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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IP over NBMA Working Group Dan Grossman 3 Internet Draft Motorola, Inc. 4 Expires: March 1999 Juha Heinanen 5 draft-ietf-ion-multiprotocol-atm-00.txt Telia 6 November, 1998 8 Multiprotocol Encapsulation over ATM Adaptation Layer 5 10 Status of this Memo 12 This document is an Internet-Draft. 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 To learn the current status of any Internet-Draft, please check the 23 ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow 24 Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), 25 munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or 26 ftp.isi.edu (US West Coast). 28 Abstract 30 This memo describes two encapsulations methods for carrying network 31 interconnect traffic over AAL type 5 over ATM. The first method 32 allows multiplexing of multiple protocols over a single ATM virtual 33 connection whereas the second method assumes that each protocol is 34 carried over a separate ATM virtual connection. 36 Copyright Notice 38 Copyright (C) The Internet Society (1998). All Rights Reserved. 40 Applicability 42 This specification is intended to be used in implementations which 43 use ATM networks to carry multiprotocol traffic among hosts, routers 44 and bridges which are ATM end systems. 46 1. Introduction 48 Asynchronous Transfer Mode (ATM) wide area, campus and local area 49 networks are used to transport IP datagrams and other connectionless 50 traffic between hosts, routers, bridges and other networking devices. 51 This memo describes two methods for carrying connectionless routed 52 and bridged Protocol Data Units (PDUs) over an ATM network. The "LLC 53 Encapsulation" method allows multiplexing of multiple protocols over 54 a single ATM virtual connection (VC). The protocol type of each PDU 55 is identified by a prefixed IEEE 802.2 Logical Link Control (LLC) 56 header. In the "VC Multiplexing" method, each ATM VC carries PDUs of 57 exactly one protocol type. When multiple protocols need to be 58 transported, there is a separate VC for each. 60 The unit of transport in ATM is a 53 octet fixed length PDU called a 61 cell. A cell consists of a 5 octet header and a 48 byte payload. 62 Variable length PDUs, including those addressed in this memo, must be 63 segmented by the transmitter to fit into the 48 octet ATM cell 64 payload, and reassembled by the receiver. This memo specifies the 65 use of the ATM Adaptation Layer type 5 (AAL5), as defined in ITU-T 66 Recommendation I.363.5 [2] for this purpose. Variable length PDUs 67 are carried in the Payload field of the AAL5 Common Part Convergence 68 Sublayer (CPCS) PDU. 70 This memo only describes how routed and bridged PDUs are carried 71 directly over the AAL5 CPCS, i.e., when the Service Specific 72 Convergence Sublayer (SSCS) of AAL5 is absent. If Frame Relay 73 Service Specific Convergence Sublayer (FR-SSCS), as defined in ITU-T 74 Recommendation I.365.1 [3], is used over the CPCS, then routed and 75 bridged PDUs are carried using the NLPID multiplexing method 76 described in RFC 2427 [4]. This encapsulation MUST be used in the 77 special case that Frame Relay Network Interworking or transparent 78 mode Service Interworking [9] are used, but is NOT RECOMMENDED for 79 other applications. Appendix A (which is for information only) shows 80 the format of the FR-SSCS-PDU as well as how IP and CLNP PDUs are 81 encapsulated over FR-SSCS according to RFC 2427. 83 If it is desired to use the facilities which are designed for the 84 Point-to-Point Protocol (PPP), and there exists a point-to-point 85 relationship between peer systems, then RFC 2364, rather than this 86 memo, applies. 88 2. Conventions 90 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, 91 SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when 92 they appear in this document, are to be interpreted as described in 93 RFC 2119 [10]. 95 3. Selection of the Multiplexing Method 97 The decision as to whether to use LLC encapsulation or VC- 98 multiplexing depends on implementation and system requirements. In 99 general, LLC encapsulation tends to require fewer VCs in a 100 multiprotocol environment. VC multiplexing tends to reduce 101 fragmentation overhead (e.g., an IPV4 datagram containing a TCP 102 control packet with neither IP nor TCP options exactly fits into a 103 single cell). 105 When two ATM end systems wish to exchange connectionless PDUs across 106 an ATM Permanent Virtual Connection (PVC), selection of the 107 multiplexing method is done by configuration. ATM connection 108 control signalling procedures are used to negotiate the encapsulation 109 method when ATM Switched Virtual Connections (SVCs) are to be used. 110 [5] and [8] specify how this negotiation is done. 112 4. AAL5 PDU Format 114 For both multiplexing methods, routed and bridged PDUs MUST be 115 encapsulated within the Payload field of an AAL5 CPCS-PDU. 117 ITU-T Recomendation I.363.5 [2] provides the complete definition of 118 the AAL5 PDU format and procedures at the sender and receiver. The 119 AAL5 message mode service, in the non-assured mode of operation MUST 120 be used. The corrupted delivery option MUST NOT be used. A 121 reassembly timer MAY be used. The following description is provided 122 for information. 124 The format of the AAL5 CPCS-PDU is shown below: 126 AAL5 CPCS-PDU Format 127 +-------------------------------+ 128 | . | 129 | . | 130 | CPCS-PDU Payload | 131 | up to 2^16 - 1 octets) | 132 | . | 133 | . | 134 +-------------------------------+ 135 | PAD ( 0 - 47 octets) | 136 +-------------------------------+ ------- 137 | CPCS-UU (1 octet ) | 138 +-------------------------------+ 139 | CPI (1 octet ) | 140 +-------------------------------+CPCS-PDU Trailer 141 | Length (2 octets) | 142 +-------------------------------| 143 | CRC (4 octets) | 144 +-------------------------------+ ------- 146 The Payload field contains user information up to 2^16 - 1 octets. 148 The PAD field pads the CPCS-PDU to fit exactly into the ATM cells 149 such that the last 48 octet cell payload created by the SAR sublayer 150 will have the CPCS-PDU Trailer right justified in the cell. 152 The CPCS-UU (User-to-User indication) field is used to transparently 153 transfer CPCS user to user information. The field is not used by the 154 multiprotocol ATM encapsulation described in this memo and MAY be set 155 to any value. 157 The CPI (Common Part Indicator) field aligns the CPCS-PDU trailer to 158 64 bits. This field MUST be coded as 0x00. 160 The Length field indicates the length, in octets, of the Payload 161 field. The maximum value for the Length field is 65535 octets. A 162 Length field coded as 0x00 is used for the abort function. 164 The CRC field is used to detect bit errors in the CPCS-PDU. A CRC-32 165 is used. 167 5. LLC Encapsulation 169 LLC Encapsulation is needed when more than one protocol might be 170 carried over the same VC. In order to allow the receiver to properly 171 process the incoming AAL5 CPCS-PDU, the Payload Field contains 172 information necessary to identify the protocol of the routed or 173 bridged PDU. In LLC Encapsulation, this information MUST be encoded 174 in an LLC header placed in front of the carried PDU. 176 Although this memo only deals with protocols that operate over LLC 177 Type 1 (unacknowledged connectionless mode) service, the same 178 encapsulation principle also applies to protocols operating over LLC 179 Type 2 (connection-mode) service. In the latter case the format and 180 contents of the LLC header would be as described in IEEE 802.1 and 181 IEEE 802.2. 183 5.1. LLC Encapsulation for Routed Protocols 185 In LLC Encapsulation, the protocol type of routed PDUs MUST be 186 identified by prefixing an IEEE 802.2 LLC header to each PDU. In 187 some cases, the LLC header MUST be followed by an IEEE 802.1a 188 SubNetwork Attachment Point (SNAP) header. In LLC Type 1 operation, 189 the LLC header MUST consist of three one octet fields: 191 +------+------+------+ 192 | DSAP | SSAP | Ctrl | 193 +------+------+------+ 195 In LLC Encapsulation for routed protocols, the Control field MUST be 196 set to 0x03, specifying a Unnumbered Information (UI) Command PDU. 198 The LLC header value 0xFE-FE-03 MUST be used to identify a routed PDU 199 in the ISO NLPID format (see [6] and Appendix B). For NLPID-formatted 200 routed PDUs, the content of the AAL5 CPCS-PDU Payload field MUST be 201 as follows: 203 Payload Format for Routed NLPID-formatted PDUs 204 +-------------------------------+ 205 | LLC 0xFE-FE-03 | 206 +-------------------------------+ 207 | NLPID (1 octet) | 208 +-------------------------------+ 209 | . | 210 | PDU | 211 | (up to 2^16 - 4 octets) | 212 | . | 213 +-------------------------------+ 215 The routed protocol MUST be identified by a one octet NLPID field 216 that is part of Protocol Data. NLPID values are administered by ISO 217 and ITU-T. They are defined in ISO/IEC TR 9577 [6] and some of the 218 currently defined ones are listed in Appendix C. 220 An NLPID value of 0x00 is defined in ISO/IEC TR 9577 as the Null 221 Network Layer or Inactive Set. Since it has no significance within 222 the context of this encapsulation scheme, a NLPID value of 0x00 MUST 223 not be used. 225 Although there is a NLPID value (0xCC) that indicates IP, the NLPID 226 format MUST NOT be used for IP. Instead, IP datagrams MUST be 227 identified by a SNAP header, as defined below. 229 The presence of a SNAP header is indicated by the LLC header value 230 0xAA-AA-03. A SNAP header is of the form 232 +------+------+------+------+------+ 233 | OUI | PID | 234 +------+------+------+------+------+ 236 The three octet Organizationally Unique Identifier (OUI) identifies 237 an organization which administers the values which might be assigned 238 to the two octet Protocol Identifier (PID). The SNAP header thus 239 uniquely identifies a routed or bridged protocol. The OUI value 240 0x00-00-00 indicates that the PID is an EtherType. 242 The format of the AAL5 CPCS-PDU Payload field for routed non-NLPID 243 Formatted PDUs MUST be as follows: 245 Payload Format for Routed non-NLPID formatted PDUs 246 +-------------------------------+ 247 | LLC 0xAA-AA-03 | 248 +-------------------------------+ 249 | OUI 0x00-00-00 | 250 +-------------------------------+ 251 | EtherType (2 octets) | 252 +-------------------------------+ 253 | . | 254 | Non-NLPID formatted PDU | 255 | (up to 2^16 - 9 octets) | 256 | . | 257 +-------------------------------+ 259 In the particular case of an IPv4 PDU, the Ethertype value is 0x08- 260 00, and the payload format MUST be: 262 Payload Format for Routed IPv4 PDUs 263 +-------------------------------+ 264 | LLC 0xAA-AA-03 | 265 +-------------------------------+ 266 | OUI 0x00-00-00 | 267 +-------------------------------+ 268 | EtherType 0x08-00 | 269 +-------------------------------+ 270 | . | 271 | IPv4 PDU | 272 | (up to 2^16 - 9 octets) | 273 | . | 274 +-------------------------------+ 276 This format is consistent with that defined in RFC 1042 [7]. 278 5.2. LLC Encapsulation for Bridged Protocols 280 In LLC Encapsulation, bridged PDUs are encapsulated by identifying 281 the type of the bridged media in the SNAP header. The presence of 282 the SNAP header MUST be indicated by the LLC header value 0xAA-AA-03. 283 The OUI value in the SNAP header MUST be the 802.1 organization code 284 0x00-80-C2. The type of the bridged media MUST be specified by the 285 two octet PID. The PID MUST also indicate whether the original Frame 286 Check Sequence (FCS) is preserved within the bridged PDU. Appendix B 287 provides a list of media type (PID) values that can be used in ATM 288 encapsulation. 290 The AAL5 CPCS-PDU Payload field carrying a bridged PDU MUST have one 291 of the following formats. Padding MUST be added after the PID field 292 if necessary in order to align the user information field of the 293 bridged PDU at a four octet boundary. 295 Payload Format for Bridged Ethernet/802.3 PDUs 296 +-------------------------------+ 297 | LLC 0xAA-AA-03 | 298 +-------------------------------+ 299 | OUI 0x00-80-C2 | 300 +-------------------------------+ 301 | PID 0x00-01 or 0x00-07 | 302 +-------------------------------+ 303 | PAD 0x00-00 | 304 +-------------------------------+ 305 | MAC destination address | 306 +-------------------------------+ 307 | | 308 | (remainder of MAC frame) | 309 | | 310 +-------------------------------+ 311 | LAN FCS (if PID is 0x00-01) | 312 +-------------------------------+ 314 The Ethernet/802.3 physical layer requires padding of frames to a minimum 315 size. A bridge that uses uses the Bridged Ethernet/802.3 encapsulation 316 format with the preserved LAN FCS MUST include padding. A bridge that uses 317 the Bridged Ethernet/802.3 encapsulation format without the preserved LAN 318 FCS MAY either include padding, or omit it. When a bridge receives a frame 319 in this format without the LAN FCS, it MUST be able to insert the necessary 320 padding (if none is already present) before forwarding to an Ethernet/802.3 321 subnetwork. 323 Payload Format for Bridged 802.4 PDUs 324 +-------------------------------+ 325 | LLC 0xAA-AA-03 | 326 +-------------------------------+ 327 | OUI 0x00-80-C2 | 328 +-------------------------------+ 329 | PID 0x00-02 or 0x00-08 | 330 +-------------------------------+ 331 | PAD 0x00-00-00 | 332 +-------------------------------+ 333 | Frame Control (1 octet) | 334 +-------------------------------+ 335 | MAC destination address | 336 +-------------------------------+ 337 | | 338 | (remainder of MAC frame) | 339 | | 340 +-------------------------------+ 341 | LAN FCS (if PID is 0x00-02) | 342 +-------------------------------+ 344 Payload Format for Bridged 802.5 PDUs 345 +-------------------------------+ 346 | LLC 0xAA-AA-03 | 347 +-------------------------------+ 348 | OUI 0x00-80-C2 | 349 +-------------------------------+ 350 | PID 0x00-03 or 0x00-09 | 351 +-------------------------------+ 352 | PAD 0x00-00-XX | 353 +-------------------------------+ 354 | Frame Control (1 octet) | 355 +-------------------------------+ 356 | MAC destination address | 357 +-------------------------------+ 358 | | 359 | (remainder of MAC frame) | 360 | | 361 +-------------------------------+ 362 | LAN FCS (if PID is 0x00-03) | 363 +-------------------------------+ 365 Since the 802.5 Access Control (AC) field has no significance outside 366 the local 802.5 subnetwork, it is treated by this encapsulation as 367 the last octet of the three octet PAD field. It MAY be set to any 368 value by the sending bridge and MUST be ignored by the receiving 369 bridge. 371 Payload Format for Bridged FDDI PDUs 372 +-------------------------------+ 373 | LLC 0xAA-AA-03 | 374 +-------------------------------+ 375 | OUI 0x00-80-C2 | 376 +-------------------------------+ 377 | PID 0x00-04 or 0x00-0A | 378 +-------------------------------+ 379 | PAD 0x00-00-00 | 380 +-------------------------------+ 381 | Frame Control (1 octet) | 382 +-------------------------------+ 383 | MAC destination address | 384 +-------------------------------+ 385 | | 386 | (remainder of MAC frame) | 387 | | 388 +-------------------------------+ 389 | LAN FCS (if PID is 0x00-04) | 390 +-------------------------------+ 392 Payload Format for Bridged 802.6 PDUs 393 +-------------------------------+ 394 | LLC 0xAA-AA-03 | 395 +-------------------------------+ 396 | OUI 0x00-80-C2 | 397 +-------------------------------+ 398 | PID 0x00-0B | 399 +---------------+---------------+ ------ 400 | Reserved | BEtag | Common 401 +---------------+---------------+ PDU 402 | BAsize | Header 403 +-------------------------------+ ------- 404 | MAC destination address | 405 +-------------------------------+ 406 | | 407 | (remainder of MAC frame) | 408 | | 409 +-------------------------------+ 410 | | 411 | Common PDU Trailer | 412 | | 413 +-------------------------------+ 415 In bridged 802.6 PDUs, the presence of a CRC-32 is indicated by the 416 CIB bit in the header of the MAC frame. Therefore, the same PID 417 value is used regardless of the presence or absence of the CRC-32 in 418 the PDU. 420 The Common Protocol Data Unit (PDU) Header and Trailer are conveyed 421 to allow pipelining at the egress bridge to an 802.6 subnetwork. 422 Specifically, the Common PDU Header contains the BAsize field, which 423 contains the length of the PDU. If this field is not available to 424 the egress 802.6 bridge, then that bridge cannot begin to transmit 425 the segmented PDU until it has received the entire PDU, calculated 426 the length, and inserted the length into the BAsize field. If the 427 field is available, the egress 802.6 bridge can extract the length 428 from the BAsize field of the Common PDU Header, insert it into the 429 corresponding field of the first segment, and immediately transmit 430 the segment onto the 802.6 subnetwork. Thus, the bridge can begin 431 transmitting the 802.6 PDU before it has received the complete PDU. 433 Note that the Common PDU Header and Trailer of the encapsulated frame 434 should not be simply copied to the outgoing 802.6 subnetwork because 435 the encapsulated BEtag value may conflict with the previous BEtag 436 value transmitted by that bridge. 438 An ingress 802.6 bridge can abort an AAL5 CPCS-PDU by setting its 439 Length field to zero. If the egress bridge has already begun 440 transmitting segments of the PDU to an 802.6 subnetwork and then 441 notices that the AAL5 CPCS-PDU has been aborted, it may immediately 442 generate an EOM cell that causes the 802.6 PDU to be rejected at the 443 receiving bridge. Such an EOM cell could, for example, contain an 444 invalid value in the Length field of the Common PDU Trailer. 446 Payload Format for BPDUs 447 +-------------------------------+ 448 | LLC 0xAA-AA-03 | 449 +-------------------------------+ 450 | OUI 0x00-80-C2 | 451 +-------------------------------+ 452 | PID 0x00-0E | 453 +-------------------------------+ 454 | | 455 | BPDU as defined by | 456 | 802.1(d) or 802.1(g) | 457 | | 458 +-------------------------------+ 460 6. VC Multiplexing 462 VC Multiplexing creates a binding between an ATM VC and the type of 463 the network protocol carried on that VC. Thus, there is no need for 464 protocol identification information to be carried in the payload of 465 each AAL5 CPCS-PDU. This reduces payload overhead and can reduce 466 per-packet processing. VC multiplexing can improve efficiency by 467 reducing the number of cells needed to carry PDUs of certain lengths. 469 For ATM PVCs, the type of the protocol to be carried over each PVC 470 MUST be determined by configuration. For ATM SVCs, the negotiations 471 specified in RFC 1755 [5] MUST be used. 473 6.1. VC Based Multiplexing of Routed Protocols 475 PDUs of routed protocols MUST be carried as the only content of the 476 Payload of the AAL5 CPCS-PDU. The format of the AAL5 CPCS-PDU 477 Payload field thus becomes: 479 Payload Format for Routed PDUs 480 +-------------------------------+ 481 | . | 482 | Carried PDU | 483 | (up to 2^16 - 1 octets) | 484 | . | 485 | . | 486 +-------------------------------+ 488 6.2. VC Based Multiplexing of Bridged Protocols 490 PDUs of bridged protocols MUST be carried in the Payload of the AAL5 491 CPCS-PDU exactly as described in section 5.2, except that only the 492 fields after the PID field MUST be included. The AAL5 CPCS-PDU 493 Payload field carrying a bridged PDU MUST, therefore, have one of the 494 following formats. 496 Payload Format for Bridged Ethernet/802.3 PDUs 497 +-------------------------------+ 498 | PAD 0x00-00 | 499 +-------------------------------+ 500 | MAC destination address | 501 +-------------------------------+ 502 | | 503 | (remainder of MAC frame) | 504 | | 505 +-------------------------------+ 506 | LAN FCS (VC dependent option) | 507 +-------------------------------+ 509 Payload Format for Bridged 802.4/802.5/FDDI PDUs 510 +-------------------------------+ 511 | PAD 0x00-00-00 or 0x00-00-XX | 512 +-------------------------------+ 513 | Frame Control (1 octet) | 514 +-------------------------------+ 515 | MAC destination address | 516 +-------------------------------+ 517 | | 518 | (remainder of MAC frame) | 519 | | 520 +-------------------------------+ 521 | LAN FCS (VC dependent option) | 522 +-------------------------------+ 524 Note that the 802.5 Access Control (AC) field has no significance 525 outside the local 802.5 subnetwork. It can thus be regarded as the 526 last octet of the three octet PAD field, which in case of 802.5 can 527 be set to any value (XX). 529 Payload Format for Bridged 802.6 PDUs 530 +---------------+---------------+ ------- 531 | Reserved | BEtag | Common 532 +---------------+---------------+ PDU 533 | BAsize | Header 534 +-------------------------------+ ------- 535 | MAC destination address | 536 +-------------------------------+ 537 | | 538 | (remainder of MAC frame) | 539 | | 540 +-------------------------------+ 541 | | 542 | Common PDU Trailer | 543 | | 544 +-------------------------------+ 546 Payload Format for BPDUs 547 +-------------------------------+ 548 | | 549 | BPDU as defined by | 550 | 802.1(d) or 802.1(g) | 551 | | 552 +-------------------------------+ 554 In case of Ethernet, 802.3, 802.4, 802.5, and FDDI PDUs the presense 555 or absence of the trailing LAN FCS shall be identified implicitly by 556 the VC, since the PID field is not included. PDUs with the LAN FCS 557 and PDUs without the LAN FCS are thus considered to belong to 558 different protocols even if the bridged media type would be the same. 560 7. Bridging in an ATM Network 562 A bridge with an ATM interface that serves as a link to one or more 563 other bridge MUST be able to flood, forward, and filter bridged PDUs. 565 Flooding is performed by sending the PDU to all possible appropriate 566 destinations. In the ATM environment this means sending the PDU 567 through each relevant VC. This may be accomplished by explicitly 568 copying it to each VC or by using a point-to-multipoint VC. 570 To forward a PDU, a bridge MUST be able to associate a destination 571 MAC address with a VC. It is unreasonable and perhaps impossible to 572 require bridges to statically configure an association of every 573 possible destination MAC address with a VC. Therefore, ATM bridges 574 must provide enough information to allow an ATM interface to 575 dynamically learn about foreign destinations beyond the set of ATM 576 stations. 578 To accomplish dynamic learning, a bridged PDU shall conform to the 579 encapsulation described within section 4. In this way, the receiving 580 ATM interface will know to look into the bridged PDU and learn the 581 association between foreign destination and an ATM station. 583 Acknowledgements 585 This memo is an update of RFC 1483, which was developed by the IP 586 over ATM working group, and edited by Juha Heinanen (then at Telecom 587 Finland). The update was developed in the IP-over-NBMA (ION) working 588 group, and Dan Grossman (Motorola). 590 This material evolved from RFCs [1] and [4] from which much of the 591 material has been adopted. Thanks to their authors T. Bradley, C. 592 Brown, A. Malis, D. Piscitello, and C. Lawrence. Other key 593 contributors to the work included Brian Carpenter of CERN, Rao 594 Cherukuri of IBM, Dan Grossman of Motorola, Joel Halpern of Network 595 Systems(now at Newbridge), Bob Hinden of Sun Mircosystems (now at 596 Nokia), and Gary Kessler of MAN Technology. 598 Security Considerations 600 The encapsulation mechanisms described in this memo are believed not 601 to have any properties that might be exploited by an attacker. 602 However, architectures and protocols operating above the 603 encapsulation layer may be subject to a variety of attacks. In 604 particular, the bridging architecture discussed in section 7 has 605 well-known vulnerabilities. 607 System security may be affected by the properties of the underlying 608 ATM network. The ATM Forum has published a security framework [11] 609 and a security specification [12]. 611 References 613 [1] Piscitello, D. and Lawrence, C., "The Transmission of IP 614 Datagrams over the SMDS Service". RFC 1209, Bell Communications 615 Research, March 1991. 617 [2] ITU-T Recommendation I.363.5, "B-ISDN ATM Adaptation Layer (AAL) 618 Type 5 Specification", August, 1996. 620 [3] ITU-T Recommendation I.365.1, "Frame Relaying 621 Service Specific Convergence Sublayer (SSCS), November, 622 1993 624 [4] Bradley, T., Brown, C., and Malis, A., "Multiprotocol 625 Interconnect over Frame Relay". RFC 2427, September 1998. 627 [5] Perez-Maher et al, "ATM Signalling Support for IP over ATM", RFC 628 1755, February 1995 630 [6] Information technology - Telecommunications and Information 631 Exchange Between Systems, "Protocol Identification in the 632 Network Layer". ISO/IEC TR 9577, October 1990. 634 [7] Postel, J. and Reynolds, J., "A Standard for the Transmission of 635 IP Datagrams over IEEE 802 Networks". RFC 1042, ISI, February, 636 1988. 638 [8] Maher, M, "IP over ATM Signalling - SIG 4.0 Update", RFC 2331, 639 ISI, April 1998 641 [9] ITU-T Recommendation I.555, "Frame Relay Bearer Service 642 Interworking", September, 1997. 644 [10] S. Bradner., "Key words for use in RFCs to Indicate Requirement 645 Levels", RFC-2119, USC/Information Sciences Institute, March 646 1997. 648 [11] The ATM Forum, "ATM Security Framework Version 1.0", af-sec- 649 0096.000, February 1998 651 [12] The ATM Forum, "ATM Security Specification v1.0", af-sec- 652 xxxx.000, December? 1998 654 Appendix A. Multiprotocol Encapsulation over FR-SSCS 656 ITU-T Recommendation I.365.1 defines a Frame Relaying Specific 657 Convergence Sublayer (FR- SSCS) to be used on the top of the Common 658 Part Convergence Sublayer CPCS) of the AAL type 5 for Frame Relay/ATM 659 interworking. The service offered by FR-SSCS corresponds to the Core 660 service for Frame Relaying as described in I.233. 662 An FR-SSCS-PDU consists of Q.922 Address field followed by Q.922 663 Information field. The Q.922 flags and the FCS are omitted, since 664 the corresponding functions are provided by the AAL. The figure 665 below shows an FR-SSCS-PDU embedded in the Payload of an AAL5 CPCS- 666 PDU. 668 FR-SSCS-PDU in Payload of AAL5 CPCS-PDU 669 +-------------------------------+ ------- 670 | Q.922 Address Field | FR-SSCS-PDU Header 671 | (2-4 octets) | 672 +-------------------------------+ ------- 673 | . | 674 | . | 675 | Q.922 Information field | FR-SSCS-PDU Payload 676 | . | 677 | . | 678 +-------------------------------+ ------- 679 | AAL5 CPCS-PDU Trailer | 680 +-------------------------------+ 682 Routed and bridged PDUs are encapsulated inside the FR-SSCS-PDU as 683 defined in RFC 2427. The Q.922 Information field starts with a Q.922 684 Control field followed by an optional Pad octet that is used to align 685 the remainder of the frame to a convenient boundary for the sender. 686 The protocol of the carried PDU is then identified by prefixing the 687 PDU by an ISO/IEC TR 9577 Network Layer Protocol ID (NLPID). 689 In the particular case of an IP PDU, the NLPID is 0xCC and the FR- 690 SSCS-PDU has the following format: 692 FR-SSCS-PDU Format for Routed IP PDUs 693 +-------------------------------+ 694 | Q.922 Addr Field | 695 | (2 or 4 octets) | 696 +-------------------------------+ 697 | 0x03 (Q.922 Control) | 698 +-------------------------------+ 699 | NLPID 0xCC | 700 +-------------------------------+ 701 | . | 702 | IP PDU | 703 | (up to 2^16 - 5 octets) | 704 | . | 705 +-------------------------------+ 707 Note that according to RFC 2427, the Q.922 Address field MUST be 708 either 2 or 4 octets, i.e., a 3 octet Address field MUST NOT be used. 710 In the particular case of a CLNP PDU, the NLPID is 0x81 and the FR- 711 SSCS-PDU has the following format: 713 FR-SSCS-PDU Format for Routed CLNP PDUs 714 +-------------------------------+ 715 | Q.922 Addr Field | 716 | (2 or 4 octets) | 717 +-------------------------------+ 718 | 0x03 (Q.922 Control) | 719 +-------------------------------+ 720 | NLPID 0x81 | 721 +-------------------------------+ 722 | . | 723 | Rest of CLNP PDU | 724 | (up to 2^16 - 5 octets) | 725 | . | 726 +-------------------------------+ 728 Note that in case of ISO protocols the NLPID field forms the first 729 octet of the PDU itself and MUST not be repeated. 731 The above encapsulation applies only to those routed protocols that 732 have a unique NLPID assigned. For other routed protocols (and for 733 bridged protocols), it is necessary to provide another mechanism for 734 easy protocol identification. This can be achieved by using an NLPID 735 value 0x80 to indicate that an IEEE 802.1a SubNetwork Attachment 736 Point (SNAP) header follows. 738 See RFC 2427 for more details related to multiprotocol encapsulation 739 over FRCS. 741 Appendix B. List of Locally Assigned values of OUI 00-80-C2 743 with preserved FCS w/o preserved FCS Media 744 ------------------ ----------------- -------------- 745 0x00-01 0x00-07 802.3/Ethernet 746 0x00-02 0x00-08 802.4 747 0x00-03 0x00-09 802.5 748 0x00-04 0x00-0A FDDI 749 0x00-05 0x00-0B 802.6 750 0x00-0D Fragments 751 0x00-0E BPDUs 753 Appendix C. Partial List of NLPIDs 755 0x00 Null Network Layer or Inactive Set (not used with ATM) 756 0x80 SNAP 757 0x81 ISO CLNP 758 0x82 ISO ESIS 759 0x83 ISO ISIS 760 0xCC Internet IP 762 Appendix D. Applications of multiprotocol encapsulation 764 Mutiprotocol encapsulation is necessary, but generally sufficient, 765 for routing and bridging over the ATM networks. Since the 766 publication of RFC 1483 (the predecessor of this memo), several 767 system specifications were developed by the IETF and the ATM Forum to 768 address various aspects of, or scenarios for, bridged or routed 769 protocols. This appendix summarizes these applications. 771 1) Point-to-point connection between routers and bridges -- 772 multiprotocol encapsulation over ATM PVCs has been used to provide a 773 simple point-to-point link between bridges and routers across an ATM 774 network. Some amount of manual configuration (e.g., in lieu of 775 INARP) was necessary in these scenarios. 777 2) Classical IP over ATM -- RFC 2225 (formerly RFC 1577) provides an 778 environment where the ATM network serves as a logical IP subnet 779 (LIS). ATM PVCs are supported, with address resolution provided by 780 INARP. For ATM SVCs, a new form of ARP, ATMARP, operates over the 781 ATM network between a host (or router) and an ATMARP server. Where 782 servers are replicated to provide higher availability or performance, 783 a Server Synchronization Cache Protocol (SCSP) defined in RFC 2335 is 784 used. Classical IP over ATM defaults to the LLC/SNAP encapsulation. 786 3) LAN Emulation -- The ATM Forum LAN Emulation specification 787 provides an environment where the ATM network is enhanced by LAN 788 Emulation Server(s) to behave as a bridged LAN. Stations obtain 789 configuration information from, and register with, a LAN Emulation 790 Configuration Server; they resolve MAC addresses to ATM addresses 791 through the services of a LAN Emulation Server; they can send 792 broadcast and multicast frames, and also send unicast frames for 793 which they have no direct VC to a Broadcast and Unicast Server. LANE 794 always uses the LLC/SNAP encapsulation. 796 4) Next Hop Resolution Protocol (NHRP) -- In some cases, the 797 constraint that Classical IP over ATM serve a single LIS limits 798 performance. NHRP, as defined in RFC 2332, extends Classical to 799 allow 'shortcuts' over a an ATM network that supports several LISs. 801 5) Multiprotocol over ATM (MPOA) -- The ATM Forum Multiprotocol over 802 ATM Specification integrates LANE and NHRP to provide a generic 803 bridging/routing environment. 805 6) IP Multicast -- RFC 2022 extends Classical IP to support IP 806 multicast. A multicast address resolution server (MARS) is used 807 possibly in conjunction with a multicast server to provide IP 808 multicast behavior over ATM point-to-multipoint and/or point to point 809 virtual connections. 811 7) PPP over ATM -- RFC 2364 extends multiprotocol over ATM to the 812 case where the encapsulated protocol is the Point-to-Point protocols. 813 Both the VC based multiplexing and LLC/SNAP encapsulations are used. 814 This approach is used when the ATM network is used as a point-to- 815 point link and PPP functions are required. 817 Author's Addresses 819 Dan Grossman 820 Motorola, Inc. 821 20 Cabot Blvd. 822 Mansfield, MA 02048 823 Email: dan@dma.isg.mot.com 825 Juha Heinanen 826 Telia Finland 827 Myyrmaentie 2 828 01600 Vantaa, Finland 829 Email: jh@telia.fi 831 Full Copyright Statement 833 Copyright (C) The Internet Society (1998). 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