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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 IP over NBMA Working Group Dan Grossman 2 Internet Draft Motorola, Inc. 3 Expires: January 2000 Juha Heinanen 4 draft-ietf-ion-multiprotocol-atm-04.txt Telia 5 July 1999 7 Multiprotocol Encapsulation over ATM Adaptation Layer 5 9 Status of this Memo 11 This document is an Internet-Draft and is in full conformance with 12 all provisions of section 10 of RFC2026. Internet-Drafts are working 13 documents of the Internet Engineering Task Force (IETF), its areas, 14 and its working groups. Note that other groups may also distribute 15 working documents as Internet-Drafts. 17 Internet-Drafts are draft documents valid for a maximum of six months 18 and may be updated, replaced, or obsoleted by other documents at any 19 time. It is inappropriate to use Internet-Drafts as reference 20 material or to cite them other than as ``work in progress.'' 22 The list of current Internet-Drafts can be accessed at 23 http://www.ietf.org/ietf/1id-abstracts.txt 25 The list of Internet-Draft Shadow Directories can be accessed at 26 http://www.ietf.org/shadow.html 28 To learn the current status of any Internet-Draft, please check the 29 ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow 30 Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), 31 munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or 32 ftp.isi.edu (US West Coast). 34 Abstract 36 This memo updates RFC 1483. It describes two encapsulations methods 37 for carrying network interconnect traffic over AAL type 5 over ATM. 38 The first method allows multiplexing of multiple protocols over a 39 single ATM virtual connection whereas the second method assumes that 40 each protocol is carried over a separate ATM virtual connection. 42 Copyright Notice 44 Copyright (C) The Internet Society (1999). All Rights Reserved. 46 Applicability 48 This specification is intended to be used in implementations which 49 use ATM networks to carry multiprotocol traffic among hosts, routers 50 and bridges which are ATM end systems. 52 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 54 1. Introduction 56 Asynchronous Transfer Mode (ATM) wide area, campus and local area 57 networks are used to transport IP datagrams and other connectionless 58 traffic between hosts, routers, bridges and other networking devices. 59 This memo describes two methods for carrying connectionless routed 60 and bridged Protocol Data Units (PDUs) over an ATM network. The "LLC 61 Encapsulation" method allows multiplexing of multiple protocols over 62 a single ATM virtual connection (VC). The protocol type of each PDU 63 is identified by a prefixed IEEE 802.2 Logical Link Control (LLC) 64 header. In the "VC Multiplexing" method, each ATM VC carries PDUs of 65 exactly one protocol type. When multiple protocols need to be 66 transported, there is a separate VC for each. 68 The unit of transport in ATM is a 53 octet fixed length PDU called a 69 cell. A cell consists of a 5 octet header and a 48 byte payload. 70 Variable length PDUs, including those addressed in this memo, must be 71 segmented by the transmitter to fit into the 48 octet ATM cell 72 payload, and reassembled by the receiver. This memo specifies the 73 use of the ATM Adaptation Layer type 5 (AAL5), as defined in ITU-T 74 Recommendation I.363.5 [2] for this purpose. Variable length PDUs are 75 carried in the Payload field of the AAL5 Common Part Convergence 76 Sublayer (CPCS) PDU. 78 This memo only describes how routed and bridged PDUs are carried 79 directly over the AAL5 CPCS, i.e., when the Service Specific 80 Convergence Sublayer (SSCS) of AAL5 is absent. If Frame Relay 81 Service Specific Convergence Sublayer (FR-SSCS), as defined in ITU-T 82 Recommendation I.365.1 [3], is used over the CPCS, then routed and 83 bridged PDUs are carried using the NLPID multiplexing method 84 described in RFC 2427 [4]. The RFC 2427 encapsulation MUST be used in 85 the special case that Frame Relay Network Interworking or transparent 86 mode Service Interworking [9] are used, but is NOT RECOMMENDED for 87 other applications. Appendix A (which is for information only) shows 88 the format of the FR-SSCS-PDU as well as how IP and CLNP PDUs are 89 encapsulated over FR-SSCS according to RFC 2427. 91 This memo also includes an optional encapsulation for use with 92 Virtual Private Networks that operate over an ATM subnet. 94 If it is desired to use the facilities which are designed for the 95 Point- to-Point Protocol (PPP), and there exists a point-to-point 96 relationship between peer systems, then RFC 2364, rather than this 97 memo, applies. 99 2. Conventions 101 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 103 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, 104 SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when 105 they appear in this document, are to be interpreted as described in 106 RFC 2119 [10]. 108 3. Selection of the Multiplexing Method 110 The decision as to whether to use LLC encapsulation or VC- 111 multiplexing depends on implementation and system requirements. In 112 general, LLC encapsulation tends to require fewer VCs in a 113 multiprotocol environment. VC multiplexing tends to reduce 114 fragmentation overhead (e.g., an IPV4 datagram containing a TCP 115 control packet with neither IP nor TCP options exactly fits into a 116 single cell). 118 When two ATM end systems wish to exchange connectionless PDUs across 119 an ATM Permanent Virtual Connection (PVC), selection of the 120 multiplexing method is done by configuration. ATM connection 121 control signalling procedures are used to negotiate the encapsulation 122 method when ATM Switched Virtual Connections (SVCs) are to be used. 123 [5] and [8] specify how this negotiation is done. 125 4. AAL5 PDU Format 127 For both multiplexing methods, routed and bridged PDUs MUST be 128 encapsulated within the Payload field of an AAL5 CPCS-PDU. 130 ITU-T Recomendation I.363.5 [2] provides the complete definition of 131 the AAL5 PDU format and procedures at the sender and receiver. The 132 AAL5 message mode service, in the non-assured mode of operation MUST 133 be used. The corrupted delivery option MUST NOT be used. A 134 reassembly timer MAY be used. The following description is provided 135 for information. 137 The format of the AAL5 CPCS-PDU is shown below: 139 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 141 AAL5 CPCS-PDU Format 142 +-------------------------------+ 143 | . | 144 | . | 145 | CPCS-PDU Payload | 146 | up to 2^16 - 1 octets) | 147 | . | 148 | . | 149 +-------------------------------+ 150 | PAD ( 0 - 47 octets) | 151 +-------------------------------+ ------- 152 | CPCS-UU (1 octet ) | 153 +-------------------------------+ 154 | CPI (1 octet ) | 155 +-------------------------------+CPCS-PDU Trailer 156 | Length (2 octets) | 157 +-------------------------------| 158 | CRC (4 octets) | 159 +-------------------------------+ ------- 161 The Payload field contains user information up to 2^16 - 1 octets. 163 The PAD field pads the CPCS-PDU to fit exactly into the ATM cells 164 such that the last 48 octet cell payload created by the SAR sublayer 165 will have the CPCS-PDU Trailer right justified in the cell. 167 The CPCS-UU (User-to-User indication) field is used to transparently 168 transfer CPCS user to user information. The field is not used by the 169 multiprotocol ATM encapsulation described in this memo and MAY be set 170 to any value. 172 The CPI (Common Part Indicator) field aligns the CPCS-PDU trailer to 173 64 bits. This field MUST be coded as 0x00. 175 The Length field indicates the length, in octets, of the Payload 176 field. The maximum value for the Length field is 65535 octets. A 177 Length field coded as 0x00 is used for the abort function. 179 The CRC field is used to detect bit errors in the CPCS-PDU. A CRC-32 180 is used. 182 5. LLC Encapsulation 184 LLC Encapsulation is needed when more than one protocol might be 185 carried over the same VC. In order to allow the receiver to properly 186 process the incoming AAL5 CPCS-PDU, the Payload Field contains 187 information necessary to identify the protocol of the routed or 188 bridged PDU. In LLC Encapsulation, this information MUST be encoded 190 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 192 in an LLC header placed in front of the carried PDU. 194 Although this memo only deals with protocols that operate over LLC 195 Type 1 (unacknowledged connectionless mode) service, the same 196 encapsulation principle also applies to protocols operating over LLC 197 Type 2 (connection-mode) service. In the latter case the format and 198 contents of the LLC header would be as described in IEEE 802.1 and 199 IEEE 802.2. 201 5.1. LLC Encapsulation for Routed Protocols 203 In LLC Encapsulation, the protocol type of routed PDUs MUST be 204 identified by prefixing an IEEE 802.2 LLC header to each PDU. In 205 some cases, the LLC header MUST be followed by an IEEE 802.1a 206 SubNetwork Attachment Point (SNAP) header. In LLC Type 1 operation, 207 the LLC header MUST consist of three one octet fields: 209 +------+------+------+ 210 | DSAP | SSAP | Ctrl | 211 +------+------+------+ 213 In LLC Encapsulation for routed protocols, the Control field MUST be 214 set to 0x03, specifying a Unnumbered Information (UI) Command PDU. 216 The LLC header value 0xFE-FE-03 MUST be used to identify a routed PDU 217 in the ISO NLPID format (see [6] and Appendix B). For NLPID-formatted 218 routed PDUs, the content of the AAL5 CPCS-PDU Payload field MUST be 219 as follows: 221 Payload Format for Routed NLPID-formatted PDUs 222 +-------------------------------+ 223 | LLC 0xFE-FE-03 | 224 +-------------------------------+ 225 | NLPID (1 octet) | 226 +-------------------------------+ 227 | . | 228 | PDU | 229 | (up to 2^16 - 4 octets) | 230 | . | 231 +-------------------------------+ 233 The routed protocol MUST be identified by a one octet NLPID field 234 that is part of Protocol Data. NLPID values are administered by ISO 235 and ITU-T. They are defined in ISO/IEC TR 9577 [6] and some of the 236 currently defined ones are listed in Appendix C. 238 An NLPID value of 0x00 is defined in ISO/IEC TR 9577 as the Null 239 Network Layer or Inactive Set. Since it has no significance within 241 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 243 the context of this encapsulation scheme, a NLPID value of 0x00 MUST 244 NOT be used. 246 Although there is a NLPID value (0xCC) that indicates IP, the NLPID 247 format MUST NOT be used for IP. Instead, IP datagrams MUST be 248 identified by a SNAP header, as defined below. 250 The presence of am IEEE 802.1a SNAP header is indicated by the LLC 251 header value 0xAA-AA-03. A SNAP header is of the form 253 +------+------+------+------+------+ 254 | OUI | PID | 255 +------+------+------+------+------+ 257 The SNAP header consists of a three octet Organizationally Unique 258 Identifier (OUI) and a two octet Protocol Identifier (PID). The OUI 259 is administered by IEEE and identifies an organization which 260 administers the values which might be assigned to the PID. The SNAP 261 header thus uniquely identifies a routed or bridged protocol. The 262 OUI value 0x00-00-00 indicates that the PID is an EtherType. 264 The format of the AAL5 CPCS-PDU Payload field for routed non-NLPID 265 Formatted PDUs MUST be as follows: 267 Payload Format for Routed non-NLPID formatted PDUs 268 +-------------------------------+ 269 | LLC 0xAA-AA-03 | 270 +-------------------------------+ 271 | OUI 0x00-00-00 | 272 +-------------------------------+ 273 | EtherType (2 octets) | 274 +-------------------------------+ 275 | . | 276 | Non-NLPID formatted PDU | 277 | (up to 2^16 - 9 octets) | 278 | . | 279 +-------------------------------+ 281 In the particular case of an IPv4 PDU, the Ethertype value is 0x08- 282 00, and the payload format MUST be: 284 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 286 Payload Format for Routed IPv4 PDUs 287 +-------------------------------+ 288 | LLC 0xAA-AA-03 | 289 +-------------------------------+ 290 | OUI 0x00-00-00 | 291 +-------------------------------+ 292 | EtherType 0x08-00 | 293 +-------------------------------+ 294 | . | 295 | IPv4 PDU | 296 | (up to 2^16 - 9 octets) | 297 | . | 298 +-------------------------------+ 300 This format is consistent with that defined in RFC 1042 [7]. 302 5.2. LLC Encapsulation for Bridged Protocols 304 In LLC Encapsulation, bridged PDUs are encapsulated by identifying 305 the type of the bridged media in the SNAP header. The presence of 306 the SNAP header MUST be indicated by the LLC header value 0xAA-AA-03. 307 The OUI value in the SNAP header MUST be the 802.1 organization code 308 0x00-80-C2. The type of the bridged media MUST be specified by the 309 two octet PID. The PID MUST also indicate whether the original Frame 310 Check Sequence (FCS) is preserved within the bridged PDU. Appendix B 311 provides a list of media type (PID) values that can be used in ATM 312 encapsulation. 314 The AAL5 CPCS-PDU Payload field carrying a bridged PDU MUST have one 315 of the following formats. The necessary number of padding octets 316 MUST be added after the PID field in order to align the 317 Ethernet/802.3 LLC Data field, 802.4 Data Unit field, 802.5 Info 318 field, FDDI Info field or 802.6 Info field (respectively) of the 319 bridged PDU to begin at a four octet boundary. The bit ordering of 320 the MAC address MUST be the same as it would be on the LAN or MAN 321 (e.g., in canoncial form for bridged Ethernet/IEEE 802.3 PDUs, but in 322 802.5/FDDI format for bridged 802.5 PDUs). 324 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 326 Payload Format for Bridged Ethernet/802.3 PDUs 327 +-------------------------------+ 328 | LLC 0xAA-AA-03 | 329 +-------------------------------+ 330 | OUI 0x00-80-C2 | 331 +-------------------------------+ 332 | PID 0x00-01 or 0x00-07 | 333 +-------------------------------+ 334 | PAD 0x00-00 | 335 +-------------------------------+ 336 | MAC destination address | 337 +-------------------------------+ 338 | | 339 | (remainder of MAC frame) | 340 | | 341 +-------------------------------+ 342 | LAN FCS (if PID is 0x00-01) | 343 +-------------------------------+ 345 The Ethernet/802.3 physical layer requires padding of frames to a 346 minimum size. A bridge that uses uses the Bridged Ethernet/802.3 347 encapsulation format with the preserved LAN FCS MUST include padding. A 348 bridge that uses the Bridged Ethernet/802.3 encapsulation format without 349 the preserved LAN FCS MAY either include padding, or omit it. When a 350 bridge receives a frame in this format without the LAN FCS, it MUST be 351 able to insert the necessary padding (if none is already present) before 352 forwarding to an Ethernet/802.3 subnetwork. 354 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 356 Payload Format for Bridged 802.4 PDUs 357 +-------------------------------+ 358 | LLC 0xAA-AA-03 | 359 +-------------------------------+ 360 | OUI 0x00-80-C2 | 361 +-------------------------------+ 362 | PID 0x00-02 or 0x00-08 | 363 +-------------------------------+ 364 | PAD 0x00-00-00 | 365 +-------------------------------+ 366 | Frame Control (1 octet) | 367 +-------------------------------+ 368 | MAC destination address | 369 +-------------------------------+ 370 | | 371 | (remainder of MAC frame) | 372 | | 373 +-------------------------------+ 374 | LAN FCS (if PID is 0x00-02) | 375 +-------------------------------+ 377 Payload Format for Bridged 802.5 PDUs 378 +-------------------------------+ 379 | LLC 0xAA-AA-03 | 380 +-------------------------------+ 381 | OUI 0x00-80-C2 | 382 +-------------------------------+ 383 | PID 0x00-03 or 0x00-09 | 384 +-------------------------------+ 385 | PAD 0x00-00-XX | 386 +-------------------------------+ 387 | Frame Control (1 octet) | 388 +-------------------------------+ 389 | MAC destination address | 390 +-------------------------------+ 391 | | 392 | (remainder of MAC frame) | 393 | | 394 +-------------------------------+ 395 | LAN FCS (if PID is 0x00-03) | 396 +-------------------------------+ 398 Since the 802.5 Access Control (AC) field has no significance outside 399 the local 802.5 subnetwork, it is treated by this encapsulation as 400 the last octet of the three octet PAD field. It MAY be set to any 401 value by the sending bridge and MUST be ignored by the receiving 403 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 405 bridge. 407 Payload Format for Bridged FDDI PDUs 408 +-------------------------------+ 409 | LLC 0xAA-AA-03 | 410 +-------------------------------+ 411 | OUI 0x00-80-C2 | 412 +-------------------------------+ 413 | PID 0x00-04 or 0x00-0A | 414 +-------------------------------+ 415 | PAD 0x00-00-00 | 416 +-------------------------------+ 417 | Frame Control (1 octet) | 418 +-------------------------------+ 419 | MAC destination address | 420 +-------------------------------+ 421 | | 422 | (remainder of MAC frame) | 423 | | 424 +-------------------------------+ 425 | LAN FCS (if PID is 0x00-04) | 426 +-------------------------------+ 428 Payload Format for Bridged 802.6 PDUs 429 +-------------------------------+ 430 | LLC 0xAA-AA-03 | 431 +-------------------------------+ 432 | OUI 0x00-80-C2 | 433 +-------------------------------+ 434 | PID 0x00-0B | 435 +---------------+---------------+ ------ 436 | Reserved | BEtag | Common 437 +---------------+---------------+ PDU 438 | BAsize | Header 439 +-------------------------------+ ------- 440 | MAC destination address | 441 +-------------------------------+ 442 | | 443 | (remainder of MAC frame) | 444 | | 445 +-------------------------------+ 446 | | 447 | Common PDU Trailer | 448 | | 449 +-------------------------------+ 451 In bridged 802.6 PDUs, the presence of a CRC-32 is indicated by the 452 CIB bit in the header of the MAC frame. Therefore, the same PID 454 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 456 value is used regardless of the presence or absence of the CRC-32 in 457 the PDU. 459 The Common Protocol Data Unit (PDU) Header and Trailer are conveyed 460 to allow pipelining at the egress bridge to an 802.6 subnetwork. 461 Specifically, the Common PDU Header contains the BAsize field, which 462 contains the length of the PDU. If this field is not available to 463 the egress 802.6 bridge, then that bridge cannot begin to transmit 464 the segmented PDU until it has received the entire PDU, calculated 465 the length, and inserted the length into the BAsize field. If the 466 field is available, the egress 802.6 bridge can extract the length 467 from the BAsize field of the Common PDU Header, insert it into the 468 corresponding field of the first segment, and immediately transmit 469 the segment onto the 802.6 subnetwork. Thus, the bridge can begin 470 transmitting the 802.6 PDU before it has received the complete PDU. 472 Note that the Common PDU Header and Trailer of the encapsulated frame 473 should not be simply copied to the outgoing 802.6 subnetwork because 474 the encapsulated BEtag value may conflict with the previous BEtag 475 value transmitted by that bridge. 477 An ingress 802.6 bridge can abort an AAL5 CPCS-PDU by setting its 478 Length field to zero. If the egress bridge has already begun 479 transmitting segments of the PDU to an 802.6 subnetwork and then 480 notices that the AAL5 CPCS-PDU has been aborted, it may immediately 481 generate an EOM cell that causes the 802.6 PDU to be rejected at the 482 receiving bridge. Such an EOM cell could, for example, contain an 483 invalid value in the Length field of the Common PDU Trailer. 485 Payload Format for BPDUs 486 +-------------------------------+ 487 | LLC 0xAA-AA-03 | 488 +-------------------------------+ 489 | OUI 0x00-80-C2 | 490 +-------------------------------+ 491 | PID 0x00-0E | 492 +-------------------------------+ 493 | | 494 | BPDU as defined by | 495 | 802.1(d) or 802.1(g) | 496 | | 497 +-------------------------------+ 499 6. VC Multiplexing 501 VC Multiplexing creates a binding between an ATM VC and the type of 502 the network protocol carried on that VC. Thus, there is no need for 503 protocol identification information to be carried in the payload of 505 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 507 each AAL5 CPCS-PDU. This reduces payload overhead and can reduce 508 per-packet processing. VC multiplexing can improve efficiency by 509 reducing the number of cells needed to carry PDUs of certain lengths. 511 For ATM PVCs, the type of the protocol to be carried over each PVC 512 MUST be determined by configuration. For ATM SVCs, the negotiations 513 specified in RFC 1755 [5] MUST be used. 515 6.1. VC Multiplexing of Routed Protocols 517 PDUs of routed protocols MUST be carried as the only content of the 518 Payload of the AAL5 CPCS-PDU. The format of the AAL5 CPCS-PDU 519 Payload field thus becomes: 521 Payload Format for Routed PDUs 522 +-------------------------------+ 523 | . | 524 | Carried PDU | 525 | (up to 2^16 - 1 octets) | 526 | . | 527 | . | 528 +-------------------------------+ 529 6.2. VC Multiplexing of Bridged Protocols 531 PDUs of bridged protocols MUST be carried in the Payload of the AAL5 532 CPCS-PDU exactly as described in section 5.2, except that only the 533 fields after the PID field MUST be included. The AAL5 CPCS-PDU 534 Payload field carrying a bridged PDU MUST, therefore, have one of the 535 following formats. 537 Payload Format for Bridged Ethernet/802.3 PDUs 538 +-------------------------------+ 539 | PAD 0x00-00 | 540 +-------------------------------+ 541 | MAC destination address | 542 +-------------------------------+ 543 | | 544 | (remainder of MAC frame) | 545 | | 546 +-------------------------------+ 547 | LAN FCS (VC dependent option) | 548 +-------------------------------+ 550 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 552 Payload Format for Bridged 802.4/802.5/FDDI PDUs 553 +-------------------------------+ 554 | PAD 0x00-00-00 or 0x00-00-XX | 555 +-------------------------------+ 556 | Frame Control (1 octet) | 557 +-------------------------------+ 558 | MAC destination address | 559 +-------------------------------+ 560 | | 561 | (remainder of MAC frame) | 562 | | 563 +-------------------------------+ 564 | LAN FCS (VC dependent option) | 565 +-------------------------------+ 567 Note that the 802.5 Access Control (AC) field has no significance 568 outside the local 802.5 subnetwork. It can thus be regarded as the 569 last octet of the three octet PAD field, which in case of 802.5 can 570 be set to any value (XX). 572 Payload Format for Bridged 802.6 PDUs 573 +---------------+---------------+ ------- 574 | Reserved | BEtag | Common 575 +---------------+---------------+ PDU 576 | BAsize | Header 577 +-------------------------------+ ------- 578 | MAC destination address | 579 +-------------------------------+ 580 | | 581 | (remainder of MAC frame) | 582 | | 583 +-------------------------------+ 584 | | 585 | Common PDU Trailer | 586 | | 587 +-------------------------------+ 589 Payload Format for BPDUs 590 +-------------------------------+ 591 | | 592 | BPDU as defined by | 593 | 802.1(d) or 802.1(g) | 594 | | 595 +-------------------------------+ 597 In case of Ethernet, 802.3, 802.4, 802.5, and FDDI PDUs the presense 598 or absence of the trailing LAN FCS shall be identified implicitly by 600 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 602 the VC, since the PID field is not included. PDUs with the LAN FCS 603 and PDUs without the LAN FCS are thus considered to belong to 604 different protocols even if the bridged media type would be the same. 606 7. Bridging in an ATM Network 608 A bridge with an ATM interface that serves as a link to one or more 609 other bridge MUST be able to flood, forward, and filter bridged PDUs. 611 Flooding is performed by sending the PDU to all possible appropriate 612 destinations. In the ATM environment this means sending the PDU 613 through each relevant VC. This may be accomplished by explicitly 614 copying it to each VC or by using a point-to-multipoint VC. 616 To forward a PDU, a bridge MUST be able to associate a destination 617 MAC address with a VC. It is unreasonable and perhaps impossible to 618 require bridges to statically configure an association of every 619 possible destination MAC address with a VC. Therefore, ATM bridges 620 must provide enough information to allow an ATM interface to 621 dynamically learn about foreign destinations beyond the set of ATM 622 stations. 624 To accomplish dynamic learning, a bridged PDU MUST conform to the 625 encapsulation described in section 4. In this way, the receiving ATM 626 interface will know to look into the bridged PDU and learn the 627 association between foreign destination and an ATM station. 629 8. Virtual Private Network (VPN) identification 631 The encapsulation defined in this section applies only to Virtual 632 Private Networks (VPNs) that operate over an ATM subnet. 634 A mechanism for globally unique identification of Virtual Private 635 multiprotocol networks is defined in [11]. The 7-octet VPN-Id 636 consists of a 3-octet VPN-related OUI (IEEE 802-1990 Organizationally 637 Unique Identifier), followed by a 4-octet VPN index which is 638 allocated by the owner of the VPN-related OUI. Typically, the VPN- 639 related OUI value is assigned to a VPN service provider, which then 640 allocates VPN index values for its customers. 642 8.1 VPN Encapsulation Header 644 The format of the VPN encapsulation header is as follows: 646 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 648 VPN Encapsulation Header 649 +-------------------------------+ 650 | LLC 0xAA-AA-03 | 651 +-------------------------------+ 652 | OUI 0x00-00-5E | 653 +-------------------------------+ 654 | PID 0x00-08 | 655 +-------------------------------+ 656 | Reserved (1 octet) | 657 +-------------------------------+ 658 | VPN related OUI (3 octets) | 659 +-------------------------------+ 660 | VPN Index (4 octets) | 661 +-------------------------------+ 662 | | 663 | (remainder of PDU) | 664 | | 665 +-------------------------------+ 667 When the encapsulation header is used, the remainder of the PDU MUST 668 be structured according to the appropiate format described in section 669 5 or 6 (i.e., the VPN encapsulation header is prepended to the PDU 670 within an AAL5 CPCS SDU). 672 8.2 LLC-encapsulated routed or bridged PDUs within a VPN 674 When a LLC-encapsulated routed or bridged PDU is sent within a VPN 675 using ATM over AAL5, a VPN encapsulation header MUST be prepended to 676 the appropriate routed or bridged PDU format defined in sections 5.1 677 and 5.2, respectively. 679 8.3 VC multiplexing of routed or bridged PDUs within a VPN 681 When a routed or bridged PDU is sent within a VPN using VC 682 multiplexing, the VPN identifier MAY either be specified a priori, 683 using ATM connection control signalling or adminstrative assignment 684 to an ATM interface, or it MAY be indicated using an encapsulation 685 header. 687 If the VPN is identified using ATM connection control signalling, all 688 PDUs carried by the ATM VC are associated with the same VPN. In 689 this case, the payload formats of routed and bridged PDUs MUST be as 690 defined in sections 6.1 and 6.2, respectively. If a PDU is received 691 containing a VPN encapsulation header when the VPN has been 692 identified using ATM signalling, the receiver MAY drop it and/or take 693 other actions which are implementation specific. Specification of 694 the mechanism in ATM connection control signalling for carrying VPN 696 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 698 identifiers is outside the scope of this Memo. 700 If a VPN identifier is administratively assigned to an ATM interface, 701 then all PDUs carried by any ATM VCs within that interface are 702 associated with that VPN. In this case, the payload formats of 703 routed and bridged PDUs MUST be as defined in sections 6.1 and 6.2, 704 respectively. If a PDU is received containing a VPN encapsulation 705 header when the VPN identifier has been administratively assigned, 706 the receiver MAY drop it and/or take other actions which are 707 implementation specific. Specification of mechanisms (such as MIBs) 708 for assigning VPN identifiers to ATM interfaces is outside the scope 709 of this Memo. 711 If the VPN identifier is to be indicated using an encapsulation 712 header, then a VPN encapsulation header MUST be prepended to the 713 appropriate routed or bridged PDU format defined in sections 6.1 and 714 6.2, respectively. 716 9. Security Considerations 718 This memo defines mechanisms for multiprotocol encapsulation over 719 ATM. There is an element of trust in any encapsulation protocol: a 720 receiver must trust that the sender has correctly identified the 721 protocol being encapsulated. There is no way to ascertain that the 722 sender did use the proper protocol identification (nor would this be 723 desirable functionality). The encapsulation mechanisms described in 724 this memo are believed not to have any other properties that might be 725 exploited by an attacker. However, architectures and protocols 726 operating above the encapsulation layer may be subject to a variety 727 of attacks. In particular, the bridging architecture discussed in 728 section 7 has the same vulnerabilities as other bridging 729 architectures. 731 System security may be affected by the properties of the underlying 732 ATM network. The ATM Forum has published a security framework [12] 733 and a security specification [13] which may be relevant. 735 Acknowledgements 737 This memo is an update of RFC 1483, which was developed by the IP 738 over ATM working group, and edited by Juha Heinanen (then at Telecom 739 Finland, now at Telia). The update was developed in the IP-over-NBMA 740 (ION) working group, and Dan Grossman (Motorola) was editor and also 741 contributed to the work on RFC 1483. 743 This material evolved from RFCs [1] and [4] from which much of the 744 material has been adopted. Thanks to their authors Terry Bradley, 745 Caralyn Brown, Andy Malis, Dave Piscitello, and C. Lawrence. Other 747 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 749 key contributors to the work included Brian Carpenter (CERN), Rao 750 Cherukuri (IBM), Joel Halpern (then at Network Systems), Bob Hinden 751 (Sun Microsystems, presently at Nokia), and Gary Kessler (MAN 752 Technology). 754 The material concerning VPNs was developed by Barbara Fox (Lucent) 755 and Bernhard Petri (Siemens). 757 References 759 [1] Piscitello, D. and Lawrence, C., "The Transmission of IP 760 Datagrams over the SMDS Service". RFC 1209, Bell Communications 761 Research, March 1991. 763 [2] ITU-T Recommendation I.363.5, "B-ISDN ATM Adaptation Layer (AAL) 764 Type 5 Specification", August, 1996. 766 [3] ITU-T Recommendation I.365.1, "Frame Relaying 767 Service Specific Convergence Sublayer (SSCS), November, 768 1993 770 [4] Brown, C., and Malis, A., "Multiprotocol Interconnect over Frame 771 Relay". RFC 2427, September 1998. 773 [5] Perez-Maher et al, "ATM Signalling Support for IP over ATM", RFC 774 1755, February 1995 776 [6] Information technology - Telecommunications and Information 777 Exchange Between Systems, "Protocol Identification in the 778 Network Layer". ISO/IEC TR 9577, October 1990. 780 [7] Postel, J. and Reynolds, J., "A Standard for the Transmission of 781 IP Datagrams over IEEE 802 Networks". RFC 1042, ISI, February, 782 1988. 784 [8] Maher, M, "IP over ATM Signalling - SIG 4.0 Update", RFC 2331, 785 ISI, April 1998 787 [9] ITU-T Recommendation I.555, "Frame Relay Bearer Service 788 Interworking", September, 1997. 790 [10] S. Bradner., "Key words for use in RFCs to Indicate Requirement 791 Levels", RFC-2119, USC/Information Sciences Institute, March 792 1997. 794 [11] Fox, B. and Gleeson, B. "Virtual Private Networks Identifier", 796 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 798 work in progress. 800 [12] The ATM Forum, "ATM Security Framework Version 1.0", af-sec- 801 0096.000, February 1998 803 [13] The ATM Forum, "ATM Security Specification v1.0", af-sec- 804 0100.001, February 1999 806 Appendix A. Multiprotocol Encapsulation over FR-SSCS 808 ITU-T Recommendation I.365.1 defines a Frame Relaying Specific 809 Convergence Sublayer (FR- SSCS) to be used on the top of the Common 810 Part Convergence Sublayer CPCS) of the AAL type 5 for Frame Relay/ATM 811 interworking. The service offered by FR-SSCS corresponds to the Core 812 service for Frame Relaying as described in I.233. 814 An FR-SSCS-PDU consists of Q.922 Address field followed by Q.922 815 Information field. The Q.922 flags and the FCS are omitted, since 816 the corresponding functions are provided by the AAL. The figure 817 below shows an FR-SSCS-PDU embedded in the Payload of an AAL5 CPCS- 818 PDU. 820 FR-SSCS-PDU in Payload of AAL5 CPCS-PDU 821 +-------------------------------+ ------- 822 | Q.922 Address Field | FR-SSCS-PDU Header 823 | (2-4 octets) | 824 +-------------------------------+ ------- 825 | . | 826 | . | 827 | Q.922 Information field | FR-SSCS-PDU Payload 828 | . | 829 | . | 830 +-------------------------------+ ------- 831 | AAL5 CPCS-PDU Trailer | 832 +-------------------------------+ 834 Routed and bridged PDUs are encapsulated inside the FR-SSCS-PDU as 835 defined in RFC 2427. The Q.922 Information field starts with a Q.922 836 Control field followed by an optional Pad octet that is used to align 837 the remainder of the frame to a convenient boundary for the sender. 838 The protocol of the carried PDU is then identified by prefixing the 839 PDU by an ISO/IEC TR 9577 Network Layer Protocol ID (NLPID). 841 In the particular case of an IP PDU, the NLPID is 0xCC and the FR- 842 SSCS-PDU has the following format: 844 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 846 FR-SSCS-PDU Format for Routed IP PDUs 847 +-------------------------------+ 848 | Q.922 Addr Field | 849 | (2 or 4 octets) | 850 +-------------------------------+ 851 | 0x03 (Q.922 Control) | 852 +-------------------------------+ 853 | NLPID 0xCC | 854 +-------------------------------+ 855 | . | 856 | IP PDU | 857 | (up to 2^16 - 5 octets) | 858 | . | 859 +-------------------------------+ 861 Note that according to RFC 2427, the Q.922 Address field MUST be 862 either 2 or 4 octets, i.e., a 3 octet Address field MUST NOT be used. 864 In the particular case of a CLNP PDU, the NLPID is 0x81 and the FR- 865 SSCS-PDU has the following format: 867 FR-SSCS-PDU Format for Routed CLNP PDUs 868 +-------------------------------+ 869 | Q.922 Addr Field | 870 | (2 or 4 octets) | 871 +-------------------------------+ 872 | 0x03 (Q.922 Control) | 873 +-------------------------------+ 874 | NLPID 0x81 | 875 +-------------------------------+ 876 | . | 877 | Rest of CLNP PDU | 878 | (up to 2^16 - 5 octets) | 879 | . | 880 +-------------------------------+ 882 Note that in case of ISO protocols the NLPID field forms the first 883 octet of the PDU itself and MUST not be repeated. 885 The above encapsulation applies only to those routed protocols that 886 have a unique NLPID assigned. For other routed protocols (and for 887 bridged protocols), it is necessary to provide another mechanism for 888 easy protocol identification. This can be achieved by using an NLPID 889 value 0x80 to indicate that an IEEE 802.1a SubNetwork Attachment 890 Point (SNAP) header follows. 892 See RFC 2427 for more details related to multiprotocol encapsulation 893 over FRCS. 895 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 897 Appendix B. List of Locally Assigned values of OUI 00-80-C2 899 with preserved FCS w/o preserved FCS Media 900 ------------------ ----------------- -------------- 901 0x00-01 0x00-07 802.3/Ethernet 902 0x00-02 0x00-08 802.4 903 0x00-03 0x00-09 802.5 904 0x00-04 0x00-0A FDDI 905 0x00-05 0x00-0B 802.6 906 0x00-0D Fragments 907 0x00-0E BPDUs 909 Appendix C. Partial List of NLPIDs 911 0x00 Null Network Layer or Inactive Set (not used with ATM) 912 0x80 SNAP 913 0x81 ISO CLNP 914 0x82 ISO ESIS 915 0x83 ISO ISIS 916 0xCC Internet IP 918 Appendix D. Applications of multiprotocol encapsulation 920 Mutiprotocol encapsulation is necessary, but generally not 921 sufficient, for routing and bridging over the ATM networks. Since 922 the publication of RFC 1483 (the predecessor of this memo), several 923 system specifications were developed by the IETF and the ATM Forum to 924 address various aspects of, or scenarios for, bridged or routed 925 protocols. This appendix summarizes these applications. 927 1) Point-to-point connection between routers and bridges -- 928 multiprotocol encapsulation over ATM PVCs has been used to provide a 929 simple point-to-point link between bridges and routers across an ATM 930 network. Some amount of manual configuration (e.g., in lieu of 931 INARP) was necessary in these scenarios. 933 2) Classical IP over ATM -- RFC 2225 (formerly RFC 1577) provides an 934 environment where the ATM network serves as a logical IP subnet 935 (LIS). ATM PVCs are supported, with address resolution provided by 936 INARP. For ATM SVCs, a new form of ARP, ATMARP, operates over the 937 ATM network between a host (or router) and an ATMARP server. Where 938 servers are replicated to provide higher availability or performance, 939 a Server Synchronization Cache Protocol (SCSP) defined in RFC 2335 is 940 used. Classical IP over ATM defaults to the LLC/SNAP encapsulation. 942 3) LAN Emulation -- The ATM Forum LAN Emulation specification 943 provides an environment where the ATM network is enhanced by LAN 944 Emulation Server(s) to behave as a bridged LAN. Stations obtain 946 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 948 configuration information from, and register with, a LAN Emulation 949 Configuration Server; they resolve MAC addresses to ATM addresses 950 through the services of a LAN Emulation Server; they can send 951 broadcast and multicast frames, and also send unicast frames for 952 which they have no direct VC to a Broadcast and Unicast Server. LANE 953 uses the VC multiplexing encapsulation foramts for Bridged 954 Etherent/802.3 (without LAN FCS) or Bridged 802.5 (without LAN FCS) 955 for the Data Direct, LE Multicast Send and Multicast Forward VCCS. 956 However, the initial PAD field described in this memo is used as an 957 LE header, and might not be set to all '0'. 959 4) Next Hop Resolution Protocol (NHRP) -- In some cases, the 960 constraint that Classical IP over ATM serve a single LIS limits 961 performance. NHRP, as defined in RFC 2332, extends Classical to 962 allow 'shortcuts' over a an ATM network that supports several LISs. 964 5) Multiprotocol over ATM (MPOA) -- The ATM Forum Multiprotocol over 965 ATM Specification integrates LANE and NHRP to provide a generic 966 bridging/routing environment. 968 6) IP Multicast -- RFC 2022 extends Classical IP to support IP 969 multicast. A multicast address resolution server (MARS) is used 970 possibly in conjunction with a multicast server to provide IP 971 multicast behavior over ATM point-to-multipoint and/or point to point 972 virtual connections. 974 7) PPP over ATM -- RFC 2364 extends multiprotocol over ATM to the 975 case where the encapsulated protocol is the Point-to-Point protocols. 976 Both the VC based multiplexing and LLC/SNAP encapsulations are used. 977 This approach is used when the ATM network is used as a point-to- 978 point link and PPP functions are required. 980 Appendix E Differences from RFC 1483 982 This memo updates RFC 1483. It was intended to remove anachronisms, 983 provide clarifications of ambiguities discovered by implementors or 984 created by changes to the base standards, and advance this work 985 through the IETF standards track process. A number of editorial 986 improvements were made, the RFC 2119 [10] conventions applied, and 987 the current RFC boilerplate added. The following substantive changes 988 were made. None of them is believed to obsolete implementations of 989 RFC 1483: 991 -- usage of NLPID encapsulation is clarified in terms of the RFC 2119 992 conventions 994 -- a pointer to RFC 2364 is added to cover the case of PPP over ATM 996 draft-ietf-ion-multiprotocol-atm-04.txt Multiprotocol over AAL5 998 -- RFC 1755 and RFC 2331 are referenced to describe how 999 encapsulations are negotiated, rather than a long-obsolete CCITT (now 1000 ITU-T) working document and references to work then in progress 1002 -- usage of AAL5 is now a reference to ITU-T I.363.5. Options 1003 created in AAL5 since the publication of RFC 1483 are selected. 1005 -- formatting of routed NLPID-formatted PDUs (which are called 1006 "routed ISO PDUs" 1007 in RFC 1483) is clarified 1009 -- clarification is provided concerning the use of padding between 1010 the PID and MAC destination address in bridged PDUs and the bit 1011 ordering of the MAC address. 1013 -- clarification is provided concerning the use of padding of 1014 Ethernet/802.3 frames 1016 -- a new encapuslation for VPNs is added 1018 -- substantive security considerations were added 1020 -- a new appendix D provides a summary of applications of 1021 multiprotocol over ATM 1023 Author's Addresses 1025 Dan Grossman 1026 Motorola, Inc. 1027 20 Cabot Blvd. 1028 Mansfield, MA 02048 1029 Email: dan@dma.isg.mot.com 1031 Juha Heinanen 1032 Telia Finland 1033 Myyrmaentie 2 1034 01600 Vantaa, Finland 1035 Email: jh@telia.fi 1037 Full Copyright Statement 1039 Copyright (C) The Internet Society (1999). 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