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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Pseudo-Wire Edge-to-Edge (PWE3) Working Group Stewart Bryant 3 Internet Draft Cisco Systems 4 Document: 5 Expires: April 2003 Prayson Pate 6 Overture Networks, Inc. 8 Editors 10 October 2002 12 PWE3 Architecture 14 Status of this Memo 16 This document is an Internet-Draft and is in full conformance with 17 all provisions of section 10 of RFC2026. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that other 21 groups may also distribute working documents as Internet-Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress". 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/ietf/1id-abstracts.txt The list of 30 Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 Abstract 35 This document describes an architecture for Pseudo Wire Emulation 36 Edge-to-Edge (PWE3). It discusses the emulation of services (such as 37 Frame Relay, ATM, Ethernet TDM and SONET/SDH) over packet switched 38 networks (PSNs) using IP or MPLS. It presents the architectural 39 framework for pseudo wires (PWs), defines terminology, specifies the 40 various protocol elements and their functions. 42 Co-Authors 44 The following are co-authors of this document: 46 Thomas K. Johnson Litchfield Communications 47 Kireeti Kompella Juniper Networks, Inc. 48 Andrew G. Malis Vivace Networks 49 Danny McPherson TCB 50 Thomas D. Nadeau Cisco Systems 51 Tricci So Caspian Networks 52 W. Mark Townsley Cisco Systems 53 Craig White Level 3 Communications, LLC. 54 Lloyd Wood Cisco Systems 55 XiPeng Xiao Redback Networks 57 Table of Contents 59 Status of this Memo.......................................... 1 61 1. Introduction............................................. 5 62 1.1 Pseudo Wire Definition............................... 5 63 1.2 PW Service Functionality............................. 6 64 1.3 Non-Goals of this document........................... 6 66 2. PWE3 Applicability....................................... 9 68 3. Protocol Layering Model.................................. 9 69 3.1 Protocol Layers...................................... 9 70 3.2 Domain of PWE3....................................... 10 71 3.3 Payload Types........................................ 10 73 4. Architecture of Pseudo-wires............................. 14 74 4.1 Network Reference Model.............................. 14 75 4.2 PWE3 Pre-processing.................................. 15 76 4.3 Maintenance Reference Model.......................... 19 77 4.4 Protocol Stack Reference Model....................... 19 78 4.5 Pre-processing Extension to Protocol Stack Reference. 79 Model................................................ 20 81 5. PW Encapsulation......................................... 21 82 5.1 Payload Convergence Layer............................ 22 83 5.2 Payload-independent PW Encapsulation Layers.......... 24 84 5.3 Fragmentation........................................ 26 85 5.4 Instantiation of the Protocol Layers................. 27 87 6. PW Demultiplexer Layer and PSN Requirements.............. 30 88 6.1 Multiplexing......................................... 30 89 6.2 Fragmentation........................................ 30 90 6.3 Length and Delivery.................................. 30 91 6.4 PW-PDU Validation.................................... 30 92 6.5 Congestion Considerations............................ 31 94 7. Control Plane............................................ 31 95 7.1 Set-up or Teardown of Pseudo-Wires................... 31 96 7.2 Status Monitoring.................................... 32 97 7.3 Notification of Pseudo-wire Status Changes........... 32 98 7.4 Keep-alive........................................... 33 99 7.5 Handling Control Messages of the Native Services..... 34 101 8. Management and Monitoring................................. 34 102 8.1 Statistics........................................... 34 103 8.2 PW SNMP MIB Architecture............................. 35 104 8.3 Connection Verification and Traceroute................ 38 106 9. IANA considerations...................................... 38 108 10. Security Considerations................................. 38 109 10.1 PW Tunnel End-Point and PW Demultiplexer Security... 38 110 10.2 Validation of PW Encapsulation...................... 39 112 1. Introduction 114 This document describes an architecture for Pseudo Wire Emulation 115 Edge-to-Edge (PWE3) in support of [XIAO]. It discusses the emulation 116 of services (such as Frame Relay, ATM, Ethernet TDM and SONET/SDH) 117 over packet switched networks (PSNs) using IP or MPLS. It presents 118 the architectural framework for pseudo wires (PWs), defines 119 terminology, specifies the various protocol elements and their 120 functions. 122 1.1 Pseudo Wire Definition 124 PWE3 is a mechanism that emulates the essential attributes of a 125 service (such as a T1 leased line or Frame Relay) over a PSN. PWE3 is 126 intended to provide only the minimum necessary functionality to 127 emulate the wire with the required degree of faithfulness for the 128 given service definition. Any required switching functionality is the 129 responsibility of a forwarder function (FWRD). Any translation or 130 other operation needing knowledge of the payload semantics is carried 131 out by native service processing (NSP) elements. The functional 132 definition of any FWRD or NSP elements is outside the scope of PWE3. 134 The required functions of PWs include encapsulating service-specific 135 bit-streams, cells or PDUs arriving at an ingress port, and carrying 136 them across a path or tunnel. In some cases it is necessary to 137 perform other operation such as managing their timing and order, to 138 emulate the behavior and characteristics of the service to the 139 required degree of faithfulness. 141 From the perspective of a Customer Edge Equipment (CE), the PW is 142 characterised as an unshared link or circuit of the chosen service. 143 In some cases, there may be deficiencies in the PW emulation that 144 impact the traffic carried over a PW, and hence limit the 145 applicability of this technology. These limitations must be fully 146 described in the appropriate service-specific documentation. It is 147 possible that these limitations may lead to the definition of more 148 than one PW emulation method, each providing a different degree of 149 faithfulness. 151 1.2 PW Service Functionality 153 PWs provide the following functions in order to emulate the behavior 154 and characteristics of the native service. 155 o Encapsulation of service-specific PDUs or circuit data arriving 156 at the ingress port (logical or physical). 157 o Carriage of the encapsulated data across a PSN tunnel. 158 o Establishment of the PW including the exchange and/or 159 distribution of the PW identifiers used by the PSN 160 tunnel endpoints. 161 o Managing the signaling, timing, order or other aspects of the 162 service at the boundaries of the PW. 163 o Service-specific status and alarm management. 165 1.3 Non-Goals of this document 167 The following are non-goals for this document: 169 o The on-the-wire specification of services encapsulation. 170 o The detailed definition of the protocols involved in PW 171 setup and maintenance. 173 The following are outside the scope of PWE3: 174 o Any multicast service not native to the emulated medium. 175 Thus, Ethernet transmission to a "multicast" IEEE-48 address 176 is in scope, while multicast services like MARS [RFC2022] that 177 are implemented on top of the medium are out of scope. 178 o Methods to signal or control the underlying PSN. 180 1.4 Terminology 182 Editor's note: Although it was decided at IETF-54 that the PWE3 183 common terminology should be published in a separate document, there 184 is case for it remaining in the architecture document. If the PWE3-WG 185 confirms the desire to have a separate document, we will remove this 186 section in the next revision. 188 This document uses the following definitions of terms. These terms 189 are illustrated in context in Figure 2. 191 Attachment Circuit The circuit or virtual circuit attaching 192 (AC) a CE to a PE. 194 Applicability Each PW service will have an Applicability 195 Statement (AS) Statement (AS) that describes the applicability 196 of PWs for that service. 198 CE-bound The traffic direction where PW-PDUs are 199 received on a PW via the PSN, processed 200 and then sent to the destination CE. 202 CE Signaling Messages sent and received by the CEs 203 control plane. It may be desirable or 204 even necessary for the PE to participate 205 in or monitor this signaling in order 206 to effectively emulate the service. 208 Customer Edge (CE) A device where one end of a service 209 originates and/or terminates. The CE is not 210 aware that it is using an emulated service 211 rather than a native service. 213 Forwarder (FWRD) A PE subsystem that selects the PW to use to 214 transmit a payload received on an AC. 216 Fragmentation The action of dividing a single PDU into 217 multiple PDUs before transmission with the 218 intent of the original PDU being reassembled 219 elsewhere in the network. Fragmentation may be 220 performed in order to allow sending of packets 221 of a larger size than the network MTU which 222 they will traverse. 224 Maximum transmission The packet size (excluding data link header) 225 unit (MTU) that an interface can be transmit without 226 needing to fragment. 228 Native Service Processing of the data received by the PE 229 Processing (NSP) from the CE before presentation to the PW 230 for transmission across the core. 232 Packet Switched Within the context of PWE3, this is a 233 Network (PSN) network using IP or MPLS as the mechanism 234 for packet forwarding. 236 Protocol Data The unit of data output to, or received 237 Unit (PDU) from, the network by a protocol layer. 239 Provider Edge (PE) A device that provides PWE3 to a CE. 241 PE-bound The traffic direction where information 242 from a CE is adapted to a PW, and PW-PDUs 243 are sent into the PSN. 245 PE/PW Maintenance Used by the PEs to set up, maintain and 246 tear down the PW. It may be coupled with 247 CE Signaling in order to effectively manage 248 the PW. 250 Pseudo Wire (PW) A mechanism that carries the essential 251 elements of an emulated service from one PE 252 to one or more other PEs over a PSN. 254 PW End Service The interface between a PE and a CE. This 255 (PWES) can be a physical interface like a T1 or 256 Ethernet, or a virtual interface like a VC 257 or VLAN. 259 Pseudo Wire A mechanism that emulates the essential 260 Emulation Edge to attributes of service (such as a T1 leased 261 Edge (PWE3) line or frame relay) over a PSN. 263 Pseudo Wire PDU A PDU sent on the PW that contains all of 264 (PW-PDU) the data and control information necessary 265 to emulate the desired service. 267 PSN Tunnel A tunnel across a PSN inside which one or 268 more PWs can be carried. 270 PSN Tunnel Used to set up, maintain and tear down the 271 Signaling underlying PSN tunnel. 273 PW Demultiplexer Data-plane method of identifying a PW 274 terminating at a PE. 276 Time Domain Synchronous bit-streams at rates defined by 277 Multiplexing (TDM) G.702. 279 Tunnel A method of transparently carrying information 280 over a network. 282 2. PWE3 Applicability 284 The PSN carrying a PW will subject payload packets to delay, jitter, 285 network transients and re-ordering. The applicability of PWE3 to a 286 particular service depends on the sensitivity of that service to 287 these effects, and the ability of the adaptation layer to mask them. 288 Some services, such as IP over FR over PWE3, may prove quite 289 resilient to IP and MPLS PSN characteristics. Other services, such as 290 the interconnection of PBX systems via PWE3, will require more 291 careful consideration of the PSN and adaptation layer 292 characteristics. In some instances, traffic engineering of the 293 underlying PSN will be required, and in some cases the constraints 294 may be such that it is not possible to provide the required service 295 guarantees. 297 3. Protocol Layering Model 299 The PWE3 protocol-layering model is intended to minimise the 300 differences between PWs operating over different PSN types. The 301 design of the protocol-layering model has the goals of making each PW 302 definition independent of the underlying PSN, and maximizing the 303 reuse of IETF protocol definitions and their implementations. 305 3.1 Protocol Layers 307 The logical protocol-layering model required to support a PW is shown 308 in Figure 1. 310 +---------------------------+ 311 | Payload | 312 +---------------------------+ 313 | Encapsulation | <==== May be empty 314 +---------------------------+ 315 | PW Demultiplexer | 316 +---------------------------+ 317 | PSN Convergence | <==== May be empty 318 +---------------------------+ 319 | PSN | 320 +---------------------------+ 321 | MAC/Data-link | 322 +---------------------------+ 323 | Physical | 324 +---------------------------+ 326 Figure 1: Logical Protocol Layering Model 328 The payload is transported over the Encapsulation Layer. The 329 Encapsulation Layer carries any information, not already present 330 within the payload itself, that is needed by the PW CE-bound PE 331 interface to send the payload to the CE via the physical interface. 332 If no information is needed beyond that in the payload itself, this 333 layer is empty. 335 This layer also provides support for real-time processing, and 336 sequencing, if needed. 338 The PW Demultiplexer Layer provides the ability to deliver multiple 339 PWs over a single PSN tunnel. The PW demultiplexer value used to 340 identify the PW in the data-plane may be unique per PE, but this is 341 not a PWE3 requirement. It must, however, be unique per tunnel 342 endpoint. If it is necessary to identify a particular tunnel, then 343 that is the responsibility of the PSN layer. 345 The PSN Convergence Layer provides the enhancements needed to make 346 the PSN conform to the assumed PSN service requirement. This layer 347 therefore provides a consistent interface to the PW, making the PW 348 independent of the PSN type. If the PSN already meets the service 349 requirements, this layer is empty. 351 The PSN header, MAC/Data-link and Physical Layer definitions are 352 outside the scope of this document. The PSN can be IPv4, IPv6 or 353 MPLS. 355 3.2 Domain of PWE3 357 PWE3 defines the Encapsulation Layer, the method of carrying various 358 payload types, and the interface to the PW Demultiplexer Layer. It 359 is expected that the other layers will be provided by tunneling 360 methods such as L2TP or MPLS over the PSN. 362 3.3 Payload Types 364 The payload is classified into the following generic types of native 365 data unit: 367 o Packet 368 o Cell 369 o Bit-stream 370 o Structured bit-stream 372 Within these generic types there are specific service types. For 373 example: 375 Generic Payload Type PW Service 376 -------------------- ---------- 377 Packet Ethernet (all types), HDLC, 378 frame-relay, ATM AAL5 PDU. 380 Cell ATM. 382 Bit-stream SONET, TDM (e.g. DS1, DS3, E1). 384 Structured bit-stream SONET, TDM. 386 3.3.1. Packet Payload 388 A packet payload is a variable-size data unit presented to the PE on 389 the AC. A packet payload may be large compared to the PSN MTU. The 390 delineation of the packet boundaries is encapsulation-specific. HDLC 391 or Ethernet PDUs can be considered as examples of packet payloads. 392 Typically a packet will be stripped of transmission overhead such as 393 HDLC flags and stuffing bits before transmission over the PW. 395 A packet payload would normally be relayed across the PW as a single 396 unit. However, there will be cases where the combined size of the 397 packet payload and its associated PWE3 and PSN headers exceeds the 398 PSN path MTU. In these cases, some fragmentation methodology needs 399 to be applied. This may, for example, be the case when a user is 400 providing the service and attaching to the service provider via an 401 Ethernet, or where nested pseudo-wires are involved. Fragmentation is 402 discussed in more detail in Section 5. 404 A packet payload may need sequencing and real-time support. 406 In some situations, the packet payload may be selected from the 407 packets presented on the emulated wire on the basis of some sub- 408 multiplexing technique. For example, one or more frame-relay PDUs 409 may be selected for transport over a particular pseudo-wire based on 410 the frame-relay Data-Link Connection Identifier (DLCI), or, in the 411 case of Ethernet payloads, on the basis of the VLAN identifier. This 412 is an FWRD function, and this selection would therefore be made 413 before the packet was presented to the PW Encapsulation Layer. 415 3.3.2. Cell Payload 417 A cell payload is created by capturing, transporting and replaying 418 groups of bits presented on the wire in a fixed-size format. The 419 delineation of the group of bits that comprise the cell is specific 420 to the encapsulation type. Two common examples of cell payloads are 421 53-octet cells carrying ATM AAL2, and the larger 188-octet MPEG 422 Transport Stream packets [ETSI]. 424 To reduce per-PSN packet overhead, multiple cells may be concatenated 425 into a single payload. The Encapsulation Layer may consider the 426 payload complete on the expiry of a timer, or after a fixed number of 427 cells have been received. The benefit of concatenating multiple PDUs 428 should be weighed against the resulting increase in jitter and the 429 larger penalty incurred by packet loss. In some cases, it may be 430 appropriate for the Encapsulation Layer to perform a silence 431 suppression or a similar compression. 433 The generic cell payload service will normally need sequence number 434 support, and may also need real-time support. The generic cell 435 payload service would not normally require fragmentation. 437 The Encapsulation Layer may apply some form of compression to some of 438 these sub-types. 440 In some instances, the cells to be incorporated in the payload may be 441 selected by filtering them from the stream of cells presented on the 442 wire. For example, an ATM PWE3 service may select cells based on 443 their VCI or VPI fields. This is an NSP function, and the selection 444 would therefore be made before the packet was presented to the PW 445 Encapsulation Layer. 447 3.3.3. Bit-stream 449 A bit-stream payload is created by capturing, transporting and 450 replaying the bit pattern on the emulated wire, without taking 451 advantage of any structure that, on inspection, may be visible within 452 the relayed traffic (i.e. the internal structure has no effect on the 453 fragmentation into packets). The Encapsulation Layer submits an 454 identical number of bits for transport in each PW-PDU. 456 This service will require sequencing and real-time support. 458 3.3.4. Structured bit-stream 460 A bit-stream payload is created by using some knowledge of the 461 underlying structure of the bit-stream to capture, transport and 462 replay the bit pattern on the emulated wire (i.e. the internal 463 structure directly effects the fragmentation into packets). 465 Two important points distinguish structured and unstructured bit- 466 streams: 468 o Some part of the original (unstructured) bit-stream are 469 stripped by, for example, the PSN-bound direction of the 470 NSP block. For example, in Structured SONET the section 471 and line overhead (and, possibly, more) may be stripped. 473 o The PW must preserve the structure across the PSN so that 474 the CE-bound NSP block can insert it correctly into the 475 reconstructed unstructured bit-stream. 477 The Encapsulation Layer may also perform silence/idle suppression or 478 similar compression on a structured bit-stream. 480 Structured bit-streams are distinguished from cells in that the 481 structures may be too long to be carried in a single packet (i.e. 482 structured SONET). Note that "short" structures are 483 indistinguishable from cells and may benefit from the use of cell 484 encapsulations. 486 This service will require sequencing and real-time support. 488 3.3.5. Principle of Minimum Intervention 490 To minimise the scope of information, and to improve the efficiency 491 of data flow through the Encapsulation Layer, the payload should be 492 transported as received with as few modifications as possible 493 [RFC1958]. 495 This minimum intervention approach decouples payload development from 496 PW development and requires fewer translations at the NSP in a system 497 with similar CE interfaces at each end. It also prevents any 498 unwanted side-effects due to subtle mis-representation of the payload 499 in the intermediate format. 501 An intervention approach can be more wire-efficient in some cases and 502 may result in fewer translations at the NSP where the CE interfaces 503 are of different types. Any intermediate format effectively becomes a 504 new framing type, requiring documentation and assured 505 interoperability. This increases the amount of work for handling the 506 protocol the intermediate format carries, and is undesirable. 508 4. Architecture of Pseudo-wires 510 This section describes the PWE3 architectural model. 512 4.1 Network Reference Model 514 Figure 2 illustrates the network reference model for point-to-point 515 PWs. 517 |<-------------- Emulated Service ---------------->| 518 | | 519 | |<------- Pseudo Wire ------>| | 520 | | | | 521 | | |<-- PSN Tunnel -->| | | 522 | PW End V V V V PW End | 523 V Service +----+ +----+ Service V 524 +-----+ | | PE1|==================| PE2| | +-----+ 525 | |----------|............PW1.............|----------| | 526 | CE1 | | | | | | | | CE2 | 527 | |----------|............PW2.............|----------| | 528 +-----+ ^ | | |==================| | | ^ +-----+ 529 ^ | +----+ +----+ | | ^ 530 | | Provider Edge 1 Provider Edge 2 | | 531 | | | | 532 Customer | | Customer 533 Edge 1 | | Edge 2 534 | | 535 | | 536 native service native service 538 Figure 2: PWE3 Network Reference Model 540 The two PEs (PE1 and PE2) need to provide one or more PWs on behalf 541 of their client CEs (CE1 and CE2) to enable the client CEs to 542 communicate over the PSN. A PSN tunnel is established to provide a 543 data path for the PW. The PW traffic is invisible to the core 544 network, and the core network is transparent to the CEs. Native data 545 units (bits, cells or packets) presented at the PW End Service (PWES) 546 are encapsulated in a PW-PDU and carried across the underlying 547 network via the PSN tunnel. The PEs perform the necesssary 548 encapsulation and decapsulation of PW-PDUs, as well as handling any 549 other functions required by the PW service, such as sequencing or 550 timing. 552 There are situations in which a particular packet payload needs to be 553 multicast so that it is received by a number of CEs. This is useful 554 when using PWs as part of a "virtual LAN" service (see, e.g., 556 [VPLS]). This can be achieved by replicating the payload and 557 transmitting the replicas on PWs, but it may also be useful to have a 558 type of PW which is inherently point-to-multipoint. In that case, 559 the PW would need to be carried through a point-to-multipoint PSN 560 tunnel, employing a multicast mechanism provided by the PSN. 562 4.2 PWE3 Pre-processing 564 In some applications, there is a need to perform operations on the 565 native data units received from the CE (including both payload and 566 signaling traffic) before they are transmitted across the PW by the 567 PE. Examples include Ethernet bridging, SONET cross-connect, 568 translation of locally-significant identifiers such as VCI/VPI, or 569 translation to another service type. These operations could be 570 carried out in external equipment, and the processed data sent to the 571 PE over one or more physical interfaces. In most cases, there are 572 cost and operational benefits in undertaking these operations within 573 the PE. This processed data is then presented to the PW via a 574 virtual interface within the PE. 576 These pre-processing operations are included in the PWE3 reference 577 model to provide a common reference point, but the detailed 578 description of these operations is outside the scope of the PW 579 definition given here. 581 PW 582 End Service 583 | 584 |<------- Pseudo Wire ------>| 585 | | 586 | |<-- PSN Tunnel -->| | 587 V V V V PW 588 +-----+----+ +----+ End Service 589 +-----+ |PREP | PE1|==================| PE2| | +-----+ 590 | | | |............PW1.............|----------| | 591 | CE1 |----| | | | | | | CE2 | 592 | | ^ | |............PW2.............|----------| | 593 +-----+ | | | |==================| | | ^ +-----+ 594 | +-----+----+ +----+ | | 595 | ^ | | 596 | | | | 597 | |<------- Emulated Service ------->| | 598 | | | 599 | Virtual physical | 600 | termination | 601 | ^ | 602 CE1 native | CE2 native 603 service | service 604 | 605 CE2 native 606 service 608 Figure 3: Pre-processing within the PWE3 Network Reference Model 610 Figure 3 shows the inter-working of one PE with pre-processing 611 (PREP), and a second without this functionality. This is a useful 612 reference point because it emphasises that the functional interface 613 between PREP and the PW is that represented by a physical interface 614 carrying the service. This effectively defines the necessary inter- 615 working specification. 617 The operation of a system in which both PEs include PREP 618 functionality is also supported. 620 The required pre-processing can be divided into two components: 621 o Forwarder (FWRD) 623 o Native Service Processing (NSP) 625 4.2.1. Forwarders 627 In some applications there is the need to selectively forward payload 628 elements from one of more ACs to one or more PWs. In such cases there 629 will also be the need to perform the inverse function on PWE3-PDUs 630 received by a PE from the PSN. This is the function of the FWRD. 632 The FWRD selects the PW based on, for example: the incoming AC, the 633 contents of the payload, or some statically- or dynamically- 634 configured forwarding information. 636 +----------------------------------------+ 637 | PE Device | 638 +----------------------------------------+ 639 Single | | | 640 PWES | | Single | PW Instance 641 <------>o Forwarder + PW Instance X<===========> 642 | | | 643 +----------------------------------------+ 645 Figure 4a: Simple point-to-point service 647 +----------------------------------------+ 648 | PE Device | 649 +----------------------------------------+ 650 Multiple| | Single | PW Instance 651 PWES | + PW Instance X<===========> 652 <------>o | | 653 | |----------------------| 654 <------>o | Single | PW Instance 655 | + PW Instance X<===========> 656 <------>o | | 657 | Forwarder |----------------------| 658 <------>o | Single | PW Instance 659 | + PW Instance X<===========> 660 <------>o | | 661 | |----------------------| Multipoint 662 | | Multipoint | PW Instance 663 | + PW Instance X<===========> 664 | | | 665 +----------------------------------------+ 667 Figure 4b: Multiple PWEs to Multiple PW Forwarding 669 Figure 4a shows a simple FWRD that performs some type of filtering 670 operation. Because the FWRD has a single input and a single output 671 interface, filtering is the only type of forwarding operation that 672 applies. Figure 4b shows a more general forwarding situation where 673 payloads are extracted from one or more PWESs and directed to one or 674 more PWs, including, in this instance, a multipoint PW. In this case 675 both filtering and direction operations may be performed on the 676 payloads. 678 4.2.2. Native Service Processing 680 In some applications some form of data or address translation, or 681 other operation requiring knowledge of the semantics of the payload, 682 will be required. This is the function of the Native Service 683 Processor (NSP). 685 The use of the NSP approach simplifies the design of the PW by 686 restricting a PW to homogeneous operation. NSP is included in the 687 reference model to provide a defined interface to this functionality. 688 The specification of the various types of NSP is outside the scope of 689 PWE3. 691 +----------------------------------------+ 692 | PE Device | 693 Multiple+----------------------------------------+ 694 PWES | | | Single | PW Instance 695 <------>o NSP # + PW Instance X<===========> 696 | | | | 697 |------| |----------------------| 698 | | | Single | PW Instance 699 <------>o NSP # + PW Instance X<===========> 700 | | | | 701 |------|Forwarder |----------------------| 702 | | | Single | PW Instance 703 <------>o NSP # + PW Instance X<===========> 704 | | | | 705 |------| |----------------------| Multipoint 706 | | | Multipoint | PW Instance 707 <------>o NSP # + PW Instance X<===========> 708 | | | | 709 +----------------------------------------+ 711 Figure 5: NSP in a Multiple PWEs to Multiple 712 PW Forwarding PE 714 Figure 5 illustrates the relationship between NSP, FWRD and PWs in a 715 PE. The NSP function may apply any transformation operation 716 (modification, injection, etc.) on the payloads as they pass between 717 the physical interface to the CE and the virtual interface to the 718 FWRD. A PE device may contain more than one FWRD. 720 This model also supports the operation of a system in which the NSP 721 functionality includes terminating the data-link and applying Network 722 Layer processing to the payload is also supported. 724 4.3 Maintenance Reference Model 726 Figure 6 illustrates the maintenance reference model for PWs. 728 |<------- CE (end-to-end) Signaling ------>| 729 | |<---- PW/PE Maintenance ----->| | 730 | | |<-- PSN Tunnel -->| | | 731 | | | Signaling | | | 732 | V V (out of scope) V V | 733 v +-----+ +-----+ v 734 +-----+ | PE1 |==================| PE2 | +-----+ 735 | |-----|.............PW1..............|-----| | 736 | CE1 | | | | | | CE2 | 737 | |-----|.............PW2..............|-----| | 738 +-----+ | |==================| | +-----+ 739 +-----+ +-----+ 740 Customer Provider Provider Customer 741 Edge 1 Edge 1 Edge 2 Edge 2 743 Figure 6: PWE3 Maintenance Reference Model 745 The following signaling mechanisms are required: 747 o The CE (end-to-end) signaling is between the CEs. This 748 signaling could be frame relay PVC status signaling, ATM SVC 749 signaling, etc. 751 o The PW/PE Maintenance is used between the PEs (or NSPs) to set 752 up, maintain and tear down PWs, including any required 753 coordination of parameters. 755 o The PSN Tunnel signaling controls the PW multiplexing and some 756 elements of the underlying PSN. Examples are L2TP control 757 protocol, MPLS LDP and RSVP-TE. The definition of the 758 information that PWE3 needs to be signaled is within the scope 759 of PWE3, but the signaling protocol itself is not. 761 4.4 Protocol Stack Reference Model 763 Figure 7 illustrates the protocol stack reference model for PWs. 765 +-----------------+ +-----------------+ 766 |Emulated Service | |Emulated Service | 767 |(e.g. TDM, ATM) |<==== Emulated Service ===>|(e.g. TDM, ATM) | 768 +-----------------+ +-----------------+ 769 | Payload | | Payload | 770 | Encapsulation |<====== Pseudo Wire ======>| Encapsulation | 771 +-----------------+ +-----------------+ 772 |PW Demultiplexer | |PW Demultiplexer | 773 | PSN Tunnel, |<======= PSN Tunnel ======>| PSN Tunnel, | 774 | PSN & Physical | | PSN & Physical | 775 | Layers | | Layers | 776 +-------+---------+ ___________ +---------+-------+ 777 | / \ | 778 +===============/ PSN \===============+ 779 \ / 780 \_____________/ 782 Figure 7: PWE3 Protocol Stack Reference Model 784 The PW provides the CE with an emulated physical or virtual 785 connection to its peer at the far end. Native service PDUs from the 786 CE are passed through an Encapsulation Layer at the sending PE, and 787 then sent over the PSN. The receiving PE removes the encapsulation 788 and restores the payload to its native format for transmission to the 789 destination CE. 791 4.5 Pre-processing Extension to Protocol Stack Reference Model 793 Figure 8 illustrates how the protocol stack reference model is 794 extended to include the provision of pre-processing (Forwarding and 795 NSP). This shows the ideal placement of the physical interface 796 relative to the CE. 798 /======================================\ 799 H Forwarder H<----Pre-processing 800 H----------------======================/ 801 H Native Service H | | 802 H Processing H | | 803 \================/ | | 804 | | | Emulated | 805 | Service | | Service | 806 | Interface | | (TDM, ATM, | 807 | (TDM, ATM, | | Ethernet, |<== Emulated Service == 808 | Ethernet, | | frame relay, | 809 | frame relay, | | etc.) | 810 | etc.) | +-----------------+ 811 | | | Payload | 812 | | | Encapsulation |<=== Pseudo Wire ====== 813 | | +-----------------+ 814 | | |PW Demultiplexer | 815 | | | PSN Tunnel, | 816 | | | PSN & Physical |<=== PSN Tunnel ======= 817 | | | Headers | 818 +----------------+ +-----------------+ 819 | Physical | | Physical | 820 +-------+--------+ +-------+---------+ 821 | | 822 | | 823 | | 824 | | 825 | | 826 | | 827 To CE <---+ +---> To PSN 829 Figure 8: Protocol Stack Reference Model with Pre-processing 831 5. PW Encapsulation 833 The PW Encapsulation Layer provides the necessary infrastructure to 834 adapt the specific payload type being transported over the PW to the 835 PW Demultiplexer Layer that is used to carry the PW over the PSN. 837 The PW Encapsulation Layer consists of three sub-layers: 839 o Payload Convergence 840 o Timing 841 o Sequencing 843 The PW Encapsulation sub-layering and its context with the protocol 844 stack are shown, in Figure 9. 846 +---------------------------+ 847 | Payload | 848 /===========================\ <------ Encapsulation 849 H Payload Convergence H Layer 850 H---------------------------H 851 H Timing H 852 H---------------------------H 853 H Sequencing H 854 \===========================/ 855 | PW Demultiplexer | 856 +---------------------------+ 857 | PSN Convergence | 858 +---------------------------+ 859 | PSN | 860 +---------------------------+ 861 | MAC/Data-link | 862 +---------------------------+ 863 | Physical | 864 +---------------------------+ 866 Figure 9: PWE3 Encapsulation Layer in Context 868 The Payload Convergence Sub-layer is highly tailored to the specific 869 payload type, but, by grouping a number of target payload types into 870 a generic class, and then providing a single convergence sub-layer 871 type common to the group, we achieve a reduction in the number of 872 payload convergence sub-layer types. This decreases implementation 873 complexity. The provision of per-packet signaling and other out-of- 874 band information (other than sequencing or timing) is undertaken by 875 this layer. 877 The Timing Layer and the Sequencing Layer provide generic services to 878 the Payload Convergence Layer for all payload types, when required. 880 5.1 Payload Convergence Layer 882 5.1.1. Encapsulation 884 The primary task of the Payload Convergence Layer is the 885 encapsulation of the payload in PW-PDUs. The native data units to be 886 encapsulated may or may not contain L2 or L1 header information. 887 This is service specific. The Payload Convergence header carries the 888 additional information needed to replay the native data units at the 889 CE-bound physical interface. The PW Demultiplexer header is not 890 considered as part of the PW header. 892 Not all the additional information needed to replay the native data 893 units need to be carried in the PW header of the PW PDUs. Some 894 information (e.g. service type of a PW) may be stored as state 895 information at the destination PE during PW set-up. 897 5.1.2. PWE3 Channel Types 899 The PW Encapsulation Layer and its associated signaling require one 900 or more of the following types of channels from its underlying PW 901 Demultiplexer and PSN Layers: 903 1. A reliable control channel for signaling line events, status 904 indications, and, in some exceptional cases, CE-CE events 905 that must be translated and sent reliably between PEs. 907 For example, this capability is needed in [PPPoL2TP] 908 (PPP negotiation has to be split between the two ends of the 909 tunnel). PWE3 may also need this type of control channel to 910 provide faithful emulation of complex data-link protocols. 912 plus one or more data channels with the following characteristics: 914 2. A high-priority, unreliable, sequenced channel. A typical use 915 is for CE-to-CE signaling. "High priority" may simply be 916 indicated via DSCP/EXP bits for priority during transit. 917 This channel type could also use a bit in the tunnel header 918 itself to indicate that packets received at the PE should be 919 processed with higher 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-path data traffic is blocked by one or 936 more of these mechanisms. In these cases unless the control channel 937 is also carried "in band" the signaling to set-up the PW will not 938 confirm the existence of an end-to-end data path. 940 In some cases there is a need to synchronize some CE events with the 941 data carried over a PW. This is especially the case with TDM 942 circuits (e.g., on-hook/off-hook events in PSTN switches). 944 PWE3 channel types that are not needed by the supported PWs need not 945 be included in such an implementation. 947 5.1.3. Quality of Service Considerations 949 Where possible, it is desirable to employ mechanisms to provide PW 950 Quality of Service (QoS) support over PSNs. 952 5.2 Payload-independent PW Encapsulation Layers 954 Two PWE3 Encapsulation Sub-layers provide common services to all 955 payload types: Sequencing and Timing. These services are optional 956 and are only used if needed by a particular PW instance. If the 957 service is not needed, the associated header may be omitted in order 958 to conserve processing and network resources. 960 There will be instances where a specific payload type will be 961 required to be transported with or without sequence and/or real-time 962 support. For example, an invariant of frame relay transport is the 963 preservation of packet order. Some frame-relay applications expect 964 in-order delivery, and may not cope with reordering of the frames. 965 However, where the frame relay service is itself only being used to 966 carry IP, it may be desirable to relax that constraint in return for 967 reduced per-packet processing cost. 969 The guiding principle is that, where possible, an existing IETF 970 protocol should be used to provide these services. Where a suitable 971 protocol is not available, the existing protocol should be extended 972 or modified to meet the PWE3 requirements, thereby making that 973 protocol available for other IETF uses. In the particular case of 974 timing, more than one general method may be necessary to provide for 975 the full scope of payload timing requirements. 977 5.2.1. Sequencing 979 The sequencing function provides three services: frame ordering, 980 frame duplication detection and frame loss detection. These services 981 allow the invariant properties of a physical wire to be emulated. 982 Support for sequencing depends on the payload type, and may be 983 omitted if not needed. 985 The size of the sequence-number space depends on the speed of the 986 emulated service, and the maximum time of the transient conditions in 987 the PSN. A sequence number space greater than approximately 2^16 may 988 therefore be needed to prevent the sequence number space wrapping 989 during the transient. 991 5.2.1.1 Frame Ordering 993 When packets carrying the PW-PDUs traverse a PSN, they may arrive out 994 of order at the destination PE. For some services, the frames 995 (control frames, data frames, or both control and data frames) must 996 be delivered in order. For such services, some mechanism must be 997 provided for ensuring in-order delivery. Providing a sequence number 998 in the sequence sub-layer header for each packet is one possible 999 approach to out-of-sequence detection. Alternatively it can be noted 1000 that sequencing is a subset of the problem of delivering timed 1001 packets, and that a single combined mechanism such as [RFC1889] may 1002 be employed. 1004 There are two possible misordering strategies: 1006 o Drop misordered PW PDUs. 1008 o Try to sort PW PDUs into the correct order. 1010 The choice of strategy will depend on: 1012 o How critical the loss of packets is to the operation of 1013 the PW (e.g. the acceptable bit error rate). 1015 o The speeds of the PW and PSN. 1017 o The acceptable delay (since delay must be introduced to 1018 reorder) 1020 o The incidence of expected misordering. 1022 5.2.1.2 Frame Duplication Detection 1024 In rare cases, packets traversing a PW may be duplicated by the 1025 underlying PSN. For some services, frame duplication is not 1026 acceptable. For such services, some mechanism must be provided to 1027 ensure that duplicated frames will not be delivered to the 1028 destination CE. The mechanism may or may not be the same as the 1029 mechanism used to ensure in-order frame delivery. 1031 5.2.1.3 Frame Loss Detection 1033 A destination PE can determine whether a frame has been lost by 1034 tracking the sequence numbers of the received PW PDUs. 1036 In some instances, a destination PE will have to presume that a PW 1037 PDU is lost if it fails to arrive within a certain time. If a PW-PDU 1038 that has been processed as lost subsequently arrives, the destination 1039 PE must discard it. 1041 5.2.2. Timing 1043 A number of native services have timing expectations based on the 1044 characteristics of the networks that they were designed to travel 1045 over, and it can be necessary for the emulated service to duplicate 1046 these network characteristics as closely as possible, e.g. in 1047 delivering native traffic with the same jitter, bit-rate and timing 1048 characteristics as it was sent. 1050 In such cases, it is necessary for the receiving PE to play out the 1051 native traffic as it was received at the sending PE. This relies on 1052 either timing information sent between the two PEs, or in some case 1053 timing information received from an external reference. 1055 The Timing Sub-layer must therefore support two timing functions: 1056 clock recovery and timed payload delivery. A particular payload type 1057 may require either or both of these services. 1059 5.2.2.1 Clock Recovery 1061 Clock recovery is the extraction of output transmission bit timing 1062 information from the delivered packet stream, and requires a phase- 1063 locking mechanism. A physical wire provides this naturally, but it 1064 is a relatively complex task to extract this from a highly jittered 1065 source such as packet stream. It is therefore desirable that an 1066 existing real-time protocol such as [RFC1889] be used for this 1067 purpose, unless it can be shown that this is unsuitable or 1068 unnecessary for a particular payload type. 1070 5.2.2.2 Timed delivery 1072 Timed delivery is the delivery of non-contiguous PW PDUs to the PW 1073 output interface with a constant phase relative to the input 1074 interface. The timing of the delivery may be relative to a clock 1075 derived from the packet stream via clock recovery, or via an external 1076 clock. 1078 5.3 Fragmentation 1080 A payload would normally be relayed across the PW as a single unit. 1081 However, there will be cases where the combined size of the payload 1082 and its associated PWE3 and PSN headers exceeds the PSN path MTU. 1083 When a packet exceeds the MTU of a given network, fragmentation and 1084 reassembly may have to be performed in order for the packet to be 1085 delivered. Since fragmentation and reassembly generally consume a 1086 large amount of network resource as compared to simply switching a 1087 packet in its entirety, efforts should be made to reduce or eliminate 1088 the need for fragmentation and reassembly as much as possible 1089 throughout a network. Of particular concern for fragmentation and 1090 reassembly are aggregation points where large numbers of pseudowires 1091 are processed (e.g. at the PE). 1093 Ideally, the equipment originating the traffic being sent over the PW 1094 will be configured to have adaptive measures (e.g. [RFC1191], 1095 [RFC1981]) in place such that it never sends a packet which must be 1096 fragmented. When this fails, the point closest to the sending host 1097 with fragmentation and reassembly capabilities should attempt to 1098 reduce the size of packets further into the network. Thus, in the 1099 reference model for PWE3 [Figure 3] fragmentation should first be 1100 performed at the CE if at all possible. If and only if the CE cannot 1101 adhere to an acceptable MTU size for the PW should the PE attempt its 1102 own fragmentation method. 1104 In cases where MTU management fails to limit the payload to a size 1105 suitable for transmission of the PW, the PE may fall back to either a 1106 generic PW fragmentation method, or, if available the fragmentation 1107 service of the underlying PSN. 1109 It is acceptable for a PE implementation to not support 1110 fragmentation. A PE that does not support fragmentation will drop 1111 packets that exceed the PSN MTU, and the management plane of the 1112 encapsulating PE may be notified. 1114 If the length of a L2/L1 frame, restored from a PW PDU, exceeds the 1115 MTU of the destination PWES, it must be dropped. In this case, the 1116 management plane of the destination PE may be notified. 1118 5.4 Instantiation of the Protocol Layers 1120 This document does not address the detailed mapping of the Protocol 1121 Layering model to existing or future IETF standards. The 1122 instantiation of the logical Protocol Layering model is shown in 1123 Figure 9. 1125 5.4.1. PWE3 over an IP PSN 1127 The protocol definition of PWE3 over an IP PSN should employ existing 1128 IETF protocols where possible. 1130 +---------------------+ +-------------------------+ 1131 | Payload |------------->| Raw payload if possible | 1132 /=====================\ +-------------------------+ 1133 H Payload Convergence H------------->| As Needed | 1134 H---------------------H +-------------------------+ 1135 H Timing H----------+-->| RTP | 1136 H---------------------H / +-------------+ | 1137 H Sequencing H----one of | | 1138 \=====================/ \ | +-----------+ 1139 | PW Demultiplexer |----------+-->| L2TP, MPLS etc. | 1140 +---------------------+ +-------------------------+ 1141 | PSN Convergence |------------->| Not needed | 1142 +---------------------+ +-------------------------+ 1143 | PSN |------------->| IP | 1144 +---------------------+ +-------------------------+ 1145 | MAC/Data-link |------------->| MAC/Data-link | 1146 +---------------------+ +-------------------------+ 1147 | Physical |------------->| Physical | 1148 +---------------------+ +-------------------------+ 1150 Figure 10: PWE3 over an IP PSN 1152 Figure 10 shows the protocol layering for PWE3 over an IP PSN. As a 1153 rule, the payload should be carried as received from the NSP, with 1154 the Payload Convergence Layer provided when needed. (It is accepted 1155 that there may sometimes be good reason not to follow this rule, but 1156 the exceptional circumstances need to be documented in the 1157 Encapsulation Layer definition for that payload type). 1159 Where appropriate, timing is provided by RTP [RFC1889], which when 1160 used also provides a sequencing service. PW Demultiplexing may be 1161 provided by a number of existing IETF tunnel protocols. Some of 1162 these tunnel protocols provide an optional sequencing service. 1163 (Sequencing is provided either by RTP, or by the PW Demultiplexer 1164 Layer, but not both). A PSN Convergence Layer is not needed, because 1165 all the tunnel protocols shown above are designed to operate directly 1166 over an IP PSN. 1168 As a special case, if the PW Demultiplexer is an MPLS label, the 1169 protocol architecture of section 5.4.2 can be used instead of the 1170 protocol architecture of this section. 1172 5.4.2. PWE3 over an MPLS PSN 1174 The MPLS ethos places importance on wire efficiency. By using a 1175 control word, some components of the PWE3 protocol layers can be 1176 compressed to increase this efficiency. 1178 +---------------------+ 1179 | Payload | 1180 /=====================\ 1181 H Payload Convergence H-----------------+ 1182 H---------------------H | 1183 H Timing H | 1184 H---------------------H | 1185 H Sequencing H-----------------| 1186 \=====================/ | 1187 | PW Demultiplexer |---+ | 1188 +---------------------+ | | 1189 | PSN Convergence |-----------------| 1190 +---------------------+ | | 1191 | PSN |-+ | v 1192 +---------------------+ | | +--------------------------------+ 1193 | MAC/Data-link | | | | Flags, Frag, Len, Seq #, etc | 1194 +---------------------+ | | +--------------------------------+ 1195 | Physical | | +->| Inner Label | 1196 +---------------------+ | +--------------------------------+ 1197 +--->| Outer Label or MPLS-in-IP encap| 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. 1215 In some networks it may be necessary to carry PWE3 over MPLS over IP. 1216 In these circumstances, the PW is encapsulated for carriage over MPLS 1217 as described in this section, and then a standard method of carrying 1218 MPLS over an IP PSN (such as GRE [RFC2784], [RFC2890]) is applied to 1219 the resultant PW-PDU. 1221 6. PW Demultiplexer Layer and PSN Requirements 1223 PWE3 places three service requirements on the protocol layers used to 1224 carry it across the PSN: 1226 o Multiplexing 1227 o Fragmentation 1228 o Length and Delivery 1230 6.1 Multiplexing 1232 The purpose of the PW Demultiplexer Layer is to allow multiple PWs to 1233 be carried in a single tunnel. This minimizes complexity and 1234 conserves resources. 1236 Some types of native service are capable of grouping multiple 1237 circuits into a "trunk", e.g. multiple ATM VCs in a VP, multiple 1238 Ethernet VLANs on a physical media, or multiple DS0 services within a 1239 T1 or E1. A PW may interconnect two end-trunks. That trunk would 1240 have a single multiplexing value. 1242 6.2 Fragmentation 1244 If the PSN provides a fragmentation and reassembly service of 1245 adequate performance, it MAY be used to obtain an effective MTU that 1246 is large enough to transport the PW PDUs. However, if the PSN does 1247 not offer an adequate service, and fragmentation at the PE cannot be 1248 avoided by any other means, then a PW-specific fragmentation method 1249 may be utilized here. See Section 5.3 for more details. 1251 6.3 Length and Delivery 1253 PDU delivery to the egress PE is the function of the PSN Layer. 1255 If the underlying PSN does not provide all the information necessary 1256 to determine the length of a PW-PDU, the Encapsulation Layer will 1257 provide it. 1259 6.4 PW-PDU Validation 1261 It is a common practice to use a CRC or similar mechanism to assure 1262 end-to-end integrity of frames. The PW service-specific mechanisms 1263 MUST define whether the packet's checksum shall be preserved across 1264 the PW or be removed from PE bound PDUs and then be re-calculated for 1265 insertion in CE bound data. 1267 The former approach saves work, while the latter saves bandwidth. For 1268 a given implementation the choice may be dictated by hardware 1269 restrictions. 1271 For protocols such as ATM and FR, the scope of the checksum is 1272 restriced to a single link. This is because the circuit identifiers 1273 (e.g. FR DLCI or ATM VPI/VCI) have only local significance and are 1274 changed on each hop or span. If the circuit identifier (and thus 1275 checksum) were going to change as a part of the PW emulation, it 1276 would be more efficient to strip and re-calculate the checksum. 1278 The service specific document for each protocol must describe the 1279 validation scheme to be used. 1281 6.5 Congestion Considerations 1283 The PSN carrying the PW may be subject to congestion. The congestion 1284 characteristics will vary with the PSN type, the network architecture 1285 and configuration, and the loading of the PSN. 1287 Each service specific document will have to specify whether it needs 1288 an appropriate mechanism for operating in the presence of this 1289 congestion, including methods of mapping between its native 1290 congestion reporting and avoidance mechanisms, and those provided by 1291 the PW. 1293 7. Control Plane 1295 This section describes PWE3 control plane services. 1297 7.1 Set-up or Teardown of Pseudo-Wires 1299 A PW must be set up before an emulated service can be established, 1300 and must be torn down when an emulated service is no longer needed. 1302 Set up or teardown of a PW can be triggered by a CLI command, from 1303 the management plane of a PE, by signaling (i.e., set-up or teardown) 1304 of a PWES, e.g., an ATM SVC, or by an auto-discovery mechanism. 1306 During the set-up process, the PEs need to exchange some information 1307 (e.g. learn each others' capabilities). The tunnel signalling 1308 protocol may be extended to provide mechanisms to enable the PEs to 1309 exchange all necessary information on behalf of the PW. 1311 Manual configuration of PWs can be considered a special kind of 1312 signaling, and is allowed. 1314 7.2 Status Monitoring 1316 Some native services have mechanisms for status monitoring. For 1317 example, ATM supports OAM for this purpose. For such services, the 1318 corresponding emulated services must specify how to perform status 1319 monitoring. 1321 7.3 Notification of Pseudo-wire Status Changes 1323 7.3.1. Pseudo-wire Up/Down Notification 1325 If a native service requires bi-directional connectivity, the 1326 corresponding emulated service can only be signaled up when the 1327 associated PWs, and PSN tunnels if any, are functional in both 1328 directions. 1330 Because the two CEs of an emulated service are not adjacent, a 1331 failure may occur at a place such that one or both physical links 1332 between the CEs and PEs remain up. For example, in Figure 2, if the 1333 physical link between CE1 and PE1 fails, the physical link between 1334 CE2 and PE2 will not be affected and will remain up. Unless CE2 is 1335 notified about the remote failure, it will continue to send traffic 1336 over the emulated service to CE1. Such traffic will be discarded at 1337 PE1. Some native services have failure notification so that when the 1338 services fail, both CEs will be notified. For such native services, 1339 the corresponding PWE3 service must provide a failure notification 1340 mechanism. 1342 Similarly, if a native service has notification mechanisms so that 1343 when a network failure is fixed, all the affected services will 1344 change status from "Down" to "Up", the corresponding emulated service 1345 must provide a similar mechanism for doing so. 1347 These mechanisms may already be built into the tunneling protocol. 1348 For example, the L2TP control protocol [RFC2661] [L2TPv3] has this 1349 capability and LDP has the ability to withdraw the corresponding MPLS 1350 label. 1352 7.3.2. Misconnection and Payload Type Mismatch 1354 With PWE3, misconnection and payload type mismatch can occur. If a 1355 misconnection occurs it can breach the integrity of the system. If a 1356 payload mismatch occurs it can disrupt the customer network. In both 1357 instances, there are security and operational concerns. 1359 The services of the underlying tunneling mechanism, and its 1360 associated control protocol, can be used to mitigate this. As part 1361 of the PW set-up a PW-TYPE identifier is exchanged. This is then used 1362 by the FWRD and NSP to verify the compatibility of the PWESs. 1364 7.3.3. Packet Loss, Corruption, and Out-of-order Delivery 1366 A PW can incur packet loss, corruption, and out-of-order delivery on 1367 the PSN path between the PEs. This can impact the working condition 1368 of an emulated service. For some payload types, packet loss, 1369 corruption, and out-of-order delivery can be mapped to either a bit 1370 error burst, or loss of carrier on the PW. If a native service has 1371 some mechanism to deal with bit error, the corresponding PWE3 service 1372 should provide a similar mechanism. 1374 7.3.4. Other Status Notification 1376 A PWE3 approach may provide a mechanism for other status 1377 notification, if any. 1379 7.3.5. Collective Status Notification 1381 Status of a group of emulated services may be affected identically by 1382 a single network incident. For example, when the physical link (or 1383 sub-network) between a CE and a PE fails, all the emulated services 1384 that go through that link (or sub-network) will fail. It is likely 1385 that there exists a group of emulated services that all terminate at 1386 a remote CE. There may also be multiple such CEs affected by the 1387 failure. Therefore, it is desirable that a single notification 1388 message be used to notify failure of the whole group of emulated 1389 services. 1391 A PWE3 approach may provide some mechanism for notifying status 1392 changes of a group of emulated circuits. One possible method is to 1393 associate each emulated service with a group ID when the PW for that 1394 emulated service is set up. Multiple emulated services can then be 1395 grouped by associating them with the same group ID. In status 1396 notification, that group ID can be used to refer all the emulated 1397 services in that group. The group ID mechanism should be a mechanism 1398 provided by the underlying tunnel signaling protocol. 1400 7.4 Keep-alive 1402 If a native service has a keep-alive mechanism, the corresponding 1403 emulated service needs to use a mechanism to propagate this across 1404 the PW. An approach following the principle of minimum intervention 1405 would be to transparently transport keep-alive messages over the PW. 1406 However, to accurately reproduce the semantics of the native 1407 mechanism, some PWs may require an alternative approach, such as 1408 piggy-backing on the PW signaling mechanism. 1410 7.5 Handling Control Messages of the Native Services 1412 Some native services use control messages for maintaining the 1413 circuits. These control messages may be in-band, e.g. Ethernet flow 1414 control or ATM performance management, or out-of-band, e.g. the 1415 signaling VC of an ATM VP. 1417 From the principle of minimum intervention, it is desirable that the 1418 PEs participate as little as possible in the signaling and 1419 maintenance of the native services. This principle should not, 1420 however, override the need to satisfactorily emulate the native 1421 service. 1423 If control messages are passed through, it may be desirable to send 1424 them using either a higher priority or a reliable channel provided by 1425 the PW Demultiplexer layer. See PWE3 Channel Types. 1427 8. Management and Monitoring 1429 This section describes the management and monitoring architecture for 1430 PWE3. 1432 8.1 Statistics 1434 The PE can tabulate statistics that help monitor the state of the 1435 network, and to help with measurement of service level agreements 1436 (SLAs). Typical counters include: 1438 o Counts of PW-PDUs sent and received, with and without errors. 1439 o Counts of sequenced PW-PDUs lost. 1440 o Counts of service PDUs sent and received, with and without 1441 errors (non-TDM). 1442 o Service-specific interface counts. 1444 These counters would be contained in a PW-specific MIB, and they 1445 should not replicate existing MIB counters. 1447 8.2 PW SNMP MIB Architecture 1449 This section describes the general architecture for SNMP MIBs used to 1450 manage PW services and the underlying PSN. The intent here is to 1451 provide a clear picture of how all of the pertinent MIBs fit together 1452 to form a cohesive management framework for deploying PWE3 services. 1454 8.2.1. MIB Layering 1456 The SNMP MIBs created for PWE3 should fit the architecture shown in 1457 Figure 12. 1459 +-----------+ +-----------+ +-----------+ 1460 Service | CEM | | Ethernet | | ATM | 1461 Layer |Service MIB| |Service MIB| ... |Service MIB| 1462 +-----------+ +-----------+ +-----------+ 1463 \ | / 1464 \ | / 1465 - - - - - - - - - - - - \ - - - | - - - - / - - - - - - - 1466 \ | / 1467 +-------------------------------------------+ 1468 Generic PW | Generic PW MIBs | 1469 Layer +-------------------------------------------+ 1470 / \ 1471 - - - - - - - - - - - - / - - - - - - - - \ - - - - - - - 1472 / \ 1473 / \ 1474 +-----------+ +-----------+ 1475 PSN VC |L2TP VC MIB| |MPLS VC MIB| 1476 Layer +-----------+ +-----------+ 1477 | | 1478 - - - - - - - - - | - - - - - - - - - - - - - - - | - - - 1479 | | 1480 +-----------+ +-----------+ 1481 PSN |L2TP MIB(s)| |MPLS MIB(s)| 1482 Layer +-----------+ +-----------+ 1484 Figure 12: Relationship of SNMP MIBs 1486 Figure 13 shows an example for a TDM PW carried over MPLS. 1488 +-----------------+ 1489 | SONET MIB | RFC2558 1490 +-----------------+ 1491 | 1492 +-----------------+ 1493 Service |SONET Service MIB| pw-cem-mib 1494 Layer +-----------------+ 1495 - - - - - - - - - - | - - - - - - - - - - - - - - - 1496 +-----------------+ 1497 Generic PW | Generic PW MIBS | pw-tc-mib 1498 Layer +-----------------+ pw-mib 1499 - - - - - - - - - - | - - - - - - - - - - - - - - - 1500 +-----------------+ 1501 PSN VC | MPLS VC MIBS | pw-mpls-mib 1502 Layer +-----------------+ 1503 - - - - - - - - - - | - - - - - - - - - - - - - - - 1504 +-----------------+ 1505 PSN | MPLS MIBs | mpls-te-mib 1506 Layer +-----------------+ mpls-lsr-mib 1508 Figure 13: Service-specific Example for MIBs 1510 Note that there is a separate MIB for each emulated service as well 1511 as one for each underlying PSN. These MIBs may be used in various 1512 combinations as needed. 1514 8.2.2. Service Layer MIBs 1516 The first layer is referred to as the Service Layer. It contains 1517 MIBs for PWE3 services such as Ethernet, ATM, circuits and Frame 1518 Relay. This layer contains those corresponding MIBs used to mate or 1519 adapt those emulated services to the underlying services. This 1520 working group should not produce any MIBs for managing the general 1521 service; rather, it should produce just those MIBs that are used to 1522 interface or adapt the emulated service onto the PWE3 management 1523 framework. For example, the standard SONET MIB [SONETMIB] is 1524 designed and maintained by another working group. Also, the SONET MIB 1525 is designed to manage the native service without PW emulation. Since 1526 the PWE3 working group is chartered to produce the corresponding 1527 adaptation MIB, in this case, it would produce the PW-CEM-MIB 1528 [PWMPLSMIB] that would be used to adapt SONET services to the 1529 underlying PSN that carries the PWE3 service. 1531 8.2.3. Generic PW MIBs 1533 The second layer is referred to as the Generic PW Layer. This layer 1534 is composed of two MIBs: the PWE-TC-MIB [PWTCMIB] and the PWE-MIB 1535 [PWMIB]. These MIBs are responsible for providing general PWE3 1536 counters and service models used for monitoring and configuration of 1537 PWE3 services over any supported PSN service. That is, this MIB 1538 provides a general model of PWE3 abstraction for management purposes. 1539 This MIB is used to interconnect the Service Layer MIBs to the PSN VC 1540 Layer MIBs. The latter will be described in the next section. This 1541 layer also provides the PW-TC-MIB [PWTCMIB]. This MIB contains 1542 common SMI textual conventions [RFC1902] that may be used by any PW 1543 MIB. 1545 8.2.4. PSN VC Layer MIBs 1547 The third layer in the PWE3 management architecture is referred to as 1548 the PSN VC layer. This layer is comprised of MIBs that are 1549 specifically designed to interface general PWE3 services (VCs) onto 1550 those underlying PSN services. In general this means that the MIB 1551 provides a means with which an operator can map the PW service onto 1552 the native PSN service. For example, in the case of MPLS, it is 1553 required that the general VC service be layered onto MPLS LSPs or 1554 Traffic Engineered (TE) Tunnels [RFC3031]. In this case, the PW- 1555 MPLS-MIB [PWMPLSMIB] was created to adapt the general PWE3 circuit 1556 services onto MPLS. Like the Service Layer described above the PWE3 1557 working group should produce these MIBs. 1559 8.2.5. PSN Layer MIBs 1561 The fourth and final layer in the PWE3 management architecture is 1562 referred to as the PSN layer. This layer is comprised of those MIBs 1563 that control the PSN service-specific services. For example, in the 1564 case of the MPLS [RFC3031] PSN service, the MPLS-LSR-MIB [LSRMIB] and 1565 the MPLS-TE-MIB [TEMIB] are used to interface the general PWE3 VC 1566 services onto native MPLS LSPs and/or TE tunnels to carry the 1567 emulated services. In addition, the MPLS-LDP-MIB [LDPMIB] may be 1568 used to reveal the MPLS labels that are distributed over the MPLS PSN 1569 in order to maintain the PW service. The MIBs in this layer are 1570 produced by other working groups that design and specify the native 1571 PSN services. These MIBs should contain the appropriate mechanisms 1572 for monitoring and configuring the PSN service such that the emulated 1573 PWE3 service will function correctly. 1575 8.3 Connection Verification and Traceroute 1577 A connection verification mechanism should be supported by PWs. 1578 Connection verification as well as other alarming mechanisms can 1579 alert the operator that a PW has lost its remote connection. The 1580 opaque nature of a PW means that it is not possible to specify a 1581 generic connection verification or traceroute mechanism that passes 1582 this status to the CEs over the PW. If connection verification 1583 status of the PW is needed by the CE, it must be mapped to the native 1584 connection status method. 1586 For troubleshooting purposes, it is sometimes desirable to know the 1587 exact functional path of a PW between PEs, thus a traceroute function 1588 capable of reporting the path taken by data packets over the PW 1589 should be provided. The opaque nature of the PW means that this 1590 traceroute information is only available within the provider network 1591 e.g. at the PEs. 1593 9. IANA considerations 1595 There are no IANA considerations for this document. 1597 10. Security Considerations 1599 PWE3 provides no means of protecting the contents or delivery of the 1600 native data units. PWE3 may, however, leverage security mechanisms 1601 provided by the PW Demultiplexer or PSN Layers, such as IPSec 1602 [RFC2401]. This section addresses the PWE3 vulnerabilities, and the 1603 mechanisms available to protect the emulated native services. 1605 The PW Tunnel End-Point, PW Demultiplexing mechanism, and the 1606 payloads of the native service are all vulnerable to attack. 1608 10.1 PW Tunnel End-Point and PW Demultiplexer Security 1610 Protection mechanisms must be considered for the PW Tunnel end-point 1611 and PW Demultiplexer mechanism in order to avoid denial-of-service 1612 attacks upon the native service, and to prevent spoofing of the 1613 native data units. Exploitation of vulnerabilities from within the 1614 PSN may be directed to the PW Tunnel end-point so that PW 1615 Demultiplexer and PSN tunnel services are disrupted. Controlling PSN 1616 access to the PW Tunnel end-point may protect against this. 1618 By restricting PW Tunnel end-point access to legitimate remote PE 1619 sources of traffic, the PE may reject traffic that would interfere 1620 with the PW Demultiplexing and PSN tunnel services. 1622 10.2 Validation of PW Encapsulation 1624 Protection mechanisms must address the spoofing of tunneled PW data. 1625 The validation of traffic addressed to the PW Demultiplexer end-point 1626 is paramount in ensuring integrity of PW encapsulation. Security 1627 protocols such as IPSec [RFC2401] may be used by the PW Demultiplexer 1628 Layer in order to maintain the integrity of the PW by authenticating 1629 data between the PW Demultiplexer End-points. IPSec may provide 1630 authentication, integrity, non-repudiation, and confidentiality of 1631 data transferred between two PEs. It cannot provide the equivalent 1632 services to the native service. 1634 Based on the type of data being transferred, the PW may indicate to 1635 the PW Demultiplexer Layer that enhanced security services are 1636 required. The PW Demultiplexer Layer may define multiple protection 1637 profiles based on the requirements of the PW emulated service. CE- 1638 to-CE signaling and control events emulated by the PW and some data 1639 types may require additional protection mechanisms. Alternatively, 1640 the PW Demultiplexer Layer may use peer authentication for every PSN 1641 packet to prevent spoofed native data units from being sent to the 1642 destination CE. 1643 Acknowledgments 1645 We thank: Sasha Vainshtein for his work on Native Service Processing 1646 and advice on bit-stream over PW services. Thomas K. Johnson for his 1647 work on the background and motivation for PWs. 1649 We also thank: Ron Bonica, Stephen Casner, Durai Chinnaiah, Jayakumar 1650 Jayakumar, Ghassem Koleyni, Eric Rosen, John Rutemiller, Scott 1651 Wainner and David Zelig for their comments and contributions. 1653 References 1655 Internet-drafts are works in progress available from 1656 1658 [ETSI] EN 300 744 Digital Video Broadcasting (DVB); Framing 1659 structure, channel coding and modulation for digital 1660 terrestrial television (DVB-T), European 1661 Telecommunications Standards Institute (ETSI) 1663 [LDP-MIB] Cucchiara, J., Sjostrand, H., and Luciani, J., 1664 "Definitions of Managed Objects for the Multiprotocol 1665 Label Switching, Label Distribution Protocol (LDP)", 1666 , work in progress, 1667 August 2001. 1669 [LSRMIB] Srinivasan et al, "MPLS Label Switch Router Management 1670 Information Base Using SMIv2", 1671 (draft-ietf-mpls-lsr-mib-08.txt, work in progress, January 1672 2002. 1674 [L2TPv3] Layer Two Tunneling Protocol (Version 3)'L2TPv3', J Lau, 1675 et. al. (work in progress). 1677 [PPPoL2TP] PPP Tunneling Using Layer Two Tunneling Protocol, 1678 J Lau et al. , 1679 work in progress. 1681 [PWMIB] Zelig et al, "Pseudo Wire (PW) Management Information Base 1682 Using SMIv2", (draft-ietf-pwe3-pw-mib-00.txt), work in 1683 progress, June 2002. 1685 [PWTCMIB] Nadeau et al, "Definitions for Textual Conventions and 1686 OBJECT-IDENTITIES for Pseudo-Wires Management" 1687 (draft-ietf-pwe3-pw-tc-mib-00.txt), work in progress, 1688 June 2002. 1690 [PWMPLSMIB] Danenberg et al, "SONET/SDH Circuit Emulation Service Over 1691 MPLS (CEM) Management Information Base Using SMIv2", 1692 (draft-ietf-pwe3-cep-mib-00.txt), work in progress, 1693 August 2001. 1695 [RFC1191] RFC-1191: Path MTU discovery. J.C. Mogul, S.E. Deering. 1697 [RFC1889] RFC-1889: RTP: A Transport Protocol for Real-Time 1698 Applications. H. Schulzrinne et. al. 1700 [RFC1902] RFC-1902: Structure of Management Information for 1701 Version 2 of the Simple Network Management Protocol 1702 (SNMPv2), Case et al, January 1996. 1704 [RFC1958] RFC-1958: Architectural Principles of the Internet, 1705 B. Carpenter et al. 1707 [RFC1981] RFC-1981: Path MTU Discovery for IP version 6. J. McCann, 1708 S. Deering, J.Mogul. 1710 [RFC2022] RFC-2022: Support for Multicast over UNI 3.0/3.1 based 1711 ATM Networks, G. Armitage. 1713 [RFC2401] RFC-2401: Security Architecture for the Internet Protocol. 1714 S. Kent, R. Atkinson. 1716 [RFC2474] RFC-2474: Definition of the Differentiated Services 1717 Field (DS Field) in the IPv4 and IPv6 Headers, 1718 K. Nichols, et. al. 1720 [RFC2661] RFC-2661: Layer Two Tunneling Protocol "L2TP". 1721 W. Townsley, et. al. 1723 [RFC2784] RFC-2784: Generic Routing Encapsulation (GRE). 1724 D. Farinacci et al. 1726 [RFC2890] RFC-2890: Key and Sequence Number Extensions to GRE. 1727 G. Dommety. 1729 [RFC3022] RFC-3022: Traditional IP Network Address Translator 1730 (Traditional NAT). P Srisuresh et al. 1732 [RFC3031] RFC3031: Multiprotocol Label Switching Architecture, 1733 E. Rosen, January 2001. 1735 [SONETMIB] K. Tesink, "Definitions of Managed Objects for the 1736 SONET/SDH Interface Type", RFC2558, March 1999. 1738 [TEMIB] Srinivasan et al, "Traffic Engineering Management 1739 Information Base Using SMIv2", 1740 (draft-ietf-mpls-te-mib-08.txt), work in progress, 1741 January 2002. 1743 [VPLS] M. Lasserre, "Virtual Private LAN Services over MPLS", 1744 draft-lasserre-vkompella-ppvpn-vpls-02.txt, work in 1745 progress, June 2002. 1747 [XIAO] Xiao et al, "Requirements for Pseudo-Wire Emulation 1748 Edge-to-Edge (PWE3)", 1749 (draft-ietf-pwe3-requirements-03.txt), X Xiao et al. 1750 work in progress, December 2002. 1752 Editors' Addresses 1754 Stewart Bryant 1755 Cisco Systems, 1756 4, The Square, 1757 Stockley Park, 1758 Uxbridge UB11 1BL, 1759 United Kingdom. Email: stbryant@cisco.com 1761 Prayson Pate 1762 Overture Networks, Inc. 1763 P. O. 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