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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: 11 August 2012 Old Dog Consulting 5 11 March 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-01.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 11 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 Domain Diversity....................................7 82 1.3.3 Existing Traffic Engineering Constraints............7 83 1.3.4 Commercial Constraints..............................7 84 1.3.5 Domain Confidentiality..............................7 85 1.3.6 Limiting Information Aggregation....................7 86 1.3.7 Domain Interconnection Discovery....................8 87 1.4 Terminology...............................................8 88 2. Examination of Existing PCE Mechanisms........................9 89 2.1 Per Domain Path Computation...............................9 90 2.2 Backward Recursive Path Computation.......................10 91 2.2.1 Applicability of BRPC when the Domain Path is not 92 Known.................................................10 93 3. Hierarchical PCE..............................................11 94 4. Hierarchical PCE Procedures...................................12 95 4.1 Objective Functions and Policy............................12 96 4.2 Maintaining Domain Confidentiality........................13 97 4.3 PCE Discovery.............................................13 98 4.4 Parent Domain Traffic Engineering Database................14 99 4.5 Determination of Destination Domain ......................14 100 4.6 Hierarchical PCE Examples.................................15 101 4.6.1 Hierarchical PCE Initial Information Exchange.......17 102 4.6.2 Hierarchical PCE End-to-End Path Computation 103 Procedure Example.........................................17 104 4.7 Hierarchical PCE Error Handling...........................19 105 4.8 Hierarchical PCEP Protocol Extensions.....................19 106 4.8.1 PCEP Request Qualifiers.............................19 107 4.8.2 Indication of H-PCE Capability......................20 108 4.8.3 Intention to Utilize Parent PCE Capabilities........20 109 4.8.4 Communication of Domain Connectivity Information....20 110 4.8.5 Domain Identifiers..................................21 111 5. Hierarchical PCE Applicability................................21 112 5.1 Antonymous Systems and Areas..............................21 113 5.2 ASON architecture (G-7715-2)..............................22 114 5.2.1 Implicit Consistency Between Hierarchical PCE and 115 G.7715.2..................................................23 116 5.2.2 Benefits of Hierarchical PCEs in ASON...............24 117 6. A Note on BGP-TE..............................................24 118 7. Management Considerations ....................................26 119 7.1 Control of Function and Policy............................26 120 7.1.1 Child PCE...........................................26 121 7.1.2 Parent PCE..........................................26 122 7.1.3 Policy Control......................................27 123 7.2 Information and Data Models...............................27 124 7.3 Liveness Detection and Monitoring.........................27 125 7.4 Verifying Correct Operation...............................27 126 7.5. Impact on Network Operation..............................28 127 8. Security Considerations ......................................28 128 9. IANA Considerations ..........................................29 129 10. Acknowledgements ............................................29 130 11. References ..................................................29 131 11.1. Normative References....................................29 132 11.2. Informative References .................................20 133 12. Authors' Addresses ..........................................31 135 1. Introduction 137 The capability to compute the routes of end-to-end inter-domain MPLS 138 Traffic Engineering (TE) and GMPLS Label Switched Paths (LSPs) is 139 expressed as requirements in [RFC4105] and [RFC4216]. This capability 140 may be realized by a Path Computation Element (PCE). The PCE 141 architecture is defined in [RFC4655]. The methods for establishing 142 and controlling inter-domain MPLS-TE and GMPLS LSPs are documented in 143 [RFC4726]. 145 In this context, a domain can be defined as a separate 146 administrative, geographic, or switching environment within the 147 network. A domain may be further defined as a zone of routing or 148 computational ability. Under these definitions a domain might be 149 categorized as an Antonymous System (AS) or an Interior Gateway 150 Protocol (IGP) area [RFC4726] and [RFC4655]. Domains are connected 151 through ingress and egress boundary nodes (BNs). A more detailed 152 definition is given in Section 1.2. 154 In a multi-domain environment, the determination of an end-to-end 155 traffic engineered path is a problem because no single point of path 156 computation is aware of all of the links and resources in each 157 domain. PCEs can be used to compute end-to-end paths using a per- 158 domain path computation technique [RFC5152]. Alternatively, the 159 backward recursive path computation (BRPC) mechanism [RFC5441] 160 allows multiple PCEs to collaborate in order to select an optimal 161 end-to-end path that crosses multiple domains. Both mechanisms 162 assume that the sequence of domains to be crossed between ingress 163 and egress in known in advance. 165 This document examines techniques to establish the optimum path when 166 the sequence of domains is not known in advance. It shows how the PCE 167 architecture can be extended to allow the optimum sequence of domains 168 to be selected, and the optimum end-to-end path to be derived. 170 The model described in this document introduces a hierarchical 171 relationship between domains. It is applicable to environments with 172 small groups of domains where visibility from the ingress Label 173 Switching Router (LSR) is limited. Applying the hierarchical PCE 174 model to large groups of domains such as the Internet, is not 175 considered feasible or desirable, and is out of scope for this 176 document. 178 This document does not specify any protocol extensions or 179 enhancements. That work is left for future protocol specification 180 documents. However, some assumptions are made about which protocols 181 will be used to provide specific functions, and guidelines to 182 future protocol developers are made in the form of requirements 183 statements. 185 1.1 Problem Statement 187 Using a PCE to compute a path between nodes within a single domain is 188 relatively straightforward. Computing an end-to-end path when the 189 source and destination nodes are located in different domains 190 requires co-operation between multiple PCEs, each responsible for 191 its own domain. 193 Techniques for inter-domain path computation described so far 194 ([RFC5152] and [RFC5441]) assume that the sequence of domains to be 195 crossed from source to destination is well known. No explanation is 196 given (for example, in [RFC4655]) of how this sequence is generated 197 or what criteria may be used for the selection of paths between 198 domains. In small clusters of domains, such as simple cooperation 199 between adjacent ISPs, this selection process is not complex. In more 200 advanced deployments (such as optical networks constructed from 201 multiple sub-domains, or in multi-AS environments) the choice of 202 domains in the end-to-end domain sequence can be critical to the 203 determination of an optimum end-to-end path. 205 This document introduces the concept of a hierarchical PCE 206 architecture and shows how to coordinate PCEs in peer domains in 207 order to derive an optimal end-to-end path. 209 The work is scoped to operate with a small group of domains, and 210 there is no intent to apply this model to a large group of domains, 211 e.g., to the Internet. 213 1.2 Definition of a Domain 215 A domain is defined in [RFC4726] as any collection of network 216 elements within a common sphere of address management or path 217 computational responsibility. Examples of such domains include 218 IGP areas and Autonomous Systems. Wholly or partially overlapping 219 domains are not within the scope of this document. 221 In the context of GMPLS, a particularly important example of a domain 222 is the Automatically Switched Optical Network (ASON) subnetwork 223 [G-8080]. In this case, computation of an end-to-end path requires 224 the selection of nodes and links within a parent domain where some 225 nodes may, in fact, be subnetworks. Furthermore, a domain might be an 226 ASON Routing Area [G-7715]. A PCE may perform the path computation 227 function of an ASON Routing Controller as described in [G-7715-2]. 228 See Section 6.2 for a further discussion of the applicability to the 229 ASON architecture. 231 This document assumes that the selection of a sequence of domains for 232 an end-to-end path is in some sense a hierarchical path computation 233 problem. That is, where one mechanism is used to determine a path 234 across a domain, a separate mechanism (or at least a separate set 235 of paradigms) is used to determine the sequence of domains. The 236 responsibility for the selection of domain interconnection can belong 237 to either or both of the mechanisms. 239 1.3 Assumptions and Requirements 241 Networks are often constructed from multiple domains. These 242 domains are often interconnected via multiple interconnect points. 243 Its assumed that the sequence of domains for an end-to-end path is 244 not always well known; that is, an application requesting end-to-end 245 connectivity has no preference for, or no ability to specify, the 246 sequence of domains to be crossed by the path. 248 The traffic engineering properties of a domain cannot be seen from 249 outside the domain. Traffic engineering aggregation or abstraction, 250 hides information and can lead to failed path setup or the selection 251 of suboptimal end-to-end paths [RFC4726]. The aggregation process 252 may also have significant scaling issues for networks with many 253 possible routes and multiple TE metrics. Flooding TE information 254 breaks confidentiality and does not scale in the routing protocol. 255 See Section 7 for a discussion of the concept of inter-domain traffic 256 engineering information exchange known as BGP-TE. 258 The primary goal of this document is to define how to derive optimal 259 end-to-end, multi-domain paths when the sequence of domains is not 260 known in advance. The solution needs to be scalable and to maintain 261 internal domain topology confidentiality while providing the optimal 262 end-to-end path. It cannot rely on the exchange of TE information 263 between domains, and for the confidentiality, scaling, and 264 aggregation reasons just cited, it cannot utilise a computation 265 element that has universal knowledge of TE properties and topology 266 of all domains. 268 The sub-sections that follow set out the primary objectives and 269 requirements to be satisfied by a PCE solution to multi-domain path 270 computation. 272 1.3.1 Metric Objectives 274 The definition of optimality is dependent on policy, and is based on 275 a single objective or a group objectives. An objective is expressed 276 as an objective function [RFC5541] and may be specified on a path 277 computation request. The following objective functions are identified 278 in this document. They define how the path metrics and TE link 279 qualities are manipulated during inter-domain path computation. The 280 list is not proscriptive and may be expanded in other documents. 282 o Minimize the cost of the path [RFC5541] 283 o Select a path using links with the minimal load [RFC5541] 284 o Select a path that leaves the maximum residual bandwidth [RFC5541] 285 o Minimize aggregate bandwidth consumption [RFC5541] 286 o Minimize the Load of the most loaded Link [RFC5541] 287 o Minimize the Cumulative Cost of a set of paths [RFC5541] 288 o Minimize or cap the number of domains crossed 289 o Disallow domain re-entry 291 See Section 5.1 for further discussion of objective functions. 293 1.3.2 Domain Diversity 295 A pair of paths are domain-diverse if they do not transit any of the 296 same domains. A pair of paths that share a common ingress and egress 297 are domain-diverse if they only share the same domains at the ingress 298 and egress (the ingress and egress domains). Domain diversity may be 299 maximized for a pair of paths by selecting paths that have the 300 smallest number of shared domains. (Note that this is not the same 301 as finding paths with the greatest number of distinct domains!) 303 Path computation should facilitate the selection of paths that share 304 ingress and egress domains, but do not share any transit domains. 305 This provides a way to reduce the risk of shared failure along any 306 path, and automatically helps to ensure path diversity for most of 307 the route of a pair of LSPs. 309 Thus, domain path selection should provide the capability to include 310 or exclude specific domains and specific boundary nodes. 312 1.3.3 Existing Traffic Engineering Constraints 314 Any solution should take advantage of typical traffic engineering 315 constraints (hop count, bandwidth, lambda continuity, path cost, 316 etc.) to meet the service demands expressed in the path computation 317 request [RFC4655]. 319 1.3.4 Commercial Constraints 321 The solution should provide the capability to include commercially 322 relevant constraints such as policy, SLAs, security, peering 323 preferences, and dollar costs. 325 Additionally it may be necessary for the service provider to 326 request that specific domains are included or excluded based on 327 commercial relationships, security implications, and reliability. 329 1.3.5 Domain Confidentiality 331 A key requirement is the ability to maintain domain confidentiality 332 when computing inter-domain end-to-end paths. It should be possible 333 for local policy to require that a PCE not disclose to any other PCE 334 the intra-domain paths it computes or the internal topology of the 335 domain it serves. This requirement should have no impact in the 336 optimality or quality of the end-to-end path that is derived. 338 1.3.6 Limiting Information Aggregation 340 In order to reduce processing overhead and to not sacrifice 341 computational detail, there should be no requirement to aggregate or 342 abstract traffic engineering link information. 344 1.3.7 Domain Interconnection Discovery 346 To support domain mesh topologies, the solution should allow the 347 discovery and selection of domain inter-connections. Pre- 348 configuration of preferred domain interconnections should also be 349 supported for network operators that have bilateral agreement, and 350 preference for the choice of points of interconnection. 352 1.4 Terminology 354 This document uses PCE terminology defined in [RFC4655], [RFC4875], 355 and [RFC5440]. Additional terms are defined below. 357 Domain Path: The sequence of domains for a path. 359 Ingress Domain: The domain that includes the ingress LSR of a path. 361 Transit Domain: A domain that has an upstream and downstream 362 neighbor domain for a specific path. 364 Egress Domain: The domain that includes the egress LSR of a path. 366 Boundary Nodes: Each Domain has entry LSRs and exit LSRs that could 367 be Area Border Routers (ABRs) or Autonomous System Border Routers 368 (ASBRs) depending on the type of domain. They are defined here more 369 generically as Boundary Nodes (BNs). 371 Entry BN of domain(n): a BN connecting domain(n-1) to domain(n) 372 on a path. 374 Exit BN of domain(n): a BN connecting domain(n) to domain(n+1) 375 on a path. 377 Parent Domain: A domain higher up in a domain hierarchy such 378 that it contains other domains (child domains) and potentially other 379 links and nodes. 381 Child Domain: A domain lower in a domain hierarchy such that it has 382 a parent domain. 384 Parent PCE: A PCE responsible for selecting a path across a parent 385 domain and any number of child domains by coordinating with child 386 PCEs and examining a topology map that shows domain inter- 387 connectivity. 389 Child PCE: A PCE responsible for computing the path across one or 390 more specific (child) domains. A child PCE maintains a relationship 391 with at least one parent PCE. 393 OF: Objective Function: A set of one or more optimization 394 criteria used for the computation of a single path (e.g., path cost 395 minimization), or the synchronized computation of a set of paths 396 (e.g., aggregate bandwidth consumption minimization). See [RFC4655] 397 and [RFC5541]. 399 2. Examination of Existing PCE Mechanisms 401 This section provides a brief overview of two existing PCE 402 cooperation mechanisms called the per-domain path computation method, 403 and the backward recursive path computation method. It describes the 404 applicability of these methods to the multi-domain problem. 406 2.1 Per-Domain Path Computation 408 The per-domain path computation method for establishing inter-domain 409 TE-LSPs [RFC5152] defines a technique whereby the path is computed 410 during the signalling process on a per-domain basis. The entry BN of 411 each domain is responsible for performing the path computation for 412 the section of the LSP that crosses the domain, or for requesting 413 that a PCE for that domain computes that piece of the path. 415 During per-domain path computation, each computation results in the 416 best path across the domain to provide connectivity to the next 417 domain in the domain sequence (usually indicated in signalling by an 418 identifier of the next domain or the identity of the next entry BN). 420 Per-domain path computation may lead to sub-optimal end-to-end paths 421 because the most optimal path in one domain may lead to the choice of 422 an entry BN for the next domain that results in a very poor path 423 across that next domain. 425 In the case that the domain path (in particular, the sequence of 426 boundary nodes) is not known, the PCE must select an exit BN based on 427 some determination of how to reach the destination that is outside 428 the domain for which the PCE has computational responsibility. 429 [RFC5152] suggest that this might be achieved using the IP shortest 430 path as advertise by BGP. Note, however, that the existence of an IP 431 forwarding path does guarantee the presence of sufficient bandwidth, 432 let alone an optimal TE path. Furthermore, in many GMPLS systems 433 inter-domain IP routing will not be present. Thus, per-domain path 434 computation may require a significant number of crankback routing 435 attempts to establish even a sub-optimal path. 437 Note also that the PCEs in each domain may have different computation 438 capabilities, may run different path computation algorithms, and may 439 apply different sets of constraints and optimization criteria, etc. 441 This can result in the end-to-end path being inconsistent and sub- 442 optimal. 444 Per-domain path computation can suit simply-connected domains where 445 the preferred points of interconnection are known. 447 2.2 Backward Recursive Path Computation 449 The Backward Recursive Path Computation (BRPC) [RFC5441] procedure 450 involves cooperation and communication between PCEs in order to 451 compute an optimal end-to-end path across multiple domains. The 452 sequence of domains to be traversed can either be determined before 453 or during the path computation. In the case where the sequence of 454 domains is known, the ingress Path Computation Client (PCC) sends a 455 path computation request to the PCE responsible for the ingress 456 domain. This request is forwarded between PCEs, domain-by-domain, to 457 the PCE responsible for the egress domain. The PCE in the egress 458 domain creates a set of optimal paths from all of the domain entry 459 BNs to the egress LSR. This set is represented as a tree of potential 460 paths called a Virtual Shortest Path Tree (VSPT), and the PCE passes 461 it back to the previous PCE on the domain path. As the VSPT is passed 462 back toward the ingress domain, each PCE computes the optimal paths 463 from its entry BNs to its exit BNs that connect to the rest of the 465 tree. It adds these paths to the VSPT and passes the VSPT on until 466 the PCE for the ingress domain is reached and computes paths from the 467 ingress LSR to connect to the rest of the tree. The ingress PCE then 468 selects the optimal end-to-end path from the tree, and returns the 469 path to the initiating PCC. 471 BRPC may suit environments where multiple connections exist between 472 domains and there is no preference for the choice of points of 473 interconnection. It is best suited to scenarios where the domain 474 path is known in advance, but can also be used when the domain path 475 is not known. 477 2.2.1. Applicability of BRPC when the Domain Path is Not Known 479 As described above, BRPC can be used to determine an optimal inter- 480 domain path when the domain sequence is known. Even when the sequence 481 of domains is not known BRPC could be used as follows. 483 o The PCC sends a request to the PCE for the ingress domain (the 484 ingress PCE). 486 o The ingress PCE sends the path computation request direct to the 487 PCE responsible for the domain containing the destination node (the 488 egress PCE). 490 o The egress PCE computes an egress VSPT and passes it to a PCE 491 responsible for each of the adjacent (potentially upstream) 492 domains. 494 o Each PCE in turn constructs a VSPT and passes it on to all of its 495 neighboring PCEs. 497 o When the ingress PCE has received a VSPT from each of its 498 neighboring domains it is able to select the optimum path. 500 Clearly this mechanism (which could be called path computation 501 flooding) has significant scaling issues. It could be improved by 502 the application of policy and filtering, but such mechanisms are not 503 simple and would still leave scaling concerns. 505 3. Hierarchical PCE 507 In the hierarchical PCE architecture, a parent PCE maintains a domain 508 topology map that contains the child domains (seen as vertices in the 509 topology) and their interconnections (links in the topology). The 510 parent PCE has no information about the content of the child domains; 511 that is, the parent PCE does not know about the resource availability 512 within the child domains, nor about the availability of connectivity 513 across each domain because such knowledge would violate the 514 confidentiality requirement and would either require flooding of full 515 information to the parent (scaling issue) or would necessitate some 516 form of aggregation. The parent PCE is aware of the TE capabilities 517 of the interconnections between child domains as these 518 interconnections are links in its own topology map. 520 Note that in the case that the domains are IGP areas, there is no 521 link between the domains (the ABRs have a presence in both 522 neighboring areas). The parent domain may choose to represent this in 523 its TED as a virtual link that is unconstrained and has zero cost, 524 but this is entirely an implementation issue. 526 Each child domain has at least one PCE capable of computing paths 527 across the domain. These PCEs are known as child PCEs and have a 528 relationship with the parent PCE. Each child PCE also knows the 529 identity of the domains that neighbor its own domain. A child PCE 530 only knows the topology of the domain that it serves and does not 531 know the topology of other child domains. Child PCEs are also not 532 aware of the general domain mesh connectivity (i.e., the domain 533 topology map) beyond the connectivity to the immediate neighbor 534 domains of the domain it serves. 536 The parent PCE builds the domain topology map either from 537 configuration or from information received from each child PCE. This 538 tells it how the domains are interconnected including the TE 539 properties of the domain interconnections. But the parent PCE does 540 not know the contents of the child domains. Discovery of the domain 541 topology and domain interconnections is discussed further in Section 542 5.3. 544 When a multi-domain path is needed, the ingress PCE sends a request 545 to the parent PCE (using the path computation element protocol, PCEP 546 [RFC5440]). The parent PCE selects a set of candidate domain paths 547 based on the domain topology and the state of the inter-domain links. 548 It then sends computation requests to the child PCEs responsible for 549 each of the domains on the candidate domain paths. These requests may 550 be sequential or parallel depending on implementation details. 552 Each child PCE computes a set of candidate path segments across its 553 domain and sends the results to the parent PCE. The parent PCE uses 554 this information to select path segments and concatenate them to 555 derive the optimal end-to-end inter-domain path. The end-to-end path 556 is then sent to the child PCE which received the initial path request 557 and this child PCE passes the path on to the PCC that issued the 558 original request. 560 4. Hierarchical PCE Procedures 562 4.1 Objective Functions and Policy 564 Deriving the optimal end-to-end domain path sequence is dependent on 565 the policy applied during domain path computation. An Objective 566 Function (OF) [RFC5541], or set of OFs, may be applied to define the 567 policy being applied to the domain path computation. 569 The OF specifies the desired outcome of the computation. It does 570 not describe the algorithm to use. When computing end-to-end inter- 571 domain paths, required OFs may include (see Section 1.3.1): 573 o Minimum cost path 574 o Minimum load path 575 o Maximum residual bandwidth path 576 o Minimize aggregate bandwidth consumption 577 o Minimize or cap the number of transit domains 578 o Disallow domain re-entry 580 The objective function may be requested by the PCC, the ingress 581 domain PCE (according to local policy), or maybe applied by the 582 parent PCE according to inter-domain policy. 584 More than one OF (or a composite OF) may be chosen to apply to a 585 single computation provided they are not contradictory. Composite OFs 586 may include weightings and preferences for the fulfilment of pieces 587 of the desired outcome. 589 4.2 Maintaining Domain Confidentiality 591 Information about the content of child domains is not shared for 592 scaling and confidentiality reasons. This means that a parent PCE is 593 aware of the domain topology and the nature of the connections 594 between domains, but is not aware of the content of the domains. 595 Similarly, a child PCE cannot know the internal topology of another 596 child domain. Child PCEs also do not know the general domain mesh 597 connectivity, this information is only known by the parent PCE. 599 As described in the earlier sections of this document, PCEs can 600 exchange path information in order to construct an end-to-end inter- 601 domain path. Each per-domain path fragment reveals information about 602 the topology and resource availability within a domain. Some 603 management domains or ASes will not want to share this information 604 outside of the domain (even with a trusted parent PCE). In order to 605 conceal the information, a PCE may replace a path segment with a 606 path-key [RFC5520]. This mechanism effectively hides the content of a 607 segment of a path. 609 4.3 PCE Discovery 611 It is a simple matter for each child PCE to be configured with the 612 address of its parent PCE. Typically, there will only be one or two 613 parents of any child. 615 The parent PCE also needs to be aware of the child PCEs for all child 616 domains that it can see. This information is most likely to be 617 configured (as part of the administrative definition of each 618 domain). 620 Discovery of the relationships between parent PCEs and child PCEs 621 does not form part of the hierarchical PCE architecture. Mechanisms 622 that rely on advertising or querying PCE locations across domain or 623 provider boundaries are undesirable for security, scaling, 624 commercial, and confidentiality reasons. 626 The parent PCE also needs to know the inter-domain connectivity. 627 This information could be configured with suitable policy and 628 commercial rules, or could be learned from the child PCEs as 629 described in Section 4. 631 In order for the parent PCE to learn about domain interconnection 632 the child PCE will report the identity of its neighbor domains. The 633 IGP in each neighbor domain can advertise its inter-domain TE 634 link capabilities [RFC5316], [RFC5392]. This information can be 635 collected by the child PCEs and forwarded to the parent PCE, or the 636 parent PCE could participate in the IGP in the child domains. 638 4.4 Parent Domain Traffic Engineering Database 640 The parent PCE maintains a domain topology map of the child domains 641 and their interconnectivity. Where inter-domain connectivity is 642 provided by TE links the capabilities of those links may also be 643 known to the parent PCE. The parent PCE maintains a traffic 644 engineering database (TED) for the parent domain in the same way that 645 any PCE does. 647 The parent domain may just be the collection of child domains and 648 their interconnectivity, may include details of the inter-domain TE 649 links, and may contain nodes and links in its own right. 651 The mechanism for building the parent TED is likely to rely heavily 652 on administrative configuration and commercial issues because the 653 network was probably partitioned into domains specifically to address 654 these issues. 656 In practice, certain information may be passed from the child domains 657 to the parent PCE to help build the parent TED. In theory, the parent 658 PCE could listen to the routing protocols in the child domains, but 659 this would violate the confidentiality and scaling issues that may be 660 responsible for the partition of the network into domains. So it is 661 much more likely that a suitable solution will involve specific 662 communication from an entity in the child domain (such as the child 663 PCE) to convey the necessary information. As already mentioned, the 664 "necessary information" relates to how the child domains are inter- 665 connected. The topology and available resources within the child 666 domain do not need to be communicated to the parent PCE: doing so 667 would violate the PCE architecture. Mechanisms for reporting this 668 information are described in the examples in Section 4.6 in abstract 669 terms as "a child PCE reports its neighbor domain connectivity to its 670 parent PCE"; the specifics of a solution are out of scope of this 671 document, but the requirements are indicated in Section 4.8. See 672 Section 6 for a brief discussion of BGP-TE. 674 In models such as ASON (see Section 5.2), it is possible to consider 675 a separate instance of an IGP running within the parent domain where 676 the participating protocol speakers are the nodes directly present in 677 that domain and the PCEs (Routing Controllers) responsible for each 678 of the child domains. 680 4.5 Determination of Destination Domain 682 The PCC asking for an inter-domain path computation is aware of the 683 identity of the destination node by definition. If it knows the 684 egress domain it can supply this information as part of the path 685 computation request. However, if it does not know the egress domain 686 this information must be known by the child PCE or known/determined 687 by the parent PCE. 689 In some specialist topologies the parent PCE could determine the 690 destination domain based on the destination address, for example from 691 configuration. However, this is not appropriate for many multi-domain 692 addressing scenarios. In IP-based multi-domain networks the 693 parent PCE may be able to determine the destination domain by 694 participating in inter-domain routing. Finally, the parent PCE could 695 issue specific requests to the child PCEs to discover if they contain 696 the destination node, but this has scaling implications. 698 For the purposes of this document, the precise mechanism of the 699 discovery of the destination domain is left out of scope. Suffice to 700 say that for each multi-domain path computation some mechanism will 701 be required to determine the location of the destination. 703 4.6 Hierarchical PCE Examples 705 The following example describes the generic hierarchical domain 706 topology. Figure 1 demonstrates four interconnected domains within a 707 fifth, parent domain. Each domain contains a single PCE: 709 o Domain 1 is the ingress domain and child PCE 1 is able to compute 710 paths within the domain. Its neighbors are Domain 2 and Domain 4. 711 The domain also contains the source LSR (S) and three egress 712 boundary nodes (BN11, BN12, and BN13). 714 o Domain 2 is served by child PCE 2. Its neighbors are Domain 1 and 715 Domain 3. The domain also contains four boundary nodes (BN21, BN22, 716 BN23, and BN24). 718 o Domain 3 is the egress domain and is served by child PCE 3. Its 719 neighbors are Domain 2 and Domain 4. The domain also contains the 720 destination LSR (D) and three ingress boundary nodes (BN31, BN32, 721 and BN33). 723 o Domain 4 is served by child PCE 4. Its neighbors are Domain 2 and 724 Domain 3. The domain also contains two boundary nodes (BN41 and 725 BN42). 727 All of these domains are contained within Domain 5 which is served 728 by the parent PCE (PCE 5). 730 ----------------------------------------------------------------- 731 | Domain 5 | 732 | ----- | 733 | |PCE 5| | 734 | ----- | 735 | | 736 | ---------------- ---------------- ---------------- | 737 | | Domain 1 | | Domain 2 | | Domain 3 | | 738 | | | | | | | | 739 | | ----- | | ----- | | ----- | | 740 | | |PCE 1| | | |PCE 2| | | |PCE 3| | | 741 | | ----- | | ----- | | ----- | | 742 | | | | | | | | 743 | | ----| |---- ----| |---- | | 744 | | |BN11+---+BN21| |BN23+---+BN31| | | 745 | | - ----| |---- ----| |---- - | | 746 | | |S| | | | | |D| | | 747 | | - ----| |---- ----| |---- - | | 748 | | |BN12+---+BN22| |BN24+---+BN32| | | 749 | | ----| |---- ----| |---- | | 750 | | | | | | | | 751 | | ---- | | | | ---- | | 752 | | |BN13| | | | | |BN33| | | 753 | -----------+---- ---------------- ----+----------- | 754 | \ / | 755 | \ ---------------- / | 756 | \ | | / | 757 | \ |---- ----| / | 758 | ----+BN41| |BN42+---- | 759 | |---- ----| | 760 | | | | 761 | | ----- | | 762 | | |PCE 4| | | 763 | | ----- | | 764 | | | | 765 | | Domain 4 | | 766 | ---------------- | 767 | | 768 ----------------------------------------------------------------- 770 Figure 1 : Sample Hierarchical Domain Topology 772 Figure 2, shows the view of the domain topology as seen by the parent 773 PCE (PCE 5). This view is an abstracted topology; PCE 5 is aware of 774 domain connectivity, but not of the internal topology within each 775 domain. 777 ---------------------------- 778 | Domain 5 | 779 | ---- | 780 | |PCE5| | 781 | ---- | 782 | | 783 | ---- ---- ---- | 784 | | |---| |---| | | 785 | | D1 | | D2 | | D3 | | 786 | | |---| |---| | | 787 | ---- ---- ---- | 788 | \ ---- / | 789 | \ | | / | 790 | ----| D4 |---- | 791 | | | | 792 | ---- | 793 | | 794 ---------------------------- 796 Figure 2 : Abstract Domain Topology as Seen by the Parent PCE 798 4.6.1 Hierarchical PCE Initial Information Exchange 800 Based on the Figure 1 topology, the following is an illustration of 801 the initial hierarchical PCE information exchange. 803 1. Child PCE 1, the PCE responsible for Domain 1, is configured 804 with the location of its parent PCE (PCE5). 806 2. Child PCE 1 establishes contact with its parent PCE. The parent 807 applies policy to ensure that communication with PCE 1 is allowed. 809 3. Child PCE 1 listens to the IGP in its domain and learns its 810 inter-domain connectivity. That is, it learns about the links 811 BN11-BN21, BN12-BN22, and BN13-BN41. 813 4. Child PCE 1 reports its neighbor domain connectivity to its parent 814 PCE. 816 5. Child PCE 1 reports any change in the resource availability on its 817 inter-domain links to its parent PCE. 819 Each child PCE performs steps 1 through 5 so that the parent PCE can 820 create a domain topology view as shown in Figure 2. 822 4.6.2 Hierarchical PCE End-to-End Path Computation Procedure 824 The procedure below is an example of a source PCC requesting an 825 end-to-end path in a multi-domain environment. The topology is 826 represented in Figure 1. It is assumed that the each child PCE has 827 connected to its parent PCE and exchanged the initial information 828 required for the parent PCE to create its domain topology view as 829 described in Section 5.6.1. 831 1. The source PCC (the ingress LSR in our example), sends a request 832 to the PCE responsible for its domain (PCE 1) for a path to the 833 destination LSR (D). 835 2. PCE 1 determines the destination is not in domain 1. 837 3. PCE 1 sends a computation request to its parent PCE (PCE 5). 839 4. The parent PCE determines that the destination is in Domain 3. 840 (See Section 5.5). 842 5. PCE 5 determines the likely domain paths according to the domain 843 interconnectivity and TE capabilities between the domains. For 844 example, assuming that the link BN12-BN22 is not suitable for the 845 requested path, three domain paths are determined: 847 S-BN11-BN21-D2-BN23-BN31-D 848 S-BN11-BN21-D2-BN24-BN32-D 849 S-BN13-BN41-D4-BN42-BN33-D 851 6. PCE 5 sends edge-to-edge path computation requests to PCE 2 852 which is responsible for Domain 2 (i.e., BN21-to-BN23 and BN21- 853 to-BN24), and to PCE 4 for Domain 4 (i.e., BN41-to-BN42). 855 7. PCE 5 sends source-to-edge path computation requests to PCE 1 856 which is responsible for Domain 1 (i.e., S-to-BN11 and S-to- 857 BN13). 859 8. PCE 5 sends edge-to-egress path computation requests to PCE3 860 which is responsible for Domain 3 (i.e., BN31-to-D, BN32-to-D, 861 and BN33-to-D). 863 9. PCE 5 correlates all the computation responses from each child 864 PCE, adds in the information about the inter-domain links, and 865 applies any requested and locally configured policies. 867 10. PCE 5 then selects the optimal end-to-end multi-domain path 868 that meets the policies and objective functions, and supplies the 869 resulting path to PCE 1. 871 11. PCE 1 forwards the path to the PCC (the ingress LSR). 873 Note that there is no requirement for steps 6, 7, and 8 to be carried 874 out in parallel or in series. Indeed, they could be overlapped with 875 step 5. This is an implementation issue. 877 4.7 Hierarchical PCE Error Handling 879 In the event that a child PCE in a domain cannot find a suitable 880 path to the egress. The child PCE should return the relevant 881 error to notify the parent PCE. Depending on the error response the 882 parent PCE can elect to: 884 o Cancel the request and send the relevant response back to the 885 initial child PCE that requested an end-to-end path; 886 o Relax the constraints associated with the initial path request; 887 o Select another candidate domain and send the path request to the 888 child PCE responsible for the domain. 890 If the parent PCE does not receive a response from a child PCE within 891 an allotted time period. The parent PCE can either: 893 o Send the path request to another child PCE in the same domain, if a 894 secondary child PCE exists; 895 o Select another candidate domain and send the path request to the 896 child PCE responsible for that domain. 898 4.8 Requirements for Hierarchical PCEP Protocol Extensions 900 This section lists the high-level requirements for extensions to the 901 PCEP to support the hierarchical PCE model. It is provided to offer 902 guidance to PCEP protocol developers in designing a solution suitable 903 for use in a hierarchical PCE framework. 905 4.8.1 PCEP Request Qualifiers 907 PCEP request (PCReq) messages are used by a PCC or a PCE to make a 908 computation request or enquiry to a PCE. The requests are qualified 909 so that the PCE knows what type of action is required. 911 Support of the hierarchical PCE architecture will introduce two new 912 qualifications as follows: 914 o It must be possible for a child PCE to indicate that the response 915 it receives from the parent PCE should consist of a domain sequence 916 only (i.e., not a fully-specified end-to-end path). This allows the 917 child PCE to initiate per-domain or backward recursive path 918 computation. 920 o A parent PCE may need to be able to ask a child PCE whether a 921 particular node address (the destination of an end-to-end path) is 922 present in the domain that the child PCE serves. 924 In PCEP, such request qualifications are carried as bit-flags in the 925 RP object carried within the PCReq message. 927 4.8.2 Indication of Hierarchical PCE Capability 929 Although parent/child PCE relationships are likely configured, it 930 will assist network operations if the parent PCE is able to indicate 931 to the child that it really is capable of acting as a parent PCE. 932 This will help to trap misconfigurations. 934 In PCEP, such capabilities are carried in the Open Object within the 935 Open message. 937 4.8.3 Intention to Utilize Parent PCE Capabilities 939 A PCE that is capable of acting as a parent PCE might not be 940 configured or willing to act as the parent for a specific child PCE. 941 This fact could be determined when the child sends a PCReq that 942 requires parental activity (such as querying other child PCEs), and 943 could result in a negative response in a PCEP Error (PCErr) message. 945 However, the expense of a poorly targeted PCReq can be avoided if 946 the child PCE indicates that it might wish to use the parent as a 947 parent (for example, on the Open message), and if the parent 948 determines at that time whether it is willing to act as a parent to 949 this child. 951 4.8.4 Communication of Domain Connectivity Information 953 Section 5.4 describes how the parent PCE needs a parent TED and 954 indicates that the information might be supplied from the child PCEs 955 in each domain. This requires a mechanism whereby information about 956 inter-domain links can be supplied by a child PCE to a parent PCE, 957 for example on a PCEP Notify (PCNtf) message. 959 The information that would be exchanged includes: 961 o Identifier of advertising child PCE 962 o Identifier of PCE's domain 963 o Identifier of the link 964 o TE properties of the link (metrics, bandwidth) 965 o Other properties of the link (technology-specific) 966 o Identifier of link end-points 967 o Identifier of adjacent domain 969 It may be desirable for this information to be periodically updated, 970 for example, when available bandwidth changes. In this case, the 971 parent PCE might be given the ability to configure thresholds in the 972 child PCE to prevent flapping of information. 974 4.8.5 Domain Identifiers 976 Domain identifiers are already present in PCEP to allow a PCE to 977 indicate which domains it serves, and to allow the representation of 978 domains as abstract nodes in paths. The wider use of domains in the 979 context of this work on hierarchical PCE will require that domains 980 can be identified in more places within objects in PCEP messages. 981 This should pose no problems. 983 However, more attention may need to be applied to the precision of 984 domain identifier definitions to ensure that it is always possible to 985 unambiguously identify a domain from its identifier. This work will 986 be necessary in configuration, and also in protocol specifications 987 (for example, an OSPF area identifier is sufficient within an 988 Autonomous System, but becomes ambiguous in a path that crosses 989 multiple Autonomous Systems). 991 5. Hierarchical PCE Applicability 993 As per [RFC4655], PCE can inherently support inter-domain path 994 computation for any definition of a domain as set out in Section 1.2 995 of this document. 997 Hierarchical PCE can be applied to inter-domain environments, 998 including Antonymous Systems and IGP areas. The hierarchical PCE 999 procedures make no distinction between, Antonymous Systems and IGP 1000 area applications, although it should be noted that the TED 1001 maintained by a parent PCE must be able to support the concept of 1002 child domains connected by inter-domain links or directly connected 1003 at boundary nodes (see Section 4). 1005 This section sets out the applicability of hierarchical PCE to three 1006 environments: 1008 o MPLS traffic engineering across multiple Autonomous Systems 1009 o MPLS traffic engineering across multiple IGP areas 1010 o GMPLS traffic engineering in the ASON architecture 1012 5.1 Antonymous Systems and Areas 1014 Networks are comprised of domains. A domain can be considered to be 1015 a collection of network elements within an AS or area that has a 1016 common sphere of address management or path computational 1017 responsibility. 1019 As networks increase in size and complexity it may be required to 1020 introduce scaling methods to reduce the amount information flooded 1021 within the network and make the network more manageable. An IGP 1022 hierarchy is designed to improve IGP scalability by dividing the 1023 IGP domain into areas and limiting the flooding scope of topology 1024 information to within area boundaries. This restricts a router's 1025 visibility to information about links and other routers within the 1026 single area. If a router needs to compute a route to destination 1027 located in another area, a method is required to compute a path 1028 across the area boundary. 1030 When an LSR within an AS or area needs to compute a path across an 1031 area or AS boundary it must also use an inter-AS computation 1032 technique. Hierarchical PCE is equally applicable to computing 1033 inter-area and inter-AS MPLS and GMPLS paths across domain 1034 boundaries. 1036 5.2 ASON Architecture 1038 The International Telecommunications Union (ITU) defines the ASON 1039 architecture in [G-8080]. [G-7715] defines the routing architecture 1040 for ASON and introduces a hierarchical architecture. In this 1041 architecture, the Routing Areas (RAs) have a hierarchical 1042 relationship between different routing levels, which means a parent 1043 (or higher level) RA can contain multiple child RAs. The 1044 interconnectivity of the lower RAs is visible to the higher level RA. 1045 Note that the RA hierarchy can be recursive. 1047 In the ASON framework, a path computation request is termed a Route 1048 Query. This query is executed before signaling is used to establish 1049 an LSP termed a Switched Connection (SC) or a Soft Permanent 1050 Connection (SPC). [G-7715-2] defines the requirements and 1051 architecture for the functions performed by Routing Controllers (RC) 1052 during the operation of remote route queries - an RC is synonymous 1053 with a PCE. For an end-to-end connection, the route may be computed 1054 by a single RC or multiple RCs in a collaborative manner (i.e., RC 1055 federations). In the case of RC federations, [G-7715-2] describes 1056 three styles during remote route query operation: 1058 o Step-by-step remote path computation 1059 o Hierarchical remote path computation 1060 o A combination of the above. 1062 In a hierarchical ASON routing environment, a child RC may 1063 communicate with its parent RC (at the next higher level of the ASON 1064 routing hierarchy) to request the computation of an end-to-end path 1065 across several RAs. It does this using a route query message (known 1066 as the abstract message RI_QUERY). The corresponding parent RC may 1067 communicate with other child RCs that belong to other child RAs at 1068 the next lower hierarchical level. Thus, a parent RC can act as 1069 either a Route Query Requester or Route Query Responder. 1071 It can be seen that the hierarchical PCE architecture fits the 1072 hierarchical ASON routing architecture well. It can be used to 1073 provide paths across subnetworks, and to determine end-to-end paths 1074 in networks constructed from multiple subnetworks or RAs. 1076 When hierarchical PCE is applied to implement hierarchical remote 1077 path computation in [G-7715-2], it is very important for operators to 1078 understand the different terminology and implicit consistency 1079 between hierarchical PCE and [G-7715-2]. 1081 5.2.1 Implicit Consistency Between Hierarchical PCE and G.7715.2 1083 This section highlights the correspondence between features of the 1084 hierarchical PCE architecture and the ASON routing architecture. 1086 (1) RC (Routing Controller) and PCE (Path Computation Element) 1088 [G-8080] describes the Routing Controller component as an 1089 abstract entity, which is responsible for responding to requests 1090 for path (route) information and topology information. It can be 1091 implemented as a single entity, or as a distributed set of 1092 entities that make up a cooperative federation. 1094 [RFC4655] describes PCE (Path Computation Element) is an entity 1095 (component, application, or network node) that is capable of 1096 computing a network path or route based on a network graph and 1097 applying computational constraints. 1099 Therefore, in the ASON architecture, a PCE can be regarded as a 1100 realizations of the RC. 1102 (2) Route Query Requester/Route Query Responder and PCC/PCE 1104 [G-7715-2] describes the Route Query Requester as a Connection 1105 Controller or Routing Controller that sends a route query message 1106 to a Routing Controller requesting one or more paths that 1107 satisfy a set of routing constraints. The Route Query Responder 1108 is a Routing Controller that performs path computation upon 1109 receipt of a route query message from a Route Query Requester, 1110 sending a response back at the end of the path computation. 1112 In the context of ASON, a Signaling Controller initiates and 1113 processes signaling messages and is closely coupled to a 1114 Signaling Protocol Speaker. A Routing Controller makes routing 1115 decisions and is usually coupled to configuration entities 1116 and/or a Routing Protocol Speaker. 1118 It can be seen that a PCC corresponds to a Route Query Requester, 1119 and a PCE corresponds to a Route Query Responder. A PCE/RC can 1120 also act as a Route Query Requester sending requests to another 1121 Route Query Responder. 1123 The PCEP path computation request (PCReq) and path computation 1124 reply (PCRep) messages between PCC and PCE correspond to the 1125 RI_QUERY and RI_UPDATE messages in [G-7715-2]. 1127 (3) Routing Area Hierarchy and Hierarchical Domain 1129 The ASON routing hierarchy model is shown in Figure 6 of 1130 [G-7715] through an example that illustrates routing area levels. 1131 If the hierarchical remote path computation mechanism of 1132 [G-7715-2] is applied in this scenario, each routing area should 1133 have at least one RC for route query function and there is a 1134 parent RC for the child RCs in each routing area. 1136 According to [G-8080], the parent RC has visibility of the 1137 structure of the lower level, so it knows the interconnectivity 1138 of the RAs in the lower level. Each child RC can compute edge-to- 1139 edge paths across its own child RA. 1141 Thus, an RA corresponds to a domain in the PCE architecture, and 1142 the hierarchical relationship between RAs corresponds to the 1143 hierarchical relationship between domains in the hierarchical PCE 1144 architecture. Furthermore, a parent PCE in a parent domain can be 1145 regarded as parent RC in a higher routing level, and a child PCE 1146 in a child domain can be regarded as child RC in a lower routing 1147 level. 1149 5.2.2 Benefits of Hierarchical PCEs in ASON 1151 RCs in an ASON environment can use the hierarchical PCE model to 1152 fully match the ASON hierarchical routing model, so the hierarchical 1153 PCE mechanisms can be applied to fully satisfy the architecture and 1154 requirements of [G-7715-2] without any changes. If the hierarchical 1155 PCE mechanism is applied in ASON, it can be used to determine end-to- 1156 end optimized paths across sub-networks and RAs before initiating 1157 signaling to create the connection. It can also improve the 1158 efficiency of connection setup to avoid crankback. 1160 6. A Note on BGP-TE 1162 The concept of exchange of TE information between Autonomous Systems 1163 (ASes) is discussed in [BGP-TE]. The information exchanged in this 1164 way could be the full TE information from the AS, an aggregation of 1165 that information, or a representation of the potential connectivity 1166 across the AS. Furthermore, that information could be updated 1167 frequently (for example, for every new LSP that is set up across the 1168 AS) or only at threshold-crossing events. 1170 There are a number of discussion points associated with the use of 1171 [BGP-TE] concerning the volume of information, the rate of churn of 1172 information, the confidentiality of information, the accuracy of 1173 aggregated or potential-connectivity information, and the processing 1174 required to generate aggregated information. The PCE architecture and 1175 the architecture enabled by [BGP-TE] make different assumptions about 1176 the operational objectives of the networks, and this document does 1177 not attempt to make one of the approaches "right" and the other 1178 "wrong". Instead, this work assumes that a decision has been made to 1179 utilize the PCE architecture. 1181 Indeed, [BGP-TE] may have some uses within the PCE model. For 1182 example, [BGP-TE] could be used as a "northbound" TE advertisement 1183 such that a PCE does not need to listen to an IGP in its domain, but 1184 has its TED populated by messages received (for example) from a 1185 Route Reflector. Furthermore, the inter-domain connectivity and 1186 connectivity capabilities that is required information for a parent 1187 PCE could be obtained as a filtered subset of the information 1188 available in [BGP-TE]. 1190 7. Management Considerations 1192 General PCE management considerations are discussed in [RFC4655]. In 1193 the case of the hierarchical PCE architecture, there are additional 1194 management considerations. 1196 The administrative entity responsible for the management of the 1197 parent PCEs must be determined. In the case of multi-domains (e.g., 1198 IGP areas or multiple ASes) within a single service provider 1199 network, the management responsibility for the parent PCE would most 1200 likely be handled by the service provider. In the case of multiple 1201 ASes within different service provider networks, it may be necessary 1202 for a third-party to manage the parent PCEs according to commercial 1203 and policy agreements from each of the participating service 1204 providers. 1206 7.1 Control of Function and Policy 1208 7.1.1 Child PCE 1210 Support of the hierarchical procedure will be controlled by the 1211 management organization responsible for each child PCE. A child PCE 1212 must be configured with the address of its parent PCE in order for 1213 it to interact with its parent PCE. The child PCE must also be 1214 authorized to peer with the parent PCE. 1216 7.1.2 Parent PCE 1217 The parent PCE must only accept path computation requests from 1218 authorized child PCEs. If a parent PCE receives requests from an 1219 unauthorized child PCE, the request should be dropped. 1221 This means that a parent PCE must be configured with the identities 1222 and security credentials of all of its child PCEs, or there must be 1223 some form of shared secret that allows an unknown child PCE to be 1224 authorized by the parent PCE. 1226 7.1.3 Policy Control 1228 It may be necessary to maintain a policy module on the parent PCE 1229 [RFC5394]. This would allow the parent PCE to apply commercially 1230 relevant constraints such as SLAs, security, peering preferences, and 1231 dollar costs. 1233 It may also be necessary for the parent PCE to limit end-to-end path 1234 selection by including or excluding specific domains based on 1235 commercial relationships, security implications, and reliability. 1237 7.2 Information and Data Models 1239 A PCEP MIB module is defined in [PCEP-MIB] that describes managed 1240 objects for modeling of PCEP communication. An additional PCEP MIB 1241 will be required to report parent PCE and child PCE information, 1242 including: 1244 o Parent PCE configuration and status, 1246 o Child PCE configuration and information, 1248 o Notifications to indicate session changes between parent PCEs and 1249 child PCEs. 1251 o Notification of parent PCE TED updates and changes. 1253 7.3 Liveness Detection and Monitoring 1255 The hierarchical procedure requires interaction with multiple PCEs. 1256 Once a child PCE requests an end-to-end path, a sequence of events 1257 occurs that requires interaction between the parent PCE and each 1258 child PCE. If a child PCE is not operational, and an alternate 1259 transit domain is not available, then a failure must be reported. 1261 7.4 Verifying Correct Operation 1263 Verifying the correct operation of a parent PCE can be performed by 1264 monitoring a set of parameters. The parent PCE implementation should 1265 provide the following parameters monitored by the parent PCE: 1267 o Number of child PCE requests. 1269 o Number of successful hierarchical PCE procedures completions on a 1270 per-PCE-peer basis. 1272 o Number of hierarchical PCE procedure completion failures on a per- 1273 PCE-peer basis. 1275 o Number of hierarchical PCE procedure requests from unauthorized 1276 child PCEs. 1278 7.5. Impact on Network Operation 1280 The hierarchical PCE procedure is a multiple-PCE path computation 1281 scheme. Subsequent requests to and from the child and parent PCEs do 1282 not differ from other path computation requests and should not have 1283 any significant impact on network operations. 1285 8. Security Considerations 1287 The hierarchical PCE procedure relies on PCEP and inherits the 1288 security requirements defined [RFC5440]. As noted in Section 7, 1289 there is a security relationship between child and parent PCEs. 1290 This relationship, like any PCEP relationship assumes 1291 preconfiguration of identities, authority, and keys, or can 1292 operate through any key distribution mechanism outside the scope of 1293 PCEP. As PCEP operates over TCP, it may make use of any TCP security 1294 mechanism. 1296 The hierarchical PCE architecture makes use of PCE policy 1297 [RFC5394] and the security aspects of the PCE communication protocol 1298 documented in [RFC5440]. It is expected that the parent PCE will 1299 require all child PCEs to use full security when communicating with 1300 the parent and that security will be maintained by not supporting the 1301 discovery by a parent of child PCEs. 1303 PCE operation also relies on information used to build the TED. 1304 Attacks on a PCE system may be achieved by falsifying or impeding 1305 this flow of information. The child PCE TEDs are constructed as 1306 described in [RFC4655] and are unchanged in this document: if the PCE 1307 listens to the IGP for this information, then normal IGP security 1308 measures may be applied, and it should be noted that an IGP routing 1309 system is generally assumed to be a trusted domain such that router 1310 subversion is not a risk. The parent PCE TED is constructed as 1311 described in this document and may involve: 1313 - multiple parent-child relationships using PCEP (as already 1314 described) 1316 - the parent PCE listening to child domain IGPs (with the same 1317 security features as a child PCE listening to its IGP) 1319 - an external mechanism (such as [BGP-TE]) which will need to be 1320 authorized and secured. 1322 Any multi-domain operation necessarily involves the exchange of 1323 information across domain boundaries. This is bound to represent a 1324 significant security and confidentiality risk especially when the 1325 child domains are controlled by different commercial concerns. PCEP 1326 allows individual PCEs to maintain confidentiality of their domain 1327 path information using Path Keys [RFC5520], and the hierarchical 1328 PCE architecture is specifically designed to enable as much isolation 1329 of domain topology and capabilities information as is possible. 1331 Further considerations of the security issues related to inter-AS 1332 path computation see [RFC5376]. 1334 9. IANA Considerations 1336 This document makes no requests for IANA action. 1338 10. Acknowledgements 1340 The authors would like to thank David Amzallag, Oscar Gonzalez de 1341 Diosm, Franz Rambach, Ramon Casellas, Olivier Dugeon, Filippo Cugini, 1342 and Dhruv Dhody for their comments and suggestions. 1344 11. References 1346 11.1 Normative References 1348 [RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation 1349 Element (PCE)-Based Architecture", RFC 4655, August 2006. 1351 [RFC5152] Vasseur, JP., Ayyangar, A., and R. Zhang, "A Per-Domain 1352 Path Computation Method for Establishing Inter-Domain 1353 Traffic Engineering (TE) Label Switched Paths (LSPs)", 1354 RFC 5152, February 2008. 1356 [RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash, 1357 "Policy-Enabled Path Computation Framework", RFC 5394, 1358 December 2008. 1360 [RFC5440] Ayyangar, A., Farrel, A., Oki, E., Atlas, A., Dolganow, 1361 A., Ikejiri, Y., Kumaki, K., Vasseur, J., and J. Roux, 1362 "Path Computation Element (PCE) Communication Protocol 1363 (PCEP)", RFC 5440, March 2009. 1365 [RFC5441] Vasseur, J.P., Ed., "A Backward Recursive PCE-based 1366 Computation (BRPC) procedure to compute shortest inter- 1367 domain Traffic Engineering Label Switched Paths", RFC 1368 5441, April 2009. 1370 [RFC5520] Brandford, R., Vasseur J.P., and Farrel A., "Preserving 1371 Topology Confidentiality in Inter-Domain Path 1372 Computation Using a Key-Based Mechanism 1373 RFC5520, April 2009. 1375 [G-8080] ITU-T Recommendation G.8080/Y.1304, Architecture for 1376 the automatically switched optical network (ASON). 1378 [G-7715] ITU-T Recommendation G.7715 (2002), Architecture 1379 and Requirements for the Automatically 1380 Switched Optical Network (ASON). 1382 [G-7715-2] ITU-T Recommendation G.7715.2 (2007), ASON 1383 routing architecture and requirements for remote route 1384 query. 1386 11.2. Informative References 1388 [RFC4105] Le Roux, J.-L., Vasseur, J.-P, and Boyle, J., 1389 "Requirements for Inter-Area MPLS Traffic Engineering", 1390 RFC 4105, June 2005. 1392 [RFC4216] Zhang, R., and Vasseur, J.-P., "MPLS Inter-Autonomous 1393 System (AS) Traffic Engineering (TE) Requirements", RFC 1394 4216, November 2005. 1396 [RFC4726] Farrel, A., Vasseur, J., and A. Ayyangar, "A Framework 1397 for Inter-Domain Multiprotocol Label Switching Traffic 1398 Engineering", RFC 4726, November 2006. 1400 [RFC4875] Aggarwal, R., Papadimitriou, D., and Yasukawa, S., 1401 "Extensions to Resource Reservation Protocol - Traffic 1402 Engineering (RSVP-TE) for Point-to-Multipoint TE Label 1403 Switched Paths (LSPs)", RFC 4875, May 2007. 1405 [RFC5152] Vasseur, JP., Ayyangar, A., and R. Zhang, "A Per-Domain 1406 Path Computation Method for Establishing Inter-Domain 1407 Traffic Engineering (TE) Label Switched Paths (LSPs)", 1408 RFC 5152, February 2008. 1410 [RFC5316] Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in 1411 Support of Inter-Autonomous System (AS) MPLS and GMPLS 1412 Traffic Engineering", RFC 5316, December 2008. 1414 [RFC5376] Bitar, N., et al., "Inter-AS Requirements for the 1415 Path Computation Element Communication Protocol 1416 (PCECP)", RFC 5376, November 2008. 1418 [RFC5392] Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in 1419 Support of Inter-Autonomous System (AS) MPLS and GMPLS 1420 Traffic Engineering", RFC 5392, January 2009. 1422 [RFC5541] Roux, J., Vasseur, J., and Y. Lee, "Encoding 1423 of Objective Functions in the Path 1424 Computation Element Communication 1425 Protocol (PCEP)", RFC5541, December 2008. 1427 [BGP-TE] Gredler, H., Medved, J, Farrel, A. and Previdi, S., 1428 "North-Bound Distribution of Link-State and TE 1429 Information using BGP", draft-gredler-idr-ls-distribution, 1430 work in progress. 1432 [PCEP-MIB] Stephan, E., Koushik, K., Zhao, Q., and King, D., "PCE 1433 Communication Protocol (PCEP) Management Information 1434 Base", work in progress. 1436 12. Authors' Addresses 1438 Daniel King 1439 Old Dog Consulting 1440 Email: daniel@olddog.co.uk 1442 Adrian Farrel 1443 Old Dog Consulting 1444 Email: adrian@olddog.co.uk 1446 Quintin Zhao 1447 Huawei Technology 1448 125 Nagog Technology Park 1449 Acton, MA 01719 1450 US 1451 Email: qzhao@huawei.com 1452 Fatai Zhang 1453 Huawei Technologies 1454 F3-5-B R&D Center, Huawei Base 1455 Bantian, Longgang District 1456 Shenzhen 518129 P.R.China 1457 Email: zhangfatai@huawei.com