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INTERNET-DRAFT                 M. Handley/J. Crowcroft/C. Bormann/J. Ott
Expires: January 2001                                  ACIRI/UCL/TZI/TZI
                                                               July 2000

           The Internet Multimedia Conferencing Architecture

Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This document is a product of the Multiparty Multimedia Session
   Control (MMUSIC) working group of the Internet Engineering Task
   Force.  Comments are solicited and should be addressed to the working
   group's mailing list at confctrl@isi.edu and/or the authors.


   This document provides an overview of multimedia conferencing on the
   Internet.  The protocols mentioned are specified elsewhere as RFCs,
   Internet-Drafts, or ITU recommendations.  Each of these
   specifications gives details of the protocol itself, how it works and
   what it does.  This document attempts to provide the reader with an
   overview of how the components fit together and of some of the
   assumptions made, as well as some statement of direction for those
   components still in a nascent stage.

   (Remove before publication:) This document is a product of the
   Multiparty Multimedia Session Control (MMUSIC) working group of the
   Internet Engineering Task Force.  Comments are solicited and should
   be addressed to the working group's mailing list at confctrl@isi.edu
   and/or the authors.

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1.  Introduction

   The Internet is not currently very good at carrying audio and video.
   This is hardly surprising as it was not designed or engineered with
   real-time traffic in mind, but there has recently been a great deal
   of interest in using the Internet for telephony services.  Part of
   this has come from pricing anomalies that make internet telephony
   somewhat artificially cheaper than traditional telephone services,
   but this is not the whole story.  The Internet itself is improving to
   better handle traffic such as audio and video, and in the medium
   term, the internet should be able to provide good quality realtime
   multimedia services, although such quality improvements are likely to
   incur additional charges.

   However, the real interest in using the internet for audio and video
   should come from the prospect for a single ubiquitous communications
   network that not only allows traditional telephony services, but also
   video, shared collaboration tools, and through IP Multicast, multi-
   party conferences and multimedia sessions that scale from small group
   meetings through to television sized audiences.  In principle, this
   may lead to a ``democratization'' of telecommunication services,
   where licenses to broadcast are not required to control physical
   access to the limited broadcast medium (although they may still be
   required for political reasons).

   It is far from clear what services will eventually emerge using such
   communications capabilities.  We can only say that the technical
   capability to have large numbers of sessions ranging in audience from
   hundreds to millions of participants, largely unlimited by geographic
   boundaries, will lead to services and social structures that do not
   exist today.  However, we can describe the basic technologies that
   are likely to bring about such changes, and in this document we
   attempt to provide such an overview.  We leave it to the reader to
   imagine the uses to which this technology will be put.

2.  The Technology

   In conjunction with computers, the term ``conferencing'' is often
   used in two different ways: firstly, to refer to bulletin boards and
   mail list style asynchronous exchanges of messages between multiple
   users; secondly, to refer to synchronous or so-called ``real-time''
   conferencing, including audio/video communication and shared tools
   such as whiteboards and other applications.  This document is about
   the architecture for this latter application, multimedia conferencing
   in an Internet environment.

   There are other infrastructures for teleconferencing in the world:
   POTS (Plain Old Telephone System) networks often provide voice
   conferencing and phone-bridges, while with ISDN, H.320 [14] can be
   used for small, strictly organized video-telephony conferencing.

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   The architecture that has evolved in the Internet is far more general
   as well as being scalable to very large groups, and permits the open
   introduction of new media and new applications as they are devised.
   As the simplest case, it also allows two persons to communicate via
   audio only, so it encompasses IP telephony.

   The determining factors of a conferencing architecture are
   communication within (possibly large) groups of humans and real-time
   delivery of information.  In the Internet, this is supported at a
   number of levels.  The remainder of this section provides an overview
   of this support, and the rest of the document describes each aspect
   in more detail.

   In a conference, information must be distributed to all the
   conference participants.  Early conferencing systems used a fan-out
   of data streams, e.g., one connection between each pair of
   participants, which means that the same information must cross some
   networks more than once.  The Internet architecture uses the more
   efficient approach of multicasting the information to all
   participants (see section 3).

   Multimedia conferences require real-time delivery of at least the
   audio and video information streams used in the conference.  In an
   ISDN context, fixed rate circuits are allocated for this purpose --
   whether their bandwidth is required at any particular instance or
   not.  On the other hand, the traditional Internet service model
   (``best effort'') cannot make the necessary quality of service
   available in congested networks.  New service models are being
   defined in the Internet together with protocols to reserve capacity
   or prioritize traffic in a more flexible way than that available with
   circuit switching (see section 4).

   In a datagram network, multimedia information must be transmitted in
   packets, some of which may be delayed more than others.  In order
   that audio and video streams be played out at the recipient in the
   correct timing, information must be transmitted that allows the
   recipient to reconstitute the timing.  A transport protocol with the
   specific functions needed for this has been defined (see section 5).
   The nature of the Internet reflects that of the world in that it is
   very heterogeneous.  Techniques exist to exploit this, and to deliver
   appropriate quality to different participants in the same conference
   according to their capabilities.

   Conference tools such as virtual whiteboards or shared editors are
   not concerned with real-time delivery of audio or video but maintain
   and update shared state between the participants.  Work on support of
   such applications in a multicast environment is in progress (section

   The humans participating in a conference generally need to have a
   specific idea of the context in which the conference is happening,
   which can be formalized as a conference policy.  Some conferences are
   essentially crowds gathered around an attraction, while others have

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   very formal guidelines on who may take part (listen in) and who may
   speak at which point.  In any case, initially the participants must
   find each other, i.e. establish communication relationships
   (conference setup, section 7).  During the conference, some
   conference control information is exchanged to implement a conference
   policy or at least to inform the crowd of who is present (section 6).

   In addition, security measures may be required to actually enforce
   the conference policy, e.g. to control who is listening and to
   authenticate contributions as actually originating from a specific
   person.  In the Internet, there is little tendency to rely on the
   traditional ``security'' of distribution offered e.g. by the phone
   system.  Instead, cryptographic methods are used for encryption and
   authentication, which need to be supported by additional conference
   setup and control mechanisms (section 8).

        Figure 1: Internet multimedia conferencing protocol stacks

   |<---       Conference Management       --->|<--- Media Agents   --->|
   |                                           |                        |
   |         Conference      |    Conference   | Audio/ |    Shared     |
   |     Setup & Discovery   |  Course Control | Video  |  Applications |

   +-------------------------+------+--------+-+--------+------------+  +
   |         S D P           |      | Distr. |  RTP /   |  Reliable  |  |
   | SAP | SIP | HTTP | SMTP | RSVP | Ctrl(1)|  RTCP    |Multicast(2)|  |
   +-----+--+--+------+------+   +--+--------+----------+------------+--+
   |   UDP  |      T C P     |   |                U D P                 |
   |                        IP + IP Multicast                           |
   |                 Integrated Services Forwarding                     |

   The protocol stacks for internet multimedia conferencing are
   illustrated in Figure 1.  Most of the protocols are not deeply
   layered unlike many protocol stacks, but rather are used alongside
   each other to produce a complete conference.

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3.  Multicast Traffic Distribution

   |Protocol      | Documentation    |  Purpose                          |
   |IP Multicast  |  RFC 1112, 2236  |  Host extensions for IP Multicast |
   |              |                  |  Multicast routing protocols:     |
   |DVMRP         |  RFC 1075        |  Dense-mode Intra-domain          |
   |PIM-SM        |  RFC 2362        |  Sparse-mode Intra-domain         |
   |PIM-DM        |  Internet Draft  |  Dense-mode Intra-domain          |
   |CBT           |  RFC 2189        |  Sparse-mode Intra-domain         |

   IP multicast provides efficient many-to-many data distribution in an
   internet environment.  It is easy to view IP multicast as simply an
   optimization for data distribution, and indeed this is the case, but
   IP multicast can also result in a different way of thinking about
   application design.  To see why this might be the case, examine the
   IP multicast service model, as described by Van Jacobson [8]:

   -    Senders just send to the group

   -    Receivers express an interest in receiving data sent to the

   -    Routers conspire to deliver data from senders to receivers

   With IP multicast, the group is indirectly identified by a single IP
   class-D multicast address.

   Several things are important about this service model from an
   architectural point of view.  Receivers do not need to know who or
   where the senders are to receive traffic from them.  Senders never
   need to know who the receivers are.  Neither senders or receivers
   need care about the network topology as the network optimizes

   An IP multicast group is scalable because information about group
   membership and group changes at the IP level are kept local to
   routers near the relevant members.  How this is performed depends on
   the particular multicast routing scheme in use local to the member,
   and although it is not a trivial task, several solutions do exist and
   therefore multicast routing will not be discussed in detail here.
   For more detailed information on multicast routing, see [7, 6, 4, 1,
   23].  Typically, as a group with s senders and r receivers increases
   in size, state in routers scales O(s) or O(1) depending on the
   routing scheme in use.  This state may be in on-tree routers for
   newer so called sparse-mode algorithms such as PIM, or in off-tree
   routers for older so-called dense-mode algorithms such as DVMRP.
   Thus the most scalable current multicast routing algorithms require
   O(1) state in on-tree routers, and hence the total routing state
   scales O(g) in a router that is on-tree for g groups.  We can also

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   envisage multicast routing schemes which require less than O(g)
   state*, but the requirement is not currently urgent, so none of these
   have yet been implemented.

   The level of indirection introduced by the IP class D address
   denominating the group solves the distributed systems binding
   problem, by pushing this task down into routing; given a multicast
   address (and UDP port), a host can send a message to the members of a
   group without needing to discover who they are.  Similarly receivers
   can ``tune in'' to multicast data sources without needing to bother
   the data source itself with any form of request.

   IP multicast is a natural solution for multi-party conferencing
   because of the efficiency of the data distribution trees, with data
   being replicated in the network at appropriate points rather than in
   end-systems.  It also avoids the need to configure special-purpose
   servers to support the session, which require support, and which
   cause traffic concentration and can be a bottleneck.  For larger
   broadcast-style sessions, it is essential that data-replication be
   carried out in a way that only requires per-receiver network-state to
   be local to each receiver, and that data-replication occurs within
   the network.  Attempting to configure a tree of application-specific
   replication servers for such broadcasts rapidly becomes a ``multicast
   routing'' problem, and thus native multicast support is a more
   appropriate solution.

3.1.  Address Allocation

    |Protocol  | Documentation     |  Purpose                         |
    |MADCAP    |  Internet Draft   |  DHCP-like client protocol       |
    |          |                   |   for address allocation         |
    |AAP       |  Internet Draft   |  Intra-domain address allocation |
    |MASC      |  Internet Draft   |  Inter-domain address allocation |
    |BGMP      |  Internet Draft   |  Inter-domain multicast routing  |

   How does an application choose a multicast address to use?

   In the absence of any other information, we can bootstrap a multicast
   application by using well-known multicast addresses.  Routing
   (unicast and multicast) and group membership protocols [5] can do
  * with IP encapsulation, not all on-tree routers need  hold  the
state for a group whose traffic they are forwarding -- traffic for
the group can be encapsulated (either unicast  of  multicast)  be-
tween  on-tree  routers  nearer  the edge of the network, reducing
some of the state burden on backbone routers.

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   just that.  However, this is not the best way of managing
   applications of which there is more than one instance at any one

   For these, we need a mechanism for allocating group addresses
   dynamically, and a directory service which can hold these allocations
   together with some key (session information for example --- see
   later), so that users can look up the address associated with the
   application.  The address allocation and directory functions should
   be distributed to scale well.

   Multicast address allocation is currently an active area of research.
   For many years multicast address allocation has been performed using
   multicast session directories (see section 7.1), but as the users and
   uses of IP multicast increase, it is becoming clear that a more
   hierarchical approach is required.

   An architecture [10] is currently being developed based around a
   well-defined API that an application can use to request an address.
   The host then requests an address from a local address allocation
   server, which in turn chooses and reserves an unallocated address
   from a range dynamically allocated to the domain.  By allocating
   addresses in a hierarchical and topologically sensitive fashion, the
   address itself can be used in a hierarchical multicast routing
   protocol currently being developed (BGMP, [29]) that will help
   multicast routing scale more gracefully that current schemes.

4.  Internet Service Models

             |Protocol  | Documentation  |  Purpose           |
             |IP        |  RFC 791       |  Internet Protocol |

   Traditionally the internet has provided so-called best-effort
   delivery of datagram traffic from senders to receivers.  No
   guarantees are made regarding when or if a datagram will be delivered
   to a receiver, however datagrams are normally only dropped when a
   router exceeds a queue size limit due to congestion.  The best-effort
   internet service model does not assume FIFO queuing, although many
   routers have implemented this.

   With best-effort service, if a link is not congested, queues will not
   build at routers, datagrams will not be discarded in routers, and
   delays will consist of serialization delays at each hop plus
   propagation delays.  With sufficiently fast link speeds,
   serialization delays are insignificant compared to propagation
  * For slow  links,  a set of mechanisms has  been  defined  that

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   If a link is congested, with best-effort service, queuing delays will
   start to influence end-to-end delays, and packets will start to be
   lost as queue size limits are exceeded.  Real-time traffic does not
   cope terribly well with packet loss levels of more than a few
   percent, although it is possible to add redundancy [12] to increase
   the levels at which loss becomes a problem.  In the last few years a
   significant amount of work has also gone into providing non-best-
   effort services that would provide a better assurance that an
   acceptable quality conference will be possible.

4.1.  Non-best effort service

   Real-time internet traffic is defined as datagrams that are delay
   sensitive.  It could be argued that all datagrams are delay sensitive
   to some extent, but for these purposes we refer only to datagrams
   where exceeding an end-to-end delay bound of a few hundred
   milliseconds renders the datagrams useless for the purpose they were
   intended.  For the purposes of this definition, TCP traffic is
   normally not considered to be real-time traffic, although there may
   be exceptions to this rule.

   On congested links, best-effort service queuing delays will adversely
   affect real-time traffic.  This does not mean that best-effort
   service cannot support real-time traffic --- merely that congested
   best-effort links seriously degrade the service provided.  For such
   congested links, a better-that-best-effort service is desirable.

   To achieve this, the service model of the routers can be modified.
   FIFO queuing can be replaced by packet forwarding strategies that
   discriminate different ``flows'' of traffic.  The idea of a flow is
   very general.  A flow might consist of ``all marketing site web
   traffic'', or ``all fileserver traffic to and from teller machines''.
   On the other hand, a flow might consist of a particular sequence of
   packets from an application in a particular machine to a peer
   application in another particular machine set up on request, or it
   might consist of all packets marked with a particular Type-of-Service

   There is really a spectrum of possibilities for non-best-effort
   service something like that shown in Figure 2.

               Figure 2: Spectrum of internet service types

   best effort            assured by                guaranteed by
   unsignalled          type of service           per-flow reservation
           prioritized by                assured by
           type of service          aggregate reservation

helps minimize serialization and link access delays [2].

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   This spectrum is intended to illustrate that between best-effort, and
   hard per-flow guarantees lie many possibilities for non-best-effort
   service, including having hard guarantees based on an aggregate
   reservation, assurances that traffic marked with a particular type-
   of-service bit will not be dropped so long as it remains in profile,
   and simpler prioritization-based services.

   Towards the right hand side of the spectrum, flows are typically
   identifiable in the Internet by the tuple: source machine,
   destination machine, source port, destination port, protocol, any of
   which could be ``ANY'' (wildcarded).

   In the multicast case, the destination is the group, and can be used
   to provide efficient aggregation.

   Flow identification is called classification and a class (which can
   contain one or more flows) has an associated service model applied.
   This can default to best effort.

   Through network management, we can imagine establishing classes of
   long lived flows -- enterprise networks (``Intranets'') often enforce
   traffic policies that distinguish priorities which can be used to
   discriminate in favor of more important traffic in the event of
   overload (though in an underloaded network, the effect of such
   policies will be invisible, and may incur no load/work in routers).

   The router service model to provide such classes with different
   treatment can be as simple as a priority queuing system, or it can be
   more elaborate.

   Although best-effort services can support real-time traffic,
   classifying real-time traffic separately from non-real-time traffic
   and giving real-time traffic priority treatment ensures that real-
   time traffic sees minimum delays.  Non-real-time TCP traffic tends to
   be elastic in its bandwidth requirements, and will then tend to fill
   any remaining bandwidth.

   We could imagine a future Internet with sufficient capacity to carry
   all of the world's telephony traffic.  Since this is a relatively
   modest capacity requirement, it might be simpler to establish
   ``POTS'' as a static class which is given some fraction of the
   capacity overall, and then within the backbone of the network no
   individual call need be given an allocation (i.e. we would no longer
   need the call setup/tear down that was needed in the legacy POTS
   which was only present due to under-provisioning of trunks, and to
   allow the trunk exchanges the option of call blocking).  The vision
   is of a network that is engineered with capacity for all of the non-
   best-effort average load sources to send without needing individual

4.2.  Reservations

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|Protocol         | Documentation  |  Purpose                              |
|RSVP             |  RFC 2205      |  Resource ReSerVation Protocol (RSVP) |
|Controlled Load  |  RFC 2211      |  Network service model                |
| Service         |                |   selected by RSVP                    |
|Guaranteed       |  RFC 2212      |  Network service model                |
| Service         |                |   selected by RSVP                    |

   For flows that may take a significant fraction of the network (i.e.
   are ``special'' and can't just be lumped under a static class), we
   need a more dynamic way of establishing these classifications.  In
   the short term, this applies to many multimedia calls since the
   Internet is largely under-provisioned at the time of writing.

   RSVP has been standardized for just this purpose.  It provides flow
   identification and classification.  Hosts and applications are
   modified to speak RSVP client language, and routers speak RSVP.

   Since most traffic requiring reservations is delivered to groups
   (e.g. TV), it is natural for the receiver to make the request for a
   reservation for a flow.  This has the added advantage that different
   receivers can make heterogeneous requests for capacity from the same
   source.  Thus RSVP can accommodate monochrome, color and HDTV
   receivers from a single source (also see section Figure 5).

   Again the routers conspire to deliver the right flows to the right

   RSVP accommodates the wildcarding noted above.

Admission Control

   If a network is provisioned such that it has excess capacity for all
   the real-time flows using it, a simple priority classification
   ensures that real-time traffic is minimally delayed.  However, if a
   network is insufficiently provisioned for the traffic in a real-time
   traffic class, then real-time traffic will be queued, and delays and
   packet loss will result.  Thus in an under-provisioned network,
   either all real-time flows will suffer, or some of them must be given

   RSVP provides a mechanism by which an admission control request can
   be made, and if sufficient capacity remains in the requested traffic
   class, then a reservation for that capacity can be put in place.

   If insufficient capacity remains, the admission request will be
   refused, but the traffic will still be forwarded with the default
   service for that traffic's traffic class.  In many cases even an

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   admission request that failed at one or more routers can still supply
   acceptable quality as it may have succeeded in installing a
   reservation in all the routers that were suffering congestion.  This
   is because other reservations may not be fully utilising their
   reserved capacity in those routers where the reservation failed.


   If a reservation involves setting aside resources for a flow, this
   will tie up resources so that other reservations may not succeed, and
   depending on whether the flow fills the reservation, other traffic is
   prevented from using the network.  Clearly some negative feedback is
   required in order to prevent pointless reservations from denying
   service to other users.  This feedback is typically in the form of

   Billing requires that the user making the reservation is properly
   authenticated so that the correct user can be charged.  Billing for
   reservations introduces a level of complexity to the internet that
   has not typically been experienced with non-reserved traffic, and
   requires network providers to have reciprocal usage-based billing
   arrangements for traffic carried between them.  It also suggests the
   use of mechanisms whereby some fraction of the bill for a link
   reservation can be charged to each of the downstream multicast

4.3.  Differentiated Services

   |Protocol                 | Documentation  |  Purpose               |
   |Differentiated Services  |  RFC 2474      |  DS Field in IP Header |
   |Differentiated Services  |  RFC 2475      |  DS Architecture       |

   Whereas RSVP asks routers to classify packets into classes to achieve
   a requested quality of services, it is also possible to explicitly
   mark packets to indicate the type of service required.  Of course,
   there has to be an incentive and mechanisms to ensure that ``high-
   priority'' is not set by everyone in all packets, and this incentive
   is provided by edge-based policing and by buying profiles of higher
   priority service.  In this context, a profile could have many forms,
   but a typical profile might be a token-bucket filter specifying a
   mean rate and a bucket size with certain time-of-day restrictions.

   This is still an active research area, but the general idea is for a
   customer to buy from their provider a profile for higher quality
   service, and the provider polices marked traffic from the site to

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   ensure that the profile is not exceeded.  Within a provider's
   network, routers give preferential services to packets marked with
   the relevant type-of-service bit.  Where providers peer, they arrange
   for an aggregate higher-quality profile to be provided, and police
   each other's aggregate if it exceeds the profile.  In this way,
   policing only needs to be performed at the edges to a provider's
   network on the assumption that within the network there is sufficient
   capacity to cope with the amount of higher-quality traffic that has
   been sold.  The remainder of the capacity can be filled with regular
   best-effort traffic.

   One big advantage of differentiated services over reservations is
   that routers do not need to keep per-flow state, or look at source
   and destination addresses to classify the traffic, and this means
   that routers can be considerably simpler.  Another big advantage is
   that the billing arrangements for differentiated services are
   pairwise between providers at boundaries -- at no time does a
   customer need to negotiate a billing arrangement with each provider
   in the path*

5.  Transport Protocols

   So-called real-time delivery of traffic requires little in the way of
   transport protocol.  In particular, real-time traffic that is sent
   over more than trivial distances is not retransmittable.

   With packet multimedia data there is no need for the different media
   comprising a conference to be carried in the same packets.  In fact
   it simplifies receivers if different media streams are carried in
   separate flows (i.e., separate transport ports and/or separate
   multicast groups).  This also allows the different media to be given
   different quality of service.  For example, under congestion, a
   router might preferentially drop video packets over audio packets.
   In addition, some sites may not wish to receive all the media flows.
   For example, a site with a slow access link may be able to
   participate in a conference using only audio and a whiteboard whereas
   other sites in the same conference with more capacity may also send
   and receive video.  This can be done because the video can be sent to
   a different multicast group than the audio and whiteboard.  This is
   first step towards coping with heterogeneity by allowing the
   receivers to decide how much traffic to receive, and hence allowing a
   conference to scale more gracefully.

5.1.  Receiver Adaptation and Synchronization

  * With  reservations  there  may  be ways to avoid this too, but
they're somewhat more difficult given the more specific nature  of
a reservation.

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                 Figure 3: Network Jitter and Packet Audio
                                                               | | |
                                                               |x| |
                                                               | | |
                                Compression                    |x| v
                                + Packetizer                   | |
                                 +--------+                 +-------+
   Microphone                    |        | 1.5 Mbit/s link |       |
               +-----+ A   A   A |        |-----------------| Router|
   /~\------+__|     |>>> >>> >>>|        |A   A   A   A   A|       |
   \_/------+  |A->D |20 ms Audio|        |-----------------|       |
               +-----+Timeslices |        |   --------->    |       |
                                 |        |                 +-------+
                                 +--------+                    |A|
                                                               | | |
                                               Shared link:    |x| |
                                               Audio traffic   |A| |
                                               interspersed w  |A| |
                                               other traffic   |x| |
                                                               |x| |
                                                               |x| |
                                Depacketizer                   |A| |
                                + Timing recovery              |A| v
                                + Decompression                | |
                                 +--------+                 +-------+
      Speaker                    |        | 1.5 Mbit/s link |       |
   |\          +-----+ A   A   A |        |-----------------| Router|
   | +---+     |     |<<< <<< <<<|        |A       A   AA A |       |
   | |   |-----|D->A |20 ms Audio|        |-----------------|       |
   | +---+     +-----+Timeslices |        |   <---------    |       |
   |/                            |        |                 +-------+
                                 +--------+                    | |
                                                               |X| |
                                                               |X| |
                                                               | | |
                                                               |X| v
                                                               | |

   Best-effort traffic is delayed by queues in routers between the
   sender and the receivers.  Even reserved priority traffic may see
   small transient queues in routers, and so packets comprising a flow
   will be delayed for different times.  Such delay variance is known as
   jitter, and is illustrated in Figure 3.

   Real-time applications such as audio and video need to be able to
   buffer real-time data at the receiver for sufficient time to remove
   the jitter added by the network and recover the original timing
   relationships between the media data.  In order to know how long to
   buffer for, each packet must carry a timestamp which gives the time

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   at the sender when the data was captured.  Note that for audio and
   video data timing recovery, it is not necessary to know the absolute
   time that the data was captured at the sender, only the time relative
   to the other data packets.

                   Figure 4: Inter-media synchronization

   Incoming packets

   ----------------     +----------------+
   V A  V   AV    A --> |   Host         |
   ----------------     |   Demuxing     |
                       /                  \
                      /                    \
           A      A  /  A           V   V   \  V
                    v                        v
      +---------------+                     +---------------+
      | Depacketizer  |  per source         | Depacketizer  |
      +---------------+  delay adaptation:  +---------------+
           v           \   45 ms    95 ms  /          v
      +------------+     \               /     +------------+
      | format     |       \           /       | format     |
      | conversion |    +------------------+   | conversion |
      +------------+    | synchronization  |   +------------+
           |            |      agent       |          |
           |            +------------------+          |
           | mix           /           \              |
           v              /             \             v
        |     |          /               \         |     |
        +-----+         /                 \        +-----+
        |     |        /                   \       |     |
        +-----+       /                     \      +-----+
        |  A  |      / 95 ms           95 ms \     |  V  |
        +-----+     /                         \    +-----+
        |  A  | <--+                           +-> |  V  |
        +-----+          /|         +--------+     +-----+
        |  A  |     +---+ |         |/------\|     |  V  |
        +-----+>>>>>|   | |         ||      ||<<<<<+-----+
                    +---+ |         |\------/|
                         \|         +--------+

   As audio and video flows will receive differing jitter and possibly
   differing quality of service, audio and video that were grabbed at
   the same time at the sender may not arrive at the receiver at the
   same time.  At the receiver, each flow will need a playout buffer to
   remove network jitter.  Inter-flow synchronization can be performed
   by adapting these playout buffers so that samples/frames that
   originated at the same time are played out at the same time (see
   figure Figure 4).  This requires that the time base of different
   flows from the same sender can be related at the receivers, e.g. by

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   making available the absolute times at which each of them was

5.2.  RTP

   |Protocol     | Documentation  |  Purpose                             |
   |RTP,RTCP     |  RFC 1889      |  packet format for realtime traffic  |
   |RTP Profile  |  RFC 1890      |  specific RTP profile for AV traffic |
   |RTP Payload  |  RFC 2032,     |  payload formats for specific codecs |
   | Formats     |   2035, etc    |                                      |

   The transport protocol for real-time flows is RTP [28].  This
   provides a standard format packet header which gives media specific
   timestamp data, as well as payload format information and sequence
   numbering amongst other things.  RTP is normally carried using UDP.
   It does not provide or require any connection setup, nor does it
   provide any enhanced reliability over UDP.  For RTP to provide a
   useful media flow, there must be sufficient capacity in the relevant
   traffic class to accommodate the traffic.  How this capacity is
   ensured is independent of RTP.

   Every original RTP source is identified by a source identifier, and
   this source id is carried in every packet.  RTP allows flows from
   several sources to be mixed in gateways to provide a single resulting
   flow.  When this happens, each mixed packet contains the source IDs
   of all the contributing sources.

   RTP media timestamp units are flow specific --- they are in units
   that are appropriate to the media flow.  For example, 8kHz sampled
   PCM encoded audio has a timestamp clock rate of 8kHz.  This means
   that inter-flow synchronization is not possible from the RTP
   timestamps alone.

   Each RTP flow is supplemented by Real-Time Control Protocol (RTCP)
   packets.  There are a number of different RTCP packet types.  RTCP
   packets provide the relationship between the realtime clock at a
   sender and the RTP media timestamps so that inter-flow
   synchronization can be performed, and they provide textual
   information to identify a sender in a conference from the source id.

5.3.  Conference Membership and Reception Feedback

   IP multicast allows sources to send to a multicast group without
   being a receiver of that group.  However, for many conferencing
   purposes it is useful to know who is listening to the conference, and
   whether the media flows are reaching receivers properly.  Accurately

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   performing both these tasks restricts the scaling of the conference.
   IP multicast means that no-one knows the precise membership of a
   multicast group at a specific time, and this information cannot be
   discovered, as to try to do so would cause an implosion of messages,
   many of which would be lost*.  Instead, RTCP provides approximate
   membership information through periodic multicast of session messages
   which, in addition to information about the recipient, also give
   information about the reception quality at that receiver.  RTCP
   session messages are restricted in rate, so that as a conference
   grows, the rate of session messages remains constant, and each
   receiver reports less often.  A member of the conference can never
   know exactly who is present at a particular time from RTCP reports,
   but does have a good approximation to the conference membership.  The
   is analogous to what happens in a real-world meeting hall; the
   meeting organizers may have an attendance list, but if people are
   coming and going all the time, they probably do not know exactly who
   is in the room at any one moment.

   Reception quality information is primarily intended for debugging
   purposes, as debugging of IP multicast problems is a difficult task.
   However, it is possible to use reception quality information for rate
   adaptive senders, although it is not clear whether this information
   is sufficiently timely to be able to adapt fast enough to transient

5.4.  Scaling Issues and Heterogeneity

   The Internet is very heterogeneous, with link speeds ranging from
   around 10 kbit/s up to around 10 Gbit/s, and very varied levels of
   congestion.  How then can a single multicast source satisfy a large
   and heterogeneous set of receivers?

  * Note that a conference policy that restricts  conference  mem-
bership can be implemented using encryption and restricted distri-
bution of encryption keys, of which more later.

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    Figure 5: Receiver adaptation: multiple layers and multicast groups

                             /~~~\           ##### 2 Mbit/s layer
                             | R |           ===== 512 kbit/s layer
                             \___/           ----- 64 kbit/s layer
          10 Mbit/s        # 10 Mbit/s
          link:           #  link
   /~~~\  #######>  +---+
   | S |  =======>  |   |
   \___/  ------->  +---+
                       \ = 1.5 Mbit/s
   Source               \ = link
                         \ =       1.5 Mbit/s
                          \ =         link
                           \ +---+=========>/~~~\
                             |   |--------->| R |
                             +---+          \___/
                10 Mbit/s  / =    \
                    link  / =      \ 128 kbit/s
                         / =        \ link
                        / =          \      10 Mbit/s
                     /~~~\            +---+     link /~~~\
                     | R |            |   |--------->| R |
                     \___/            +---+          \___/
                                            \ 10 Mbit/s
                                             \ link
                                               | R |

   In addition to each receiver performing its own adaptation to jitter,
   if the sender layers [22] its video (or audio) stream, different
   receivers can choose to receive different amounts of traffic and
   hence different qualities.  To do this the sender must code they
   video as a base layer (the lowest quality that might be acceptable)
   and a number of enhancement layers, each of which adds more quality
   at the expense of more bandwidth.  With video, these additional
   layers might increase the framerate or increase the spatial
   resolution of the images or both.  Each layer is sent to a different
   multicast group, and receivers can decide individually how many
   layers to subscribe to.  This is illustrated in Figure 5.  Of course,
   if they are going to respond to congestion in this way, then we also
   need to arrange that the receivers in a conference behind a common
   bottleneck tend to respond together to prevent de-synchronized
   experiments by different receivers from having the net effect that
   too many layers are always being drawn through a common bottleneck.
   RLM [21] is one way that this might be achieved, although there is

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   continuing research in this area.

6.  Conference Control

|Protocol | Documentation            |  Purpose                            |
|H.323    | ITU recommendation H.323 | Tightly coupled conference setup    |
|         |                          |    and control                      |
|H.332    | ITU recommendation H.332 | Loosely coupled extensions to H.323 |

   Conferences come in many shapes and sizes, but there are only really
   two models for conference control: light-weight sessions and tightly
   coupled conferencing.  For both models, rendezvous mechanisms are
   needed.  Note that the conference control model is orthogonal to
   issues of quality of service and network resource reservation, and it
   is also orthogonal to the mechanism for discovering the conference.

   Light-weight sessions are multicast based multimedia conferences that
   lack explicit conference membership control and explicit conference
   control mechanisms.  Typically a lightweight session consists of a
   number of many-to-many media streams supported using RTP and RTCP
   using IP multicast*.  Typically, the only conference control
   information needed during the course of a light-weight session is
   that distributed in the RTCP session information, i.e. an approximate
   membership list with some attributes per member.

   Tightly coupled conferences may also be multicast based and use RTP
   and RTCP, but in addition they have an explicit conference membership
   mechanism and may have an explicit conference control mechanism that
   provides facilities such as floor control.

   The most widely used tightly coupled conference control protocols
   suitable for Internet use are those belonging to the ITU's H.323
   family [16].  However it should be noted that this is inappropriate
   for larger conferences where scaling problems will be introduced by
   the conference control mechanisms.  The Simple Conference Control
   Protocol (SCCP) [18] has been proposed as a more scalable distributed
   conference control protocol.

   In order to try and address large conferences, the ITU is currently
   standardising H.332 [17], which is essentially a small tightly
   coupled H.323 conference with a larger lightweight-sessions-style
  * There is some confusion on the term session,  which  is  some-
times  used  for  a  conference  and  sometimes for a single media
stream transported by RTP.  In this document, we prefer to use the
less ambiguous term conference except where existing protocols use
the term session.

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   conference listening in as passive participants.  It is not yet clear
   whether H.332 will see large scale acceptance, as its benefits over a
   simple lightweight session are not terribly obvious.  It seems likely
   that lightweight sessions combined with stream authentication (see
   section 8.3) might be a more appropriate solution for many potential

6.1.  Controlling Multimedia Servers

       |Protocol | Documentation |  Purpose                        |
       |RTSP     |  RFC 2326     |  Remote control and AV playback |
       |         |               |    and recording servers        |

   The Real-Time Stream-control Protocol (RTSP) provides a standard way
   to remote control a multimedia server.  While primarily aimed at web-
   based media-on-demand services, RTSP is also well suited to provide
   VCR-like controls for audio and video streams, and to provide
   playback and record functionality of RTP data streams.  A client can
   specify that an RTSP server plays a recorded multimedia session into
   an existing multicast-based conference, or can specify that the
   server should join the conference and record it.

6.2.  Protocols for Non-A/V Applications

   Applications other than audio and video have evolved in Internet
   conferencing, e.g. Imm, Wb [8], NTE [11].  Such applications can be
   used to substitute for meeting aids in physical conferences
   (whiteboards, projectors) or replace visual and auditory cues that
   are lost in teleconferences (e.g., a speaker list application); they
   also can enable new styles of joint work.

   Most non-A/V applications have in common that the application
   protocol is about establishing and updating a shared state.  Loss of
   information is often not acceptable, so some form of multicast
   reliability is required.  The applications' requirements differ: Some
   applications make per-participant additions to the shared state that
   are orthogonal to each other (e.g., whiteboards), some evolve a more
   closely interrelated common state (e.g., additions to a speaker list
   must be properly sequenced).  Some applications can make use of added
   bandwidth/react to congestion in an elastic way, others transport
   data that, although not strictly real-time, is time-critical.

   In the IRTF research group on Reliable Multicast [13], work is in
   progress on common protocol elements that can be used in such
   applications.  At the time of writing, some aspects of reliable
   multicast are not well-understood, such as the proper way to provide

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   congestion control in a multi-sender multicast environment.  As
   congestion control is considered an essential element, standards
   track protocols are not expected before this can be solved.

7.  Conference Setup

   There are two basic forms of conference discovery mechanism.  These
   are session advertisement and session invitation.  Session
   advertisements are provided using a session directory, and inviting a
   user to join a session is provided using a session invitation
   protocol such as SIP or H.323.

7.1.  Session Directories

     |Protocol  | Documentation    |  Purpose                         |
     |SDP       |  RFC 2327        |  Session description format      |
     |SAP       |  Internet draft  |  Multicast session announcements |

   The rendezvous mechanism for many light-weight sessions is a
   multicast based session directory.  This ``broadcasts'' session
   descriptions [9] to all the potential session participants.  These
   session descriptions provide an advertisement that the session will
   exist, and also provide sufficient information including multicast
   addresses, ports, media formats and session times so that a receiver
   of the session description can join the session.  The session
   description protocol (SDP) describes the content and format of a
   multimedia session, and the session announcement protocol (SAP) is
   used to distribute it to all potential session recipients.

   This mechanism can also be applied to advertised tightly coupled
   sessions, and only requires that additional information about the
   mechanism to use to join the session is given.  However, as the
   number of sessions in the session directory grows, we expect that
   only larger-scale public sessions will be announced in this manner,
   and smaller, more private sessions will tend to use direct invitation
   rather than advertisement.

7.2.  Session Invitation

|Protocol  | Documentation  |  Purpose                                     |
|SIP       |  RFC 2543      |  initiating multimedia calls and conferences |

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   Not all sessions are advertised, and even those that are advertised
   may require a mechanism to explicitly invite a user to join a
   session.  Such a mechanism is required regardless of whether the
   session is a lightweight session or a more tightly coupled session,
   although the invitation system must specify the mechanism to be used
   to join the session.

   As users are mobile, it is important that such an invitation
   mechanism is capable of locating and inviting a user in a location
   independent manner.  Thus user addresses need to be used as a level
   of indirection rather than routing a call to a specific terminal.
   The invitation mechanism should also provide for alternative
   responses, such as leaving a message or being referred to another
   user, should the invited user be unavailable.

   The Session Initiation Protocol (SIP) provides a mechanism whereby a
   user can be invited to participate in a conference.  SIP does not
   care whether the session is already ongoing, or is just being
   created, and it doesn't care whether the conference is a small
   tightly coupled session or a huge broadcast -- it merely conveys an
   invitation to a user in a timely manner, inviting them to
   participate, and provides enough information for them to be able to
   know what sort of session to expect.  Thus although SIP can be used
   to make telephone-style calls, it is by no means restricted to that
   style of conference.

8.  Security

   There is a temptation to believe that multicast is inherently less
   private than unicast communication since the traffic visits so many
   more places in the network.  In fact, this is not the case except
   with broadcast and prune type multicast routing protocols [4].
   However, IP multicast does make it simple for a host to anonymously
   join a multicast group and receive traffic destined to that group
   without the other senders' and receivers' knowledge.  If the
   application requirement (conference policy) is to communicate between
   some defined set of users, then strict privacy can only be enforced
   in any case through adequate end-to-end encryption.

   RTP specifies a standard way to encrypt RTP and RTCP packets using
   private key encryption schemes such as DES [24].  It also specifies a
   standard mechanism to manipulate plain text keys using MD5 [25] so
   that the resulting bit string can be used as a DES key.  This allows
   simple out-of-band mechanisms such as privacy-enhanced mail to be
   used for encryption key exchange.

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8.1.  Authentication and Key Distribution

   |Protocol  | Documentation              |  Purpose                  |
   |PGP       |  RFC 1991                  |  public key cryptography  |
   |X.509     |  ITU recommendation X.509  |  directory authentication |

   Key distribution is closely tied to authentication.  Conference or
   session directory keys can be securely distributed using public-key
   cryptography on a one-to-one basis (by email, a directory service, or
   by an explicit conference setup mechanism), but this is only as good
   as the certification mechanism used to certify that a key given by a
   user is the correct public key for that user.  Such certification
   mechanisms [3] are however not specific to conferencing, and it looks
   likely that certificates such as those provided by PGP will be most
   widely used in the near term.

   Session keys can be distributed using encrypted Session Descriptions
   carried in SIP session invitations, or in encrypted session
   announcements as described below.  Neither of these mechanisms
   provide for changing keys during a session as might be required in
   some tightly coupled sessions, but they are probably sufficient for
   many used in the context of lightweight sessions.

   Even without privacy requirements in the conference policy, strong
   authentication of a user is required if making a network reservation
   results in usage based billing.

8.2.  Encrypted Session Announcements

     |Protocol  | Documentation    |  Purpose                         |
     |SAP       |  Internet draft  |  multicast session announcements |

   Session Directories can make encrypted session announcements using
   private key encryption, and carry the encryption keys to be used for
   each of the conference media streams in the session.  Whilst this
   does not solve the key distribution problem, it does allow a single
   conference to be announced more than once to more than one key-group,
   where each group holds a different session directory key, so that the
   two groups can be brought together into a single conference without
   having to know each other's keys.

8.3.  Secured ``Broadcasts''

   While private-key encryption is sufficient to exclude non-members

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            Figure 6: Joining a light-weight multimedia session

   User A     |                                             |
   creates    |  SDP/SAP                                    |
   conference |----------->                                 |
              |                                             |User B
              |  SDP/SAP                            IGMP    |starts
              |----------->               IGMP /--<---------|session
              |                 IGMP /-<------/             |directory
              |-----------<---------/                       |
              |                                             |
              |  SDP/SAP                                    |
              |                                             |
   User A     |                                             |
   starts     |    RTP                                      |
   sending    |===========>                                 |
              |    RTCP                                     |
              |----------->                                 |
              |                                             |
              |    RTP                                      |
              |===========>                                 |
              |                                             |
              |    RTP                                      |User B
              |===========>                         IGMP    |joins
              |                           IGMP /--<---------|conference
              |                 IGMP /-<------/             |
              |-----------<---------/                       |User's App
              |                                     RTCP    |Sends RTCP
              |    RTP                         /--<---------|Session
              |<-----------------------------/              |
              |    RSVP Path Message                        |
              |                                             |User's App
              |    RTP                          /-----------|makes
              |    RSVP RESV Message    /-----/             |
              |<-----------------------/                    |
              |                                             |
              |    RTP                                      |Quality
              |============================================>|of Service
              |                                             |improves

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   from sending or receiving multicast conference traffic, it does mean
   that all members of a session are equal.  This is normally acceptable
   for multi-way conferences, but will not be acceptable for many
   broadcasters who require the ability to ensure that only they can
   send, perhaps in addition to ensuring that only their paid customers
   can receive.  This is nicely illustrated by the multicast of the
   Rolling Stones concert in 1994 which was billed as being the first
   live concert on the Mbone.  In fact, this honour goes to a little
   known band called Severe Tire Damage who had multicast an impromptu
   concert a year previously.  To make their point, just before the
   Stones were due to go on stage, Severe Tire Damage suddenly started
   broadcasting one of their songs live to the same multicast group.
   Clearly commercial broadcasters want to avoid occurrences like this

   Such secured broadcasts can be performed by encrypting a hash
   (digitally signing) of each packet with the senders private key of a
   public-private key pair.  The public key is then given to the
   receivers, and they discard (and prune if possible) any packets that
   are unsigned.  The problem with this is that even encrypting a 128
   bit hash with a public key algorithm can be relatively expensive to
   perform at high packet rates sometimes seen with video.  The use of
   public-key cryptography for this purpose has not yet been
   standardized, but some such mechanism will clearly be needed before
   the Mbone becomes an acceptable environment for commercial

9.  Summary

   This document is an attempt to gather together in one place the set
   of assumptions behind the design of the Internet Multimedia
   Conferencing architecture, and the services that are provided to
   support it.

   Figure 6 shows an example time sequence involved in setting up a
   light-weight session between two sites.  In this case, site A creates
   a session advertisement, and some time later starts sending a media
   stream even though there may be no receiver at that time.  Some time
   later, site B joins the session (the multicast routing protocol here
   is PIM), and starts to receive the traffic.  At the earliest
   opportunity site B also makes an RSVP reservation to ensure the flow
   quality is satisfactory.  This example should be taken as
   illustrative only -- there are different ways to join sessions, and
   different ways to get improved quality of service.

   The lightweight sessions model for Internet multimedia conferencing
   may not be appropriate for all conferences, but for those sessions
   that do not require tightly-coupled conference control, it provides
   an elegant style of conferencing that scales from two participants to
   millions of participants.  It achieves this scaling by virtue of the
   way that multicast routing is receiver driven, keeping essential
   information about receivers local to those receivers.  Each new

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   participant only adds state close to them in the network.  It also
   scales by not requiring explicit conference join mechanisms; if
   everyone were to need to know exactly who is in the session at any
   time, the scaling would be severely adversely affected.  RTCP
   provides membership information that is accurate when the group is
   small, and increasingly only a statistical representation of the
   membership as the group grows.  Security is handled through the use
   of encryption rather than through the control of data distribution.

   For those that require tightly coupled conferences, solutions such as
   H.323 are emerging there too.

   There are still many parts of this architecture that are incomplete,
   and are still the subject of active research.  In particular,
   differentiated services for better-than-best-effort service show
   great promise to provide a more scalable alternative to individual
   reservations.  Multicast routing scales well to large groups, but
   scales less well to large numbers of groups; we expect this will
   become the subject of significant research over the next few years.
   Multicast congestion control mechanisms are still a research topic,
   although in the last year several schemes have emerged that show
   promise.  Layered codecs show great promise to allow conferences to
   scale in the face of heterogeneity, but the join and leave mechanisms
   that allow them to perform receiver-based congestion control are
   still being examined.  We have several working examples of reliable-
   multicast-based shared applications; the next few years should see
   the start of standardization work in this area as appropriate
   multicast congestion control mechanisms emerge.  Finally a complete
   security architecture for conferencing would be very desirable;
   currently we have many parts of the solution, but are still waiting
   for an appropriate key-distribution architecture to emerge from the
   security research community.

   The Internet Multimedia Conferencing architecture and the Mbone have
   come a long way from their early beginnings on the DARTnet testbed in
   1992.  The picture is not yet finished, but it has now taken shape
   sufficiently that we can see the form it will take.  Whether or not
   the Internet does evolve into the single communications network that
   is used for most telephone, television, and other person-to-person
   communication, only time will tell.  However, we believe that it is
   becoming clear that if the industry decides that this should be the
   case, the Internet should be up to the task.

10.  Acknowledgments

   Acknowledgments are due to the End-to-End Research Group, the Int-
   serv, RSVP, MMUSIC and AVT working groups of the IETF, and discussion
   with colleagues at UCL.  The earliest clear exposition of some of the
   ideas here was presented at ACM SIGCOMM 1994 in London by Van

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11.  Authors' Addresses

   Mark Handley
   AT&T Center for Internet Research at ICSI
   1947 Center St, Suite 600
   Berkeley, CA 94704
   EMail: mjh@aciri.org

   Jon Crowcroft,
   Department of Computer Science
   University College London
   Gower Street,
   London WC1E 6BT, UK.
   Email: j.crowcroft@cs.ucl.ac.uk

   Carsten Bormann, Joerg Ott
   Universitaet Bremen TZI
   Postfach 330440
   D-28334 Bremen, GERMANY.
   Email: cabo@tzi.org, jo@tzi.org


   [1]  A. Ballardie, P. Francis, J. Crowcroft, ``An Architecture for
        Scalable Inter-Domain Multicast Routing'', ACM SIGCOMM 1993, pp

   [2]  C. Bormann, ``Providing integrated services over low-bitrate
        links,'' RFC2689, September 1999.

   [3]  CCITT (Consultative Committee on International Telegraphy and
        Telephony). ``Recommendation X.509: The Directory --
        Authentication Framework.'' 1988.

   [4]  S. Deering, C. Partridge, D. Waitzman, ``Distance Vector
        Multicast Routing Protocol'', RFC 1075,  Nov 1988.

   [5]  Steve Deering, ``Multicast Routing in Internetworks and Extended
        LANs'', ACM SIGCOMM 88, August 1988, pp 55-64 and ``Host
        Extensions for IP Multicasting'', RFC 1112.

   [6]  S. Deering, D. Estrin, D. Farinacci, V. Jacobson, C-G. Liu, L.
        Wei ``An Architecture for Wide Area Multicast Routing'' ACM
        SIGCOMM 1994, pp 126-135.

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INTERNET-DRAFT          Conferencing Architecture              July 2000

   [7]  Estrin, Farinacci, Helmy, Thaler, Deering, Handley, Jacobson,
        Liu, Sharma, Wei, ``Protocol Independent Multicast-Sparse Mode
        (PIM-SM): Protocol Specification'', RFC 2362.

   [8]  S. Floyd, V. Jacobson, S. McCanne, C-G. Liu, L. Zhang, ``A
        Reliable Multicast Framework for Light-weight Sessions and
        Application Level Framing'' ACM SIGCOMM 1995, pp 342-356.

   [9]  M. Handley, V. Jacobson, ``SDP: Session Description Protocol''
        INTERNET-DRAFT, Dec 1997.

   [10] M. Handley, D. Thaler, D. Estrin, ``The Internet Multicast
        Address Allocation Architecture'', INTERNET-DRAFT, Dec 1997.

   [11] M. Handley, J. Crowcroft, ``Network Text Editor (NTE): A
        scalable shared text editor for the MBone'', ACM SIGCOMM 1997.

   [12] V. Hardman, A. Sasse, M. Handley, A. Watson, ``Reliable Audio
        for Use over the Internet'' Proc INET '95, Hawaii, Internet
        Society, Reston, VA, 1995.

   [13] IRTF Research Group on Reliable Multicast,

   [14] ITU ``Recommendation H.320: Narrow-band visual telephone systems
        and terminal equipment'', ITU, Geneva, 1997

   [15] ITU ``Recommendation T.124 -- Generic Conference Control'', ITU,

   [16] ITU ``Recommendation H.323: Visual telephone systems and
        equipment for local area networks which provide a non guaranteed
        quality of service'', ITU, Geneva, 1996

   [17] ITU ``Recommendation H.332: H.323 Extended for Loosely-Coupled
        conferences'', ITU, Geneva

   [18] C. Bormann, J. Ott, C. Reichert, ``Simple Conference Control
        Protocol'' INTERNET-DRAFT, June 1996.

   [19] V. Jacobson, ``Congestion Avoidance and Control'', ACM SIGCOMM

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INTERNET-DRAFT          Conferencing Architecture              July 2000


   [20] J. Linn, ``Privacy Enhancement for Internet Electronic Mail:
        Part I: Message Encryption and Authentication Procedures'', RFC
        1421, Feb 1993

   [21] S. McCanne, V. Jacobson and M. Vetterli, ``Receiver-driven
        Layered Multicast''. ACM SIGCOMM 1996, pp. 117-130.

   [22] S. McCanne, M. Vetterli, ``Joint Source/Channel Coding for
        Multicast Packet Video''. Proceedings of the IEEE International
        Conference on Image Processing. October, 1995. Washington, DC.

   [23] J. Moy, ``Multicast Extensions to OSPF'', RFC 1584, March 1994.

   [24] National Institute of Standards and Technology (NIST), ``FIPS
        Publication 46-1: Data Encryption Standard'', January 22, 1988

   [25] Rivest, R., ``The MD5 Message-Digest Algorithm'', RFC 1321, MIT
        Laboratory for Computer Science and RSA Data Security, Inc.,
        April 1992

   [26] Schooler, E., A Distributed Architecture for Multimedia
        Conference Control, ISI Research Report ISI/RR-91-289, November
        1991.  ftp://ftp.isi.edu/pub/hpcc-papers/mmc/mmcc.ps

   [27] Schulzrinne, H., ``Personal Mobility for Multimedia Services in
        the Internet'' IMDS'96, March 4-6 1996.

   [28] H. Schulzrinne, S. Casner, R. Frederick and V. Jacobson ``RTP: A
        Transport Protocol for Real-Time Applications'' RFC 1889.

   [29] D. Thaler, D. Estrin, D. Meyer, ``Border Gateway Multicast
        Protocol'', INTERNET-DRAFT, Oct 1997.

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