Network Working Group Eric C. Rosen
Internet Draft Cisco Systems, Inc.
Expiration Date: August 2002
February 2002
Single-Sided Signaling for L2VPNs
draft-rosen-ppvpn-l2-signaling-01.txt
Status of this Memo
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Abstract
[MARTINISIG] contains a proposal for using LDP [RFC 3036] to signal
point-to-point VCs (sometimes also known as "pseudowires") across an
MPLS network. However, that draft requires that each endpoint have
apriori knowledge of the IP address other endpoint, and that both
endpoints have apriori knowledge of a common VC identifier. As a
consequence, each VC needs to be provisioned at both endpoints. In
this draft, we extend the [MARTINISIG] signaling technique so as to
eliminate the requirement for apriori knowledge of a common VC
identifier, and to eliminate the requirement that each endpoint be
known to the other. We then show the signaling can then be used for
setting up a Virtual Private LAN Service [VPLS1, VPLS2] or a full-
mesh of point-to-point VCs.
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Table of Contents
1 Specification of Requirements .......................... 2
2 Introduction ........................................... 3
3 Attachment Identifiers ................................. 5
4 Signaling .............................................. 6
4.1 Procedures ............................................. 6
4.2 FEC Element ........................................... 8
5 Individual Point-to-Point VCs .......................... 9
6 Virtual Private LAN Service ............................ 9
6.1 Provisioning ........................................... 9
6.2 Auto-Discovery ......................................... 10
6.2.1 BGP-based auto-discovery ............................... 10
6.2.2 DNS-based auto-discovery ............................... 11
6.3 Signaling .............................................. 11
7 Colored Pools: Mesh of Point-to-Point VCs .............. 11
7.1 Provisioning ........................................... 12
7.2 Auto-Discovery ......................................... 12
7.2.1 BGP-based auto-discovery ............................... 12
7.2.2 DNS-based Auto-Discovery ............................... 13
7.3 Signaling .............................................. 13
8 IETF Sub-IP Area Positioning ........................... 13
9 Security Considerations ................................ 14
10 Acknowledgments ........................................ 14
11 References ............................................. 14
12 Author's Information ................................... 15
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
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2. Introduction
In this paper we will use some terminology drawn from [L2VPN].
- Attachment VC: A VC (virtual circuit) connecting a CE (customer
edge) device to a PE (provider edge) device.
- Pseudowire: a VC connecting two attachment VCs (previously called
"emulated VC"; the term has been changed for alignment with the
work in the PWE3 WG).
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
the pseudowire itself is bidirectional.)
In order for the signaling to proceed, each endpoint has apriori
knowledge of (a) the address of the other endpoint, and (b) a VCid.
On a given pseudowire, the same VCid must be used for both LSPs.
In this context, "apriori knowledge" simply means information that
must be known prior to the initiation of signaling.
Each LSP is uniquely identified by the triple <transmitter,
responder, vcid>. (The VCid 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>
Each endpoint must also have apriori knowledge, for each pseudowire,
of the local Attachment VC to which that pseudowire is to be bound.
Unfortunately, this requires that PE1 and PE2 be explicitly
provisioned with the elements of these tuples; there does not seem to
be any way to provide an auto-discovery mechanism which eliminates
the need to provision the VCid at both endpoints, or which eliminates
the need for each endpoint to have apriori knowledge of the other's
IP address. Essentially this means that each pseudowire must be
individually provisioned both of its endpoint PEs.
The purpose of this paper is to extend the signaling of [MARTINISIG]
so that the need for this provisioning is eliminated. In particular,
we would like to be able to set up a pseudowire while requiring that
only ONE endpoint need have apriori knowledge of the IP address of
the other. (Note that if one endpoint has no apriori knowledge of
the other, it CANNOT have apriori knowledge of a VCid that the other
will use for a particular pseudowire; if one doesn't know the other
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endpoint, one cannot be sure that the VCid is unique in the context
of an LDP session to that endpoint.) There may be some identifier of
which both endpoints need to have apriori knowledge (e.g., a VPN-id,
or a VLAN identifier), but this will not be the identifier of any
individual VC.
The procedures described herein, together with an auto-discovery
procedure, will enable signaling to proceed without requiring either
endpoint to be provisioned even with the address of the other
endpoint, as the latter may be provided through the auto-discovery
procedure.
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.
(Note that although the need to provision each individual pseudowire
VC at both endpoints is eliminated, it may still be necessary to
provision the Attachment VCs. If, e.g,. the Attachment VCs pass
through a switched network, elimination of the provisioning step for
them is impossible.)
We will still require that each endpoint have apriori knowledge, for
each of its Attachment VCs, that that circuit is to be bound to a
pseudowire, but we will not require that there be apriori knowledge
of which pseudowire is bound to which Attachment VC.
In [MARTINISIG], the VCid is used for two purposes:
- to indicate that a given pseudowire is bound to a given
Attachment VC (i.e., the Attachment VC is configured with the
VCid and remote PE address);
- to indicate in LDP signaling that a given LSP in one direction is
part of the same VC as a given LSP in the other direction.
So it is necessary to have another means of achieving these
functions.
Note that although this draft is entitled "Single-sided signaling",
that is really something of a misnomer, as both endpoints do
participate in the signaling . A more accurate title would be
"Signaling Based on the Existence of Apriori Knowledge at only One
Endpoint".
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3. Attachment Identifiers
We propose to extend [MARTINISIG] by adding a new FEC type
(provisionally type 129, subject to assignment by IANA) which,
instead of specifying a VCid, may the specify following
identification fields:
- Target Attachment Identifier (TAI);
- Source Attachment Identifier (SAI);
- Attachment Group Identifier (AGI).
Each Attachment VC will be configured with an Attachment Identifier
and/or with an Attachment Group Identifier.
An Attachment Identifier is unique in the context of a single
PE, but different PEs may use different AIs for different things.
An Attachment Identifier may be used, at a given PE, to identify:
- A single Attachment VC, e.g., corresponding to an ATM VPI/VCI.
This is how it would be used when setting up individual point-
to-point VCs, as is done in [MARTINISIG].
- A pool of Attachment VCs, e.g., corresponding to a set of
Attachment VCs which connect the PE to a particular CE. This is
how it would be used when one wants to have a VC going from CE1
(at PE1) to CE2 (at PE2), but one doesn't care which CE1-PE1
connection gets bound to which CE2-PE2 connection.
- A virtual Attachment VC, e.g., corresponding to an ethernet
switching function for a particular VLAN. This is how it would
be used when providing a transparent LAN service, in which a set
of Virtual Switches for a particular VLAN have to be meshed via
point-to-point VCs.
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 intention is that an LSP will now be identified by:
<PE1, AI1, PE2, AI2>,
the LSP in the opposite direction will be identified by:
<PE2, AI2, PE1, AI1>,
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and an pseudowire is a pair of such LSPs.
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.)
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".
When an Attachment VC or pool is provisioned, it is configured
with an Attachment Identifier, or an Attachment Group Identifier, or
both.
4. Signaling
4.1. Procedures
In order for PE1 to begin signaling PE2, PE1 must know the address
of the remote PE2, and a TAI. While this information may be
configured at PE1, it is expected that it would be learned
dynamically via some autodiscovery procedure. To PE1, the auto-
discovery process is a function which maps from an AGI to a set of
<PE, AI> pairs.
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 type includes the following:
- SAI
- AGI, if one has been configured.
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- TAI (SAI at the remote PE).
What happens when PE2 receives such a Label Mapping message?
First, PE2 interprets the TAI. Exactly how PE2 does this mapping is
a local matter. The TAI might, e.g., be a string which
identifies a particular Attachment VC, such as "ATM3VPI4VCI5", or
it might, e.g., be a string such as "Fred" which is associated by
configuration with a particular Attachment VC, or it might be a VPN-
id of some sort, depending on the application.
If PE2 cannot map the TAI to anything, 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 TAI maps to an Attachment VC (or pool) which is NOT
configured with the same AGI that is specified in the Label Mapping
message, a corresponding Label Release message, with a Status Code
meaning "no AGI in common" is sent to PE1, and the processing of the
Mapping message is complete.
The AGI of an Attachment VC (or pool) and the Label Mapping
Message's AGI are considered to be the same if and only if:
- The Attachment VC or pool is configured with an AGI, and that
same AGI occurs in the message, and
- The Attachment VC or pool is not configured with an AGI, no AGI
occurs in the message.
If the TAI maps to a single Attachment VC, and that Attachment VC 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 VC bound to different PE", and the processing of
the Mapping message is complete.
If the TAI maps to a single Attachment VC, and that Attachment VC 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 SAI field of the Mapping message, then PE2
sends a Label Release message to PE1, with a Status Code meaning
"Attachment VC bound to different remote Attachment VC", and the
processing of the Mapping message is complete.
If the TAI maps to a pool of Attachment VCs, and there is already an
pseudowire connecting an Attachment VC in that pool to an Attachment
VC at PE1, and the AI at PE1 of that pseudowire is the same as the
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SAI of the Label Mapping message, then PE2 sends a Label Release
message to PE1, with a Status Code meaning "Attachment VC bound to
different remote Attachment VC", and the processing of the Mapping
message is complete.
If PE2 is still processing the Label Mapping message at this point,
then it accepts the label (provided of course that there are no
"standard" LDP errors). Then it has to make sure that an LSP is set
up in the opposite (PE1-->PE2) direction. If it has already signaled
for an 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.
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 as extended in [VPLS2] and
[shah].
C and VC info length are as defined in [MARTINISIG].
Parameters are:
- SAI, encoded as a one byte length field followed by the SAI.
- TAI, encoded as a one byte length field followed by the TAI.
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- AGI, encoded as a one byte length field followed by the AGI.
- Interface parameters, as defined in [MARTINISIG].
5. Individual Point-to-Point VCs
One model of operation which the above procedures can support is the
following. Each PE is configured with the necessary set of
Attachment VCs, and each Attachment VC is configured with a VPN-id, a
local name (which is unique relative to the VPN-id), and a remote
name. The intent is that the local Attachment VC is supposed to be
connected to the remote Attachment VC with the specified remote name.
As part of an auto-discovery procedure, each PE advertises its <VPN-
id, local name> pairs. Each PE compares its local <VPN-id, remote
name> pairs with the <VPN-id, local name> pairs advertised by the
other PEs. If PE1 has a local <VPN-id, remote name> pair with value
<V, fred>, and PE2 has a local <VPN-id, local name> pair with value
<V, fred>, PE1 will thus be able to discover that it needs to connect
to PE2, and will use "fred" as the TAI. The SAI is not used.
In this model of operation, the Attachment VCs and the pseudowires
are point-to-point. The primary benefit of this model of operation
is that it enables you to move an Attachment VC from one PE to
another without having to reconfigure the remote endpoint.
6. Virtual Private LAN Service
In the VPLS model of operation, the Attachment VCs can be though of
as LAN interfaces which attach to "virtual LAN switches". The VPLS
service [VPLS1, VPLS2] requires that a pseudowire be created between
each pair of virtual LAN switches that are in a common VPLS. Each PE
device may have a number of virtual LAN switches, where each of those
virtual LAN switches belongs to a different VPLS.
6.1. Provisioning
Each VPLS must have a globally unique identifier. Every virtual LAN
switch must be configured with the identifier of the VPLS to which it
belongs.
Each virtual LAN switch must also have a unique identifier, but this
can be formed automatically by concatenating its VPLS identifier with
the IP address of the PE router in which that virtual LAN switch
exists.
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6.2. Auto-Discovery
6.2.1. 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 virtual LAN switch 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 virtual LAN
switches 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 virtual LAN switch 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 virtual LAN switches, 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 virtual LAN switch 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
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PE routers which have virtual LAN switches 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 virtual LAN switches 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 virtual LAN switch 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.
6.2.2. DNS-based auto-discovery
[DNS-LDP-VPLS] includes a proposal for using DNS-based auto-
discovery.
6.3. Signaling
The AI of a particular virtual LAN interface is identical to the RT
described above in section 6.2.1. (If DNS-based auto-discovery is
being used, this would likely be just an RFC2685 VPN-id encoded as an
RT.)
As the AI is the same at both ends of the pseudowire, only the TAI
field need be provided; the SAI field can be omitted. The AGI field
is omitted as well.
This means that the TAI field of the FEC element will consist of a
length byte whose value is 8, followed by the 8-byte RT. The SAI and
AGI fields will consist solely of a length byte whose value is 0.
7. Colored Pools: Mesh of Point-to-Point VCs
In the "Colored Pools" model of operation, each PE may contain
several pools of Attachment VCs, 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.
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This model of operation can be used to provide the same service as in
[BGP-SIGNALING]. However, the pseudowire setup is done with LDP
rather than BGP, and there is reduced provisioning, as the
configuration of CE-ids and label ranges is not required.
7.1. Provisioning
Each pool is configured, and associated with:
- a set of Attachment VCs; whether these Attachment VCs 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 in the simplest case is simply a VPN-id of some
sort;
- a globally unique identifier.
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 VC from each of the two pools. If
each pool is a set of Attachment VCs 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 make use of the auto-
discovery scheme of [BGP-AUTO], the colors must be encodable as RTs,
and the unique pool identifiers must be encodable as RDs.
7.2. Auto-Discovery
7.2.1. 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 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 an RT (Route Target) and the unique
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identifier of a pool must be encodable as an RD (Route
Distinguisher), as discussed in [BGP-AUTO]. If a full-mesh
configuration is desired, the RT can be, though it need not be, the
encoding of an RFC 2685 VPN-id.
The NLRI for a given pool would consist of its RD prepended to a
loopback address of the PE router in which it exists.
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 RD.
7.2.2. DNS-based Auto-Discovery
The use of DNS-based auto-discovery for the colored pool model of
operation is for further study.
7.3. Signaling
When a PE sends a Label Mapping message to set up a VC between two
pools, the local pool's unique identifier is encoded as the SAI and
the remote pool's unique identifier is encoded as the TAI.
8. 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.
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9. 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.
10. Acknowledgments
Thanks to Dan Tappan and Ted Qian for their comments. Thanks to
Tissa Senevirathne, Hamid Ould-Brahim and Yakov Rekhter for
discussing the auto-discovery issues.
11. 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-01.txt, November 2001
[DNS-LDP-VPLS] "DNS/LDP Based VPLS", Heinanen, draft-heinanen-dns-
ldp-vpls-00.txt, January 2002
[L2VPN] "An Architecture for L2VPNs", Rosen et. al., draft-ietf-
ppvpn-l2vpn-00.txt, July 2001.
[MARTINISIG] "Transport of Layer 2 Frames Over MPLS", Martini et.
al., draft-martini-l2circuit-trans-mpls-08.txt, November 2001.
[RFC2547bis], "BGP/MPLS VPNs", Rosen, Rekhter, et. al., draft-ietf-
ppvpn-rfc2547bis-01.txt, January 2002
[RFC3036] "LDP Specification", January 2001.
[VPLS1] "Requirements for Virtual Private LAN Services (VPLS)",
Augustyn et. al., draft-augustyn-vpls-requirements-00.txt, October
2001.
[VPLS2] "Transparent VLAN Services over MPLS", Laserre, et. al.,
draft-lasserre-vkompella-ppvpn-vpls-00.txt, November 2001.
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12. Author's Information
Eric C. Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: erosen@cisco.com
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