[Docs] [txt|pdf] [Tracker] [Email] [Nits]

Versions: 00 draft-ietf-l2tpext-pwe3-ip

Internet Engineering Task Force                             C. Pignataro
Internet-Draft                                                    W. Luo
Expires: February 24, 2006                           Cisco Systems, Inc.
                                                         August 23, 2005


                    IP Layer 2 Transport over L2TPv3
                draft-pignataro-l2tpext-pwe3-ipl2-00.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
   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.

   This Internet-Draft will expire on February 24, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The Layer 2 Tunneling Protocol, Version 3, (L2TPv3) defines a
   protocol for tunneling a variety of data link protocols over IP
   networks.  These data link protocols must be of the same technology
   and the L2TPv3 session carries frames of that technology.  However,
   if it is known beforehand that the frames to be transported only
   carry IP datagrams as payload, it is possible to provide L2TPv3
   sessions that connects unlike data link technologies (e.g., ethernet
   and PPP).  This is achieved by terminating the Layer 2 protocol in



Pignataro & Luo         Expires February 24, 2006               [Page 1]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


   the LCCE and transporting only the IP datagrams over L2TPv3, such
   that the L2TPv3 session links uniform Pseudowire terminations (i.e.
   IP Layer 2 Transport Pseudowire Type) and an NSP function provides
   interworking.  This document describes the control messaging and
   encapsulation specifics of how to tunnel IP datagrams directly over
   L2TPv3 to allow for connecting different data link technologies in an
   interworking fashion.

Requirements Language

   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].


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Abbreviations  . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  4

   2.  Control Connection Establishment . . . . . . . . . . . . . . .  5

   3.  Layer 2 Link Status Notification and Session Establishment . .  6
     3.1.  L2TPv3 Session Establishment . . . . . . . . . . . . . . .  6
     3.2.  L2TPv3 Session Teardown  . . . . . . . . . . . . . . . . .  8
     3.3.  L2TPv3 Session Maintenance . . . . . . . . . . . . . . . .  8
     3.4.  Use of Circuit Status AVP for IP Layer 2 Pseudowires . . .  8

   4.  Encapsulation  . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  Data Packet Encapsulation  . . . . . . . . . . . . . . . .  9
     4.2.  Data Packet Sequencing . . . . . . . . . . . . . . . . . . 10
     4.3.  MTU Considerations . . . . . . . . . . . . . . . . . . . . 10

   5.  Point-to-Point Address Resolution Considerations . . . . . . . 11
     5.1.  Static Address Resolution  . . . . . . . . . . . . . . . . 11
     5.2.  Dynamic ARP Mediation  . . . . . . . . . . . . . . . . . . 12
       5.2.1.  CE IP Address AVP usage  . . . . . . . . . . . . . . . 13

   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15

   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
     7.1.  Pseudowire Type  . . . . . . . . . . . . . . . . . . . . . 15
     7.2.  Control Message Attribute Value Pairs (AVPs) . . . . . . . 15
     7.3.  Result Code AVP Values . . . . . . . . . . . . . . . . . . 15

   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
     8.1.  Preventing forwarding loops with Ethernet and VLAN ACs . . 16



Pignataro & Luo         Expires February 24, 2006               [Page 2]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 17

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
   Intellectual Property and Copyright Statements . . . . . . . . . . 19













































Pignataro & Luo         Expires February 24, 2006               [Page 3]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


1.  Introduction

   [RFC3931] defines a base protocol for Layer 2 Tunneling over IP
   networks.  This document defines the specifics necessary for
   tunneling IP datagrams over L2TPv3 in order to provide interworking
   between diverse data link technologies.  Such emulated circuits are
   referred to as IP Layer 2 Transport Pseudowires (IPL2PWs).  When it
   is known that the Layer 2 frames to be transported only carry IP
   datagrams as payload, interworking between different Layer 2
   technologies can be achieved by only transporting the IP datagrams
   over L2TPv3.  Such an interface is referred to as an IP Layer 2
   interface.

   Protocol specifics defined in this document for L2TPv3 IPL2PWs
   include those necessary for simple point-to-point (e.g., between two
   L2TPv3 nodes) IPL2PW signaling, IP datagram encapsulation, address
   resolution considerations and simple interface up and interface down
   notifications.  This document also defines a new AVP to be used in
   point-to-point IPL2PWs.

   The reader is expected to be very familiar with the terminology and
   protocol constructs defined in [RFC3931].

1.1.  Abbreviations

   AC     Attachment Circuit

   CE     Customer Edge (Typically also the L2TPv3 Remote System).

   IPL2PW IP Layer 2 Transport Pseudowires

   LAC    L2TP Access Concentrator (See [RFC3931])

   LCCE   L2TP Control Connection Endpoint (See [RFC3931])

   NSP    Native Service Processing

   PE     Provider Edge (Typically also the LCCE).

   PW     Pseudowire

1.2.  Requirements

   The Pseudowire architecture [RFC3985] defines a Native Service
   Processing (NSP) function to restrict a Pseudowire to homogeneous
   operation by having the NSP perform actions that need knowledge of
   the semantics of the payload.




Pignataro & Luo         Expires February 24, 2006               [Page 4]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


   The following figure depicts the PW termination and NSP function
   within an LCCE:

                  +---------------------------------------+
                  |                 LCCE                  |
                  +-+   +-----+   +------+   +------+   +-+
                  |P|   |IP L2|   |  PW  |   | PSN  |   |P|
      Layer 2 <==>|h|<=>| NSP |<=>| Term |<=>|Tunnel|<=>|h|<==> PSN
    Interface     |y|   |     |   |      |   |      |   |y|
                  +-+   +-----+   +------+   +------+   +-+
                  |                                       |
                  +---------------------------------------+

   Figure 1: Requirements for IPL2PWs

   In IP Layer 2 Transport, the NSP function acts as the interface
   between the AC and the PW termination, and performs the following
   operations:

   o  Terminates the data link layer.

   o  Extracts IP datagrams from the Layer 2 frames and injects them
      into the PW.

   o  Drops non-IP payloads.

   o  Performs Address Resolution mediation and proxy functions
      (optionally).

   To the right of the NSP, only IP datagrams are forwarded to the PW
   termination point.  The PW termination point receives raw IP
   datagrams and delivers them unaltered to the PW termination point on
   the remote LCCE, providing an IP Layer 2 Transport emulation service
   that allows interworking between different Layer 2 technologies.


2.  Control Connection Establishment

   In order to tunnel IP datagrams as to provide interworking of Layer 2
   Attachment Circuits over an IP packet switched network using L2TPv3,
   an L2TPv3 Control Connection MUST first be established as described
   in [RFC3931].  The L2TPv3 SCCRQ Control Message and corresponding
   SCCRP Control Message MUST include the IPL2 Pseudowire Type of 0x000B
   (See IANA Considerations Section), in the Pseudowire Capabilities
   List as defined in 5.4.3 of [RFC3931].  This identifies the control
   connection as able to establish L2TP sessions to support IP Layer 2
   Transport Pseudowires (IPL2PWs).




Pignataro & Luo         Expires February 24, 2006               [Page 5]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


   An LCCE MUST be able to uniquely identify itself in the SCCRQ and
   SCCRP messages via a globally unique value.  This is advertised via
   the structured Router ID AVP [RFC3931], though the unstructured
   Hostname AVP [RFC3931] MAY be used to identify LCCEs as well.


3.  Layer 2 Link Status Notification and Session Establishment

   This section specifies how the status of the Layer 2 interface that
   only encapsulates IP is reported between two LCCEs, and the
   associated L2TP session creation and deletion that occurs.

3.1.  L2TPv3 Session Establishment

   Associating an IP Layer 2 interface with a PW and its transition to
   "Ready" or "Up" results in the establishment of an L2TP session via
   the standard three-way handshake described in Section 3.4.1 of
   [RFC3931].  For the purposes of this discussion, the action of
   locally associating an IP Layer 2 interface (i.e., an interface
   running a Layer 2 carrying IP as payload) with a PW by local
   configuration or otherwise is referred to as "provisioning" the
   interface.  The transition of the interface to "ready" or "up" will
   be referred to as the interface becoming ACTIVE.  The transition of
   the interface to "not-ready" or "down" will be referred to as the
   interfacing becoming INACTIVE.

   An LCCE MAY initiate the session immediately upon association with an
   IP Layer 2 interface, or wait until the interface becomes ACTIVE
   before attempting to establish an L2TP session.

   The Circuit Status AVP (see Section 3.4) MUST be present in the ICRQ,
   ICRP messages and MAY be present in the SLI message for IPL2PWs.



















Pignataro & Luo         Expires February 24, 2006               [Page 6]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


   Following is an example of the L2TP messages exchanged for an IPL2PW
   which is initiated after an Layer 2 interface is provisioned and
   becomes ACTIVE.

         LCCE (LAC) A                     LCCE (LAC) B
      ------------------               ------------------
      IP L2 Interface Provisioned
                                       IP L2 Interface Provisioned
      IP L2 Interface ACTIVE

                   ICRQ (status = 0x03) ---->

                                       IP L2 Interface ACTIVE

                   <---- ICRP (status = 0x03)

      L2TP session established,
      OK to send data into tunnel

                   ICCN ----->
                                       L2TP session established,
                                       OK to send data into tunnel

   In the example above, an ICRQ is sent after the interface is
   provisioned and becomes ACTIVE.  The Circuit Status AVP indicates
   that this link is ACTIVE and New (0x03).  The Remote End ID AVP
   [RFC3931] MUST be present in the ICRQ in order to identify the IP
   Layer 2 Transport AC (together with the identity of the LCCE itself
   as defined in Section 2) to associate the L2TP session with.  The
   Remote End ID AVP defined in [RFC3931] is of opaque form and variable
   length, though one MUST at a minimum support use of an unstructured
   four-octet value that is known to both LCCEs (either by direct
   configuration, or some other means).  The exact method of how this
   value is configured, retrieved, discovered, or otherwise determined
   at each LCCE is outside the scope of this document.

   As with the ICRQ, the ICRP is sent only after the associated Layer 2
   interface transitions to ACTIVE as well.  If LCCE B had not been
   provisioned for the interface identified in the ICRQ, a CDN would
   have been immediately returned indicating that the associated link
   was not provisioned or available at this LCCE.  LCCE A SHOULD then
   exhibit a periodic retry mechanism.  If so, the period and maximum
   number of retries MUST be configurable.

   An Implementation MAY send an ICRQ or ICRP before an IP Layer 2
   interface is ACTIVE, as long as the Circuit Status AVP reflects that
   the link is INACTIVE and an SLI is sent when the IP Layer 2 interface
   becomes ACTIVE (see Section 3.3).



Pignataro & Luo         Expires February 24, 2006               [Page 7]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


   The ICCN is the final stage in the session establishment, confirming
   the receipt of the ICRP with acceptable parameters to allow
   bidirectional traffic.

3.2.  L2TPv3 Session Teardown

   In the event a link is removed (unprovisioned) at either LCCE, the
   associated L2TP session MUST be torn down via the CDN message defined
   in Section 3.4.3 of [RFC3931].  General Result Codes regarding L2TP
   session establishment are defined in [RFC3931].

3.3.  L2TPv3 Session Maintenance

   IPL2PWs over L2TP make use of the Set Link Info (SLI) control message
   defined in [RFC3931] to signal Layer 2 link status notifications
   between PEs.  The SLI message is a single message that is sent over
   the L2TP control channel, signaling the interface state change.

   The SLI message MUST be sent any time there is a status change of the
   Active value 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 PE 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).

   All sessions established by a given control connection utilize the
   L2TP Hello facility defined in [RFC3931] for session keepalive.  This
   gives all sessions basic dead peer and path detection between PEs.

3.4.  Use of Circuit Status AVP for IP Layer 2 Pseudowires

   IP Layer 2 Transport reports Circuit Status with the Circuit Status
   AVP defined in [RFC3931], Attribute Type 71.

   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 N (New) bit SHOULD be set to one (1) if the Circuit Status



Pignataro & Luo         Expires February 24, 2006               [Page 8]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


   indication is for a new IP Layer 2 interface, zero (0) otherwise.

   The A (Active) bit indicates whether the IP Layer 2 interface is
   ACTIVE (1) or INACTIVE (0).


4.  Encapsulation

4.1.  Data Packet Encapsulation

   IPL2PWs use the default encapsulations defined in [RFC3931] for
   demultiplexing, sequencing, and flags.

   The L2TPv3 encapsulation carrying an IP datagram is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Session ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Cookie (optional, maximum 64 bits)...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Default L2-Specific Sublayer (optional)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          IP datagram                          |
   .                                                               .
   .                                                               .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Because Layer 2 frames of different encapsulation are normalized into
   IP datagrams when transporting over the IP over L2TP Pseudowire, it
   becomes possible to interconnect a pair of disparate attachment
   circuits.  In consequence, the IPL2PW does not have any operational
   mode contraints, (i.e., it can either operate in an "interface to
   interface" fashion, "virtual circuit to virtual circuit" fashion, or
   hybrid "interface to virtual circuit" fashion).  For example one AC
   can be a Frame-Relay DLCI while the other AC can be a PPP interface;
   or one AC can be an ATM PVC, while the other AC can be an ethernet
   VLAN.

   The Layer 2 is terminated on the LAC, the IPv4 or IPv6 datagram
   extracted and transported over the PW.  If a non-IP packet is
   received over the AC, it MUST be dropped and not transported over the
   PW.



Pignataro & Luo         Expires February 24, 2006               [Page 9]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


4.2.  Data Packet Sequencing

   Data Packet Sequencing MAY be enabled for IPL2PWs.  The sequencing
   mechanisms described in Section 4.6.1 of [RFC3931] MUST be used for
   signaling sequencing support.  IPL2PW over L2TP MUST request the
   presence of the L2TPv3 Default L2-Specific Sublayer defined in
   Section 4.6 of [RFC3931] when sequencing is enabled, and MAY request
   its presence at all times.

   It should be noted that the following two values for the Data
   Sequencing AVP, Attribute Type 70, have the exact same meaning for
   IPL2PWs:

      0 - No incoming data packets require sequencing.

      1 - Only non-IP data packets require sequencing.

   As such, the value of 1 SHOULD NOT be used for IPL2PWs.
   Additionally, the requirement of sequencing MUST be signaled with the
   following value:

      2 - All incoming data packets require sequencing.

   This value implies that all IPv4 and IPv6 datagrams transported
   include sequencing as described in Section 4.6.1 of [RFC3931].

4.3.  MTU Considerations

   With L2TPv3 as the tunneling protocol, the packet resulted from the
   encapsulation is N bytes longer than the raw IP datagram transported.

   The value of N depends on the following fields:

      L2TP Session Header:
         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,  L2TPv3 data messages are over IP;
      N = 16-28, L2TPv3 data messages are over UDP;
      (N does not include the IP header).

   The MTU and fragmentation implications resulting from this are
   discussed in Section 4.1.4 of [RFC3931].




Pignataro & Luo         Expires February 24, 2006              [Page 10]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


5.  Point-to-Point Address Resolution Considerations

   Different data link layers implement different address resolution
   mechanisms.

   The following sections describe the two address resolution
   operational modes: the REQUIRED Static Address Resolution mode (see
   Section 5.1) and the OPTIONAL Dynamic ARP Mediation mode (see
   Section 5.2).

5.1.  Static Address Resolution

   An IPL2PW is intended to provide point-to-point connectivity between
   two CE devices.  When dynamic ARP mediation procedures are not used,
   the following considerations and requirements apply to the specific
   data link layers.

   o  Ethernet
      Because only one CE device is expected to be attached to the
      Ethernet port, the LAC SHOULD act as proxy ARP for the segment
      responding with its own MAC address to all ARP requests.  The LAC
      MUST provide a configuration option to turn off this behavior, for
      the cases where more than one CE device may be connected to the
      Ethernet port.  Additionally, the LAC MAY provide an option to
      configure the remote CE's IP address and when doing so the LAC
      MUST respond with its own MAC address (source hardware address) to
      ARP requests for this configured IP address (destination protocol
      address) only.  If neither of these options are provided, address
      resolution is achieved by configuring static ARP entries for the
      locally attached devices.

   o  VLAN
      Same as Ethernet.

   o  PPP
      The LAC MUST provide an option to configure the remote CE's IP
      address and use it in IPCP negotiations.

   o  Frame Relay
      Because it is expected that the CE device treats the link as
      point-to-point, no specific address resolution requirements are
      needed.  For the cases where the CE device may treat the link as
      multi-point, the LAC MAY provide an option to configure the remote
      CE's IP address and use it when replying to Inverse ARP messages
      from the local CE; additionally, when the CE device treats the
      link as multi-point, address resolution can be achieved by a
      static Inverse ARP configuration at the CE device.




Pignataro & Luo         Expires February 24, 2006              [Page 11]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


   o  ATM
      Same as Frame Relay

   o  HDLC
      The CE MUST treat the link as point-to-point.

   Note that for Ethernet and VLAN links, the PE device MUST discover
   the MAC address of the locally attached CE device, and SHOULD use the
   procedures in [I-D.ietf-l2vpn-arp-mediation] to do so.

5.2.  Dynamic ARP Mediation

   The IP Layer 2 NSP function MAY implement dynamic ARP mediation
   mechanisms and procedures to signal the CE IP addresses between PE
   devices for point-to-point IPL2PWs, and SHOULD follow [I-D.ietf-
   l2vpn-arp-mediation] if doing so.  The dynamic ARP mediation defined
   in [I-D.ietf-l2vpn-arp-mediation] outlines a three-step procedure for
   PE devices:

   1.  Discover the IP addresses of the locally attached CE device.

   2.  Distribute those IP Addresses to the remote PE.

   3.  Notify the locally attached CE of the remote CE's IP address.

   Additionally, if the AC data link layer is Ethernet or VLAN, the PE
   device also needs to discover the MAC address of the locally attached
   CE device and SHOULD use the procedures in [I-D.ietf-l2vpn-arp-
   mediation].

   When using the Dynamic ARP Mediation signaling procedures for L2TPv3
   IPL2PWs, the CE IP Address AVP, Attribute Type AVP-TBD-1, is used to
   distribute the CE IP address to the remote PE.

   The Attribute Value field for this AVP has the following format:

   CE IP Address AVP (ICRQ, ICRP, SLI)

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Address Family        |         CE IP Address ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           ... (4 or 16 octets)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Address Family is a two octet quantity containing a value from
   the ADDRESS FAMILY NUMBERS in [RFC1700] that encodes the address



Pignataro & Luo         Expires February 24, 2006              [Page 12]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


   contained in the CE IP Address field.

   The following address encodings to be used in this AVP are hereby
   defined:

   +----------------+----------------------------+
   | Address Family |      Address Encoding      |
   +----------------+----------------------------+
   |    IPv4 (1)    |  4 octet full IPv4 address |
   |    IPv6 (2)    | 16 octet full IPv6 address |
   +----------------+----------------------------+

   Table 1

   The CE IP Address encodes the IP address of the CE attached to the
   advertising PE.  The encoding depends on the Address Family field,
   either a 4 octet IPv4 address or a 16 octet IPv6 address.

   The Length of this AVP is either 12 (when encoding an IPv4 address)
   or 24 (when encoding an IPv6 address).  The M bit for this AVP MUST
   be set to 0 (zero).  This AVP MAY be hidden (the H bit MAY be 1 or
   0).

5.2.1.  CE IP Address AVP usage

   The presence of this AVP in an ICRQ or ICRP message indicates that
   the LCCE is willing to perform ARP mediation procedures.  A null CE
   IP Address value indicates that the LCCE has not yet learned the IP
   address of his attached Remote System for the given address family.
   This AVP MUST be included in SLI messages sent asynchronously when
   the IP address of the local Remote System is discovered, with a non-
   null value denoting the IP address of the Remote System.



















Pignataro & Luo         Expires February 24, 2006              [Page 13]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


   The following example depicts the L2TP signaling messages exchanged
   for an IPL2PW establishment using the dynamic ARP Mediation
   procedures assuming an IPv4 address family.

         LCCE (LAC) A                     LCCE (LAC) B
      ------------------               ------------------
      IP L2 Interface Provisioned
      with ARP Mediation
                                       IP L2 Interface Provisioned
                                       with ARP Mediation
      IP Layer 2 Interface ACTIVE

                 ICRQ (IP address = NULL) ---->

                                       IP Layer 2 Interface ACTIVE

                 <---- ICRP (IP address = NULL)

      L2TP session established,
      OK to send data into tunnel

                 ICCN ----->
                                       L2TP session established,
                                       OK to send data into tunnel

      CE A's IP Address learned.

                 SLI (IP address = CE_A) ---->

                                       CE B's IP Address learned.

                 <---- SLI (IP address = CE_B)

   Likewise, this AVP MAY be re-advertised with a null CE IP Address
   value in an SLI message to indicate that the CE IP Address has become
   unavailable or is no longer valid.  This mechanism serves as a CE IP
   Address withdrawal.

   Continuing with the same example, the following figure depicts the
   L2TP signaling message sent when PE A discovers that CE A's IP
   Address is no longer valid.

         LCCE (LAC) A                     LCCE (LAC) B
      ------------------               ------------------
      CE A's IP Address invalidated.

                 SLI (IP address = NULL) ---->




Pignataro & Luo         Expires February 24, 2006              [Page 14]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


   If an ICRQ or ICRP message is received containing the CE IP Address
   AVP and the receiving LCCE is not capable, configured or willing to
   perform ARP mediation procedures, the session MUST be rejected via a
   CDN mesage with the following General Result Code:

   RC-TBD1: Mismatching ARP Mediation

   If an ICRP message is received lacking the CE IP Address AVP when the
   respective ICRQ was sent including this AVP, the session MUST be
   rejected via a CDN mesage with the same General Result Code.


6.  Acknowledgements

   This document heavily borrows and adapts format and text from the
   "HDLC Frames over L2TPv3" Internet-Draft, and we would like to
   acknowledge its authors and contributors.

   Many thanks to Maria Alice Dos Santos and George Wilkie for providing
   a thorough review and valuable comments and suggestions.


7.  IANA Considerations

7.1.  Pseudowire Type

   The signaling mechanisms defined in this document rely upon the
   assignment of an IP Layer 2 Transport Pseudowire Type (see Pseudowire
   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]).  The IP Layer 2
   Transport Pseudowire Type is defined in Section 2 of this
   specification:

   0x000B  IP Layer 2 Transport Pseudowire Type.

7.2.  Control Message Attribute Value Pairs (AVPs)

   A new AVP appears in Section 5.2 which needs assignment by IANA as
   described in Section 2.2 of [RFC3438].

   AVP-TBD-1: CE IP Address AVP

7.3.  Result Code AVP Values

   A new L2TP Result Code for the CDN message appears in Section 5.2.1
   which need assignment by IANA as described in Section 2.3 of
   [RFC3438].



Pignataro & Luo         Expires February 24, 2006              [Page 15]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


   RC-TBD1: Mismatching ARP Mediation


8.  Security Considerations

   The IP Layer 2 Transport over L2TPv3 is subject to the security
   considerations defined in [RFC3931] and [I-D.ietf-l2vpn-arp-
   mediation].  Additionaly, Section 8.1 includes a specific
   consideration to carrying IP Layer 2 using the Static Address
   Resolution procedures not present when carrying other data link
   types.

8.1.  Preventing forwarding loops with Ethernet and VLAN ACs

   IPL2PWs are intended to provide point-to-point connectivity between
   two CE devices, and therefore it is expected that the CE devices be
   configured with a subnet mask of at least 30-bits.  In an IPL2PW
   where the ACs are either Ethernet or Ethernet VLAN, an undesired side
   effect arises if the CE device is configured with a subnet mask
   shorter than 30-bits and the LCCE acts as a proxy ARP for the segment
   responding to all ARP requests with its own MAC Address: If an IP
   Address falls within the prefix in the segment but is not at either
   end of the PW, IP datagrams destined to it would loop back and forth
   until the TTL field expires.  To prevent this unwanted behavior, the
   mechanisms defined in Section 5.1 MUST be used.  They are copied
   verbatim as follows:

      Because only one CE device is expected to be attached to the
      Ethernet port, the LAC SHOULD act as proxy ARP for the segment
      responding with its own MAC address to all ARP requests.  The LAC
      MUST provide a configuration option to turn off this behavior, for
      the cases where more than one CE device may be connected to the
      Ethernet port.  Additionally, the LAC MAY provide an option to
      configure the remote CE's IP address and when doing so the LAC
      MUST respond with its own MAC address (source hardware address) to
      ARP requests for this configured IP address (destination protocol
      address) only.  If neither of these options are provided, address
      resolution is achieved by configuring static ARP entries for the
      locally attached devices.


9.  References

9.1.  Normative References

   [I-D.ietf-l2vpn-arp-mediation]
              Shah, H., "ARP Mediation for IP Interworking of Layer 2
              VPN", draft-ietf-l2vpn-arp-mediation-03 (work in



Pignataro & Luo         Expires February 24, 2006              [Page 16]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


              progress), August 2005.

   [RFC1700]  Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1700,
              October 1994.

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

   [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
              Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

9.2.  Informative References

   [RFC3438]  Townsley, W., "Layer Two Tunneling Protocol (L2TP)
              Internet Assigned Numbers Authority (IANA) Considerations
              Update", BCP 68, RFC 3438, December 2002.

   [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
              Edge (PWE3) Architecture", RFC 3985, March 2005.
































Pignataro & Luo         Expires February 24, 2006              [Page 17]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


Authors' Addresses

   Carlos Pignataro
   Cisco Systems, Inc.
   7025 Kit Creek Road
   PO Box 14987
   Research Triangle Park, NC  27709
   USA

   Email: cpignata@cisco.com


   Wei Luo
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email: luo@cisco.com
































Pignataro & Luo         Expires February 24, 2006              [Page 18]


Internet-Draft      IP Layer 2 Transport over L2TPv3         August 2005


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
   http://www.ietf.org/ipr.

   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-ipr@ietf.org.


Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


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.


Acknowledgment

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




Pignataro & Luo         Expires February 24, 2006              [Page 19]


Html markup produced by rfcmarkup 1.126, available from https://tools.ietf.org/tools/rfcmarkup/