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rfc:rfc4448

Network Working Group L. Martini, Ed. Request for Comments: 4448 E. Rosen Category: Standards Track Cisco Systems, Inc.

                                                           N. El-Aawar
                                           Level 3 Communications, LLC
                                                              G. Heron
                                                               Tellabs
                                                            April 2006
 Encapsulation Methods for Transport of Ethernet over MPLS Networks

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 An Ethernet pseudowire (PW) is used to carry Ethernet/802.3 Protocol
 Data Units (PDUs) over an MPLS network.  This enables service
 providers to offer "emulated" Ethernet services over existing MPLS
 networks.  This document specifies the encapsulation of
 Ethernet/802.3 PDUs within a pseudowire.  It also specifies the
 procedures for using a PW to provide a "point-to-point Ethernet"
 service.

Martini, et al. Standards Track [Page 1] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

Table of Contents

 1. Introduction ....................................................3
 2. Specification of Requirements ...................................6
 3. Applicability Statement .........................................6
 4. Details Specific to Particular Emulated Services ................7
    4.1. Ethernet Tagged Mode .......................................7
    4.2. Ethernet Raw Mode ..........................................8
    4.3. Ethernet-Specific Interface Parameter LDP Sub-TLV ..........8
    4.4. Generic Procedures .........................................9
         4.4.1. Raw Mode vs. Tagged Mode ............................9
         4.4.2. MTU Management on the PE/CE Links ..................11
         4.4.3. Frame Ordering .....................................11
         4.4.4. Frame Error Processing .............................11
         4.4.5. IEEE 802.3x Flow Control Interworking ..............11
    4.5. Management ................................................12
    4.6. The Control Word ..........................................12
    4.7. QoS Considerations ........................................13
 5. Security Considerations ........................................14
 6. PSN MTU Requirements ...........................................14
 7. Normative References ...........................................15
 8. Informative References .........................................15
 9. Significant Contributors .......................................17
 Appendix A. Interoperability Guidelines ...........................20
    A.1. Configuration Options .....................................20
    A.2. IEEE 802.3x Flow Control Considerations ...................21
 Appendix B. QoS Details ...........................................21
    B.1. Adaptation of 802.1Q CoS to PSN CoS .......................22
    B.2. Drop Precedence ...........................................23

Martini, et al. Standards Track [Page 2] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

1. Introduction

 An Ethernet pseudowire (PW) allows Ethernet/802.3 [802.3] Protocol
 Data Units (PDUs) to be carried over a Multi-Protocol Label Switched
 [MPLS-ARCH] network.  In addressing the issues associated with
 carrying an Ethernet PDU over a packet switched network (PSN), this
 document assumes that a pseudowire (PW) has been set up by using a
 control protocol such as the one as described in [PWE3-CTRL].  The
 design of Ethernet pseudowire described in this document conforms to
 the pseudowire architecture described in [RFC3985].  It is also
 assumed in the remainder of this document that the reader is familiar
 with RFC 3985.
 The Pseudowire Emulation Edge-to-Edge (PWE3) Ethernet PDU consists of
 the Destination Address, Source Address, Length/Type, MAC Client
 Data, and padding extracted from a MAC frame as a concatenated octet
 sequence in their original order [PDU].
 In addition to the Ethernet PDU format used within the pseudowire,
 this document discusses:
  1. Procedures for using a PW in order to provide a pair of Customer

Edge (CE) routers with an emulated (point-to-point) Ethernet

      service, including the procedures for the processing of Provider
      Edge (PE)-bound and CE-bound Ethernet PDUs [RFC3985]
  1. Ethernet-specific quality of service (QoS) and security

considerations

  1. Inter-domain transport considerations for Ethernet PW
 The following two figures describe the reference models that are
 derived from [RFC3985] to support the Ethernet PW emulated services.

Martini, et al. Standards Track [Page 3] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

          |<-------------- Emulated Service ---------------->|
          |                                                  |
          |          |<------- Pseudowire ------->|          |
          |          |                            |          |
          |          |    |<-- 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
             |                                            |
             |                                            |
    Attachment Circuit (AC)                    Attachment Circuit (AC)
    native Ethernet service                    native Ethernet service
       Figure 1: PWE3 Ethernet/VLAN Interface Reference Configuration
 The "emulated service" shown in Figure 1 is, strictly speaking, a
 bridged LAN; the PEs have MAC interfaces, consume MAC control frames,
 etc.  However, the procedures specified herein only support the case
 in which there are two CEs on the "emulated LAN".  Hence we refer to
 this service as "emulated point-to-point Ethernet".  Specification of
 the procedures for using pseudowires to emulate LANs with more than
 two CEs are out of the scope of the current document.
 +-------------+                                +-------------+
 |  Emulated   |                                |  Emulated   |
 |  Ethernet   |                                |  Ethernet   |
 | (including  |         Emulated Service       | (including  |
 |  VLAN)      |<==============================>|  VLAN)      |
 |  Services   |                                |  Services   |
 +-------------+           Pseudowire           +-------------+
 |Demultiplexer|<==============================>|Demultiplexer|
 +-------------+                                +-------------+
 |    PSN      |            PSN Tunnel          |    PSN      |
 |   MPLS      |<==============================>|   MPLS      |
 +-------------+                                +-------------+
 |  Physical   |                                |  Physical   |
 +-----+-------+                                +-----+-------+
       Figure 2: Ethernet PWE3 Protocol Stack Reference Model

Martini, et al. Standards Track [Page 4] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

 For the purpose of this document, PE1 will be defined as the ingress
 router, and PE2 as the egress router.  A layer 2 PDU will be received
 at PE1, encapsulated at PE1, transported, decapsulated at PE2, and
 transmitted out on the attachment circuit of PE2.
 An Ethernet PW emulates a single Ethernet link between exactly two
 endpoints.  The mechanisms described in this document are agnostic to
 that which is beneath the "Pseudowire" level in Figure 2, concerning
 itself only with the "Emulated Service" portion of the stack.
 The following reference model describes the termination point of each
 end of the PW within the PE:
         +-----------------------------------+
         |                PE                 |
 +---+   +-+  +-----+  +------+  +------+  +-+
 |   |   |P|  |     |  |PW ter|  | PSN  |  |P|
 |   |<==|h|<=| NSP |<=|minati|<=|Tunnel|<=|h|<== From PSN
 |   |   |y|  |     |  |on    |  |      |  |y|
 | C |   +-+  +-----+  +------+  +------+  +-+
 | E |   |                                   |
 |   |   +-+  +-----+  +------+  +------+  +-+
 |   |   |P|  |     |  |PW ter|  | PSN  |  |P|
 |   |==>|h|=>| NSP |=>|minati|=>|Tunnel|=>|h|==> To PSN
 |   |   |y|  |     |  |on    |  |      |  |y|
 +---+   +-+  +-----+  +------+  +------+  +-+
         |                                   |
         +-----------------------------------+
                     ^         ^         ^
                     |         |         |
                     A         B         C
         Figure 3: PW Reference Diagram
 The PW terminates at a logical port within the PE, defined at point B
 in the above diagram.  This port provides an Ethernet MAC service
 that will deliver each Ethernet frame that is received at point A,
 unaltered, to the point A in the corresponding PE at the other end of
 the PW.
 The Native Service Processing (NSP) function includes frame
 processing that is required for the Ethernet frames that are
 forwarded to the PW termination point.  Such functions may include
 stripping, overwriting or adding VLAN tags, physical port
 multiplexing and demultiplexing, PW-PW bridging, L2 encapsulation,
 shaping, policing, etc.  These functions are specific to the Ethernet
 technology, and may not be required for the PW emulation service.

Martini, et al. Standards Track [Page 5] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

 The points to the left of A, including the physical layer between the
 CE and PE, and any adaptation (NSP) functions between it and the PW
 terminations, are outside of the scope of PWE3 and are not defined
 here.
 "PW Termination", between A and B, represents the operations for
 setting up and maintaining the PW, and for encapsulating and
 decapsulating the Ethernet frames as necessary to transmit them
 across the MPLS network.
 An Ethernet PW operates in one of two modes: "raw mode" or "tagged
 mode".  In tagged mode, each frame MUST contain at least one 802.1Q
 [802.1Q] VLAN tag, and the tag value is meaningful to the NSPs at the
 two PW termination points.  That is, the two PW termination points
 must have some agreement (signaled or manually configured) on how to
 process the tag.  On a raw mode PW, a frame MAY contain an 802.1Q
 VLAN tag, but if it does, the tag is not meaningful to the NSPs, and
 passes transparently through them.
 Additional terminology relevant to pseudowires and Layer 2 Virtual
 Private Networking may be found in [RFC4026].

2. Specification of Requirements

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

3. Applicability Statement

 The Ethernet PW emulation allows a service provider to offer a "port
 to port" Ethernet-based service across an MPLS packet switched
 network (PSN) while the Ethernet VLAN PW emulation allows an
 "Ethernet VLAN to VLAN" based service across an MPLS packet switched
 network (PSN).
 The Ethernet or Ethernet VLAN PW has the following characteristics in
 relationship to the respective native service:
  1. An Ethernet PW connects two Ethernet ACs while an Ethernet VLAN

PW connects two Ethernet VLAN ACs, supporting bidirectional

      transport of variable length Ethernet frames.  The ingress
      Native Service Processing (NSP) function strips the preamble and
      frame check sequence (FCS) from the Ethernet frame and
      transports the frame in its entirety across the PW.  This is
      done regardless of the presence of the 802.1Q tag in the frame.
      The egress NSP function receives the Ethernet frame from the PW
      and regenerates the preamble or FCS before forwarding the frame

Martini, et al. Standards Track [Page 6] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

      to the attachment circuit.  Since the FCS is not transported
      across either Ethernet or Ethernet VLAN PWs, payload integrity
      transparency may be lost.  The OPTIONAL method described in
      [FCS] can be used to achieve payload integrity transparency on
      Ethernet or Ethernet VLAN PWs.
  1. For an Ethernet VLAN PW, VLAN tag rewrite can be achieved by NSP

at the egress PE, which is outside the scope of this document.

  1. The Ethernet or Ethernet VLAN PW only supports homogeneous

Ethernet frame type across the PW; both ends of the PW must be

      either tagged or untagged.  Heterogeneous frame type support
      achieved with NSP functionality is outside the scope of this
      document.
  1. Ethernet port or Ethernet VLAN status notification is provided

using the PW Status TLV in the Label Distribution Protocol (LDP)

      status notification message.  Loss of connectivity between PEs
      can be detected by the LDP session closing, or by using [VCCV]
      mechanisms.  The PE can convey these indications back to its
      attached Remote System.
  1. The maximum frame size that can be supported is limited by the

PSN MTU minus the MPLS header size, unless fragmentation and

      reassembly are used [FRAG].
  1. The packet switched network may reorder, duplicate, or silently

drop packets. Sequencing MAY be enabled in the Ethernet or

      Ethernet VLAN PW to detect lost, duplicate, or out-of-order
      packets on a per-PW basis.
  1. The faithfulness of an Ethernet or Ethernet VLAN PW may be

increased by leveraging Quality of Service features of the PEs

      and the underlying PSN.  (See Section 4.7, "QoS
      Considerations".)

4. Details Specific to Particular Emulated Services

4.1. Ethernet Tagged Mode

 The Ethernet frame will be encapsulated according to the procedures
 defined later in this document for tagged mode.  It should be noted
 that if the VLAN identifier is modified by the egress PE, the
 Ethernet spanning tree protocol might fail to work properly.  If this
 issue is of significance, the VLAN identifier MUST be selected in
 such a way that it matches on the attachment circuits at both ends of
 the PW.

Martini, et al. Standards Track [Page 7] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

 If the PE detects a failure on the Ethernet physical port, or the
 port is administratively disabled, it MUST send a PW status
 notification message for all PWs associated with the port.
 This mode uses service-delimiting tags to map input Ethernet frames
 to respective PWs and corresponds to PW type 0x0004 "Ethernet Tagged
 Mode" [IANA].

4.2. Ethernet Raw Mode

 The Ethernet frame will be encapsulated according to the procedures
 defined later in this document for raw mode.  If the PE detects a
 failure on the Ethernet input port, or the port is administratively
 disabled, the PE MUST send an appropriate PW status notification
 message to the corresponding remote PE.
 In this mode, all Ethernet frames received on the attachment circuit
 of PE1 will be transmitted to PE2 on a single PW.  This service
 corresponds to PW type 0x0005 "Ethernet" [IANA].

4.3. Ethernet-Specific Interface Parameter LDP Sub-TLV

 This LDP sub-Type Length Value [LDP] specifies interface-specific
 parameters.  When applicable, it MUST be used to validate that the
 PEs, and the ingress and egress ports at the edges of the circuit,
 have the necessary capabilities to interoperate with each other.  The
 Interface parameter TLV is defined in [PWE3-CTRL], the IANA registry
 with initial values for interface parameter sub-TLV types is defined
 in [IANA], but the Ethernet-specific interface parameters are
 specified as follows:
  1. 0x06 Requested VLAN ID Sub-TLV
      An Optional 16-bit value indicating the requested VLAN ID.  This
      parameter MUST be used by a PE that is incapable of rewriting
      the 802.1Q Ethernet VLAN tag on output.  If the ingress PE
      receives this request, it MUST rewrite the VLAN ID contained
      inside the VLAN Tag at the input to match the requested VLAN ID.
      If this is not possible, and the VLAN ID does not already match
      the configured ingress VLAN ID, the PW MUST not be enabled.
      This parameter is applicable only to PW type 0x0004.

Martini, et al. Standards Track [Page 8] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

4.4. Generic Procedures

 When the NSP/Forwarder hands a frame to the PW termination function:
  1. The preamble (if any) and FCS are stripped off.
  1. The control word as defined in Section 4.6, "The Control Word",

is, if necessary, prepended to the resulting frame. The

      conditions under which the control word is or is not used are
      specified below.
  1. The proper pseudowire demultiplexer (PW Label) is prepended to

the resulting packet.

  1. The proper tunnel encapsulation is prepended to the resulting

packet.

  1. The packet is transmitted.
 The way in which the proper tunnel encapsulation and pseudowire
 demultiplexer is chosen depends on the procedures that were used to
 set up the pseudowire.
 The tunnel encapsulation depends on how the MPLS PSN is set up.  This
 can include no label, one label, or multiple labels.  The proper
 pseudowire demultiplexer is an MPLS label whose value is determined
 by the PW setup and maintenance protocols.
 When a packet arrives over a PW, the tunnel encapsulation and PW
 demultiplexer are stripped off.  If the control word is present, it
 is processed and stripped off.  The resulting frame is then handed to
 the Forwarder/NSP.  Regeneration of the FCS is considered to be an
 NSP responsibility.

4.4.1. Raw Mode vs. Tagged Mode

 When the PE receives an Ethernet frame, and the frame has a VLAN tag,
 we can distinguish two cases:
    1. The tag is service-delimiting.  This means that the tag was
       placed on the frame by some piece of service provider-operated
       equipment, and the tag is used by the service provider to
       distinguish the traffic.  For example, LANs from different
       customers might be attached to the same service provider
       switch, which applies VLAN tags to distinguish one customer's
       traffic from another's, and then forwards the frames to the PE.

Martini, et al. Standards Track [Page 9] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

    2. The tag is not service-delimiting.  This means that the tag was
       placed in the frame by a piece of customer equipment, and is
       not meaningful to the PE.
 Whether or not the tag is service-delimiting is determined by local
 configuration on the PE.
 If an Ethernet PW is operating in raw mode, service-delimiting tags
 are NEVER sent over the PW.  If a service-delimiting tag is present
 when the frame is received from the attachment circuit by the PE, it
 MUST be stripped (by the NSP) from the frame before the frame is sent
 to the PW.
 If an Ethernet PW is operating in tagged mode, every frame sent on
 the PW MUST have a service-delimiting VLAN tag.  If the frame as
 received by the PE from the attachment circuit does not have a
 service-delimiting VLAN tag, the PE must prepend the frame with a
 dummy VLAN tag before sending the frame on the PW.  This is the
 default operating mode.  This is the only REQUIRED mode.
 In both modes, non-service-delimiting tags are passed transparently
 across the PW as part of the payload.  It should be noted that a
 single Ethernet packet may contain more than one tag.  At most, one
 of these tags may be service-delimiting.  In any case, the NSP
 function may only inspect the outermost tag for the purpose of
 adapting the Ethernet frame to the pseudowire.
 In both modes, the service-delimiting tag values have only local
 significance, i.e., are meaningful only at a particular PE-CE
 interface.  When tagged mode is used, the PE that receives a frame
 from the PW may rewrite the tag value, or may strip the tag entirely,
 or may leave the tag unchanged, depending on its configuration.  When
 raw mode is used, the PE that receives a frame may or may not need to
 add a service-delimiting tag before transmitting the frame on the
 attachment circuit; however, it MUST not rewrite or remove any tags
 that are already present.
 The following table illustrates the operations that might be
 performed at input from the attachment circuit:
 +-----------------------------------------------------------+
 |       Tag-> |  service delimiting | non service delimiting|
 |-------------+---------------------+-----------------------|
 |   Raw Mode  | 1st VLAN Tag Removed| no operation performed|
 |-------------+---------------------+-----------------------|
 | Tagged Mode | NO OP or Tag Added  |     Tag Added         |
 +-----------------------------------------------------------+

Martini, et al. Standards Track [Page 10] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

4.4.2. MTU Management on the PE/CE Links

 The Ethernet PW MUST NOT be enabled unless it is known that the MTUs
 of the CE-PE links are the same at both ends of the PW.  If an egress
 router receives an encapsulated layer 2 PDU whose payload length
 (i.e., the length of the PDU itself without any of the encapsulation
 headers) exceeds the MTU of the destination layer 2 interface, the
 PDU MUST be dropped.

4.4.3. Frame Ordering

 In general, applications running over Ethernet do not require strict
 frame ordering.  However, the IEEE definition of 802.3 [802.3]
 requires that frames from the same conversation in the context of
 link aggregation (clause 43) are delivered in sequence.  Moreover,
 the PSN cannot (in the general case) be assumed to provide or to
 guarantee frame ordering.  An Ethernet PW can, through use of the
 control word, provide strict frame ordering.  If this option is
 enabled, any frames that get misordered by the PSN will be dropped or
 reordered by the receiving PW endpoint.  If strict frame ordering is
 a requirement for a particular PW, this option MUST be enabled.

4.4.4. Frame Error Processing

 An encapsulated Ethernet frame traversing a pseudowire may be
 dropped, corrupted, or delivered out-of-order.  As described in
 [PWE3-REQ], frame loss, corruption, and out-of-order delivery are
 considered to be a "generalized bit error" of the pseudowire.  PW
 frames that are corrupted will be detected at the PSN layer and
 dropped.
 At the ingress of the PW, the native Ethernet frame error processing
 mechanisms MUST be enabled.  Therefore, if a PE device receives an
 Ethernet frame containing hardware-level Cyclic Redundancy Check
 (CRC) errors, framing errors, or a runt condition, the frame MUST be
 discarded on input.  Note that defining this processing is part of
 the NSP function and is outside the scope of this document.

4.4.5. IEEE 802.3x Flow Control Interworking

 In a standard Ethernet network, the flow control mechanism is
 optional and typically configured between the two nodes on a point-
 to-point link (e.g., between the CE and the PE).  IEEE 802.3x PAUSE
 frames MUST NOT be carried across the PW.  See Appendix A for notes
 on CE-PE flow control.

Martini, et al. Standards Track [Page 11] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

4.5. Management

 The Ethernet PW management model follows the general PW management
 model defined in [RFC3985] and [PWE3-MIB].  Many common PW management
 facilities are provided here, with no additional Ethernet specifics
 necessary.  Ethernet-specific parameters are defined in an additional
 MIB module, [PW-MIB].

4.6. The Control Word

 The control word defined in this section is based on the Generic PW
 MPLS Control Word as defined in [PWE3-CW].  It provides the ability
 to sequence individual frames on the PW, avoidance of equal-cost
 multiple-path load-balancing (ECMP) [RFC2992], and Operations and
 Management (OAM) mechanisms including VCCV [VCCV].
 [PWE3-CW] states, "If a PW is sensitive to packet misordering and is
 being carried over an MPLS PSN that uses the contents of the MPLS
 payload to select the ECMP path, it MUST employ a mechanism which
 prevents packet misordering." This is necessary because ECMP
 implementations may examine the first nibble after the MPLS label
 stack to determine whether the labelled packet is IP or not.  Thus,
 if the source MAC address of an Ethernet frame carried over the PW
 without a control word present begins with 0x4 or 0x6, it could be
 mistaken for an IPv4 or IPv6 packet.  This could, depending on the
 configuration and topology of the MPLS network, lead to a situation
 where all packets for a given PW do not follow the same path.  This
 may increase out-of-order frames on a given PW, or cause OAM packets
 to follow a different path than actual traffic (see Section 4.4.3,
 "Frame Ordering").
 The features that the control word provides may not be needed for a
 given Ethernet PW.  For example, ECMP may not be present or active on
 a given MPLS network, strict frame sequencing may not be required,
 etc.  If this is the case, the control word provides little value and
 is therefore optional.  Early Ethernet PW implementations have been
 deployed that do not include a control word or the ability to process
 one if present.  To aid in backwards compatibility, future
 implementations MUST be able to send and receive frames without the
 control word present.
 In all cases, the egress PE MUST be aware of whether the ingress PE
 will send a control word over a specific PW.  This may be achieved by
 configuration of the PEs, or by signaling, as defined in [PWE3-CTRL].

Martini, et al. Standards Track [Page 12] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

 The control word is defined as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0|   Reserved            |       Sequence Number         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 In the above diagram, the first 4 bits MUST be set to 0 to indicate
 PW data.  The rest of the first 16 bits are reserved for future use.
 They MUST be set to 0 when transmitting, and MUST be ignored upon
 receipt.
 The next 16 bits provide a sequence number that can be used to
 guarantee ordered frame delivery.  The processing of the sequence
 number field is OPTIONAL.
 The sequence number space is a 16-bit, unsigned circular space.  The
 sequence number value 0 is used to indicate that the sequence number
 check algorithm is not used.  The sequence number processing
 algorithm is found in [PWE3-CW].

4.7. QoS Considerations

 The ingress PE MAY consider the user priority (PRI) field [802.1Q] of
 the VLAN tag header when determining the value to be placed in a QoS
 field of the encapsulating protocol (e.g., the EXP fields of the MPLS
 label stack).  In a similar way, the egress PE MAY consider the QoS
 field of the encapsulating protocol (e.g., the EXP fields of the MPLS
 label stack) when queuing the frame for transmission towards the CE.
 A PE MUST support the ability to carry the Ethernet PW as a best-
 effort service over the MPLS PSN.  PRI bits are kept transparent
 between PE devices, regardless of the QoS support of the PSN.
 If an 802.1Q VLAN field is added at the PE, a default PRI setting of
 zero MUST be supported, a configured default value is recommended, or
 the value may be mapped from the QoS field of the PSN, as referred to
 above.
 A PE may support additional QoS support by means of one or more of
 the following methods:
      i.  One class of service (CoS) per PW End Service (PWES), mapped
          to a single CoS PW at the PSN.
     ii.  Multiple CoS per PWES mapped to a single PW with multiple
          CoS at the PSN.
    iii.  Multiple CoS per PWES mapped to multiple PWs at the PSN.

Martini, et al. Standards Track [Page 13] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

 Examples of the cases above and details of the service mapping
 considerations are described in Appendix B.
 The PW guaranteed rate at the MPLS PSN level is PW service provider
 policy based on agreement with the customer, and may be different
 from the Ethernet physical port rate.

5. Security Considerations

 The Ethernet pseudowire type is subject to all of the general
 security considerations discussed in [RFC3985] and [PWE3-CTRL].
 The Ethernet pseudowire is transported on an MPLS PSN; therefore, the
 security of the pseudowire itself will only be as good as the
 security of the MPLS PSN.  The MPLS PSN can be secured by various
 methods, as described in [MPLS-ARCH].
 Security achieved by access control of MAC addresses is out of the
 scope of this document.  Additional security requirements related to
 the use of PW in a switching (virtual bridging) environment are not
 discussed here as they are not within the scope of this document.

6. PSN MTU Requirements

 The MPLS PSN MUST be configured with an MTU that is large enough to
 transport a maximum-sized Ethernet frame that has been encapsulated
 with a control word, a pseudowire demultiplexer, and a tunnel
 encapsulation.  With MPLS used as the tunneling protocol, for
 example, this is likely to be 8 or more bytes greater than the
 largest frame size.  The methodology described in [FRAG] MAY be used
 to fragment encapsulated frames that exceed the PSN MTU.  However, if
 [FRAG] is not used and if the ingress router determines that an
 encapsulated layer 2 PDU exceeds the MTU of the PSN tunnel through
 which it must be sent, the PDU MUST be dropped.

Martini, et al. Standards Track [Page 14] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

7. Normative References

 [PWE3-CW]    Bryant, S., Swallow, G., and D. McPherson, "Pseudowire
              Emulation Edge-to-Edge (PWE3) Control Word for Use over
              an MPLS PSN", RFC 4385, February 2006.
 [IANA]       Martini, L., "IANA Allocations for Pseudowire Edge to
              Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
 [PWE3-CTRL]  Martini, L., El-Aawar, N., Heron, G., Rosen, E., Tappan,
              D., and T. Smith, "Pseudowire Setup and Maintenance
              using the Label Distribution Protocol (LDP)", RFC 4447,
              April 2006.
 [MPLS-ARCH]  Rosen, E., Viswanathan, A., and R. Callon,
              "Multiprotocol Label Switching Architecture", RFC 3031,
              January 2001.
 [802.3]      IEEE802.3-2005, ISO/IEC 8802-3: 2000 (E), "IEEE Standard
              for Information technology -- Telecommunications and
              information exchange between systems -- Local and
              metropolitan
               area networks -- Specific requirements -- Part 3:
              Carrier Sense Multiple Access with Collision Detection
              (CSMA/CD) Access Method and Physical Layer
              Specifications", 2005.
 [802.1Q]     ANSI/IEEE Standard 802.1Q-2005, "IEEE Standards for
              Local and Metropolitan Area Networks: Virtual Bridged
              Local Area Networks", 2005.
 [PDU]        IEEE Std 802.3, 1998 Edition, "Part 3: Carrier sense
              multiple access with collision detection (CSMA/CD)
              access method and physical layer specifications" figure
              3.1, 1998
 [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

8. Informative References

 [RFC3985]    Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
              Edge (PWE3) Architecture", RFC 3985, March 2005.
 [PW-MIB]     Zelig, D. and T. Nadeau, "Ethernet Pseudo Wire (PW)
              Management Information Base", Work in Progress, February
              2006.

Martini, et al. Standards Track [Page 15] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

 [PWE3-REQ]   Xiao, X., McPherson, D., and P. Pate, "Requirements for
              Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC 3916,
              September 2004.
 [PWE3-MIB]   Zelig, D., Ed. and T. Nadeau, Ed., "Pseudo Wire (PW)
              Management Information Base", Work in Progress, February
              2006.
 [LDP]        Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
              and B. Thomas, "LDP Specification", RFC 3036, January
              2001.
 [FRAG]       Malis, A. and W. Townsley, "PWE3 Fragmentation and
              Reassembly", Work in Progress, February 2005.
 [FCS]        Malis, A., Allan, D., and N. Del Regno, "PWE3 Frame
              Check Sequence Retention", Work in Progress, September
              2005.
 [VCCV]       Nadeau, T., Ed. and R. Aggarwal, Ed., "Pseudo Wire
              Virtual Circuit Connectivity Verification (VCCV)", Work
              in Progress, August 2005.
 [RFC2992]    Hopps, C., "Analysis of an Equal-Cost Multi-Path
              Algorithm", RFC 2992, November 2000.
 [RFC4026]    Andersson, L. and T. Madsen, "Provider Provisioned
              Virtual Private Network (VPN) Terminology", RFC 4026,
              March 2005.
 [L2TPv3]     Lau, J., Townsley, M., and I. Goyret, "Layer Two
              Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931,
              March 2005.

Martini, et al. Standards Track [Page 16] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

9. Significant Contributors

 Andrew G. Malis
 Tellabs
 90 Rio Robles Dr.
 San Jose, CA 95134
 EMail: Andy.Malis@tellabs.com
 Dan Tappan
 Cisco Systems, Inc.
 1414 Massachusetts Avenue
 Boxborough, MA 01719
 EMail: tappan@cisco.com
 Steve Vogelsang
 ECI Telecom
 Omega Corporate Center
 1300 Omega Drive
 Pittsburgh, PA 15205
 EMail: stephen.vogelsang@ecitele.com
 Vinai Sirkay
 Reliance Infocomm
 Dhirubai Ambani Knowledge City
 Navi Mumbai 400 709
 India
 EMail: vinai@sirkay.com
 Vasile Radoaca
 Nortel Networks
 600  Technology Park
 Billerica MA 01821
 EMail: vasile@nortelnetworks.com

Martini, et al. Standards Track [Page 17] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

 Chris Liljenstolpe
 Alcatel
 11600 Sallie Mae Dr.
 9th Floor
 Reston, VA 20193
 EMail: chris.liljenstolpe@alcatel.com
 Kireeti Kompella
 Juniper Networks
 1194 N. Mathilda Ave
 Sunnyvale, CA 94089
 EMail: kireeti@juniper.net
 Tricci So
 Nortel Networks 3500 Carling Ave.,
 Nepean, Ontario,
 Canada, K2H 8E9.
 EMail: tso@nortelnetworks.com
 XiPeng Xiao
 Riverstone Networks
 5200 Great America Parkway
 Santa Clara, CA 95054
 EMail: xxiao@riverstonenet.com
 Christopher O.  Flores
 T-Systems
 10700 Parkridge Boulevard
 Reston, VA 20191
 USA
 EMail: christopher.flores@usa.telekom.de
 David Zelig
 Corrigent Systems
 126, Yigal Alon St.
 Tel Aviv, ISRAEL
 EMail: davidz@corrigent.com

Martini, et al. Standards Track [Page 18] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

 Raj Sharma
 Luminous Networks, Inc.
 10460 Bubb Road
 Cupertino, CA 95014
 EMail: raj@luminous.com
 Nick Tingle
 TiMetra Networks
 274 Ferguson Drive
 Mountain View, CA 94043
 EMail: nick@timetra.com
 Sunil Khandekar
 TiMetra Networks
 274 Ferguson Drive
 Mountain View, CA 94043
 EMail: sunil@timetra.com
 Loa Andersson
 TLA-group
 EMail: loa@pi.se

Martini, et al. Standards Track [Page 19] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

Appendix A. Interoperability Guidelines

A.1. Configuration Options

 The following is a list of the configuration options for a point-to-
 point Ethernet PW based on the reference points of Figure 3:
  1. ————-|—————|—————|——————

Service and | Encap on C |Operation at B | Remarks

 Encap on A    |               |ingress/egress |
 --------------|---------------|---------------|------------------
 1) Raw        | Raw - Same as |               |
               | A             |               |
               |               |               |
 --------------|---------------|---------------|------------------
 2) Tag1       | Tag2          |Optional change| VLAN can be
               |               |of VLAN value  | 0-4095
               |               |               | Change allowed in
               |               |               | both directions
 --------------|---------------|---------------|------------------
 3) No Tag     | Tag           |Add/remove Tag | Tag can be
               |               |field          | 0-4095
               |               |               | (note i)
               |               |               |
 --------------|---------------|---------------|------------------
 4) Tag        | No Tag        |Remove/add Tag | (note ii)
               |               |field          |
               |               |               |
               |               |               |
 --------------|---------------|---------------|------------------
                    Figure 4: Configuration Options
 Allowed combinations:
 Raw and other services are not allowed on the same NSP virtual port
 (A).  All other combinations are allowed, except that conflicting
 VLANs on (A) are not allowed.  Note that in most point-to-point PW
 applications the NSP virtual port is the same entity as the physical
 port.
 Notes:
      i.  Mode #3 MAY be limited to adding VLAN NULL only, since
          change of VLAN or association to specific VLAN can be done
          at the PW CE-bound side.

Martini, et al. Standards Track [Page 20] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

     ii.  Mode #4 exists in layer 2 switches, but is not recommended
          when operating with PW since it may not preserve the user's
          PRI bits.  If there is a need to remove the VLAN tag (for
          TLS at the other end of the PW), it is recommended to use
          mode #2 with tag2=0 (NULL VLAN) on the PW and use mode #3 at
          the other end of the PW.

A.2. IEEE 802.3x Flow Control Considerations

 If the receiving node becomes congested, it can send a special frame,
 called the PAUSE frame, to the source node at the opposite end of the
 connection.  The implementation MUST provide a mechanism for
 terminating PAUSE frames locally (i.e., at the local PE).  It MUST
 operate as follows: PAUSE frames received on a local Ethernet port
 SHOULD cause the PE device to buffer, or to discard, further Ethernet
 frames for that port until the PAUSE condition is cleared.
 Optionally, the PE MAY simply discard PAUSE frames.
 If the PE device wishes to pause data received on a local Ethernet
 port (perhaps because its own buffers are filling up or because it
 has received notification of congestion within the PSN), then it MAY
 issue a PAUSE frame on the local Ethernet port, but MUST clear this
 condition when willing to receive more data.

Appendix B. QoS Details

 Section 4.7, "QoS Considerations", describes various modes for
 supporting PW QOS over the PSN.  Examples of the above for a point-
 to-point VLAN service are:
  1. The classification to the PW is based on VLAN field, but the

user PRI bits are mapped to different CoS markings (and network

      behavior) at the PW level.  An example of this is a PW mapped to
      an E-LSP in an MPLS network.
  1. The classification to the PW is based on VLAN field and the PRI

bits, and frames with different PRI bits are mapped to different

      PWs.  An example is to map a PWES to different L-LSPs in MPLS
      PSN in order to support multiple CoS over an L-LSP-capable
      network, or to map a PWES to multiple L2TPv3 sessions [L2TPv3].
      The specific value to be assigned at the PSN for various CoS is
      out of the scope of this document.

Martini, et al. Standards Track [Page 21] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

B.1. Adaptation of 802.1Q CoS to PSN CoS

 It is not required that the PSN will have the same CoS definition of
 CoS as defined in [802.1Q], and the mapping of 802.1Q CoS to PSN CoS
 is application specific and depends on the agreement between the
 customer and the PW provider.  However, the following principles
 adopted from 802.1Q, Table 8-2, MUST be met when applying the set of
 PSN CoS based on user's PRI bits.
  1. ———————————

|#of available classes of service|

  1. ————||—+—+—+—+—+—+—+—|

User || 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |

 Priority     ||   |   |   |   |   |   |   |   |
 ===============================================
 0 Best Effort|| 0 | 0 | 0 | 1 | 1 | 1 | 1 | 2 |
 (Default)    ||   |   |   |   |   |   |   |   |
 ------------ ||---+---+---+---+---+---+---+---|
 1 Background || 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
              ||   |   |   |   |   |   |   |   |
 ------------ ||---+---+---+---+---+---+---+---|
 2 Spare      || 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 |
              ||   |   |   |   |   |   |   |   |
 ------------ ||---+---+---+---+---+---+---+---|
 3 Excellent  || 0 | 0 | 0 | 1 | 1 | 2 | 2 | 3 |
 Effort       ||   |   |   |   |   |   |   |   |
 ------------ ||---+---+---+---+---+---+---+---|
 4 Controlled || 0 | 1 | 1 | 2 | 2 | 3 | 3 | 4 |
 Load         ||   |   |   |   |   |   |   |   |
 ------------ ||---+---+---+---+---+---+---+---|
 5 Interactive|| 0 | 1 | 1 | 2 | 3 | 4 | 4 | 5 |
 Multimedia   ||   |   |   |   |   |   |   |   |
 ------------ ||---+---+---+---+---+---+---+---|
 6 Interactive|| 0 | 1 | 2 | 3 | 4 | 5 | 5 | 6 |
 Voice        ||   |   |   |   |   |   |   |   |
 ------------ ||---+---+---+---+---+---+---+---|
 7 Network    || 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
 Control      ||   |   |   |   |   |   |   |   |
 ------------ ||---+---+---+---+---+---+---+---|
                   Figure 5: IEEE 802.1Q CoS Mapping

Martini, et al. Standards Track [Page 22] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

B.2. Drop Precedence

 The 802.1P standard does not support drop precedence; therefore, from
 the PW PE-bound point of view there is no mapping required.  It is,
 however, possible to mark different drop precedence for different PW
 frames based on the operator policy and required network behavior.
 This functionality is not discussed further here.
 PSN QoS support and signaling of QoS are out of the scope of this
 document.

Authors' Addresses

 Luca Martini, Editor
 Cisco Systems, Inc.
 9155 East Nichols Avenue, Suite 400
 Englewood, CO, 80112
 EMail: lmartini@cisco.com
 Nasser El-Aawar
 Level 3 Communications, LLC.
 1025 Eldorado Blvd.
 Broomfield, CO, 80021
 EMail: nna@level3.net
 Giles Heron
 Tellabs
 Abbey Place
 24-28 Easton Street
 High Wycombe
 Bucks
 HP11 1NT
 UK
 EMail: giles.heron@tellabs.com
 Eric C. Rosen
 Cisco Systems, Inc.
 1414 Massachusetts Avenue
 Boxborough, MA 01719
 EMail: erosen@cisco.com

Martini, et al. Standards Track [Page 23] RFC 4448 Encapsulation of Ethernet over MPLS April 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

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Martini, et al. Standards Track [Page 24]

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