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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-04) exists of draft-ietf-rtgwg-cl-framework-01 == Outdated reference: A later version (-06) exists of draft-ietf-rtgwg-cl-use-cases-01 Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 RTGWG C. Villamizar, Ed. 3 Internet-Draft OCCNC, LLC 4 Intended status: Informational D. McDysan, Ed. 5 Expires: August 9, 2013 Verizon 6 S. Ning 7 Tata Communications 8 A. Malis 9 Verizon 10 L. Yong 11 Huawei USA 12 February 5, 2013 14 Requirements for MPLS Over a Composite Link 15 draft-ietf-rtgwg-cl-requirement-09 17 Abstract 19 There is often a need to provide large aggregates of bandwidth that 20 are best provided using parallel links between routers or MPLS LSR. 21 In core networks there is often no alternative since the aggregate 22 capacities of core networks today far exceed the capacity of a single 23 physical link or single packet processing element. 25 The presence of parallel links, with each link potentially comprised 26 of multiple layers has resulted in additional requirements. Certain 27 services may benefit from being restricted to a subset of the 28 component links or a specific component link, where component link 29 characteristics, such as latency, differ. Certain services require 30 that an LSP be treated as atomic and avoid reordering. Other 31 services will continue to require only that reordering not occur 32 within a microflow as is current practice. 34 Current practice related to multipath is described briefly in an 35 appendix. 37 Status of this Memo 39 This Internet-Draft is submitted 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). Note that other groups may also distribute 44 working documents as Internet-Drafts. The list of current Internet- 45 Drafts is at http://datatracker.ietf.org/drafts/current/. 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." 52 This Internet-Draft will expire on August 9, 2013. 54 Copyright Notice 56 Copyright (c) 2013 IETF Trust and the persons identified as the 57 document authors. All rights reserved. 59 This document is subject to BCP 78 and the IETF Trust's Legal 60 Provisions Relating to IETF Documents 61 (http://trustee.ietf.org/license-info) in effect on the date of 62 publication of this document. Please review these documents 63 carefully, as they describe your rights and restrictions with respect 64 to this document. Code Components extracted from this document must 65 include Simplified BSD License text as described in Section 4.e of 66 the Trust Legal Provisions and are provided without warranty as 67 described in the Simplified BSD License. 69 Table of Contents 71 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 72 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 73 2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 4 74 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 75 4. Network Operator Functional Requirements . . . . . . . . . . . 5 76 4.1. Availability, Stability and Transient Response . . . . . . 5 77 4.2. Component Links Provided by Lower Layer Networks . . . . . 6 78 4.3. Parallel Component Links with Different Characteristics . 8 79 5. Derived Requirements . . . . . . . . . . . . . . . . . . . . . 10 80 6. Management Requirements . . . . . . . . . . . . . . . . . . . 11 81 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 82 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 83 9. Security Considerations . . . . . . . . . . . . . . . . . . . 12 84 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 85 10.1. Normative References . . . . . . . . . . . . . . . . . . . 13 86 10.2. Informative References . . . . . . . . . . . . . . . . . . 13 87 Appendix A. ITU-T G.800 Composite Link Definitions and 88 Terminology . . . . . . . . . . . . . . . . . . . . . 14 89 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15 91 1. Introduction 93 The purpose of this document is to describe why network operators 94 require certain functions in order to solve certain business problems 95 (Section 2). The intent is to first describe why things need to be 96 done in terms of functional requirements that are as independent as 97 possible of protocol specifications (Section 4). For certain 98 functional requirements this document describes a set of derived 99 protocol requirements (Section 5). Appendix A provides a summary of 100 G.800 terminology used to define a composite link. 102 1.1. Requirements Language 104 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 105 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 106 document are to be interpreted as described in RFC 2119 [RFC2119]. 108 2. Assumptions 110 The services supported include pseudowire based services (RFC 3985 111 [RFC3985]), including VPN services, Internet traffic encapsulated by 112 at least one MPLS label (RFC 3032 [RFC3032]), and dynamically 113 signaled MPLS (RFC 3209 [RFC3209] or RFC 5036 [RFC5036]) or MPLS-TP 114 LSPs (RFC 5921 [RFC5921]). The MPLS LSPs supporting these services 115 may be point-to-point, point-to-multipoint, or multipoint-to- 116 multipoint. 118 The locations in a network where these requirements apply are a Label 119 Edge Router (LER) or a Label Switch Router (LSR) as defined in RFC 120 3031 [RFC3031]. 122 The IP DSCP cannot be used for flow identification since L3VPN 123 requires Diffserv transparency (see RFC 4031 5.5.2 [RFC4031]), and in 124 general network operators do not rely on the DSCP of Internet 125 packets. 127 3. Definitions 129 ITU-T G.800 Based Composite and Component Link Definitions: 130 Section 6.9.2 of ITU-T-G.800 [ITU-T.G.800] defines composite and 131 component links as summarized in Appendix A. The following 132 definitions for composite and component links are derived from 133 and intended to be consistent with the cited ITU-T G.800 134 terminology. 136 Composite Link: A composite link is a logical link composed of a 137 set of parallel point-to-point component links, where all 138 links in the set share the same endpoints. A composite link 139 may itself be a component of another composite link, but only 140 a strict hierarchy of links is allowed. 142 Component Link: A point-to-point physical link (including one or 143 more link layer) or a logical link that preserves ordering in 144 the steady state. A component link may have transient out of 145 order events, but such events must not exceed the network's 146 specific NPO. Examples of a physical link are: any set of 147 link layers over a WDM wavelength or any supportable 148 combination of Ethernet PHY, PPP, SONET or OTN over a 149 physical link. Examples of a logical link are: MPLS LSP, 150 Ethernet VLAN, MPLS-TP LSP. A set of link layers supported 151 over pseudowire is a logical link that appears to the client 152 to be a physical link. 154 Flow: A sequence of packets that must be transferred in order on one 155 component link. 157 Flow identification: The label stack and other information that 158 uniquely identifies a flow. Other information in flow 159 identification may include an IP header, PW control word, 160 Ethernet MAC address, etc. Note that an LSP may contain one or 161 more Flows or an LSP may be equivalent to a Flow. Flow 162 identification is used to locally select a component link, or a 163 path through the network toward the destination. 165 Network Performance Objective (NPO): Numerical values for 166 performance measures, principally availability, latency, and 167 delay variation. See [I-D.ietf-rtgwg-cl-use-cases] for more 168 details. 170 4. Network Operator Functional Requirements 172 The Functional Requirements in this section are grouped in 173 subsections starting with the highest priority. 175 4.1. Availability, Stability and Transient Response 177 Limiting the period of unavailability in response to failures or 178 transient events is extremely important as well as maintaining 179 stability. The transient period between some service disrupting 180 event and the convergence of the routing and/or signaling protocols 181 MUST occur within a time frame specified by NPO values. 182 [I-D.ietf-rtgwg-cl-use-cases] provides references and a summary of 183 service types requiring a range of restoration times. 185 FR#1 The solution SHALL provide a means to summarize some routing 186 advertisements regarding the characteristics of a composite 187 link such that the routing protocol converges within the 188 timeframe needed to meet the network performance objective. A 189 composite link CAN be announced in conjunction with detailed 190 parameters about its component links, such as bandwidth and 191 latency. The composite link SHALL behave as a single IGP 192 adjacency. 194 FR#2 The solution SHALL ensure that all possible restoration 195 operations happen within the timeframe needed to meet the NPO. 196 The solution may need to specify a means for aggregating 197 signaling to meet this requirement. 199 FR#3 The solution SHALL provide a mechanism to select a path for a 200 flow across a network that contains a number of paths comprised 201 of pairs of nodes connected by composite links in such a way as 202 to automatically distribute the load over the network nodes 203 connected by composite links while meeting all of the other 204 mandatory requirements stated above. The solution SHOULD work 205 in a manner similar to that of current networks without any 206 composite link protocol enhancements when the characteristics 207 of the individual component links are advertised. 209 FR#4 If extensions to existing protocols are specified and/or new 210 protocols are defined, then the solution SHOULD provide a means 211 for a network operator to migrate an existing deployment in a 212 minimally disruptive manner. 214 FR#5 Any automatic LSP routing and/or load balancing solutions MUST 215 NOT oscillate such that performance observed by users changes 216 such that an NPO is violated. Since oscillation may cause 217 reordering, there MUST be means to control the frequency of 218 changing the component link over which a flow is placed. 220 FR#6 Management and diagnostic protocols MUST be able to operate 221 over composite links. 223 Existing scaling techniques used in MPLS networks apply to MPLS 224 networks which support Composite Links. Scalability and stability 225 are covered in more detail in [I-D.ietf-rtgwg-cl-framework]. 227 4.2. Component Links Provided by Lower Layer Networks 229 Case 3 as defined in [ITU-T.G.800] involves a component link 230 supporting an MPLS layer network over another lower layer network 231 (e.g., circuit switched or another MPLS network (e.g., MPLS-TP)). 232 The lower layer network may change the latency (and/or other 233 performance parameters) seen by the MPLS layer network. Network 234 Operators have NPOs of which some components are based on performance 235 parameters. Currently, there is no protocol for the lower layer 236 network to inform the higher layer network of a change in a 237 performance parameter. Communication of the latency performance 238 parameter is a very important requirement. Communication of other 239 performance parameters (e.g., delay variation) is desirable. 241 FR#7 In order to support network NPOs and provide acceptable user 242 experience, the solution SHALL specify a protocol means to 243 allow a lower layer server network to communicate latency to 244 the higher layer client network. 246 FR#8 The precision of latency reporting SHOULD be configurable. A 247 reasonable default SHOULD be provided. Implementations SHOULD 248 support precision of at least 10% of the one way latencies for 249 latency of 1 ms or more. 251 FR#9 The solution SHALL provide a means to limit the latency on a 252 per LSP basis between nodes within a network to meet an NPO 253 target when the path between these nodes contains one or more 254 pairs of nodes connected via a composite link. 256 The NPOs differ across the services, and some services have 257 different NPOs for different QoS classes, for example, one QoS 258 class may have a much larger latency bound than another. 259 Overload can occur which would violate an NPO parameter (e.g., 260 loss) and some remedy to handle this case for a composite link 261 is required. 263 FR#10 If the total demand offered by traffic flows exceeds the 264 capacity of the composite link, the solution SHOULD define a 265 means to cause the LSPs for some traffic flows to move to some 266 other point in the network that is not congested. These 267 "preempted LSPs" may not be restored if there is no 268 uncongested path in the network. 270 The intent is to measure the predominant latency in uncongested 271 service provider networks, where geographic delay dominates and is on 272 the order of milliseconds or more. The argument for including 273 queuing delay is that it reflects the delay experienced by 274 applications. The argument against including queuing delay is that 275 it if used in routing decisions it can result in routing instability. 276 This tradeoff is discussed in detail in 277 [I-D.ietf-rtgwg-cl-framework]. 279 4.3. Parallel Component Links with Different Characteristics 281 Corresponding to Case 1 of [ITU-T.G.800], as one means to provide 282 high availability, network operators deploy a topology in the MPLS 283 network using lower layer networks that have a certain degree of 284 diversity at the lower layer(s). Many techniques have been developed 285 to balance the distribution of flows across component links that 286 connect the same pair of nodes. When the path for a flow can be 287 chosen from a set of candidate nodes connected via composite links, 288 other techniques have been developed. Refer to the Appendices in 289 [I-D.ietf-rtgwg-cl-use-cases] for a description of existing 290 techniques and a set of references. 292 FR#11 The solution SHALL measure traffic on a labeled traffic flow 293 and dynamically select the component link on which to place 294 this flow in order to balance the load so that no component 295 link in the composite link between a pair of nodes is 296 overloaded. 298 FR#12 When a traffic flow is moved from one component link to 299 another in the same composite link between a set of nodes (or 300 sites), it MUST be done so in a minimally disruptive manner. 302 FR#13 Load balancing MAY be used during sustained low traffic 303 periods to reduce the number of active component links for the 304 purpose of power reduction. 306 FR#14 The solution SHALL provide a means to identify flows whose 307 rearrangement frequency needs to be bounded by a configured 308 value. 310 FR#15 The solution SHALL provide a means that communicates whether 311 the flows within an LSP can be split across multiple component 312 links. The solution SHOULD provide a means to indicate the 313 flow identification field(s) which can be used along the flow 314 path which can be used to perform this function. 316 FR#16 The solution SHALL provide a means to indicate that a traffic 317 flow shall select a component link with the minimum latency 318 value. 320 FR#17 The solution SHALL provide a means to indicate that a traffic 321 flow shall select a component link with a maximum acceptable 322 latency value as specified by protocol. 324 FR#18 The solution SHALL provide a means to indicate that a traffic 325 flow shall select a component link with a maximum acceptable 326 delay variation value as specified by protocol. 328 FR#19 The solution SHALL provide a means local to a node that 329 automatically distributes flows across the component links in 330 the composite link such that NPOs are met. 332 FR#20 The solution SHALL provide a means to distribute flows from a 333 single LSP across multiple component links to handle at least 334 the case where the traffic carried in an LSP exceeds that of 335 any component link in the composite link. As defined in 336 section 3, a flow is a sequence of packets that must be 337 transferred on one component link. 339 FR#21 The solution SHOULD support the use case where a composite 340 link itself is a component link for a higher order composite 341 link. For example, a composite link comprised of MPLS-TP bi- 342 directional tunnels viewed as logical links could then be used 343 as a component link in yet another composite link that 344 connects MPLS routers. 346 FR#22 The solution MUST support an optional means for LSP signaling 347 to bind an LSP to a particular component link within a 348 composite link. If this option is not exercised, then an LSP 349 that is bound to a composite link may be bound to any 350 component link matching all other signaled requirements, and 351 different directions of a bidirectional LSP can be bound to 352 different component links. 354 FR#23 The solution MUST support a means to indicate that both 355 directions of co-routed bidirectional LSP MUST be bound to the 356 same component link. 358 A minimally disruptive change implies that as little disruption as is 359 practical occurs. Such a change can be achieved with zero packet 360 loss. A delay discontinuity may occur, which is considered to be a 361 minimally disruptive event for most services if this type of event is 362 sufficiently rare. A delay discontinuity is an example of a 363 minimally disruptive behavior corresponding to current techniques. 365 A delay discontinuity is an isolated event which may greatly exceed 366 the normal delay variation (jitter). A delay discontinuity has the 367 following effect. When a flow is moved from a current link to a 368 target link with lower latency, reordering can occur. When a flow is 369 moved from a current link to a target link with a higher latency, a 370 time gap can occur. Some flows (e.g., timing distribution, PW 371 circuit emulation) are quite sensitive to these effects. A delay 372 discontinuity can also cause a jitter buffer underrun or overrun 373 affecting user experience in real time voice services (causing an 374 audible click). These sensitivities may be specified in an NPO. 376 As with any load balancing change, a change initiated for the purpose 377 of power reduction may be minimally disruptive. Typically the 378 disruption is limited to a change in delay characteristics and the 379 potential for a very brief period with traffic reordering. The 380 network operator when configuring a network for power reduction 381 should weigh the benefit of power reduction against the disadvantage 382 of a minimal disruption. 384 5. Derived Requirements 386 This section takes the next step and derives high-level requirements 387 on protocol specification from the functional requirements. 389 DR#1 The solution SHOULD attempt to extend existing protocols 390 wherever possible, developing a new protocol only if this adds 391 a significant set of capabilities. 393 DR#2 A solution SHOULD extend LDP capabilities to meet functional 394 requirements (without using TE methods as decided in 395 [RFC3468]). 397 DR#3 Coexistence of LDP and RSVP-TE signaled LSPs MUST be supported 398 on a composite link. Other functional requirements should be 399 supported as independently of signaling protocol as possible. 401 DR#4 When the nodes connected via a composite link are in the same 402 MPLS network topology, the solution MAY define extensions to 403 the IGP. 405 DR#5 When the nodes are connected via a composite link are in 406 different MPLS network topologies, the solution SHALL NOT rely 407 on extensions to the IGP. 409 DR#6 The solution SHOULD support composite link IGP advertisement 410 that results in convergence time better than that of 411 advertising the individual component links. The solution SHALL 412 be designed so that it represents the range of capabilities of 413 the individual component links such that functional 414 requirements are met, and also minimizes the frequency of 415 advertisement updates which may cause IGP convergence to occur. 417 Examples of advertisement update triggering events to be 418 considered include: LSP establishment/release, changes in 419 component link characteristics (e.g., latency, up/down state), 420 and/or bandwidth utilization. 422 DR#7 When a worst case failure scenario occurs, the number of 423 RSVP-TE LSPs to be resignaled will cause a period of 424 unavailability as perceived by users. The resignaling time of 425 the solution MUST meet the NPO objective for the duration of 426 unavailability. The resignaling time of the solution MUST NOT 427 increase significantly as compared with current methods. 429 6. Management Requirements 431 MR#1 Management Plane MUST support polling of the status and 432 configuration of a composite link and its individual composite 433 link and support notification of status change. 435 MR#2 Management Plane MUST be able to activate or de-activate any 436 component link in a composite link in order to facilitate 437 operation maintenance tasks. The routers at each end of a 438 composite link MUST redistribute traffic to move traffic from 439 a de-activated link to other component links based on the 440 traffic flow TE criteria. 442 MR#3 Management Plane MUST be able to configure a LSP over a 443 composite link and be able to select a component link for the 444 LSP. 446 MR#4 Management Plane MUST be able to trace which component link a 447 LSP is assigned to and monitor individual component link and 448 composite link performance. 450 MR#5 Management Plane MUST be able to verify connectivity over each 451 individual component link within a composite link. 453 MR#6 Component link fault notification MUST be sent to the 454 management plane. 456 MR#7 Composite link fault notification MUST be sent to management 457 plane and distribute via link state message in the IGP. 459 MR#8 Management Plane SHOULD provide the means for an operator to 460 initiate an optimization process. 462 MR#9 An operator initiated optimization MUST be performed in a 463 minimally disruptive manner as described in Section 4.3. 465 MR#10 Any statement which requires the solution to support some new 466 functionality through use of the words new functionality, 467 SHOULD be interpretted as follows. The implementation either 468 MUST or SHOULD support the new functionality depending on the 469 use of either MUST or SHOULD in the requirements statement. 470 The implementation SHOULD in most or all cases allow any new 471 functionality to be individually enabled or disabled through 472 configuration. 474 7. Acknowledgements 476 Frederic Jounay of France Telecom and Yuji Kamite of NTT 477 Communications Corporation co-authored a version of this document. 479 A rewrite of this document occurred after the IETF77 meeting. 480 Dimitri Papadimitriou, Lou Berger, Tony Li, the former WG chairs John 481 Scuder and Alex Zinin, the current WG chair Alia Atlas, and others 482 provided valuable guidance prior to and at the IETF77 RTGWG meeting. 484 Tony Li and John Drake have made numerous valuable comments on the 485 RTGWG mailing list that are reflected in versions following the 486 IETF77 meeting. 488 Iftekhar Hussain and Kireeti Kompella made comments on the RTGWG 489 mailing list after IETF82 that identified a new requirement. 490 Iftekhar Hussain made numerous valuable comments on the RTGWG mailing 491 list that resulted in improvements to document clarity. 493 In the interest of full disclosure of affiliation and in the interest 494 of acknowledging sponsorship, past affiliations of authors are noted. 495 Much of the work done by Ning So occurred while Ning was at Verizon. 496 Much of the work done by Curtis Villamizar occurred while at 497 Infinera. Infinera continues to sponsor this work on a consulting 498 basis. 500 8. IANA Considerations 502 This memo includes no request to IANA. 504 9. Security Considerations 506 This document specifies a set of requirements. The requirements 507 themselves do not pose a security threat. If these requirements are 508 met using MPLS signaling as commonly practiced today with 509 authenticated but unencrypted OSPF-TE, ISIS-TE, and RSVP-TE or LDP, 510 then the requirement to provide additional information in this 511 communication presents additional information that could conceivably 512 be gathered in a man-in-the-middle confidentiality breach. Such an 513 attack would require a capability to monitor this signaling either 514 through a provider breach or access to provider physical transmission 515 infrastructure. A provider breach already poses a threat of numerous 516 tpes of attacks which are of far more serious consequence. Encrption 517 of the signaling can prevent or render more difficult any 518 confidentiality breach that otherwise might occur by means of access 519 to provider physical transmission infrastructure. 521 10. References 523 10.1. Normative References 525 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 526 Requirement Levels", BCP 14, RFC 2119, March 1997. 528 10.2. Informative References 530 [I-D.ietf-rtgwg-cl-framework] 531 Ning, S., McDysan, D., Osborne, E., Yong, L., and C. 532 Villamizar, "Composite Link Framework in Multi Protocol 533 Label Switching (MPLS)", draft-ietf-rtgwg-cl-framework-01 534 (work in progress), August 2012. 536 [I-D.ietf-rtgwg-cl-use-cases] 537 Ning, S., Malis, A., McDysan, D., Yong, L., and C. 538 Villamizar, "Composite Link Use Cases and Design 539 Considerations", draft-ietf-rtgwg-cl-use-cases-01 (work in 540 progress), August 2012. 542 [ITU-T.G.800] 543 ITU-T, "Unified functional architecture of transport 544 networks", 2007, . 547 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 548 Label Switching Architecture", RFC 3031, January 2001. 550 [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., 551 Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack 552 Encoding", RFC 3032, January 2001. 554 [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., 555 and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP 556 Tunnels", RFC 3209, December 2001. 558 [RFC3468] Andersson, L. and G. Swallow, "The Multiprotocol Label 559 Switching (MPLS) Working Group decision on MPLS signaling 560 protocols", RFC 3468, February 2003. 562 [RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to- 563 Edge (PWE3) Architecture", RFC 3985, March 2005. 565 [RFC4031] Carugi, M. and D. McDysan, "Service Requirements for Layer 566 3 Provider Provisioned Virtual Private Networks (PPVPNs)", 567 RFC 4031, April 2005. 569 [RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP 570 Specification", RFC 5036, October 2007. 572 [RFC5921] Bocci, M., Bryant, S., Frost, D., Levrau, L., and L. 573 Berger, "A Framework for MPLS in Transport Networks", 574 RFC 5921, July 2010. 576 Appendix A. ITU-T G.800 Composite Link Definitions and Terminology 578 Composite Link: 579 Section 6.9.2 of ITU-T-G.800 [ITU-T.G.800] defines composite link 580 in terms of three cases, of which the following two are relevant 581 (the one describing inverse (TDM) multiplexing does not apply). 582 Note that these case definitions are taken verbatim from section 583 6.9, "Layer Relationships". 585 Case 1: "Multiple parallel links between the same subnetworks 586 can be bundled together into a single composite link. Each 587 component of the composite link is independent in the sense 588 that each component link is supported by a separate server 589 layer trail. The composite link conveys communication 590 information using different server layer trails thus the 591 sequence of symbols crossing this link may not be preserved. 592 This is illustrated in Figure 14." 594 Case 3: "A link can also be constructed by a concatenation of 595 component links and configured channel forwarding 596 relationships. The forwarding relationships must have a 1:1 597 correspondence to the link connections that will be provided 598 by the client link. In this case, it is not possible to 599 fully infer the status of the link by observing the server 600 layer trails visible at the ends of the link. This is 601 illustrated in Figure 16." 603 Subnetwork: A set of one or more nodes (i.e., LER or LSR) and links. 604 As a special case it can represent a site comprised of multiple 605 nodes. 607 Forwarding Relationship: Configured forwarding between ports on a 608 subnetwork. It may be connectionless (e.g., IP, not considered 609 in this draft), or connection oriented (e.g., MPLS signaled or 610 configured). 612 Component Link: A topolological relationship between subnetworks 613 (i.e., a connection between nodes), which may be a wavelength, 614 circuit, virtual circuit or an MPLS LSP. 616 Authors' Addresses 618 Curtis Villamizar (editor) 619 OCCNC, LLC 621 Email: curtis@occnc.com 623 Dave McDysan (editor) 624 Verizon 625 22001 Loudoun County PKWY 626 Ashburn, VA 20147 628 Email: dave.mcdysan@verizon.com 630 So Ning 631 Tata Communications 633 Email: ning.so@tatacommunications.com 635 Andrew Malis 636 Verizon 637 60 Sylvan Road 638 Waltham, MA 02451 640 Phone: +1 781-466-2362 641 Email: andrew.g.malis@verizon.com 642 Lucy Yong 643 Huawei USA 644 5340 Legacy Dr. 645 Plano, TX 75025 647 Phone: +1 469-277-5837 648 Email: lucy.yong@huawei.com