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'PPPoL2TP' -- Possible downref: Non-RFC (?) normative reference: ref. 'PWMIB' -- Possible downref: Non-RFC (?) normative reference: ref. 'PWTCMIB' -- Possible downref: Non-RFC (?) normative reference: ref. 'PWMPLSMIB' ** Obsolete normative reference: RFC 1883 (Obsoleted by RFC 2460) ** Obsolete normative reference: RFC 1889 (Obsoleted by RFC 3550) ** Obsolete normative reference: RFC 1902 (Obsoleted by RFC 2578) ** Downref: Normative reference to an Informational RFC: RFC 1958 ** Obsolete normative reference: RFC 1981 (Obsoleted by RFC 8201) ** Obsolete normative reference: RFC 2338 (Obsoleted by RFC 3768) ** Obsolete normative reference: RFC 2401 (Obsoleted by RFC 4301) ** Downref: Normative reference to an Informational RFC: RFC 3022 ** Obsolete normative reference: RFC 2558 (ref. 'SONETMIB') (Obsoleted by RFC 3592) -- Possible downref: Non-RFC (?) normative reference: ref. 'TEMIB' -- Possible downref: Non-RFC (?) normative reference: ref. 'TFRC' -- Possible downref: Non-RFC (?) normative reference: ref. 'VPLS' == Outdated reference: draft-ietf-pwe3-requirements has been published as RFC 3916 ** Downref: Normative reference to an Informational draft: draft-ietf-pwe3-requirements (ref. 'XIAO') Summary: 12 errors (**), 0 flaws (~~), 8 warnings (==), 14 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Pseudo-Wire Edge-to-Edge (PWE3) Working Group Stewart Bryant 2 Internet Draft Cisco Systems 3 Document: 4 Expires: December 2003 Prayson Pate 5 Overture Networks, Inc. 7 Editors 9 June 2003 11 PWE3 Architecture 13 Status of this Memo 15 This document is an Internet-Draft and is in full conformance with 16 all provisions of section 10 of RFC2026. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that other 20 groups may also distribute working documents as Internet-Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress". 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt The list of 29 Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 Abstract 34 This document describes an architecture for Pseudo Wire Emulation 35 Edge-to-Edge (PWE3). It discusses the emulation of services (such as 36 Frame Relay, ATM, Ethernet, TDM and SONET/SDH) over packet switched 37 networks (PSNs) using IP or MPLS. It presents the architectural 38 framework for pseudo wires (PWs), defines terminology, specifies the 39 various protocol elements and their functions. 41 Co-Authors 43 The following are co-authors of this document: 45 Thomas K. Johnson Litchfield Communications 46 Kireeti Kompella Juniper Networks, Inc. 47 Andrew G. Malis Vivace Networks 48 Thomas D. Nadeau Cisco Systems 49 Tricci So Caspian Networks 50 W. Mark Townsley Cisco Systems 51 Craig White Level 3 Communications, LLC. 52 Lloyd Wood Cisco Systems 53 XiPeng Xiao Redback Networks 55 Conventions used in this document 57 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 58 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 59 document are to be interpreted as described in [RFC2119]. 61 Table of Contents 63 1. Introduction............................................. 5 64 1.1 Pseudo Wire Definition............................... 5 65 1.2 PW Service Functionality............................. 6 66 1.3 Non-Goals of this document........................... 6 67 1.4 Terminology.......................................... 6 69 2. PWE3 Applicability....................................... 9 71 3. Protocol Layering Model.................................. 9 72 3.1 Protocol Layers...................................... 9 73 3.2 Domain of PWE3....................................... 11 74 3.3 Payload Types........................................ 11 76 4. Architecture of Pseudo-wires............................. 14 77 4.1 Network Reference Model.............................. 14 78 4.2 PWE3 Pre-processing.................................. 15 79 4.3 Maintenance Reference Model.......................... 19 80 4.4 Protocol Stack Reference Model....................... 19 81 4.5 Pre-processing Extension to Protocol Stack Reference 82 Model................................................ 20 84 5. PW Encapsulation......................................... 21 85 5.1 Payload Convergence Layer............................ 22 86 5.2 Payload-independent PW Encapsulation Layers.......... 24 87 5.3 Fragmentation........................................ 27 88 5.4 Instantiation of the Protocol Layers................. 27 90 6. PW Demultiplexer Layer and PSN Requirements.............. 32 91 6.1 Multiplexing......................................... 32 92 6.2 Fragmentation........................................ 32 93 6.3 Length and Delivery.................................. 32 94 6.4 PW-PDU Validation.................................... 33 95 6.5 Congestion Considerations............................ 33 97 7. Control Plane............................................ 34 98 7.1 Set-up or Teardown of Pseudo-Wires................... 34 99 7.2 Status Monitoring.................................... 34 100 7.3 Notification of Pseudo-wire Status Changes........... 35 101 7.4 Keep-alive........................................... 36 102 7.5 Handling Control Messages of the Native Services..... 36 104 8. Management and Monitoring................................. 37 105 8.1 Status and Statistics................................ 37 106 8.2 PW SNMP MIB Architecture............................. 37 107 8.3 Connection Verification and Traceroute................ 41 109 9. IANA considerations...................................... 41 111 10. Security Considerations................................. 41 113 1. Introduction 115 This document describes an architecture for Pseudo Wire Emulation 116 Edge-to-Edge (PWE3) in support of [XIAO]. It discusses the emulation 117 of services (such as Frame Relay, ATM, Ethernet, TDM and SONET/SDH) 118 over packet switched networks (PSNs) using IP or MPLS. It presents 119 the architectural framework for pseudo wires (PWs), defines 120 terminology, specifies the various protocol elements and their 121 functions. 123 1.1 Pseudo Wire Definition 125 PWE3 is a mechanism that emulates the essential attributes of a 126 service (such as a T1 leased line or Frame Relay) over a PSN. PWE3 is 127 intended to provide only the minimum necessary functionality to 128 emulate the wire with the required degree of faithfulness for the 129 given service definition. Any required switching functionality is the 130 responsibility of a forwarder function (FWRD). Any translation or 131 other operation needing knowledge of the payload semantics is carried 132 out by native service processing (NSP) elements. The functional 133 definition of any FWRD or NSP elements is outside the scope of PWE3. 135 The required functions of PWs include encapsulating service-specific 136 bit-streams, cells or PDUs arriving at an ingress port, and carrying 137 them across a IP path or MPLS tunnel. In some cases it is necessary 138 to perform other operation such as managing their timing and order, 139 to emulate the behavior and characteristics of the service to the 140 required degree of faithfulness. 142 From the perspective of a Customer Edge Equipment (CE), the PW is 143 characterised as an unshared link or circuit of the chosen service. 144 In some cases, there may be deficiencies in the PW emulation that 145 impact the traffic carried over a PW, and hence limit the 146 applicability of this technology. These limitations must be fully 147 described in the appropriate service-specific documentation. 149 For each service type, there will be one default mode of operation 150 that all PEs offering that service type MUST support. However, 151 OPTIONAL modes MAY be defined to improve the faithfulness of the 152 emulated service, if it can be clearly demonstrated that the 153 additional complexity associated with the OPTIONAL mode is offset by 154 the value it offers to PW users. 156 1.2 PW Service Functionality 158 PWs provide the following functions in order to emulate the behavior 159 and characteristics of the native service. 160 o Encapsulation of service-specific PDUs or circuit data arriving 161 at the PE-bound port (logical or physical). 162 o Carriage of the encapsulated data across a PSN tunnel. 163 o Establishment of the PW including the exchange and/or 164 distribution of the PW identifiers used by the PSN 165 tunnel endpoints. 166 o Managing the signaling, timing, order or other aspects of the 167 service at the boundaries of the PW. 168 o Service-specific status and alarm management. 170 1.3 Non-Goals of this document 172 The following are non-goals for this document: 174 o The on-the-wire specification of PW encapsulations. 175 o The detailed definition of the protocols involved in PW 176 set-up and maintenance. 178 The following are outside the scope of PWE3: 179 o Any multicast service not native to the emulated medium. 180 Thus, Ethernet transmission to a "multicast" IEEE-48 address 181 is in scope, while multicast services like MARS [RFC2022] that 182 are implemented on top of the medium are out of scope. 183 o Methods to signal or control the underlying PSN. 185 1.4 Terminology 187 This document uses the following definitions of terms. These terms 188 are illustrated in context in Figure 2. 190 Attachment Circuit The circuit or virtual circuit attaching 191 (AC) a CE to a PE. 193 CE-bound The traffic direction where PW-PDUs are 194 received on a PW via the PSN, processed 195 and then sent to the destination CE. 197 CE Signaling Messages sent and received by the CEs 198 control plane. It may be desirable or 199 even necessary for the PE to participate 200 in or monitor this signaling in order 201 to effectively emulate the service. 203 Control Word (CW) A four octet header used in some encapsulations 204 to carry per packet information when the PSN 205 is MPLS. 207 Customer Edge (CE) A device where one end of a service 208 originates and/or terminates. The CE is not 209 aware that it is using an emulated service 210 rather than a native service. 212 Forwarder (FWRD) A PE subsystem that selects the PW to use to 213 transmit a payload received on an AC. 215 Fragmentation The action of dividing a single PDU into 216 multiple PDUs before transmission with the 217 intent of the original PDU being reassembled 218 elsewhere in the network. Fragmentation MAY be 219 performed in order to allow sending of packets 220 of a larger size than the network MTU which 221 they will traverse. 223 Maximum transmission The packet size (excluding data link header) 224 unit (MTU) that an interface can transmit without 225 needing to fragment. 227 Native Service Processing of the data received by the PE 228 Processing (NSP) from the CE before presentation to the PW 229 for transmission across the core, or 230 processing of the data received from a PW 231 by a PE before it is output on the AC. 232 NSP functionality is defined by standards 233 bodies other than the IETF, such as ITU-T, 234 ANSI, ATMF, etc.) 236 Packet Switched Within the context of PWE3, this is a 237 Network (PSN) network using IP or MPLS as the mechanism 238 for packet forwarding. 240 Protocol Data The unit of data output to, or received 241 Unit (PDU) from, the network by a protocol layer. 243 Provider Edge (PE) A device that provides PWE3 to a CE. 245 PE-bound The traffic direction where information 246 from a CE is adapted to a PW, and PW-PDUs 247 are sent into the PSN. 249 PE/PW Maintenance Used by the PEs to set up, maintain and 250 tear down the PW. It may be coupled with 251 CE Signaling in order to effectively manage 252 the PW. 254 Pseudo Wire (PW) A mechanism that carries the essential 255 elements of an emulated service from one PE 256 to one or more other PEs over a PSN. 258 PW End Service The interface between a PE and a CE. This 259 (PWES) can be a physical interface like a T1 or 260 Ethernet, or a virtual interface like a VC 261 or VLAN. 263 Pseudo Wire A mechanism that emulates the essential 264 Emulation Edge to attributes of service (such as a T1 leased 265 Edge (PWE3) line or frame relay) over a PSN. 267 Pseudo Wire PDU A PDU sent on the PW that contains all of 268 (PW-PDU) the data and control information necessary 269 to emulate the desired service. 271 PSN Tunnel A tunnel across a PSN inside which one or 272 more PWs can be carried. 274 PSN Tunnel Used to set up, maintain and tear down the 275 Signaling underlying PSN tunnel. 277 PW Demultiplexer Data-plane method of identifying a PW 278 terminating at a PE. 280 Time Domain Time Division Multiplexing. Frequently used 281 Multiplexing (TDM) to refer to the synchronous bit-streams at 282 rates defined by G.702. 284 Tunnel A method of transparently carrying information 285 over a network. 287 2. PWE3 Applicability 289 The PSN carrying a PW will subject payload packets to loss, delay, 290 delay variation, and re-ordering. During a network transient there 291 may be a sustained period of impaired service. The applicability of 292 PWE3 to a particular service depends on the sensitivity of that 293 service (or the CE implementation) to these effects, and the ability 294 of the adaptation layer to mask them. Some services, such as IP over 295 FR over PWE3, may prove quite resilient to IP and MPLS PSN 296 characteristics. Other services, such as the interconnection of PBX 297 systems via PWE3, will require more careful consideration of the PSN 298 and adaptation layer characteristics. In some instances, traffic 299 engineering of the underlying PSN will be required, and in some 300 cases, the constraints may be such that it is not possible to provide 301 the required service guarantees. 303 3. Protocol Layering Model 305 The PWE3 protocol-layering model is intended to minimise the 306 differences between PWs operating over different PSN types. The 307 design of the protocol-layering model has the goals of making each PW 308 definition independent of the underlying PSN, and maximizing the 309 reuse of IETF protocol definitions and their implementations. 311 3.1 Protocol Layers 313 The logical protocol-layering model required to support a PW is shown 314 in Figure 1. 316 +---------------------------+ 317 | Payload | 318 +---------------------------+ 319 | Encapsulation | <==== MAY be null 320 +---------------------------+ 321 | PW Demultiplexer | 322 +---------------------------+ 323 | PSN Convergence | <==== MAY be null 324 +---------------------------+ 325 | PSN | 326 +---------------------------+ 327 | Data-link | 328 +---------------------------+ 329 | Physical | 330 +---------------------------+ 332 Figure 1: Logical Protocol Layering Model 334 The payload is transported over the Encapsulation Layer. The 335 Encapsulation Layer carries any information, not already present 336 within the payload itself, that is needed by the PW CE-bound PE 337 interface to send the payload to the CE via the physical interface. 338 If no information is needed beyond that in the payload itself, this 339 layer is empty. 341 This layer also provides support for real-time processing, and 342 sequencing, if needed. 344 The PW Demultiplexer Layer provides the ability to deliver multiple 345 PWs over a single PSN tunnel. The PW demultiplexer value used to 346 identify the PW in the data-plane may be unique per PE, but this is 347 not a PWE3 requirement. It MUST, however, be unique per tunnel 348 endpoint. If it is necessary to identify a particular tunnel, then 349 that is the responsibility of the PSN layer. 351 The PSN Convergence Layer provides the enhancements needed to make 352 the PSN conform to the assumed PSN service requirement. This layer 353 therefore provides a consistent interface to the PW, making the PW 354 independent of the PSN type. If the PSN already meets the service 355 requirements, this layer is empty. 357 The PSN header, MAC/Data-link and Physical Layer definitions are 358 outside the scope of this document. The PSN can be IPv4, IPv6 or 359 MPLS. 361 3.2 Domain of PWE3 363 PWE3 defines the Encapsulation Layer, the method of carrying various 364 payload types, and the interface to the PW Demultiplexer Layer. It 365 is expected that the other layers will be provided by tunneling 366 methods such as L2TP or MPLS over the PSN. 368 3.3 Payload Types 370 The payload is classified into the following generic types of native 371 data unit: 373 o Packet 374 o Cell 375 o Bit-stream 376 o Structured bit-stream 378 Within these generic types there are specific service types. For 379 example: 381 Generic Payload Type PW Service 382 -------------------- ---------- 383 Packet Ethernet (all types), HDLC framing, 384 frame-relay, ATM AAL5 PDU. 386 Cell ATM. 388 Bit-stream Unstructured E1, T1, E3, T3. 390 Structured bit-stream SONET/SDH (e.g. SPE, VT, NxDS0). 392 3.3.1. Packet Payload 394 A packet payload is a variable-size data unit presented to the PE on 395 the AC. A packet payload may be large compared to the PSN MTU. The 396 delineation of the packet boundaries is encapsulation-specific. HDLC 397 or Ethernet PDUs can be considered as examples of packet payloads. 398 Typically a packet will be stripped of transmission overhead such as 399 HDLC flags and stuffing bits before transmission over the PW. 401 A packet payload would normally be relayed across the PW as a single 402 unit. However, there will be cases where the combined size of the 403 packet payload and its associated PWE3 and PSN headers exceeds the 404 PSN path MTU. In these cases, some fragmentation methodology needs 405 to be applied. This may, for example, be the case when a user is 406 providing the service and attaching to the service provider via 407 Ethernet, or where nested pseudo-wires are involved. Fragmentation is 408 discussed in more detail in Section 5.3 410 A packet payload may need sequencing and real-time support. 412 In some situations, the packet payload MAY be selected from the 413 packets presented on the emulated wire on the basis of some sub- 414 multiplexing technique. For example, one or more frame-relay PDUs 415 may be selected for transport over a particular pseudo-wire based on 416 the frame-relay Data-Link Connection Identifier (DLCI), or, in the 417 case of Ethernet payloads, using a suitable MAC bridge filter. This 418 is an FWRD function, and this selection would therefore be made 419 before the packet was presented to the PW Encapsulation Layer. 421 3.3.2. Cell Payload 423 A cell payload is created by capturing, transporting and replaying 424 groups of octets presented on the wire in a fixed-size format. The 425 delineation of the group of bits that comprise the cell is specific 426 to the encapsulation type. Two common examples of cell payloads are 427 ATM 53-octet cells, and the larger 188-octet MPEG Transport Stream 428 packets [DVB]. 430 To reduce per-PSN packet overhead, multiple cells MAY be concatenated 431 into a single payload. The Encapsulation Layer MAY consider the 432 payload complete on the expiry of a timer, after a fixed number of 433 cells have been received or when a significant cell (e.g. an ATM OAM 434 cell) has been received. The benefit of concatenating multiple PDUs 435 should be weighed against a possible increase in packet delay 436 variation and the larger penalty incurred by packet loss. In some 437 cases, it may be appropriate for the Encapsulation Layer to perform 438 some type of compression, such as silence suppression or voice 439 compression. 441 The generic cell payload service will normally need sequence number 442 support, and may also need real-time support. The generic cell 443 payload service would not normally require fragmentation. 445 The Encapsulation Layer MAY apply some form of compression to some of 446 these sub-types (e.g. idle cells MAY be suppressed). 448 In some instances, the cells to be incorporated in the payload MAY be 449 selected by filtering them from the stream of cells presented on the 450 wire. For example, an ATM PWE3 service may select cells based on 451 their VCI or VPI fields. This is an FWRD function, and the selection 452 would therefore be made before the packet was presented to the PW 453 Encapsulation Layer. 455 3.3.3. Bit-stream 457 A bit-stream payload is created by capturing, transporting and 458 replaying the bit pattern on the emulated wire, without taking 459 advantage of any structure that, on inspection, may be visible within 460 the relayed traffic (i.e. the internal structure has no effect on the 461 fragmentation into packets). 463 In some instances it is possible to apply suppression to bit-streams. 464 For example, E1 and T1 send "all-ones" to indicate failure. This 465 condition can be detected without any knowledge of the structure of 466 the bit-stream, and transmission of packetized data suppressed. 468 This service will require sequencing and real-time support. 470 3.3.4. Structured bit-stream 472 A structured bit-stream payload is created by using some knowledge of 473 the underlying structure of the bit-stream to capture, transport and 474 replay the bit pattern on the emulated wire. 476 Two important points distinguish structured and unstructured bit- 477 streams: 479 o Some parts of the original bit-stream MAY be stripped in the 480 PSN-bound direction by NSP block. For example, in Structured 481 SONET the section and line overhead (and, possibly more) may 482 be stripped. A framer is required to enable such stripping. 483 It is also required for frame/payload alignment for 484 fractional T1/E1 applications. 486 o The PW MUST preserve the structure across the PSN so that 487 the CE-bound NSP block can insert it correctly into the 488 reconstructed unstructured bit-stream. The stripped 489 information (such as SONET pointer justifications) may 490 appear in the encapsulation layer to facilitate this 491 reconstitution. 493 As an option, the Encapsulation Layer MAY also perform silence/idle 494 suppression or similar compression on a structured bit-stream. 496 Structured bit-streams are distinguished from cells in that the 497 structures may be too long to be carried in a single packet. Note 498 that "short" structures are indistinguishable from cells and may 499 benefit from the use of methods described in section 3.3.2. 501 This service REQUIRES sequencing and real-time support. 503 3.3.5. Principle of Minimum Intervention 505 To minimise the scope of information, and to improve the efficiency 506 of data flow through the Encapsulation Layer, the payload SHOULD be 507 transported as received with as few modifications as possible 508 [RFC1958]. 510 This minimum intervention approach decouples payload development from 511 PW development and requires fewer translations at the NSP in a system 512 with similar CE interfaces at each end. It also prevents unwanted 513 side-effects due to subtle misrepresentation of the payload in the 514 intermediate format. 516 An approach which does intervene can be more wire-efficient in some 517 cases and may result in fewer translations at the NSP where the CE 518 interfaces are of different types. Any intermediate format 519 effectively becomes a new framing type, requiring documentation and 520 assured interoperability. This increases the amount of work for 521 handling the protocol the intermediate format carries, and is 522 undesirable. 524 4. Architecture of Pseudo-wires 526 This section describes the PWE3 architectural model. 528 4.1 Network Reference Model 530 Figure 2 illustrates the network reference model for point-to-point 531 PWs. 533 |<-------------- Emulated Service ---------------->| 534 | | 535 | |<------- Pseudo Wire ------>| | 536 | | | | 537 | | |<-- PSN Tunnel -->| | | 538 | PW End V V V V PW End | 539 V Service +----+ +----+ Service V 540 +-----+ | | PE1|==================| PE2| | +-----+ 541 | |----------|............PW1.............|----------| | 542 | CE1 | | | | | | | | CE2 | 543 | |----------|............PW2.............|----------| | 544 +-----+ ^ | | |==================| | | ^ +-----+ 545 ^ | +----+ +----+ | | ^ 546 | | Provider Edge 1 Provider Edge 2 | | 547 | | | | 548 Customer | | Customer 549 Edge 1 | | Edge 2 550 | | 551 | | 552 native service native service 554 Figure 2: PWE3 Network Reference Model 556 The two PEs (PE1 and PE2) need to provide one or more PWs on behalf 557 of their client CEs (CE1 and CE2) to enable the client CEs to 558 communicate over the PSN. A PSN tunnel is established to provide a 559 data path for the PW. The PW traffic is invisible to the core 560 network, and the core network is transparent to the CEs. Native data 561 units (bits, cells or packets) presented to the PW End Service (PWES) 562 are encapsulated in a PW-PDU and carried across the underlying 563 network via the PSN tunnel. The PEs perform the necessary 564 encapsulation and decapsulation of PW-PDUs, as well as handling any 565 other functions required by the PW service, such as sequencing or 566 timing. A PE MAY provide multiple PWESs. 568 4.2 PWE3 Pre-processing 570 In some applications, there is a need to perform operations on the 571 native data units received from the CE (including both payload and 572 signaling traffic) before they are transmitted across the PW by the 573 PE. Examples include Ethernet bridging, SONET cross-connect, 574 translation of locally-significant identifiers such as VCI/VPI, or 575 translation to another service type. These operations could be 576 carried out in external equipment, and the processed data sent to the 577 PE over one or more physical interfaces. In most cases, there are 578 cost and operational benefits in undertaking these operations within 579 the PE. This processed data is then presented to the PW via a 580 virtual interface within the PE. 582 These pre-processing operations are included in the PWE3 reference 583 model to provide a common reference point, but the detailed 584 description of these operations is outside the scope of the PW 585 definition given here. 587 PW 588 End Service 589 | 590 |<------- Pseudo Wire ------>| 591 | | 592 | |<-- PSN Tunnel -->| | 593 V V V V PW 594 +-----+----+ +----+ End Service 595 +-----+ |PREP | PE1|==================| PE2| | +-----+ 596 | | | |............PW1.............|----------| | 597 | CE1 |----| | | | | | | CE2 | 598 | | ^ | |............PW2.............|----------| | 599 +-----+ | | | |==================| | | ^ +-----+ 600 | +-----+----+ +----+ | | 601 | ^ | | 602 | | | | 603 | |<------- Emulated Service ------->| | 604 | | | 605 | Virtual physical | 606 | termination | 607 | ^ | 608 CE1 native | CE2 native 609 service | service 610 | 611 CE2 native 612 service 614 Figure 3: Pre-processing within the PWE3 Network Reference Model 616 Figure 3 shows the inter-working of one PE with pre-processing 617 (PREP), and a second without this functionality. This is a useful 618 reference point because it emphasises that the functional interface 619 between PREP and the PW is that represented by a physical interface 620 carrying the service. This effectively defines the necessary inter- 621 working specification. 623 The operation of a system in which both PEs include PREP 624 functionality is also supported. 626 The required pre-processing can be divided into two components: 627 o Forwarder (FWRD) 629 o Native Service Processing (NSP) 631 4.2.1. Forwarders 633 In some applications there is the need to selectively forward payload 634 elements from one of more ACs to one or more PWs. In such cases there 635 will also be the need to perform the inverse function on PWE3-PDUs 636 received by a PE from the PSN. This is the function of the FWRD. 638 The FWRD selects the PW based on, for example: the incoming AC, the 639 contents of the payload, or some statically and/or dynamically 640 configured forwarding information. 642 +----------------------------------------+ 643 | PE Device | 644 +----------------------------------------+ 645 Single | | | 646 PWES | | Single | PW Instance 647 <------>o Forwarder + PW Instance X<===========> 648 | | | 649 +----------------------------------------+ 651 Figure 4a: Simple point-to-point service 653 +----------------------------------------+ 654 | PE Device | 655 +----------------------------------------+ 656 Multiple| | Single | PW Instance 657 PWES | + PW Instance X<===========> 658 <------>o | | 659 | |----------------------| 660 <------>o | Single | PW Instance 661 | Forwarder + PW Instance X<===========> 662 <------>o | | 663 | |----------------------| 664 <------>o | Single | PW Instance 665 | + PW Instance X<===========> 666 <------>o | | 667 +----------------------------------------+ 669 Figure 4b: Multiple PWES to Multiple PW Forwarding 671 Figure 4a shows a simple FWRD that performs some type of filtering 672 operation. Because the FWRD has a single input and a single output 673 interface, filtering is the only type of forwarding operation that 674 applies. Figure 4b shows a more general forwarding situation where 675 payloads are extracted from one or more PWESs and directed to one or 676 more PWs, including, in this instance, a multipoint PW. In this case 677 both filtering and direction operations MAY be performed on the 678 payloads. 680 4.2.2. Native Service Processing 682 In some applications some form of data or address translation, or 683 other operation requiring knowledge of the semantics of the payload, 684 will be required. This is the function of the Native Service 685 Processor (NSP). 687 The use of the NSP approach simplifies the design of the PW by 688 restricting a PW to homogeneous operation. NSP is included in the 689 reference model to provide a defined interface to this functionality. 690 The specification of the various types of NSP is outside the scope of 691 PWE3. 693 +----------------------------------------+ 694 | PE Device | 695 Multiple+----------------------------------------+ 696 PWES | | | Single | PW Instance 697 <------>o NSP # + PW Instance X<===========> 698 | | | | 699 |------| |----------------------| 700 | | | Single | PW Instance 701 <------>o NSP #Forwarder + PW Instance X<===========> 702 | | | | 703 |------| |----------------------| 704 | | | Single | PW Instance 705 <------>o NSP # + PW Instance X<===========> 706 | | | | 707 +----------------------------------------+ 709 Figure 5: NSP in a Multiple PWEs to Multiple 710 PW Forwarding PE 712 Figure 5 illustrates the relationship between NSP, FWRD and PWs in a 713 PE. The NSP function MAY apply any transformation operation 714 (modification, injection, etc.) on the payloads as they pass between 715 the physical interface to the CE and the virtual interface to the 716 FWRD. A PE device MAY contain more than one FWRD. 718 This model also supports the operation of a system in which the NSP 719 functionality includes terminating the data-link, and applying 720 Network Layer processing to the payload is also supported. 722 4.3 Maintenance Reference Model 724 Figure 6 illustrates the maintenance reference model for PWs. 726 |<------- CE (end-to-end) Signaling ------>| 727 | |<---- PW/PE Maintenance ----->| | 728 | | |<-- PSN Tunnel -->| | | 729 | | | Signaling | | | 730 | V V (out of scope) V V | 731 v +-----+ +-----+ v 732 +-----+ | PE1 |==================| PE2 | +-----+ 733 | |-----|.............PW1..............|-----| | 734 | CE1 | | | | | | CE2 | 735 | |-----|.............PW2..............|-----| | 736 +-----+ | |==================| | +-----+ 737 +-----+ +-----+ 738 Customer Provider Provider Customer 739 Edge 1 Edge 1 Edge 2 Edge 2 741 Figure 6: PWE3 Maintenance Reference Model 743 The following signaling mechanisms are REQUIRED: 745 o The CE (end-to-end) signaling is between the CEs. This 746 signaling could be frame relay PVC status signaling, ATM SVC 747 signaling, TDM CAS signaling, etc. 749 o The PW/PE Maintenance is used between the PEs (or NSPs) to set 750 up, maintain and tear down PWs, including any required 751 coordination of parameters. 753 o The PSN Tunnel signaling controls the PW multiplexing and some 754 elements of the underlying PSN. Examples are L2TP control 755 protocol, MPLS LDP and RSVP-TE. The definition of the 756 information that PWE3 needs to be signaled is within the scope 757 of PWE3, but the signaling protocol itself is not. 759 4.4 Protocol Stack Reference Model 761 Figure 7 illustrates the protocol stack reference model for PWs. 763 +-----------------+ +-----------------+ 764 |Emulated Service | |Emulated Service | 765 |(e.g. TDM, ATM) |<==== Emulated Service ===>|(e.g. TDM, ATM) | 766 +-----------------+ +-----------------+ 767 | Payload | | Payload | 768 | Encapsulation |<====== Pseudo Wire ======>| Encapsulation | 769 +-----------------+ +-----------------+ 770 |PW Demultiplexer | |PW Demultiplexer | 771 | PSN Tunnel, |<======= PSN Tunnel ======>| PSN Tunnel, | 772 | PSN & Physical | | PSN & Physical | 773 | Layers | | Layers | 774 +-------+---------+ ___________ +---------+-------+ 775 | / \ | 776 +===============/ PSN \===============+ 777 \ / 778 \_____________/ 780 Figure 7: PWE3 Protocol Stack Reference Model 782 The PW provides the CE with an emulated physical or virtual 783 connection to its peer at the far end. Native service PDUs from the 784 CE are passed through an Encapsulation Layer at the sending PE, and 785 then sent over the PSN. The receiving PE removes the encapsulation 786 and restores the payload to its native format for transmission to the 787 destination CE. 789 4.5 Pre-processing Extension to Protocol Stack Reference Model 791 Figure 8 illustrates how the protocol stack reference model is 792 extended to include the provision of pre-processing (Forwarding and 793 NSP). This shows the placement of the physical interface relative to 794 the CE. 796 /======================================\ 797 H Forwarder H<----Pre-processing 798 H----------------======================/ 799 H Native Service H | | 800 H Processing H | | 801 \================/ | | 802 | | | Emulated | 803 | Service | | Service | 804 | Interface | | (TDM, ATM, | 805 | (TDM, ATM, | | Ethernet, |<== Emulated Service == 806 | Ethernet, | | frame relay, | 807 | frame relay, | | etc.) | 808 | etc.) | +-----------------+ 809 | | | Payload | 810 | | | Encapsulation |<=== Pseudo Wire ====== 811 | | +-----------------+ 812 | | |PW Demultiplexer | 813 | | | PSN Tunnel, | 814 | | | PSN & Physical |<=== PSN Tunnel ======= 815 | | | Headers | 816 +----------------+ +-----------------+ 817 | Physical | | Physical | 818 +-------+--------+ +-------+---------+ 819 | | 820 | | 821 | | 822 | | 823 | | 824 | | 825 To CE <---+ +---> To PSN 827 Figure 8: Protocol Stack Reference Model with Pre-processing 829 5. PW Encapsulation 831 The PW Encapsulation Layer provides the necessary infrastructure to 832 adapt the specific payload type being transported over the PW to the 833 PW Demultiplexer Layer that is used to carry the PW over the PSN. 835 The PW Encapsulation Layer consists of three sub-layers: 837 o Payload Convergence 838 o Timing 839 o Sequencing 841 The PW Encapsulation sub-layering and its context with the protocol 842 stack are shown, in Figure 9. 844 +---------------------------+ 845 | Payload | 846 /===========================\ <------ Encapsulation 847 H Payload Convergence H Layer 848 H---------------------------H 849 H Timing H 850 H---------------------------H 851 H Sequencing H 852 \===========================/ 853 | PW Demultiplexer | 854 +---------------------------+ 855 | PSN Convergence | 856 +---------------------------+ 857 | PSN | 858 +---------------------------+ 859 | Data-link | 860 +---------------------------+ 861 | Physical | 862 +---------------------------+ 864 Figure 9: PWE3 Encapsulation Layer in Context 866 The Payload Convergence Sub-layer is highly tailored to the specific 867 payload type, but, by grouping a number of target payload types into 868 a generic class, and then providing a single convergence sub-layer 869 type common to the group, we achieve a reduction in the number of 870 payload convergence sub-layer types. This decreases implementation 871 complexity. The provision of per-packet signaling and other out-of- 872 band information (other than sequencing or timing) is undertaken by 873 this layer. 875 The Timing Layer and the Sequencing Layer provide generic services to 876 the Payload Convergence Layer for all payload types that require 877 them. 879 5.1 Payload Convergence Layer 881 5.1.1. Encapsulation 883 The primary task of the Payload Convergence Layer is the 884 encapsulation of the payload in PW-PDUs. The native data units to be 885 encapsulated MAY contain a L2 header or L1 overhead. This is service 886 specific. The Payload Convergence header carries the additional 887 information needed to replay the native data units at the CE-bound 888 physical interface. The PW Demultiplexer header is not considered as 889 part of the PW header. 891 Not all the additional information needed to replay the native data 892 units need to be carried in the PW header of the PW PDUs. Some 893 information (e.g. service type of a PW) MAY be stored as state 894 information at the destination PE during PW set-up. 896 5.1.2. PWE3 Channel Types 898 The PW Encapsulation Layer and its associated signaling require one 899 or more of the following types of channels from its underlying PW 900 Demultiplexer and PSN Layers: 902 1. A reliable control channel for signaling line events, status 903 indications, and, in some exceptional cases, CE-CE events 904 that must be translated and sent reliably between PEs. 906 For example, this capability is needed in [PPPoL2TP] 907 (PPP negotiation has to be split between the two ends of the 908 tunnel). PWE3 may also need this type of control channel to 909 provide faithful emulation of complex data-link protocols. 911 plus one or more data channels with the following characteristics: 913 2. A high-priority, unreliable, sequenced channel. A typical use 914 is for CE-to-CE signaling. "High priority" may simply be 915 indicated via the DSCP bits for IP or the EXP bits for MPLS, 916 giving the packet priority during transit. This channel type 917 could also use a bit in the tunnel header itself to indicate 918 that packets received at the PE SHOULD be processed with higher 919 priority [RFC2474]. 921 3. A sequenced channel for data traffic that is sensitive to 922 packet reordering (one classification for use could be for 923 any non-IP traffic). 925 4. An un-sequenced channel for data traffic insensitive to packet 926 order. 928 The data channels (2, 3 and 4 above) SHOULD be carried "in band" with 929 one another to as much of a degree as is reasonably possible on a 930 PSN. 932 Where end-to-end connectivity may be disrupted by address translation 933 [RFC3022], access-control lists, firewalls etc., there exists the 934 possibility that the control channel may be able to pass traffic and 935 set-up the PW, but the PW data traffic is blocked by one or more of 936 these mechanisms. In these cases unless the control channel is also 937 carried "in band" the signaling to set-up the PW will not confirm the 938 existence of an end-to-end data path. 940 In some cases there is a need to synchronize CE events with the data 941 carried over a PW. This is especially the case with TDM circuits 942 (e.g., the on-hook/off-hook events in PSTN switches might be carried 943 over a reliable control channel, whilst the associated bit-stream is 944 carried over a sequenced data channel). 946 PWE3 channel types that are not needed by the supported PWs need not 947 be included in such an implementation. 949 5.1.3. Quality of Service Considerations 951 Where possible, it is desirable to employ mechanisms to provide PW 952 Quality of Service (QoS) support over PSNs. 954 5.2 Payload-independent PW Encapsulation Layers 956 Two PWE3 Encapsulation Sub-layers provide common services to all 957 payload types: Sequencing and Timing. These services are optional 958 and are only used if needed by a particular PW instance. If the 959 service is not needed, the associated header MAY be omitted in order 960 to conserve processing and network resources. 962 There will be instances where a specific payload type will be 963 required to be transported with or without sequence and/or real-time 964 support. For example, an invariant of frame relay transport is the 965 preservation of packet order. Some frame-relay applications expect 966 in-order delivery, and may not cope with reordering of the frames. 967 However, where the frame relay service is itself only being used to 968 carry IP, it may be desirable to relax that constraint in return for 969 reduced per-packet processing cost. 971 The guiding principle is that, where possible, an existing IETF 972 protocol SHOULD be used to provide these services. Where a suitable 973 protocol is not available, the existing protocol should be extended 974 or modified to meet the PWE3 requirements, thereby making that 975 protocol available for other IETF uses. In the particular case of 976 timing, more than one general method may be necessary to provide for 977 the full scope of payload timing requirements. 979 5.2.1. Sequencing 981 The sequencing function provides three services: frame ordering, 982 frame duplication detection and frame loss detection. These services 983 allow the emulation of the invariant properties of a physical wire. 984 Support for sequencing depends on the payload type, and MAY be 985 omitted if not needed. 987 The size of the sequence-number space depends on the speed of the 988 emulated service, and the maximum time of the transient conditions in 989 the PSN. A sequence number space greater than 2^16 may therefore be 990 needed to prevent the sequence number space wrapping during the 991 transient. 993 5.2.1.1 Frame Ordering 995 When packets carrying the PW-PDUs traverse a PSN, they may arrive out 996 of order at the destination PE. For some services, the frames 997 (control frames, data frames, or both control and data frames) MUST 998 be delivered in order. For such services, some mechanism MUST be 999 provided for ensuring in-order delivery. Providing a sequence number 1000 in the sequence sub-layer header for each packet is one possible 1001 approach to out-of-sequence detection. Alternatively it can be noted 1002 that sequencing is a subset of the problem of delivering timed 1003 packets, and that a single combined mechanism such as [RFC1889] MAY 1004 be employed. 1006 There are two possible misordering strategies: 1008 o Drop misordered PW PDUs. 1010 o Try to sort PW PDUs into the correct order. 1012 The choice of strategy will depend on: 1014 o How critical the loss of packets is to the operation of 1015 the PW (e.g. the acceptable bit error rate). 1017 o The speeds of the PW and PSN. 1019 o The acceptable delay (since delay must be introduced to 1020 reorder) 1022 o The incidence of expected misordering. 1024 5.2.1.2 Frame Duplication Detection 1026 In rare cases, packets traversing a PW may be duplicated by the 1027 underlying PSN. For some services frame duplication is not 1028 acceptable. For such services, some mechanism MUST be provided to 1029 ensure that duplicated frames will not be delivered to the 1030 destination CE. The mechanism MAY be the same as the mechanism used 1031 to ensure in-order frame delivery. 1033 5.2.1.3 Frame Loss Detection 1035 A destination PE can determine whether a frame has been lost by 1036 tracking the sequence numbers of the received PW PDUs. 1038 In some instances, a destination PE will have to presume that a PW 1039 PDU is lost if it fails to arrive within a certain time. If a PW-PDU 1040 that has been processed as lost subsequently arrives, the destination 1041 PE MUST discard it. 1043 5.2.2. Timing 1045 A number of native services have timing expectations based on the 1046 characteristics of the networks that they were designed to travel 1047 over, and it can be necessary for the emulated service to duplicate 1048 these network characteristics as closely as possible, e.g. in 1049 delivering native traffic with bit-rate, jitter, wander and delay 1050 characteristics similar to those received at the sending PE. 1052 In such cases, it is necessary for the receiving PE to play out the 1053 native traffic as it was received at the sending PE. This relies on 1054 either timing information sent between the two PEs, or in some case 1055 timing information received from an external reference. 1057 The Timing Sub-layer must therefore support two timing functions: 1058 clock recovery and timed payload delivery. A particular payload type 1059 may require either or both of these services. 1061 5.2.2.1 Clock Recovery 1063 Clock recovery is the extraction of output transmission bit timing 1064 information from the delivered packet stream, and requires a suitable 1065 mechanism. A physical wire carries the timing information natively, 1066 but it is a relatively complex task to extract timing from a highly 1067 jittered source such as packet stream. It is therefore desirable 1068 that an existing real-time protocol such as [RFC1889] be used for 1069 this purpose, unless it can be shown that this is unsuitable or 1070 unnecessary for a particular payload type. 1072 5.2.2.2 Timed delivery 1074 Timed delivery is the delivery of non-contiguous PW PDUs to the PW 1075 output interface with a constant phase relative to the input 1076 interface. The timing of the delivery may be relative to a clock 1077 derived from the packet stream received over the PSN clock recovery, 1078 or with reference to an external clock. 1080 5.3 Fragmentation 1082 A payload would ideally be relayed across the PW as a single unit. 1083 However, there will be cases where the combined size of the payload 1084 and its associated PWE3 and PSN headers exceeds the PSN path MTU. 1085 When a packet size exceeds the MTU of a given network, fragmentation 1086 and reassembly have to be performed in order for the packet to be 1087 delivered. Since fragmentation and reassembly generally consume a 1088 considerable network resources as compared to simply switching a 1089 packet in its entirety, efforts SHOULD be made to reduce or eliminate 1090 the need for fragmentation and reassembly throughout a network to the 1091 extent possible. Of particular concern for fragmentation and 1092 reassembly are aggregation points where large numbers of PWs are 1093 processed (e.g. at the PE). 1095 Ideally, the equipment originating the traffic being sent over the PW 1096 will be configured to have adaptive measures (e.g. [RFC1191], 1097 [RFC1981]) in place that ensure that packets that need to be 1098 fragmented are not sent. When this fails, the point closest to the 1099 sending host with fragmentation and reassembly capabilities SHOULD 1100 attempt to reduce the size of packets to satisfy the PSN MTU. Thus, 1101 in the reference model for PWE3 [Figure 3] fragmentation SHOULD first 1102 be performed at the CE if at all possible. If and only if the CE 1103 cannot adhere to an acceptable MTU size for the PW should the PE 1104 attempt its own fragmentation method. 1106 In cases where MTU management fails to limit the payload to a size 1107 suitable for transmission of the PW, the PE MAY fall back to either a 1108 generic PW fragmentation method, or, if available the fragmentation 1109 service of the underlying PSN. 1111 It is acceptable for a PE implementation not to support 1112 fragmentation. A PE that does not support fragmentation will drop 1113 packets that exceed the PSN MTU, and the management plane of the 1114 encapsulating PE MAY be notified. 1116 If the length of a L2/L1 frame, restored from a PW PDU, exceeds the 1117 MTU of the destination PWES, it MUST be dropped. In this case, the 1118 management plane of the destination PE MAY be notified. 1120 5.4 Instantiation of the Protocol Layers 1122 This document does not address the detailed mapping of the Protocol 1123 Layering model to existing or future IETF standards. The 1124 instantiation of the logical Protocol Layering model is shown in 1125 Figure 9. 1127 5.4.1. PWE3 over an IP PSN 1129 The protocol definition of PWE3 over an IP PSN SHOULD employ existing 1130 IETF protocols where possible. 1132 +---------------------+ +-------------------------+ 1133 | Payload |------------->| Raw payload if possible | 1134 /=====================\ +-------------------------+ 1135 H Payload Convergence H-----------+->| As Needed | 1136 H---------------------H / +-------------------------+ 1137 H Timing H---------/--->| RTP | 1138 H---------------------H / +-------------+ | 1139 H Sequencing H----one of | | 1140 \=====================/ \ | +-----------+ 1141 | PW Demultiplexer |---------+--->| L2TP, MPLS etc. | 1142 +---------------------+ +-------------------------+ 1143 | PSN Convergence |------------->| Not needed | 1144 +---------------------+ +-------------------------+ 1145 | PSN |------------->| IP | 1146 +---------------------+ +-------------------------+ 1147 | Data-link |------------->| Data-link | 1148 +---------------------+ +-------------------------+ 1149 | Physical |------------->| Physical | 1150 +---------------------+ +-------------------------+ 1152 Figure 10: PWE3 over an IP PSN 1154 Figure 10 shows the protocol layering for PWE3 over an IP PSN. As a 1155 rule, the payload SHOULD be carried as received from the NSP, with 1156 the Payload Convergence Layer provided when needed. (It is accepted 1157 that there MAY sometimes be good reason not to follow this rule, but 1158 the exceptional circumstances need to be documented in the 1159 Encapsulation Layer definition for that payload type). 1161 Where appropriate, timing is provided by RTP [RFC1889], which when 1162 used also provides a sequencing service. PW Demultiplexing may be 1163 provided by a number of existing IETF tunnel protocols. Some of 1164 these tunnel protocols provide an optional sequencing service. 1165 (Sequencing is provided either by RTP, or by the PW Demultiplexer 1166 Layer, but not both). A PSN Convergence Layer is not needed, because 1167 all the tunnel protocols shown above are designed to operate directly 1168 over an IP PSN. 1170 As a special case, if the PW Demultiplexer is an MPLS label, the 1171 protocol architecture of section 5.4.2 can be used instead of the 1172 protocol architecture of this section. 1174 5.4.2. PWE3 over an MPLS PSN 1176 The MPLS ethos places importance on wire efficiency. By using a 1177 control word, some components of the PWE3 protocol layers can be 1178 compressed to increase this efficiency. 1180 +---------------------+ 1181 | Payload | 1182 /=====================\ +--------------------------------+ 1183 H Payload Convergence H--+------>| Flags, Frag, Len, Seq #, etc | 1184 H---------------------H | +--------------------------------+ 1185 H Timing H--------->| RTP | 1186 H---------------------H | +--------------------------------+ 1187 H Sequencing H--+ | MPLS Payload Type Ident | 1188 \=====================/ | +--------------------------------+ 1189 | PW Demultiplexer |--------->| PW Label | 1190 +---------------------+ | +--------------------------------+ 1191 | PSN Convergence |--+ +--->| Outer Label or MPLS-in-IP encap| 1192 +---------------------+ | +--------------------------------+ 1193 | PSN |-----+ 1194 +---------------------+ 1195 | Data-link | 1196 +---------------------+ 1197 | Physical | 1198 +---------------------+ 1200 Figure 11: PWE3 over an MPLS PSN using a control word 1202 Figure 11 shows the protocol layering for PWE3 over an MPLS PSN. An 1203 inner MPLS label is used to provide the PW demultiplexing function. 1204 A control word is used to carry most of the information needed by the 1205 PWE3 Encapsulation Layer and the PSN Convergence Layer in a compact 1206 format. The flags in the control word provide the necessary payload 1207 convergence. A sequence field provides support for both in-order 1208 payload delivery and (supported by a fragmentation control method) a 1209 PSN fragmentation service within the PSN Convergence Layer. Ethernet 1210 pads all frames to a minimum size of 64 bytes. The MPLS header does 1211 not include a length indicator. Therefore to allow PWE3 to be carried 1212 in MPLS to correctly pass over an Ethernet data-link, a length 1213 correction field is needed in the control word. Where the design of 1214 the control word would alias an IP packet, an MPLS Payload Type 1215 Identifier should be interposed between the PW label and the control 1216 word (see 5.4.4). As with an IP PSN, where appropriate, timing is 1217 provided by RTP [RFC1889]. 1219 In some networks it may be necessary to carry PWE3 over MPLS over IP. 1220 In these circumstances, the PW is encapsulated for carriage over MPLS 1221 as described in this section, and then a method of carrying MPLS over 1222 an IP PSN (such as GRE [RFC2784], [RFC2890]) is applied to the 1223 resultant PW-PDU. 1225 5.4.3. PW over MPLS Generic Control Word 1227 To allow accurate packet inspection in an MPLS PSN, and/or to operate 1228 correctly over MPLS PSNs that have deployed equal-cost multiple-path 1229 load-balancing (ECMP), a PW packet MUST NOT alias an IP packet. IP 1230 packets are carried in MPLS label stacks without any protocol 1231 identifier. Historic values of the IP version number [RFC791] 1232 [RFC1881] are therefore used to distinguish between IP and non-IP 1233 MPLS payloads. 1235 To disambiguate the PW from an IP flow the PW SHOULD employ either 1236 the generic PW control word shown in Figure 12, or an MPLS payload 1237 type identifier. Note that an MPLS payload with bits 0..3 = 4 is an 1238 IPv4 packet and an MPLS payload with bits 0..3 = 6 is an IPv6 packet. 1240 0 1 2 3 1241 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 1242 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1243 |0 0 0 0| Specified by PW Encapsulation | 1244 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1246 Figure 12: Generic PW Control Word 1248 The PW set-up protocol determines whether a PW uses a control word. 1249 When a control word is used, it SHOULD have the following preferred 1250 form: 1252 0 1 2 3 1253 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 1254 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1255 |0 0 0 0| Flags |FRG| Length | Sequence Number | 1256 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1258 Figure 13: MPLS Preferred Control Word 1260 The meaning of the fields of the MPLS Preferred Control Word (Figure 1261 13) is as follows: 1263 Flags (bits 4 to 7): 1264 These bits are available for per payload signaling. Their 1265 definition is encapsulation specific. 1267 FRG (bits 8 and 9): 1269 These bits are used when fragmenting a PW payload. Their use 1270 is defined in [FRAG]. When the PW is of a type that will 1271 never need payload fragmentation, these bits may be used as 1272 general purpose flags. 1274 Length (bits 10 to 15): 1275 The length field is used to determine the size of a PW 1276 payload that might have been padded to the minimum Ethernet 1277 MAC frame size during its transit across the PSN. If the 1278 MPLS payload (defined as the CW + the PW payload + any 1279 additional PW headers is less than 46 bytes, the length MUST 1280 be set to the length of the MPLS payload. If the MPLS 1281 payload is between 46 bytes and 63 bytes the implementation 1282 MAY either set to the length to the length of the MPLS 1283 payload, or it MAY set it to 0. If the length of the MPLS 1284 payload is greater than 63 bytes the length MUST be set to 0. 1286 Sequence number (Bit 16 to 31): 1287 If the sequence number is not used, it is set to zero by 1288 the sender and ignored by the receiver. Otherwise it 1289 specifies the sequence number of a packet. A circular list 1290 of sequence numbers is used. A sequence number takes a value 1291 from 1 to 65535 (2**16-1). If the payload is an OAM packet 1292 the sequence number MAY be used to mark the position in the 1293 sequence, in which case it has the same value as the last 1294 data PDU sent. The use of the sequence number is optional 1295 for OAM payloads. 1297 5.4.4. MPLS Payload Identifier 1299 If technical considerations result in a PW control word that may 1300 alias an IP packet, the control word SHOULD be preceeded by an MPLS 1301 payload type identifier. 1303 The MPLS payload type is defined as follows: 1305 0 1 2 3 1306 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 1307 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1308 |0 0 0 1| reserved = 0 | PPP DLL Protocol Number | 1309 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1310 | As defined by PPP DLL protocol definition | 1311 | | 1312 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1314 Figure 14: MPLS Payload Type Identifier 1315 PPP DLL Protocol Number [16:31]: 1316 These numbers are assigned by IANA. 1318 Bits 4 to 15 inclusive are reserved for future use and must be zero. 1320 6. PW Demultiplexer Layer and PSN Requirements 1322 PWE3 places three service requirements on the protocol layers used to 1323 carry it across the PSN: 1325 o Multiplexing 1326 o Fragmentation 1327 o Length and Delivery 1329 6.1 Multiplexing 1331 The purpose of the PW Demultiplexer Layer is to allow multiple PWs to 1332 be carried in a single tunnel. This minimizes complexity and 1333 conserves resources. 1335 Some types of native service are capable of grouping multiple 1336 circuits into a "trunk", e.g. multiple ATM VCs in a VP, multiple 1337 Ethernet VLANs on a physical media, or multiple DS0 services within a 1338 T1 or E1. A PW MAY interconnect two end-trunks. That trunk would 1339 have a single multiplexing identifier. 1341 When a MPLS label is used as a PW Demultiplexer setting of the TTL 1342 value [RFC3032] in the PW label is application specific, however in a 1343 strict point to point application the TTL SHOULD be set to 2. 1345 6.2 Fragmentation 1347 If the PSN provides a fragmentation and reassembly service of 1348 adequate performance, it MAY be used to obtain an effective MTU that 1349 is large enough to transport the PW PDUs. See Section 5.3 for a full 1350 discussion of the PW fragmentation issues. 1352 6.3 Length and Delivery 1354 PDU delivery to the egress PE is the function of the PSN Layer. 1356 If the underlying PSN does not provide all the information necessary 1357 to determine the length of a PW-PDU, the Encapsulation Layer MUST 1358 provide it. 1360 6.4 PW-PDU Validation 1362 It is a common practice to use an error detection mechanism such as a 1363 CRC or similar mechanism to assure end-to-end integrity of frames. 1364 The PW service-specific mechanisms MUST define whether the packet's 1365 checksum shall be preserved across the PW, or be removed from PE- 1366 bound PDUs and then be re-calculated for insertion in CE-bound data. 1368 The former approach saves work, while the latter saves bandwidth. For 1369 a given implementation the choice may be dictated by hardware 1370 restrictions, which may not allow the preservation of the checksum. 1372 For protocols such as ATM and FR, the scope of the checksum is 1373 restricted to a single link. This is because the circuit identifiers 1374 (e.g. FR DLCI or ATM VPI/VCI) have only local significance and are 1375 changed on each hop or span. If the circuit identifier (and thus 1376 checksum) were going to change as a part of the PW emulation, it 1377 would be more efficient to strip and re-calculate the checksum. 1379 The service specific document for each protocol MUST describe the 1380 validation scheme to be used. 1382 6.5 Congestion Considerations 1384 The PSN carrying the PW may be subject to congestion. The congestion 1385 characteristics will vary with the PSN type, the network architecture 1386 and configuration, and the loading of the PSN. 1388 Where the traffic carried over the PW is known to be TCP friendly 1389 (by, for example, packet inspection), packet discard in the PSN will 1390 trigger the necessary reduction in offered load, and no additional 1391 congestion avoidance action is necessary. 1393 If the PW is operating over a PSN that provides enhanced delivery, 1394 the PEs SHOULD monitor packet loss to ensure that the service that 1395 was requested is actually being delivered. If it is not, then the PE 1396 SHOULD assume that the PSN is providing a best-effort service, and 1397 SHOULD use the best-effort service congestion avoidance measures 1398 described below. 1400 If best-effort service is being used and the traffic is not known to 1401 be TCP friendly, the PEs SHOULD monitor packet loss to ensure that 1402 the packet loss rate is within acceptable parameters. Packet loss is 1403 considered acceptable if a TCP flow across the same network path and 1404 experiencing the same network conditions would achieve an average 1405 throughput, measured on a reasonable timescale, that is not less than 1406 the PW flow is achieving. This condition can be satisfied by 1407 implementing a rate-limiting measure in the NSP, or by shutting down 1408 one or more PWs. The choice of which approach to use depends upon 1409 the type of traffic being carried. Where congestion is avoided by 1410 shutting down a PW, a suitable mechanism MUST be provided to prevent 1411 it immediately returning to service, causing a series of congestion 1412 pulses. 1414 The comparison to TCP cannot be specified exactly, but is intended as 1415 an "order-of-magnitude" comparison in timescale and throughput. The 1416 timescale on which TCP throughput is measured is the round-trip time 1417 of the connection. In essence, this requirement states that it is not 1418 acceptable to deploy an application (using PWE3 or any other 1419 transport protocol) on the best-effort Internet which consumes 1420 bandwidth arbitrarily and does not compete fairly with TCP within an 1421 order of magnitude. One method of determining an acceptable PW 1422 bandwidth is described in [TFRC]. 1424 7. Control Plane 1426 This section describes PWE3 control plane services. 1428 7.1 Set-up or Teardown of Pseudo-Wires 1430 A PW MUST be set up before an emulated service can be established, 1431 and MUST be torn down when an emulated service is no longer needed. 1433 Set up or teardown of a PW can be triggered by an operator command, 1434 from the management plane of a PE, by signaling (i.e., set-up or 1435 teardown) of a PWES, e.g., an ATM SVC, or by an auto-discovery 1436 mechanism. 1438 During the set-up process, the PEs need to exchange some information 1439 (e.g. learn each other's capabilities). The tunnel signaling 1440 protocol MAY be extended to provide mechanisms to enable the PEs to 1441 exchange all necessary information on behalf of the PW. 1443 Manual configuration of PWs can be considered a special kind of 1444 signaling, and is allowed. 1446 7.2 Status Monitoring 1448 Some native services have mechanisms for status monitoring. For 1449 example, ATM supports OAM for this purpose. For such services, the 1450 corresponding emulated services MUST specify how to perform status 1451 monitoring. 1453 7.3 Notification of Pseudo-wire Status Changes 1455 7.3.1. Pseudo-wire Up/Down Notification 1457 If a native service REQUIRES bi-directional connectivity, the 1458 corresponding emulated service can only be signaled as being up when 1459 the associated PWs, and PSN tunnels if any, are functional in both 1460 directions. 1462 Because the two CEs of an emulated service are not adjacent, a 1463 failure may occur at a place such that one or both physical links 1464 between the CEs and PEs remain up. For example, in Figure 2, if the 1465 physical link between CE1 and PE1 fails, the physical link between 1466 CE2 and PE2 will not be affected and will remain up. Unless CE2 is 1467 notified about the remote failure, it will continue to send traffic 1468 over the emulated service to CE1. Such traffic will be discarded at 1469 PE1. Some native services have failure notification so that when the 1470 services fail, both CEs will be notified. For such native services, 1471 the corresponding PWE3 service MUST provide a failure notification 1472 mechanism. 1474 Similarly, if a native service has notification mechanisms so that 1475 when a network failure is fixed, all the affected services will 1476 change status from "Down" to "Up", the corresponding emulated service 1477 MUST provide a similar mechanism for doing so. 1479 These mechanisms may already be built into the tunneling protocol. 1480 For example, the L2TP control protocol [RFC2661] [L2TPv3] has this 1481 capability and LDP has the ability to withdraw the corresponding MPLS 1482 label. 1484 7.3.2. Misconnection and Payload Type Mismatch 1486 With PWE3, misconnection and payload type mismatch can occur. If a 1487 misconnection occurs it can breach the integrity of the system. If a 1488 payload mismatch occurs it can disrupt the customer network. In both 1489 instances, there are security and operational concerns. 1491 The services of the underlying tunneling mechanism, and its 1492 associated control protocol, can be used to mitigate this. As part 1493 of the PW set-up a PW-TYPE identifier is exchanged. This is then used 1494 by the FWRD and NSP to verify the compatibility of the PWESs. 1496 7.3.3. Packet Loss, Corruption, and Out-of-order Delivery 1498 A PW can incur packet loss, corruption, and out-of-order delivery on 1499 the PSN path between the PEs. This can impact the working condition 1500 of an emulated service. For some payload types, packet loss, 1501 corruption, and out-of-order delivery can be mapped to either a bit 1502 error burst, or loss of carrier on the PW. If a native service has 1503 some mechanism to deal with bit error, the corresponding PWE3 service 1504 should provide a similar mechanism. 1506 7.3.4. Other Status Notification 1508 A PWE3 approach MAY provide a mechanism for other status 1509 notification, if any are needed. 1511 7.3.5. Collective Status Notification 1513 Status of a group of emulated services may be affected identically by 1514 a single network incident. For example, when the physical link (or 1515 sub-network) between a CE and a PE fails, all the emulated services 1516 that go through that link (or sub-network) will fail. It is likely 1517 that there exists a group of emulated services that all terminate at 1518 a remote CE. There may also be multiple such CEs affected by the 1519 failure. Therefore, it is desirable that a single notification 1520 message be used to notify failure of the whole group of emulated 1521 services. 1523 A PWE3 approach MAY provide some mechanism for notifying status 1524 changes of a group of emulated circuits. One possible method is to 1525 associate each emulated service with a group ID when the PW for that 1526 emulated service is set up. Multiple emulated services can then be 1527 grouped by associating them with the same group ID. In status 1528 notification, that group ID can be used to refer all the emulated 1529 services in that group. The group ID mechanism should be a mechanism 1530 provided by the underlying tunnel signaling protocol. 1532 7.4 Keep-alive 1534 If a native service has a keep-alive mechanism, the corresponding 1535 emulated service MUST provide a mechanism to propagate this across 1536 the PW. An approach following the principle of minimum intervention 1537 would be to transparently transport keep-alive messages over the PW. 1538 However, to accurately reproduce the semantics of the native 1539 mechanism, some PWs MAY REQUIRE an alternative approach, such as 1540 piggy-backing on the PW signaling mechanism. 1542 7.5 Handling Control Messages of the Native Services 1544 Some native services use control messages for circuit maintenance. 1545 These control messages MAY be in-band, e.g. Ethernet flow control, 1546 ATM performance management, or TDM tone signaling, or they MAY be 1547 out-of-band, e.g. the signaling VC of an ATM VP, or TDM CCS 1548 signaling. 1550 From the principle of minimum intervention, it is desirable that the 1551 PEs participate as little as possible in the signaling and 1552 maintenance of the native services. This principle SHOULD NOT, 1553 however, override the need to satisfactorily emulate the native 1554 service. 1556 If control messages are passed through, it may be desirable to send 1557 them using either a higher priority or a reliable channel provided by 1558 the PW Demultiplexer layer. See PWE3 Channel Types. 1560 8. Management and Monitoring 1562 This section describes the management and monitoring architecture for 1563 PWE3. 1565 8.1 Status and Statistics 1567 The PE should report the status of the interface and tabulate 1568 statistics that help monitor the state of the network, and to help 1569 with measurement of service level agreements (SLAs). Typical counters 1570 include: 1572 o Counts of PW-PDUs sent and received, with and without errors. 1573 o Counts of sequenced PW-PDUs lost. 1574 o Counts of service PDUs sent and received over the PSN, with 1575 and without errors (non-TDM). 1576 o Service-specific interface counts. 1577 o One way delay and delay variation. 1579 These counters would be contained in a PW-specific MIB, and they 1580 should not replicate existing MIB counters. 1582 8.2 PW SNMP MIB Architecture 1584 This section describes the general architecture for SNMP MIBs used to 1585 manage PW services and the underlying PSN. The intent here is to 1586 provide a clear picture of how all of the pertinent MIBs fit together 1587 to form a cohesive management framework for deploying PWE3 services. 1589 8.2.1. MIB Layering 1591 The SNMP MIBs created for PWE3 should fit the architecture shown in 1592 Figure 15. 1594 +-----------+ +-----------+ +-----------+ 1595 Service | CEM | | Ethernet | | ATM | 1596 Layer |Service MIB| |Service MIB| ... |Service MIB| 1597 +-----------+ +-----------+ +-----------+ 1598 \ | / 1599 \ | / 1600 - - - - - - - - - - - - \ - - - | - - - - / - - - - - - - 1601 \ | / 1602 +-------------------------------------------+ 1603 Generic PW | Generic PW MIBs | 1604 Layer +-------------------------------------------+ 1605 / \ 1606 - - - - - - - - - - - - / - - - - - - - - \ - - - - - - - 1607 / \ 1608 / \ 1609 +-----------+ +-----------+ 1610 PSN VC |L2TP VC MIB| |MPLS VC MIB| 1611 Layer +-----------+ +-----------+ 1612 | | 1613 - - - - - - - - - | - - - - - - - - - - - - - - - | - - - 1614 | | 1615 +-----------+ +-----------+ 1616 PSN |L2TP MIB(s)| |MPLS MIB(s)| 1617 Layer +-----------+ +-----------+ 1619 Figure 15: Relationship of SNMP MIBs 1621 Figure 16 shows an example for a SONET PW carried over MPLS. 1623 +-----------------+ 1624 | SONET MIB | RFC2558 1625 +-----------------+ 1626 | 1627 +-----------------+ 1628 Service |SONET Service MIB| pw-cem-mib 1629 Layer +-----------------+ 1630 - - - - - - - - - - | - - - - - - - - - - - - - - - 1631 +-----------------+ 1632 Generic PW | Generic PW MIBS | pw-tc-mib 1633 Layer +-----------------+ pw-mib 1634 - - - - - - - - - - | - - - - - - - - - - - - - - - 1635 +-----------------+ 1636 PSN VC | MPLS VC MIBS | pw-mpls-mib 1637 Layer +-----------------+ 1638 - - - - - - - - - - | - - - - - - - - - - - - - - - 1639 +-----------------+ 1640 PSN | MPLS MIBs | mpls-te-mib 1641 Layer +-----------------+ mpls-lsr-mib 1643 Figure 16: Service-specific Example for MIBs 1645 Note that there is a separate MIB for each emulated service as well 1646 as one for each underlying PSN. These MIBs MAY be used in various 1647 combinations as needed. 1649 8.2.2. Service Layer MIBs 1651 The first layer is referred to as the Service Layer. It contains 1652 MIBs for PWE3 services such as Ethernet, ATM, circuits and Frame 1653 Relay. This layer contains those corresponding MIBs used to mate or 1654 adapt those emulated services to the underlying services. This 1655 working group should not produce any MIBs for managing the general 1656 service; rather, it should produce just those MIBs that are used to 1657 interface or adapt the emulated service onto the PWE3 management 1658 framework. For example, the standard SONET MIB [SONETMIB] is 1659 designed and maintained by another working group. Also, the SONET MIB 1660 is designed to manage the native service without PW emulation. Since 1661 the PWE3 working group is chartered to produce the corresponding 1662 adaptation MIB, in this case, it would produce the PW-CEM-MIB 1663 [PWMPLSMIB] that would be used to adapt SONET services to the 1664 underlying PSN that carries the PWE3 service. 1666 8.2.3. Generic PW MIBs 1668 The second layer is referred to as the Generic PW Layer. This layer 1669 is composed of two MIBs: the PWE-TC-MIB [PWTCMIB] and the PWE-MIB 1670 [PWMIB]. These MIBs are responsible for providing general PWE3 1671 counters and service models used for monitoring and configuration of 1672 PWE3 services over any supported PSN service. That is, this MIB 1673 provides a general model of PWE3 abstraction for management purposes. 1674 This MIB is used to interconnect the Service Layer MIBs to the PSN VC 1675 Layer MIBs. The latter will be described in the next section. This 1676 layer also provides the PW-TC-MIB [PWTCMIB]. This MIB contains 1677 common SMI textual conventions [RFC1902] that MAY be used by any PW 1678 MIB. 1680 8.2.4. PSN VC Layer MIBs 1682 The third layer in the PWE3 management architecture is referred to as 1683 the PSN VC layer. This layer is comprised of MIBs that are 1684 specifically designed to interface general PWE3 services (VCs) onto 1685 those underlying PSN services. In general this means that the MIB 1686 provides a means with which an operator can map the PW service onto 1687 the native PSN service. For example, in the case of MPLS, it is 1688 required that the general VC service be layered onto MPLS LSPs or 1689 Traffic Engineered (TE) Tunnels [RFC3031]. In this case, the PW- 1690 MPLS-MIB [PWMPLSMIB] was created to adapt the general PWE3 circuit 1691 services onto MPLS. Like the Service Layer described above the PWE3 1692 working group should produce these MIBs. 1694 8.2.5. PSN Layer MIBs 1696 The fourth and final layer in the PWE3 management architecture is 1697 referred to as the PSN layer. This layer is comprised of those MIBs 1698 that control the PSN service-specific services. For example, in the 1699 case of the MPLS [RFC3031] PSN service, the MPLS-LSR-MIB [LSRMIB] and 1700 the MPLS-TE-MIB [TEMIB] are used to interface the general PWE3 VC 1701 services onto native MPLS LSPs and/or TE tunnels to carry the 1702 emulated services. In addition, the MPLS-LDP-MIB [LDPMIB] MAY be 1703 used to reveal the MPLS labels that are distributed over the MPLS PSN 1704 in order to maintain the PW service. The MIBs in this layer are 1705 produced by other working groups that design and specify the native 1706 PSN services. These MIBs should contain the appropriate mechanisms 1707 for monitoring and configuring the PSN service such that the emulated 1708 PWE3 service will function correctly. 1710 8.3 Connection Verification and Traceroute 1712 A connection verification mechanism should be supported by PWs. 1713 Connection verification as well as other alarm mechanisms can alert 1714 the operator that a PW has lost its remote connection. The opaque 1715 nature of a PW means that it is not possible to specify a generic 1716 connection verification or traceroute mechanism that passes this 1717 status to the CEs over the PW. If connection verification status of 1718 the PW is needed by the CE, it MUST be mapped to the native 1719 connection status method. 1721 For troubleshooting purposes, it is sometimes desirable to know the 1722 exact functional path of a PW between PEs. This is provided by the 1723 traceroute service of the underlying PSN. The opaque nature of the 1724 PW means that this traceroute information is only available within 1725 the provider network, e.g., at the PEs. 1727 9. IANA considerations 1729 The control word PID bits need to be assigned by IANA. 1731 10. Security Considerations 1733 PWE3 provides no means of protecting the integrity, confidentiality 1734 or delivery of the native data units. The use of PWE3 can therefore 1735 expose a particular environment to additional security threats. 1736 Assumptions that might be appropriate when all communicating systems 1737 are interconnected via a point to point or circuit-switched network 1738 may no longer hold when they are interconnected using an emulated 1739 wire carried over some types of PSN. It is outside the scope of this 1740 specification, to fully analyze and review the risks of PWE3, 1741 particularly as these risks will depend on the PSN. An example should 1742 make the concern clear. A number of IETF standards employ relatively 1743 weak security mechanisms when communicating nodes are expected to be 1744 connected to the same local area network. The Virtual Router 1745 Redundancy Protocol [RFC2338] is one instance. The relatively weak 1746 security mechanisms represent a greater vulnerability in an emulated 1747 Ethernet connected via a PW. 1749 Exploitation of vulnerabilities from within the PSN may be directed 1750 to the PW Tunnel end-point so that PW Demultiplexer and PSN tunnel 1751 services are disrupted. Controlling PSN access to the PW Tunnel 1752 end-point is one way to protect against this. By restricting PW 1753 Tunnel end-point access to legitimate remote PE sources of traffic, 1754 the PE may reject traffic that would interfere with the PW 1755 Demultiplexing and PSN tunnel services. 1757 Protection mechanisms MUST also address the spoofing of tunneled PW 1758 data. The validation of traffic addressed to the PW Demultiplexer 1759 end-point is paramount in ensuring integrity of PW encapsulation. 1760 Security protocols such as IPSec [RFC2401] MAY be used by the PW 1761 Demultiplexer Layer in order to maintain the integrity of the PW by 1762 authenticating data between the PW Demultiplexer End-points. 1764 IPSec MAY provide authentication, integrity, non-repudiation, and 1765 confidentiality of data transferred between two PEs. It cannot 1766 provide the equivalent services to the native service. 1768 Based on the type of data being transferred, the PW MAY indicate to 1769 the PW Demultiplexer Layer that enhanced security services are 1770 required. The PW Demultiplexer Layer MAY define multiple protection 1771 profiles based on the requirements of the PW emulated service. CE- 1772 to-CE signaling and control events emulated by the PW and some data 1773 types may require additional protection mechanisms. Alternatively, 1774 the PW Demultiplexer Layer may use peer authentication for every PSN 1775 packet to prevent spoofed native data units from being sent to the 1776 destination CE. 1778 Acknowledgments 1780 We thank: Sasha Vainshtein for his work on Native Service Processing 1781 and advice on bit-stream over PW services. Thomas K. Johnson for his 1782 work on the background and motivation for PWs. 1784 We also thank: Ron Bonica, Stephen Casner, Durai Chinnaiah, Jayakumar 1785 Jayakumar, Ghassem Koleyni, Danny McPherson, Eric Rosen, John 1786 Rutemiller, Scott Wainner and David Zelig for their comments and 1787 contributions. 1789 References 1791 Internet-drafts are works in progress available from 1792 1794 [DVB] EN 300 744 Digital Video Broadcasting (DVB); Framing 1795 structure, channel coding and modulation for digital 1796 terrestrial television (DVB-T), European 1797 Telecommunications Standards Institute (ETSI) 1799 [FRAG] Malis and Townsley, "PWE3 Fragmentation and 1800 Reassembly", , 1801 work in progress, October 2002. 1803 [LDPMIB] Cucchiara, J., Sjostrand, H., and Luciani, J., 1804 "Definitions of Managed Objects for the Multiprotocol 1805 Label Switching, Label Distribution Protocol (LDP)", 1806 , work in progress, 1807 October 2002. 1809 [LSRMIB] Srinivasan et al, "MPLS Label Switch Router Management 1810 Information Base Using SMIv2", 1811 , work in progress, 1812 October 2002. 1814 [L2TPv3] Layer Two Tunneling Protocol (Version 3)'L2TPv3', J Lau, 1815 et. al. , work 1816 in progress, January 2003. 1818 [PPPoL2TP] PPP Tunneling Using Layer Two Tunneling Protocol, 1819 J Lau et al. , 1820 work in progress, June 2002. 1822 [PWMIB] Zelig et al, "Pseudo Wire (PW) Management Information 1823 Base Using SMIv2", , 1824 work in progress, June 2002. 1826 [PWTCMIB] Nadeau et al, "Definitions for Textual Conventions and 1827 OBJECT-IDENTITIES for Pseudo-Wires Management" 1828 , work in progress, 1829 June 2002. 1831 [PWMPLSMIB] Danenberg et al, "SONET/SDH Circuit Emulation Service 1832 Over MPLS (CEM) Management Information Base Using 1833 SMIv2", , work in 1834 progress, October 2002. 1836 [RFC791] RFC-791: DARPA Internet Program, Protocol Specification, 1837 ISI, September 1981. 1839 [RFC1191] RFC-1191: Path MTU discovery. J.C. Mogul, S.E. Deering. 1841 [RFC1883] RFC-1883: Internet Protocol, Version 6 (IPv6), 1842 S. Deering, et al, December 1995 1844 [RFC1889] RFC-1889: RTP: A Transport Protocol for Real-Time 1845 Applications. H. Schulzrinne et. al. 1847 [RFC1902] RFC-1902: Structure of Management Information for 1848 Version 2 of the Simple Network Management Protocol 1849 (SNMPv2), Case et al, January 1996. 1851 [RFC1958] RFC-1958: Architectural Principles of the Internet, 1852 B. Carpenter et al. 1854 [RFC1981] RFC-1981: Path MTU Discovery for IP version 6. J. McCann, 1855 S. Deering, J. Mogul. 1857 [RFC2022] RFC-2022: Support for Multicast over UNI 3.0/3.1 based 1858 ATM Networks, G. Armitage. 1860 [RFC2119] RFC-2119, BCP-14: Key words for use in RFCs to Indicate 1861 Requirement Levels, S. Bradner. 1863 [RFC2338] RFC-2338: Virtual Router Redundancy Protocol, 1864 S. Knight, M. Shand et. al. 1866 [RFC2401] RFC-2401: Security Architecture for the Internet 1867 Protocol. S. Kent, R. Atkinson. 1869 [RFC2474] RFC-2474: Definition of the Differentiated Services 1870 Field (DS Field) in the IPv4 and IPv6 Headers, 1871 K. Nichols, et. al. 1873 [RFC2661] RFC-2661: Layer Two Tunneling Protocol "L2TP". 1874 W. Townsley, et. al. 1876 [RFC2784] RFC-2784: Generic Routing Encapsulation (GRE). 1877 D. Farinacci et al. 1879 [RFC2890] RFC-2890: Key and Sequence Number Extensions to GRE. 1880 G. Dommety. 1882 [RFC3022] RFC-3022: Traditional IP Network Address Translator 1883 (Traditional NAT). P Srisuresh et al. 1885 [RFC3031] RFC3031: Multiprotocol Label Switching Architecture, 1886 E. Rosen, January 2001. 1888 [RFC3032] RFC3032: MPLS Label Stack Encoding, E. Rosen, 1889 January 2001. 1891 [SONETMIB] K. Tesink, "Definitions of Managed Objects for the 1892 SONET/SDH Interface Type", RFC2558, March 1999. 1894 [TEMIB] Srinivasan et al, "Traffic Engineering Management 1895 Information Base Using SMIv2", 1896 , work in progress, 1897 November 2002. 1899 [TFRC] M. Handley et al, "TCP Friendly Rate Control (TFRC): 1900 Protocol Specification" , 1901 work in progress, October 2002. 1903 [VPLS] M. Lasserre, "Virtual Private LAN Services over MPLS", 1904 , work in 1905 progress, January 2003. 1907 [XIAO] Xiao et al, "Requirements for Pseudo-Wire Emulation 1908 Edge-to-Edge (PWE3)", 1909 (draft-ietf-pwe3-requirements-04.txt), X Xiao et al. 1910 work in progress, December 2002. 1912 Editors' Addresses 1914 Stewart Bryant 1915 Cisco Systems, 1916 4, The Square, 1917 Stockley Park, 1918 Uxbridge UB11 1BL, 1919 United Kingdom. Email: stbryant@cisco.com 1921 Prayson Pate 1922 Overture Networks, Inc. 1923 507 Airport Boulevard 1924 Morrisville, NC, USA 27560 Email: prayson.pate@overturenetworks.com 1926 Full copyright statement 1928 Copyright (C) The Internet Society (2002). 1929 All Rights Reserved. 1931 This document and translations of it may be copied and 1932 furnished to others, and derivative works that comment 1933 on or otherwise explain it or assist in its implementation 1934 may be prepared, copied, published and distributed, in 1935 whole or in part, without restriction of any kind, 1936 provided that the above copyright notice and this 1937 paragraph are included on all such copies and derivative works. 1938 However, this document itself may not be modified in any way, 1939 such as by removing the copyright notice or references to the 1940 Internet Society or other Internet organizations, except as 1941 needed for the purpose of developing Internet standards in 1942 which case the procedures for copyrights defined in the 1943 Internet Standards process must be followed, or as required to 1944 translate it into languages other than English. 1946 The limited permissions granted above are perpetual and will 1947 not be revoked by the Internet Society or its successors or assigns. 1949 This document and the information contained herein is provided 1950 on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET 1951 ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR 1952 IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE 1953 USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS 1954 OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS 1955 FOR A PARTICULAR PURPOSE.