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RFC 4664 - Framework for Layer 2 Virtual Private Networks (L2VPN


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Network Working Group                                  L. Andersson, Ed.
Request for Comments: 4664                                      Acreo AB
Category: Informational                                    E. Rosen, Ed.
                                                     Cisco Systems, Inc.
                                                          September 2006

        Framework for Layer 2 Virtual Private Networks (L2VPNs)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document provides a framework for Layer 2 Provider Provisioned
   Virtual Private Networks (L2VPNs).  This framework is intended to aid
   in standardizing protocols and mechanisms to support interoperable
   L2VPNs.

Table of Contents

   1. Introduction ....................................................3
      1.1. Conventions Used in This Document ..........................3
      1.2. Objectives and Scope of the Document .......................3
      1.3. Layer 2 Virtual Private Networks ...........................3
      1.4. Terminology ................................................4
   2. Models ..........................................................5
      2.1. Reference Model for VPWS ...................................5
           2.1.1. Entities in the VPWS Reference Model ................5
      2.2. Reference Model for VPLS ...................................6
           2.2.1. Entities in the VPLS Reference Model ................8
      2.3. Reference Model for Distributed VPLS-PE or VPWS-PE .........9
           2.3.1. Entities in the Distributed PE Reference Models .....9
      2.4. VPWS-PE and VPLS-PE ........................................9
   3. Functional Components of L2 VPN .................................9
      3.1. Types of L2VPN ............................................10
           3.1.1. Virtual Private Wire Service (VPWS) ................10
           3.1.2. Virtual Private LAN Service (VPLS) .................10
           3.1.3. IP-Only LAN-Like Service (IPLS) ....................11
      3.2. Generic L2VPN Transport Functional Components .............11
           3.2.1. Attachment Circuits ................................11
           3.2.2. Pseudowires ........................................12
           3.2.3. Forwarders .........................................14
           3.2.4. Tunnels ............................................15
           3.2.5. Encapsulation ......................................16
           3.2.6. Pseudowire Signaling ...............................16
                  3.2.6.1. Point-to-Point Signaling ..................18
                  3.2.6.2. Point-to-Multipoint Signaling .............18
                  3.2.6.3. Inter-AS Considerations ...................19
           3.2.7. Service Quality ....................................20
                  3.2.7.1. Quality of Service (QoS) ..................20
                  3.2.7.2. Resiliency ................................21
           3.2.8. Management .........................................22
      3.3. VPWS ......................................................22
           3.3.1. Provisioning and Auto-Discovery ....................23
                  3.3.1.1. Attachment Circuit Provisioning ...........23
                  3.3.1.2. PW Provisioning for Arbitrary
                           Overlay Topologies ........................23
                  3.3.1.3. Colored Pools PW Provisioning Model .......25
           3.3.2. Requirements on Auto-Discovery Procedures ..........27
           3.3.3. Heterogeneous Pseudowires ..........................28
      3.4. VPLS Emulated LANs ........................................29
           3.4.1. VPLS Overlay Topologies and Forwarding .............31
           3.4.2. Provisioning and Auto-Discovery ....................33
           3.4.3. Distributed PE .....................................33
           3.4.4. Scaling Issues in VPLS Deployment ..................36
      3.5. IP-Only LAN-Like Service (IPLS) ...........................36

   4. Security Considerations ........................................37
      4.1. Provider Network Security Issues ..........................37
      4.2. Provider-Customer Network Security Issues .................39
      4.3. Customer Network Security Issues ..........................39
   5. Acknowledgements ...............................................40
   6. Normative References ...........................................41
   7. Informative References .........................................41

1.  Introduction

1.1.  Conventions Used in This Document

   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 RFC 2119 [RFC2119].

1.2.  Objectives and Scope of the Document

   This document provides a framework for Layer 2 Provider Provisioned
   Virtual Private Networks (L2VPNs).  This framework is intended to aid
   in standardizing protocols and mechanisms to support interoperable
   L2VPNs.

   The term "provider provisioned VPNs" refers to Virtual Private
   Networks (VPNs) for which the Service Provider (SP) participates in
   management and provisioning of the VPN.

   Requirements for L2VPNs can be found in [RFC4665].

   This document provides reference models for L2VPNs and discusses the
   functional components of L2VPNs.  Specifically, this includes
   discussion of the technical issues that are important in the design
   of standards and mechanisms for L2VPNs, including those standards and
   mechanisms needed for interworking and security.

   This document discusses a number of different technical approaches to
   L2VPNs.  It tries to show how the different approaches are related,
   and to clarify the issues that may lead one to select one approach
   instead of another.  However, this document does not attempt to
   select any particular approach.

1.3.  Layer 2 Virtual Private Networks

   There are two fundamentally different kinds of Layer 2 VPN service
   that a service provider could offer to a customer: Virtual Private
   Wire Service (VPWS) and Virtual Private LAN Service (VPLS).  There is
   also the possibility of an IP-only LAN-like Service (IPLS).

   A VPWS is a VPN service that supplies an L2 point-to-point service.
   As this is a point-to-point service, there are very few scaling
   issues with the service as such.  Scaling issues might arise from the
   number of end-points that can be supported on a particular PE.

   A VPLS is an L2 service that emulates LAN service across a Wide Area
   Network (WAN).  With regard to the amount of state information that
   must be kept at the edges in order to support the forwarding
   function, it has the scaling characteristics of a LAN.  Other scaling
   issues might arise from the number of end-points that can be
   supported on a particular PE.  (See Section 3.4.4.)

   Note that VPLS uses a service that does not have native multicast
   capability to emulate a service that does have native multicast
   capability.  As a result, there will be scalability issues with
   regard to the handling of multicast traffic in VPLS.

   A VPLS service may also impose longer delays and provide less
   reliable transport than would a native LAN service.  The standard LAN
   control protocols may not have been designed for such an environment
   and may experience scaling problems when run in that environment.

1.4.  Terminology

   The list of the technical terms used when discussing L2VPNs may be
   found in the companion document [RFC4026].

2.  Models

2.1.  Reference Model for VPWS

   The VPWS reference model is shown in Figure 1.

                  Attachment        PSN           Attachment
                   Circuits        tunnel          Circuits
                                     +
           +-----+                 pseudo                    +-----+
           |     |                  wire                     |     |
           | CE1 |--+                                     +--| CE2 |
           |     |  |    +-----+   +-----+     +-----+    |  |     |
           +-----+  +----|---- |   |  P  |     | ----+----+  +-----+
                         |VPWS\---|-----|-----|/VPWS|
                         | PE1 |===|=====|=====| PE2 |
                         |    /|---|-----|-----|\\    |
           +-----+  +----|---- |   |     |     | ----|----+  +-----+
           |     |  |    +-----+   +-----+     +-----+    |  |     |
           | CE3 |--+                                     +--| CE4 |
           |     |                                           |     |
           +-----+                                           +-----+

                                    Figure 1

2.1.1.  Entities in the VPWS Reference Model

   The P, PE (VPWS-PE), and CE devices and the PSN tunnel are defined in
   [RFC4026].  The attachment circuit and pseudowire are discussed in
   Section 3.  The PE does a simple mapping between the PW and
   attachment circuit based on local information; i.e., the PW
   demultiplexor and incoming/outgoing logical/physical port.

2.2.  Reference Model for VPLS

   The following diagram shows a VPLS reference model where PE devices
   that are VPLS-capable provide a logical interconnect such that CE
   devices belonging to a specific VPLS appear to be on a single bridged
   Ethernet.  A VPLS can contain a single VLAN or multiple tagged VLANs.

   The VPLS reference model is shown in Figures 2 and 3.

           +-----+                                  +-----+
           + CE1 +--+                           +---| CE2 |
           +-----+  |    ...................    |   +-----+
            VPLS A  |  +----+           +----+  |    VPLS A
                    |  |VPLS|           |VPLS|  |
                    +--| PE |--Routed---| PE |-+
                       +----+  Backbone +----+
                      /  .       |         .  \     _   /\_
           +-----+   /   .       |         .   \   / \ /   \     +-----+
           + CE  +--+    .       |         .    +--\ Access \----| CE  |
           +-----+       .    +----+       .       | Network |   +-----+
            VPLS B       .....|VPLS|........        \       /     VPLS B
                              | PE |     ^           -------
                              +----+     |
                                |        |
                                |        |
                             +-----+     |
                             | CE3 |     +-- Emulated LAN
                             +-----+
                              VPLS A

                                    Figure 2

                         |-----Routed Backbone-----|
                         |     (P Routers)         |PSN Tunnels,
   Emulated LAN          |                         |Pseudowires
 .......................................................................
 .                       |                         |                   .
 . |---------------------|----|           |--------|-----------------| .
 . | --------------------|--- |           | -------|---------------- | .
 . |      VPLS Forwarder      |           |      VPLS Forwarder      | .
 . | ----------|------------- |           | ----------|------------- | .
 ..|.................................................................|..
   |           | Emulated LAN |           |           | Emulated LAN |
   |           | Interface    | VPLS-PEs  |           | Interface    |
   |           |              |  <---->   |           |              |
   | ----------|------------  |           | ----------|------------  |
   | |       Bridge        |  |           | |       Bridge        |  |
   | -|--------|---------|--  |           | ---|-------|---------|-  |
   |--|--------|---------|----|           |----|-------|---------|---|
      |        |         |                     |       |         |
      |        | Access  |                     |       | Access  |
      |        | Networks|                     |       | Networks|
      |        |         |                     |       |         |
      |        |         |                     |       |         |
           CE devices                                CE devices

                                Figure 3

   From Figure 3, we see that in VPLS, a CE device attaches, possibly
   through an access network, to a "bridge" module of a VPLS-PE.  Within
   the VPLS-PE, the bridge module attaches, through an "Emulated LAN
   Interface", to an Emulated LAN.  For each VPLS, there is an Emulated
   LAN instance.  Figure 3 shows some internal structure to the Emulated
   LAN: it consists of "VPLS Forwarder" modules connected by
   pseudowires, where the pseudowires may be traveling through PSN
   tunnels over a routed backbone.

   A "VPLS instance" consists of a set of VPLS Forwarders (no more than
   one per PE) connected by pseudowires.

   The functionality that the bridge module must support depends on the
   service that is being offered by the SP to its customers, as well as
   on various details of the SP's network.  At a minimum, the bridge
   module must be able to learn MAC addresses, and to "age them out", in
   the standard manner.  However, if the PE devices have backdoor
   connections with each other via a Layer 2 network, they may need to
   be full IEEE bridges ([IEEE8021D]), running a spanning tree with each
   other.  Specification of the precise functionality that the bridge

   modules must have in particular circumstances is, however, out of
   scope of the current document.

   This framework specifies that each "bridge module" have a single
   "Emulated LAN interface".  It does not specify the number of bridge
   modules that a VPLS-PE may contain, nor does it specify the number of
   VPLS instances that may attach to a bridge module over a single
   "Emulated LAN interface".

   Thus the framework is compatible with at least the following three
   models:

      - Model 1

        A VPLS-PE contains a single bridge module and supports a single
        VPLS instance.  The VPLS instance is an Emulated LAN; if that
        Emulated LAN contains VLANs, 802.1Q [IEEE8021Q] tagging must be
        used to indicate which packets are in which VLANs.

      - Model 2

        A VPLS-PE contains a single bridge module, but supports multiple
        VPLS instances.  Each VPLS instance is thought of as a VLAN (in
        effect, an "Emulated VLAN"), and the set of VPLS instances are
        treated as a set of VLANs on a common LAN.  Since each VLAN uses
        a separate set of PWs, there is no need for 802.1Q tagging.

      - Model 3

        A VPLS-PE contains an arbitrary number of bridge modules, each
        of which attaches to a single VPLS instance.

        There may be other models as well, some of which are
        combinations of the 3 models above.  Different models may have
        different characteristics, and different scopes of
        applicability.

        Each VPLS solution should specify the model or models that it is
        supporting.  Each solution should also specify the necessary
        bridge functionality that its bridge modules must support.

        This framework does not specify the way in which bridge control
        protocols are used on the Emulated LANs.

2.2.1.  Entities in the VPLS Reference Model

   The PE (VPLS-PE) and CE devices are defined in [RFC4026].

2.3.  Reference Model for Distributed VPLS-PE or VPWS-PE

                  VPLS-PE/VPWS-PE
                   Functionality       . . . . . . .
               . . . . . . . . . . .   .           .
               .                   .   .           .
       +----+  .  +----+    +----+ .   .  Service  .
       | CE |--.--|U-PE|----|N-PE|-.---.  Provider .
       +----+  .  +----+    +----+ .   .  Backbone .
               . . . . . . . . . . .   .           .

2.3.1.  Entities in the Distributed PE Reference Models

   A VPLS-PE or a VPWS-PE functionality may be distributed to more than
   one device.  The device closer to the customer/user is called the
   User-facing PE (U-PE), and the device closer to the core network is
   called Network-facing PE (N-PE).

   For further discussion, see Section 3.4.3.

   The terms "U-PE" and "N-PE" are defined in [RFC4026].

2.4.  VPWS-PE and VPLS-PE

   The VPWS-PE and VPLS-PE are functionally very similar, in that they
   both use forwarders to map attachment circuits to pseudowires.  The
   only difference is that while the forwarder in a VPWS-PE does a one-
   to-one mapping between the attachment circuit and pseudowire, the
   forwarder in a VPLS-PE is a Virtual Switching Instance (VSI) that
   maps multiple attachment circuits to multiple pseudowires (for
   further discussion, see Section 3).

3.  Functional Components of L2 VPN

   This section specifies a functional model for L2VPN, which allows one
   to break an L2VPN architecture down into its functional components.
   This exhibits the roles played by the various protocols and
   mechanisms, and thus makes it easier to understand the differences
   and similarities between various proposed L2VPN architectures.

   Section 3.1 contains an overview of some different types of L2VPNs.
   In Section 3.2, functional components that are common to the
   different types are discussed.  Then, there is a section for each of
   the L2VPN service types being considered.  The latter sections
   discuss functional components, which may be specific to particular
   L2VPN types, and type-specific features of the generic components.

3.1.  Types of L2VPN

   The types of L2VPN are distinguished by the characteristics of the
   service that they offer to the customers of the Service Provider
   (SP).

3.1.1.  Virtual Private Wire Service (VPWS)

   In a VPWS, each CE device is presented with a set of point-to-point
   virtual circuits.

   The other end of each virtual circuit is another CE device.  Frames
   transmitted by a CE on such a virtual circuit are received by the CE
   device at the other end-point of the virtual circuit.  Forwarding
   from one CE device to another is not affected by the content of the
   frame, but is fully determined by the virtual circuit on which the
   frame is transmitted.  The PE thus acts as a virtual circuit switch.

   This type of L2VPN has long been available over ATM and Frame Relay
   backbones.  Providing this type of L2VPN over MPLS and/or IP
   backbones is the current topic.

   Requirements for this type of L2VPN are specified in [RFC4665].

3.1.2.  Virtual Private LAN Service (VPLS)

   In a VPLS, each CE device has one or more LAN interfaces that lead to
   a "virtual backbone".

   Two CEs are connected to the same virtual backbone if and only if
   they are members of the same VPLS instance (i.e., same VPN).  When a
   CE transmits a frame, the PE that receives it examines the MAC
   Destination Address field in order to determine how to forward the
   frame.  Thus, the PE functions as a bridge.  As Figure 3 indicates,
   if a set of PEs support a common VPLS instance, then there is an
   Emulated LAN, corresponding to that VPLS instance, to which each of
   those PE bridges attaches (via an emulated interface).  From the
   perspective of a CE device, the virtual backbone is the set of PE
   bridges and the Emulated LAN on which they reside.  Thus to a CE
   device, the LAN that attaches it to the PE is extended transparently
   over the routed MPLS and/or IP backbone.

   The PE bridge function treats the Emulated LAN as it would any other
   LAN to which it has an interface.  Forwarding decisions are made in
   the manner that is normal for bridges, which is based on MAC Source
   Address learning.

   VPLS is like VPWS in that forwarding is done without any
   consideration of the Layer3 header.  VPLS is unlike VPWS in that:

      - VPLS allows the PE to use addressing information in a frame's L2
        header to determine how to forward the frame; and

      - VPLS allows a single CE/PE connection to be used for
        transmitting frames to multiple remote CEs; in this particular
        respect, VPLS resembles L3VPN more than VPWS.

   Requirements for this type of L2VPN are specified in [RFC4665].

3.1.3.  IP-Only LAN-Like Service (IPLS)

   An IPLS is very like a VPLS, except that:

      - it is assumed that the CE devices are hosts or routers, not
        switches; and

      - it is assumed that the service will only carry IP packets and
        supporting packets such as ICMP and ARP (in the case of IPv4) or
        Neighbor Discovery (in the case of IPv6); Layer 2 packets that
        do not contain IP are not supported.

   While this service is a functional subset of the VPLS service, it is
   considered separately because it may be possible to provide it using
   different mechanisms, which may allow it to run on certain hardware
   platforms that cannot support the full VPLS functionality.

3.2.  Generic L2VPN Transport Functional Components

   All L2VPN types must transport "frames" across the core network
   connecting the PEs.  In all L2VPN types, a PE (PE1) receives a frame
   from a CE (CE1), and then transports the frame to a PE (PE2), which
   then transports the frame to a CE (CE2).  In this section, we discuss
   the functional components that are necessary to transport L2 frames
   in any type of L2VPN service.

3.2.1.  Attachment Circuits

   In any type of L2VPN, a CE device attaches to a PE device via some
   sort of circuit or virtual circuit.  We will call this an "Attachment
   Circuit" (AC).  We use this term very generally; an Attachment
   Circuit may be a Frame Relay DLCI, an ATM VPI/VCI, an Ethernet port,
   a VLAN, a PPP connection on a physical interface, a PPP session from

   an L2TP tunnel, an MPLS LSP, etc.  The CE device may be a router, a
   switch, a host, or just about anything, which the customer needs
   hooked up to the VPN.  An AC carries a frame between CE and PE, or
   vice versa.

   Procedures for setting up and maintaining the ACs are out of scope of
   this architecture.

   These procedures are generally specified as part of the specification
   of the particular Attachment Circuit technology.

   Any given frame will traverse an AC from a CE to a PE, and then on
   another AC from a PE to a CE.

   We refer to the former AC as the frame's "ingress AC" and to the
   latter AC as the frame's "egress AC".  Note that this notion of
   "ingress AC" and "egress AC" is relative to a specific frame and
   denotes nothing more than the frame's direction of travel while it is
   on that AC.

3.2.2.  Pseudowires

   A "Pseudowire" (PW) is a relation between two PE devices.  Whereas an
   AC is used to carry a frame from CE to PE, a PW is used to carry a
   frame between two PEs.  We use the term "pseudowire" in the sense of
   [RFC3985].

   Setting up and maintaining the PWs is the job of the PEs.  State
   information for a particular PW is maintained at the two PEs that are
   its endpoints, but not at other PEs, and not in the backbone routers
   (P routers).

   Pseudowires may be point-to-point, multipoint-to-point, or point-to-
   multipoint.  In this framework, point-to-point PWs are always
   considered bidirectional; multipoint-to-point and point-to-multipoint
   PWs are always considered unidirectional.  Multipoint-to-point PWs
   can be used only when the PE receiving a frame does not need to
   infer, from the PW on which the frame was received, the identity of
   the frame's ingress AC.  Point-to-multipoint PWs may be useful when
   frames need to be multicast.

   Procedures for setting up and maintaining point-to-multipoint PWs are
   not considered in this version of this framework.

   Any given frame travels first on its ingress AC, then on a PW, and
   then on its egress AC.

   Multicast frames may be replicated by a PE, so of course the
   information carried in multicast frames may travel on more than one
   PW and more than one egress AC.

   Thus with respect to a given frame, a PW may be said to associate a
   number of ACs.  If these ACs are of the same technology (e.g., both
   ATM, both Ethernet, both Frame Relay), the PW is said to provide
   "homogeneous transport"; otherwise it is said to provide
   "heterogeneous transport".  Heterogeneous transport requires that
   some sort of interworking function be applied.  There are at least
   three different approaches to interworking:

       1.  One of the CEs may perform the interworking locally.  For
           example, if CE1 attaches to PE1 via ATM, but CE2 attaches to
           PE2 via Ethernet, then CE1 may decide to send/receive
           Ethernet frames over ATM, using the RFC 2684, "LLC
           Encapsulation for Bridged Protocols".  In such a case, PE1
           would need to know that it is to terminate the ATM VC
           locally, and only to send/receive Ethernet frames over the
           PW.

       2.  One of the PEs may perform the interworking.  For example, if
           CE1 attaches to PE1 via ATM, but CE2 attaches to PE2 via
           Frame Relay, PE1 may provide the "ATM/FR Service
           Interworking" function.  This would be transparent to the
           CEs, and the PW would carry only Frame Relay frames.

       3.  IPLS could be used.  In this case, the "frames" carried by
           the PW are IP datagrams, and the two PEs need to cooperate in
           order to spoof various L2-specific procedures used by IP (see
           Section 3.5).

   If heterogeneous PWs are used, the setup protocol must ensure that
   each endpoint knows the MTU of the remote AC.  If the two ACs do not
   have the same MTU, one of the following three procedures must be
   carried out:

      - The PW is not allowed to come up.

      - The endpoint at the AC with the larger MTU must reduce the AC's
        MTU so that it is the same as the MTU of the remote AC.

      - The two endpoints must agree to use a specified
        fragmentation/reassembly procedure.

3.2.3.  Forwarders

   In all types of L2VPN, a PE (say, PE1) receives a frame over an AC
   and forwards it over a PW to another PE (say, PE2).  PE2 then
   forwards the frame out on another AC.

   The case in which PE1 and PE2 are the same device is an important
   case to handle correctly, in order to provide the L2VPN service
   properly.  However, as this case does not require any protocol, we do
   not address it further in this document.

   When PE1 receives a frame on a particular AC, it must determine the
   PW on which the frame must be forwarded.  In general, this is done by
   considering:

      - the incoming AC;

      - possibly the contents of the frame's Layer2 header; and

      - possibly some forwarding information that may be statically or
        dynamically maintained.

   If dynamic or static forwarding information is considered, the
   information is specific to a particular L2VPN instance (i.e., to a
   particular VPN).

   Similarly, when PE2 receives a frame on a particular PW, it must
   determine the AC on which the frame must be forwarded.  This is done
   by considering:

      - the incoming PW;

      - possibly the contents of the frame's Layer2 header; and

      - possibly some forwarding information that may be statically or
        dynamically maintained.

   If dynamic or static forwarding information is considered, the
   information is specific to a particular L2VPN instance (i.e., to a
   particular VPN).

   The procedures used to make the forwarding decision are known as a
   "forwarder".  We may think of a PW as being "bound", at each of its
   endpoints, to a forwarder.  The forwarder in turn "binds" the PWs to
   ACs.  Different types of L2VPN have different types of forwarders.

   For instance, a forwarder may bind a single AC to a single PW,
   ignoring all frame contents and using no other forwarding
   information.  Or a forwarder may bind an AC to a set of PWs and ACs,
   moving individual frames from AC to PW, from a PW to an AC or from AC
   to AC by comparing information from the frame's Layer2 header to
   information in a forwarding database.  This is discussed in more
   detail below, as we consider the different L2VPN types.

3.2.4.  Tunnels

   A PW is carried in a "tunnel" from PE1 to PE2.  We assume that an
   arbitrary number of PWs may be carried in a single tunnel; the only
   requirement is that the PWs all terminate at PE2.

   We do not even require that all the PWs in the tunnel originate at
   PE1; the tunnels may be multipoint-to-point tunnels.  Nor do we
   require that all PWs between the same pair of PEs travel in the same
   tunnel.  All we require is that when a frame traveling through such a
   tunnel arrives at PE2, PE2 will be able to associate it with a
   particular PW.

   (While one can imagine tunneling techniques that only allow one PW
   per tunnel, they have evident scalability problems, and we do not
   consider them further.)

   A variety of different tunneling technologies may be used for the
   PE-PE tunnels.  All that is really required is that the tunneling
   technologies allow the proper demultiplexing of the contained PWs.
   The tunnels might be MPLS LSPs, L2TP tunnels, IPsec tunnels, MPLS-
   in-IP tunnels, etc.  Generally the tunneling technology will require
   the use of an encapsulation that contains a demultiplexor field,
   where the demultiplexor field is used to identify a particular PW.
   Procedures for setting up and maintaining the tunnels are not within
   the scope of this framework.  (But see Section 3.2.6, "Pseudowire
   Signaling".)

   If there are multiple tunnels from PE1 to PE2, it may be desirable to
   assign a particular PE1-PE2 PW to a particular tunnel based on some
   particular characteristics of the PW and/or the tunnel.  For example,
   perhaps different tunnels are associated with different QoS
   characteristics, and different PWs require different QoS.  Procedures
   for specifying how to assign PWs to tunnels are out of scope of the
   current framework.

   Though point-to-point PWs are bidirectional, the tunnels in which
   they travel need not be either bidirectional or point-to-point.  For
   example, a point-to-point PW may travel within a unidirectional
   multipoint-to-point MPLS LSP.

3.2.5.  Encapsulation

   As L2VPN packets are carried in pseudowires, standard pseudowire
   encapsulation formats and techniques (as specified by the IETF's PWE3
   WG) should be used wherever applicable.

   Generally the PW encapsulations will themselves be encapsulated
   within a tunnel encapsulation, as determined by the specification of
   the tunneling protocol.

   It may be necessary to define additional PW encapsulations to cover
   areas that are of importance for L2VPN, but that may not be within
   the scope of PWE3.  Heterogeneous transport may be an instance of
   this.

3.2.6.  Pseudowire Signaling

   Procedures for setting up and maintaining the PWs themselves are
   within the scope of this framework.  This includes procedures for
   distributing demultiplexor field values, even though the
   demultiplexor field, strictly speaking, belongs to the tunneling
   protocol and not to the PW.

   The signaling for a point-to-point pseudowire must perform the
   following functions:

      - Distribution of the demultiplexor.

        Since many PWs may be carried in a single tunnel, the tunneling
        protocol must assign a demultiplexor value to each PW.  These
        demultiplexors must be unique with respect to a given tunnel
        (or, with some tunneling technologies, unique at the egress PE).
        Generally, the PE that is the egress of the tunnel will select
        the demultiplexor values and will distribute them to the PE(s)
        which is (are) the ingress(es) of the tunnel.  This is the
        essential part of the PW setup procedure.

        Note that, as is usually the case in tunneling architectures,
        the demultiplexor field belongs to the tunneling protocol, not
        to the protocol being tunneled.  For this reason, the PW setup
        protocols may be extensions of the control protocols for setting
        up the tunnels.

      - Selection of the Forwarder at the remote PE.

        The signaling protocol must contain enough information to enable
        the remote PE to select the proper forwarder to which the PW is
        to be bound.  We can call this information the "Remote Forwarder

        Selector".  The information that is required will depend on the
        type of L2VPN being provided and on the provisioning model being
        used (see Sections 3.3.1 and 3.4.2).  The Remote Forwarder
        Selector may uniquely identify a particular Forwarder, or it may
        identify an attribute of Forwarders.  In the latter case, it
        would select whichever Forwarder has been provisioned with that
        attribute.

      - Supporting pseudowire emulations.

        To the extent that a particular PW must emulate the signaling of
        a particular Layer2 technology, the PW signaling must provide
        the necessary functions.

      - Distribution of state changes.

        Changes in the state of an AC may need to be reflected in
        changes to the state of the PW to which the AC is bound, and
        vice versa.  The specification as to which changes need to be
        reflected in what way would generally be within the province of
        the PWE3 WG.

      - Establishing pseudowire characteristics.

        To the extent that one or more characteristics of a PW must be
        known to and/or agreed upon by both endpoints, the signaling
        must allow for the necessary interaction.

   As specified above, signaling for point-to-point PWs must pass enough
   information to allow a remote PE to properly bind a PW to a
   Forwarder, and to associate a particular demultiplexor value with
   that PW.  Once the two PEs have done the proper PW/Forwarder
   bindings, and have agreed on the demultiplexor values, the PW may be
   considered set up.  If it is necessary to negotiate further
   characteristics or parameters of a particular PW, or to pass status
   information for a particular PW, the PW may be identified by the
   demultiplexor value.

   Signaling procedures for point-to-point pseudowires are most commonly
   point-to-point procedures that are executed by the two PW endpoints.
   There are, however, proposals to use point-to-multipoint signaling
   for setting up point-to-point pseudowires, so this is included in the
   framework.  When PWs are themselves point-to-multipoint, it is also
   possible to use either point-to-point signaling or point-to-
   multipoint signaling to set them up.  This is discussed in the
   remainder of this section.

3.2.6.1.  Point-to-Point Signaling

   There are several ways to do the necessary point-to-point signaling.
   Among them are:

      - LDP

        LDP [RFC3036] extensions can be defined for pseudowire
        signaling.  This form of signaling can be used for pseudowires
        that are to be carried in MPLS "tunnels", or in MPLS-in-
        something-else tunnels.

      - L2TP

        L2TP [RFC2661] can be used for pseudowire signaling, resulting
        in pseudowires that are carried as "sessions" within L2TP
        tunnels.  Pseudowire-specific extensions to L2TP may also be
        needed.

   Other methods may be possible as well.

   It is possible to have one control connection between a pair of PEs,
   which is used to control many PWs.

   The use of point-to-point signaling for setting up point-to-point PWs
   is straightforward.  Multipoint-to-point PWs can also be set up by
   point-to-point signaling, as the remote PEs do not necessarily need
   to know whether the PWs are multipoint-to-point or point-to-point.
   In some signaling procedures, the same demultiplexor value may be
   assigned to all the remote PEs.

3.2.6.2.  Point-to-Multipoint Signaling

   Consider the following conditions:

      - It is necessary to set up a set of PWs, all of which have the
        same characteristics.

      - It is not necessary to use the PW signaling protocol to pass PW
        state changes.

      - For each PW in the set, the same value of the Remote Forwarder
        Selector can be used.

   Call these the "Environmental Conditions".

   Suppose also that there is some mechanism by which, given a range of
   demultiplexor values, each of a set of PEs can make a unique and

   deterministic selection of a single value from within that range.
   Call this the "Demultiplexor Condition".  Alternatively, suppose that
   one is trying to set up a multipoint-to-point PW rather than to set
   up a point-to-point PW.  Call this the "Multipoint Condition".

   If:

      - The Environmental Conditions hold; and

      - Either

         * the Demultiplexor Condition holds, or

         * the Multipoint Condition holds,

   then for a given set of PWs that terminate at egress PE1, the
   information that PE1 needs to send to the ingress PE(s) of each
   pseudowire in the set is exactly the same.  All the ingress PE(s)
   receive the same Forwarder Selector value.  They all receive the same
   set of PW parameters (if any).  And either they all receive the same
   demultiplexor value (if the PW is multipoint-to-point) or they all
   receive a range of demultiplexor values from which each can choose a
   unique demultiplexor value for itself.

   Rather than connect to each ingress PE and replicate the same
   information, it may make sense either to multicast the information,
   or to send the information once to a "reflector", which will then
   take responsibility for distributing the information to the other
   PEs.

   We refer to this sort of technique as "point-to-multipoint"
   signaling.  It would, for example, be possible to use BGP [RFC1771]
   to do the signaling, with PEs that are BGP peers not of each other,
   but of one or more BGP route reflectors [RFC2796].

3.2.6.3.  Inter-AS Considerations

   Pseudowires may need to run from a PE in one Service Provider's
   network to a PE in another Service Provider's network.  This has the
   following implications:

      - The signaling protocol that sets up the PWs must be able to
        cross network boundaries.  Of course, all IP-based protocols
        have this capability.

      - The two PEs at the PW endpoints must be addressable and routable
        from each other.

      - The signaling protocol needs to allow each PW endpoint to
        authenticate the other.  To make use of the authentication
        capability, there would also need to be some method of key
        distribution that is acceptable to both administrations.

3.2.7.  Service Quality

   Service Quality refers to the ability for the network to deliver a
   Service level Specification (SLS) for service attributes such as
   protection, security, and Quality of Service (QoS).  The service
   quality provided depends on the subscriber's requirements and can be
   characterized by a number of performance metrics.

   The necessary Service Quality must be provided on the ACs, as well as
   on the PWs.  Mechanisms for providing Service Quality on the PWs may
   be PW-specific or tunnel-specific; in the latter case, the assignment
   of a PW to a tunnel may depend on the Service Quality.

3.2.7.1.  Quality of Service (QoS)

   QoS describes the queuing behavior applied to a particular "flow", in
   order to achieve particular goals of precedence, throughput, delay,
   jitter, etc.

   Based on the customer Service Level Agreement (SLA), traffic from a
   customer can be prioritized, policed, and shaped for QoS
   requirements.  The queuing and forwarding policies can preserve the
   packet order and QoS parameters of customer traffic.  The class of
   services can be mapped from information in the customer frames, or it
   can be independent of the frame content.

   QoS functions can be listed as follows:

      - Customer Traffic Prioritization: L2VPN services could be best
        effort or QoS guaranteed.  Traffic from one customer might need
        to be prioritized over others when sharing same network
        resources.  This requires capabilities within the L2VPN solution
        to classify and mark priority to QoS guaranteed customer
        traffic.

      - Proper queuing behavior would be needed at the egress AC, and
        possibly within the backbone network as well.  If queuing
        behavior must be controlled within the backbone network, the
        control might be based on CoS information in the MPLS or IP
        header, or it might be achieved by nesting particular tunnels
        within particular traffic engineering tunnels.

      - Policing: This ensures that a user of L2VPN services uses
        network resources within the limits of the agreed SLA.  Any
        excess L2VPN traffic can be rejected or handled differently
        based on provider policy.

      - Policing would generally be applied at the ingress AC.

      - Shaping: Under some cases, the random nature of L2VPN traffic
        might lead to sub-optimal utilization of network resources.
        Through queuing and forwarding mechanisms, the traffic can be
        shaped without altering the packet order.

      - Shaping would generally be applied at the ingress AC.

3.2.7.2.  Resiliency

   Resiliency describes the ability of the L2VPN infrastructure to
   protect a flow from network outage, so that service remains available
   in the presence of failures.

   L2VPN, like any other service, is subject to failures such as link,
   trunk, and node failures, both in the SP's core network
   infrastructure and on the ACs.

   It is desirable that the failure be detected "immediately" and that
   protection mechanisms allow fast restoration times to make L2VPN
   service almost transparent to these failures to the extent possible,
   based on the level of resiliency.  Restoration should take place
   before the CEs can react to the failure.  Essential aspects of
   providing resiliency are:

      - Link/Node failure detection: Mechanisms within the L2VPN service
        should allow for link or node failures that impact the service,
        and that should be detected immediately.

      - Resiliency policy: The way in which a detected failure is
        handled will depend on the restoration policy of the SLA
        associated with the L2VPN service specification.  It may need to
        be handled immediately, or it may need to be handled only if no
        other critical failure needs protection resources, or it may be
        completely ignored if it is within the bounds of the "acceptable
        downtime" allowed by the L2VPN service.

      - Restoration Mechanisms: The L2VPN solutions could allow for
        physical level protection, logical level protection, or both.
        For example, by connecting customers over redundant and

        physically separate ACs to different provider customer-facing
        devices, one AC can be maintained as active, and the other could
        be marked as a backup; upon the failure detection across the
        primary AC, the backup could become active.

   To a great extent, resiliency is a matter of having appropriate
   failure and recovery mechanisms in the network core, including
   "ordinary" adaptive routing as well as "fast reroute" capabilities.
   The ability to support redundant ACs between CEs and PEs also plays a
   role.

3.2.8.  Management

   An L2VPN solution can provide mechanisms to manage and monitor
   different L2VPN components.  From a Service Level Agreement (SLA)
   perspective, L2VPN solutions could allow monitoring of L2VPN service
   characteristics and offer mechanisms used by Service Providers to
   report such monitored statistical data.  Trouble-shooting and
   verification of operational and maintenance activities of L2VPN
   services are essential requirements for Service Providers.

3.3.  VPWS

   A VPWS is an L2VPN service in which each forwarder binds exactly one
   AC to exactly one PW.  Frames received on the AC are transmitted on
   the PW; frames received on the PW are transmitted on the AC.  The
   content of a frame's Layer2 header plays no role in the forwarding
   decision, except insofar as the Layer2 header contents are used to
   associate the frame with a particular AC (e.g., the DLCI field of a
   Frame Relay frame identifies the AC).

   A particular combination of <AC, PW, AC> forms a "virtual circuit"
   between two CE devices.

   A particular VPN (VPWS instance) may be thought of as a collection of
   such virtual circuits, or as an "overlay" of PWs on the MPLS or IP
   backbone.  This creates an overlay topology that is in effect the
   "virtual backbone" of a particular VPN.

   Whether two virtual circuits are said to belong to the same VPN or
   not is an administrative matter based on the agreements between the
   SPs and their customers.  This may impact the provisioning model
   (discussed below).  It may also affect how particular PWs are
   assigned to tunnels, the way QoS is assigned to particular ACs and
   PWs, etc.

   Note that VPWS makes use of point-to-point PWs exclusively.

3.3.1.  Provisioning and Auto-Discovery

   Provisioning a VPWS is a matter of:

       1.  Provisioning the ACs;

       2.  Providing the PEs with the necessary information to enable
          them to set up PWs between ACs to result in the desired
          overlay topology; and

       3.  Configuring the PWs with any necessary characteristics.

3.3.1.1.  Attachment Circuit Provisioning

   In many cases, the ACs must be individually provisioned on the PE
   and/or CE.  This will certainly be the case if the CE/PE attachment
   technology is a switched network, such as ATM or FR, and the VCs are
   PVCs rather than SVCs.  It is also the case whenever the individual
   Attachment Circuits need to be given specific parameters (e.g., QoS
   parameters, guaranteed bandwidth parameters) that differ from circuit
   to circuit.

   There are also cases in which ACs might not have to be individually
   provisioned.  For example, if an AC is just an MPLS LSP running
   between a CE and a PE, it could be set up as the RESULT of setting up
   a PW rather than having to be provisioned BEFORE the PW can be set
   up.  The same may apply whenever the AC is a Switched Virtual Circuit
   of any sort, though in this case, various policy controls might need
   to be provisioned; e.g., limiting the number of ACs that can be set
   up between a given CE and a given PE.

   Issues such as whether the Attachment Circuits need to be
   individually provisioned or not, whether they are Switched VCs or
   Permanent VCs, and what sorts of policy controls may be applied are
   implementation and deployment issues and are considered to be out of
   scope of this framework.

3.3.1.2.  PW Provisioning for Arbitrary Overlay Topologies

   In order to support arbitrary overlay topologies, it is necessary to
   allow the provisioning of individual PWs.  In this model, when a PW
   is provisioned on a PE device, it is locally bound to a specific AC.
   It is also provisioned with information that identifies a specific AC
   at a remote PE.

   There are basically two variations of this provisioning model:

      - Two-sided provisioning

        With two-sided provisioning, each PE that is at the end of a PW
        is provisioned with the following information:

          * Identifier of the Local AC to which the PW is to be bound

          * PW type and parameters

          * IP address of the remote PE (i.e., the PE that is to be at
            the remote end of the PW)

          * Identifier that is meaningful to the remote PE, and that can
            be passed in the PW signaling protocol to enable the remote
            PE to bind the PW to the proper AC.  This can be an
            identifier of the PW or an identifier of the remote AC.  If
            a PW identifier is used, it must be unique at each of the
            two PEs.  If an AC identifier is used, it need only be
            unique at the remote PE.

        This identifier is then used as the Remote Forwarder Selector
        when signaling is done (see 3.2.6.1).

      - Single-sided provisioning

        With single-sided provisioning, a PE at one end of a PW is
        provisioned with the following information:

          * Identifier of the Local AC to which the PW is to be bound

          * PW type and parameters

          * Globally unique identifier of remote AC

        This identifier is then used as the Forwarder Selector when
        signaling is done (see section 3.2.6.1).

        In this provisioning model, the IP address of the remote PE is
        not provisioned.  Rather, the assumption is that an auto-
        discovery scheme will be used to map the globally unique
        identifier to the IP address of the remote PE, along with an
        identifier (perhaps unique only at the latter PE) for an AC at
        that PE.  The PW signaling protocol can then make a connection
        to the remote PE, passing the AC identifier, so that the remote
        PE binds the PW to the proper AC.

        This scheme requires provisioning of the PW at only one PE, but
        it does not eliminate the need (if there is a need) to provision
        the ACs at both PEs.

   These provisioning models fit well with the use of point-to-point
   signaling.  When each PW is individually provisioned, as the
   conditions necessary for the use of point-to-multipoint signaling do
   not hold.

3.3.1.3.  Colored Pools PW Provisioning Model

   Suppose that at each PE, sets of ACs are gathered together into
   "pools", and that each such pool is assigned a "color".  (For
   example, a pool might contain all and only the ACs from this PE to a
   particular CE.) Now suppose that we impose the following rule:
   whenever PE1 and PE2 have a pool of the same color, there will be a
   PW between PE1 and PE2 that is bound at PE1 to an arbitrarily chosen
   AC from that pool, and at PE2 to an arbitrarily chosen AC from that
   pool.  (We do not rule out the case where a single PE has multiple
   pools of a given color.)

   For example, each pool in a particular PE might represent a
   particular CE device, for which the ACs in the pool are the ACs
   connecting that CE to that PE.  The color might be a VPN-id.
   Application of this provisioning model would then lead to a full CE-
   to-CE mesh within the VPN, where every CE in the VPN has a virtual
   circuit to every other CE within the VPN.

   More specifically, to provision VPWS according to this model, one
   provisions a set of pools and configures each pool with the following
   information:

      - The set of ACs that belong to the pool (with no AC belonging to
        more than one pool)

      - The color

      - A pool identifier that is unique at least relative to the color.

        An auto-discovery procedure is then used to map each color into
        a list of ordered pairs <IP address of PE, pool id>.  The
        occurrence of a pair <X, Y> on this list means that the PE at IP
        address X has a pool with pool id Y, which is of the specified
        color.

        This information can be used to support several different
        signaling techniques.  One possible technique proceeds as
        follows:

      - A PE finds that it has a pool of color C.

      - Using auto-discovery, it obtains the set of ordered pairs <X,Y>
        for color C.

      - For each such pair <X,Y>, it:

        * removes an AC from the pool;

        * binds the AC to a particular PW; and

        * signals PE X via point-to-point signaling that the PW is to be
          bound to an AC from pool Y.

   Another possible signaling technique is the following:

      - A PE finds that it has a pool of color C, containing n ACs.

      - It binds each AC to a PW, creating a set of PWs.  This set of
        PWs is then organized into a sequence.  (For instance, each PW
        may be associated with a demultiplexor field value, and the PWs
        may then be sequenced according to the numerical value of their
        respective demultiplexors.)

      - Using auto-discovery, it obtains the list of PE routers that
        have one or more pools of color C.

      - It signals each such PE router, specifying the sequence Q of
        PWs.

      - If PE X receives such a signal and PE X has a pool Y of the
        specified color, it:

        * removes an AC from the pool; and

        * binds the AC to the PW that is the "Yth" PW in the sequence Q.

   This presumes, of course, that the pool identifiers are or can be
   uniquely mapped into small ordinal numbers; assigning the pool
   identifiers in this way becomes a requirement of the provisioning
   system.

   Note that since this technique signals the same information to all
   the remote PEs, it can be supported via point-to-multipoint
   signaling.

   This provisioning model can be applied as long as the following
   conditions hold:

      - There is no need to provision different characteristics for the
        different PWs;

      - It makes no difference which pairs of ACs are bound together by
        PWs, as long as both ACs in the pair come from like-colored
        pools; and

      - It is possible to construct the desired overlay topology simply
        by assigning colors to the pools.  (This is certainly simple if
        a full mesh is desired, or if a hub and spoke configuration is
        desired; creating arbitrary topologies is less simple, and is
        perhaps not always possible.)

3.3.2.  Requirements on Auto-Discovery Procedures

   Some of the requirements for auto-discovery procedures can be deduced
   from the above.

   To support the single-sided provisioning model, auto-discovery must
   be able to map a globally unique identifier (of a PW or of an
   Attachment Circuit) to an IP address of a PE.

   To support the colored pools provisioning model, auto-discovery must
   enable a PE to determine the set of other PEs that contain pools of
   the same color.

   These requirements enable the auto-discovery scheme to provide the
   information, which the PEs need to set up the PWs.

   There are additional requirements on the auto-discovery procedures
   that cannot simply be deduced from the provisioning model:

      - Particular signaling schemes may require additional information
        before they can proceed and hence may impose additional
        requirements on the auto-discovery procedures.

      - A given Service Provider may support several different types of
        signaling procedures, and thus the PEs may need to learn, via
        auto-discovery, which signaling procedures to use.

      - Changes in the configuration of a PE should be reflected by the
        auto-discovery procedures, within a timely manner, and without
        the need to explicitly reconfigure any other PE.

      - The auto-configuration procedures must work across service
        provider boundaries.  This rules out, e.g., use of schemes that
        piggyback the auto-discovery information on the backbone's IGP.

3.3.3.  Heterogeneous Pseudowires

   Under certain circumstances, it may be desirable to have a PW that
   binds two ACs that use different technologies (e.g., one is ATM, one
   is Ethernet).  There are a number of different ways, depending on the
   AC types, in which this can be done.  For example:

      - If one AC is ATM and one is FR, then standard ATM/FR Network
        Interworking can be used.  In this case, the PW might be
        signaled for ATM, where the Interworking function occurs between
        the PW and the FR AC.

      - A common encapsulation can be used on both ACs, if for example,
        one AC is Ethernet and one is FR, an "Ethernet over FR"
        encapsulation can be used on the latter.  In this case, the PW
        could be signaled for Ethernet, with processing of the Ethernet
        over FR encapsulation local to the PE with the FR AC.

      - If it is known that the two ACs attach to IP routers or hosts
        and carry only IP traffic, then one could use a PW that carries
        the IP packets, and the respective Layer2 encapsulations would
        be local matters for the two PEs.  However, if one of the ACs is
        a LAN and one is a point-to-point link, care would have to be
        taken to ensure that procedures such as ARP and Inverse ARP are
        properly handled; this might require some signaling, and some
        proxy functions.  Further, if the CEs use a routing algorithm
        that has different procedures for LAN interfaces than those for
        point-to-point interfaces, additional mechanisms may be required
        to ensure proper interworking.

3.4.  VPLS Emulated LANs

   A VPLS is an L2VPN service in which:

      - the ACs attach CE devices to PE bridge modules; and

      - each PE bridge module is attached via an "emulated LAN
        interface" to an "emulated LAN".

   This is shown in Figure 3.

   In this section, we examine the functional decomposition of the VPLS
   Emulated LAN.  An Emulated LAN's ACs are the "emulated LAN
   interfaces" attaching PE bridge modules to the "VPLS Forwarder"
   modules (see Figure 3).  The payload on the ACs consists of ethernet
   frames, with or without VLAN headers.

   A given VPLS Forwarder in a given PE will have multiple ACs only if
   there are multiple bridge modules in that PE that attach to that
   Forwarder.  This scenario is included in the Framework, though
   discussion of its utility is out of scope.

   The set of VPLS Forwarders within a single VPLS are connected via
   PWs.  Two VPLS Forwarders will have a PW between them only if those
   two Forwarders are part of the same VPLS.  (There may be a further
   restriction that two VPLS Forwarders have a PW between them only if
   those two Forwarders belong to the same VLAN in the same VPN.)  A
   particular set of interconnected VPLS Forwarders is what constitutes
   a VPLS Emulated LAN.

   On a real LAN, any frame transmitted by one entity is received by all
   the others.  A VPLS Emulated LAN, however, behaves somewhat
   differently.  When a VPLS Forwarder receives a unicast frame over one
   of its Emulated LAN interfaces, the Forwarder does not necessarily
   send the frame to all the other Forwarders on that Emulated LAN.  A
   unicast frame needs to be sent to only one other Forwarder in order
   to be properly delivered to its destination MAC address.  If the
   transmitting Forwarder knows which other Forwarder needs to receive a
   particular unicast frame, it will send the frame to just that one
   Forwarder.  This forwarding optimization is an important part of any
   attempt to provide a VPLS service over a wide-area or metropolitan
   area network.

   In effect, then, each Forwarder behaves as a "Virtual Switch
   Instance" (VSI), maintaining a forwarding table that maps MAC
   addresses to PWs.  The VSI is populated in much the same way that a
   standard bridge populates its forwarding table.  The VPLS Forwarders
   do MAC Source Address (SA) learning on frames received on PWs from

   other Forwarders and must also do the related set of procedures, such
   as aging out address entries.  Frames with unknown DAs or multicast
   DAs must be "broadcast" by one Forwarder to all the others (on the
   same emulated LAN).  There are, however, a few important differences
   between the VPLS Forwarder VSI and the standard bridge forwarding
   function:

      - A VPLS Forwarder never learns the MAC SAs of frames that it
        receives on its ACs; it only learns the MAC SAs of frames that
        are received on PWs from other VPLS Forwarders; and

      - The VPLS Forwarders of a particular emulated LAN do not
        participate in a spanning tree protocol with each other.  A
        "split horizon" technique is used to prevent forwarding loops.

   These points are discussed further in the next section.

   Note that the PE bridge modules that are on a given Emulated LAN may
   or may not run a spanning tree protocol with each other over the
   Emulated LAN; whether they do so or not is outside the scope of the
   VPLS specifications.  The PE bridge modules will do MAC address
   learning on the ACs.  The PE bridge modules also do MAC address
   learning on the Emulated LAN interfaces, but do not do MAC address
   learning on the PWs, as the PWs are "hidden" behind the Emulated LAN
   interface.  Conceptually, the PE bridge module's forwarding table and
   the VPLS Forwarder's VSI are distinct entities.  (Of course,
   particular implementations might combine these into a single table,
   but that is beyond the scope of this document.)

   A further issue arises if the PE bridges run bridge control protocols
   with each other over the Emulated LAN.  Bridge control protocols are
   generally designed to run in over a real LAN and may presume, for
   their proper functioning, certain characteristics of the LAN, such as
   low latency and sequential delivery.  If the Emulated LAN does not
   provide these characteristics, the control protocols may not perform
   as expected unless special mechanisms are provided for carrying the
   control frames.

   It should be noted that changes in the spanning tree (if any) of a
   customer network, or in the spanning tree (if any) of the PE bridges,
   may cause certain MAC addresses to change their location from one PE
   to another.  These changes may not be visible to the VPLS Forwarders,
   which means that those MAC addresses might become unreachable until
   they are aged out of the first PE's VSI.  If this is not acceptable,
   some mechanism for communicating such changes to the VPLS Forwarders
   must be provided.

3.4.1.  VPLS Overlay Topologies and Forwarding

   Within a single VPLS, the VPLS Forwarders are interconnected by PWs.
   The set of PWs thus forms an "overlay topology".

   The VPLS Forwarder VSIs are populated by means of MAC address
   learning.  That is, the VSI keeps track of which MAC SAs have been
   received over which PWs.  The presumption, of course, is that if a
   particular MAC address appears as the SA of a frame received over a
   particular PW, then frames that carry that MAC address in the DA
   field should be sent to the VSI that is at the remote end of the PW.
   In order for this presumption to be true, there must be a unique VSI
   at the remote end of the PW, which means that VSIs cannot be
   interconnected by means of multipoint-to-point PWs.  The PWs are
   necessarily either point-to-point or, possibly, point-to-multipoint.

   MAC learning over a point-to-point PW is done via the standard
   techniques as specified by IEEE, where the PW is treated by the VPLS
   Forwarder as a "bridge port".  Of course, if a MAC address is learned
   from a point-to-multipoint PW, the VSI must indicate that packets to
   that address are to be sent over a point-to-point PW that leads to
   the root of that point-to-multipoint PW.

   The VSI forwarding decisions must be coordinated so that loop-free
   forwarding over the overlay topology is ensured.

   There are several possible types of overlay topologies:

      - Full mesh

        In a full mesh, every VSI in a given VPLS has exactly one
        point-to-point PW to every other VSI in that same VPLS.

        In this topology, loop free forwarding of frames is ensured by
        the following rule: if a VSI receives a frame, over a PW, from
        another VSI, it MUST NOT forward that frame over ANY other PW to
        any other VSI.  This ensures that once a frame traverses the
        Emulated LAN, it must be sent off the Emulated LAN.

        If a VSI receives, on one of its Emulated LAN interfaces, a
        unicast frame with a known DA, the frame is sent on exactly one
        point-to-point PW.

        If a VSI receives, on one of its Emulated LAN interfaces, a
        multicast frame or a unicast frame with an unknown DA, it sends
        a copy of the frame to each other VSI in the same Emulated LAN.
        This can be done by replicating the frame and sending a copy
        over each point-to-point PW.  Alternatively, the full mesh of

        point-to-point PWs may be augmented with point-to-multipoint
        PWs, where each VSI in a VPLS is the transmitter on a single
        point-to-multipoint PW, and the receivers on that PW are all the
        other VSIs in that VPLS.

      - Tree structured

        In a tree structured topology, every VSI in a particular VPLS is
        provisioned to be at a particular level in the tree.  A given
        VSI has at most one pseudowire leading to a higher level.  The
        root of the tree is considered the highest level.

        In this topology, loop free forwarding of frames is ensured by
        the following rule: if a frame is received over a pseudowire
        from a higher level, it may not be sent over a pseudowire that
        leads to a higher level.

      - Tree with Meshed Highest Level

        In this variant of the tree-structured topology, there may be
        more than one VSI at the highest level, but the set of VSIs that
        are at the highest level must be fully meshed.  To ensure loop
        free forwarding, we need to impose the rule that a frame can be
        sent on a pseudowire to the same or higher level only if it
        arrived over a pseudowire from a lower level, and that frames
        arriving over PWs from the same level cannot be sent on PWs to
        the same level.

   Other overlay topologies are also possible; e.g., an arbitrary
   partial mesh of PWs among the VSIs of a VPLS.  Loop-freedom could
   then be assured by, for example, running a spanning tree on the
   overlay.  These topologies are not further considered in this
   framework.

   Note that loop freedom in the overlay topology does not necessarily
   ensure loop freedom in the overall customer LAN that contains the
   VPLS.  It does not even ensure loop freedom among the PE bridge
   modules.  It ensures only that when a frame is sent on the Emulated
   LAN, the frame will not loop endlessly before (or instead of) leaving
   the Emulated LAN.

   Improper configuration of the customer LAN or PE bridge modules may
   cause frames to loop, and frames that fall into such loops may
   transit the overlay topology multiple times.  Procedures that enable
   the PE to detect and/or prevent such loops may be advisable.

3.4.2.  Provisioning and Auto-Discovery

   Each VPLS must be assigned a globally unique identifier.  This can be
   thought of as a VPN-id.

   The ACs attaching the CEs to the PEs must be provisioned on both the
   PEs and the CEs.  A VSI for that VPLS must be provisioned on the PE,
   and the local ACs of that VPLS must be associated with that VSI.  The
   VSI must be provisioned with the identifier of the VPLS to which it
   belongs.

   An auto-discovery scheme may be used by a PE to map a VPLS identifier
   into the set of remote PEs that have VSIs in that VPLS.  Once this
   set is determined, the PE can use pseudowire signaling to set up a PW
   to each of those VSIs.  The VPLS identifier would serve as the
   signaling protocol's Forwarder Selector.  This would result in a full
   mesh of PWs among the VSIs in a particular VPLS.

   If a single VPLS contains multiple VLANs, then it may be desirable to
   limit connectivity so that two VSIs are connected only if they have a
   VLAN in common.

   In this case, each VSI would need to be provisioned with one or more
   VLAN ids, and the auto-discovery scheme would need to map a VPLS
   identifier into pairs of <PE, VLAN id>.

   If a fully meshed topology of VSIs is not desired, then each VSI
   needs to be provisioned with additional information specifying its
   placement in the topology.  This information would also need to be
   provided by the auto-discovery scheme.

   Alternatively, the single-sided provisioning method discussed in
   Section 3.3.1.2 could be used.  As this is more complicated, it would
   only be used if it were necessary to associate individual PWs with
   individual characteristics.  For example, if different guaranteed
   bandwidths were needed between different pairs of sites within a
   VPLS, the PWs would have to be provisioned individually.

3.4.3.  Distributed PE

   Often, when a VPLS type of service is provided, the CE devices attach
   to a provider-managed CPE device.  This provider-managed CPE device
   may attach to CEs of multiple customers, especially if, for example,
   there are multiple customers occupying the same building.  However,
   this device is really part of the SP's network, hence may be
   considered a PE device.

   In some scenarios in which a VPLS type of service is provided, the CE
   devices attach to a provider-managed intermediary device.  This
   provider-managed device may attach to CEs of multiple customers.
   This may arise if there are multiple customers occupying the same
   building.  This device is really part of the SP's network and may for
   that reason be considered to be a PE device; however, in the simplest
   case, it is performing only aggregation and none of the function
   associated with a VPLS.

   Relative to the VPLS there are three different possibilities for
   allocate functions to a device in such a position in the provider
   network:

      - it can perform aggregation and pure Layer2 service only, in
        which case it does not really play the role of a PE device in a
        VPLS service.  In this case the intermediary system must connect
        to devices that perform VPLS PE functionality; the intermediary
        device itself is not part of the VPLS architecture and has hence
        not been named in this architecture.

      - it can perform all the PE functions relevant for a VPLS.  In
        such a case, the device is called VPLS-PE, see [RFC4026].  This
        type of device will be connected to the core (P) routers.

      The PE functionality for a VPLS may be distributed between two
      devices, one "low-end" closer to the customer that performs, for
      example, the MAC-address learning and forwarding decisions, and
      one "high-end" that performs the control functions; e.g.,
      establishing tunnels, PWs, and VCs.  We call the low-end device
      the User-Facing PE (U-PE) and the high-end device the Network-
      Facing PE (N-PE).

      It is conceivable that the U-PE may be placed very close to the
      customer; e.g., in a building with more than one customer.  The
      N-PE will presumably be placed on the SP's premises.

      The distributed case is potentially of interest for a number of
      possible reasons:

      - The N-PE may be a device that cannot easily implement the VSI
        functionality described above.  For example, perhaps the N-PE is
        a router that cannot perform the high speed MAC learning that is
        needed in order to implement a VSI forwarder.  At the same time,
        the U-PE may need to be a low-cost device that also cannot
        implement the full set of VPLS functions.

        This leads one to investigate further if there are sensible ways
        to split the VPLS PE functionality between the U-PE and the N-
        PE.

      - Generally, in the L2VPN architecture, the PEs are expected to
        participate as peers in the backbone routing protocol.  Since
        the number of U-PEs is potentially very large relative to the
        number of N-PEs, this may be undesirable as a matter of scaling
        the backbone routing protocol.

      - The U-PE may be a relatively inexpensive device that is unable
        to participate in the full range of signaling and/or auto-
        discovery procedures that are needed in order to provide the
        VPLS service.

   The VPLS functionality can be distributed between U-PE and N-PE in a
   number of different ways, and a number of different proposals have
   been made.  They all presume that the U-PE will maintain a VSI
   forwarder, connected by PWs to the remote VSIs; the N-PE thus does
   not need to perform the VSI forwarding function.  The proposals tend
   to differ with respect to the following questions:

      - Should the U-PEs perform full PW signaling to set up the PWs to
        remote VSIs, or should the N-PEs do this signaling?

        Since the U-PEs need to be able to send packets on PWs to remote
        VSIs and receive packets on PWs from remote VSIs, if the PW
        signaling is done by the N-PE, there would have to be some form
        of "lightweight" (presumably) signaling between N-PE and U-PE
        that allows the PWs to be extended from N-PE to U-PE.

      - Should the U-PEs do their own auto-discovery, or should this be
        done by the N-PEs?

        In the latter case, the U-PEs may need to have some means of
        telling the N-PEs which VPLSes they are interested in, and the
        N-PEs must have some means of passing the results of the auto-
        discovery process to the U-PE.

        Whether it makes sense to split auto-discovery in this manner
        may depend on the particular auto-discovery protocol used.  One
        would not expect the U-PEs to participate in, if for example, a
        BGP-based auto-discovery scheme, but perhaps they would be
        expected to participate in a RADIUS-based auto-discovery scheme.

      - If a U-PE does not participate in routing but is redundantly
        connected to two different N-PEs, can the U-PE still make an
        intelligent choice of the best N-PE to use as the "next hop" for

        traffic destined to a particular remote VSI? If not, can this
        choice be made as the result of some other sort of interaction
        between N-PE and U-PE, or does this choice need to be
        established by provisioning?

      - If a U-PE does not participate in routing but does participate
        in full PW signaling, and if MPLS is being used, how can an N-PE
        send a U-PE the labels that the U-PE needs in order to be able
        to send traffic to its signaling peers?  (If the U-PE did
        participate in routing, this would happen automatically.)

      - When a frame must be multicast, should the replication be done
        by the N-PE or the U-PE?

        These questions are not all independent; the way one answers
        some of them may influence the way one answers others.

3.4.4.  Scaling Issues in VPLS Deployment

   In general, the PSN supports a VPLS solution with a tunnel from each
   VPLS-PE to every other VPLS-PE participating in the same VPLS
   instance.  Strictly, VPLS-PEs with more than one VPLS instance in
   common only need one tunnel, but for resource allocation reasons it
   might be necessary to establish several tunnels.  For each VPLS
   service on a given VPLS-PE, it needs to establish one pseudowire to
   every other VPLS-PE participating in that VPLS service.  In total
   n*(n-1) pseudowires must be setup between the VPLS-PE routers.  In
   large scale deployment this obviously creates scaling problems.  One
   way to address the scaling problems is to use hierarchy.

3.5.  IP-Only LAN-Like Service (IPLS)

   If, instead of providing a general VPLS service, one wishes to
   provide a VPLS that is used only to connect IP routers or hosts
   (i.e., the CE devices are all assumed to be IP routers or hosts),
   then it is possible to make certain simplifications.

   In this environment, all Ethernet frames sent from a particular CE to
   a particular PE on a particular Attachment Circuit will have the same
   MAC Source Address.  Thus, rather than use address learning in the
   data plane to learn the MAC addresses, the PE can use the control
   plane to learn the MAC address.  This allows the PE to be implemented
   on devices that are not capable of doing MAC address learning in the
   data plane.

   To eliminate the need for MAC address learning on the PWs as well as
   on the ACs, the pseudowire signaling protocol would have to carry the
   MAC address from one pseudowire endpoint to the other.  In the case

   of IPv4, Each PE would perform proxy ARP to its directly attached
   CEs.  In the case of IPv6, each PE would send proxy Neighbor and/or
   Router Advertisements.

   Eliminating the need to do MAC address learning on the PWs eliminates
   the need for the PWs to be point-to-point.  Multipoint-to-point PWs
   could be used instead.

   Unlike a VPLS, all the ACs in an IPLS would not necessarily have to
   carry Ethernet frames; only the IP packets would need to be passed
   across the network, not their Layer 2 wrappers.  However, if there
   are protocols that are specific to the Layer 2, but that provide, for
   example, address resolution services for Layer 3, it may then be
   necessary to "translate" (or otherwise interwork) one of these Layer
   2 protocols to the other.  For example, if an IPLS instance has an
   ethernet AC and a Frame Relay AC, and IPv4 is running on both,
   interworking between ARP and Inverse ARP might be required.

   The set of routing protocols that could be carried across the IPLS
   might also be restricted.

   An IPLS instance must have a particular IPLS-wide MTU; if there are
   different kinds of AC in an IPLS instance, and those different kinds
   of AC support different MTUs, all ACS must enforce the IPLS-wide MTU;
   an AC that cannot do this must not be allowed to join the IPLS
   instance.

4.  Security Considerations

   The security considerations section of the L2VPN requirements
   document [RFC4665] addresses a number of areas that are potentially
   insecure aspects of the L2VPN.  These relate to both control plane
   and data plane security issues that may arise in the following areas:

      - issues fully contained in the provider network

      - issues fully contained in the customer network

      - issues in the customer-provider interface network

   These three areas are addressed below.

4.1.  Provider Network Security Issues

   This section discusses security issues that only impact the SP's
   equipment.

   There are security issues having to do with the control connections
   that are used on a PE-PE basis for setting up and maintaining the
   pseudowires.

   A PE should not engage with another PE in a control connection unless
   it has some confidence that the peer is really a PE to which it
   should be setting up PWs.  Otherwise, L2PVN traffic may go to the
   wrong place.  If control packets are maliciously and undetectably
   altered while in flight, denial of service, or alteration of the
   expected quality of service, may result.

   If peers discover each other dynamically (via some auto-discovery
   procedure), this presupposes that the auto-discovery procedures are
   themselves adequately trusted.

   PEs should not accept control connections from arbitrary entities; a
   PE either should be configured with its peers or should learn them
   from a trusted auto-configuration procedure.  If the peer is required
   to be within the same SP's network, then access control filters at
   the borders of that network can be used to prevent spoofing of the
   peer's source address.  If the peer is from another SP's network,
   then setting up such filters may be difficult or even impossible,
   depending on the way in which the two SPs are connected.  Even if the
   access filters can be set up, the level of assurance that they
   provide will be lower.

   Thus, for inter-SP control connections, it is advisable to use some
   sort of cryptographic authentication procedure.  Control protocols
   which used TCP may use the TCP MD5 option to provide a measure of
   PE-PE authentication; this requires at least one shared secret
   between SPs.  The use of IPsec between PEs is also possible and
   provides a greater degree of assurance, though at a greater cost.

   Any other security considerations that apply to the control protocol
   in general will also apply when the control protocol is used for
   setting up PWs.  If the control protocol uses UDP messages, it may be
   advisable to have some protection against spoofed UDP messages that
   appear to be from a valid peer; this requires further study.

   To limit the effect of Denial of Service attacks on a PE, some means
   of limiting the rate of processing of control plane traffic may be
   desirable.

   Unlike authentication and integrity, privacy of the signaling
   messages is not usually considered very important.  If it is needed,
   the signaling messages can be sent through an IPsec connection.

   If the PE cannot efficiently handle high volumes of multicast traffic
   for sustained periods, then it may be possible to launch a denial of
   service attack on a VPLS service by sending a PE a large number of
   frames that have either a multicast address or an unknown MAC address
   in their MAC Destination Address fields.  A similar denial of service
   attack can be mounted by sending a PE a large number of frames with
   bogus MAC Source Address fields.  The bogus addresses can fill the
   MAC address tables in the PEs, with the result that frames destined
   to the real MAC addresses always get flooded (i.e., multicast).  Note
   that this flooding can remove the (weak) confidentiality property of
   this or any other bridged network.

4.2.  Provider-Customer Network Security Issues

   There are a number of security issues related to the access network
   between the provider and the customer.  This is also traditionally a
   network that is hard to protect physically.

   Typical security issues on the provider-customer interface include
   the following:

      - Ensuring that the correct customer interface is configured

      - Preventing unauthorized access to the PE

      - Preventing unauthorized access to a specific PE port

      - Ensuring correct service delimiting fields (VLAN, DLCI, etc.)

   As the access network for an L2VPN service is necessarily a Layer 2
   network, it is preferable to use authentication mechanisms that do
   not presuppose any IP capabilities on the CE device.

   There are existing Layer 2 protocols and best current practices to
   guard against these security issues.  For example, IEEE 802.1x
   defines authentication at the link level for access through an
   ethernet bridge; the Frame Relay Forum defines LMI extensions for
   authentication (FRF.17).

4.3.  Customer Network Security Issues

   Even if all CE devices are properly authorized to attach to their PE
   devices, misconfiguration of the PE may interconnect CEs that are not
   supposed to be in the same L2VPN.

   In a VPWS, the CEs may run IPsec to authenticate each other.  Other
   Layer 3 or Layer 4 protocols may have their own authentication
   methods.

   In a VPLS, CE-to-CE IPsec is even more problematic, as IPsec does not
   well support the multipoint configuration that is provided by the
   VPLS service.

   There may be alternative methods for achieving a degree of CE-to-CE
   authentication, if the L2VPN signaling protocol can carry opaque
   objects between the CEs, either inband (over the L2VPN) or out-of-
   band, through the participation of the signaling protocol.  This is
   for further study.

   The L2VPN procedures do not provide authentication, integrity, or
   privacy for the customer's traffic; if this is needed, it becomes the
   responsibility of the customer.  For customers who really need these
   features or who do not trust their service providers to provide the
   level of security that they need, the L2VPN framework discussed in
   this document may not be satisfactory.  Such customers may consider
   alternative L2VPN schemes that are based not on an overlay of PWs,
   but on an overlay of IPsec tunnels whose endpoints are at the
   customer sites; however, such alternatives are not discussed in this
   document.

   If there is CE-to-CE control traffic (e.g., BPDUs) on whose integrity
   the customer's own Layer 2 network depends, it may be advisable to
   send the control traffic using some more secure mechanism than is
   used for the data traffic.

   In general, any means of mounting a denial of service attack on
   bridged networks generally can also be used to mount a denial of
   service attack on the VPLS service for a particular customer.  We
   have discussed here only those attacks that rely on features of the
   VPLS service that are not shared by bridged networks in general.

5.  Acknowledgements

   This document is the outcome of discussions within a Layer 2 VPN
   design team, all of whose members could be considered co-authors.
   Specifically, the co-authors are Loa Andersson, Waldemar Augustyn,
   Marty Borden, Hamid Ould-Brahim, Juha Heinanen, Kireeti Kompella,
   Vach Kompella, Marc Lasserre, Pascal Menezes, Vasile Radoaca, Eric
   Rosen, and Tissa Senevirathne.

   The authors would like to thank Marco Carugi for cooperation in
   setting up context, working directions, and taking time for
   discussions in this space; Tove Madsen and Pekka Savola for valuable
   input and reviews; and Norm Finn, Matt Squires, and Ali Sajassi for
   valuable discussion of the VPLS issues.

6.  Normative References

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

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

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

   [RFC4665]    Augustyn, W., Ed. and Y. Serbest, Ed., "Service
                Requirements for Layer 2 Provider-Provisioned Virtual
                Private Networks (L2VPNs)", RFC 4665, September 2006.

7. Informative References

   [IEEE8021D]  IEEE 802.1D-2003, "IEEE Standard for Local and
                Metropolitan Area Networks:  Media Access Control (MAC)
                Bridges"

   [IEEE8021Q]  IEEE 802.1Q-1998, "IEEE Standards for Local and
                Metropolitan Area Networks:  Virtual Bridged Local Area
                Networks"

   [RFC1771]    Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
                (BGP-4)", RFC 1771, March 1995.

   [RFC2661]    Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
                G., and B. Palter, "Layer Two Tunneling Protocol
                "L2TP"", RFC 2661, August 1999.

   [RFC2796]    Bates, T., Chandra, R., and E. Chen, "BGP Route
                Reflection - An Alternative to Full Mesh IBGP", RFC
                2796, April 2000.

   [RFC3036]    Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
                and B. Thomas, "LDP Specification", RFC 3036, January
                2001.

Authors' Addresses

   Loa Andersson
   Acreo AB

   EMail: loa@pi.se

   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719

   EMail: erosen@cisco.com

   Waldemar Augustyn

   EMail: waldemar@wdmsys.com

   Marty Borden

   EMail: mborden@acm.org

   Juha Heinanen
   Song Networks, Inc.
   Hallituskatu 16
   33200 Tampere, Finland

   EMail: jh@song.fi

   Kireeti Kompella
   Juniper Networks, Inc.
   1194 N. Mathilda Ave
   Sunnyvale, CA 94089

   EMail: kireeti@juniper.net

   Vach Kompella
   TiMetra Networks
   274 Ferguson Dr.
   Mountain View, CA 94043

   EMail: vach.kompella@alcatel.com

   Marc Lasserre
   Riverstone Networks
   5200 Great America Pkwy
   Santa Clara, CA 95054

   EMail: mlasserre@lucent.com

   Pascal Menezies

   EMail: pascalm1@yahoo.com

   Hamid Ould-Brahim
   Nortel Networks
   P O Box 3511 Station C
   Ottawa, ON K1Y 4H7, Canada

   EMail: hbrahim@nortelnetworks.com

   Vasile Radoaca
   Nortel Networks
   600 Technology Park
   Billerica, MA 01821

   EMail: radoaca@hotmail.com

   Tissa Senevirathne
   1567 Belleville Way
   Sunnyvale CA 94087

   EMail: tsenevir@hotmail.com

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