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Versions: 00 01 02                                                      
                                                          Himanshu Shah
                                                         Tenor Networks
   PPVPN Working Group
   Internet Draft
   draft-shah-ppvpn-ipls-00.txt                              Eric Rosen
                                                   Francois Le Faucheur
   October 2002                                           Cisco Systems
   Expires: March 2003
                                                            Giles Heron

                                                          Vasile Radoaca
                                                        Nortel Networks

                        IP over LAN Service (IPLS)

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   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
   The list of Internet-Draft Shadow Directories can be accessed at


   A Virtual Private LAN Service (VPLS) [VPLS] is used to interconnect
   systems across a wide-area or metropolitan-area network, making it
   appear to those systems as if they are interconnected on a private
   LAN.  The systems which are interconnected in this way may
   themselves be LAN switches.  If, however, the interconnected systems
   are NOT LAN switches, but rather are IP hosts or IP routers, certain
   simplifications are possible.  We call this simplified type of
   virtual private LAN service an ôIP over LAN Serviceö (IPLS).  In
   IPLS, as in VPLS, LAN interfaces are run in promiscuous mode, and
   frames are forwarded based on their MAC Destination Addresses.

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   However, the maintenance of the MAC forwarded tables is done via
   signaling, rather than via the ôMAC Address Learningö procedures of
   IEEE 802.1D.  Further, Address Resolution Protocol (ARP) messages
   are proxied, rather than being carried transparently. This draft
   specifies the protocols and procedures for support of the IPLS

1.0 Boiler Plate for Sub-IP Area Drafts



   Belongs in PPVPN


   This document describes a mechanism to assist in Provider-
   Provisioned Layer 2 VPNs.


   This document provides a detailed description for IP over LAN
   Service (IPLS), which is discussed in the L2 PPVPN Framework [PPVPN-
   FWK]. The VPLS [VPLS] services of L2VPN require PE devices to
   function as MAC learning bridges. IPLS is a solution for a specific
   topology where MAC learning capabilities are not required for VPLS
   services, because user data traffic is restricted to IP, and the CE
   devices are not LAN switches.

2.0 Overview

   As emphasized in [VPLS], Ethernet has become popular as an access
   technology in Metropolitan and Wide Area Networks. [VPLS] describes
   how geographically dispersed customer LANs can be interconnected
   over a service providerÆs network using Layer 2 VPNs. The VPLS
   service is provided by Provider Edge (PE) devices, and it is
   provided to Customer Edge (CE) devices. The VPLS architecture
   provides such services by incorporating bridging functions such as
   MAC address learning in the PE devices.

   There are Provider Edge platforms, both existing and forthcoming,
   which have been designed primarily to be IP routers, rather than to
   be LAN switches. It can be fairly straightforward to add a MAC
   address lookup capability to these platforms, and to run their LAN
   interfaces in promiscuous mode, so that they can forward frames

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   based on the MAC Destination Address of the frame.  It is less
   straightforward to add the IEEE 802.1D MAC Address learning
   capability to these platforms.  However, in scenarios where the CE
   devices are NOT LAN switches, but rather are IP hosts or IP routers,
   it is possible to provide the virtual private LAN service without
   requiring IEEE 802.1D MAC address learning/aging on the PE.  Due to
   these restrictions, we dub such a service an ôIP LAN Serviceö, or
   IPLS.  The purpose of this draft is to specify the IPLS.

   Consequently, IPLS allows a service provider to provide a VPLS-like
   service by using PE routers that are not designed to perform general
   LAN bridging functions. However one must be willing to accept the
   restriction that the Virtual LAN service be used for IP traffic
   only, and not used to interconnect CE devices that are themselves
   LAN switches. This seems like an acceptable restriction in many
   environments, given that IP is the predominant type of traffic in
   today's networks.

   In IPLS, a PE device implements multi-point LAN connectivity for IP
   traffic using the following key functions:

     1. Discovery: Each Provider Edge (PE) device discovers IP/MAC
        address associations for the locally attached Customer Edge
        (CE) devices, for each IPLS instance configured on the PE

     2. Pseudowires (PW) for Unicast Traffic: For each locally attached
        CE device in a given IPLS instance, a PE device sets up a
        pseudo-wire (VC-LSP) to each of the other PEs that supports the
        same IPLS instance.

        For instance, if PEx and PEy both support IPLS I, and PEy is
        locally attached to CEw and CEz, PEy will initiate the setup of
        two pseudowires between itself and PEx.  One of these will be
        used to carry unicast traffic from any of PExÆs CE devices to
        CEw.  The other will be used to carry unicast traffic from any
        of PExÆs CE devices to CEz.

        Note that these pseudowires carry traffic only in one
        direction.  Further, while the pseudowire implicitly identifies
        the destination CE of the traffic, it does not identify the
        source CE; packets from many CEs may be freely intermixed on a
        given pseudowire.

     3. Pseudowires for Multicast Traffic:  In addition, every PE
        supporting a given IPLS instance will set up a special
        ômulticast pseudowireö to every other PE in that IPLS instance.
        If, in the above example, one of PExÆs CE devices sends a
        multicast packet, PEx would forward the multicast packet to PEy
        on the special multicast pseudowire.  PEy would then send a
        copy of that packet to CEw and a copy to CEz.

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        Thus when a PE sends a multicast packet across the network, it
        sends one copy to each remote PE (supporting the given IPLS
        instance).  If a particular remote PE has more than one CE
        device in that IPLS instance, the remote PE must replicate the
        packet and send one copy to each of its local CEs.

        As with the pseudowires that are used for unicast traffic,
        packets travel in only one direction on these pseudowires, and
        packets from different sources may be freely intermixed.

     4. Signaling:  The necessary pseudowires can be set up and
        maintained using the LDP-based signaling procedures described
        in [PWE3-CONTROL] and/or [ROSEN-SIG].  Use of other signaling
        procedures is for further study.

        A PE may assign the same label to each of the unicast
        pseudowires that leads to a given CE device, in effect creating
        a multipoint-to-point pseudowire.

        Similarly, a PE may assign the same label to each of the
        multicast pseudowires for a given IPLS instances, in effect
        creating a multipoint-to-point pseudowire.

        When setting up a pseudowire to be used for unicast traffic,
        the PE must also signal the IP address and the MAC address of
        the corresponding CE device.

     5. Proxy ARP: Distribution of IP/MAC address associations to
        remote PE devices via PW signaling enables each PE device to
        function as a proxy ARP server for CE devices attached to other
        PE devices. This makes it possible for any CE device to ARP for
        the MAC addresses of remote CE devices.

     6. Forwarding:  A PE device programs its Forwarding Information
        Base using the CE MAC addresses and VC labels signaled through
        the PW signaling. Unicast IP traffic from the local CEs is then
        switched to the proper VC-LSP based on the destination MAC
        address. Multicast IP traffic from the local CEs is replicated
        by the local PE over all the multicast VC-LSPs for that IPLS
        instance and is then replicated by each remote PE onto all its
        Attachment Circuits for that IPLS instance.

   Both VPLS [VPLS] and IPLS require the ingress PE to forward a frame
   based on its destination MAC address. However, two key differences
   between VPLS and IPLS can already be noted from the above

     . In VPLS, MAC entries are placed in the FIB of the ingress PE as
        a result of IEEE 802.1D MAC address learning (which occurs in
        the data plane) while in IPLS MAC entries are placed in the FIB
        as a result of pseudowire signaling operations (control plane).

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     . In VPLS, the egress PE looks up a frameÆs MAC destination
        address to determine the customer-facing interface out which
        the frame must be sent; in IPLS, the choice of interface is
        based entirely on the VC-label.

   The following sections describe the details of the IPLS scheme.

2.1 Terminology

        IPLS            IP over LAN service (class of VPLS for IP

        IPLS network   A collection of PE nodes supporting the IPLS
                       service and the mechanisms described in this
                       document, including the Extended LDP based PW
                       signaling between them.

        IPLS Service   A single service instance of IPLS emulating a
                        LAN segment for IP data traffic.

        MPt-Pt PW      Multipoint-to-Point Pseudowire. A pseudowire
                        that carries traffic from remote PE devices to
                        a PE device that signals the pseudowire. The
                        signaling PE device advertises the same VC-
                        label to all remote PE devices that participate
                        in the IPLS service instance. In IPLS, for a
                        given IPLS instance, a MPt-Pt PW used for IP
                        unicast traffic is established by a PE for each
                        CE device locally attached to that PE. It is a
                        unidirectional tree whose leaves consist of the
                        remote PE peers (which connect at least one
                        Attachment Circuit associated with the same
                        IPLS instance) and whose root is the signaling
                        PE. Traffic flows from the leaves towards the

        Multicast PW   Multicast Pseudowire. A special kind of MPt-Pt
                        PW that carries only IP multicast/broadcast
                        traffic. In the IPLS architecture, for each
                        IPLS instance supported by a PE, that PE device
                        establishes exactly one Multicast PW.

        CE             Customer Edge device. In this document, a CE is
                        any IP node (host or router) connected to the
                        IPLS LAN service.

        Replication Tree The collection of all pseudowires and
                        attachment circuits that are members of an IPLS
                        service instance on a given PE. When a
                        multicast/broadcast packet is received by the
                        PE on an attachment circuit, the PE device
                        sends a copy of the packet to every pseudowire

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                        and attachment circuit of the replication tree,
                        excluding the attachment circuit on which the
                        packet was received.

3.0 Topology

The Customer Edge (CE) devices are IP nodes (hosts or routers) that are
connected to PE devices either directly, or via an Ethernet network. We
assume that the PE/CE connection may be regarded by the PE as an
ôinterfaceö to which one or more CEs are attached.  This interface may
be the physical LAN interface or a VLAN.  The Provider Edge (PE)
routers are MPLS Label Edge Routers (LERs) that serve as pseudowire

      +----+                                              +----+
      + S1 +---+      ...........................     +---| S2 |
      +----+ | |      .                         .     |   +----+
       IPa   | |   +----+                    +----+   |    IPe
             + +---| PE1|---MPLS and/or IP---| PE2|---+
            / \    +----+         |Network   +----+   |
      +----+   +---+  .           |             .     |   +----+
      + S1 +   | S1|  .         +----+          .     +---| S2 |
      +----+   +---+  ..........| PE3|...........         +----+
       IPb       IPc            +----+                     IPf
                                | S3 |

   In the above diagram, an IPLS instance is shown with three sites:
   site S1, site S2 and site S3. In site S3, the CE device is directly
   connected to its PE.  In the other two sites, there are multiple CEs
   connected to a single PE. More precisely, the CEs at these sites are
   on an Ethernet network (or VLAN), and the PE is attached to that
   same Ethernet network or VLAN).  We impose the following
   restriction:  if one or more CEs attach to a PE by virtue of being
   on a common LAN or VLAN, there MUST NOT be more than one PE on that
   LAN or VLAN.

   PE1, PE2 and PE3 are shown to be connected via an MPLS network;
   however, other tunneling technologies, such as GRE, L2TP, etc.,
   could also be used to carry the pseudowires.

   An IPLS instance is a single broadcast domain, such that each IP end
   station (e.g., IPa) appears to be co-located with other IP end
   stations (e.g., IPb though IPf) on the same subnet. The IPLS service
   is transparent to the CE devices and requires no changes to them.

4.0 Configuration

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   Each PE router is configured with one or more IPLS service
   instances, and each IPLS service instance is associated with a
   unique VPN-Id. For a given IPLS service instance, a set of
   Attachment Circuits is identified. Each Attachment Circuit can be
   associated with only one IPLS instance. An Attachment Circuit, in
   this document, is either a customer-facing Ethernet port, or a
   particular VLAN (identified by an IEEE 802.1Q VLAN ID) on a
   customer-facing Ethernet port.

   The PE router can optionally be configured with a local MAC address
   to be used as source MAC address when packets are forwarded from a
   pseudowire to an Attachment Circuit. By default, a PE uses the MAC
   address of the customer-facing Ethernet interface for this purpose.

5.0 Discovery

   The discovery process includes:
     . Remote PE discovery
     . VPN (i.e., IPLS) membership discovery
     . IP CE end station discovery

   This draft does not discuss the remote PE discovery or VPN
   membership discovery. This information can either be user configured
   or can be obtained using auto-discovery techniques described in
   [DNS-Discovery] or [BGP-Discovery]. However, the discovery of the CE
   is an important operational step in the IPLS model and is described

5.1 CE discovery

   Each PE actively detects the presence of local CEs by snooping IP
   and ARP frames received over the Attachment Circuits. During the
   discovery phase, the PE examines each broadcast/multicast Ethernet
   frame. For IP frames (for example IGP discovery/multicast/broadcast
   packets), the CEÆs (source) MAC address is extracted from the
   Ethernet header and the (source) IP address is obtained from the IP
   header. For ARP frames, the source MAC and IP address are determined
   from the ARP PDU.

   For each CE, the PE maintains a <Attachment Circuit identification
   info, VPN-Id, IP address, MAC address> tuple.

   Once discovered, the presence/liveness of a CE is monitored
   continuously by examining the received ARP frames and by
   periodically generating ARP requests. The absence of an ARP response
   from a CE after a configurable number of such ARP requests, is
   interpreted as a loss of connectivity with the CE.

6.0 Pseudowire Creation

6.1 Receive Unicast Multipoint-to-point Pseudowire

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   As the PE discovers each locally attached CE, a unicast Multipoint-
   to-point Pseudowire (MPt-Pt PW) associated exclusively with that CE
   is created by distributing the CEÆs IP address and MAC address along
   with a VC-Label to all the remote PE peers that participate in the
   same IPLS instance. Note that the same value of a VC-label should be
   distributed to all the remote PE peers for a given CE. The MP-Pt PW
   thus created is used by remote PEs to send unicast IP traffic to a
   specific CE.

   (The same functionality can be provided by a set of point-to-point
   PWs, so the PE is not required to send the same VC-label to all the
   other PEs.  For convenience however, we will speak in the following
   only of multipoint-to-point PWs, without pointing out each time that
   a set of point-to-point PWs could be used instead.)

   The PE forwards a frame received over this MPt-Pt PW to the
   associated attachment circuit.

6.2 Receive Replication Multipoint-to-point PseudoWire

   When a PE is configured to participate in an IPLS instance, it
   advertises a "multicast" VC-label to every other PE that is a member
   of the same IPLS. The advertised VC-label value is the same for each
   PE, which creates a multipoint-to-point pseudowire for IP multicast
   traffic. There is only one multicast MPt-Pt PW per PE for each IPLS
   instance and this pseudowire is used exclusively to carry
   multicast/broadcast IP traffic from the remote PEs to this PE for
   this IPLS instance.

   Note that no special functionality is expected from this pseudowire.
   We sometimes call it a ômulticast pseudowireö because we use it only
   to carry multicast traffic.  The pseudowire itself need not provide
   any different service than any of the unicast pseudowires.

   In particular, the Receive multicast MPt-PT PW does not perform any
   replication of frames itself. Rather, it is there to signify to the
   PE that the PE needs to replicate a copy of a frame received over
   this MPt-Pt PW onto all the attachment circuits that are associated
   with the IPLS instance of the MPt-Pt PW.

   The use of pseudowires, which are specially optimized for multicast,
   is for further study.

6.3 Send Multicast Replication tree

   The PE creates a send replication tree for each IPLS instance, which
   consists of the collection of all attachment circuits and all the
   ômulticastö pseudowires of this IPLS instance.

   Any broadcast/multicast frame received over an attachment circuit is
   replicated to all the other attachment circuits and all pseudowires
   of the send replication tree of the IPLS instance of the incoming
   Attachment Circuit.

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7.0 Proxy ARP

   As part of the signaling of the unicast multipoint-to-point pseudo-
   wire (See Section 8), each PE distributes to its remote PE peers the
   CE IP address/MAC address associations that it has discovered. The
   remote PE peers then build and maintain a database of these

   When a PE receives an ARP request from a local CE for a remote CE,
   it searches for the destination IP address in the database
   associated with the CEÆs IPLS instance. If a match is found, the PE
   sends an ARP response with the MAC address of the remote CE. This
   enables the local CE to send unicast IP frames addressed directly to
   the MAC address of the remote CE.

8.0 Signaling

   IPLS uses PW signaling based on LDP as specified in [PWE3-CONTROL]
   and [ROSEN-SIG] to exchange layer-2 cross-connect information for a
   given VPN. The cross-connect information is represented as a new LDP
   FEC element (VC-FEC in [PWE3-CONTROL] with a FEC element type of
   128, new FEC element in [ROSEN-SIG] with a FEC element type of 129),
   which LDP then distributes to remote peers in downstream-unsolicited
   mode. This document proposes extensions to the new FEC element to
   support the IPLS as a new circuit type and to include the IP address
   and MAC address information.

8.1 IPLS PW Signaling

   The IPLS VPN type is advertised in the VC-Type field of the VC FEC
   as the value 0x000D.

   The ôinterface parameterö field in the VC FEC is defined in [PWE3-
   CONTROL] and [ROSEN-SIG] 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     |    Variable Length Value      |
      |                         Variable Length Value                 |
      |                             "                                 |

      The following parameters of the VC FEC are already defined:
      Parameter ID   Length    Description

          0x01         4       Interface MTU in octets.
          0x02         4       Maximum Number of concatenated ATM cells.
          0x03   up to 82      Optional Interface Description string.
          0x04         4       CEM [8] Payload Bytes.

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          0x05         4       CEM options.

      The Length field is defined as the length of the interface
   parameter including the parameter id and length field itself.

   We propose the following additional parameters for the VC FEC:

          0x06         4       IP address of CE
          0x07         6       MAC address of CE
          0x08         1       Multicast Label Flag

   These parameters are used to transport CE IP address/MAC address
   associations when establishing the MPt-Pt unicast PWs and to
   establish the multicast multipoint-to-point pseudowire label. The IP
   address and MAC address interface parameter denotes the IP and MAC
   address of a CE. The Multicast Label Flag value of 1 indicates that
   the advertised VC-Label represents a ômulticastö PW. The Multicast
   Label Flag value of 0 indicates that advertised VC-label represents
   a ôunicastö PW. As explained earlier, the term ômulticast PWö only
   means that the PW carries IP broadcast/multicast traffic and does
   not refer to a multicast LSP in the traditional sense.

   The Multicast Label flag must be zero, if present, when the IP and
   MAC address parameters are present (and their value is non-zero).

   We recommend two options that are currently present as the interface
   parameter field in the VC FEC, for signaling such ôVC associated
     1. The entire ôinterface parameterö field is either removed or
        duplicated from the VC FEC to the ôoptional parameterö field of
        the LDPÆs Label Mapping Message.
     2. A new VC Status FEC is introduced that accompanies the VC FEC
        in the LDPÆs Label Mapping Message. The ôinterface parameterö
        field of the VC FEC is then either removed or duplicated from
        the VC FEC to the VC Status FEC.

   We intend to work with authors of [PWE3-Control] and [ROSEN-SIG] to
   find the most suitable solution for extensions (such as IP address,
   IP address and MAC address, interface status, etc.) that are
   generic, in a backward-compatible fashion.

   The new FEC specified in [ROSEN-SIG] contains an Attachment Group
   Identifier (AGI) field, a Source Attachment Individual Identifier
   field (SAII) and a Target Attachment Individual Identifier field
   (TAII). The VPN-Id configured on the advertising PE to identify the
   IPLS instance is advertised in the AGI field. The SAII and TAAI
   fields are set to the null value.

8.2 Signaling Advertisement Processing

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   A PE should process a received [PWE3-CONTROL] advertisement with VC-
   type of IPLS as follows,
        - Verify the IPLS VPN membership by matching the VPN-Id
          signaled in the AGI field with the all the VPN-Ids configured
          on the PE. Discard and release the VC label if VPN-Id is not
        - Distribute the received IP address-to-MAC address binding by
          sending a gratuitous ARP response on all the attachment
          circuits associated with the VPN-Id.
        - Program the Forwarding Information Base (FIB) such that when
          a packet is received from an attachment circuit with its
          destination MAC address matching the advertised MAC address,
          the packet is forwarded out over the tunnel to the
          advertising PE with the advertised VC-label as the inner
        - When the advertised VC-label is ômulticastö, add the VC-label
          to the multicast replication tree for the VPN-Id. This
          enables sending a copy of a multicast/broadcast IP frame from
          the attachment circuit to this Pseudowire.

8.3 Requesting for IP to MAC binding

   It is possible that in some cases, some CEs may remain undetected in
   the absence of any multicast/broadcast IP or ARP packet generation.
   If another CE needs to converse with a CE in this undetected set, it
   will proceed to generate ARP requests. The Proxy ARP scheme
   described so far will be unable to resolve the ARP request, since
   the address to be resolved would not have been discovered yet.

   In order to address such situations, an optional Address Resolution
   Request TLV is included in the Label Mapping message. This TLV
   contains an IP address parameter that represents the destination IP
   address that needs to be resolved. The PE may use some intelligent
   mechanisms (e.g., the number of ARP requests received for unknown IP
   destination within a certain interval exceeds a threshold) to detect
   the need for such advertisement. When the need is detected, the PE
   generates Label Mapping Messages to all remote PEs in the IPLS, with
   the IP address parameter in the Address Resolution Request TLV set
   to the destination IP address to be resolved.

   A PE that supports the Address Resolution Request TLV must, on
   receiving a Label Mapping message with this TLV, generate an ARP
   request message using the received IP address as the destination,
   and some already known IP and MAC address as the source (in the ARP
   PDU) on all Attachment Circuits associated with the IPLS instance.

   In essence, this is a request to remote PEs to generate an ARP
   request on their Attachment Circuits to locate a specific CE and
   advertise a Label Mapping message back to the requesting PE. This
   can be seen as reverting to the usual full broadcasting of ARP
   messages throughout the Emulated LAN in case Proxy ARP fails.

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9 Forwarding

9.1 Non-IP traffic

   In an IPLS VPN, only IP traffic is forwarded by a PE. ARP frames are
   directed to the control plane in the PE and the rest of the frames
   are dropped silently.  If the CEs must pass non-IP traffic to each
   other, they must do so through IP tunnels that terminate at the CEs

9.2 Unicast IP Traffic

   In IPLS, IP traffic is forwarded based on the destination MAC
   address of the layer 2 frame (and not based on the IP Header).

   To do so, the PE uses a Forwarding Information Base (FIB), which is
   programmed for the considered IPLS instance, in the following
        - Whenever a CE (and its MAC address) is discovered locally by
          the PE on an Attachment Circuit, the PE programs its FIB such
          that frames received on other attachment circuits with a
          destination address equal to the CE MAC address, are
          forwarded onto the corresponding attachment circuit.
        - When advertising the corresponding unicast PW, the PE
          programs the FIB such that packets received onto the
          advertised unicast pseudo-wire are forwarded onto the
          attachment circuit associated with the CE corresponding to
          the advertised MAC and IP addresses.
        - As discussed in section 8.2, on receipt of a PW signaling
          advertisement, the FIB is programmed by the PE receiving the
          advertisement such that frames received on an Attachment
          Circuit with a destination address equal to the advertised
          MAC address, are forwarded onto the advertised unicast

   Using the FIB of the IPLS instance to which the attachment circuit
   belongs, the PE forwards a unicast IP frame to the Attachment
   Circuit or pseudowire based on the destination MAC address
   information, if received from an Attachment Circuit, or to the
   Attachment Circuit based on the VC label, if received from a

   When a unicast destination MAC address is not recognized in the FIB,
   the IP frame is dropped.

9.3 Broadcasts and Multicast forwarding

   When the destination MAC address is either a broadcast or multicast,
   a copy of the frame is sent to the control plane for CE discovery
   purposes (see section 5.1).

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   When a multicast/broadcast IP frame is received from an Attachment
   Circuit, a PE replicates it onto the Send Multicast Replication Tree
   (See section 6.3). When a multicast/broadcast IP frame is received
   from a pseudowire, the PE forwards it to all attachment circuits
   associated with the IPLS VPN instance involved.

   It is important to note that PEs participating in an IPLS VPN are
   responsible for translating a multicast IP address to a multicast
   Ethernet MAC address when forwarding frames from a ômulticastö
   pseudowire to the Attachment Circuits. (The translation consists of
   recognizing the multicast IP address (224.x1.x2.x3) and appending
   the least significant three bytes of the IP address to 0x01-00-05 to
   construct the MAC address, e.g., 0x01-00-5E-x1-x2-x3 [RFC-1112]).

   All other IP packets received over the ômulticastö MPt-Pt PW (such
   as directed broadcasts, subnet broadcasts, etc) are forwarded over
   Attachment Circuits using a broadcast MAC address.

9.4 Encapsulation

   The Ethernet MAC header of a frame received from an Attachment
   Circuit is stripped before forwarding the frame to the appropriate
   pseudowire. However, the MAC header is retained when a unicast or
   broadcast IP frame is directed to one or more Attachment Circuit(s).
   An IP frame received over a pseudowire is prepended with a MAC
   header before transmitting it on the appropriate Attachment
   Circuit(s). The fields in the MAC header are filled in as follows:
        - The destination MAC address is the MAC address associated
          with the VC label in the FIB when the pseudowire is unicast
        - The destination MAC address is a multicast MAC address
          derived from the IP multicast address or the broadcast MAC
          address when the VC label is ômulticastö
        - The source MAC address is the PEÆs own local MAC address or a
          MAC address which has been specially configured on the PE for
          this use.
        - The Ethernet Type field is 0x0800
        - The frame may get IEEE802.1Q tagged based on the VLAN
          information associated with the Attachment Circuit.

   An FCS field is appended to the frame.

10.0   Attaching to IPLS via ATM or FR

   In addition to (i) an Ethernet port and a (ii) combination of
   Ethernet port and a VLAN ID, an Attachment Circuit to IPLS may also
   be (iii) an ATM or FR VC carrying encapsulated bridged Ethernet
   frames or (iv) the combination of an ATM or FR VC and a VLAN ID.

   The ATM/FR VC is just used as a way to transport Ethernet frames
   between a customer site and the PE. The PE terminates the ATM/FR VC
   and operates on the encapsulated Ethernet frames exactly as if those
   were received on a local Ethernet interface. Operation of an IPLS
   over ATM/FR VC is exactly as described above, with the exception

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   that the attachment circuit is then identified via the ATM VCI/VPI
   or Frame Relay DLCI (instead of via a local Ethernet port ID), or a
   combination of those with a VLAN ID.

11.0 VPLS vs IPLS

   The VPLS approach proposed in [VPLS] provides VPN services for IP as
   well as other protocols. The IPLS approach described in this draft
   is similar to VPLS in many respects:
        - It provides a Provider Provisioned Virtual LAN service with
          multipoint capability where a CE connected via a single
          attachment circuit can reach many remote CEs
        - It appears as a broadcast domain and a single subnet
        - forwarding is based on destination MAC addresses

   However, unlike VPLS, IPLS is restricted to IP traffic only. By
   restricting the scope of the service to the predominant type of
   traffic in today's environment, IPLS eliminates the need for service
   provider edge routers to implement some bridging functions such as
   MAC address learning in the data path (by, instead, distributing MAC
   information in the control plane). Thus this solution offers a
   number of benefits:

        - Facilitates Virtual LAN services in instances where PE
          devices cannot or cannot efficiently (or are specifically
          configured not to) perform MAC address learning.
        - Does not require flooding of ARP frames.
        - Encapsulation is more efficient (MAC header is stripped)
          while traversing the backbone network.
        - PE devices are not burdened with the processing overhead
          associated with traditional bridging (e.g., STP processing,
          etc.). Note however that some of these overheads (e.g., STP
          processing) could optionally be turned-off with a VPLS
          solution in the case where it is known that only IP devices
          are interconnected.
        - Loops (perhaps through backdoor links) are minimized since a
          PE could easily reject (via label release) a duplicate IP to
          MAC address advertisement.

12.0 Acknowledgements

   Authors would like to thank Nigel Burmeister and others at Tenor
   Networks for their valuable comments.

13.0 Security Considerations

   The security aspects of this solution will be discussed at a later

14.0 References

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   [L2VPN-FMWK] Andersson, draft-ietf-ppvpn-l2-framework-01.txt, PPVPN
   L2 Framework, August 2002, (work in progress).

   [PWE3-CONTROL] Martini et. Al., ôTransport of Layer 2 Frames Over
   MPLSö, draft-ietf-pwe3-control-protocol-00.txt, August 2002 (work in

   [PWE3-ETH-ENCAP] Martini et. Al., ôEncapsulation Methods for
   Transport of Ethernet Frames over IP/MPLS Networksö, draft-ietf-
   pwe3-ethernet-encap-00.txt, August 2002 (work in progress)

   [ROSEN-SIG] Rosen, ôLDP-based Signaling for L2VPNsö, draft-rosen-
   ppvpn-l2-signaling-02.txt. September 2002. (work in progress).

   [VPLS] Lasserre et al, ôVirtual Private LN Service over MPLSö,
   draft-lasserre-vkompella-ppvpn-vpls-02.txt, June 2002 (work in

   [DNS-Discovery] "DNS/LDP Based VPLS", Heinanen, draft-heinanen-dns-
   ldp-vpls-00.txt, June 2002

   [BGP-Discovery] ôUsing BGP as an Auto-Discovery Mechanism for
   Network Based VPNsö, Ould-Brahim et al., draft-ietf-ppvpn-bgpvpn-
   auto-02.txt, February 2002, (work in progress).

   [ARP] Plummer, D., "An Ethernet Address Resolution Protocol:  Or
   Converting Network Protocol Addresses to 48.bit Ethernet
   Addresses for Transmission on Ethernet Hardware", STD 37, RFC 826,
   November 1982.

   [PROXY-ARP] Postel, J., "Multi-LAN Address Resolution", RFC 925,
   October 1984.

   [RFC-1112] Deering, S., ôHost Extensions for IP Multicastingö, RFC
   1112, August, 1989.

15.0. Intellectual Property Considerations

   Tenor Networks may seek patent or other intellectual property
   protection for some or all of the technologies disclosed in this
   document.  If any standards arising from this document are or become
   protected by one or more patents assigned to Tenor Networks,
   Tenor intends to disclose those patents and license them on
   reasonable and non-discriminatory terms.

Author's Address

   Himanshu Shah
   Tenor Networks
   100 Nagog Park

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   Acton, MA 01720
   Email: hshah@tenornetworks.com
   Email: arvind@tenornetworks.com

   Eric Rosen
   Cisco Systems
   300 Apollo Drive,
   Chelmsford, MA 01824
   Email: erosen@cisco.com

   Giles Heron
   PacketExchange Ltd.
   The Truman Brewery
   91 Brick Lane
   United Kingdom
   Email: giles@packetexchange.net

   Francois Le Faucheur
   Cisco Systems, Inc.
   Village d'Entreprise Green Side - Batiment T3
   400, Avenue de Roumanille
   06410 Biot-Sophia Antipolis
   Email: flefauch@cisco.com

   Vasile Radoaca
   Nortel Networks
   Email: vasile@nortelnetworks.com

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