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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Luca Martini 3 Internet Draft Nasser El-Aawar 4 Expiration Date: October 2002 Level 3 Communications, LLC. 6 Giles Heron Steve Vogelsang 7 PacketExchange Ltd. Laurel Networks, Inc. 9 Chris Liljenstolpe Andrew G. Malis 10 Cable & Wireless Vinai Sirkay 11 Vivace Networks, Inc. 12 Daniel Tappan 13 Eric C. Rosen Kireeti Kompella 14 Jayakumar Jayakumar Juniper Networks 15 Cisco Systems, Inc. 17 Dimitri Stratton Vlachos 18 Mazu Networks, Inc. 19 April 2002 21 Encapsulation Methods for Transport of ATM Cells/Frame Over IP and MPLS Networks 23 draft-martini-atm-encap-mpls-00.txt 25 Status of this Memo 27 This document is an Internet-Draft and is in full conformance with 28 all provisions of Section 10 of RFC2026. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF), its areas, and its working groups. Note that other 32 groups may also distribute working documents as Internet-Drafts. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 The list of current Internet-Drafts can be accessed at 40 http://www.ietf.org/ietf/1id-abstracts.txt. 42 The list of Internet-Draft Shadow Directories can be accessed at 43 http://www.ietf.org/shadow.html. 45 Abstract 47 This document describes methods for encapsulating the Protocol Data 48 Units (PDUs) of layer 2 protocols such as ATM fortransport across an 49 MPLS or IP network. 51 Table of Contents 53 1 Specification of Requirements .......................... 2 54 2 Introduction ........................................... 3 55 3 General encapsulation method ........................... 4 56 3.1 The Control Word ....................................... 4 57 3.1.1 Setting the sequence number ............................ 5 58 3.1.2 Processing the sequence number ......................... 5 59 3.2 MTU Requirements ....................................... 6 60 4 ATM .................................................... 7 61 4.1 ATM AAL5 CPCS-SDU Mode ................................. 7 62 4.2 ATM Cell Mode .......................................... 8 63 4.2.1 OAM Cell Support ....................................... 10 64 4.2.2 CLP bit to Quality of Service mapping .................. 10 65 5 Using an MPLS Label as the Demultiplexer Field ......... 10 66 5.1 MPLS Shim EXP Bit Values ............................... 11 67 5.2 MPLS Shim S Bit Value .................................. 11 68 5.3 MPLS Shim TTL Values ................................... 11 69 6 Security Considerations ................................ 11 70 7 Intellectual Property Disclaimer ....................... 11 71 8 References ............................................. 11 72 9 Author Information ..................................... 12 74 1. Specification of Requirements 76 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 77 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 78 document are to be interpreted as described in RFC 2119 80 2. Introduction 82 In an MPLS or IP network, it is possible to use control protocols 83 such as those specified in [1] to set up "emulated virtual circuits" 84 that carry the the Protocol Data Units of layer 2 protocols across 85 the network. A number of these emulated virtual circuits may be 86 carried in a single tunnel. This requires of course that the layer 2 87 PDUs be encapsulated. We can distinguish three layers of this 88 encapsulation: 90 - the "tunnel header", which contains the information needed to 91 transport the PDU across the IP or MPLS network; this is header 92 belongs to the tunneling protocol, e.g., MPLS, GRE, L2TP. 94 - the "demultiplexer field", which is used to distinguish 95 individual emulated virtual circuits within a single tunnel; this 96 field must be understood by the tunneling protocol as well; it 97 may be, e.g., an MPLS label or a GRE key field. 99 - the "emulated VC encapsulation", which contains the information 100 about the enclosed layer 2 PDU which is necessary in order to 101 properly emulate the corresponding layer 2 protocol. 103 This document specifies the emulated VC encapsulation for ATM cells 104 and ATM AAL5 SDUs. Although different layer 2 protocols require 105 different information to be carried in this encapsulation, an attempt 106 has been made to make the encapsulation as common as possible for all 107 layer 2 protocols. Other layer 2 protocols are described in separate 108 documents. [5] [6] [7] 110 This document also specifies the way in which the demultiplexer field 111 is added to the emulated VC encapsulation when an MPLS label is used 112 as the demultiplexer field. 114 QoS related issues are not discussed in this draft 116 For the purpose of this document R1 will be defined as the ingress 117 router, and R2 as the egress router. A layer 2 PDU will be received 118 at R1, encapsulated at R1, transported, decapsulated at R2, and 119 transmitted out of R2. 121 3. General encapsulation method 123 In most cases, it is not necessary to transport the layer 2 124 encapsulation across the network; rather, the layer 2 header can be 125 stripped at R1, and reproduced at R2. This is done using information 126 carried in the control word (see below), as well as information that 127 may already have been signaled from R1 to R2. 129 3.1. The Control Word 131 There are three requirements that may need to be satisfied when 132 transporting layer 2 protocols over an IP or MPLS backbone: 134 -i. Sequentiality may need to be preserved. 135 -ii. Small packets may need to be padded in order to be 136 transmitted on a medium where the minimum transport unit is 137 larger than the actual packet size. 138 -iii. Control bits carried in the header of the layer 2 frame may 139 need to be transported. 141 The control word defined here addresses all three of these 142 requirements. For some protocols this word is REQUIRED, and for 143 others OPTIONAL. For protocols where the control word is OPTIONAL 144 implementations MUST support sending no control word, and MAY support 145 sending a control word. 147 In all cases the egress router must be aware of whether the ingress 148 router will send a control word over a specific virtual circuit. 149 This may be achieved by configuration of the routers, or by 150 signaling, for example as defined in [1]. 152 The control word is defined as follows: 154 0 1 2 3 155 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 156 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 157 | Rsvd | Flags |0 0| Length | Sequence Number | 158 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 160 In the above diagram the first 4 bits are reserved for future use. 161 They MUST be set to 0 when transmitting, and MUST be ignored upon 162 receipt. 164 The next 4 bits provide space for carrying protocol specific flags. 165 These are defined in the protocol-specific details below. 167 The next 2 bits MUST be set to 0 when transmitting. 169 The next 6 bits provide a length field, which is used as follows: If 170 the packet's length (defined as the length of the layer 2 payload 171 plus the length of the control word) is less than 64 bytes, the 172 length field MUST be set to the packet's length. Otherwise the length 173 field MUST be set to zero. The value of the length field, if non- 174 zero, can be used to remove any padding. When the packet reaches the 175 service provider's egress router, it may be desirable to remove the 176 padding before forwarding the packet. 178 The next 16 bits provide a sequence number that can be used to 179 guarantee ordered packet delivery. The processing of the sequence 180 number field is OPTIONAL. 182 The sequence number space is a 16 bit, unsigned circular space. The 183 sequence number value 0 is used to indicate an unsequenced packet. 185 3.1.1. Setting the sequence number 187 For a given emulated VC, and a pair of routers R1 and R2, if R1 188 supports packet sequencing then the following procedures should be 189 used: 191 - the initial packet transmitted on the emulated VC MUST use 192 sequence number 1 193 - subsequent packets MUST increment the sequence number by one for 194 each packet 195 - when the transmit sequence number reaches the maximum 16 bit 196 value (65535) the sequence number MUST wrap to 1 198 If the transmitting router R1 does not support sequence number 199 processing, then the sequence number field in the control word MUST 200 be set to 0. 202 3.1.2. Processing the sequence number 204 If a router R2 supports receive sequence number processing, then the 205 following procedures should be used: 207 When an emulated VC is initially set up, the "expected sequence 208 number" associated with it MUST be initialized to 1. 210 When a packet is received on that emulated VC, the sequence number 211 should be processed as follows: 213 - if the sequence number on the packet is 0, then the packet passes 214 the sequence number check 216 - otherwise if the packet sequence number >= the expected sequence 217 number and the packet sequence number - the expected sequence 218 number < 32768, then the packet is in order. 220 - otherwise if the packet sequence number < the expected sequence 221 number and the expected sequence number - the packet sequence 222 number >= 32768, then the packet is in order. 224 - otherwise the packet is out of order. 226 If a packet passes the sequence number check, or is in order then, it 227 can be delivered immediately. If the packet is in order, then the 228 expected sequence number should be set using the algorithm: 230 expected_sequence_number := packet_sequence_number + 1 mod 2**16 231 if (expected_sequence_number = 0) then expected_sequence_number := 1; 233 Packets which are received out of order MAY be dropped or reordered 234 at the discretion of the receiver. 236 If a router R2 does not support receive sequence number processing, 237 then the sequence number field MAY be ignored. 239 3.2. MTU Requirements 241 The network MUST be configured with an MTU that is sufficient to 242 transport the largest encapsulation frames. If MPLS is used as the 243 tunneling protocol, for example, this is likely to be 12 or more 244 bytes greater than the largest frame size. Other tunneling protocols 245 may have longer headers and require larger MTUs. If the ingress 246 router determines that an encapsulated layer 2 PDU exceeds the MTU of 247 the tunnel through which it must be sent, the PDU MUST be dropped. If 248 an egress router receives an encapsulated layer 2 PDU whose payload 249 length (i.e., the length of the PDU itself without any of the 250 encapsulation headers), exceeds the MTU of the destination layer 2 251 interface, the PDU MUST be dropped. 253 4. ATM 255 Two encapsulations are supported for ATM transport: one for ATM AAL5 256 and another for ATM cells. 258 The AAL5 CPCS-SDU encapsulation consists of the REQUIRED control 259 word, and the AAL5 CPCS-SDU. The ATM cell encapsulation consists of 260 an OPTIONAL control word, a 4 byte ATM cell header, and the ATM cell 261 payload. 263 4.1. ATM AAL5 CPCS-SDU Mode 265 In ATM AAL5 mode the ingress router is required to reassemble AAL5 266 CPCS-SDUs from the incoming VC and transport each CPCS-SDU as a 267 single packet. No AAL5 trailer is transported. The control word is 268 REQUIRED; its use, however, is optional, although desirable. Use of 269 the control word means that the ingress and egress LSRs follow the 270 procedures below. If an ingress LSR chooses not to use the control 271 word, it MUST set the flags in the control word to 0; if an egress 272 LSR chooses to ignore the control word, it MUST set the ATM control 273 bits to 0. 275 The EFCI and CLP bits are carried across the network in the control 276 word. The edge routers that implement this document MAY, when either 277 adding or removing the encapsulation described herein, change the 278 EFCI bit from zero to one in order to reflect congestion in the 279 network that is known to the edge routers, and the CLP bit from zero 280 to one to reflect marking from edge policing of the ATM Sustained 281 Cell Rate. The EFCI and CLP bits MUST NOT be changed from one to 282 zero. 284 The AAL5 CPCS-SDU is prepended by the following header: 286 0 1 2 3 287 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 288 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 289 | Rsvd |T|E|L|C| Length | Sequence Number | 290 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 291 | ATM AAL5 CPCS-SDU | 292 | " | 293 | " | 294 | " | 295 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 296 * T (transport type) bit 298 Bit (T) of the control word indicates whether the packet contains 299 an ATM cell or an AAL5 CPCS-SDU. If set the packet contains an 300 ATM cell, encapsulated according to the ATM cell mode section 301 below, otherwise it contains an AAL5 CPCS-SDU. The ability to 302 transport an ATM cell in the AAL5 mode is intended to provide a 303 means of enabling OAM functionality over the AAL5 VC. 305 * E ( EFCI ) Bit 307 The ingress router, R1, SHOULD set this bit to 1 if the EFCI bit 308 of the final cell of those that transported the AAL5 CPCS-SDU is 309 set to 1, or if the EFCI bit of the single ATM cell to be 310 transported in the packet is set to 1. Otherwise this bit 311 SHOULD be set to 0. The egress router, R2, SHOULD set the EFCI 312 bit of all cells that transport the AAL5 CPCS-SDU to the value 313 contained in this field. 315 * L ( CLP ) Bit 317 The ingress router, R1, SHOULD set this bit to 1 if the CLP bit 318 of any of the ATM cells that transported the AAL5 CPCS-SDU is set 319 to 1, or if the CLP bit of the single ATM cell to be transported 320 in the packet is set to 1. Otherwise this bit SHOULD be set to 321 0. The egress router, R2, SHOULD set the CLP bit of all cells 322 that transport the AAL5 CPCS-SDU to the value contained in this 323 field. 325 * C ( Command / Response Field ) Bit 327 When FRF.8.1 Frame Relay / ATM PVC Service Interworking [3] 328 traffic is being transported, the CPCS-UU Least Significant Bit 329 (LSB) of the AAL5 CPCS-SDU may contain the Frame Relay C/R bit. 330 The ingress router, R1, SHOULD copy this bit to the C bit of the 331 control word. The egress router, R2, SHOULD copy the C bit to the 332 CPCS-UU Least Significant Bit (LSB) of the AAL5 CPCS PDU. 334 4.2. ATM Cell Mode 336 In this encapsulation mode ATM cells are transported individually 337 without a SAR process. The ATM cell encapsulation consists of an 338 OPTIONAL control word, and one or more ATM cells - each consisting of 339 a 4 byte ATM cell header and the 48 byte ATM cell payload. This ATM 340 cell header is defined as in the FAST encapsulation [4] section 341 3.1.1, but without the trailer byte. The length of each frame, 342 without the encapsulation headers, is a multiple of 52 bytes long. 344 The maximum number of ATM cells that can be fitted in a frame, in 345 this fashion, is limited only by the network MTU and by the ability 346 of the egress router to process them. The ingress router MUST NOT 347 send more cells than the egress router is willing to receive. The 348 number of cells that the egress router is willing to receive may 349 either be configured in the ingress router or may be signaled, for 350 example using the methods described in [1]. The number of cells 351 encapsulated in a particular frame can be inferred by the frame 352 length. The control word is OPTIONAL. If the control word is used 353 then the flag bits in the control word are not used, and MUST be set 354 to 0 when transmitting, and MUST be ignored upon receipt. 356 The EFCI and CLP bits are carried across the network in the ATM cell 357 header. The edge routers that implement this document MAY, when 358 either adding or removing the encapsulation described herein, change 359 the EFCI bit from zero to one in order to reflect congestion in the 360 network that is known to the edge router, and the CLP bit from zero 361 to one to reflect marking from edge policing of the ATM Sustained 362 Cell Rate. The EFCI and CLP bits SHOULD NOT be changed from one to 363 zero. 365 This diagram illustrates an encapsulation of two ATM cells: 367 0 1 2 3 368 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 369 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 370 | Control word ( Optional ) | 371 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 372 | VPI | VCI | PTI |C| 373 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 374 | ATM Payload ( 48 bytes ) | 375 | " | 376 | " | 377 | " | 378 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 379 | VPI | VCI | PTI |C| 380 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 381 | ATM Payload ( 48 bytes ) | 382 | " | 383 | " | 384 | " | 385 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 387 * VPI 389 The ingress router MUST copy the VPI field from the incoming cell 390 into this field. For particular emulated VCs, the egress router 391 MAY generate a new VPI and ignore the VPI contained in this 392 field. 394 * VCI 396 The ingress router MUST copy the VCI field from the incoming ATM 397 cell header into this field. For particular emulated VCs, the 398 egress router MAY generate a new VCI. 400 * PTI & CLP ( C bit ) 402 The PTI and CLP fields are the PTI and CLP fields of the incoming 403 ATM cells. The cell headers of the cells within the packet are 404 the ATM headers (without HEC) of the incoming cell. 406 4.2.1. OAM Cell Support 408 OAM cells MAY be transported on the VC LSP. An egress router that 409 does not support transport of OAM cells MUST discard frames that 410 contain an ATM cell with the high-order bit of the PTI field set to 411 1. A router that supports transport of OAM cells MUST follow the 412 procedures outlined in [4] section 8 for mode 0 only, in addition to 413 the applicable procedures specified in [1]. 415 4.2.2. CLP bit to Quality of Service mapping 417 The ingress router MAY consider the CLP bit when determining the 418 value to be placed in the Quality of Service fields (e.g. the EXP 419 fields of the MPLS label stack) of the encapsulating protocol. This 420 gives the network visibility of the CLP bit. Note however that cells 421 from the same VC MUST NOT be reordered. 423 5. Using an MPLS Label as the Demultiplexer Field 425 To use an MPLS label as the demultiplexer field, a 32-bit label stack 426 entry [2] is simply prepended to the emulated VC encapsulation, and 427 hence will appear as the bottom label of an MPLS label stack. This 428 label may be called the "VC label". The particular emulated VC 429 identified by a particular label value must be agreed by the ingress 430 and egress LSRs, either by signaling (e.g, via the methods of [1]) or 431 by configuration. Other fields of the label stack entry are set as 432 follows. 434 5.1. MPLS Shim EXP Bit Values 436 If it is desired to carry Quality of Service information, the Quality 437 of Service information SHOULD be represented in the EXP field of the 438 VC label. If more than one MPLS label is imposed by the ingress LSR, 439 the EXP field of any labels higher in the stack SHOULD also carry the 440 same value. 442 5.2. MPLS Shim S Bit Value 444 The ingress LSR, R1, MUST set the S bit of the VC label to a value of 445 1 to denote that the VC label is at the bottom of the stack. 447 5.3. MPLS Shim TTL Values 449 The ingress LSR, R1, SHOULD set the TTL field of the VC label to a 450 value of 2. 452 6. Security Considerations 454 This document specifies only encapsulations, and not the protocols 455 used to carry the encapsulated packets across the network. Each such 456 protocol may have its own set of security issues, but those issues 457 are not affected by the encapsulations specified herein. 459 7. Intellectual Property Disclaimer 461 This document is being submitted for use in IETF standards 462 discussions. 464 8. References 466 [1] "Transport of Layer 2 Frames Over MPLS", draft-martini- 467 l2circuit-trans-mpls-09.txt. ( work in progress ) 469 [2] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D. Tappan, G. 470 Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032 472 [3] "Frame Relay / ATM PVC Service Interworking Implementation 473 Agreement", Frame Relay Forum 2000. 475 [4] "Frame Based ATM over SONET/SDH Transport (FAST)," 2000. 477 [5] "Encapsulation Methods for Transport of PPP/HDLC Frames Over IP 478 and MPLS Networks", draft-martini-ppp-hdlc-encap-mpls-00.txt. ( work 479 in progress ) 481 [6] "Encapsulation Methods for Transport of Ethernet Frames Over IP 482 and MPLS Networks", draft-martini-ethernet-encap-mpls-00.txt. ( work 483 in progress ) 485 [7] "Encapsulation Methods for Transport of Frame-Relay Over IP and 486 MPLS Networks", draft-martini-frame-encap-mpls-00.txt. ( work in 487 progress ) 489 9. Author Information 491 Luca Martini 492 Level 3 Communications, LLC. 493 1025 Eldorado Blvd. 494 Broomfield, CO, 80021 495 e-mail: luca@level3.net 497 Nasser El-Aawar 498 Level 3 Communications, LLC. 499 1025 Eldorado Blvd. 500 Broomfield, CO, 80021 501 e-mail: nna@level3.net 503 Giles Heron 504 PacketExchange Ltd. 505 The Truman Brewery 506 91 Brick Lane 507 LONDON E1 6QL 508 United Kingdom 509 e-mail: giles@packetexchange.net 511 Dimitri Stratton Vlachos 512 Mazu Networks, Inc. 513 125 Cambridgepark Drive 514 Cambridge, MA 02140 515 e-mail: d@mazunetworks.com 516 Dan Tappan 517 Cisco Systems, Inc. 518 250 Apollo Drive 519 Chelmsford, MA, 01824 520 e-mail: tappan@cisco.com 522 Jayakumar Jayakumar, 523 Cisco Systems Inc. 524 225, E.Tasman, MS-SJ3/3, 525 San Jose , CA, 95134 526 e-mail: jjayakum@cisco.com 528 Eric Rosen 529 Cisco Systems, Inc. 530 250 Apollo Drive 531 Chelmsford, MA, 01824 532 e-mail: erosen@cisco.com 534 Steve Vogelsang 535 Laurel Networks, Inc. 536 Omega Corporate Center 537 1300 Omega Drive 538 Pittsburgh, PA 15205 539 e-mail: sjv@laurelnetworks.com 541 Andrew G. Malis 542 Vivace Networks, Inc. 543 2730 Orchard Parkway 544 San Jose, CA 95134 545 e-mail: Andy.Malis@vivacenetworks.com 547 Vinai Sirkay 548 Vivace Networks, Inc. 549 2730 Orchard Parkway 550 San Jose, CA 95134 551 e-mail: sirkay@technologist.com 552 Chris Liljenstolpe 553 Cable & Wireless 554 11700 Plaza America Drive 555 Reston, VA 20190 556 e-mail: chris@cw.net 558 Kireeti Kompella 559 Juniper Networks 560 1194 N. Mathilda Ave 561 Sunnyvale, CA 94089 562 e-mail: kireeti@juniper.net