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'ISIS-GRACE') (Obsoleted by RFC 5306) ** Obsolete normative reference: RFC 2740 (ref. 'OSPFv3') (Obsoleted by RFC 5340) Summary: 6 errors (**), 0 flaws (~~), 12 warnings (==), 7 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group D. Katz 3 Internet Draft Juniper Networks 4 D. Ward 5 Cisco Systems 6 Expires: December, 2006 June, 2006 8 Generic Application of BFD 9 draft-ietf-bfd-generic-02.txt 11 Status of this Memo 13 By submitting this Internet-Draft, each author represents that any 14 applicable patent or other IPR claims of which he or she is aware 15 have been or will be disclosed, and any of which he or she becomes 16 aware will be disclosed, in accordance with Section 6 of BCP 79. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that other 20 groups may also distribute working documents as Internet-Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/1id-abstracts.html 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html 33 Copyright Notice 35 Copyright (C) The Internet Society (2006). All Rights Reserved. 37 Abstract 39 This document describes the generic application of the Bidirectional 40 Forwarding Detection (BFD) protocol. Comments on this draft should 41 be directed to rtg-bfd@ietf.org. 43 Conventions used in this document 45 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 46 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 47 document are to be interpreted as described in RFC-2119 [KEYWORDS]. 49 1. Introduction 51 The Bidirectional Forwarding Detection protocol [BFD] provides a 52 liveness detection mechanism that can be utilized by other network 53 components for which their integral liveness mechanisms are either 54 too slow, inappropriate, or nonexistent. Other drafts have detailed 55 the use of BFD with specific encapsulations ([BFD-1HOP], [BFD-MULTI], 56 [BFD-MPLS]). As the utility of BFD has become understood, there have 57 been calls to specify BFD interactions with a growing list of network 58 functions. Rather than producing a long series of short documents on 59 the application of BFD, it seemed worthwhile to describe the 60 interactions between BFD and other network functions in a broad way. 62 This document describes the generic application of BFD. Specific 63 protocol applications are provided for illustrative purposes. 65 2. Overview 67 The Bidirectional Forwarding Detection (BFD) specification defines a 68 protocol with simple and specific semantics. Its sole purpose is to 69 verify connectivity between a pair of systems, for a particular data 70 protocol across a path (which may be of any technology, length, or 71 OSI layer). The promptness of the detection of a path failure can be 72 controlled by trading off protocol overhead and system load with 73 detection times. 75 BFD is *not* intended to directly provide control protocol liveness 76 information; those protocols have their own means and vagaries. 77 Rather, control protocols can use the services provided by BFD to 78 inform their operation. BFD can be viewed as a service provided by 79 the layer in which it is running. 81 The service interface with BFD is straightforward. The application 82 supplies session parameters (neighbor address, time parameters, 83 protocol options), and BFD provides the session state, of which the 84 most interesting transitions are to and from the Up state. The 85 application is expected to bootstrap the BFD session, as BFD has no 86 discovery mechanism. 88 An implementation SHOULD establish only a single BFD session per data 89 protocol path, regardless of the number of applications that wish to 90 utilize it. There is no additional value in having multiple BFD 91 sessions to the same endpoints. If multiple applications request 92 different session parameters, it is a local issue as to how to 93 resolve the parameter conflicts. BFD in turn will notify all 94 applications bound to a session when a session state change occurs. 96 BFD should be viewed as having an advisory role to the protocol or 97 protocols or other network functions with which it is interacting, 98 which will then use their own mechanisms to effect any state 99 transitions. The interaction is very much at arm's length, which 100 keeps things simple and decoupled. In particular, BFD explicitly 101 does not carry application-specific information, partly for 102 architectural reasons, and partly because BFD may have curious and 103 unpredictable latency characteristics and as such makes a poor 104 transport mechanism. 106 It is important to remember that the interaction between BFD and its 107 client applications has essentially no interoperability issues, 108 because BFD is acting in an advisory nature (similar to hardware 109 signaling the loss of light on a fiber optic circuit, for example) 110 and existing mechanisms in the client applications are used in 111 reaction to BFD events. In fact, BFD may interact with only one of a 112 pair of systems for a particular client application without any ill 113 effect. 115 3. Control Protocol Interactions 117 Very common client applications of BFD are control protocols, such as 118 routing protocols. The object when BFD interacts with a control 119 protocol is to advise the control protocol of the connectivity of the 120 data protocol. In the case of routing protocols, for example, this 121 allows the connectivity failure to trigger the rerouting of traffic 122 around the failed path more quickly than the native detection 123 mechanisms. 125 3.1. Session Establishment 127 BFD sessions are typically bootstrapped by the control protocol, 128 using the mechanism (discovery, configuration) used by the control 129 protocol to find neighbors. In most cases it is not desirable to 130 preclude the control protocol from establishing an adjacency if the 131 BFD session cannot be established (usually because the neighbor does 132 not support BFD.) 134 If the control protocol carries signaling that indicates the 135 willingness of each system to establish a BFD session, the lack of a 136 BFD session MAY be used to block establishment of a control protocol 137 adjacency. 139 If it appears that the neighboring system does not support BFD 140 (because the control protocol adjacency was established but no BFD 141 Control packets have been received from the neighbor) a system MAY 142 increase the interval between transmitted BFD Control packets beyond 143 the minimum specified in [BFD]. This will have negligible impact on 144 BFD session establishment if the neighbor decides to run BFD after 145 all, since BFD Control packets will be sent on an event-driven basis 146 once the first packet is seen from the neighbor. 148 The setting of BFD's various timing parameters and modes are not 149 subject to standardization. Note that all protocols sharing a 150 session will operate using the same parameters. The mechanism for 151 choosing the parameters among those desired by the various protocols 152 are outside the scope of this specification. It is generally useful 153 to choose the parameters resulting in the shortest detection time; a 154 particular client application can always apply hysteresis to the 155 notifications from BFD if it desires longer detection times. 157 3.2. Reaction to BFD Session State Changes 159 The mechanism by which the control protocol reacts to a path failure 160 signaled by BFD depends on the capabilities of the protocol. 162 3.2.1. Control Protocols with a Single Data Protocol 164 A control protocol that is tightly bound to a single failing data 165 protocol SHOULD take action to ensure that data traffic is no longer 166 directed to the failing path. Note that this should not be 167 interpreted as BFD replacing the control protocol liveness mechanism, 168 if any, as the control protocol may rely on mechanisms not verified 169 by BFD (multicast, for instance) so BFD most likely cannot detect all 170 failures that would impact the control protocol. However, a control 171 protocol MAY choose to use BFD session state information to more 172 rapidly detect an impending control protocol failure, particularly if 173 the control protocol operates in band (over the data protocol.) 175 Therefore, when a BFD session transitions from Up to Down, action 176 SHOULD be taken in the control protocol to signal the lack of 177 connectivity for the data protocol over which BFD is running. If the 178 control protocol has an explicit mechanism for announcing path state, 179 a system SHOULD use that mechanism rather than impacting the 180 connectivity of the control protocol, particularly if the control 181 protocol operates out of band from the failed data protocol. 182 However, if such a mechanism is not available, a control protocol 183 timeout SHOULD be emulated for the associated neighbor. 185 3.2.2. Control Protocols with Multiple Data Protocols 187 Slightly different mechanisms are used if the control protocol 188 supports the routing of multiple data protocols, depending on whether 189 the control protocol supports separate topologies for each data 190 protocol. 192 3.2.2.1. Shared Topologies 194 With a shared topology, if one of the data protocols fails (as 195 signaled by the associated BFD session), it is necessary to consider 196 the path to have failed for all data protocols. Otherwise, there is 197 no way for the control protocol to turn away traffic for the failed 198 data protocol (and such traffic would be black holed indefinitely.) 200 Therefore, when a BFD session transitions from Up to Down, action 201 SHOULD be taken in the control protocol to signal the lack of 202 connectivity for all data protocols sharing the topology. If this 203 cannot be signaled otherwise, a control protocol timeout SHOULD be 204 emulated for the associated neighbor. 206 3.2.2.2. Independent Topologies 208 With individual routing topologies for each data protocol, only the 209 failed data protocol needs to be rerouted around the failed path. 211 Therefore, when a BFD session transitions from Up to Down, action 212 SHOULD be taken in the control protocol to signal the lack of 213 connectivity for the data protocol over which BFD is running. 214 Generally this can be done without impacting the connectivity of 215 other data protocols (since otherwise it is very difficult to support 216 separate topologies for multiple data protocols.) 218 3.2.3. Partial BFD Deployments 220 Note that it is possible in some failure scenarios for the network to 221 be in a state such that the control protocol comes up, but the BFD 222 session cannot be established, and, more particularly, data cannot be 223 forwarded. To avoid this situation, it would be beneficial to not 224 allow the control protocol to establish a neighbor adjacency. 225 However, this would preclude the operation of the control protocol in 226 an environment in which not all systems support BFD. 228 Therefore, if a BFD session is not in Up state (possibly because the 229 remote system does not support BFD), it is OPTIONAL to preclude the 230 establishment of a control protocol neighbor adjacency. The choice 231 of whether to do so SHOULD be controlled by means outside the scope 232 of this specification, such as configuration or other mechanisms. If 233 a neighbor adjacency is established for the control protocol but the 234 corresponding BFD session is not in Up state (implying that the 235 neighbor does not support BFD) implementations MAY raise the BFD 236 transmit and receive intervals beyond the minimum of one second 237 specified in [BFD] in order to minimize extraneous traffic. 239 3.3. Interactions with Graceful Restart Mechanisms 241 A number of control protocols support Graceful Restart mechanisms. 242 These mechanisms are designed to allow a control protocol to restart 243 without perturbing network connectivity state (lest it appear that 244 the system and/or all of its links had failed.) They are predicated 245 on the existence of a separate forwarding plane that does not 246 necessarily share fate with the control plane in which the routing 247 protocols operate. In particular, the assumption is that the 248 forwarding plane can continue to function while the protocols restart 249 and sort things out. 251 BFD implementations announce via the Control Plane Independent (C) 252 bit whether or not BFD shares fate with the control plane. This 253 information is used to determine the actions to be taken in 254 conjunction with Graceful Restart. If BFD does not share its fate 255 with the control plane on either system, it can be used to determine 256 whether Graceful Restart in a control protocol is NOT viable (the 257 forwarding plane is not operating.) 259 If the control protocol has a Graceful Restart mechanism, BFD may be 260 used in conjunction with this mechanism. The interaction between BFD 261 and the control protocol depends on the capabilities of the control 262 protocol, and whether or not BFD shares fate with the control plane. 263 In particular, it may be desirable for a BFD session failure to abort 264 the Graceful Restart process and allow the failure to be visible to 265 the network. 267 3.3.1. BFD Fate Independent of the Control Plane 269 If BFD is implemented in the forwarding plane and does not share fate 270 with the control plane on either system (the "C" bit is set in the 271 BFD Control packets in both directions), control protocol restarts 272 should not affect the BFD Session. In this case, a BFD session 273 failure implies that data can no longer be forwarded, so any Graceful 274 Restart in progress at the time of the BFD session failure SHOULD be 275 aborted in order to avoid black holes, and a topology change SHOULD 276 be signaled in the control protocol. 278 3.3.2. BFD Shares Fate with the Control Plane 280 If BFD shares fate with the control plane on either system (the "C" 281 bit is clear in either direction), a BFD session failure cannot be 282 disentangled from other events taking place in the control plane. In 283 many cases, the BFD session will fail as a side effect of the restart 284 taking place. As such, it would be best to avoid aborting any 285 Graceful Restart taking place, if possible (since otherwise BFD and 286 Graceful Restart cannot coexist.) 288 There is some risk in doing so, since a simultaneous failure or 289 restart of the forwarding plane will not be detected, but this is 290 always an issue when BFD shares fate with the control plane. 292 3.3.2.1. Control Protocols with Planned Restart Signaling 294 Some control protocols can signal a planned restart prior to the 295 restart taking place. In this case, if a BFD session failure occurs 296 during the restart, such a planned restart SHOULD NOT be aborted and 297 the session failure SHOULD NOT result in a topology change being 298 signaled in the control protocol. 300 3.3.2.2. Control Protocols Without Planned Restart Signaling 302 Control protocols that cannot signal a planned restart depend on the 303 recently restarted system to signal the Graceful Restart prior to the 304 control protocol adjacency timeout. In most cases, whether the 305 restart is planned or unplanned, it is likely that the BFD session 306 will time out prior to the onset of Graceful Restart, in which case a 307 topology change SHOULD be signaled in the control protocol as 308 specified in section 3.2. 310 However, if the restart is in fact planned, an implementation MAY 311 adjust the BFD session timing parameters prior to restarting in such 312 a way that the detection time in each direction is longer than the 313 restart period of the control protocol, providing the restarting 314 system the same opportunity to enter Graceful Restart as it would 315 have without BFD. The restarting system SHOULD NOT send any BFD 316 Control packets until there is a high likelihood that its neighbors 317 know a Graceful Restart is taking place, as the first BFD Control 318 packet will cause the BFD session to fail. 320 3.4. Interactions with Multiple Control Protocols 322 If multiple control protocols wish to establish BFD sessions with the 323 same remote system for the same data protocol, all MUST share a 324 single BFD session. 326 If hierarchical or dependent layers of control protocols are in use 327 (say, OSPF and IBGP), it may not be useful for more than one of them 328 to interact with BFD. In this example, because IBGP is dependent on 329 OSPF for its routing information, the faster failure detection 330 relayed to IBGP may actually be detrimental. The cost of a peer 331 state transition is high in BGP, and OSPF will naturally heal the 332 path through the network if it were to receive the failure detection. 334 In general, it is best for the protocol at the lowest point in the 335 hierarchy to interact with BFD, and then to use existing interactions 336 between the control protocols to effect changes as necessary. This 337 will provide the fastest possible failure detection and recovery in a 338 network. 340 4. Interactions With Non-Protocol Functions 342 BFD session status may be used to affect other system functions that 343 are not protocol-based (for example, static routes.) If the path to 344 a remote system fails, it may be desirable to avoid passing traffic 345 to that remote system, so the local system may wish to take internal 346 measures to accomplish this (such as withdrawing a static route and 347 withdrawing that route from routing protocols.) 349 Bootstrapping of the BFD session in the non-protocol case is likely 350 to be derived from configuration information. 352 There is no need to exchange endpoints or discriminator values via 353 any mechanism other than configuration (via Operational Support 354 Systems or any other means) as the endpoints must be known and 355 configured by the same means. 357 5. Data Protocols and Demultiplexing 359 BFD is intended to protect a single "data protocol" and is 360 encapsulated within that protocol. A pair of systems may have 361 multiple BFD sessions over the same topology if they support (and are 362 encapsulated by) different protocols. For example, if two systems 363 have IPv4 and IPv6 running across the same link between them, these 364 are considered two separate paths and require two separate BFD 365 sessions. 367 This same technique is used for more fine-grained paths. For 368 example, if multiple differentiated services [DIFFSERV] are being 369 operated on over IPv4, an independent BFD session may be run for each 370 service level. The BFD Control packets must be marked in the same 371 way as the data packets, partly to ensure as much fate sharing as 372 possible between BFD and data traffic, and also to demultiplex the 373 initial packet if the discriminator values have not been exchanged. 375 6. Other Application Issues 377 BFD can provide liveness detection for OAM-like functions in 378 tunneling and pseudowire protocols. Running BFD inside the tunnel is 379 recommended, as it exercises more aspects of the path. One way to 380 accommodate this is to address BFD packets based on the tunnel 381 endpoints, assuming that they are numbered. 383 If a planned outage is to take place on a path over which BFD is run, 384 it is preferable to take down the BFD session by going into ADMIN 385 DOWN state prior to the outage. 387 7. Interoperability Issues 389 The BFD protocol itself is designed so that it will always 390 interoperate at a basic level; asynchronous mode is mandatory and is 391 always available, and other modes and functions are negotiated at run 392 time. Since the service provided by BFD is identical regardless of 393 the variants used, the particular choice of BFD options has no 394 bearing on interoperability. 396 The interaction between BFD and other protocols and control functions 397 is very loosely coupled. The actions taken are based on existing 398 mechanisms in those protocols and functions, so interoperability 399 problems are very unlikely unless BFD is applied in contradictory 400 ways (such as a BFD session failure causing one implementation to go 401 down and another implementation to come up.) In fact, BFD may be 402 advising one system for a particular control function but not the 403 other; the only impact of this would be potentially asymmetric 404 control protocol failure detection. 406 8. Specific Protocol Interactions (Non-Normative) 408 As noted above, there are no interoperability concerns regarding 409 interactions between BFD and control protocols. However, there is 410 enough concern and confusion in this area so that it is worthwhile to 411 provide examples of interactions with specific protocols. 413 Since the interactions do not affect interoperability, they are non- 414 normative. 416 8.1. BFD Interactions with OSPFv2, OSPFv3, and IS-IS 418 The two versions of OSPF ([OSPFv2] and [OSPFv3]), as well as IS-IS 419 [ISIS], all suffer from an architectural limitation, namely that 420 their Hello protocols are limited in the granularity of their failure 421 detection times. In particular, OSPF has a minimum detection time of 422 two seconds, and IS-IS has a minimum detection time of one second. 424 BFD may be used to achieve arbitrarily small detection times for 425 these protocols by supplementing the Hello protocols used in each 426 case. 428 8.1.1. Session Establishment 430 The most obvious choice for triggering BFD session establishment with 431 these protocols would be to use the discovery mechanism inherent in 432 the Hello protocols in OSPF and IS-IS to bootstrap the establishment 433 of the BFD session. Any BFD sessions established to support OSPF and 434 IS-IS across a single IP hop must operate in accordance with 435 [BFD-1HOP]. 437 8.1.2. Reaction to BFD State Changes 439 The basic mechanisms are covered in section 3 above. At this time, 440 OSPFv2 and OSPFv3 carry routing information for a single data 441 protocol (IPv4 and IPv6, respectively) so when it is desired to 442 signal a topology change after a BFD session failure, this should be 443 done by tearing down the corresponding OSPF neighbor. 445 ISIS may be used to support only one data protocol, or multiple data 446 protocols. [ISIS] specifies a common topology for multiple data 447 protocols, but work is underway to support multiple topologies. If 448 multiple data protocols are advertised in the ISIS Hello, and 449 independent topologies are in use, the failing data protocol should 450 no longer be advertised in ISIS Hello packets in order to signal a 451 lack of connectivity for that protocol. Otherwise, a failing BFD 452 session should be signaled by simulating an ISIS adjacency failure. 454 OSPF has a planned restart signaling mechanism, whereas ISIS does 455 not. The appropriate mechanisms outlined in section 3.3 should be 456 used. 458 8.1.3. OSPF Virtual Links 460 If it is desired to use BFD for failure detction of OSPF Virtual 461 Links, the mechanism described in [BFD-MULTI] MUST be used, since 462 OSPF Virtual Links may traverse an arbitrary number of hops. BFD 463 Authentication SHOULD be used and is strongly encouraged. 465 8.2. Interactions with BGP 467 BFD may be useful with EBGP sessions [BGP] in order to more rapidly 468 trigger topology changes in the face of path failure. As noted in 469 section 3.4, it is generally unwise for IBGP sessions to interact 470 with BFD if the underlying IGP is already doing so. 472 EBGP sessions being advised by BFD may establish either a one hop 473 [BFD-1HOP] or a multihop [BFD-MULTIHOP] session, depending on whether 474 the neighbor is immediately adjacent or not. The BFD session should 475 be established to the BGP neighbor (as opposed to any other Next Hop 476 advertised in BGP.) 478 [BGP-GRACE] describes a Graceful Restart mechanism for BGP. If 479 Graceful Restart is not taking place on an EBGP session, and the 480 corresponding BFD session fails, the EBGP session should be torn down 481 in accordance with section 3.2. If Graceful Restart is taking place, 482 the basic procedures in section 3.3 applies. BGP Graceful Restart 483 does not signal planned restarts, so section 3.3.2.2 applies. If 484 Graceful Restart is aborted due to the rules described in section 485 3.3, the "receiving speaker" should act as if the "restart timer" 486 expired (as described in [BGP-GRACE].) 488 8.3. Interactions with RIP 490 The RIP protocol [RIP] is somewhat unique in that, at least as 491 specified, neighbor adjacency state is not stored per se. Rather, 492 installed routes contain a next hop address, which in most cases is 493 the address of the advertising neighbor (but may not be.) 495 In the case of RIP, when the BFD session associated with a neighbor 496 fails, an expiration of the "timeout" timer for each route installed 497 from the neighbor (for which the neighbor is the next hop) should be 498 simulated. 500 Note that if a BFD session fails, and a route is received from that 501 neighbor with a next hop address that is not the address of the 502 neighbor itself, the route will linger until it naturally times out 503 (after 180 seconds. However, if an implementation keeps track of all 504 of the routes received from each neighbor, all of the routes from the 505 neighbor corresponding to the failed BFD session should be timed out, 506 regardless of the next hop specified therein, and thereby avoiding 507 the lingering route problem. 509 Normative References 511 [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection", 512 draft-ietf-bfd-base-05.txt, June, 2006. 514 [BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single 515 Hop)", draft-ietf-bfd-v4v6-1hop-05.txt, June, 2006. 517 [BFD-MPLS] Aggarwal, R., and Kompella, K., "BFD for MPLS LSPs", 518 draft-ietf-bfd-mpls-03.txt, June, 2006. 520 [BFD-MULTI] Katz, D., and Ward, D., "BFD for Multihop Paths", draft- 521 ietf-bfd-multihop-04.txt, June, 2006. 523 [BGP] Rekhter, Y., Li, T. et al, "A Border Gateway Protocol 4 524 (BGP-4)", RFC 4271, January, 2006. 526 [BGP-GRACE] Sangli, S., Rekhter, Y., et al, "Graceful Restart 527 Mechanism for BGP", draft-ietf-idr-restart-12.txt, June, 2006. 529 [DIFFSERV] Nichols, K. et al, "Definition of the Differentiated 530 Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 531 2474, December, 1998. 533 [ISIS] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual 534 environments", RFC 1195, December 1990. 536 [ISIS-GRACE] Shand, M., and Ginsberg, L., "Restart signaling for IS- 537 IS", RFC 3847, July 2004. 539 [KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate 540 Requirement Levels", RFC 2119, March 1997. 542 [OSPFv2] Moy, J., "OSPF Version 2", RFC 2328, April 1998. 544 [OSPFv3] Coltun, R., et al, "OSPF for IPv6", RFC 2740, December 1999. 546 [OSPF-GRACE] Moy, J., et al, "Graceful OSPF Restart", RFC 3623, 547 November 2003. 549 [RIP] Malkin, G., "RIP Version 2", RFC 2453, November, 1998. 551 Security Considerations 553 This specification does not raise any additional security issues 554 beyond those of the specifications referred to in the list of 555 normative references. 557 IANA Considerations 559 This document has no actions for IANA. 561 Authors' Addresses 563 Dave Katz 564 Juniper Networks 565 1194 N. Mathilda Ave. 566 Sunnyvale, California 94089-1206 USA 567 Phone: +1-408-745-2000 568 Email: dkatz@juniper.net 570 Dave Ward 571 Cisco Systems 572 170 W. Tasman Dr. 573 San Jose, CA 95134 USA 574 Phone: +1-408-526-4000 575 Email: dward@cisco.com 577 Changes from the previous draft 579 Control protocol-specific interactions were moved from [BFD-1HOP] to 580 this document, and brief mentions of BGP and RIP were added. 582 IPR Disclaimer 584 The IETF takes no position regarding the validity or scope of any 585 Intellectual Property Rights or other rights that might be claimed to 586 pertain to the implementation or use of the technology described in 587 this document or the extent to which any license under such rights 588 might or might not be available; nor does it represent that it has 589 made any independent effort to identify any such rights. 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