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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (August 4, 2015) is 3180 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) ** Obsolete normative reference: RFC 4601 (Obsoleted by RFC 7761) ** Downref: Normative reference to an Informational RFC: RFC 4925 Summary: 3 errors (**), 0 flaws (~~), 4 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group M. Xu 3 Internet-Draft Y. Cui 4 Expires: February 5, 2016 J. Wu 5 S. Yang 6 Tsinghua University 7 C. Metz 8 G. Shepherd 9 Cisco Systems 10 August 4, 2015 12 Softwire Mesh Multicast 13 draft-ietf-softwire-mesh-multicast-10 15 Abstract 17 The Internet needs to support IPv4 and IPv6 packets. Both address 18 families and their related protocol suites support multicast of the 19 single-source and any-source varieties. During IPv6 transition, 20 there will be scenarios where a backbone network running one IP 21 address family internally (referred to as internal IP or I-IP) will 22 provide transit services to attached client networks running another 23 IP address family (referred to as external IP or E-IP). It is 24 expected that the I-IP backbone will offer unicast and multicast 25 transit services to the client E-IP networks. 27 Softwire Mesh is a solution to E-IP unicast and multicast support 28 across an I-IP backbone. This document describes the mechanisms for 29 supporting Internet-style multicast across a set of E-IP and I-IP 30 networks supporting softwire mesh. 32 Status of This Memo 34 This Internet-Draft is submitted in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at http://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on February 5, 2016. 49 Copyright Notice 51 Copyright (c) 2015 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents 56 (http://trustee.ietf.org/license-info) in effect on the date of 57 publication of this document. Please review these documents 58 carefully, as they describe your rights and restrictions with respect 59 to this document. Code Components extracted from this document must 60 include Simplified BSD License text as described in Section 4.e of 61 the Trust Legal Provisions and are provided without warranty as 62 described in the Simplified BSD License. 64 This document may contain material from IETF Documents or IETF 65 Contributions published or made publicly available before November 66 10, 2008. The person(s) controlling the copyright in some of this 67 material may not have granted the IETF Trust the right to allow 68 modifications of such material outside the IETF Standards Process. 69 Without obtaining an adequate license from the person(s) controlling 70 the copyright in such materials, this document may not be modified 71 outside the IETF Standards Process, and derivative works of it may 72 not be created outside the IETF Standards Process, except to format 73 it for publication as an RFC or to translate it into languages other 74 than English. 76 Table of Contents 78 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 79 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 80 3. Scenarios of Interest . . . . . . . . . . . . . . . . . . . . 6 81 3.1. IPv4-over-IPv6 . . . . . . . . . . . . . . . . . . . . . 6 82 3.2. IPv6-over-IPv4 . . . . . . . . . . . . . . . . . . . . . 7 83 4. IPv4-over-IPv6 Mechanism . . . . . . . . . . . . . . . . . . 9 84 4.1. Mechanism Overview . . . . . . . . . . . . . . . . . . . 9 85 4.2. Group Address Mapping . . . . . . . . . . . . . . . . . . 9 86 4.3. Source Address Mapping . . . . . . . . . . . . . . . . . 10 87 4.4. Routing Mechanism . . . . . . . . . . . . . . . . . . . . 11 88 5. IPv6-over-IPv4 Mechanism . . . . . . . . . . . . . . . . . . 12 89 5.1. Mechanism Overview . . . . . . . . . . . . . . . . . . . 12 90 5.2. Group Address Mapping . . . . . . . . . . . . . . . . . . 12 91 5.3. Source Address Mapping . . . . . . . . . . . . . . . . . 12 92 5.4. Routing Mechanism . . . . . . . . . . . . . . . . . . . . 13 93 6. Control Plane Functions of AFBR . . . . . . . . . . . . . . . 14 94 6.1. E-IP (*,G) State Maintenance . . . . . . . . . . . . . . 14 95 6.2. E-IP (S,G) State Maintenance . . . . . . . . . . . . . . 14 96 6.3. I-IP (S',G') State Maintenance . . . . . . . . . . . . . 14 97 6.4. E-IP (S,G,rpt) State Maintenance . . . . . . . . . . . . 15 98 6.5. Inter-AFBR Signaling . . . . . . . . . . . . . . . . . . 15 99 6.6. SPT Switchover . . . . . . . . . . . . . . . . . . . . . 17 100 6.7. Other PIM Message Types . . . . . . . . . . . . . . . . . 17 101 6.8. Other PIM States Maintenance . . . . . . . . . . . . . . 17 102 7. Data Plane Functions of AFBR . . . . . . . . . . . . . . . . 17 103 7.1. Process and Forward Multicast Data . . . . . . . . . . . 17 104 7.2. Selecting a Tunneling Technology . . . . . . . . . . . . 18 105 7.3. TTL . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 106 7.4. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 18 107 8. Security Considerations . . . . . . . . . . . . . . . . . . . 18 108 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 109 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 110 10.1. Normative References . . . . . . . . . . . . . . . . . . 19 111 10.2. Informative References . . . . . . . . . . . . . . . . . 19 112 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 20 113 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 115 1. Introduction 117 The Internet needs to support IPv4 and IPv6 packets. Both address 118 families and their related protocol suites support multicast of the 119 single-source and any-source varieties. During IPv6 transition, 120 there will be scenarios where a backbone network running one IP 121 address family internally (referred to as internal IP or I-IP) will 122 provide transit services to attached client networks running another 123 IP address family (referred to as external IP or E-IP). 125 The preferred solution is to leverage the multicast functions 126 inherent in the I-IP backbone, to efficiently forward client E-IP 127 multicast packets inside an I-IP core tree, which roots at one or 128 more ingress AFBR nodes and branches out to one or more egress AFBR 129 leaf nodes. 131 [RFC4925] outlines the requirements for the softwires mesh scenario 132 including the multicast. It is straightforward to envisage that 133 client E-IP multicast sources and receivers will reside in different 134 client E-IP networks connected to an I-IP backbone network. This 135 requires that the client E-IP source-rooted or shared tree should 136 traverse the I-IP backbone network. 138 One method to accomplish this is to re-use the multicast VPN approach 139 outlined in [RFC6513]. MVPN-like schemes can support the softwire 140 mesh scenario and achieve a "many-to-one" mapping between the E-IP 141 client multicast trees and the transit core multicast trees. The 142 advantage of this approach is that the number of trees in the I-IP 143 backbone network scales less than linearly with the number of E-IP 144 client trees. Corporate enterprise networks and by extension 145 multicast VPNs have been known to run applications that create too 146 many (S,G) states. Aggregation at the edge contains the (S,G) states 147 that need to be maintained by the network operator supporting the 148 customer VPNs. The disadvantage of this approach is the possible 149 inefficient bandwidth and resource utilization when multicast packets 150 are delivered to a receiver AFBR with no attached E-IP receivers. 152 Internet-style multicast is somewhat different in that the trees are 153 relatively sparse and source-rooted. The need for multicast 154 aggregation at the edge (where many customer multicast trees are 155 mapped into a few or one backbone multicast trees) does not exist and 156 to date has not been identified. Thus the need for a basic or closer 157 alignment with E-IP and I-IP multicast procedures emerges. 159 A framework on how to support such methods is described in [RFC5565]. 160 In this document, a more detailed discussion supporting the "one-to- 161 one" mapping schemes for the IPv6 over IPv4 and IPv4 over IPv6 162 scenarios will be discussed. 164 2. Terminology 166 An example of a softwire mesh network supporting multicast is 167 illustrated in Figure 1. A multicast source S is located in one E-IP 168 client network, while candidate E-IP group receivers are located in 169 the same or different E-IP client networks that all share a common 170 I-IP transit network. When E-IP sources and receivers are not local 171 to each other, they can only communicate with each other through the 172 I-IP core. There may be several E-IP sources for some multicast 173 group residing in different client E-IP networks. In the case of 174 shared trees, the E-IP sources, receivers and RPs might be located in 175 different client E-IP networks. In a simple case the resources of 176 the I-IP core are managed by a single operator although the inter- 177 provider case is not precluded. 179 ._._._._. ._._._._. 180 | | | | -------- 181 | E-IP | | E-IP |--|Source S| 182 | network | | network | -------- 183 ._._._._. ._._._._. 184 | | 185 AFBR upstream AFBR 186 | | 187 __+____________________+__ 188 / : : : : \ 189 | : : : : | E-IP Multicast 190 | : I-IP transit core : | packets should 191 | : : : : | get across the 192 | : : : : | I-IP transit core 193 \_._._._._._._._._._._._._./ 194 + + 195 downstream AFBR downstream AFBR 196 | | 197 ._._._._ ._._._._ 198 -------- | | | | -------- 199 |Receiver|-- | E-IP | | E-IP |--|Receiver| 200 -------- |network | |network | -------- 201 ._._._._ ._._._._ 203 Figure 1: Softwire Mesh Multicast Framework 205 Terminology used in this document: 207 o Address Family Border Router (AFBR) - A dual-stack router 208 interconnecting two or more networks using different IP address 209 families. In the context of softwire mesh multicast, the AFBR runs 210 E-IP and I-IP control planes to maintain E-IP and I-IP multicast 211 states respectively and performs the appropriate encapsulation/ 212 decapsulation of client E-IP multicast packets for transport across 213 the I-IP core. An AFBR will act as a source and/or receiver in an 214 I-IP multicast tree. 216 o Upstream AFBR: The AFBR router that is located on the upper reaches 217 of a multicast data flow. 219 o Downstream AFBR: The AFBR router that is located on the lower 220 reaches of a multicast data flow. 222 o I-IP (Internal IP): This refers to the form of IP (i.e., either 223 IPv4 or IPv6) that is supported by the core (or backbone) network. 224 An I-IPv6 core network runs IPv6 and an I-IPv4 core network runs 225 IPv4. 227 o E-IP (External IP): This refers to the form of IP (i.e. either IPv4 228 or IPv6) that is supported by the client network(s) attached to the 229 I-IP transit core. An E-IPv6 client network runs IPv6 and an E-IPv4 230 client network runs IPv4. 232 o I-IP core tree: A distribution tree rooted at one or more AFBR 233 source nodes and branched out to one or more AFBR leaf nodes. An 234 I-IP core tree is built using standard IP or MPLS multicast signaling 235 protocols operating exclusively inside the I-IP core network. An 236 I-IP core tree is used to forward E-IP multicast packets belonging to 237 E-IP trees across the I-IP core. Another name for an I-IP core tree 238 is multicast or multipoint softwire. 240 o E-IP client tree: A distribution tree rooted at one or more hosts 241 or routers located inside a client E-IP network and branched out to 242 one or more leaf nodes located in the same or different client E-IP 243 networks. 245 o uPrefix64: The /96 unicast IPv6 prefix for constructing 246 IPv4-embedded IPv6 source address. 248 o Inter-AFBR signaling: A mechanism used by downstream AFBRs to send 249 PIM messages to the upstream AFBR. 251 3. Scenarios of Interest 253 This section describes the two different scenarios where softwires 254 mesh multicast will apply. 256 3.1. IPv4-over-IPv6 257 ._._._._. ._._._._. 258 | IPv4 | | IPv4 | -------- 259 | Client | | Client |--|Source S| 260 | network | | network | -------- 261 ._._._._. ._._._._. 262 | | 263 AFBR upstream AFBR 264 | | 265 __+____________________+__ 266 / : : : : \ 267 | : : : : | 268 | : IPv6 transit core : | 269 | : : : : | 270 | : : : : | 271 \_._._._._._._._._._._._._./ 272 + + 273 downstream AFBR downstream AFBR 274 | | 275 ._._._._ ._._._._ 276 -------- | IPv4 | | IPv4 | -------- 277 |Receiver|-- | Client | | Client |--|Receiver| 278 -------- | network| | network| -------- 279 ._._._._ ._._._._ 281 Figure 2: IPv4-over-IPv6 Scenario 283 In this scenario, the E-IP client networks run IPv4 and I-IP core 284 runs IPv6. This scenario is illustrated in Figure 2. 286 Because of the much larger IPv6 group address space, it will not be a 287 problem to map individual client E-IPv4 tree to a specific I-IPv6 288 core tree. This simplifies operations on the AFBR because it becomes 289 possible to algorithmically map an IPv4 group/source address to an 290 IPv6 group/source address and vice-versa. 292 The IPv4-over-IPv6 scenario is an emerging requirement as network 293 operators build out native IPv6 backbone networks. These networks 294 naturally support native IPv6 services and applications but it is 295 with near 100% certainty that legacy IPv4 networks handling unicast 296 and multicast should be accommodated. 298 3.2. IPv6-over-IPv4 299 ._._._._. ._._._._. 300 | IPv6 | | IPv6 | -------- 301 | Client | | Client |--|Source S| 302 | network | | network | -------- 303 ._._._._. ._._._._. 304 | | 305 AFBR upstream AFBR 306 | | 307 __+____________________+__ 308 / : : : : \ 309 | : : : : | 310 | : IPv4 transit core : | 311 | : : : : | 312 | : : : : | 313 \_._._._._._._._._._._._._./ 314 + + 315 downstream AFBR downstream AFBR 316 | | 317 ._._._._ ._._._._ 318 -------- | IPv6 | | IPv6 | -------- 319 |Receiver|-- | Client | | Client |--|Receiver| 320 -------- | network| | network| -------- 321 ._._._._ ._._._._ 323 Figure 3: IPv6-over-IPv4 Scenario 325 In this scenario, the E-IP Client Networks run IPv6 while the I-IP 326 core runs IPv4. This scenario is illustrated in Figure 3. 328 IPv6 multicast group addresses are longer than IPv4 multicast group 329 addresses. It will not be possible to perform an algorithmic IPv6 - 330 to - IPv4 address mapping without the risk of multiple IPv6 group 331 addresses mapped to the same IPv4 address resulting in unnecessary 332 bandwidth and resource consumption. Therefore additional efforts 333 will be required to ensure that client E-IPv6 multicast packets can 334 be injected into the correct I-IPv4 multicast trees at the AFBRs. 335 This clear mismatch in IPv6 and IPv4 group address lengths means that 336 it will not be possible to perform a one-to-one mapping between IPv6 337 and IPv4 group addresses unless the IPv6 group address is scoped. 339 As mentioned earlier, this scenario is common in the MVPN 340 environment. As native IPv6 deployments and multicast applications 341 emerge from the outer reaches of the greater public IPv4 Internet, it 342 is envisaged that the IPv6 over IPv4 softwire mesh multicast scenario 343 will be a necessary feature supported by network operators. 345 4. IPv4-over-IPv6 Mechanism 347 4.1. Mechanism Overview 349 Routers in the client E-IPv4 networks contain routes to all other 350 client E-IPv4 networks. Through the set of known and deployed 351 mechanisms, E-IPv4 hosts and routers have discovered or learnt of 352 (S,G) or (*,G) IPv4 addresses. Any I-IPv6 multicast state 353 instantiated in the core is referred to as (S',G') or (*,G') and is 354 certainly separated from E-IPv4 multicast state. 356 Suppose a downstream AFBR receives an E-IPv4 PIM Join/Prune message 357 from the E-IPv4 network for either an (S,G) tree or a (*,G) tree. 358 The AFBR can translate the E-IPv4 PIM message into an I-IPv6 PIM 359 message with the latter being directed towards I-IP IPv6 address of 360 the upstream AFBR. When the I-IPv6 PIM message arrives at the 361 upstream AFBR, it should be translated back into an E-IPv4 PIM 362 message. The result of these actions is the construction of E-IPv4 363 trees and a corresponding I-IP tree in the I-IP network. 365 In this case it is incumbent upon the AFBR routers to perform PIM 366 message conversions in the control plane and IP group address 367 conversions or mappings in the data plane. It becomes possible to 368 devise an algorithmic one-to-one IPv4-to-IPv6 address mapping at 369 AFBRs. 371 4.2. Group Address Mapping 373 For IPv4-over-IPv6 scenario, a simple algorithmic mapping between 374 IPv4 multicast group addresses and IPv6 group addresses is supported. 375 [RFC7371] has already defined an applicable format. Figure 4 is the 376 reminder of the format: 378 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 379 | 0-------------32--40--48--56--64--72--80--88--96-----------127| 380 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 381 | MPREFIX64 |group address | 382 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 384 Figure 4: IPv4-Embedded IPv6 Multicast Address Format 386 The MPREFIX64 for SSM mode is also defined in [RFC7371] : 388 o ff3x:0:8000::/96 ('x' is any valid scope) 389 With this scheme, each IPv4 multicast address can be mapped into an 390 IPv6 multicast address (with the assigned prefix), and each IPv6 391 multicast address with the assigned prefix can be mapped into IPv4 392 multicast address. 394 4.3. Source Address Mapping 396 There are two kinds of multicast --- ASM and SSM. Considering that 397 I-IP network and E-IP network may support different kind of 398 multicast, the source address translation rules could be very complex 399 to support all possible scenarios. But since SSM can be implemented 400 with a strict subset of the PIM-SM protocol mechanisms [RFC4601], we 401 can treat I-IP core as SSM-only to make it as simple as possible, 402 then there remains only two scenarios to be discussed in detail: 404 o E-IP network supports SSM 406 One possible way to make sure that the translated I-IPv6 PIM 407 message reaches upstream AFBR is to set S' to a virtual IPv6 408 address that leads to the upstream AFBR. Figure 5 is the 409 recommended address format based on [RFC6052]: 411 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 412 | 0-------------32--40--48--56--64--72--80--88--96-----------127| 413 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 414 | prefix |v4(32) | u | suffix |source address | 415 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 416 |<------------------uPrefix64------------------>| 418 Figure 5: IPv4-Embedded IPv6 Virtual Source Address Format 420 In this address format, the "prefix" field contains a "Well-Known" 421 prefix or an ISP-defined prefix. An existing "Well-Known" prefix 422 is 64:ff9b, which is defined in [RFC6052]; "v4" field is the IP 423 address of one of upstream AFBR's E-IPv4 interfaces; "u" field is 424 defined in [RFC4291], and MUST be set to zero; "suffix" field is 425 reserved for future extensions and SHOULD be set to zero; "source 426 address" field stores the original S. We call the overall /96 427 prefix ("prefix" field and "v4" field and "u" field and "suffix" 428 field altogether) "uPrefix64". 430 o E-IP network supports ASM 431 The (S,G) source list entry and the (*,G) source list entry only 432 differ in that the latter have both the WC and RPT bits of the 433 Encoded-Source-Address set, while the former all cleared (See 434 Section 4.9.5.1 of [RFC4601]). So we can translate source list 435 entries in (*,G) messages into source list entries in (S'G') 436 messages by applying the format specified in Figure 5 and clearing 437 both the WC and RPT bits at downstream AFBRs, and translate them 438 back at upstream AFBRs vice-versa. 440 4.4. Routing Mechanism 442 In the mesh multicast scenario, routing information is required to be 443 distributed among AFBRs to make sure that PIM messages that a 444 downstream AFBR propagates reach the right upstream AFBR. 446 To make it feasible, the /32 prefix in "IPv4-Embedded IPv6 Virtual 447 Source Address Format" must be known to every AFBR, and every AFBR 448 should not only announce the IP address of one of its E-IPv4 449 interfaces presented in the "v4" field to other AFBRs by MPBGP, but 450 also announce the corresponding uPrefix64 to the I-IPv6 network. 451 Since every IP address of upstream AFBR's E-IPv4 interface is 452 different from each other, every uPrefix64 that AFBR announces should 453 be different either, and uniquely identifies each AFBR. "uPrefix64" 454 is an IPv6 prefix, and the distribution of it is the same as the 455 distribution in the traditional mesh unicast scenario. But since 456 "v4" field is an E-IPv4 address, and BGP messages are NOT tunneled 457 through softwires or through any other mechanism as specified in 458 [RFC5565], AFBRs MUST be able to transport and encode/decode BGP 459 messages that are carried over I-IPv6, whose NLRI and NH are of 460 E-IPv4 address family. 462 In this way, when a downstream AFBR receives an E-IPv4 PIM (S,G) 463 message, it can translate this message into (S',G') by looking up the 464 IP address of the corresponding AFBR's E-IPv4 interface. Since the 465 uPrefix64 of S' is unique, and is known to every router in the I-IPv6 466 network, the translated message will eventually arrive at the 467 corresponding upstream AFBR, and the upstream AFBR can translate the 468 message back to (S,G). When a downstream AFBR receives an E-IPv4 PIM 469 (*,G) message, S' can be generated according to the format specified 470 in Figure 4, with "source address" field set to *(the IPv4 address of 471 RP). The translated message will eventually arrive at the 472 corresponding upstream AFBR. Since every PIM router within a PIM 473 domain must be able to map a particular multicast group address to 474 the same RP (see Section 4.7 of [RFC4601]), when this upstream AFBR 475 checks the "source address" field of the message, it'll find the IPv4 476 address of RP, so this upstream AFBR judges that this is originally a 477 (*,G) message, then it translates the message back to the (*,G) 478 message and processes it. 480 5. IPv6-over-IPv4 Mechanism 482 5.1. Mechanism Overview 484 Routers in the client E-IPv6 networks contain routes to all other 485 client E-IPv6 networks. Through the set of known and deployed 486 mechanisms, E-IPv6 hosts and routers have discovered or learnt of 487 (S,G) or (*,G) IPv6 addresses. Any I-IP multicast state instantiated 488 in the core is referred to as (S',G') or (*,G') and is certainly 489 separated from E-IP multicast state. 491 This particular scenario introduces unique challenges. Unlike the 492 IPv4-over-IPv6 scenario, it's impossible to map all of the IPv6 493 multicast address space into the IPv4 address space to address the 494 one-to-one Softwire Multicast requirement. To coordinate with the 495 "IPv4-over-IPv6" scenario and keep the solution as simple as 496 possible, one possible solution to this problem is to limit the scope 497 of the E-IPv6 source addresses for mapping, such as applying a "Well- 498 Known" prefix or an ISP-defined prefix. 500 5.2. Group Address Mapping 502 To keep one-to-one group address mapping simple, the group address 503 range of E-IP IPv6 can be reduced in a number of ways to limit the 504 scope of addresses that need to be mapped into the I-IP IPv4 space. 506 A recommended multicast address format is defined in [RFC7371]. The 507 high order bits of the E-IPv6 address range will be fixed for mapping 508 purposes. With this scheme, each IPv4 multicast address can be 509 mapped into an IPv6 multicast address(with the assigned prefix), and 510 each IPv6 multicast address with the assigned prefix can be mapped 511 into IPv4 multicast address. 513 5.3. Source Address Mapping 515 There are two kinds of multicast --- ASM and SSM. Considering that 516 I-IP network and E-IP network may support different kind of 517 multicast, the source address translation rules could be very complex 518 to support all possible scenarios. But since SSM can be implemented 519 with a strict subset of the PIM-SM protocol mechanisms [RFC4601], we 520 can treat I-IP core as SSM-only to make it as simple as possible, 521 then there remains only two scenarios to be discussed in detail: 523 o E-IP network supports SSM 524 To make sure that the translated I-IPv4 PIM message reaches the 525 upstream AFBR, we need to set S' to an IPv4 address that leads to 526 the upstream AFBR. But due to the non-"one-to-one" mapping of 527 E-IPv6 to I-IPv4 unicast address, the upstream AFBR is unable to 528 remap the I-IPv4 source address to the original E-IPv6 source 529 address without any constraints. 531 We apply a fixed IPv6 prefix and static mapping to solve this 532 problem. A recommended source address format is defined in 533 [RFC6052]. Figure 6 is the reminder of the format: 535 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 536 | 0-------------32--40--48--56--64--72--80--88--96-----------127| 537 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 538 | uPrefix64 |source address | 539 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 541 Figure 6: IPv4-Embedded IPv6 Source Address Format 543 In this address format, the "uPrefix64" field starts with a "Well- 544 Known" prefix or an ISP-defined prefix. An existing "Well-Known" 545 prefix is 64:ff9b/32, which is defined in [RFC6052]; "source 546 address" field is the corresponding I-IPv4 source address. 548 o E-IP network supports ASM 550 The (S,G) source list entry and the (*,G) source list entry only 551 differ in that the latter have both the WC and RPT bits of the 552 Encoded-Source-Address set, while the former all cleared (See 553 Section 4.9.5.1 of [RFC4601]). So we can translate source list 554 entries in (*,G) messages into source list entries in (S',G') 555 messages by applying the format specified in Figure 5 and setting 556 both the WC and RPT bits at downstream AFBRs, and translate them 557 back at upstream AFBRs vice-versa. Here, the E-IPv6 address of RP 558 MUST follow the format specified in Figure 6. RP' is the upstream 559 AFBR that locates between RP and the downstream AFBR. 561 5.4. Routing Mechanism 563 In the mesh multicast scenario, routing information is required to be 564 distributed among AFBRs to make sure that PIM messages that a 565 downstream AFBR propagates reach the right upstream AFBR. 567 To make it feasible, the /96 uPrefix64 must be known to every AFBR, 568 every E-IPv6 address of sources that support mesh multicast MUST 569 follow the format specified in Figure 6, and the corresponding 570 upstream AFBR of this source should announce the I-IPv4 address in 571 "source address" field of this source's IPv6 address to the I-IPv4 572 network. Since uPrefix64 is static and unique in IPv6-over-IPv4 573 scenario, there is no need to distribute it using BGP. The 574 distribution of "source address" field of multicast source addresses 575 is a pure I-IPv4 process and no more specification is needed. 577 In this way, when a downstream AFBR receives a (S,G) message, it can 578 translate the message into (S',G') by simply taking off the prefix in 579 S. Since S' is known to every router in I-IPv4 network, the 580 translated message will eventually arrive at the corresponding 581 upstream AFBR, and the upstream AFBR can translate the message back 582 to (S,G) by appending the prefix to S'. When a downstream AFBR 583 receives a (*,G) message, it can translate it into (S',G') by simply 584 taking off the prefix in *(the E-IPv6 address of RP). Since S' is 585 known to every router in I-IPv4 network, the translated message will 586 eventually arrive at RP'. And since every PIM router within a PIM 587 domain must be able to map a particular multicast group address to 588 the same RP (see Section 4.7 of [RFC4601]), RP' knows that S' is the 589 mapped I-IPv4 address of RP, so RP' will translate the message back 590 to (*,G) by appending the prefix to S' and propagate it towards RP. 592 6. Control Plane Functions of AFBR 594 The AFBRs are responsible for the following functions: 596 6.1. E-IP (*,G) State Maintenance 598 When an AFBR wishes to propagate a Join/Prune(*,G) message to an I-IP 599 upstream router, the AFBR MUST translate Join/Prune(*,G) messages 600 into Join/Prune(S',G') messages following the rules specified above, 601 then send the latter. 603 6.2. E-IP (S,G) State Maintenance 605 When an AFBR wishes to propagate a Join/Prune(S,G) message to an I-IP 606 upstream router, the AFBR MUST translate Join/Prune(S,G) messages 607 into Join/Prune(S',G') messages following the rules specified above, 608 then send the latter. 610 6.3. I-IP (S',G') State Maintenance 612 It is possible that there runs a non-transit I-IP PIM-SSM in the I-IP 613 transit core. Since the translated source address starts with the 614 unique "Well-Known" prefix or the ISP-defined prefix that should not 615 be used otherwise, mesh multicast won't influence non-transit PIM-SM 616 multicast at all. When one AFBR receives an I-IP (S',G') message, it 617 should check S'. If S' starts with the unique prefix, it means that 618 this message is actually a translated E-IP (S,G) or (*,G) message, 619 then the AFBR should translate this message back to E-IP PIM message 620 and process it. 622 6.4. E-IP (S,G,rpt) State Maintenance 624 When an AFBR wishes to propagate a Join/Prune(S,G,rpt) message to an 625 I-IP upstream router, the AFBR MUST do as specified in Section 6.5 626 and Section 6.6. 628 6.5. Inter-AFBR Signaling 630 Assume that one downstream AFBR has joined a RPT of (*,G) and a SPT 631 of (S,G), and decide to perform a SPT switchover. According to 632 [RFC4601], it should propagate a Prune(S,G,rpt) message along with 633 the periodical Join(*,G) message upstream towards RP. Unfortunately, 634 routers in I-IP transit core are not supposed to understand (S,G,rpt) 635 messages since I-IP transit core is treated as SSM-only. As a 636 result, this downstream AFBR is unable to prune S from this RPT, then 637 it will receive two copies of the same data of (S,G). In order to 638 solve this problem, we introduce a new mechanism for downstream AFBRs 639 to inform upstream AFBRs of pruning any given S from RPT. 641 When a downstream AFBR wishes to propagate a (S,G,rpt) message 642 upstream, it should encapsulate the (S,G,rpt) message, then unicast 643 the encapsulated message to the corresponding upstream AFBR, which we 644 call "RP'". 646 When RP' receives this encapsulated message, it should decapsulate 647 this message as what it does in the unicast scenario, and get the 648 original (S,G,rpt) message. The incoming interface of this message 649 may be different from the outgoing interface which propagates 650 multicast data to the corresponding downstream AFBR, and there may be 651 other downstream AFBRs that need to receive multicast data of (S,G) 652 from this incoming interface, so RP' should not simply process this 653 message as specified in [RFC4601] on the incoming interface. 655 To solve this problem, and keep the solution as simple as possible, 656 we introduce an "interface agent" to process all the encapsulated 657 (S,G,rpt) messages the upstream AFBR receives, and prune S from the 658 RPT of group G when no downstream AFBR wants to receive multicast 659 data of (S,G) along the RPT. In this way, we do insure that 660 downstream AFBRs won't miss any multicast data that they needs, at 661 the cost of duplicated multicast data of (S,G) along the RPT received 662 by SPT-switched-over downstream AFBRs, if there exists at least one 663 downstream AFBR that hasn't yet sent Prune(S,G,rpt) messages to the 664 upstream AFBR. The following diagram shows an example of how an 665 "interface agent" may be implemented: 667 +----------------------------------------+ 668 | | 669 | +-----------+----------+ | 670 | | PIM-SM | UDP | | 671 | +-----------+----------+ | 672 | ^ | | 673 | | | | 674 | | v | 675 | +----------------------+ | 676 | | I/F Agent | | 677 | +----------------------+ | 678 | PIM ^ | multicast | 679 | messages | | data | 680 | | +-------------+---+ | 681 | +--+--|-----------+ | | 682 | | v | v | 683 | +--------- + +----------+ | 684 | | I-IP I/F | | I-IP I/F | | 685 | +----------+ +----------+ | 686 | ^ | ^ | | 687 | | | | | | 688 +--------|-----|----------|-----|--------+ 689 | v | v 691 Figure 7: Interface Agent Implementation Example 693 In this example, the interface agent has two responsibilities: In the 694 control plane, it should work as a real interface that has joined 695 (*,G) in representative of all the I-IP interfaces who should have 696 been outgoing interfaces of (*,G) state machine, and process the 697 (S,G,rpt) messages received from all the I-IP interfaces. The 698 interface agent maintains downstream (S,G,rpt) state machines of 699 every downstream AFBR, and submits Prune(S,G,rpt) messages to the 700 PIM-SM module only when every (S,G,rpt) state machine is at Prune(P) 701 or PruneTmp(P') state, which means that no downstream AFBR wants to 702 receive multicast data of (S,G) along the RPT of G. Once a (S,G,rpt) 703 state machine changes to NoInfo(NI) state, which means that the 704 corresponding downstream AFBR has changed it mind to receive 705 multicast data of (S,G) along the RPT again, the interface agent 706 should send a Join(S,G,rpt) to PIM-SM module immediately; In the data 707 plane, upon receiving a multicast data packet, the interface agent 708 should encapsulate it at first, then propagate the encapsulated 709 packet onto every I-IP interface. 711 NOTICE: There may exist an E-IP neighbor of RP' that has joined the 712 RPT of G, so the per-interface state machine for receiving E-IP Join/ 713 Prune(S,G,rpt) messages should still take effect. 715 6.6. SPT Switchover 717 After a new AFBR expresses its interest in receiving traffic destined 718 for a multicast group, it will receive all the data from the RPT at 719 first. At this time, every downstream AFBR will receive multicast 720 data from any source from this RPT, in spit of whether they have 721 switched over to SPT of some source(s) or not. 723 To minimize this redundancy, it's recommended that every AFBR's 724 SwitchToSptDesired(S,G) function employs the "switch on first packet" 725 policy. In this way, the delay of switchover to SPT is kept as 726 little as possible, and after the moment that every AFBR has 727 performed the SPT switchover for every S of group G, no data will be 728 forwarded in the RPT of G, thus no more redundancy will be produced. 730 6.7. Other PIM Message Types 732 Apart from Join or Prune, there exists other message types including 733 Register, Register-Stop, Hello and Assert. Register and Register- 734 Stop messages are sent by unicast, while Hello and Assert messages 735 are only used between dierctly linked routers to negotiate with each 736 other. It's not necessary to translate them for forwarding, thus the 737 process of these messages is out of scope for this document. 739 6.8. Other PIM States Maintenance 741 Apart from states mentioned above, there exists other states 742 including (*,*,RP) and I-IP (*,G') state. Since we treat I-IP core 743 as SSM-only, the maintenance of these states is out of scope for this 744 document. 746 7. Data Plane Functions of AFBR 748 7.1. Process and Forward Multicast Data 750 On receiving multicast data from upstream routers, the AFBR looks up 751 its forwarding table to check the IP address of each outgoing 752 interface. If there exists at least one outgoing interface whose IP 753 address family is different from the incoming interface, the AFBR 754 should encapsulate/decapsulate this packet and forward it to such 755 outgoing interface(s), then forward the data to other outgoing 756 interfaces without encapsulation/decapsulation. 758 When a downstream AFBR that has already switched over to SPT of S 759 receives an encapsulated multicast data packet of (S,G) along the 760 RPT, it should silently drop this packet. 762 7.2. Selecting a Tunneling Technology 764 Choosing tunneling technology depends on the policies configured at 765 AFBRs. It's recommended that all AFBRs use the same technology, 766 otherwise some AFBRs may not be able to decapsulate encapsulated 767 packets from other AFBRs that use a different tunneling technology. 769 7.3. TTL 771 Processing of TTL depends on the tunneling technology, and is out of 772 scope of this document. 774 7.4. Fragmentation 776 The encapsulation performed by upstream AFBR will increase the size 777 of packets. As a result, the outgoing I-IP link MTU may not 778 accommodate the extra size. As it's not always possible for core 779 operators to increase the MTU of every link. Fragmentation and 780 reassembling of encapsulated packets MUST be supported by AFBRs. 782 8. Security Considerations 784 The AFBR routers could maintain secure communications within Security 785 Architecture for the Internet Protocol as described in [RFC4301] . To 786 protect against unwanted forged PIM protocol messages, the PIM 787 messages can be authenticated using IPsec as described in [RFC4601] . 789 But some schemes, which will cause heavy burden on routers, may be 790 used by attackers as a tool when they carry out DDoS attack. 791 Compared with [RFC4301] , the security concerns should be more 792 carefully considered. The attackers can set up many multicast trees 793 in the edge networks, causing too many multicast states in the core 794 network. 796 9. IANA Considerations 798 When AFBRs perform address mapping, they should follow some 799 predefined rules, especially the IPv6 prefix for source address 800 mapping should be predefined, such that ingress AFBRs and egress 801 AFBRs can complete the mapping procedure correctly. The IPv6 prefix 802 for translation can be unified within only the transit core, or 803 within global area. In the later condition, the prefix should be 804 assigned by IANA. 806 10. References 808 10.1. Normative References 810 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 811 Architecture", RFC 4291, DOI 10.17487/RFC4291, February 812 2006, . 814 [RFC4301] Kent, S. and K. Seo, "Security Architecture for the 815 Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, 816 December 2005, . 818 [RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas, 819 "Protocol Independent Multicast - Sparse Mode (PIM-SM): 820 Protocol Specification (Revised)", RFC 4601, 821 DOI 10.17487/RFC4601, August 2006, 822 . 824 [RFC4925] Li, X., Ed., Dawkins, S., Ed., Ward, D., Ed., and A. 825 Durand, Ed., "Softwire Problem Statement", RFC 4925, 826 DOI 10.17487/RFC4925, July 2007, 827 . 829 [RFC5565] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh 830 Framework", RFC 5565, DOI 10.17487/RFC5565, June 2009, 831 . 833 [RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. 834 Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052, 835 DOI 10.17487/RFC6052, October 2010, 836 . 838 [RFC6513] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/ 839 BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February 840 2012, . 842 10.2. Informative References 844 [RFC7371] Boucadair, M. and S. Venaas, "Updates to the IPv6 845 Multicast Addressing Architecture", RFC 7371, 846 DOI 10.17487/RFC7371, September 2014, 847 . 849 Appendix A. Acknowledgements 851 Wenlong Chen, Xuan Chen, Alain Durand, Yiu Lee, Jacni Qin and Stig 852 Venaas provided useful input into this document. 854 Authors' Addresses 856 Mingwei Xu 857 Tsinghua University 858 Department of Computer Science, Tsinghua University 859 Beijing 100084 860 P.R. China 862 Phone: +86-10-6278-5822 863 Email: xmw@cernet.edu.cn 865 Yong Cui 866 Tsinghua University 867 Department of Computer Science, Tsinghua University 868 Beijing 100084 869 P.R. China 871 Phone: +86-10-6278-5822 872 Email: cuiyong@tsinghua.edu.cn 874 Jianping Wu 875 Tsinghua University 876 Department of Computer Science, Tsinghua University 877 Beijing 100084 878 P.R. China 880 Phone: +86-10-6278-5983 881 Email: jianping@cernet.edu.cn 883 Shu Yang 884 Tsinghua University 885 Department of Computer Science, Tsinghua University 886 Beijing 100084 887 P.R. China 889 Phone: +86-10-6278-5822 890 Email: yangshu@csnet1.cs.tsinghua.edu.cn 891 Chris Metz 892 Cisco Systems 893 170 West Tasman Drive 894 San Jose, CA 95134 895 USA 897 Phone: +1-408-525-3275 898 Email: chmetz@cisco.com 900 Greg Shepherd 901 Cisco Systems 902 170 West Tasman Drive 903 San Jose, CA 95134 904 USA 906 Phone: +1-541-912-9758 907 Email: shep@cisco.com