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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Engineering Task Force PIM WG 2 INTERNET-DRAFT Mark Handley/UCL 3 draft-ietf-pim-bidir-06.txt Isidor Kouvelas/Cisco 4 Tony Speakman/Cisco 5 Lorenzo Vicisano/Cisco 6 12 April 2004 7 Expires: October 2004 9 Bi-directional Protocol Independent Multicast (BIDIR-PIM) 11 Status of this Document 13 This document is an Internet-Draft and is in full conformance with all 14 provisions of Section 10 of RFC2026. 16 Internet-Drafts are working documents of the Internet Engineering Task 17 Force (IETF), its areas, and its working groups. Note that other groups 18 may also distribute working documents as Internet-Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference material 23 or to cite them other than as "work in progress." 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/ietf/1id-abstracts.txt 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 This document is a product of the IETF PIM WG. Comments should be 32 addressed to the authors, or the WG's mailing list at 33 pim@catarina.usc.edu. 35 Abstract 37 This document discusses Bi-directional PIM, a variant of PIM 38 Sparse-Mode [4] that builds bi-directional shared trees 39 connecting multicast sources and receivers. Bi-directional 40 trees are built using a fail-safe Designated Forwarder (DF) 41 election mechanism operating on each link of a multicast 42 topology. With the assistance of the DF, multicast data is 43 natively forwarded from sources to the Rendezvous-Point and 44 hence along the shared tree to receivers without requiring 45 source-specific state. The DF election takes place at RP 46 discovery time and provides the route to the RP thus 47 eliminating the requirement for data-driven protocol events. 49 Table of Contents 51 1. Introduction. . . . . . . . . . . . . . . . . . . . . . 5 52 2. Terminology . . . . . . . . . . . . . . . . . . . . . . 5 53 2.1. Definitions. . . . . . . . . . . . . . . . . . . . . 6 54 2.2. Pseudocode Notation. . . . . . . . . . . . . . . . . 8 55 3. Protocol Specification. . . . . . . . . . . . . . . . . 8 56 3.1. BIDIR-PIM Protocol State . . . . . . . . . . . . . . 9 57 3.1.1. General Purpose State . . . . . . . . . . . . . . 9 58 3.1.2. RPA State . . . . . . . . . . . . . . . . . . . . 10 59 3.1.3. Group State . . . . . . . . . . . . . . . . . . . 10 60 3.1.4. State Summarization Macros. . . . . . . . . . . . 11 61 3.2. PIM Neighbor Discovery . . . . . . . . . . . . . . . 12 62 3.3. Data Packet Forwarding Rules . . . . . . . . . . . . 13 63 3.3.1. Upstream Forwarding at RP . . . . . . . . . . . . 14 64 3.3.2. Source-Only Branches. . . . . . . . . . . . . . . 14 65 3.3.3. Directly Connected Sources. . . . . . . . . . . . 15 66 3.4. PIM Join/Prune Messages. . . . . . . . . . . . . . . 15 67 3.4.1. Receiving (*,G) Join/Prune Messages . . . . . . . 15 68 3.4.2. Sending Join/Prune Messages . . . . . . . . . . . 18 69 3.5. Designated Forwarder (DF) Election . . . . . . . . . 21 70 3.5.1. DF Requirements . . . . . . . . . . . . . . . . . 21 71 3.5.2. DF Election description . . . . . . . . . . . . . 22 72 3.5.2.1. Bootstrap Election . . . . . . . . . . . . . . 22 73 3.5.2.2. Loser Metric Changes . . . . . . . . . . . . . 23 74 3.5.2.3. Winner Metric Changes. . . . . . . . . . . . . 24 75 3.5.2.4. Winner Loses Path. . . . . . . . . . . . . . . 24 76 3.5.2.5. Late Router Starting Up. . . . . . . . . . . . 25 77 3.5.2.6. Winner Dies. . . . . . . . . . . . . . . . . . 25 78 3.5.3. Election Protocol Specification . . . . . . . . . 25 79 3.5.3.1. Election State . . . . . . . . . . . . . . . . 25 80 3.5.3.2. Election Messages. . . . . . . . . . . . . . . 26 81 3.5.3.3. Election Events. . . . . . . . . . . . . . . . 27 82 3.5.3.4. Election Actions . . . . . . . . . . . . . . . 28 83 3.5.3.5. Election State Transitions . . . . . . . . . . 28 84 3.5.4. Election Reliability Enhancements . . . . . . . . 32 85 3.5.5. Missing Pass. . . . . . . . . . . . . . . . . . . 32 86 3.5.6. Periodic Winner Announcement. . . . . . . . . . . 32 87 3.6. Timers Counters and Constants. . . . . . . . . . . . 32 88 3.7. BIDIR PIM Packet Formats . . . . . . . . . . . . . . 36 89 3.7.1. DF Election Packet Formats. . . . . . . . . . . . 36 90 3.7.2. Backoff Message . . . . . . . . . . . . . . . . . 37 91 3.7.3. Pass Message. . . . . . . . . . . . . . . . . . . 38 92 3.7.4. Bidir Capable PIM-Hello Option. . . . . . . . . . 39 93 4. RP Discovery. . . . . . . . . . . . . . . . . . . . . . 39 94 5. Security Considerations . . . . . . . . . . . . . . . . 39 95 5.1. Attacks Based on Forged Messages . . . . . . . . . . 39 96 5.1.1. Election of an Incorrect DF . . . . . . . . . . . 40 97 5.1.2. Preventing Election Convergence . . . . . . . . . 41 98 5.2. Non-cryptographic Authentication Mechanisms. . . . . 41 99 5.2.1. Basic Access Control. . . . . . . . . . . . . . . 41 100 5.3. Authentication Using IPsec . . . . . . . . . . . . . 41 101 5.4. Denial of Service Attacks. . . . . . . . . . . . . . 41 102 6. Change history. . . . . . . . . . . . . . . . . . . . . 42 103 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . 42 104 8. Authors' Addresses. . . . . . . . . . . . . . . . . . . 42 105 9. Normative References. . . . . . . . . . . . . . . . . . 43 106 10. Informative References . . . . . . . . . . . . . . . . 43 107 11. Index. . . . . . . . . . . . . . . . . . . . . . . . . 45 109 1. Introduction 111 This document specifies Bi-directional PIM (BIDIR-PIM), a variant of PIM 112 Sparse-Mode (PIM-SM) [4] that builds bi-directional shared trees 113 connecting multicast sources and receivers. 115 PIM-SM constructs uni-directional shared trees that are used to forward 116 data from senders to receivers of a multicast group. PIM-SM also allows 117 the construction of source specific trees, but this capability is not 118 related to the protocol described in this document. 120 The shared tree for each multicast group is rooted at a multicast router 121 called the Rendezvous Point (RP). Different multicast groups can use 122 separate RPs within a PIM domain. 124 In unidirectional PIM-SM, there are two possible methods for 125 distributing data packets on the shared tree. These differ in the way 126 packets are forwarded from a source to the RP: 128 o Initially when a source starts transmitting, its first hop router 129 encapsulates data packets in special control messages (Registers) 130 which are unicast to the RP. After reaching the RP the packets are 131 decapsulated and distributed on the shared tree. 133 o A transition from the above distribution mode can be made at a later 134 stage. This is achieved by building source specific state on all 135 routers along the path between the source and the RP. This state is 136 then used to natively forward packets from that source. 138 Both these mechanisms suffer from problems. Encapsulation results in 139 significant processing, bandwidth and delay overheads. Forwarding using 140 source specific state has additional protocol and memory requirements. 142 Bi-directional PIM dispenses with both encapsulation and source state by 143 allowing packets to be natively forwarded from a source to the RP using 144 shared tree state. In contrast to PIM-SM this mode of forwarding does 145 not require any data-driven events. 147 The protocol specification in this document assumes familiarity with the 148 PIM-SM specification in [4]. Portions of the BIDIR-PIM protocol 149 operation that are identical to that of PIM-SM are only defined by 150 reference. 152 2. Terminology 154 In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", 155 "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and 156 "OPTIONAL" are to be interpreted as described in RFC 2119 and indicate 157 requirement levels for compliant BIDIR-PIM implementations. 159 2.1. Definitions 161 This specification uses a number of terms to refer to the roles of 162 routers participating in BIDIR-PIM. The following terms have special 163 significance for BIDIR-PIM: 165 MRIB Multicast Routing Information Base. This is the multicast 166 topology table, which is typically derived from the unicast 167 routing table, or routing protocols such as MBGP that carry 168 multicast-specific topology information. It is used by PIM for 169 establishing the RPF interface (used in the forwarding rules). In 170 PIM-SM the MRIB is also used to make decisions regarding where to 171 forward Join/Prune messages whereas in BIDIR-PIM it is used as a 172 source for routing metrics for the DF election process. 174 Rendezvous Point Address (RPA): 175 An RPA is an address that has been configured to be used as the 176 root of the distribution tree for a range of multicast groups. The 177 RPA must be routable from all routers in the PIM domain. The RPA 178 does not need to correspond to an address for an interface of a 179 real router. In this respect BIDIR-PIM differs from PIM-SM that 180 requires an actual router to be configured as the Rendezvous Point 181 (RP). Join messages from receivers for a BIDIR-PIM group propagate 182 hop-by-hop towards the RPA. 184 Rendezvous Point Link (RPL): 185 An RPL for a particular RPA is the physical link to which the RPA 186 belongs. In BIDIR-PIM all multicast traffic to groups mapping to a 187 specific RPA is forwarded on the RPL of that RPA. The RPL is 188 special within a BIDIR-PIM domain as it is the only link on which 189 a Designated Forwarder election does not take place (see DF 190 definition below). 192 Upstream 193 Towards the root (RPA) of the tree. The direction used by packets 194 traveling from sources to the RPL. 196 Downstream 197 Away from the root of the tree. The direction on which packets 198 travel from the RPL to receivers. 200 Designated Forwarder (DF): 201 The protocol presented in this document is largely based on the 202 concept of a Designated Forwarder (DF). A single DF exists for 203 each RPA on every link within a BIDIR-PIM domain (this includes 204 both multi-access and point-to-point links). The only exception is 205 the RPL on which no DF exists. The DF is the router on the link 206 with the best route to the RPA (determined by comparing MRIB 207 provided metrics). A DF for a given RPA is in charge of forwarding 208 downstream traffic onto its link, and forwarding upstream traffic 209 from its link towards the RPL. It does this for all the bi- 210 directional groups that map to the RPA. The DF on a link is also 211 responsible for processing Join messages from downstream routers 212 on the link as well as ensuring that packets are forwarded to 213 local receivers (discovered through a local membership mechanism 214 such as MLD [3] or IGMP [2]). 216 RPF Interface 217 RPF stands for "Reverse Path Forwarding". The RPF Interface of a 218 router with respect to an address is the interface that the MRIB 219 indicates should be used to reach that address. In the case of a 220 BIDIR-PIM multicast group, the RPF interface is determined by 221 looking up the RPA in the MRIB. The RPF information determines the 222 interface of the router that would be used to send packets towards 223 the RPL for the group. 225 RPF Neighbor 226 The RPF Neighbor of a router with respect to an address is the 227 neighbor that the MRIB indicates should be used to reach that 228 address. Note that in BIDIR-PIM, the RPF neighbor for a group is 229 not necessarily the router on the RPF interface that Join messages 230 for that group would be directed to (Join messages are only 231 directed to the DF on the RPF interface for the group). 233 TIB Tree Information Base. This is the collection of state at a PIM 234 router that has been created by receiving PIM Join/Prune messages, 235 PIM DF election messages and IGMP or MLD information from local 236 hosts. It essentially stores the state of all multicast 237 distribution trees at that router. 239 MFIB Multicast Forwarding Information Base. The TIB holds all the 240 state that is necessary to forward multicast packets at a router. 241 However, although this specification defines forwarding in terms 242 of the TIB, to actually forward packets using the TIB is very 243 inefficient. Instead a real router implementation will normally 244 build an efficient MFIB from the TIB state to perform forwarding. 245 How this is done is implementation-specific, and is not discussed 246 in this document. 248 2.2. Pseudocode Notation 250 We use set notation in several places in this specification. 252 A (+) B 253 is the union of two sets A and B. 255 A (-) B 256 is the elements of set A that are not in set B. 258 NULL 259 is the empty set or list. 261 In addition we use C-like syntax: 263 = denotes assignment of a variable. 265 == denotes a comparison for equality. 267 != denotes a comparison for inequality. 269 Braces { and } are used for grouping. 271 3. Protocol Specification 273 The specification of BIDIR-PIM is broken into several parts: 275 o Section 3.1 details the protocol state stored. 277 o Section 3.2 defines the BIDIR-PIM extensions to the PIM-SM [4] 278 neighbour discovery mechanism. 280 o Section 3.3 specifies the data packet forwarding rules. 282 o Section 3.4 specifies the BIDIR-PIM Join/Prune generation and 283 processing rules. 285 o Designated Forwarder (DF) election is specified in Section 3.5. 287 o PIM packet formats are specified in Section 3.7. 289 o A summary of BIDIR-PIM timers and their default values is given in 290 Section 3.6. 292 3.1. BIDIR-PIM Protocol State 294 This section specifies all the protocol state that a BIDIR-PIM 295 implementation should maintain in order to function correctly. We term 296 this state the Tree Information Base or TIB, as it holds the state of 297 all the multicast distribution trees at this router. In this 298 specification we define PIM mechanisms in terms of the TIB. However, 299 only a very simple implementation would actually implement packet 300 forwarding operations in terms of this state. Most implementations will 301 use this state to build a multicast forwarding table, which would then 302 be updated when the relevant state in the TIB changes. 304 Although we specify precisely the state to be kept, this does not mean 305 that an implementation of BIDIR-PIM needs to hold the state in this 306 form. This is actually an abstract state definition, which is needed in 307 order to specify the router's behavior. A BIDIR-PIM implementation is 308 free to hold whatever internal state it requires, and will still be 309 conformant with this specification so long as it results in the same 310 externally visible protocol behavior as an abstract router that holds 311 the following state. 313 We divide TIB state into two sections: 315 RPA state 316 State that maintains the DF election information for each RPA. 318 Group state 319 State that maintains a group-specific tree for groups that map to a 320 given RPA. 322 The state that should be kept is described below. Of course, 323 implementations will only maintain state when it is relevant to 324 forwarding operations - for example, the "NoInfo" state might be assumed 325 from the lack of other state information, rather than being held 326 explicitly. 328 3.1.1. General Purpose State 330 A router holds the following state that is not specific to a RPA or 331 group: 333 Neighbor State: 335 For each neighbor: 337 o Neighbor's Gen ID. 339 o Neighbor liveness timer (NLT) 341 o Other information from neighbor's Hello 343 For more information on Hello information look at section 3.2 as well as 344 the PIM-SM specification in [4]. 346 3.1.2. RPA State 348 A router maintains a multicast-group to RPA mapping which is built 349 through static configuration or by using an automatic RP discovery 350 mechanism like BSR or AUTO-RP (see section 4). For each BIDIR-PIM RPA a 351 router holds the following state: 353 o RPA (actual address) 355 Designated Forwarder (DF) State: 357 For each router interface: 359 Acting DF information: 361 o DF IP Address 363 o DF metric 365 Election information: 367 o Election State 369 o DF election-Timer (DFT) 371 o Message-Count (MC) 373 Current best offer: 375 o IP address of best offering router 377 o Best offering router metric 379 Designated Forwarder state is described in section 3.5. 381 3.1.3. Group State 383 For every group G a router keeps the following state: 385 Group state: 387 For each interface: 389 Local Membership: 391 o State: One of {"NoInfo", "Include"} 393 PIM Join/Prune State: 395 o State: One of {"NoInfo" (NI), "Join" (J), 396 "PrunePending" (PP)} 398 o Prune Pending Timer (PPT) 400 o Join/Prune Expiry Timer (ET) 402 Not interface specific: 404 o Upstream Join/Prune Timer (JT) 406 o Last RPA Used 408 Local membership is the result of the local membership mechanism (such 409 as IGMP [2]) running on that interface. This information is used by the 410 pim_include(*,G) macro described in section 3.1.4. 412 PIM Join/Prune state is the result of receiving PIM (*,G) Join/Prune 413 messages on this interface, and is specified in section 3.4.1. The state 414 is used by the macros that calculate the outgoing interface list in 415 section 3.1.4, and in the JoinDesired(G) macro (defined in section 416 3.4.2) that is used in deciding whether a Join(*,G) should be sent 417 upstream. 419 The upstream Join/Prune timer is used to send out periodic Join(*,G) 420 messages, and to override Prune(*,G) messages from peers on an upstream 421 LAN interface. 423 The last RPA used must be stored because if the group to RPA mapping 424 changes (see RP Set changes in [4]) then state must be torn down and 425 rebuilt for groups whose RPA changes. 427 3.1.4. State Summarization Macros 429 Using this state, we define the following "macro" definitions which we 430 will use in the descriptions of the state machines and pseudocode in the 431 following sections. 433 olist(G) = 434 RPF_interface(RPA(G)) (+) joins(G) (+) pim_include(G) 436 RPF_interface(RPA) is the interface the MRIB indicates would be used to 437 route packets to RPA. The olist(G) is the list of interfaces on which 438 packets to group G must be forwarded. 440 The macro pim_include(G) indicates the interfaces to which traffic might 441 be forwarded because of hosts that are local members on that interface. 443 pim_include(G) = 444 { all interfaces I such that: 445 I_am_DF(RPA(G),I) AND local_receiver_include(G,I) } 447 The clause "I_am_DF(RPA,I)" is TRUE if the router is in the Win or 448 Backoff states in the DF election state machine (described in section 449 3.5) for the given RPA on interface I. Otherwise it is FALSE. 451 The clause "local_receiver_include(G,I)" is true if the IGMP module, MLD 452 module or other local membership mechanism has determined that there are 453 local members on interface I that desire to receive traffic sent to 454 group G. 456 The set "joins(G)" is the set of all interfaces on which the router has 457 received (*,G) Joins: 459 joins(G) = 460 { all interfaces I such that 461 I_am_DF(RPA(G),I) AND 462 DownstreamJPState(G,I) is either Joined or PrunePending } 464 DownstreamJPState(G,I) is the state of the finite state machine in 465 section 3.4.1. 467 RPF_DF(RPA) is the neighbor that Join messages must be sent to in order 468 to build the group shared tree rooted at the RPL for the given RPA. This 469 is the Designated-Forwarder on the RPF_interface(RPA). 471 3.2. PIM Neighbor Discovery 473 PIM routers exchange PIM-Hello messages with their neighboring PIM 474 routers. These messages are used to update the Neighbor State described 475 in section 3.1. The procedures for generating and processing Hello 476 messages as well as maintaining Neighbor State are specified in the PIM- 477 SM [4] documentation. 479 Bidir PIM introduces the Bidir_Capable PIM-Hello option that MUST be 480 included in all Hello messages from a Bidir-PIM capable router. The 481 Bidir_Capable option advertises the router's ability to participate in 482 the Bidir-PIM protocol. The format of the Bidir_Capable option is 483 described in section 3.7. 485 If a Bidir PIM router receives a PIM-Hello message that does not contain 486 the Bidir_Capable option from one of its neighbours, the error must be 487 logged to the router administrator in a rate-limited manner. 489 3.3. Data Packet Forwarding Rules 491 For groups mapping to a given RPA, the following responsibilities are 492 uniquely assigned to the DF for that RPA on each link: 494 o The DF is the only router that forwards packets traveling downstream 495 onto the link. 497 o The DF is the only router that picks-up upstream traveling packets off 498 the link to forward towards the RPL. 500 Non-DF routers on a link, that use that link as their RPF interface to 501 reach the RPA, may perform the following forwarding actions for 502 bidirectional groups: 504 o Forward packets from the link towards downstream receivers. 506 o Forward packets from downstream sources onto the link (provided they 507 are the DF for the downstream link from which the packet was picked- 508 up). 510 The BIDIR-PIM packet forwarding rules are defined below in pseudocode. 512 iif is the incoming interface of the packet. 513 G is the destination address of the packet (group address). 514 RPA is the Rendezvous Point Address for this group. 516 First we check to see whether the packet should be accepted based on TIB 517 state and the interface that the packet arrived on. A packet is accepted 518 if it arrives on the RPF_interface to reach the RPA (downstream 519 traveling packet) or if the router is the DF on the interface the packet 520 arrives (upstream traveling packet). 522 If the packet should be forwarded we build an outgoing interface list 523 for the packet. 525 Finally we remove the incoming interface from the outgoing interface 526 list we've created, and if the resulting outgoing interface list is not 527 empty, we forward the packet out of those interfaces. 529 On receipt on a data to G on interface iif: 531 if( iif == RPF_interface(RPA) || I_am_DF(RPA,I) ) { 532 oiflist = olist(G) (-) iif 533 forward packet on all interfaces in oiflist 534 } 536 3.3.1. Upstream Forwarding at RP 538 When configuring a BIDIR-PIM domain it is possible to assign the 539 Rendezvous Point Address (RPA) such that it does not belong to a 540 physical box but instead is simply a routable address. Routers that have 541 interfaces on the RPL that the RPA belongs to will upstream forward 542 traffic onto the link. Joins from receivers in the domain will propagate 543 hop-by-hop till they reach one of the routers connected to the RPL where 544 they will terminate (as there will be no DF elected on the RPL). 546 If instead the administrator chooses to configure the RPA to be the 547 address of an interface of a specific router then nothing changes. That 548 router must still upstream forward traffic on to the RPL and behave no 549 differently than any other router with an interface on the RPL. 551 To configure a BIDIR-PIM network to operate in a mode similar to that of 552 PIM-SM where a single router (the RP) is acting as the root of the 553 distribution tree, the RPA address can be configured to be the loopback 554 interface of a router. 556 3.3.2. Source-Only Branches 558 Source-only branches of the distribution tree for a group G are branches 559 which do not lead to any receivers, but which are used to forward 560 packets traveling upstream from sources towards the RPL. Routers along 561 source-only branches only have the RPF_interface to the RPA in their 562 olist for G and hence do not need to maintain any group specific state. 563 Upstream forwarding can be performed using only RPA specific state. An 564 implementation may decide to maintain group state for source-only 565 branches for accounting or performance reasons. However, doing so 566 requires data-driven events thus sacrificing one of tha main benefits of 567 Bidir PIM. 569 3.3.3. Directly Connected Sources 571 A major advantage of using a Designated Forwarder in BIDIR-PIM compared 572 to PIM-SM is that special treatment is no longer required for sources 573 that are directly connected to a router. Data from such sources does not 574 need to be differentiated from other multicast traffic and will 575 automatically be picked up by the DF and forwarded upstream. This 576 removes the need for performing a directly-connected-source check for 577 data to groups that do not have existing state. 579 3.4. PIM Join/Prune Messages 581 BIDIR-PIM Join/Prune messages are used to construct group specific 582 distribution trees between receivers and the RPL. Joins are originated 583 by last-hop routers that are elected as the DF on an interface with 584 directly connected receivers. The Joins propagate hop-by-hop towards the 585 RPA till they reach a router connected to the RPL. 587 A BIDIR-PIM Join/Prune message consists of a list of Joined and Pruned 588 Groups. When processing a received Join/Prune message, each Joined or 589 Pruned Group is effectively considered individually by applying the 590 following state machines. When considering a Join/Prune message whose 591 PIM Destination field addresses this router, (*,G) Joins and Prunes can 592 affect the downstream state machine. When considering a Join/Prune 593 message whose PIM Destination field addresses another router, most Join 594 or Prune entries could affect the upstream state machine. 596 3.4.1. Receiving (*,G) Join/Prune Messages 598 When a router receives a Join(*,G) or Prune(*,G) it must first check to 599 see whether the RP address in the message matches RPA(G) (the router's 600 idea of what the Rendezvous Point Address is). If the RP address in the 601 message does not match RPA(G) the Join or Prune MUST be silently 602 dropped. 604 The per-interface state-machine for receiving (*,G) Join/Prune Messages 605 is given below. There are three states: 607 NoInfo (NI) 608 The interface has no (*,G) Join state and no timers running. 610 Join (J) 611 The interface has (*,G) Join state. If the router is the DF on 612 this interface (I_am_DF(RPA(G),I) is TRUE), the Join state 613 will cause us to forward packets destined for G on this 614 interface. 616 PrunePending (PP) 617 The router has received a Prune(*,G) on this interface from a 618 downstream neighbor and is waiting to see whether the prune 619 will be overridden by another downstream router. For 620 forwarding purposes, the PrunePending state functions exactly 621 like the Join state. 623 In addition the state-machine uses two timers: 625 ExpiryTimer (ET) 626 This timer is restarted when a valid Join(*,G) is received. 627 Expiry of the ExpiryTimer causes the interface state to revert 628 to NoInfo for this group. 630 PrunePendingTimer (PPT) 631 This timer is set when a valid Prune(*,G) is received. Expiry 632 of the PrunePendingTimer causes the interface state to revert 633 to NoInfo for this group. 635 +-----------------------------------+ 636 | Figures omitted from text version | 637 +-----------------------------------+ 639 Figure 1: Downstream group per-interface state-machine 641 In tabular form, the group per-interface state-machine is: 643 +----------+------------------------------------------------------------+ 644 | | Event | 645 | +----------+------------+-----------+------------+-----------+ 646 Prev State |Receive |Receive |Prune |Expiry Stop Being | 647 | |Join(*,G) |Prune(*,G) |Pending |Timer DF on I | 648 | | | |Timer |Expires | | 649 | | | |Expires | | | 650 +----------+----------+------------+-----------+------------+-----------+ 651 | |-> J state|- |- |- + | 652 NoInfo |start | | | | | 653 (NI) |Expiry | | | | | 654 | |Timer | | | | | 655 +----------+----------+------------+-----------+------------+-----------+ 656 | |-> J state|-> PP state |- |-> NI state +> NI state | 657 Join (J) |restart |start Prune | | | | 658 | |Expiry |Pending | | | | 659 | |Timer |Timer | | | | 660 +----------+----------+------------+-----------+------------+-----------+ 661 | |-> J state|-> PP state |-> NI state|-> NI state +> NI state | 662 | |restart | |Send Prune-| | | 663 Prune |Expiry | |Echo(*,G) | | | 664 Pending |Timer; | | | | | 665 (PP) |stop Prune| | | | | 666 | |Pending | | | | | 667 | |Timer | | | | | 668 +----------+----------+------------+-----------+------------+-----------+ 670 The transition events "Receive Join(*,G)" and "Receive Prune(*,G)" imply 671 receiving a Join or Prune targeted to this router's address on the 672 received interface. If the destination address is not correct, these 673 state transitions in this state machine must not occur, although seeing 674 such a packet may cause state transitions in other state machines. 676 On unnumbered interfaces on point-to-point links, the router's address 677 should be the same as the source address it chose for the hello packet 678 it sent over that interface. However on point-to-point links we also 679 recommend that PIM messages with a destination address of all zeros are 680 also accepted. 682 The transition event "Stop being DF" implies a DF re-election taking 683 place on this router interface for RPA(G) and the router changing status 684 from being the active DF to being a non-DF router (the value of the 685 I_am_DF macro changing to FALSE). 687 When ExpiryTimer is started or restarted, it is set to the HoldTime from 688 the triggering received Join/Prune message. 690 When PrunePendingTimer is started, it is set to the 691 J/P_Override_Interval if the router has more than one neighbor on that 692 interface; otherwise it is set to zero causing it to expire immediately. 694 The action "Send PruneEcho(*,G)" is triggered when the router stops 695 forwarding on an interface as a result of a prune. A PruneEcho(*,G) is 696 simply a Prune(*,G) message sent by the upstream router to itself on a 697 LAN. Its purpose is to add additional reliability so that if a Prune 698 that should have been overridden by another router is lost locally on 699 the LAN, then the PruneEcho may be received and cause the override to 700 happen. A PruneEcho(*,G) need not be sent when the router has only one 701 neighbour on the link. 703 3.4.2. Sending Join/Prune Messages 705 The downstream per-interface state-machines described above hold join 706 state from downstream PIM routers. This state then determines whether a 707 router needs to propagate a Join(*,G) upstream towards the RPA. Such 708 Join(*,G) messages are sent on the RPF_interface towards the RPA and are 709 targeted at the DF on that interface. 711 If a router wishes to propagate a Join(*,G) upstream, it must also watch 712 for messages on its upstream interface from other routers on that 713 subnet, and these may modify its behavior. If it sees a Join(*,G) to 714 the correct upstream neighbor, it should suppress its own Join(*,G). If 715 it sees a Prune(*,G) to the correct upstream neighbor, it should be 716 prepared to override that prune by sending a Join(*,G) almost 717 immediately. Finally, if it sees the Generation ID (see PIM-SM 718 specification [4]) of the correct upstream neighbor change, it knows 719 that the upstream neighbor has lost state, and it should be prepared to 720 refresh the state by sending a Join(*,G) almost immediately. 722 In addition changes in the next hop towards the RPA trigger a prune off 723 from the old next hop, and join towards the new next hop. Such a change 724 can be caused by the following two events: 726 o The MRIB indicates that the RPF Interface towards the RPA has 727 changed. In this case the DF on the new RPF_interface becomes 728 the new RPF Neighbour. 730 o There is a DF re-election on the RPF_interface and a new router 731 emerges as the DF. 733 The upstream (*,G) state-machine only contains two states: 735 Not Joined 736 The downstream state-machines indicate that the router does not 737 need to join the RPA tree for this group. 739 Joined 740 The downstream state-machines indicate that the router would like 741 to join the RPA tree for this group. 743 In addition, one timer JT(G) is kept which is used to trigger the 744 sending of a Join(*,G) to the upstream next hop towards the RPA (the DF 745 on the RPF_interface for RPA(G)). 747 +-----------------------------------+ 748 | Figures omitted from text version | 749 +-----------------------------------+ 751 Figure 2: Upstream group state-machine 753 In tabular form, the state machine is: 755 +----------------------+------------------------------------------------+ 756 | | Event | 757 | Prev State +------------------------+-----------------------+ 758 | | JoinDesired(G) | JoinDesired(G) | 759 | | ->True | ->False | 760 +----------------------+------------------------+-----------------------+ 761 | | -> J state | - | 762 | NotJoined (NJ) | Send Join(*,G); | | 763 | | Set Timer to | | 764 | | t_periodic | | 765 +----------------------+------------------------+-----------------------+ 766 | Joined (J) | - | -> NJ state | 767 | | | Send Prune(*,G) | 768 +----------------------+------------------------+-----------------------+ 770 In addition, we have the following transitions which occur within the 771 Joined state: 773 +-----------------------------------------------------------------------+ 774 | In Joined (J) State | 775 +-----------------+-----------------+-----------------+-----------------+ 776 |Timer Expires | See Join(*,G) | See Prune(*,G) | RPF_DF(RPA(G)) | 777 | | to | to | changes | 778 | | RPF_DF(RPA(G)) | RPF_DF(RPA(G)) | | 779 +-----------------+-----------------+-----------------+-----------------+ 780 |Send | Increase Timer | Decrease Timer | Decrease Timer | 781 |Join(*,G); Set | to | to t_override | to t_override | 782 |Timer to | t_suppressed | | | 783 |t_periodic | | | | 784 +-----------------+-----------------+-----------------+-----------------+ 786 +-----------------------------------------------------------------------+ 787 | In Joined (J) State | 788 +-------------------------------------+---------------------------------+ 789 | Change of RPF_DF(RPA(G)) | RPF_DF(RPA(G)) GenID | 790 | | changes | 791 +-------------------------------------+---------------------------------+ 792 | Send Join(*,G) to new | Decrease Timer to | 793 | DF; Send Prune(*,G) to | t_override | 794 | old DF; set Timer to | | 795 | t_periodic | | 796 +-------------------------------------+---------------------------------+ 797 This state machine uses the following macro: 799 bool JoinDesired(G) { 800 if (olist(G) (-) RPF_interface(RPA(G))) != NULL 801 return TRUE 802 else 803 return FALSE 804 } 806 3.5. Designated Forwarder (DF) Election 808 This section presents a fail-safe mechanism for electing a per-RPA 809 designated router on each link in a BIDIR-PIM domain. We call this 810 router the Designated Forwarder (DF). The DF election does not take 811 place on the RPL for a RPA. 813 3.5.1. DF Requirements 815 The DF election chooses the best router on a link to assume the 816 responsibility of forwarding traffic between the RPL and the link for 817 the range of multicast groups served by the RPA. Different multicast 818 groups that share a common RPA share the same upstream direction. 819 Hence, the election of an upstream forwarder on each link does not have 820 to be a group specific decision but instead can be RPA-specific. As the 821 number of RPAs is typically small, the number of elections that have to 822 be performed is significantly reduced by this observation. 824 To optimise tree creation, it is desirable that the winner of the 825 election process should be the router on the link with the "best" 826 unicast routing metric (as reported by the MRIB) to reach the RPA. When 827 comparing metrics from different unicast routing protocols, we use the 828 same comparison rules used by the PIM-SM assert process [4]. 830 The election process needs to take place when information on a new RPA 831 initially becomes available. The result can be re-used as new bidir 832 groups that map to the same RPA are encountered. There are however some 833 conditions under which an update to the election is required: 835 o There is a change in unicast metric to reach the RPA for any of 836 the routers on the link. 838 o The interface on which the RPA is reachable (RPF Interface) 839 changes to an interface for which the router was previously the 840 DF. 842 o A new PIM neighbor starts up on a link that must participate in 843 the elections and be informed of current outcome. 845 o The elected DF fails (detected through neighbor information 846 timeout or MRIB RPF change at downstream router). 848 The election process has to be robust enough to ensure with very high 849 probability that all routers on the link have a consistent view of the 850 DF. This is because with the forwarding rules described in section 3.3 851 if multiple routers end-up thinking that they should be responsible for 852 forwarding, loops may result. To reduce the possibility of this 853 occurrence to a minimum, the election algorithm has been biased towards 854 discarding DF information and suspending forwarding during periods of 855 ambiguity. 857 3.5.2. DF Election description 859 This section gives an outline of the DF election process. It does not 860 provide the definitive specification for the DF election. If any 861 discrepancy exists between section 3.5.3 and this section, the 862 specification in section 3.5.3 is to be assumed correct. 864 To perform the election of the DF for a particular RPA, routers on a 865 link need to exchange their unicast routing metric information for 866 reaching the RPA. Routers advertise their own metrics in Offer, Winner, 867 Backoff and Pass messages. The advertised metric is calculated using the 868 RPF Interface and metric to reach the RPA available through the MRIB. 869 When a router is participating in a DF election for an RPA on the 870 interface that its MRIB indicates as the RPF Interface then that router 871 MUST always advertise an infinite metric in its election messages. When 872 a router is participating in a DF election on an interface other than 873 the MRIB indicated RPF Interface then it MUST advertise the MRIB 874 provided metrics in its election messages. 876 In the election protocol described below, many message exchanges are 877 repeated Election_Robustness times for reliability. In all those cases 878 the message retransmissions are spaced in time by a small random 879 interval. All of the following description is specific to the election 880 on a single link for a single RPA. 882 3.5.2.1. Bootstrap Election 884 Initially when no DF has been elected, routers finding out about a new 885 RPA start participating in the election by sending Offer messages. 886 Offer messages include the router's metric to reach the RPA. Offers are 887 periodically retransmitted with a period of Offer_Interval. 889 If a router hears a better offer than its own from a neighbor, it stops 890 participating in the election for a period of Election_Robustness * 891 Offer_Interval thus giving a chance to the neighbour with the better 892 metric to be elected DF. If during this period no winner is elected, the 893 router restarts the election from the beginning. If at any point during 894 the initial election a router receives an out of order offer with worse 895 metrics than its own, then it restarts the election from the beginning. 897 The result should be that all routers except the best candidate stop 898 advertising their offers. 900 A router assumes the role of the DF after having advertised its metrics 901 Election_Robustness times without receiving any offer from any other 902 neighbor. At that point it transmits a Winner message which declares to 903 every other router on the link the identity of the winner and the 904 metrics it is using. 906 Routers receiving a winner message stop participating in the election 907 and record the identity and metrics of the winner. If the local metrics 908 are better than those of the winner then the router records the identity 909 of the winner (accepting it as the acting DF) but re-initiates the 910 election to try and take over. 912 3.5.2.2. Loser Metric Changes 914 Whenever the unicast metric to a RPA changes at a non-DF router to a 915 value that is better than that previously advertised by the acting DF, 916 the router with the new better metric should take action to eventually 917 assume forwarding responsibility. When the metric change is detected, 918 the non-DF router with the now better metric restarts the DF election 919 process by sending Offer messages with this new metric. Note that at 920 any point during an election if no response is received after 921 Election_Robustness retransmissions of an offer, a router assumes the 922 role of the DF following the usual Winner announcement procedure. 924 Upon receipt of an offer that is worse than its current metric, the DF 925 will respond with a Winner message declaring its status and advertising 926 its better metric. Upon receiving the Winner message, the originator of 927 the Offer records the identity of the DF and aborts the election. 929 Upon receipt of an offer that is better than its current metric, the DF 930 records the identity and metrics of the offering router and responds 931 with a Backoff message. This instructs the offering router to hold off 932 for a short period of time while the unicast routing stabilises and 933 other routers get a chance to put in their offers. The Backoff message 934 includes the offering router's new metric and address. All routers on 935 the link that have pending offers with metrics worse than those in the 936 backoff message (including the original offering router) will hold 937 further offers for a period of time defined in the Backoff message. 939 If during the Backoff_Period, a third router sends a new better offer, 940 the Backoff message is repeated for the new offer and the Backoff_Period 941 restarted. 943 Before the Backoff_Period expires, the acting DF nominates the router 944 having made the best offer as the new DF using a Pass message. This 945 message includes the IDs and metrics of both the old and new DFs. The 946 old DF stops performing its tasks at the time the Pass message 947 transmission is made. The new DF assumes the role of the DF as soon as 948 it receives the Pass message. All other routers on the link take note of 949 the new DF and its metric. Note that this event constitutes an RPF 950 Neighbour change which may trigger Join messages to the new DF (see 951 section 3.4). 953 3.5.2.3. Winner Metric Changes 955 If the DF's routing metric to reach the RPA changes to a worse value, it 956 sends a set of Election_Robustness randomly spaced Winner messages on 957 the link, advertising the new metric. Routers that receive this 958 announcement but have a better metric may respond with an Offer message 959 which results in the same handoff procedure described above. All 960 routers assume the DF has not changed until they see a Pass or Winner 961 message indicating the change. 963 There is no pressure to make this handoff quickly if the acting DF still 964 has a path to the RPL. The old path may now be suboptimal but it will 965 still work while the re-election is in progress. 967 If the routing metric at the DF changes to a better value, a single 968 Winner message is sent advertising the new metric. 970 3.5.2.4. Winner Loses Path 972 If a router's RPF Interface to the RPA switches to be on a link for 973 which it is acting as the DF, then it can no longer provide forwarding 974 services for that link. It therefore immediately stops being the DF and 975 restarts the election. As its path to the RPA is through the link, an 976 infinite metric is used in the Offer message it sends. 978 Note: At this stage the old DF will have a hint at a possible RPF 979 neighbor on the link indicated by the new MRIB next-hop. The old DF 980 could use this next-hop hint in a Pass message but this adds unnecessary 981 complication to the election process. 983 3.5.2.5. Late Router Starting Up 985 A late router starting up after the DF election process has completed 986 will have no immediate knowledge of the election outcome. As a result, 987 it will start advertising its metric in Offer messages. As soon as this 988 happens, the currently elected DF will respond with a Winner message if 989 its metric is better than the metric in the Offer message, or with a 990 Backoff message if its metric worse than the metric in the Offer 991 message. 993 3.5.2.6. Winner Dies 995 Whenever the DF dies, a new DF has to be elected. The speed at which 996 this can be achieved depends on whether there are any downstream routers 997 on the link. 999 If there are downstream routers, typically their MRIB reported next-hop 1000 before the DF dies will be the DF itself. They will therefore notice 1001 either a change in the metric for the route to the RPA or a change in 1002 next-hop away from the DF and can restart the election by transmitting 1003 Offer messages. If according to the MRIB the RPA is now reachable 1004 through the same link via another upstream router, an infinite metric 1005 will be used in the Offer. 1007 If no downstream routers are present, the only way for other upstream 1008 routers to detect a DF failure is by the timeout of the PIM neighbor 1009 information, which will take significantly longer. 1011 3.5.3. Election Protocol Specification 1013 This section provides the definitive specification for the DF election 1014 process. If any discrepancy exists between section 3.5.2 and this 1015 section, the specification in this section is to be assumed correct. 1017 3.5.3.1. Election State 1019 The DF election state is maintained per RPA for each multicast enabled 1020 interface I on the router as introduced in section 3.1. 1022 The state machine has the following four states: 1024 Offer 1025 Initial election state. When in the Offer state a router 1026 thinks it can eventually become the winner and periodically 1027 generates Offer messages. 1029 Lose In this state the router knows that there either is a 1030 different election winner or that no router on the link has a 1031 path to the RP. 1033 Winner 1034 The router is the acting DF without any contest. 1036 Backoff 1037 The router is the acting DF but another router has made a bid 1038 to take over. 1040 In the state machine a router is considered to be an acting DF if it is 1041 in the Win or Backoff states. 1043 The operation of the election protocol makes use of the variables and 1044 timers described below: 1046 Acting DF information 1047 Used to store the identity and advertised metrics of the 1048 election winner that is the currently acting DF. 1050 DF election-Timer (DFT) 1051 Used to schedule transmission of Offer, Winner and Pass 1052 messages. 1054 Message-Count (MC) 1055 Used to maintain the number of times an Offer or Winner 1056 message has been transmitted. 1058 Best-Offer 1059 Used by the DF to record the identity and advertised metrics 1060 of the router has made the last offer for use when sending the 1061 Pass message. 1063 3.5.3.2. Election Messages 1065 The election process uses the following PIM control messages the packet 1066 format of which is described in section 3.7: 1068 Offer (OfferingID, Metric) 1069 Sent by routers that believe they have a better metric to the 1070 RPA than the metric that has been on offer so far. 1072 Winner (DF-ID, DF-Metric) 1073 Sent by a router when assuming the role of the DF or when re- 1074 asserting in response to worse offers. 1076 Backoff (DF-ID, DF-Metric, OfferingID, OfferMetric, 1077 BackoffInterval) 1078 Used by the DF to acknowledge better offers. It instructs 1079 other routers with equal or worse offers to wait till the DF 1080 passes responsibility to the sender of the offer. 1082 Pass (Old-DF-ID, Old-DF-Metric, New-DF-ID, New-DF-Metric) 1083 Used by the old DF to pass forwarding responsibility to a 1084 router that has previously made an offer. The Old-DF-Metric 1085 is the current metric of the DF at the time the pass is sent. 1087 Note that when a router is participating in a DF election for an RPA on 1088 the interface that its MRIB indicates as the RPF Interface then that 1089 router MUST always advertise an infinite metric in its election 1090 messages. When a router is participating in a DF election on an 1091 interface other than the MRIB indicated RPF Interface then it MUST 1092 advertise the MRIB provided metrics in its election messages. 1094 3.5.3.3. Election Events 1096 During protocol operation the following events can take place: 1098 Control message reception 1099 Reception of one of the four control DF election messages 1100 (Offer, Winner, Backoff and Pass). When a control message is 1101 received and actions are specified on a condition that metrics 1102 are Better or Worse the comparison must be performed as 1103 follows: 1105 o On receipt of an Offer or Winner message compare our current 1106 metrics for the RPA with the metrics advertised for the 1107 sender of the message. 1109 o On receipt of a Backoff or Pass message compare our current 1110 metrics for the RPA with the metrics advertised for the 1111 target of the message. 1113 Path to RPA lost 1114 Losing the path to the RPA can happen in two ways. The first 1115 happens when the route learned through the MRIB is withdrawn 1116 and the MRIB no longer reports an available route to reach the 1117 RPA. The second case happens when the next-hop information 1118 reported by the MRIB changes to indicate a next-hop that is 1119 reachable through the router interface under consideration. 1120 Clearly as the router is using the interface as its RPF 1121 Interface it cannot offer forwarding services towards the RPL 1122 to other routers on that link. 1124 Metric reported by the MRIB to reach the RPA changes 1125 This event is triggered when the MRIB supplied information for 1126 the RPA changes and the new information provides a path to the 1127 RPA. If the new MRIB information either reports no route or 1128 reports a next-hop interface through the interface for which 1129 the DF election is taking place then the "Path to RPA lost" 1130 event triggers instead. In specific states the event may be 1131 further filtered by specifying whether it is expected of the 1132 metric to become better or worse and which stored metric the 1133 new MRIB information must be compared against. The new 1134 information must be compared with either the router's old 1135 metric, the stored DF metric or the stored Best Offer metric. 1137 Election-Timer (DFT) Expiration 1138 Expiration of the DFT election timer can cause message 1139 transmission and state transitions. The event might be further 1140 qualified by specifying the value of the Message Count (MC) as 1141 well as the current existence of a path to the RPA (as defined 1142 above). 1144 Detection of DF failure 1145 Detection of DF failure can occur through the timeout of PIM 1146 neighbor state. 1148 3.5.3.4. Election Actions 1150 The DF election state machine action descriptions use the following 1151 notation in addition to the pseudocode notation described earlier in 1152 this spec. 1154 ?= denotes the operation of lowering a timer to a new value. If 1155 the timer is not running then it is started using the new 1156 value. If the timer is running with an expiration lower than 1157 the new value, then the timer is not altered. 1159 When an action of "set DF to Sender or Target" is encountered during 1160 receipt of a Winner, Pass or Backoff message it means the following: 1162 o On receipt of a Winner message set the DF to be the originator of 1163 the message and record its metrics. 1165 o On receipt of a Pass message set the DF to be the target of the 1166 message and record its metrics. 1168 o On receipt of a Backoff message set the DF to be the originator 1169 of the message and record its metrics. 1171 3.5.3.5. Election State Transitions 1173 When a Designated Forwarder election is initiated the starting state is 1174 the Offer state, the message counter (MC) is set to zero and the DF 1175 election Timer (DFT) is set to OPlow (see section 3.6 for a definition 1176 of timer values). 1178 +-----------------------------------+ 1179 | Figures omitted from text version | 1180 +-----------------------------------+ 1182 Figure 3: Designated Forwarder election state-machine 1184 In tabular form, the state machine is: 1186 +-------------++--------------------------------------------------------+ 1187 | || Event | 1188 | Prev State ++------------------+------------------+------------------+ 1189 | || Recv better | Recv better | Recv better | 1190 | || Pass / Win | Backoff | Offer | 1191 +-------------++------------------+------------------+------------------+ 1192 | || -> Lose | - | - | 1193 | Offer || DF = Sender or | DFT = BOperiod | DFT = OPhigh; | 1194 | || Target; Stop | + OPlow; MC = | MC = 0 | 1195 | || DFT | 0 | | 1196 +-------------++------------------+------------------+------------------+ 1197 | || - | - | -> Offer | 1198 | Lose || DF = Sender or | DF = Sender | DFT = OPhigh; | 1199 | || Target | | MC = 0 | 1200 +-------------++------------------+------------------+------------------+ 1201 | || -> Lose | -> Lose | -> Backoff | 1202 | || DF = Sender or | DF = Sender; | Set Best to | 1203 | Win || Target; Stop | Stop DFT | Sender; Send | 1204 | || DFT | | Backoff; DFT = | 1205 | || | | BOperiod | 1206 +-------------++------------------+------------------+------------------+ 1207 | || -> Lose | -> Lose | - | 1208 | || DF = Sender or | DF = Sender; | Set Best to | 1209 | Backoff || Target; Stop | Stop DFT | Sender; Send | 1210 | || DFT | | Backoff; DFT = | 1211 | || | | BOperiod | 1212 +-------------++------------------+------------------+------------------+ 1213 +-----------++----------------------------------------------------------+ 1214 | || Event | 1215 | ++-------------+--------------+--------------+--------------+ 1216 |Prev State ||Recv Backoff | Recv Pass | Recv Worse | Recv worse | 1217 | ||for us | for us | Pass / Win / | Offer | 1218 | || | | Backoff | | 1219 +-----------++-------------+--------------+--------------+--------------+ 1220 | ||- | -> Win | - | - | 1221 | ||DFT = | Stop DFT | Set DF to | DFT ?= | 1222 |Offer ||BOperiod + | | Sender or | OPlow; MC = | 1223 | ||OPlow; MC = | | Target; DFT | 0 | 1224 | ||0 | | ?= OPlow; MC | | 1225 | || | | = 0 | | 1226 +-----------++-------------+--------------+--------------+--------------+ 1227 | ||-> Offer | -> Offer | -> Offer | -> Offer | 1228 | ||DF = Sender; | DF = Sender; | DF = Sender | DFT = OPlow; | 1229 |Lose ||DFT = OPlow; | DFT = OPlow; | or Target; | MC = 0 | 1230 | ||MC = 0 | MC = 0 | DFT = OPlow; | | 1231 | || | | MC = 0 | | 1232 +-----------++-------------+--------------+--------------+--------------+ 1233 | ||-> Offer | -> Offer | -> Offer | - | 1234 | ||DF = Sender; | DF = Sender; | DF = Sender | Send Winner | 1235 |Win ||DFT = OPlow; | DFT = OPlow; | or Target; | | 1236 | ||MC = 0 | MC = 0 | DFT = OPlow; | | 1237 | || | | MC = 0 | | 1238 +-----------++-------------+--------------+--------------+--------------+ 1239 | ||-> Offer | -> Offer | -> Offer | -> Win | 1240 | ||DF = Sender; | DF = Sender; | DF = Sender | Send Winner; | 1241 |Backoff ||DFT = OPlow; | DFT = OPlow; | or Target; | Stop DFT | 1242 | ||MC = 0 | MC = 0 | DFT = OPlow; | | 1243 | || | | MC = 0 | | 1244 +-----------++-------------+--------------+--------------+--------------+ 1246 +-----------------------------------------------------------------------+ 1247 | In Offer State | 1248 +-----------------------+-----------------------+-----------------------+ 1249 | DFT Expires and MC | DFT Expires and MC | DFT Expires and MC | 1250 | is less than | is equal to | is equal to | 1251 | Robustness | Robustness and we | Robustness and | 1252 | | have path to RPA | there is no path | 1253 | | | to RPA | 1254 +-----------------------+-----------------------+-----------------------+ 1255 | - | -> Win | -> Lose | 1256 | Send Offer; DFT = | Send Winner | Set DF to None | 1257 | OPlow; MC = MC + 1 | | | 1258 +-----------------------+-----------------------+-----------------------+ 1259 +-----------------------------------------------------------------------+ 1260 | In Offer State | 1261 +-----------------------------------------------------------------------+ 1262 | Metric changes and is now worse | 1263 +-----------------------------------------------------------------------+ 1264 | DFT ?= OPlow | 1265 | MC = 0 | 1266 +-----------------------------------------------------------------------+ 1268 +-----------------------------------------------------------------------+ 1269 | In Lose State | 1270 +--------------------------------+--------------------------------------+ 1271 | Detect DF Failure | Metric changes and now | 1272 | | is better than DF | 1273 +--------------------------------+--------------------------------------+ 1274 | -> Offer | -> Offer | 1275 | DF = None; DFT = | DFT = OPlow_int; MC = 0 | 1276 | OPlow_int; MC = 0 | | 1277 +--------------------------------+--------------------------------------+ 1279 +-----------------------------------------------------------------------+ 1280 | In Win State | 1281 +-----------------------+------------------------+----------------------+ 1282 | Metric changes and | Timer Expires and | Path to RPA lost | 1283 | is now worse | MC is less than | | 1284 | | Robustness | | 1285 +-----------------------+------------------------+----------------------+ 1286 | - | - | -> Offer | 1287 | DFT = OPlow; MC = | Send Winner; DFT = | Set DF to None; | 1288 | 0 | OPlow; MC = MC + 1 | DFT = OPlow; MC = | 1289 | | | 0 | 1290 +-----------------------+------------------------+----------------------+ 1292 +-----------------------------------------------------------------------+ 1293 | In Backoff State | 1294 +-----------------------+------------------------+----------------------+ 1295 | Metric changes and | Timer Expires | Path to RPA lost | 1296 | is now better than | | | 1297 | Best | | | 1298 +-----------------------+------------------------+----------------------+ 1299 | -> Win | -> Lose | -> Offer | 1300 | Stop Timer | Send Pass; Set DF | Set DF to None; | 1301 | | to stored Best | DFT = OPlow; MC = | 1302 | | | 0 | 1303 +-----------------------+------------------------+----------------------+ 1304 3.5.4. Election Reliability Enhancements 1306 For the correct operation of BIDIR-PIM it is very important to avoid 1307 situations where two routers consider themselves to be Designated 1308 Forwarders for the same link. The two precautions below are not required 1309 for correct operation but can help diagnose anomalies and correct them. 1311 3.5.5. Missing Pass 1313 After a DF has been elected, a router whose metrics change to become 1314 better than the DF will attempt to take over. If during the re-election 1315 the acting DF has a condition that causes it to lose all of the election 1316 messages (like a CPU overload), the new candidate will transmit three 1317 offers and assume the role of the forwarder resulting in two DFs on the 1318 link. This situation is pathological and should be corrected by fixing 1319 the overloaded router. It is desirable that such an event can be 1320 detected by a network administrator. 1322 When a router becomes the DF for a link without receiving a Pass message 1323 from the known old DF, the PIM neighbor information for the old DF can 1324 be marked to this effect. Upon receiving the next PIM Hello message from 1325 the old DF, the router can retransmit Winner messages for all the RPAs 1326 for which it is acting as the DF. The anomaly may also be logged by the 1327 router in a rate-limited manner to alert the operator. 1329 3.5.6. Periodic Winner Announcement 1331 An additional degree of safety can be achieved by having the DF for each 1332 RPA periodically announce its status in a Winner message. Transmission 1333 of the periodic Winner message can be restricted to occur only for RPAs 1334 which have active groups, thus avoiding the periodic control traffic in 1335 areas of the network without senders or receivers for a particular RPA. 1337 3.6. Timers Counters and Constants 1339 BIDIR-PIM maintains the following timers, as discussed in section 3.1. 1340 All timers are countdown timers - they are set to a value and count down 1341 to zero, at which point they typically trigger an action. Of course 1342 they can just as easily be implemented as count-up timers, where the 1343 absolute expiry time is stored and compared against a real-time clock, 1344 but the language in this specification assumes that they count downwards 1345 to zero. 1347 Per Rendezvous-Point Address (RPA): 1349 Per interface (I): 1351 DF Election Timer: DFT(RPA,I) 1353 Per Group (G): 1355 Upstream Join Timer: JT(G) 1357 Per interface (I): 1359 Join Expiry Timer: ET(G,I) 1361 PrunePending Timer: PPT(G,I) 1363 When timers are started or restarted, they are set to default values. 1364 This section summarizes those default values. 1366 Timer Name: DF Election Timer (DFT) 1368 +--------------------+-------------------------+------------------------+ 1369 | Value Name | Value | Explanation | 1370 +--------------------+-------------------------+------------------------+ 1371 | Offer_Period | 100 ms | Interval to wait | 1372 | | | between repeated | 1373 | | | Offer and Winner | 1374 | | | messages. | 1375 +--------------------+-------------------------+------------------------+ 1376 | Backoff_Period | 1 sec | Period that acting | 1377 | | | DF waits between | 1378 | | | receiving a better | 1379 | | | Offer and sending | 1380 | | | the Pass message | 1381 | | | to transfer DF | 1382 | | | responsibility. | 1383 +--------------------+-------------------------+------------------------+ 1384 | OPLow | rand(0.5, 1) * | Range of actual | 1385 | | Offer_Period | randomised value | 1386 | | | used between | 1387 | | | repeated messages. | 1388 +--------------------+-------------------------+------------------------+ 1389 | OPHigh | Election_Robustness | Interval to wait | 1390 | | * Offer_Period | in order to give a | 1391 | | | chance to a router | 1392 | | | with a better | 1393 | | | Offer to become | 1394 | | | the DF. | 1395 +--------------------+-------------------------+------------------------+ 1397 Timer Names: Join Expiry Timer (ET(G,I)) 1399 +----------------+----------------+-------------------------------------+ 1400 | Value Name | Value | Explanation | 1401 +----------------+----------------+-------------------------------------+ 1402 | J/P HoldTime | from message | Hold Time from Join/Prune Message | 1403 +----------------+----------------+-------------------------------------+ 1404 Timer Names: Prune Pending Timer (PPT(G,I)) 1406 +--------------------------+--------------------+-----------------------+ 1407 | Value Name | Value | Explanation | 1408 +--------------------------+--------------------+-----------------------+ 1409 | J/P Override Interval | Default: 3 secs | Short period after | 1410 | | | a join or prune to | 1411 | | | allow other | 1412 | | | routers on the LAN | 1413 | | | to override the | 1414 | | | join or prune | 1415 +--------------------------+--------------------+-----------------------+ 1417 Note that the value of the J/P Override Interval is interface specific 1418 and depends on both the Propagation_Delay and the Override_Interval 1419 values that may change when Hello messages are received [4]. 1421 Timer Names: Upstream Join Timer (JT(G)) 1423 +-------------+--------------------+------------------------------------+ 1424 |Value Name |Value |Explanation | 1425 +-------------+--------------------+------------------------------------+ 1426 |t_periodic |Default: 60 secs |Period between Join/Prune Messages | 1427 +-------------+--------------------+------------------------------------+ 1428 |t_suppressed |rand(1.1 * |Suppression period when someone | 1429 | |t_periodic, 1.4 * |else sends a J/P message so we | 1430 | |t_periodic) |don't need to do so. | 1431 +-------------+--------------------+------------------------------------+ 1432 |t_override |rand(0, 0.9 * J/P |Randomized delay to prevent | 1433 | |Override Interval) |response implosion when sending a | 1434 | | |join message to override someone | 1435 | | |else's prune message. | 1436 +-------------+--------------------+------------------------------------+ 1438 For more information about these values refer to the PIM-SM [4] 1439 documentation. 1441 Constant Name: DF Election Robustness 1443 +--------------------------+-------------------+------------------------+ 1444 | Constant Name | Value | Explanation | 1445 +--------------------------+-------------------+------------------------+ 1446 | Election_Robustness | Default: 3 | Minimum number of | 1447 | | | election messages | 1448 | | | that must be lost | 1449 | | | in order for | 1450 | | | election to fail. | 1451 +--------------------------+-------------------+------------------------+ 1453 3.7. BIDIR PIM Packet Formats 1455 This section describes the details of the packet formats for BIDIR-PIM 1456 control messages. BIDIR-PIM shares a number of control messages in 1457 common with PIM-SM [4]. These include the Hello and Join/Prune messages 1458 as well as the format for the Encoded-Unicast address. For details on 1459 the format of these packets please refer to the PIM-SM documentation. 1460 Here we will only define the additional packets that are introduced by 1461 BIDIR-PIM. These are the packets used in the DF election process as 1462 well as the Bidir_Capable PIM-Hello option. 1464 3.7.1. DF Election Packet Formats 1466 All PIM control messages have IP protocol number 103. 1468 BIDIR-PIM messages are multicast with TTL 1 to the `ALL-PIM-ROUTERS' 1469 group `224.0.0.13'. 1471 All DF election BIDIR-PIM control messages share the common header 1472 below: 1474 0 1 2 3 1475 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1476 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1477 |PIM Ver| Type |Subtype| Rsvd | Checksum | 1478 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1479 | Encoded-Unicast-RP-Address | 1480 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1481 | Sender Metric Preference | 1482 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1483 | Sender Metric | 1484 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1485 PIM Ver 1486 PIM Version number is 2. 1488 Type All DF-Election PIM control messages share the PIM message Type of 1489 10. 1491 Subtype 1492 Subtypes for DF election messages are: 1494 1 = Offer 1495 2 = Winner 1496 3 = Backoff 1497 4 = Pass 1499 Rsvd Set to zero on transmission. Ignored upon receipt. 1501 Checksum 1502 The checksum is standard IP checksum, i.e. the 16-bit one's 1503 complement of the one's complement sum of the entire PIM message. 1504 For computing the checksum, the checksum field is zeroed. 1506 RP-Address 1507 The bidir RPA for which the election is taking place (note that the 1508 length of this field is more than 32 bits). 1510 Sender Metric Preference 1511 Preference value assigned to the unicast routing protocol that the 1512 message sender used to obtain the route to the RPA. 1514 Sender Metric 1515 The unicast routing table metric used by the message sender to 1516 reach the RPA. The metric is in units applicable to the unicast 1517 routing protocol used. 1519 In addition to the fields defined above the Backoff and Pass messages 1520 have the extra fields described below. 1522 3.7.2. Backoff Message 1524 The Backoff message uses the following fields in addition to the common 1525 election message format described above. 1527 0 1 2 3 1528 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1529 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1530 | Encoded-Unicast-Offering-Address | 1531 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1532 | Offering Metric Preference | 1533 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1534 | Offering Metric | 1535 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1536 | Interval | 1537 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1539 Offering Address 1540 The address of the router that made the last (best) Offer (note 1541 that the length of this field is more than 32 bits). 1543 Offering Metric Preference 1544 Preference value assigned to the unicast routing protocol that the 1545 offering router used to obtain the route to the RPA. 1547 Offering Metric 1548 The unicast routing table metric used by the offering router to 1549 reach the RPA. The metric is in units applicable to the unicast 1550 routing protocol used. 1552 Interval 1553 The backoff interval in milliseconds to be used by routers with 1554 worse metrics than the offering router. 1556 3.7.3. Pass Message 1558 The Pass message uses the following fields in addition to the common 1559 election fields described above. 1561 0 1 2 3 1562 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1563 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1564 | Encoded-Unicast-New-Winner-Address | 1565 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1566 | New Winner Metric Preference | 1567 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1568 | New Winner Metric | 1569 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1570 New Winner Address 1571 The address of the router that made the last (best) Offer (note 1572 that the length of this field is more than 32 bits). 1574 New Winner Metric Preference 1575 Preference value assigned to the unicast routing protocol that the 1576 offering router used to obtain the route to the RPA. 1578 New Winner Metric 1579 The unicast routing table metric used by the offering router to 1580 reach the RPA. The metric is in units applicable to the unicast 1581 routing protocol used. 1583 3.7.4. Bidir Capable PIM-Hello Option 1585 BIDIR-PIM introduces one new PIM-Hello option. 1587 o OptionType 22: Bidir Capable 1589 0 1 2 3 1590 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 1591 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1592 | Type = 22 | Length = 0 | 1593 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1595 4. RP Discovery 1597 Routers discover that a range of multicast group addresses operates in 1598 bi-directional mode and the address of the Rendezvous-Point address 1599 (RPA) serving the group range either through static configuration or 1600 using an automatic RP discovery mechanism like the PIM Bootsrtap 1601 mechanism (BSR). [9] or Auto-RP. 1603 5. Security Considerations 1605 The IPsec [5] authentication header MAY be used to provide data 1606 integrity protection and group-wise data origin authentication of BIDIR- 1607 PIM protocol messages. Authentication of BIDIR-PIM messages can protect 1608 against unwanted behaviour caused by unauthorized or altered BIDIR-PIM 1609 messages. 1611 5.1. Attacks Based on Forged Messages 1613 As in PIM Sparse-Mode, the extent of possible damage depends on the type 1614 of counterfeit messages accepted. BIDIR-PIM only uses link-local 1615 multicast messages sent to the ALL_PIM_ROUTERS address, hence attacks 1616 can only be carried out by directly connected nodes, or with the 1617 complicity of directly connected routers. 1619 Some of the BIDIR-PIM protocol messages (Join/Prune and Hello) are 1620 identical, both in format and functionality, to the respective messages 1621 used in PIM-SM. Security considerations for these messages are to be 1622 found in [4]. Other messages (DF-election messages) are specific to 1623 BIDIR-PIM and will be discussed in the following paragraphs. 1625 By forging DF-election messages an attacker can disrupt the election of 1626 the Designated Forwarder on a link in two different ways: 1628 5.1.1. Election of an Incorrect DF 1630 An attacker can force its election as DF by participating in a regular 1631 election and advertising the best metric to reach the RPA. An attacker 1632 can also try to force the election of another router as DF by sending an 1633 Offer, Winner or Pass message and impersonating another router. In some 1634 cases (e.g. the Offer) multiple messages might be needed to carry out an 1635 attack. 1637 In the case of Offer or Winner messages the attacker will have to 1638 impersonate the node that it wants to have become the DF. In the case of 1639 the Pass it will have to impersonate the current DF. This type of attack 1640 causes the wrong DF to be recorded in all nodes apart from the one that 1641 is being impersonated. This node typically will be able to detect the 1642 anomaly and, possibly, restart a new election. 1644 A more sophisticated attacker might carry out a concurrent DoS attack on 1645 the node being impersonated, so that it will not be able to detect the 1646 forged packets and/or take countermeasures. 1648 All attacks based on impersonation can be detected by all routers and 1649 avoided if the source of DF-election messages can be authenticated. 1650 When authentication is available, spoofed messages MUST be discarded and 1651 a rate-limited warning message SHOULD be logged. 1653 A more subtle attacker could use MAC-level addresses to partition the 1654 set of recipients of DF-election messages and create an inconsistent DF 1655 view on the link. For example the attacker could use unicast MAC 1656 addresses for its forged DF-election messages. To prevent this type of 1657 attack, BIDIR-PIM routers SHOULD check the destination MAC address of 1658 received DF-election messages. This however is ineffective on links 1659 that do not support layer-2 multicast delivery. 1661 Source authentication is also sufficient to prevent this kind of attack. 1663 5.1.2. Preventing Election Convergence 1665 By forging DF election messages, an attacker can prevent the election 1666 from converging thus disrupting the establishment of multicast 1667 forwarding trees. There are many ways to achieve this. The simplest is 1668 by sending an infinite sequence of Offer messages (the metric used in 1669 the messages is not important). 1671 5.2. Non-cryptographic Authentication Mechanisms 1673 A BIDIR-PIM router SHOULD provide an option to limit the set of 1674 neighbors from which it will accept Join/Prune, Assert, and DF-election 1675 messages. Either static configuration of IP addresses or an IPsec 1676 security association may be used. Furthermore, a PIM router SHOULD NOT 1677 accept protocol messages from a router from which it has not yet 1678 received a valid Hello message. 1680 5.2.1. Basic Access Control 1682 In a PIM-SM domain, when all routers are trusted, it is possible to 1683 implement a basic form of access control for both sources and receivers: 1684 Receivers can be validated by the last-hop DR and sources can be 1685 validated by the first-hop DR and/or the RP. 1687 In BIDIR-PIM this is generally feasible only for receivers, as sources 1688 can send to the multicast group without the need for routers to detect 1689 their activity and create source-specific state. However it is possible 1690 to modify the standard BIDIR-PIM behaviour, in a backward compatible 1691 way, to allow per-source access control. The tradeoff would be protocol 1692 simplicity, memory and processing requirements. 1694 5.3. Authentication Using IPsec 1696 The IPsec [5] transport mode using the Authentication Header (AH) is the 1697 RECOMMENDED method to prevent the above attacks against BIDIR-PIM. 1699 It is RECOMMENDED that IPsec authentication be applied to all BIDIR-PIM 1700 protocol messages. The specification on how this is done is to be found 1701 in [4]. specifically the authentication of PIM-SM link-local messages, 1702 described in [4] applies to all BIDIR-PIM messages as well. 1704 5.4. Denial of Service Attacks 1706 The denial of service attack based on forged Join described in [4] also 1707 apply to BIDIR-PIM. 1709 6. Change history 1711 >From 05 to 06: 1713 Minor editorial corrections. 1715 >From 03 to 05: 1717 RP concept replaced by RP Address (RPA) and RP Link (RPL). No DF 1718 election on RPL. RP forwards upstream on RPL. Accept joins even if not 1719 DF but do not forward. Added event description for DF election state 1720 machine. Security considerations by Lorenzo.Removed comparison with 1721 Dino's draft. 1723 >From 02 to 03: 1725 Consistency fixes in DF election tables to match state transition 1726 diagram pointed out by Apoorva. 1728 >From 00 to 01: 1730 The differences between this version (-01) of the BIDIR-PIM 1731 specification and draft-ietf-pim-bidir-new-00.txt are mostly in the 1732 format of the information presented. As BIDIR-PIM has many similarities 1733 in operation to Sparse-Mode PIM, the earlier version of this spec relied 1734 heavily on the now obsolete PIM-SM [8] specification. This revision 1735 removes this dependency and instead references the new Sparse-Mode 1736 documentation [4] where necessary. In addition the method in which the 1737 protocol specification is presented has been updated to follow the 1738 format of [4]. 1740 7. Acknowledgments 1742 The bidir proposal in this draft is heavily based on the ideas and text 1743 presented by Estrin and Farinacci in [7]. The main difference between 1744 the two proposals is in the method chosen for upstream forwarding. 1746 We would also like to thank John Zwiebel at Procket, Deborah Estrin at 1747 ISI/USC as well as Nidhi Bhaskar, Yiqun Cai, Toerless Eckert, Apoorva 1748 Karan, Rajitha Sumanasekera and Beau Williamson at cisco for their 1749 contributions and comments to this draft. 1751 8. Authors' Addresses 1753 Mark Handley 1754 Computer Science Department 1755 University College London 1756 M.Handley@cs.ucl.ac.uk 1758 Isidor Kouvelas 1759 Cisco Systems 1760 kouvelas@cisco.com 1762 Tony Speakman 1763 Cisco Systems 1764 speakman@cisco.com 1766 Lorenzo Vicisano 1767 Cisco Systems 1768 lorenzo@cisco.com 1770 9. Normative References 1772 [1] S.E. Deering, "Host extensions for IP multicasting", RFC 1112, Aug 1773 1989. 1775 [2] B. Cain, S Deering, W. Fenner, I Kouvelas, A. Thyagarajan, "Internet 1776 Group Management Protocol, Version 3", RFC 3376. 1778 [3] S. Deering, W. Fenner, B. Haberman, "Multicast Listener Discovery 1779 (MLD) for IPv6", RFC 2710. 1781 [4] B. Fenner, M. Handley, H. Holbrook, I. Kouvelas "Protocol 1782 Independent Multicast - Sparse Mode (PIM-SM): Protocol 1783 Specification (Revised)", Work In Progress, , 2004. 1786 [5] S. Kent, R. Atkinson, "Security Architecture for the Internet 1787 Protocol.", RFC 2401. 1789 10. Informative References 1791 [6] T. Bates , R. Chandra , D. Katz , Y. Rekhter, "Multiprotocol 1792 Extensions for BGP-4", RFC 2283 1794 [7] D. Estrin, D. Farinacci, "Bi-directional Shared Trees in PIM-SM", 1795 , May 1999. 1797 [8] D. Estrin et al, "Protocol Independent Multicast-Sparse Mode (PIM- 1798 SM): Protocol Specification", RFC 2362, Nov 1999. 1800 [9] W. Fenner, M. Handley, R. Kermode and D. Thaler, "Bootstrap Router 1801 (BSR) Mechanism for PIM Sparse Mode", Work in progress , 2003. 1804 11. Index 1805 DF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7,21 1806 Downstream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1807 DownstreamJPState(G,I) . . . . . . . . . . . . . . . . . . . . . . . 12 1808 ET(G,I). . . . . . . . . . . . . . . . . . . . . . . . . . . . .11,16,34 1809 ET(RPA,I). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1810 I_am_DF(RPA,I) . . . . . . . . . . . . . . . . . . . . . . . . .12,14,17 1811 J/P_HoldTime . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 1812 J/P_Override_Interval. . . . . . . . . . . . . . . . . . . . . . . 18,35 1813 JoinDesired(G) . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 1814 joins(G) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1815 JT(*,G). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 1816 JT(G). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11,35 1817 local_receiver_include(G,I). . . . . . . . . . . . . . . . . . . . . 12 1818 MFIB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1819 NLT(N,I) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1820 Offer_Period . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 1821 olist(G) . . . . . . . . . . . . . . . . . . . . . . . . . . . .12,14,20 1822 OT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 1823 pim_include(G) . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1824 PPT(G,I) . . . . . . . . . . . . . . . . . . . . . . . . . . . .11,16,35 1825 RPA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1826 RPF_interface(RPA) . . . . . . . . . . . . . . . . . . . . . . . . 12,14 1827 RPL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1828 TIB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1829 t_override . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20,35 1830 t_periodic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20,35 1831 t_suppressed . . . . . . . . . . . . . . . . . . . . . . . . . . . 20,35 1832 Upstream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6