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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 4618

Network Working Group                                       Luca Martini
Internet Draft                                             Eric C. Rosen
Expiration Date: June 2006                           Cisco Systems, Inc.

Giles Heron
Andrew G. Malis

                                                           December 2005

   Encapsulation Methods for Transport of PPP/HDLC Over MPLS Networks


Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   A Pseudowire (PW) can be used to carry PPP, or HDLC Protocol Data
   Units over an MPLS network without terminating the PPP/HDLC protocol.
   This enables service providers to offer "emulated" HDLC, or PPP link
   services over existing MPLS networks. This document specifies the

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   encapsulation of PPP/HDLC PDUs within a pseudo wire.

Table of Contents

    1      Specification of Requirements  ..........................   2
    2      Introduction  ...........................................   2
    3      General encapsulation method  ...........................   5
    3.1    The Control Word  .......................................   5
    3.1.1  Setting the sequence number  ............................   7
    3.2    MTU Requirements  .......................................   7
    4      Protocol-Specific Details  ..............................   8
    4.1    HDLC  ...................................................   8
    4.2    Frame Relay Port Mode  ..................................   8
    4.3    PPP  ....................................................  10
    5      Using an MPLS Label as the Demultiplexer Field  .........  11
    5.1    MPLS Shim EXP Bit Values  ...............................  11
    5.2    MPLS Shim S Bit Value  ..................................  11
    6      Congestion Control  .....................................  11
    7      IANA Considerations  ....................................  12
    8      Security Considerations  ................................  12
    9      Intellectual Property Statement  ........................  12
   10      Full Copyright Statement  ...............................  13
   11      Normative References  ...................................  13
   12      Informative References  .................................  14
   13      Author Information  .....................................  14
   14      Contributing Author Information  ........................  15

1. Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119

2. Introduction

   A PPP/HDLC Pseudowire (PW) allows PPP/HDLC Protocol Data Units (PDUs)
   to be carried over an MPLS network. In addressing the issues
   associated with carrying a PPP/HDLC PDU over an MPLS network, 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 using the signaling protocol such as that defined
   in [CONTROL].

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   The following figure describe the reference models which are derived
   from [ARCH] to support the HDLC/PPP PW emulated services.

         |<-------------- Emulated Service ---------------->|
         |                                                  |
         |          |<------- Pseudo Wire ------>|          |
         |          |                            |          |
         |          |    |<-- PSN Tunnel -->|    |          |
         |          V    V                  V    V          |
         V   AC     +----+                  +----+    AC    V
   +-----+    |     | PE1|==================| PE2|     |    +-----+
   |     |----------|............PW1.............|----------|     |
   | CE1 |    |     |    |                  |    |     |    | CE2 |
   |     |----------|............PW2.............|----------|     |
   +-----+  ^ |     |    |==================|    |     | ^  +-----+
         ^  |       +----+                  +----+     | |  ^
         |  |   Provider Edge 1         Provider Edge 2  |  |
         |  |                                            |  |
   Customer |                                            | Customer
   Edge 1   |                                            | Edge 2
            |                                            |
            |                                            |
      native HDLC/PPP service                   native HDLC/PPP service

      Figure 1: PWE3 HDLC/PPP Interface Reference Configuration

   This document specifies the emulated PW encapsulation for PPP, and
   HDLC, however QoS related issues are not discussed in this document.
   For the purpose of the discussion in 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

   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          C

           Figure 2: 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 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 native
   frame technology , and may not be required for the PW emulation

   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

   "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.

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3. General encapsulation method

   This section describes the general encapsulation format for PPP and
   HDLC frames over MPLS pseudo wires.

    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
   |               PSN Transport Header (As Required)              |
   |                     Pseudo Wire Header                        |
   |                     Control Word                              |
   |                     PPP/HDLC Service Payload                  |

     Figure 3: General format for PPP/HDLC encapsulation over PSNs

   The PSN Transport Header depends on the particular tunneling
   technology in use. This header is used to transport the encapsulated
   PPP/HDLC information through the packet switched core.

   The Pseudo Wire Header identifies a particular PPP/HDLC service on a
   tunnel.  In case of MPLS the Pseudo Wire Header is the MPLS label at
   the bottom of the MPLS label stack.

   The Control Word is inserted before the PPP/HDLC service payload. It
   may contain a length and sequence number.

3.1. The Control Word

   There are three requirements that may need to be satisfied when
   transporting layer 2 protocols over an MPLS backbone:

        -i. Sequentiality may need to be preserved.
       -ii. Small packets may need to be padded in order to be
            transmitted on a medium where the minimum transport unit is
            larger than the actual packet size.
      -iii. Control bits carried in the header of the layer 2 frame may
            need to be transported.
       -iv. Creating an in-band associated channel for operation and
            maintenance communications.

   The Control Word defined in this section is based on the Generic PW
   MPLS Control Word as defined in [CW]. It provides the ability to
   sequence individual frames on the PW, avoidance of equal-cost

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   multiple-path load-balancing (ECMP) [RFC2992], and 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 due to the fact that
   ECMP implementations may examine the first nibble after the MPLS
   label stack to determine whether the labeled packet is IP or not.
   Thus, if the PPP protocol number of an ppp 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.

   The features that the control word provides may not be needed for a
   given PPP/HDLC 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 PPP/HDLC 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 ingress PE MUST be aware of whether the egress 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 [CONTROL].

   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|0 0 0 0|Res|   Length  |     Sequence Number           |

                   Figure 4: MPLS PWE3 Control Word

   In the above diagram the first 4 bits are set to 0 in indicate a CW

   The next 4 bits provide space for carrying protocol specific flags.
   These are not used for HDLC/PPP and they MUST be set to 0 when
   transmitting, and MUST be ignored upon receipt.

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   The next 2 bits are reserved for future use , and MUST be ignored.

   The next 6 bits provide a length field, which is used as follows: If
   the packet's length (defined as the length of the layer 2 payload
   plus the length of the control word) is less than 64 bytes, the
   length field MUST be set to the packet's length. Otherwise the length
   field MUST be set to zero.  The value of the length field, if not
   zero, is used to remove any padding that may have been added by the
   MPLS network. When the packet reaches the service provider's egress
   router any padding MUST be removed before forwarding the packet.

   The next 16 bits provide a sequence number that can be used to
   guarantee ordered packet 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 packet.

3.1.1. Setting the sequence number

   The procedures described in section 4 of [CW] MUST be followed to
   process the sequence number field.

3.2. MTU Requirements

   The network MUST be configured with an MTU that is sufficient to
   transport the largest encapsulation frames. When MPLS is used as the
   tunneling protocol, for example, this is likely to be 12 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 then 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.

   If a packet is received, on the attachment circuit, that exceeds the
   interface MTU subTLV value [CONTROL] , it MUST be dropped.

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4. Protocol-Specific Details

4.1. HDLC

   HDLC mode provides port to port transport of HDLC encapsulated
   traffic. The HDLC PDU is transported in its entirety, including the
   HDLC address, control and protocol fields, but excluding HDLC flags
   and the FCS. Bit/Byte stuffing is undone. The control word is
   OPTIONAL. If the control word is used then the flag bits in the
   control word are not used, and MUST be set to 0 when transmitting,
   and MUST be ignored upon receipt.

   When the PE detects a status change in the attachment circuit status,
   such as an attachment circuit physical link failure, or the AC is
   administratively disabled, the PE MUST send the appropriate PW status
   notification message that corresponds to the HDLC AC status. In a
   similar manner, the local PW status MUST also be reflected in a
   respective PW status notification message as described in [CONTROL].

   The HDLC mode is suitable for port to port transport of Frame Relay
   UNI or NNI traffic. It must be noted, however, that this mode is
   transparent to the FECN, BECN and DE bits. Since all packets are
   passed in a largely transparent manner over the HDLC PW, any protocol
   which has HDLC-like framing may utilize the HDLC PW mode, including
   PPP, Frame-Relay, X.25, etc.  Exceptions include cases where direct
   access to the HDLC interface is required, or modes which operate on
   the flags, FCS, or bit/byte unstuffing that is performed before
   sending the HDLC PDU over the PW. An example of this is PPP ACCM

   The PW of type 0x0006 "HDLC" will be used to transport HDLC frames.
   The IANA allocation registry of "Pseudowire Type" is defined in the
   iana allocation document for PWs [IANA] along with initial allocated

4.2. Frame Relay Port Mode

   Figure 5 illustrates the concept of frame relay port mode or many-
   to-one mapping which is an OPTIONAL capability.

   Figure 5 (a) shows two frame relay devices physically connected with
   a frame relay UNI or NNI. Between their two ports P1 and P2, n frame
   relay VCs are configured.

   Figure 5 (b) shows the replacement of the physical frame relay
   interface with a pair of PEs and a PW between them. The interface
   between a FR device and a PE is either a FR UNI or NNI. The set of n

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   FR VCs between the two FR ports P1 and P2 which are controlled by the
   same signaling channel using DLCI=0, are mapped into one PW. The
   standard frame relay Link management Interface (LMI) procedures
   happen directly between the CEs. Hence with port mode we have many-
   to-one mapping between FR VCs and a PW.

              +------+                          +-------+
              | FR   |                          |   FR  |
              |device|         FR UNI/NNI       | device|
              |    [P1]------------------------[P2]     |
              |      |      carrying n FR VCs   |       |
              +------+                          +-------+

                 [Pn]: A port

            Figure 5a FR interface between two FR devices

                    |                              |
                     +----+                  +----+
   +------+          |    |     One PW       |    |         +------+
   |      |          |    |==================|    |         |      |
   |  FR  |    FR    | PE1| carrying n FR VCs| PE2|    FR   |  FR  |
   |device|----------|    |                  |    |---------|device|
   | CE1  | UNI/NNI  |    |                  |    | UNI/NNI | CE2  |
   +------+          +----+                  +----+         +------+
          |                                                 |
                                  n FR VCs

         Figure 5b Pseudo-wires replacing the FR interface

   FR VCs are not visible individually to a PE; there is no
   configuration of individual FR VC in a PE. A PE processes the set of
   FR VCs assigned to a port as an aggregate.

   FR port mode provides transport between two PEs of a complete FR
   frame using the same encapsulation as described above for HDLC mode.

   Al thought frame relay port mode shares the same encapsulation as
   HDLC mode, a different PW type is allocated in [IANA]: 0x000F
   Frame-Relay Port mode.

   All other aspects of this PW type are identical to the HDLC PW
   encapsulation described above.

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4.3. PPP

   PPP mode provides point to point transport of PPP encapsulated
   traffic, as specified in [PPP]. The PPP PDU is transported in its
   entirety, including the protocol field (whether compressed using
   Protocol Field Compression or not), but excluding any media-specific
   framing information, such as HDLC address and control fields or FCS.
   Since media-specific framing is not carried the following options
   will not operate correctly if the PPP peers attempt to negotiate

     - Frame Check Sequence (FCS) Alternatives
     - Address-and-Control-Field-Compression (ACFC)
     - Asynchronous-Control-Character-Map (ACCM)

   Note also that PW LSP Interface MTU negotiation as specified in
   [CONTROL] is not affected by PPP MRU advertisement. Thus if a PPP
   peer sends a PDU with a length in excess of that negotiated for the
   PW tunnel that PDU will be discarded by the ingress router.

   If the OPTIONAL control word is used then the flag bits in the
   control word are not used, and MUST be set to 0 when transmitting,
   and MUST be ignored upon receipt.

   When the PE detects a status change in the attachment circuit
   (AC)status, such as an attachment circuit physical link failure, or
   the AC is administratively disabled, the PE MUST send the appropriate
   PW status notification message that corresponds to the PPP AC status.
   It should be noted that PPP negotiation status is transparent to the
   PW, and MUST not be communicated to the remote MPLS PE. In a similar
   manner, the local PW status MUST also be reflected in a respective PW
   status notification message as described in [CONTROL].

   A PW of type 0x0007 "PPP" will be used to transport PPP frames.

   The IANA allocation registry of "Pseudowire Type" is defined in the
   iana allocation document for PWs [IANA] along with initial allocated

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5. Using an MPLS Label as the Demultiplexer Field

   To use an MPLS label as the demultiplexer field, a 32-bit label stack
   entry [MPLSENCAP] is simply prepended to the emulated PW
   encapsulation, and hence will appear as the bottom label of an MPLS
   label stack. This label may be called the "PW label". The particular
   emulated PW identified by a particular label value must be agreed by
   the ingress and egress LSRs, either by signaling (e.g, via the
   methods of [CONTROL]) or by configuration. Other fields of the label
   stack entry are set as follows.

5.1. MPLS Shim EXP Bit Values

   If it is desired to carry Quality of Service information, the Quality
   of Service information SHOULD be represented in the EXP field of the
   PW label.  If more than one MPLS label is imposed by the ingress LSR,
   the EXP field of any labels higher in the stack MUST also carry the
   same value.

5.2. MPLS Shim S Bit Value

   The ingress LSR, PE1, MUST set the S bit of the PW label to a value
   of 1 to denote that the PW label is at the bottom of the stack.

6. Congestion Control

   As explained in [ARCH], the PSN carrying the PW may be subject to
   congestion, with congestion characteristics depending on PSN type,
   network architecture, configuration, and loading. During congestion
   the PSN may exhibit packet loss that will impact the service carried
   by the PPP/HLDC PW.  In addition, since PPP/HDLC PWs carry an
   unspecified type of services across the PSN, they cannot behave in a
   TCP-friendly manner prescribed by [RFC2914]. In the presence of
   services that reduce transmission rate, PPP/HDLC PWs will thus
   consume more than their fair share and SHOULD be halted.

   Whenever possible, PPP/HDLC PWs should be run over traffic-engineered
   PSNs providing bandwidth allocation and admission control mechanisms.
   IntServ-enabled domains providing the Guaranteed Service (GS) or
   DiffServ-enabled domains using EF (expedited forwarding) are examples
   of traffic-engineered PSNs. Such PSNs will minimize loss and delay
   while providing some degree of isolation of the PPP/HDLC PW's effects
   from neighboring streams.

   The PEs SHOULD monitor for congestion (by using explicit congestion

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   notification, [VCCV], or by measuring packet loss) in order to ensure
   that the service using the PPP/HDLC PW may be maintained. When
   significant congestion is detected the PPP/HDLCPW SHOULD be
   administratively disabled.  If the PW has been set up using the
   protocol defined in [CONTROL], then procedures specified in [CONTROL]
   for status notification can be used to disable packet transmission on
   the ingress PE from the egress PE. The PW may be restarted by manual
   intervention, or by automatic means after an appropriate waiting

7. IANA Considerations

   This document has no IANA Actions.

8. Security Considerations

   The PPP, and HDLC pseudowire type is subject to all of the general
   security considerations discussed in [ARCH][CONTROL]. This document
   specifies only encapsulations, and not the protocols that may be used
   to carry the encapsulated packets across the MPLS network. Each such
   protocol may have its own set of security issues, but those issues
   are not affected by the encapsulations specified herein.

9. Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-

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   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.

10. Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   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

11. Normative References

   [CONTROL] "Pseudowire Setup and Maintenance using LDP",
        Martini, L., et al., draft-ietf-pwe3-control-protocol-16.txt,
        ( work in progress ), April 2005
   [MPLSENCAP] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D.
   Tappan, G. Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032

[IANA] "IANA Allocations for pseudo Wire Edge to Edge Emulation (PWE3)"
   Martini,Townsley, draft-ietf-pwe3-iana-allocation-09.txt (work in
   progress), April 2004

[CW] "PWE3 Control Word for use over an MPLS PSN", S. Bryant, G. Swal-
   low, D. McPherson, draft-ietf-pwe3-cw-06.txt, ( work in progress ),
   October 2005.

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12. Informative References

   [PPP] "The Point-to-Point Protocol (PPP)", RFC 1661.

   [ATM] "Encapsulation Methods for Transport of ATM Cells/Frame Over
        MPLS Networks", draft-ietf-pwe3-atm-encap-02.txt
        ( work in progress )

   [ETHER] "Encapsulation Methods for Transport of Ethernet Frames Over
        MPLS Networks", draft-ietf-pwe3-ethernet-encap-06.txt.
        ( work in progress )

   [FRAME] "Frame Relay over Pseudo-Wires",
        draft-ietf-pwe3-frame-relay-01.txt. (work in progress )

   [ARCH] "PWE3 Architecture" Bryant, et al.,RFC3985

   [FRAG] "PWE3 Fragmentation and Reassembly", A. Malis,W. M. Townsley,
        draft-ietf-pwe3-fragmentation-08.txt ( work in progress )
        February 2005

   [ARCH] "PWE3 Architecture", Bryant, et al., RFC3985.

   [VCCV] Nadeau, T., et al."Pseudo Wire Virtual Circuit Connection
        Verification (VCCV)", Internet Draft
        draft-ietf-pwe3-vccv-08.txt, October 2005. (work in progress)

   [RFC2992] RFC-2992:  Analysis of an Equal-Cost Multi-Path
        Algorithm, C. Hopps, November 2000

   [RFC4026] Andersson, L., Madsen, T. "Provider Provisioned
        Virtual Private Network (VPN) Terminology", RFC 4026,
        March 2005.

   [RFC2914]  S. Floyd, "Congestion Control Principles" RFC 2914

13. Author Information

   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO, 80112
   e-mail: lmartini@cisco.com

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   Giles Heron
   Abbey Place
   24-28 Easton Street
   High Wycombe
   HP11 1NT
   e-mail: giles.heron@tellabs.com

   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719
   E-mail: erosen@cisco.com

   Andrew G. Malis
   90 Rio Robles Dr.
   San Jose, CA 95134
   e-mail: Andy.Malis@tellabs.com

14. Contributing Author Information

   Yeongil Seo
   463-1 KT Technology Lab
   Jeonmin-dong Yusung-gu
   Daegeon, Korea
   email: syi1@kt.co.kr

   Toby Smith
   Laurel Networks, Inc.
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   e-mail: tob@laurelnetworks.com

Martini, et al.                                                [Page 15]

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