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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 1332, 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' ** Obsolete normative reference: RFC 1293 (ref. '11') (Obsoleted by RFC 2390) -- Possible downref: Non-RFC (?) normative reference: ref. '12' -- Possible downref: Non-RFC (?) normative reference: ref. '13' -- Possible downref: Non-RFC (?) normative reference: ref. '14' Summary: 14 errors (**), 0 flaws (~~), 8 warnings (==), 12 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group C. Brown 3 INTERNET DRAFT Bay Networks, Inc. 4 Obsoletes: 1294, 1490 A. Malis 5 Ascom Nexion, Inc. 6 August 5, 1996 7 Expires February 5, 1997 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 ds.internic.net (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 Starburst Communications. Comments and 51 contributions from many sources, especially those from Ray Samora of 52 Proteon, Ken Rehbehn of Netrix Corporation, Fred Baker and Charles 53 Carvalho of Cisco Systems and Mostafa Sherif of AT&T have been 54 incorporated into this document. Special thanks to Dory Leifer of 55 University of Michigan for his contributions to the resolution of 56 fragmentation issues (though it was deleted in the final version) and 57 Floyd Backes and Laura Bridge of 3Com for their contributions to the 58 bridging descriptions. This document could not have been completed 59 without the expertise of the IP over Large Public Data Networks 60 working group of the IETF. 62 Modifications from RFC 1490 64 Some language changes were necessary to clarify RFC 1490. None of 65 these changes impacted the technical aspects of this document, but 66 were required to keep diagrams and language specific and consistent. 67 Specifics of these changes will not be listed here. Below are listed 68 those changes which were significant. 70 a) The requirement for stations to accept SNAP encapsulated protocols 71 for which a NLPID was available, was removed. RFC 1490 indicated 72 that, if a protocol, such as IP, had a designated NLPID value, it 73 must be used. Later the document required stations to accept a 74 SNAP encapsulated version of this same protocol. This is clearly 75 inconsistent. A compliant station must send and accept the NLPID 76 encapsulated version of such a protocol. It MAY accept the SNAP 77 encapsulation but should not be required to do so as these frames 78 are noncompliant. 80 b) Fragmentation was removed. To date there are no interoperable 81 implementations of the fragmentation algorithm presented in RFC 82 1490. Additionally, there have been several suggestions that the 83 proposed mechanisms are insufficient for some frame relay 84 applications. To this end, fragmentation was removed from this 85 document. Work will continue elsewhere on the fragmentation 86 issue. 88 c) The address resolution presented in RFC 1490 referred only to 89 PVC environments and is insufficient for SVC environments. 90 Therefore the section title was changed to reflect this. Further 91 work on SVC address resolution will take place in the ION working 92 group. 94 d) The encapsulation for Source Routing BPDUs was added, and the 95 lists in Appendix A were augmented. 97 e) The use of canonical and non-canonical MAC destination addresses 98 in the bridging encapsulations was clarified. 100 1. Conventions and Acronyms 102 The following language conventions are used in the items of 103 specification in this document: 105 o Must, Shall or Mandatory -- the item is an absolute 106 requirement of the specification. 108 o Should or Recommended -- the item should generally be 109 followed for all but exceptional circumstances. 111 o May or Optional -- the item is truly optional and may be 112 followed or ignored according to the needs of the 113 implementor. 115 All drawings in this document are drawn with the left-most bit as the 116 high order bit for transmission. For example, the drawings might be 117 labeled as: 119 0 1 2 3 4 5 6 7 bits 120 +---+---+---+---+---+---+---+ 122 +---------------------------+ 123 | flag (7E hexadecimal) | 124 +---------------------------+ 125 | Q.922 Address* | 126 +-- --+ 127 | | 128 +---------------------------+ 129 : : 130 : : 131 +---------------------------+ 133 Drawings that would be too large to fit onto one page if each octet 134 were presented on a single line are drawn with two octets per line. 135 These are also drawn with the left-most bit as the high order bit for 136 transmission. There will be a "+" to distinguish between octets as 137 in the following example. 139 |--- octet one ---|--- octet two ---| 140 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 141 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 143 +--------------------------------------------+ 144 | Organizationally Unique | 145 +-- +--------------------+ 146 | Identifier | Protocol | 147 +-----------------------+--------------------+ 148 | Identifier | 149 +-----------------------+ 151 The following are common acronyms used throughout this document. 153 BECN - Backward Explicit Congestion Notification 154 BPDU - Bridge Protocol Data Unit 155 C/R - Command/Response bit 156 DCE - Data Communication Equipment 157 DE - Discard Eligibility bit 158 DTE - Data Terminal Equipment 159 FECN - Forward Explicit Congestion Notification 160 PDU - Protocol Data Unit 161 PTT - Postal Telephone & Telegraph 162 SNAP - Subnetwork Access Protocol 164 2. Introduction 166 The following discussion applies to those devices which serve as end 167 stations (DTEs) on a public or private Frame Relay network (for 168 example, provided by a common carrier or PTT. It will not discuss 169 the behavior of those stations that are considered a part of the 170 Frame Relay network (DCEs) other than to explain situations in which 171 the DTE must react. 173 The Frame Relay network provides a number of virtual circuits that 174 form the basis for connections between stations attached to the same 175 Frame Relay network. The resulting set of interconnected devices 176 forms a private Frame Relay group which may be either fully 177 interconnected with a complete "mesh" of virtual circuits, or only 178 partially interconnected. In either case, each virtual circuit is 179 uniquely identified at each Frame Relay interface by a Data Link 180 Connection Identifier (DLCI). In most circumstances, DLCIs have 181 strictly local significance at each Frame Relay interface. 183 The specifications in this document are intended to apply to both 184 switched and permanent virtual circuits. 186 3. Frame Format 188 All protocols must encapsulate their packets within a Q.922 Annex A 189 frame [1,2]. Additionally, frames shall contain information 190 necessary to identify the protocol carried within the protocol data 191 unit (PDU), thus allowing the receiver to properly process the 192 incoming packet. The format shall be as follows: 194 +---------------------------+ 195 | flag (7E hexadecimal) | 196 +---------------------------+ 197 | Q.922 Address* | 198 +-- --+ 199 | | 200 +---------------------------+ 201 | Control (UI = 0x03) | 202 +---------------------------+ 203 | Pad (when required) (0x00)| 204 +---------------------------+ 205 | NLPID | 206 +---------------------------+ 207 | . | 208 | . | 209 | . | 210 | Data | 211 | . | 212 | . | 213 +---------------------------+ 214 | Frame Check Sequence | 215 +-- . --+ 216 | (two octets) | 217 +---------------------------+ 218 | flag (7E hexadecimal) | 219 +---------------------------+ 221 * Q.922 addresses, as presently defined, are two octets and 222 contain a 10-bit DLCI. In some networks Q.922 addresses 223 may optionally be increased to three or four octets. 225 The control field is the Q.922 control field. The UI (0x03) value is 226 used unless it is negotiated otherwise. The use of XID (0xAF or 227 0xBF) is permitted and is discussed later. 229 The pad field is used to align the data portion (beyond the 230 encapsulation header) of the frame to a two octet boundary. If 231 present, the pad is a single octet and must have a value of zero. 232 Explicit directions of when to use the pad field are discussed later 233 in this document. 235 The Network Level Protocol ID (NLPID) field is administered by ISO 236 and the ITU. It contains values for many different protocols 237 including IP, CLNP, and IEEE Subnetwork Access Protocol (SNAP)[10]. 238 This field tells the receiver what encapsulation or what protocol 239 follows. Values for this field are defined in ISO/IEC TR 9577 [3]. A 240 NLPID value of 0x00 is defined within ISO/IEC TR 9577 as the Null 241 Network Layer or Inactive Set. Since it cannot be distinguished from 242 a pad field, and because it has no significance within the context of 243 this encapsulation scheme, a NLPID value of 0x00 is invalid under the 244 Frame Relay encapsulation. Appendix A contains a list of some of the 245 more commonly used NLPID values. 247 There is no commonly implemented minimum maximum frame size for Frame 248 Relay. A network must, however, support at least a 262 octet 249 maximum. Generally, the maximum will be greater than or equal to 250 1600 octets, but each Frame Relay provider will specify an 251 appropriate value for its network. A Frame Relay DTE, therefore, 252 must allow the maximum acceptable frame size to be configurable. 254 The minimum frame size allowed for Frame Relay is five octets between 255 the opening and closing flags assuming a two octet Q.922 address 256 field. This minimum increases to six octets for three octet Q.922 257 address and seven octets for the four octet Q.922 address format. 259 4. Interconnect Issues 261 There are two basic types of data packets that travel within the 262 Frame Relay network: routed packets and bridged packets. These 263 packets have distinct formats and therefore, must contain an 264 indicator that the destination may use to correctly interpret the 265 contents of the frame. This indicator is embedded within the NLPID 266 and SNAP header information. 268 For those protocols that do not have a NLPID already assigned, it is 269 necessary to provide a mechanism to allow easy protocol 270 identification. There is a NLPID value defined indicating the 271 presence of a SNAP header. 273 A SNAP header is of the form: 275 +--------------------------------------------+ 276 | Organizationally Unique | 277 +-- +--------------------+ 278 | Identifier | Protocol | 279 +-----------------------+--------------------+ 280 | Identifier | 281 +-----------------------+ 283 The three-octet Organizationally Unique Identifier (OUI) identifies 284 an organization which administers the meaning of the Protocol 285 Identifier (PID) which follows. Together they identify a distinct 286 protocol. Note that OUI 0x00-00-00 specifies that the following PID 287 is an Ethertype. 289 4.1. Routed Frames 291 Some protocols will have an assigned NLPID, but because the NLPID 292 numbering space is limited, not all protocols have specific NLPID 293 values assigned to them. When packets of such protocols are routed 294 over Frame Relay networks, they are sent using the NLPID 0x80 (which 295 indicates the presence of a SNAP header) followed by SNAP. If the 296 protocol has an Ethertype assigned, the OUI is 0x00-00-00 (which 297 indicates an Ethertype follows), and PID is the Ethertype of the 298 protocol in use. 300 When a SNAP header is present as described above, a one octet pad is 301 used to align the protocol data on a two octet boundary as shown 302 below. 304 Format of Routed Frames 305 with a SNAP Header 306 +-------------------------------+ 307 | Q.922 Address | 308 +---------------+---------------+ 309 | Control 0x03 | pad 0x00 | 310 +---------------+---------------+ 311 | NLPID 0x80 | Organization- | 312 +---------------+ | 313 | ally Unique Identifier (OUI) | 314 +-------------------------------+ 315 | Protocol Identifier (PID) | 316 +-------------------------------+ 317 | | 318 | Protocol Data | 319 | | 320 +-------------------------------+ 321 | FCS | 322 +-------------------------------+ 324 In the few cases when a protocol has an assigned NLPID (see Appendix 325 A), 48 bits can be saved using the format below: 327 Format of Routed NLPID Protocol 328 +-------------------------------+ 329 | Q.922 Address | 330 +---------------+---------------+ 331 | Control 0x03 | NLPID | 332 +---------------+---------------+ 333 | Protocol Data | 334 +-------------------------------+ 335 | FCS | 336 +-------------------------------+ 338 When using the NLPID encapsulation format as described above, the pad 339 octet is not used. 341 In the case of ISO protocols, the NLPID is considered to be the first 342 octet of the protocol data. It is unnecessary to repeat the NLPID in 343 this case. The single octet serves both as the demultiplexing value 344 and as part of the protocol data (refer to "Other Protocols over 345 Frame Relay for more details). Other protocols, such as IP, have a 346 NLPID defined (0xCC), but it is not part of the protocol itself. 348 Format of Routed IP Datagram 349 +-------------------------------+ 350 | Q.922 Address | 351 +---------------+---------------+ 352 | Control 0x03 | NLPID 0xCC | 353 +---------------+---------------+ 354 | IP Datagram | 355 +-------------------------------+ 356 | FCS | 357 +-------------------------------+ 359 4.2. Bridged Frames 361 The second type of Frame Relay traffic is bridged packets. These 362 packets are encapsulated using the NLPID value of 0x80 indicating 363 SNAP. As with other SNAP encapsulated protocols, there will be one 364 pad octet to align the data portion of the encapsulated frame. The 365 SNAP header which follows the NLPID identifies the format of the 366 bridged packet. The OUI value used for this encapsulation is the 367 802.1 organization code 0x00-80-C2. The PID portion of the SNAP 368 header (the two bytes immediately following the OUI) specifies the 369 form of the MAC header, which immediately follows the SNAP header. 370 Additionally, the PID indicates whether the original FCS is preserved 371 within the bridged frame. 373 As specified in RFC 1638 [4], non-canonical MAC destination addresses 374 are used for encapsulated IEEE 802.5 and FDDI frames, and canonical 375 MAC destination addresses are used for the remaining encapsulations 376 defined in this section. 378 The 802.1 organization has reserved the following values to be used 379 with Frame Relay: 381 PID Values for OUI 0x00-80-C2 383 with preserved FCS w/o preserved FCS Media 384 ------------------ ----------------- ---------------- 385 0x00-01 0x00-07 802.3/Ethernet 386 0x00-02 0x00-08 802.4 387 0x00-03 0x00-09 802.5 388 0x00-04 0x00-0A FDDI 389 0x00-0B 802.6 391 In addition, the PID value 0x00-0E, when used with OUI 0x00-80-C2, 392 identifies Bridge Protocol Data Units (BPDUs) as defined by 393 802.1(d) or 802.1(g) [12], and the PID value 0x00-0F identifies 394 Source Routing BPDUs. 396 A packet bridged over Frame Relay will, therefore, have one of the 397 following formats: 399 Format of Bridged Ethernet/802.3 Frame 400 +-------------------------------+ 401 | Q.922 Address | 402 +---------------+---------------+ 403 | Control 0x03 | pad 0x00 | 404 +---------------+---------------+ 405 | NLPID 0x80 | OUI 0x00 | 406 +---------------+ --+ 407 | OUI 0x80-C2 | 408 +-------------------------------+ 409 | PID 0x00-01 or 0x00-07 | 410 +-------------------------------+ 411 | MAC destination address | 412 : : 413 | | 414 +-------------------------------+ 415 | (remainder of MAC frame) | 416 +-------------------------------+ 417 | LAN FCS (if PID is 0x00-01) | 418 +-------------------------------+ 419 | FCS | 420 +-------------------------------+ 421 Format of Bridged 802.4 Frame 422 +-------------------------------+ 423 | Q.922 Address | 424 +---------------+---------------+ 425 | Control 0x03 | pad 0x00 | 426 +---------------+---------------+ 427 | NLPID 0x80 | OUI 0x00 | 428 +---------------+ --+ 429 | OUI 0x80-C2 | 430 +-------------------------------+ 431 | PID 0x00-02 or 0x00-08 | 432 +---------------+---------------+ 433 | pad 0x00 | Frame Control | 434 +---------------+---------------+ 435 | MAC destination address | 436 : : 437 | | 438 +-------------------------------+ 439 | (remainder of MAC frame) | 440 +-------------------------------+ 441 | LAN FCS (if PID is 0x00-02) | 442 +-------------------------------+ 443 | FCS | 444 +-------------------------------+ 445 Format of Bridged 802.5 Frame 446 +-------------------------------+ 447 | Q.922 Address | 448 +---------------+---------------+ 449 | Control 0x03 | pad 0x00 | 450 +---------------+---------------+ 451 | NLPID 0x80 | OUI 0x00 | 452 +---------------+ --+ 453 | OUI 0x80-C2 | 454 +-------------------------------+ 455 | PID 0x00-03 or 0x00-09 | 456 +---------------+---------------+ 457 | pad 0x00 | Frame Control | 458 +---------------+---------------+ 459 | MAC destination address | 460 : : 461 | | 462 +-------------------------------+ 463 | (remainder of MAC frame) | 464 +-------------------------------+ 465 | LAN FCS (if PID is 0x00-03) | 466 | | 467 +-------------------------------+ 468 | FCS | 469 +-------------------------------+ 470 Format of Bridged FDDI Frame 471 +-------------------------------+ 472 | Q.922 Address | 473 +---------------+---------------+ 474 | Control 0x03 | pad 0x00 | 475 +---------------+---------------+ 476 | NLPID 0x80 | OUI 0x00 | 477 +---------------+ --+ 478 | OUI 0x80-C2 | 479 +-------------------------------+ 480 | PID 0x00-04 or 0x00-0A | 481 +---------------+---------------+ 482 | pad 0x00 | Frame Control | 483 +---------------+---------------+ 484 | MAC destination address | 485 : : 486 | | 487 +-------------------------------+ 488 | (remainder of MAC frame) | 489 +-------------------------------+ 490 | LAN FCS (if PID is 0x00-04) | 491 | | 492 +-------------------------------+ 493 | FCS | 494 +-------------------------------+ 495 Format of Bridged 802.6 Frame 496 +-------------------------------+ 497 | Q.922 Address | 498 +---------------+---------------+ 499 | Control 0x03 | pad 0x00 | 500 +---------------+---------------+ 501 | NLPID 0x80 | OUI 0x00 | 502 +---------------+ --+ 503 | OUI 0x80-C2 | 504 +-------------------------------+ 505 | PID 0x00-0B | 506 +---------------+---------------+ ------- 507 | Reserved | BEtag | Common 508 +---------------+---------------+ PDU 509 | BAsize | Header 510 +-------------------------------+ ------- 511 | MAC destination address | 512 : : 513 | | 514 +-------------------------------+ 515 | (remainder of MAC frame) | 516 +-------------------------------+ 517 | | 518 +- Common PDU Trailer -+ 519 | | 520 +-------------------------------+ 521 | FCS | 522 +-------------------------------+ 524 Note that in bridge 802.6 PDUs, there is only one choice for the PID 525 value, since the presence of a CRC-32 is indicated by the CIB bit in 526 the header of the MAC frame. 528 The Common Protocol Data Unit (CPDU) Header and Trailer are conveyed 529 to allow pipelining at the egress bridge to an 802.6 subnetwork. 530 Specifically, the CPDU Header contains the BAsize field, which 531 contains the length of the PDU. If this field is not available to 532 the egress 802.6 bridge, then that bridge cannot begin to transmit 533 the segmented PDU until it has received the entire PDU, calculated 534 the length, and inserted the length into the BAsize field. If the 535 field is available, the egress 802.6 bridge can extract the length 536 from the BAsize field of the Common PDU Header, insert it into the 537 corresponding field of the first segment, and immediately transmit 538 the segment onto the 802.6 subnetwork. Thus, the bridge can begin 539 transmitting the 802.6 PDU before it has received the complete PDU. 541 One should note that the Common PDU Header and Trailer of the 542 encapsulated frame should not be simply copied to the outgoing 802.6 543 subnetwork because the encapsulated BEtag value may conflict with the 544 previous BEtag value transmitted by that bridge. 546 Format of BPDU Frame 547 +-------------------------------+ 548 | Q.922 Address | 549 +-------------------------------+ 550 | Control 0x03 | 551 +-------------------------------+ 552 | PAD 0x00 | 553 +-------------------------------+ 554 | NLPID 0x80 | 555 +-------------------------------+ 556 | OUI 0x00-80-C2 | 557 +-------------------------------+ 558 | PID 0x00-0E | 559 +-------------------------------+ 560 | | 561 | BPDU as defined by | 562 | 802.1(d) or 802.1(g)[12] | 563 | | 564 +-------------------------------+ 565 | FCS | 566 +-------------------------------+ 568 Format of Source Routing BPDU Frame 569 +-------------------------------+ 570 | Q.922 Address | 571 +-------------------------------+ 572 | Control 0x03 | 573 +-------------------------------+ 574 | PAD 0x00 | 575 +-------------------------------+ 576 | NLPID 0x80 | 577 +-------------------------------+ 578 | OUI 0x00-80-C2 | 579 +-------------------------------+ 580 | PID 0x00-0F | 581 +-------------------------------+ 582 | | 583 | Source Routing BPDU | 584 | | 585 | | 586 +-------------------------------+ 587 | FCS | 588 +-------------------------------+ 590 5. Data Link Layer Parameter Negotiation 592 Frame Relay stations may choose to support the Exchange 593 Identification (XID) specified in Appendix III of Q.922 [1]. This 594 XID exchange allows the following parameters to be negotiated at the 595 initialization of a Frame Relay circuit: maximum frame size N201, 596 retransmission timer T200, and the maximum number of outstanding 597 Information (I) frames K. 599 A station may indicate its unwillingness to support acknowledged mode 600 multiple frame operation by specifying a value of zero for the 601 maximum window size, K. 603 If this exchange is not used, these values must be statically 604 configured by mutual agreement of Data Link Connection (DLC) 605 endpoints, or must be defaulted to the values specified in Section 606 5.9 of Q.922: 608 N201: 260 octets 610 K: 3 for a 16 Kbps link, 611 7 for a 64 Kbps link, 612 32 for a 384 Kbps link, 613 40 for a 1.536 Mbps or above link 615 T200: 1.5 seconds [see Q.922 for further details] 617 If a station supporting XID receives an XID frame, it shall respond 618 with an XID response. In processing an XID, if the remote maximum 619 frame size is smaller than the local maximum, the local system shall 620 reduce the maximum size it uses over this DLC to the remotely 621 specified value. Note that this shall be done before generating a 622 response XID. 624 The following diagram describes the use of XID to specify non-use of 625 acknowledged mode multiple frame operation. 627 Non-use of Acknowledged Mode Multiple Frame Operation 628 +---------------+ 629 | Address | (2,3 or 4 octets) 630 | | 631 +---------------+ 632 | Control 0xAF | 633 +---------------+ 634 | format 0x82 | 635 +---------------+ 636 | Group ID 0x80 | 637 +---------------+ 638 | Group Length | (2 octets) 639 | 0x00-0E | 640 +---------------+ 641 | 0x05 | PI = Frame Size (transmit) 642 +---------------+ 643 | 0x02 | PL = 2 644 +---------------+ 645 | Maximum | (2 octets) 646 | Frame Size | 647 +---------------+ 648 | 0x06 | PI = Frame Size (receive) 649 +---------------+ 650 | 0x02 | PL = 2 651 +---------------+ 652 | Maximum | (2 octets) 653 | Frame Size | 654 +---------------+ 655 | 0x07 | PI = Window Size 656 +---------------+ 657 | 0x01 | PL = 1 658 +---------------+ 659 | 0x00 | 660 +---------------+ 661 | 0x09 | PI = Retransmission Timer 662 +---------------+ 663 | 0x01 | PL = 1 664 +---------------+ 665 | 0x00 | 666 +---------------+ 667 | FCS | (2 octets) 668 | | 669 +---------------+ 671 6. Address Resolution for PVCs 673 Though address resolution is required in both PVC and SVC environments, this 674 document will only describe address resolution as it applies to PVCs. SVC 675 operation will be discussed in future documents. 677 There are situations in which a Frame Relay station may wish to 678 dynamically resolve a protocol address over PVCs. This may be 679 accomplished using the standard Address Resolution Protocol (ARP) [6] 680 encapsulated within a SNAP encoded Frame Relay packet as follows: 682 +-----------------------+-----------------------+ 683 | Q.922 Address | 684 +-----------------------+-----------------------+ 685 | Control (UI) 0x03 | pad 0x00 | 686 +-----------------------+-----------------------+ 687 | NLPID 0x80 | | SNAP Header 688 +-----------------------+ OUI 0x00-00-00 + Indicating 689 | | ARP 690 +-----------------------+-----------------------+ 691 | PID 0x0806 | 692 +-----------------------+-----------------------+ 693 | ARP packet | 694 | . | 695 | . | 696 | . | 697 +-----------------------+-----------------------+ 699 Where the ARP packet has the following format and values: 701 Data: 702 ar$hrd 16 bits Hardware type 703 ar$pro 16 bits Protocol type 704 ar$hln 8 bits Octet length of hardware address (n) 705 ar$pln 8 bits Octet length of protocol address (m) 706 ar$op 16 bits Operation code (request or reply) 707 ar$sha noctets source hardware address 708 ar$spa moctets source protocol address 709 ar$tha noctets target hardware address 710 ar$tpa moctets target protocol address 712 ar$hrd - assigned to Frame Relay is 15 decimal 713 (0x000F) [7]. 715 ar$pro - see assigned numbers for protocol ID number for 716 the protocol using ARP. (IP is 0x0800). 718 ar$hln - length in bytes of the address field (2, 3, or 4) 720 ar$pln - protocol address length is dependent on the 721 protocol (ar$pro) (for IP ar$pln is 4). 723 ar$op - 1 for request and 2 for reply. 725 ar$sha - Q.922 source hardware address, with C/R, FECN, 726 BECN, and DE set to zero. 728 ar$tha - Q.922 target hardware address, with C/R, FECN, 729 BECN, and DE set to zero. 731 Because DLCIs within most Frame Relay networks have only local 732 significance, an end station will not have a specific DLCI assigned 733 to itself. Therefore, such a station does not have an address to put 734 into the ARP request or reply. Fortunately, the Frame Relay network 735 does provide a method for obtaining the correct DLCIs. The solution 736 proposed for the locally addressed Frame Relay network below will 737 work equally well for a network where DLCIs have global significance. 739 The DLCI carried within the Frame Relay header is modified as it 740 traverses the network. When the packet arrives at its destination, 741 the DLCI has been set to the value that, from the standpoint of the 742 receiving station, corresponds to the sending station. For example, 743 in figure 1 below, if station A were to send a message to station B, 744 it would place DLCI 50 in the Frame Relay header. When station B 745 received this message, however, the DLCI would have been modified by 746 the network and would appear to B as DLCI 70. 748 ~~~~~~~~~~~~~~~ 749 ( ) 750 +-----+ ( ) +-----+ 751 | |-50------(--------------------)---------70-| | 752 | A | ( ) | B | 753 | |-60-----(---------+ ) | | 754 +-----+ ( | ) +-----+ 755 ( | ) 756 ( | ) <---Frame Relay 757 ~~~~~~~~~~~~~~~~ network 758 80 759 | 760 +-----+ 761 | | 762 | C | 763 | | 764 +-----+ 765 Figure 1 767 Lines between stations represent data link connections (DLCs). 768 The numbers indicate the local DLCI associated with each 769 connection. 771 DLCI to Q.922 Address Table for Figure 1 773 DLCI (decimal) Q.922 address (hex) 774 50 0x0C21 775 60 0x0CC1 776 70 0x1061 777 80 0x1401 779 For authoritative description of the correlation between DLCI and 780 Q.922 [1] addresses, the reader should consult Q.922. A summary 781 of the correlation is included here for convenience. The 782 translation between DLCI and Q.922 address is based on a two byte 783 address length using the Q.922 encoding format. The format is: 785 8 7 6 5 4 3 2 1 786 +------------------------+---+--+ 787 | DLCI (high order) |c/r|ea| 788 +--------------+----+----+---+--+ 789 | DLCI (lower) |FECN|BECN|DE |EA| 790 +--------------+----+----+---+--+ 792 For ARP and its variants, the FECN, BECN, C/R and DE bits are 793 assumed to be 0. 795 When an ARP message reaches a destination, all hardware addresses 796 will be invalid. The address found in the frame header will, 797 however, be correct. Though it does violate the purity of layering, 798 Frame Relay may use the address in the header as the sender hardware 799 address. It should also be noted that the target hardware address, 800 in both ARP request and reply, will also be invalid. This should not 801 cause problems since ARP does not rely on these fields and in fact, 802 an implementation may zero fill or ignore the target hardware address 803 field entirely. 805 As an example of how this address replacement scheme may work, refer 806 to figure 1. If station A (protocol address pA) wished to resolve 807 the address of station B (protocol address pB), it would format an 808 ARP request with the following values: 810 ARP request from A 811 ar$op 1 (request) 812 ar$sha unknown 813 ar$spa pA 814 ar$tha undefined 815 ar$tpa pB 817 Because station A will not have a source address associated with it, 818 the source hardware address field is not valid. Therefore, when the 819 ARP packet is received, it must extract the correct address from the 820 Frame Relay header and place it in the source hardware address field. 821 This way, the ARP request from A will become: 823 ARP request from A as modified by B 824 ar$op 1 (request) 825 ar$sha 0x1061 (DLCI 70) from Frame Relay header 826 ar$spa pA 827 ar$tha undefined 828 ar$tpa pB 830 Station B's ARP will then be able to store station A's protocol 831 address and Q.922 address association correctly. Next, station B 832 will form a reply message. Many implementations simply place the 833 source addresses from the ARP request into the target addresses and 834 then fills in the source addresses with its addresses. In this case, 835 the ARP response would be: 837 ARP response from B 838 ar$op 2 (response) 839 ar$sha unknown 840 ar$spa pB 841 ar$tha 0x1061 (DLCI 70) 842 ar$tpa pA 844 Again, the source hardware address is unknown and when the request is 845 received, station A will extract the address from the Frame Relay 846 header and place it in the source hardware address field. Therefore, 847 the response will become: 849 ARP response from B as modified by A 850 ar$op 2 (response) 851 ar$sha 0x0C21 (DLCI 50) 852 ar$spa pB 853 ar$tha 0x1061 (DLCI 70) 854 ar$tpa pA 856 Station A will now correctly recognize station B having protocol 857 address pB associated with Q.922 address 0x0C21 (DLCI 50). 859 Reverse ARP (RARP) [8] will work in exactly the same way. Still 860 using figure 1, if we assume station C is an address server, the 861 following RARP exchanges will occur: 863 RARP request from A RARP request as modified by C 864 ar$op 3 (RARP request) ar$op 3 (RARP request) 865 ar$sha unknown ar$sha 0x1401 (DLCI 80) 866 ar$spa undefined ar$spa undefined 867 ar$tha 0x0CC1 (DLCI 60) ar$tha 0x0CC1 (DLCI 60) 868 ar$tpa pC ar$tpa pC 870 Station C will then look up the protocol address corresponding to 871 Q.922 address 0x1401 (DLCI 80) and send the RARP response. 873 RARP response from C RARP response as modified by A 874 ar$op 4 (RARP response) ar$op 4 (RARP response) 875 ar$sha unknown ar$sha 0x0CC1 (DLCI 60) 876 ar$spa pC ar$spa pC 877 ar$tha 0x1401 (DLCI 80) ar$tha 0x1401 (DLCI 80) 878 ar$tpa pA ar$tpa pA 880 This means that the Frame Relay interface must only intervene in the 881 processing of incoming packets. 883 In the absence of suitable multicast, ARP may still be implemented. 884 To do this, the end station simply sends a copy of the ARP request 885 through each relevant DLC, thereby simulating a broadcast. 887 The use of multicast addresses in a Frame Relay environment is 888 presently under study by Frame Relay providers. At such time that 889 the issues surrounding multicasting are resolved, multicast 890 addressing may become useful in sending ARP requests and other 891 "broadcast" messages. 893 Because of the inefficiencies of broadcasting in a Frame Relay 894 environment, a new address resolution variation was developed. It is 895 called Inverse ARP [11] and describes a method for resolving a 896 protocol address when the hardware address is already known. In 897 Frame Relay's case, the known hardware address is the DLCI. Using 898 Inverse ARP for Frame Relay follows the same pattern as ARP and RARP 899 use. That is the source hardware address is inserted at the 900 receiving station. 902 In our example, station A may use Inverse ARP to discover the 903 protocol address of the station associated with its DLCI 50. The 904 Inverse ARP request would be as follows: 906 InARP Request from A (DLCI 50) 907 ar$op 8 (InARP request) 908 ar$sha unknown 909 ar$spa pA 910 ar$tha 0x0C21 (DLCI 50) 911 ar$tpa unknown 913 When Station B receives this packet, it will modify the source 914 hardware address with the Q.922 address from the Frame Relay header. 915 This way, the InARP request from A will become: 917 ar$op 8 (InARP request) 918 ar$sha 0x1061 919 ar$spa pA 920 ar$tha 0x0C21 921 ar$tpa unknown. 923 Station B will format an Inverse ARP response and send it to station 924 A as it would for any ARP message. 926 7. IP over Frame Relay 928 Internet Protocol [9] (IP) datagrams sent over a Frame Relay network 929 conform to the encapsulation described previously. Within this 930 context, IP could be encapsulated in two different ways. 932 1. NLPID value indicating IP 934 +-----------------------+-----------------------+ 935 | Q.922 Address | 936 +-----------------------+-----------------------+ 937 | Control (UI) 0x03 | NLPID 0xCC | 938 +-----------------------+-----------------------+ 939 | IP packet | 940 | . | 941 | . | 942 | . | 943 +-----------------------+-----------------------+ 945 2. NLPID value indicating SNAP 947 +-----------------------+-----------------------+ 948 | Q.922 Address | 949 +-----------------------+-----------------------+ 950 | Control (UI) 0x03 | pad 0x00 | 951 +-----------------------+-----------------------+ 952 | NLPID 0x80 | | SNAP Header 953 +-----------------------+ OUI = 0x00-00-00 + Indicating 954 | | IP 955 +-----------------------+-----------------------+ 956 | PID 0x0800 | 957 +-----------------------+-----------------------+ 958 | IP packet | 959 | . | 960 | . | 961 | . | 962 +-----------------------+-----------------------+ 964 Although both of these encapsulations are supported under the given 965 definitions, it is advantageous to select only one method as the 966 appropriate mechanism for encapsulating IP data. Therefore, IP data 967 shall be encapsulated using the NLPID value of 0xCC indicating IP as 968 shown in option 1 above. This (option 1) is more efficient in 969 transmission (48 fewer bits), and is consistent with the 970 encapsulation of IP in X.25. 972 8. Other Protocols over Frame Relay 974 As with IP encapsulation, there are alternate ways to transmit 975 various protocols within the scope of this definition. To eliminate 976 the conflicts, the SNAP encapsulation is only used if no NLPID value 977 is defined for the given protocol. 979 As an example of how this works, ISO CLNP has a NLPID defined (0x81). 980 Therefore, the NLPID field will indicate ISO CLNP and the data packet 981 will follow immediately. The frame would be as follows: 983 +---------------------------------------------+ 984 | Q.922 Address | 985 +----------------------+----------------------+ 986 | Control (UI) 0x03 | NLPID 0x81 (CLNP) | 987 +----------------------+----------------------+ 988 | remainder of CLNP packet | 989 | . | 990 | . | 991 +---------------------------------------------+ 993 In this example, the NLPID is used to identify the data packet as 994 CLNP. It is also considered part of the CLNP packet and as such, the 995 NLPID should not be removed before being sent to the upper layers for 996 processing. The NLPID is not duplicated. 998 Other protocols, such as IPX, do not have a NLPID value defined. As 999 mentioned above, IPX would be encapsulated using the SNAP header. In 1000 this case, the frame would be as follows: 1002 +---------------------------------------------+ 1003 | Q.922 Address | 1004 +----------------------+----------------------+ 1005 | Control (UI) 0x03 | pad 0x00 | 1006 +----------------------+----------------------+ 1007 | NLPID 0x80 (SNAP) | OUI - 0x00 00 00 | 1008 +----------------------+ | 1009 | | 1010 +---------------------------------------------+ 1011 | PID 0x8137 | 1012 +---------------------------------------------+ 1013 | IPX packet | 1014 | . | 1015 | . | 1016 +---------------------------------------------+ 1018 9. Bridging Model for Frame Relay 1020 The model for bridging in a Frame Relay network is identical to the 1021 model for remote bridging as described in IEEE P802.1g "Remote MAC 1022 Bridging" [13] and supports the concept of "Virtual Ports". Remote 1023 bridges with LAN ports receive and transmit MAC frames to and from 1024 the LANs to which they are attached. They may also receive and 1025 transmit MAC frames through virtual ports to and from other remote 1026 bridges. A virtual port may represent an abstraction of a remote 1027 bridge's point of access to one, two or more other remote bridges. 1029 Remote Bridges are statically configured as members of a remote 1030 bridge group by management. All members of a remote bridge group are 1031 connected by one or more virtual ports. The set of remote MAC bridges 1032 in a remote bridge group provides actual or *potential* MAC layer 1033 interconnection between a set of LANs and other remote bridge groups 1034 to which the remote bridges attach. 1036 In a Frame Relay network there must be a full mesh of Frame Relay VCs 1037 between bridges of a remote bridge group. If the frame relay network 1038 is not a full mesh, then the bridge network must be divided into 1039 multiple remote bridge groups. 1041 The frame relay VCs that interconnect the bridges of a remote bridge 1042 group may be combined or used individually to form one or more 1043 virtual bridge ports. This gives flexibility to treat the Frame 1044 Relay interface either as a single virtual bridge port, with all VCs 1045 in a group, or as a collection of bridge ports (individual or grouped 1046 VCs). 1048 When a single virtual bridge port provides the interconnectivity for 1049 all bridges of a given remote bridge group (i.e. all VCs are combined 1050 into a single virtual port), the standard Spanning Tree Algorithm may 1051 be used to determine the state of the virtual port. When more than 1052 one virtual port is configured within a given remote bridge group 1053 then an "extended" Spanning Tree Algorithm is required. Such an 1054 extended algorithm is defined in IEEE 802.1g [13]. The operation of 1055 this algorithm is such that a virtual port is only put into backup if 1056 there is a loop in the network external to the remote bridge group. 1058 The simplest bridge configuration for a Frame Relay network is the 1059 LAN view where all VCs are combined into a single virtual port. 1060 Frames, such as BPDUs, which would be broadcast on a LAN, must be 1061 flooded to each VC (or multicast if the service is developed for 1062 Frame Relay services). Flooding is performed by sending the packet to 1063 each relevant DLC associated with the Frame Relay interface. The VCs 1064 in this environment are generally invisible to the bridge. That is, 1065 the bridge sends a flooded frame to the frame relay interface and 1066 does not "see" that the frame is being forwarded to each VC 1067 individually. If all participating bridges are fully connected (full 1068 mesh) the standard Spanning Tree Algorithm will suffice in this 1069 configuration. 1071 Typically LAN bridges learn which interface a particular end station 1072 may be reached on by associating a MAC address with a bridge port. 1073 In a Frame Relay network configured for the LAN-like single bridge 1074 port (or any set of VCs grouped together to form a single bridge 1075 port), however, the bridge must not only associated a MAC address 1076 with a bridge port, but it must also associate it with a connection 1077 identifier. For Frame Relay networks, this connection identifier is 1078 a DLCI. It is unreasonable and perhaps impossible to require bridges 1079 to statically configure an association of every possible destination 1080 MAC address with a DLC. Therefore, Frame Relay LAN-modeled bridges 1081 must provide a mechanism to allow the Frame Relay bridge port to 1082 dynamically learn the associations. To accomplish this dynamic 1083 learning, a bridged packet shall conform to the encapsulation 1084 described within section 7. In this way, the receiving Frame Relay 1085 interface will know to look into the bridged packet to gather the 1086 appropriate information. 1088 A second Frame Relay bridging approach, the point-to-point view, 1089 treats each Frame Relay VC as a separate bridge port. Flooding and 1090 forwarding packets are significantly less complicated using the 1091 point-to-point approach because each bridge port has only one 1092 destination. There is no need to perform artificial flooding or to 1093 associate DLCIs with destination MAC addresses. Depending upon the 1094 interconnection of the VCs, an extended Spanning Tree algorithm may 1095 be required to permit all virtual ports to remain active as long as 1096 there are no true loops in the topology external to the remote bridge 1097 group. 1099 It is also possible to combine the LAN view and the point-to-point 1100 view on a single Frame Relay interface. To do this, certain VCs are 1101 combined to form a single virtual bridge port while other VCs are 1102 independent bridge ports. 1104 The following drawing illustrates the different possible bridging 1105 configurations. The dashed lines between boxes represent virtual 1106 circuits. 1108 +-------+ 1109 -------------------| B | 1110 / -------| | 1111 / / +-------+ 1112 / | 1113 +-------+/ \ +-------+ 1114 | A | -------| C | 1115 | |-----------------------| | 1116 +-------+\ +-------+ 1117 \ 1118 \ +-------+ 1119 \ | D | 1120 -------------------| | 1121 +-------+ 1123 Since there is less than a full mesh of VCs between the bridges in 1124 this example, the network must be divided into more than one remote 1125 bridge group. A reasonable configuration is to have bridges A, B, 1126 and C in one group, and have bridges A and D in a second. 1128 Configuration of the first bridge group combines the VCs 1129 interconnection the three bridges (A, B, and C) into a single virtual 1130 port. This is an example of the LAN view configuration. The second 1131 group would also be a single virtual port which simply connects 1132 bridges A and D. In this configuration the standard Spanning Tree 1133 Algorithm is sufficient to detect loops. 1135 An alternative configuration has three individual virtual ports in 1136 the first group corresponding to the VCs interconnecting bridges A, B 1137 and C. Since the application of the standard Spanning Tree Algorithm 1138 to this configuration would detect a loop in the topology, an 1139 extended Spanning Tree Algorithm would have to be used in order for 1140 all virtual ports to be kept active. Note that the second group 1141 would still consist of a single virtual port and the standard 1142 Spanning Tree Algorithm could be used in this group. 1144 Using the same drawing, one could construct a remote bridge scenario 1145 with three bridge groups. This would be an example of the point-to- 1146 point case. Here, the VC connecting A and B, the VC connecting A and 1147 C, and the VC connecting A and D are all bridge groups with a single 1148 virtual port. 1150 10. Appendix A 1152 List of Commonly Used NLPIDs 1154 0x00 Null Network Layer or Inactive Set 1155 (not used with Frame Relay) 1156 0x80 SNAP 1157 0x81 ISO CLNP 1158 0x82 ISO ESIS 1159 0x83 ISO ISIS 1160 0xB0 FRF.9 Data Compression [14] 1161 0xCC Internet IP 1163 List of PIDs of OUI 00-80-C2 1165 with preserved FCS w/o preserved FCS Media 1166 ------------------ ----------------- -------------- 1167 0x00-01 0x00-07 802.3/Ethernet 1168 0x00-02 0x00-08 802.4 1169 0x00-03 0x00-09 802.5 1170 0x00-04 0x00-0A FDDI 1171 0x00-0B 802.6 1172 0x00-0D Fragments 1173 0x00-0E BPDUs as defined by 1174 802.1(d) or 1175 802.1(g)[12]. 1176 0x00-0F Source Routing BPDUs 1178 11. Appendix B - Connection Oriented Procedures 1180 This Appendix contains additional information and instructions for 1181 using ITU Q.933 and other ITU standards for encapsulating data over 1182 frame relay. The information contained here is similar (and in some 1183 cases identical) to that found in Annex F to ANSI T1.617A. The 1184 authoritative source for this information is in Annex F and is 1185 repeated here only for convenience. 1187 The Network Level Protocol ID (NLPID) field is administered by ISO 1188 and the ITU. It contains values for many different protocols 1189 including IP, CLNP (ISO 8473), ITU Q.933, and ISO 8208. A figure 1190 summarizing a generic encapsulation technique over frame relay 1191 networks follows. The scheme's flexibility consists in the 1192 identification of multiple alternative to identify different 1193 protocols used either by 1195 - end-to-end systems or 1196 - LAN to LAN bride and routers or 1197 - a combination of the above. 1199 over frame relay networks. 1201 Q.922 control 1202 | 1203 | 1204 -------------------------------------------- 1205 | | 1206 UI I Frame 1207 | | 1208 --------------------------------- -------------- 1209 | 0x08 | 0x81 |0xCC | 0x80 |..01.... |..10.... 1210 | | | | | | 1211 Q.933 CLNP IP SNAP ISO 8208 ISO 8208 1212 | | Modulo 8 Modulo 128 1213 | | 1214 -------------------- OUI 1215 | | | 1216 L2 ID L3 ID ------- 1217 | User | | 1218 | specified | | 1219 | 0x70 802.3 802.6 1220 | 1221 ------------------- 1222 |0x51 |0x4E | |0x4C 1223 | | | | 1224 7776 Q.922 Others 802.2 1226 For those protocols which do not have a NLPID assigned or do not have 1227 a SNAP encapsulation, the NLPID value of 0x08, indicating ITU 1228 Recommendation Q.933 should be used. The four octets following the 1229 NLPID include both layer 2 and layer 3 protocol identification. The 1230 code points for most protocols are currently defined in ANSI T1.617 1231 low layer compatibility information element. There is also an escape 1232 for defining non-standard protocols. 1234 Format of Other Protocols 1235 using Q.933 NLPID 1236 +-------------------------------+ 1237 | Q.922 Address | 1238 +---------------+---------------+ 1239 | Control 0x03 | NLPID 0x08 | 1240 +---------------+---------------+ 1241 | L2 Protocol ID | 1242 | octet 1 | octet 2 | 1243 +---------------+---------------+ 1244 | L3 Protocol ID | 1245 | octet 1 | octet 2 | 1246 +---------------+---------------+ 1247 | Protocol Data | 1248 +-------------------------------+ 1249 | FCS | 1250 +-------------------------------+ 1252 ISO 8802/2 with user specified 1253 layer 3 1254 +-------------------------------+ 1255 | Q.922 Address | 1256 +---------------+---------------+ 1257 | Control 0x03 | NLPID 0x08 | 1258 +---------------+---------------+ 1259 | 802/2 0x4C | 0x80 | 1260 +---------------+---------------+ 1261 |User Spec. 0x70| Note 1 | 1262 +---------------+---------------+ 1263 | DSAP | SSAP | 1264 +---------------+---------------+ 1265 | Control (Note 2) | 1266 +-------------------------------+ 1267 | Remainder of PDU | 1268 +-------------------------------+ 1269 | FCS | 1270 +-------------------------------+ 1272 Note 1: Indicates the code point for user specified 1273 layer 3 protocol. 1275 Note 2: Control field is two octets for I-format and 1276 S-format frames (see 88002/2) 1278 Encapsulations using I frame (layer 2) 1279 The Q.922 I frame is for supporting layer 3 protocols which require 1280 acknowledged data link layer (e.g., ISO 8208). The C/R bit will be 1281 used for command and response indications. 1283 Format of ISO 8208 frame 1284 Modulo 8 1285 +-------------------------------+ 1286 | Q.922 Address | 1287 +---------------+---------------+ 1288 | ....Control I frame | 1289 +---------------+---------------+ 1290 | 8208 packet (modulo 8) Note 3 | 1291 | | 1292 +-------------------------------+ 1293 | FCS | 1294 +-------------------------------+ 1296 Note 3: First octet of 8208 packet also identifies the 1297 NLPID which is "..01....". 1299 Format of ISO 8208 frame 1300 Modulo 128 1301 +-------------------------------+ 1302 | Q.922 Address | 1303 +---------------+---------------+ 1304 | ....Control I frame | 1305 +---------------+---------------+ 1306 | 8208 packet (modulo 128) | 1307 | Note 4 | 1308 +-------------------------------+ 1309 | FCS | 1310 +-------------------------------+ 1312 Note 4: First octet of 8208 packet also identifies the 1313 NLPID which is "..10....". 1315 12. References 1317 [1] International Telecommunication Union, "ISDN Data Link Layer 1318 Specification for Frame Mode Bearer Services", ITU-T 1319 Recommendation Q.922, 1992. 1321 [2] American National Standard For Telecommunications - Integrated 1322 Services Digital Network - Signaling Specification for Frame 1323 Relay Bearer Service for DSS 1, ANSI T1.617a, 1994. 1325 [3] Information technology - Telecommunications and Information 1326 Exchange between systems - Protocol Identification in the Network 1327 Layer, ISO/IEC TR 9577: 1992. 1329 [4] F. Baker, R. Bowen, "PPP Bridging Control Protocol (BCP)", RFC 1330 1638, ACC, June 1994. 1332 [5] International Standard, Information Processing Systems - Local 1333 Area Networks - Logical Link Control, ISO 8802-2, ANSI/IEEE, 1334 Second Edition, 1994-12-30. 1336 [6] Plummer, D., "An Ethernet Address Resolution Protocol - or - 1337 Converting Network Protocol Addresses to 48.bit Ethernet Address 1338 for Transmission on Ethernet Hardware", STD 37, RFC 826, MIT, 1339 November 1982. 1341 [7] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700, 1342 USC/Information Sciences Institute, October 1994 1344 [8] Finlayson, R., Mann, R., Mogul, J., and M. Theimer, "A Reverse 1345 Address Resolution Protocol", STD 38, RFC 903, Stanford 1346 University, June 1984. 1348 [9] Postel, J. and Reynolds, J., "A Standard for the Transmission of 1349 IP Datagrams over IEEE 802 Networks", RFC 1042, USC/Information 1350 Sciences Institute, February 1988. 1352 [10] IEEE, "IEEE Standard for Local and Metropolitan Area Networks: 1353 Overview and architecture", IEEE Standard 802-1990. 1355 [11] Bradley, T., and C. Brown, "Inverse Address Resolution Protocol", 1356 RFC 1293, Wellfleet Communications, Inc., January 1992. 1358 [12] IEEE, "IEEE Standard for Local and Metropolitan Networks: Media 1359 Access Control (MAC) Bridges", IEEE Standard 802.1D-1990. 1361 [13] PROJECT 802 - LOCAL AND METROPOLITAN AREA NETWORKS, Draft 1362 Standard 802.1G: Remote MAC Bridging, Draft 13, May 22, 1995. 1364 [14] Frame Relay Forum, "Data Compression Over Frame Relay 1365 Implementation Agreement", FRF.9, January 22, 1996. 1367 13. Security Considerations 1369 Security issues are not discussed in this memo. 1371 14. Authors' Addresses 1373 Caralyn Brown 1374 Bay Networks 1375 3 Federal Street 1376 Billerica, MA 01821 1377 Phone: (508) 436-3835 1378 Email: cbrown@baynetworks.com 1380 Andrew G. Malis 1381 Ascom Nexion, Inc. 1382 289 Great Road 1383 Acton, MA 01720 1385 Phone: (508) 266-4522 1386 Email: malis@nexen.com