Mboned                                                    M. Abrahamsson
Internet-Draft                                                 T-Systems
Intended status: Informational                                  T. Chown
Expires: January 7, 2017                                            Jisc
                                                             L. Giuliano
                                                  Juniper Networks, Inc.
                                                            July 6, 2016

                        Multicast Service Models


   The draft provides a high-level overview of multicast service and
   deployment models, principally the Any-Source Multicast (ASM) and
   Source-Specific Multicast (SSM) models, and aims to provoke
   discussion of applicability of the models to certain scenarios.  This
   initial draft is by no means comprehensive.  Comments on the initial
   content, and what further content would be appropriate, or indeed
   whether the draft is of value, are welcomed.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on January 7, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Multicast service models  . . . . . . . . . . . . . . . . . .   3
   3.  Multicast building blocks . . . . . . . . . . . . . . . . . .   4
     3.1.  Multicast addressing  . . . . . . . . . . . . . . . . . .   4
     3.2.  Host signalling . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Multicast snooping  . . . . . . . . . . . . . . . . . . .   4
   4.  ASM service model protocols . . . . . . . . . . . . . . . . .   5
     4.1.  Protocol Independent Multicast, Dense Mode (PIM-DM) . . .   5
     4.2.  Protocol Independent Multicast, Sparse Mode (PIM-SM)  . .   5
       4.2.1.  Inter-domain PIM-SM, and MSDP . . . . . . . . . . . .   5
     4.3.  Bidirectional PIM (BIDIR-PIM) . . . . . . . . . . . . . .   6
     4.4.  IPv6 PIM-SM with Embedded RP  . . . . . . . . . . . . . .   6
   5.  SSM service model protocols . . . . . . . . . . . . . . . . .   6
     5.1.  Source Specific Multicast (PIM-SSM) . . . . . . . . . . .   6
   6.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  ASM Deployment  . . . . . . . . . . . . . . . . . . . . .   7
     6.2.  SSM Deployment  . . . . . . . . . . . . . . . . . . . . .   7
     6.3.  Other considerations  . . . . . . . . . . . . . . . . . .   8
       6.3.1.  Scalability, and multicast domains  . . . . . . . . .   9
       6.3.2.  Reliable multicast  . . . . . . . . . . . . . . . . .   9
       6.3.3.  Inter-domain multicast peering  . . . . . . . . . . .   9
       6.3.4.  Layer 2 multicast domains . . . . . . . . . . . . . .   9
       6.3.5.  Anything else?  . . . . . . . . . . . . . . . . . . .   9
   7.  Use case examples . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     12.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   IP Multicast has been deployed in various forms, both within private
   networks and on the wider Internet.  While a number of service models
   have been published individually, and in many cases revised over
   time, there is, we believe, no high-level guidance in the form of an
   Informational RFC documenting the models, their advantages and

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   disadvantages, and their appropriateness to certain scenarios.  This
   document aims to fill that gap.

   This initial version of the document is not complete.  There are
   other topics that can be included.  The aim of this initial version
   is to determine whether this work is deemed of value within the IETF
   mboned WG.

2.  Multicast service models

   The general IP multicast service model [RFC1112] is that senders send
   to a multicast IP address, receivers express an interest in traffic
   sent to a given multicast address, and that routers figure out how to
   deliver traffic from the senders to the receivers.

   The benefit of IP multicast is that it enables delivery of content
   such that any multicast packet sent from a source to a given
   multicast group address appears once and only once on any path
   between a sender and an interested receiver that has joined that
   multicast group.  A reserved range of addresses (for either IPv4 or
   IPv6) is used for multicast group communication.

   Two high-level flavours of this service model have evolved over time.
   In Any-Source Multicast (ASM), any number of sources may transmit
   multicast packets, and those sources may come and go over the course
   of a multicast session without being known a priori.  In ASM,
   receivers express interest in a given multicast group address.  In
   Source-Specific Multicast (SSM) the specific source(s) that may send
   traffic to the group are known in advance.  In SSM, receivers express
   interest in a given multicast address and specific source(s).

   Senders transmit multicast packets without knowing where receivers
   are, or how many there are.  Receivers are able to signal to on-link
   routers their desire to receive multicast content sent to a given
   multicast group, and in the case of SSM from specific sender IP
   addresses.  They may discover the group (and sender IP) information
   in a number of different ways.  They may also signal their desire to
   no longer receive multicast traffic for a given group (and sender

   Multicast routing protocols are used to establish the multicast
   forwarding paths (tree) between a sender and a set of receivers.
   Each router would typically maintain multicast forwarding state for a
   given group (and potentially sender IP), such that it knows which
   interfaces to forward (and where necessary replicate) multicast
   packets to.

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   Multicast packet forwarding is generally not considered a reliable
   service.  It is typically unidirectional, but a bidirectional
   multicast delivery mechanism also exists.

3.  Multicast building blocks

   In this section we describe general multicast building blocks that
   are applicable to both ASM and SSM deployment.

3.1.  Multicast addressing

   IANA has reserved specific ranges of IPv4 and IPv6 address space for
   multicast addressing.

   Guidelines for IPv4 multicast address assignments can be found in
   [RFC5771].  IPv4 has no explicit multicast address format; a specific
   portion of the overall IPv4 address space is reserved for multicast
   use (

   Guidelines for IPv6 multicast address assignments can be found in
   [RFC2375] and [RFC3307].  The IPv6 multicast address format is
   described in [RFC4291].  An IPv6 multicast group address will lie
   within ff00::/8.

3.2.  Host signalling

   A host wishing to signal interest in receiving (or no longer
   receiving) multicast to a given multicast group (and potentially from
   a specific sender IP) may do so by sending a packet using one of the
   protocols described below on an appropriate interface.

   For IPv4, a host may use Internet Group Management Protocol Version 2
   (IGMPv2) [RFC2236] to signal interest in a given group.  IGMPv3
   [RFC3376] has the added capability of specifying interest in
   receiving multicast packets from specific sources.

   For IPv6, a host may use Multicast Listener Discovery Protocol (MLD)
   [RFC2710] to signal interest in a given group.  MLDv2 [RFC3810] has
   the added capability of specifying interest in receiving multicast
   packets from specific sources.

   Further guidance on IGMPv3 and MLDv2 is given in [RFC4604].

3.3.  Multicast snooping

   Is this appropriate in this document?  There is discussion in

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4.  ASM service model protocols

4.1.  Protocol Independent Multicast, Dense Mode (PIM-DM)

   PIM-DM is detailed in [RFC3973].  It operates by flooding multicast
   messages to all routers within the network in which it is configured.
   This ensures multicast data packets reach all interested receivers
   behind edge routers.  Prune messages are used by routers to tell
   upstream routers to (temporarily) stop forwarding multicast for
   groups for which they have no known receivers.

   PIM-DM remains an Experimental protocol since its publication in

4.2.  Protocol Independent Multicast, Sparse Mode (PIM-SM)

   The most recent revision of PIM-SM is detailed in [RFC7761].  PIM-SM
   is, as the name suggests, well-suited to scenarios where the subnets
   with receivers are sparsely distributed throughout the network.  PIM-
   SM supports any number of senders for a given multicast group, which
   do not need to be known in advance, and which may come and go through
   the session.  PIM-SM does not use a flooding phase, making it more
   scalable and efficient than PIM-DM, but this means PIM-SM needs a
   mechanism to construct the multicast forwarding tree (and associated
   forwarding tables in the routers) without flooding the network.

   To achieve this, PIM-SM introduces the concept of a Rendezvous Point
   (RP) for a PIM domain.  All routers in a PIM-SM domain are then
   configured to use specific RP(s).  Such configuration may be
   performed by a variety of methods, including Anycast-RP [RFC4610].

   A sending host's Designated Router encapsulates multicast packets to
   the RP, and a receiving host's Designated Router can forward PIM JOIN
   messages to the RP, in so doing forming what is known as the
   Rendezvous Point Tree (RPT).  Optimisation of the tree may then
   happen once the receiving host's router is aware of the sender's IP,
   and a source-specific JOIN message may be sent towards it, in so
   doing forming the Shortest Path Tree (SPT).  Unnecessary RPT paths
   are removed after the SPT is established.

4.2.1.  Inter-domain PIM-SM, and MSDP

   PIM-SM can in principle operate over any network in which the
   cooperating routers are configured with RPs.  But in general, PIM-SM
   for a given domain will use an RP configured for that domain.  There
   is thus a challenge in enabling PIM-SM to work between multiple
   domains, i.e. to allow an RP in one domain to learn the existence of
   a source in another domain, such that a receiver's router in one

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   domain can know to forward a PIM JOIN towards a source's Designated
   Router in another domain.  The solution to this problem is to use an
   inter-RP signalling protocol known as Multicast Source Discovery
   Protocol (MSDP).  [RFC3618].

   Deployment scenarios for MSDP are given in [RFC4611].  MSDP remains
   an Experimental protocol since its publication in 2003.  MSDP was not
   replicated for IPv6.

4.3.  Bidirectional PIM (BIDIR-PIM)

   BIDIR-PIM is detailed in [RFC5015].  In contrast to PIM-SM, it can
   establish bi-directional multicast forwarding trees between multicast
   sources and receivers.

   Add more...

4.4.  IPv6 PIM-SM with Embedded RP

   Within a single PIM domain, PIM-SM for IPv6 works largely the same as
   it does for IPv4.  However, the size of the IPv6 address (128 bits)
   allows a different mechanism for multicast routers to determine the
   RP for a given multicast group address.  Embedded-RP [RFC3956]
   specifies a method to embed the unicast RP IP address in an IPv6
   multicast group address, allowing routers supporting the protocol to
   determine the RP for the group without any prior configuration.

   Embedded-RP allows PIM-SM operation across any network in which there
   is an end-to-end path of routers supporting the protocol.  By
   embedding the RP address in this way, multicast for a given group can
   operate inter-domain without the need for an explicit source
   discovery protocol (i.e. without MSDP for IPv6).  It would be
   desirable that the RP would be located close to the sender(s) in the

5.  SSM service model protocols

5.1.  Source Specific Multicast (PIM-SSM)

   PIM-SSM is detailed in [RFC4607].  In contrast to PIM-SM, PIM-SSM
   benefits from assuming that source(s) are known about in advance,
   i.e. the source IP address is known (by some out of band mechanism),
   and thus the receiver's router can send a PIM JOIN directly towards
   the sender, without needing to use an RP.

   IPv4 addresses in the 232/8 ( to range are
   designated as source-specific multicast (SSM) destination addresses
   and are reserved for use by source-specific applications and

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   protocols.  For IPv6, the address prefix FF3x::/32 is reserved for
   source-specific multicast use.

6.  Discussion

   In this section we discuss the applicability of the ASM and SSM
   models described above, and their associated protocols, to a range of
   deployment scenarios.  The context is framed in a campus / enterprise
   environment, but the draft could broaden its scope to other
   environments (thoughts?).

6.1.  ASM Deployment

   PIM-DM remains an Experimental protocol, that appears to be rarely
   used in campus or enterprise environments.  Open question: what are
   the use cases for PIM-DM today?

   In campus scenarios, PIM-SM is in common use.  The configuration and
   management of an RP is not onerous.  However, if interworking with
   external PIM domains in IPv4 multicast deployments is needed, MSDP is
   required to exchange information between domain RPs about sources.
   MSDP remains an Experimental protocol, and can be a complex and
   fragile protocol to administer and troubleshoot.  MSDP is also
   specific to IPv4; it was not carried forward to IPv6.

   PIM-SM is a general purpose protocol that can handle all use cases.
   In particular, it is well-suited to cases where one or more sources
   may came and go during a multicast session.  For cases where a
   single, persistent source is used, PIM-SM has unnecessary complexity.

   As stated above, MSDP was not taken forward to IPv6.  Instead, IPv6
   has Embedded-RP, which allows the RP address for a multicast group to
   be embedded in the group address, making RP discovery automatic, if
   all routers on the path between a receiver and a sender support the
   protocol.  Embedded-RP is well-suited for lightweight ad-hoc
   deployments.  However, it does rely on a single RP for an entire
   group.  Embedded-RP was run successfully between European and US
   academic networks during the 6NET project in 2004/05.  Its usage
   generally remains constrained to academic networks.

   BIDIR-PIM is designed, as the name suggests, for bidirectional use

6.2.  SSM Deployment

   As stated in RFC4607, SSM is particularly well-suited to
   dissemination-style applications with one or more senders whose
   identities are known (by some mechanism) before the application

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   begins.  PIM-SSM is therefore very well-suited to applications such
   as IP TV.

   Some benefits of PIM-SSM are presented in RFC 4607:

      "Elimination of cross-delivery of traffic when two sources
      simultaneously use the same source-specific destination address;

      Avoidance of the need for inter-host coordination when choosing
      source-specific addresses, as a consequence of the above;

      Avoidance of many of the router protocols and algorithms that are
      needed to provide the ASM service model."

   A significant benefit of SSM is its reduced complexity through
   eliminating network-based source discovery.  This means no RPs,
   shared trees, SPT switchover, PIM registers, MSDP or data-driven
   state creation.  It is really just a small subset of PIM-SM, plus
   IGMPv3.  This makes it radically simpler to manage, troubleshoot and

   SSM is considered more secure in that it supports access control,
   i.e. you only get packets from the sources you explicitly ask for, as
   opposed to ASM where anyone can decide to send traffic to a PIM-SM
   group address.

   It is often thought that ASM is required for multicast applications
   where there are multiple sources.  However, RFC4607 also describes
   how SSM can be used instead of PIM-SM for multi-party applications:

      "SSM can be used to build multi-source applications where all
      participants' identities are not known in advance, but the multi-
      source "rendezvous" functionality does not occur in the network
      layer in this case.  Just like in an application that uses unicast
      as the underlying transport, this functionality can be implemented
      by the application or by an application-layer library."

   A disadvantage of SSM is that it requires hosts using SSM and (edge)
   routers with SSM receivers to support the new(er) IGMPv3 and MLDv2
   protocols.  The slow delivery of support in some OSes has meant that
   adoption of SSM has also been slower than might have been expected,
   or hoped.

6.3.  Other considerations

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6.3.1.  Scalability, and multicast domains

   One of the challenges in wider-scale multicast deployment is its
   scalability, if it is expected that multicast-enabled routers are
   required to hold state for large numbers of multicast sources/groups.

   In practice, the number of groups a given router needs to hold state
   for is limited by the propagation of the multicast messages for any
   given group, e.g. because only a specific connected set of routers
   are multicast-enabled, or because multicast scope borders have been
   configured between multicast-enabled routers for access control
   purposes.  Further, protocol policy/filters are typically used to
   limit state, as well as access control.

   IPv4 multicast has no explicit indication of scope boundaries within
   its multicast address format.  The prefix is reserved for
   private use within a network, as per [RFC2365], and is believed to be
   in common usage.  Other scopes within this range are defined, e.g.
   Organizational Local Scope, but whether this is in common use is

   In contrast, IPv6 has specific flag bits reserved to indicate the
   scope of an address, e.g. link (0x2), site (0x5), organisation (0x8)
   or global (0xe), as described in [RFC7346].  Such explicit scoping
   makes configuration of scope boundaries a simpler, cleaner process.

6.3.2.  Reliable multicast

   Do we want to go here, and if so which protocols should we mention?
   FLUTE [RFC6726] might be one example.

6.3.3.  Inter-domain multicast peering

   Interdomain peering best practices are documented in

6.3.4.  Layer 2 multicast domains

   Open question - do we want to look at L2 models, e.g. as might be
   applied at an IXP?

6.3.5.  Anything else?

   Anything else to add here?

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7.  Use case examples

   Aim to add 2-3 deployment examples here, if deemed useful.  Perhaps
   one PIM-SM/MSDP/Anycast-RP, one Embedded-RP, one SSM?

8.  Conclusions

   Do we wish to make a very strong recommendation here for the SSM
   service model, and thus for PIM-SSM, even in multi-source

   Is this document Informational or BCP?  Currently assumed

9.  Security Considerations

   Do we need general text on multicast security here, or not?

10.  IANA Considerations

   This document currently makes no request of IANA.

   Note to RFC Editor: this section may be removed upon publication as
   an RFC.

11.  Acknowledgments

   TBC if draft progresses...

12.  References

12.1.  Normative References

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, DOI 10.17487/RFC1112, August 1989,

   [RFC2236]  Fenner, W., "Internet Group Management Protocol, Version
              2", RFC 2236, DOI 10.17487/RFC2236, November 1997,

   [RFC2365]  Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
              RFC 2365, DOI 10.17487/RFC2365, July 1998,

   [RFC2375]  Hinden, R. and S. Deering, "IPv6 Multicast Address
              Assignments", RFC 2375, DOI 10.17487/RFC2375, July 1998,

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   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710,
              DOI 10.17487/RFC2710, October 1999,

   [RFC3307]  Haberman, B., "Allocation Guidelines for IPv6 Multicast
              Addresses", RFC 3307, DOI 10.17487/RFC3307, August 2002,

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, DOI 10.17487/RFC3376, October 2002,

   [RFC3618]  Fenner, B., Ed. and D. Meyer, Ed., "Multicast Source
              Discovery Protocol (MSDP)", RFC 3618,
              DOI 10.17487/RFC3618, October 2003,

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,

   [RFC3956]  Savola, P. and B. Haberman, "Embedding the Rendezvous
              Point (RP) Address in an IPv6 Multicast Address",
              RFC 3956, DOI 10.17487/RFC3956, November 2004,

   [RFC3973]  Adams, A., Nicholas, J., and W. Siadak, "Protocol
              Independent Multicast - Dense Mode (PIM-DM): Protocol
              Specification (Revised)", RFC 3973, DOI 10.17487/RFC3973,
              January 2005, <http://www.rfc-editor.org/info/rfc3973>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <http://www.rfc-editor.org/info/rfc4291>.

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,

   [RFC4610]  Farinacci, D. and Y. Cai, "Anycast-RP Using Protocol
              Independent Multicast (PIM)", RFC 4610,
              DOI 10.17487/RFC4610, August 2006,

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   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,

   [RFC5771]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
              IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
              DOI 10.17487/RFC5771, March 2010,

   [RFC6726]  Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen,
              "FLUTE - File Delivery over Unidirectional Transport",
              RFC 6726, DOI 10.17487/RFC6726, November 2012,

   [RFC7346]  Droms, R., "IPv6 Multicast Address Scopes", RFC 7346,
              DOI 10.17487/RFC7346, August 2014,

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <http://www.rfc-editor.org/info/rfc7761>.

12.2.  Informative References

   [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
              "Considerations for Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,

   [RFC4604]  Holbrook, H., Cain, B., and B. Haberman, "Using Internet
              Group Management Protocol Version 3 (IGMPv3) and Multicast
              Listener Discovery Protocol Version 2 (MLDv2) for Source-
              Specific Multicast", RFC 4604, DOI 10.17487/RFC4604,
              August 2006, <http://www.rfc-editor.org/info/rfc4604>.

   [RFC4611]  McBride, M., Meylor, J., and D. Meyer, "Multicast Source
              Discovery Protocol (MSDP) Deployment Scenarios", BCP 121,
              RFC 4611, DOI 10.17487/RFC4611, August 2006,

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              Tarapore, P., Sayko, R., Shepherd, G., Eckert, T., and R.
              Krishnan, "Use of Multicast Across Inter-Domain Peering
              Points", draft-ietf-mboned-interdomain-peering-bcp-03
              (work in progress), May 2016.

Authors' Addresses

   Mikael Abrahamsson

   Email: mikael.abrahamsson@t-systems.se

   Tim Chown
   Lumen House, Library Avenue
   Harwell Oxford, Didcot  OX11 0SG
   United Kingdom

   Email: tim.chown@jisc.ac.uk

   Lenny Giuliano
   Juniper Networks, Inc.
   2251 Corporate Park Drive
   Hemdon, Virginia  20171
   United States

   Email: lenny@juniper.net

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