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Versions: (draft-andersson-ppvpn-l2-framework) 00 01 02 03

PPVPN Working Group                                     Loa Andersson
Internet-Draft                                             Utfors AB
                                                  Design team editor

Expiration Date: January 2003

                                                        01 August, 2002

                         PPVPN L2 Framework
                <draft-ietf-ppvpn-l2-framework-00.txt>

Status of this Memo

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

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

For potential updates to the above required-text see:
http://www.ietf.org/ietf/1id-guidelines.txt

Summary for Sub-IP related Internet Drafts

RELATED DOCUMENTS:

This being a Layer 2 vpn framework document, almost every document that
has been sent to the ppvpn working group is related, at least in that
they address provider provisioned vpn's. Even more closely related are
the documents that address L2 vpn's. The reference section includes a
list of the document we found that most useful to illustrate the issues
we discuss in this document.

WHERE DOES IT FIT IN THE PICTURE OF THE SUB-IP WORK

This ID is intended for the PPVPN WG.



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WHY IS IT TARGETED AT THIS WG(s)

PPVPN deals with provider provisioned VPNs. This document provides and a
framework and architecture for Layer 2 Provider Provisioned Virtual
Private Network services, a class of Provider Provisioned Virtual
Private Networks services.

JUSTIFICATION

This document is a framework for Layer 2 VPNs, one of the main topics on
the PPVPN WG charter, and is considered instrumental in progressing the
standards work within the PPVPN group.

Abstract

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

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].

Contents

1.  Introduction........................................................ 4
  1.1  Objectives and Scope of the Document ............................ 4
  1.2  Layer 2 Virtual Private Networks ................................ 5
  1.3  Terminology ..................................................... 5

2.  Models.............................................................. 6
  2.1  Reference Model for VPWS ........................................ 6
  2.2  Reference Model for VPLS ........................................ 6
  2.3  Reference Model for distributed  VPLS-PE or VPWS-PE.............. 7
  2.4  VPWS-PE and VPLS-PE ............................................. 8

3.  Functional Components of L2 VPN .................................... 8
  3.1  Types of L2VPN................................................... 8
  3.2  Generic L2VPN Transport Functional Components................... 10
  3.3  VPWS............................................................ 20
  3.4  VPLS............................................................ 26
  3.5  IP-only LAN-like Service (IPLS) ................................ 32

4.  Security Considerations ........................................... 33
  4.1  System security ................................................ 33
  4.2  Access Control.................................................. 33


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     4.3  Endpoint authentication ..................................... 33
     4.4  Data Integrity............................................... 33
     4.5  Confidentiality ............................................. 33
     4.6  User data and Control data .................................. 33

5.  References......................................................... 33

6.  Acknowledgements .................................................. 36

7.  Authors Contact ................................................... 37




































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1.  Introduction

1.1  Objectives and Scope of the Document

This document provides a framework for Layer 2 Provider Provisioned
Virtual Private Networks (PPVPNs).  This framework is intended to aid in
standardizing protocols and mechanisms to support interoperable Layer2
PPVPNs.

The PPVPN WG group works with both Layer 3 PPVPNs and Layer 2 PPVPNs. A
framework for L3 VPNs is found in [L3VPN-FW]. This document provides the
same type of framework for Layer 2 PPVPNs as the Layer 3 framework does
for Layer 3 PPVPNs.

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.

There are multiple ways in which a provider can participate in a VPN,
and there are therefore multiple different types of PPVPNs.  The
framework document discusses Layer2 VPNs (as defined in section 1.2).
It also describes technical issues related to VPNs in which the provider
participates in provisioning for provider edge and customer edge
devices.

First, this document discusses reference models for Layer 2 PPVPNs. Then
the functional components of Layer2 PPVPN operations are discussed.

Specifically, this includes discussion of the technical issues, which
are important in the design of standards and mechanisms for support of
Layer 2 PPVPNs.  Furthermore, technical aspects of Layer2 PPVPNs
interworking is clarified.  Finally, security issues as they apply to
Layer2 PPVPNs are addressed.

Requirements for Layer 3 VPNs are found in [L3VPN-REQ] and for Layer 2
VPNs, for VPLS and VPWS, in [L2VPN-REQ].

This document has "inherited" a substantial content from "An
Architecture for L2VPNs" [L2VPN-ARCH].

This document does not make choices, and does not select any particular
approach to support VPNs.




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1.2  Layer 2 Virtual Private Networks

As Layer 2 provider provisioned VPN solutions has attracted more and
more interest, several solutions has been proposed to the PPVPN WG. This
document addresses the generic components relevant for every Layer 2 VPN
but will not make any recommendations on the relative merits of how the
different components are implemented.

In [ANDERSSON-METRICS] parameters and metrics that could be used to
compare different Layer 2 VPN solutions and how they could be evaluated
when a L2 VPN has to meet different set of requirements is discussed.
The parameters to be considered in evaluating L2 VPN implementations in
different environments are e.g. scaling, cost, inter-domain
reachability, provisioning, flexibility, integration and migration from
existing infrastructure and services, value-added services, cost, etc.

Currently we see two kinds of services that a service provider could
offer to a customer by means of Layer 2 VPNs. Virtual Private Wire
Service(VPWS)and a Virtual Private LAN Service (VPLS). The possibility
of an IP-only LAN-like Service (IPLS) is opened up, but is very much for
future study.

A VPWS is a VPN service that supplies a L2 point-to-point service. Being
a point-to-point service where 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 in all respects emulates LAN across a Wide
Area Network (WAN). Thus it also has all 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.

1.3   Terminology

This document list some terms and concepts that are specific to the L2
VPN framework, terms and concepts generally applicable to the PPVPN area
will be found in [ANDERSSON-TERM].












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2.  Models

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



2.1.1 Entities in the VPWS reference model

The P, PE (VPWS-PE) and CE devices and the PSN tunnel as defined in
[ANDERSSON-TERM]. Attachment circuit and pseudo wire as discussed in
section 3. The PE does a simple mapping between the PW and attachment
circuit based on local information, i.e. the PW de-multiplexor 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 connected by a single logical
Ethernet bridge. A VPLS can contain a single VLAN or multiple, tagged
VLANs.








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        +-----+                                  +-----+
      + CE1 +--+                           +---| CE2 |
      +-----+  |    ...................    |    +-----+
       VPLS A  |  +----+           +----+  |    VPLS A
               |  |VPLS|           |VPLS|  |
               +--| PE |--Service--| PE |-+
                  +----+  Provider +----+
                 /  .      Backbone    .  \      -   /\-_
      +-----+   /   .       |          .   \   / \ /   \     +-----+
      + CE   +--+    .       |          .    +-- \ Access \----| CE  |
      +-----+       .     +----+       .       | Network |    +-----+
       VPLS B       .....|VPLS|........         \       /     VPLS B
                         | PE |     ^           -------
                         +----+     |
                           |        |
                           |        |
                        +-----+     |
                        | CE3 |     +-- Logical bridge
                        +-----+
                         VPLS A

This reference model is adapted from [L2VPN-REQ].  The only difference
is that the VPLS-PE is explicitly named.

2.2.1 Entities in the VPLS reference model

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

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 VPLS-PE or VPWS-PE reference
     model

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 User facing PE



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(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 U-PE and N-PE are defined in [ANDERSSON-TERM].

2.4  VPWS-PE and VPLS-PE

The VPWS-PE and VPLS-PE are functionally very similar, the they both use
forwarders to map attachment circuits to pseudo-wires. The only
differences is that while the forwarder in a VPWS-PE does a one-to-one
mapping between the attachment circuit and psedo-wire, the forwarder in
a VPLS-PE is a Virtual Switching Instance (VSI) that maps multiple
attachment circuits to multiple pseudo-wires (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
allows us to exhibit the roles played by the various protocols and
mechanisms, and thus to make it easier to understand the differences and
similarities between various proposed L2VPN architectures.

Section 3.1 contains an overview of some different types of L2VPN.  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, as well as
discussing 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,



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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 [L2VPN-REQ].

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.

This is determined using standard LAN bridging techniques, such as MAC
Source Address Learning. (Thus unlike VPWS, VPLS allows the use of
addressing information in a frame's L2 header to determine the CE to
which a frame should be sent.)  This allows a LAN to be extended
transparently over an MPLS and/or IP backbone.

VPLS is like VPWS in that forwarding is done without any consideration
of the Layer3 header. Unlike VPWS, VPLS allows a single CE/PE connection
to be used for transmitting frames to multiple remote CEs.  In this
respect, VPLS is more like L3VPN.

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

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

  -  it is assumed that the service will only carry IP packets, and
     supporting packets such as ICMP and ARP; Layer2 packets which 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.



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3.2  Generic L2VPN Transport Functional Components

All L2VPN types must transport "frames" across the core network
connecting the PE's. In all L2VPN types, a PE (PE1) receives a frame
from a CE (CE1), 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, which 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 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 [PWE3-
FW].

Setting up and maintaining the PWs is the job of the PEs.  State
information for a particular PW is maintained at the two PEs which 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


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to be bidirectional; multipoint-to-point and point-to-multipoint PWs are
always considered to be unidirectional. Multipoint-to-point PWs can be
used only when the PE receiving a frame from the PW does not need to
know where the frame came from. 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, 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 RFC2684 "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 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).

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.


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However, as this case does not require any protocol, we do not further
address it 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 which 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 which 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.





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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.)

There are a variety of different tunneling technologies which 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.


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It may be necessary to define additional PW encapsulations to cover
areas that are of importance for L2VPN, but 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 rather than 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 which 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 (see
        sections 3.3.1 and 3.4.1) being used.  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.

  -  Support pseudowire emulations.




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        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.

  -  Establish 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 to have been set up.
If it is necessary to negotiate further characteristics or parameters of
a particular PW, or to passing 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 extensions can be defined for pseudowire signaling. See for
        example [MARTINI-SIGNALING], [ROSEN-L2-SIGNALING].  This form of
        signaling can be used for pseudowires which are to be carried in
        MPLS "tunnels", or in MPLS-in-something -else tunnels (e.g.,
        [MPLS-IP], [MPLS-GRE]).


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  -  L2TP

          L2TP [L2TP-BASE] 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, e.g., see [L2TP-FR].

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 situation:

  -  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 a point-to-point
PW. Call this the "Multipoint Condition".

If:

  -  The Environmental Conditions hold, and

  -  Either

          *  the Demultiplexor Condition holds, or


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        *  the Multipoint Condition holds,

then for a given set of PWs which terminate at egress PE1, the
information which 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 connecting to each ingress PE and replicating 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 to do the signaling, with
the PEs being BGP peers not of each other, but of one or more BGP route
reflectors.

Such a scheme, based Multi-protocol Extensions to BGP, is proposed
in [KOMPELLA-L2VPN].

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 means that the
signaling to set up the PEs must be able to cross network boundaries.
All known proposals for signaling are able to do this. It is especially
advisable to use some form of authentication between the two PW
endpoints in this case.

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.




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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
customer can be prioritized, policed and shaped for QoS requirements.
The queuing and forwarding po licies 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.



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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
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
         should be detected immediately.

  -  Resiliency policy: The way in which a detected failure is handled
         will depend upon 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 while 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.





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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 n o role in the forwarding decision, except
insofar as the Layer2 header contents are used to associate the frame
with a particular AC (as, 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.

VPWS solutions are found e.g. in [DIRLDP], [KOMPELLA-L2VPN] and [ROSEN-
L2-SIGNALING].

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.

  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



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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.  E.g., if an AC is just an MPLS LSP running between a CE
and a PE, it could be set up as the RESULT of a PW being set up, 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 which is to be at
           the remote end of the PW)

        *  Identifier which is meaningful to the remote PE, and which
           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 pseudowire (as in [MARTINI-SIGNALING]), or
           an identifier of the remote AC (as in [ROSEN-L2-SIGNALING]).
           If a PW identifier is used, it must be unique at each of the



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             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.  (See, for example, [ROSEN-L2-
          SIGNALING].)

          This scheme requires provisioning of the PW at only one PE, but
          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 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 which is
bound at PE1 to an arbitrarily chosen AC from that pool, and at PE2 to


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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, with the ACs in the pool being 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

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

This sort of technique is discussed in [ROSEN-L2-SIGNALING].

Another possible signaling technique is the following:



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  -  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

            *  binds the AC to the PW which 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
sort of scheme is discussed in [KOMPELLA-L2VPN].

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, and

  -  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 perhaps not
     always possible.)





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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.

Examples of suitable auto-discovery procedures can be found in Examples
of suitable auto-discovery procedures can be found in [KOMPELLA-L2VPN],
[BGP-AUTO] and [ROSEN-L2-SIGNALING], and [DNS-LDP-VPLS].

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., the 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


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     for ATM, with the Interworking function occurring between the PW
     and the FR AC.

  -  A common encapsulation can be used on both ACs, e.g., if 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 the processing of the Ethernet over FR encapsulation
     being 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 which 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 such procedures 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 for point-to-point interfaces,
     additional mechanisms may be required to ensure proper
     interworking.  These issues are discussed in Fel! Hittar inte
     referensk„lla..

3.4  VPLS

A VPLS is an L2VPN service in which:

  -  The Forwarders bind multiple ACs to multiple PWs

  -  Each Forwarder behaves as a "Virtual Switch Instance" (VSI), which
     performs standard LAN (i.e., Ether net) bridging functions. These
     include maintenance of a forwarding table by means of MAC address
     learning, and broadcasting of frames with unknown MAC Destination
     Addresses.

An AC connects a CE to a VSI.  Multiple CEs may be connected to a single
VSI.  The payload on the ACs must be Ethernet frames, with or without
VLAN headers. An AC may run over any medium that can carry Ethernet
frames, either natively or in some encapsulation.

The set of VSIs within a single VPLS are connected via PWs; two VSIs
will have a PW between them only if those two VSIs are part of the same
VPN. There may be a further restriction that two VSIs have a PW between
them only if those two VSIs are part of the same VLAN in the same VPN.

When the CE device is itself a LAN switch, the VSI may or may not be
visible as a LAN switch to the CE.  That is, it may send and receive
BPDUs to and from the CE, or it may simply pass a CE's BPDUs to the



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other CEs, without ever sending a BPDU of its own to the CE.  Different
VPLS solutions may differ in this respect.

VPLS solutions are found e.g. in [DNS-LDP-VPLS], [LASSERRE-VKOMPELLA-
VPLS] and [KOMPELLA-VPLS].

3.4.1 VPLS Overlay Topologies and Forwarding

A VPLS emulates a LAN, in that all frame forwarding decisions are based
only on the frame's MAC Destination Address (DA), the frame's "incoming
port", and the contents of the forwarding table.  For this purpose, both
PWs and ACs are considered to be ports.  The VSI forwarding decision
maps a MAC DA and incoming port to an outgoing port.

In order to use MAC address learning to populate the forwarding table,
the PWs must be point-to-point or point-to-multipoint PWs. (Point -to-
multipoint PWs may be useful when it is necessary to multicast a frame;
the alternative would be replication of the frame by the PE, and
transmission of each replica over a set of point-to-point PWs.)  There
is no use for multipoint-to-point PWs.

MAC learning over a point-to-point PW is done via standard techniques,
considering the PW to be a port.  But MAC addresses learned over a
point-to-multipoint PW whose root is PE1 would have to be treated as if
they had been learned over the point-to-point PW which comes from PE1.

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 frame is received over a PW, do not
        forward it over ANY other PW.

        Multicast and unknown DA packets are replicated and sent over
        all ports other than the one from which they were received.
        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



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          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 to be 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
          which 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 pseudo wire from a lower level, and 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 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.
Improper configuration of the customer LAN (outside the limits of the
VPLS) may cause looping, 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.




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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.

Examples of suitable auto-discovery procedures can be found in
[KOMPELLA-L2VPN], [BGP-AUTO] and [ROSEN-L2-SIGNALING], and [DNS-LDP-
VPLS].

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 individually provisioned.

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, e.g., there are
multiple customers occupying the same building. However, this device is
really part of the SP's network, hence may be considered to be a PE
device.

In some scenarios when 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
in a situation there 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 only
performing aggregation and none of the function associated with a VPLS.




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Relative to the VPLS there are three different possibilities to 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 [ANDERSSON -TERM]. 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 e.g. 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 User-Facing PE (U-PE) and the high-end device 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. In [KOMPELLA-
DTLS], these are referred to as Multi-Tenant Units, but the resulting
acronym is already used for something else. In [SAJASSI-VPLS] this type
of device is called PE-CLE. [MOHAN-LPE] introduces another yet another
naming scheme, the U-PE is called PE-Edge and the N-PE is called PE-
Core.

The N-PE, in [KOMPELLA-DTLS] called L2PE and in [SAJASSI-VPLS] called
PE-POP, 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. E.g., perhaps the N-PE is a router
     which 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



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     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 ([KOMPELLA-DTLS], [MOHAN -LPE], [SAJASSI-VPLS]). 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 VPLS's 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-PE to participate in BGP auto-discovery, e.g., but
     perhaps they would be expected to participate in DNS auto-
     discovery.

  -  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 the the N-PE
     send the the U-Pes the labels that the U-PE needs to send traffic




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     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 every other VPLS-PE participating in the same VPLS instance.
Strictly, VPLS-PE's 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. An solution addressing the scaling problems was
addressed in an Internet Draft by S Khandekar et.al. called
"Hierarchical This has been addressed "Hierarchical Virtual Private LAN
Service", this work has latter been included in [LASSERRE-VKOMPELLA-
VPLS].

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 using address learning in the data
plane to learn the MAC addresses, the PE can use the control plane to
learn the MAC address. (See Fel! Hittar inte referensk„lla. for a
discussion of this.) 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. Each PE would perform
proxy ARP to its directly attached CEs.

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.


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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 Layer2 wrappers. However, this might
require "translation" between "ARP" and "Inverse ARP". The set of
routing protocols which could be carried across the IPLS might also be
restricted. A fuller discussion of the advantages, disadvantages, and
restrictions may be found in  Fel! Hittar inte referensk„lla..

4.  Security Considerations

Security considerations will be addressed in a future version of this
document.

4.1   System security

This is for a future version of this document.

4.2   Access Control

This is for a future version of this document.

4.3   Endpoint authentication

This is for a future version of this document.

4.4   Data Integrity

This is for a future version of this document.

4.5   Confidentiality

This is for a future version of this document.

4.6  User data and Control data

This is for a future version of this document.

5.  References

[rfc2026]
     Bradner, S. "The Internet Standards Process -- Revision 3", RFC
     2026, October 1996.




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[rfc2119]
     Bradner, S. "Key words for use in RFCs to Indicate Requirement
     Levels", RFC 2119, March 1997.

[ANDERSSON-METRICS]
     Andersson, L. "Parameters and related metrics to compare PPVPN
     Layer 2 solutions", draft-andersson-ppvpn-metrics -00.txt, Work
     in Progrss, Internet Draft, Feb 2002.

[ANDERSSON-TERM]
     Andersson, L. and Madsen T. "VPN Terminology", draft-andersson-
     ppvpn-terminology-00.txt", Work in Progress, Internet Draft,
     Feb 2002.

[BGP-AUTO]
     Ould-Brahim, H. et.al. "Using BGP as an Auto-Discovery
     Mechanism for Network-based VPNs", Ould-Brahim et al, draft-
     ietf-ppvpn-bgpvpn-auto-01.txt, Work in Progress, Internet
     Draft, Nov 2001

[DIRLDP]
     Heinanen, J, "Directory/LDP Based Unidirectional Virtual
     Circuit VPNs" draft-heinanen-dirldp-uni-vc-vpns-01.txt, Work in
     Progress, Internet Draft, Nov 2001

[DNS-LDP-VPLS]
     Heinanen, J, "DNS/LDP Based VPLS", draft-heinanen -dns-ldp-vpls-
     00.txt, Work in Progress, Internet Draft, Jan 2002

[KOMPELLA-DTLS]
     Kompella, K et.al. "Decoupled Virtual private LAN Services",
     draft-kompella-ppvpn-dtls-01.txt, December 2001

[KOMPELLA-L2VPN]
     Kompella, K. et.al. "Layer 2 VPNs Over Tunnels", draft-
     kompella-ppvpn-l2vpn-01.txt, Nov 2001

 [KOMPELLA-VPLS]
     Kompella, K. "Virtual Private LAN Service", draft -kompella-
     ppvpn-vpls-00.txt, Work in Progress, Internet Draft, Nov 2001

[L2TP-BASE]
     Lau, J. "Layer Two Tunneling Protocol (Version 3) L2TPv3"
     draft-ietf-l2tpext-l2tp-base-02.txt", Work in Progress,
     Internet Draft, March 2002.

[L2TP-FR]
     Townsley, W. M. et.al. ""Frame Relay Pseudowire Extensions for



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     L2TP", draft-ietf-l2tpext-pwe3-fr-00.txt, Work in Progress,
     Internet Draft, Feb 2002.

[L2VPN-ARCH]
     Rosen, E. et.al. "An Architecture for L2VPNs", draft-ietf-
     ppvpn-l2vpn-00.txt, Work in Progress, Internet Draft Jul 2001.

[L2VPN-REQ]
     Augustyn, W. et.al "Requirements for Layer 2 Virtual Private
     Network Services (L2VPN)", draft-augustyn-ppvpn-l2vpn-
     requirements-00.txt, Work in Progress, Internet Draft, June
     2002.

[L3VPN-FW]
     Callon, R. et.al. "A Framework for Layer 3 Provider Provisioned
     Virtual Private Networks", draft-ietf-ppvpn-framework-05.txt,
     Work in Progress, Internet Draft, April 2002

[L3VPN-REQ]
     Carugi, M. et.al. "Service requirements for Layer 3 Provider
     Provisioned Virtual Private Networks" draft-ietf-ppvpn-
     requirements-04.txt, Work in Progress, Internet Draft, Feb
     2002.

[LASSERRE-VKOMPELLA-VPLS]
     Lasserre, M. et.al. "Virtual Private LAN Services over MPLS",
     draft-lasserre-vkompella-ppvpn-vpls-01.txt, Work in progress,
     Internet Draft, Mar 2002.

[MARTINI-SIGNALING]
     Martini, L. et.al. "Transport of Layer 2 Frames Over MPLS",
     draft-martini-l2circuit-trans-mpls -08.txt, Work in Progress,
     Internet Draft, Nov 2001

[MOHAN-LPE]
     Mohan, D. et.al. "VPLS/LPE L2VPNs: Virtual Private LAN Services
     using Logical PE Architecture ", draft-ouldbrahim -l2vpn-lpe-
     02.txt, Work in Progress, Internet Draft, Mar 2002

[MPLS-GRE]
     Rekhter, Y. "MPLS Label Stack Encapsulation in GRE", Rekhter et
     al, draft-rekhter-mpls-over-gre-03.txt, Work in Progress,
     Internet Draft Sep, 2001.

[MPLS-IP]
     Worster, T. et.al. "MPLS Label Stack Encapsulation in IP",
     Worster et al,   draft-worster-mpls-in-ip-05.txt, Work in
     Progress, Internet Draft, Jul 2001.



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[PWE3-FW]
     Pate, P. editor, "Framework for Pseudo Wire Emulation Edge-to-
     Edge", Pate, P. editor, draft-ietf -pwe3-framework -00.txt, Work
     in Progress, Internet draft, Feb 2002

[ROSEN-L2-SIGNALING]
     Rosen, E. "Single-Sided Signaling for L2VPNs", draft-rosen-
     ppvpn-l2-signaling-01.txt, Work in Progress, Internet Draft,
     Feb 2002

 [SAJASSI-VPLS]
     Sajassi, A. "VPLS Architectures", draft-sajassi-vpls-
     architectures-00.txt , Work in Progress, Internet Draft, Feb
     2002.

[SHAH-INTER]
     Shah, H. et.al. "IP address resolution for IP interworking of
     Layer 2 VPN", draft-shah-l2vpn-arp -resolve-00.txt, Work in
     Progress, Internet Draft, Jan 2002

[SHAH-SIG]
     Shah, H. et.al. "Signaling between PE and L2PE/MTU for
     Decoupled VPLS and Hierarchical VPLS", draft-shah -ppvpn-vpls -
     pe-mtu-signaling-00.txt, Work in Progress, Internet Draft, Feb
     2002.

6.  Acknowledgements

This document is the outcome of discussions within the PPVPN Layer 2 VPN
design team. The members of the design team are

Eric Rosen,        Cisco Systems        erosen@cisco.com
Hamid Ould-Brahim, Nortel Networks      hbrahim@nortelnetworks.com
Juha Heinanen,     Song Networks        jh@lohi.eng.song.fi
Kireeti Kompella,  Juniper Networks     kireeti@juniper.net
Loa Andersson,     Utfors AB,           loa.andersson@utfors.se
Marc Lasserre,     Riverstone Networks  marc@riverstonenet.com
Marty Borden,      Atrica               mborden@atrica.com
Pascal Menezes,    Terrabeam            Pascal.Menezes@Terabeam.com
Vach Kompella,     Timetra Networks     vkompella@timetra.com
Vasile Radoaca     Nortel Networks      vasile@nortelnetworks.com
Waldemar Augustyn,                      waldemar@nxp.com

The team would like to thank Marco Carugi for cooperation in setting up
context, working directions and taking time for discussions in this
space. The team would also like to thank Tove Madsen for valuable input
and reviews.



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7.  Authors Contact

Loa Andersson (editor)
Utfors AB
P.O Box 525
SE-169 29 Solna
tel: +46 8 52 70 50 38
email: loa.andersson@utfors.se





































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