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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). 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 Intended status: Proposed Standard D. Ward 5 Cisco 7 Expires: January, 2013 June 1, 2012 9 BFD for Multipoint Networks 10 draft-ietf-bfd-multipoint-01.txt 12 Status of this Memo 14 This Internet-Draft is submitted to IETF in full conformance with the 15 provisions of BCP 78 and BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that other 19 groups may also distribute working documents as Internet-Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/1id-abstracts.html 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html 32 Copyright Notice 34 Copyright (c) 2011 IETF Trust and the persons identified as the 35 document authors. All rights reserved. 37 This document is subject to BCP 78 and the IETF Trust's Legal 38 Provisions Relating to IETF Documents 39 (http://trustee.ietf.org/license-info) in effect on the date of 40 publication of this document. Please review these documents 41 carefully, as they describe your rights and restrictions with respect 42 to this document. Code Components extracted from this document must 43 include Simplified BSD License text as described in Section 4.e of 44 the Trust Legal Provisions and are provided without warranty as 45 described in the Simplified BSD License. 47 Abstract 49 This document describes extensions to the Bidirectional Forwarding 50 Detection (BFD) protocol for its use in multipoint and multicast 51 networks. Comments on this draft should be directed to 52 rtg-bfd@ietf.org. 54 Conventions used in this document 56 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 57 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 58 document are to be interpreted as described in RFC-2119 [KEYWORDS]. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 64 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 65 4. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 6 66 4.1 Multipoint BFD Control Packets . . . . . . . . . . . . . . 6 67 4.2 Session Model . . . . . . . . . . . . . . . . . . . . . . 6 68 4.3 Session Failure Semantics . . . . . . . . . . . . . . . . 7 69 4.4 State Variables . . . . . . . . . . . . . . . . . . . . . 7 70 4.4.1 New State Variables . . . . . . . . . . . . . . . . . 7 71 4.4.2 State Variable Initialization and Maintenance . . . . 9 72 4.5 Controlling Multipoint BFD Options . . . . . . . . . . . 10 73 4.6 State Machine . . . . . . . . . . . . . . . . .. . . . . 10 74 4.7 Session Establishment . . . . . . . . . . . . . . . . . 11 75 4.8 Discriminators and Packet Demultiplexing . . . . . . . . 12 76 4.9 Controlling Tail Packet Transmission . . . . . . . . . . 12 77 4.10 Bringing Up and Shutting Down Multipoint BFD Service . 13 78 4.11 Soliciting the Tails . . . . . . . . . . . . . . . . . 14 79 4.12 Verifying Connectivity to Specific Tails . . . . . . . 14 80 4.13 Timer Manipulation . . . . . . . . . . . . . . . . . . 15 81 4.14 Detection Times . . . . . . . . . . . . . . . . . . . . 15 82 4.15 State Maintenance for Down/AdminDown Sessions . . . . . 16 83 4.15.1 MultipointHead Sessions . . . . . . . . . . . . . 16 84 4.15.2 MultipointTail Sessions . . . . . . . . . . . . . 16 85 4.15.3 MultipointClient Sessions . . . . . . . . . . . . 17 86 4.16 Base Specification Text Replacement . . . . . . . . . . 17 87 4.16.1 Reception of BFD Control Packets . . . . . . . . 17 88 4.16.2 Demultiplexing of BFD Control Packets . . . . . . 20 89 4.16.3 Transmitting BFD Control Packets . . . . . . . . 21 90 5. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . 25 91 6. Operational Scenarios . . . . . . . . . . . . . . . . . . . 25 92 6.1 No Head Notification . . . . . . . . . . . . . . . . . . 26 93 6.2 Unreliable Head Notification . . . . . . . . . . . . . . 26 94 6.3 Semi-reliable Head Notification and Tail Solicitation . 26 95 6.4 Reliable Head Notification . . . . . . . . . . . . . . . 27 96 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 28 97 8. Security Considerations . . . . . . . . . . . . . . . . . . 28 98 9. Normative References . . . . . . . . . . . . . . . . . . . . 28 99 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 28 100 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 28 101 Changes from the previous draft . . . . . . . . . . . . . . . . 29 103 1. Introduction 105 The Bidirectional Forwarding Detection protocol [BFD] specifies a 106 method for verifying unicast connectivity between a pair of systems. 107 This document defines a method for using BFD to provide verification 108 of multipoint or multicast connectivity between a multipoint sender 109 (the "head") and a set of one or more multipoint receivers (the 110 "tails"). 112 As multipoint transmissions are inherently unidirectional, this 113 mechanism purports only to verify this unidirectional connectivity. 114 Although this seems in conflict with the "Bidirectional" in BFD, it 115 is a natural fit for that protocol. 117 This application of BFD allows for the tails to detect a lack of 118 connectivity from the head. As an option, the tail may unreliably 119 notify the head of the lack of multipoint connectivity. As a further 120 option, this notification can be made reliable. Notification to the 121 head can be enabled for all tails, or for only a subset of the tails. 123 Multipoint BFD verifies only the head-to-tail connectivity over the 124 multipoint path. Although it may use unicast paths in both 125 directions, Multipoint BFD does not verify those paths (and in fact 126 it is preferable if unicast paths share as little fate with the 127 multipoint path as is feasible.) 129 Virtually all options and timing parameters are controlled by the 130 head. This is particularly important if head notifications are 131 enabled, since there are obvious scaling concerns in that case. 133 Throughout this document, the term "multipoint" is defined as a 134 mechanism by which one or more systems receive packets sent by a 135 single sender. This specifically includes such things as IP 136 multicast and point-to-multipoint MPLS. 138 This document effectively modifies and adds to the base BFD 139 specification. It is the intention of the authors to fold these 140 extensions into the base specification at the appropriate time. 142 2. Goals 144 The primary goal of this mechanism is to allow tails to rapidly 145 detect the fact that multipoint connectivity from the head has 146 failed. An optional goal is for the head to reasonably rapidly have 147 knowledge of tails that have lost connectivity from the head. 149 Since scaling is a primary concern (particularly state implosion 150 toward the head), it is a goal that the head be in control of all 151 timing aspects of the mechanism, and that BFD packets from the tails 152 to the head not be synchronized. 154 Another goal is for the mechanism to work on any multicast or 155 multipoint medium. 157 A further goal is to support multiple, overlapping multipoint paths, 158 as well as multipoint paths with multiple heads, and to allow point- 159 to-point BFD sessions to operate simultaneously among the systems 160 participating in Multipoint BFD. 162 A final goal is to integrate multipoint operation into the base 163 specification in such a way as to make it relatively easy to support 164 both multipoint and point-to-point operation in a single 165 implementation. 167 It is a non-goal for this protocol to verify point-to-point 168 connectivity between the head and any tails. This can be done 169 independently (and with no penalty in protocol overhead) by using 170 point-to-point BFD. 172 3. Overview 174 The heart of this protocol is the periodic transmission of BFD 175 Control packets along a multipoint path, from the head to all tails 176 on the tree. The contents of the BFD packets provide the means for 177 the tails to calculate the detection time for path failure. If no 178 BFD Control packets are received by a tail for a detection time, the 179 tail declares the path to have failed. For some applications this is 180 the only mechanism necessary; the head can remain ignorant of the 181 tails. In this mode, the tails never send any BFD traffic to the 182 head. 184 If the head wishes to be alerted to the tails' connectivity (or lack 185 thereof), there are a number of options. First, if all that is 186 needed is an unreliable failure notification, the head can direct the 187 tails to transmit unicast BFD Control packets back to the head when 188 the path fails. 190 If the head wishes to know the identity of the tails on the 191 multipoint path, it may solicit membership by sending a multipoint 192 BFD Control packet with the Poll (P) bit set, which will induce the 193 tails to return a unicast BFD Control packet with the Final (F) bit 194 set. The head can then create BFD session state for each of the 195 tails that have multipoint connectivity. If the head sends such a 196 packet on occasion, it can keep track of which tails answer, thus 197 providing a somewhat reliable mechanism for detecting which tails 198 fail to respond (implying a loss of multipoint connectivity.) 200 If the head wishes a reliable indication of the tails' connectivity, 201 it may do all of the above, but if it detects that a tail did not 202 answer the previous multipoint poll, it may initiate a Demand mode 203 Poll Sequence as a unicast to the tail. This covers the case where 204 either the multipoint poll or the single reply thereto is lost in 205 transit. If desired, the head may Poll one or more tails proactively 206 to track the tails' connectivity. 208 If some tails are more equal than others, in the sense that the head 209 needs to detect the lack of multipoint connectivity to a subset of 210 tails at a different rate, the head may transmit unicast BFD Polls to 211 that subset of tails. In this case, the timing may be independent on 212 a tail-by-tail basis. 214 Individual tails may be configured so that they never send BFD 215 control packets to the head, even when the head wishes notification 216 of path failure from the tail. Such tails will never be known to the 217 head, but will still be able to detect multipoint path failures from 218 the head. 220 Although this document describes a single head and a set of tails 221 spanned by a single multipoint path, the protocol is capable of 222 supporting (and discriminating between) more than one multipoint path 223 at both heads and tails. Furthermore, the same head and tail may 224 share multiple multipoint paths, and a multipoint path may have 225 multiple heads. 227 4. Protocol Details 229 This section describes the operation of Multipoint BFD in detail. 231 4.1. Multipoint BFD Control Packets 233 Multipoint BFD Control packets (packets sent by the head over a 234 multipoint path) are explicitly marked as such, via the setting of 235 the M bit (added to the latest revision of the BFD base 236 specification. This means that Multipoint BFD does not depend on the 237 recipient of a packet to know whether the packet was received over a 238 multipoint path. This can be useful in scenarios where this 239 information may not be available to the recipient. 241 4.2. Session Model 243 Multipoint BFD is modeled as a set of sessions of different types. 244 The elements of procedure differ slightly for each type. 246 Point-to-point sessions, as described in [BFD], are of type 247 PointToPoint. 249 The head has a session of type MultipointHead that is bound to a 250 multipoint path. Multipoint BFD Control packets are sent by this 251 session over the multipoint path, and no BFD Control packets are 252 received by it. 254 If the head is keeping track of some or all of the tails, it has a 255 session of type MultipointClient per tail that it cares about. All 256 of the MultipointClient sessions for tails on a particular particular 257 multipoint path are grouped with the MultipointHead session to which 258 the clients are listening. A BFD Poll Sequence may be sent over such 259 a session to a tail if the head wishes to verify connectivity. These 260 sessions receive any BFD Control packets sent by the tails, and never 261 transmit periodic BFD Control packets other than Poll Sequences 262 (since periodic transmission is always done by the MultipointHead 263 session.) 264 Each tail has a session of type MultipointTail associated with a 265 multipoint path. These sessions receive BFD Control packets from the 266 head, both as multipoint packets (the MultipointHead session) and as 267 unicast packets (the MultipointClient session, if it exists.) Any 268 BFD Control packets sent to the head are sent over this session. 270 4.3. Session Failure Semantics 272 The semantics of session failure are subtle enough to warrant further 273 explanation. 275 MultipointHead sessions cannot fail (since they are controlled 276 administratively.) 278 If a MultipointTail session fails, it means that the tail definitely 279 has lost contact with the head (or the head has been administratively 280 disabled) and the tail should take appropriate action. 282 If a MultipointClient session receives a BFD Control packet from the 283 tail with state Down or AdminDown, the head reliably knows that the 284 tail has lost multipoint connectivity. If the Detection Time expires 285 on a MultipointClient session, it is ambiguous as to whether the 286 multipoint connectivity failed or whether there was a unicast path 287 problem in one direction or the other, so the head does not reliably 288 know the tail state. 290 4.4. State Variables 292 Multipoint BFD introduces some new state variables, and modifies the 293 usage of a few existing ones. 295 4.4.1. New State Variables 297 A number of state variables are added to the base specification in 298 support of Multipoint BFD. 300 bfd.SessionType 302 The type of this session. Allowable values are: 304 PointToPoint: Classic point-to-point BFD. 306 MultipointHead: A session on the head responsible for the 307 periodic transmission of multipoint BFD Control packets 308 along the multipoint path. 310 MultipointClient: A session on the head that tracks the 311 state of an individual tail, when desirable. 313 MultipointTail: A multipoint session on a tail. 315 This variable MUST be initialized to the appropriate type when 316 the session is created, according to the rules in section 4.16. 318 bfd.SilentTail 320 If 1, a tail will never transmit any BFD Control packets to the 321 head under any circumstances. If 0, a tail may send packets to 322 the head according to other parts of this specification. This 323 allows tails to be provisioned to always be silent, even when 324 the head is soliciting traffic from the tails. This can be 325 useful, for example, in deployments of a large number of tails 326 when the head wishes to track the state of a subset of them. 327 This variable MUST be initialized based on configuration. 329 This variable is only pertinent when bfd.SessionType is 330 MultipointTail. 332 bfd.ReportTailDown 334 Set to 1 if the head wishes tails to notify the head, via 335 periodic BFD Control packets, when they see the BFD session 336 fail. If 0, the tail will never send periodic BFD Control 337 packets, and the head will not be notified of session failures 338 by the tails. This variable MUST be initialized based on 339 configuration. 341 This variable is only pertinent when bfd.SessionType is 342 MultipointHead or MultipointClient. 344 bfd.UnicastRcvd 346 Set to 1 if a tail receives a unicast BFD Control packet from 347 the head. This variable MUST be set to zero if the session 348 transitions from Up state to some other state. 350 This variable MUST be initialized to zero. 352 This variable is only pertinent when Bfd.SessionType is 353 MultipointTail. 355 4.4.2. State Variable Initialization and Maintenance 357 Some state variables defined in section 6.8.1 of the BFD base 358 specification need to be initialized or manipulated differently 359 depending on the session type. 361 bfd.LocalDiscr 363 For session type MultipointClient, this variable MUST always 364 match the value of bfd.LocalDiscr in the associated 365 MultipointHead session. 367 bfd.DesiredMinTxInterval 369 For session type MultipointClient, this variable MUST always 370 match the value of bfd.DesiredMinTxInterval in the associated 371 MultipointHead session. 373 bfd.RequiredMinRxInterval 375 This variable MUST always be 0 for session type MultipointHead 376 if bfd.ReportTailDown is 0. 378 It should be noted that for sessions of type MultipointTail, 379 this variable only affects the rate of unicast Polls sent by 380 the head; the rate of multipoint packets is necessarily 381 unaffected by it. 383 bfd.DemandMode 385 This variable MUST be initialized to 1 for session types 386 MultipointHead and MultipointClient, and MUST be initialized to 387 0 for session type MultipointTail. 389 bfd.DetectMult 391 For session type MultipointClient, this variable MUST always 392 match the value of bfd.DetectMult in the associated 393 MultipointHead session. 395 4.5. Controlling Multipoint BFD Options 397 The state variables defined above are used to choose which 398 operational options are active. 400 The most basic form of operation, in which BFD Control packets flow 401 only from the head and no tracking is desired of tail state at the 402 head, is accomplished by setting bfd.ReportTailDown to 0 in the 403 MultipointHead session. 405 If the head wishes to know the identity of the tails, it sends 406 multipoint Polls as needed. Previously known tails that don't 407 respond to the Polls will be detected. 409 If the head wishes to be notified by the tails when they lose 410 connectivity, it sets bfd.ReportTailDown to 1 in either the 411 MultipointHead session (if such notification is desired from all 412 tails) or in the MultipointClient session (if notification is desired 413 from a particular tail.) Note that the setting of this variable in a 414 MultipointClient session for a particular tail overrides the setting 415 in the MultipointHead session. 417 If the head wishes to verify the state of a tail on an ongoing basis, 418 it sends a Poll Sequence from the MultipointClient session associated 419 with that tail as needed. 421 If the head wants to more quickly be alerted to a session failure 422 from a particular tail, it sends a BFD Control packet from the 423 MultipointClient session associated with that tail. This has the 424 effect of eliminating the initial delay that the tail would otherwise 425 insert prior to transmission of the packet. 427 If a tail wishes to operate silently (sending no BFD Control packets 428 to the head) it sets bfd.SilentTail to 1 in the MultipointTail 429 session. This allows a tail to be silent independent of the settings 430 on the head. 432 4.6. State Machine 434 The BFD state machine works slightly differently in the multipoint 435 application. In particular, since there is a many-to-one mapping, 436 three-way handshakes for session establishment and teardown are 437 neither possible nor appropriate. As such there is no Init state. 439 The following diagram provides an overview of the state machine for 440 session types MultipointClient and MultipointTail. The notation on 441 each arc represents the state of the remote system (as received in 442 the State field in the BFD Control packet) or indicates the 443 expiration of the Detection Timer. 445 DOWN, ADMIN DOWN, 446 +------+ TIMER +------+ 447 +----| |<---------------------| |----+ 448 DOWN,| | DOWN | | UP | |UP 449 ADMIN DOWN,+--->| |--------------------->| |<---+ 450 TIMER +------+ UP +------+ 452 Sessions of type MultipointHead never receive packets and have no 453 Detection Timer, and as such all state transitions are 454 administratively driven. 456 4.7. Session Establishment 458 Unlike Point-to-point BFD, Multipoint BFD provides a form of 459 discovery mechanism for tails to discover the head, and vice versa. 460 The minimum amount of a priori information required both on the head 461 and tails is the binding to the multipoint path over which BFD is 462 running. The head transmits Multipoint BFD packets on that tree, and 463 the tails listen for BFD packets on that tree. All other information 464 MAY be determined dynamically. 466 A session of type MultipointHead is created for each multipoint path 467 over which the head wishes to run BFD. This session runs in the 468 Active role. Except when terminating BFD service, this session is 469 always in state Up and always operates in Demand mode. No received 470 packets are ever demultiplexed to the MultipointHead session. In 471 this sense it is a degenerate form of a session. 473 Sessions on the tail MAY be established dynamically, based on the 474 receipt of a Multipoint BFD Control packet from the head, and are of 475 type MultipointTail. Tail sessions always take the Passive role. 477 If BFD Control packets are received at the head, they are 478 demultiplexed to sessions of type MultipointClient, which represent 479 the set of tails that the head is interested in tracking. These 480 sessions will typically also be established dynamically based on the 481 receipt of BFD Control packets. The head has broad latitude in 482 choosing which tails to track, if any, without affecting the basic 483 operation of the protocol. The head directly controls whether or not 484 tails are allowed to send BFD Control packets back to the head. 486 4.8. Discriminators and Packet Demultiplexing 488 The use of Discriminators is somewhat different in Multipoint BFD 489 than in Point-to-point BFD. 491 The head sends Multipoint BFD Control packets over the MultipointHead 492 session with My Discr set to a value bound to the multipoint path, 493 and with Your Discr set to zero. The tails MUST demultiplex these 494 packets based on a combination of the source address and My Discr, 495 which together uniquely identify the head and the multipoint path. 497 When the tails send BFD Control packets to the head from the 498 MultipointTail session, the contents of Your Discr (the discriminator 499 received from the head) will not be sufficient for the head to 500 demultiplex the packet, since the same value will be received from 501 all tails on the multicast tree. In this case, the head MUST 502 demultiplex packets based on the source address and the value of Your 503 Discr, which together uniquely identify the tail and the multipoint 504 path. 506 When the head sends unicast BFD Control packets to a tail from a 507 MultipointClient session, the value of Your Discr will be valid, and 508 the tail MUST demultiplex the packet based solely on Your Discr. 510 Note that, unlike PointToPoint sessions, the discriminator values on 511 all multipoint session types MUST NOT be changed during the life of a 512 session. This is a side effect of the more complex demultiplexing 513 scheme. 515 4.9. Controlling Tail Packet Transmission 517 As the fan-in from the tails to the head may be very large, it is 518 critical that the flow of BFD Control packets from the tails is 519 controlled. 521 The head always operates in Demand mode. This means that no tail 522 will send an asynchronous BFD Control packet as long as the session 523 is Up. 525 The value of Required Min Rx Interval received by a tail in a unicast 526 BFD Control packet, if any, always takes precedence over the value 527 received in Multipoint BFD Control packets. This allows the packet 528 rate from individual tails to be controlled separately as desired by 529 sending a BFD Control packet from the corresponding MultipointClient 530 session. This also eliminates the random delay prior to transmission 531 from the tail that would otherwise be inserted, reducing the latency 532 of reporting a failure to the head. 534 If the head wishes to suppress traffic from the tails when they 535 detect a session failure, it MAY set bfd.RequiredMinRxInterval to 536 zero, which is a reserved value that indicates that the sender wishes 537 to receive no periodic traffic. This can be set in the 538 MultipointHead session (suppressing traffic from all tails) or it can 539 be set in a MultipointClient session (suppressing traffic from only a 540 single tail.) 542 Any tail may be provisioned to never send *any* BFD Control packets 543 to the head by setting bfd.SilentTail to 1. This provides a 544 mechanism by which only a subset of tails report their session status 545 to the head. 547 4.10. Bringing Up and Shutting Down Multipoint BFD Service 549 Because there is no three-way handshake in Multipoint BFD, a newly 550 started head (that does not have any previous state information 551 available) SHOULD start with bfd.SessionState set to Down and with 552 bfd.RequiredMinRxInterval set to zero in the MultipointHead session. 553 The session SHOULD remain in this state for a time equal to 554 (bfd.DesiredMinTxInterval * bfd.DetectMult). This will ensure that 555 all MultipointTail sessions are reset (so long as the restarted head 556 is using the same or larger value of bfd.DesiredMinTxInterval than it 557 did previously.) 559 Multipoint BFD service is brought up by administratively setting 560 bfd.SessionState to Up in the MultipointHead session. 562 A head may wish to shut down its BFD service in a controlled fashion. 563 This is desirable because the tails need not wait a detection time 564 prior to declaring the multipoint session to be down (and taking 565 whatever action is necessary in that case.) 567 To shut down a multipoint session a head MUST administratively set 568 bfd.SessionState in the MultipointHead session to either Down or 569 AdminDown and SHOULD set bfd.RequiredMinRxInterval to zero (to keep 570 the tails from sending any BFD Control packets back.) The session 571 SHOULD send BFD Control packets in this state for a period equal to 572 (bfd.DesiredMinTxInterval * bfd.DetectMult). The tail SHOULD destroy 573 all MultipointClient sessions associated with the MultipointHead 574 session. 576 The semantic difference between Down and AdminDown state is for 577 further discussion. 579 4.11. Soliciting the Tails 581 If the head wishes to know the identities of the tails, the 582 MultipointHead session MAY send a BFD Control packet as specified in 583 section 4.16.3, with the Poll (P) bit set to 1. This will cause all 584 of the tails to reply with a unicast BFD Control Packet, randomized 585 across one packet interval. 587 The decision as to when to send a multipoint Poll is outside the 588 scope of this specification. However, it must never be sent more 589 often than the regular multipoint BFD Control packet. Since the tail 590 will treat a multipoint Poll like any other multipoint BFD Control 591 packet, Polls may be sent in lieu of non-Poll packets. 593 Soliciting the tails also starts the Detection Timer for each 594 associated MultipointClient session, which will cause those sessions 595 to time out if the associated tails do not respond. 597 Note that for this mechanism to work properly, the Detection Time 598 (which is equal to bfd.DesiredMinTxInterval) MUST be greater than the 599 round trip time of BFD Control packets from the head to the tail (via 600 the multipoint path) and back (via a unicast path.) See section 4.14 601 for more details. 603 4.12. Verifying Connectivity to Specific Tails 605 If the head wishes to verify connectivity to a specific tail, the 606 corresponding MultipointClient session MAY send a BFD Poll Sequence 607 to said tail. This might be done in reaction to the expiration of 608 the Detection Timer (the tail didn't respond to a multipoint Poll), 609 or it might be done on a proactive basis. 611 The interval between transmitted packets in the Poll Sequence MUST be 612 calculated as specified in the base specification (the greater of 613 bfd.DesiredMinTxInterval and bfd.RemoteMinRxInterval.) 615 The value transmitted in Required Min RX Interval will be used by the 616 tail (rather than the value received in any multipoint packet) when 617 it transmits BFD Control packets to the head notifying it of a 618 session failure, and the transmitted packets will not be delayed. 619 This value can potentially be set much lower than in the multipoint 620 case, in order to speed up notification to the head, since the value 621 will be used only by the single tail. This value (and the lack of 622 delay) are "sticky", in that once the tail receives it, it will 623 continue to use it indefinitely. Therefore, if the head no longer 624 wishes to single out the tail, it SHOULD reset the timer to the 625 default by sending a Poll Sequence with the same value of Required 626 Min Rx Interval as is carried in the multipoint packets, or it MAY 627 reset the tail session by sending a Poll Sequence with state 628 AdminDown (after the completion of which the session will come back 629 up.) 631 Note that a failure of the head to receive a response to a Poll 632 Sequence does not necessarily mean that the tail has lost multipoint 633 connectivity, though a reply to a Poll Sequence does reliably 634 indicate connectivity or lack thereof (by virtue of the tail's state 635 not being Up in the BFD Control packet.) 637 4.13. Timer Manipulation 639 Because of the one-to-many mapping, a session of type MultipointHead 640 SHOULD NOT initiate a Poll Sequence in conjunction with timer value 641 changes. As such, such a session cannot wait for a Final before 642 increasing the transmit interval; such a session SHOULD send 643 bfd.DetectMult packets at the old transmit interval before using the 644 higher value in order to avoid false detection timeouts at the tails. 646 Since MultipointHead sessions do not calculate detection times, the 647 value of bfd.RequiredMinRxInterval may be changed at any time. 649 4.14. Detection Times 651 Multipoint BFD is inherently asymmetric. As such, each session type 652 has a different approach to detection times. 654 Since the MultipointHead session never receives packets, it does not 655 calculate a detection time. 657 MultipointClient sessions at the head are always in Demand mode, and 658 as such only care about detection time in two cases. First, if a 659 Poll Sequence is being sent on a MultipointClient session, the 660 detection time on this session is calculated according to the base 661 specification, that is, the transmission interval multiplied by 662 bfd.DetectMult. Second, when a multipoint Poll is sent to solicit 663 tail replies, the detection time on all associated MultipointClient 664 sessions that aren't currently sending Poll Sequences is set to a 665 value greater than or equal to bfd.RequiredMinRxInterval (one packet 666 time.) This value can be made arbitrarily large in order to ensure 667 that the detection time is greater than the BFD round trip time 668 between the head and the tail with no ill effects, other than 669 delaying the detection of unresponsive tails. Note that a detection 670 time expiration on a MultipointClient session at the head, while 671 indicating a BFD session failure, cannot be construed to mean that 672 the tail is not hearing multipoint packets from the head. 674 MultipointTail sessions cannot influence the transmission rate of the 675 MultipointHead session using the Required Min Rx Interval field 676 because of its one-to-many nature. As such, the Detection Time 677 calculation for a MultipointTail session does not use 678 bfd.RequiredMinRxInterval in the calculation. The detection time is 679 calculated as the product of the last received values of Desired Min 680 TX Interval and Detect Mult. 682 The value of bfd.DetectMult may be changed at any time on any session 683 type. 685 4.15. State Maintenance for Down/AdminDown Sessions 687 The length of time session state is kept after the session goes down 688 determines how long the session will continue to send BFD Control 689 packets (since no packets can be sent after the session is 690 destroyed.) 692 4.15.1. MultipointHead Sessions 694 When a MultipointHead session transitions to states Down or 695 AdminDown, the state SHOULD be maintained for a period equal to 696 (bfd.DesiredMinTxInterval * bfd.DetectMult) to ensure that the tails 697 more quickly detect the session going down (by continuing to transmit 698 BFD Control packets with the new state.) 700 4.15.2. MultipointTail Sessions 702 If bfd.SilentTail is 1, or bfd.RemoteMinRxInterval is zero, 703 MultipointTail sessions MAY be destroyed immediately upon leaving Up 704 state, since they will transmit no further packets. 706 Otherwise, MultipointTail sessions MUST be maintained as long as BFD 707 Control packets are being received by it (which by definition will 708 indicate that the head is not Up.) 710 MultipointTail sessions MUST be maintained after a Detection Time 711 expiration for at least the longer of an additional Detection Time 712 and the transmission of the first (delayed) BFD Control packet to the 713 head. The state MAY be maintained longer than this, but the session 714 MUST NOT transmit periodic BFD Control packets for a period longer 715 than the negotiated transmit interval multiplied by bfd.DetectMult; 716 after this time either the session MUST be destroyed or 717 bfd.RemoteMinRxInterval MUST be set to zero to suppress packet 718 transmission. 720 4.15.3. MultipointClient Sessions 722 If the MultipointHead session is going down (which only happens 723 administratively), all associated MultipointClient sessions SHOULD be 724 destroyed as they are superfluous. 726 If a MultipointClient session goes down due to the receipt of an 727 unsolicited BFD Control packet from the tail with state Down or 728 AdminDown (not in response to a Poll), and tail connectivity 729 verification is not being done, the session MAY be destroyed. If 730 verification is desired, the session SHOULD send a Poll Sequence and 731 the session SHOULD be maintained. 733 If the tail replies to a Poll Sequence with state Down or AdminDown, 734 it means that the tail session is definitely down. In this case, the 735 session MAY be destroyed. 737 If the Detection Time expires on a MultipointClient session (meaning 738 that the tail did not reply to a Poll Sequence) the session MAY be 739 destroyed. 741 4.16. Base Specification Text Replacement 743 The following sections are meant to replace the corresponding 744 sections in the base specification. 746 4.16.1. Reception of BFD Control Packets 748 The following procedure replaces section 6.8.6 of [BFD]. 750 When a BFD Control packet is received, the following procedure MUST 751 be followed, in the order specified. If the packet is discarded 752 according to these rules, processing of the packet MUST cease at that 753 point. 755 If the version number is not correct (1), the packet MUST be 756 discarded. 758 If the Length field is less than the minimum correct value (24 if 759 the A bit is clear, or 26 if the A bit is set), the packet MUST be 760 discarded. 762 If the Length field is greater than the payload of the 763 encapsulating protocol, the packet MUST be discarded. 765 If the Detect Mult field is zero, the packet MUST be discarded. 767 If the My Discriminator field is zero, the packet MUST be 768 discarded. 770 Demultiplex the packet to a session according to section 4.16.2 771 below. The result is either a session of the proper type, or the 772 packet is discarded (and packet processing MUST cease.) 774 If the A bit is set and no authentication is in use (bfd.AuthType 775 is zero), the packet MUST be discarded. 777 If the A bit is clear and authentication is in use (bfd.AuthType 778 is nonzero), the packet MUST be discarded. 780 If the A bit is set, the packet MUST be authenticated under the 781 rules of section 6.7, based on the authentication type in use 782 (bfd.AuthType.) This may cause the packet to be discarded. 784 Set bfd.RemoteDiscr to the value of My Discriminator. 786 Set bfd.RemoteState to the value of the State (Sta) field. 788 Set bfd.RemoteDemandMode to the value of the Demand (D) bit. 790 If bfd.SessionType is MultipointTail 792 If bfd.UnicastRcvd is 0 or the M bit is clear, set 793 bfd.RemoteMinRxInterval to the value of Required Min RX 794 Interval. 796 If the M bit is clear, set bfd.UnicastRcvd to 1. 798 Else (not MultipointTail) 800 Set bfd.RemoteMinRxInterval to the value of Required Min RX 801 Interval. 803 If the Required Min Echo RX Interval field is zero, the 804 transmission of Echo packets, if any, MUST cease. 806 If a Poll Sequence is being transmitted by the local system and 807 the Final (F) bit in the received packet is set, the Poll Sequence 808 MUST be terminated. 810 If bfd.SessionType is PointToPoint or MultipointClient, update the 811 transmit interval as described in [BFD] section 6.8.2. 813 If bfd.SessionType is PointToPoint, update the Detection Time as 814 described in [BFD] section 6.8.4. Otherwise, update the Detection 815 Time as described in section 4.14 above. 817 If bfd.SessionState is AdminDown 818 Discard the packet 820 If received state is AdminDown 821 If bfd.SessionState is not Down 822 Set bfd.LocalDiag to 3 (Neighbor signaled session down) 823 Set bfd.SessionState to Down 825 Else 827 If bfd.SessionState is Down 828 If bfd.SessionType is PointToPoint 829 If received State is Down 830 Set bfd.SessionState to Init 831 Else if received State is Init 832 Set bfd.SessionState to Up 834 Else (bfd.SessionType is not PointToPoint) 835 If received State is Up 836 Set bfd.SessionState to Up 838 Else if bfd.SessionState is Init 839 If received State is Init or Up 840 Set bfd.SessionState to Up 842 Else (bfd.SessionState is Up) 843 If received State is Down 844 Set bfd.LocalDiag to 3 (Neighbor signaled session down) 845 Set bfd.SessionState to Down 847 Check to see if Demand mode should become active or not (see [BFD] 848 section 6.6). 850 If bfd.RemoteDemandMode is 1, bfd.SessionState is Up, and 851 bfd.RemoteSessionState is Up, Demand mode is active on the remote 852 system and the local system MUST cease the periodic transmission 853 of BFD Control packets (see section 4.16.3.) 855 If bfd.RemoteDemandMode is 0, or bfd.SessionState is not Up, or 856 bfd.RemoteSessionState is not Up, Demand mode is not active on the 857 remote system and the local system MUST send periodic BFD Control 858 packets (see section 4.16.3.) 860 If the Poll (P) bit is set, and bfd.SilentTail is zero, send a BFD 861 Control packet to the remote system with the Poll (P) bit clear, 862 and the Final (F) bit set (see section 4.16.3.) 864 If the packet was not discarded, it has been received for purposes 865 of the Detection Time expiration rules in [BFD] section 6.8.4. 867 4.16.2. Demultiplexing BFD Control Packets 869 This section is part of the replacement for [BFD] section 6.8.6, 870 separated for clarity. 872 If the Multipoint (M) bit is set 874 If the Your Discriminator field is nonzero, the packet MUST be 875 discarded. 877 Select a session based on the source address and the My 878 Discriminator field. If a session is found, and 879 bfd.SessionType is not MultipointTail, the packet MUST be 880 discarded. If a session is not found, a new session of type 881 MultipointTail MAY be created, or the packet MAY be discarded. 882 This choice is outside the scope of this specification. 884 Else (Multipoint bit is clear) 886 If the Your Discriminator field is nonzero 888 Select a session based on the value of Your Discriminator. 889 If no session is found, the packet MUST be discarded. 891 If bfd.SessionType is MulticastHead 893 Select a session based on the source address and the 894 value of Your Discriminator. If no session is found, a 895 new session of type MultipointClient MAY be created, or 896 the packet MAY be discarded. This choice is outside the 897 scope of this specification. 899 If bfd.SessionType is not MulticastClient, the packet 900 MUST be discarded. 902 Else (Your Discriminator is zero) 904 If the State field is not Down or AdminDown, the packet MUST 905 be discarded. 907 Otherwise, the session MUST be selected based on some 908 combination of other fields, possibly including source 909 addressing information, the My Discriminator field, and the 910 interface over which the packet was received. The exact 911 method of selection is application-specific and is thus 912 outside the scope of this specification. 914 If a matching session is found, and bfd.SessionType is not 915 PointToPoint, the packet MUST be discarded. 917 If a matching session is not found, a new session of type 918 PointToPoint may be created, or the packet may be discarded. 919 This choice is outside the scope of this specification. 921 If the State field is Init and bfd.SessionType is not 922 PointToPoint, the packet MUST be discarded. 924 4.16.3. Transmitting BFD Control Packets 926 The following procedure replaces section 6.8.7 of [BFD]. 928 BFD Control packets MUST be transmitted periodically at the rate 929 determined according to [BFD] section 6.8.2, except as specified in 930 this section. 932 A system MUST NOT transmit any BFD Control packets if bfd.RemoteDiscr 933 is zero and the system is taking the Passive role. 935 A system MUST NOT transmit any BFD Control packets if bfd.SilentTail 936 is 1. 938 A system MUST NOT periodically transmit BFD Control packets if Demand 939 mode is active on the remote system (bfd.RemoteDemandMode is 1, 940 bfd.SessionState is Up, and bfd.RemoteSessionState is Up) and a Poll 941 Sequence is not being transmitted. 943 A system MUST NOT periodically transmit BFD Control packets if 944 bfd.RemoteMinRxInterval is zero. 946 A system MUST NOT periodically transmit BFD Control packets if 947 bfd.SessionType is MulticastClient and a Poll Sequence is not being 948 transmitted. 950 If bfd.SessionType is MultipointHead, the transmit interval MUST be 951 set to bfd.DesiredMinTxInterval (this should happen automatically, as 952 bfd.RemoteMinRxInterval will be zero.) 954 If bfd.SessionType is not MultipointHead, the transmit interval MUST 955 be recalculated whenever bfd.DesiredMinTxInterval changes, or 956 whenever bfd.RemoteMinRxInterval changes, and is equal to the greater 957 of those two values. See [BFD] sections 6.8.2 and 6.8.3 for details 958 on transmit timers. 960 If bfd.SessionType is MulticastTail and periodic transmission of BFD 961 Control packets is just starting (due to Demand mode not being active 962 on the remote system), the first packet to be transmitted MUST be 963 delayed by a random amount of time between zero and (0.9 * 964 bfd.RemoteMinRxInterval). 966 If a BFD Control packet is received with the Poll (P) bit set to 1, 967 the receiving system MUST transmit a BFD Control packet with the Poll 968 (P) bit clear and the Final (F) bit, without respect to the 969 transmission timer or any other transmission limitations, without 970 respect to the session state, and without respect to whether Demand 971 mode is active on either system. A system MAY limit the rate at 972 which such packets are transmitted. If rate limiting is in effect, 973 the advertised value of Desired Min TX Interval MUST be greater than 974 or equal to the interval between transmitted packets imposed by the 975 rate limiting function. If the Multipoint (M) bit is set in the 976 received packet, the packet transmission MUST be delayed by a random 977 amount of time between zero and (0.9 * bfd.RemoteMinRxInterval). 978 Otherwise, the packet MUST be transmitted as soon as practicable. 980 A system MUST NOT set the Demand (D) bit if bfd.SessionType is 981 MultipointTail. 983 A system MUST NOT set the Demand (D) bit if bfd.SessionType is 984 MultipointClient or PointToPoint unless bfd.DemandMode is 1, 985 bfd.SessionState is Up, and bfd.RemoteSessionState is Up. 987 If bfd.SessionType is PointToPoint or MultipointHead, a BFD Control 988 packet SHOULD be transmitted during the interval between periodic 989 Control packet transmissions when the contents of that packet would 990 differ from that in the previously transmitted packet (other than the 991 Poll and Final bits) in order to more rapidly communicate a change in 992 state. 994 The contents of transmitted BFD Control packets MUST be set as 995 follows: 997 Version 999 Set to the current version number (1). 1001 Diagnostic (Diag) 1003 Set to bfd.LocalDiag. 1005 State (Sta) 1007 Set to the value indicated by bfd.SessionState. 1009 Poll (P) 1011 Set to 1 if the local system is sending a Poll Sequence or is a 1012 session of type MultipointHead soliciting the identities of the 1013 tails, or 0 if not. 1015 Final (F) 1017 Set to 1 if the local system is responding to a Control packet 1018 received with the Poll (P) bit set, or 0 if not. 1020 Control Plane Independent (C) 1022 Set to 1 if the local system's BFD implementation is independent 1023 of the control plane (it can continue to function through a 1024 disruption of the control plane.) 1026 Authentication Present (A) 1028 Set to 1 if authentication is in use on this session (bfd.AuthType 1029 is nonzero), or 0 if not. 1031 Demand (D) 1033 Set to bfd.DemandMode if bfd.SessionState is Up and 1034 bfd.RemoteSessionState is Up. Set to 1 if bfd.SessionType is 1035 MultipointHead or MultipointClient. Otherwise it is set to 0. 1037 Multipoint (M) 1039 Set to 1 if bfd.SessionType is MultipointHead. Otherwise it is 1040 set to 0. 1042 Detect Mult 1044 Set to bfd.DetectMult. 1046 Length 1048 Set to the appropriate length, based on the fixed header length 1049 (24) plus any Authentication Section. 1051 My Discriminator 1053 Set to bfd.LocalDiscr. 1055 Your Discriminator 1057 Set to bfd.RemoteDiscr. 1059 Desired Min TX Interval 1061 Set to bfd.DesiredMinTxInterval. 1063 Required Min RX Interval 1065 Set to bfd.RequiredMinRxInterval. 1067 Required Min Echo RX Interval 1069 Set to the minimum required Echo packet receive interval for this 1070 session. If this field is set to zero, the local system is 1071 unwilling or unable to loop back BFD Echo packets to the remote 1072 system, and the remote system will not send Echo packets. 1074 Authentication Section 1076 Included and set according to the rules in section 6.7 if 1077 authentication is in use (bfd.AuthType is nonzero.) Otherwise 1078 this section is not present. 1080 5. Assumptions 1082 If head notification is to be used, it is assumed that a multipoint 1083 BFD packet encapsulation contains enough information so that a tail 1084 can address a unicast BFD packet to the head. 1086 If head notification is to be used, it is assumed that is that there 1087 is bidirectional unicast communication available (at the same 1088 protocol layer within which BFD is being run) between the tail and 1089 head. 1091 For the head to know reliably that a tail has lost multipoint 1092 connectivity, the unicast paths in both directions between that tail 1093 and the head must remain operational when the multipoint path fails. 1094 It is thus desirable that unicast paths not share fate with the 1095 multipoint path to the extent possible if the head wants reliable 1096 knowledge of tail state. 1098 Since the normal BFD three-way handshake is not used in this 1099 application, a tail transitioning from state Up to Down and back to 1100 Up again may not be reliably detected at the head. 1102 If authentication is in use, all tails must be configured to have a 1103 common authentication key in order to receive the multipoint BFD 1104 Control packets. 1106 6. Operational Scenarios 1108 It is worth analyzing how this protocol reacts to various scenarios. 1109 There are three path components present, namely, the multipoint path, 1110 the forward unicast path (from head to a particular tail), and the 1111 reverse unicast path (from a tail to the head.) There are also four 1112 options as to how the head is notified about failures from the tail. 1114 6.1. No Head Notification 1116 Since the only path used in this scenario is the multipoint path, 1117 none of the others matter. A failure in the multipoint path will 1118 result in the tail noticing the failure within a detection time, and 1119 the head will remain ignorant of the tail state. 1121 6.2. Unreliable Head Notification 1123 In this scenario, the tail sends back unsolicicted BFD packets in 1124 response to the detection of a multipoint path failure. It uses the 1125 reverse unicast path, but not the forward unicast path. 1127 If the multipoint path fails but the reverse unicast path stays up, 1128 the tail will detect the failure within a detection time, and the 1129 head will know about it within one reverse packet time (since the 1130 notification is delayed.) 1132 If both the multipoint path and the reverse unicast paths fail, the 1133 tail will detect the failure but the head will remain unaware of it. 1135 6.3. Semi-reliable Head Notification and Tail Solicitation 1137 In this scenario, the head sends occasional multipoint Polls in 1138 addition to (or in lieu of) non-Poll multipoint BFD Control packets, 1139 expecting the tails to reply with Final. This also uses the reverse 1140 unicast path, but not the forward unicast path. 1142 If the multipoint path fails but the reverse unicast path stays up, 1143 the tail will detect the failure within a detection time, and the 1144 head will know about it within one reverse packet time (the 1145 notification is delayed to avoid synchronization of the tails.) 1147 If both the multipoint path and the reverse unicast paths fail, the 1148 tail will detect the failure but the head will remain unaware of this 1149 fact. 1151 If the reverse unicast path fails but the multipoint path stays up, 1152 the head will see the BFD session fail, but the state of the 1153 multipoint path will be unknown to the head. The tail will continue 1154 to receive multipoint data traffic. 1156 If either the multipoint Poll or the unicast reply is lost in 1157 transit, the head will see the BFD session fail, but the state of the 1158 multipoint path will be unknown to the head. The tail will continue 1159 to receive multipoint data traffic. 1161 6.4. Reliable Head Notification 1163 In this scenario, the head sends occasional multipoint Polls in 1164 addition to (or in lieu of) non-Poll multipoint BFD control packets, 1165 expecting the tails to reply with Final. If a tail that had 1166 previously replied to a multipoint Poll fails to reply (or if the 1167 head simply wishes to verify tail connectivity,) the head issues a 1168 unicast Poll Sequence to the tail. This scenario makes use of all 1169 three paths. 1171 If the multipoint path fails but the two unicast paths stay up, the 1172 tail will detect the failure within a detection time, and the head 1173 will know about it within one reverse packet time (since the 1174 notification is delayed.) Note that the reverse packet time may be 1175 smaller in this case if the head has previously issued a unicast Poll 1176 (since the tail will not delay transmission of the notification in 1177 this case.) 1179 If both the multipoint path and the reverse unicast paths fail 1180 (regardless of the state of the forward unicast path), the tail will 1181 detect the failure but the head will remain unaware of this fact. 1182 The head will detect a BFD session failure to the tail but cannot 1183 make a determination about the state of the tail's multipoint 1184 connectivity. 1186 If the forward unicast path fails but the reverse unicast path stays 1187 up, the head will detect a BFD session failure to the tail if it 1188 happens to send a unicast Poll sequence, but cannot make a 1189 determination about the state of the tail's multipoint connectivity. 1190 If the multipoint path to the tail fails prior to any unicast Poll 1191 being sent, the tail will detect the failure within a detection time, 1192 and the head will know about it within one reverse packet time (since 1193 the notification is delayed.) 1195 If the multipoint path stays up but the reverse unicast path fails, 1196 the head will see the BFD session fail if it happens to send a Poll 1197 Sequence, but the state of the multipoint path will be unknown to the 1198 head. The tail will continue to receive multipoint data traffic. 1200 If the multipoint path and the reverse unicast path both stay up but 1201 the forward unicast path fails, neither side will notice so long as a 1202 unicast Poll Sequence is never sent by the head. If the head sends a 1203 unicast Poll Sequence, the head will see the BFD session fail, but 1204 the state of the multipoint path will be unknown to the head. The 1205 tail will continue to receive multipoint data traffic. 1207 7. IANA Considerations 1209 This document has no actions for IANA. 1211 8. Security Considerations 1213 This specification does not raise any additional security issues 1214 beyond those of the specifications referred to in the list of 1215 normative references. 1217 9. Normative References 1219 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate 1220 Requirement Levels", RFC 2119, March 1997 1222 [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding 1223 Detection", RFC 5880, June 2010 1225 Contributors 1227 Rahul Aggarwal of Juniper Networks and George Swallow of Cisco 1228 Systems provided the initial idea for this specification and 1229 contributed to its development. 1231 Authors' Addresses 1233 Dave Katz 1234 Juniper Networks 1235 1194 N. Mathilda Ave. 1236 Sunnyvale, California 94089-1206 USA 1237 Phone: +1-408-745-2000 1238 Email: dkatz@juniper.net 1240 Dave Ward 1241 Cisco 1242 170 West Tasman Dr. 1243 San Jose, CA 95134 1244 Phone: +1-408-526-4000 1245 Email: dward@cisco.com 1247 Changes from the previous draft 1249 This is a reissue of the previous version. There are only minor 1250 editorial changes. 1252 This document expires in January, 2013.