<pre>Internet Engineering Task Force (IETF)                      H. Shah, Ed.
Request for Comments: 6575                                         Ciena
Category: Standards Track                                  E. Rosen, Ed.
ISSN: 2070-1721                                            G. Heron, Ed.
                                                                   Cisco
                                                        V. Kompella, Ed.
                                                          Alcatel-Lucent
                                                               June 2012


            <span class="h1">Address Resolution Protocol (ARP) Mediation for</span>
                    <span class="h1">IP Interworking of Layer 2 VPNs</span>

Abstract

   The Virtual Private Wire Service (VPWS), detailed in <a href="./rfc4664">RFC 4664</a>,
   provides point-to-point connections between pairs of Customer Edge
   (CE) devices.  It does so by binding two Attachment Circuits (each
   connecting a CE device with a Provider Edge (PE) device) to a
   pseudowire (connecting the two PEs).  In general, the Attachment
   Circuits must be of the same technology (e.g., both Ethernet or both
   ATM), and the pseudowire must carry the frames of that technology.
   However, if it is known that the frames' payload consists solely of
   IP datagrams, it is possible to provide a point-to-point connection
   in which the pseudowire connects Attachment Circuits of different
   technologies.  This requires the PEs to perform a function known as
   "Address Resolution Protocol (ARP) Mediation".  ARP Mediation refers
   to the process of resolving Layer 2 addresses when different
   resolution protocols are used on either Attachment Circuit.  The
   methods described in this document are applicable even when the CEs
   run a routing protocol between them, as long as the routing protocol
   runs over IP.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc6575">http://www.rfc-editor.org/info/rfc6575</a>.





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Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
      <a href="#section-1.1">1.1</a>. Conventions Used in This Document ..........................<a href="#page-4">4</a>
   <a href="#section-2">2</a>. ARP Mediation (AM) Function .....................................<a href="#page-5">5</a>
   <a href="#section-3">3</a>. IP Layer 2 Interworking Circuit .................................<a href="#page-6">6</a>
   <a href="#section-4">4</a>. IP Address Discovery Mechanisms .................................<a href="#page-6">6</a>
      <a href="#section-4.1">4.1</a>. Discovery of IP Addresses of Locally Attached IPv4 CE ......<a href="#page-7">7</a>
           <a href="#section-4.1.1">4.1.1</a>. Monitoring Local Traffic ............................<a href="#page-7">7</a>
           <a href="#section-4.1.2">4.1.2</a>. CE Devices Using ARP ................................<a href="#page-7">7</a>
           <a href="#section-4.1.3">4.1.3</a>. CE Devices Using Inverse ARP ........................<a href="#page-8">8</a>
           <a href="#section-4.1.4">4.1.4</a>. CE Devices Using PPP ................................<a href="#page-9">9</a>
           <a href="#section-4.1.5">4.1.5</a>. Router Discovery Method ............................<a href="#page-10">10</a>
           <a href="#section-4.1.6">4.1.6</a>. Manual Configuration ...............................<a href="#page-10">10</a>
      <a href="#section-4.2">4.2</a>. How a CE Learns the IPv4 Address of a Remote CE ...........<a href="#page-10">10</a>
           <a href="#section-4.2.1">4.2.1</a>. CE Devices Using ARP ...............................<a href="#page-11">11</a>
           <a href="#section-4.2.2">4.2.2</a>. CE Devices Using Inverse ARP .......................<a href="#page-11">11</a>
           <a href="#section-4.2.3">4.2.3</a>. CE Devices Using PPP ...............................<a href="#page-11">11</a>
      <a href="#section-4.3">4.3</a>. Discovery of IP Addresses of IPv6 CE Devices ..............<a href="#page-11">11</a>
           <a href="#section-4.3.1">4.3.1</a>. Distinguishing Factors between IPv4 and IPv6 .......<a href="#page-11">11</a>
           <a href="#section-4.3.2">4.3.2</a>. Requirements for PEs ...............................<a href="#page-12">12</a>
           <a href="#section-4.3.3">4.3.3</a>. Processing of Neighbor Solicitations ...............<a href="#page-12">12</a>
           <a href="#section-4.3.4">4.3.4</a>. Processing of Neighbor Advertisements ..............<a href="#page-13">13</a>
           <a href="#section-4.3.5">4.3.5</a>. Processing Inverse Neighbor Solicitations (INSs) ...<a href="#page-14">14</a>
           4.3.6. Processing of Inverse Neighbor
                  Advertisements (INAs) ..............................<a href="#page-15">15</a>
           <a href="#section-4.3.7">4.3.7</a>. Processing of Router Solicitations .................<a href="#page-15">15</a>
           <a href="#section-4.3.8">4.3.8</a>. Processing of Router Advertisements ................<a href="#page-15">15</a>
           <a href="#section-4.3.9">4.3.9</a>. Duplicate Address Detection ........................<a href="#page-16">16</a>
           <a href="#section-4.3.10">4.3.10</a>. CE Address Discovery for CEs Attached Using PPP ...<a href="#page-16">16</a>
   <a href="#section-5">5</a>. CE IPv4 Address Signaling between PEs ..........................<a href="#page-16">16</a>
      <a href="#section-5.1">5.1</a>. When to Signal an IPv4 Address of a CE ....................<a href="#page-16">16</a>
      <a href="#section-5.2">5.2</a>. LDP-Based Distribution of CE IPv4 Addresses ...............<a href="#page-17">17</a>



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   <a href="#section-6">6</a>. IPv6 Capability Advertisement ..................................<a href="#page-20">20</a>
      <a href="#section-6.1">6.1</a>. PW Operational Down on Stack Capability Mismatch ..........<a href="#page-21">21</a>
      <a href="#section-6.2">6.2</a>. Stack Capability Fallback .................................<a href="#page-21">21</a>
   <a href="#section-7">7</a>. IANA Considerations ............................................<a href="#page-22">22</a>
      <a href="#section-7.1">7.1</a>. LDP Status Messages .......................................<a href="#page-22">22</a>
      <a href="#section-7.2">7.2</a>. Interface Parameters ......................................<a href="#page-22">22</a>
   <a href="#section-8">8</a>. Security Considerations ........................................<a href="#page-22">22</a>
      <a href="#section-8.1">8.1</a>. Control Plane Security ....................................<a href="#page-23">23</a>
      <a href="#section-8.2">8.2</a>. Data Plane Security .......................................<a href="#page-24">24</a>
   <a href="#section-9">9</a>. Acknowledgements ...............................................<a href="#page-24">24</a>
   <a href="#section-10">10</a>. Contributors ..................................................<a href="#page-24">24</a>
   <a href="#section-11">11</a>. References ....................................................<a href="#page-25">25</a>
      <a href="#section-11.1">11.1</a>. Normative References .....................................<a href="#page-25">25</a>
      <a href="#section-11.2">11.2</a>. Informative References ...................................<a href="#page-26">26</a>
   <a href="#appendix-A">Appendix A</a>.  Use of IGPs with IP L2 Interworking L2VPNs ...........<a href="#page-27">27</a>
      <a href="#appendix-A.1">A.1</a>. OSPF ......................................................<a href="#page-27">27</a>
      <a href="#appendix-A.2">A.2</a>. RIP .......................................................<a href="#page-27">27</a>
      <a href="#appendix-A.3">A.3</a>. IS-IS .....................................................<a href="#page-28">28</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   Layer 2 Virtual Private Networks (L2VPNs) are constructed over a
   Service Provider IP/MPLS backbone but are presented to the Customer
   Edge (CE) devices as Layer 2 networks.  In theory, L2VPNs can carry
   any Layer 3 protocol, but in many cases, the Layer 3 protocol is IP.
   Thus, it makes sense to consider procedures that are optimized for
   IP.

   In a typical implementation, illustrated in the diagram below, the CE
   devices are connected to the Provider Edge (PE) devices via
   Attachment Circuits (ACs).  The ACs are Layer 2 circuits.  In a pure
   L2VPN, if traffic sent from CE1 via AC1 reaches CE2 via AC2, both ACs
   would have to be of the same type (i.e., both Ethernet, both Frame
   Relay, etc.).  However, if it is known that only IP traffic will be
   carried, the ACs can be of different technologies, provided that the
   PEs provide the appropriate procedures to allow the proper transfer
   of IP packets.














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                                           +-----+
                              +------ -----| CE3 |
                              |AC3         +-----+
                           +-----+
                     ......| PE3 |...........
                     .     +-----+          .
                     .        |             .
                     .        |             .
      +-----+ AC1 +-----+    Service      +-----+ AC2 +-----+
      | CE1 |-----| PE1 |--- Provider ----| PE2 |-----| CE2 |
      +-----+     +-----+    Backbone     +-----+     +-----+
                     .                      .
                     ........................

   A CE, which is connected via a given type of AC, may use an IP
   address resolution procedure that is specific to that type of AC.
   For example, an Ethernet-attached IPv4 CE would use ARP [<a href="./rfc826" title="&quot;Ethernet Address Resolution Protocol: Or Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware&quot;">RFC826</a>] and
   a Frame-Relay-attached CE might use Inverse ARP [<a href="./rfc2390" title="&quot;Inverse Address Resolution Protocol&quot;">RFC2390</a>].  If we are
   to allow the two CEs to have a Layer 2 connection between them, even
   though each AC uses a different Layer 2 technology, the PEs must
   intercept and "mediate" the Layer-2-specific address resolution
   procedures.

   In this document, we specify the procedures for VPWS services
   [<a href="./rfc4664" title="&quot;Framework for Layer 2 Virtual Private Networks (L2VPNs)&quot;">RFC4664</a>], which the PEs need to implement in order to mediate the IP
   address resolution mechanism.  We call these procedures "ARP
   Mediation".  Consider a Virtual Private Wire Service (VPWS)
   constructed between CE1 and CE2 in the diagram above.  If AC1 and AC2
   are of different technologies, e.g., AC1 is Ethernet and AC2 is Frame
   Relay (FR), then ARP requests coming from CE1 cannot be passed
   transparently to CE2.  PE1 MUST interpret the meaning of the ARP
   requests and mediate the necessary information with PE2 before
   responding.

   This document uses the term "ARP" to mean any protocol that is used
   to resolve IP addresses to link-layer addresses.  For instance, in
   IPv4, ARP and Inverse ARP protocols are used for address resolution
   while in IPv6, Neighbor Discovery [<a href="./rfc4861" title="&quot;Neighbor Discovery for IP version 6 (IPv6)&quot;">RFC4861</a>] and Inverse Neighbor
   Discovery [<a href="./rfc3122" title="&quot;Extensions to IPv6 Neighbor Discovery for Inverse Discovery Specification&quot;">RFC3122</a>] based on ICMPv6 are used for address resolution.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Conventions Used in This Document</span>

   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 [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].






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<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  ARP Mediation (AM) Function</span>

   The ARP Mediation (AM) function is an element of a PE node that deals
   with the IP address resolution for CE devices connected via a VPWS
   L2VPN.  By placing this function in the PE node, ARP Mediation is
   transparent to the CE devices.

   For a given point-to-point connection between a pair of CEs, the ARP
   Mediation procedure depends on whether the packets being forwarded
   are IPv4 or IPv6.  A PE that is to perform ARP Mediation for IPv4
   packets MUST perform the following logical steps:

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

   2.  Terminate.  Do not forward ARP and Inverse ARP requests from the
       CE device at the local PE.

   3.  Distribute the IP address to the remote PE using pseudowire
       control signaling.

   4.  Notify the locally attached CE of the IP address of the remote
       CE.

   5.  Respond appropriately to ARP and Inverse ARP requests from the
       local CE device using the IP address of the remote CE and the
       hardware address of the local PE.

   A PE that is to perform ARP Mediation for IPv6 packets MUST perform
   the following logical steps:

   1.  Discover the IPv6 addresses of the locally attached CE device,
       together with those of the remote CE device.

   2.  Perform the following steps:

       a.  Intercept Neighbor Discovery (ND) and Inverse Neighbor
           Discovery (IND) packets received from the local CE device.

       b.  From these ND and IND packets, learn the IPv6 configuration
           of the CE.

       c.  Forward the ND and IND packets over the pseudowire to the
           remote PE.








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   3.  Intercept Neighbor Discovery and Inverse Neighbor Discovery
       packets received over the pseudowire from the remote PE, possibly
       modifying them (if required for the type of outgoing AC) before
       forwarding to the local CE and learning information about the
       IPv6 configuration of the remote CE.

   Details for the procedures described above are given in the following
   sections.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  IP Layer 2 Interworking Circuit</span>

   The IP Layer 2 Interworking Circuit refers to interconnection of the
   Attachment Circuit with the IP Layer 2 Transport pseudowire that
   carries IP datagrams as the payload.  The ingress PE removes the data
   link header of its local Attachment Circuit and transmits the payload
   (an IP packet) over the pseudowire with or without the optional
   control word.  If the IP packet arrives at the ingress PE with
   multiple data link headers (for example, in the case of bridged
   Ethernet PDU on an ATM Attachment Circuit), all data link headers
   MUST be removed from the IP packet before transmission over the
   pseudowire (PW).  The egress PE encapsulates the IP packet with the
   data link header used on its local Attachment Circuit.

   The encapsulation for the IP Layer 2 Transport pseudowire is
   described in [<a href="./rfc4447" title="&quot;Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)&quot;">RFC4447</a>].  The "IP Layer 2 Interworking Circuit"
   pseudowire is also referred to as "IP pseudowire" in this document.

   In the case of an IPv6 L2 Interworking Circuit, the egress PE MAY
   modify the contents of Neighbor Discovery or Inverse Neighbor
   Discovery packets before encapsulating the IP packet with the data
   link header.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  IP Address Discovery Mechanisms</span>

   An IP Layer 2 Interworking Circuit enters monitoring state
   immediately after configuration.  During this state, it performs two
   functions:

   o  Discovery of the CE IP device(s)

   o  Establishment of the PW

   The establishment of the PW occurs independently from local CE IP
   address discovery.  During the period when the PW has been
   established but the local CE IP device has not been discovered, only
   broadcast/multicast IP frames are propagated between the Attachment
   Circuit and pseudowire; unicast IP datagrams are dropped.  The IP
   destination address is used to classify unicast/multicast packets.



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   Unicast IP frames are propagated between the AC and pseudowire only
   when CE IP devices on both Attachment Circuits have been discovered
   and notified and proxy functions have completed.

   The need to wait for address resolution completion before unicast IP
   traffic can flow is simple.

   o  PEs do not perform routing operations.

   o  The destination IP address in the packet is not necessarily that
      of the attached CE.

   o  On a broadcast link, there is no way to find out the Media Access
      Control (MAC) address of the CE based on the destination IP
      address of the packet.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Discovery of IP Addresses of Locally Attached IPv4 CE</span>

   A PE MUST support manual configuration of IPv4 CE addresses.  This
   section also describes automated mechanisms by which a PE MAY also
   discover an IPv4 CE address.

<span class="h4"><a class="selflink" id="section-4.1.1" href="#section-4.1.1">4.1.1</a>.  Monitoring Local Traffic</span>

   The PE devices MAY learn the IP addresses of the locally attached CEs
   from any IP traffic, such as link-local multicast packets (e.g.,
   destined to 224.0.0.x), and are not restricted to the operations
   below.

<span class="h4"><a class="selflink" id="section-4.1.2" href="#section-4.1.2">4.1.2</a>.  CE Devices Using ARP</span>

   If a CE device uses ARP to determine the IP-address-to-MAC-address
   binding of its neighbor, the PE processes the ARP requests to learn
   the IP address of the local CE for the local Attachment Circuit.

   The method described in this document only supports the case where
   there is a single CE per Attachment Circuit.  However, customer-
   facing access topologies may exist whereby more than one CE appears
   to be connected to the PE on a single Attachment Circuit.  For
   example, this could be the case when CEs are connected to a shared
   LAN that connects to the PE.  In such a case, the PE MUST select one
   local CE.  The selection could be based on manual configuration or
   the PE MAY optionally use the following selection criteria.  In
   either case, manual configuration of the IP address of the local CE
   (and its MAC address) MUST be supported.






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   o  Wait to learn the IP address of the remote CE (through PW
      signaling) and then select the local CE that is sending the
      request for IP address of the remote CE.

   o  Augment cross-checking with the local IP address learned through
      listening for link-local multicast packets (as per <a href="#section-4.1.1">Section 4.1.1</a>).

   o  Augment cross-checking with the local IP address learned through
      the Router Discovery Protocol (as described in <a href="#section-4.1.5">Section 4.1.5</a>).

   o  There is still a possibility that the local PE may not receive an
      IP address advertisement from the remote PE, and there may exist
      multiple local IP routers that attempt to 'connect' to remote CEs.
      In this situation, the local PE MAY use some other criteria to
      select one IP device from many (such as "the first ARP received"),
      or an operator MAY configure the IP address of the local CE.  Note
      that the operator does not have to configure the IP address of the
      remote CE (as that would be learned through pseudowire signaling).

   Once the local and remote CEs have been discovered for the given
   Attachment Circuit, the local PE responds with its own MAC address to
   any subsequent ARP requests from the local CE with a destination IP
   address matching the IP address of the remote CE.

   The local PE signals the IP address of the local CE to the remote PE
   and MAY initiate an unsolicited ARP response to notify the IP-
   address-to-MAC-address binding for the remote CE to the local CE
   (again using its own MAC address).

   Once the ARP Mediation function is completed (i.e., the PE device
   knows both the local and remote CE IP addresses), unicast IP frames
   are propagated between the AC and the established PW.

   The PE MAY periodically generate ARP request messages for the IP
   address of the CE as a means of verifying the continued existence of
   the IP address and its MAC address binding.  The absence of a
   response from the CE device for a given number of retries could be
   used as a trigger for withdrawal of the IP address advertisement to
   the remote PE.  The local PE would then re-enter the address
   resolution phase to rediscover the IP address of the attached CE.
   Note that this "heartbeat" scheme is needed only where the failure of
   a CE device may otherwise be undetectable.

<span class="h4"><a class="selflink" id="section-4.1.3" href="#section-4.1.3">4.1.3</a>.  CE Devices Using Inverse ARP</span>

   If a CE device uses Inverse ARP to determine the IP address of its
   neighbor, the attached PE processes the Inverse ARP request from the
   Attachment Circuit and responds with an Inverse ARP reply containing



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   the IP address of the remote CE, if the address is known.  If the PE
   does not yet have the IP address of the remote CE, it does not
   respond, but records the IP address of the local CE and the circuit
   information.  Subsequently, when the IP address of the remote CE
   becomes available, the PE MAY initiate an Inverse ARP request as a
   means of notifying the local CE of the IP address of the remote CE.

   This is the typical mode of operation for Frame Relay and ATM
   Attachment Circuits.  If the CE does not use Inverse ARP, the PE can
   still discover the IP address of the local CE using the mechanisms
   described in Sections <a href="#section-4.1.1">4.1.1</a> and <a href="#section-4.1.5">4.1.5</a>.

<span class="h4"><a class="selflink" id="section-4.1.4" href="#section-4.1.4">4.1.4</a>.  CE Devices Using PPP</span>

   The IP Control Protocol [<a href="./rfc1332" title="&quot;The PPP Internet Protocol Control Protocol (IPCP)&quot;">RFC1332</a>] describes a procedure to establish
   and configure IP on a point-to-point connection, including the
   negotiation of IP addresses.  When such an Attachment Circuit is
   configured for IP interworking, PPP negotiation is not performed end-
   to-end between CE devices.  Instead, PPP negotiation takes place
   between the CE and its local PE.  The PE performs proxy PPP
   negotiation and informs the attached CE of the IP address of the
   remote CE during IP Control Protocol (IPCP) negotiation using the IP-
   Address option (0x03).

   When a PPP link completes Link Control Protocol (LCP) negotiations,
   the local PE MAY perform the following IPCP actions:

   o  The PE learns the IP address of the local CE from the Configure-
      Request received with the IP-Address option (0x03).  If the IP
      address is non-zero, the PE records the address and responds with
      Configure-Ack.  However, if the IP address is zero, the PE
      responds with Configure-Reject (as this is a request from the CE
      to assign it an IP address).  Also, the IP-Address option is set
      with a zero value in the Configure-Reject response to instruct the
      CE not to include that option in any subsequent Configure-Request.

   o  If the PE receives a Configure-Request without the IP-Address
      option, it responds with a Configure-Ack.  In this case, the PE is
      unable to learn the IP address of the local CE using IPCP; hence,
      it MUST rely on other means as described in Sections <a href="#section-4.1.1">4.1.1</a> and
      4.1.5.  Note that in order to employ other learning mechanisms,
      the IPCP negotiations MUST have reached the open state.

   o  If the PE does not know the IP address of the remote CE, it sends
      a Configure-Request without the IP-Address option.






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   o  If the PE knows the IP address of the remote CE, it sends a
      Configure-Request with the IP-Address option containing the IP
      address of the remote CE.

   The IPCP IP-Address option MAY be negotiated between the PE and the
   local CE device.  Configuration of other IPCP options MAY be
   rejected.  Other Network Control Protocols (NCPs), with the exception
   of the Compression Control Protocol (CCP) and the Encryption Control
   Protocol (ECP), MUST be rejected.  The PE device MAY reject
   configuration of the CCP and ECP.

<span class="h4"><a class="selflink" id="section-4.1.5" href="#section-4.1.5">4.1.5</a>.  Router Discovery Method</span>

   In order to learn the IP address of the CE device for a given
   Attachment Circuit, the PE device MAY execute the Router Discovery
   Protocol [<a href="./rfc1256" title="&quot;ICMP Router Discovery Messages&quot;">RFC1256</a>] whereby a Router Discovery Request (ICMP - Router
   Solicitation) message is sent using a source IP address of zero.  The
   IP address of the CE device is extracted from the Router Discovery
   Response (ICMP - Router Advertisement) message from the CE.  It is
   possible that the response contains more than one router address with
   the same preference level, in which case, some heuristics (such as
   first on the list) are necessary.  The use of the Router Discovery
   method by the PE is optional.

<span class="h4"><a class="selflink" id="section-4.1.6" href="#section-4.1.6">4.1.6</a>.  Manual Configuration</span>

   In some cases, it may not be possible to discover the IP address of
   the local CE device using the mechanisms described in Sections <a href="#section-4.1.1">4.1.1</a>
   to 4.1.5.  In such cases, manual configuration MAY be used.  All
   implementations of this document MUST support manual configuration of
   the IPv4 address of the local CE.  This is the only REQUIRED mode for
   a PE to support.

   The support for configuration of the IP address of the remote CE is
   OPTIONAL.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  How a CE Learns the IPv4 Address of a Remote CE</span>

   Once the local PE has received the IP address information of the
   remote CE from the remote PE, it will either initiate an address
   resolution request or respond to an outstanding request from the
   attached CE device.

   In the event that the IPv4 address of the remote CE is manually
   configured, the address resolution can begin immediately as receipt
   of remote IP address of the CE becomes unnecessary.





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<span class="h4"><a class="selflink" id="section-4.2.1" href="#section-4.2.1">4.2.1</a>.  CE Devices Using ARP</span>

   When the PE learns the IP address of the remote CE as described in
   <a href="#section-5.1">Section 5.1</a>, it may or may not already know the IP address of the
   local CE.  If the IP address is not known, the PE MUST wait until it
   is acquired through one of the methods described in Sections <a href="#section-4.1.1">4.1.1</a>,
   4.1.2, and 4.1.5.  If the IP address of the local CE is known, the PE
   MAY choose to generate an unsolicited ARP message to notify the local
   CE about the binding of the IP address of the remote CE with the PE's
   own MAC address.

   When the local CE generates an ARP request, the PE MUST proxy the ARP
   response [<a href="./rfc925" title="&quot;Multi-LAN address resolution&quot;">RFC925</a>] using its own MAC address as the source hardware
   address and the IP address of the remote CE as the source protocol
   address.  The PE MUST respond only to those ARP requests whose
   destination protocol address matches the IP address of the remote CE.

<span class="h4"><a class="selflink" id="section-4.2.2" href="#section-4.2.2">4.2.2</a>.  CE Devices Using Inverse ARP</span>

   When the PE learns the IP address of the remote CE, it SHOULD
   generate an Inverse ARP request.  If the Attachment Circuit requires
   activation (e.g., Frame Relay), the PE SHOULD activate it first
   before the Inverse ARP request.  It should be noted that the PE might
   never receive the response to its own request, nor see any Inverse
   ARP request from the CE, in cases where the CE is pre-configured with
   the IP address of the remote CE or where the use of Inverse ARP has
   not been enabled.  In either case, the CE has used other means to
   learn the IP address of its neighbor.

<span class="h4"><a class="selflink" id="section-4.2.3" href="#section-4.2.3">4.2.3</a>.  CE Devices Using PPP</span>

   When the PE learns the IP address of the remote CE, it SHOULD
   initiate a Configure-Request and set the IP-Address option to the IP
   address of the remote CE.  This notifies the local CE of the IP
   address of the remote CE.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Discovery of IP Addresses of IPv6 CE Devices</span>

<span class="h4"><a class="selflink" id="section-4.3.1" href="#section-4.3.1">4.3.1</a>.  Distinguishing Factors between IPv4 and IPv6</span>

   IPv4 uses ARP and Inverse ARP to resolve IP address and link-layer
   associations.  Since these are dedicated address resolution
   protocols, and not IP packets, they cannot be carried on an IP
   pseudowire.  They MUST be processed locally and the IPv4 address
   information they carry signaled between the PEs using the pseudowire
   control plane.  IPv6 uses ICMPv6 extensions to resolve IP address and





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   link address associations.  As these are IPv6 packets, they can be
   carried on an IP pseudowire; therefore, no IPv6 address signaling is
   required.

<span class="h4"><a class="selflink" id="section-4.3.2" href="#section-4.3.2">4.3.2</a>.  Requirements for PEs</span>

   A PE device that supports IPv6 MUST be capable of the following:

   o  Intercepting ICMPv6 Neighbor Discovery [<a href="./rfc4861" title="&quot;Neighbor Discovery for IP version 6 (IPv6)&quot;">RFC4861</a>] and Inverse
      Neighbor Discovery [<a href="./rfc3122" title="&quot;Extensions to IPv6 Neighbor Discovery for Inverse Discovery Specification&quot;">RFC3122</a>] packets received over the AC as well
      as over the PW,

   o  Recording the IPv6 interface addresses and CE link-layer addresses
      present in these packets,

   o  Possibly modifying these packets as dictated by the data link type
      of the egress AC (described in the following sections), and

   o  Forwarding them towards the original destination.

   The PE MUST also be capable of generating packets in order to
   interwork between Neighbor Discovery (ND) and Inverse Neighbor
   Discovery (IND).  This is specified in Sections <a href="#section-4.3.3">4.3.3</a> to <a href="#section-4.3.6">4.3.6</a>.

   If an IP PW is used to interconnect CEs that use IPv6 Router
   Discovery [<a href="./rfc4861" title="&quot;Neighbor Discovery for IP version 6 (IPv6)&quot;">RFC4861</a>], a PE device MUST also be capable of intercepting
   and processing those Router Discovery packets.  This is required in
   order to translate between different link-layer addresses.  If a
   Router Discovery message contains a link-layer address, then the PE
   MAY also use this message to discover the link-layer address and IPv6
   interface address.  This is described in more detail in Sections
   4.3.7 and 4.3.8.

   The PE device MUST learn a list of CE IPv6 interface addresses for
   its directly attached CE and another list of CE IPv6 interface
   addresses for the far-end CE.  The PE device MUST also learn the
   link-layer address of the local CE and be able to use it when
   forwarding traffic between the local and far-end CEs.  The PE MAY
   also wish to monitor the source link-layer address of data packets
   received from the CE and discard packets not matching its learned CE
   link-layer address.

<span class="h4"><a class="selflink" id="section-4.3.3" href="#section-4.3.3">4.3.3</a>.  Processing of Neighbor Solicitations</span>

   A Neighbor Solicitation received on an AC from a local CE SHOULD be
   inspected to determine and learn an IPv6 interface address (if
   provided, this will not be the case for Duplicate Address Detection)
   and any link-layer address provided.  The packet MUST then be



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   forwarded over the pseudowire unmodified.  A Neighbor Solicitation
   received over the pseudowire SHOULD be inspected to determine and
   learn an IPv6 interface address for the far-end CE.  If a source
   link-layer address option is present, the PE MUST remove it.  The PE
   MAY substitute an appropriate link-layer address option, specifying
   the link-layer address of the PE interface attached to the local AC.
   Note that if the local AC is Ethernet, failure to substitute a link-
   layer address option may mean that the CE has no valid link-layer
   address with which to transmit data packets.

   When a PE with a local AC, which is of the type point-to-point Layer
   2 circuit, e.g., FR, ATM or PPP, receives a Neighbor Solicitation
   from a far-end PE over the pseudowire, after learning the IP address
   of the far-end CE, the PE MAY use one of the following procedures:

   1.  Forward the Neighbor Solicitation to the local CE after replacing
       the source link-layer address with the link-layer address of the
       local AC.

   2.  Send an Inverse Neighbor Solicitation to the local CE, specifying
       the far-end CE's IP address and the link-layer address of the PE
       interface attached to local AC.

   3.  Reply to the far-end PE with a Neighbor Advertisement, using the
       IP address of the local CE as the source address and an
       appropriate link-layer address option that specifies the link-
       layer address of the PE interface attached to local AC.  As
       described in <a href="#section-4.3.10">Section 4.3.10</a>, the IP address of the local CE is
       learned through IPv6 Control Protocol (IPv6CP) in the case of PPP
       and through Neighbor Solicitation in other cases.

<span class="h4"><a class="selflink" id="section-4.3.4" href="#section-4.3.4">4.3.4</a>.  Processing of Neighbor Advertisements</span>

   A Neighbor Advertisement received on an AC from a local CE SHOULD be
   inspected to determine and learn an IPv6 interface address and any
   link-layer address provided.  The packet MUST then be forwarded over
   the IP pseudowire unmodified.

   A Neighbor Advertisement received over the pseudowire SHOULD be
   inspected to determine and learn an IPv6 interface address for the
   far-end CE.  If a source link-layer address option is present, the PE
   MUST remove it.  The PE MAY substitute an appropriate link-layer
   address option, specifying the link-layer address of the PE interface
   attached to local AC.  Note that if the local AC is Ethernet, failure
   to substitute a link-layer address option may mean that the local AC
   has no valid link-layer address with which to transmit data packets.





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   When a PE with a local AC that is of the type point-to-point Layer 2
   circuit, such as ATM, FR, or PPP, receives a Neighbor Advertisement
   over the pseudowire, in addition to learning the remote CE's IPv6
   address, it SHOULD perform the following steps:

   o  If the AC supports Inverse Neighbor Discovery (IND) and the PE had
      already processed an Inverse Neighbor Solicitation (INS) from the
      local CE, it SHOULD send an Inverse Neighbor Advertisement (INA)
      on the local AC using source IP address information received in an
      ND advertisement (ND-ADV) and its own local AC link-layer
      information.

   o  If the PE has not received any Inverse Neighbor Solicitation (INS)
      from the local CE and the AC supports Inverse Neighbor Discovery
      (IND), it SHOULD send an INS on the local AC using source IP
      address information received in the INA together with its own
      local AC link-layer information.

<span class="h4"><a class="selflink" id="section-4.3.5" href="#section-4.3.5">4.3.5</a>.  Processing Inverse Neighbor Solicitations (INSs)</span>

   An INS received on an AC from a local CE SHOULD be inspected to
   determine and learn the IPv6 addresses and the link-layer addresses.
   The packet MUST then be forwarded over the pseudowire unmodified.

   An INS received over the pseudowire SHOULD be inspected to determine
   and learn one or more IPv6 addresses for the far-end CE.  If the
   local AC supports IND (e.g., a switched Frame Relay AC), the packet
   SHOULD be forwarded to the local CE after modifying the link-layer
   address options to match the type of the local AC.

   If the local AC does not support IND, processing of the packet
   depends on whether the PE has learned at least one interface address
   for its directly attached CE.

   o  If it has learned at least one IPv6 address for the CE, the PE
      MUST discard the Inverse Neighbor Solicitation (INS) and generate
      an Inverse Neighbor Advertisement (INA) back into the pseudowire.
      The destination address of the INA is the source address from the
      INS; the source address is one of the local CE's interface
      addresses; and all the local CE's interface addresses that have
      been learned so far SHOULD be included in the Target Address List.
      The Source and Target link-layer addresses are copied from the
      INS.  In addition, the PE SHOULD generate ND advertisements on the
      local AC using the IPv6 address of the remote CE and the link-
      layer address of the local PE.






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   o  If it has not learned at least one IPv6 and link-layer address of
      its directly connected CE, the INS MUST continue to be discarded
      until the PE learns an IPv6 and link-layer address from the local
      CE (through receiving, for example, a Neighbor Solicitation).
      After this has occurred, the PE will be able to respond to INS
      messages received over the pseudowire as described above.

<span class="h4"><a class="selflink" id="section-4.3.6" href="#section-4.3.6">4.3.6</a>.  Processing of Inverse Neighbor Advertisements (INAs)</span>

   An INA received on an AC from a local CE SHOULD be inspected to
   determine and learn one or more IPv6 addresses for the CE.  It MUST
   then be forwarded unmodified over the pseudowire.

   An INA received over the pseudowire SHOULD be inspected to determine
   and learn one or more IPv6 addresses for the far-end CE.

   If the local AC supports IND (e.g., a Frame Relay AC), the packet MAY
   be forwarded to the local CE after modifying the link-layer address
   options to match the type of the local AC.

   If the local AC does not support IND, the PE MUST discard the INA and
   generate a Neighbor Advertisement (NA) towards its local CE.  The
   source IPv6 address of the NA is the source IPv6 address from the
   INA; the destination IPv6 address is the destination IPv6 address
   from the INA; and the link-layer address is that of the local AC on
   the PE.

<span class="h4"><a class="selflink" id="section-4.3.7" href="#section-4.3.7">4.3.7</a>.  Processing of Router Solicitations</span>

   A Router Solicitation received on an AC from a local CE SHOULD be
   inspected to determine and learn an IPv6 address for the CE and, if
   present, the link-layer address of the CE.  It MUST then be forwarded
   unmodified over the pseudowire.

   A Router Solicitation received over the pseudowire SHOULD be
   inspected to determine and learn an IPv6 address for the far-end CE.
   If a source link-layer address option is present, the PE MUST remove
   it.  The PE MAY substitute a source link-layer address option
   specifying the link-layer address of its local AC.  The packet is
   then forwarded to the local CE.

<span class="h4"><a class="selflink" id="section-4.3.8" href="#section-4.3.8">4.3.8</a>.  Processing of Router Advertisements</span>

   A Router Advertisement received on an AC from a local CE SHOULD be
   inspected to determine and learn an IPv6 address for the CE and, if
   present, the link-layer address of the CE.  It MUST then be forwarded
   unmodified over the pseudowire.




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   A Router Advertisement received over the pseudowire SHOULD be
   inspected to determine and learn an IPv6 address for the far-end CE.
   If a source link-layer address option is present, the PE MUST remove
   it.  The PE MAY substitute a source link-layer address option
   specifying the link-layer address of its local AC.  If an MTU option
   is present, the PE MAY reduce the specified MTU if the MTU of the
   pseudowire is less than the value specified in the option.  The
   packet is then forwarded to the local CE.

<span class="h4"><a class="selflink" id="section-4.3.9" href="#section-4.3.9">4.3.9</a>.  Duplicate Address Detection</span>

   Duplicate Address Detection [<a href="./rfc4862" title="&quot;IPv6 Stateless Address Autoconfiguration&quot;">RFC4862</a>] allows IPv6 hosts and routers
   to ensure that the addresses assigned to interfaces are unique on a
   link.  As with all Neighbor Discovery packets, those used in
   Duplicate Address Detection will simply flow through the pseudowire,
   being inspected at the PEs at each end.  Processing is performed as
   detailed in Sections <a href="#section-4.3.3">4.3.3</a> and <a href="#section-4.3.4">4.3.4</a>.  However, the source IPv6
   address of Neighbor Solicitations used in Duplicate Address Detection
   is the unspecified address, so the PEs cannot learn the CE's IPv6
   interface address (nor would it make sense to do so, given that at
   least one address is tentative at that time).

<span class="h4"><a class="selflink" id="section-4.3.10" href="#section-4.3.10">4.3.10</a>.  CE Address Discovery for CEs Attached Using PPP</span>

   The IPv6 Control Protocol (IPv6CP) [<a href="./rfc5072" title="&quot;IP Version 6 over PPP&quot;">RFC5072</a>] describes a procedure
   for establishing and configuring IPv6 on a point-to-point connection,
   including the negotiation of a link-local interface identifier.  As
   in the case of IPv4, when such an AC is configured for IP
   interworking, PPP negotiation is not performed end-to-end between CE
   devices.  Instead, PPP negotiation takes place between the CE and its
   local PE.  The PE performs proxy PPP negotiation and informs the
   attached CE of the link-local identifier of its local interface using
   the Interface-Identifier option (0x01).  This local interface
   identifier is used by stateless address autoconfiguration [<a href="./rfc4862" title="&quot;IPv6 Stateless Address Autoconfiguration&quot;">RFC4862</a>].

   When a PPP link completes IPv6CP negotiations and the PPP link is
   open, a PE MAY discover the IPv6 unicast address of the CE using any
   of the mechanisms described above.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  CE IPv4 Address Signaling between PEs</span>

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  When to Signal an IPv4 Address of a CE</span>

   A PE device advertises the IPv4 address of the attached CE only when
   the encapsulation type of the pseudowire is IP Layer2 Transport (the
   value 0x000B, as defined in [<a href="./rfc4446" title="&quot;IANA Allocations for Pseudowire Edge to Edge Emulation (PWE3)&quot;">RFC4446</a>]).  The IP Layer2 transport PW
   is also referred to as IP PW and is used interchangeably in this
   document.  It is quite possible that the IPv4 address of a CE device



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   is not available at the time the PW labels are signaled.  For
   example, in Frame Relay, the CE device sends an Inverse ARP request
   only when the Data Link Connection Identifier (DLCI) is active.  If
   the PE signals the DLCI to be active only when it has received the
   IPv4 address along with the PW Forwarding Equivalence Class (FEC)
   from the remote PE, a deadlock situation arises.  In order to avoid
   such problems, the PE MUST be prepared to advertise the PW FEC before
   the IPv4 address of the CE is known; hence,the PE uses an IPv4
   address value zero.  When the IPv4 address of the CE device does
   become available, the PE re-advertises the PW FEC along with the IPv4
   address of the CE.

   Similarly, if the PE detects that an IP address of a CE is no longer
   valid (by methods described above), the PE MUST re-advertise the PW
   FEC with a null IP address to denote the withdrawal of the IP address
   of the CE.  The receiving PE then waits for notification of the
   remote IP address.  During this period, propagation of unicast IPv4
   traffic is suspended, but multicast IPv4 traffic can continue to flow
   between the AC and the pseudowire.

   If two CE devices are locally attached to the PE on disparate AC
   types (for example, one CE connected to an Ethernet port and the
   other to a Frame Relay port), the IPv4 addresses are learned in the
   same manner as described above.  However, since the CE devices are
   local, the distribution of IPv4 addresses for these CE devices is a
   local step.

   Note that the PEs discover the IPv6 addresses of the remote CE by
   intercepting Neighbor Discovery and Inverse Neighbor Discovery
   packets that have been passed in-band through the pseudowire.  Hence,
   there is no need to communicate the IPv6 addresses of the CEs through
   LDP signaling.

   If the pseudowire is carrying both IPv4 and IPv6 traffic, the
   mechanisms used for IPv6 and IPv4 SHOULD NOT interact.  In
   particular, just because a PE has learned a link-layer address for
   IPv6 traffic by intercepting a Neighbor Advertisement from its
   directly connected CE, it SHOULD NOT assume that it can use that
   link-layer address for IPv4 traffic until that fact is confirmed by
   reception of, for example, an IPv4 ARP message from the CE.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  LDP-Based Distribution of CE IPv4 Addresses</span>

   [<a id="ref-RFC4447">RFC4447</a>] uses Label Distribution Protocol (LDP) transport to
   exchange PW FECs in the Label Mapping message in the Downstream
   Unsolicited (DU) mode.  The PW FEC comes in two flavors, with some





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   common fields between them: PWid and Generalized ID FEC elements.
   The discussions below refer to these common fields for IP L2
   Interworking encapsulation.

   In addition to PW FEC, this document uses an IP Address List TLV (as
   defined in [<a href="./rfc5036" title="&quot;LDP Specification&quot;">RFC5036</a>]) that is to be included in the optional
   parameter field of the Label Mapping message when advertising the PW
   FEC for the IP Layer2 Transport.  The use of optional parameters in
   the Label Mapping message to extend the attributes of the PW FEC is
   specified in [<a href="./rfc4447" title="&quot;Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)&quot;">RFC4447</a>].

   As defined in [<a href="./rfc4447" title="&quot;Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)&quot;">RFC4447</a>], when processing a received PW FEC, the PE
   matches the PW ID and PW type with the locally configured PW ID and
   PW Type.  If there is a match and if the PW Type is IP Layer2
   Transport, the PE further checks for the presence of an Address List
   TLV [<a href="./rfc5036" title="&quot;LDP Specification&quot;">RFC5036</a>] in the optional parameter TLVs.  The processing of the
   Address List TLV is as follows.

   o  If a PE is configured for an AC to a CE enabled for IPv4 or dual-
      stack IPv4/IPv6, the PE SHOULD advertise an Address List TLV with
      address family type of IPv4 address.  The PE SHOULD process the
      IPv4 Address List TLV as described in this document.  The PE MUST
      advertise and process IPv6 capability using the procedures
      described in <a href="#section-6">Section 6</a>.

   o  If a PE does not receive any IPv4 address in the Address List TLV,
      it MAY assume IPv4 behavior.  The address resolution for IPv4 MUST
      then depend on local manual configuration.  In the case of
      mismatched configuration whereby one PE has manual configuration
      while the other does not, the IP address to link-layer address
      mapping remains unresolved, resulting in unsuccessful propagation
      of IPv4 traffic to the local CE.

   o  If a PE is configured for an AC to a CE enabled for IPv6 only, the
      PE MUST advertise IPv6 capability using the procedures described
      in <a href="#section-6">Section 6</a>.  In addition, by virtue of not setting the manual
      configuration for IPv4 support, IPv6-only support is realized.

   We use the Address List TLV [<a href="./rfc5036" title="&quot;LDP Specification&quot;">RFC5036</a>] to signal the IPv4 address of
   the local CE.  This IP Address List TLV is included in the optional
   parameter field of the Label Mapping message.

   The Address List TLV is only used for IPv4 addresses.








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   The fields of the IP Address List TLV are set as follows:

   Length
      Set to 6 to encompass 2 bytes of Address Family field and 4 bytes
      of Addresses field (because a single IPv4 address is used).

   Address Family
      Set to 1 to indicate IPv4 as defined in [<a href="./rfc5036" title="&quot;LDP Specification&quot;">RFC5036</a>].

   Addresses
      Contains a single IPv4 address that is the address of the CE
      attached to the advertising PE.

   The address in the Addresses field is set to all zeros to denote that
   the advertising PE has not learned the IPv4 address of its local CE.
   Any non-zero address value denotes the IPv4 address of the
   advertising PE's attached CE device.

   The IPv4 address of the CE is also supplied in the optional
   parameters field of the LDP Notification message along with the PW
   FEC.  The LDP Notification message is used to signal any change in
   the status of the CE's IPv4 address.

   The encoding of the LDP Notification message 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0|   Notification (0x0001)     |      Message Length           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Message ID                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Status TLV                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 IP Address List TLV (as defined above)        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 PWid FEC or Generalized ID FEC                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Status TLV status code is set to 0x0000002C "IP address of CE",
   to indicate that an IP address update follows.  Since this
   notification does not refer to any particular message, the Message ID
   field is set to 0.

   The PW FEC TLV SHOULD NOT include the interface parameters as they
   are ignored in the context of this message.





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<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  IPv6 Capability Advertisement</span>

   A Stack Capability Interface Parameter sub-TLV is signaled by the two
   PEs so that they can agree which network protocol(s) they SHOULD be
   using.  As discussed earlier, the use of the Address List TLV
   signifies support for IPv4 stack, so the Stack Capability Interface
   Parameter sub-TLV is used to indicate whether support for IPv6 stack
   is required on a given IP PW.

   The Stack Capability Interface Parameter sub-TLV is part of the
   interface parameters.  The proposed format for the Stack Capability
   Interface Parameter sub-TLV 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Parameter ID  |     Length    |       Stack Capability        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Parameter ID = 0x16

   Length = 4

   The Stack Capability field is a bit field.  Only one bit is defined
   in this document.  When bit zero (the least significant bit with
   bitmask 0x0001) is set, it indicates IPv6 Stack Capability.

   The presence of the Stack Capability Interface Parameter sub-TLV is
   relevant only when the PW type is IP PW.  A PE that supports IPv6 on
   an IP PW MUST signal the Stack Capability Interface Parameter sub-TLV
   in the initial Label Mapping message for the PW.  The PE nodes
   compare the value advertised by the remote PE with the local
   configuration and only use a capability that is supported by both.

   The behavior of a PE that does not understand an Interface Parameter
   sub-TLV is specified in <a href="./rfc4447#section-5.5">Section&nbsp;5.5 of RFC 4447</a> [<a href="./rfc4447" title="&quot;Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)&quot;">RFC4447</a>].

   In some deployment scenarios, it may be desirable to take a PW
   operationally down if there is a mismatch of the Stack Capability
   between the PEs.  In other deployment scenarios, an operator may wish
   the IP version supported by both PEs to fall back to IPv4 if one of
   the PEs does not support IPv6.  The following procedures MUST be
   followed for each of these cases.








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<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  PW Operational Down on Stack Capability Mismatch</span>

   If a PE that supports IPv6 and has not yet sent a Label Mapping
   message receives an initial Label Mapping message from the far-end PE
   that does not include the Stack Capability Interface Parameter sub-
   TLV, or one is received but it is not set to the 'IPv6 Stack
   Capability' value, then the PE supporting this procedure MUST NOT
   send a Label Mapping message for this PW.

   If a PE that supports IPv6 has already sent an initial Label Mapping
   message for the PW and does not receive a Stack Capability Interface
   Parameter sub-TLV in the Label Mapping message from the far-end PE,
   or one is received but it is not set to 'IPv6 Stack Capability', the
   PE supporting this procedure MUST withdraw its PW label with the LDP
   status code meaning "IP Address type mismatch" (Status Code
   0x0000004A).  However, subsequently, if the configuration was to
   change at the far-end PE and a Stack Capability Interface Parameter
   sub-TLV in the Label Mapping message is received from the far-end PE,
   the local PE MUST re-advertise the Label Mapping message for the PW.

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  Stack Capability Fallback</span>

   If a PE that supports IPv6 and has not yet sent a Label Mapping
   message receives an initial Label Mapping message from the far-end PE
   that does not include the Stack Capability Interface Parameter sub-
   TLV, or one is received but it is not set to the 'IPv6 Stack
   Capability' value, then it MAY send a Label Mapping message for this
   PW but MUST NOT include the Stack Capability Interface Parameter sub-
   TLV.

   If a PE that supports IPv6 and has already sent a Label Mapping
   message for the PW with the Stack Capability Interface Parameter sub-
   TLV but does not receive a Stack Capability Interface Parameter sub-
   TLV from the far-end PE in the initial Label Mapping message (or one
   is received but it is not set to the 'IPv6 Stack Capability' value),
   the PE following this procedure MUST send a Label Withdraw for its PW
   label with the LDP status code meaning "Wrong IP Address type"
   (Status Code 0x000004B) followed by a Label Mapping message that does
   not include the Stack Capability Interface Parameter sub-TLV.  If a
   Label Withdraw message with the "Wrong IP Address Type" status code
   is received by a PE, it SHOULD treat this as a normal Label Withdraw
   but MUST NOT respond with a Label Release.  It MUST continue to wait
   for the next control message for the PW as specified in <a href="./rfc4447#section-6.2">Section&nbsp;6.2
   of RFC 4447</a> [<a href="./rfc4447" title="&quot;Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)&quot;">RFC4447</a>].







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<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  IANA Considerations</span>

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  LDP Status Messages</span>

   This document uses new LDP status codes.  IANA already maintains a
   registry of name "Status Code Name Space" defined by [<a href="./rfc5036" title="&quot;LDP Specification&quot;">RFC5036</a>].  The
   following values have been assigned:

      0x0000002C "IP Address of CE"
      0x0000004A "IP Address Type Mismatch"
      0x0000004B "Wrong IP Address Type"

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  Interface Parameters</span>

   This document proposes a new Interface Parameters sub-TLV, that has
   been assigned from the 'Pseudowire Interface Parameters Sub-TLV type
   Registry'.  The following value has been assigned for the Parameter
   ID:

      0x16   "Stack Capability"

   IANA has also set up a registry of "L2VPN PE stack Capabilities".
   This is a 16-bit field.  Stack Capability bitmask 0x0001 is specified
   in <a href="#section-6">Section 6</a> of this document.  The remaining bitfield values
   (0x0002,..,0x8000) are to be assigned by IANA using the "IETF Review"
   policy defined in [<a href="./rfc5226" title="">RFC5226</a>].

   L2VPN PE Stack Capabilities:

   Bit (Value)       Description
   ===============   ========================
   Bit 0  (0x0001) - IPv6 stack capability
   Bit 1  (0x0002) - Unassigned
   Bit 2  (0x0004) - Unassigned
            .
            .
            .

   Bit 14 (0x4000) - Unassigned
   Bit 15 (0x8000) - Unassigned

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Security Considerations</span>

   The security aspect of this solution is addressed for two planes: the
   control plane and the data plane.






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<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Control Plane Security</span>

   Control plane security pertains to establishing the LDP connection
   and to pseudowire signaling and CE IP address distribution over that
   LDP connection.  For greater security, the LDP connection between two
   trusted PEs MUST be secured by each PE verifying the incoming
   connection against the configured address of the peer and
   authenticating the LDP messages, as described in <a href="./rfc5036#section-2.9">Section&nbsp;2.9 of
   [RFC5036]</a>.  Pseudowire signaling between two secure LDP peers does
   not pose a security issue but mis-wiring could occur due to
   configuration error.  However, the fact that the pseudowire will only
   be established if the two PEs have matching configurations (e.g., PW
   ID, PW type, and MTU) provides some protection against mis-wiring due
   to configuration errors.

   Learning the IP address of the appropriate CE can be a security
   issue.  It is expected that the Attachment Circuit to the local CE
   will be physically secured.  If this is a concern, the PE MUST be
   configured with the IP and MAC address of the CE when connected with
   Ethernet, IP and virtual circuit information (DLCI or VPI/VCI
   (Virtual Path Identifier / Virtual Circuit Identifier) when connected
   over Frame Relay or ATM, and IP address only when connected over PPP.
   During ARP/Inverse ARP frame processing, the PE MUST verify the
   received information against local configuration before forwarding
   the information to the remote PE to protect against hijacking of the
   connection.

   For IPv6, the preferred means of security is Secure Neighbor
   Discovery (SEND) [<a href="./rfc3971" title="&quot;SEcure Neighbor Discovery (SEND)&quot;">RFC3971</a>].  SEND provides a mechanism for securing
   Neighbor Discovery packets over media (such as wireless links) that
   may be insecure and open to packet interception and substitution.
   SEND is based upon cryptographic signatures of Neighbor Discovery
   packets.  These signatures allow the receiving node to detect packet
   modification and confirm that a received packet originated from the
   claimed source node.  SEND is incompatible with the Neighbor
   Discovery packet modifications described in this document.  As such,
   SEND cannot be used for Neighbor Discovery across an ARP Mediation
   pseudowire.  PEs taking part in IPv6 ARP Mediation MUST remove all
   SEND packet options from Neighbor Discovery packets before forwarding
   into the pseudowire.  If the CE devices are configured to accept only
   SEND Neighbor Discovery packets, Neighbor Discovery will fail.  Thus,
   the CE devices MUST be configured to accept non-SEND packets, even if
   they treat them with lower priority than SEND packets.  Because SEND
   cannot be used in combination with IPv6 ARP Mediation, it is
   suggested that IPv6 ARP Mediation only be used with secure Attachment
   Circuits.  An exception to this recommendation applies to an
   implementation that supports the SEND Proxy [<a href="./rfc6496" title="&quot;Secure Proxy ND Support for SEcure Neighbor Discovery (SEND)&quot;">RFC6496</a>], which allows a
   device such as PE to act as an ND proxy as described in [<a href="./rfc6496" title="&quot;Secure Proxy ND Support for SEcure Neighbor Discovery (SEND)&quot;">RFC6496</a>].



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<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Data Plane Security</span>

   The data traffic between CE and PE is not encrypted, and it is
   possible that in an insecure environment, a malicious user may tap
   into the CE-to-PE connection and generate traffic using the spoofed
   destination MAC address on the Ethernet Attachment Circuit.  In order
   to avoid such hijacking, the local PE may verify the source MAC
   address of the received frame against the MAC address of the admitted
   connection.  The frame is forwarded to the PW only when authenticity
   is verified.  When spoofing is detected, the PE MUST sever the
   connection with the local CE, tear down the PW, and start over.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Acknowledgements</span>

   The authors would like to thank Yetik Serbest, Prabhu Kavi, Bruce
   Lasley, Mark Lewis, Carlos Pignataro, and others who participated in
   the discussions related to this document.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Contributors</span>

   This document is the combined effort of many who have contributed,
   carefully reviewed, and provided technical clarifications.  This
   includes the individuals listed in this section and those listed in
   the Editors' Addresses.

   Matthew Bocci
   Alcatel-Lucent
   EMail: Mathew.bocci@alcatel-lucent.com

   Tiberiu Grigoriu
   Alcatel-Lucent
   EMail: Tiberiu.Grigoriu@alcatel-lucent.com

   Neil Hart
   Alcatel-Lucent
   EMail: Neil.Hart@alcatel-lucent.com

   Andrew Dolganow
   Alcatel-Lucent
   EMail: Andrew.Dolganow@alcatel-lucent.com

   Shane Amante
   Level 3
   EMail: Shane@castlepoint.net

   Toby Smith
   Google
   EMail: tob@google.com



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   Andrew G. Malis
   Verizon
   EMail: Andy.g.Malis@verizon.com

   Steven Wright
   Bell South Corp
   EMail: steven.wright@bellsouth.com

   Waldemar Augustyn
   Consultant
   EMail: waldemar@wdmsys.com

   Arun Vishwanathan
   Juniper Networks
   EMail: arunvn@juniper.net

   Ashwin Moranganti
   IneoQuest Technologies
   EMail: Ashwin.Moranganti@Ineoquest.com

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  References</span>

<span class="h3"><a class="selflink" id="section-11.1" href="#section-11.1">11.1</a>.  Normative References</span>

   [<a id="ref-RFC826">RFC826</a>]   Plummer, D., "Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              <a href="./rfc826">RFC 826</a>, November 1982.

   [<a id="ref-RFC2390">RFC2390</a>]  Bradley, T., Brown, C., and A. Malis, "Inverse Address
              Resolution Protocol", <a href="./rfc2390">RFC 2390</a>, September 1998.

   [<a id="ref-RFC4447">RFC4447</a>]  Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
              G. Heron, "Pseudowire Setup and Maintenance Using the
              Label Distribution Protocol (LDP)", <a href="./rfc4447">RFC 4447</a>, April 2006.

   [<a id="ref-RFC4446">RFC4446</a>]  Martini, L., "IANA Allocations for Pseudowire Edge to Edge
              Emulation (PWE3)", <a href="https://www.rfc-editor.org/bcp/bcp116">BCP 116</a>, <a href="./rfc4446">RFC 4446</a>, April 2006.

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-RFC5036">RFC5036</a>]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", <a href="./rfc5036">RFC 5036</a>, October 2007.

   [<a id="ref-RFC4861">RFC4861</a>]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", <a href="./rfc4861">RFC 4861</a>,
              September 2007.



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   [<a id="ref-RFC3122">RFC3122</a>]  Conta, A., "Extensions to IPv6 Neighbor Discovery for
              Inverse Discovery Specification", <a href="./rfc3122">RFC 3122</a>, June 2001.

   [<a id="ref-RFC4862">RFC4862</a>]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", <a href="./rfc4862">RFC 4862</a>, September 2007.

   [<a id="ref-RFC3971">RFC3971</a>]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", <a href="./rfc3971">RFC 3971</a>, March 2005.

   [<a id="ref-RFC5226">RFC5226</a>]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc5226">RFC 5226</a>,
              May 2008.

<span class="h3"><a class="selflink" id="section-11.2" href="#section-11.2">11.2</a>.  Informative References</span>

   [<a id="ref-RFC4664">RFC4664</a>]  Andersson, L., Ed., and E. Rosen, Ed., "Framework for
              Layer 2 Virtual Private Networks (L2VPNs)", <a href="./rfc4664">RFC 4664</a>,
              September 2006.

   [<a id="ref-RFC1332">RFC1332</a>]  McGregor, G., "The PPP Internet Protocol Control Protocol
              (IPCP)", <a href="./rfc1332">RFC 1332</a>, May 1992.

   [<a id="ref-RFC5072">RFC5072</a>]  Varada, S., Ed., Haskins, D., and E. Allen, "IP Version 6
              over PPP", <a href="./rfc5072">RFC 5072</a>, September 2007.

   [<a id="ref-RFC925">RFC925</a>]   Postel, J., "Multi-LAN address resolution", <a href="./rfc925">RFC 925</a>,
              October 1984.

   [<a id="ref-RFC1256">RFC1256</a>]  Deering, S., Ed., "ICMP Router Discovery Messages", <a href="./rfc1256">RFC</a>
              <a href="./rfc1256">1256</a>, September 1991.

   [<a id="ref-RFC5309">RFC5309</a>]  Shen, N., Ed., and A. Zinin, Ed., "Point-to-Point
              Operation over LAN in Link State Routing Protocols", <a href="./rfc5309">RFC</a>
              <a href="./rfc5309">5309</a>, October 2008.

   [<a id="ref-RFC6496">RFC6496</a>]   Krishnan, S., Laganier, J., Bonola, M., and A. Garcia-
              Martinez, "Secure Proxy ND Support for SEcure Neighbor
              Discovery (SEND)", <a href="./rfc6496">RFC 6496</a>, February 2012.













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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Use of IGPs with IP L2 Interworking L2VPNs</span>

   In an IP L2 interworking L2VPN, when an IGP on a CE connected to a
   broadcast link is cross-connected with an IGP on a CE connected to a
   point-to-point link, there are routing protocol related issues that
   MUST be addressed.  The link state routing protocols are cognizant of
   the underlying link characteristics and behave accordingly when
   establishing neighbor adjacencies, representing the network topology,
   and passing protocol packets.  The point-to-point operations of the
   routing protocols over a LAN are discussed in [<a href="./rfc5309" title="&quot;Point-to-Point Operation over LAN in Link State Routing Protocols&quot;">RFC5309</a>].

<span class="h3"><a class="selflink" id="appendix-A.1" href="#appendix-A.1">A.1</a>.  OSPF</span>

   The OSPF protocol treats a broadcast link type with a special
   procedure that engages in Neighbor Discovery to elect a designated
   router and a backup designated router (DR and BDR, respectively),
   with which each other router on the link forms adjacencies.  However,
   these procedures are neither applicable nor understood by OSPF
   running on a point-to-point link.  By cross-connecting two neighbors
   with disparate link types, an IP L2 interworking L2VPN may experience
   connectivity issues.

   Additionally, the link type specified in the router Link State
   Advertisement (LSA) will not match for the two cross-connected
   routers.

   Finally, each OSPF router generates network LSAs when connected to a
   broadcast link such as Ethernet, receipt of which by an OSPF router
   that believes itself to be connected to a point-to-point link further
   adds to the confusion.

   Fortunately, the OSPF protocol provides a configuration option
   (ospfIfType) whereby OSPF will treat the underlying physical
   broadcast link as a point-to-point link.

   It is strongly recommended that all OSPF protocols on CE devices
   connected to Ethernet interfaces use this configuration option when
   attached to a PE that is participating in an IP L2 Interworking VPN.

<span class="h3"><a class="selflink" id="appendix-A.2" href="#appendix-A.2">A.2</a>.  RIP</span>

   The RIP protocol broadcasts RIP advertisements every 30 seconds.  If
   the multicast/broadcast traffic snooping mechanism is used as
   described in <a href="#section-4.1">Section 4.1</a>, the attached PE can learn the local CE
   router's IP address from the IP header of its advertisements.  No
   special configuration is required for RIP in this type of Layer 2 IP
   Interworking L2VPN.




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<span class="h3"><a class="selflink" id="appendix-A.3" href="#appendix-A.3">A.3</a>.  IS-IS</span>

   The IS-IS protocol does not encapsulate its PDUs in IP; hence, it
   cannot be supported in IP L2 Interworking L2VPNs.

Editors' Addresses

   Himanshu Shah (editor)
   Ciena
   EMail: hshah@ciena.com

   Eric Rosen (editor)
   Cisco Systems
   EMail: erosen@cisco.com

   Giles Heron (editor)
   Cisco Systems
   EMail: giheron@cisco.com

   Vach Kompella (editor)
   Alcatel-Lucent
   EMail: vach.kompella@alcatel-lucent.com





























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