and a border LSR. VNTM-LSR communication can rely on existing GMPLS- 665 TE MIB modules. PCE-VNTM communication will be detailed in further 666 revisions of this document. 668 In the higher-layer signaling trigger model, no VNTM functions are 669 required, and no such communications are required. 671 If VNTM functions are not supported in a multi-layer network, the 672 higher-layer signaling trigger model has to be chosen. 674 Oki et al Expires January 2008 11 675 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 677 The inclusion of VNTM functionality allows better coordination of 678 cross-network LSP tunnels and application of network-wide policy 679 that is far harder to apply in the trigger model since it requires 680 the coordination of policy between multiple border LSRs. 682 5.2. Border LSR Functions: 684 In the higher-layer signaling trigger model, a border LSR must have 685 some additional functions. It needs to trigger lower-layer signaling 686 when a higher-layer path message suggests that lower-layer LSP setup 687 is necessary. Note that, if virtual TE links are used, the border 688 LSRs must be capable of triggered signaling. 690 If the ERO in the higher-layer Path message uses a mono-layer path 691 or specifies a loose hop, the border LSR receiving the Path message 692 must obtain a lower-layer route either by consulting a PCE or by 693 using its own computation engine. If the ERO in the higher-layer 694 Path message uses a multi-layer path, the border LSR must judge 695 whether lower-layer signaling is needed. 697 In the cooperation model, no additional function for triggered 698 signaling is required in border LSRs except when virtual TE links 699 are used. Therefore, if these additional functions are not supported 700 in border LSRs, where a border LSR is controlled by VNTM to set up a 701 lower-layer LSP, the cooperation model has to be chosen. 703 5.3. Complete Inter-Layer LSP Setup Time: 705 The complete inter-layer LSP setup time includes inter-layer path 706 computation, signaling, and the communication time between PCC and 707 PCE, PCE and VNTM, and VNTM and LSR. In the cooperation model, the 708 additional communication steps are required compared with the 709 higher-layer signaling trigger model. On the other hand, the 710 cooperation model provides better control at the cost of a longer 711 service setup time. 713 Note that, in terms of higher-layer signaling time, in the higher- 714 layer signaling trigger model, the required time from when higher- 715 layer signaling starts to when it is completed, is more than that of 716 the cooperation model except when a virtual TE link is included. 717 This is because the former model requires lower-layer signaling to 718 take place during the higher-layer signaling. A higher-layer ingress 719 LSR has to wait for more time until the higher-layer signaling is 720 completed. A higher-layer ingress LSR is required to be tolerant of 721 longer path setup times. 723 5.4. Network Complexity 725 If the higher and lower layer networks have multiple interconnects 726 then optimal path computation for end-to-end LSPs that cross the 727 layer boundaries is non-trivial. The higher layer LSP must be routed 728 to the correct layer border nodes to achieve optimality in both 729 layers. 731 Where the lower layer LSPs are advertised into the higher layer 732 network as TE links, the computation can be resolved in the higher 733 layer network. Care needs to be taken in the allocation of TE 734 metrics (i.e., costs) to the lower layer LSPs as they are advertised 735 as TE links into the higher layer network, and this might be a 736 function for a VNT Manager component. Similarly, attention should be 738 Oki et al Expires January 2008 12 739 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 741 given to the fact that the LSPs crossing the lower-layer network 742 might share points of common failure (e.g., they might traverse the 743 same link in the lower-layer network) and the shared risk link 744 groups (SRLGs) for the TE links advertised in the higher-layer must 745 be set accordingly. 747 In the single PCE model an end-to-end path can be found in a single 748 computation because there is full visibility into both layers and 749 all possible paths through all layer interconnects can be considered. 751 Where PCEs cooperate to determine a path, an iterative computation 752 model such as [BRPC] can be used to select an optimal path across 753 layers. 755 When non-cooperating mono-layer PCEs, each of which is in a separate 756 layer, are used with the triggered LSP model, it is not possible to 757 determine the best border LSRs, and connectivity cannot even be 758 guaranteed. In this case, signaling crankback techniques [CRANK] can 759 be used to eventually achieve connectivity, but optimality is far 760 harder to achieve. In this model, a PCE that is requested by an 761 ingress LSR to compute a path expects a border LSR to setup a lower- 762 layer path triggered by high-layer signaling when there is no TE 763 link between border LSRs. 765 5.5. Separation of Layer Management 767 Many network operators may want to provide a clear separation 768 between the management of the different layer networks. In some 769 cases, the lower layer network may come from a separate commercial 770 arm of an organization or from a different corporate body entirely. 771 In these cases, the policy applied to the establishment of LSPs in 772 the lower-layer network and to the advertisement of these LSPs as TE 773 links in the higher-layer network will reflect commercial agreements 774 and security concerns (see next section). Since the capacity of the 775 LSPs in the lower-layer network are likely to be significantly 776 larger than those in the client higher-layer network (multiplex- 777 server model), the administrator of the lower-layer network may want 778 to exercise caution before allowing a single small demand in the 779 higher layer to tie up valuable resources in the lower layer. 781 The necessary policy points for this separation of administration 782 and management are more easily achieved through the VNTM approach 783 than by using triggered signaling. In effect, the VNTM is the 784 coordination point for all lower layer LSPs and can be closely tied 785 to a human operator as well as to policy and billing. Such a model 786 can also be achieved using triggered signaling. 788 6. Security Considerations 790 Inter-layer traffic engineering with PCE raises new security issues 791 in both inter-layer path control models. 793 In the cooperation model between PCE and VNTM, when the PCE judges a 794 new lower-layer LSP, communications between PCE and VNTM and between 795 VNTM and a border LSR are needed. In this case, there are some 796 security concerns that need to be addressed for these communications. 797 These communications should have some security mechanisms to ensure 798 authenticity, privacy and integrity. In particular, it is important 799 to protect against false triggers for LSP setup in the lower-layer 800 network. 802 Oki et al Expires January 2008 13 803 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 805 In the higher-layer signaling trigger model, there are several 806 security concerns. First, PCE may inform PCC, which is located in 807 the higher-layer network, of multi-layer path information that 808 includes an ERO in the lower-layer network, while the PCC may not 809 have TE topology visibility into the lower-layer network. This 810 raises a security concern, where lower-layer hop information is 811 known to transit LSRs supporting a higher-layer LSP. Some security 812 mechanisms to ensure authenticity, privacy and integrity may be used. 814 Security issues may also exist when a single PCE is granted full 815 visibility of TE information that applies to multiple layers. 817 7. Acknowledgment 819 We would like to thank Kohei Shiomoto, Ichiro Inoue, Julien Meuric, 820 Jean-Francois Peltier, Young Lee, and Ina Minei for their useful 821 comments. 823 8. References 825 8.1. Normative Reference 827 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 828 Label Switching Architecture", RFC 3031, January 2001. 829 [RFC3945] Mannie, E., "Generalized Multi-Protocol Label Switching 830 Architecture", RFC 3945, October 2004. 832 [RFC4206] Kompella, K., and Rekhter, Y., "Label Switched Paths (LSP) 833 Hierarchy with Generalized Multi-Protocol Label Switching (GMPLS) 834 Traffic Engineering (TE)", RFC 4206, October 2005. 836 [RFC4655] A. Farrel, JP. Vasseur and J. Ash, "A Path Computation 837 Element (PCE)-Based Architecture", RFC 4655, August 2006. 839 8.2. Informative Reference 841 [MLN-REQ] K. Shiomoto et al., "Requirements for GMPLS-based multi- 842 region networks (MRN)", draft-ietf-ccamp-gmpls-mln-reqs (work in 843 progress). 845 [PCE-INTER-LAYER-REQ] E. Oki et al., "PCC-PCE Communication 846 Requirements for Inter-Layer Traffic Engineering," draft-ietf-pce- 847 inter-layer-req (work in progress). 849 [BRPC] JP. Vasseur et al., "A Backward Recursive PCE-based 850 Computation (BRPC) procedure to compute shortest inter-domain 851 Traffic Engineering Label Switched Paths", draft-ietf-pce-brpc (work 852 in progress). 854 [CRANK] A. Farrel et al., "Crankback Signaling Extensions for MPLS 855 and GMPLS RSVP-TE", draft-ietf-ccamp-crankback (work in progress). 857 9. Authors' Addresses 859 Eiji Oki 860 NTT 861 3-9-11 Midori-cho, 862 Musashino-shi, Tokyo 180-8585, Japan 863 Email: oki.eiji@lab.ntt.co.jp 865 Oki et al Expires January 2008 14 866 draft-ietf-pce-inter-layer-frwk-04.txt July 2007 868 Jean-Louis Le Roux 869 France Telecom R&D, 870 Av Pierre Marzin, 871 22300 Lannion, France 872 Email: jeanlouis.leroux@orange-ftgroup.com 874 Adrian Farrel 875 Old Dog Consulting 876 Email: adrian@olddog.co.uk 878 10. 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