Pseudo-Wire Edge-to-Edge (PWE3) Working Group Stewart Bryant Internet Draft Lloyd Wood Document: Mark Townsley Expires: November 2002 Cisco Systems Ltd Danny McPherson TCB May 2002 Protocol Layering in PWE3 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This draft proposes a unified protocol layering approach for pseudo- wire emulation edge-to-edge (PWE3). It adopts the principle that PWE3 should be a single transport type operating over a common packet- switched network (PSN) service model using, wherever possible, existing IETF protocols. The draft defines the protocol layering model for pseudo-wires, guidelines for the design of a specific encapsulation type, and the service requirements on the underlying PSN tunneling mechanism. Bryant, et al. Informational [Page 1] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 Table of Contents 1. Introduction............................................. 3 2. Terminology.............................................. 3 3. Protocol Layering Model.................................. 5 3.1 Protocol Layers...................................... 5 3.2 Domain of PWE3....................................... 7 3.3 Payload Types........................................ 7 4. Architecture of Pseudo-wires............................. 10 4.1 Network Reference Model.............................. 10 4.2 PWE3 Pre-processing.................................. 11 4.3 Maintenance Reference Model.......................... 15 4.4 Protocol Stack Reference Model....................... 15 4.5 Pre-processing Extension to Protocol Stack Reference Model................................................ 16 5. PW Encapsulation......................................... 17 5.1 Payload Convergence Layer............................ 18 5.2 Payload-independent PW Encapsulation Layers.......... 20 5.3 Fragmentation........................................ 22 5.4 Instantiation of the Protocol Layers................. 23 6. PW Demultiplexer Layer and PSN Requirements.............. 25 6.1 Multiplexing......................................... 26 6.2 Fragmentation........................................ 26 6.3 Length and Delivery.................................. 26 7. Control Plane............................................ 26 7.1 Set-up or Teardown of Pseudo-Wires................... 26 7.2 Status Monitoring.................................... 27 7.3 Notification of Pseudo-wire Status Changes........... 27 7.4 Keep-alive........................................... 29 7.5 Handling Control Messages of the Native Services..... 29 8. IANA considerations...................................... 29 9. Security Considerations.................................. 29 9.1 PW Tunnel End-Point and PW Demultiplexer Security.... 30 9.2 Validation of PW Encapsulation....................... 30 9.3 End-to-End Security.................................. 30 Bryant, et al. Informational [Page 2] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 1. Introduction This document presents a unified protocol layering approach for pseudo-wire emulation edge-to-edge (PWE3). Wherever possible, existing IETF protocols [RFC-1958] are used. PWE3 is intended to provide only the necessary and sufficient functionality to emulate the wire with the required degree of faithfulness for the given service definition. A pseudo-wire (PW) may be point-to-point, multipoint-to-point or point-to-multipoint. Any required switching functionality, is the responsibility of a forwarder function. Any translation or other operation needing a knowledge of the payload semantics is carried out by native service processing (NSP) elements. The functional definition of any forwarder of NSP elements is outside the scope of PWE3. This document defines the protocol layering model for pseudo-wires (PW), guidelines for the design of a pseudowire encapsulations, and the service requirements on the underlying PSN tunneling mechanism. 2. Terminology This document uses the following definition of terms. These terms are illustrated in context in Figure 2. Attachment Circuit The circuit or virtual circuit attaching (AC) a CE to a PE. CE-bound The traffic direction where PW-PDUs are received on a PW via the PSN, processed and then sent to the destination CE. CE Signaling Messages sent and received by the CEs control plane. It may be desirable or even necessary for the PE to participate in or monitor this signaling in order to effectively emulate the service. Customer Edge (CE) A device where one end of a service originates and/or terminates. The CE is not aware that it is using an emulated service rather than a native service. Bryant, et al. Informational [Page 3] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 Forwarder A PE sub-system that determines which PW a payload received on an AC must be sent over. Fragmentation When a packet MTU is greater than that supported by the PSN, either the PSN packet or the payload is fragmented into smaller data units which are transmitted separately and reassembled elsewhere in the network. Inter-working Interactions between networks, between end systems, or between parts thereof, with the aim of providing a functional entity capable of supporting an end-to-end communication. Inter-working A function that facilitates inter-working Function (IWF) between two dissimilar services. NSP may perform the IWF function. Native Service Processing of the data received by the PE Processing (NSP) from the CE before presentation to the PW for transmission across the core. Packet Switched A network using IP or MPLS as the mechanism Network (PSN) for packet forwarding. Protocol Data The unit of data output to, or received Unit (PDU) from, the network by a protocol layer. Provider Edge (PE) A device that provides PWE3 to a CE. PE-bound The traffic direction where information from a CE is adapted to a PW, and PW-PDUs are sent into the PSN. PE/PW Maintenance Used by the PEs to set up, maintain and tear down the PW. It may be coupled with CE Signaling in order to effectively manage the PW. Pseudo Wire (PW) A mechanism that carries the essential elements of an emulated service from one PE to one or more other PEs over a PSN. PW End Service The interface between a PE and a CE. This (PWES) can be a physical interface like a T1 or Ethernet, or a virtual interface like a VC Bryant, et al. Informational [Page 4] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 or VLAN. Pseudo Wire A mechanism that emulates the essential Emulation Edge to attributes of service (such as a T1 leased Edge (PWE3) line or frame relay) over a PSN. Pseudo Wire PDU A PDU sent on the PW that contains all of (PW-PDU) the data and control information necessary to emulate the desired service. PSN Tunnel A tunnel across a PSN inside which one or more PWs can be carried. PSN Tunnel Used to set up, maintain and tear down the Signaling underlying PSN tunnel. PW Demultiplexer Data-plane method of identifying PW terminating at a PE. Tunnel A method of transparently carrying information over a network. 3. Protocol Layering Model The PWE3 protocol-layering model is intended to minimise the differences between PWs operating over different PSN types. The design of the protocol-layering model thus has the goals of making each PW definition independent of the underlying PSN, and maximizing the reuse of IETF protocol definitions and their implementations. 3.1 Protocol Layers The logical protocol-layering model required to support a PW is shown in Figure 1. Bryant, et al. Informational [Page 5] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 +---------------------------+ | Payload | +---------------------------+ | Encapsulation | <==== May be empty +---------------------------+ | PW Demultiplexer | +---------------------------+ | PSN Convergence | <==== May be empty +---------------------------+ | PSN | +---------------------------+ | MAC/Data-link | +---------------------------+ | Physical | +---------------------------+ Figure 1: Logical Protocol Layering Model The payload is transported over the Encapsulation Layer. The Encapsulation Layer carries any information, not already present within the payload itself, that is needed by the PW CE-bound PE interface to send the payload to the CE via the physical interface. If no information is needed beyond that in the payload itself, this layer is empty. If needed, this layer also provides support for real-time processing, and also sequencing, if needed. The PW Demultiplexer Layer provides the ability to deliver multiple PWs over a single PSN tunnel. The PW demultiplexer value used to identify the PW in the data-plane may be unique per PE, but this is not a PWE3 requirement. It must however be unique per tunnel. If it is necessary to identify a particular tunnel, then that is the responsibility of the PSN layer. The PSN Convergence Layer provides the enhancements needed to make the PSN conform to the assumed PSN service requirement. This layer therefore provides a consistent interface to the PW, making the PW independent of the PSN type. If the PSN already meets the service requirements, this layer is empty. The PSN header, MAC/Data-link and Physical Layer definitions are outside the scope of this document. The PSN can be any PSN type defined by the IETF. These are currently IPv4, IPv6 and MPLS. Bryant, et al. Informational [Page 6] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 3.2 Domain of PWE3 PWE3 defines the Encapsulation Layer, the method of carrying various payload types, and the interface to the PW Demultiplexer Layer. It is expected that the other layers will be provided by tunneling methods such as L2TP or MPLS over the PSN. 3.3 Payload Types The payload is classified into the following generic types of native data unit: o Bit-stream o Structured bit-stream o Cell o Packet Within these generic types there are specific service types. For example: Generic Payload Type PW Service -------------------- ---------- Bit-stream SONET, TDM (e.g. DS1, DS3, E1). Structured bit-stream SONET, TDM. Cell ATM. Packet Ethernet (all types), HDLC, frame-relay, ATM AAL5 PDU. 3.3.1. Bit-stream A bit-stream payload is created by capturing, transporting and replaying the bit pattern on the emulated wire, without taking advantage of any structure that, on inspection, may be visible within the relayed traffic. The Encapsulation Layer submits an identical number of bits for transport in each PW-PDU. This service will require sequencing and real-time support. 3.3.2. Structured bit-stream A bit-stream payload is created by using some knowledge of the underlying structure of the bit-stream to capture, transport and replay the bit pattern on the emulated wire. Bryant, et al. Informational [Page 7] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 Two important points distinguish structured and unstructured bit- streams: o Some part of the original (unstructured) bit stream are stripped by, for example, the PSN-bound direction of the NSP block. For example, in Structured SONET the section and line overhead (and, possibly, more) may be stripped. o The PW must preserve the structure across the PSN so that the CE-bound NSP block can insert it correctly into the reconstructed unstructured bit stream. The Encapsulation Layer may also perform silence/idle suppression or similar compression on a structured bit stream. Structured bit streams are distinguished from cells in that the structures may be too long to be carried in a single packet (i.e. structured SONET). Note that "short" structures are indistinguishable from cells and may benefit from the use of cell encapsulations. This service will require sequencing and real-time support. 3.3.3. Cell Payload A cell payload is created by capturing, transporting and replaying groups of bits presented on the wire in a fixed-size format. The delineation of the group of bits that comprise the cell is specific to the encapsulation type. Two common examples of cell payloads are 53-octet cells carrying ATM AAL2, and the larger 188-octet MPEG Transport Stream packets [ETSI]. To reduce per-PSN packet overhead, multiple cells may be concatenated into a single payload. The Encapsulation Layer may consider the payload complete on the expiry of a timer, or after a fixed number of cells have been received. The benefit of concatenating multiple PDUs should be weighed against the resulting increase in jitter and the larger penalty incurred by packet loss. In some cases, it may be appropriate for the Encapsulation Layer to perform a silence suppression or a similar compression. The generic cell payload service will normally need sequence number support, and may also need real-time support. The generic cell payload service would not normally require fragmentation. The Encapsulation Layer may apply some form of compression to some of these sub-types. Bryant, et al. Informational [Page 8] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 In some instances, the cells to be incorporated in the payload may be selected by filtering them from the stream of cells presented on the wire. For example, an ATM PWE3 service may select cells based on their VCI or VPI fields. This is an NSP function, and the selection would therefore be made before the packet was presented to the PW Encapsulation Layer. 3.3.4. Packet Payload A packet payload is a variable-size data unit presented to the PE on the AC. A packet payload may be large compared to the PSN MTU. The delineation of the packet boundaries is encapsulation-specific. HDLC or Ethernet PDUs can be considered as examples of packet payloads. Typically a packet will be stripped of transmission overhead such as HDLC flags and stuffing bits before transmission over the PW. A packet payload would normally be relayed across the PW as a single unit. However, there will be cases where the combined size of the packet payload and its associated PWE3 and PSN headers exceeds the PSN path MTU. In this case some fragmentation methodology needs to be applied. This is likely to be the case when a user is providing the service and attaching to the service provider via an Ethernet, or where nested pseudo-wires are involved. Fragmentation is discussed in more detail in Section 5. A packet payload may need sequencing and real-time support. In some situations, the packet payload may be selected from the packets presented on the emulated wire on the basis of some sub- multiplexing technique. For example, one or more frame-relay PDUs may be selected for transport over a particular pseudo-wire based on the frame-relay Data-Link Connection Identifier (DLCI), or, in the case of Ethernet payloads, on the basis of the VLAN identifier. This is an NSP function, and this selection would therefore be made before the packet was presented to the PW Encapsulation Layer. 3.3.5. Principle of Minimum Intervention To minimise the scope of information, and to improve the efficiency of data flow through the Encapsulation Layer, the payload should be transported as received with as few modifications as possible [RFC- 1958]. This minimum intervention approach decouples payload development from PW development and requires fewer translations at the NSP in a system with similar CE interfaces at each end. It also prevents any unwanted side-effects due to subtle mis-representation of the payload in the intermediate format. Bryant, et al. Informational [Page 9] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 An intervention approach can be more wire-efficient in some cases and may result in fewer translations at the NSP where the the CE interfaces are of different types. The intermediate format is effectively a new framing type. 4. Architecture of Pseudo-wires This section describes the PWE3 architectural model. 4.1 Network Reference Model Figure 2 illustrates the network reference model for point-to-point PWs. |<-------------- Emulated Service ---------------->| | | | |<------- Pseudo Wire ------>| | | | | | | | |<-- PSN Tunnel -->| | | | PW End V V V V PW End | V Service +----+ +----+ Service V +-----+ | | PE1|==================| PE2| | +-----+ | |----------|............PW1.............|----------| | | CE1 | | | | | | | | CE2 | | |----------|............PW2.............|----------| | +-----+ ^ | | |==================| | | ^ +-----+ ^ | +----+ +----+ | | ^ | | Provider Edge 1 Provider Edge 2 | | | | | | Customer | | Customer Edge 1 | | Edge 2 | | | | native service native service Figure 2: PWE3 Network Reference Model The two PEs (PE1 and PE2) need to provide one or more PWs on behalf of their client CEs (CE1 and CE2) to enable the client CEs to communicate over the PSN. A PSN tunnel is established to provide a data path for the PW. The PW traffic is invisible to the core network, and the core network is transparent to the CEs. Native data units (bits, cells or packets) presented at the PW End Service (PWES) Bryant, et al. Informational [Page 10] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 are encapsulated in a PW-PDU and carried across the underlying network via the PSN tunnel. The PEs perform the necesssary encapsulation and decapsulation of PW-PDUs, as well as handling any other functions required by the PW service, such as sequencing or timing. There are situations in which a particular packet payload needs to be multicast so that it is received by a number of CEs. This is useful when using PWs as part of a "virtual LAN" service (see, e.g., [VPLS]). This can be achieved by replicating the payload and transmitting the replicas on PWs, but it may also be useful to have a type of PW which is inherently point-to-multipoint. In that case, the PW would need to be carried through a point-to-multipoint PSN tunnel, employing a multicast mechanism provided by the PSN. 4.2 PWE3 Pre-processing In some applications, there is a need to perform operations on the native data units received from the CE (including both payload and signalling traffic) before they are transmitted across the PW by the PE. Examples include Ethernet bridging, SONET cross-connect, translation of locally-significant identifiers such as VCI/VPI, or translation to another service type. These operations could be carried out in external equipment, and the processed data sent to the PE over one or more physical interfaces. In most cases, there are cost and operational benefits in undertaking these operations within the PE. This processed data is then presented to the PW via a virtual interface within the PE. These pre-processing operations are included in the PWE3 reference model to provide a common reference point, but the detailed description of these operations is outside the scope of the PW definition given here. Bryant, et al. Informational [Page 11] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 PW End Service | |<------- Pseudo Wire ------>| | | | |<-- PSN Tunnel -->| | V V V V PW +-----+----+ +----+ End Service +-----+ |PREP | PE1|==================| PE2| | +-----+ | | | |............PW1.............|----------| | | CE1 |----| | | | | | | CE2 | | | ^ | |............PW2.............|----------| | +-----+ | | | |==================| | | ^ +-----+ | +-----+----+ +----+ | | | ^ | | | | | | | |<------- Emulated Service ------->| | | | | | Virtual physical | | termination | | ^ | CE1 native | CE2 native service | service | CE2 native service Figure 3: Pre-processing within the PWE3 Network Reference Model Figure 3 shows the inter-working of one PE with pre-processing (PREP), and a second without this functionality. This is a useful reference point because it emphasises that the functional interface between PREP and the PW is that represented by a physical interface carrying the service. This effectively defines the necessary inter- working specification. The operation of a system in which both PEs include PREP functionality is also supported. The required pre-processing can be divided into two components: o Forwarding (FWD) o Native Service Processing (NSP) 4.2.1. Forwarders In some applications there is the need to selectively forward payload elements from one of more ACs to one or more PWs. In such cases there Bryant, et al. Informational [Page 12] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 will also be the need to perform the inverse function on PWE3-PDUs received by a PE from the PSN. This is the function of the Forwarder (FWD). The forwarder selects the PW based on for example: the incoming AC, the contents of the payload, or some statically or dynamically configured forwarding information. +----------------------------------------+ | PE Device | +----------------------------------------+ Single | | | PWES | | Single | PW Instance <------>o Forwarder + PW IWF X<===========> | | Instance | +----------------------------------------+ Figure 4a: Simple point-to-point service +----------------------------------------+ | PE Device | +----------------------------------------+ Multiple| | Single | PW Instance PWES | + PW IWF X<===========> <------>o | Instance | | |----------------------| <------>o | Single | PW Instance | + PW IWF X<===========> <------>o | Instance | | Forwarder |----------------------| <------>o | Single | PW Instance | + PW IWF X<===========> <------>o | Instance | | |----------------------| Multipoint | | Multipoint | PW Instance | + PW IWF X<===========> | | Instance | +----------------------------------------+ Figure 4b: Multiple PWEs to Multiple PW Forwarding Figure 4a shows a simple forwarder that performs some type of filtering operation. Figure 4b shows a more general forwarding situation where payloads are extracted from one or more PWESs and directed to one or more PWs, including, in this instance, a multipoint PW. Bryant, et al. Informational [Page 13] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 4.2.2. Native Service Processing In some applications some form of data or address translation, or other operation requiring knowledge of the semantics of the payload, will be required. This is the function of the Native Service Processor (NSP). The use of the NSP approach simplifies the design of the PW by restricting a PW to homogeneous operation. NSP is included in the reference model to provide a defined interface to this functionality. The specification of the various types of NSP is outside the scope of PWE3. +----------------------------------------+ | PE Device | Multiple+----------------------------------------+ PWES | | | Single | PW Instance <------>o NSP # + PW IWF X<===========> | | | Instance | |------| |----------------------| | | | Single | PW Instance <------>o NSP # + PW IWF X<===========> | | | Instance | |------|Forwarder |----------------------| | | | Single | PW Instance <------>o NSP # + PW IWF X<===========> | | | Instance | |------| |----------------------| Multipoint | | | Multipoint | PW Instance <------>o NSP # + PW IWF X<===========> | | | Instance | +----------------------------------------+ Figure 5: NSP in a Multiple PWEs to Multiple PW Forwarding PE Figure 5 illustrates the relationship between NSP, Forwarding and PWs in a PE. The NSP function may apply any transformation operation (modification, injection, etc.) on the payloads as they pass between the physical interface to the CE and the virtual interface to the Forwarder. A PE device may contain more than one Forwarder. The operation of a system in which the NSP functionality includes terminating the data-link and applying network layer processing to the payload is also supported. Bryant, et al. Informational [Page 14] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 4.3 Maintenance Reference Model Figure 6 illustrates the maintenance reference model for PWs. |<------- CE (end-to-end) Signaling ------>| | |<---- PW/PE Maintenance ----->| | | | |<-- PSN Tunnel -->| | | | | | Signaling | | | | V V (out of scope) V V | v +-----+ +-----+ v +-----+ | PE1 |==================| PE2 | +-----+ | |-----|.............PW1..............|-----| | | CE1 | | | | | | CE2 | | |-----|.............PW2..............|-----| | +-----+ | |==================| | +-----+ +-----+ +-----+ Customer Provider Provider Customer Edge 1 Edge 1 Edge 2 Edge 2 Figure 6: PWE3 Maintenance Reference Model The following signaling mechanisms are required: o The CE (end-to-end) signaling is between the CEs. This signaling could be frame relay PVC status signaling, ATM SVC signaling, etc. o The PW/PE Maintenance is used between the PEs (or NSPs) to set up, maintain and tear down PWs, including any required coordination of parameters. o The PSN Tunnel signaling controls the PW multiplexing and some elements of the underlying PSN. Examples are L2TP control protocol, MPLS LDP and RSVP-TE. This type of signaling is not within the scope of PWE3. 4.4 Protocol Stack Reference Model Figure 7 illustrates the protocol stack reference model for PWs. Bryant, et al. Informational [Page 15] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 +-----------------+ +-----------------+ |Emulated Service | |Emulated Service | |(e.g. TDM, ATM) |<==== Emulated Service ===>|(e.g. TDM, ATM) | +-----------------+ +-----------------+ | Payload | | Payload | | Encapsulation |<====== Pseudo Wire ======>| Encapsulation | +-----------------+ +-----------------+ |PW Demultiplexer | |PW Demultiplexer | | PSN Tunnel, |<======= PSN Tunnel ======>| PSN Tunnel, | | PSN & Physical | | PSN & Physical | | Layers | | Layers | +-------+---------+ ___________ +---------+-------+ | / \ | +===============/ PSN \================+ \ / \_____________/ Figure 7: PWE3 Protocol Stack Reference Model The PW provides the CE with an emulated physical or virtual connection to its peer at the far end. Native data units from the CE are passed through an encapsulation layer at the sending PE, and then sent over the PSN. The receiving PE removes the encapsulation and restores the payload to its native format for transmission to the destination CE. 4.5 Pre-processing Extension to Protocol Stack Reference Model Figure 8 illustrates how the protocol stack reference model is extended to include the provision of pre-processing (Fowarding and NSP). This shows the ideal placement of the physical interface relative to the CE. Bryant, et al. Informational [Page 16] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 /======================================\ H Forwarder H<----Pre-processing H----------------======================/ H Native Service H | | H Processing H | | \================/ | | | | | Emulated | | Service | | Service | | Interface | | (TDM, ATM, | | (TDM, ATM, | | Ethernet, |<== Emulated Service == | Ethernet, | | frame relay, | | frame relay, | | etc.) | | etc.) | +-----------------+ | | | Payload | | | | Encapsulation |<=== Pseudo Wire ====== | | +-----------------+ | | |PW Demultiplexer | | | | PSN Tunnel, | | | | PSN & Physical |<=== PSN Tunnel ======= | | | Headers | +----------------+ +-----------------+ | Physical | | Physical | +-------+--------+ +-------+---------+ | | | | | | | | | | | | To CE <---+ +---> To PSN Figure 8: Protocol Stack Reference Model with Pre-processing 5. PW Encapsulation The PW Encapsulation Layer provides the necessary infrastructure to adapt the specific payload type being transported over the PW to the PW Demultiplexer Layer that is used to carry the PW over the PSN. The PW Encapsulation Layer consists of three sub-layers: o Payload Convergence o Timing o Sequencing Bryant, et al. Informational [Page 17] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 The PW Encapsulation sub-layering and its context with the protocol stack are shown, in Figure 9. +---------------------------+ | Payload | /===========================\ <------ Encapsulation H Payload Convergence H Layer H---------------------------H H Timing H H---------------------------H H Sequencing H \===========================/ | PW Demultiplexer | +---------------------------+ | PSN Convergence | +---------------------------+ | PSN | +---------------------------+ | MAC/Data-link | +---------------------------+ | Physical | +---------------------------+ Figure 9: PWE3 Encapsulation Layer in Context The Payload Convergence Sub-layer is highly tailored to the specific payload type, but, by grouping a number of target payload types into a generic class, and then providing a single convergence sub-layer type common to the group, we achieve a reduction in the number of payload convergence sub-layer types. This decreases implementation complexity. The provision of per-packet signalling and other out-of- band information (other than sequencing or timing) is undertaken by this layer. The Timing Layer and the Sequencing Layer provide generic services to the Payload Convergence Layer for all payload types, when required. 5.1 Payload Convergence Layer 5.1.1. Encapsulation The primary task of the Payload Convergence Layer is the encapsulation of the payload in PW-PDUs. The native data units to be encapsulated may or may not contain L2 or L1 header information. This is service specific. The Payload Convergence header carries the additional information needed to replay the native data units at the CE-bound physical interface. The PW Demultiplexer header is not considered as part of the PW header. Bryant, et al. Informational [Page 18] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 Not all the additional information needed to replay the native data units need to be carried in the PW header of the PW PDUs. Some information (e.g. service type of a PW) may be stored as state information at the destination PE during PW set-up. 5.1.2. PWE3 Channel Types The PW Encapsulation Layer and its associated signaling require one or more of the following types of channel from its underlying PW Demultiplexer and PSN Layers: 1. A reliable control channel for signaling line events, status indications, and, in some exceptional cases, CE-CE events which must be translated and sent reliably between PEs. For example, this capability is needed in [PPPoL2TP] (PPP negotiation has to be split between the two ends of the tunnel). PWE3 may also need this type of control channel to provide faithful emulation of complex data-link protocols. plus one or more data channels with the following characteristics: 2. A high-priority, unreliable, sequenced channel. A typical use is for CE-to-CE signaling. "High priority" may simply be indicated via DSCP/EXP bits for priority during transit. This channel type could also use a bit in the tunnel header itself to indicate that packets received at the PE should be processed with higher priority. 3. A sequenced channel for data traffic that is sensitive to packet reordering (one classification for use could be for any non-IP traffic). 4. An un-sequenced channel for data traffic insensitive to packet order. The data channels (2, 3 and 4 above) should be carried "in band" with one another to as much of a degree as is reasonably possible on a PSN. Where end-to-end connectivity may be disrupted by address translation [RFC3022], access-control lists, firewalls etc., there exists the possibility that the control channel may be able to pass traffic and set up the PW, but the PW data-path data traffic is blocked by one or more of these mechanisms. In these cases unless the control channel is also carried "in band" the signalling to set-up the PW will not confirms the existence of an end-to-end data path. Bryant, et al. Informational [Page 19] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 In some cases there is a need to synchronize some CE events with the data carried over a PW. This is especially the case with TDM circuits (e.g., on-hook/off-hook events in PSTN switches). PWE3 channel types that are not needed by the supported PWs need not be included in such an implementation. 5.1.3. Quality of Service Considerations Where possible, it is desirable to employ mechanisms to provide PW Quality of Service (QoS) support over PSNs. Specification of a QoS design common to all PW Service types needs further investigation. 5.2 Payload-independent PW Encapsulation Layers Two PWE3 Encapsulation Sub-layers provide common services to all payload types: Sequencing and Timing. These services are optional and are only used if needed by a particular PW instance. If the service is not needed, the associated header may be omitted in order to conserve processing and network resources. There will be instances where a specific payload type will be required to be transported with or without sequence and/or real-time support. For example, an invariant of frame relay transport is the preservation of packet order. Some frame-relay applications expect in-order delivery, and may not cope with reordering of the frames. However, where the frame relay service is itself only being used to carry IP, it may be desirable to relax that constraint in return for reduced per-packet processing cost. The guiding principle is that, where possible, an existing IETF protocol should be used to provide these services. Where a suitable protocol is not available, the existing protocol should be extended or modified to meet the PWE3 requirements, thereby making that protocol available for other IETF uses. In the particular case of timing, more than one general method may be necessary to provide for the full scope of payload timing requirements. 5.2.1. Sequencing The sequencing function provides three services: frame ordering, frame duplication detection and frame loss detection. These services allow the invariant properties of a physical wire to be emulated. Support for sequencing depends on the payload type, and may be omitted if not needed. The size of the sequence-number space depends on the speed of the emulated service, and the maximum time of the transient conditions in Bryant, et al. Informational [Page 20] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 the PSN. A sequence number space greater than approximately 2^16 may therefore be needed to prevent the sequence number space wrapping during the transient. 5.2.1.1 Frame Ordering When packets carrying the PW-PDUs traverse a PSN, they may arrive out of order at the destination PE. For some services, the frames (control frames, data frames, or both control and data frames) must be delivered in order. For such services, some mechanism must be provided for ensuring in-order delivery. Providing a sequence number in the sequence sub-layer header for each packet is one possible approach to out-of-sequence detection. Alternatively it can be noted that sequencing is a subset of the problem of delivering timed packets, and that a single combined mechanism such as [RTP] may be employed. There are two possible misordering strategies: o Drop misordered PW PDUs. o Try to sort PW PDUs into the correct order. The choice of strategy will depend on: o How critical the loss of packets is to the operation of the PW (e.g. the acceptable bit error rate). o The speeds of the PW and PSN. o The acceptable delay (since delay must be introduced to reorder) o The incidence of expected misordering. 5.2.1.2 Frame Duplication Detection In rare cases, packets traversing a PW may be duplicated by the underlying PSN. For some services, frame duplication is not acceptable. For such services, some mechanism must be provided to ensure that duplicated frames will not be delivered to the destination CE. The mechanism may or may not be the same as the mechanism used to ensure in-order frame delivery. 5.2.1.3 Frame Loss Detection A destination PE can determine whether a frame has been lost by tracking the sequence numbers of the received PW PDUs. Bryant, et al. Informational [Page 21] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 In some instances, a destination PE will have to presume that a PW PDU is lost if it fails to arrive within a certain time. If a PW-PDU that has been processed as lost subsequently arrives, the destination PE must discard it. 5.2.2. Timing A number of native services have timing expectations based on the characteristics of the networks that they were designed to travel over, and it can be necessary for the emulated service to duplicate these network characteristics as closely as possible, e.g. in delivering native traffic with the same jitter, bit-rate and timing characteristics as it was sent. In such cases, it is necessary for the receiving PE to play out the native traffic as it was received at the sending PE. This relies on either timing information sent between the two PEs, or in some case timing information received from an external reference. The Timing Sub-layer must therefore support two timing functions: clock recovery and timed payload delivery. A particular payload type may require either or both of these services. 5.2.2.1 Clock Recovery Clock recovery is the extraction of output transmission bit timing information from the delivered packet stream, and requires a phase- locking mechanism. A physical wire provides this naturally, but it is a relatively complex task to extract this from a highly jittered source such as packet stream. It is therefore desirable that an existing real-time protocol such as [RTP] be used for this purpose, unless it can be shown that this is unsuitable or unnecessary for a particular payload type. 5.2.2.2 Timed delivery Timed delivery is the delivery of non-contiguous PW PDUs to the PW output interface with a constant phase relative to the input interface. The timing of the delivery may be relative to a clock derived from the packet stream via clock recovery, or via an external clock. 5.3 Fragmentation A payload would normally be relayed across the PW as a single unit. However, there will be cases where the combined size of the payload and its associated PWE3 and PSN headers exceeds the PSN path MTU. When a packet exceeds the MTU of a given network, fragmentation and Bryant, et al. Informational [Page 22] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 reassembly may have to be performed in order for the packet to be delivered. Since fragmentation and reassembly generally consume a large amount of network resource as compared to simply switching a packet in its entirety, efforts should be made to reduce or eliminate the need for fragmentation and reassembly as much as possible throughout a network. Of particular concern for fragmentation and reassembly are aggregation points where large numbers of pseudowires are processed (e.g. at the PE). Ideally, the equipment originating the traffic being sent over the PW will be configured to have adaptive measures [e.g. [RFC1191], [RFC1981]] in place such that it never sends a packet which must be fragmented. When this fails, the point closest to the sending host with fragmentation and reassembly capabilities should attempt to reduce the size of packets further into the network. Thus, in the reference model for PWE3 [Figure 3] fragmentation should first be performed at the CE if at all possible. If and only if the CE cannot adhere to an acceptable MTU size for the PW should the PE attempt its own fragmentation methods. Further, if an adequate general fragmentation method exists (e.g. as part of the native protocol being carried by the PW, or the PSN itself) then this should be employed when possible. Examining fragmentation mechanisms for PWE3 is a current work item. Further study may establish the need for a common PWE3 specific method. It is acceptable for a PE implementation not to support fragmentation. A PE that does not support fragmentation will drop packets that exceed the PSN MTU, and the management plane of the encapsulating PE may be notified. If the length of a L2/L1 frame, restored from a PW PDU, exceeds the MTU of the destination PWES, it must be dropped. In this case, the management plane of the destination PE may be notified. 5.4 Instantiation of the Protocol Layers This document does not address the detailed mapping of the Protocol Layering model to existing or future IETF standards. The instantiation of the logical Protocol Layering model is shown in Figure 9. 5.4.1. PWE3 over an IP PSN The protocol definition of PWE3 over an IP PSN therefore should employ existing IETF protocols where possible. Bryant, et al. Informational [Page 23] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 +---------------------+ +-------------------------+ | Payload |------------->| Raw payload if possible | /=====================\ +-------------------------+ H Payload Convergence H------------->| As Needed | H---------------------H +-------------------------+ H Timing H----------+-->| RTP | H---------------------H / +-------------+ | H Sequencing H----one of | | \=====================/ \ | +-----------+ | PW Demultiplexer |----------+-->| L2TP, GRE, IPSec, MPLS | +---------------------+ +-------------------------+ | PSN Convergence |------------->| Not needed | +---------------------+ +-------------------------+ | PSN |------------->| IP | +---------------------+ +-------------------------+ | MAC/Data-link |------------->| MAC/Data-link | +---------------------+ +-------------------------+ | Physical |------------->| Physical | +---------------------+ +-------------------------+ Figure 10: PWE3 over an IP PSN Figure 10 shows the protocol layering for PWE3 over an IP PSN. As a rule, the payload should be carried as received from the NSP, with the Payload Convergence Layer provided when needed. (It is accepted that there may sometimes be good reason not to follow this rule, but the exceptional circumstances need to be documented in the encapsulation layer definition for that payload type). Where appropriate, timing is provided by RTP, which when used also provides a sequencing service. PW demultiplexing may be provided by a number of existing IETF tunnel protocols. Some of these tunnel protocols provide an optional sequencing service. (Sequencing is provided either by RTP, or by the PW Demultiplexer Layer, but not both). A PSN convergence layer is not needed, because all the tunnel protocols shown above are designed to operate directly over an IP PSN. As a special case, if the PW demultiplexer label is MPLS, the protocol architecture of section 5.4.2 can be used instead of the protocol architecture of this section. 5.4.2. PWE3 over an MPLS PSN The MPLS ethos places importance on wire efficiency. By using a control word, some components of the PWE3 protocol layers can be compressed to increase wire efficiency. Bryant, et al. Informational [Page 24] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 +---------------------+ | Payload | /=====================\ H Payload Convergence H-----------------+ H---------------------H | H Timing H | H---------------------H | H Sequencing H------------------------------------+ \=====================/ | | | PW Demultiplexer |---+ | | +---------------------+ | | | | PSN Convergence |-----------------------+----+----+ | +---------------------+ | | | | | | | PSN |-+ | v v v v v +---------------------+ | | +--------------------------------+ | MAC/Data-link | | | | Flags |Frag| Len |Seq # | +---------------------+ | | +--------------------------------+ | Physical | | +->| Inner Label | +---------------------+ | +--------------------------------+ +--->| Outer Label or MPLS-in-IP encap| +--------------------------------+ Figure 11: PWE3 over an MPLS PSN using a control word Figure 11 shows the protocol layering for PWE3 over an MPLS PSN. An inner MPLS label is used to provide the PW demultiplexing function. A control word is used to carry most of the information needed by the PWE3 Encapsulation Layer and the PSN Convergence Layer in a compact format. The flags in the control word provide the necessary payload convergence. A sequence field provides support for both in-order payload delivery and (supported by fragmentation control bits) a PSN fragmentation service within the PSN Convergence Layer. To allow PWE3 carried in MPLS to correctly pass over an Ethernet data-link, a length correction field is needed in the control word. In some networks it may be necessary to carry PWE3 over MPLS over IP. In these circumstances, the PW is encapsulated for carriage over MPLS as described in this section, and then a standard method of carrying MPLS over an IP PSN is applied to the resultant PW-PDU. 6. PW Demultiplexer Layer and PSN Requirements PWE3 places three service requirements on the protocol layers used to carry it across the PSN: Bryant, et al. Informational [Page 25] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 o Multiplexing o Fragmentation o Length and Delivery 6.1 Multiplexing The purpose of the PW Demultiplexer Layer is to allow multiple PWs to be carried in a single tunnel. This minimizes complexity and conserves resources. Some types of native service are capable of grouping multiple circuits into a "trunk", e.g. multiple ATM VCs in a VP, multiple Ethernet VLANs in a port, or multiple DS0 services within a T1 or E1. A PW may interconnect two end-trunks. That trunk would have a single multiplexing value. 6.2 Fragmentation Fragmentation is discussed in Section 5.3. If the PSN provides a fragmentation service of adequate performance, that mechanism may be used by the PE to fragment and reassemble PW PDUs which exceed the PSN MTU. This fragmentation service is transparent to the PW Encapsulation Layer. 6.3 Length and Delivery PDU delivery to the egress PE is the function of the PSN Layer. If the underlying PSN does not provide all the information necessary to determine the length of a PW-PDU, the encapsulation layer will provide it. 7. Control Plane This section describes PWE3 control plane services. 7.1 Set-up or Teardown of Pseudo-Wires A PW must be set up before an emulated service can be established, and must be torn down when an emulated service is no longer needed. Set up or teardown of a PW can be triggered by a CLI command, from the management plane of a PE, by signaling (i.e., set-up or teardown) Bryant, et al. Informational [Page 26] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 of a PWES, e.g., an ATM SVC, or by an auto-discovery mechanism e.g. [BGPAUTO]. During the set-up process, the PEs need to exchange some information (e.g. learn each others' capabilities). The tunneling control protocol may be extended to provide mechanisms to enable the PEs to exchange all necessary information on behalf of the PW. Manual configuration of PWs can be considered a special kind of signaling, and is explicitly allowed. 7.2 Status Monitoring Some native services have mechanisms for status monitoring. For example, ATM supports OAM for this purpose. For such services, the corresponding emulated services must specify how to perform status monitoring. 7.3 Notification of Pseudo-wire Status Changes 7.3.1. Pseudo-wire Up/Down Notification If a native service requires bi-directional connectivity, the corresponding emulated service can only be signaled up when the associated PWs, and PSN tunnels if any, are functional in both directions. Because the two CEs of an emulated service are not adjacent, a failure may occur at a place such that one or both physical links between the CEs and PEs remain up. For example, in Figure 2, if the physical link between CE1 and PE1 fails, the physical link between CE2 and PE2 will not be affected and will remain up. Unless CE2 is notified about the remote failure, it will continue to send traffic over the emulated service to CE1. Such traffic will be discarded at PE1. Some native services have failure notification so that when the services fail, both CEs will be notified. For such native services, the corresponding PWE3 service must provide a failure notification mechanism. Similarly, if a native service has notification mechanisms so that when a network failure is fixed, all the affected services will change status from "Down" to "Up", the corresponding emulated service must provide a similar mechanism for doing so. These mechanisms may already be built into the tunneling protocol. For example, the L2TP control protocol has this capability and LDP has the ability to withdraw the corresponding MPLS label. Bryant, et al. Informational [Page 27] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 7.3.2. Misconnection and Payload Type Mismatch With PWE3, misconnection and payload type mismatch can occur. If a misconnection occurs it can breach the integrity of the system. If a payload mismatch occurs it can disrupt the customer network. In both instances, there are security concerns. The services of the underlying tunneling mechanism, and its associated control protocol, can be used to mitigate this. This area needs further study. 7.3.3. Packet Loss, Corruption, and Out-of-order Delivery A PW can incur packet loss, corruption, and out-of-order delivery on the PSN path between the PEs. This can impact the working condition of an emulated service. For some payload types, packet loss, corruption, and out-of-order delivery can be mapped to either a bit error burst, or loss of carrier on the PW. If a native service has some mechanism to deal with bit error, the corresponding PWE3 service should provide a similar mechanism. 7.3.4. Other Status Notification A PWE3 approach may provide a mechanism for other status notification, if any. 7.3.5. Collective Status Notification Status of a group of emulated services may be affected identically by a single network incident. For example, when the physical link (or sub-network) between a CE and a PE fails, all the emulated services that go through that link (or sub-network) will fail. It is likely that there exists a group of emulated services that all terminate at a remote CE. There may also be multiple such CEs affected by the failure. Therefore, it is desirable that a single notification message be used to notify failure of the whole group of emulated services. A PWE3 approach may provide some mechanism for notifying status changes of a group of emulated circuits. One possible method is to associate each emulated service with a group ID when the PW for that emulated service is set up. Multiple emulated services can then be grouped by associating them with the same group ID. In status notification, that group ID can be used to refer all the emulated services in that group. This should be a mechanism provided by the underlying tunneling Bryant, et al. Informational [Page 28] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 protocol. 7.4 Keep-alive If a native service has a keep-alive mechanism, the corresponding emulated service needs to use a mechanism to propagate this across the PW. An approach following the principle of minimum intervention would be to transparently transport keep-alive messages over the PW. However, to accurately reproduce the semantics of the native mechanism, some PWs may require an alternative approach, such as piggy-backing on the PW signalling mechanism. 7.5 Handling Control Messages of the Native Services Some native services use control messages for maintaining the circuits. These control messages may be in-band, e.g. Ethernet flow control or ATM performance management, or out-of-band, e.g. the signaling VC of an ATM VP. From the principle of minimum intervention, it is desirable that the PEs participate as little as possible in the signaling and maintenance of the native services. This principle should not, however, override the need to satisfactorily emulate the native service. If control messages are passed through, it may be desirable to send them using a reliable channel provided by the PW Demultiplexer layer. See Bearer Channel Types. 8. IANA considerations There are no IANA considerations for this document. 9. Security Considerations PWE3 provides no means of protecting the contents or delivery of the native data units. PWE3 may, however, leverage security mechanisms provided by the PW Demultiplexer or PSN Layers, such as IPSec [RFC2401]. This section addresses the PWE3 vulnerabilities, and the mechanisms available to protect the emulated native services. The PW Tunnel End-Point, PW demultiplexing mechanism, and the Bryant, et al. Informational [Page 29] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 payloads of the native service are all vulnerable to attack. The security aspects of PWE3 need further study. 9.1 PW Tunnel End-Point and PW Demultiplexer Security Protection mechanisms must be considered for the PW Tunnel end-point and PW Demultiplexer mechanism in order to avoid denial-of-service attacks upon the native service, and to prevent spoofing of the native data units. Exploitation of vulnerabilities from within the PSN may be directed to the PW Tunnel end-point so that PW Demultiplexer and PSN tunnel services are disrupted. Controlling PSN access to the PW Tunnel end-point may protect against this. By restricting PW Tunnel end-point access to legitimate remote PE sources of traffic, the PE may reject traffic that would interfere with the PW demultiplexing and PSN tunnel services. 9.2 Validation of PW Encapsulation Protection mechanisms must address the spoofing of tunneled PW data. The validation of traffic addressed to the PW demultiplexer end-point is paramount in ensuring integrity of PW encapsulation. Security protocols such as IPSec [RFC2401] may be used by the PW Demultiplexer Layer in order to maintain the integrity of the PW by authenticating data between the PW Demultiplexer End-points. IPSec may provide authentication, integrity, non-repudiation, and confidentiality of data transferred between two PE. It cannot provide the equivalent services to the native service. Based on the type of data being transferred, the PW may indicate to the PW Demultiplexer Layer that enhanced security services are required. The PW Demultiplexer Layer may define multiple protection profiles based on the requirements of the PW emulated service. CE- to-CE signaling and control events emulated by the PW and some data types may require additional protection mechanisms. Alternatively, the PW Demultiplexer Layer may use peer authentication for every PSN packet to prevent spoofed native data units from being sent to the destination CE. 9.3 End-to-End Security Protection of the PW encapsulated data stream between PEs should not be considered equivalent to end-to-end security, because the CE-PE interface and the PE processing element remain unprotected. PW service emulation does not preclude the application of additional security mechanisms, such as IPSec, that are implemented end-to-end. Likewise, end-to-end security mechanisms applied in the native Bryant, et al. Informational [Page 30] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 service do not protect the PW demultiplexing and PSN tunnel services provided by the PE for PW encapsulation. Acknowledgments We thank Sasha Vainshtein for his work on Native Service Processing and advice on bit-stream over PW services. We thank Scott Wainner Stephen Casner, Andy Malis and Eric Rosen for their comments and contributions. References Internet-drafts are works in progress available from [BGPAUTO] Using BGP as an Auto-Discovery Mechanism for Network-based VPNs. Ould-Brahim et al. , work in progress. [ETSI] EN 300 744 Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television (DVB-T), European Telecommunications Standards Institute (ETSI) [PPPoL2TP] PPP Tunneling Using Layer Two Tunneling Protocol, J Lau et al. , work in progress. [RFC1191] RFC-1191: Path MTU discovery. J.C. Mogul, S.E. Deering. [RFC1958] RFC-1958: Architectural Principles of the Internet, B. Carpenter et al. [RFC1981] RFC-1981: Path MTU Discovery for IP version 6. J. McCann, S. Deering, J.Mogul. [RFC2401] RFC-2401: Security Architecture for the Internet Protocol. S. Kent, R. Atkinson. [RFC3022] RFC-3022: Traditional IP Network Address Translator (Traditional NAT). P Srisuresh et al. [RTP] RFC-1889: A Transport Protocol for Real-Time Applications, H. Schulzrinne et al. Bryant, et al. Informational [Page 31] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 Authors' Addresses Stewart Bryant Cisco Systems, 4, The Square, Stockley Park, Uxbridge UB11 1BL, United Kingdom. Email: stbryant@cisco.com Danny McPherson TCB. Email: danny@tcb.net W. Mark Townsley Cisco Systems, 7025 Kit Creek Road, PO Box 14987, Research Triangle Park, NC 27709 Email: mark@townsley.net Lloyd Wood Cisco Systems, 4, The Square, Stockley Park, Uxbridge UB11 1BL, United Kingdom. Email: lwood@cisco.com Full copyright statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. Bryant, et al. Informational [Page 32] INTERNET DRAFT draft-ietf-pwe3-protocol-layer-00.txt May 2002 This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Bryant, et al. Informational [Page 33]