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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 RFC 4448

Network Working Group                                       Luca Martini
Internet Draft                                           Nasser El-Aawar
Expiration Date: August 2003                Level 3 Communications, LLC.

Giles Heron                                                Eric C. Rosen
PacketExchange Ltd.                                  Cisco Systems, Inc.




                                                           February 2003


Encapsulation Methods for Transport of Ethernet Frames Over IP/MPLS Networks


                 draft-ietf-pwe3-ethernet-encap-02.txt

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

   An Ethernet Pseudowire (PW) is used to carry Ethernet/802.3 Protocol
   Data Units over an IP or MPLS network. This enables service providers
   to offer "emulated" ethernet services over existing IP or 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.





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Table of Contents

    1      Specification of Requirements  ..........................   2
    2      Introduction  ...........................................   2
    3      Requirements for Ethernet Pseudo-Wires Emulating Point-to-Point Ethernet Links  4
    3.1    Frame Processing at the PW Endpoints  ...................   6
    3.1.1  Encapsulation  ..........................................   6
    3.1.2  Tagged Mode  ............................................   6
    3.1.3  MTU Management on the PE/CE Links  ......................   6
    3.1.4  Frame Ordering  .........................................   6
    3.1.5  Frame Error Processing  .................................   7
    3.1.6  IEEE 802.3x Flow Control Interworking  ..................   7
    3.2    PW Setup and Maintenance  ...............................   7
    3.3    Management  .............................................   7
    3.4    The Control Word  .......................................   8
    3.4.1  Setting the sequence number  ............................   8
    3.4.2  Processing the sequence number  .........................   9
    3.5    QoS Considerations  .....................................   9
    3.6    Security Considerations  ................................  10
    3.7    PSN MTU Requirements  ...................................  10
    4      Intellectual Property Disclaimer  .......................  11
    5      References  .............................................  11
    6      Author Information  .....................................  12
           Appendix A - Interoperability Guidelines  ...............  15
           Appendix B - QoS Details  ...............................  16





1. 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


2. Introduction

   An Ethernet Pseudowire (PW) allows Ethernet/802.3 Protocol Data Units
   (PDUs) to be carried over an IP network or an MPLS network. In
   addressing the issues associated with carrying an Ethernet PDU over a
   PSN, this document assumes that a Pseudowire (PW) has been set up by
   some means outside the scope of this document. This may be via manual
   configuration, or a signaling protocol such as that defined in
   [PWE3-CTRL] or [L2TPv3]. As described in [PWE3-FRAME], this PW may be



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   tunneled through an MPLS, IPv4 or IPv6 PSN.

   In addition to the Ethernet PDU format used within the pseudowire,
   this document discusses:

     - Procedures for using a PW in order to provide a pair of CEs with
       an emulated (point-to-point) ethernet service, including the
       procedures for the processing of PE-bound and CE-bound ethernet
       PDUs. [PWE3-FRAME]

     - Ethernet-specific QoS and security considerations

     - Inter-domain transport considerations for Ethernet PW

   The following two figures describe the reference models which are
   derived from [PWE3-FRAME] to support the Ethernet PW emulated
   services.

           Native    |<----- Pseudo Wire ---->|  Native
          Ethernet   |                        |  Ethernet
             or      |  |<-- PSN Tunnel -->|  |    or
           VLAN      V  V                  V  V   VLAN
          Service  +----+                  +----+ Service
   +----+    |     | PE1|==================| PE2|     |    +----+
   |    |----------|............PW1.............|----------|    |
   | CE1|    |     |    |                  |    |     |    |CE2 |
   |    |----------|............PW2.............|----------|    |
   +----+    |     |    |==================|    |     |    +----+
        ^          +----+                  +----+     |    ^
        |      Provider Edge 1         Provider Edge 2     |
        |                                                  |
        |<-------------- Emulated 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 scope of the current document.









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   +-------------+                                +-------------+
   |  Emulated   |                                |  Emulated   |
   |  Ethernet   |                                |  Ethernet   |
   | (including  |         Emulated Service       | (including  |
   |  VLAN)      |<==============================>|  VLAN)      |
   |  Services   |                                |  Services   |
   +-------------+           Pseudo Wire          +-------------+
   |Demultiplexer|<==============================>|Demultiplexor|
   +-------------+                                +-------------+
   |    PSN      |            PSN Tunnel          |    PSN      |
   | MPLS or IP  |<==============================>| MPLS or IP  |
   +-------------+                                +-------------+
   |  Physical   |                                |  Physical   |
   +-----+-------+                                +-----+-------+

      Figure 2: Ethernet PWE3 Protocol Stack Reference Model

   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 of PE2.


3. Requirements for Ethernet Pseudo-Wires Emulating Point-to-Point
   Ethernet Links

   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 "Pseudo Wire" 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:


















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           +-----------------------------------+
           |                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

           Figure 3: PW reference diagram

   The PW terminates at a logical port within the PE, defined at point A
   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 "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.

   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 according to the PSN type in use.

   An ethernet PW can operate in one of two modes: "raw mode" or "tagged
   mode".  In tagged mode, each frame MUST contain an 802.1Q VLAN tag,
   and the tag value is meaningful to the NSPs at the two PW endpoints.
   That is, the two endpoints 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



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   meaningful to the NSPs, and passes transparently through them.


3.1. Frame Processing at the PW Endpoints

3.1.1. Encapsulation

   The entire Ethernet frame without any preamble or FCS is transported
   as a single frame over the Pseudowire.  Note that when using the
   signaling procedures defined in [PWE3-CRTL] or [L2TPv3], a "Raw Mode"
   PW should be signaled as being of type "Ethernet".


3.1.2. Tagged Mode

   The ethernet frame may contain an 802.1Q tag, in this case the PE
   MUST signal that the PW will is transporting ethernet frames
   including 802.1Q tags. In this case all frames in a PW MUST have the
   same 802.1Q tag value.  Note that the tag may be overwritten by the
   NSP function at ingress or at egress. Note that when using the
   signaling procedures defined in [PWE3-CRTL] or [L2TPv3], a "Tagged
   Mode" PW should be signaled as being of type "Ethernet VLAN".


3.1.3. 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.


3.1.4. 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 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 which get misordered by the PSN will be dropped
   by the receiving PW endpoint. If strict frame ordering is a
   requirement for a particular PW, this option MUST be enabled.










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3.1.5. Frame Error Processing

   An encapsulated Ethernet frame traversing a psuedo-wire may be
   dropped, corrupted or delivered out-of-order. As described in [PWE3-
   REQ], frame-loss, corruption, and out-of-order delivery is considered
   to be a "generalized bit error" of the psuedo-wire. 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 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 draft.


3.1.6. IEEE 802.3x Flow Control Interworking

   In a standard gigabit 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.


3.2. PW Setup and Maintenance

   This document assumes that a mechanism exists to set up the ethernet
   PW.  Maintenance of the PW (e.g. keepalives, status updates, etc) is
   generally tied closely to the PW Setup mechanisms. [PWE3-CTRL] and
   [L2TPv3] define two mechanisms for setup and maintenance of Ethernet
   PWs.


3.3. Management

   The Ethernet PW management model follows the general management
   defined in [PWE3-FRAME] 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-ENET-MIB].

   As specified in [PWE3-FRAME], an implementation SHOULD support the
   generic and specific PW MIB modules for PW set-up and monitoring.
   Other mechanisms for PW set up (command line interface for example)
   MAY be supported.





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3.4. The Control Word

   When carrying Ethernet over an IP or MPLS backbone sequentiality may
   need to be preserved.  The OPTIONAL control word defined here
   addresses this requirement.  Implementations MUST support sending no
   control word, and MAY support sending a control word.

   In all cases the egress router must be aware of whether the ingress
   router will send a control word over a specific virtual circuit.
   This may be achieved by configuration of the routers, or by
   signaling, for example as defined in [PWE3-CRTL].

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Reserved            |       Sequence Number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In the above diagram 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 an unsequenced frame.


3.4.1. Setting the sequence number

   For a given PW, and a pair of routers PE11 and PE2, if PE11 supports
   frame sequencing then the following procedures should be used:

     - the initial frame transmitted on the PW MUST use sequence number
       1
     - subsequent frames MUST increment the sequence number by one for
       each frame
     - when the transmit sequence number reaches the maximum 16 bit
       value (65535) the sequence number MUST wrap to 1

   If the transmitting router PE1 does not support sequence number
   processing, then the sequence number field in the control word MUST
   be set to 0.




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3.4.2. Processing the sequence number

   If a router PE2 supports receive sequence number processing, then the
   following procedures should be used:

   When a PW is initially set up, the "expected sequence number"
   associated with it MUST be initialized to 1.

   When a frame is received on that PW, the sequence number should be
   processed as follows:

     - if the sequence number on the frame is 0, then the frame passes
       the sequence number check

     - otherwise if the frame sequence number >= the expected sequence
       number and the frame sequence number - the expected sequence
       number < 32768, then the frame is in order.

     - otherwise if the frame sequence number < the expected sequence
       number and the expected sequence number - the frame sequence
       number >= 32768, then the frame is in order.

     - otherwise the frame is out of order.

   If a frame passes the sequence number check, or is in order then, it
   can be delivered immediately. If the frame is in order, then the
   expected sequence number should be set using the algorithm:

   expected_sequence_number := frame_sequence_number + 1 mod 2**16
   if (expected_sequence_number = 0) then expected_sequence_number := 1;

   Packets which are received out of order MAY be dropped or reordered
   at the discretion of the receiver.

   If a router PE2 does not support receive sequence number processing,
   then the sequence number field MAY be ignored.


3.5. 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 or the DSCP of an IP packet).  In a similar way, the
   egress PE MAY consider the QoS field of the PSN's encapsulating
   protocol when queuing the frame for CE-bound.

   A PE MUST support the ability to carry the Ethernet PW as a best



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   effort service over the 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 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.

   Examples of the cases above and details of the service mapping
   considerations are described in Appendix B.

   The PW guaranteed rate at the PSN level is PW provider policy based
   on agreement with the customer, and may be different from the
   Ethernet physical port rate.


3.6. Security Considerations

   The ethernet pseudowire type is subject to all of the general
   security considerations discussed in [PWE3-ARCH].

   Security achieved by access control of MAC addresses is out of 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 draft.


3.7. PSN MTU Requirements

   The PSN MUST be configured with an MTU that is large enough to
   transport a maximum sized ethernet frame which has been encapsulated
   with a control word, a pseudowire demultiplexor, and a tunnel
   encapsulation.  If MPLS is used as the tunneling protocol, for
   example, this is likely to be 8 or more bytes greater than the
   largest frame size. Other tunneling protocols may have longer headers
   and require larger MTUs. If the ingress router determines that an
   encapsulated layer 2 PDU exceeds the MTU of the tunnel through which
   it must be sent, the PDU MUST be dropped. If an egress router
   receives an encapsulated layer 2 PDU whose payload length (i.e., the



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   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. Intellectual Property Disclaimer

   This document is being submitted for use in IETF standards
   discussions.


5. References

   [PWE3-CRTL] "Transport of Layer 2 Frames Over MPLS",
        Martini, L., et al., draft-ietf-pwe3-control-protocol-01.txt,
        ( work in progress ), February 2003.

   [PWE3-ARCH] "Protocol Layering in PWE3"
        Bryant, et al., draft-ietf-pwe3-protocol-layer-00.txt
        ( work in progress ), November 2002.

   [PWE3-REQ] "Requirements for Pseudo Wire Emulation Edge-to-Edge
        (PWE3)", Xiao, X., McPherson, D., Pate, P., White, C.,
        Kompella, K., Gill, V., Nadeau, T.,
        draft-pwe3-requirements-03.txt, ( work in progress ), June 2002.

   [PWE3-FRAME] "Framework for Pseudo Wire Emulation Edge-to-Edge
        (PWE3)", Pate, P., Xiao, X., So, T., Malis, A., Nadeau, T.,
        White, C., Kompella, K., Johnson, T., Bryant, S.,
        draft-pate-pwe3-framework-03.txt, ( work in progress ),
        June 2002.

   [PW-MIB] "Pseudo Wire (PW) Management Information Base using SMIv2",
        Zelig, D., Mantin, S., Nadeau, T., Danenberg, D.,
        draft-zelig-pw-mib-02.txt, ( work in progress), February 2002.

   [PW-ENET-MIB] "Ethernet Pseudo Wire (PW) Management Information
        Base", Zelig, D., Nadeau, T., draft-zelig-pw-enet-mib-00.txt,
        ( work in progress ) February 2002.

   [802.3] IEEE, 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", 2000.

   [802.1Q] ANSI/IEEE Standard 802.1Q, "IEEE Standards for Local and



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        Metropolitan Area Networks: Virtual Bridged Local Area
        Networks", 1998.

   [L2TPv3] J. Lau, M. Townsley, A. Valencia, G. Zorn, I. Goyret,
        G. Pall, A. Rubens, B. Palter, Layer Two Tunneling Protocol
        (Version 3) "L2TPv3", work in progress,
        draft-ietf-l2tpext-l2tp-base-03.txt, June 2002.


6. Author Information


   Luca Martini
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021
   e-mail: luca@level3.net


   Nasser El-Aawar
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021
   e-mail: nna@level3.net


   Giles Heron
   PacketExchange Ltd.
   The Truman Brewery
   91 Brick Lane
   LONDON E1 6QL
   United Kingdom
   e-mail: giles@packetexchange.net


   Dan Tappan
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   e-mail: tappan@cisco.com


   Eric Rosen
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   e-mail: erosen@cisco.com




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   Steve Vogelsang
   Laurel Networks, Inc.
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   e-mail: sjv@laurelnetworks.com


   Andrew G. Malis
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134
   e-mail: Andy.Malis@vivacenetworks.com


   Vinai Sirkay
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134
   e-mail: sirkay@technologist.com


   Vasile Radoaca
   Nortel Networks
   600  Technology Park
   Billerica MA 01821
   e-mail: vasile@nortelnetworks.com


   Chris Liljenstolpe
   Cable & Wireless
   11700 Plaza America Drive
   Reston, VA 20190
   e-mail: chris@cw.net


   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA 94089
   e-mail: kireeti@juniper.net


   Tricci So
   e-mail: tricciso@yahoo.ca





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   XiPeng Xiao
   Redback Networks
   300 Holger Way,
   San Jose, CA 95134
   e-mail: xipeng@redback.com


   Chris Flores
   Austin, Texas
   e-mail: chris_flores@hotmail.com


   David Zelig
   Corrigent Systems
   126, Yigal Alon St.
   Tel Aviv, ISRAEL
   e-mail: davidz@corrigent.com


   Raj Sharma
   Luminous Netwokrs, Inc.
   10460 Bubb Road
   Cupertino, CA 95014
   e-mail: raj@luminous.com


   Nick Tingle
   TiMetra Networks
   274 Ferguson Drive
   Mountain View, CA 94043
   e-mail: nick@timetra.com


   Sunil Khandekar
   TiMetra Networks
   274 Ferguson Drive
   Mountain View, CA 94043
   email: sunil@timetra.com


   Loa Andersson
   Utfors
   P.O. Box 525,
   SE-169 29 Solna, Sweden
   e-mail: loa.andersson@utfors.se





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Appendix A - Interoperability Guidelines

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:

   --------------|---------------|---------------|------------------
   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 physical port (A).
   All other combinations are allowed, except that conflicting VLANs on
   (A) are not allowed.

   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.







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       -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.


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 3.7 describes various modes for supporting PW QOS over the
   PSN.  Examples of the above for a point to point VLAN service are:

     - The classification to the PW is based on VLAN field only,
       regardless of the user PRI bits.  The PW is assigned a specific
       COS (marking, scheduling, etc.)  at the tunnel level.

     - The classification to the PW is based on VLAN field, but the PRI
       bits of the user is mapped to different COS marking (and network
       behavior) at the PW level.  Examples are DiffServ coding in case
       of IP PSN, and E-LSP in MPLS PSN.

     - 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 multiple L2TPv3 sessions [L2TPv3].




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       The specific value to be assigned at the PSN for various COS is
       out of scope for this document.


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 set of PSN
   COS based on user's PRI bits.

                ----------------------------------
                |#of available classes of service|
   -------------||---|---|---|---|---|---|---|---|
   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 Service Mapping






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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 is out of scope of this
   document.









































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