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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: January 2004 Prayson Pate 5 Overture Networks, Inc. 7 Editors 9 August 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 Riverstone 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.................................. 33 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.................................... 35 100 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..... 37 104 8. Management and Monitoring................................. 37 105 8.1 Status and Statistics................................ 37 106 8.2 PW SNMP MIB Architecture............................. 38 107 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 PE-bound The traffic direction where information 241 from a CE is adapted to a PW, and PW-PDUs 242 are sent into the PSN. 244 PE/PW Maintenance Used by the PEs to set up, maintain and 245 tear down the PW. It may be coupled with 246 CE Signaling in order to effectively manage 247 the PW. 249 Protocol Data The unit of data output to, or received 250 Unit (PDU) from, the network by a protocol layer. 252 Provider Edge (PE) A device that provides PWE3 to a CE. 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 Pseudo Wire A mechanism that emulates the essential 259 Emulation Edge to attributes of service (such as a T1 leased 260 Edge (PWE3) line or frame relay) over a PSN. 262 Pseudo Wire PDU A PDU sent on the PW that contains all of 263 (PW-PDU) the data and control information necessary 264 to emulate the desired service. 266 PSN Tunnel A tunnel across a PSN inside which one or 267 more PWs can be carried. 269 PSN Tunnel Used to set up, maintain and tear down the 270 Signaling underlying PSN tunnel. 272 PW Demultiplexer Data-plane method of identifying a PW 273 terminating at a PE. 275 PW End Service The interface between a PE and a CE. This 276 (PWES) can be a physical interface like a T1 or 277 Ethernet, or a virtual interface like a VC 278 or VLAN. 280 PWE3 Payload Type A identifier used to distinguish between 281 Identifier an MPLS IP payload and a CW that is not 282 (PWE3-PID) ECMP safe. 284 Time Domain Time Division Multiplexing. Frequently used 285 Multiplexing (TDM) to refer to the synchronous bit-streams at 286 rates defined by G.702. 288 Tunnel A method of transparently carrying information 289 over a network. 291 2. PWE3 Applicability 293 The PSN carrying a PW will subject payload packets to loss, delay, 294 delay variation, and re-ordering. During a network transient there 295 may be a sustained period of impaired service. The applicability of 296 PWE3 to a particular service depends on the sensitivity of that 297 service (or the CE implementation) to these effects, and the ability 298 of the adaptation layer to mask them. Some services, such as IP over 299 FR over PWE3, may prove quite resilient to IP and MPLS PSN 300 characteristics. Other services, such as the interconnection of PBX 301 systems via PWE3, will require more careful consideration of the PSN 302 and adaptation layer characteristics. In some instances, traffic 303 engineering of the underlying PSN will be required, and in some 304 cases, the constraints may be such that it is not possible to provide 305 the required service guarantees. 307 3. Protocol Layering Model 309 The PWE3 protocol-layering model is intended to minimise the 310 differences between PWs operating over different PSN types. The 311 design of the protocol-layering model has the goals of making each PW 312 definition independent of the underlying PSN, and maximizing the 313 reuse of IETF protocol definitions and their implementations. 315 3.1 Protocol Layers 317 The logical protocol-layering model required to support a PW is shown 318 in Figure 1. 320 +---------------------------+ 321 | Payload | 322 +---------------------------+ 323 | Encapsulation | <==== MAY be null 324 +---------------------------+ 325 | PW Demultiplexer | 326 +---------------------------+ 327 | PSN Convergence | <==== MAY be null 328 +---------------------------+ 329 | PSN | 330 +---------------------------+ 331 | Data-link | 332 +---------------------------+ 333 | Physical | 334 +---------------------------+ 336 Figure 1: Logical Protocol Layering Model 338 The payload is transported over the Encapsulation Layer. The 339 Encapsulation Layer carries any information, not already present 340 within the payload itself, that is needed by the PW CE-bound PE 341 interface to send the payload to the CE via the physical interface. 342 If no information is needed beyond that in the payload itself, this 343 layer is empty. 345 This layer also provides support for real-time processing, and 346 sequencing, if needed. 348 The PW Demultiplexer Layer provides the ability to deliver multiple 349 PWs over a single PSN tunnel. The PW demultiplexer value used to 350 identify the PW in the data-plane may be unique per PE, but this is 351 not a PWE3 requirement. It MUST, however, be unique per tunnel 352 endpoint. If it is necessary to identify a particular tunnel, then 353 that is the responsibility of the PSN layer. 355 The PSN Convergence Layer provides the enhancements needed to make 356 the PSN conform to the assumed PSN service requirement. This layer 357 therefore provides a consistent interface to the PW, making the PW 358 independent of the PSN type. If the PSN already meets the service 359 requirements, this layer is empty. 361 The PSN header, MAC/Data-link and Physical Layer definitions are 362 outside the scope of this document. The PSN can be IPv4, IPv6 or 363 MPLS. 365 3.2 Domain of PWE3 367 PWE3 defines the Encapsulation Layer, the method of carrying various 368 payload types, and the interface to the PW Demultiplexer Layer. It 369 is expected that the other layers will be provided by tunneling 370 methods such as L2TP or MPLS over the PSN. 372 3.3 Payload Types 374 The payload is classified into the following generic types of native 375 data unit: 377 o Packet 378 o Cell 379 o Bit-stream 380 o Structured bit-stream 382 Within these generic types there are specific service types. For 383 example: 385 Generic Payload Type PW Service 386 -------------------- ---------- 387 Packet Ethernet (all types), HDLC framing, 388 frame-relay, ATM AAL5 PDU. 390 Cell ATM. 392 Bit-stream Unstructured E1, T1, E3, T3. 394 Structured bit-stream SONET/SDH (e.g. SPE, VT, NxDS0). 396 3.3.1. Packet Payload 398 A packet payload is a variable-size data unit presented to the PE on 399 the AC. A packet payload may be large compared to the PSN MTU. The 400 delineation of the packet boundaries is encapsulation-specific. HDLC 401 or Ethernet PDUs can be considered as examples of packet payloads. 402 Typically a packet will be stripped of transmission overhead such as 403 HDLC flags and stuffing bits before transmission over the PW. 405 A packet payload would normally be relayed across the PW as a single 406 unit. However, there will be cases where the combined size of the 407 packet payload and its associated PWE3 and PSN headers exceeds the 408 PSN path MTU. In these cases, some fragmentation methodology needs 409 to be applied. This may, for example, be the case when a user is 410 providing the service and attaching to the service provider via 411 Ethernet, or where nested pseudo-wires are involved. Fragmentation is 412 discussed in more detail in Section 5.3 414 A packet payload may need sequencing and real-time support. 416 In some situations, the packet payload MAY be selected from the 417 packets presented on the emulated wire on the basis of some sub- 418 multiplexing technique. For example, one or more frame-relay PDUs 419 may be selected for transport over a particular pseudo-wire based on 420 the frame-relay Data-Link Connection Identifier (DLCI), or, in the 421 case of Ethernet payloads, using a suitable MAC bridge filter. This 422 is a forwarder function, and this selection would therefore be made 423 before the packet was presented to the PW Encapsulation Layer. 425 3.3.2. Cell Payload 427 A cell payload is created by capturing, transporting and replaying 428 groups of octets presented on the wire in a fixed-size format. The 429 delineation of the group of bits that comprise the cell is specific 430 to the encapsulation type. Two common examples of cell payloads are 431 ATM 53-octet cells, and the larger 188-octet MPEG Transport Stream 432 packets [DVB]. 434 To reduce per-PSN packet overhead, multiple cells MAY be concatenated 435 into a single payload. The Encapsulation Layer MAY consider the 436 payload complete on the expiry of a timer, after a fixed number of 437 cells have been received or when a significant cell (e.g. an ATM OAM 438 cell) has been received. The benefit of concatenating multiple PDUs 439 should be weighed against a possible increase in packet delay 440 variation and the larger penalty incurred by packet loss. In some 441 cases, it may be appropriate for the Encapsulation Layer to perform 442 some type of compression, such as silence suppression or voice 443 compression. 445 The generic cell payload service will normally need sequence number 446 support, and may also need real-time support. The generic cell 447 payload service would not normally require fragmentation. 449 The Encapsulation Layer MAY apply some form of compression to some of 450 these sub-types (e.g. idle cells MAY be suppressed). 452 In some instances, the cells to be incorporated in the payload MAY be 453 selected by filtering them from the stream of cells presented on the 454 wire. For example, an ATM PWE3 service may select cells based on 455 their VCI or VPI fields. This is a forwader function, and the 456 selection would therefore be made before the packet was presented to 457 the PW Encapsulation Layer. 459 3.3.3. Bit-stream 461 A bit-stream payload is created by capturing, transporting and 462 replaying the bit pattern on the emulated wire, without taking 463 advantage of any structure that, on inspection, may be visible within 464 the relayed traffic (i.e. the internal structure has no effect on the 465 fragmentation into packets). 467 In some instances it is possible to apply suppression to bit-streams. 468 For example, E1 and T1 send "all-ones" to indicate failure. This 469 condition can be detected without any knowledge of the structure of 470 the bit-stream, and transmission of packetized data suppressed. 472 This service will require sequencing and real-time support. 474 3.3.4. Structured bit-stream 476 A structured bit-stream payload is created by using some knowledge of 477 the underlying structure of the bit-stream to capture, transport and 478 replay the bit pattern on the emulated wire. 480 Two important points distinguish structured and unstructured bit- 481 streams: 483 o Some parts of the original bit-stream MAY be stripped in the 484 PSN-bound direction by NSP block. For example, in Structured 485 SONET the section and line overhead (and, possibly more) may 486 be stripped. A framer is required to enable such stripping. 487 It is also required for frame/payload alignment for 488 fractional T1/E1 applications. 490 o The PW MUST preserve the structure across the PSN so that 491 the CE-bound NSP block can insert it correctly into the 492 reconstructed unstructured bit-stream. The stripped 493 information (such as SONET pointer justifications) may 494 appear in the encapsulation layer to facilitate this 495 reconstitution. 497 As an option, the Encapsulation Layer MAY also perform silence/idle 498 suppression or similar compression on a structured bit-stream. 500 Structured bit-streams are distinguished from cells in that the 501 structures may be too long to be carried in a single packet. Note 502 that "short" structures are indistinguishable from cells and may 503 benefit from the use of methods described in section 3.3.2. 505 This service REQUIRES sequencing and real-time support. 507 3.3.5. Principle of Minimum Intervention 509 To minimise the scope of information, and to improve the efficiency 510 of data flow through the Encapsulation Layer, the payload SHOULD be 511 transported as received with as few modifications as possible 512 [RFC1958]. 514 This minimum intervention approach decouples payload development from 515 PW development and requires fewer translations at the NSP in a system 516 with similar CE interfaces at each end. It also prevents unwanted 517 side-effects due to subtle misrepresentation of the payload in the 518 intermediate format. 520 An approach which does intervene can be more wire-efficient in some 521 cases and may result in fewer translations at the NSP where the CE 522 interfaces are of different types. Any intermediate format 523 effectively becomes a new framing type, requiring documentation and 524 assured interoperability. This increases the amount of work for 525 handling the protocol the intermediate format carries, and is 526 undesirable. 528 4. Architecture of Pseudo-wires 530 This section describes the PWE3 architectural model. 532 4.1 Network Reference Model 534 Figure 2 illustrates the network reference model for point-to-point 535 PWs. 537 |<-------------- Emulated Service ---------------->| 538 | | 539 | |<------- Pseudo Wire ------>| | 540 | | | | 541 | | |<-- PSN Tunnel -->| | | 542 | PW End V V V V PW End | 543 V Service +----+ +----+ Service V 544 +-----+ | | PE1|==================| PE2| | +-----+ 545 | |----------|............PW1.............|----------| | 546 | CE1 | | | | | | | | CE2 | 547 | |----------|............PW2.............|----------| | 548 +-----+ ^ | | |==================| | | ^ +-----+ 549 ^ | +----+ +----+ | | ^ 550 | | Provider Edge 1 Provider Edge 2 | | 551 | | | | 552 Customer | | Customer 553 Edge 1 | | Edge 2 554 | | 555 | | 556 native service native service 558 Figure 2: PWE3 Network Reference Model 560 The two PEs (PE1 and PE2) need to provide one or more PWs on behalf 561 of their client CEs (CE1 and CE2) to enable the client CEs to 562 communicate over the PSN. A PSN tunnel is established to provide a 563 data path for the PW. The PW traffic is invisible to the core 564 network, and the core network is transparent to the CEs. Native data 565 units (bits, cells or packets) presented to the PW End Service (PWES) 566 are encapsulated in a PW-PDU and carried across the underlying 567 network via the PSN tunnel. The PEs perform the necessary 568 encapsulation and decapsulation of PW-PDUs, as well as handling any 569 other functions required by the PW service, such as sequencing or 570 timing. A PE MAY provide multiple PWESs. 572 4.2 PWE3 Pre-processing 574 In some applications, there is a need to perform operations on the 575 native data units received from the CE (including both payload and 576 signaling traffic) before they are transmitted across the PW by the 577 PE. Examples include Ethernet bridging, SONET cross-connect, 578 translation of locally-significant identifiers such as VCI/VPI, or 579 translation to another service type. These operations could be 580 carried out in external equipment, and the processed data sent to the 581 PE over one or more physical interfaces. In most cases, there are 582 cost and operational benefits in undertaking these operations within 583 the PE. This processed data is then presented to the PW via a 584 virtual interface within the PE. 586 These pre-processing operations are included in the PWE3 reference 587 model to provide a common reference point, but the detailed 588 description of these operations is outside the scope of the PW 589 definition given here. 591 PW 592 End Service 593 | 594 |<------- Pseudo Wire ------>| 595 | | 596 | |<-- PSN Tunnel -->| | 597 V V V V PW 598 +-----+----+ +----+ End Service 599 +-----+ |PREP | PE1|==================| PE2| | +-----+ 600 | | | |............PW1.............|----------| | 601 | CE1 |----| | | | | | | CE2 | 602 | | ^ | |............PW2.............|----------| | 603 +-----+ | | | |==================| | | ^ +-----+ 604 | +-----+----+ +----+ | | 605 | ^ | | 606 | | | | 607 | |<------- Emulated Service ------->| | 608 | | | 609 | Virtual physical | 610 | termination | 611 | ^ | 612 CE1 native | CE2 native 613 service | service 614 | 615 CE2 native 616 service 618 Figure 3: Pre-processing within the PWE3 Network Reference Model 620 Figure 3 shows the inter-working of one PE with pre-processing 621 (PREP), and a second without this functionality. This is a useful 622 reference point because it emphasises that the functional interface 623 between PREP and the PW is that represented by a physical interface 624 carrying the service. This effectively defines the necessary inter- 625 working specification. 627 The operation of a system in which both PEs include PREP 628 functionality is also supported. 630 The required pre-processing can be divided into two components: 631 o Forwarder (FWRD) 633 o Native Service Processing (NSP) 635 4.2.1. Forwarders 637 In some applications there is the need to selectively forward payload 638 elements from one of more ACs to one or more PWs. In such cases there 639 will also be the need to perform the inverse function on PWE3-PDUs 640 received by a PE from the PSN. This is the function of the forwarder. 642 The forwarder selects the PW based on, for example: the incoming AC, 643 the contents of the payload, or some statically and/or dynamically 644 configured forwarding information. 646 +----------------------------------------+ 647 | PE Device | 648 +----------------------------------------+ 649 Single | | | 650 PWES | | Single | PW Instance 651 <------>o Forwarder + PW Instance X<===========> 652 | | | 653 +----------------------------------------+ 655 Figure 4a: Simple point-to-point service 657 +----------------------------------------+ 658 | PE Device | 659 +----------------------------------------+ 660 Multiple| | Single | PW Instance 661 PWES | + PW Instance X<===========> 662 <------>o | | 663 | |----------------------| 664 <------>o | Single | PW Instance 665 | Forwarder + PW Instance X<===========> 666 <------>o | | 667 | |----------------------| 668 <------>o | Single | PW Instance 669 | + PW Instance X<===========> 670 <------>o | | 671 +----------------------------------------+ 673 Figure 4b: Multiple PWES to Multiple PW Forwarding 675 Figure 4a shows a simple forwarder that performs some type of 676 filtering operation. Because the forwarder has a single input and a 677 single output interface, filtering is the only type of forwarding 678 operation that applies. Figure 4b shows a more general forwarding 679 situation where payloads are extracted from one or more PWESs and 680 directed to one or more PWs, including, in this instance, a 681 multipoint PW. In this case both filtering and direction operations 682 MAY be performed on the payloads. 684 4.2.2. Native Service Processing 686 In some applications some form of data or address translation, or 687 other operation requiring knowledge of the semantics of the payload, 688 will be required. This is the function of the Native Service 689 Processor (NSP). 691 The use of the NSP approach simplifies the design of the PW by 692 restricting a PW to homogeneous operation. NSP is included in the 693 reference model to provide a defined interface to this functionality. 694 The specification of the various types of NSP is outside the scope of 695 PWE3. 697 +----------------------------------------+ 698 | PE Device | 699 Multiple+----------------------------------------+ 700 PWES | | | Single | PW Instance 701 <------>o NSP # + PW Instance X<===========> 702 | | | | 703 |------| |----------------------| 704 | | | Single | PW Instance 705 <------>o NSP #Forwarder + PW Instance X<===========> 706 | | | | 707 |------| |----------------------| 708 | | | Single | PW Instance 709 <------>o NSP # + PW Instance X<===========> 710 | | | | 711 +----------------------------------------+ 713 Figure 5: NSP in a Multiple PWEs to Multiple 714 PW Forwarding PE 716 Figure 5 illustrates the relationship between NSP, forwarder and PWs 717 in a PE. The NSP function MAY apply any transformation operation 718 (modification, injection, etc.) on the payloads as they pass between 719 the physical interface to the CE and the virtual interface to the 720 forwarder. A PE device MAY contain more than one forwarder. 722 This model also supports the operation of a system in which the NSP 723 functionality includes terminating the data-link, and applying 724 Network Layer processing to the payload is also supported. 726 4.3 Maintenance Reference Model 728 Figure 6 illustrates the maintenance reference model for PWs. 730 |<------- CE (end-to-end) Signaling ------>| 731 | |<---- PW/PE Maintenance ----->| | 732 | | |<-- PSN Tunnel -->| | | 733 | | | Signaling | | | 734 | V V (out of scope) V V | 735 v +-----+ +-----+ v 736 +-----+ | PE1 |==================| PE2 | +-----+ 737 | |-----|.............PW1..............|-----| | 738 | CE1 | | | | | | CE2 | 739 | |-----|.............PW2..............|-----| | 740 +-----+ | |==================| | +-----+ 741 +-----+ +-----+ 742 Customer Provider Provider Customer 743 Edge 1 Edge 1 Edge 2 Edge 2 745 Figure 6: PWE3 Maintenance Reference Model 747 The following signaling mechanisms are REQUIRED: 749 o The CE (end-to-end) signaling is between the CEs. This 750 signaling could be frame relay PVC status signaling, ATM SVC 751 signaling, TDM CAS signaling, etc. 753 o The PW/PE Maintenance is used between the PEs (or NSPs) to set 754 up, maintain and tear down PWs, including any required 755 coordination of parameters. 757 o The PSN Tunnel signaling controls the PW multiplexing and some 758 elements of the underlying PSN. Examples are L2TP control 759 protocol, MPLS LDP and RSVP-TE. The definition of the 760 information that PWE3 needs to be signaled is within the scope 761 of PWE3, but the signaling protocol itself is not. 763 4.4 Protocol Stack Reference Model 765 Figure 7 illustrates the protocol stack reference model for PWs. 767 +-----------------+ +-----------------+ 768 |Emulated Service | |Emulated Service | 769 |(e.g. TDM, ATM) |<==== Emulated Service ===>|(e.g. TDM, ATM) | 770 +-----------------+ +-----------------+ 771 | Payload | | Payload | 772 | Encapsulation |<====== Pseudo Wire ======>| Encapsulation | 773 +-----------------+ +-----------------+ 774 |PW Demultiplexer | |PW Demultiplexer | 775 | PSN Tunnel, |<======= PSN Tunnel ======>| PSN Tunnel, | 776 | PSN & Physical | | PSN & Physical | 777 | Layers | | Layers | 778 +-------+---------+ ___________ +---------+-------+ 779 | / \ | 780 +===============/ PSN \===============+ 781 \ / 782 \_____________/ 784 Figure 7: PWE3 Protocol Stack Reference Model 786 The PW provides the CE with an emulated physical or virtual 787 connection to its peer at the far end. Native service PDUs from the 788 CE are passed through an Encapsulation Layer at the sending PE, and 789 then sent over the PSN. The receiving PE removes the encapsulation 790 and restores the payload to its native format for transmission to the 791 destination CE. 793 4.5 Pre-processing Extension to Protocol Stack Reference Model 795 Figure 8 illustrates how the protocol stack reference model is 796 extended to include the provision of pre-processing (Forwarding and 797 NSP). This shows the placement of the physical interface relative to 798 the CE. 800 /======================================\ 801 H Forwarder H<----Pre-processing 802 H----------------======================/ 803 H Native Service H | | 804 H Processing H | | 805 \================/ | | 806 | | | Emulated | 807 | Service | | Service | 808 | Interface | | (TDM, ATM, | 809 | (TDM, ATM, | | Ethernet, |<== Emulated Service == 810 | Ethernet, | | frame relay, | 811 | frame relay, | | etc.) | 812 | etc.) | +-----------------+ 813 | | | Payload | 814 | | | Encapsulation |<=== Pseudo Wire ====== 815 | | +-----------------+ 816 | | |PW Demultiplexer | 817 | | | PSN Tunnel, | 818 | | | PSN & Physical |<=== PSN Tunnel ======= 819 | | | Headers | 820 +----------------+ +-----------------+ 821 | Physical | | Physical | 822 +-------+--------+ +-------+---------+ 823 | | 824 | | 825 | | 826 | | 827 | | 828 | | 829 To CE <---+ +---> To PSN 831 Figure 8: Protocol Stack Reference Model with Pre-processing 833 5. PW Encapsulation 835 The PW Encapsulation Layer provides the necessary infrastructure to 836 adapt the specific payload type being transported over the PW to the 837 PW Demultiplexer Layer that is used to carry the PW over the PSN. 839 The PW Encapsulation Layer consists of three sub-layers: 841 o Payload Convergence 842 o Timing 843 o Sequencing 845 The PW Encapsulation sub-layering and its context with the protocol 846 stack are shown, in Figure 9. 848 +---------------------------+ 849 | Payload | 850 /===========================\ <------ Encapsulation 851 H Payload Convergence H Layer 852 H---------------------------H 853 H Timing H 854 H---------------------------H 855 H Sequencing H 856 \===========================/ 857 | PW Demultiplexer | 858 +---------------------------+ 859 | PSN Convergence | 860 +---------------------------+ 861 | PSN | 862 +---------------------------+ 863 | Data-link | 864 +---------------------------+ 865 | Physical | 866 +---------------------------+ 868 Figure 9: PWE3 Encapsulation Layer in Context 870 The Payload Convergence Sub-layer is highly tailored to the specific 871 payload type, but, by grouping a number of target payload types into 872 a generic class, and then providing a single convergence sub-layer 873 type common to the group, we achieve a reduction in the number of 874 payload convergence sub-layer types. This decreases implementation 875 complexity. The provision of per-packet signaling and other out-of- 876 band information (other than sequencing or timing) is undertaken by 877 this layer. 879 The Timing Layer and the Sequencing Layer provide generic services to 880 the Payload Convergence Layer for all payload types that require 881 them. 883 5.1 Payload Convergence Layer 885 5.1.1. Encapsulation 887 The primary task of the Payload Convergence Layer is the 888 encapsulation of the payload in PW-PDUs. The native data units to be 889 encapsulated MAY contain a L2 header or L1 overhead. This is service 890 specific. The Payload Convergence header carries the additional 891 information needed to replay the native data units at the CE-bound 892 physical interface. The PW Demultiplexer header is not considered as 893 part of the PW header. 895 Not all the additional information needed to replay the native data 896 units need to be carried in the PW header of the PW PDUs. Some 897 information (e.g. service type of a PW) MAY be stored as state 898 information at the destination PE during PW set-up. 900 5.1.2. PWE3 Channel Types 902 The PW Encapsulation Layer and its associated signaling require one 903 or more of the following types of channels from its underlying PW 904 Demultiplexer and PSN Layers: 906 1. A reliable control channel for signaling line events, status 907 indications, and, in some exceptional cases, CE-CE events 908 that must be translated and sent reliably between PEs. 910 For example, this capability is needed in [PPPoL2TP] 911 (PPP negotiation has to be split between the two ends of the 912 tunnel). PWE3 may also need this type of control channel to 913 provide faithful emulation of complex data-link protocols. 915 plus one or more data channels with the following characteristics: 917 2. A high-priority, unreliable, sequenced channel. A typical use 918 is for CE-to-CE signaling. "High priority" may simply be 919 indicated via the DSCP bits for IP or the EXP bits for MPLS, 920 giving the packet priority during transit. This channel type 921 could also use a bit in the tunnel header itself to indicate 922 that packets received at the PE SHOULD be processed with higher 923 priority [RFC2474]. 925 3. A sequenced channel for data traffic that is sensitive to 926 packet reordering (one classification for use could be for 927 any non-IP traffic). 929 4. An un-sequenced channel for data traffic insensitive to packet 930 order. 932 The data channels (2, 3 and 4 above) SHOULD be carried "in band" with 933 one another to as much of a degree as is reasonably possible on a 934 PSN. 936 Where end-to-end connectivity may be disrupted by address translation 937 [RFC3022], access-control lists, firewalls etc., there exists the 938 possibility that the control channel may be able to pass traffic and 939 set-up the PW, but the PW data traffic is blocked by one or more of 940 these mechanisms. In these cases unless the control channel is also 941 carried "in band" the signaling to set-up the PW will not confirm the 942 existence of an end-to-end data path. 944 In some cases there is a need to synchronize CE events with the data 945 carried over a PW. This is especially the case with TDM circuits 946 (e.g., the on-hook/off-hook events in PSTN switches might be carried 947 over a reliable control channel, whilst the associated bit-stream is 948 carried over a sequenced data channel). 950 PWE3 channel types that are not needed by the supported PWs need not 951 be included in such an implementation. 953 5.1.3. Quality of Service Considerations 955 Where possible, it is desirable to employ mechanisms to provide PW 956 Quality of Service (QoS) support over PSNs. 958 5.2 Payload-independent PW Encapsulation Layers 960 Two PWE3 Encapsulation Sub-layers provide common services to all 961 payload types: Sequencing and Timing. These services are optional 962 and are only used if needed by a particular PW instance. If the 963 service is not needed, the associated header MAY be omitted in order 964 to conserve processing and network resources. 966 There will be instances where a specific payload type will be 967 required to be transported with or without sequence and/or real-time 968 support. For example, an invariant of frame relay transport is the 969 preservation of packet order. Some frame-relay applications expect 970 in-order delivery, and may not cope with reordering of the frames. 971 However, where the frame relay service is itself only being used to 972 carry IP, it may be desirable to relax that constraint in return for 973 reduced per-packet processing cost. 975 The guiding principle is that, where possible, an existing IETF 976 protocol SHOULD be used to provide these services. Where a suitable 977 protocol is not available, the existing protocol should be extended 978 or modified to meet the PWE3 requirements, thereby making that 979 protocol available for other IETF uses. In the particular case of 980 timing, more than one general method may be necessary to provide for 981 the full scope of payload timing requirements. 983 5.2.1. Sequencing 985 The sequencing function provides three services: frame ordering, 986 frame duplication detection and frame loss detection. These services 987 allow the emulation of the invariant properties of a physical wire. 988 Support for sequencing depends on the payload type, and MAY be 989 omitted if not needed. 991 The size of the sequence-number space depends on the speed of the 992 emulated service, and the maximum time of the transient conditions in 993 the PSN. A sequence number space greater than 2^16 may therefore be 994 needed to prevent the sequence number space wrapping during the 995 transient. 997 5.2.1.1 Frame Ordering 999 When packets carrying the PW-PDUs traverse a PSN, they may arrive out 1000 of order at the destination PE. For some services, the frames 1001 (control frames, data frames, or both control and data frames) MUST 1002 be delivered in order. For such services, some mechanism MUST be 1003 provided for ensuring in-order delivery. Providing a sequence number 1004 in the sequence sub-layer header for each packet is one possible 1005 approach to out-of-sequence detection. Alternatively it can be noted 1006 that sequencing is a subset of the problem of delivering timed 1007 packets, and that a single combined mechanism such as [RFC3550] MAY 1008 be employed. 1010 There are two possible misordering strategies: 1012 o Drop misordered PW PDUs. 1014 o Try to sort PW PDUs into the correct order. 1016 The choice of strategy will depend on: 1018 o How critical the loss of packets is to the operation of 1019 the PW (e.g. the acceptable bit error rate). 1021 o The speeds of the PW and PSN. 1023 o The acceptable delay (since delay must be introduced to 1024 reorder) 1026 o The incidence of expected misordering. 1028 5.2.1.2 Frame Duplication Detection 1030 In rare cases, packets traversing a PW may be duplicated by the 1031 underlying PSN. For some services frame duplication is not 1032 acceptable. For such services, some mechanism MUST be provided to 1033 ensure that duplicated frames will not be delivered to the 1034 destination CE. The mechanism MAY be the same as the mechanism used 1035 to ensure in-order frame delivery. 1037 5.2.1.3 Frame Loss Detection 1039 A destination PE can determine whether a frame has been lost by 1040 tracking the sequence numbers of the received PW PDUs. 1042 In some instances, a destination PE will have to presume that a PW 1043 PDU is lost if it fails to arrive within a certain time. If a PW-PDU 1044 that has been processed as lost subsequently arrives, the destination 1045 PE MUST discard it. 1047 5.2.2. Timing 1049 A number of native services have timing expectations based on the 1050 characteristics of the networks that they were designed to travel 1051 over, and it can be necessary for the emulated service to duplicate 1052 these network characteristics as closely as possible, e.g. in 1053 delivering native traffic with bit-rate, jitter, wander and delay 1054 characteristics similar to those received at the sending PE. 1056 In such cases, it is necessary for the receiving PE to play out the 1057 native traffic as it was received at the sending PE. This relies on 1058 either timing information sent between the two PEs, or in some case 1059 timing information received from an external reference. 1061 The Timing Sub-layer must therefore support two timing functions: 1062 clock recovery and timed payload delivery. A particular payload type 1063 may require either or both of these services. 1065 5.2.2.1 Clock Recovery 1067 Clock recovery is the extraction of output transmission bit timing 1068 information from the delivered packet stream, and requires a suitable 1069 mechanism. A physical wire carries the timing information natively, 1070 but it is a relatively complex task to extract timing from a highly 1071 jittered source such as packet stream. It is therefore desirable 1072 that an existing real-time protocol such as [RFC3550] be used for 1073 this purpose, unless it can be shown that this is unsuitable or 1074 unnecessary for a particular payload type. 1076 5.2.2.2 Timed delivery 1078 Timed delivery is the delivery of non-contiguous PW PDUs to the PW 1079 output interface with a constant phase relative to the input 1080 interface. The timing of the delivery may be relative to a clock 1081 derived from the packet stream received over the PSN clock recovery, 1082 or with reference to an external clock. 1084 5.3 Fragmentation 1086 A payload would ideally be relayed across the PW as a single unit. 1087 However, there will be cases where the combined size of the payload 1088 and its associated PWE3 and PSN headers exceeds the PSN path MTU. 1089 When a packet size exceeds the MTU of a given network, fragmentation 1090 and reassembly have to be performed in order for the packet to be 1091 delivered. Since fragmentation and reassembly generally consume a 1092 considerable network resources as compared to simply switching a 1093 packet in its entirety, efforts SHOULD be made to reduce or eliminate 1094 the need for fragmentation and reassembly throughout a network to the 1095 extent possible. Of particular concern for fragmentation and 1096 reassembly are aggregation points where large numbers of PWs are 1097 processed (e.g. at the PE). 1099 Ideally, the equipment originating the traffic being sent over the PW 1100 will be configured to have adaptive measures (e.g. [RFC1191], 1101 [RFC1981]) in place that ensure that packets that need to be 1102 fragmented are not sent. When this fails, the point closest to the 1103 sending host with fragmentation and reassembly capabilities SHOULD 1104 attempt to reduce the size of packets to satisfy the PSN MTU. Thus, 1105 in the reference model for PWE3 [Figure 3] fragmentation SHOULD first 1106 be performed at the CE if at all possible. If and only if the CE 1107 cannot adhere to an acceptable MTU size for the PW should the PE 1108 attempt its own fragmentation method. 1110 In cases where MTU management fails to limit the payload to a size 1111 suitable for transmission of the PW, the PE MAY fall back to either a 1112 generic PW fragmentation method, or, if available the fragmentation 1113 service of the underlying PSN. 1115 It is acceptable for a PE implementation not to support 1116 fragmentation. A PE that does not support fragmentation will drop 1117 packets that exceed the PSN MTU, and the management plane of the 1118 encapsulating PE MAY be notified. 1120 If the length of a L2/L1 frame, restored from a PW PDU, exceeds the 1121 MTU of the destination PWES, it MUST be dropped. In this case, the 1122 management plane of the destination PE MAY be notified. 1124 5.4 Instantiation of the Protocol Layers 1126 This document does not address the detailed mapping of the Protocol 1127 Layering model to existing or future IETF standards. The 1128 instantiation of the logical Protocol Layering model is shown in 1129 Figure 9. 1131 5.4.1. PWE3 over an IP PSN 1133 The protocol definition of PWE3 over an IP PSN SHOULD employ existing 1134 IETF protocols where possible. 1136 +---------------------+ +-------------------------+ 1137 | Payload |------------->| Raw payload if possible | 1138 /=====================\ +-------------------------+ 1139 H Payload Convergence H-----------+->| As Needed | 1140 H---------------------H / +-------------------------+ 1141 H Timing H---------/--->| RTP | 1142 H---------------------H / +-------------+ | 1143 H Sequencing H----one of | | 1144 \=====================/ \ | +-----------+ 1145 | PW Demultiplexer |---------+--->| L2TP, MPLS etc. | 1146 +---------------------+ +-------------------------+ 1147 | PSN Convergence |------------->| Not needed | 1148 +---------------------+ +-------------------------+ 1149 | PSN |------------->| IP | 1150 +---------------------+ +-------------------------+ 1151 | Data-link |------------->| Data-link | 1152 +---------------------+ +-------------------------+ 1153 | Physical |------------->| Physical | 1154 +---------------------+ +-------------------------+ 1156 Figure 10: PWE3 over an IP PSN 1158 Figure 10 shows the protocol layering for PWE3 over an IP PSN. As a 1159 rule, the payload SHOULD be carried as received from the NSP, with 1160 the Payload Convergence Layer provided when needed. (It is accepted 1161 that there MAY sometimes be good reason not to follow this rule, but 1162 the exceptional circumstances need to be documented in the 1163 Encapsulation Layer definition for that payload type). 1165 Where appropriate, timing is provided by RTP [RFC3550], which when 1166 used also provides a sequencing service. PW Demultiplexing may be 1167 provided by a number of existing IETF tunnel protocols. Some of 1168 these tunnel protocols provide an optional sequencing service. 1169 (Sequencing is provided either by RTP, or by the PW Demultiplexer 1170 Layer, but not both). A PSN Convergence Layer is not needed, because 1171 all the tunnel protocols shown above are designed to operate directly 1172 over an IP PSN. 1174 As a special case, if the PW Demultiplexer is an MPLS label, the 1175 protocol architecture of section 5.4.2 can be used instead of the 1176 protocol architecture of this section. 1178 5.4.2. PWE3 over an MPLS PSN 1180 The MPLS ethos places importance on wire efficiency. By using a 1181 control word, some components of the PWE3 protocol layers can be 1182 compressed to increase this efficiency. 1184 +---------------------+ 1185 | Payload | 1186 /=====================\ 1187 H Payload Convergence H--+ 1188 H---------------------H | +--------------------------------+ 1189 H Timing H--------->| RTP | 1190 H---------------------H | +--------------------------------+ 1191 H Sequencing H--+------>| Flags, Frag, Len, Seq #, etc | 1192 \=====================/ | +--------------------------------+ 1193 | PW Demultiplexer |----+ | PWE3 Payload Type Identifier | 1194 +---------------------+ | | +--------------------------------+ 1195 | PSN Convergence |--+ +---->| PW Label | 1196 +---------------------+ +--------------------------------+ 1197 | PSN |--------->| Outer Label or MPLS-in-IP encap| 1198 +---------------------+ +--------------------------------+ 1199 | Data-link | 1200 +---------------------+ 1201 | Physical | 1202 +---------------------+ 1204 Figure 11: PWE3 over an MPLS PSN using a control word 1206 Figure 11 shows the protocol layering for PWE3 over an MPLS PSN. An 1207 inner MPLS label is used to provide the PW demultiplexing function. 1208 A control word is used to carry most of the information needed by the 1209 PWE3 Encapsulation Layer and the PSN Convergence Layer in a compact 1210 format. The flags in the control word provide the necessary payload 1211 convergence. A sequence field provides support for both in-order 1212 payload delivery and (supported by a fragmentation control method) a 1213 PSN fragmentation service within the PSN Convergence Layer. Ethernet 1214 pads all frames to a minimum size of 64 bytes. The MPLS header does 1215 not include a length indicator. Therefore to allow PWE3 to be carried 1216 in MPLS to correctly pass over an Ethernet data-link, a length 1217 correction field is needed in the control word. Where the design of 1218 the control word would alias an IP packet, a PWE3 Payload Type 1219 Identifier (PWE3 PID) should be interposed between the PW label and 1220 the control word (see 5.4.4). As with an IP PSN, where appropriate, 1221 timing is provided by RTP [RFC3550]. 1223 In some networks it may be necessary to carry PWE3 over MPLS over IP. 1224 In these circumstances, the PW is encapsulated for carriage over MPLS 1225 as described in this section, and then a method of carrying MPLS over 1226 an IP PSN (such as GRE [RFC2784], [RFC2890]) is applied to the 1227 resultant PW-PDU. 1229 5.4.3. PW over MPLS Generic Control Word 1231 To allow accurate packet inspection in an MPLS PSN, and/or to operate 1232 correctly over MPLS PSNs that have deployed equal-cost multiple-path 1233 load-balancing (ECMP), a PW packet MUST NOT alias an IP packet. IP 1234 packets are carried in MPLS label stacks without any protocol 1235 identifier. Historic values of the IP version number [RFC791] 1236 [RFC1883] are therefore used to distinguish between IP and non-IP 1237 MPLS payloads. 1239 To disambiguate the PW from an IP flow the PW SHOULD employ either 1240 the generic PW control word shown in Figure 12, or a PWE3 PID. Note 1241 that an MPLS payload with bits 0..3 = 4 is an IPv4 packet and an MPLS 1242 payload with bits 0..3 = 6 is an IPv6 packet. 1244 0 1 2 3 1245 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 1246 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1247 |0 0 0 0| Specified by PW Encapsulation | 1248 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1250 Figure 12: Generic PW Control Word 1252 The PW set-up protocol determines whether a PW uses a control word. 1253 When a control word is used, it SHOULD have the following preferred 1254 form: 1256 0 1 2 3 1257 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 1258 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1259 |0 0 0 0| Flags |FRG| Length | Sequence Number | 1260 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1262 Figure 13: MPLS Preferred Control Word 1264 The meaning of the fields of the MPLS Preferred Control Word (Figure 1265 13) is as follows: 1267 Flags (bits 4 to 7): 1268 These bits are available for per payload signaling. Their 1269 definition is encapsulation specific. 1271 FRG (bits 8 and 9): 1272 These bits are used when fragmenting a PW payload. Their use 1273 is defined in [FRAG]. When the PW is of a type that will 1274 never need payload fragmentation, these bits may be used as 1275 general purpose flags. 1277 Length (bits 10 to 15): 1278 The length field is used to determine the size of a PW 1279 payload that might have been padded to the minimum Ethernet 1280 MAC frame size during its transit across the PSN. If the 1281 MPLS payload (defined as the CW + the PW payload + any 1282 additional PW headers is less than 46 bytes, the length MUST 1283 be set to the length of the MPLS payload. If the MPLS 1284 payload is between 46 bytes and 63 bytes the implementation 1285 MAY either set to the length to the length of the MPLS 1286 payload, or it MAY set it to 0. If the length of the MPLS 1287 payload is greater than 63 bytes the length MUST be set to 0. 1289 Sequence number (Bit 16 to 31): 1290 If the sequence number is not used, it is set to zero by 1291 the sender and ignored by the receiver. Otherwise it 1292 specifies the sequence number of a packet. A circular list 1293 of sequence numbers is used. A sequence number takes a value 1294 from 1 to 65535 (2**16-1). If the payload is an OAM packet 1295 the sequence number MAY be used to mark the position in the 1296 sequence, in which case it has the same value as the last 1297 data PDU sent. The use of the sequence number is optional 1298 for OAM payloads. 1300 5.4.4. PWE3 Payload Type Identifier 1302 If technical considerations result in a PW control word that may 1303 alias an IP packet, the control word SHOULD be preceded by an PWE3 1304 payload type identifier (PWE3 PID). 1306 The PWE3 PID is defined as follows: 1308 0 1 2 3 1309 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 1310 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1311 |0 0 0 1| reserved = 0 | PA | Protocol ID | 1312 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1313 | As defined by PPP DLL protocol definition | 1314 | | 1315 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1317 Figure 14: PWE3 PID 1319 The meaning of the fields of the PWE3 PID (Figure 14) is as follows: 1321 PA protocol authority for the user plane or the control plane 1322 protocol ID 1323 0 = PPP DLL 1324 1-15 = Reserved 1326 Protocol ID 1327 Protocol ID following the format defined by the protocol 1328 authority identified in PA. 1330 Bits 4 to 11 inclusive are reserved for future use and must be zero. 1332 6. PW Demultiplexer Layer and PSN Requirements 1334 PWE3 places three service requirements on the protocol layers used to 1335 carry it across the PSN: 1337 o Multiplexing 1338 o Fragmentation 1339 o Length and Delivery 1341 6.1 Multiplexing 1343 The purpose of the PW Demultiplexer Layer is to allow multiple PWs to 1344 be carried in a single tunnel. This minimizes complexity and 1345 conserves resources. 1347 Some types of native service are capable of grouping multiple 1348 circuits into a "trunk", e.g. multiple ATM VCs in a VP, multiple 1349 Ethernet VLANs on a physical media, or multiple DS0 services within a 1350 T1 or E1. A PW MAY interconnect two end-trunks. That trunk would 1351 have a single multiplexing identifier. 1353 When a MPLS label is used as a PW Demultiplexer setting of the TTL 1354 value [RFC3032] in the PW label is application specific, however in a 1355 strict point to point application the TTL SHOULD be set to 2. 1357 6.2 Fragmentation 1359 If the PSN provides a fragmentation and reassembly service of 1360 adequate performance, it MAY be used to obtain an effective MTU that 1361 is large enough to transport the PW PDUs. See Section 5.3 for a full 1362 discussion of the PW fragmentation issues. 1364 6.3 Length and Delivery 1366 PDU delivery to the egress PE is the function of the PSN Layer. 1368 If the underlying PSN does not provide all the information necessary 1369 to determine the length of a PW-PDU, the Encapsulation Layer MUST 1370 provide it. 1372 6.4 PW-PDU Validation 1374 It is a common practice to use an error detection mechanism such as a 1375 CRC or similar mechanism to assure end-to-end integrity of frames. 1376 The PW service-specific mechanisms MUST define whether the packet's 1377 checksum shall be preserved across the PW, or be removed from PE- 1378 bound PDUs and then be re-calculated for insertion in CE-bound data. 1380 The former approach saves work, while the latter saves bandwidth. For 1381 a given implementation the choice may be dictated by hardware 1382 restrictions, which may not allow the preservation of the checksum. 1384 For protocols such as ATM and FR, the scope of the checksum is 1385 restricted to a single link. This is because the circuit identifiers 1386 (e.g. FR DLCI or ATM VPI/VCI) have only local significance and are 1387 changed on each hop or span. If the circuit identifier (and thus 1388 checksum) were going to change as a part of the PW emulation, it 1389 would be more efficient to strip and re-calculate the checksum. 1391 The service specific document for each protocol MUST describe the 1392 validation scheme to be used. 1394 6.5 Congestion Considerations 1396 The PSN carrying the PW may be subject to congestion. The congestion 1397 characteristics will vary with the PSN type, the network architecture 1398 and configuration, and the loading of the PSN. 1400 Where the traffic carried over the PW is known to be TCP friendly 1401 (by, for example, packet inspection), packet discard in the PSN will 1402 trigger the necessary reduction in offered load, and no additional 1403 congestion avoidance action is necessary. 1405 If the PW is operating over a PSN that provides enhanced delivery, 1406 the PEs SHOULD monitor packet loss to ensure that the service that 1407 was requested is actually being delivered. If it is not, then the PE 1408 SHOULD assume that the PSN is providing a best-effort service, and 1409 SHOULD use the best-effort service congestion avoidance measures 1410 described below. 1412 If best-effort service is being used and the traffic is not known to 1413 be TCP friendly, the PEs SHOULD monitor packet loss to ensure that 1414 the packet loss rate is within acceptable parameters. Packet loss is 1415 considered acceptable if a TCP flow across the same network path and 1416 experiencing the same network conditions would achieve an average 1417 throughput, measured on a reasonable timescale, that is not less than 1418 the PW flow is achieving. This condition can be satisfied by 1419 implementing a rate-limiting measure in the NSP, or by shutting down 1420 one or more PWs. The choice of which approach to use depends upon 1421 the type of traffic being carried. Where congestion is avoided by 1422 shutting down a PW, a suitable mechanism MUST be provided to prevent 1423 it immediately returning to service, causing a series of congestion 1424 pulses. 1426 The comparison to TCP cannot be specified exactly, but is intended as 1427 an "order-of-magnitude" comparison in timescale and throughput. The 1428 timescale on which TCP throughput is measured is the round-trip time 1429 of the connection. In essence, this requirement states that it is not 1430 acceptable to deploy an application (using PWE3 or any other 1431 transport protocol) on the best-effort Internet which consumes 1432 bandwidth arbitrarily and does not compete fairly with TCP within an 1433 order of magnitude. One method of determining an acceptable PW 1434 bandwidth is described in [RFC3448]. 1436 7. Control Plane 1438 This section describes PWE3 control plane services. 1440 7.1 Set-up or Teardown of Pseudo-Wires 1442 A PW MUST be set up before an emulated service can be established, 1443 and MUST be torn down when an emulated service is no longer needed. 1445 Set up or teardown of a PW can be triggered by an operator command, 1446 from the management plane of a PE, by signaling (i.e., set-up or 1447 teardown) of a PWES, e.g., an ATM SVC, or by an auto-discovery 1448 mechanism. 1450 During the set-up process, the PEs need to exchange some information 1451 (e.g. learn each other's capabilities). The tunnel signaling 1452 protocol MAY be extended to provide mechanisms to enable the PEs to 1453 exchange all necessary information on behalf of the PW. 1455 Manual configuration of PWs can be considered a special kind of 1456 signaling, and is allowed. 1458 7.2 Status Monitoring 1460 Some native services have mechanisms for status monitoring. For 1461 example, ATM supports OAM for this purpose. For such services, the 1462 corresponding emulated services MUST specify how to perform status 1463 monitoring. 1465 7.3 Notification of Pseudo-wire Status Changes 1467 7.3.1. Pseudo-wire Up/Down Notification 1469 If a native service REQUIRES bi-directional connectivity, the 1470 corresponding emulated service can only be signaled as being up when 1471 the associated PWs, and PSN tunnels if any, are functional in both 1472 directions. 1474 Because the two CEs of an emulated service are not adjacent, a 1475 failure may occur at a place such that one or both physical links 1476 between the CEs and PEs remain up. For example, in Figure 2, if the 1477 physical link between CE1 and PE1 fails, the physical link between 1478 CE2 and PE2 will not be affected and will remain up. Unless CE2 is 1479 notified about the remote failure, it will continue to send traffic 1480 over the emulated service to CE1. Such traffic will be discarded at 1481 PE1. Some native services have failure notification so that when the 1482 services fail, both CEs will be notified. For such native services, 1483 the corresponding PWE3 service MUST provide a failure notification 1484 mechanism. 1486 Similarly, if a native service has notification mechanisms so that 1487 when a network failure is fixed, all the affected services will 1488 change status from "Down" to "Up", the corresponding emulated service 1489 MUST provide a similar mechanism for doing so. 1491 These mechanisms may already be built into the tunneling protocol. 1492 For example, the L2TP control protocol [RFC2661] [L2TPv3] has this 1493 capability and LDP has the ability to withdraw the corresponding MPLS 1494 label. 1496 7.3.2. Misconnection and Payload Type Mismatch 1498 With PWE3, misconnection and payload type mismatch can occur. If a 1499 misconnection occurs it can breach the integrity of the system. If a 1500 payload mismatch occurs it can disrupt the customer network. In both 1501 instances, there are security and operational concerns. 1503 The services of the underlying tunneling mechanism, and its 1504 associated control protocol, can be used to mitigate this. As part 1505 of the PW set-up a PW-TYPE identifier is exchanged. This is then used 1506 by the forwarder and the NSP to verify the compatibility of the 1507 PWESs. 1509 7.3.3. Packet Loss, Corruption, and Out-of-order Delivery 1511 A PW can incur packet loss, corruption, and out-of-order delivery on 1512 the PSN path between the PEs. This can impact the working condition 1513 of an emulated service. For some payload types, packet loss, 1514 corruption, and out-of-order delivery can be mapped to either a bit 1515 error burst, or loss of carrier on the PW. If a native service has 1516 some mechanism to deal with bit error, the corresponding PWE3 service 1517 should provide a similar mechanism. 1519 7.3.4. Other Status Notification 1521 A PWE3 approach MAY provide a mechanism for other status 1522 notification, if any are needed. 1524 7.3.5. Collective Status Notification 1526 Status of a group of emulated services may be affected identically by 1527 a single network incident. For example, when the physical link (or 1528 sub-network) between a CE and a PE fails, all the emulated services 1529 that go through that link (or sub-network) will fail. It is likely 1530 that there exists a group of emulated services that all terminate at 1531 a remote CE. There may also be multiple such CEs affected by the 1532 failure. Therefore, it is desirable that a single notification 1533 message be used to notify failure of the whole group of emulated 1534 services. 1536 A PWE3 approach MAY provide some mechanism for notifying status 1537 changes of a group of emulated circuits. One possible method is to 1538 associate each emulated service with a group ID when the PW for that 1539 emulated service is set up. Multiple emulated services can then be 1540 grouped by associating them with the same group ID. In status 1541 notification, that group ID can be used to refer all the emulated 1542 services in that group. The group ID mechanism should be a mechanism 1543 provided by the underlying tunnel signaling protocol. 1545 7.4 Keep-alive 1547 If a native service has a keep-alive mechanism, the corresponding 1548 emulated service MUST provide a mechanism to propagate this across 1549 the PW. An approach following the principle of minimum intervention 1550 would be to transparently transport keep-alive messages over the PW. 1551 However, to accurately reproduce the semantics of the native 1552 mechanism, some PWs MAY REQUIRE an alternative approach, such as 1553 piggy-backing on the PW signaling mechanism. 1555 7.5 Handling Control Messages of the Native Services 1557 Some native services use control messages for circuit maintenance. 1558 These control messages MAY be in-band, e.g. Ethernet flow control, 1559 ATM performance management, or TDM tone signaling, or they MAY be 1560 out-of-band, e.g. the signaling VC of an ATM VP, or TDM CCS 1561 signaling. 1563 From the principle of minimum intervention, it is desirable that the 1564 PEs participate as little as possible in the signaling and 1565 maintenance of the native services. This principle SHOULD NOT, 1566 however, override the need to satisfactorily emulate the native 1567 service. 1569 If control messages are passed through, it may be desirable to send 1570 them using either a higher priority or a reliable channel provided by 1571 the PW Demultiplexer layer. See PWE3 Channel Types. 1573 8. Management and Monitoring 1575 This section describes the management and monitoring architecture for 1576 PWE3. 1578 8.1 Status and Statistics 1580 The PE should report the status of the interface and tabulate 1581 statistics that help monitor the state of the network, and to help 1582 with measurement of service level agreements (SLAs). Typical counters 1583 include: 1585 o Counts of PW-PDUs sent and received, with and without errors. 1586 o Counts of sequenced PW-PDUs lost. 1587 o Counts of service PDUs sent and received over the PSN, with 1588 and without errors (non-TDM). 1589 o Service-specific interface counts. 1590 o One way delay and delay variation. 1592 These counters would be contained in a PW-specific MIB, and they 1593 should not replicate existing MIB counters. 1595 8.2 PW SNMP MIB Architecture 1597 This section describes the general architecture for SNMP MIBs used to 1598 manage PW services and the underlying PSN. The intent here is to 1599 provide a clear picture of how all of the pertinent MIBs fit together 1600 to form a cohesive management framework for deploying PWE3 services. 1602 8.2.1. MIB Layering 1604 The SNMP MIBs created for PWE3 should fit the architecture shown in 1605 Figure 15. 1607 +-----------+ +-----------+ +-----------+ 1608 Service | CEM | | Ethernet | | ATM | 1609 Layer |Service MIB| |Service MIB| ... |Service MIB| 1610 +-----------+ +-----------+ +-----------+ 1611 \ | / 1612 \ | / 1613 - - - - - - - - - - - - \ - - - | - - - - / - - - - - - - 1614 \ | / 1615 +-------------------------------------------+ 1616 Generic PW | Generic PW MIBs | 1617 Layer +-------------------------------------------+ 1618 / \ 1619 - - - - - - - - - - - - / - - - - - - - - \ - - - - - - - 1620 / \ 1621 / \ 1622 +-----------+ +-----------+ 1623 PSN VC |L2TP VC MIB| |MPLS VC MIB| 1624 Layer +-----------+ +-----------+ 1625 | | 1626 - - - - - - - - - | - - - - - - - - - - - - - - - | - - - 1627 | | 1628 +-----------+ +-----------+ 1629 PSN |L2TP MIB(s)| |MPLS MIB(s)| 1630 Layer +-----------+ +-----------+ 1632 Figure 15: Relationship of SNMP MIBs 1634 Figure 16 shows an example for a SONET PW carried over MPLS. 1636 +-----------------+ 1637 | SONET MIB | RFC2558 1638 +-----------------+ 1639 | 1640 +-----------------+ 1641 Service |SONET Service MIB| pw-cem-mib 1642 Layer +-----------------+ 1643 - - - - - - - - - - | - - - - - - - - - - - - - - - 1644 +-----------------+ 1645 Generic PW | Generic PW MIBS | pw-tc-mib 1646 Layer +-----------------+ pw-mib 1647 - - - - - - - - - - | - - - - - - - - - - - - - - - 1648 +-----------------+ 1649 PSN VC | MPLS VC MIBS | pw-mpls-mib 1650 Layer +-----------------+ 1651 - - - - - - - - - - | - - - - - - - - - - - - - - - 1652 +-----------------+ 1653 PSN | MPLS MIBs | mpls-te-mib 1654 Layer +-----------------+ mpls-lsr-mib 1656 Figure 16: Service-specific Example for MIBs 1658 Note that there is a separate MIB for each emulated service as well 1659 as one for each underlying PSN. These MIBs MAY be used in various 1660 combinations as needed. 1662 8.2.2. Service Layer MIBs 1664 The first layer is referred to as the Service Layer. It contains 1665 MIBs for PWE3 services such as Ethernet, ATM, circuits and Frame 1666 Relay. This layer contains those corresponding MIBs used to mate or 1667 adapt those emulated services to the underlying services. This 1668 working group should not produce any MIBs for managing the general 1669 service; rather, it should produce just those MIBs that are used to 1670 interface or adapt the emulated service onto the PWE3 management 1671 framework. For example, the standard SONET MIB [RFC2558] is designed 1672 and maintained by another working group. Also, the SONET MIB is 1673 designed to manage the native service without PW emulation. Since 1674 the PWE3 working group is chartered to produce the corresponding 1675 adaptation MIB, in this case, it would produce the PW-CEM-MIB 1676 [PWMPLSMIB] that would be used to adapt SONET services to the 1677 underlying PSN that carries the PWE3 service. 1679 8.2.3. Generic PW MIBs 1681 The second layer is referred to as the Generic PW Layer. This layer 1682 is composed of two MIBs: the PWE-TC-MIB [PWTCMIB] and the PWE-MIB 1683 [PWMIB]. These MIBs are responsible for providing general PWE3 1684 counters and service models used for monitoring and configuration of 1685 PWE3 services over any supported PSN service. That is, this MIB 1686 provides a general model of PWE3 abstraction for management purposes. 1687 This MIB is used to interconnect the Service Layer MIBs to the PSN VC 1688 Layer MIBs. The latter will be described in the next section. This 1689 layer also provides the PW-TC-MIB [PWTCMIB]. This MIB contains 1690 common SMI textual conventions [RFC1902] that MAY be used by any PW 1691 MIB. 1693 8.2.4. PSN VC Layer MIBs 1695 The third layer in the PWE3 management architecture is referred to as 1696 the PSN VC layer. This layer is comprised of MIBs that are 1697 specifically designed to interface general PWE3 services (VCs) onto 1698 those underlying PSN services. In general this means that the MIB 1699 provides a means with which an operator can map the PW service onto 1700 the native PSN service. For example, in the case of MPLS, it is 1701 required that the general VC service be layered onto MPLS LSPs or 1702 Traffic Engineered (TE) Tunnels [RFC3031]. In this case, the PW- 1703 MPLS-MIB [PWMPLSMIB] was created to adapt the general PWE3 circuit 1704 services onto MPLS. Like the Service Layer described above the PWE3 1705 working group should produce these MIBs. 1707 8.2.5. PSN Layer MIBs 1709 The fourth and final layer in the PWE3 management architecture is 1710 referred to as the PSN layer. This layer is comprised of those MIBs 1711 that control the PSN service-specific services. For example, in the 1712 case of the MPLS [RFC3031] PSN service, the MPLS-LSR-MIB [LSRMIB] and 1713 the MPLS-TE-MIB [TEMIB] are used to interface the general PWE3 VC 1714 services onto native MPLS LSPs and/or TE tunnels to carry the 1715 emulated services. In addition, the MPLS-LDP-MIB [LDPMIB] MAY be 1716 used to reveal the MPLS labels that are distributed over the MPLS PSN 1717 in order to maintain the PW service. The MIBs in this layer are 1718 produced by other working groups that design and specify the native 1719 PSN services. These MIBs should contain the appropriate mechanisms 1720 for monitoring and configuring the PSN service such that the emulated 1721 PWE3 service will function correctly. 1723 8.3 Connection Verification and Traceroute 1725 A connection verification mechanism should be supported by PWs. 1726 Connection verification as well as other alarm mechanisms can alert 1727 the operator that a PW has lost its remote connection. The opaque 1728 nature of a PW means that it is not possible to specify a generic 1729 connection verification or traceroute mechanism that passes this 1730 status to the CEs over the PW. If connection verification status of 1731 the PW is needed by the CE, it MUST be mapped to the native 1732 connection status method. 1734 For troubleshooting purposes, it is sometimes desirable to know the 1735 exact functional path of a PW between PEs. This is provided by the 1736 traceroute service of the underlying PSN. The opaque nature of the 1737 PW means that this traceroute information is only available within 1738 the provider network, e.g., at the PEs. 1740 9. IANA considerations 1742 The control word PID bits need to be assigned by IANA. 1744 10. Security Considerations 1746 PWE3 provides no means of protecting the integrity, confidentiality 1747 or delivery of the native data units. The use of PWE3 can therefore 1748 expose a particular environment to additional security threats. 1749 Assumptions that might be appropriate when all communicating systems 1750 are interconnected via a point to point or circuit-switched network 1751 may no longer hold when they are interconnected using an emulated 1752 wire carried over some types of PSN. It is outside the scope of this 1753 specification, to fully analyze and review the risks of PWE3, 1754 particularly as these risks will depend on the PSN. An example should 1755 make the concern clear. A number of IETF standards employ relatively 1756 weak security mechanisms when communicating nodes are expected to be 1757 connected to the same local area network. The Virtual Router 1758 Redundancy Protocol [RFC2338] is one instance. The relatively weak 1759 security mechanisms represent a greater vulnerability in an emulated 1760 Ethernet connected via a PW. 1762 Exploitation of vulnerabilities from within the PSN may be directed 1763 to the PW Tunnel end-point so that PW Demultiplexer and PSN tunnel 1764 services are disrupted. Controlling PSN access to the PW Tunnel 1765 end-point is one way to protect against this. By restricting PW 1766 Tunnel end-point access to legitimate remote PE sources of traffic, 1767 the PE may reject traffic that would interfere with the PW 1768 Demultiplexing and PSN tunnel services. 1770 Protection mechanisms MUST also address the spoofing of tunneled PW 1771 data. The validation of traffic addressed to the PW Demultiplexer 1772 end-point is paramount in ensuring integrity of PW encapsulation. 1773 Security protocols such as IPSec [RFC2401] MAY be used by the PW 1774 Demultiplexer Layer in order to maintain the integrity of the PW by 1775 authenticating data between the PW Demultiplexer End-points. 1777 IPSec MAY provide authentication, integrity, non-repudiation, and 1778 confidentiality of data transferred between two PEs. It cannot 1779 provide the equivalent services to the native service. 1781 Based on the type of data being transferred, the PW MAY indicate to 1782 the PW Demultiplexer Layer that enhanced security services are 1783 required. The PW Demultiplexer Layer MAY define multiple protection 1784 profiles based on the requirements of the PW emulated service. CE- 1785 to-CE signaling and control events emulated by the PW and some data 1786 types may require additional protection mechanisms. Alternatively, 1787 the PW Demultiplexer Layer may use peer authentication for every PSN 1788 packet to prevent spoofed native data units from being sent to the 1789 destination CE. 1791 Acknowledgments 1793 We thank: Sasha Vainshtein for his work on Native Service Processing 1794 and advice on bit-stream over PW services. Thomas K. Johnson for his 1795 work on the background and motivation for PWs. 1797 We also thank: Ron Bonica, Stephen Casner, Durai Chinnaiah, Jayakumar 1798 Jayakumar, Ghassem Koleyni, Danny McPherson, Eric Rosen, John 1799 Rutemiller, Scott Wainner and David Zelig for their comments and 1800 contributions. 1802 Normative References 1804 Internet-drafts are works in progress available from 1805 1807 [FRAG] Malis and Townsley, "PWE3 Fragmentation and 1808 Reassembly", , 1809 work in progress, June 2003. 1811 [L2TPv3] Layer Two Tunneling Protocol (Version 3)'L2TPv3', J Lau, 1812 et. al. , work 1813 in progress, June 2003. 1815 [RFC791] RFC-791: DARPA Internet Program, Protocol Specification, 1816 ISI, September 1981. 1818 [RFC1883] RFC-1883: Internet Protocol, Version 6 (IPv6), 1819 S. Deering, et al, December 1995 1821 [RFC1902] RFC-1902: Structure of Management Information for 1822 Version 2 of the Simple Network Management Protocol 1823 (SNMPv2), Case et al, January 1996. 1825 [RFC2119] RFC-2119, BCP-14: Key words for use in RFCs to Indicate 1826 Requirement Levels, S. Bradner. 1828 [RFC2401] RFC-2401: Security Architecture for the Internet 1829 Protocol. S. Kent, R. Atkinson. 1831 [RFC2474] RFC-2474: Definition of the Differentiated Services 1832 Field (DS Field) in the IPv4 and IPv6 Headers, 1833 K. Nichols, et. al. 1835 [RFC2558] K. Tesink, "Definitions of Managed Objects for the 1836 SONET/SDH Interface Type", RFC2558, March 1999. 1838 [RFC2661] RFC-2661: Layer Two Tunneling Protocol "L2TP". 1839 W. Townsley, et. al. 1841 [RFC2784] RFC-2784: Generic Routing Encapsulation (GRE). 1842 D. Farinacci et al. 1844 [RFC2890] RFC-2890: Key and Sequence Number Extensions to GRE. 1845 G. Dommety. 1847 [RFC3031] RFC3031: Multiprotocol Label Switching Architecture, 1848 E. Rosen, January 2001. 1850 [RFC3032] RFC3032: MPLS Label Stack Encoding, E. Rosen, 1851 January 2001. 1853 [RFC3550] RFC-3550: RTP: A Transport Protocol for Real-Time 1854 Applications. H. Schulzrinne et. al. 1856 Informative References 1858 Internet-drafts are works in progress available from 1859 1861 [DVB] EN 300 744 Digital Video Broadcasting (DVB); Framing 1862 structure, channel coding and modulation for digital 1863 terrestrial television (DVB-T), European 1864 Telecommunications Standards Institute (ETSI) 1866 [LDPMIB] Cucchiara, J., Sjostrand, H., and Luciani, J., 1867 "Definitions of Managed Objects for the Multiprotocol 1868 Label Switching, Label Distribution Protocol (LDP)", 1869 , work in progress, 1870 June 2003. 1872 [LSRMIB] Srinivasan et al, "MPLS Label Switch Router Management 1873 Information Base Using SMIv2", 1874 , work in progress, 1875 June 2003. 1877 [PPPoL2TP] PPP Tunneling Using Layer Two Tunneling Protocol, 1878 J Lau et al. , 1879 work in progress, June 2002. 1881 [PWMIB] Zelig et al, "Pseudo Wire (PW) Management Information 1882 Base Using SMIv2", , 1883 work in progress, June 2003. 1885 [PWTCMIB] Nadeau et al, "Definitions for Textual Conventions and 1886 OBJECT-IDENTITIES for Pseudo-Wires Management" 1887 , work in progress, 1888 June 2003. 1890 [PWMPLSMIB] Danenberg et al, "SONET/SDH Circuit Emulation Service 1891 Over MPLS (CEM) Management Information Base Using 1892 SMIv2", , work in 1893 progress, October 2002. 1895 [RFC1191] RFC-1191: Path MTU discovery. J.C. Mogul, S.E. Deering. 1897 [RFC1958] RFC-1958: Architectural Principles of the Internet, 1898 B. Carpenter et al. 1900 [RFC1981] RFC-1981: Path MTU Discovery for IP version 6. J. McCann, 1901 S. Deering, J. Mogul. 1903 [RFC2022] RFC-2022: Support for Multicast over UNI 3.0/3.1 based 1904 ATM Networks, G. Armitage. 1906 [RFC2338] RFC-2338: Virtual Router Redundancy Protocol, 1907 S. Knight, M. Shand et. al. 1909 [RFC3022] RFC-3022: Traditional IP Network Address Translator 1910 (Traditional NAT). P Srisuresh et al. 1912 [RFC3448] RFC3448: TCP Friendly Rate Control (TFRC): Protocol 1913 Specification, M. Handley et al. January 2003. 1915 [TEMIB] Srinivasan et al, "Traffic Engineering Management 1916 Information Base Using SMIv2", 1917 , work in progress, 1918 June 2003. 1920 [XIAO] Xiao et al, "Requirements for Pseudo-Wire Emulation 1921 Edge-to-Edge (PWE3)", 1922 (draft-ietf-pwe3-requirements-06.txt), X Xiao et al. 1923 work in progress, June 2002. 1925 Editors' Addresses 1927 Stewart Bryant 1928 Cisco Systems, 1929 250, Longwater, 1930 Green Park, 1931 Reading, RG2 6GB, 1932 United Kingdom. Email: stbryant@cisco.com 1934 Prayson Pate 1935 Overture Networks, Inc. 1936 507 Airport Boulevard 1937 Morrisville, NC, USA 27560 Email: prayson.pate@overturenetworks.com 1939 Full copyright statement 1941 Copyright (C) The Internet Society (2002). 1942 All Rights Reserved. 1944 This document and translations of it may be copied and 1945 furnished to others, and derivative works that comment 1946 on or otherwise explain it or assist in its implementation 1947 may be prepared, copied, published and distributed, in 1948 whole or in part, without restriction of any kind, 1949 provided that the above copyright notice and this 1950 paragraph are included on all such copies and derivative works. 1952 However, this document itself may not be modified in any way, 1953 such as by removing the copyright notice or references to the 1954 Internet Society or other Internet organizations, except as 1955 needed for the purpose of developing Internet standards in 1956 which case the procedures for copyrights defined in the 1957 Internet Standards process must be followed, or as required to 1958 translate it into languages other than English. 1960 The limited permissions granted above are perpetual and will 1961 not be revoked by the Internet Society or its successors or assigns. 1963 This document and the information contained herein is provided 1964 on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET 1965 ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR 1966 IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE 1967 USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS 1968 OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS 1969 FOR A PARTICULAR PURPOSE.