Network Working Group Eric C. Rosen
Internet Draft Cisco Systems, Inc.
Expiration Date: March 2003
September 2002
LDP-based Signaling for L2VPNs
draft-rosen-ppvpn-l2-signaling-02.txt
Status of this Memo
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Abstract
[MARTINISIG] specifies a way of using LDP [RFC3036] to set up and
maintain pseudowires [PWE3-FR]. It requires that each endpoint have
apriori knowledge of the IP address other endpoint, and that both
endpoints have apriori knowledge of a common 32-bit pseudowire
identifier. While this is adequate for the case in which pseudowires
are provisioned individually within a single Service Provider's
network, there are a variety of PPVPN provisioning models [L2VPN-FW]
for which it is not adequate. In particular it is not adequate if it
is desired to provision a pseudowire at only one endpoint, or if it
is desired to use auto-discovery mechanisms to provision a mesh of
pseudowires. It also is not adequate for inter-provider Virtual
Private LAN Services (VPLS), or for distributed VPLS. The current
draft extends [MARTINISIG] so that LDP-based signaling can be used
for these cases.
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Table of Contents
1 Specification of Requirements ...................... 3
2 Introduction ....................................... 3
2.1 Deficiencies of Martini Signaling .................. 3
2.2 Protocol Framework ................................. 5
2.2.1 Endpoint Identification ............................ 5
2.2.2 Association of two LSPs as one Pseudowire .......... 6
3 Attachment Identifiers ............................. 6
4 Signaling .......................................... 7
4.1 Procedures ......................................... 8
4.2 FEC Element ........................................ 9
5 Applications ....................................... 10
5.1 Individual Point-to-Point VCs ...................... 10
5.1.1 Provisioning Models ................................ 10
5.1.1.1 Double Sided Provisioning .......................... 10
5.1.1.2 Single Sided Provisioning with Discovery ........... 10
5.1.2 Signaling .......................................... 11
5.2 Virtual Private LAN Service ........................ 11
5.2.1 Provisioning ....................................... 12
5.2.2 Auto-Discovery ..................................... 12
5.2.2.1 BGP-based auto-discovery ........................... 12
5.2.2.2 DNS-based auto-discovery ........................... 13
5.2.3 Signaling .......................................... 13
5.3 Colored Pools: Full Mesh of Point-to-Point VCs ..... 14
5.3.1 Provisioning ....................................... 14
5.3.2 Auto-Discovery ..................................... 15
5.3.2.1 BGP-based auto-discovery ........................... 15
5.3.2.2 DNS-based Auto-Discovery ........................... 16
5.3.3 Signaling .......................................... 16
5.4 Colored Pools: Partial Mesh ........................ 16
5.5 Distributed VPLS ................................... 17
5.5.1 Signaling .......................................... 18
5.5.2 Provisioning and Discovery ......................... 20
5.5.3 Non-distributed VPLS as a sub-case ................. 20
5.5.4 An Inter-Provider Application of Distributed VPLS Signaling 20
5.5.5 Splicing and the Data Plane ........................ 21
6 Backwards Compatibility ............................ 22
7 IETF Sub-IP Area Positioning ....................... 22
8 Security Considerations ............................ 22
9 Acknowledgments .................................... 23
10 References ......................................... 23
11 Author's Information ............................... 24
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1. Specification of Requirements
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. Introduction
We make free use of terminology from [L2VPN-FW], [L2VPN-TERM], and
[PWE3-FR], in particular the terms "Attachment Circuit",
"pseudowire", "PE", "CE".
2.1. Deficiencies of Martini Signaling
In [MARTINISIG], a pseudowire consists of two LSPs (Label Switched
Paths), one in each direction. Each endpoint initiates the setup of
the LSP that carries packets in the "incoming" direction. (Note that
although each LSP is unidirectional, the pseudowire itself is
bidirectional.)
Each LSP is uniquely identified by the triple <transmitter,
responder, VCid>. (The VCid is a 32-bit quantity which must be
unique in the context of a single LDP session between PE1 and PE2.)
A pseudowire is a pair of LSPs:
<PE1, PE2, VCid_x, VC_type_y>, <PE2, PE1, VCid_x, VC_type_y>
In order for the signaling to proceed, each endpoint must have
apriori knowledge of (a) the IP address of the other endpoint, and
(b) the 32-bit VCid. For a given pseudowire, the same VCid must be
used when setting up both of the LSPs. In this context, "apriori
knowledge" simply means information that must be known prior to the
initiation of signaling.
Each endpoint must also have apriori knowledge, for each pseudowire,
of the local Attachment Circuit to which that pseudowire is to be
bound.
This gives rise to a number of problems:
- In VPLS provisioning, (a) the PE devices are provisioned with
VPN-ids, (b) auto-discovery is used to allow a given PE to
discover other PEs with which it has a VPN-id in common, (c) a
mesh of pseudowires is then set up among these PEs.
As this provisioning model does not assign any identifiers to the
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pseudowires, other than the VPN-id itself, the only way to use
[MARTINISIG] to set up these pseudowires is to treat the VPN-id
as if it were VCid. However, [MARTINISIG] only allows 32 bits
for encoding a VCid. This is not adequate for VPLS. To
accommodate inter-provider VPLS, VPN-ids must be globally unique.
There are a number of schemes for assigning globally unique VPN-
ids, but in general they require more than 32 bits for a VPN-id.
(E.g., [RFC2685] uses 7 bytes; [RFC2547bis] uses 8 bytes for the
quantities that play the role of VPN-id; [DNS-L2TP-VPLS] uses
arbitrarily long DNS names.) An extension is needed to allow
VPN-ids longer than 32 bits to be carried by the signaling
protocol.
- In distributed VPLS [e.g. L2VPN-FW section 3.4.3], for a given
VPN-id there may be more than one pseudowire between a given pair
of nodes. This makes it impossible to treat the VPN-id as a
pseudowire identifier; the VPN-id can be at most part of the
pseudowire identifier.
- In the VPWS "colored pools provisioning model" of [L2VPN-FW]
section 3.3.1.3, or the provisioning model of [BGP-SIGNALING],
the basic identification mechanisms are endpoint identifiers,
rather than pseudowire identifiers. A pseudowire can then be
identified by a pair of endpoint identifiers. Encoding a pair of
endpoint identifiers into a single 32-bit VCid field would be
very restrictive.
- It is sometimes desirable for all the pseudowire signaling
information to be provisioned at one end of the pseudowire,
without any need to provision the other end in advance of
signaling. This would be necessary, for example, if one were
using the pseudowires to emulate Switched VCs rather than
Permanent VCs. This immediately rules out any signaling technique
in which both endpoints need apriori knowledge of a common
identifier.
These problems can be eliminated with a small number of relatively
minor extensions to [MARTINISIG]. The purpose of this paper is to
specify those extensions, and to show how the resulting protocol can
be used together with an auto-discovery mechanism to support a large
set of L2VPN provisioning models.
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 LDP-based signaling mechanisms can be integrated with BGP-
based auto-discovery is covered in some detail. Later revisions of
this draft will provide equivalent detail for other discovery
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mechanisms.
2.2. Protocol Framework
2.2.1. 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. We use the term "Attachment Identifier", or "AI", to
refer to a quantity whose purpose is to identify a Forwarder.
In [MARTINISIG], the only identifier is the VCid. The implicit
endpoint identifiers are "the Forwarder that is configured to be the
endpoint of the pseudowire identified by the specified VCid". We
propose to replace the single VCid of [MARTINISIG] with a pair of
Attachment Identifiers, one for each of the two endpoints.
Although this draft discusses only LDP-based signaling, it is
possible that the very same Attachment Identifier mechanism will be
useful for L2TP-based signaling. This issue is for further study.
Different Forwarders support different applications. The particular
application for which a given PW is used will depend on the Forwarder
that is identified when setting up the PW. There is, for example, no
signaling to distinguish a PW used in VPLS from a PW used in a
point-to-point service, as this can be inferred from the identity of
the Forwarder.
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2.2.2. Association of two LSPs as one 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.
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.
3. Attachment Identifiers
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).
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:
<PE2, <AGI, AII2>, PE1, <AGI, AII1>>;
a pseudowire is a pair of such LSPs.
When a signaling message is sent from PE1 to PE2, and PE1 needs to
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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 a Label Mapping
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".
4. Signaling
An LDP Label Mapping message contains a FEC TLV, a Label TLV, and
zero or more optional parameter TLVs. [MARTINISIG] defines a FEC
TLV, containing a single FEC element, containing a VC type, a fixed
length group id, a fixed length VCid, and variable length "interface
parameters".
We propose to extend [MARTINISIG] by adding a new FEC type
(provisionally type 129, subject to assignment by IANA) in which the
group id and and VCid are eliminated, and their place taken by
variable length SAII, AGI, and TAII fields. In other respects, the
Label Mapping messages will be the same.
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4.1. Procedures
In order for PE1 to begin signaling PE2, PE1 must know the address of
the remote PE2, and a TAI. This information may have been configured
at PE1, or it may have been learned dynamically via some
autodiscovery procedure.
To begin the signaling procedure, a PE (PE1) that has knowledge of
the other endpoint (PE2) initiates the setup of the LSP in the
incoming (PE2-->PE1) direction by sending a Label Mapping message
containing the new FEC type. The FEC element includes the SAII, AGI,
and TAII.
What happens when PE2 receives such a Label Mapping message?
PE2 interprets the message as a request to set up a PW whose endpoint
(at PE2) is the Forwarder identified by the TAI. From the
perspective of the signaling protocol, exactly how PE2 maps AIs to
Forwarders is a local matter. In some VPWS provisioning models, the
TAI might, e.g., be a string which identifies a particular Attachment
Circuit, such as "ATM3VPI4VCI5", or it might, e.g., be a string such
as "Fred" which is associated by configuration with a particular
Attachment Circuit. In VPLS, the TAI would be a VPN-id, identifying
a particular VPLS instance.
If PE2 cannot map the TAI to one of its Forwarders, then PE2 sends a
Label Release message to PE1, with a Status Code meaning "invalid
TAI", and the processing of the Mapping message is complete.
If the Label Mapping Message has a valid TAI, PE2 must decide whether
to accept it or not. The procedures for so deciding will depend on
the particular type of Forwarder identified by the TAI. As the
details are specific to the type of Forwarder, they are specified in
later sections where we discuss the different provisioning models
that can be supported.
Of course, the Label Mapping message may be rejected due to standard
LDP error conditions as detailed in [LDP].
If PE2 decides to accept the Label Mapping message, then it has to
make sure that an LSP is set up in the opposite (PE1-->PE2)
direction. If it has already signaled for the corresponding LSP in
that direction, nothing more need be done. Otherwise, it must
initiate such signaling by sending a Label Mapping message to PE1.
This is very similar to the Label Mapping message PE2 received, but
with the SAI and TAI reversed.
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4.2. FEC Element
FEC element type 129 is used. The FEC element is encoded as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 129 |C| VC Type |VC info Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameters |
| " |
| " |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
VC Type is as defined in [MARTINISIG] and its various extensions
[e.g., VPLS2].
C and VC info length are as defined in [MARTINISIG].
Parameters are:
- SAII, encoded as a one byte length field followed by the SAI.
- TAII, encoded as a one byte length field followed by the TAI.
- AGI, encoded as a one byte length field followed by the AGI.
- Interface parameters, as defined in [MARTINISIG].
The SAII, TAII, and AGI are simply carried as octet strings. The
length byte specifies the size of the field, excluding the length
byte itself. The null string can be sent by setting the length byte
to 0.
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5. Applications
In this section, we specify the way in which the above procedures are
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.
5.1. 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.
5.1.1. Provisioning Models
5.1.1.1. 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.
5.1.1.2. 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. A null SAII is sent.
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.
5.1.2. Signaling
Signaling is as specified in section 4 above, with the addition of
the following:
When a PE receives a Label Mapping Message, and the TAI identifiers 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.
These errors could occur as the result of misconfigurations.
5.2. Virtual Private LAN Service
In the VPLS application [VPLS1, VPLS2], 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 [VPLS1,
VPLS2] 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.
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5.2.1. 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.
5.2.2. Auto-Discovery
5.2.2.1. 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 a VPLS 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
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.
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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.
5.2.2.2. DNS-based auto-discovery
[DNS-LDP-VPLS] includes a proposal for using DNS-based auto-
discovery.
5.2.3. 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.
If the VPN-id is a DNS name, the first byte of the AGI field
(immediately following the length) will be 0x90. This distinguishes
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it from any RD. The DNS name itself then follows.
Note that it is not possible using this technique to set up more than
one PW per pair of VSIs.
5.3. 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. (This application was originally suggested in [BGP-SIGNALING];
we show here that it can be done with LDP-based signaling.)
5.3.1. Provisioning
Each pool is configured, and associated with:
- 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
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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.
5.3.2. Auto-Discovery
5.3.2.1. 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.)
- An SAFI specified by IANA specifically for a Colored Pool L2VPN
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.
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5.3.2.2. DNS-based Auto-Discovery
The use of DNS-based auto-discovery for the colored pool model of
operation is for further study.
5.3.3. Signaling
When a PE sends a Label Mapping 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 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 message, then PE2 sends a Label Release message to PE1,
with a Status Code meaning "Attachment Circuit bound to different
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
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.
5.4. 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 5.3.3
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5.5. 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:
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. In distributed VPLS,
there will be three PWs from A to E. Call these A-E/1, A-E/2, and A-
E/3. There will be two PWs from E to F. Call these E-F/1 and E-F/2.
And there will be three more PWs, one 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.
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- 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.
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.
5.5.1. Signaling
The signaling to support Distributed VPLS can be done with the
mechanisms described in this paper. However, the procedures for VPLS
(section 5.2.3) 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
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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 of its U-PEs, relative to a VPLS. 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 has two 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, as specified in section 4. 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 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.
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5.5.2. 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
the local list and the remote list for each N-PE.
5.5.3. Non-distributed VPLS as a sub-case
A PE which is providing "non-distributed VPLS" (i.e., a PE which
peforms 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
5.2.3 above, except that SAII and TAII values of 1 are used instead
of SAII and TAII values of null.
5.5.4. An Inter-Provider Application of Distributed 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.
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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.
This can be achieved using the techniques of section 5.5, 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.
5.5.5. 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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6. Backwards Compatibility
It may be desirable to have nodes which can use either the procedures
described herein, or the unaltered procedures of [MARTINISIG]. In
that case, the procedures described herein would be used if and only
if both sides were capable of using them.
This can be done by defining a new TLV for the LDP Label Mapping
message. Call it the "Extended L2 Signaling TLV". A node which can
support the messages and procedures of this draft as well as the
messages and procedures of [MARTINISIG] would, if so configured,
initiate signaling using the [MARTINISIG] messages, but including the
Extended L@ Signaling TLV in the LDP Mapping Message.
If the other node does not understand this TLV, it will simply ignore
it, and [MARTINISIG] will be used.
When a node which supports this backwards compatibility feature
receives an LDP mapping message containing a [MARTINISIG] FEC, but
with the Extended L2 Signaling TLV, it will send a corresponding
Label Release message, and will re-initiate signaling of that
pseudowire with the messages described in this draft.
7. IETF Sub-IP Area Positioning
This draft is targeted at both the PPVPN WG and the MPLS WG. It
appears to be in the province of the PPVPN WG to consider the
requirements of signaling to support layer 2 VPNs. Specification in
detail of the actual extensions to LDP would appear to be the
province of the MPLS WG.
8. Security Considerations
The signaling procedures specified herein require that a node
initiate and/or accept LDP sessions with entities that are not
necessarily directly connected to that node. It would be advisable
for a given node to use access control to restrict the set of nodes
that can set up LDP sessions with it, and it would be advisable to
use some form of authentication to guarantee that the remote endpoint
of an LDP session is the entity that it claims to be. Using the TCP
MD5 option may be adequate, or alternatively IPsec can be used.
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9. Acknowledgments
Thanks to Dan Tappan, Ted Qian, Bruce Davie, Ali Sajassi, Wei Luo,
and Skip Booth for their comments, criticisms, and helpful
suggestions.
Thanks to Tissa Senevirathne, Hamid Ould-Brahim and Yakov Rekhter for
discussing the auto-discovery issues.
10. References
[BGP-AUTO] "Using BGP as an Auto-Discovery Mechanism for Network-
based VPNs", Ould-Brahim et. al., draft-ietf-ppvpn-bgpvpn-auto-
02.txt, February 2002.
[BGP-SIGNALING] "Layer 2 VPNs over Tunnels", Kompella et. al.,
draft-kompella-ppvpn-l2vpn-02.txt, June 2002
[DNS-L2TP-VPLS] "DNS/LDP Based VPLS", Heinanen, draft-heinanen-dns-
ldp-vpls-00.txt, June 2002
[L2VPN-FW] "PPVPN L2 Framework", Andersson et. al., draft-ietf-
ppvpn-l2-framework-00.txt, August 2002
[L2VPN-TERM] "PPVPN Terminology", Andersson, Madsen, draft-
andersson-ppvpn-terminology-01.txt, June 2002
[LDP] "LDP Specification", Andersson, et. al., RFC 3036, January 2001
[MARTINISIG] "Transport of Layer 2 Frames Over MPLS", Martini et.
al., draft-martini-l2circuit-trans-mpls-10.txt, August 2002
[PWE3-FR] " Framework for Pseudo Wire Emulation Edge-to-Edge ",
draft-pate-pwe3-framework-03.txt, January 2002
[RFC2547bis], "BGP/MPLS VPNs", Rosen, Rekhter, et. al., draft-ietf-
ppvpn-rfc2547bis-02.txt, July 2002
[RFC2685] "Virtual Private Networks Identifier", Fox, Gleeson,
September 1999
[RFC3036] "LDP Specification", January 2001
[VPLS1] "Requirements for Virtual Private Network Services", Augustyn
et. al., draft-augustyn-ppvpn-l2vpn-requirements-00.txt, June 2002
[VPLS2] "Transparent VLAN Services over MPLS", Laserre, et. al.,
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draft-lasserre-vkompella-ppvpn-vpls-02.txt, June 2002
11. Author's Information
Eric C. Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: erosen@cisco.com
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