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Versions: 00 01 02 03 04 05 06 07 08 RFC 6074
Network Working Group Eric C. Rosen
Internet Draft Wei Luo
Expiration Date: March 2005 Cisco Systems, Inc.
Vasile Radoaca
Nortel Networks
September 2004
Provisioning Models and Endpoint Identifiers in L2VPN Signaling
draft-ietf-l2vpn-signaling-02.txt
Status of this Memo
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Abstract
There are a number of different kinds of "Provider Provisioned Layer
2 VPNs" (L2VPNs). The different kinds of L2VPN may have different
"provisioning models", i.e., different models for what information
needs to be configured in what entities. Once configured, the
provisioning information is distributed by a "discovery process".
When the discovery process is complete, a signaling protocol is
automatically invoked. The signaling protocol sets up the mesh of
Pseudowires (PWs) that form the (virtual) backbone of the L2VPN. Any
PW signaling protocol needs to have a method which allows each PW
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endpoint to identify the other; thus a PW signaling protocol will
have the notion of an endpoint identifier. The semantics of the
endpoint identifiers which the signaling protocol uses for a
particular type of L2VPN are determined by the provisioning model.
This document specifies a number of L2VPN provisioning models, and
further specifies the semantic structure of the endpoint identifiers
required by each provisioning model. It discusses the way in which
the endpoint identifiers are distributed by the discovery process,
especially when the discovery process is based upon the Border
Gateway Protocol (BGP). It then specifies how the endpoint
identifiers are carried in the two signaling protocols that are used
to set up PWs, the Label Distribution Protocol (LDP) and the Layer 2
Tunneling Protocol (L2TPv3).
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Contents
1 Introduction ......................................... 4
2 Signaling Protocol Framework ......................... 5
2.1 Endpoint Identification .............................. 5
2.2 Creating a Single Bidirectional Pseudowire ........... 6
2.3 Attachment Identifiers and Forwarders ................ 7
3 Applications ......................................... 8
3.1 Individual Point-to-Point VCs ........................ 9
3.1.1 Provisioning Models .................................. 9
3.1.1.1 Double Sided Provisioning ............................ 9
3.1.1.2 Single Sided Provisioning with Discovery ............. 9
3.1.2 Signaling ............................................ 10
3.2 Virtual Private LAN Service .......................... 11
3.2.1 Provisioning ......................................... 11
3.2.2 Auto-Discovery ....................................... 11
3.2.2.1 BGP-based auto-discovery ............................. 11
3.2.3 Signaling ............................................ 13
3.2.4 Pseudowires as VPLS Attachment Circuits .............. 13
3.3 Colored Pools: Full Mesh of Point-to-Point VCs ....... 13
3.3.1 Provisioning ......................................... 13
3.3.2 Auto-Discovery ....................................... 14
3.3.2.1 BGP-based auto-discovery ............................. 14
3.3.3 Signaling ............................................ 15
3.4 Colored Pools: Partial Mesh .......................... 16
3.5 Distributed VPLS ..................................... 16
3.5.1 Signaling ............................................ 18
3.5.2 Provisioning and Discovery ........................... 19
3.5.3 Non-distributed VPLS as a sub-case ................... 20
3.5.4 Inter-Provider Application of Dist. VPLS Signaling ... 20
3.5.5 Splicing and the Data Plane .......................... 21
4 Security Considerations .............................. 22
5 Acknowledgments ...................................... 22
6 References ........................................... 22
7 Author's Information ................................. 23
8 Intellectual Property Statement ...................... 24
9 Full Copyright Statement ............................. 24
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1. Introduction
[L2VPN-FW] describes a number of different ways in which sets of
pseudowires may be combined together into "Provider Provisioned Layer
2 VPNs" (L2 PPVPNs, or L2VPNs), resulting in a number of different
kinds of L2VPN. Different kinds of L2VPN may have different
"provisioning models", i.e., different models for what information
needs to be configured in what entities. Once configured, the
provisioning information is distributed by a "discovery process", and
once the information is discovered, the signaling protocol is
automatically invoked to set up the required pseudowires. The
semantics of the endpoint identifiers which the signaling protocol
uses for a particular type of L2VPN are determined by the
provisioning model. That is, different kinds of L2VPN, with different
provisioning models, require different kinds of endpoint identifiers.
This document specifies a number of PPVPN provisioning models, and
specifies the semantic structure of the endpoint identifiers required
for each provisioning model.
Either LDP (as specified in [LDP] and extended in [PWE3-CONTROL]) or
L2TP version 3 (as specified in [L2TP-BASE] and extended in [L2TP-
L2VPN] can be used as signaling protocols to set up and maintain
pseudowires (PWs) [PWE3-ARCH]. Any protocol which sets up connections
must provide a way for each endpoint of the connection to identify
the other; each PW signaling protocol thus provides a way to identify
the PW endpoints. Since each signaling protocol needs to support
all the different kinds of L2VPN and provisioning models, the
signaling protocol must have a very general way of representing
endpoint identifiers, and it is necessary to specify rules for
encoding each particular kind of endpoint identifier into the
relevant fields of each signaling protocol. This document specifies
how to encode the endpoint identifiers of each provisioning model
into the LDP and L2TPv3 signaling protocols.
We make free use of terminology from [L2VPN-FW], [L2VPN-TERM], and
[PWE3-ARCH], in particular the terms "Attachment Circuit",
"pseudowire", "PE", "CE".
Section 2 provides an overview of the relevant aspects of [PWE3-
CONTROL] and [L2TP-L2VPN].
Section 3 details various provisioning models and relates them to the
signaling process and to the discovery process.
We do not specify an auto-discovery procedure in this draft, but we
do specify the information which needs to be obtained via auto-
discovery in order for the signaling procedures to begin. The way in
which the signaling mechanisms can be integrated with BGP-based
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auto-discovery is covered in some detail.
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
2. Signaling Protocol Framework
2.1. Endpoint Identification
Per [L2VPN-FW], a pseudowire can be thought of as a relationship
between a pair of "Forwarders". In simple instances of VPWS, a
Forwarder binds a pseudowire to a single Attachment Circuit, such
that frames received on the one are sent on the other, and vice
versa. In VPLS, a Forwarder binds a set of pseudowires to a set of
Attachment Circuits; when a frame is received from any member of that
set, a MAC address table is consulted (and various 802.1d procedures
executed) to determine the member or members of that set on which the
frame is to be transmitted. In more complex scenarios, Forwarders
may bind PWs to PWs, thereby "splicing" two PWs together; this is
needed, e.g., to support distributed VPLS.
In simple VPWS, where a Forwarder binds exactly one PW to exactly one
Attachment Circuit, a Forwarder can be identified by identifying its
Attachment Circuit. In simple VPLS, a Forwarder can be identified by
identifying its PE device and its VPN.
To set up a PW between a pair of Forwarders, the signaling protocol
must allow the Forwarder at one endpoint to identify the Forwarder at
the other. In [PWE3-CONTROL], the term "Attachment Identifier", or
"AI", to refer to a quantity whose purpose is to identify a
Forwarder. In [L2TP-L2VPN], the term "Forwarder Identifier" is used
for the same purpose. In the context of this document, "Attachment
Identifier" and "Forwarder Identifier" are used interchangably.
[PWE3-CONTROL] specifies two FEC elements which can be used for when
setting up pseudowires, the PWid FEC element, and the Generalized Id
FEC element. The PWid FEC element carries only one Forwarder
identifier; it can be thus be used only when both forwarders have the
same identifier, and when that identifier can be coded as a 32-bit
quantity. The Generalized Id FEC element carries two Forwarder
identifiers, one for each of the two Forwarders being connected.
Each identifier is known as an Attachment Identifier, and a signaling
message carries both a "Source Attachment Identifier" (SAI) and a
"Target Attachment Identifier" (TAI).
The Generalized ID FEC element also provides some additional
structuring of the identifiers. It is assumed that the SAI and TAI
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will sometimes have a common part, called the "Attachment Group
Identifier" (AGI), such that the SAI and TAI can each be thought of
as the concatenation of the AGI with an "Attachment Individual
Identifier" (AII). So the pair of identifiers is encoded into three
fields: AGI, Source AII (SAII), and Target AII (TAII). The SAI is
the concatenation of the AGI and the SAII, while the TAI is the
concatenation of the AGI and the TAII.
Similiarly, [L2TP-L2VPN] allows using one or two Forwarder
Identifiers to set up pseudowires. If only the target Forwarder
Identifier is used in L2TP signaling messages, both the source and
target Forwarders are assumed to have the same value. If both the
source and target Forwarder Identifers are carried in L2TP siganling
messages, each Forwarder uses a locally significant identifier value.
The Forwarder Identifier in [L2TP-L2VPN] is an equivalent term as
Attachment Identifer in [PWE3-CONTROL]. A Forwarder Identifier also
consists of an Attachment Group Identifier and an Attachment
Individual Identifier. Unlike the Generalized ID FEC element, the
AGI and AII are carried in distinct L2TP Attribute-Value-Pairs
(AVPs). The AGI is encoded in the AGI AVP, and the SAII and TAII are
encoded in the Local End ID AVP and the Remote End ID AVP
respectively. The source Forwarder Identifier is the concatenation
of the AGI and SAII, while the target Forwarder Identifier is the
concatenation of the AGI and TAII.
In applications that group sets of PWs into "Layer 2 Virtual Private
Networks", the AGI can be thought of as a "VPN Identifier".
It should be noted that while different forwarders support different
applications, the type of application (e.g., VPLS vs. VPWS) cannot
necessarily be inferred from the forwarders' identifiers. A router
receiving a signaling message with a particular TAI will have to be
able to determine which of its local forwarders is identified by that
TAI, and to determine the application provided by that forwarder.
But other nodes may not be able to infer the application simply by
inspection of the signaling messages.
2.2. Creating a Single Bidirectional Pseudowire
In any form of LDP-based signaling, each PW endpoint must initiate
the creation of a unidirectional LSP. A PW is a pair of such LSPs.
In most of the PPVPN provisioning models, the two endpoints of a
given PW can simultaneously initiate the signaling for it. They must
therefore have some way of determining when a given pair of LSPs are
intended to be associated together as a single PW.
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The way in which this association is done is different for the
various different L2VPN services and provisioning models. The
details appear in later sections.
L2TP signaling inherently establishes a bidirectional session that
carries a PW between two PW endpoints. The two endpoints can also
simultaneously initiate the signaling for a given PW. It is possible
that two PWs can be established for a pair of Forwarders.
In order to avoid setting up duplicated pseudowires between two
Forwarders, each PE must be able to independently detect such a
pseudowire tie. The procedures of detecting a pseudowire tie is
described in [L2TP-L2VPN]
2.3. Attachment Identifiers and Forwarders
Every Forwarder in a PE must be associated with an Attachment
Identifier (AI), either through configuration or through some
algorithm. The Attachment Identifier must be unique in the context
of the PE router in which the Forwarder resides. The combination <PE
router, AI> must be globally unique.
It is frequently convenient to a set of Forwarders as being members
of a particular "group", where PWs may only be set up among members
of a group. In such cases, it is convenient to identify the
Forwarders relative to the group, so that an Attachment Identifier
would consist of an Attachment Group Identifier (AGI) plus an
Attachment Individual Identifier (AII).
IT MUST BE UNDERSTOOD THAT THIS NOTION OF "GROUP" HAS NOTHING
WHATSOEVER TO DO WITH THE "GROUP ID" THAT IS PART OF THE PWID FEC IN
[PWE3-CONTROL].
An Attachment Group Identifier may be thought of as a VPN-id, or
a VLAN identifier, some attribute which is shared by all the
Attachment VCs (or pools thereof) which are allowed to be connected.
The details for how to construct the AGI and AII fields identifying
the pseudowire endpoints in particular provisioning models are
discussed later in this paper.
We can now consider an LSP to be identified by:
<PE1, <AGI, AII1>, PE2, <AGI, AII2>>,
and the LSP in the opposite direction will be identified by:
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<PE2, <AGI, AII2>, PE1, <AGI, AII1>>;
a pseudowire is a pair of such LSPs. In the case of using L2TP
signaling, these refer to the two directions of an L2TP session.
When a signaling message is sent from PE1 to PE2, and PE1 needs to
refer to an Attachment Identifier which has been configured on
one of its own Attachment VCs (or pools), the Attachment
Identifier is called a "Source Attachment Identifier". If PE1
needs to refer to an Attachment Identifier which has been
configured on one of PE2's Attachment VCs (or pools), the
Attachment Identifier is called a "Target Attachment Identifier".
(So an SAI at one endpoint is a TAI at the remote endpoint, and vice
versa.)
In the signaling protocol, we define encodings for the following
three fields:
- Attachment Group Identifier (AGI)
- Source Attachment Individual Identifier (SAII)
- Target Attachment Individual Identifier (TAII)
If the AGI is non-null, then the SAI consists of the AGI together
with the SAII, and the TAI consists of the TAII together with the
AGI. If the AGI is null, then the SAII and TAII are the SAI and TAI
respectively.
The intention is that the PE which receives an LDP Label Mapping
message or an L2TP Incoming Call Request (ICRQ) message containing a
TAI will be able to map that TAI uniquely to one of its Attachment
VCs (or pools). The way in which a PE maps a TAI to an Attachment
VC (or pool) should be a local matter. So as far as the signaling
procedures are concerned, the TAI is really just an arbitrary string
of bytes, a "cookie".
3. Applications
In this section, we specify the way in which the pseudowire signaling
using the notion of source and target Forwarder is applied for a
number of different applications. For some of the applications, we
specify the way in which different provisioning models can be used.
However, this is not meant to be an exhaustive list of the
applications, or an exhaustive list of the provisioning models that
can be applied to each application.
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3.1. Individual Point-to-Point VCs
The signaling specified in this document can be used to set up
individually provisioned point-to-point pseudowires. In this
application, each Forwarder binds a single PW to a single Attachment
Circuit. Each PE must be provisioned with the necessary set of
Attachment Circuits, and then certain parameters must be provisioned
for each Attachment Circuit.
3.1.1. Provisioning Models
3.1.1.1. Double Sided Provisioning
In this model, the Attachment Circuit must be provisioned with a
local name, a remote PE address, and a remote name. During
signaling, the local name is sent as the SAII, the remote name as the
TAII, and the AGI is null. If two Attachment Circuits are to be
connected by a PW, the local name of each must be the remote name of
the other.
Note that if the local name and the remote name are the same, the
PWid FEC element can be used instead of the Generalized ID FEC
element in the LDP based signaling.
With L2TP signaling, the local name is sent in Local End ID AVP, the
remote name in Remote End ID AVP. The AGI AVP is optional. If
present, it contains a zero-length AGI value. If the local name and
the remote name are the same, Local End ID AVP can be omitted from
L2TP signaling messages.
3.1.1.2. Single Sided Provisioning with Discovery
In this model, each Attachment Circuit must be provisioned with a
local name. The local name consists of a VPN-id (signaled as the
AGI) and an Attachment Individual Identifier which is unique relative
to the AGI. If two Attachment circuits are to be connected by a PW,
only one of them needs to be provisioned with a remote name (which of
course is the local name of the other Attachment Circuit). Neither
needs to be provisioned with the address of the remote PE, but both
must have the same VPN-id.
As part of an auto-discovery procedure, each PE advertises its <VPN-
id, local AII> pairs. Each PE compares its local <VPN-id, remote
AII> pairs with the <VPN-id, local AII> pairs advertised by the other
PEs. If PE1 has a local <VPN-id, remote AII> pair with value <V,
fred>, and PE2 has a local <VPN-id, local AII> pair with value <V,
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fred>, PE1 will thus be able to discover that it needs to connect to
PE2. When signaling, it will use "fred" as the TAII, and will use V
as he AGI. PE1's local name for the Attachment Circuit is sent as
the SAII.
The primary benefit of this provisioning model when compared to
Double Sided Provisioning is that it enables one to move an
Attachment Circuit from one PE to another without having to
reconfigure the remote endpoint.
3.1.2. Signaling
The LDP-based signaling is as specified in [PWE3-CONTROL], with the
addition of the following:
When a PE receives a Label Mapping Message, and the TAI identifies a
particular Attachment Circuit which is configured to be bound to a
point-to-point PW, then the following checks must be made.
If the Attachment Circuit is already bound to a pseudowire (including
the case where only one of the two LSPs currently exists), and the
remote endpoint is not PE1, then PE2 sends a Label Release message to
PE1, with a Status Code meaning "Attachment Circuit bound to
different PE", and the processing of the Mapping message is complete.
If the Attachment Circuit is already bound to a pseudowire (including
the case where only one of the two LSPs currently exists), but the AI
at PE1 is different than that specified in the AGI/SAII fields of the
Mapping message then PE2 sends a Label Release message to PE1, with a
Status Code meaning "Attachment Circuit bound to different remote
Attachment Circuit", and the processing of the Mapping message is
complete.
Similarly with the L2TP-based signaling, when a PE receives an ICRQ
message, and the TAI identifies a particular Attachment Circuit which
is configured to be bound to a point-to-point PW, it performs the
following checks.
If the Attachment Circuit is already bound to a pseudowire, and the
remote endpoint is not PE1, then PE2 sends a Call Disconnect Notify
(CDN) message to PE1, with a Status Code meaning "Attachment Circuit
bound to different PE", and the processing of the ICRQ message is
complete.
If the Attachment Circuit is already bound to a pseudowire, but the
pseudowire is bound to a Forwarder on PE1 with the AI different than
that specified in the SAI fields of the ICRQ message, then PE2 sends
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a CDN message to PE1, with a Status Code meaning "Attachment Circuit
bound to different remote Attachment Circuit", and the processing of
the ICRQ message is complete.
These errors could occur as the result of misconfigurations.
3.2. Virtual Private LAN Service
In the VPLS application [L2VPN-REQ, VPLS], the Attachment Circuits
can be though of as LAN interfaces which attach to "virtual LAN
switches", or, in the terminology of [L2VPN-FW], "Virtual Switching
Instances" (VSIs). Each Forwarder is a VSI that attaches to a number
of PWs and a number of Attachment Circuits. The VPLS service
[L2VPN-REQ, VPLS] requires that a single pseudowire be created
between each pair of VSIs that are in the same VPLS. Each PE device
may have a multiple VSIs, where each VSI belongs to a different VPLS.
3.2.1. Provisioning
Each VPLS must have a globally unique identifier, which we call a
VPN-id. Every VSI must be configured with the VPN-id of the VPLS to
which it belongs.
Each VSI must also have a unique identifier, but this can be formed
automatically by concatenating its VPN-id with the IP address of its
PE router.
3.2.2. Auto-Discovery
3.2.2.1. BGP-based auto-discovery
The framework for BGP-based auto-discovery for a VPLS service is as
specified in [BGP-AUTO], section 3.2.
The AFI/SAFI used would be:
- An AFI specified by IANA for L2VPN. (This is the same for all
L2VPN schemes.)
- An SAFI specified by IANA specifically for an L2VPN (VPLS or
VPWS) service whose pseudowires are set up using the procedures
described in the current document.
In order to use BGP-based auto-discovery as specified in [BGP-AUTO],
the globally unique identifier associated with a VPLS must be
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encodable as an 8-byte Route Distinguisher (RD). If the globally
unique identifier for a VPLS is an RFC2685 VPN-id, it can be encoded
as an RD as specified in [BGP-AUTO]. However, any other method of
assigning a unique identifier to a VPLS and encoding it as an RD
(using the encoding techniques of [RFC2547bis]) will do.
Each VSI needs to have a unique identifier, which can be encoded as a
BGP NLRI. This is formed by prepending the RD (from the previous
paragraph) to an IP address of the PE containing the virtual LAN
switch.
(Note that it is not strictly necessary for all the VSIs in the same
VPLS to have the same RD, all that is really necessary is that the
NLRI uniquely identify a virtual LAN switch.)
Each VSI needs to be associated with one or more Route Target (RT)
Extended Communities, as discussed in [BGP-AUTO}. These control the
distribution of the NLRI, and hence will control the formation of the
overlay topology of pseudowires that constitutes a particular VPLS.
Auto-discovery proceeds by having each PE distribute, via BGP, the
NLRI for each of its VSIs, with itself as the BGP next hop, and with
the appropriate RT for each such NLRI. Typically, each PE would be a
client of a small set of BGP route reflectors, which would
redistribute this information to the other clients.
If a PE has a VSI with a particular RT, it can then receive all the
NLRI which have that same RT, and from the BGP next hop attribute of
these NLRI will learn the IP addresses of the other PE routers which
have VSIs with the same RT. The considerations of [RFC2547bis]
section 4.3.3 on the use of route reflectors apply.
If a particular VPLS is meant to be a single fully connected LAN, all
its VSIs will have the same RT, in which case the RT could be (though
it need not be) an encoding of the VPN-id. If a particular VPLS
consists of multiple VLANs, each VLAN must have its own unique RT. A
VSI can be placed in multiple VLANS (or even in multiple VPLSes) by
assigning it multiple RTs.
Note that hierarchical VPLS can be set up by assigning multiple RTs
to some of the virtual LAN switches; the RT mechanism allows one to
have complete control over the pseudowire overlay which constitutes
the VPLS topology.
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3.2.3. Signaling
It is necessary to create Attachment Identifiers which identify the
VSIs. Given that each VPLS has at most one VSI per PE, and that only
one PW is permitted between any pair of VSIs, a VSI can be uniquely
identified (relative to its PE) by the VPN-id of its VPLS. Therefore
the signaling messages can encode the VPN-id in the AGI field, and
use the null values of the SAII and TAII fields.
The VPN-id may be encoded as an [RFC2547bis] RD, in which case the
AGI field consist of a length field of value 8, followed by the 8
bytes of the RD. If the VPN-id is an RFC2685 VPN-id, it should be
encoded as an RD (as specified in [BGP-AUTO]), and then the RD should
be carried in the AGI field.
Note that it is not possible using this technique to set up more than
one PW per pair of VSIs.
3.2.4. Pseudowires as VPLS Attachment Circuits
It is also possible using this technique to set up a PW which
attaches at one endpoint to a VSI, but at the other endpoint only to
an Attachment Circuit. However, in this case there may be more than
one PW between a pair of PEs, so that AIIs cannot be null. Rather,
each such PW must have AII which is unique relative to the VPN-id.
This value would be carried in both the SAII and the TAII field of
the signaling messages.
3.3. Colored Pools: Full Mesh of Point-to-Point VCs
In the "Colored Pools" model of operation, each PE may contain
several pools of Attachment Circuits, each pool associated with a
particular VPN. A PE may contain multiple pools per VPN, as each
pool may correspond to a particular CE device. It may be desired to
create one pseudowire between each pair of pools that are in the same
VPN; the result would be to create a full mesh of CE-CE VCs for each
VPN.
3.3.1. Provisioning
Each pool is configured, and associated with:
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- a set of Attachment Circuits; whether these Attachment Circuits
must themselves be provisioned, or whether they can be auto-
allocated as needed, is independent of and orthogonal to the
procedures described in this document;
- a "color", which can be thought of as a VPN-id of some sort;
- a relative pool identifier, which is unique relative to the
color.
The pool identifier, and color, taken together, constitute a globally
unique identifier for the pool. Thus if there are n pools of a given
color, their pool identifiers can be (though they do not need to be)
the numbers 1-n.
The semantics are that a pseudowire will be created between every
pair of pools that have the same color, where each such pseudowire
will be bound to one Attachment Circuit from each of the two pools.
If each pool is a set of Attachment Circuits leading to a single CE
device, then the layer 2 connectivity among the CEs is controlled by
the way the colors are assigned to the pools. To create a full mesh,
the "color" would just be a VPN-id.
Optionally, a particular Attachment Circuit may be configured with
the relative pool identifier of a remote pool. Then that Attachment
Circuit would be bound to a particular pseudowire only if that
pseudowire's remote endpoint is the pool with that relative pool
identifier. With this option, the same pairs of Attachment Circuits
will always be bound via pseudowires.
3.3.2. Auto-Discovery
3.3.2.1. BGP-based auto-discovery
The framework for BGP-based auto-discovery for a colored pool service
is as specified in [BGP-AUTO], section 3.2.
The AFI/SAFI used would be:
- An AFI specified by IANA for L2VPN. (This is the same for all
L2VPN schemes.)
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- An SAFI specified by IANA specifically for an L2VPN (VPLS or
VPWS) service whose pseudowires are set up using the procedures
described in the current document.
In order to use BGP-based auto-discovery, the color associated with a
colored pool must be encodable as both an RT (Route Target) and an RD
(Route Distinguisher). The globally unique identifier of a pool must
be encodable as NLRI; the color would be encoded as the RD and the
pool identifier as a four-byte quantity which is appended to the RD
to create the NLRI.
Auto-discovery procedures by having each PE distribute, via BGP, the
NLRI for each of its pools, with itself as the BGP next hop, and with
the RT that encodes the pool's color. If a given PE has a pool with
a particular color (RT), it must receive, via BGP, all NLRI with that
same color (RT). Typically, each PE would be a client of a small set
of BGP route reflectors, which would redistribute this information to
the other clients.
If a PE has a pool with a particular color, it can then receive all
the NLRI which have that same color, and from the BGP next hop
attribute of these NLRI will learn the IP addresses of the other PE
routers which have pools switches with the same color. It also
learns the unique identifier of each such remote pool, as this is
encoded in the NLRI. The remote pool's relative identifier can be
extracted from the NLRI and used in the signaling, as specified
below.
3.3.3. Signaling
When a PE sends a Label Mapping message or an ICRQ message to set up
a PW between two pools, it encodes the color as the AGI, the local
pool's relative identifier as the SAII, and the remote pool's
relative identifier as the TAII.
When PE2 receives a Label Mapping message or an ICRQ message from
PE1, and the TAI identifies to a pool, and there is already an
pseudowire connecting an Attachment Circuit in that pool to an
Attachment Circuit at PE1, and the AI at PE1 of that pseudowire is
the same as the SAI of the Label Mapping or ICRQ message, then PE2
sends a Label Release or CDN message to PE1, with a Status Code
meaning "Attachment Circuit already bound to remote Attachment
Circuit". This prevents the creation of multiple pseudowires between
a given pair of pools.
Note that the signaling itself only identifies the remote pool to
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which the pseudowire is to lead, not the remote Attachment Circuit
which is to be bound to the the pseudowire. However, the remote PE
may examine the SAII field to determine which Attachment Circuit
should be bound to the pseudowire.
3.4. Colored Pools: Partial Mesh
The procedures for creating a partial mesh of pseudowires among a set
of colored pools are substantially the same as those for creating a
full mesh, with the following exceptions:
- Each pool is optionally configured with a set of "import RTs" and
"export RTs";
- During BGP-based auto-discovery, the pool color is still encoded
in the RD, but if the pool is configured with a set of "export
RTs", these are are encoded in the RTs of the BGP Update
messages, INSTEAD the color.
- If a pool has a particular "import RT" value X, it will create a
PW to every other pool which has X as one of its "export RTs".
The signaling messages and procedures themselves are as in
section 3.3.3.
3.5. Distributed VPLS
In Distributed VPLS ([L2VPN-FW], [DTLS], [LPE]), the VPLS
functionality of a PE router is divided among two systems: a U-PE and
an N-PE. The U-PE sits between the user and the N-PE. VSI
functionality (e.g., MAC address learning and bridging) is performed
on the U-PE. A number of U-PEs attach to an N-PE. For each VPLS
supported by a U-PE, the U-PE maintains a pseudowire to each other
U-PE in the same VPLS. However, the U-PEs do not maintain signaling
control connections with each other. Rather, each U-PE has only a
single signaling connection, to its N-PE. In essence, each U-PE-to-
U-PE pseudowire is composed of three pseudowires spliced together:
one from U-PE to N-PE, one from N-PE to N-PE, and one from N-PE to
U-PE.
Consider for example the following topology:
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U-PE A-----| |----U-PE C
| |
| |
N-PE E--------N-PE F
| |
| |
U-PE B-----| |-----U-PE D
where the four U-PEs are in a common VPLS. We now illustrate how PWs
get spliced together in the above topology in order to establish the
necessary PWs from U-PE A to the other U-PEs.
There are three PWs from A to E. Call these A-E/1, A-E/2, and A-E/3.
In order to connect A properly to the other U-PEs, there must be two
PWs from E to F (call these E-F/1 and E-F/2), one PW from E to B (E-
B/1), one from F to C (F-C/1), and one from F to D (F-D/1).
The N-PEs must then splice these pseudowires together to get the
equivalent of what the non-distributed VPLS signaling mechanism would
provide:
- PW from A to B: A-E/1 gets spliced to E-B/1.
- PW from A to C: A-E/2 gets spliced to E-F/1 gets spliced to F-
C/1.
- PW from A to D: A-E/3 gets spliced to E-F/2 gets spliced to F-
D/1.
It doesn't matter which PWs get spliced together, as long as the
result is one from A to each of B, C, and D.
Similarly, there are additional PWs which must get spliced together
to properly interconnect U-PE B with U-PEs C and D, and to
interconnect U-PE C with U-PE D.
One can see that distributed VPLS does not reduce the number of
pseudowires per U-PE, but it does reduce the number of control
connections per U-PE. Whether this is worthwhile depends, of course,
on what the bottleneck is.
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3.5.1. Signaling
The signaling to support Distributed VPLS can be done with the
mechanisms described in this paper. However, the procedures for VPLS
(section 3.2.3) presuppose that, between a pair of PEs, there is only
one PW per VPLS. In distributed VPLS, this isn't so. In the
topology above, for example, there are two PWs between A and E for
the same VPLS. For distributed VPLS therefore, one cannot identify
the Forwarders merely by using the VPN-id as the AGI, while using
null values of the SAII and TAII. Rather, the SAII and TAII must be
used to identify particular U-PE devices.
At a given N-PE, the directly attached U-PEs in a given VPLS can be
numbered from 1 to n. This number identifies the U-PE relative to a
particular VPN-id and a particular PE. (That is, to uniquely
identify the U-PE, the N-PE, the VPN-id, and the U-PE number must be
known.)
As a result of configuration/discovery, each U-PE must be given a
list of <j, IP address> pairs. Each element in this list tells the
U-PE to set up j PWs to the specified IP address. When the U-PE
signals to the N-PE, it sets the AGI to the proper-VPN-id, and sets
the SAII to the PW number, and sets the TAII to null.
In the above example, U-PE A would be told <3, E>, telling it to set
up 3 PWs to E. When signaling, A would set the AGI to the proper
VPN-id, and would set the SAII to 1, 2, or 3, depending on which of
the three PWs it is signaling.
As a result of configuration/discovery, each N-PE must be given the
following information for each VPLS:
- A "Local" list: {<j, IP address>}, where each element tells it to
set up j PWs to the locally attached U-PE at the specified
address. The number of elements in this list will be n, the
number of locally attached U-PEs in this VPLS. In the above
example, E would be given the local list: {<3, A>, <3, B>},
telling it to set up 3 PWs to A and 3 to B.
- A local numbering, relative to the particular VPLS and the
particular N-PE, of its U-PEs. In the above example, E could be
told that U-PE A is 1, and U-PE B is 2.
- A "Remote" list: {<IP address, k>}, telling it to set up k PWs,
for each U-PE, to the specified IP address. Each of these IP
addresses identifies a N-PE, and k specifies the number of U-PEs
at that N-PE which are in the VPLS. In the above example, E
would be given the remote list: {<2, F>}. Since N-PE E has two
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U-PEs, this tells it to set up 4 PWs to N-PE F, 2 for each of its
E's U-PEs.
The signaling of a PW from N-PE to U-PE is based on the local list
and the local numbering of U-PEs. When signaling a particular PW
from an N-PE to a U-PE, the AGI is set to the proper VPN-id, and SAII
is set to null, and the TAII is set to the PW number (relative to
that particular VPLS and U-PE). In the above example, when E signals
to A, it would set the TAII to be 1, 2, or 3, respectively, for the
three PWs it must set up to A. It would similarly signal three PWs
to B.
The LSP signaled from U-PE to N-PE is associated with an LSP from N-
PE to U-PE in the usual manner. A PW between a U-PE and an N-PE is
known as a "U-PW".
The signaling of a PW from N-PE to N-PE is based on the remote list.
When signaling a particular PW from an N-PE to an N-PE, the AGI is
set to the appropriate VPN-id. The remote list specifies the number
of PWs to set up, per local U-PE, to a particular remote N-PE. If
there are n such PWs, they are distinguished by the setting of the
TAII, which will be a number from 1 to n inclusive. The SAII is set
to the local number of the U-PE. In the above example, E would set
up 4 PWs to F. The SAII/TAII fields would be set to 1/1, 1/2, 2/1,
and 2/2 respectively. A PW between two N-PEs is known as an "N-PW".
Each U-PW must be "spliced" to an N-PW. This is based on the remote
list. If the remote list contains an element <i, F>, then i U-PWs
from each local U-PE must be spliced to i N-PWs from the remote N-PE
F. It does not matter which U-PWs are spliced to which N-PWs, as
long as this constraint is met.
If an N-PE has more than one local U-PE for a given VPLS, it must
also ensure that a U-PW from each such U-PE is spliced to a U-PW
from each of the other U-PEs.
3.5.2. Provisioning and Discovery
Every N-PE must be provisioned with the set of VPLS instances it
supports, a VPN-id for each one, and a list of local U-PEs for each
such VPLS. As part of the discovery procedure, the N-PE advertises
the number of U-PEs for each VPLS.
Auto-discovery (e.g., BGP-based) can be used to discover all the
other N-PEs in the VPLS, and for each, the number of U-PEs local to
that N-PE. From this, one can compute the total number of U-PEs in
the VPLS. This information is sufficient to enable one to compute
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the local list and the remote list for each N-PE.
3.5.3. Non-distributed VPLS as a sub-case
A PE which is providing "non-distributed VPLS" (i.e., a PE which
performs both the U-PE and N-PE functions) can interoperate with N-
PE/U-PE pairs that are providing distributed VPLS. The "non-
distributed PE" simply advertises, in the discovery procedure, that
it has one local U-PE per VPLS. And of course, the non-distributed
PE does no splicing.
If every PE in a VPLS is providing non-distributed VPLS, and thus
every PE advertises itself as an N-PE with one local U-PE, the
resultant signaling is exactly the same as that specified in section
3.2.3 above, except that SAII and TAII values of 1 are used instead
of SAII and TAII values of null. (A PE providing non-distributed
VPLS should therefore treat AII values of 1 the same as it treats AII
values of null.)
3.5.4. Inter-Provider Application of Dist. VPLS Signaling
Consider the following topology:
PE A ---- Network 1 ----- Border ----- Border ----- Network 2 ---- PE B
Router 12 Router 21 |
|
PE C
where A, B, and C are PEs in a common VPLS, but Networks 1 and 2 are
networks of different Service Providers. Border Router 12 is
Network 1's border router to network 2, and Border Router 21 is
Network 2's border router to Network 1. We suppose further that the
PEs are not "distributed", i.e, that each provides both the U-PE and
N-PE functions.
In this topology, one needs two inter-provider pseudowires: A-B and
A-C.
Suppose a Service Provider decides, for whatever reason, that it does
not want each of its PEs to have a control connection to any PEs in
the other network. Rather, it wants the inter-provider control
connections to run only between the two border routers.
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This can be achieved using the techniques of section 3.5, where the
PEs behave like U-PEs, and the BRs behave like N-PEs. In the example
topology, PE A would behave like a U-PE which is locally attached to
BR12; PEs B and C would be have like U-PEs which are locally attached
to BR21; and the two BRs would behave like N-PEs.
As a result, the PW from A to B would consist of three segments: A-
BR12, BR12-BR21, and BR21-B. The border routers would have to splice
the corresponding segments together.
This requires the PEs within a VPLS to be numbered from 1-n (relative
to that VPLS) within a given network.
3.5.5. Splicing and the Data Plane
Splicing two PWs together is quite straightforward in the MPLS data
plane, as moving a packet from one PW directly to another is just a
label replace operation on the PW label. When a PW consists of two
PWs spliced together, it is assumed that the data will go to the node
where the splicing is being done, i.e., that the data path will
include the control points.
In some cases, it may be desired to have the data go on a more direct
route from one "true endpoint" to another, without necessarily
passing through the splice points. This could be done by means of a
new LDP TLV carried in the LDP mapping message; call it the "direct
route" TLV. A direct route TLV would be placed in an LDP Label
Mapping message by the LSP's "true endpoint". The TLV would specify
the IP address of the true endpoint, and would also specify a label,
representing the pseudowire, which is assigned by that endpoint.
When PWs are spliced together at intermediate control points, this
TLV would simply be passed upstream. Then when a frame is first put
on the pseudowire, it can be given this pseudowire label, and routed
to the true endpoint, thereby possibly bypassing the intermediate
control points.
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4. Security Considerations
This document describes a number of different L2VPN provisioning
models, and specifies the endpoint identifiers that are required to
support each of the provisioning models. It also specifies how those
endpoint identifiers are mapped into fields of auto-discovery
protocols and signaling protocols.
The security considerations related to the signaling and auto-
discovery protocols are discussed in the relevant protocol
specifications ([BGP-AUTO], [L2TP-BASE], [L2TP-L2VPN], [LDP], [PWE3-
CONTROL]).
The security considerations related to the particular kind of L2VPN
service being supported are discussed in [L2VPN-REQS], [L2VPN-FW],
and [VPLS].
The way in which endpoint identifiers are mapped into protocol fields
does not create any additional security issues.
5. Acknowledgments
Thanks to Dan Tappan, Ted Qian, Bruce Davie, Ali Sajassi, Skip Booth,
and Francois LeFaucheur for their comments, criticisms, and helpful
suggestions.
Thanks to Tissa Senevirathne, Hamid Ould-Brahim and Yakov Rekhter for
discussing the auto-discovery issues.
Thanks to Vach Kompella for a continuing discussion of the proper
semantics of the generalized identifiers.
6. References
[BGP-AUTO] "Using BGP as an Auto-Discovery Mechanism for Network-
based VPNs", Ould-Brahim et. al., draft-ietf-l3vpn-bgpvpn-auto-
04.txt, May 2004
[L2TP-BASE] "Layer Two Tunneling Protocol (Version 3)", Lau et. al.,
draft-ietf-l2tpext-l2tp-base-14.txt, June 2004
[L2TP-L2VPN] "L2VPN Extensions for L2TP", Luo, draft-ietf-l2tpext-
l2vpn-01.txt, Jul 2004
[L2VPN-FW] "L2VPN Framework", Andersson et. al., draft-ietf-l2vpn-
l2-framework-05.txt, June 2004
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[L2VPN-REQ] "Service Requirements for Layer 2 Provider Provisioned
Virtual Private Network Services", Augustyn, Serbest, et. al.,
draft-ietf-l2vpn-requirements-02.txt, September 2004
[L2VPN-TERM] "PPVPN Terminology", Andersson, Madsen, draft-ietf-
l3vpn-ppvpn-terminology-04.txt, September 2004
[LDP] "LDP Specification", Andersson, et. al., RFC 3036, Jan 2001
[PWE3-ARCH] "PWE3 Architecture", Bryant, Pate, et. al., draft-ietf-
pwe3-arch-07.txt, March 2004
[PWE3-CONTROL] "Pseudowire Setup and Maintenance using LDP", Martini,
et. al., draft-ietf-pwe3-control-protocol-09.txt, September 2004
[RFC2547bis], "BGP/MPLS IP VPNs", Rosen, Rekhter, et. al., draft-
ietf-l3vpn-rfc2547bis-02.txt, September 2004
[RFC2685] "Virtual Private Networks Identifier", Fox, Gleeson,
September 1999
[VPLS] "Virtual Private LAN Services over MPLS", Laserre, et. al.,
draft-ietf-l2vpn-vpls-ldp-05.txt, September 2004
7. Author's Information
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
E-mail: erosen@cisco.com
Wei Luo
Cisco Systems, Inc.
170 W. Tasman Drive
San Jose, CA 95134
E-mail: luo@cisco.com
Vasile Radoaca
Nortel Networks
600 Technology Park
Billerica, MA 01821
Phone: (781) 856-0590/978-288-6097
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8. Intellectual Property Statement
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Copyright (C) The Internet Society (2004). This document is subject
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This document and the information contained herein are provided on an
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Rosen, et al. [Page 24]
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