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Checking references for intended status: Informational ---------------------------------------------------------------------------- -- Obsolete informational reference (is this intentional?): RFC 5316 (Obsoleted by RFC 9346) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group D. King (Ed.) 2 Internet-Draft Old Dog Consulting 3 Intended Status: Informational A. Farrel (Ed.) 4 Expires: 29 January 2013 Old Dog Consulting 5 29 August 2012 7 The Application of the Path Computation Element Architecture to the 8 Determination of a Sequence of Domains in MPLS and GMPLS 10 draft-ietf-pce-hierarchy-fwk-05.txt 12 Abstract 14 Computing optimum routes for Label Switched Paths (LSPs) across 15 multiple domains in MPLS Traffic Engineering (MPLS-TE) and GMPLS 16 networks presents a problem because no single point of path 17 computation is aware of all of the links and resources in each 18 domain. A solution may be achieved using the Path Computation 19 Element (PCE) architecture. 21 Where the sequence of domains is known a priori, various techniques 22 can be employed to derive an optimum path. If the domains are 23 simply-connected, or if the preferred points of interconnection are 24 also known, the Per-Domain Path Computation technique can be used. 25 Where there are multiple connections between domains and there is 26 no preference for the choice of points of interconnection, the 27 Backward Recursive Path Computation Procedure (BRPC) can be used to 28 derive an optimal path. 30 This document examines techniques to establish the optimum path when 31 the sequence of domains is not known in advance. The document 32 shows how the PCE architecture can be extended to allow the optimum 33 sequence of domains to be selected, and the optimum end-to-end path 34 to be derived through the use of a hierarchical relationship between 35 domains. 37 Status of this Memo 39 This Internet-Draft is submitted to IETF in full conformance with the 40 provisions of BCP 78 and BCP 79. 42 Internet-Drafts are working documents of the Internet Engineering 43 Task Force (IETF), its areas, and its working groups. Note that 44 other groups may also distribute working documents as Internet- 45 Drafts. 47 Internet-Drafts are draft documents valid for a maximum of six months 48 and may be updated, replaced, or obsoleted by other documents at any 49 time. It is inappropriate to use Internet-Drafts as reference 50 material or to cite them other than as "work in progress." 51 The list of current Internet-Drafts can be accessed at 52 http://www.ietf.org/ietf/1id-abstracts.txt. 54 The list of Internet-Draft Shadow Directories can be accessed at 55 http://www.ietf.org/shadow.html. 57 This Internet-Draft will expire on 29 August 2012. 59 Copyright Notice 61 Copyright (c) 2012 IETF Trust and the persons identified as the 62 document authors. All rights reserved. 64 This document is subject to BCP 78 and the IETF Trust's Legal 65 Provisions Relating to IETF Documents 66 (http://trustee.ietf.org/license-info) in effect on the date of 67 publication of this document. Please review these documents 68 carefully, as they describe your rights and restrictions with respect 69 to this document. Code Components extracted from this document must 70 include Simplified BSD License text as described in Section 4.e of 71 the Trust Legal Provisions and are provided without warranty as 72 described in the Simplified BSD License. 74 Contents 76 1. Introduction..................................................3 77 1.1 Problem Statement.........................................4 78 1.2 Definition of a Domain............. ......................5 79 1.3 Assumptions and Requirements..............................5 80 1.3.1 Metric Objectives...................................6 81 1.3.2 Diversity...........................................6 82 1.3.2.1 Physical Diversity..........................6 83 1.3.2.2 Domain Diversity............................7 84 1.3.3 Existing Traffic Engineering Constraints............7 85 1.3.4 Commercial Constraints..............................7 86 1.3.5 Domain Confidentiality..............................7 87 1.3.6 Limiting Information Aggregation....................8 88 1.3.7 Domain Interconnection Discovery....................8 89 1.4 Terminology...............................................8 90 2. Examination of Existing PCE Mechanisms........................9 91 2.1 Per Domain Path Computation...............................9 92 2.2 Backward Recursive Path Computation.......................10 93 2.2.1 Applicability of BRPC when the Domain Path is not 94 Known.................................................10 95 3. Hierarchical PCE..............................................11 96 4. Hierarchical PCE Procedures...................................12 97 4.1 Objective Functions and Policy............................12 98 4.2 Maintaining Domain Confidentiality........................13 99 4.3 PCE Discovery.............................................13 100 4.4 Parent Domain Traffic Engineering Database................14 101 4.5 Determination of Destination Domain ......................15 102 4.6 Hierarchical PCE Examples.................................15 103 4.6.1 Hierarchical PCE Initial Information Exchange.......17 104 4.6.2 Hierarchical PCE End-to-End Path Computation 105 Procedure Example.........................................17 106 4.7 Hierarchical PCE Error Handling...........................19 107 4.8 Hierarchical PCEP Protocol Extensions.....................19 108 4.8.1 PCEP Request Qualifiers.............................19 109 4.8.2 Indication of H-PCE Capability......................20 110 4.8.3 Intention to Utilize Parent PCE Capabilities........20 111 4.8.4 Communication of Domain Connectivity Information....20 112 4.8.5 Domain Identifiers..................................21 113 5. Hierarchical PCE Applicability................................21 114 5.1 autonomous Systems and Areas..............................21 115 5.2 ASON architecture (G-7715-2)..............................22 116 5.2.1 Implicit Consistency Between Hierarchical PCE and 117 G.7715.2..................................................23 118 5.2.2 Benefits of Hierarchical PCEs in ASON...............24 119 6. A Note on BGP-TE..............................................24 120 6.1 Use of BGP for TED Synchronization........................25 121 7. Management Considerations ....................................25 122 7.1 Control of Function and Policy............................25 123 7.1.1 Child PCE...........................................25 124 7.1.2 Parent PCE..........................................26 125 7.1.3 Policy Control......................................26 126 7.2 Information and Data Models...............................26 127 7.3 Liveness Detection and Monitoring.........................26 128 7.4 Verifying Correct Operation...............................26 129 7.5. Impact on Network Operation..............................27 130 8. Security Considerations ......................................27 131 9. IANA Considerations ..........................................28 132 10. Acknowledgements ............................................28 133 11. References ..................................................28 134 11.1. Normative References....................................28 135 11.2. Informative References .................................29 136 12. Authors' Addresses ..........................................12 138 1. Introduction 140 The capability to compute the routes of end-to-end inter-domain MPLS 141 Traffic Engineering (TE) and GMPLS Label Switched Paths (LSPs) is 142 expressed as requirements in [RFC4105] and [RFC4216]. This capability 143 may be realized by a Path Computation Element (PCE). The PCE 144 architecture is defined in [RFC4655]. The methods for establishing 145 and controlling inter-domain MPLS-TE and GMPLS LSPs are documented in 146 [RFC4726]. 148 In this context, a domain can be defined as a separate 149 administrative, geographic, or switching environment within the 150 network. A domain may be further defined as a zone of routing or 151 computational ability. Under these definitions a domain might be 152 categorized as an autonomous System (AS) or an Interior Gateway 153 Protocol (IGP) area [RFC4726] and [RFC4655]. Domains are connected 154 through ingress and egress boundary nodes (BNs). A more detailed 155 definition is given in Section 1.2. 157 In a multi-domain environment, the determination of an end-to-end 158 traffic engineered path is a problem because no single point of path 159 computation is aware of all of the links and resources in each 160 domain. PCEs can be used to compute end-to-end paths using a per- 161 domain path computation technique [RFC5152]. Alternatively, the 162 backward recursive path computation (BRPC) mechanism [RFC5441] 163 allows multiple PCEs to collaborate in order to select an optimal 164 end-to-end path that crosses multiple domains. Both mechanisms 165 assume that the sequence of domains to be crossed between ingress 166 and egress is known in advance. 168 This document examines techniques to establish the optimum path when 169 the sequence of domains is not known in advance. It shows how the PCE 170 architecture can be extended to allow the optimum sequence of domains 171 to be selected, and the optimum end-to-end path to be derived. 173 The model described in this document introduces a hierarchical 174 relationship between domains. It is applicable to environments with 175 small groups of domains where visibility from the ingress Label 176 Switching Router (LSR) is limited. Applying the hierarchical PCE 177 model to large groups of domains such as the Internet, is not 178 considered feasible or desirable, and is out of scope for this 179 document. 181 This document does not specify any protocol extensions or 182 enhancements. That work is left for future protocol specification 183 documents. However, some assumptions are made about which protocols 184 will be used to provide specific functions, and guidelines to 185 future protocol developers are made in the form of requirements 186 statements. 188 1.1 Problem Statement 190 Using a PCE to compute a path between nodes within a single domain is 191 relatively straightforward. Computing an end-to-end path when the 192 source and destination nodes are located in different domains 193 requires co-operation between multiple PCEs, each responsible for 194 its own domain. 196 Techniques for inter-domain path computation described so far 197 ([RFC5152] and [RFC5441]) assume that the sequence of domains to be 198 crossed from source to destination is well known. No explanation is 199 given (for example, in [RFC4655]) of how this sequence is generated 200 or what criteria may be used for the selection of paths between 201 domains. In small clusters of domains, such as simple cooperation 202 between adjacent ISPs, this selection process is not complex. In more 203 advanced deployments (such as optical networks constructed from 204 multiple sub-domains, or in multi-AS environments) the choice of 205 domains in the end-to-end domain sequence can be critical to the 206 determination of an optimum end-to-end path. 208 1.2 Definition of a Domain 210 A domain is defined in [RFC4726] as any collection of network 211 elements within a common sphere of address management or path 212 computational responsibility. Examples of such domains include 213 IGP areas and Autonomous Systems. Wholly or partially overlapping 214 domains are not within the scope of this document. 216 In the context of GMPLS, a particularly important example of a domain 217 is the Automatically Switched Optical Network (ASON) subnetwork 218 [G-8080]. In this case, a domain might be an ASON Routing Area 219 [G-7715]. Furthermore, computation of an end-to-end path requires 220 the selection of nodes and links within a routing area where some 221 nodes may, in fact, be subnetworks. A PCE may perform the path 222 computation function of an ASON Routing Controller as described in 223 [G-7715-2]. See Section 5.2 for a further discussion of the 224 applicability to the ASON architecture. 226 This document assumes that the selection of a sequence of domains for 227 an end-to-end path is in some sense a hierarchical path computation 228 problem. That is, where one mechanism is used to determine a path 229 across a domain, a separate mechanism (or at least a separate set 230 of paradigms) is used to determine the sequence of domains. The 231 responsibility for the selection of domain interconnection can belong 232 to either or both of the mechanisms. 234 1.3 Assumptions and Requirements 236 Networks are often constructed from multiple domains. These 237 domains are often interconnected via multiple interconnect points. 238 Its assumed that the sequence of domains for an end-to-end path is 239 not always well known; that is, an application requesting end-to-end 240 connectivity has no preference for, or no ability to specify, the 241 sequence of domains to be crossed by the path. 243 The traffic engineering properties of a domain cannot be seen from 244 outside the domain. Traffic engineering aggregation or abstraction, 245 hides information and can lead to failed path setup or the selection 246 of suboptimal end-to-end paths [RFC4726]. The aggregation process 247 may also have significant scaling issues for networks with many 248 possible routes and multiple TE metrics. Flooding TE information 249 breaks confidentiality and does not scale in the routing protocol. 250 See Section 6 for a discussion of the concept of inter-domain traffic 251 engineering information exchange known as BGP-TE. 253 The primary goal of this document is to define how to derive optimal 254 end-to-end, multi-domain paths when the sequence of domains is not 255 known in advance. The solution needs to be scalable and to maintain 256 internal domain topology confidentiality while providing the optimal 257 end-to-end path. It cannot rely on the exchange of TE information 258 between domains, and for the confidentiality, scaling, and 259 aggregation reasons just cited, it cannot utilize a computation 260 element that has universal knowledge of TE properties and topology 261 of all domains. 263 The sub-sections that follow set out the primary objectives and 264 requirements to be satisfied by a PCE solution to multi-domain path 265 computation. 267 1.3.1 Metric Objectives 269 The definition of optimality is dependent on policy, and is based on 270 a single objective or a group objectives. An objective is expressed 271 as an objective function [RFC5541] and may be specified on a path 272 computation request. The following objective functions are identified 273 in this document. They define how the path metrics and TE link 274 qualities are manipulated during inter-domain path computation. The 275 list is not proscriptive and may be expanded in other documents. 277 o Minimize the cost of the path [RFC5541] 278 o Select a path using links with the minimal load [RFC5541] 279 o Select a path that leaves the maximum residual bandwidth [RFC5541] 280 o Minimize aggregate bandwidth consumption [RFC5541] 281 o Minimize the Load of the most loaded Link [RFC5541] 282 o Minimize the Cumulative Cost of a set of paths [RFC5541] 283 o Minimize or cap the number of domains crossed 284 o Disallow domain re-entry 286 See Section 4.1 for further discussion of objective functions. 288 1.3.2 Diversity 290 1.3.2.1 Physical Diversity 292 Within a Carrier's carrier environment MPLS services may share common 293 underlying equipment and resources, including optical fiber and 294 nodes. An MPLS service request may require a path for traffic that is 295 physically disjointed from another service. Thus, if a physical link 296 or node fails on one of the disjoint paths, not all traffic is lost. 298 1.3.2.2 Domain Diversity 300 A pair of paths are domain-diverse if they do not transit any of the 301 same domains. A pair of paths that share a common ingress and egress 302 are domain-diverse if they only share the same domains at the ingress 303 and egress (the ingress and egress domains). Domain diversity may be 304 maximized for a pair of paths by selecting paths that have the 305 smallest number of shared domains. (Note that this is not the same 306 as finding paths with the greatest number of distinct domains!) 308 Path computation should facilitate the selection of paths that share 309 ingress and egress domains, but do not share any transit domains. 310 This provides a way to reduce the risk of shared failure along any 311 path, and automatically helps to ensure path diversity for most of 312 the route of a pair of LSPs. 314 Thus, domain path selection should provide the capability to include 315 or exclude specific domains and specific boundary nodes. 317 1.3.3 Existing Traffic Engineering Constraints 319 Any solution should take advantage of typical traffic engineering 320 constraints (hop count, bandwidth, lambda continuity, path cost, 321 etc.) to meet the service demands expressed in the path computation 322 request [RFC4655]. 324 1.3.4 Commercial Constraints 326 The solution should provide the capability to include commercially 327 relevant constraints such as policy, SLAs, security, peering 328 preferences, and monetary costs. 330 Additionally it may be necessary for the service provider to 331 request that specific domains are included or excluded based on 332 commercial relationships, security implications, and reliability. 334 1.3.5 Domain Confidentiality 336 A key requirement is the ability to maintain domain confidentiality 337 when computing inter-domain end-to-end paths. It should be possible 338 for local policy to require that a PCE not disclose to any other PCE 339 the intra-domain paths it computes or the internal topology of the 340 domain it serves. This requirement should have no impact in the 341 optimality or quality of the end-to-end path that is derived. 343 1.3.6 Limiting Information Aggregation 345 In order to reduce processing overhead and to not sacrifice 346 computational detail, there should be no requirement to aggregate or 347 abstract traffic engineering link information. 349 1.3.7 Domain Interconnection Discovery 351 To support domain mesh topologies, the solution should allow the 352 discovery and selection of domain inter-connections. Pre- 353 configuration of preferred domain interconnections should also be 354 supported for network operators that have bilateral agreement, and 355 preference for the choice of points of interconnection. 357 1.4 Terminology 359 This document uses PCE terminology defined in [RFC4655], [RFC4726], 360 and [RFC5440]. Additional terms are defined below. 362 Domain Path: The sequence of domains for a path. 364 Ingress Domain: The domain that includes the ingress LSR of a path. 366 Transit Domain: A domain that has an upstream and downstream 367 neighbor domain for a specific path. 369 Egress Domain: The domain that includes the egress LSR of a path. 371 Boundary Nodes: Each Domain has entry LSRs and exit LSRs that could 372 be Area Border Routers (ABRs) or Autonomous System Border Routers 373 (ASBRs) depending on the type of domain. They are defined here more 374 generically as Boundary Nodes (BNs). 376 Entry BN of domain(n): a BN connecting domain(n-1) to domain(n) 377 on a path. 379 Exit BN of domain(n): a BN connecting domain(n) to domain(n+1) 380 on a path. 382 Parent Domain: A domain higher up in a domain hierarchy such 383 that it contains other domains (child domains) and potentially other 384 links and nodes. 386 Child Domain: A domain lower in a domain hierarchy such that it has 387 a parent domain. 389 Parent PCE: A PCE responsible for selecting a path across a parent 390 domain and any number of child domains by coordinating with child 391 PCEs and examining a topology map that shows domain inter- 392 connectivity. 394 Child PCE: A PCE responsible for computing the path across one or 395 more specific (child) domains. A child PCE maintains a relationship 396 with at least one parent PCE. 398 OF: Objective Function: A set of one or more optimization 399 criteria used for the computation of a single path (e.g., path cost 400 minimization), or the synchronized computation of a set of paths 401 (e.g., aggregate bandwidth consumption minimization). See [RFC4655] 402 and [RFC5541]. 404 2. Examination of Existing PCE Mechanisms 406 This section provides a brief overview of two existing PCE 407 cooperation mechanisms called the per-domain path computation method, 408 and the backward recursive path computation method. It describes the 409 applicability of these methods to the multi-domain problem. 411 2.1 Per-Domain Path Computation 413 The per-domain path computation method for establishing inter-domain 414 TE-LSPs [RFC5152] defines a technique whereby the path is computed 415 during the signalling process on a per-domain basis. The entry BN of 416 each domain is responsible for performing the path computation for 417 the section of the LSP that crosses the domain, or for requesting 418 that a PCE for that domain computes that piece of the path. 420 During per-domain path computation, each computation results in the 421 best path across the domain to provide connectivity to the next 422 domain in the domain sequence (usually indicated in signalling by an 423 identifier of the next domain or the identity of the next entry BN). 425 Per-domain path computation may lead to sub-optimal end-to-end paths 426 because the most optimal path in one domain may lead to the choice of 427 an entry BN for the next domain that results in a very poor path 428 across that next domain. 430 In the case that the domain path (in particular, the sequence of 431 boundary nodes) is not known, the path computing entity must select 432 an exit BN based on some determination of how to reach the 433 destination that is outside the domain for which the path computing 434 entity has computational responsibility. [RFC5152] suggest that this 435 might be achieved using the IP shortest path as advertise by BGP. 436 Note, however, that the existence of an IP forwarding path does not 437 guarantee the presence of sufficient bandwidth, let alone an optimal 438 TE path. Furthermore, in many GMPLS systems inter-domain IP routing 439 will not be present. Thus, per-domain path computation may require a 440 significant number of crankback routing attempts to establish even a 441 sub-optimal path. 443 Note also that the path computing entities in each domain may have 444 different computation capabilities, may run different path 445 computation algorithms, and may apply different sets of constraints 446 and optimization criteria, etc. 448 This can result in the end-to-end path being inconsistent and sub- 449 optimal. 451 Per-domain path computation can suit simply-connected domains where 452 the preferred points of interconnection are known. 454 2.2 Backward Recursive Path Computation 456 The Backward Recursive Path Computation (BRPC) [RFC5441] procedure 457 involves cooperation and communication between PCEs in order to 458 compute an optimal end-to-end path across multiple domains. The 459 sequence of domains to be traversed can either be determined before 460 or during the path computation. In the case where the sequence of 461 domains is known, the ingress Path Computation Client (PCC) sends a 462 path computation request to a PCE responsible for the ingress 463 domain. This request is forwarded between PCEs, domain-by-domain, to 464 a PCE responsible for the egress domain. The PCE in the egress 465 domain creates a set of optimal paths from all of the domain entry 466 BNs to the egress LSR. This set is represented as a tree of potential 467 paths called a Virtual Shortest Path Tree (VSPT), and the PCE passes 468 it back to the previous PCE on the domain path. As the VSPT is passed 469 back toward the ingress domain, each PCE computes the optimal paths 470 from its entry BNs to its exit BNs that connect to the rest of the 471 tree. It adds these paths to the VSPT and passes the VSPT on until 472 the PCE for the ingress domain is reached and computes paths from the 473 ingress LSR to connect to the rest of the tree. The ingress PCE then 474 selects the optimal end-to-end path from the tree, and returns the 475 path to the initiating PCC. 477 BRPC may suit environments where multiple connections exist between 478 domains and there is no preference for the choice of points of 479 interconnection. It is best suited to scenarios where the domain 480 path is known in advance, but can also be used when the domain path 481 is not known. 483 2.2.1. Applicability of BRPC when the Domain Path is Not Known 485 As described above, BRPC can be used to determine an optimal inter- 486 domain path when the domain sequence is known. Even when the sequence 487 of domains is not known BRPC could be used as follows. 489 o The PCC sends a request to a PCE for the ingress domain (the 490 ingress PCE). 492 o The ingress PCE sends the path computation request direct to a 493 PCE responsible for the domain containing the destination node (the 494 egress PCE). 496 o The egress PCE computes an egress VSPT and passes it to a PCE 497 responsible for each of the adjacent (potentially upstream) 498 domains. 500 o Each PCE in turn constructs a VSPT and passes it on to all of its 501 neighboring PCEs. 503 o When the ingress PCE has received a VSPT from each of its 504 neighboring domains it is able to select the optimum path. 506 Clearly this mechanism (which could be called path computation 507 flooding) has significant scaling issues. It could be improved by 508 the application of policy and filtering, but such mechanisms are not 509 simple and would still leave scaling concerns. 511 3. Hierarchical PCE 513 In the hierarchical PCE architecture, a parent PCE maintains a domain 514 topology map that contains the child domains (seen as vertices in the 515 topology) and their interconnections (links in the topology). The 516 parent PCE has no information about the content of the child domains; 517 that is, the parent PCE does not know about the resource availability 518 within the child domains, nor about the availability of connectivity 519 across each domain because such knowledge would violate the 520 confidentiality requirement and would either require flooding of full 521 information to the parent (scaling issue) or would necessitate some 522 form of aggregation. The parent PCE is aware of the TE capabilities 523 of the interconnections between child domains as these 524 interconnections are links in its own topology map. 526 Note that in the case that the domains are IGP areas, there is no 527 link between the domains (the ABRs have a presence in both 528 neighboring areas). The parent domain may choose to represent this in 529 its TED as a virtual link that is unconstrained and has zero cost, 530 but this is entirely an implementation issue. 532 Each child domain has at least one PCE capable of computing paths 533 across the domain. These PCEs are known as child PCEs and have a 534 relationship with the parent PCE. Each child PCE also knows the 535 identity of the domains that neighbor its own domain. A child PCE 536 only knows the topology of the domain that it serves and does not 537 know the topology of other child domains. Child PCEs are also not 538 aware of the general domain mesh connectivity (i.e., the domain 539 topology map) beyond the connectivity to the immediate neighbor 540 domains of the domain it serves. 542 The parent PCE builds the domain topology map either from 543 configuration or from information received from each child PCE. This 544 tells it how the domains are interconnected including the TE 545 properties of the domain interconnections. But the parent PCE does 546 not know the contents of the child domains. Discovery of the domain 547 topology and domain interconnections is discussed further in Section 548 4.3. 550 When a multi-domain path is needed, the ingress PCE sends a request 551 to the parent PCE (using the path computation element protocol, PCEP 552 [RFC5440]). The parent PCE selects a set of candidate domain paths 553 based on the domain topology and the state of the inter-domain links. 554 It then sends computation requests to the child PCEs responsible for 555 each of the domains on the candidate domain paths. These requests may 556 be sequential or parallel depending on implementation details. 558 Each child PCE computes a set of candidate path segments across its 559 domain and sends the results to the parent PCE. The parent PCE uses 560 this information to select path segments and concatenate them to 561 derive the optimal end-to-end inter-domain path. The end-to-end path 562 is then sent to the child PCE which received the initial path request 563 and this child PCE passes the path on to the PCC that issued the 564 original request. 566 Specific deployment and implementation scenarios are out of scope of 567 this document. However the hierarchical PCE architecture described 568 does support the function of parent PCE and child PCE being 569 implemented as a common PCE. 571 4. Hierarchical PCE Procedures 573 4.1 Objective Functions and Policy 575 Deriving the optimal end-to-end domain path sequence is dependent on 576 the policy applied during domain path computation. An Objective 577 Function (OF) [RFC5541], or set of OFs, may be applied to define the 578 policy being applied to the domain path computation. 580 The OF specifies the desired outcome of the computation. It does 581 not describe the algorithm to use. When computing end-to-end inter- 582 domain paths, required OFs may include (see Section 1.3.1): 584 o Minimum cost path 585 o Minimum load path 586 o Maximum residual bandwidth path 587 o Minimize aggregate bandwidth consumption 588 o Minimize or cap the number of transit domains 589 o Disallow domain re-entry 591 The objective function may be requested by the PCC, the ingress 592 domain PCE (according to local policy), or applied by the parent PCE 593 according to inter-domain policy. 595 More than one OF (or a composite OF) may be chosen to apply to a 596 single computation provided they are not contradictory. Composite OFs 597 may include weightings and preferences for the fulfilment of pieces 598 of the desired outcome. 600 4.2 Maintaining Domain Confidentiality 602 Information about the content of child domains is not shared for 603 scaling and confidentiality reasons. This means that a parent PCE is 604 aware of the domain topology and the nature of the connections 605 between domains, but is not aware of the content of the domains. 606 Similarly, a child PCE cannot know the internal topology of another 607 child domain. Child PCEs also do not know the general domain mesh 608 connectivity, this information is only known by the parent PCE. 610 As described in the earlier sections of this document, PCEs can 611 exchange path information in order to construct an end-to-end inter- 612 domain path. Each per-domain path fragment reveals information about 613 the topology and resource availability within a domain. Some 614 management domains or ASes will not want to share this information 615 outside of the domain (even with a trusted parent PCE). In order to 616 conceal the information, a PCE may replace a path segment with a 617 path-key [RFC5520]. This mechanism effectively hides the content of a 618 segment of a path. 620 4.3 PCE Discovery 622 It is a simple matter for each child PCE to be configured with the 623 address of its parent PCE. Typically, there will only be one or two 624 parents of any child. 626 The parent PCE also needs to be aware of the child PCEs for all child 627 domains that it can see. This information is most likely to be 628 configured (as part of the administrative definition of each 629 domain). 631 Discovery of the relationships between parent PCEs and child PCEs 632 does not form part of the hierarchical PCE architecture. Mechanisms 633 that rely on advertising or querying PCE locations across domain or 634 provider boundaries are undesirable for security, scaling, 635 commercial, and confidentiality reasons. 637 The parent PCE also needs to know the inter-domain connectivity. 638 This information could be configured with suitable policy and 639 commercial rules, or could be learned from the child PCEs as 640 described in Section 4.4. 642 In order for the parent PCE to learn about domain interconnection 643 the child PCE will report the identity of its neighbor domains. The 644 IGP in each neighbor domain can advertise its inter-domain TE 645 link capabilities [RFC5316], [RFC5392]. This information can be 646 collected by the child PCEs and forwarded to the parent PCE, or the 647 parent PCE could participate in the IGP in the child domains. 649 4.4 Parent Domain Traffic Engineering Database 651 The parent PCE maintains a domain topology map of the child domains 652 and their interconnectivity. Where inter-domain connectivity is 653 provided by TE links the capabilities of those links may also be 654 known to the parent PCE. The parent PCE maintains a traffic 655 engineering database (TED) for the parent domain in the same way that 656 any PCE does. 658 The parent domain may just be the collection of child domains and 659 their interconnectivity, may include details of the inter-domain TE 660 links, and may contain nodes and links in its own right. 662 The mechanism for building the parent TED is likely to rely heavily 663 on administrative configuration and commercial issues because the 664 network was probably partitioned into domains specifically to address 665 these issues. 667 In practice, certain information may be passed from the child domains 668 to the parent PCE to help build the parent TED. In theory, the parent 669 PCE could listen to the routing protocols in the child domains, but 670 this would violate the confidentiality and scaling issues that may be 671 responsible for the partition of the network into domains. So it is 672 much more likely that a suitable solution will involve specific 673 communication from an entity in the child domain (such as the child 674 PCE) to convey the necessary information. As already mentioned, the 675 "necessary information" relates to how the child domains are inter- 676 connected. The topology and available resources within the child 677 domain do not need to be communicated to the parent PCE: doing so 678 would violate the PCE architecture. Mechanisms for reporting this 679 information are described in the examples in Section 4.6 in abstract 680 terms as "a child PCE reports its neighbor domain connectivity to its 681 parent PCE"; the specifics of a solution are out of scope of this 682 document, but the requirements are indicated in Section 4.8. See 683 Section 6 for a brief discussion of BGP-TE. 685 In models such as ASON (see Section 5.2), it is possible to consider 686 a separate instance of an IGP running within the parent domain where 687 the participating protocol speakers are the nodes directly present in 688 that domain and the PCEs (Routing Controllers) responsible for each 689 of the child domains. 691 4.5 Determination of Destination Domain 693 The PCC asking for an inter-domain path computation is aware of the 694 identity of the destination node by definition. If it knows the 695 egress domain it can supply this information as part of the path 696 computation request. However, if it does not know the egress domain 697 this information must be known by the child PCE or known/determined 698 by the parent PCE. 700 In some specialist topologies the parent PCE could determine the 701 destination domain based on the destination address, for example from 702 configuration. However, this is not appropriate for many multi-domain 703 addressing scenarios. In IP-based multi-domain networks the 704 parent PCE may be able to determine the destination domain by 705 participating in inter-domain routing. Finally, the parent PCE could 706 issue specific requests to the child PCEs to discover if they contain 707 the destination node, but this has scaling implications. 709 For the purposes of this document, the precise mechanism of the 710 discovery of the destination domain is left out of scope. Suffice to 711 say that for each multi-domain path computation some mechanism will 712 be required to determine the location of the destination. 714 4.6 Hierarchical PCE Examples 716 The following example describes the generic hierarchical domain 717 topology. Figure 1 demonstrates four interconnected domains within a 718 fifth, parent domain. Each domain contains a single PCE: 720 o Domain 1 is the ingress domain and child PCE 1 is able to compute 721 paths within the domain. Its neighbors are Domain 2 and Domain 4. 722 The domain also contains the source LSR (S) and three egress 723 boundary nodes (BN11, BN12, and BN13). 725 o Domain 2 is served by child PCE 2. Its neighbors are Domain 1 and 726 Domain 3. The domain also contains four boundary nodes (BN21, BN22, 727 BN23, and BN24). 729 o Domain 3 is the egress domain and is served by child PCE 3. Its 730 neighbors are Domain 2 and Domain 4. The domain also contains the 731 destination LSR (D) and three ingress boundary nodes (BN31, BN32, 732 and BN33). 734 o Domain 4 is served by child PCE 4. Its neighbors are Domain 2 and 735 Domain 3. The domain also contains two boundary nodes (BN41 and 736 BN42). 738 All of these domains are contained within Domain 5 which is served 739 by the parent PCE (PCE 5). 741 ----------------------------------------------------------------- 742 | Domain 5 | 743 | ----- | 744 | |PCE 5| | 745 | ----- | 746 | | 747 | ---------------- ---------------- ---------------- | 748 | | Domain 1 | | Domain 2 | | Domain 3 | | 749 | | | | | | | | 750 | | ----- | | ----- | | ----- | | 751 | | |PCE 1| | | |PCE 2| | | |PCE 3| | | 752 | | ----- | | ----- | | ----- | | 753 | | | | | | | | 754 | | ----| |---- ----| |---- | | 755 | | |BN11+---+BN21| |BN23+---+BN31| | | 756 | | - ----| |---- ----| |---- - | | 757 | | |S| | | | | |D| | | 758 | | - ----| |---- ----| |---- - | | 759 | | |BN12+---+BN22| |BN24+---+BN32| | | 760 | | ----| |---- ----| |---- | | 761 | | | | | | | | 762 | | ---- | | | | ---- | | 763 | | |BN13| | | | | |BN33| | | 764 | -----------+---- ---------------- ----+----------- | 765 | \ / | 766 | \ ---------------- / | 767 | \ | | / | 768 | \ |---- ----| / | 769 | ----+BN41| |BN42+---- | 770 | |---- ----| | 771 | | | | 772 | | ----- | | 773 | | |PCE 4| | | 774 | | ----- | | 775 | | | | 776 | | Domain 4 | | 777 | ---------------- | 778 | | 779 ----------------------------------------------------------------- 781 Figure 1 : Sample Hierarchical Domain Topology 783 Figure 2, shows the view of the domain topology as seen by the parent 784 PCE (PCE 5). This view is an abstracted topology; PCE 5 is aware of 785 domain connectivity, but not of the internal topology within each 786 domain. 788 ---------------------------- 789 | Domain 5 | 790 | ---- | 791 | |PCE5| | 792 | ---- | 793 | | 794 | ---- ---- ---- | 795 | | |---| |---| | | 796 | | D1 | | D2 | | D3 | | 797 | | |---| |---| | | 798 | ---- ---- ---- | 799 | \ ---- / | 800 | \ | | / | 801 | ----| D4 |---- | 802 | | | | 803 | ---- | 804 | | 805 ---------------------------- 807 Figure 2 : Abstract Domain Topology as Seen by the Parent PCE 809 4.6.1 Hierarchical PCE Initial Information Exchange 811 Based on the Figure 1 topology, the following is an illustration of 812 the initial hierarchical PCE information exchange. 814 1. Child PCE 1, the PCE responsible for Domain 1, is configured 815 with the location of its parent PCE (PCE5). 817 2. Child PCE 1 establishes contact with its parent PCE. The parent 818 applies policy to ensure that communication with PCE 1 is allowed. 820 3. Child PCE 1 listens to the IGP in its domain and learns its 821 inter-domain connectivity. That is, it learns about the links 822 BN11-BN21, BN12-BN22, and BN13-BN41. 824 4. Child PCE 1 reports its neighbor domain connectivity to its parent 825 PCE. 827 5. Child PCE 1 reports any change in the resource availability on its 828 inter-domain links to its parent PCE. 830 Each child PCE performs steps 1 through 5 so that the parent PCE can 831 create a domain topology view as shown in Figure 2. 833 4.6.2 Hierarchical PCE End-to-End Path Computation Procedure 835 The procedure below is an example of a source PCC requesting an 836 end-to-end path in a multi-domain environment. The topology is 837 represented in Figure 1. It is assumed that the each child PCE has 838 connected to its parent PCE and exchanged the initial information 839 required for the parent PCE to create its domain topology view as 840 described in Section 4.6.1. 842 1. The source PCC (the ingress LSR in our example), sends a request 843 to the PCE responsible for its domain (PCE 1) for a path to the 844 destination LSR (D). 846 2. PCE 1 determines the destination is not in domain 1. 848 3. PCE 1 sends a computation request to its parent PCE (PCE 5). 850 4. The parent PCE determines that the destination is in Domain 3. 851 (See Section 4.5). 853 5. PCE 5 determines the likely domain paths according to the domain 854 interconnectivity and TE capabilities between the domains. For 855 example, assuming that the link BN12-BN22 is not suitable for the 856 requested path, three domain paths are determined: 858 S-BN11-BN21-D2-BN23-BN31-D 859 S-BN11-BN21-D2-BN24-BN32-D 860 S-BN13-BN41-D4-BN42-BN33-D 862 6. PCE 5 sends edge-to-edge path computation requests to PCE 2 863 which is responsible for Domain 2 (i.e., BN21-to-BN23 and BN21- 864 to-BN24), and to PCE 4 for Domain 4 (i.e., BN41-to-BN42). 866 7. PCE 5 sends source-to-edge path computation requests to PCE 1 867 which is responsible for Domain 1 (i.e., S-to-BN11 and S-to- 868 BN13). 870 8. PCE 5 sends edge-to-egress path computation requests to PCE3 871 which is responsible for Domain 3 (i.e., BN31-to-D, BN32-to-D, 872 and BN33-to-D). 874 9. PCE 5 correlates all the computation responses from each child 875 PCE, adds in the information about the inter-domain links, and 876 applies any requested and locally configured policies. 878 10. PCE 5 then selects the optimal end-to-end multi-domain path 879 that meets the policies and objective functions, and supplies the 880 resulting path to PCE 1. 882 11. PCE 1 forwards the path to the PCC (the ingress LSR). 884 Note that there is no requirement for steps 6, 7, and 8 to be carried 885 out in parallel or in series. Indeed, they could be overlapped with 886 step 5. This is an implementation issue. 888 4.7 Hierarchical PCE Error Handling 890 In the event that a child PCE in a domain cannot find a suitable 891 path to the egress, the child PCE should return the relevant 892 error to notify the parent PCE. Depending on the error response the 893 parent PCE can elect to: 895 o Cancel the request and send the relevant response back to the 896 initial child PCE that requested an end-to-end path; 897 o Relax some of the constraints associated with the initial path 898 request; 899 o Select another candidate domain and send the path request to the 900 child PCE responsible for the domain. 902 If the parent PCE does not receive a response from a child PCE within 903 an allotted time period. The parent PCE can elect to: 905 o Cancel the request and send the relevant response back to the 906 initial child PCE that requested an end-to-end path; 907 o Send the path request to another child PCE in the same domain, if a 908 secondary child PCE exists; 909 o Select another candidate domain and send the path request to the 910 child PCE responsible for that domain. 912 The parent PCE may also want to prune any unresponsive child PCE 913 domain paths from the candidate set. 915 4.8 Requirements for Hierarchical PCEP Protocol Extensions 917 This section lists the high-level requirements for extensions to the 918 PCEP to support the hierarchical PCE model. It is provided to offer 919 guidance to PCEP protocol developers in designing a solution suitable 920 for use in a hierarchical PCE framework. 922 4.8.1 PCEP Request Qualifiers 924 PCEP request (PCReq) messages are used by a PCC or a PCE to make a 925 computation request or enquiry to a PCE. The requests are qualified 926 so that the PCE knows what type of action is required. 928 Support of the hierarchical PCE architecture will introduce two new 929 qualifications as follows: 931 o It must be possible for a child PCE to indicate that the response 932 it receives from the parent PCE should consist of a domain sequence 933 only (i.e., not a fully-specified end-to-end path). This allows the 934 child PCE to initiate per-domain or backward recursive path 935 computation. 937 o A parent PCE may need to be able to ask a child PCE whether a 938 particular node address (the destination of an end-to-end path) is 939 present in the domain that the child PCE serves. 941 In PCEP, such request qualifications are carried as bit-flags in the 942 RP object within the PCReq message. 944 4.8.2 Indication of Hierarchical PCE Capability 946 Although parent/child PCE relationships are likely configured, it 947 will assist network operations if the parent PCE is able to indicate 948 to the child that it really is capable of acting as a parent PCE. 949 This will help to trap misconfigurations. 951 In PCEP, such capabilities are carried in the Open Object within the 952 Open message. 954 4.8.3 Intention to Utilize Parent PCE Capabilities 956 A PCE that is capable of acting as a parent PCE might not be 957 configured or willing to act as the parent for a specific child PCE. 958 This fact could be determined when the child sends a PCReq that 959 requires parental activity (such as querying other child PCEs), and 960 could result in a negative response in a PCEP Error (PCErr) message. 962 However, the expense of a poorly targeted PCReq can be avoided if 963 the child PCE indicates that it might wish to use the parent-capable 964 as a parent (for example, on the Open message), and if the 965 parent-capable determines at that time whether it is willing to act 966 as a parent to this child. 968 4.8.4 Communication of Domain Connectivity Information 970 Section 4.4 describes how the parent PCE needs a parent TED and 971 indicates that the information might be supplied from the child PCEs 972 in each domain. This requires a mechanism whereby information about 973 inter-domain links can be supplied by a child PCE to a parent PCE, 974 for example on a PCEP Notify (PCNtf) message. 976 The information that would be exchanged includes: 978 o Identifier of advertising child PCE 979 o Identifier of PCE's domain 980 o Identifier of the link 981 o TE properties of the link (metrics, bandwidth) 982 o Other properties of the link (technology-specific) 983 o Identifier of link end-points 984 o Identifier of adjacent domain 986 It may be desirable for this information to be periodically updated, 987 for example, when available bandwidth changes. In this case, the 988 parent PCE might be given the ability to configure thresholds in the 989 child PCE to prevent flapping of information. 991 4.8.5 Domain Identifiers 993 Domain identifiers are already present in PCEP to allow a PCE to 994 indicate which domains it serves, and to allow the representation of 995 domains as abstract nodes in paths. The wider use of domains in the 996 context of this work on hierarchical PCE will require that domains 997 can be identified in more places within objects in PCEP messages. 998 This should pose no problems. 1000 However, more attention may need to be applied to the precision of 1001 domain identifier definitions to ensure that it is always possible to 1002 unambiguously identify a domain from its identifier. This work will 1003 be necessary in configuration, and also in protocol specifications 1004 (for example, an OSPF area identifier is sufficient within an 1005 Autonomous System, but becomes ambiguous in a path that crosses 1006 multiple Autonomous Systems). 1008 5. Hierarchical PCE Applicability 1010 As per [RFC4655], PCE can inherently support inter-domain path 1011 computation for any definition of a domain as set out in Section 1.2 1012 of this document. 1014 Hierarchical PCE can be applied to inter-domain environments, 1015 including autonomous Systems and IGP areas. The hierarchical PCE 1016 procedures make no distinction between, autonomous Systems and IGP 1017 area applications, although it should be noted that the TED 1018 maintained by a parent PCE must be able to support the concept of 1019 child domains connected by inter-domain links or directly connected 1020 at boundary nodes (see Section 3). 1022 This section sets out the applicability of hierarchical PCE to three 1023 environments: 1025 o MPLS traffic engineering across multiple Autonomous Systems 1026 o MPLS traffic engineering across multiple IGP areas 1027 o GMPLS traffic engineering in the ASON architecture 1029 5.1 autonomous Systems and Areas 1031 Networks are comprised of domains. A domain can be considered to be 1032 a collection of network elements within an AS or area that has a 1033 common sphere of address management or path computational 1034 responsibility. 1036 As networks increase in size and complexity it may be required to 1037 introduce scaling methods to reduce the amount information flooded 1038 within the network and make the network more manageable. An IGP 1039 hierarchy is designed to improve IGP scalability by dividing the 1040 IGP domain into areas and limiting the flooding scope of topology 1041 information to within area boundaries. This restricts a router's 1042 visibility to information about links and other routers within the 1043 single area. If a router needs to compute a route to destination 1044 located in another area, a method is required to compute a path 1045 across the area boundary. 1047 When an LSR within an AS or area needs to compute a path across an 1048 area or AS boundary it must also use an inter-AS computation 1049 technique. Hierarchical PCE is equally applicable to computing 1050 inter-area and inter-AS MPLS and GMPLS paths across domain 1051 boundaries. 1053 5.2 ASON Architecture 1055 The International Telecommunications Union (ITU) defines the ASON 1056 architecture in [G-8080]. [G-7715] defines the routing architecture 1057 for ASON and introduces a hierarchical architecture. In this 1058 architecture, the Routing Areas (RAs) have a hierarchical 1059 relationship between different routing levels, which means a parent 1060 (or higher level) RA can contain multiple child RAs. The 1061 interconnectivity of the lower RAs is visible to the higher level RA. 1062 Note that the RA hierarchy can be recursive. 1064 In the ASON framework, a path computation request is termed a Route 1065 Query. This query is executed before signaling is used to establish 1066 an LSP termed a Switched Connection (SC) or a Soft Permanent 1067 Connection (SPC). [G-7715-2] defines the requirements and 1068 architecture for the functions performed by Routing Controllers (RC) 1069 during the operation of remote route queries - an RC is synonymous 1070 with a PCE. For an end-to-end connection, the route may be computed 1071 by a single RC or multiple RCs in a collaborative manner (i.e., RC 1072 federations). In the case of RC federations, [G-7715-2] describes 1073 three styles during remote route query operation: 1075 o Step-by-step remote path computation 1076 o Hierarchical remote path computation 1077 o A combination of the above. 1079 In a hierarchical ASON routing environment, a child RC may 1080 communicate with its parent RC (at the next higher level of the ASON 1081 routing hierarchy) to request the computation of an end-to-end path 1082 across several RAs. It does this using a route query message (known 1083 as the abstract message RI_QUERY). The corresponding parent RC may 1084 communicate with other child RCs that belong to other child RAs at 1085 the next lower hierarchical level. Thus, a parent RC can act as 1086 either a Route Query Requester or Route Query Responder. 1088 It can be seen that the hierarchical PCE architecture fits the 1089 hierarchical ASON routing architecture well. It can be used to 1090 provide paths across subnetworks, and to determine end-to-end paths 1091 in networks constructed from multiple subnetworks or RAs. 1093 When hierarchical PCE is applied to implement hierarchical remote 1094 path computation in [G-7715-2], it is very important for operators to 1095 understand the different terminology and implicit consistency 1096 between hierarchical PCE and [G-7715-2]. 1098 5.2.1 Implicit Consistency Between Hierarchical PCE and G.7715.2 1100 This section highlights the correspondence between features of the 1101 hierarchical PCE architecture and the ASON routing architecture. 1103 (1) RC (Routing Controller) and PCE (Path Computation Element) 1105 [G-8080] describes the Routing Controller component as an 1106 abstract entity, which is responsible for responding to requests 1107 for path (route) information and topology information. It can be 1108 implemented as a single entity, or as a distributed set of 1109 entities that make up a cooperative federation. 1111 [RFC4655] describes PCE (Path Computation Element) is an entity 1112 (component, application, or network node) that is capable of 1113 computing a network path or route based on a network graph and 1114 applying computational constraints. 1116 Therefore, in the ASON architecture, a PCE can be regarded as a 1117 realizations of the RC. 1119 (2) Route Query Requester/Route Query Responder and PCC/PCE 1121 [G-7715-2] describes the Route Query Requester as a Connection 1122 Controller or Routing Controller that sends a route query message 1123 to a Routing Controller requesting one or more paths that 1124 satisfy a set of routing constraints. The Route Query Responder 1125 is a Routing Controller that performs path computation upon 1126 receipt of a route query message from a Route Query Requester, 1127 sending a response back at the end of the path computation. 1129 In the context of ASON, a Signaling Controller initiates and 1130 processes signaling messages and is closely coupled to a 1131 Signaling Protocol Speaker. A Routing Controller makes routing 1132 decisions and is usually coupled to configuration entities 1133 and/or a Routing Protocol Speaker. 1135 It can be seen that a PCC corresponds to a Route Query Requester, 1136 and a PCE corresponds to a Route Query Responder. A PCE/RC can 1137 also act as a Route Query Requester sending requests to another 1138 Route Query Responder. 1140 The PCEP path computation request (PCReq) and path computation 1141 reply (PCRep) messages between PCC and PCE correspond to the 1142 RI_QUERY and RI_UPDATE messages in [G-7715-2]. 1144 (3) Routing Area Hierarchy and Hierarchical Domain 1146 The ASON routing hierarchy model is shown in Figure 6 of 1147 [G-7715] through an example that illustrates routing area levels. 1148 If the hierarchical remote path computation mechanism of 1149 [G-7715-2] is applied in this scenario, each routing area should 1150 have at least one RC for route query function and there is a 1151 parent RC for the child RCs in each routing area. 1153 According to [G-8080], the parent RC has visibility of the 1154 structure of the lower level, so it knows the interconnectivity 1155 of the RAs in the lower level. Each child RC can compute edge-to- 1156 edge paths across its own child RA. 1158 Thus, an RA corresponds to a domain in the PCE architecture, and 1159 the hierarchical relationship between RAs corresponds to the 1160 hierarchical relationship between domains in the hierarchical PCE 1161 architecture. Furthermore, a parent PCE in a parent domain can be 1162 regarded as parent RC in a higher routing level, and a child PCE 1163 in a child domain can be regarded as child RC in a lower routing 1164 level. 1166 5.2.2 Benefits of Hierarchical PCEs in ASON 1168 RCs in an ASON environment can use the hierarchical PCE model to 1169 fully match the ASON hierarchical routing model, so the hierarchical 1170 PCE mechanisms can be applied to fully satisfy the architecture and 1171 requirements of [G-7715-2] without any changes. If the hierarchical 1172 PCE mechanism is applied in ASON, it can be used to determine end-to- 1173 end optimized paths across sub-networks and RAs before initiating 1174 signaling to create the connection. It can also improve the 1175 efficiency of connection setup to avoid crankback. 1177 6. A Note on BGP-TE 1179 The concept of exchange of TE information between Autonomous Systems 1180 (ASes) is discussed in [BGP-TE]. The information exchanged in this 1181 way could be the full TE information from the AS, an aggregation of 1182 that information, or a representation of the potential connectivity 1183 across the AS. Furthermore, that information could be updated 1184 frequently (for example, for every new LSP that is set up across the 1185 AS) or only at threshold-crossing events. 1187 There are a number of discussion points associated with the use of 1188 [BGP-TE] concerning the volume of information, the rate of churn of 1189 information, the confidentiality of information, the accuracy of 1190 aggregated or potential-connectivity information, and the processing 1191 required to generate aggregated information. The PCE architecture and 1192 the architecture enabled by [BGP-TE] make different assumptions about 1193 the operational objectives of the networks, and this document does 1194 not attempt to make one of the approaches "right" and the other 1195 "wrong". Instead, this work assumes that a decision has been made to 1196 utilize the PCE architecture. 1198 6.1 Use of BGP for TED Synchronization 1200 Indeed, [BGP-TE] may have some uses within the PCE model. For 1201 example, [BGP-TE] could be used as a "northbound" TE advertisement 1202 such that a PCE does not need to listen to an IGP in its domain, but 1203 has its TED populated by messages received (for example) from a 1204 Route Reflector. Furthermore, the inter-domain connectivity and 1205 connectivity capabilities that is required information for a parent 1206 PCE could be obtained as a filtered subset of the information 1207 available in [BGP-TE]. This scenario is discussed further in 1208 [PCE-AREA-AS]. 1210 7. Management Considerations 1212 General PCE management considerations are discussed in [RFC4655]. In 1213 the case of the hierarchical PCE architecture, there are additional 1214 management considerations. 1216 The administrative entity responsible for the management of the 1217 parent PCEs must be determined. In the case of multi-domains (e.g., 1218 IGP areas or multiple ASes) within a single service provider 1219 network, the management responsibility for the parent PCE would most 1220 likely be handled by the service provider. In the case of multiple 1221 ASes within different service provider networks, it may be necessary 1222 for a third-party to manage the parent PCEs according to commercial 1223 and policy agreements from each of the participating service 1224 providers. 1226 7.1 Control of Function and Policy 1228 7.1.1 Child PCE 1230 Support of the hierarchical procedure will be controlled by the 1231 management organization responsible for each child PCE. A child PCE 1232 must be configured with the address of its parent PCE in order for 1233 it to interact with its parent PCE. The child PCE must also be 1234 authorized to peer with the parent PCE. 1236 7.1.2 Parent PCE 1238 The parent PCE must only accept path computation requests from 1239 authorized child PCEs. If a parent PCE receives requests from an 1240 unauthorized child PCE, the request should be dropped. 1242 This means that a parent PCE must be configured with the identities 1243 and security credentials of all of its child PCEs, or there must be 1244 some form of shared secret that allows an unknown child PCE to be 1245 authorized by the parent PCE. 1247 7.1.3 Policy Control 1249 It may be necessary to maintain a policy module on the parent PCE 1250 [RFC5394]. This would allow the parent PCE to apply commercially 1251 relevant constraints such as SLAs, security, peering preferences, and 1252 monetary costs. 1254 It may also be necessary for the parent PCE to limit end-to-end path 1255 selection by including or excluding specific domains based on 1256 commercial relationships, security implications, and reliability. 1258 7.2 Information and Data Models 1260 A PCEP MIB module is defined in [PCEP-MIB] that describes managed 1261 objects for modeling of PCEP communication. An additional PCEP MIB 1262 will be required to report parent PCE and child PCE information, 1263 including: 1265 o Parent PCE configuration and status, 1267 o Child PCE configuration and information, 1269 o Notifications to indicate session changes between parent PCEs and 1270 child PCEs. 1272 o Notification of parent PCE TED updates and changes. 1274 7.3 Liveness Detection and Monitoring 1276 The hierarchical procedure requires interaction with multiple PCEs. 1277 Once a child PCE requests an end-to-end path, a sequence of events 1278 occurs that requires interaction between the parent PCE and each 1279 child PCE. If a child PCE is not operational, and an alternate 1280 transit domain is not available, then a failure must be reported. 1282 7.4 Verifying Correct Operation 1284 Verifying the correct operation of a parent PCE can be performed by 1285 monitoring a set of parameters. The parent PCE implementation should 1286 provide the following parameters monitored by the parent PCE: 1288 o Number of child PCE requests. 1290 o Number of successful hierarchical PCE procedures completions on a 1291 per-PCE-peer basis. 1293 o Number of hierarchical PCE procedure completion failures on a per- 1294 PCE-peer basis. 1296 o Number of hierarchical PCE procedure requests from unauthorized 1297 child PCEs. 1299 7.5. Impact on Network Operation 1301 The hierarchical PCE procedure is a multiple-PCE path computation 1302 scheme. Subsequent requests to and from the child and parent PCEs do 1303 not differ from other path computation requests and should not have 1304 any significant impact on network operations. 1306 8. Security Considerations 1308 The hierarchical PCE procedure relies on PCEP and inherits the 1309 security requirements defined [RFC5440]. As noted in Section 7, 1310 there is a security relationship between child and parent PCEs. 1311 This relationship, like any PCEP relationship assumes 1312 pre-configuration of identities, authority, and keys, or can 1313 operate through any key distribution mechanism outside the scope of 1314 PCEP. As PCEP operates over TCP, it may make use of any TCP security 1315 mechanism. 1317 The hierarchical PCE architecture makes use of PCE policy 1318 [RFC5394] and the security aspects of the PCE communication protocol 1319 documented in [RFC5440]. It is expected that the parent PCE will 1320 require all child PCEs to use full security when communicating with 1321 the parent and that security will be maintained by not supporting the 1322 discovery by a parent of child PCEs. 1324 PCE operation also relies on information used to build the TED. 1325 Attacks on a PCE system may be achieved by falsifying or impeding 1326 this flow of information. The child PCE TEDs are constructed as 1327 described in [RFC4655] and are unchanged in this document: if the PCE 1328 listens to the IGP for this information, then normal IGP security 1329 measures may be applied, and it should be noted that an IGP routing 1330 system is generally assumed to be a trusted domain such that router 1331 subversion is not a risk. The parent PCE TED is constructed as 1332 described in this document and may involve: 1334 - multiple parent-child relationships using PCEP (as already 1335 described) 1337 - the parent PCE listening to child domain IGPs (with the same 1338 security features as a child PCE listening to its IGP) 1340 - an external mechanism (such as [BGP-TE]) which will need to be 1341 authorized and secured. 1343 Any multi-domain operation necessarily involves the exchange of 1344 information across domain boundaries. This is bound to represent a 1345 significant security and confidentiality risk especially when the 1346 child domains are controlled by different commercial concerns. PCEP 1347 allows individual PCEs to maintain confidentiality of their domain 1348 path information using Path Keys [RFC5520], and the hierarchical 1349 PCE architecture is specifically designed to enable as much isolation 1350 of domain topology and capabilities information as is possible. 1352 Further considerations of the security issues related to inter-AS 1353 path computation see [RFC5376]. 1355 9. IANA Considerations 1357 This document makes no requests for IANA action. 1359 10. Acknowledgements 1361 The authors would like to thank David Amzallag, Oscar Gonzalez de 1362 Dios, Franz Rambach, Ramon Casellas, Olivier Dugeon, Filippo Cugini, 1363 Dhruv Dhody and Julien Meuric for their comments and suggestions. 1365 11. References 1367 11.1 Normative References 1369 [RFC4655] Farrel, A., Vasseur, J., Ash, J., "A Path Computation 1370 Element (PCE)-Based Architecture", RFC 4655, August 2006. 1372 [RFC5152] Vasseur, JP., Ayyangar, A., and R. Zhang, "A Per-Domain 1373 Path Computation Method for Establishing Inter-Domain 1374 Traffic Engineering (TE) Label Switched Paths (LSPs)", 1375 RFC 5152, February 2008. 1377 [RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash, 1378 "Policy-Enabled Path Computation Framework", RFC 5394, 1379 December 2008. 1381 [RFC5440] Ayyangar, A., Farrel, A., Oki, E., Atlas, A., Dolganow, 1382 A., Ikejiri, Y., Kumaki, K., Vasseur, J., and J. Roux, 1383 "Path Computation Element (PCE) Communication Protocol 1384 (PCEP)", RFC 5440, March 2009. 1386 [RFC5441] Vasseur, J.P., Ed., "A Backward Recursive PCE-based 1387 Computation (BRPC) procedure to compute shortest inter- 1388 domain Traffic Engineering Label Switched Paths", RFC 1389 5441, April 2009. 1391 [RFC5520] Brandford, R., Vasseur J.P., and Farrel A., "Preserving 1392 Topology Confidentiality in Inter-Domain Path 1393 Computation Using a Key-Based Mechanism 1394 RFC5520, April 2009. 1396 11.2. Informative References 1398 [RFC4105] Le Roux, JL., Vasseur, J., Boyle, J., 1399 "Requirements for Inter-Area MPLS Traffic Engineering", 1400 RFC 4105, June 2005. 1402 [RFC4216] Zhang, R., and Vasseur, J., "MPLS Inter-Autonomous 1403 System (AS) Traffic Engineering (TE) Requirements", RFC 1404 4216, November 2005. 1406 [RFC4726] Farrel, A., Vasseur, J., Ayyangar, A., "A Framework 1407 for Inter-Domain Multiprotocol Label Switching Traffic 1408 Engineering", RFC 4726, November 2006. 1410 [RFC5152] Vasseur, JP., Ayyangar, A., Zhang, R., "A Per-Domain 1411 Path Computation Method for Establishing Inter-Domain 1412 Traffic Engineering (TE) Label Switched Paths (LSPs)", 1413 RFC 5152, February 2008. 1415 [RFC5316] Chen, M., Zhang, R., Duan, X., "ISIS Extensions in 1416 Support of Inter-Autonomous System (AS) MPLS and GMPLS 1417 Traffic Engineering", RFC 5316, December 2008. 1419 [RFC5376] Bitar, N., et al., "Inter-AS Requirements for the 1420 Path Computation Element Communication Protocol 1421 (PCECP)", RFC 5376, November 2008. 1423 [RFC5392] Chen, M., Zhang, R., Duan, X., "OSPF Extensions in 1424 Support of Inter-Autonomous System (AS) MPLS and GMPLS 1425 Traffic Engineering", RFC 5392, January 2009. 1427 [RFC5541] Le Roux, J., Vasseur, J., Lee, Y., "Encoding 1428 of Objective Functions in the Path Computation Element 1429 Communication Protocol (PCEP)", RFC5541, December 2008. 1431 [G-8080] ITU-T Recommendation G.8080/Y.1304, Architecture for 1432 the automatically switched optical network (ASON). 1434 [G-7715] ITU-T Recommendation G.7715 (2002), Architecture 1435 and Requirements for the Automatically 1436 Switched Optical Network (ASON). 1438 [G-7715-2] ITU-T Recommendation G.7715.2 (2007), ASON 1439 routing architecture and requirements for remote route 1440 query. 1442 [BGP-TE] Gredler, H., Medved, J, Farrel, A. Previdi, S., 1443 "North-Bound Distribution of Link-State and TE 1444 Information using BGP", draft-gredler-idr-ls-distribution, 1445 work in progress. 1447 [PCE-AREA-AS] King, D., Meuric, J., Dugeon, O., Zhao, Q., Gonzalez de 1448 Dios, O., "Applicability of the Path Computation Element 1449 to Inter-Area and Inter-AS MPLS and GMPLS Traffic 1450 Engineering", draft-ietf-pce-inter-area-as-applicability, 1451 work in progress. 1453 [PCEP-MIB] Stephan, E., Koushik, K., Zhao, Q., King, D., "PCE 1454 Communication Protocol (PCEP) Management Information 1455 Base", work in progress. 1457 12. Authors' Addresses 1459 Daniel King 1460 Old Dog Consulting 1461 UK 1463 Email: daniel@olddog.co.uk 1465 Adrian Farrel 1466 Old Dog Consulting 1467 UK 1469 Email: adrian@olddog.co.uk 1471 Quintin Zhao 1472 Huawei Technology 1473 125 Nagog Technology Park 1474 Acton, MA 01719 1475 US 1477 Email: qzhao@huawei.com 1478 Fatai Zhang 1479 Huawei Technologies 1480 F3-5-B R&D Center, Huawei Base 1481 Bantian, Longgang District 1482 Shenzhen 518129 P.R.China 1484 Email: zhangfatai@huawei.com