Network Working Group T. Morin
Internet-Draft France Telecom R&D
Intended status: Informational B. Niven-Jenkins
Expires: July 19, 2007 BT
Y. Kamite
NTT Communications
R. Zang
BT
N. Leymann
T-Systems
January 15, 2007
Considerations about Multicast BGP/MPLS Standardization
draft-morin-l3vpn-mvpn-considerations-00
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Copyright Notice
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Abstract
The current proposal for multicast in BGP/MPLS includes multiple
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alternative mechanisms for some of the required building blocks of
the solution. The aim of this document is to leverage requirements
to identify the key elements and help move forward solution design,
toward the definition of a standard having a well defined set of
mandatory procedures. The different proposed alternative mechanisms
are examined in the light of requirements identified for multicast in
L3VPNs, and suggestions are made about which of these mechanisms
standardization should favor. Issues related to existing deployments
of early implementations are also addressed.
Requirements Language
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 [RFC2119].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Examining alternatives mechanisms for MVPN functions . . . . . 3
3.1. MVPN auto-discovery . . . . . . . . . . . . . . . . . . . 3
3.2. S-PMSI Signaling . . . . . . . . . . . . . . . . . . . . . 5
3.3. PE-PE Transmission of C-Multicast Routing . . . . . . . . 7
3.4. Encapsulation techniques for P-multicast trees . . . . . . 10
3.5. Inter-AS deployments options . . . . . . . . . . . . . . . 11
4. Existing deployments . . . . . . . . . . . . . . . . . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
8. Informative References . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. Introduction
The current proposal for multicast in BGP/MPLS
[I-D.ietf-l3vpn-2547bis-mcast] includes multiple alternative
mechanisms for some of the required building blocks of the solution.
However, it does not identify the core set of mechanisms which must
be implemented in order to ensure interoperability. This may lead to
a situation where implementations may support different subsets of
the available optional mechanisms leading to implementations that do
not interoperate.
The aim of this document is to leverage the already expressed
requirements [I-D.ietf-l3vpn-ppvpn-mcast-reqts] to identify the
mechanisms that the authors believe are good candidates for being
part of a core set of mandatory mechanisms which can be used to
provide a base for interoperable solutions.
This document will go through the different building blocks of the
solution and provide the authors' recommendations as to which
mechanisms the authors' favor for each building block, while
considering the requirements already defined and the goal of a fully-
interoperable standard.
Considering the history of the multicast VPN proposals and
implementations, the authors also consider it useful to depict how
existing deployments of early implementations
[I-D.rosen-vpn-mcast][I-D.raggarwa-l3vpn-2547-mvpn] can fit in the
picture, and provide suggestions in this respect.
2. Terminology
Please refer to [I-D.ietf-l3vpn-2547bis-mcast] and
[I-D.ietf-l3vpn-ppvpn-mcast-reqts].
3. Examining alternatives mechanisms for MVPN functions
3.1. MVPN auto-discovery
Section 5.2.10 of [I-D.ietf-l3vpn-ppvpn-mcast-reqts] states "The
operation of a multicast VPN solution SHALL be as light as possible
and providing automatic configuration and discovery SHOULD be a
priority when designing a multicast VPN solution. Particularly the
operational burden of setting up multicast on a PE or for a VR/VRF
SHOULD be as low as possible".
The current solution document [I-D.ietf-l3vpn-2547bis-mcast]
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addresses this requirement by proposing two different mechanisms for
MVPN auto-discovery:
1. BGP-based auto-discovery (described in section 4).
2. Discovery using PIM running on a MI-PMSI implemented with a
shared tree using multicast ASM, or MP2MP LDP with the same
common tree identifier configured in all VRFs of an MVPN.
It is the recommendation of the authors that BGP-based auto-discovery
is the preferred solution for auto-discovery and should be supported
by all implementations while PIM/shared-tree based auto-discovery
should be optionally considered for migration purpose only.
Part of the rationale for this recommendation is also based on
section 5.2.10 of [I-D.ietf-l3vpn-ppvpn-mcast-reqts] which states "as
far as possible, the design of a solution SHOULD carefully consider
the number of protocols within the core network: if any additional
protocols are introduced compared with the unicast VPN service, the
balance between their advantage and operational burden SHOULD be
examined thoroughly".
BGP is indeed the auto-discovery protocol used in unicast (RFC4364)
VPNs and therefore the use of BGP-based auto-discovery within
multicast VPNs avoids the introduction of an additional auto-
discovery protocol that would require additional OAM processes and
tools. Service providers with deployed unicast (RFC4364) VPNs
already have extensive deployment and operations experience of using
BGP as an auto-discovery protocol including OAM processes and tools.
Such processes and tools will require modifications in order to
support multicast auto-discovery but those modifications are
anticipated to be less than those required to develop new processes
and tools for a specific auto-discovery protocol. Additionally, BGP
supports MD5 authentication of its peers for additional security. In
contrast, there are no obvious authentication mechanisms to secure
PIM communications in any known implementation.
Furthermore, PIM based discovery is only applicable to deployments
using a shared tree on an MI-PMSI, whereas BGP-based auto-discovery
does not place any restrictions on the type of multicast trees that
can be used. BGP-based auto-discovery is independent of the type of
P-multicast tree used thus satisfying the requirement in section
5.2.4.1 of [I-D.ietf-l3vpn-ppvpn-mcast-reqts] that "a multicast VPN
solution SHOULD be designed so that control and forwarding planes are
not interdependent". Additionaly, it is to be noted that a number of
service providers have chosen to use SSM-based trees for the default
MDTs within their current deploymentsn, therefore relying already on
BGP-based auto-discovery.
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When shared trees are used, the use of BGP autodiscovery would allow
inconsistencies in the addresses/identifiers used for the shared
trees to be detected (e.g. the same shared tree identifier being used
for different VPNs with distinct BGP route targets). This is
particularly attractive in the context of inter-AS VPNs where the
impact of any misconfiguration could be magnified and where a single
service provider may not operate all the ASs.
The authors note that, in order to support the coexistence of both
protocols (for example during migration scenarios), implementations
could support both alternatives by providing a per-VRF configuration
knob that would allow recognizing new PIM neighbors based on the
reception of PIM hellos on a shared P-multicast tree, even for
neighbors that did not advertise a BGP auto discovery route.
3.2. S-PMSI Signaling
The current solution document [I-D.ietf-l3vpn-2547bis-mcast] proposes
two mechanisms for S-PMSI Signaling:
1. A new UDP-based TLV protocol specifically for S-PMSI signaling
(described in section 7.2.1).
2. A BGP-based mechanism for S-PMSI signaling (described in section
7.2.2).
It is the recommendation of the authors that BGP is the preferred
solution for S-PMSI signaling and should be supported by all
implementations while the UDP-based S-PMSI signaling protocol should
be considered optional.
Part of the rationale for this recommendation is similar to that for
BGP-based auto-discovery and is based on section 5.2.10 of
[I-D.ietf-l3vpn-ppvpn-mcast-reqts] and the desire to avoid
introducing and deploying additional protocols unless strictly
necessary.
Furthermore:
o The BGP-based S-PMSI signaling mechanism can be used within MVPNs
using either a UI-PMSI or a MI-PMSI while the UDP-based protocol
is restricted to use within MVPNs using an MI-PMSI.
o The BGP-based S-PMSI signaling mechanism can be efficiently used
in an inter-AS option B deployment context while the use of the
UDP-based protocol does not preserve AS routing independence when
used in an inter-AS option B context (i.e. the decision by a PE in
an AS to use an S-PMSI for a given customer flow will impact
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routing state in other ASes). Co-existence with unicast inter-AS
VPN options is strongly encouraged by section 5.2.6 of
[I-D.ietf-l3vpn-ppvpn-mcast-reqts].
o The BGP-based S-PMSI signaling supports all the functionality and
all the operational contexts that are supported by UDP-based
protocol (and more).
o BGP supports MD5 authentication of its peers for additional
security. In contrast, there are no obvious authentication
mechanisms to secure PIM communications in any known
implementation.
Therefore, it is the opinion of the authors that BGP is the preferred
solution for performing S-PMSI signaling.
However, the authors recognize that the UDP-based protocol has been
in deployment for some time and would recommend that implementations
supporting both protocols optionally provide a per-VRF configuration
knob to allow an implementation to use the UDP-based TLV protocol for
S-PMSI signaling for specific VRFs in order to support the
coexistence of both protocols (for example during migration
scenarios).
Apart from such migration-facilitating mechanisms, the authors
specifically do not recommend extending the already proposed UDP-
based TLV protocal to new types of P-multicast trees.
Section 7.2.2.3 of [I-D.ietf-l3vpn-2547bis-mcast] proposes two
approaches for how a source PE can decide when to start transmitting
customer multicast traffic on a S-PMSI:
1. The source PE sends multicast packets for the <C-S, C-G> on both
the I-PMSI P-multicast tree and the S-PMSI P-multicast tree
simultaneously for a pre-configured period of time, letting the
receiver PEs select the new tree for reception, before switching
to only the S-PMSI.
2. The source PE waits for a pre-configured period of time after
advertising the <C-S, C-G> entry bound to the S-PMSI before fully
switching the traffic onto the S-PMSI-bound P-multicast tree.
The first alternative has essentially two drawbacks:
o <C-S,C-G> traffic is sent twice for some periode of time, for a
stream for which bandwidth optimization just happen to be
considered useful
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o it is unlikely that the switch can happen without packet loss or
duplicates, as soon as the transit delay of the I-PMSI P-multicast
tree and the S-PMSI P-multicast tree are different
By contrast, the second alternative has none of these drawbacks, and
would meet the requirement made in section 5.1.3 of requirements
stating that "[...] a multicast VPN solution SHOULD as much as
possible ensure that client multicast traffic packets are neither
lost nor duplicated, even when changes occur in the way a client
multicast data stream is carried over the provider network". The
second alternative also happen to be the one used in existing
deployments.
For these reasons, it is the author recomandation to mandate the
implementation of this second alternative for switching to S-PMSI.
3.3. PE-PE Transmission of C-Multicast Routing
The current solution document [I-D.ietf-l3vpn-2547bis-mcast] proposes
multiple mechanisms for PE-PE transmission of customer multicast
routing information:
1. Full per-MVPN PIM peering across an MI-PMSI (described in section
5.2.1).
2. Lightweight PIM peering across an MI-PMSI (described in section
5.2.2)
3. The unicasting of PIM C-Join/Prune messages (described in section
5.2.3)
4. The use of BGP for carrying C-Multicast routing (described in
section 5.3).
The first mechanism (full per-MVPN PIM peering across an MI-PMSI) is
the mechanism used by [I-D.rosen-vpn-mcast] and therefore it is
deployed and operating in MVPNs today. The authors recognize that
because full per-MVPN PIM peering has been in deployment for some
time support for this mechanism may be required for backwards
compatibility and in order to facilitate migration towards
alternative PE-PE protocols.
Section 4.2.4 of [I-D.ietf-l3vpn-ppvpn-mcast-reqts] recommends that
"a multicast VPN solution SHOULD support several hundreds of PEs per
multicast VPN, and MAY usefully scale up to thousands" and section
4.2.5 states that "a solution SHOULD scale up to thousands of PEs
having multicast service enabled".
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At those scales of multicast deployment within a large VPN, the first
and third mechanisms require PEs to maintain a large number of PIM
adjacencies with other PEs within the same MVPN and to regularly
exchange PIM hellos with each other and refresh C-Join/Prune state.
The third mechanism would reduce the amount of C-Join/Prune
processing by PEs when they are not the upstream neighbor for a given
multicast flow, but would :
a. require explicit tracking related state to be maintained by PEs
when they are the upstream PE for a said customer flow, and
b. require refresh reduction mechanisms to be used to be applicable.
The second mechanism (lightweight PIM peering across an MI-PMSI)
would operate in a similar manner to full per-MVPN PIM peering except
that PIM hellos are not transmitted and PIM C-Join/Prune refresh
reduction would be used, thereby improving scalability.
The fourth mechanism (the use of BGP for carrying C-Multicast
routing) has to solve roughly the same intrinsic scalability related
difficulties as the other three mechanisms, but because it is based
on TCP it has no refresh-related scalability limit, and inherits some
BGP features that are expected to improve scalability through, for
instance, providing a means to offload some of the processing burden
associated with client multicast routing onto BGP route reflectors.
Furthermore, co-existence with unicast inter-AS VPN options, and an
equal level of security for multicast and unicast including in an
inter-AS context, are specifically mentioned in sections 5.2.6, 5.2.8
and 5.2.12 of [I-D.ietf-l3vpn-ppvpn-mcast-reqts]. The first three
mechanisms impose direct PE to PE communications which means that
they do not apply well to an inter-AS option B context, because of
security and robustness issues that are involved by such a level of
reachability and interaction between PEs in different ASes. Their
use in an inter-AS context is possible, but not without limitations
or additional engineering design trade-offs depending upon the
interconnect types. By comparison, the fourth option (the use of BGP
for carrying C-Multicast routing) does not have any of the above
limitations related to inter-AS deployments, and also provides an
additional alternative to facilitate such deployments through the
possibility of using segmented inter-AS trees.
Moreover, mechanisms one and two are restricted to use within MVPNs
using an MI-PMSI, thereby necessitating:
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a. The use of a P-multicast tree technique that allows shared trees
(for example PIM-SM in ASM mode or MP2MP LDP).
b. The use of one P-multicast tree per PE per VPN, even for PEs that
do not have sources in their directly attached sites for that
VPN.
By comparision, the fourth mechanism doesn't impose either of these
restrictions.
Additionally, the fourth mechanism (the use of BGP for carrying
C-Multicast routing) would appear to fit well with the current
unicast architecture as BGP is the customer routing distribution
protocol used in unicast VPNs and therefore using BGP for customer
routing distribution within multicast VPNs avoids the introduction of
an additional protocol that would require additional OAM processes
and tools. Service provider's with deployed unicast (RFC4364) VPNs
already have extensive deployment and operations experience of using
BGP as a customer routing distribution protocol including OAM
processes and tools. Such processes and tools will require
modification in order to support customer multicast routing but those
modifications are anticipated to be less than those required to
develop new processes and tools for a distinct customer routing
protocol. It should be noted that because PIM will be used as the
CE-PE customer routing distribution protocol, service providers will
still need OAM processes and tools in order to manage the PIM
protocol, so this rationale only applies to a subset of the tools and
processes already in place. Additionally, BGP supports MD5
authentication of its peers for additional security. In contrast,
there are no obvious authentication mechanisms to secure PIM
communications in any known implementation.
An illustrative example of the benefit brought by consistency with
unicast design is how the "extranet" feature can be implemented :
when BGP-based mechanisms are used, the well defined and well
understood BGP route target import/export semantics are just reused.
By contrast, it is not specified how implementing the same feature
would be done in the context of other alternative mechanism, and
unclear if this is possible without significant engineering trade-
offs. Note that the support for such a feature is stated as a MUST
in sections 5.1.6 of [I-D.ietf-l3vpn-ppvpn-mcast-reqts].
Section 5.2.10 of requiremnts [I-D.ietf-l3vpn-ppvpn-mcast-reqts]
states that "as far as possible, the design of a solution SHOULD
carefully consider the number of protocols within the core network:
if any additional protocols are introduced compared with the unicast
VPN service, the balance between their advantage and operational
burden SHOULD be examined thoroughly". Considering that the
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recommendation of the authors would be BGP for auto-discovery and
S-PMSI signalling, the choice of BGP for customer multicast routing
would be consistent with the protocol choice for unicast VPNs and
would adequately address this requirement.
BGP-based customer multicast routing clearly presents some advantages
over the PIM-based alternatives. However it has yet to be deployed
within an operational MVPN, and service providers currently lack
experience of its implementation. By contrast, experience proves
that PIM-based mechanisms are operationaly viable, but with well
identified limitations. Some of those limitations may be improved
upon (although there is currently no clearly defined proposal to do
this), however some of the limitations appear to be intrinsic to the
approach.
Moreover to improve the clarity of the proposed specifications,
considering that neither hello suppression nor refresh reduction
procedures are currently specified or documented and that it is not
clear what the impact to the PIM state machine of these additional
procedures may be, the authors recommend that the proposals for
lightweight PIM peering across an MI-PMSI (the second mechanism) and
for the unicasting of PIM C-Join/Prune messages (the third mechanism)
be removed from the current solution document
[I-D.ietf-l3vpn-2547bis-mcast] until the additional PIM hello and
refresh reduction procedures have been well specified and both their
impact and benefit on an MPVN deployment is understood.
Consequently, at the present time and until there is experience with
all of the proposed mechanisms it is not clear which of the above
mechanisms should be recommended as the preferred solution to
implementers. However, it would appear prudent for implementations
to consider supporting both the fourth (BGP-based) and first (full
per-MPVN PIM peering) mechanisms. Further experience on both
implementation is likely to be required before some best practice can
be defined.
3.4. Encapsulation techniques for P-multicast trees
In this section the authors will not make any restricting
recommendations as the appropriateness of a specific core (P) data
plane technology will depend on a large number of factors, for
example the service provider's currently deployed unicast data plane,
many of which are service provider specific.
However, implementations should not unreasonably restrict the data
plane technology that can be used, and should not force the use of
the same technology for different VPNs attached to a single PE.
Initial implementations may only support a reduced set of
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encapsulation techniques and data plane technologies but this should
not be considered a limiting factor that hinders future support for
other encapsulation techniques, data plane technologies or
interoperability.
Section 5.2.4.1 of [I-D.ietf-l3vpn-ppvpn-mcast-reqts] states "In a
multicast VPN solution extending a unicast L3 PPVPN solution,
consistency in the tunneling technology has to be favored: such a
solution SHOULD allow the use of the same tunneling technology for
multicast as for unicast. Deployment consistency, ease of operation
and potential migrations are the main motivations behind this
requirement."
Current unicast VPN deployments use a variety of LDP, RSVP-TE and
GRE/IP for encapsulating customer packets for transport across the
provider core of VPN services. It is recommended that
implementations support the three corresponding multicast tree
encapsulations techniques, namely: mLDP, P2MP RSVP-TE and GRE/IP
multicast in order to allow the same encapsulations to be used for
unicast and multicast traffic as well as facilitating migration from
[I-D.rosen-vpn-mcast] to an MPLS label based encapsulation.
3.5. Inter-AS deployments options
Section 5.2.6 of [I-D.ietf-l3vpn-ppvpn-mcast-reqts] states "A
solution MUST support inter-AS multicast VPNs, and SHOULD support
inter-provider multicast VPNs. Considerations about coexistence with
unicast inter-AS VPN Options A, B and C (as described in section 10
of [RFC4364]) are strongly encouraged." and "A multicast VPN solution
SHOULD provide inter-AS mechanisms requiring the least possible
coordination between providers, and keep the need for detailed
knowledge of providers' networks to a minimum - all this being in
comparison with corresponding unicast VPN options."
Section 8 of [I-D.ietf-l3vpn-2547bis-mcast] addresses these
requirements by proposing two approaches for mVPN inter-AS
deployments:
1. Segmented inter-AS tunnels where each AS constructs its own
separate multicast tunnels which are then 'stitched' together by
the ASBRs (described in section 8.2).
2. Non-segmented inter-AS tunnels where the multicast tunnels are
end-to-end across ASes, so even though the PEs belonging to a
given MVPN may be in different ASes the ASBRs play no special
role and function merely as P routers (described in section 8.1).
Section 5.2.6 of [I-D.ietf-l3vpn-ppvpn-mcast-reqts] also states
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"Within each service provider the service provider SHOULD be able on
its own to pick the most appropriate tunneling mechanism to carry
(multicast) traffic among PEs (just like what is done today for
unicast)". However, the applicability of segmented or non-segmented
inter-AS tunnels to a given deployment or inter-provider interconnect
will depend on a number of factors specific to each service provider.
It is therefore the recommendation of the authors that implementation
should support both segmented and non-segmented inter-AS tunnels,
although it is recognised that initial implementations may only
support one or the other.
Additionally, the authors note that the proposed BGP-based approaches
for S-PMSI signaling and C-multicast routing information distribution
provide a good fit with both segmented and non-segmented inter-AS
tunnels. In contrast the UDP-TLV based approach for S-PSMI signaling
appears to be incompatible with segmented inter-AS tunnels, and it is
unclear if the proposed PIM-based approaches for C-multicast routing
information distribution would be fully applicable to segmented
inter-AS tunnels.
4. Existing deployments
Some suggestions provided in this document can be used to
incrementally modify currently deployed implementations without
hindering these deployments, and without hindering the consistency of
the standardized solution : optionnaly providing a per-VRF tunable to
support a mode of operation compatible with currently deployed
implementations, while using at the same time the recommended
approach with implementation supporting the standard.
In cases where this may not be easily doable, a recommended approach
would be to provide a per-VRF tunable allowing incremental per-VPN
migration of the mechanisms used by a PE device, which would allow
migration with some per-VPN interruption of service (e.g. on non-peak
hours).
Mechanisms allowing "live" migration by providing concurrent use of
multiple alternatives for a said PE and a said VPN, is not seen as a
priority considering the expected associated implementation
complexity. However, if there happen to be cases where they could be
relatively simple and viable, such mechanisms may help improve
migration management.
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5. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
6. Security Considerations
This document does not by itself raise any particular security issue.
7. Acknowledgements
[To be completed]
8. Informative References
[I-D.ietf-l3vpn-2547bis-mcast]
Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP
VPNs", draft-ietf-l3vpn-2547bis-mcast-03 (work in
progress), October 2006.
[I-D.ietf-l3vpn-ppvpn-mcast-reqts]
Morin, T., "Requirements for Multicast in L3 Provider-
Provisioned VPNs", draft-ietf-l3vpn-ppvpn-mcast-reqts-10
(work in progress), November 2006.
[I-D.raggarwa-l3vpn-2547-mvpn]
Aggarwal, R., "Base Specification for Multicast in BGP/
MPLS VPNs", draft-raggarwa-l3vpn-2547-mvpn-00 (work in
progress), June 2004.
[I-D.rosen-vpn-mcast]
Rosen, E., "Multicast in MPLS/BGP VPNs",
draft-rosen-vpn-mcast-08 (work in progress),
December 2004.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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Authors' Addresses
Thomas Morin
France Telecom R&D
2 rue Pierre Marzin
Lannion 22307
France
Email: thomas.morin@orange-ftgroup.com
Ben Niven-Jenkins
BT
208 Callisto House, Adastral Park
Ipswich, Suffolk IP5 3RE
UK
Email: benjamin.niven-jenkins@bt.com
Yuji Kamite
NTT Communications Corporation
Tokyo Opera City Tower
3-20-2 Nishi Shinjuku, Shinjuku-ku
Tokyo 163-1421
Japan
Email: y.kamite@ntt.com
Raymond
BT
2160 E. Grand Ave.
El Segundo CA 90025
USA
Email: raymond.zhang@bt.com
Nicolai Leymann
T-Systems International GmbH
Goslarer Ufer
Berlin 3510589
Germany
Email: nicolai.leymann@t-systems.com
Morin, et al. Expires July 19, 2007 [Page 14]
Internet-Draft Multicast VPN Considerations January 2007
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