Network Working Group K. Kompella (Juniper)
Internet Draft Y. Rekhter (Juniper)
V. Kompella (TiMetra)
Expires: November 2003 J. Achirica (Telefonica)
draft-ietf-l2vpn-vpls-bgp-00.txt L. Andersson (Utfors)
G. Heron (PacketExchange)
S. Khandekar (TiMetra)
M. Lasserre (Riverstone)
P. Lin (Yipes)
P. Menezes (Terabeam)
A. Moranganti (Appian)
H. Ould-Brahim (Nortel)
S. Yeong-il (Korea Tel)
Virtual Private LAN Service
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
Virtual Private LAN Service (VPLS), also known as Transparent LAN
Service, and Virtual Private Switched Network service, is a useful
Service Provider offering. The service offered is a Layer 2 VPN;
however, in the case of VPLS, the customers in the VPN are connected
by a multipoint network, in contrast to the usual Layer 2 VPNs, which
are point-to-point in nature.
This document describes the functions required to offer VPLS, and
proposes a mechanism for signaling a VPLS, as well as for forwarding
VPLS frames across a packet switched network.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [1].
The terms P (Provider router, VPN-unaware), PE (Provider Edge
router), VE (VPLS Edge device), CE (Customer Edge device), etc. are
defined in [2].
1. Introduction
Virtual Private LAN Service (VPLS), also known as Transparent LAN
Service, and Virtual Private Switched Network service, is a useful
service offering. A Virtual Private LAN appears in (almost) all
respects as a LAN to customers of a Service Provider. However, in a
VPLS, the customers are not all connected to a single LAN; the
customers may be spread across a metro or wide area. In essence, a
VPLS glues several individual LANs across a metro area to appear and
function as a single LAN [3].
This document describes the functions needed to offer VPLS, and goes
on to propose a mechanism for signaling a VPLS, as well as a
mechanism for transport of VPLS frames over tunnels across a packet
switched network. The signaling mechanism is taken from [4]; BGP is
used as the control plane protocol. This document also discusses
deployment options, in particular, the notion of decoupling functions
across devices.
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Alternative approaches include: [4], which allows one to build a
Layer 2 VPN with Ethernet as the interconnect; and [5], which allows
one to set up an Ethernet connection across a packet switched
network. Both of these, however, offer point-to-point Ethernet
services. What distinguishes VPLS from the above two is that a VPLS
offers a multipoint service. A mechanism for setting up pseudowires
for VPLS using LDP is defined in [6].
1.1. Scope of this Document
This document has four major parts: defining a VPLS functional model;
defining a control plane for setting up VPLS; defining the data plane
for VPLS (encapsulation and forwarding of data); and defining various
deployment scenarios.
The functional model underlying VPLS is laid out in section 2. This
describes the service being offered, the network components that
interact to provide the service, and at a high level their
interactions.
The control plane proposed here (section 3) uses BGP to establish
VPLS service, i.e., for autodiscovery of VPLS members and for the
setup and teardown of the pseudowires that constitute a given VPLS.
Section 3 also describes how a VPLS that spans Autonomous System
boundaries is set up. Using BGP as the control plane for VPNs is not
new (see [4], [7] and [8]): what is described here is identical to
what is in [4], which itself is based on the mechanisms proposed in
[7].
The forwarding plane and the actions that a participating PE must
take is described in section 4.
In section 5, the notion of 'decoupled' operation is defined, and the
interaction of decoupled and non-decoupled PEs is described.
Decoupling allows for more flexible deployment of VPLS.
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2. Functional Model
This will be described with reference to Figure 1.
Figure 1: Example of a VPLS
-----
/ A1 \
---- ____CE1 |
/ \ -------- -------- / | |
| A2 CE2- / \ / PE1 \ /
\ / \ / \___/ | \ -----
---- ---PE2 | \
| | \ -----
| Service Provider Network | \ / \
| | CE5 A5 |
| ___ | / \ /
|----| \ / \ PE4_/ -----
|L2PE|--PE3 / \ /
|----| -------- -------
---- / | ----
/ \/ \ / \ CE = Customer Edge Device
| A3 CE3 --CE4 A4 | PE = Provider Edge Router
\ / \ / L2PE = Layer 2 Aggregation
---- ----
2.1. Terminology
(NOTE: the terminology here has not quite been fully harmonized with
the terminology in the VPN terminology document [2] or the PWE3
framework document; this will be done in a later version.)
The terminology of [4] is used, with the addition of "L2PE", a Layer
2 PE device used for Layer 2 aggregation. An L2PE is owned and
operated by the Service Provider (as is the PE). PE and L2PE devices
are "VPLS-aware", which means that they know that a VPLS service is
being offered. We will call these VPLS edge devices, which could be
either a PE or an L2PE, a VE.
In contrast, the CE device (which may be owned and operated by either
the SP or the customer) is VPLS-unaware; as far as the CE is
concerned, it is connected to the other CEs in the VPLS via a Layer 2
switched network. This means that there should be no changes to a CE
device, either to the hardware or the software, in order to offer
VPLS.
A CE device may be connected to a PE or an L2PE via Layer 2 switches
that are VPLS-unaware. From a VPLS point of view, such Layer 2
switches are invisible, and hence will not be discussed further.
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Furthermore, an L2PE may be connected to a PE via Layer 2 and Layer 3
devices; this will be discussed further in a later section.
The term "demultiplexor" refers to an identifier in a data packet
that identifies both the VPLS to which the packet belongs as well as
the ingress PE. In this document, the demultiplexor is an MPLS
label.
The term "VPLS" will refer to the service as well as a particular
instantiation of the service (i.e., an emulated LAN); it should be
clear from the context which usage is intended.
2.2. Assumptions
The Service Provider Network is a packet switched network. The PEs
are assumed to be full meshed with tunnels over which packets that
belong to a service (such as VPLS) are encapsulated and forwarded.
These tunnels can be IP tunnels, such as GRE, or MPLS tunnels,
established by RSVP-TE or LDP. These tunnels are established
independently of the services offered over them; the signaling and
establishment of these tunnels are not discussed in this document.
"Flooding" and MAC address "learning" (see section 4) are an integral
part of VPLS. However, these activities are private to an SP device,
i.e., in the VPLS described below, no SP device requests another SP
device to flood packets or learn MAC addresses on its behalf.
All the PEs participating in a VPLS are assumed to be fully meshed,
i.e., every (ingress) PE can send a VPLS packet to the egress PE(s)
directly, without the need for an intermediate PE. This assumption
reduces (but does not eliminate) the need to run Spanning Tree
Protocol among the PEs.
2.3. Interactions
VPLS is a successful "LAN Service" if CE devices that belong to VPLS
V can interact through the SP network as if they were connected by a
LAN. VPLS is "private" if CE devices that belong to different VPLSs
cannot interact. VPLS is "virtual" if multiple VPLSs can be offered
over a common packet switched network.
PE devices interact to "discover" who all participate in the same
VPLS (i.e., are attached to CE devices that belong to the same VPLS),
and to exchange demultiplexors. These interactions are control-
driven, not data-driven.
L2PEs interact with PEs to establish connections with remote PEs or
L2PEs in the same VPLS. Again, this interaction is control-driven.
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3. Control Plane
There are two primary functions of the VPLS control plane:
autodiscovery, and setup and teardown of the pseudowires that
constitute the VPLS, often called signaling. The first two
subsections describe these functions. The last subsection describes
the setting up of pseudowires that span Autonomous Systems.
3.1. Autodiscovery
Autodiscovery refers to the process of finding all the PEs that
participate in a given VPLS. A PE can either be configured with the
identities of all the other PEs in a given VPLS, or the PE can
autodiscover the other PEs.
The former approach is fairly configuration-intensive, especially
since it is required (in this and other VPLS approaches) that the PEs
participating in a given VPLS are fully meshed (i.e., every pair of
PEs in a given VPLS establish pseudowires to each other).
Furthermore, when the topology of a VPLS changes (i.e., a PE is added
to, or removed from the VPLS), the VPLS configuration on all PEs in
that VPLS must be changed.
In the autodiscovery approach, each PE "discovers" which other PEs
are part of a given VPLS by means of some protocol. In this
approach, each PE's configuration consists only of the identity of
the VPLS that each customer belongs to, not the identity of every
other PE in that VPLS. Moreover, when the topology of a VPLS
changes, only the affected PE's configuration changes; other PEs
automatically find out about the change and adapt.
3.1.1. Functions
A PE that participates in a given VPLS V must be able to tell all
other PEs in VPLS V that it is also a member of V. A PE must also
have a means of declaring that it no longer participates in a VPLS.
To do both of these, the PE must have a means of identifying a VPLS
and a means by which to communicate to all other PEs.
L2PE devices also need to know what constitutes a given VPLS;
however, they don't need the same level of detail. The PE (or PEs)
to which an L2PE is connected gives the L2PE an abstraction of the
VPLS; this is described in section 5. One protocol mechanism to
achieve this is described in [9].
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3.1.2. Protocol Specification
The specific mechanism for autodiscovery described here, borrowed
essentially unchanged from [4] and [7], uses BGP extended communities
[10] to identify a VPLS. This mechanism is described more
generically in [8]. The specific extended community used is the
Route Target, whose format is described in [10]. The semantics of
the use of Route Targets is described in [7]; their use in VPLS is
identical.
As it has been assumed that VPLSs are fully meshed, a single Route
Target RT suffices for a given VPLS V, and in effect that RT is the
identifier for VPLS V.
A PE announces (typically via I-BGP) that it belongs to VPLS V by
annotating its NLRIs for V (see next subsection) with Route Target
RT, and acts on this by accepting NLRIs from other PEs that have
Route Target RT. A PE announces that it no longer participates in V
by withdrawing all NLRIs that it had advertised with Route Target RT.
3.2. Signaling
Once discovery is done, each pair of PEs in a VPLS must be able to
establish (and tear down) pseudowires to each other, i.e., exchange
(and withdraw) demultiplexors. This process is known as signaling.
Signaling is also used to initiate "relearning", and to transmit
certain characteristics of the PE regarding a given VPLS.
Recall that a demultiplexor is used to distinguish among several
different streams of traffic carried over a tunnel, each stream
possibly representing a different service. In the case of VPLS, the
demultiplexor not only says to which specific VPLS a packet belongs,
but also identifies the ingress PE. The former information is used
for forwarding the packet; the latter information is used for
learning MAC addresses. The demultiplexor described here is an MPLS
label, even though the PE-to-PE tunnels may not be MPLS tunnels.
3.2.1. Setup and Teardown
A BGP NLRI, the VPLS NLRI, is used to exchange demultiplexors, using
the mechanism described in [4].
A PE advertises a VPLS NLRI for each VPLS that it participates in.
If the PE is doing learning and flooding, i.e., it is the VE, it
announces a single set of VPLS NLRIs for each VPLS that it is in. If
the PE is connected to several L2PEs, it announces one set of VPLS
NLRIs for each L2PE. A hybrid scheme is also possible, where the PE
learns MAC addresses on some interfaces (over which it is directly
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connected to CEs) and delegates learning on other interfaces (over
which it is connected to L2PEs). In this case, the PE would announce
one set of VPLS NLRIs for each L2PE that has customer ports in a
given VPLS, and one set for itself, if it has customer ports in that
VPLS.
Each set of NLRIs defines the demultiplexors for a range of other PEs
in the VPLS. Ideally, a single NLRI suffices for all PEs in a VPLS;
however, there are cases (such as a newly added PE) where the pre-
existing NLRI does not have enough labels. In such cases,
advertising an additional NLRI for the same VPLS serves to add labels
for the new PEs without disrupting service to the pre-existing PEs.
If service disruption is acceptable (or when the PE restarts its BGP
process), a PE MAY consider coalescing all NLRIs for a VPLS into a
single NLRI.
If a PE X is part of VPLS V, and X receives a VPLS NLRI for V from PE
Y that includes a demultiplexor that X can use, X sets up its ends of
a pair of pseudowires between X and Y. X may also have to advertise
a new NLRI for V that includes a demultiplexor that Y can use, if its
pre-existing NLRI for V did not include a demultiplexor for Y.
If Y withdraws its NLRI for V that X was using, then X tears down its
ends of the pseudowires between X and Y.
The format of the VPLS NLRI is given below; it is essentially
identical to the L2 VPN NLRI [4]. The AFI and SAFI are the same as
for the L2 VPN NLRI.
Figure 2: BGP NLRI for VPLS Information
+------------------------------------+
| Length (2 octets) |
+------------------------------------+
| Route Distinguisher (8 octets) |
+------------------------------------+
| VE ID (2 octets) |
+------------------------------------+
| Label-block Offset (2 octets) |
+------------------------------------+
| Label Base (3 octets) |
+------------------------------------+
| Variable TLVs (0 to N octets) |
| ... |
+------------------------------------+
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3.2.2. Relearning MAC Addresses
Once a MAC address has been learned by VE A, VE A no longer needs to
flood packets destined to that MAC address; instead VE A can send
those packets directly to the VE "owning" that MAC address, say B.
However, it is possible that a CE "move" from VE B to VE C; one
example scenario is that the CE is dual-homed to VE B and C, and the
link over which the CE is attached to VE B goes down. In this case,
VE B may want to signal to other VEs in the VPLS that MAC addresses
that they learned from VE B (for the given VPLS) are no longer valid.
While aging timers will eventually enforce this, they may often be
too slow. The Relearn Sequence Number (RSN) TLV will help speed up
relearning.
The RSN TLV is an optional TLV with Type TBD, Length 4 and Value a 32
bit RSN, a monotonically increasing unsigned number. When an RSN TLV
is received, the RSN number is compared against the existing one for
that VPLS and PE. If the new number is higher than the previous one,
or no previous RSN exists, the PE SHOULD flush all existing MAC
address to VC bindings for that VPLS and PE.
3.2.3. Signaling PE Capabilities
The Encaps Type and Control Flags are encoded in an extended
attribute, just as in [4]. The community type remains the same.
There is a new Encaps Type for VPLS (TBD).
Figure 3: layer2-info extended community
+------------------------------------+
| Extended community type (2 octets) |
+------------------------------------+
| Encaps Type (1 octet) |
+------------------------------------+
| Control Flags (1 octet) |
+------------------------------------+
| Layer-2 MTU (2 octet) |
+------------------------------------+
| Reserved (2 octets) |
+------------------------------------+
There are three new control flags, Q, F and P defined for VPLS. Q
says whether qualified learning occurs (1) or not (0); F which says
whether the PE is capable of flooding (1) or not (0). P indicates
that the PE will strip the outermost VLAN from the layer 2 customer
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frame on ingress, and push a VLAN on egress.
Figure 4: Control Flags Bit Vector
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| MBZ |P|Q|F|C|S| (MBZ = MUST Be Zero)
+-+-+-+-+-+-+-+-+
3.3. Inter-Provider VPLS
As in [4] and [7], the above autodiscovery and signaling functions
are typically announced via I-BGP. This assumes that all the sites
in a VPLS are connected to PEs in a single Autonomous System (AS).
However, sites in a VPLS may occasionally connect to PEs in different
ASes. This leads to two issues: a) there would not be an I-BGP
connection between those PEs, so some means of signaling inter-AS is
needed; and b) there may not be PE-to-PE tunnels between the ASes.
The former problem is solved in [7], Section 10. Three methods are
suggested; of these, the last two Just Work for Inter-Provider VPLS.
Method (b) requires an I-BGP peering between the PEs in AS1 and ASBR1
in AS1, an E-BGP peering between ASBR1 and ASBR2 in AS2, and I-BGP
peerings between ASBR2 and the PEs in AS2. Method (c) requires a
multi-hop E-BGP peering between the PEs (or preferably, a Route
Reflector) in AS1 and the PEs (or Route Reflector) in AS2.
The latter is easy if the PE-to-PE tunnels are IP. If the tunnels
are MPLS, labeled IPv4 distribution of PE loopback addresses by ASBRs
(as described in part (c) of Section 10 of [7]) can be used to create
PE-to-PE MPLS LSPs that traverses the ASes.
4. Data Plane
This section discusses two aspects of the data plane for PEs and
L2PEs implementing VPLS: encapsulation and forwarding.
4.1. Encapsulation
Ethernet frames received from CE devices are encapsulated for
transmission over the packet switched network connecting the PEs.
The encapsulation is as in [11], with one change: a PE that sets the
P bit in the Control Flags strips the outermost VLAN from an Ethernet
frame received from a CE before encapsulating it, and pushes a VLAN
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onto a decapsulated frame before sending it to a CE.
4.2. Forwarding
Forwarding of VPLS packets is based on the interface over which the
packet is received, which determines which VPLS the packet belongs
to, and the destination MAC address. The former mapping is
determined by configuration. The latter is the focus of this
section.
4.2.1. MAC address learning
As was mentioned earlier, the key distinguishing feature of VPLS is
that it is a multipoint service. This means that the entire Service
Provider network should appear as a single logical learning bridge
for each VPLS that the SP network supports. The logical ports for
the SP "bridge" are the connections from the SP edge, be it a PE or
an L2PE, to the CE. Just as a learning bridge learns MAC addresses
on its ports, the SP bridge must learn MAC addresses at its VEs [3].
Learning consists of associating source MAC addresses of packets with
the ports on which they arrive; call this association the Forwarding
Information Base (FIB). The FIB is used for forwarding packets. For
example, suppose the bridge receives a packet with source MAC address
S on port P. If subsequently, the bridge receives a packet with
destination MAC address S, it knows that it should send the packet
out on port P.
There are two modes of learning: qualified and unqualified learning.
In qualified learning, the learning decisions at the VE are based on
the customer ethernet packet's MAC address and VLAN tag, if one
exists. If no VLAN tag exists, the default VLAN is assumed.
Effectively, within one VPLS, there are multiple logical FIBs, one
for each customer VLAN tag identified in a customer packet.
In unqualified learning, learning is based on a customer ethernet
packet's MAC address only. In other words, at any VE, there is only
one FIB per VPLS.
Every VE must have at least one FIB for each VPLS that it
participates in.
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4.2.2. Flooding
When a bridge receives a packet to a destination that is not in its
FIB, it floods the packet on all the other ports. Similarly, a VE
will flood packets to an unknown destination to all other VEs in the
VPLS.
In Figure 1 above, if CE2 sent an Ethernet frame to PE2, and the
destination MAC address on the frame was not in PE2's FIB (for that
VPLS), then PE2 would be responsible for flooding that frame to every
other PE in the same VPLS. On receiving that frame, PE1 would be
responsible for further flooding the frame to CE1 and CE5 (unless PE1
knew which CE "owned" that MAC address).
On the other hand, if PE3 received the frame, it could delegate
further flooding of the frame to its L2PE. If PE3 was connected to 2
L2PEs, it would announce that it has two L2PEs. PE3 could either
announce that it is incapable of flooding, in which case it would
receive two frames, one for each L2PE, or it could announce that it
is capable of flooding, in which case it would receive one copy of
the frame, which it would then send to both L2PEs.
5. Deployment Scenarios
In deploying a network that supports VPLS, the SP must decide whether
the VPLS-aware device closest to the customer (the VE) is an L2PE or
a PE. The default case described in this document is that the VE is
a PE. However, there are a number of reasons that the VE might be an
L2PE, i.e., a device that does layer 2 functions such as MAC address
learning and flooding, and some limited layer 3 functions such as
communicating to its PE, but doesn't do full-fledged discovery and
PE-to-PE signaling [12].
As both of these cases have benefits, one would like to be able to
"mix and match" these scenarios. The signaling mechanism presented
here allows this. PE1 may be directly connected to CE devices; PE2
may be connected to L2PEs that are connected to CEs; and PE3 may be
connected directly to a customer over some interfaces and to L2PEs
over others. All these PEs do discovery and signaling in the same
manner. How they do learning and forwarding depends on whether or
not there is an L2PE; however, this is a local matter, and is not
signaled.
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6. Normative References
[ 1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
[ 2] Andersson, L. and T. Madsen, "PPVPN Terminology", work in
progress.
[ 3] Andersson, L. (Editor), "PPVPN L2 Framework", work in progress.
[ 4] Kompella, K., et al, "MPLS-based Layer 2 VPNs", work in
progress.
[ 7] Rosen, E., et al, "BGP/MPLS VPNs", work in progress.
[10] Sangli, S., D. Tappan, and Y. Rekhter, "BGP Extended Communities
Attribute", (work in progress).
[11] Martini, L., et al, "Encapsulation Methods for Transport of
Layer 2 Frames Over IP and MPLS Networks", work in progress.
[12] Kompella, K., et al, "Decoupled TLS", work in progress.
7. Informative References
[ 5] Martini, L., et al, "Transport of Layer 2 Frames Over MPLS",
work in progress.
[ 6] Kompella, V., et al, "Virtual Private Switched Network Services
over an MPLS Network", work in progress.
[ 8] Ould-Brahim, H. et al, "Using BGP as an Auto-Discovery Mechanism
for Network-based VPNs", work in progress.
[ 9] Shah, H. et al, "Signaling between PE and L2PE/MTU for Decoupled
VPLS and Hierarchical VPLS", work in progress.
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Security Considerations
To be filled in in a later version.
IANA Considerations
To be filled in in a later version.
Acknowledgments
Thanks to Joe Regan and Alfred Nothaft for their contributions.
Authors' Addresses
Yeong-il,Seo
Korea Telecom
Telecommunication Network Laboratory
Member of Technical Staff
463-1 Junmin-dong, Yusung-gu, Taejeon, Korea
Tel : +82-42-870-8333 / FAX : +82-42-870-8339
Mobile : 016-235-0135 / E-mail : syi@hana.ne.kr
Hamid Ould-Brahim
Nortel Networks
P O Box 3511 Station C
Ottawa ON K1Y 4H7 Canada
Phone: +1 (613) 765 3418
Email: hbrahim@nortelnetworks.com
Ashwin Moranganti
Appian Communications
email: amoranganti@appiancom.com
phone: 978 206-7248
Pascal Menezes
Terabeam Networks, Inc.
14833 NE 87th St.
Redmond, WA, USA
phone: (206) 686-2001
email: Pascal.Menezes@Terabeam.com
Pierre Lin
Yipes Communications, Inc.
114 Sansome St.
San Francisco CA 94104
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email: pierre.lin@yipes.com
Marc Lasserre
Riverstone Networks
5200 Great America Parkway
Santa Clara CA 95054
marc@riverstonenet.com
Sunil Khandekar
TiMetra Networks
274 Ferguson Dr.
Mountain View, CA 94043
Giles Heron
PacketExchange Ltd.
The Truman Brewery
91 Brick Lane
LONDON E1 6QL
United Kingdom
Email: giles@packetexchange.net
Loa Andersson
Utfors AB
Box 525, 169 29 Solna
Sweden
phone: +46 8 5270 5038
email: loa.andersson@utfors.se
Javier Achirica
Telefonica Data
javier.achirica@telefonica-data.com
Vach Kompella
TiMetra Networks
274 Ferguson Dr.
Mountain View, CA 94043
Email: vkompella@timetra.com
Yakov Rekhter
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
yakov@juniper.net
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
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kireeti@juniper.net
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This document and the information contained herein is provided on an
Kompella (Editor) [Page 16]
Internet Draft Virtual Private LAN Service May 2003
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement
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Kompella (Editor) [Page 17]
