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Network Working Group L. Andersson, Ed.
Request for Comments: 4664 Acreo AB
Category: Informational E. Rosen, Ed.
Cisco Systems, Inc.
September 2006
Framework for Layer 2 Virtual Private Networks (L2VPNs)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document provides a framework for Layer 2 Provider Provisioned
Virtual Private Networks (L2VPNs). This framework is intended to aid
in standardizing protocols and mechanisms to support interoperable
L2VPNs.
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................3
1.2. Objectives and Scope of the Document .......................3
1.3. Layer 2 Virtual Private Networks ...........................3
1.4. Terminology ................................................4
2. Models ..........................................................5
2.1. Reference Model for VPWS ...................................5
2.1.1. Entities in the VPWS Reference Model ................5
2.2. Reference Model for VPLS ...................................6
2.2.1. Entities in the VPLS Reference Model ................8
2.3. Reference Model for Distributed VPLS-PE or VPWS-PE .........9
2.3.1. Entities in the Distributed PE Reference Models .....9
2.4. VPWS-PE and VPLS-PE ........................................9
3. Functional Components of L2 VPN .................................9
3.1. Types of L2VPN ............................................10
3.1.1. Virtual Private Wire Service (VPWS) ................10
3.1.2. Virtual Private LAN Service (VPLS) .................10
3.1.3. IP-Only LAN-Like Service (IPLS) ....................11
3.2. Generic L2VPN Transport Functional Components .............11
3.2.1. Attachment Circuits ................................11
3.2.2. Pseudowires ........................................12
3.2.3. Forwarders .........................................14
3.2.4. Tunnels ............................................15
3.2.5. Encapsulation ......................................16
3.2.6. Pseudowire Signaling ...............................16
3.2.6.1. Point-to-Point Signaling ..................18
3.2.6.2. Point-to-Multipoint Signaling .............18
3.2.6.3. Inter-AS Considerations ...................19
3.2.7. Service Quality ....................................20
3.2.7.1. Quality of Service (QoS) ..................20
3.2.7.2. Resiliency ................................21
3.2.8. Management .........................................22
3.3. VPWS ......................................................22
3.3.1. Provisioning and Auto-Discovery ....................23
3.3.1.1. Attachment Circuit Provisioning ...........23
3.3.1.2. PW Provisioning for Arbitrary
Overlay Topologies ........................23
3.3.1.3. Colored Pools PW Provisioning Model .......25
3.3.2. Requirements on Auto-Discovery Procedures ..........27
3.3.3. Heterogeneous Pseudowires ..........................28
3.4. VPLS Emulated LANs ........................................29
3.4.1. VPLS Overlay Topologies and Forwarding .............31
3.4.2. Provisioning and Auto-Discovery ....................33
3.4.3. Distributed PE .....................................33
3.4.4. Scaling Issues in VPLS Deployment ..................36
3.5. IP-Only LAN-Like Service (IPLS) ...........................36
4. Security Considerations ........................................37
4.1. Provider Network Security Issues ..........................37
4.2. Provider-Customer Network Security Issues .................39
4.3. Customer Network Security Issues ..........................39
5. Acknowledgements ...............................................40
6. Normative References ...........................................41
7. Informative References .........................................41
1. Introduction
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.2. Objectives and Scope of the Document
This document provides a framework for Layer 2 Provider Provisioned
Virtual Private Networks (L2VPNs). This framework is intended to aid
in standardizing protocols and mechanisms to support interoperable
L2VPNs.
The term "provider provisioned VPNs" refers to Virtual Private
Networks (VPNs) for which the Service Provider (SP) participates in
management and provisioning of the VPN.
Requirements for L2VPNs can be found in [RFC4665].
This document provides reference models for L2VPNs and discusses the
functional components of L2VPNs. Specifically, this includes
discussion of the technical issues that are important in the design
of standards and mechanisms for L2VPNs, including those standards and
mechanisms needed for interworking and security.
This document discusses a number of different technical approaches to
L2VPNs. It tries to show how the different approaches are related,
and to clarify the issues that may lead one to select one approach
instead of another. However, this document does not attempt to
select any particular approach.
1.3. Layer 2 Virtual Private Networks
There are two fundamentally different kinds of Layer 2 VPN service
that a service provider could offer to a customer: Virtual Private
Wire Service (VPWS) and Virtual Private LAN Service (VPLS). There is
also the possibility of an IP-only LAN-like Service (IPLS).
A VPWS is a VPN service that supplies an L2 point-to-point service.
As this is a point-to-point service, there are very few scaling
issues with the service as such. Scaling issues might arise from the
number of end-points that can be supported on a particular PE.
A VPLS is an L2 service that emulates LAN service across a Wide Area
Network (WAN). With regard to the amount of state information that
must be kept at the edges in order to support the forwarding
function, it has the scaling characteristics of a LAN. Other scaling
issues might arise from the number of end-points that can be
supported on a particular PE. (See Section 3.4.4.)
Note that VPLS uses a service that does not have native multicast
capability to emulate a service that does have native multicast
capability. As a result, there will be scalability issues with
regard to the handling of multicast traffic in VPLS.
A VPLS service may also impose longer delays and provide less
reliable transport than would a native LAN service. The standard LAN
control protocols may not have been designed for such an environment
and may experience scaling problems when run in that environment.
1.4. Terminology
The list of the technical terms used when discussing L2VPNs may be
found in the companion document [RFC4026].
2. Models
2.1. Reference Model for VPWS
The VPWS reference model is shown in Figure 1.
Attachment PSN Attachment
Circuits tunnel Circuits
+
+-----+ pseudo +-----+
| | wire | |
| CE1 |--+ +--| CE2 |
| | | +-----+ +-----+ +-----+ | | |
+-----+ +----|---- | | P | | ----+----+ +-----+
|VPWS\---|-----|-----|/VPWS|
| PE1 |===|=====|=====| PE2 |
| /|---|-----|-----|\\ |
+-----+ +----|---- | | | | ----|----+ +-----+
| | | +-----+ +-----+ +-----+ | | |
| CE3 |--+ +--| CE4 |
| | | |
+-----+ +-----+
Figure 1
2.1.1. Entities in the VPWS Reference Model
The P, PE (VPWS-PE), and CE devices and the PSN tunnel are defined in
[RFC4026]. The attachment circuit and pseudowire are discussed in
Section 3. The PE does a simple mapping between the PW and
attachment circuit based on local information; i.e., the PW
demultiplexor and incoming/outgoing logical/physical port.
2.2. Reference Model for VPLS
The following diagram shows a VPLS reference model where PE devices
that are VPLS-capable provide a logical interconnect such that CE
devices belonging to a specific VPLS appear to be on a single bridged
Ethernet. A VPLS can contain a single VLAN or multiple tagged VLANs.
The VPLS reference model is shown in Figures 2 and 3.
+-----+ +-----+
+ CE1 +--+ +---| CE2 |
+-----+ | ................... | +-----+
VPLS A | +----+ +----+ | VPLS A
| |VPLS| |VPLS| |
+--| PE |--Routed---| PE |-+
+----+ Backbone +----+
/ . | . \ _ /\_
+-----+ / . | . \ / \ / \ +-----+
+ CE +--+ . | . +--\ Access \----| CE |
+-----+ . +----+ . | Network | +-----+
VPLS B .....|VPLS|........ \ / VPLS B
| PE | ^ -------
+----+ |
| |
| |
+-----+ |
| CE3 | +-- Emulated LAN
+-----+
VPLS A
Figure 2
|-----Routed Backbone-----|
| (P Routers) |PSN Tunnels,
Emulated LAN | |Pseudowires
.......................................................................
. | | .
. |---------------------|----| |--------|-----------------| .
. | --------------------|--- | | -------|---------------- | .
. | VPLS Forwarder | | VPLS Forwarder | .
. | ----------|------------- | | ----------|------------- | .
..|.................................................................|..
| | Emulated LAN | | | Emulated LAN |
| | Interface | VPLS-PEs | | Interface |
| | | <----> | | |
| ----------|------------ | | ----------|------------ |
| | Bridge | | | | Bridge | |
| -|--------|---------|-- | | ---|-------|---------|- |
|--|--------|---------|----| |----|-------|---------|---|
| | | | | |
| | Access | | | Access |
| | Networks| | | Networks|
| | | | | |
| | | | | |
CE devices CE devices
Figure 3
From Figure 3, we see that in VPLS, a CE device attaches, possibly
through an access network, to a "bridge" module of a VPLS-PE. Within
the VPLS-PE, the bridge module attaches, through an "Emulated LAN
Interface", to an Emulated LAN. For each VPLS, there is an Emulated
LAN instance. Figure 3 shows some internal structure to the Emulated
LAN: it consists of "VPLS Forwarder" modules connected by
pseudowires, where the pseudowires may be traveling through PSN
tunnels over a routed backbone.
A "VPLS instance" consists of a set of VPLS Forwarders (no more than
one per PE) connected by pseudowires.
The functionality that the bridge module must support depends on the
service that is being offered by the SP to its customers, as well as
on various details of the SP’s network. At a minimum, the bridge
module must be able to learn MAC addresses, and to "age them out", in
the standard manner. However, if the PE devices have backdoor
connections with each other via a Layer 2 network, they may need to
be full IEEE bridges ([IEEE8021D]), running a spanning tree with each
other. Specification of the precise functionality that the bridge
modules must have in particular circumstances is, however, out of
scope of the current document.
This framework specifies that each "bridge module" have a single
"Emulated LAN interface". It does not specify the number of bridge
modules that a VPLS-PE may contain, nor does it specify the number of
VPLS instances that may attach to a bridge module over a single
"Emulated LAN interface".
Thus the framework is compatible with at least the following three
models:
- Model 1
A VPLS-PE contains a single bridge module and supports a single
VPLS instance. The VPLS instance is an Emulated LAN; if that
Emulated LAN contains VLANs, 802.1Q [IEEE8021Q] tagging must be
used to indicate which packets are in which VLANs.
- Model 2
A VPLS-PE contains a single bridge module, but supports multiple
VPLS instances. Each VPLS instance is thought of as a VLAN (in
effect, an "Emulated VLAN"), and the set of VPLS instances are
treated as a set of VLANs on a common LAN. Since each VLAN uses
a separate set of PWs, there is no need for 802.1Q tagging.
- Model 3
A VPLS-PE contains an arbitrary number of bridge modules, each
of which attaches to a single VPLS instance.
There may be other models as well, some of which are
combinations of the 3 models above. Different models may have
different characteristics, and different scopes of
applicability.
Each VPLS solution should specify the model or models that it is
supporting. Each solution should also specify the necessary
bridge functionality that its bridge modules must support.
This framework does not specify the way in which bridge control
protocols are used on the Emulated LANs.
2.2.1. Entities in the VPLS Reference Model
The PE (VPLS-PE) and CE devices are defined in [RFC4026].
2.3. Reference Model for Distributed VPLS-PE or VPWS-PE
VPLS-PE/VPWS-PE
Functionality . . . . . . .
. . . . . . . . . . . . .
. . . .
+----+ . +----+ +----+ . . Service .
| CE |--.--|U-PE|----|N-PE|-.---. Provider .
+----+ . +----+ +----+ . . Backbone .
. . . . . . . . . . . . .
2.3.1. Entities in the Distributed PE Reference Models
A VPLS-PE or a VPWS-PE functionality may be distributed to more than
one device. The device closer to the customer/user is called the
User-facing PE (U-PE), and the device closer to the core network is
called Network-facing PE (N-PE).
For further discussion, see Section 3.4.3.
The terms "U-PE" and "N-PE" are defined in [RFC4026].
2.4. VPWS-PE and VPLS-PE
The VPWS-PE and VPLS-PE are functionally very similar, in that they
both use forwarders to map attachment circuits to pseudowires. The
only difference is that while the forwarder in a VPWS-PE does a one-
to-one mapping between the attachment circuit and pseudowire, the
forwarder in a VPLS-PE is a Virtual Switching Instance (VSI) that
maps multiple attachment circuits to multiple pseudowires (for
further discussion, see Section 3).
3. Functional Components of L2 VPN
This section specifies a functional model for L2VPN, which allows one
to break an L2VPN architecture down into its functional components.
This exhibits the roles played by the various protocols and
mechanisms, and thus makes it easier to understand the differences
and similarities between various proposed L2VPN architectures.
Section 3.1 contains an overview of some different types of L2VPNs.
In Section 3.2, functional components that are common to the
different types are discussed. Then, there is a section for each of
the L2VPN service types being considered. The latter sections
discuss functional components, which may be specific to particular
L2VPN types, and type-specific features of the generic components.
3.1. Types of L2VPN
The types of L2VPN are distinguished by the characteristics of the
service that they offer to the customers of the Service Provider
(SP).
3.1.1. Virtual Private Wire Service (VPWS)
In a VPWS, each CE device is presented with a set of point-to-point
virtual circuits.
The other end of each virtual circuit is another CE device. Frames
transmitted by a CE on such a virtual circuit are received by the CE
device at the other end-point of the virtual circuit. Forwarding
from one CE device to another is not affected by the content of the
frame, but is fully determined by the virtual circuit on which the
frame is transmitted. The PE thus acts as a virtual circuit switch.
This type of L2VPN has long been available over ATM and Frame Relay
backbones. Providing this type of L2VPN over MPLS and/or IP
backbones is the current topic.
Requirements for this type of L2VPN are specified in [RFC4665].
3.1.2. Virtual Private LAN Service (VPLS)
In a VPLS, each CE device has one or more LAN interfaces that lead to
a "virtual backbone".
Two CEs are connected to the same virtual backbone if and only if
they are members of the same VPLS instance (i.e., same VPN). When a
CE transmits a frame, the PE that receives it examines the MAC
Destination Address field in order to determine how to forward the
frame. Thus, the PE functions as a bridge. As Figure 3 indicates,
if a set of PEs support a common VPLS instance, then there is an
Emulated LAN, corresponding to that VPLS instance, to which each of
those PE bridges attaches (via an emulated interface). From the
perspective of a CE device, the virtual backbone is the set of PE
bridges and the Emulated LAN on which they reside. Thus to a CE
device, the LAN that attaches it to the PE is extended transparently
over the routed MPLS and/or IP backbone.
The PE bridge function treats the Emulated LAN as it would any other
LAN to which it has an interface. Forwarding decisions are made in
the manner that is normal for bridges, which is based on MAC Source
Address learning.
VPLS is like VPWS in that forwarding is done without any
consideration of the Layer3 header. VPLS is unlike VPWS in that:
- VPLS allows the PE to use addressing information in a frame’s L2
header to determine how to forward the frame; and
- VPLS allows a single CE/PE connection to be used for
transmitting frames to multiple remote CEs; in this particular
respect, VPLS resembles L3VPN more than VPWS.
Requirements for this type of L2VPN are specified in [RFC4665].
3.1.3. IP-Only LAN-Like Service (IPLS)
An IPLS is very like a VPLS, except that:
- it is assumed that the CE devices are hosts or routers, not
switches; and
- it is assumed that the service will only carry IP packets and
supporting packets such as ICMP and ARP (in the case of IPv4) or
Neighbor Discovery (in the case of IPv6); Layer 2 packets that
do not contain IP are not supported.
While this service is a functional subset of the VPLS service, it is
considered separately because it may be possible to provide it using
different mechanisms, which may allow it to run on certain hardware
platforms that cannot support the full VPLS functionality.
3.2. Generic L2VPN Transport Functional Components
All L2VPN types must transport "frames" across the core network
connecting the PEs. In all L2VPN types, a PE (PE1) receives a frame
from a CE (CE1), and then transports the frame to a PE (PE2), which
then transports the frame to a CE (CE2). In this section, we discuss
the functional components that are necessary to transport L2 frames
in any type of L2VPN service.
3.2.1. Attachment Circuits
In any type of L2VPN, a CE device attaches to a PE device via some
sort of circuit or virtual circuit. We will call this an "Attachment
Circuit" (AC). We use this term very generally; an Attachment
Circuit may be a Frame Relay DLCI, an ATM VPI/VCI, an Ethernet port,
a VLAN, a PPP connection on a physical interface, a PPP session from
an L2TP tunnel, an MPLS LSP, etc. The CE device may be a router, a
switch, a host, or just about anything, which the customer needs
hooked up to the VPN. An AC carries a frame between CE and PE, or
vice versa.
Procedures for setting up and maintaining the ACs are out of scope of
this architecture.
These procedures are generally specified as part of the specification
of the particular Attachment Circuit technology.
Any given frame will traverse an AC from a CE to a PE, and then on
another AC from a PE to a CE.
We refer to the former AC as the frame’s "ingress AC" and to the
latter AC as the frame’s "egress AC". Note that this notion of
"ingress AC" and "egress AC" is relative to a specific frame and
denotes nothing more than the frame’s direction of travel while it is
on that AC.
3.2.2. Pseudowires
A "Pseudowire" (PW) is a relation between two PE devices. Whereas an
AC is used to carry a frame from CE to PE, a PW is used to carry a
frame between two PEs. We use the term "pseudowire" in the sense of
[RFC3985].
Setting up and maintaining the PWs is the job of the PEs. State
information for a particular PW is maintained at the two PEs that are
its endpoints, but not at other PEs, and not in the backbone routers
(P routers).
Pseudowires may be point-to-point, multipoint-to-point, or point-to-
multipoint. In this framework, point-to-point PWs are always
considered bidirectional; multipoint-to-point and point-to-multipoint
PWs are always considered unidirectional. Multipoint-to-point PWs
can be used only when the PE receiving a frame does not need to
infer, from the PW on which the frame was received, the identity of
the frame’s ingress AC. Point-to-multipoint PWs may be useful when
frames need to be multicast.
Procedures for setting up and maintaining point-to-multipoint PWs are
not considered in this version of this framework.
Any given frame travels first on its ingress AC, then on a PW, and
then on its egress AC.
Multicast frames may be replicated by a PE, so of course the
information carried in multicast frames may travel on more than one
PW and more than one egress AC.
Thus with respect to a given frame, a PW may be said to associate a
number of ACs. If these ACs are of the same technology (e.g., both
ATM, both Ethernet, both Frame Relay), the PW is said to provide
"homogeneous transport"; otherwise it is said to provide
"heterogeneous transport". Heterogeneous transport requires that
some sort of interworking function be applied. There are at least
three different approaches to interworking:
1. One of the CEs may perform the interworking locally. For
example, if CE1 attaches to PE1 via ATM, but CE2 attaches to
PE2 via Ethernet, then CE1 may decide to send/receive
Ethernet frames over ATM, using the RFC 2684, "LLC
Encapsulation for Bridged Protocols". In such a case, PE1
would need to know that it is to terminate the ATM VC
locally, and only to send/receive Ethernet frames over the
PW.
2. One of the PEs may perform the interworking. For example, if
CE1 attaches to PE1 via ATM, but CE2 attaches to PE2 via
Frame Relay, PE1 may provide the "ATM/FR Service
Interworking" function. This would be transparent to the
CEs, and the PW would carry only Frame Relay frames.
3. IPLS could be used. In this case, the "frames" carried by
the PW are IP datagrams, and the two PEs need to cooperate in
order to spoof various L2-specific procedures used by IP (see
Section 3.5).
If heterogeneous PWs are used, the setup protocol must ensure that
each endpoint knows the MTU of the remote AC. If the two ACs do not
have the same MTU, one of the following three procedures must be
carried out:
- The PW is not allowed to come up.
- The endpoint at the AC with the larger MTU must reduce the AC’s
MTU so that it is the same as the MTU of the remote AC.
- The two endpoints must agree to use a specified
fragmentation/reassembly procedure.
3.2.3. Forwarders
In all types of L2VPN, a PE (say, PE1) receives a frame over an AC
and forwards it over a PW to another PE (say, PE2). PE2 then
forwards the frame out on another AC.
The case in which PE1 and PE2 are the same device is an important
case to handle correctly, in order to provide the L2VPN service
properly. However, as this case does not require any protocol, we do
not address it further in this document.
When PE1 receives a frame on a particular AC, it must determine the
PW on which the frame must be forwarded. In general, this is done by
considering:
- the incoming AC;
- possibly the contents of the frame’s Layer2 header; and
- possibly some forwarding information that may be statically or
dynamically maintained.
If dynamic or static forwarding information is considered, the
information is specific to a particular L2VPN instance (i.e., to a
particular VPN).
Similarly, when PE2 receives a frame on a particular PW, it must
determine the AC on which the frame must be forwarded. This is done
by considering:
- the incoming PW;
- possibly the contents of the frame’s Layer2 header; and
- possibly some forwarding information that may be statically or
dynamically maintained.
If dynamic or static forwarding information is considered, the
information is specific to a particular L2VPN instance (i.e., to a
particular VPN).
The procedures used to make the forwarding decision are known as a
"forwarder". We may think of a PW as being "bound", at each of its
endpoints, to a forwarder. The forwarder in turn "binds" the PWs to
ACs. Different types of L2VPN have different types of forwarders.
For instance, a forwarder may bind a single AC to a single PW,
ignoring all frame contents and using no other forwarding
information. Or a forwarder may bind an AC to a set of PWs and ACs,
moving individual frames from AC to PW, from a PW to an AC or from AC
to AC by comparing information from the frame’s Layer2 header to
information in a forwarding database. This is discussed in more
detail below, as we consider the different L2VPN types.
3.2.4. Tunnels
A PW is carried in a "tunnel" from PE1 to PE2. We assume that an
arbitrary number of PWs may be carried in a single tunnel; the only
requirement is that the PWs all terminate at PE2.
We do not even require that all the PWs in the tunnel originate at
PE1; the tunnels may be multipoint-to-point tunnels. Nor do we
require that all PWs between the same pair of PEs travel in the same
tunnel. All we require is that when a frame traveling through such a
tunnel arrives at PE2, PE2 will be able to associate it with a
particular PW.
(While one can imagine tunneling techniques that only allow one PW
per tunnel, they have evident scalability problems, and we do not
consider them further.)
A variety of different tunneling technologies may be used for the
PE-PE tunnels. All that is really required is that the tunneling
technologies allow the proper demultiplexing of the contained PWs.
The tunnels might be MPLS LSPs, L2TP tunnels, IPsec tunnels, MPLS-
in-IP tunnels, etc. Generally the tunneling technology will require
the use of an encapsulation that contains a demultiplexor field,
where the demultiplexor field is used to identify a particular PW.
Procedures for setting up and maintaining the tunnels are not within
the scope of this framework. (But see Section 3.2.6, "Pseudowire
Signaling".)
If there are multiple tunnels from PE1 to PE2, it may be desirable to
assign a particular PE1-PE2 PW to a particular tunnel based on some
particular characteristics of the PW and/or the tunnel. For example,
perhaps different tunnels are associated with different QoS
characteristics, and different PWs require different QoS. Procedures
for specifying how to assign PWs to tunnels are out of scope of the
current framework.
Though point-to-point PWs are bidirectional, the tunnels in which
they travel need not be either bidirectional or point-to-point. For
example, a point-to-point PW may travel within a unidirectional
multipoint-to-point MPLS LSP.
3.2.5. Encapsulation
As L2VPN packets are carried in pseudowires, standard pseudowire
encapsulation formats and techniques (as specified by the IETF’s PWE3
WG) should be used wherever applicable.
Generally the PW encapsulations will themselves be encapsulated
within a tunnel encapsulation, as determined by the specification of
the tunneling protocol.
It may be necessary to define additional PW encapsulations to cover
areas that are of importance for L2VPN, but that may not be within
the scope of PWE3. Heterogeneous transport may be an instance of
this.
3.2.6. Pseudowire Signaling
Procedures for setting up and maintaining the PWs themselves are
within the scope of this framework. This includes procedures for
distributing demultiplexor field values, even though the
demultiplexor field, strictly speaking, belongs to the tunneling
protocol and not to the PW.
The signaling for a point-to-point pseudowire must perform the
following functions:
- Distribution of the demultiplexor.
Since many PWs may be carried in a single tunnel, the tunneling
protocol must assign a demultiplexor value to each PW. These
demultiplexors must be unique with respect to a given tunnel
(or, with some tunneling technologies, unique at the egress PE).
Generally, the PE that is the egress of the tunnel will select
the demultiplexor values and will distribute them to the PE(s)
which is (are) the ingress(es) of the tunnel. This is the
essential part of the PW setup procedure.
Note that, as is usually the case in tunneling architectures,
the demultiplexor field belongs to the tunneling protocol, not
to the protocol being tunneled. For this reason, the PW setup
protocols may be extensions of the control protocols for setting
up the tunnels.
- Selection of the Forwarder at the remote PE.
The signaling protocol must contain enough information to enable
the remote PE to select the proper forwarder to which the PW is
to be bound. We can call this information the "Remote Forwarder
Selector". The information that is required will depend on the
type of L2VPN being provided and on the provisioning model being
used (see Sections 3.3.1 and 3.4.2). The Remote Forwarder
Selector may uniquely identify a particular Forwarder, or it may
identify an attribute of Forwarders. In the latter case, it
would select whichever Forwarder has been provisioned with that
attribute.
- Supporting pseudowire emulations.
To the extent that a particular PW must emulate the signaling of
a particular Layer2 technology, the PW signaling must provide
the necessary functions.
- Distribution of state changes.
Changes in the state of an AC may need to be reflected in
changes to the state of the PW to which the AC is bound, and
vice versa. The specification as to which changes need to be
reflected in what way would generally be within the province of
the PWE3 WG.
- Establishing pseudowire characteristics.
To the extent that one or more characteristics of a PW must be
known to and/or agreed upon by both endpoints, the signaling
must allow for the necessary interaction.
As specified above, signaling for point-to-point PWs must pass enough
information to allow a remote PE to properly bind a PW to a
Forwarder, and to associate a particular demultiplexor value with
that PW. Once the two PEs have done the proper PW/Forwarder
bindings, and have agreed on the demultiplexor values, the PW may be
considered set up. If it is necessary to negotiate further
characteristics or parameters of a particular PW, or to pass status
information for a particular PW, the PW may be identified by the
demultiplexor value.
Signaling procedures for point-to-point pseudowires are most commonly
point-to-point procedures that are executed by the two PW endpoints.
There are, however, proposals to use point-to-multipoint signaling
for setting up point-to-point pseudowires, so this is included in the
framework. When PWs are themselves point-to-multipoint, it is also
possible to use either point-to-point signaling or point-to-
multipoint signaling to set them up. This is discussed in the
remainder of this section.
3.2.6.1. Point-to-Point Signaling
There are several ways to do the necessary point-to-point signaling.
Among them are:
- LDP
LDP [RFC3036] extensions can be defined for pseudowire
signaling. This form of signaling can be used for pseudowires
that are to be carried in MPLS "tunnels", or in MPLS-in-
something-else tunnels.
- L2TP
L2TP [RFC2661] can be used for pseudowire signaling, resulting
in pseudowires that are carried as "sessions" within L2TP
tunnels. Pseudowire-specific extensions to L2TP may also be
needed.
Other methods may be possible as well.
It is possible to have one control connection between a pair of PEs,
which is used to control many PWs.
The use of point-to-point signaling for setting up point-to-point PWs
is straightforward. Multipoint-to-point PWs can also be set up by
point-to-point signaling, as the remote PEs do not necessarily need
to know whether the PWs are multipoint-to-point or point-to-point.
In some signaling procedures, the same demultiplexor value may be
assigned to all the remote PEs.
3.2.6.2. Point-to-Multipoint Signaling
Consider the following conditions:
- It is necessary to set up a set of PWs, all of which have the
same characteristics.
- It is not necessary to use the PW signaling protocol to pass PW
state changes.
- For each PW in the set, the same value of the Remote Forwarder
Selector can be used.
Call these the "Environmental Conditions".
Suppose also that there is some mechanism by which, given a range of
demultiplexor values, each of a set of PEs can make a unique and
deterministic selection of a single value from within that range.
Call this the "Demultiplexor Condition". Alternatively, suppose that
one is trying to set up a multipoint-to-point PW rather than to set
up a point-to-point PW. Call this the "Multipoint Condition".
If:
- The Environmental Conditions hold; and
- Either
* the Demultiplexor Condition holds, or
* the Multipoint Condition holds,
then for a given set of PWs that terminate at egress PE1, the
information that PE1 needs to send to the ingress PE(s) of each
pseudowire in the set is exactly the same. All the ingress PE(s)
receive the same Forwarder Selector value. They all receive the same
set of PW parameters (if any). And either they all receive the same
demultiplexor value (if the PW is multipoint-to-point) or they all
receive a range of demultiplexor values from which each can choose a
unique demultiplexor value for itself.
Rather than connect to each ingress PE and replicate the same
information, it may make sense either to multicast the information,
or to send the information once to a "reflector", which will then
take responsibility for distributing the information to the other
PEs.
We refer to this sort of technique as "point-to-multipoint"
signaling. It would, for example, be possible to use BGP [RFC1771]
to do the signaling, with PEs that are BGP peers not of each other,
but of one or more BGP route reflectors [RFC2796].
3.2.6.3. Inter-AS Considerations
Pseudowires may need to run from a PE in one Service Provider’s
network to a PE in another Service Provider’s network. This has the
following implications:
- The signaling protocol that sets up the PWs must be able to
cross network boundaries. Of course, all IP-based protocols
have this capability.
- The two PEs at the PW endpoints must be addressable and routable
from each other.
- The signaling protocol needs to allow each PW endpoint to
authenticate the other. To make use of the authentication
capability, there would also need to be some method of key
distribution that is acceptable to both administrations.
3.2.7. Service Quality
Service Quality refers to the ability for the network to deliver a
Service level Specification (SLS) for service attributes such as
protection, security, and Quality of Service (QoS). The service
quality provided depends on the subscriber’s requirements and can be
characterized by a number of performance metrics.
The necessary Service Quality must be provided on the ACs, as well as
on the PWs. Mechanisms for providing Service Quality on the PWs may
be PW-specific or tunnel-specific; in the latter case, the assignment
of a PW to a tunnel may depend on the Service Quality.
3.2.7.1. Quality of Service (QoS)
QoS describes the queuing behavior applied to a particular "flow", in
order to achieve particular goals of precedence, throughput, delay,
jitter, etc.
Based on the customer Service Level Agreement (SLA), traffic from a
customer can be prioritized, policed, and shaped for QoS
requirements. The queuing and forwarding policies can preserve the
packet order and QoS parameters of customer traffic. The class of
services can be mapped from information in the customer frames, or it
can be independent of the frame content.
QoS functions can be listed as follows:
- Customer Traffic Prioritization: L2VPN services could be best
effort or QoS guaranteed. Traffic from one customer might need
to be prioritized over others when sharing same network
resources. This requires capabilities within the L2VPN solution
to classify and mark priority to QoS guaranteed customer
traffic.
- Proper queuing behavior would be needed at the egress AC, and
possibly within the backbone network as well. If queuing
behavior must be controlled within the backbone network, the
control might be based on CoS information in the MPLS or IP
header, or it might be achieved by nesting particular tunnels
within particular traffic engineering tunnels.
- Policing: This ensures that a user of L2VPN services uses
network resources within the limits of the agreed SLA. Any
excess L2VPN traffic can be rejected or handled differently
based on provider policy.
- Policing would generally be applied at the ingress AC.
- Shaping: Under some cases, the random nature of L2VPN traffic
might lead to sub-optimal utilization of network resources.
Through queuing and forwarding mechanisms, the traffic can be
shaped without altering the packet order.
- Shaping would generally be applied at the ingress AC.
3.2.7.2. Resiliency
Resiliency describes the ability of the L2VPN infrastructure to
protect a flow from network outage, so that service remains available
in the presence of failures.
L2VPN, like any other service, is subject to failures such as link,
trunk, and node failures, both in the SP’s core network
infrastructure and on the ACs.
It is desirable that the failure be detected "immediately" and that
protection mechanisms allow fast restoration times to make L2VPN
service almost transparent to these failures to the extent possible,
based on the level of resiliency. Restoration should take place
before the CEs can react to the failure. Essential aspects of
providing resiliency are:
- Link/Node failure detection: Mechanisms within the L2VPN service
should allow for link or node failures that impact the service,
and that should be detected immediately.
- Resiliency policy: The way in which a detected failure is
handled will depend on the restoration policy of the SLA
associated with the L2VPN service specification. It may need to
be handled immediately, or it may need to be handled only if no
other critical failure needs protection resources, or it may be
completely ignored if it is within the bounds of the "acceptable
downtime" allowed by the L2VPN service.
- Restoration Mechanisms: The L2VPN solutions could allow for
physical level protection, logical level protection, or both.
For example, by connecting customers over redundant and
physically separate ACs to different provider customer-facing
devices, one AC can be maintained as active, and the other could
be marked as a backup; upon the failure detection across the
primary AC, the backup could become active.
To a great extent, resiliency is a matter of having appropriate
failure and recovery mechanisms in the network core, including
"ordinary" adaptive routing as well as "fast reroute" capabilities.
The ability to support redundant ACs between CEs and PEs also plays a
role.
3.2.8. Management
An L2VPN solution can provide mechanisms to manage and monitor
different L2VPN components. From a Service Level Agreement (SLA)
perspective, L2VPN solutions could allow monitoring of L2VPN service
characteristics and offer mechanisms used by Service Providers to
report such monitored statistical data. Trouble-shooting and
verification of operational and maintenance activities of L2VPN
services are essential requirements for Service Providers.
3.3. VPWS
A VPWS is an L2VPN service in which each forwarder binds exactly one
AC to exactly one PW. Frames received on the AC are transmitted on
the PW; frames received on the PW are transmitted on the AC. The
content of a frame’s Layer2 header plays no role in the forwarding
decision, except insofar as the Layer2 header contents are used to
associate the frame with a particular AC (e.g., the DLCI field of a
Frame Relay frame identifies the AC).
A particular combination of <AC, PW, AC> forms a "virtual circuit"
between two CE devices.
A particular VPN (VPWS instance) may be thought of as a collection of
such virtual circuits, or as an "overlay" of PWs on the MPLS or IP
backbone. This creates an overlay topology that is in effect the
"virtual backbone" of a particular VPN.
Whether two virtual circuits are said to belong to the same VPN or
not is an administrative matter based on the agreements between the
SPs and their customers. This may impact the provisioning model
(discussed below). It may also affect how particular PWs are
assigned to tunnels, the way QoS is assigned to particular ACs and
PWs, etc.
Note that VPWS makes use of point-to-point PWs exclusively.
3.3.1. Provisioning and Auto-Discovery
Provisioning a VPWS is a matter of:
1. Provisioning the ACs;
2. Providing the PEs with the necessary information to enable
them to set up PWs between ACs to result in the desired
overlay topology; and
3. Configuring the PWs with any necessary characteristics.
3.3.1.1. Attachment Circuit Provisioning
In many cases, the ACs must be individually provisioned on the PE
and/or CE. This will certainly be the case if the CE/PE attachment
technology is a switched network, such as ATM or FR, and the VCs are
PVCs rather than SVCs. It is also the case whenever the individual
Attachment Circuits need to be given specific parameters (e.g., QoS
parameters, guaranteed bandwidth parameters) that differ from circuit
to circuit.
There are also cases in which ACs might not have to be individually
provisioned. For example, if an AC is just an MPLS LSP running
between a CE and a PE, it could be set up as the RESULT of setting up
a PW rather than having to be provisioned BEFORE the PW can be set
up. The same may apply whenever the AC is a Switched Virtual Circuit
of any sort, though in this case, various policy controls might need
to be provisioned; e.g., limiting the number of ACs that can be set
up between a given CE and a given PE.
Issues such as whether the Attachment Circuits need to be
individually provisioned or not, whether they are Switched VCs or
Permanent VCs, and what sorts of policy controls may be applied are
implementation and deployment issues and are considered to be out of
scope of this framework.
3.3.1.2. PW Provisioning for Arbitrary Overlay Topologies
In order to support arbitrary overlay topologies, it is necessary to
allow the provisioning of individual PWs. In this model, when a PW
is provisioned on a PE device, it is locally bound to a specific AC.
It is also provisioned with information that identifies a specific AC
at a remote PE.
There are basically two variations of this provisioning model:
- Two-sided provisioning
With two-sided provisioning, each PE that is at the end of a PW
is provisioned with the following information:
* Identifier of the Local AC to which the PW is to be bound
* PW type and parameters
* IP address of the remote PE (i.e., the PE that is to be at
the remote end of the PW)
* Identifier that is meaningful to the remote PE, and that can
be passed in the PW signaling protocol to enable the remote
PE to bind the PW to the proper AC. This can be an
identifier of the PW or an identifier of the remote AC. If
a PW identifier is used, it must be unique at each of the
two PEs. If an AC identifier is used, it need only be
unique at the remote PE.
This identifier is then used as the Remote Forwarder Selector
when signaling is done (see 3.2.6.1).
- Single-sided provisioning
With single-sided provisioning, a PE at one end of a PW is
provisioned with the following information:
* Identifier of the Local AC to which the PW is to be bound
* PW type and parameters
* Globally unique identifier of remote AC
This identifier is then used as the Forwarder Selector when
signaling is done (see section 3.2.6.1).
In this provisioning model, the IP address of the remote PE is
not provisioned. Rather, the assumption is that an auto-
discovery scheme will be used to map the globally unique
identifier to the IP address of the remote PE, along with an
identifier (perhaps unique only at the latter PE) for an AC at
that PE. The PW signaling protocol can then make a connection
to the remote PE, passing the AC identifier, so that the remote
PE binds the PW to the proper AC.
This scheme requires provisioning of the PW at only one PE, but
it does not eliminate the need (if there is a need) to provision
the ACs at both PEs.
These provisioning models fit well with the use of point-to-point
signaling. When each PW is individually provisioned, as the
conditions necessary for the use of point-to-multipoint signaling do
not hold.
3.3.1.3. Colored Pools PW Provisioning Model
Suppose that at each PE, sets of ACs are gathered together into
"pools", and that each such pool is assigned a "color". (For
example, a pool might contain all and only the ACs from this PE to a
particular CE.) Now suppose that we impose the following rule:
whenever PE1 and PE2 have a pool of the same color, there will be a
PW between PE1 and PE2 that is bound at PE1 to an arbitrarily chosen
AC from that pool, and at PE2 to an arbitrarily chosen AC from that
pool. (We do not rule out the case where a single PE has multiple
pools of a given color.)
For example, each pool in a particular PE might represent a
particular CE device, for which the ACs in the pool are the ACs
connecting that CE to that PE. The color might be a VPN-id.
Application of this provisioning model would then lead to a full CE-
to-CE mesh within the VPN, where every CE in the VPN has a virtual
circuit to every other CE within the VPN.
More specifically, to provision VPWS according to this model, one
provisions a set of pools and configures each pool with the following
information:
- The set of ACs that belong to the pool (with no AC belonging to
more than one pool)
- The color
- A pool identifier that is unique at least relative to the color.
An auto-discove |
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