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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (July 23, 1998) is 9402 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) == Unused Reference: '5' is defined on line 1357, but no explicit reference was found in the text -- Possible downref: Non-RFC (?) normative reference: ref. '1' -- Possible downref: Non-RFC (?) normative reference: ref. '2' -- Possible downref: Non-RFC (?) normative reference: ref. '3' ** Obsolete normative reference: RFC 1638 (ref. '4') (Obsoleted by RFC 2878) -- Possible downref: Non-RFC (?) normative reference: ref. '5' ** Obsolete normative reference: RFC 1700 (ref. '7') (Obsoleted by RFC 3232) -- Possible downref: Non-RFC (?) normative reference: ref. '10' -- Possible downref: Non-RFC (?) normative reference: ref. '11' -- Possible downref: Non-RFC (?) normative reference: ref. '12' -- Possible downref: Non-RFC (?) normative reference: ref. '13' -- Possible downref: Non-RFC (?) normative reference: ref. '14' -- Possible downref: Non-RFC (?) normative reference: ref. '15' -- Possible downref: Non-RFC (?) normative reference: ref. '18' -- Possible downref: Non-RFC (?) normative reference: ref. '19' Summary: 13 errors (**), 0 flaws (~~), 9 warnings (==), 16 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group C. Brown 3 INTERNET DRAFT Consultant 4 Obsoletes: 1294, 1490 A. Malis 5 Ascend Communications, Inc. 6 July 23, 1998 7 Expires January 22, 1998 9 Multiprotocol Interconnect over Frame Relay 11 Status of this Memo 13 This document is an Internet-Draft. Internet-Drafts are working 14 documents of the Internet Engineering Task Force (IETF), its areas, 15 and its working groups. Note that other groups may also distribute 16 working documents as Internet-Drafts. 18 Internet-Drafts are draft documents valid for a maximum of six months 19 and may be updated, replaced, or obsoleted by other documents at any 20 time. It is inappropriate to use Internet-Drafts as reference 21 material or to cite them other than as ``work in progress.'' 23 To learn the current status of any Internet-Draft, please check the 24 ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow 25 Directories on ftp.ietf.org (US East Coast), nic.nordu.net 26 (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific 27 Rim). 29 This draft specifies an IAB standards track protocol for the Internet 30 community, and requests discussion and suggestions for improvements. 31 Please refer to the current edition of the "IAB Official Protocol 32 Standards" for the standardization state and status of this protocol. 33 Distribution of this memo is unlimited. 35 Abstract 37 This memo describes an encapsulation method for carrying network 38 interconnect traffic over a Frame Relay backbone. It covers aspects 39 of both Bridging and Routing. 41 Systems with the ability to transfer both the encapsulation method 42 described in this document, and others must have a priori knowledge 43 of which virtual circuits will carry which encapsulation method and 44 this encapsulation must only be used over virtual circuits that have 45 been explicitly configured for its use. 47 Acknowledgments 49 This document could not have been completed without the support of 50 Terry Bradley of Avici Systems, Inc.. Comments and contributions 51 from many sources, especially those from Ray Samora of Proteon, Ken 52 Rehbehn of Visual Networks, Fred Baker and Charles Carvalho of Cisco 53 Systems, and Mostafa Sherif of AT&T have been incorporated into this 54 document. Special thanks to Dory Leifer of University of Michigan for 55 his contributions to the resolution of fragmentation issues (though 56 it was deleted in the final version) and Floyd Backes and Laura 57 Bridge of 3Com for their contributions to the bridging descriptions. 58 This document could not have been completed without the expertise of 59 the IP over Large Public Data Networks and the IP over NBMA working 60 groups of the IETF. 62 Modifications from RFC 1490 64 RFC 1490 has been widely implemented and used, and has been adopted 65 by the Frame Relay Forum in FRF.3.1 [15] and by the ITU in Q.933 [2]. 66 This section describes updates to RFC 1490 that have been made as a 67 result of this implementation and interoperability experience, and 68 which reflect current implementation practice. 70 Some language changes were necessary to clarify RFC 1490. None of 71 these changes impacted the technical aspects of this document, but 72 were required to keep diagrams and language specific and consistent. 73 Specifics of these changes will not be listed here. Below are listed 74 those changes which were significant. 76 a) The requirement for stations to accept SNAP encapsulated protocols 77 for which a NLPID was available, was removed. RFC 1490 indicated 78 that, if a protocol, such as IP, had a designated NLPID value, it 79 must be used. Later the document required stations to accept a 80 SNAP encapsulated version of this same protocol. This is clearly 81 inconsistent. A compliant station must send and accept the NLPID 82 encapsulated version of such a protocol. It MAY accept the SNAP 83 encapsulation but should not be required to do so as these frames 84 are noncompliant. 86 b) Fragmentation was removed. To date there are no interoperable 87 implementations of the fragmentation algorithm presented in RFC 88 1490. Additionally, there have been several suggestions that the 89 proposed mechanisms are insufficient for some frame relay 90 applications. To this end, fragmentation was removed from this 91 document, and has been replaced by the fragmentation specified in 92 FRF.12 [18]. 94 c) The address resolution presented in RFC 1490 referred only to 95 PVC environments and is insufficient for SVC environments. 96 Therefore the section title was changed to reflect this. Further 97 work on SVC address resolution will take place in the ION working 98 group. 100 d) The encapsulation for Source Routing BPDUs was added, and the 101 lists in Appendix A were augmented. 103 e) The use of canonical and non-canonical MAC destination addresses 104 in the bridging encapsulations was clarified. 106 f) Explicit support for multiple IP addresses mapped to a single 107 Frame Relay DLCI. 109 g) The Inverse ARP description was moved to the Inverse ARP 110 specification [11]. 112 h) A new security section was added. 114 1. Conventions and Acronyms 116 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, 117 SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this 118 document, are to be interpreted as described in [16]. 120 All drawings in this document are drawn with the left-most bit as the 121 high order bit for transmission. For example, the drawings might be 122 labeled as: 124 0 1 2 3 4 5 6 7 bits 125 +---+---+---+---+---+---+---+ 127 +---------------------------+ 128 | flag (7E hexadecimal) | 129 +---------------------------+ 130 | Q.922 Address* | 131 +-- --+ 132 | | 133 +---------------------------+ 134 : : 135 : : 136 +---------------------------+ 138 Drawings that would be too large to fit onto one page if each octet 139 were presented on a single line are drawn with two octets per line. 140 These are also drawn with the left-most bit as the high order bit for 141 transmission. There will be a "+" to distinguish between octets as 142 in the following example. 144 |--- octet one ---|--- octet two ---| 145 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 146 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 148 +--------------------------------------------+ 149 | Organizationally Unique | 150 +-- +--------------------+ 151 | Identifier | Protocol | 152 +-----------------------+--------------------+ 153 | Identifier | 154 +-----------------------+ 156 The following are common acronyms used throughout this document. 158 BECN - Backward Explicit Congestion Notification 159 BPDU - Bridge Protocol Data Unit 160 C/R - Command/Response bit 161 DCE - Data Communication Equipment 162 DE - Discard Eligibility bit 163 DTE - Data Terminal Equipment 164 FECN - Forward Explicit Congestion Notification 165 PDU - Protocol Data Unit 166 PTT - Postal Telephone & Telegraph 167 SNAP - Subnetwork Access Protocol 169 2. Introduction 171 The following discussion applies to those devices which serve as end 172 stations (DTEs) on a public or private Frame Relay network (for 173 example, provided by a common carrier or PTT. It will not discuss 174 the behavior of those stations that are considered a part of the 175 Frame Relay network (DCEs) other than to explain situations in which 176 the DTE must react. 178 The Frame Relay network provides a number of virtual circuits that 179 form the basis for connections between stations attached to the same 180 Frame Relay network. The resulting set of interconnected devices 181 forms a private Frame Relay group which may be either fully 182 interconnected with a complete "mesh" of virtual circuits, or only 183 partially interconnected. In either case, each virtual circuit is 184 uniquely identified at each Frame Relay interface by a Data Link 185 Connection Identifier (DLCI). In most circumstances, DLCIs have 186 strictly local significance at each Frame Relay interface. 188 The specifications in this document are intended to apply to both 189 switched and permanent virtual circuits. 191 3. Frame Format 193 All protocols must encapsulate their packets within a Q.922 Annex A 194 frame [1]. Additionally, frames shall contain information necessary 195 to identify the protocol carried within the protocol data unit (PDU), 196 thus allowing the receiver to properly process the incoming packet. 197 The format shall be as follows: 199 +---------------------------+ 200 | flag (7E hexadecimal) | 201 +---------------------------+ 202 | Q.922 Address* | 203 +-- --+ 204 | | 205 +---------------------------+ 206 | Control (UI = 0x03) | 207 +---------------------------+ 208 | Pad (when required) (0x00)| 209 +---------------------------+ 210 | NLPID | 211 +---------------------------+ 212 | . | 213 | . | 214 | . | 215 | Data | 216 | . | 217 | . | 218 +---------------------------+ 219 | Frame Check Sequence | 220 +-- . --+ 221 | (two octets) | 222 +---------------------------+ 223 | flag (7E hexadecimal) | 224 +---------------------------+ 226 * Q.922 addresses, as presently defined, are two octets and 227 contain a 10-bit DLCI. In some networks Q.922 addresses 228 may optionally be increased to three or four octets. 230 The control field is the Q.922 control field. The UI (0x03) value is 231 used unless it is negotiated otherwise. The use of XID (0xAF or 232 0xBF) is permitted and is discussed later. 234 The pad field is used to align the data portion (beyond the 235 encapsulation header) of the frame to a two octet boundary. If 236 present, the pad is a single octet and must have a value of zero. 237 Explicit directions of when to use the pad field are discussed later 238 in this document. 240 The Network Level Protocol ID (NLPID) field is administered by ISO 241 and the ITU. It contains values for many different protocols 242 including IP, CLNP, and IEEE Subnetwork Access Protocol (SNAP)[10]. 243 This field tells the receiver what encapsulation or what protocol 244 follows. Values for this field are defined in ISO/IEC TR 9577 [3]. A 245 NLPID value of 0x00 is defined within ISO/IEC TR 9577 as the Null 246 Network Layer or Inactive Set. Since it cannot be distinguished from 247 a pad field, and because it has no significance within the context of 248 this encapsulation scheme, a NLPID value of 0x00 is invalid under the 249 Frame Relay encapsulation. Appendix A contains a list of some of the 250 more commonly used NLPID values. 252 There is no commonly implemented minimum maximum frame size for Frame 253 Relay. A network must, however, support at least a 262 octet 254 maximum. Generally, the maximum will be greater than or equal to 255 1600 octets, but each Frame Relay provider will specify an 256 appropriate value for its network. A Frame Relay DTE, therefore, 257 must allow the maximum acceptable frame size to be configurable. 259 The minimum frame size allowed for Frame Relay is five octets between 260 the opening and closing flags assuming a two octet Q.922 address 261 field. This minimum increases to six octets for three octet Q.922 262 address and seven octets for the four octet Q.922 address format. 264 4. Interconnect Issues 266 There are two basic types of data packets that travel within the 267 Frame Relay network: routed packets and bridged packets. These 268 packets have distinct formats and therefore, must contain an 269 indicator that the destination may use to correctly interpret the 270 contents of the frame. This indicator is embedded within the NLPID 271 and SNAP header information. 273 For those protocols that do not have a NLPID already assigned, it is 274 necessary to provide a mechanism to allow easy protocol 275 identification. There is a NLPID value defined indicating the 276 presence of a SNAP header. 278 A SNAP header is of the form: 280 +--------------------------------------------+ 281 | Organizationally Unique | 282 +-- +--------------------+ 283 | Identifier | Protocol | 284 +-----------------------+--------------------+ 285 | Identifier | 286 +-----------------------+ 288 The three-octet Organizationally Unique Identifier (OUI) identifies 289 an organization which administers the meaning of the Protocol 290 Identifier (PID) which follows. Together they identify a distinct 291 protocol. Note that OUI 0x00-00-00 specifies that the following PID 292 is an Ethertype. 294 4.1. Routed Frames 296 Some protocols will have an assigned NLPID, but because the NLPID 297 numbering space is limited, not all protocols have specific NLPID 298 values assigned to them. When packets of such protocols are routed 299 over Frame Relay networks, they are sent using the NLPID 0x80 (which 300 indicates the presence of a SNAP header) followed by SNAP. If the 301 protocol has an Ethertype assigned, the OUI is 0x00-00-00 (which 302 indicates an Ethertype follows), and PID is the Ethertype of the 303 protocol in use. 305 When a SNAP header is present as described above, a one octet pad is 306 used to align the protocol data on a two octet boundary as shown 307 below. 309 Format of Routed Frames 310 with a SNAP Header 311 +-------------------------------+ 312 | Q.922 Address | 313 +---------------+---------------+ 314 | Control 0x03 | pad 0x00 | 315 +---------------+---------------+ 316 | NLPID 0x80 | Organization- | 317 +---------------+ | 318 | ally Unique Identifier (OUI) | 319 +-------------------------------+ 320 | Protocol Identifier (PID) | 321 +-------------------------------+ 322 | | 323 | Protocol Data | 324 | | 325 +-------------------------------+ 326 | FCS | 327 +-------------------------------+ 329 In the few cases when a protocol has an assigned NLPID (see Appendix 330 A), 48 bits can be saved using the format below: 332 Format of Routed NLPID Protocol 333 +-------------------------------+ 334 | Q.922 Address | 335 +---------------+---------------+ 336 | Control 0x03 | NLPID | 337 +---------------+---------------+ 338 | Protocol Data | 339 +-------------------------------+ 340 | FCS | 341 +-------------------------------+ 343 When using the NLPID encapsulation format as described above, the pad 344 octet is not used. 346 In the case of ISO protocols, the NLPID is considered to be the first 347 octet of the protocol data. It is unnecessary to repeat the NLPID in 348 this case. The single octet serves both as the demultiplexing value 349 and as part of the protocol data (refer to "Other Protocols over 350 Frame Relay for more details). Other protocols, such as IP, have a 351 NLPID defined (0xCC), but it is not part of the protocol itself. 353 Format of Routed IP Datagram 354 +-------------------------------+ 355 | Q.922 Address | 356 +---------------+---------------+ 357 | Control 0x03 | NLPID 0xCC | 358 +---------------+---------------+ 359 | IP Datagram | 360 +-------------------------------+ 361 | FCS | 362 +-------------------------------+ 364 4.2. Bridged Frames 366 The second type of Frame Relay traffic is bridged packets. These 367 packets are encapsulated using the NLPID value of 0x80 indicating 368 SNAP. As with other SNAP encapsulated protocols, there will be one 369 pad octet to align the data portion of the encapsulated frame. The 370 SNAP header which follows the NLPID identifies the format of the 371 bridged packet. The OUI value used for this encapsulation is the 372 802.1 organization code 0x00-80-C2. The PID portion of the SNAP 373 header (the two bytes immediately following the OUI) specifies the 374 form of the MAC header, which immediately follows the SNAP header. 375 Additionally, the PID indicates whether the original FCS is preserved 376 within the bridged frame. 378 Following the precedent in RFC 1638 [4], non-canonical MAC destination 379 addresses are used for encapsulated IEEE 802.5 and FDDI frames, and 380 canonical MAC destination addresses are used for the remaining 381 encapsulations defined in this section. 383 The 802.1 organization has reserved the following values to be used 384 with Frame Relay: 386 PID Values for OUI 0x00-80-C2 388 with preserved FCS w/o preserved FCS Media 389 ------------------ ----------------- ---------------- 390 0x00-01 0x00-07 802.3/Ethernet 391 0x00-02 0x00-08 802.4 392 0x00-03 0x00-09 802.5 393 0x00-04 0x00-0A FDDI 394 0x00-0B 802.6 396 In addition, the PID value 0x00-0E, when used with OUI 0x00-80-C2, 397 identifies Bridge Protocol Data Units (BPDUs) as defined by 398 802.1(d) or 802.1(g) [12], and the PID value 0x00-0F identifies 399 Source Routing BPDUs. 401 A packet bridged over Frame Relay will, therefore, have one of the 402 following formats: 404 Format of Bridged Ethernet/802.3 Frame 405 +-------------------------------+ 406 | Q.922 Address | 407 +---------------+---------------+ 408 | Control 0x03 | pad 0x00 | 409 +---------------+---------------+ 410 | NLPID 0x80 | OUI 0x00 | 411 +---------------+ --+ 412 | OUI 0x80-C2 | 413 +-------------------------------+ 414 | PID 0x00-01 or 0x00-07 | 415 +-------------------------------+ 416 | MAC destination address | 417 : : 418 | | 419 +-------------------------------+ 420 | (remainder of MAC frame) | 421 +-------------------------------+ 422 | LAN FCS (if PID is 0x00-01) | 423 +-------------------------------+ 424 | FCS | 425 +-------------------------------+ 426 Format of Bridged 802.4 Frame 427 +-------------------------------+ 428 | Q.922 Address | 429 +---------------+---------------+ 430 | Control 0x03 | pad 0x00 | 431 +---------------+---------------+ 432 | NLPID 0x80 | OUI 0x00 | 433 +---------------+ --+ 434 | OUI 0x80-C2 | 435 +-------------------------------+ 436 | PID 0x00-02 or 0x00-08 | 437 +---------------+---------------+ 438 | pad 0x00 | Frame Control | 439 +---------------+---------------+ 440 | MAC destination address | 441 : : 442 | | 443 +-------------------------------+ 444 | (remainder of MAC frame) | 445 +-------------------------------+ 446 | LAN FCS (if PID is 0x00-02) | 447 +-------------------------------+ 448 | FCS | 449 +-------------------------------+ 450 Format of Bridged 802.5 Frame 451 +-------------------------------+ 452 | Q.922 Address | 453 +---------------+---------------+ 454 | Control 0x03 | pad 0x00 | 455 +---------------+---------------+ 456 | NLPID 0x80 | OUI 0x00 | 457 +---------------+ --+ 458 | OUI 0x80-C2 | 459 +-------------------------------+ 460 | PID 0x00-03 or 0x00-09 | 461 +---------------+---------------+ 462 | pad 0x00 | Frame Control | 463 +---------------+---------------+ 464 | MAC destination address | 465 : : 466 | | 467 +-------------------------------+ 468 | (remainder of MAC frame) | 469 +-------------------------------+ 470 | LAN FCS (if PID is 0x00-03) | 471 | | 472 +-------------------------------+ 473 | FCS | 474 +-------------------------------+ 475 Format of Bridged FDDI Frame 476 +-------------------------------+ 477 | Q.922 Address | 478 +---------------+---------------+ 479 | Control 0x03 | pad 0x00 | 480 +---------------+---------------+ 481 | NLPID 0x80 | OUI 0x00 | 482 +---------------+ --+ 483 | OUI 0x80-C2 | 484 +-------------------------------+ 485 | PID 0x00-04 or 0x00-0A | 486 +---------------+---------------+ 487 | pad 0x00 | Frame Control | 488 +---------------+---------------+ 489 | MAC destination address | 490 : : 491 | | 492 +-------------------------------+ 493 | (remainder of MAC frame) | 494 +-------------------------------+ 495 | LAN FCS (if PID is 0x00-04) | 496 | | 497 +-------------------------------+ 498 | FCS | 499 +-------------------------------+ 500 Format of Bridged 802.6 Frame 501 +-------------------------------+ 502 | Q.922 Address | 503 +---------------+---------------+ 504 | Control 0x03 | pad 0x00 | 505 +---------------+---------------+ 506 | NLPID 0x80 | OUI 0x00 | 507 +---------------+ --+ 508 | OUI 0x80-C2 | 509 +-------------------------------+ 510 | PID 0x00-0B | 511 +---------------+---------------+ ------- 512 | Reserved | BEtag | Common 513 +---------------+---------------+ PDU 514 | BAsize | Header 515 +-------------------------------+ ------- 516 | MAC destination address | 517 : : 518 | | 519 +-------------------------------+ 520 | (remainder of MAC frame) | 521 +-------------------------------+ 522 | | 523 +- Common PDU Trailer -+ 524 | | 525 +-------------------------------+ 526 | FCS | 527 +-------------------------------+ 529 Note that in bridge 802.6 PDUs, there is only one choice for the PID 530 value, since the presence of a CRC-32 is indicated by the CIB bit in 531 the header of the MAC frame. 533 The Common Protocol Data Unit (CPDU) Header and Trailer are conveyed 534 to allow pipelining at the egress bridge to an 802.6 subnetwork. 535 Specifically, the CPDU Header contains the BAsize field, which 536 contains the length of the PDU. If this field is not available to 537 the egress 802.6 bridge, then that bridge cannot begin to transmit 538 the segmented PDU until it has received the entire PDU, calculated 539 the length, and inserted the length into the BAsize field. If the 540 field is available, the egress 802.6 bridge can extract the length 541 from the BAsize field of the Common PDU Header, insert it into the 542 corresponding field of the first segment, and immediately transmit 543 the segment onto the 802.6 subnetwork. Thus, the bridge can begin 544 transmitting the 802.6 PDU before it has received the complete PDU. 546 One should note that the Common PDU Header and Trailer of the 547 encapsulated frame should not be simply copied to the outgoing 802.6 548 subnetwork because the encapsulated BEtag value may conflict with the 549 previous BEtag value transmitted by that bridge. 551 Format of BPDU Frame 552 +-------------------------------+ 553 | Q.922 Address | 554 +-------------------------------+ 555 | Control 0x03 | 556 +-------------------------------+ 557 | PAD 0x00 | 558 +-------------------------------+ 559 | NLPID 0x80 | 560 +-------------------------------+ 561 | OUI 0x00-80-C2 | 562 +-------------------------------+ 563 | PID 0x00-0E | 564 +-------------------------------+ 565 | | 566 | BPDU as defined by | 567 | 802.1(d) or 802.1(g)[12] | 568 | | 569 +-------------------------------+ 570 | FCS | 571 +-------------------------------+ 573 Format of Source Routing BPDU Frame 574 +-------------------------------+ 575 | Q.922 Address | 576 +-------------------------------+ 577 | Control 0x03 | 578 +-------------------------------+ 579 | PAD 0x00 | 580 +-------------------------------+ 581 | NLPID 0x80 | 582 +-------------------------------+ 583 | OUI 0x00-80-C2 | 584 +-------------------------------+ 585 | PID 0x00-0F | 586 +-------------------------------+ 587 | | 588 | Source Routing BPDU | 589 | | 590 | | 591 +-------------------------------+ 592 | FCS | 593 +-------------------------------+ 595 5. Data Link Layer Parameter Negotiation 597 Frame Relay stations may choose to support the Exchange 598 Identification (XID) specified in Appendix III of Q.922 [1]. This 599 XID exchange allows the following parameters to be negotiated at the 600 initialization of a Frame Relay circuit: maximum frame size N201, 601 retransmission timer T200, and the maximum number of outstanding 602 Information (I) frames K. 604 A station may indicate its unwillingness to support acknowledged mode 605 multiple frame operation by specifying a value of zero for the 606 maximum window size, K. 608 If this exchange is not used, these values must be statically 609 configured by mutual agreement of Data Link Connection (DLC) 610 endpoints, or must be defaulted to the values specified in Section 611 5.9 of Q.922: 613 N201: 260 octets 615 K: 3 for a 16 Kbps link, 616 7 for a 64 Kbps link, 617 32 for a 384 Kbps link, 618 40 for a 1.536 Mbps or above link 620 T200: 1.5 seconds [see Q.922 for further details] 622 If a station supporting XID receives an XID frame, it shall respond 623 with an XID response. In processing an XID, if the remote maximum 624 frame size is smaller than the local maximum, the local system shall 625 reduce the maximum size it uses over this DLC to the remotely 626 specified value. Note that this shall be done before generating a 627 response XID. 629 The following diagram describes the use of XID to specify non-use of 630 acknowledged mode multiple frame operation. 632 Non-use of Acknowledged Mode Multiple Frame Operation 633 +---------------+ 634 | Address | (2,3 or 4 octets) 635 | | 636 +---------------+ 637 | Control 0xAF | 638 +---------------+ 639 | format 0x82 | 640 +---------------+ 641 | Group ID 0x80 | 642 +---------------+ 643 | Group Length | (2 octets) 644 | 0x00-0E | 645 +---------------+ 646 | 0x05 | PI = Frame Size (transmit) 647 +---------------+ 648 | 0x02 | PL = 2 649 +---------------+ 650 | Maximum | (2 octets) 651 | Frame Size | 652 +---------------+ 653 | 0x06 | PI = Frame Size (receive) 654 +---------------+ 655 | 0x02 | PL = 2 656 +---------------+ 657 | Maximum | (2 octets) 658 | Frame Size | 659 +---------------+ 660 | 0x07 | PI = Window Size 661 +---------------+ 662 | 0x01 | PL = 1 663 +---------------+ 664 | 0x00 | 665 +---------------+ 666 | 0x09 | PI = Retransmission Timer 667 +---------------+ 668 | 0x01 | PL = 1 669 +---------------+ 670 | 0x00 | 671 +---------------+ 672 | FCS | (2 octets) 673 | | 674 +---------------+ 676 6. Address Resolution for PVCs 678 This document only describes address resolution as it applies to PVCs. 679 SVC operation will be discussed in future documents. 681 There are situations in which a Frame Relay station may wish to 682 dynamically resolve a protocol address over PVCs. This may be 683 accomplished using the standard Address Resolution Protocol (ARP) [6] 684 encapsulated within a SNAP encoded Frame Relay packet as follows: 686 +-----------------------+-----------------------+ 687 | Q.922 Address | 688 +-----------------------+-----------------------+ 689 | Control (UI) 0x03 | pad 0x00 | 690 +-----------------------+-----------------------+ 691 | NLPID 0x80 | | SNAP Header 692 +-----------------------+ OUI 0x00-00-00 + Indicating 693 | | ARP 694 +-----------------------+-----------------------+ 695 | PID 0x0806 | 696 +-----------------------+-----------------------+ 697 | ARP packet | 698 | . | 699 | . | 700 | . | 701 +-----------------------+-----------------------+ 703 Where the ARP packet has the following format and values: 705 Data: 706 ar$hrd 16 bits Hardware type 707 ar$pro 16 bits Protocol type 708 ar$hln 8 bits Octet length of hardware address (n) 709 ar$pln 8 bits Octet length of protocol address (m) 710 ar$op 16 bits Operation code (request or reply) 711 ar$sha noctets source hardware address 712 ar$spa moctets source protocol address 713 ar$tha noctets target hardware address 714 ar$tpa moctets target protocol address 716 ar$hrd - assigned to Frame Relay is 15 decimal 717 (0x000F) [7]. 719 ar$pro - see assigned numbers for protocol ID number for 720 the protocol using ARP. (IP is 0x0800). 722 ar$hln - length in bytes of the address field (2, 3, or 4) 724 ar$pln - protocol address length is dependent on the 725 protocol (ar$pro) (for IP ar$pln is 4). 727 ar$op - 1 for request and 2 for reply. 729 ar$sha - Q.922 source hardware address, with C/R, FECN, 730 BECN, and DE set to zero. 732 ar$tha - Q.922 target hardware address, with C/R, FECN, 733 BECN, and DE set to zero. 735 Because DLCIs within most Frame Relay networks have only local 736 significance, an end station will not have a specific DLCI assigned 737 to itself. Therefore, such a station does not have an address to put 738 into the ARP request or reply. Fortunately, the Frame Relay network 739 does provide a method for obtaining the correct DLCIs. The solution 740 proposed for the locally addressed Frame Relay network below will 741 work equally well for a network where DLCIs have global significance. 743 The DLCI carried within the Frame Relay header is modified as it 744 traverses the network. When the packet arrives at its destination, 745 the DLCI has been set to the value that, from the standpoint of the 746 receiving station, corresponds to the sending station. For example, 747 in figure 1 below, if station A were to send a message to station B, 748 it would place DLCI 50 in the Frame Relay header. When station B 749 received this message, however, the DLCI would have been modified by 750 the network and would appear to B as DLCI 70. 752 ~~~~~~~~~~~~~~~ 753 ( ) 754 +-----+ ( ) +-----+ 755 | |-50------(--------------------)---------70-| | 756 | A | ( ) | B | 757 | |-60-----(---------+ ) | | 758 +-----+ ( | ) +-----+ 759 ( | ) 760 ( | ) <---Frame Relay 761 ~~~~~~~~~~~~~~~~ network 762 80 763 | 764 +-----+ 765 | | 766 | C | 767 | | 768 +-----+ 770 Figure 1 772 Lines between stations represent data link connections (DLCs). 773 The numbers indicate the local DLCI associated with each 774 connection. 776 DLCI to Q.922 Address Table for Figure 1 778 DLCI (decimal) Q.922 address (hex) 779 50 0x0C21 780 60 0x0CC1 781 70 0x1061 782 80 0x1401 784 For authoritative description of the correlation between DLCI and 785 Q.922 [1] addresses, the reader should consult that specification. 786 A summary of the correlation is included here for convenience. The 787 translation between DLCI and Q.922 address is based on a two byte 788 address length using the Q.922 encoding format. The format is: 790 8 7 6 5 4 3 2 1 791 +------------------------+---+--+ 792 | DLCI (high order) |C/R|EA| 793 +--------------+----+----+---+--+ 794 | DLCI (lower) |FECN|BECN|DE |EA| 795 +--------------+----+----+---+--+ 797 For ARP and its variants, the FECN, BECN, C/R and DE bits are 798 assumed to be 0. 800 When an ARP message reaches a destination, all hardware addresses 801 will be invalid. The address found in the frame header will, 802 however, be correct. Though it does violate the purity of layering, 803 Frame Relay may use the address in the header as the sender hardware 804 address. It should also be noted that the target hardware address, 805 in both ARP request and reply, will also be invalid. This should not 806 cause problems since ARP does not rely on these fields and in fact, 807 an implementation may zero fill or ignore the target hardware address 808 field entirely. 810 As an example of how this address replacement scheme may work, refer 811 to figure 1. If station A (protocol address pA) wished to resolve 812 the address of station B (protocol address pB), it would format an 813 ARP request with the following values: 815 ARP request from A 816 ar$op 1 (request) 817 ar$sha unknown 818 ar$spa pA 819 ar$tha undefined 820 ar$tpa pB 822 Because station A will not have a source address associated with it, 823 the source hardware address field is not valid. Therefore, when the 824 ARP packet is received, it must extract the correct address from the 825 Frame Relay header and place it in the source hardware address field. 826 This way, the ARP request from A will become: 828 ARP request from A as modified by B 829 ar$op 1 (request) 830 ar$sha 0x1061 (DLCI 70) from Frame Relay header 831 ar$spa pA 832 ar$tha undefined 833 ar$tpa pB 835 Station B's ARP will then be able to store station A's protocol 836 address and Q.922 address association correctly. Next, station B 837 will form a reply message. Many implementations simply place the 838 source addresses from the ARP request into the target addresses and 839 then fills in the source addresses with its addresses. In this case, 840 the ARP response would be: 842 ARP response from B 843 ar$op 2 (response) 844 ar$sha unknown 845 ar$spa pB 846 ar$tha 0x1061 (DLCI 70) 847 ar$tpa pA 849 Again, the source hardware address is unknown and when the response 850 is received, station A will extract the address from the Frame Relay 851 header and place it in the source hardware address field. Therefore, 852 the response will become: 854 ARP response from B as modified by A 855 ar$op 2 (response) 856 ar$sha 0x0C21 (DLCI 50) 857 ar$spa pB 858 ar$tha 0x1061 (DLCI 70) 859 ar$tpa pA 861 Station A will now correctly recognize station B having protocol 862 address pB associated with Q.922 address 0x0C21 (DLCI 50). 864 Reverse ARP (RARP) [8] works in exactly the same way. Still using 865 figure 1, if we assume station C is an address server, the following 866 RARP exchanges will occur: 868 RARP request from A RARP request as modified by C 869 ar$op 3 (RARP request) ar$op 3 (RARP request) 870 ar$sha unknown ar$sha 0x1401 (DLCI 80) 871 ar$spa undefined ar$spa undefined 872 ar$tha 0x0CC1 (DLCI 60) ar$tha 0x0CC1 (DLCI 60) 873 ar$tpa pC ar$tpa pC 875 Station C will then look up the protocol address corresponding to 876 Q.922 address 0x1401 (DLCI 80) and send the RARP response. 878 RARP response from C RARP response as modified by A 879 ar$op 4 (RARP response) ar$op 4 (RARP response) 880 ar$sha unknown ar$sha 0x0CC1 (DLCI 60) 881 ar$spa pC ar$spa pC 882 ar$tha 0x1401 (DLCI 80) ar$tha 0x1401 (DLCI 80) 883 ar$tpa pA ar$tpa pA 885 This means that the Frame Relay interface must only intervene in the 886 processing of incoming packets. 888 In the absence of suitable multicast, ARP may still be implemented. 889 To do this, the end station simply sends a copy of the ARP request 890 through each relevant DLC, thereby simulating a broadcast. 892 The use of multicast addresses in a Frame Relay environment, as 893 specified by [19], is presently being considered by Frame Relay 894 providers. In time, multicast addressing may become useful in 895 sending ARP requests and other "broadcast" messages. 897 Because of the inefficiencies of emulating broadcasting in a Frame 898 Relay environment, a new address resolution variation was developed. 899 It is called Inverse ARP [11] and describes a method for resolving a 900 protocol address when the hardware address is already known. In 901 Frame Relay's case, the known hardware address is the DLCI. Support 902 for Inverse ARP is not required to implement this specification, but 903 it has proven useful for Frame Relay interface autoconfiguration. 904 See [11] for its description and an example of its use with Frame 905 Relay. 907 Stations must be able to map more than one IP address in the same IP 908 subnet (CIDR address prefix) to a particular DLCI on a Frame Relay 909 interface. This need arises from applications such as remote access, 910 where servers must act as ARP proxies for many dial-in clients, each 911 assigned a unique IP address while sharing bandwidth on the same DLC. 912 The dynamic nature of such applications result in frequent address 913 association changes with no affect on the DLC's status as reported by 914 Frame Relay PVC Status Signaling. 916 As with any other interface that utilizes ARP, stations may learn the 917 associations between IP addresses and DLCIs by processing unsolicited 918 ("gratuitous") ARP requests that arrive on the DLC. If one station 919 (perhaps a terminal server or remote access server) wishes to inform 920 its peer station on the other end of a Frame Relay DLC of a new 921 association between an IP address and that PVC, it should send an 922 unsolicited ARP request with the source IP address equal to the 923 destination IP address, and both set to the new IP address being used 924 on the DLC. This allows a station to "announce" new client 925 connections on a particular DLCI. The receiving station must store 926 the new association, and remove any old existing association, if 927 necessary, from any other DLCI on the interface. 929 7. IP over Frame Relay 931 Internet Protocol [9] (IP) datagrams sent over a Frame Relay network 932 conform to the encapsulation described previously. Within this 933 context, IP could be encapsulated in two different ways. 935 1. NLPID value indicating IP 937 +-----------------------+-----------------------+ 938 | Q.922 Address | 939 +-----------------------+-----------------------+ 940 | Control (UI) 0x03 | NLPID 0xCC | 941 +-----------------------+-----------------------+ 942 | IP packet | 943 | . | 944 | . | 945 | . | 946 +-----------------------+-----------------------+ 948 2. NLPID value indicating SNAP 950 +-----------------------+-----------------------+ 951 | Q.922 Address | 952 +-----------------------+-----------------------+ 953 | Control (UI) 0x03 | pad 0x00 | 954 +-----------------------+-----------------------+ 955 | NLPID 0x80 | | SNAP Header 956 +-----------------------+ OUI = 0x00-00-00 + Indicating 957 | | IP 958 +-----------------------+-----------------------+ 959 | PID 0x0800 | 960 +-----------------------+-----------------------+ 961 | IP packet | 962 | . | 963 | . | 964 | . | 965 +-----------------------+-----------------------+ 967 Although both of these encapsulations are supported under the given 968 definitions, it is advantageous to select only one method as the 969 appropriate mechanism for encapsulating IP data. Therefore, IP data 970 shall be encapsulated using the NLPID value of 0xCC indicating IP as 971 shown in option 1 above. This (option 1) is more efficient in 972 transmission (48 fewer bits), and is consistent with the 973 encapsulation of IP in X.25. 975 8. Other Protocols over Frame Relay 977 As with IP encapsulation, there are alternate ways to transmit 978 various protocols within the scope of this definition. To eliminate 979 the conflicts, the SNAP encapsulation is only used if no NLPID value 980 is defined for the given protocol. 982 As an example of how this works, ISO CLNP has a NLPID defined (0x81). 983 Therefore, the NLPID field will indicate ISO CLNP and the data packet 984 will follow immediately. The frame would be as follows: 986 +---------------------------------------------+ 987 | Q.922 Address | 988 +----------------------+----------------------+ 989 | Control (UI) 0x03 | NLPID 0x81 (CLNP) | 990 +----------------------+----------------------+ 991 | remainder of CLNP packet | 992 | . | 993 | . | 994 +---------------------------------------------+ 996 In this example, the NLPID is used to identify the data packet as 997 CLNP. It is also considered part of the CLNP packet and as such, the 998 NLPID should not be removed before being sent to the upper layers for 999 processing. The NLPID is not duplicated. 1001 Other protocols, such as IPX, do not have a NLPID value defined. As 1002 mentioned above, IPX would be encapsulated using the SNAP header. In 1003 this case, the frame would be as follows: 1005 +---------------------------------------------+ 1006 | Q.922 Address | 1007 +----------------------+----------------------+ 1008 | Control (UI) 0x03 | pad 0x00 | 1009 +----------------------+----------------------+ 1010 | NLPID 0x80 (SNAP) | OUI - 0x00 00 00 | 1011 +----------------------+ | 1012 | | 1013 +---------------------------------------------+ 1014 | PID 0x8137 | 1015 +---------------------------------------------+ 1016 | IPX packet | 1017 | . | 1018 | . | 1019 +---------------------------------------------+ 1021 9. Bridging Model for Frame Relay 1023 The model for bridging in a Frame Relay network is identical to the 1024 model for remote bridging as described in IEEE P802.1g "Remote MAC 1025 Bridging" [13] and supports the concept of "Virtual Ports". Remote 1026 bridges with LAN ports receive and transmit MAC frames to and from 1027 the LANs to which they are attached. They may also receive and 1028 transmit MAC frames through virtual ports to and from other remote 1029 bridges. A virtual port may represent an abstraction of a remote 1030 bridge's point of access to one, two or more other remote bridges. 1032 Remote Bridges are statically configured as members of a remote 1033 bridge group by management. All members of a remote bridge group are 1034 connected by one or more virtual ports. The set of remote MAC bridges 1035 in a remote bridge group provides actual or *potential* MAC layer 1036 interconnection between a set of LANs and other remote bridge groups 1037 to which the remote bridges attach. 1039 In a Frame Relay network there must be a full mesh of Frame Relay VCs 1040 between bridges of a remote bridge group. If the frame relay network 1041 is not a full mesh, then the bridge network must be divided into 1042 multiple remote bridge groups. 1044 The frame relay VCs that interconnect the bridges of a remote bridge 1045 group may be combined or used individually to form one or more 1046 virtual bridge ports. This gives flexibility to treat the Frame 1047 Relay interface either as a single virtual bridge port, with all VCs 1048 in a group, or as a collection of bridge ports (individual or grouped 1049 VCs). 1051 When a single virtual bridge port provides the interconnectivity for 1052 all bridges of a given remote bridge group (i.e. all VCs are combined 1053 into a single virtual port), the standard Spanning Tree Algorithm may 1054 be used to determine the state of the virtual port. When more than 1055 one virtual port is configured within a given remote bridge group 1056 then an "extended" Spanning Tree Algorithm is required. Such an 1057 extended algorithm is defined in IEEE 802.1g [13]. The operation of 1058 this algorithm is such that a virtual port is only put into backup if 1059 there is a loop in the network external to the remote bridge group. 1061 The simplest bridge configuration for a Frame Relay network is the 1062 LAN view where all VCs are combined into a single virtual port. 1063 Frames, such as BPDUs, which would be broadcast on a LAN, must be 1064 flooded to each VC (or multicast if the service is developed for 1065 Frame Relay services). Flooding is performed by sending the packet to 1066 each relevant DLC associated with the Frame Relay interface. The VCs 1067 in this environment are generally invisible to the bridge. That is, 1068 the bridge sends a flooded frame to the frame relay interface and 1069 does not "see" that the frame is being forwarded to each VC 1070 individually. If all participating bridges are fully connected (full 1071 mesh) the standard Spanning Tree Algorithm will suffice in this 1072 configuration. 1074 Typically LAN bridges learn which interface a particular end station 1075 may be reached on by associating a MAC address with a bridge port. 1076 In a Frame Relay network configured for the LAN-like single bridge 1077 port (or any set of VCs grouped together to form a single bridge 1078 port), however, the bridge must not only associated a MAC address 1079 with a bridge port, but it must also associate it with a connection 1080 identifier. For Frame Relay networks, this connection identifier is 1081 a DLCI. It is unreasonable and perhaps impossible to require bridges 1082 to statically configure an association of every possible destination 1083 MAC address with a DLC. Therefore, Frame Relay LAN-modeled bridges 1084 must provide a mechanism to allow the Frame Relay bridge port to 1085 dynamically learn the associations. To accomplish this dynamic 1086 learning, a bridged packet shall conform to the encapsulation 1087 described within section 4.2. In this way, the receiving Frame Relay 1088 interface will know to look into the bridged packet to gather the 1089 appropriate information. 1091 A second Frame Relay bridging approach, the point-to-point view, 1092 treats each Frame Relay VC as a separate bridge port. Flooding and 1093 forwarding packets are significantly less complicated using the 1094 point-to-point approach because each bridge port has only one 1095 destination. There is no need to perform artificial flooding or to 1096 associate DLCIs with destination MAC addresses. Depending upon the 1097 interconnection of the VCs, an extended Spanning Tree algorithm may 1098 be required to permit all virtual ports to remain active as long as 1099 there are no true loops in the topology external to the remote bridge 1100 group. 1102 It is also possible to combine the LAN view and the point-to-point 1103 view on a single Frame Relay interface. To do this, certain VCs are 1104 combined to form a single virtual bridge port while other VCs are 1105 independent bridge ports. 1107 The following drawing illustrates the different possible bridging 1108 configurations. The dashed lines between boxes represent virtual 1109 circuits. 1111 +-------+ 1112 -------------------| B | 1113 / -------| | 1114 / / +-------+ 1115 / | 1116 +-------+/ \ +-------+ 1117 | A | -------| C | 1118 | |-----------------------| | 1119 +-------+\ +-------+ 1120 \ 1121 \ +-------+ 1122 \ | D | 1123 -------------------| | 1124 +-------+ 1126 Since there is less than a full mesh of VCs between the bridges in 1127 this example, the network must be divided into more than one remote 1128 bridge group. A reasonable configuration is to have bridges A, B, 1129 and C in one group, and have bridges A and D in a second. 1131 Configuration of the first bridge group combines the VCs 1132 interconnection the three bridges (A, B, and C) into a single virtual 1133 port. This is an example of the LAN view configuration. The second 1134 group would also be a single virtual port which simply connects 1135 bridges A and D. In this configuration the standard Spanning Tree 1136 Algorithm is sufficient to detect loops. 1138 An alternative configuration has three individual virtual ports in 1139 the first group corresponding to the VCs interconnecting bridges A, B 1140 and C. Since the application of the standard Spanning Tree Algorithm 1141 to this configuration would detect a loop in the topology, an 1142 extended Spanning Tree Algorithm would have to be used in order for 1143 all virtual ports to be kept active. Note that the second group 1144 would still consist of a single virtual port and the standard 1145 Spanning Tree Algorithm could be used in this group. 1147 Using the same drawing, one could construct a remote bridge scenario 1148 with three bridge groups. This would be an example of the point-to- 1149 point case. Here, the VC connecting A and B, the VC connecting A and 1150 C, and the VC connecting A and D are all bridge groups with a single 1151 virtual port. 1153 10. Appendix A 1155 List of Commonly Used NLPIDs 1157 0x00 Null Network Layer or Inactive Set 1158 (not used with Frame Relay) 1159 0x08 Q.933 [2] 1160 0x80 SNAP 1161 0x81 ISO CLNP 1162 0x82 ISO ESIS 1163 0x83 ISO ISIS 1164 0x8E IPv6 1165 0xB0 FRF.9 Data Compression [14] 1166 0xB1 FRF.12 Fragmentation [18] 1167 0xCC IPv4 1168 0xCF PPP in Frame Relay [17] 1170 List of PIDs of OUI 00-80-C2 1172 with preserved FCS w/o preserved FCS Media 1173 ------------------ ----------------- -------------- 1174 0x00-01 0x00-07 802.3/Ethernet 1175 0x00-02 0x00-08 802.4 1176 0x00-03 0x00-09 802.5 1177 0x00-04 0x00-0A FDDI 1178 0x00-0B 802.6 1179 0x00-0D Fragments 1180 0x00-0E BPDUs as defined by 1181 802.1(d) or 1182 802.1(g)[12]. 1183 0x00-0F Source Routing BPDUs 1185 11. Appendix B - Connection Oriented Procedures 1187 This Appendix contains additional information and instructions for 1188 using ITU Recommendation Q.933 [2] and other ITU standards for 1189 encapsulating data over frame relay. The information contained here 1190 is similar (and in some cases identical) to that found in Annex E to 1191 ITU Q.933. The authoritative source for this information is in Annex 1192 E and is repeated here only for convenience. 1194 The Network Level Protocol ID (NLPID) field is administered by ISO 1195 and the ITU. It contains values for many different protocols 1196 including IP, CLNP (ISO 8473), ITU Q.933, and ISO 8208. A figure 1197 summarizing a generic encapsulation technique over frame relay 1198 networks follows. The scheme's flexibility consists in the 1199 identification of multiple alternative to identify different 1200 protocols used either by 1201 - end-to-end systems or 1202 - LAN to LAN bride and routers or 1203 - a combination of the above. 1205 over frame relay networks. 1207 Q.922 control 1208 | 1209 | 1210 -------------------------------------------- 1211 | | 1212 UI I Frame 1213 | | 1214 --------------------------------- -------------- 1215 | 0x08 | 0x81 |0xCC | 0x80 |..01.... |..10.... 1216 | | | | | | 1217 Q.933 CLNP IP SNAP ISO 8208 ISO 8208 1218 | | Modulo 8 Modulo 128 1219 | | 1220 -------------------- OUI 1221 | | | 1222 L2 ID L3 ID ------- 1223 | User | | 1224 | Specified | | 1225 | 0x70 802.3 802.6 1226 | 1227 --------------------------- 1228 |0x51 |0x4E | |0x4C |0x50 1229 | | | | | 1230 7776 Q.922 Others 802.2 User 1231 Specified 1233 For those protocols which do not have a NLPID assigned or do not have 1234 a SNAP encapsulation, the NLPID value of 0x08, indicating ITU 1235 Recommendation Q.933 should be used. The four octets following the 1236 NLPID include both layer 2 and layer 3 protocol identification. The 1237 code points for most protocols are currently defined in ITU Q.933 low 1238 layer compatibility information element. The code points for "User 1239 Specified" are described in Frame Relay Forum FRF.3.1 [15]. There is 1240 also an escape for defining non-standard protocols. 1242 Format of Other Protocols 1243 using Q.933 NLPID 1244 +-------------------------------+ 1245 | Q.922 Address | 1246 +---------------+---------------+ 1247 | Control 0x03 | NLPID 0x08 | 1248 +---------------+---------------+ 1249 | L2 Protocol ID | 1250 | octet 1 | octet 2 | 1251 +---------------+---------------+ 1252 | L3 Protocol ID | 1253 | octet 1 | octet 2 | 1254 +---------------+---------------+ 1255 | Protocol Data | 1256 +-------------------------------+ 1257 | FCS | 1258 +-------------------------------+ 1260 ISO 8802/2 with user specified 1261 layer 3 1262 +-------------------------------+ 1263 | Q.922 Address | 1264 +---------------+---------------+ 1265 | Control 0x03 | NLPID 0x08 | 1266 +---------------+---------------+ 1267 | 802/2 0x4C | 0x80 | 1268 +---------------+---------------+ 1269 |User Spec. 0x70| Note 1 | 1270 +---------------+---------------+ 1271 | DSAP | SSAP | 1272 +---------------+---------------+ 1273 | Control (Note 2) | 1274 +-------------------------------+ 1275 | Remainder of PDU | 1276 +-------------------------------+ 1277 | FCS | 1278 +-------------------------------+ 1280 Note 1: Indicates the code point for user specified 1281 layer 3 protocol. 1283 Note 2: Control field is two octets for I-format and 1284 S-format frames (see 88002/2) 1286 Encapsulations using I frame (layer 2) 1287 The Q.922 I frame is for supporting layer 3 protocols which require 1288 acknowledged data link layer (e.g., ISO 8208). The C/R bit will be 1289 used for command and response indications. 1291 Format of ISO 8208 frame 1292 Modulo 8 1293 +-------------------------------+ 1294 | Q.922 Address | 1295 +---------------+---------------+ 1296 | ....Control I frame | 1297 +---------------+---------------+ 1298 | 8208 packet (modulo 8) Note 3 | 1299 | | 1300 +-------------------------------+ 1301 | FCS | 1302 +-------------------------------+ 1304 Note 3: First octet of 8208 packet also identifies the 1305 NLPID which is "..01....". 1307 Format of ISO 8208 frame 1308 Modulo 128 1309 +-------------------------------+ 1310 | Q.922 Address | 1311 +---------------+---------------+ 1312 | ....Control I frame | 1313 +---------------+---------------+ 1314 | 8208 packet (modulo 128) | 1315 | Note 4 | 1316 +-------------------------------+ 1317 | FCS | 1318 +-------------------------------+ 1320 Note 4: First octet of 8208 packet also identifies the 1321 NLPID which is "..10....". 1323 12. Security Considerations 1325 This document defines mechanisms for identifying the multiprotocol 1326 encapsulation of datagrams over Frame Relay. There is obviously an 1327 element in trust in any encapsulation protocol - a receiver must 1328 trust that the sender has correctly identified the protocol being 1329 encapsulated. In general, there is no way for a receiver to try to 1330 ascertain that the sender did indeed use the proper protocol 1331 identification, nor would this be desired functionality. 1333 It also specifies the use of ARP and RARP with Frame Relay, and is 1334 subject to the same security constraints that affect ARP and similar 1335 address resolution protocols. Because authentication is not a part 1336 of ARP, there are known security issues relating to its use (e.g., 1337 host impersonation). No additional security mechanisms have been 1338 added to ARP or RARP for use with Frame Relay networks. 1340 13. References 1342 [1] International Telecommunication Union, "ISDN Data Link Layer 1343 Specification for Frame Mode Bearer Services", ITU-T 1344 Recommendation Q.922, 1992. 1346 [2] International Telecommunication Union, "Signalling Specifications 1347 for Frame Mode Switched and Permanent Virtual Connection Control 1348 and Status Monitoring", ITU-T Recommendation Q.933, 1995. 1350 [3] Information technology - Telecommunications and Information 1351 Exchange between systems - Protocol Identification in the Network 1352 Layer, ISO/IEC TR 9577: 1992. 1354 [4] F. Baker, R. Bowen, "PPP Bridging Control Protocol (BCP)", RFC 1355 1638, ACC, June 1994. 1357 [5] International Standard, Information Processing Systems - Local 1358 Area Networks - Logical Link Control, ISO 8802-2, ANSI/IEEE, 1359 Second Edition, 1994-12-30. 1361 [6] D. Plummer, "An Ethernet Address Resolution Protocol - or - 1362 Converting Network Protocol Addresses to 48.bit Ethernet Address 1363 for Transmission on Ethernet Hardware", STD 37, RFC 826, MIT, 1364 November 1982. 1366 [7] J. Reynolds, J. Postel, "Assigned Numbers", STD 2, RFC 1700, 1367 USC/Information Sciences Institute, October 1994 1369 [8] R. Finlayson, R. Mann, J. Mogul, M. Theimer, "A Reverse Address 1370 Resolution Protocol", STD 38, RFC 903, Stanford University, June 1371 1984. 1373 [9] J. Postel, J. Reynolds, "A Standard for the Transmission of IP 1374 Datagrams over IEEE 802 Networks", RFC 1042, USC/Information 1375 Sciences Institute, February 1988. 1377 [10] IEEE, "IEEE Standard for Local and Metropolitan Area Networks: 1378 Overview and architecture", IEEE Standard 802-1990. 1380 [11] T. Bradley, C. Brown, A. Malis, "Inverse Address Resolution 1381 Protocol", RFC TBD, August 1998. 1383 [12] IEEE, "IEEE Standard for Local and Metropolitan Networks: Media 1384 Access Control (MAC) Bridges", IEEE Standard 802.1D-1990. 1386 [13] ISO/IEC 15802-5 : 1998 (IEEE Standard 802.1G), Remote Media 1387 Access Control (MAC) Bridging, March 12, 1997. 1389 [14] Frame Relay Forum, "Data Compression Over Frame Relay 1390 Implementation Agreement", FRF.9, January 22, 1996. 1392 [15] Frame Relay Forum, "Multiprotocol Encapsulation Implementation 1393 Agreement", FRF.3.1, June 22, 1995. 1395 [16] S. Bradner, "Key words for use in RFCs to Indicate Requirement 1396 Levels", BCP 14, RFC 2119, Harvard University, March 1997. 1398 [17] W. Simpson, "PPP in Frame Relay", RFC 1973, Daydreamer, June 1399 1996. 1401 [18] Frame Relay Forum, "Frame Relay Fragmentation Implementation 1402 Agreement", FRF.12, December 1997. 1404 [19] Frame Relay Forum, "Frame Relay PVC Multicast Service and 1405 Protocol Implementation Agreement", FRF.7, October 21, 1994. 1407 14. Authors' Addresses 1409 Caralyn Brown 1410 Consultant 1411 Email: cbrown@juno.com 1413 Andrew Malis 1414 Ascend Communications, Inc. 1415 1 Robbins Road 1416 Westford, MA 01886 1417 Phone: (978) 952-7414 1418 Email: malis@ascend.com