Internet DRAFT - draft-ietf-ppvpn-l2-framework
draft-ietf-ppvpn-l2-framework
Network Working Group Loa Andersson
Internet Draft
Expiration Date: August 2003 Eric Rosen
Cisco Systems, Inc.
Editors
February 2003
L2VPN Framework
draft-ietf-ppvpn-l2-framework-03.txt
Status of this Memo
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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.
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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 ................... 4
1.4 Terminology ........................................ 5
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 . 8
2.3.1 Entities in the Distributed PE Reference Models .... 8
2.4 VPWS-PE and VPLS-PE ................................ 8
3 Functional Components of L2 VPN .................... 9
3.1 Types of L2VPN ..................................... 9
3.1.1 Virtual Private Wire Service (VPWS) ................ 9
3.1.2 Virtual Private LAN Service (VPLS) ................. 10
3.1.3 IP-only LAN-like Service (IPLS) .................... 10
3.2 Generic L2VPN Transport Functional Components ...... 11
3.2.1 Attachment Circuits ................................ 11
3.2.2 Pseudowires ........................................ 11
3.2.3 Forwarders ......................................... 13
3.2.4 Tunnels ............................................ 14
3.2.5 Encapsulation ...................................... 15
3.2.6 Pseudowire Signaling ............................... 15
3.2.6.1 Point-to-Point Signaling ........................... 17
3.2.6.2 Point-to-Multipoint Signaling ...................... 17
3.2.6.3 Inter-AS Considerations ............................ 19
3.2.7 Service Quality .................................... 19
3.2.7.1 Quality of Service (QoS) ........................... 19
3.2.7.2 Resiliency ......................................... 20
3.2.8 Management ......................................... 21
3.3 VPWS ............................................... 21
3.3.1 Provisioning and Auto-Discovery .................... 22
3.3.1.1 Attachment Circuit Provisioning .................... 22
3.3.1.2 PW Provisioning for Arbitrary Overlay Topologies ... 23
3.3.1.3 Colored Pools PW Provisioning Model ................ 24
3.3.2 Requirements on Auto-Discovery Procedures .......... 26
3.3.3 Heterogeneous Pseudowires .......................... 27
3.4 VPLS Emulated LANs ................................. 28
3.4.1 VPLS Overlay Topologies and Forwarding ............. 30
3.4.2 Provisioning and Auto-Discovery .................... 32
3.4.3 Distributed PE ..................................... 33
3.4.4 Scaling issues in VPLS deployment .................. 35
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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 .......... 38
4.3 Customer Network Security Issues ................... 39
5 References ......................................... 39
6 Authors and Acknowledgments ........................ 42
7 Authors' Contact Information ....................... 43
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 (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 1.3).
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.
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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.
1.3. 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.
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1.4. 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].
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 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.
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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
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|-----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 "VPLS Forwarder" modules (one per VPLS-PE per VPLS
instance) connected by Pseudowires, where the Pseudowires may be
traveling through PSN tunnels over a routed backbone.
The functionality which the bridge module must support depends on the
service which is being offered by the SP to its customers, as well as
on various details of the SP's network. At 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, running spanning tree with each other.
Specification of the precise functionality which the bridge modules
must have in particular circumstances is, however, out of scope of
the current document.
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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 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 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 [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 pseudo-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.)
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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,
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].
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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 is indicated in Figure
3, 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 which 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 which is normal for bridges, 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;
- 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 [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
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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 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].
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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 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.
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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. 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
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"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.)
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.
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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 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.
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- 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 3.4.2) 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.
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-
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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]).
- 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.
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- 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
* 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].
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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.
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 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.
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- 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
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
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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.
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 (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.
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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 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
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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
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
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* 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 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
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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:
- 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.)
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- 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.)
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.
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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
for ATM, with the Interworking function occurring between the PW
and the FR AC.
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- 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 [SHAH-INTER].
3.4. VPLS Emulated LANs
A VPLS is an L2VPN service in which:
- the ACs attach CE devices to PE bridge modules,
- 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 which 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.
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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 need 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 in which maps MAC addresses to
PWs. The VSI is populated in much the same was as a standard bridge
populates its forwarding table. The VPLS Forwarders do MAC SA
learning on frames received on PWs from other Forwarders, and must
also do the related set of procedures, such as the aging out of
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 which it
receives on its ACs, it only learns the MAC SAs of frames which
are received on PWs from other VPLS Forwarders;
- 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 which are on a given Emulated LAN may
or may not run 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 does arise in the case where the PE bridges do run
bridge control protocols with each other over the Emulated LAN.
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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.
VPLS solutions are found e.g. in [DNS-LDP-VPLS], [LASSERRE-VKOMPELLA-
VPLS] and [KOMPELLA-VPLS].
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 which carry that MAC address in the DA
field should be sent to the VSI which 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 which 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:
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- 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 unknown DA, it send 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 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 pseudowire from a lower level, and frames arriving
over PWs from the same level cannot be sent on PWs to the same
level.
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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. 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
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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.
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
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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 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:
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- 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 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
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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 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 [SHAH-INTER] 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.
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 [SHAH-INTER].
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4. Security Considerations
The security considerations section of the L2VPN requirements
document [L2VPN-REQ] 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 should either be configured with its peers, or 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 which they
provide will be lower.
Thus for inter-SP control connections, it is advisable to use some
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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.
When LDP [RFC3036] is used as the control protocol, the LDP security
considerations discussed in [RFC3036] are applicable. Note that LDP
uses both UDP as well as TCP. It may be advisable to have some
protection against spoofed UDP messages which appear to be from a
valid peer; this requires further study.
Future versions of this document will discuss the security
considerations that apply to other signaling protocols.
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.
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:
- Preventing unauthorized members joining an L2VPN
- Ensuring correct customer interface configured
- Ensuring correct service delimiting fields (VLAN, DLCI, etc.)
- Preventing unauthorized access to PE
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
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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 which 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.
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.
5. References
[RFC2026]
Bradner, S. "The Internet Standards Process -- Revision 3", RFC
2026, October 1996.
[RFC2119]
Bradner, S. "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[RFC3036]
Andersson, L et.al. "LDP Speficification", RFC 3036, Jan 2001
[ANDERSSON-METRICS]
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Internet Draft draft-ietf-ppvpn-l2-framework-03.txt February 2003
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
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-
Andersson, et al. [Page 40]
Internet Draft draft-ietf-ppvpn-l2-framework-03.txt February 2003
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.
[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
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Internet Draft draft-ietf-ppvpn-l2-framework-03.txt February 2003
[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. Authors and Acknowledgments
This document is the outcome of discussions within a PPVPN Layer 2
VPN design team, all of whose members could be considered to be 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 for valuable input and
reviews, and Norm Finn, Matt Squires, and Ali Sajassi for valuable
discussion of the VPLS issues.
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7. Authors' Contact Information
Loa Andersson
Email: loa@pi.se
Eric C. Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
Email: erosen@cisco.com
Waldemar Augustyn
Email: waldemar@nxp.com
Marty Borden
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: vkompella@timetra.com
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Marc Lasserre
Riverstone Networks
5200 Great America Pkwy
Santa Clara, CA 95054
Email: marc@riverstonenet.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: vasile@nortelnetworks.com
Tissa Senevirathne
1567 Belleville Way
Sunnyvale CA 94087
Email: tsenevir@hotmail.com
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