idnits 2.17.1 draft-ietf-softwire-mesh-multicast-22.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (June 18, 2018) is 2111 days in the past. Is this intentional? 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 Softwire WG M. Xu 3 Internet-Draft Y. Cui 4 Intended status: Standards Track J. Wu 5 Expires: December 20, 2018 Tsinghua University 6 S. Yang 7 Oudmon Tech 8 C. Metz 9 Cisco Systems 10 June 18, 2018 12 IPv4 Multicast over an IPv6 Multicast in Softwire Mesh Network 13 draft-ietf-softwire-mesh-multicast-22 15 Abstract 17 During the transition to IPv6, there will be scenarios where a 18 backbone network internally running one IP address family (referred 19 to as the internal IP or I-IP family), connects client networks 20 running another IP address family (referred to as the external IP or 21 E-IP family). In such cases, the I-IP backbone needs to offer both 22 unicast and multicast transit services to the client E-IP networks. 24 This document describes a mechanism for supporting multicast across 25 backbone networks where the I-IP and E-IP protocol families differ. 26 The document focuses on IPv4-over-IPv6 scenario, due to lack of real- 27 world use cases for IPv6-over-IPv4 scenario. 29 Status of This Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at https://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on December 20, 2018. 46 Copyright Notice 48 Copyright (c) 2018 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (https://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 64 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5 65 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 66 4. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 67 5. Mesh Multicast Mechanism . . . . . . . . . . . . . . . . . . 7 68 5.1. Mechanism Overview . . . . . . . . . . . . . . . . . . . 7 69 5.2. Group Address Mapping . . . . . . . . . . . . . . . . . . 7 70 5.3. Source Address Mapping . . . . . . . . . . . . . . . . . 8 71 5.4. Routing Mechanism . . . . . . . . . . . . . . . . . . . . 9 72 6. Control Plane Functions of AFBR . . . . . . . . . . . . . . . 10 73 6.1. E-IP (*,G) and (S,G) State Maintenance . . . . . . . . . 10 74 6.2. I-IP (S',G') State Maintenance . . . . . . . . . . . . . 10 75 6.3. E-IP (S,G,rpt) State Maintenance . . . . . . . . . . . . 10 76 6.4. Inter-AFBR Signaling . . . . . . . . . . . . . . . . . . 10 77 6.5. SPT Switchover . . . . . . . . . . . . . . . . . . . . . 13 78 6.6. Other PIM Message Types . . . . . . . . . . . . . . . . . 13 79 6.7. Other PIM States Maintenance . . . . . . . . . . . . . . 13 80 7. Data Plane Functions of the AFBR . . . . . . . . . . . . . . 13 81 7.1. Process and Forward Multicast Data . . . . . . . . . . . 13 82 7.2. TTL . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 83 7.3. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 14 84 8. Packet Format and Translation . . . . . . . . . . . . . . . . 14 85 9. Softwire Mesh Multicast Encapsulation . . . . . . . . . . . . 15 86 10. Security Considerations . . . . . . . . . . . . . . . . . . . 16 87 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 88 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 89 12.1. Normative References . . . . . . . . . . . . . . . . . . 16 90 12.2. Informative References . . . . . . . . . . . . . . . . . 17 91 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 17 92 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 94 1. Introduction 96 During the transition to IPv6, there will be scenarios where a 97 backbone network internally running one IP address family (referred 98 to as the internal IP or I-IP family), connects client networks 99 running another IP address family (referred to as the external IP or 100 E-IP family). 102 One solution is to leverage the multicast functions inherent in the 103 I-IP backbone to efficiently forward client E-IP multicast packets 104 inside an I-IP core tree. The I-IP tree is rooted at one or more 105 ingress Address Family Border Routers (AFBRs) [RFC5565] and branches 106 out to one or more egress AFBRs. 108 [RFC4925] outlines the requirements for the softwire mesh scenario 109 and includes support for multicast traffic. It is likely that client 110 E-IP multicast sources and receivers will reside in different client 111 E-IP networks connected to an I-IP backbone network. This requires 112 the client E-IP source-rooted or shared tree to traverse the I-IP 113 backbone network. 115 This could be accomplished by re-using the multicast VPN approach 116 outlined in [RFC6513]. MVPN-like schemes can support the softwire 117 mesh scenario and achieve a "many-to-one" mapping between the E-IP 118 client multicast trees and the transit core multicast trees. The 119 advantage of this approach is that the number of trees in the I-IP 120 backbone network scales less than linearly with the number of E-IP 121 client trees. Corporate enterprise networks, and by extension 122 multicast VPNs, have been known to run applications that create too 123 many (S,G) states [RFC7899]. Aggregation at the edge contains the 124 (S,G) states for customer's VPNs and these need to be maintained by 125 the network operator. The disadvantage of this approach is the 126 possibility of inefficient bandwidth and resource utilization when 127 multicast packets are delivered to a receiving AFBR with no attached 128 E-IP receivers. 130 [RFC8114] provides a solution for delivering IPv4 multicast services 131 over an IPv6 network. But it mainly focuses on the DS-lite [RFC6333] 132 scenario. This document describes a detailed solution for the IPv4- 133 over-IPv6 softwire mesh scenario, where client networks run IPv4 and 134 the backbone network runs IPv6. 136 Internet-style multicast is somewhat different to the [RFC8114] 137 scenario in that the trees are source-rooted and relatively sparse. 138 The need for multicast aggregation at the edge (where many customer 139 multicast trees are mapped into one or more backbone multicast trees) 140 does not exist and to date has not been identified. Thus the need 141 for alignment between the E-IP and I-IP multicast mechanisms emerges. 143 [RFC5565] describes the "Softwire Mesh Framework". This document 144 provides a more detailed description of how one-to-one mapping 145 schemes ([RFC5565], Section 11.1) for IPv4-over-IPv6 multicast can be 146 achieved. 148 Figure 1 shows an example of how a softwire mesh network can support 149 multicast traffic. A multicast source S is located in one E-IP 150 client network, while candidate E-IP group receivers are located in 151 the same or different E-IP client networks that all share a common 152 I-IP transit network. When E-IP sources and receivers are not local 153 to each other, they can only communicate with each other through the 154 I-IP core. There may be several E-IP sources for a single multicast 155 group residing in different client E-IP networks. In the case of 156 shared trees, the E-IP sources, receivers and rendezvous points (RPs) 157 might be located in different client E-IP networks. In the simplest 158 case, a single operator manages the resources of the I-IP core, 159 although the inter-operator case is also possible and so not 160 precluded. 162 +---------+ +---------+ 163 | | | | +--------+ 164 | E-IP | | E-IP +--+Source S| 165 | network | | network | +--------+ 166 +---+-----+ +--+------+ 167 | | 168 +-+--------+ +-------+--+ 169 | | | upstream | 170 +-| AFBR +--+ AFBR |-+ 171 | +----------+ +----------+ | 172 | | E-IP Multicast 173 | I-IP transit core | packets are forwarded 174 | | across the I-IP 175 | +----------+ +----------+ | transit core 176 +-|dowstream | |downstream|-+ 177 | AFBR |--| AFBR | 178 +--+-------+ +--------+-+ 179 | | 180 +---+----+ +---+----+ 181 +--------+ | | | | +--------+ 182 |Receiver+---+ E-IP | | E-IP +--+Receiver| 183 +--------+ |network | |network | +--------+ 184 +--------+ +--------+ 186 Figure 1: Softwire Mesh Multicast Framework 188 2. Requirements Language 190 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 191 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 192 document are to be interpreted as described in [RFC2119]. 194 3. Terminology 196 Terminology used in this document: 198 o Address Family Border Router (AFBR) - A router interconnecting two 199 or more networks using different IP address families. Besides, in 200 the context of softwire mesh multicast, the AFBR runs E-IP and I-IP 201 control planes to maintain E-IP and I-IP multicast states 202 respectively and performs the appropriate encapsulation/decapsulation 203 of client E-IP multicast packets for transport across the I-IP core. 204 An AFBR will act as a source and/or receiver in an I-IP multicast 205 tree. 207 o Upstream AFBR: An AFBR that is located on the upper reaches of a 208 multicast data flow. 210 o Downstream AFBR: An AFBR that is located on the lower reaches of a 211 multicast data flow. 213 o I-IP (Internal IP): This refers to IP address family that is 214 supported by the core network. In this document, the I-IP is IPv6. 216 o E-IP (External IP): This refers to the IP address family that is 217 supported by the client network(s) attached to the I-IP transit core. 218 In this document, the I-IP is IPv6. 220 o I-IP core tree: A distribution tree rooted at one or more AFBR 221 source nodes and branched out to one or more AFBR leaf nodes. An 222 I-IP core tree is built using standard IP or MPLS multicast signaling 223 protocols operating exclusively inside the I-IP core network. An 224 I-IP core tree is used to forward E-IP multicast packets belonging to 225 E-IP trees across the I-IP core. Another name for an I-IP core tree 226 is multicast or multipoint softwire. 228 o E-IP client tree: A distribution tree rooted at one or more hosts 229 or routers located inside a client E-IP network and branched out to 230 one or more leaf nodes located in the same or different client E-IP 231 networks. 233 o uPrefix46: The /96 unicast IPv6 prefix for constructing an 234 IPv4-embedded IPv6 unicast address [RFC6052]. 236 o mPrefix46: The /96 multicast IPv6 prefix for constructing an 237 IPv4-embedded IPv6 multicast address. 239 o PIMv4, PIMv6: refer to [RFC8114]. 241 o Inter-AFBR signaling: A mechanism used by downstream AFBRs to send 242 PIMv6 messages to the upstream AFBR. 244 4. Scope 246 This document focuses on the IPv4-over-IPv6 scenario, as shown in the 247 following diagram: 249 +---------+ +---------+ 250 | IPv4 | | IPv4 | +--------+ 251 | Client | | Client |--+Source S| 252 | Network | | Network | +--------+ 253 +----+----+ +----+----+ 254 | | 255 +--+-------+ +-------+--+ 256 | | | Upstream | 257 +-+ AFBR +--+ AFBR |-+ 258 | +----------+ +----------+ | 259 | | 260 | IPv6 transit core | 261 | | 262 | +----------+ +----------+ | 263 +-+Downstream+--+Downstream+-+ 264 | AFBR | | AFBR | 265 +--+-------+ +-------+--+ 266 | | 267 +----+----+ +----+----+ 268 +--------+ | IPv4 | | IPv4 | +--------+ 269 |Receiver+--+ Client | | Client +--+Receiver| 270 +--------+ | Network | | Network | +--------+ 271 +---------+ +---------+ 273 Figure 2: IPv4-over-IPv6 Scenario 275 In Figure 2, the E-IP client networks run IPv4 and the I-IP core runs 276 IPv6. 278 Because of the much larger IPv6 group address space, the client E-IP 279 tree can be mapped to a specific I-IP core tree. This simplifies 280 operations on the AFBR because it becomes possible to algorithmically 281 map an IPv4 group/source address to an IPv6 group/source address and 282 vice-versa. 284 The IPv4-over-IPv6 scenario is an emerging requirement as network 285 operators build out native IPv6 backbone networks. These networks 286 support native IPv6 services and applications but in many cases, 287 support for legacy IPv4 unicast and multicast services will also need 288 to be accommodated. 290 5. Mesh Multicast Mechanism 292 5.1. Mechanism Overview 294 Routers in the client E-IP networks have routes to all other client 295 E-IP networks. Through PIMv4 messages, E-IP hosts and routers have 296 discovered or learnt of (S,G) or (*,G) IPv4 addresses. Any I-IP 297 multicast state instantiated in the core is referred to as (S',G') or 298 (*,G') and is separated from E-IP multicast state. 300 Suppose a downstream AFBR receives an E-IP PIM Join/Prune message 301 from the E-IP network for either an (S,G) tree or a (*,G) tree. The 302 AFBR translates the PIMv4 message into an PIMv6 message with the 303 latter being directed towards the I-IP IPv6 address of the upstream 304 AFBR. When the PIMv6 message arrives at the upstream AFBR, it is 305 translated back into an PIMv4 message. The result of these actions 306 is the construction of E-IP trees and a corresponding I-IP tree in 307 the I-IP network. An example of the packet format and translation is 308 provided in Section 8. 310 In this case, it is incumbent upon the AFBRs to perform PIM message 311 conversions in the control plane and IP group address conversions or 312 mappings in the data plane. The AFBRs perform an algorithmic, one- 313 to-one mapping of IPv4-to-IPv6. 315 5.2. Group Address Mapping 317 A simple algorithmic mapping between IPv4 multicast group addresses 318 and IPv6 group addresses is performed. Figure 3 is provided as a 319 reminder of the format: 321 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 322 | 0-------------32--40--48--56--64--72--80--88--96-----------127| 323 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 324 | mPrefix46 | group address | 325 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 327 Figure 3: IPv4-Embedded IPv6 Multicast Address Format 329 An IPv6 multicast prefix (mPrefix46) is provisioned on each AFBR. 330 AFBRs will prepend the prefix to an IPv4 multicast group address when 331 translating it to an IPv6 multicast group address. 333 The construction of the mPrefix46 for SSM is the same as the 334 construction of the mPrefix64 described in Section 5 of [RFC8114]. 336 With this scheme, each IPv4 multicast address can be mapped into an 337 IPv6 multicast address (with the assigned prefix), and each IPv6 338 multicast address with the assigned prefix can be mapped into an IPv4 339 multicast address. The group address translation algorithm can be 340 referred in Section 5.2 of [RFC8114]. 342 5.3. Source Address Mapping 344 There are two kinds of multicast: ASM and SSM. Considering that the 345 I-IP network and E-IP network may support different kinds of 346 multicast, the source address translation rules needed to support all 347 possible scenarios may become very complex. But since SSM can be 348 implemented with a strict subset of the PIM-SM protocol mechanisms 349 [RFC7761], we can treat the I-IP core as SSM-only to make it as 350 simple as possible. There then remain only two scenarios to be 351 discussed in detail: 353 o E-IP network supports SSM 355 One possible way to make sure that the translated PIMv6 message 356 reaches upstream AFBR is to set S' to a virtual IPv6 address that 357 leads to the upstream AFBR. The unicast adddress translation 358 should be achieved according to [RFC6052] 360 o E-IP network supports ASM 362 The (S,G) source list entry and the (*,G) source list entry differ 363 only in that the latter has both the WildCard (WC) and RPT bits of 364 the Encoded-Source-Address set, while with the former, the bits 365 are cleared (See Section 4.9.5.1 of [RFC7761]). As a result, the 366 source list entries in (*,G) messages can be translated into 367 source list entries in (S',G') messages by clearing both the WC 368 and RPT bits at downstream AFBRs, and vice-versa for the reverse 369 translation at upstream AFBRs. 371 5.4. Routing Mechanism 373 With mesh multicast, PIMv6 messages originating from a downstream 374 AFBR need to be propogated to the correct upstream AFBR, and every 375 AFBR needs the /96 prefix in "IPv4-Embedded IPv6 Virtual Source 376 Address Format". 378 To achieve this, every AFBR MUST announce the address of one of its 379 E-IPv4 interfaces in the "v4" field alongside the corresponding 380 uPreifx64. The announcement MUST be sent to the other AFBRs through 381 MBGP [RFC4760]. Every uPrefix46 that an AFBR announces MUST be 382 unique. "uPrefix46" is an IPv6 prefix, and the distribution 383 mechanism is the same as the traditional mesh unicast scenario. 385 As the "v4" field is an E-IP address, and BGP messages are not 386 tunneled through softwires or any other mechanism specified in 387 [RFC5565], AFBRs MUST be able to transport and encode/decode BGP 388 messages that are carried over the I-IP, and whose NLRI and NH are of 389 the E-IP address family. 391 In this way, when a downstream AFBR receives an E-IP PIM (S,G) 392 message, it can translate this message into (S',G') by looking up the 393 IP address of the corresponding AFBR's E-IP interface. Since the 394 uPrefix46 of S' is unique, and is known to every router in the I-IP 395 network, the translated message will be forwarded to the 396 corresponding upstream AFBR, and the upstream AFBR can translate the 397 message back to (S,G). 399 When a downstream AFBR receives an E-IP PIM (*,G) message, S' can be 400 generated according to the format specified in Figure 3, with the 401 "source address" field set to * (wildcard value). The translated 402 message will be forwarded to the corresponding upstream AFBR. Since 403 every PIM router within a PIM domain MUST be able to map a particular 404 multicast group address to the same RP (see Section 4.7 of 405 [RFC7761]), when the upstream AFBR checks the "source address" field 406 of the message, it finds the IPv4 address of the RP, and ascertains 407 that this is originally a (*,G) message. This is then translated 408 back to the (*,G) message and processed. 410 6. Control Plane Functions of AFBR 412 AFBRs are responsible for the following functions: 414 6.1. E-IP (*,G) and (S,G) State Maintenance 416 E-IP (*,G) and (S,G) state maintenance for an AFBR is the same as 417 E-IP (*,G) and (S,G) state maintenance for an mAFTR described in 418 Section 7.2 of [RFC8114] 420 6.2. I-IP (S',G') State Maintenance 422 It is possible that the I-IP transit core runs another, non-transit, 423 I-IP PIM-SSM instance. Since the translated source address starts 424 with the unique "Well-Known" prefix or the ISP-defined prefix that 425 MUST NOT be used by another service provider, mesh multicast will not 426 influence non-transit PIM-SSM multicast at all. When an AFBR 427 receives an I-IP (S',G') message, it MUST check S'. If S' starts 428 with the unique prefix, then the message is actually a translated 429 E-IP (S,G) or (*,G) message, and the AFBR translate this message back 430 to a PIMv4 message and process it. 432 6.3. E-IP (S,G,rpt) State Maintenance 434 When an AFBR wishes to propagate a Join/Prune(S,G,rpt) message to an 435 I-IP upstream router, the AFBR MUST operate as specified in 436 Section 6.5 and Section 6.6. 438 6.4. Inter-AFBR Signaling 440 Assume that one downstream AFBR has joined an RPT of (*,G) and an SPT 441 of (S,G), and decided to perform an SPT switchover. According to 442 [RFC7761], it SHOULD propagate a Prune(S,G,rpt) message along with 443 the periodical Join(*,G) message upstream towards the RP. However, 444 routers in the I-IP transit core do not process (S,G,rpt) messages 445 since the I-IP transit core is treated as SSM-only. As a result, the 446 downstream AFBR is unable to prune S from this RPT, so it will 447 receive two copies of the same data for (S,G). In order to solve 448 this problem, we introduce a new mechanism for downstream AFBRs to 449 inform upstream AFBRs of pruning any given S from an RPT. 451 When a downstream AFBR wishes to propagate an (S,G,rpt) message 452 upstream, it SHOULD encapsulate the (S,G,rpt) message, then send the 453 encapsulated unicast message to the corresponding upstream AFBR, 454 which we call "RP'". 456 When RP' receives this encapsulated message, it SHOULD decapsulate 457 the message as in the unicast scenario, and retrieve the original 458 (S,G,rpt) message. The incoming interface of this message may be 459 different to the outgoing interface which propagates multicast data 460 to the corresponding downstream AFBR, and there may be other 461 downstream AFBRs that need to receive multicast data of (S,G) from 462 this incoming interface, so RP' SHOULD NOT simply process this 463 message as specified in [RFC7761] on the incoming interface. 465 To solve this problem, we introduce an "interface agent" to process 466 all the encapsulated (S,G,rpt) messages the upstream AFBR receives. 467 The interface agent's RP' SHOULD prune S from the RPT of group G when 468 no downstream AFBR is subscribed to receive multicast data of (S,G) 469 along the RPT. 471 In this way, we ensure that downstream AFBRs will not miss any 472 multicast data that they need. The cost of this is that multicast 473 data for (S,G) will be duplicated along the RPT received by AFBRs 474 affected by the SPT switch over, if at least one downstream AFBR 475 exists that has not yet sent Prune(S,G,rpt) messages to the upstream 476 AFBR. 478 In certain deployment scenarios (e.g. if there is only a single 479 downstream router), the interface agent function is not required. 481 The mechanism used to achieve this is left to the implementation. 482 The following diagram provides one possible solution for an 483 "interface agent" implementation: 485 +----------------------------------------+ 486 | | 487 | +-----------+----------+ | 488 | | PIM-SM | UDP | | 489 | +-----------+----------+ | 490 | ^ | | 491 | | | | 492 | | v | 493 | +----------------------+ | 494 | | I/F Agent | | 495 | +----------------------+ | 496 | PIM ^ | multicast | 497 | messages | | data | 498 | | +-------------+---+ | 499 | +--+--|-----------+ | | 500 | | v | v | 501 | +--------- + +----------+ | 502 | | I-IP I/F | | I-IP I/F | | 503 | +----------+ +----------+ | 504 | ^ | ^ | | 505 | | | | | | 506 +--------|-----|----------|-----|--------+ 507 | v | v 509 Figure 4: Interface Agent Implementation Example 511 Figure 4 shows an example of an interface agent implementation using 512 UDP encapsulation. The interface agent has two responsibilities: In 513 the control plane, it SHOULD work as a real interface that has joined 514 (*,G), representing of all the I-IP interfaces which are outgoing 515 interfaces of the (*,G) state machine, and process the (S,G,rpt) 516 messages received from all the I-IP interfaces. 518 The interface agent maintains downstream (S,G,rpt) state machines for 519 every downstream AFBR, and submits Prune (S,G,rpt) messages to the 520 PIM-SM module only when every (S,G,rpt) state machine is in the 521 Prune(P) or PruneTmp(P') state, which means that no downstream AFBR 522 is subscribed to receive multicast data for (S,G) along the RPT of G. 523 Once a (S,G,rpt) state machine changes to NoInfo(NI) state, which 524 means that the corresponding downstream AFBR has switched to receive 525 multicast data of (S,G) along the RPT again, the interface agent 526 SHOULD send a Join (S,G,rpt) to the PIM-SM module immediately. 528 In the data plane, upon receiving a multicast data packet, the 529 interface agent SHOULD encapsulate it at first, then propagate the 530 encapsulated packet from every I-IP interface. 532 NOTICE: It is possible that an E-IP neighbor of RP' has joined the 533 RPT of G, so the per-interface state machine for receiving E-IP Join/ 534 Prune (S,G,rpt) messages SHOULD be preserved. 536 6.5. SPT Switchover 538 After a new AFBR requests the receipt of traffic destined for a 539 multicast group, it will receive all the data from the RPT at first. 540 At this time, every downstream AFBR will receive multicast data from 541 any source from this RPT, in spite of whether they have switched over 542 to an SPT or not. 544 To minimize this redundancy, it is recommended that every AFBR's 545 SwitchToSptDesired(S,G) function employs the "switch on first packet" 546 policy. In this way, the delay in switchover to SPT is kept as small 547 as possible, and after the moment that every AFBR has performed the 548 SPT switchover for every S of group G, no data will be forwarded in 549 the RPT of G, thus no more unnecessary duplication will be produced. 551 6.6. Other PIM Message Types 553 In addition to Join or Prune, other message types exist, including 554 Register, Register-Stop, Hello and Assert. Register and Register- 555 Stop messages are sent by unicast, while Hello and Assert messages 556 are only used between directly linked routers to negotiate with each 557 other. It is not necessary to translate these for forwarding, thus 558 the processing of these messages is out of scope for this document. 560 6.7. Other PIM States Maintenance 562 In addition to states mentioned above, other states exist, including 563 (*,*,RP) and I-IP (*,G') state. Since we treat the I-IP core as SSM- 564 only, the maintenance of these states is out of scope for this 565 document. 567 7. Data Plane Functions of the AFBR 569 7.1. Process and Forward Multicast Data 571 Refer to Section 7.4 of [RFC8114]. If there is at least one outgoing 572 interface whose IP address family is different from the incoming 573 interface, the AFBR MUST encapsulate this packet with 574 mPrefix46-derived and uPrefix46-derived IPv6 address to form an IPv6 575 multicast packet. 577 7.2. TTL 579 Processing of TTL information in protocol headers depends on the 580 tunneling technology, and it is out of scope of this document. 582 7.3. Fragmentation 584 The encapsulation performed by an upstream AFBR will increase the 585 size of packets. As a result, the outgoing I-IP link MTU may not 586 accommodate the larger packet size. As it is not always possible for 587 core operators to increase the MTU of every link. Fragmentation 588 after encapsulation and reassembling of encapsulated packets MUST be 589 supported by AFBRs [RFC5565]. 591 8. Packet Format and Translation 593 Because the PIM-SM Specification is independent of the underlying 594 unicast routing protocol, the packet format in Section 4.9 of 595 [RFC7761] remains the same, except that the group address and source 596 address MUST be translated when traversing an AFBR. 598 For example, Figure 5 shows the register-stop message format in the 599 IPv4 and IPv6 address families. 601 0 1 2 3 602 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 1 603 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 604 |PIM Ver| Type | Reserved | Checksum | 605 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 606 | IPv4 Group Address (Encoded-Group format) | 607 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 608 | IPv4 Source Address (Encoded-Unicast format) | 609 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 610 (1). IPv4 Register-Stop Message Format 612 0 1 2 3 613 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 1 614 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 615 |PIM Ver| Type | Reserved | Checksum | 616 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 617 | IPv6 Group Address (Encoded-Group format) | 618 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 619 | IPv6 Source Address (Encoded-Unicast format) | 620 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 621 (2). IPv6 Register-Stop Message Format 623 Figure 5: Register-Stop Message Format 625 In Figure 5, the semantics of fields "PIM Ver", "Type", "Reserved", 626 and "Checksum" can be referred in Section 4.9 of [RFC7761]. 628 IPv4 Group Address (Encoded-Group format): The encoded-group format 629 of the IPv4 group address described in Section 4.2. 631 IPv4 Source Address (Encoded-Group format): The encoded-unicast 632 format of the IPv4 source address described in Section 4.3. 634 IPv6 Group Address (Encoded-Group format): The encoded-group format 635 of the IPv6 group address described in Section 4.2. 637 IPv6 Source Address (Encoded-Group format): The encoded-unicast 638 format of the IPv6 source address described in Section 4.3. 640 9. Softwire Mesh Multicast Encapsulation 642 Softwire mesh multicast encapsulation does not require the use of any 643 one particular encapsulation mechanism. Rather, it MUST accommodate 644 a variety of different encapsulation mechanisms, and allow the use of 645 encapsulation mechanisms mentioned in [RFC4925]. Additionally, all 646 of the AFBRs attached to the I-IP network MUST implement the same 647 encapsulation mechanism. 649 10. Security Considerations 651 The security concerns raised in [RFC4925] and [RFC7761] are 652 applicable here. 654 The additional workload associated with some schemes could be 655 exploited by an attacker to perform a DDoS attack. 657 Compared with [RFC4925], the security concerns SHOULD be considered 658 more carefully: an attacker could potentially set up many multicast 659 trees in the edge networks, causing too many multicast states in the 660 core network. To defend against these attacks, BGP policies SHOULD 661 be carefully configured, e.g., AFBRs only accept Well-Known prefix 662 advertisements from trusted peers. 664 11. IANA Considerations 666 This document includes no request to IANA. 668 12. References 670 12.1. Normative References 672 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 673 Requirement Levels", BCP 14, RFC 2119, 674 DOI 10.17487/RFC2119, March 1997, 675 . 677 [RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, 678 "Multiprotocol Extensions for BGP-4", RFC 4760, 679 DOI 10.17487/RFC4760, January 2007, 680 . 682 [RFC5565] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh 683 Framework", RFC 5565, DOI 10.17487/RFC5565, June 2009, 684 . 686 [RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. 687 Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052, 688 DOI 10.17487/RFC6052, October 2010, 689 . 691 [RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual- 692 Stack Lite Broadband Deployments Following IPv4 693 Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011, 694 . 696 [RFC6513] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/ 697 BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February 698 2012, . 700 [RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I., 701 Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent 702 Multicast - Sparse Mode (PIM-SM): Protocol Specification 703 (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March 704 2016, . 706 [RFC7899] Morin, T., Ed., Litkowski, S., Patel, K., Zhang, Z., 707 Kebler, R., and J. Haas, "Multicast VPN State Damping", 708 RFC 7899, DOI 10.17487/RFC7899, June 2016, 709 . 711 [RFC8114] Boucadair, M., Qin, C., Jacquenet, C., Lee, Y., and Q. 712 Wang, "Delivery of IPv4 Multicast Services to IPv4 Clients 713 over an IPv6 Multicast Network", RFC 8114, 714 DOI 10.17487/RFC8114, March 2017, 715 . 717 12.2. Informative References 719 [RFC4925] Li, X., Ed., Dawkins, S., Ed., Ward, D., Ed., and A. 720 Durand, Ed., "Softwire Problem Statement", RFC 4925, 721 DOI 10.17487/RFC4925, July 2007, 722 . 724 Appendix A. Acknowledgements 726 Wenlong Chen, Xuan Chen, Alain Durand, Yiu Lee, Jacni Qin and Stig 727 Venaas provided useful input into this document. 729 Authors' Addresses 731 Mingwei Xu 732 Tsinghua University 733 Department of Computer Science, Tsinghua University 734 Beijing 100084 735 P.R. China 737 Phone: +86-10-6278-5822 738 Email: xumw@tsinghua.edu.cn 739 Yong Cui 740 Tsinghua University 741 Department of Computer Science, Tsinghua University 742 Beijing 100084 743 P.R. China 745 Phone: +86-10-6278-5822 746 Email: cuiyong@tsinghua.edu.cn 748 Jianping Wu 749 Tsinghua University 750 Department of Computer Science, Tsinghua University 751 Beijing 100084 752 P.R. China 754 Phone: +86-10-6278-5983 755 Email: jianping@cernet.edu.cn 757 Shu Yang 758 Oudmon Tech 759 OUDMON Technology Co.,ltd 760 Shenzhen 518057 761 P.R. China 763 Phone: +86-755-2601-3697 764 Email: yangshu@oudmon.com 766 Chris Metz 767 Cisco Systems 768 170 West Tasman Drive 769 San Jose, CA 95134 770 USA 772 Phone: +1-408-525-3275 773 Email: chmetz@cisco.com