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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 4719

Network Working Group                               Rahul Aggarwal
Internet Draft                                      Juniper Networks
Expiration Date: February 2007                      W. Mark Townsley
                                                    Maria A. Dos Santos
                                                    Cisco Systems
                                                    Editors
                                                    August 2006

                Transport of Ethernet Frames over L2TPv3

                draft-ietf-l2tpext-pwe3-ethernet-09.txt

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.

   Internet-Drafts are working documents of the Internet Engineering
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   Drafts.

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   http://www.ietf.org/1id-abstracts.html

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Abstract

   This document describes the transport of Ethernet frames over Layer 2
   Tunneling Protocol (L2TPv3). This includes the transport of Ethernet
   port to port frames as well as the transport of Ethernet VLAN frames.
   The mechanism described in this document can be used in the creation
   of Pseudo Wires to transport Ethernet frames over an IP network.









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Contributors

   Following is the complete list of contributors to this document.

   Rahul Aggarwal
   Juniper Networks

   Xipeng Xiao
   Riverstone Networks

   W. Mark Townsley
   Stewart Bryant
   Maria Alice Dos Santos
   Cisco Systems

   Cheng-Yin Lee
   Alcatel

   Tissa Senevirathne
   Consultant

   Mitsuru Higashiyama
   Anritsu Corporation

Table of Contents


   Status of this Memo..........................................    1

   1. Introduction..............................................    3
      1.1 Abbreviations.........................................    3
      1.2 L2TPv3 Control Message Types..........................    4
      1.3 Requirements..........................................    4

   2. PW Establishment..........................................    5
      2.1 LCCE-LCCE Control Connection Establishment............    5
      2.2 PW Session Establishment..............................    5
      2.3 PW Session Monitoring.................................    6

   3. Packet Processing.........................................    8
      3.1 Encapsulation.........................................    8
      3.2 Sequencing............................................    8
      3.3 MTU Handling..........................................    8

   4. Applicability Statement...................................    9

   5. Congestion Control........................................   11




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   6. Security Considerations...................................   12

   7. IANA Considerations.......................................   12

   8. Acknowledgements..........................................   12

   9. References................................................   12
      9.1 Normative References..................................   12
      9.2 Informative References................................   13

   10. Author Information.......................................   13


Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  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 [RFC2119].


1. Introduction

   L2TPv3 can be used as a control protocol and for data encapsulation
   to set up Pseudo Wires (PW) for transporting layer 2 Packet Data
   Units across an IP network [RFC3931]. This document describes the
   transport of Ethernet frames over L2TPv3 including the PW
   establishment and data encapsulation.

   The term "Ethernet" in this draft is used with the intention to
   include all such protocols that are reasonably similar in their
   packet format to IEEE 802.3 [802.3], including variants or extensions
   which may or may not necessarily be sanctioned by IEEE (including
   such things as jumbo frames, etc).  The term "VLAN" in this draft is
   used with the intention to include all virtual LAN tagging protocols
   such as IEEE 802.1Q [802.1Q], 802.1ad [802.1ad], etc.


1.1 Abbreviations


   AC      Attachment Circuit (See [RFC3985])
   CE      Customer Edge (Typically also the L2TPv3 Remote System)
   LCCE    L2TP Control Connection Endpoint (See [RFC3931])
   NSP     Native Service Processing (See [RFC3985])
   PE      Provider Edge (Typically also the LCCE) (See [RFC3985])
   PSN     Packet Switched Network (See [RFC3985])



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   PW      Pseudo-Wire (See [RFC3985])
   PWE3    Pseudo-Wire Emulation Edge to Edge (Working Group)


1.2 L2TPv3 Control Message Types

   Relevant L2TPv3 control message types (See [RFC3931]) are listed for
   reference.


   SCCRQ   L2TPv3 Start-Control-Connection-Request control message
   SCCRP   L2TPv3 Start-Control-Connection-Reply control message
   SCCCN   L2TPv3 Start-Control-Connection-Connected control message
   STOPCCN L2TPv3 Stop-Control-Connection-Notification control message
   ICRQ    L2TPv3 Incoming-Call-Request control message
   ICRP    L2TPv3 Incoming-Call-Reply control message
   ICCN    L2TPv3 Incoming-Call-Connected control message
   OCRQ    L2TPv3 Outgoing-Call-Request control message
   OCRP    L2TPv3 Outgoing-Call-Reply control message
   OCCN    L2TPv3 Outgoing-Call-Connected control message
   CDN     L2TPv3 Call-Disconnect-Notify control message
   SLI     L2TPv3 Set-Link-Info control message



1.3 Requirements

   An Ethernet PW emulates a single Ethernet link between exactly two
   endpoints. The following figure depicts the PW termination relative
   to the NSP and PSN tunnel within a LCCE [RFC3985]. The Ethernet
   interface may be connected to one or more Remote Systems (an L2TPv3
   Remote System is referred to as Customer Edge (CE) in this and
   associated PWE3 documents). The LCCE may or may not be a PE.

                 +---------------------------------------+
                 |                 LCCE                  |
                 +-+   +-----+   +------+   +------+   +-+
                 |P|   |     |   |PW ter|   | PSN  |   |P|
   Ethernet  <==>|h|<=>| NSP |<=>|minati|<=>|Tunnel|<=>|h|<==> PSN
   Interface     |y|   |     |   |on    |   |      |   |y|
                 +-+   +-----+   +------+   +------+   +-+
                 |                                       |
                 +---------------------------------------+
                       Figure 1: PW termination


   The PW termination point receives untagged (also referred to as
   'raw') or tagged Ethernet frames and delivers them unaltered to the



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   PW termination point on the remote LCCE. Hence it can provide
   untagged or tagged Ethernet link emulation service.

   The "NSP" function includes packet processing needed to translate the
   Ethernet frames that arrive at the CE-LCCE interface to/from the
   Ethernet frames that are applied to the PW termination point. Such
   functions may include stripping, overwriting or adding VLAN tags.
   The NSP functionality can be used in conjunction with local
   provisioning to provide heterogeneous services where the CE-LCCE
   encapsulations at the two ends may be different.

   The physical layer between the CE and LCCE, and any adaptation (NSP)
   functions between it and the PW termination, are outside of the scope
   of PWE3 and are not defined here.


2. PW Establishment

   With L2TPv3 as the tunneling protocol, Ethernet PWs are L2TPv3
   sessions. An L2TP control connection has to be set up first between
   the two LCCEs. Individual PWs can then be established as L2TP
   sessions.


2.1 LCCE-LCCE Control Connection Establishment

   The two LCCEs that wish to set up Ethernet PWs MUST establish a L2TP
   control connection first as described in [RFC3931]. Hence an Ethernet
   PW type must be included in the Pseudo Wire Capabilities List as
   defined in [RFC3931]. The type of PW can be either "Ethernet port" or
   "Ethernet VLAN". This indicates that the control connection can
   support the establishment of Ethernet PWs. Note that there are two
   Ethernet PW types required.  For connecting an Ethernet port to
   another Ethernet port, the PW Type MUST be "Ethernet port"; for
   connecting an Ethernet VLAN to another Ethernet VLAN, the PW Type
   MUST be "Ethernet VLAN".


2.2 PW Session Establishment

   The provisioning of an Ethernet port or Ethernet VLAN and its
   association with a PW triggers the establishment of an L2TP session
   via the standard Incoming Call three-way handshake described in
   Section 3.4.1 of [RFC3931].

   Note that an L2TP Outgoing Call is essentially a method of
   controlling the originating point of an SVC, allowing it to be
   established from any reachable L2TP-enabled device able to perform



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   outgoing calls.  The Outgoing Call model and its corresponding OCRQ,
   OCRP and OCCN control messages are mainly used within the dial arena
   with L2TPv2 today and has not been found applicable for PW
   applications yet.

   The following are the signaling elements needed for the Ethernet PW
   establishment:

   a) Pseudo Wire Type: The type of a Pseudo Wire can be either
   "Ethernet port" or "Ethernet VLAN". Each LCCE signals its Pseudo Wire
   type in the Pseudowire Type AVP [RFC3931]. The assigned values for
   "Ethernet port" and "Ethernet VLAN" Pseudo Wire types are captured in
   the "IANA Considerations" of this document. The Pseudowire Type AVP
   MUST be present in the ICRQ.

   b) Pseudo Wire ID: Each PW is associated with a Pseudo Wire ID. The
   two LCCEs of a PW have the same Pseudo Wire ID for it. The Remote End
   Identifier AVP [RFC3931] is used to convey the Pseudo Wire ID. The
   Remote End Identifier AVP MUST be present in the ICRQ in order for
   the remote LCCE to determine the PW to associate the L2TP session
   with.  An implementation MUST support a Remote End Identifier of four
   octets known to both LCCEs either by manual configuration or some
   other means. Additional Remote End Identifier formats which MAY be
   supported are outside the scope of this document.

   c) The Circuit Status AVP [RFC3931] MUST be included in ICRQ and ICRP
   to indicate the circuit status of the Ethernet port or Ethernet VLAN.
   For ICRQ and ICRP, the Circuit Status AVP MUST indicate that the
   circuit status is for a new circuit (refer to N bit in Section
   2.3.3).  An Implementation MAY send an ICRQ or ICRP before an
   Ethernet interface is ACTIVE, as long as the Circuit Status AVP
   (refer to A bit in Section 2.3.3) in the ICRQ or ICRP reflects the
   correct status of the Ethernet port or Ethernet VLAN link. Subsequent
   circuit status change of the Ethernet port or Ethernet VLAN MUST be
   conveyed in the Circuit Status AVP in ICCN or SLI control messages.
   For ICCN and SLI (refer to Section 2.3.2), the Circuit Status AVP
   MUST indicate that the circuit status is for an existing circuit
   (refer to N bit in Section 2.3.3) and reflect the current status of
   the link (refer to A bit in Section 2.3.3).


2.3 PW Session Monitoring









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2.3.1. Control Connection Keep-alive

   The working status of a PW is reflected by the state of the L2TPv3
   session. If the corresponding L2TPv3 session is down, the PW
   associated with it MUST be shut down. The control connection keep-
   alive mechanism of L2TPv3 can serve as a link status monitoring
   mechanism for the set of PWs associated with a Control Connection.


2.3.2. SLI Message

   In addition to the control connection keep-alive mechanism of L2TPv3,
   Ethernet PW over L2TP makes use of the Set Link Info (SLI) control
   message defined in [RFC3931]. The SLI message is used to signal
   Ethernet link status notifications between LCCEs. This can be useful
   to indicate Ethernet interface state changes without bringing down
   the L2TP session.  Note that change in the Ethernet interface state
   will trigger a SLI message for each PW associated with that Ethernet
   interface.  This may be one Ethernet Port PW or more than one
   Ethernet VLAN PW.  The SLI message MUST be sent any time there is a
   status change of any values identified in the Circuit Status AVP. The
   only exception to this is the initial ICRQ, ICRP and CDN messages
   which establish and teardown the L2TP session itself.  The SLI
   message may be sent from either LCCE at any time after the first ICRQ
   is sent (and perhaps before an ICRP is received, requiring the peer
   to perform a reverse Session ID lookup).


2.3.3. Use of Circuit Status AVP for Ethernet

   Ethernet PW reports Circuit Status with the Circuit Status AVP
   defined in [RFC3931]. For reference, this AVP is shown below:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Reserved        |N|A|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Value is a 16 bit mask with the two least significant bits
   defined and the remaining bits reserved for future use. Reserved bits
   MUST be set to 0 when sending, and ignored upon receipt.

   The A (Active) bit indicates whether the Ethernet interface is ACTIVE
   (1) or INACTIVE (0).

   The N (New) bit indicates whether the circuit status is for a new (1)
   Ethernet circuit or an existing (0) Ethernet circuit.



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3. Packet Processing


3.1 Encapsulation

   The encapsulation described in this section refers to the
   functionality performed by the PW termination point depicted in
   figure 1, unless otherwise indicated.

   The entire Ethernet frame, without the preamble or FCS, is
   encapsulated in L2TPv3 and is sent as a single packet by the ingress
   LCCE. This is done regardless of whether an VLAN tag is present in
   the Ethernet frame or not. For Ethernet port to port mode, the remote
   LCCE simply decapsulates the L2TP payload and sends it out on the
   appropriate interface without modifying the Ethernet header. For
   Ethernet VLAN to VLAN mode, the remote LCCE MAY rewrite the VLAN tag.
   As described in section 1, the VLAN tag modification is an NSP
   function.

   The Ethernet PW over L2TP is homogeneous with respect to packet
   encapsulation i.e. both the ends of the PW are either untagged or
   tagged. The Ethernet PW can still be used to provide heterogeneous
   services using NSP functionality at the ingress and/or egress LCCE.
   The definition of such NSP functionality is outside the scope of this
   document.

   The maximum length of the Ethernet frame carried as the PWE payload
   is irrelevant as far as the PWE is concerned. If anything, that value
   would only be relevant when quantifying the faithfulness of the
   emulation.


3.2 Sequencing

   Data packet sequencing MAY be enabled for Ethernet PWs. The
   sequencing mechanisms described in [RFC3931] MUST be used for
   signaling sequencing support.


3.3 MTU Handling

   With L2TPv3 as the tunneling protocol, the IP packet resulting from
   the encapsulation is M + N bytes longer than Ethernet frame without
   the preamble or FCS. Here M is the length of the IP header along with
   associated options and extension headers, and the value of N depends
   on the following fields:

      L2TP Session Header:



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         Flags, Ver, Res - 4 octets (L2TPv3 over UDP only)
         Session ID      - 4 octets
         Cookie Size     - 0, 4 or 8 octets
         L2-Specific Sublayer - 0 or 4 octets (i.e., using sequencing)

      Hence the range for N in octets is:

         N = 4-16,  for L2TPv3 data messages over IP;
         N = 16-28, for L2TPv3 data messages over UDP;
         (N does not include the IP header).

   Fragmentation in the PSN can occur when using Ethernet over L2TP,
   unless proper configuration and management of MTU sizes are in place
   between the Customer Edge (CE) router, Provider Edge (PE) router, and
   across the PSN. This is not specific only to Ethernet over L2TPv3,
   and the base L2TPv3 specification [RFC3931] provides general
   recommendations with respect to fragmentation and reassembly in
   section 4.1.4. "PWE3 Fragmentation and Reassembly" [L2TPFRAG]
   expounds on this topic further, including a fragmentation and
   reassembly mechanism within L2TP itself in the event that no other
   option is available.  Implementations MUST follow these guidelines
   with respect to Fragmentation and Reassembly.


4. Applicability Statement

   The Ethernet PW emulation allows a service provider to offer a "port
   to port" Ethernet based service across an IP packet switched network
   (PSN) while the Ethernet VLAN PW emulation allows an "Ethernet VLAN
   to VLAN" based service across an IP packet switched network (PSN).

   The Ethernet or Ethernet VLAN PW emulation has the following
   characteristics in relationship to the respective native service:

   o Ethernet PW connects two Ethernet ACs while Ethernet VLAN PW
     connects two Ethernet VLAN ACs, supporting bi-directional
     transport of variable length Ethernet frames.  The ingress LCCE
     strips the preamble and FCS from the Ethernet frame and transports
     the frame in its entirety across the PW.  This is done regardless
     of the presence of the VLAN tag in the frame.  The egress LCCE
     receives the Ethernet frame from the PW and regenerates the
     preamble and FCS before forwarding the frame to the attached Remote
     System (See Section 3.1).  Since FCS is not being transported
     across either Ethernet or Ethernet VLAN PWs, payload integrity
     transparency may be lost.  To achieve payload integrity
     transparency on Ethernet or Ethernet VLAN PWs using L2TP over IP
     or L2TP over UDP/IP, the L2TPv3 session can utilize IPsec as
     specified in Section 4.1.3 of [RFC3931].



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   o While architecturally [RFC3985] outside the scope of the L2TPv3 PW
     itself, if VLAN tags are present, the NSP may rewrite VLAN tags on
     ingress or egress from the PW (see section 3.1).

   o 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 (See Section
     3.1).

   o Ethernet port or Ethernet VLAN status notification is provided
     using the Circuit Status AVP in SLI message (See Section 2.3.1).
     Loss of connectivity between LCCEs can be detected by the L2TPv3
     keep-alive mechanism (see Section 2.3.1 in [RFC3931]).  The LCCE
     can convey these indications back to its attached Remote System.

   o The maximum frame size that can be supported is limited by the PSN
     MTU minus the L2TPv3 header size, unless fragmentation and
     reassembly is used (see Section 3.3 and Section 4.1.4 of
     [RFC3931]).

   o The packet switched network may reorder, duplicate, or silently
     drop packets.  Sequencing may be enabled in the Ethernet or
     Ethernet VLAN PW for some or all packets to detect lost,
     duplicate, or out-of-order packets on a per-session basis
     (see Section 3.2).

   o The faithfulness of an Ethernet or Ethernet VLAN PW may be
     increased by leveraging Quality of Service features of the LCCEs
     and the underlying PSN.  For example for Ethernet 802.1Q [802.1Q]
     VLAN transport, the ingress LCCE MAY consider the user priority
     field (i.e. 802.1P) of the VLAN tag for traffic classification
     and QoS treatments, such as determining the DS field [RFC2474] of
     the encapsulating IP header.  Similarly, the egress LCCE MAY
     consider the DS field of the encapsulating IP header when
     rewriting the user priority field of the VLAN tag or queuing the
     Ethernet frame before forwarding the frame to the Remote System.
     The mapping between the user priority field and the IP header DS
     field as well as the Quality of Service model deployed are
     application specific and are outside the scope of this document.











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5. Congestion Control

   As explained in [RFC3985], 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 Ethernet or Ethernet VLAN PW. In addition, since Ethernet or
   Ethernet VLAN PWs carry a variety of services across the PSN,
   including but not restricted to TCP/IP, they may or may not behave in
   a TCP-friendly manner prescribed by [RFC2914] and thus consume more
   than their fair share.

   Whenever possible, Ethernet or Ethernet VLAN 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 Ethernet or Ethernet VLAN PW's effects from neighboring streams.

   LCCEs SHOULD monitor for congestion (by using explicit congestion
   notification, or by measuring packet loss) in order to ensure that
   the service using the Ethernet or Ethernet VLAN PW may be maintained.
   When severe congestion is detected (for example when enabling
   Sequencing and detecting that the packet loss is higher than a
   threshold) the Ethernet or Ethernet VLAN PW SHOULD be halted by
   tearing down the L2TP session via a CDN message.  The PW may be
   restarted by manual intervention, or by automatic means after an
   appropriate waiting time.  Note that the thresholds and time periods
   for shutdown and possible automatic recovery need to be carefully
   configured. This is necessary to avoid loss of service due to
   temporary congestion, and to prevent oscillation between the
   congested and halted states.

   This specification offers no congestion control and is not TCP
   friendly [TFRC]. Future works for PW congestion control (being
   studied by the PWE3 Working Group) will provide congestion control
   for all PW types including Ethernet and Ethernet VLAN PWs.













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6. Security Considerations

   Ethernet over L2TPv3 is subject to all of the general security
   considerations outlined in [RFC3931].

7. IANA Considerations

   The signaling mechanisms defined in this document rely upon the
   allocation of following Ethernet Pseudowire Types (see Pseudo Wire
   Capabilities List as defined in 5.4.3 of [RFC3931] and L2TPv3
   Pseudowire Types in 10.6 of [RFC3931]) by the IANA (number space
   created as part of publication of [RFC3931]):

         Pseudowire Types
         ----------------

         0x0004  Ethernet VLAN Pseudowire Type
         0x0005  Ethernet Pseudowire Type


8. Acknowledgements

   This draft evolves from the draft, "Ethernet Pseudo Wire Emulation
   Edge-to-Edge". We would like to thank its authors, T.So, X.Xiao, L.
   Anderson, C. Flores, N. Tingle, S. Khandekar, D. Zelig and G. Heron
   for their contribution. We would also like to thank S. Nanji, the
   author of the draft, "Ethernet Service for Layer Two Tunneling
   Protocol", for writing the first Ethernet over L2TP draft.

   Thanks to Carlos Pignataro for providing a thorough review and
   helpful input.


9. References


9.1 Normative References

   [RFC3931]    J. Lau, M. Townsley, I. Goyret, "Layer Two Tunneling
                Protocol (Version 3)", RFC3931, March 2005.

   [RFC2119]    S. Bradner, "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [L2TPFRAG]   A. Malis, M. Townsley, "PWE3 Fragmentation and
                Reassembly", draft-ietf-pwe3-fragmentation-10.txt





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9.2 Informative References

   [RFC3985]    S. Bryant, P. Pate, "Pseudo Wire Emulation Edge-to-Edge
                (PWE3) Architecture", RFC3985, March 2005

   [RFC2914]    S. Floyd, "Congestion Control Principles", BCP 41,
                RFC 2914, September 2000.

   [RFC2474]    K. Nichols, S. Blake, F. Baker, D. Black, "Definition of
                the Differentiated Services Field (DS Field) in the IPv4
                and IPv6 Headers", RFC2474, December 1998

   [802.3]      IEEE, "IEEE std 802.3 -2005/Cor 1-2006 IEEE Standard for
                Information Technology - Telecommuincations and
                Information Exchange Between Systems - Local and
                Metropolitan Area Networks", IEEE Std 802.3-2005/Cor
                1-2006 (Corrigendum to IEEE Std 802.3-2005)

   [802.1Q]     IEEE, "IEEE standard for local and metropolitan area
                networks virtual bridged local area networks", IEEE
                Std 802.1Q-2005 (Incorporates IEEE Std 802.1Q1998, IEEE
                Std 802.1u-2001, IEEE Std 802.1v-2001, and IEEE Std
                802.1s-2002)

   [802.1ad]    IEEE, "IEEE Std 802.1ad - 2005 IEEE Standard for Local
                and metropolitan area networks - virtual Bridged Local
                Area Networks, Amendment 4: Provider Bridges", IEEE
                Std 802.1ad-2005 (Amendment to IEEE Std 8021Q-2005)

   [TFRC]       M. Handley, S. Floyd, J. Padhye, J. Widmer, "TCP
                Friendly Rate Control (TFRC): Protocol Specification",
                RFC3448, January 2003



10. Author Information


   Rahul Aggarwal
   Juniper Networks
   1194 North Mathilda Avenue
   Sunnyvale, CA 94089
   e-mail: rahul@juniper.net

   W. Mark Townsley
   Cisco Systems
   7025 Kit Creek Road
   PO Box 14987



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   Research Triangle Park, NC 27709
   e-mail: mark@townsley.net

   Maria Alice Dos Santos
   Cisco Systems
   170 W Tasman Dr
   San Jose, CA 95134
   e-mail: mariados@cisco.com



Intellectual Property Statement

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   Copyright (C) The Internet Society (2006).



Aggarwal, Townsley, et al.  Standards Track                    [Page 14]

INTERNET DRAFT  draft-ietf-l2tpext-pwe3-ethernet-09.txt      August 2006


   This document is subject to the rights, licenses and restrictions
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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.











































Aggarwal, Townsley, et al.  Standards Track                    [Page 15]


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