The MECCANO Internet Multimedia Conferencing Architecture


Igor Bokun, ACC
Carsten Bormann, UB/TZI
Tarik Cicic, UiO
Peter Kirstein, UCL (Editor)
Colin Krawchuck, HPLB
Lambros Lambrinos, UCL
Martin Mauve, UM
Jörg Ott, TELES
Luc Ottavj, INRIA
Colin Perkins, UCL
Andreas Rozek, RUS
Klaus Stuhlmueller, UEN
Thierry Turletti, INRIA


This document provides an overview of the Architecture for multimedia conferencing on the Internet used in the MECCANO project. The protocols mentioned are all 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.

Parts of this document are strongly based on a draft provided for the Multiparty Multimedia Session Control (MMUSIC) working group of the Internet Engineering Task Force. Nevertheless it covers a number of areas not tackled in that draft - or even relevant to the IETF. It is designed to be a guide to what we are trying to address in the MECCANO project.

The first version of this document was delivered as R3.1; this version is updated from time to time as the situation changes.

Keyword list:

Architecture, Multimedia Conferencing, Mbone, Security.


Table of Contents

1 Introduction *

2 Multicast Conferencing Architecture overview *

3 Multimedia traffic over IP *

3.1 Multicast Traffic Distribution *

3.1.1 The Nature of Multicast *

3.1.2 Address Allocation *

3.2 Internet Service Models *

3.2.1 Non-best effort service *

3.2.2 Reservations *

3.2.3 Differentiated Services *

3.3 Network Support in MECCANO *

3.3.1 Reasonable Quality Internet *

3.3.2 Quality of Service Considerations *

3.3.3 Support for Other Access Networks *

4 Audio/Video transport *

4.1 Transport Protocols *

4.1.1 Receiver Adaptation and Synchronisation *

4.1.2 Transport for Real-Time Flows (RTP) *

4.1.3 Conference Membership and Reception Feedback *

4.1.4 Scaling Issues and Heterogeneity *

4.2 Codec Technology in the Context of Multicast Conferencing *

4.2.1 The Nature of Codecs *

4.2.2 Software Video Coding *

4.2.3 Redundancy, Quality and Robustness *

5 Non-A/V Applications *

5.1 Transport Protocols *

5.2 Application Sharing and Shared Workspaces *

5.2.1 Common Issues *

5.2.2 Application Sharing *

5.2.3 Workspace Sharing *

6 Conference coordination and control *

6.1 Conference and Server Control *

6.1.1 Conference Control in the IETF and ITU *

6.1.2 Controlling Multimedia Servers *

6.1.3 Audio and Video Stream Control and Management in CORBA *

6.2 Conference Set-up *

6.2.1 Session Announcements *

6.2.2 Session Invitation *

6.2.3 Off-line Mechanisms *

6.3 Inter-process Control *

6.3.1 The Message Bus Goals *

6.3.2 Mbus Transport Services *

6.3.3 Semantic Concept *

7 ITU Conferencing Architecture overview *

8 Gateways and Relays *

8.1 Introduction *

8.2 The Gateway Architecture *

8.2.1 Building Block Approach *

8.2.2 Mbus Entities for control *

8.3 AudioGate *

8.4 StarGate *

8.5 Multicast-Unicast reflector *

8.5.1 Introduction *

8.5.2 Architecture *

8.5.3 Usage example *

8.6 The UCL Transcoding Gateway *

8.7 Firewall Gateways *

9 Security *

9.1 Introduction *

9.2 Encryption of Media Streams *

9.3 Encrypted and Authenticated Session Descriptions *

9.3.1 Authentication and Key Distribution *

9.3.2 Encrypted Session Announcements using only Symmetric Encryption *

9.3.3 Encrypted Session Announcements using Public Key Systems *

10 Recording and Replay *

10.1 Introduction *

10.2 Recording Multimedia Conferences over the Mbone *

10.2.1 Recorder Requirements *

10.2.2 The General Problem *

10.2.3 Recording Caches *

10.2.4 Recording Encrypted Streams *

10.2.5 Storage Formats versus Transmission Formats *

10.3 Multimedia players *

10.3.1 Example applications *

10.3.2 Player requirements *

10.3.3 Subsequent Data Retrieval *

10.4 Multimedia Recording and Playback systems *

10.4.1 Generic Systems *

10.4.2 Standards for Recorders and Players *

10.5 Existing Multimedia Recorders and Players using Multicast without Caches. *

10.5.1 The Server *

10.5.2 The Recorder *

10.5.3 The Browser *

10.5.4 The Player *

10.5.5 The ACC Video Server *

11 Summary *

12 Acknowledgements *

13 References *


  1. Introduction
  2. In conjunction with computers, the term conferencing is often used in two different ways. One refers to bulletin boards and mail list asynchronous exchanges of messages between multiple users; secondly, one may 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 [h320] can be used for small, strictly organised video-telephony conferencing. This latter infrastructure will be called ITU Conferencing in this report.

    The use of the term architecture in the title of this document should be understood neither as meaning plans for building specific software nor an inflexible environment in which conferencing must operate; it is a description of the context in which the project is working and the requirements for individual elements of that conferencing environment. It also includes the type of components which the MECCANO partners are committed to provide. Moreover, this document does not intend to give a complete bibliography of multimedia conferencing, or even Mbone multicast conferencing tools. It references mainly those used within the MECCANO project. We use directly many tools deriving from the earlier activities of the Lawrence Berkeley Laboratories (LBL) [lbl] and the European projects MICE [mice] and MERCI [merci]. We are fully aware of the later work of MASH [mash], MATES [mates] and MARRATECH [marra]. The first is insufficiently modular for us to include parts; the commercial nature of the latter two has made it difficult to integrate them with parts of our system, in order to provide the extension we need for security, loss-resistance and multiple platforms.

    Most of this Deliverable is about the architecture of multicast conferencing and data delivery as being developed in the Internet Engineering Task Force [ietf]. The Mbone is used to describe that portion of the Internet, which supports multicast data distribution; while this is currently being used mainly for conferencing and the distribution of multimedia broadcasting, there are many other uses of the technology. For the sake of notation only, this form of conferencing will be called Mbone Conferencing. There used to be a clean divide between this conferencing architecture and that being pursued in the ITU as exemplified by H.320 for ISDN conferencing; the former used the Internet, the latter the ISDN or POTS. Recently the distinction has become somewhat blurred, because at the lower transport levels the ITU has adopted the same procedures as IETF [h323]; this means that ITU-style conferences can also use the Internet. There are still some differences, but there is also clear overlap. The architectural environment of the ITU mechanisms do not scale as well, but for small, tightly controlled, conferences they are a quite viable alternative.

    Since most of the MECCANO project is about the use of Mbone conferencing, more detail will be given of this technology. However, one part of the project is concerned with the provision of facilities to allow ITU-conferencing workstations to join Mbone conferences and to allow Mbone-conferencing workstations to join ITU conferences. For this reason enough detail will be provided to define the requirements for the gateways needed. In addition, we will attempt to indicate over what range of parameters we consider it desirable to use the ITU conferencing concepts - and even where some of those concepts could be added to Mbone conferencing.

    Sections 2, 3, 4 and 6 are based on "The Internet Multimedia Conferencing Architecture", an overview of Mbone conferencing being prepared both as a paper and a submission to the MMUSIC group of the IETF [han99-3]. There are some differences between the current draft of that paper and the relevant sections of this document. These reflect that the purpose of this Deliverable is to define the MECCANO architecture. This is closely aligned to the activity of the MMUSIC group, but there are differences of emphasis. Moreover, we address issues that are not addressed in that paper, and often are not even appropriate to the IETF. There are two reasons for referring to that here. First we wish to acknowledge our substantial debt to the authors of [han99-3]; secondly, we do not imply that all the statements here have the approval of the authors of [han99-3].

    The Mbone architecture is only one of the architectures being considered in the MECCANO project. Another is the ITU one, exemplified by H.323. This follows a much more conventional ITU approach, which is sender driven. An earlier complete independence of protocol structures in the H.320 family has become more similar in the H.323 version because both use the same underlying transport protocols. The methods of control and initiation are very different, but it is meaningful to consider H.323 workstations joining Mbone conferences and vice-versa - without undue processing on the media streams themselves during the running conferences. Section 7 gives the salient properties of the ITU architecture.

    There are many other areas being explored in the project, most of which have architectural considerations. In Section 8 we consider the various gateways being considered; some of these are between the Mbone and the ITU worlds; others are between different types of technology. Others are concerned with overcoming the justified fears that many organisations have to allowing multicast into their organisations. In Section 9 we consider the mechanisms which are required to provide privacy in conferences, and to allow authentication both of participants and the activities of conference organisers.

    Other sections deal with the architectural implications of various tools and components. Thus in Section 3 we consider aspects of supporting various network types, in Sections 4 and 5 we consider the architectures of the Mbone tools and in Section 10 those of the recording and replay media servers. Finally, some conclusions are drawn in Section 11.

  3. Multicast Conferencing Architecture overview
  4. The architecture that has evolved in the Internet is general as well as being scalable to very large groups; it 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; i.e. it encompasses IP telephony.

    The determining factors of conferencing architecture are communication between (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 (cf. Section 3.1).

    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 (QoS) available in congested networks. New service models are being defined in the Internet together with protocols to reserve capacity or prioritise traffic in a more flexible way than that available with circuit switching (cf. Section 3.2).

    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 (cf. Section 4.1). The Internet is a very heterogeneous world. Techniques exist to exploit this, and to deliver appropriate quality to different participants in the same conference according to their capabilities.

    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 formalised as a conference policy. Some conferences are essentially crowds gathered around an attraction, while others have 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 set-up, Section 6.2). During the conference, some conference control information is exchanged to implement a conference policy or at least to inform the participants of who is present.

    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 purporting to originate 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 set-up and control mechanisms (See Section 9).


    Figure 1 Internet multimedia conferencing protocol stacks


    Most of the protocol stacks for Internet multimedia conferencing are shown in Fig. 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. For secure conferencing, there may be additional protocols for group management. This question is addressed in Section 9.

  5. Multimedia traffic over IP
    1. Multicast Traffic Distribution
      1. The Nature of Multicast
      2. IP multicast provides efficient many-to-many data distribution in an Internet environment. It is easy to view IP multicast as simply an optimisation for data distribution; 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 Jacobson [jacobson95]:

        • Senders just send to the group
        • Receivers express an interest in receiving data sent to the group

        • 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 nor receivers need care about the network topology as the network optimises delivery.

        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; such servers require support, cause traffic concentration and can be a bottleneck. For larger broadcast-style sessions, it is essential that data-replication is carried out in a way that requires only that per-receiver network-state is 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; thus native multicast support is a more appropriate solution.

        There are a number of IETF documents outlining the requirements of Hosts and multicast routing. The most important, defining the Host extensions for IP multicast, are [ipm]. There are many mechanisms, which have been proposed for multicast routing; some of these are described in [dvmrp], [pimsm], [pimdm] and [bal98]. It is beyond the scope of this Deliverable to discuss the differences and advantages of the different proposals.

      3. Address Allocation
      4. There is an important question on how an application chooses which 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 [deer88-1] can do just that. However, this is not the best way of managing applications of which there is more than one instance at any one time.

        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 (cf. Section 6.2), but as the users and uses of IP multicast increase, it is becoming clear that a more hierarchical approach is required.

        An architecture [han99-2] 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, [thal98]) that will help multicast routing scale more gracefully that current schemes.

        A number of specific documents giving methods for address allocation are given in [estr99] and [phil98]. It is relevant also to consider the extensions required to resource discovery protocols using multicast [patel99].

    2. Internet Service Models
    3. 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 serialisation delays at each hop plus propagation delays. With sufficiently fast link speeds, serialisation delays are insignificant compared to propagation delays. For slow links, a set of mechanisms has been defined that helps minimise serialisation and link access delay is low.

      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. High quality real-time multimedia traffic does not cope well with packet loss levels of more than a few percent unless steps are taken to mitigate its effects. One such step is the use of redundant encoding [redenc] to raise the level 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.

      1. Non-best effort service
      2. Real-time Internet traffic is defined as that carried by 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-than-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 bit.

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


        Figure 2 Spectrum of Internet service types


        This spectrum is intended to illustrate that between best-effort and hard per-flow guarantees lie many possibilities for non-best-effort service. These include 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 prioritisation-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; 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. For example, Enterprise networks (Intranets) often enforce traffic policies that distinguish priorities which can be used to discriminate in favour 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 (POTS). 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. In that case, within the backbone of the network no individual call need be given an allocation. We would no longer need the call set-up/tear down that was needed in the legacy POTS; this 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 reservations.

      3. Reservations
        1. RSVP
        2. For flows that may take a significant fraction of the network (i.e. are special and cannot 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 standardised 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, colour and HDTV receivers from a single source (also see Section 4.2). Again the routers conspire to deliver the right flows to the right locations. RSVP accommodates the wildcarding noted above.

          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 priority.

          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 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.

          A number of specific documents describing the RSVP protocols are: [rsvp], [rsvp-cls] and [rsvp-gs]

        3. Billing
        4. If a reservation involves setting aside resources for a flow, this will tie up resources so that other reservations may not succeed; then, depending on whether the flow fills the reservation, other traffic may be 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.

          Billing requires that the user making the reservation be 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 receivers.

      4. Differentiated Services
      5. 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; 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 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; 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. With reservations there may be ways to avoid this too, but they are somewhat more difficult given the more specific nature of a reservation. A good overview of the network service model for Differentiated Services (DiffServe) is given in [difserv].

    4. Network Support in MECCANO
    5. Network support is not supposed to be a major part of the MECCANO project, but it is an essential ingredient to provide adequate service. We describe here some of the requirements and components we consider indispensable for MECCANO services. It is essential that all the main nodes participating in MECCANO conferences have reasonable connection to the Internet, and that the Internet has reasonable performance.

      1. Reasonable Quality Internet
      2. The meaning of reasonable, is difficult to determine; it depends on the performance desired. The key parameters are packets/sec, variability in packet arrival (jitter in ms), mean transit time of packets, stability of the Internet Connectivity of certain routes, availability of multicast in the Internet Provider. These parameters have different impact on the different services.

        In international services, the different paths often have very different performance characteristics, moreover, alternate routing may be difficult to arrange automatically - particularly with multicast. Hence it is essential that the links are stable enough to have few outages of many seconds during a typical session. If longer and more frequent interruptions of service take place, it is very difficult to run a conference.

        The normal Internet topology with multicast provision is called the Mbone. This is a single topology and has no differentiation between different users. Even if all conferees have access to the Mbone, there remains the question of whether the performance is adequate, and whether the demands on the bandwidth are too great. If all the above are in order, nothing needs to be done to compensate. Unfortunately, particularly the international Mbone is very variable between countries, and they have different policies on how much bandwidth they provide - both nationally and internationally. Thus we often need to supplement normal Mbone by unicast tunnels. If this is done, we have to be very careful not to upset the total Mbone topology.

        With the present European networks, the Mbone is often inadequate for reasonable conferencing. Hence we often construct a significant part of the topology from unicast routes. Both in MERCI and in MECCANO, we have used the experimental high-speed networks originally JAMES, and more recently the VPN capability of TEN-155 to construct a high-quality backbone. We must then construct reflectors at strategic points to ensure full multicast facilities. If we still have access to the Mbone at any site, we must ensure, by route filtering and scoping, that the appropriate Mbone topology is not disturbed.

        The next parameter is throughput - in Kbps or packets/sec per audio stream; here the requirements depend on the media and quality desired. For audio tools like RAT, typical bandwidths needed are 8 - 32 Kbps; on good channels, with modern codecs, the higher of these give good quality audio. For video using tools like VIC, the corresponding rates are 50 Kbps - 3 Mbps. At the lower bandwidths, with slowly varying scenes, a few frames/sec can be achieved with QCIF, at the higher rates, full motion can be achieved at reasonable definition. The optimal media data rates for specific bandwidth depend also on the level and nature of channel errors. With audio, data arriving delayed or not at all is dropped. However, it is possible to provide deliberate redundancy in the talk-spurts to compensate for the loss. With video, similar errors will impact a whole frame. For this reason, while inter-frame coding will give much better performance at low error rates, it can be very inefficient at high ones. For this reason, most current implementations of tools like VIC do not currently use inter-frame coding; therefore higher data rates are needed for a given quality. We have done many measurements on data quality as a function of packet loss. We find loss rates of 15% still give reasonable quality of audio, with losses up to 40% tolerable with good use of redundancy. With no inter-frame coding, loss rates of up to 20% give quite tolerable video.

        The next parameter is variability of delay, called jitter. In most of the audio tools, one can trade tolerance to jitter against delay. A jitter above a hundred ms in a replayed packet is so annoying, that this may be taken as an approximate cut-off point. If most packets have a variability of arrival between 50 and 150 ms, for example, then one can deliberately put in a delay of 150 ms in the replay buffer. Any packets arriving more than 150 ms late will be dropped. Similar considerations apply to video. If the delay is made too long, it is very annoying for fully interactive sessions; it is much less serious for one-way activities like lectures. A jitter above the cut-off point is exactly equivalent to packet loss.

        The considerations for shared work-space tools are different. Here because the individual operations must be kept consistent, some form of Reliable multicast is normally used. Now it is the total rate of updates, packet loss, and connectivity that are the important factors. Overmuch packet loss will increase the traffic level on the network; the load is normally much less than that due to the audio and video.

      3. Quality of Service Considerations
      4. So far we have considered standard use of the Internet. In fact there are many activities to introduce mechanisms for providing "Quality of Service" (QoS), as mentioned in Section 3.2. Some of these rely on mechanisms introduced at the access point of the networks; most DiffServe algorithms come into this category. Others rely on setting up reservations throughout the networks; most IntServ algorithms come into this category. So far none of the regular National or International IPv4 networks used in MECCANO support such mechanisms. With the next generation of Internet Protocols, those based on IPv6 [ipv6], there is support for this facility at the IP level. For this reason, there is starting to be support on experimental networks running IPv6 for QoS. There is no reason why the same support cannot be provided on IPv4 networks; it is just not happening.

        We would like to explore in MECCANO the scope for improving the quality of conferencing by providing QoS support. In view of the above, there will be a deliberate activity in the project to support IPv6 and its QoS enhancements. This work will have many aspects. Clearly it will be necessary to provide IPv6-enabled routers; at the very least they can apply DiffServ algorithms at the borders to the WANs. It will be necessary to tunnel through any intermediate networks that support only IPv4. When VPNs are used, as with some of the TEN-155 activity, it may be possible to use IPv6 throughout.

        It is possible to provide IPv4 enabled applications inside the local areas of conferees, and then to change to IPv6 at the edges to the wide-area. This approach will be followed initially, providing only very crude clues to where QoS is to be used - like favouring audio over video. This can be achieved by prioritising streams in the router based on the multicast group used with techniques such as Class-Based Queueing (CBQ) [cbq]. Another aspect will be to make the applications able to signal their own needs for QoS; thus audio codecs may decide which of their streams require the better service, and to mark them accordingly. The lower level software may then assign the different streams to different multicast groups; the routers may then apply the relevant QoS policies. Because IPv6 provides support for QoS at the IP level, many operators will not support QoS except on IPv6-enabled parts of their networks. For this reason, another part of the MECCANO project will be to ensure that the applications themselves can support IPv6 directly. In many cases the implementation work required will be carried out in other projects; the results will be used experimentally also in MECCANO.

      5. Support for Other Access Networks
      6. Of course we support the usual aggregation of LAN and WAN technologies which support multicast, and are normally encountered in organisations. In addition, we will support two others: unicast ISDN and Direct Broadcast Satellite access. The salient point of each is discussed below.

        1. ISDN and Unicast Access
        2. Many organisations, particularly industrial ones, do not support multicast - or will not let it get through their firewalls. For this reason one useful device that will be provided is one that does unicast<->multicast conversion. Further details are given in Section 8. Others will want to participate in MECCANO via unicast ISDN; this may be because their organisation will not allow direct Internet access from inside their organisations, or may want to participate from home. For this reason, we will provide such access facilities; the gateways needs to provide them are discussed in Section 8.5.

        3. DBS Access
        4. The nature of participation in multimedia conferences is that it is possible to participate at different levels of service - and therefore of bandwidth need. It is possible to participate with only audio - but get much more out of it if one can also receive video or high quality presentation material. Moreover, we have already said that the availability of network services is very variable; in many areas of Western European countries, and in even larger areas when one goes further East, the bandwidth available to individuals through the Internet will not support even medium quality multimedia. For this reason, there is considerable scope for supporting access mechanisms which are not symmetric. In the future, we expect that mobile radio, xDSL and Cable TV access will become very useful in this context; during the MECCANO project, we will not use such mechanisms since we do not have access to them. We will, however, make use of DBS services.

          In a DBS, equipment at the up-link site is a normal participant of the conference, and receives all the multicast data streams; the up-link then retransmits all the normal multicast digital stream via the satellite channel. Any participants via the DBS system have a DBS receiving terminal which has a built-in multicast router, and a separate Internet link. If there is no nearby multicast facility on the Mbone, a unicast tunnel may need to be set up. Normally, as discussed in Section 3.1, routers must prune any unicast traffic down-stream from them towards leaf nodes; this uses the symmetric nature of most Internet connections. In the DBS case, alternative mechanisms must be used. The full details of the technology will be reported in the Deliverables of WP6, but an overview is given below. Two approaches to solve the problem were considered. The first one is based on routing protocol modification and the other on tunneling.

          In the first approach each routing protocol is modified in order to take into account the unidirectional aspect of the underlying network. Modifications to protocols such as RIP, OSPF and DVMRP were proposed and implemented before the start of the MECCANO project. The modified routing protocols are operational. The experimentation is described in [udlr]. However, The second approach was more attractive: a link layer tunneling mechanism that hides the link uni-directionality and allows transparent functioning of all upper layer protocols (routing and above).

          Tunneling is a means to construct virtual networks by encapsulating the data. Broadly speaking, data is sent by the network layer instead of the data-link layer. This generally allows new protocols experiments and provides a quick simple solution to various routing problems by building a kind of virtual network on top of the actual one. The aim of using tunneling is to make routing protocols work over unidirectional links without having to provide any modification to them.

          The tunneling approach adds a layer between the network interface and the routing software on both feed and receiver sides (or between some intermediate gateways), resulting in the emulation of a bi-directional satellite link where only an unidirectional link is available. Packet encapsulation is hiding the actual topology of the network in order to elude the routing protocols and make them behave as if there exists a bi-directional satellite link. The tunneling mechanism we proposed is described in detail in an Internet draft of the udlr working group [duros99]. Here follows a short description of the solution.

          Basically, routing traffic is sent from the receivers to the feed on the virtual link and is later captured by the added layer interface, which encapsulates it and sends it on the actual reverse link. The feed station then decapsulates it and transmits it to the routing protocols as if this was coming from the virtual satellite link. As a receiver needs to sends a routing message to a feed, the packet is encapsulated in an IP packet whose IP source address is the receiver bi-directional address and IP destination address is the feed bi-directional address. The datagram is then sent to the end point of the tunnel via the terrestrial network. As it is received by the feed, the payload of the datagram is decapsulated. The new IP packet that is obtained is routed according it its destination address. The packet is then routed locally and not forwarded if the destination address is the feed itself. The IP stack passes the packet to higher level, in our case the routing protocol.

          Similarly to routing protocols modifications, as they discover feeds dynamically, receivers should be capable of setting up tunnels dynamically as they boot up. The only way a receiver can learn the tunnel end point is to define a new simple protocol. Feeds periodically advertise their tunnel end point (IP address) over the satellite network. As a receiver gets this message, it checks if the tunnel exists, if not it creates a tunnel and uses it.

          Routing protocols usually have mechanisms based on timeouts to detect if a directly connected network is down. If the satellite network is not operational and receivers keep on sending their routings messages via regular connections, feeds will continue to send packets on its unidirectional interface. This is undesirable behaviour because it will create a black hole. In order to prevent this, when receivers have not received routing messages from the satellite network for a defined time, they turn their tunnel interface off. As a result, feeds receiving no routing messages from receivers delete in their routing tables all destinations reachable via the satellite network.

          Having a tunnel between a receiver and a feed is very attractive because the unidirectional link is totally hidden to applications. As far as routing protocols are concerned they have to be correctly configured. For instance, for RIP, a feed must announce infinite distance vectors to receivers, this way receivers do not take into account destinations advertised by feeds.

          In the MECCANO project, we will provide as good quality links as we can to any available DBS up-links. There will certainly be one at INRIA, but there may be ones also at other sites. We will also equip a number of sites with DBS receivers; this will certainly include most of the MECCANO partners, but may include other sites too. There will therefore be a DBS overlay in addition to any other network facilities provided.

  6. Audio/Video transport
    1. Transport Protocols
    2. 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 the 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.

      1. Receiver Adaptation and Synchronisation
      2. 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 Fig. 3



        Figure 3 Network Jitter and Packet Audio


        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, each packet must carry a timestamp, which gives the time 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 synchronisation


        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. Adapting these playout buffers so that samples/frames that originated at the same time are played out at the same time (see Fig. 4) can perform inter-flow synchronisation.

        This requires that the time base of different flows from the same sender can be related at the receivers, e.g. by making available the absolute times at which each of them was captured.

      3. Transport for Real-Time Flows (RTP)
      4. The transport protocol for real-time flows is RTP [schu96-1]. 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 set-up, 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 synchronisation 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 real-time clock at a sender and the RTP media timestamps so that inter-flow synchronisation can be performed, and they provide textual information to identify a sender in a conference from the source id.

        There are a number of detailed documents on the RTP protocol, which has been adopted both for ITU and Mbone conferencing. Some of these are:

        • [schu96-1], which gives the packet format for real-time traffic used in RTP and RTCP.
        • [schu96-2], which specifies the RTP profile for AV traffic
        • [rtpf] There are a whole series of reports, which specify the payload formats for specific codecs.
      5. Conference Membership and Reception Feedback
      6. 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 are listening to the conference, and whether the media flows are reaching receivers properly. Accurately 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; to try to do so would cause an implosion of messages, many of which would be lost. A conference policy that restricts conference membership can be implemented using encryption and restricted distribution of encryption keys; this is discussed further in Section 9. However, RTCP provides approximate membership information through periodic multicast of session messages; in addition to information about the recipient, these 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 organisers 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 congestion.

      7. Scaling Issues and Heterogeneity
      8. The Internet is very heterogeneous, with link speeds ranging from 14.4 Kbps up to 1.2 Gbps, and very varied levels of congestion. How then can a single multicast source satisfy a large and heterogeneous set of receivers?


        Figure 5 Receiver adaptation using multiple layers and multiple multicast groups


        In addition to each receiver performing its own adaptation to jitter, if the sender layers [mcca95] 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 frame rate 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 Fig. 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. This will prevent de-synchronised experiments by different receivers from having the net effect that too many layers are always being drawn through a common bottleneck. Receiver-driven Layered Multicast (RLM) [mcca96] is one way that this might be achieved, although there is continuing research in this area.

    3. Codec Technology in the Context of Multicast Conferencing
      1. The Nature of Codecs
      2. Fundamental to the transmission of audio and video streams over digital networks is the use of coders and decoders; their combination is called a codec. These are devices that sample the analogue signals, and process the resulting digital streams. This processing, which is done in the codec, will require variable amounts of processing power, and produce output with different properties. The algorithms used in different codecs are beyond the scope of this Deliverable. There is a detailed discussion of different audio and video codecs in [acoder] and [vcoder]. Because it is inevitable that there are some losses in data transmission, various techniques are invoked to overcome the impact of errors. Many of these need not be discussed here; they impact strongly the interaction between the coder and the decoder, but have little impact on the conferencing architecture. There are some techniques, however, which do have such impact - particularly if the network characteristics are variable.

        One aspect is that different codec algorithms have different compression factors; i.e. for a given picture, different amounts of data are generated. If one part of the network is able to transmit at one speed without undue network error, and another has a lower capacity, it may be necessary to use different coding algorithms in the two regions. To mediate between the two may require decoding and re-coding (though this may be possible completely in the digital domain). Devices that carry out this process are called transcoders [amir98].

        Another property of codecs is that they may be scaleable, producing different layers of coding. A receiver may process one, some or all of these layers. With well-structured layered coding, processing one layer will provide a minimal quality of media; processing more layers will provide progressively better quality. If all the layers are sent over one multicast group, then a layered codec may not be architecturally different from other codecs for the purpose of this paper. However, if it is easy for an intermediate node to recognise the different layers, then it may be easy to provide digitally the equivalent of transcoding. It is also possible to send different layers to different multicast groups. By subscribing only to some groups, a receiver may avoid overloading the network, or its own processor. Alternately, by passing only certain multicast groups, an active element in the network may ensure the protection of a lower capacity region. Both these mechanisms do have impact on the architecture of multicast conferencing. In addition, if the network does not support hierarchical protection, unequal error protection has to be added at the transmitter such that the more important layers are protected by a stronger channel code [scalvico, rs].

      3. Software Video Coding
      4. Since digital video has by far the largest data rate of all media, it is essential to use scalable video coding. This is not only true for video transmission but also for the processing at encoder and decoder. Especially at the encoder side, which is usually much more complex than the decoder, scalable software is essential, if it should be used on various platforms. A scalable coder should choose its coding algorithms dynamically according to the available processing power and produce hierarchical bit streams for different transmission rates [scalvico].

      5. Redundancy, Quality and Robustness
      6. Codec algorithms are deeply concerned with providing the optimal digital rendering of the media streams in terms of certain criteria. These criteria certainly include faithful reproduction, minimising bandwidth, reducing computation, and robustness to errors. The robustness can be provided by providing various forms of redundancy [rosen98]. The redundancy may be independent of the contents; there are various codes which can recover from successive bit errors, for example. Alternately one can use knowledge of the characteristics of the media to limit the amount of redundancy transmitted. For example, with speech, if the previous audio frame is similar in characteristics to the current frame we do not bother to send a redundant copy; receiver-driven error concealment will provide an acceptable alternative in case of loss. In a similar manner, we could use knowledge of the characteristics of a speech signal to determine the priority to insert into the IPv6 flow label header. This would allow a router with class-based queuing to give priority to those packets which are perceptually more important. This is the audio analogue to giving different QoS to MPEG P and I frames [mpeg].

  7. Non-A/V Applications
    1. Transport Protocols
    2. Applications other than audio and video have evolved in Internet conferencing, conferencing, ranging from shared text editors (NTE [han97]) to shared whiteboards (WB [floy95], DLB [gey98]) and support for dynamic 3D objects (TeCo3D [mau99]). 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. raising hands [mates], voting [mates, mpoll] and a speaker list); they can also enable new styles of joint work.

      Non-A/V applications currently have vastly different design philosophies. This leads to a multitude of architectures and proprietary protocols both at the transport and at the application level. It is therefore a challenging task to combine them into a homogeneous teleconferencing toolset. Especially the development of generic services, like recording and playback of conferencing sessions, is currently impossible. These problems can be traced to two related areas: reliability and application layer protocols for non-A/V applications.

      Many 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. However, the applications' requirements differ. Some applications just need a guarantee that each application data unit (ADU) eventually arrives; others require that the ordering of the ADUs transmitted by a single participant will be received in order. Applications might even demand that the total order of all ADUs send by all participants is preserved. Additionally applications have different requirements on the timeliness with which the packets need to be delivered.

      Closely related to the reliability requirements of an application is the problem of which part of the system is responsible for providing the reliability. On the one hand there are approaches to provide reliability at the transport level (layer 4). These approaches basically provide an interface similar to TCP. The positive aspect of realising reliability in this way is the simplicity of the interface and a very clean (layered) software architecture.

      On the other hand approaches exist which require the application to be network aware and help with repairing packet loss. This is especially desirable if the repair mechanism does not rely exclusively on the simple retransmission of the lost packet(s) but also on application level information. In a shared whiteboard, for example, it might be desirable to repair packet loss, which relates to the state of the pages visible to the local user on a higher priority than packet loss for other pages. An additional benefit of being network aware is the possibility of mapping ADUs to transport PDUs on a one-to-one basis (application level framing, ALF [clark90]). If each transport PDU carries information which is useful to the application independent of any other transport PDUs, the application can usually process them out of order, significantly increasing the efficiency for some applications.

      A possible solution to the problem of diverse approaches to provide reliability in a multicast environment could be a flexible framework for reliable multicast protocols. As proposed by the Reliable multicast Framing Protocol Internet-Draft [crow98], a framing protocol could be used to provide a similar service to different reliable multicast approaches as RTP provides to different A/V encodings. Ideally this would result in a flexible, common API for reliable multicast where each application can choose from a set of services (such as one-to-many bulk transfer, or many-to-many user interaction).

      The question of how to realise the reliability for different applications, given the wide range of reliability requirements, is one of the topics where work is still in progress in the IRTF research group on Reliable multicast [rmrg]. Other aspects of reliable multicast, which are not well understood, include how to provide congestion control in a multicast environment. As these issues are considered essential elements, standards track protocols are not expected before these can be solved.

      The second reason why non-A/V applications are so diverse, and services like generic recording are currently not possible, is the lack of a commonly accepted application level protocol (an RTP-like protocol framework for non-A/V applications). While the need for such a protocol has been expressed by many application developers, it is currently not addressed by any standards track activities.

    3. Application Sharing and Shared Workspaces
    4. Most conferencing solutions, which allow for collaboration between participants in a conference, provide either a single application distributed amongst the participants (Application Sharing) or a distributed data set on which all may work (Workspace Sharing).

      Some conferencing tools allow additionally for text-based communication exchange, file transmission, acquisition of distributed information and other mechanisms to distribute or collect (non-audiovisual) information in the context of a running conference.

      As shown in Fig. 12, Application Sharing basically works by distributing the user interface of a single application. While that program is still running on a single machine, it may now be seen by multiple conference participants. If permitted by the user actually running the application, it may also be operated by several users - either simultaneously or sequentially, depending on the chosen floor control policy.



      Figure 12 Data flow in an Application Sharing scenario


      On the other hand, Workspace Sharing tools distribute a common data set among the participants of a conference, as illustrated in Fig. 13. Each partner has a local representation of the shared workspace and may modify it at will (depending on the access control policy). By the exchange of messages, the sites try to achieve a consistent workspace state.



      Figure 13 Data flow in a Workspace Sharing scenario


      The shared workspaces described within this chapter allow several participants to modify simultaneously one or multiple documents in the context of a running conference (synchronous collaboration). There are other (asynchronous) systems [bscw][linkworks] defining a workspace as a set of documents (in terms of files created by an arbitrary non-distributed application) which can be consecutively modified by the members of a working group. In contrast to a mere distributed file system, these tools implement versioning and access control, support routing and workflow mechanisms, provide electronic signatures for individual documents, allow for approval and disapproval of modifications, keep track of deadlines and the status of documents and inform group members of any document changes they might be interested in.

      1. Common Issues
      2. Tools for Application Sharing and Workspace Sharing share a number of common issues.

        1. (Adaptable) Reliability of Message Transfer
        2. In contrast to real-time multimedia streams, where lost packets just degrade the perceivable quality of an audio/video transmission, application and workspace-sharing techniques usually require a certain level of transport reliability, at least. Otherwise, users would be faced with incomplete documents or antiquated application states and actually be unable to collaborate.

          Only few applications (e.g. multicast file transfer) need every packet be delivered reliably (and in the original order), most systems want all participants to share a common and actual status (e.g. the current position of a shared mouse pointer) - packets carrying obsolete status information (e.g., former pointer positions) may get lost without adverse consequences.

          Some tools designed for Distributed Interactive Simulation assign priorities to certain status records (depending on their importance for a successful collaboration) and require only high-priority information to be delivered reliably. While a successful reception of low-priority data may increase the quality, it is not vital for the simulation as a whole.

          In the extreme case, some battlefield simulations do without any mechanism to achieve a reliable transmission, since most aspects of such a simulation can be calculated from physical models and only behavioural changes have to be transmitted. As these are rare compared to the capacity of the networks and computers used, they can be sent multiple times assuming that eventually every participant will have got the information at least once.

        3. Scalability
        4. Most application and workspace-sharing tools support a small number of users only - with numbers ranging from two to a few tens - often limited by the transport protocol used. multicast applications have the advantage, in principle, of supporting an unlimited number of participants, as long as these sites only receive packets passively from other active participants and do not try to send packets themselves. In practice, current techniques for reliable multicast still limit the number of participants to a few hundred.

        5. Floor and Access Control
        6. If multiple conference participants may operate a shared application or modify objects in a shared workspace, there is a need to coordinate these activities.

          Some systems use the concept of an explicit floor holder to restrict activities to a single user - with varying policies for the initial assignment of the floor and different (social) protocols for passing it between session participants.

          Multicast tools usually do without such a concept as it does not mesh well with the idea of lightweight multicast sessions. In a multicast environment it is often better to forbid or permit the modification of certain objects - or not to implement any access restrictions at all.

        7. Consistency of State Information among Session Participants
        8. Packet loss and different packet transit times may lead to inconsistent states at individual sites causing problems if these partners start operating a shared application or modifying a shared workspace themselves based on their improper state information.

          Floor-controlled systems often check (and establish) workspace consistency when passing the floor to a new floor-holder - other session partners may continue with an inconsistent state as they must not apply any modifications.

          Centralised approaches rely on one (or multiple) central servers holding the actual state which all other session participants (clients) are bound upon. Clients may use these servers to update their own status information or check for consistency before requesting a status change themselves.

          Multicast tools (without any access restrictions) often use global clocks to identify the latest status of a given object and impose a strict ordering on operations from several users. In combination with a message counter (and some heartbeat mechanism) to detect packet loss these clocks help individual sites to converge to a common (workspace) state.

        9. Integration of late joining Participants into a running Conference
        10. Usually, not all conference partners participate in a conference from the beginning - it often happens, that some users join a session which is already running. For an application sharing session this implies that the late user first has to receive the actual contents of the shared screen in order to be able to interpret mouse movements and operations properly. Similarly, in a shared workspace environment the late user has to obtain the actual workspace status before she/he will be able to apply any modifications.

        11. Behaviour in case of Network Partitioning
        12. A special case occurs, when certain sites become disconnected temporarily from the remaining group, e.g. because of a router failure. In a floor-controlled system this might lead to the situation, that the floor holder gets disconnected and, thus, prevents all other participants from operating a shared application or modifying a shared workspace. Some tools solve this problem by automatically or manually reassigning the floor to a new participant (e.g. in an application-sharing session the floor is usually given back to the person actually running the shared application).

        13. Synchronisation with other Media Streams
        14. When used in the context of an audio/video conference, it is sometimes necessary to synchronise the application or workspace-sharing session with A/V streams transmitted simultaneously to prevent a speaker talking about a state not yet seen by his/her listeners. At present, however, we know of only one system which provides such synchronisation, the commercial product MarratechPro [marra].

        15. Recordability
        16. If an application or workspace-sharing tool requires some kind of transport reliability, it is no longer sufficient for a conference recorder just to store any incoming packet - instead, it has to understand the protocol, at least, in order to detect packet loss and take the appropriate measures (such as to request the retransmission of any lost packet).

          Depending on the actual application, a conference recorder might need even more intelligence - e.g. for late joining a running conference and acquiring enough information to successfully perform the recording from then on.

          Additional paradigm-specific issues are mentioned in the next two sections.

      3. Application Sharing
      4. As mentioned above, application sharing basically works by distributing the user interface of a single application; any changes in the appearance of a running application (e.g., a moving mouse pointer, new contents of a document window, any dialogue windows or menu boxes that appear, or interface elements that change their look when being used) are reported to all participants of a sharing session. If the session further supports the remote operation of a locally running program, any mouse movements and key presses performed by a remote conference partner must be sent back and fed into the application as if they had originated from the local machine. It depends on the chosen policy whether all other partners may simultaneously operate a shared program or whether the right to control it is restricted to one participant at a time. The former simplifies sessions with frequently changing operators, the latter avoids any confusion resulting from multiple users trying to operate an application simultaneously.

        The most important advantage of application sharing is its concept of working with arbitrary group-unaware applications. As a consequence, there is no need for the development of new (group-aware) tools - the user may continue using his/her favourite legacy application instead. That is, provided that the programmer has used officially documented programming interfaces only. It is often the case, where this has not been done, that it is not possible to share action games and other real-time applications because they rely on special programming tricks.

        Session set-up and control is provided by a different program (a session manager) which is independent of the shared application itself. Sometimes, this session manager also comes with additional group-aware tools such as a shared whiteboard, a text-based message exchange (chat) or a file transfer feature.

        Depending on how they distribute the visual appearance of a user interface, application-sharing environments can be classified into two main categories: view-sharing and primitive-sharing environments.

        A view-sharing (sometimes called screen-sharing) environment takes screen snapshots from the system running the shared application and sends them as a bitmap to all other participants. Such a technique is simple to implement and relatively easy to keep platform-independent. An important implementation of this concept is the Virtual Network Computing (VNC) [vnc], developed by AT&T Laboratories Cambridge (see below).

        However, most implementations follow the primitive-sharing paradigm and distribute graphics primitives which then have to be rendered at each site individually. Due to its similarity to the way in which the X Window system works, so called X multiplexers were the first systems which implemented this approach. Today, numerous such systems exist (XTV [aw99], Hewlett-Packard's SharedX [garf], Sun's SharedApp [sunf] to mention just a few examples) for the UNIX platform and for IBM-compatible PCs running Windows (Microsoft NetMeeting [netm] and other T.120-based tools, see below), but most of them are platform-dependent as the graphics primitives used for distributing an user interface usually resemble corresponding library calls and their parameters on a given platform. An important exception from this rule is the JVTOS system [froitz] which was developed during the RACE project CIO to provide cross-platform application sharing between Sun and SGI workstations, Apple Macintosh computers [wolf] and IBM-compatible PCs running Windows.

        1. Issues
        2. This section addresses a few problems of application-sharing environments in addition to those mentioned in Section 5.2.1. See also [begole].

          Screen resolution and colour model

          Since application sharing works by distributing the graphical user interface of an application, each partner's screen resolution and colour depth should be compatible with that of the system running the shared application. Otherwise, parts of a program's window might fall outside the available screen area or colours may look considerably different at certain sites.

          Centralised design

          Application sharing sometimes suffers from its client-server model: a single (server) site running the actual program distributes its user interface to one or multiple client sites which may for their part operate the shared application - if permitted. Any files needed while running the application have to be stored on the server first - and any results from the run have to be distributed to the participants again (which is usually done outside the sharing session). If the central server gets disconnected, other partners are unable to continue with their work and have to wait for the server to become available again. Similarly, working with an application is limited to the life-time of a conference session for remote partners - unless they get all the files and install the application locally as well, there is no possibility to continue with their work as soon as the session has finished.

          What-you-see-is-what-I-see (WYSIWIS)

          Both an advantage as well as a disadvantage of application sharing is the what-you-see-is-what-I-see effect: every conference participant has the same view of a shared application, there is no possibility of browsing through a document or experiment with other settings without requiring all other partners to follow these activities. While such a behaviour is explicitly desired during tutorial lectures, it may become cumbersome in distributed workgroup sessions.

          No concurrent work

          An important side-effect of WYSIWIS behaviour is the restriction to a single position in a document. There is no possibility for the members of a group to work on different parts of a document simultaneously by means of application sharing.

          Despite these problems, application sharing is still the most important form of computer-supported collaboration as it can be used with many (collaboration-unaware) legacy applications.

        3. Standards and Implementations
        4. We briefly describe the ITU T.120 family of standards and some implementations.


          The International Telecommunication Union (ITU) T.120 standard family [t120] contains a series of communication and application protocols and services that provide support for real-time, multipoint data communications - including a broad range of collaborative applications, such as desktop data conferencing, multi-user applications, and multi-player games.

          While the standard itself also supports multipoint data delivery (even using IP multicast by means of a multicast adaptation protocol (MAP), tools with T.120 data conferencing capabilities still need a Multipoint Control Unit (MCU) for sessions with more than two participants as the ITU standard family H.32x used for audio/video conferencing still lacks multicast functionality.


          NetMeeting [netm] is an integrated H.32x and T.120-compliant conferencing system including audio/video conferencing, a text-based chat feature, a simple whiteboard and both program and desktop sharing. Over the internet, conference partners may be called by specifying their (numeric or symbolic) IP address or by looking them up (based on their electronic mail address) in an Internet Location Server.

          While most of the components can also be used in conferences with more than two participants, audio/video communication can only be done between two partners (although several of these pairs can exist simultaneously during a session).

          Since NetMeeting offers a number of features, runs stable, has an intuitive graphical user interface and is available free of charge, it has become the de-facto standard for ITU-based video and data conferences.

          CIO JVTOS, MaX

          The Joint-Viewing and Tele-Operation System JVTOS is a cross-platform application-sharing tool based on the X Windows protocol. Due to its design and by means of X protocol converters it is possible to view and control X Windows applications (running on a Sun, HP or SGI workstation) from a Macintosh computer or an IBM-compatible PC running Windows, or vice-versa [ciojvtos, max].

          Virtual Network Computing (VNC)

          An important implementation of the screen-sharing paradigm is the Virtual Network Computing system available from AT&T Labs Cambridge. Instead of transmitting graphics primitives in order to share the user interface of a single application, a VNC server shares its complete desktop (on a bitmap basis) with one or multiple clients which may for their part operate the whole server - if permitted. Servers exist for a number of platforms including SunOS, LINUX, Windows, MacOS and even WindowsCE; clients can be run on any platform supporting Java as VNC also contains a Java-based client implementation [vnc].

      5. Workspace Sharing
      6. In contrast to application-sharing environments, workspace-sharing tools distribute a common data set (the workspace) among the participants of a conference, allowing every partner to view (and possibly edit) his/her own part of a shared document independent of other users - if permitted. Every site has its own local representation of a shared workspace and may continue working with it even after other sites have become disconnected or left a conference. One of the primary duties of a workspace-sharing protocol is therefore to keep the individual (local) representations consistent with those at other sites and let them all converge to a common global shared workspace.

        In principle, people participating in a workspace-sharing session may use different applications to work on the common data set. For the time being, however, workspaces are still application-specific: in order to be able to examine and modify a shared workspace the same tool has to be used at each site.

        The lifetime of a shared workspace is determined by the period of at least one running instance of the corresponding application - i.e., as soon as all conference partners have left the conference, the workspace is lost. While some tools allow saving and (re) loading the contents of a workspace, there are no means for merging any changes applied to the saved data outside any session.

        A new approach, which is currently being developed in the context of MECCANO, decouples the shared workspace from its applications and allows its objects to be deposited in an independent persistent store. A number of these stores then form a flexibly extensible federated workspace providing an environment for generic workspace sharing.

        1. Issues
        2. This section briefly describes a few problems shared-workspace applications have to deal with apart from those already listed in Section 5.2.1.

          Application-specific Workspaces and Protocols

          Most of today's workspace-sharing tools use their own proprietary data and protocol specifications. The workspace itself (and the set of operations defined for its objects) is then only implicitly defined in terms of that protocol making it difficult to use (parts of) existing implementations for different purposes (i.e. other kinds of documents).

          Behaviour in case of Network Partitioning

          All systems allowing each group to continue with their conference (although with a smaller number of participants) when a network gets partitioned face the problem of having to recombine the states of every subsession and form a common state as soon as the individual groups become connected again. Several solutions exist which are based on a global time associated with every object's state and enforce a consistent workspace by using younger state information only. Other approaches exploit the similarity of this problem to the situation of a late joining person with a non-empty workspace which has to be made consistent again.

          Incompatibilities between versions of a protocol (or workspace)

          Shared-workspace applications often run into problems when the specification of objects within a workspace and/or the sharing protocol itself are changed and not all participants immediately upgrade to the new version. Today's tools require the whole workspace (i.e. all instances of the corresponding application) to be shut down and then restarted using the upgraded version only.

        3. Implementations and Developments
        4. For MECCANO, workspace sharing is of higher importance than application sharing. Within the project, the following tools are used and/or developed:

          Shared Whiteboard (wb and wbd)

          The whiteboard wb was originally developed at LBL as a test environment for scalable reliable multicast (SRM) [floy95]. As the source of wb was never released and the exact protocol never published, wbd was developed for the PC platform by Julian Highfield at Loughborough University from an analysis of the wb protocol [rasmus] performed by Lars Rasmusson in 1995. Wb has become an important tool for visual presentations over the Mbone - despite the lack of many important features. Although there are numerous other shared whiteboards today, none of them has ever found such a widespread use.

          UCL Network Text Editor (NTE)

          NTE is a shared text editor designed for use on the Mbone. Many people can (if they wish) edit the same document simultaneously. Unless a conference participant locks a block of text, anyone else in the session can edit that text or delete it [nte].

          Authoring-on-the-fly (AOFwb)

          AOFwb is the combination of a graphical whiteboard with a tool for tele-presentations. Presentations may be prepared using AOFwb and then distributed among the receivers where a companion tool (AOFrec) is responsible for receiving and displaying these documents - page changes and telepointer movements are still controlled by AOFwb [lien98].

          Distributed Lecture Board (dlb)

          Basically, the Distributed Lecture Board dlb [gey98] is an enhanced whiteboard tailored to the needs of synchronous teleteaching. Different media are integrated in an easy-to-use interface. The dlb provides flexibility for the use of media, support for collaborative group work and will be integrated in an overall teaching environment which will support most of the synchronous teleteaching requirements (construction, transmission, recording, retrieval, playback, and preparation of lectures and teaching materials).

          dlb uses a SGML-like format for its documents and extends the classical functionality of a shared whiteboard by supporting annotations and providing mechanisms for voting, online feedback and attention. It is also network-compatible with the AOF whiteboard.

          The most interesting feature, however, is the possibility to record and playback dlb sessions using a VCR on-demand service.


          TeleCanvas [rozek96] is a Java-based (and therefore rather platform-independent) shared whiteboard that tries to combine a hierarchical workspace model with a simple transaction model in order to realise a distributed undo/redo feature. A multi-centred approach guarantees workspace consistency combined with strict access control while still maintaining scalability with respect to the number of passive (i.e. listening) number of participants.


          Based on the lessons learned from TeleCanvas, a new approach to generic workspace sharing is currently being developed in the context of MECCANO. An application-independent specification of the shared workspace allows any tool that conforms to this specification to work on a distributed data set. Servers allow for the persistent storage of workspace data, and distributed garbage collection mechanisms remove objects which are no longer used. A protoype-based inheritance scheme as part of the data specification helps keeping different versions of the workspace compatible with each other even when these are used within the same session. A local caching concept combined with well-defined behaviour in case of transaction failures (which is consistent with a user's expectation) provides a high degree of robustness against network and server failures while still offering enough responsivity to be used for interactive applications.

          As for TeleCanvas, a separate transaction model forms the basis for

          • distributed undo/redo mechanism;
          • robust combination of different workspace states after a temporary network partition;
          • simplified recording and playback of a given session.

          The design and implementation of TeleStore are being done as part of a PhD thesis.

  8. Conference coordination and control
    1. Conference and Server Control
      1. Conference Control in the IETF and ITU
      2. Conferences come in many shapes and sizes, but there are really only two models for conference control: lightweight 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.

        Lightweight 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. (There is some confusion on the term session, which is sometimes used for a conference and sometimes for a single media stream transported by RTP. In this paper, we prefer to use the less ambiguous term conference except where existing protocols use the term session.) Typically, the only conference control information needed during the course of a lightweight session is that distributed in the RTCP session information, i.e. an approximate membership lists 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 is those belonging to the ITU's H.323 family [h323]. However it should be noted that this is inappropriate for large sessions where scaling problems will be introduced by the conference control mechanisms.

        In order to address this, the ITU has standardised H.332 [h332]; this is essentially a small tightly coupled H.323 conference with a larger lightweight-sessions-style 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 9.3) might be a more appropriate solution for many potential customers.

      3. Controlling Multimedia Servers
      4. The Real-Time Stream-control Protocol (RTSP) [schu98] provides a standard way to remotely control a multimedia server, such as those discussed in Section 10. 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.

      5. Audio and Video Stream Control and Management in CORBA
      6. An alternate approach is the one adopted for many traditional distributed object computing (DOC) middleware by the communications industry. This uses CORBA and Java RMI support/response as the semantics for distributed applications. Since real-time multimedia applications require transmission of continuous streams of audio and video packets, there are stringent performance requirements for streaming data; these often preclude DOC middleware from being used as the transport mechanism for multimedia applications. For instance, the CORBA Internet Inter-ORB Protocol (IIOP) implemented over TCP is inefficient for audio and video transmission and makes multicast distribution impossible. However, the stream establishment and control components of distributed multimedia applications can benefit greatly from the portability and flexibility provided by middleware. To address these issues, the Object Management Group (OMG) has defined a specification for the control and management of A/V streams, based on the CORBA reference model.

        The CORBA A/V streaming specification defines a model for implementing a multimedia-streaming framework. This model integrates

        • well-defined modules, interfaces, and semantics for stream establishment and control


        • efficient transport-level mechanisms for data transmission.

        The OMG environment does not include multicast; however, this is not necessarily a requirement for the A/V control - provided the media transport protocol does allow multicast. The framework provides the following flexibility:

        Stream endpoint creation strategies: Many performance-sensitive multimedia applications require fine-grained control over the strategies governing the creation of stream components.

        Transport protocol: The OMG streaming service makes no assumptions about the transport protocol used for data streaming. Consequently, the stream establishment components in the A/V streaming service provided flexible mechanisms that allow applications to define and use multiple transport endpoints, such as sockets and TLI, and multiple protocols, such as TCP, UDP, RTP, or ATM.

        Stream control interfaces: The A/V streaming framework provides flexible mechanisms that allow designers of streaming services to define their own stream control interfaces. In particular the existing stream control protocols such as RTSP may be used for implementation of their operations.

        Managing states of stream supplier and consumer: An important design challenge for developers is designing flexible applications whose states can be extended.

        In summary the OMG A/V streaming model goals are the following: definition of standardised stream establishment and control mechanisms, supporting multiple transport protocols, and various types of sources and sinks.

    2. Conference Set-up
    3. There are several basic forms of conference discovery mechanism. The conferences can be announced in a broadcast mode, individuals can be invited in real time, or information about the session can be provided off-line – by putting the information in a depository or sending it by e-mail. Each is described briefly below. Information on the security aspects of Session Initiation is provided in Section 9.3.

      1. Session Announcements
      2. One method of announcing sessions is to multicast the session description on a well-known multicast port, with a specific scope, using the Session Announcement Protocol (SAP) [han99-4]. The announcement includes some information (like the Organiser of the conference), some authentication information and the Session Description defined in the Session Description Protocol (SDP) [han98]. People wishing to participate in a particular conference must then listen for the SAP announcement, and start up their tools with the SDP details provided. A number of automated tools (e.g. SDR [sdr]) have been developed which can receive the multicast session descriptions, browse through all sessions currently being announced and then start up the relevant media tools. An important aspect is that if the announcement of the message is received, there is a high probability that the session itself can be joined; this is because the advertisement uses the same scope as the session.



        Figure 6 Joining a lightweight multimedia session


        This mechanism can also be applied to advertised tightly coupled sessions, requiring only additional information about the mechanism to use to join the session. 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; smaller, more private, sessions will tend to use direct invitation rather than advertisement.

      3. Session Invitation
      4. Not all sessions are advertised, and even those that are advertised may require a mechanism to invite explicitly, but in real-time, 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.

        Since users may be 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) [han99-1] 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. It does not 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.

      5. Off-line Mechanisms
      6. It is also possible to use off-line mechanisms for providing the information on up-coming sessions. One method is to send the information by e-mail; mechanisms for parsing e-mail and starting sessions automatically have been provided [hin96]. While this is an adequate method, it really needs to be provided in a generic way, so that it can be parsed automatically by the mail systems. Since a special MIME type has been defined for specifying SDP, it would be best to use this for all implementations passing SDP in messages. It is then possible to provide a MIME Plug-in for each e-mail tool, to parse automatically this particular MIME type and launch the tool. We plan to provide this functionality in MECCANO from a WWW browser. It is also desirable to provide a mechanism for obtaining listings of all sessions currently available, or announced for a certain interval.

        Alternately the information can be put into a repository known to the potential participants. The information can then be extracted at will by the potential participants. This mechanism is convenient if potential participants can be expected to access the directory sufficiently often. A combination of the use of ordinary e-mail to announce the existence of a conference, together with a directory mechanism, will probably be the most popular for a large class of applications. Specialised Web-based tools already exist which allow the browsing through lists of conferences, together with client plug-ins that can extract the relevant Session Description and start up the tools. There is still a potential problem mentioned in Section 3.1.2. If the Session Announcements are made privately, the address allocation is not necessarily unique; two independent announcements may be made of different conferences on the same multicast address/port. If the address allocation by the announcer is made randomly, this is unlikely to occur – and will be easily detected. Moreover, the MMUSIC group is addressing this problem at the moment; any solutions it develops will also be incorporated in the MECCANO architecture. Multicast sessions are scope-limited; hence although an announcement may be retrieved, it does not follow that the retriever can participate in a particular session. Finally, use of a repository can lead to a single point of failure – the availability or accessibility of the repository. It is possible to ameliorate this problem by putting the announcements in several depositories; this is comparable with the running of a primary and several secondary DNSs in the Internet.

        Neither of these mechanisms will scale well to very large conferences, because of the potential number of messages or depository accesses. However this is just the environment where SAP is particularly useful. In any case, it is not clear that we have the tools to manage really large conferences.

    4. Inter-process Control
    5. Many of the activities in multimedia conferencing involve synchronisation or control between different processes in the same or different processors. A number of general-purpose systems have been developed to meet this need like CORBA [cor]. These can be used, in fact one of the systems mentioned in Section 10.5 does indeed use CORBA. However for most of the purposes described in this report, the performance of CORBA is too poor, and the functionality too general purpose, rather than tailored to the multimedia applications. For this reason we use, in general, Mbus which is designed to synchronise and mediate between real-time processes in one machine – or between machines where negligible delay or error can be expected. The characteristics of Mbus are discussed below.

      1. The Message Bus Goals
      2. The Message Bus (Mbus) [ott] infrastructure has been designed to simplify development of complex communication systems intended to enable and augment interactive human-to-human (tele)co-operation. Such system may include (typically workstation-based) user terminals (as designed in WP4) as well as various types of interworking units (WP6) and other management entities (partially developed in the context of WP7 but to a large extent beyond the scope of the MECCANO project).

        In our abstract system concept, the functionality of any conferencing system considered in MECCANO can be broken into a variety of components. These include but are not limited to media engines, user interfaces, conference control, and modules providing administrative mechanisms. Depending on the type of conference and the type of system, some or all these components may need to act in a closely co-ordinated fashion. Individual tools may be separated logically and physically into their respective engines and user interfaces that need to communicate to convey user actions and system responses back and forth. Systems may also have various media engines controlled by a single user interface and thus require mechanisms to simultaneously control these engines. Also, media engines may need to co-ordinate themselves e.g. to synchronise audio playback with presentation of video streams to achieve lip synchronisation. Finally, in tightly coupled conferences or IP telephone calls, dedicated entities may perform conference control functions and may need to closely interact with other system components to make them adapts their behaviour according to the conference state. Altogether, local co-ordination is needed to achieve coherent system behaviour in response to user actions as well as interaction with other conferencing systems.

        While horizontal control protocols (such as SCCP) are intended to synchronise state between communicating systems (inter-system protocol), vertical control protocols serve intra-system co-ordination. The Message Bus shall provide an infrastructure for vertical co-ordination based upon mechanisms for inter-process communication (IPC).

        It is designed to satisfy the following needs:

        • support a modular system design;

        • simplify using building blocks from different sources through well-defined interfaces;

        • enable efficient independent development and testing;

        • allow for independence of programming languages;
        • maximise re-usability of components in different systems as well as different system types;

        • support separation of engines from user interface;

        • enable easy system extensibility (at run time);

        • not to prescribe system design or mandate certain components;

        • allow for efficient and low-overhead communication;

        • be robust against partial system failures; and

        • support monitoring of system functions for debugging.

        The Mbus infrastructure logically consists of two components: a transport infrastructure that provides message transfer, addressing, and basic bootstrap as well as awareness mechanisms and a semantic layer that is defined through the abstract services of the communicating modules. Within the MECCANO project, semantic layers is defined for call and rudimentary conference control services, control of selected media engines and their user interfaces, and for simple user preferences/policies.

        Systems to be built based upon the Mbus infrastructure include multimedia terminals and various types of (multimedia) gateways.

      3. Mbus Transport Services
      4. As a basic service, the Message Bus provides local (intra-system) exchange of messages between components that attach to the Mbus (Mbus entities). A local system is typically expected to comprise exactly a single host, but a local system may also extend across a network link and include several hosts sharing the tasks; a local system must not extend beyond a single link. Message exchange takes place using UDP datagrams; UDP datagrams are either sent via unicast to a single Mbus entity or are multicast using host-local or link-local scope (depending on far the system reaches).

        For point-to-point communications, message delivery is optionally performed reliably; i.e. acknowledgements and retransmissions take place at the Mbus transport layer. All multicast communication is performed unreliably. Point-to-point and multicast communication are not distinguished by the transmission mechanism employed at the IP layer but rather by the qualification of the Mbus destination address (briefly described below).

        In addition to basic message transmission, the Mbus transport provides mechanisms for Mbus entities to automatically determine the availability and the address other Mbus entities as well as an entity's failure. Based upon these functions, a bootstrap procedure is defined that allows Mbus entities to determine whether all other entities they depend on are present (without bearing the risk of deadlocks).

        It should be noted that Mbus entities are logical components. In particular, a process or a thread may represent an arbitrary number of Mbus entities (which may even communicate with one another via the Mbus).

        A key concept of the Mbus is its flexible and extensible naming and addressing scheme. Mbus entities are identified by n-tuples. Each tuple component is represented by an (attribute: value) peer. Currently defined attributes include conference (conf), media (media), module type (module), application name (app), and application instance (instance). Individual or all tuple components may be omitted or wildcarded (*) to implement broadcast, multicast, and anycast services. Full qualification of an address (i.e. non-wildcarded presence of all defined components) denotes a Mbus unicast address. Furthermore, the Mbus provides message authentication and encryption as inherent transport features. Message authentication is needed to prevent malicious entities from taking control or at least disturbing a user's system but also to prevent accidental interpretation of other users' message, e.g. in case multicast transport addresses of two users accidentally match and their transmission scopes overlap. As the Mbus may also be used to convey personal information or keying material (IVs, keys) between Mbus entities, the messages also need to be encrypted to prevent other entities from eavesdropping.

        A set of user-specific resources (specified in a file, a registry, etc.) contains all the user-specific configuration information for all Mbus entities representing this user. The resource information includes user id, authentication/encryption keys, multicast address/port for message exchange among others. These facilities combine to allow the Mbus to support multiple sessions of a user per host (including cross-session co-ordination) as well as any number of users on the same host or link (preventing accidental cross-user interaction).

      5. Semantic Concept
      6. As stated above, we can identify various component types that attach to the Mbus to form an integrated system. The rather intuitive overview can be formalised to yield a finer granularity of component types that is sufficient to build all kinds of systems within the scope of the MECCANO project. The following (logical) component types are defined:

        • (media) engines providing the functionality necessary for telecooperation (such as audio or video communications, shared workspace or editor, etc.);

        • control (protocol) engines managing interactions with remote users (e.g. providing means for call/conference set-up, floor control, mutual awareness in a conference, etc.);

        • graphical user interfaces (GUIs) as suitable means for a human user to access the system functionality implemented by the (control and media) engines;

        • policy modules that control automated system behaviour (e.g. based upon user preferences, administrative settings, call/conference processing scripts, etc.);

        • applets that provide an abstraction from specific implementations of background/backend services (such as address resolution, certificate validation, user authentication, directory services, etc.); and

        • an Mbus controller that may combine any number of the aforementioned modules and add specific interpretation/processing to create a particular integrated system type.

        Particular instantiations of these (abstract) components are implemented within MECCANO to build the complex components to be delivered. Control protocol engines being implemented include an H.323 engine and a SIP engine as well as a module to communicate via ISDN lines. Media engines include at least the Robust Audio Tool (separate engine and user interface) with an Mbus interface. Various user interfaces for integrated terminals are likely to be developed. Finally, Mbus controllers are designed to combine the aforementioned components to form two important deliverables:

        An Mbus architecture framework document is in progress and will be submitted as an internet-draft for discussion in the multiparty multimedia session control (MMUSIC) working group of the IETF. The Mbus transport specification is essentially complete and has two implementations, one at UCL and another at Bremen University.

        The basic Mbus operation and bootstrap procedures have been defined, together with specific commands for control of audio and RTP related functions. Basic call signalling has been defined, with specific commands for H.323, SIP, and ISDN lines to come. An internet draft, which is being written to document this, will involve extension of the Mbus semantics to further cover basic call/gateway control, including protocol specific extensions, floor control, dynamic control of media session, etc.

  9. ITU Conferencing Architecture overview
  10. The ITU-T Working Group of most relevance to multimedia communications and conferencing is ITU-T SG16, also referred to as Study Group for Multimedia. In the past they defined a series of recommendations for multimedia conferencing called H.323. H.323 is currently considered the standard for controlling multimedia communications in small groups as well as for IP Telephony. The core Recommendations include H.323 (system and procedures), H.225.0 (message formats, encodings), and H.245 (capability/media descriptions, handling of media streams). H.235 covers security mechanisms for H.323, the H.450.x standards address Supplementary Services for multimedia communications, and H.341 defines Management Information Bases. Other recommendations address remote device control and text conversation (transport and semantics in both cases).

    The mechanisms of H.323 conferencing are somewhat different to those of the Mbone described earlier in this section. Here the basic model is that one party initiates a communication with one other party. The extension to multi-party conferencing comes from establishing communications through reflectors, called Multiplexing Control Units (MCUs), which coordinate the conference and optionally replicate the data to all the participating parties. Considerable attention to has been paid to enabling H.323 interoperability with the pure Mbone-based applications, in spite of the fairly fundamental differences between the conference models. The previous ITU standards (H.320 and H.324) differed even more; they had, for example, quite different formats at the media transport level. The foundation for interworking was laid when the ITU adopted the RTP standards unchanged as the basis for its H.323 media transmission.

    H.323 interfaces well to the loosely-coupled conferencing concept prevalent in the IETF through H.332 that defines how to set up H.323 conferences, map their parameters to SAP/SDP, and announce the conferences on the Mbone. Under the recommendations there are considerations of how to deal with ISDN, Internet and ATM. There are also detailed recommendations of media encodings to be supported. These include some of the audio and video encodings supported in the Mbone conferencing. They have long advocated the H.261 coding used by VIC and other Mbone tools; they are now studying advanced video coding schemes (particularly including error resilience schemes as in the 1998 revision of H.263 (also referred to as H.263+).

    SG16 has developed the T.120 series of Recommendations used for data conferencing across arbitrary networks including the Internet. The T.120 infrastructure provides a platform that creates a multipoint communication environment from a set of point-to-point connections by ordering them in a tree structure and offers a rich set of conference control functions. Also, use of native multicast networks is supported. On top of this infrastructure, T.120 application protocols provide means for telecollaboration through shared whiteboards, file transfer, application sharing, and text chat. Recent developments include support for T.120-specific security features and the design of a semantic meeting room model (including different conferencing styles, roles, associated privileges, etc.). Most of today's commercial shared workspaces and particularly application sharing systems are based on the T.120 series of Recommendations: these include Microsoft NetMeeting [netm] and Intel ProShare [prosh].

    Because of the convergence of many of the SG16 and IETF endeavours, we have two activities under MECCANO auspices. The first is to implement a gateway to allow ITU H.323 workstations to participate in Mbone conferences and vice-versa. Such a gateway will be controlled, in early versions, by a message passing control bus called Mbus. An important part of the project is to provide a sufficiently general specification of Mbus to allow it to be used in the Mbone-H.323 gateway. In addition, the requirements for trunking gateways at carrier class size have been recognised by both the IETF and the ITU SG16 communities. Such trunking gateways act as interface points to the traditional telephone network (not only for the last mile) while the carrier infrastructure interconnecting the gateways is entirely IP-based. This has led SG16 to embark on the design of H.248 (formerly known as H.GCP), and the IETF community design of similar facilities in a new IETF MEGACO Working Group. The design of H.248 is now related to, and increasingly co-ordinated with, the IETF MEGACO WG, H.225.0 Annex G and a variety of Annexes for more efficient communication procedures and simpler endpoints. The functional range of H.248 / MECAGO is likely to impact the design of the Mbus semantics layer for call signalling and control in gateways. H.225.0, Annex G, covers inter-domain exchange of addressing information and (potentially) other characteristics of specific devices (such as gateways) and so may also impact the design of the MECCANO gateway.

    The second of the MECCANO activities is to ensure the continued alignment of the two standardisation activities - partially by our participation in both. There is also a need to consider whether the Mbus itself should be the subject of IETF standardisation; thus our Mbus activity is also part of the alignment procedure.

  11. Gateways and Relays
    1. Introduction
    2. The variety of different network technologies, workstation capabilities and conference system technologies preclude the adoption of a single conferencing system, at a single speed, with homogeneous facilities. Many mechanisms have been suggested, and some even implemented, to address system heterogeneity. At some level these may be independent of any direct intervention inside the network. For example, if layered coding is used, all workstations support multicast, a disadvantaged receiver may just not subscribe to all the relevant multicast groups. However the above constraints are too prescriptive, and the section considers what can be done in more heterogeneous environments.

      There have been recent moves to consider Active Networks [tennen96], in which each node can do reasonably complex packet manipulations at the IP level. We consider this much too radical for deployment in the MECCANO project. Here we are prepared to put in Active Components, but only at the Application level at boundaries between technologies. This approach can be termed the use of Active Service nodes. In advanced environments, such nodes may be cloned at will automatically and instantiated in an optimal manner throughout the network. The requisite technology is still in the research stage and such automated deployment will not be considered in the MECCANO architecture. We will consider, however, several sorts of Active Service nodes.

      All these gateways have a common approach, though they have quite different purposes. The common aspects of this approach, including the requirements, protocol conversion, and signalling conversion needed are considered in Section 8.2. In Sections 8.3 and 8.4 we will consider specific implementations of gateways to provide part (AudioGate) or all (StarGate) of the functionality described in Section 7. Here it is assumed that the IETF multicast procedures are used on one side of the gateway, and the ITU ones operate on the other. In Sections 8.5 and 8.6 we review two tools which provide multicast to unicast conversion within the Internet environment. The first is a simple reflector. The second, the UCL Transcoding Gateway (UTG), is a device that is located near a specific change in network technology. While it assumes that Mbone technology is used in the tools on both sides of the UTG, it provides, in addition to the multicast to unicast conversion, video and audio multiplexing and media conversion (e.g. transcoding and packet filtering). An earlier implementation of such functionality may be found in the mTunnel [mates] and LiveGate [livegate] applications. It is also possible to arrange for the UTG to act as a multicast node from the viewpoint of terminating some multicast groups and allowing clients to subscribe to a limited range of groups.

      Finally, a component frequently deployed at administrative boundaries is a Firewall. Both the facilities provided in such devices and the constraints that these impose are discussed in Section 8.7. Here we suggest also mechanisms that may allow multimedia conferencing to be deployed, even if there are firewalls in place.

    3. The Gateway Architecture
    4. The gateway architecture follows the overall system architecture outlined in the MECCANO Deliverable D3.1 and employs the components and control mechanisms described in Deliverable D7.1.



      Figure 7: Mbus Architecture for Endpoint, Gateways, and Management Systems


      An overview of this system architecture is depicted in Fig. 7 with the following Mbus entities being envisioned so far:

      • Media engines providing the functionality necessary for telecooperation (such as audio or video communications, shared workspace or editor, etc.);
      • Control (protocol) engines managing interactions with remote users (e.g. providing means for call/conference set-up, floor control, mutual awareness in a conference, etc.);
      • graphical user interfaces (GUIs) as suitable means for a human user to access the system functionality implemented by the (control and media) engines;
      • Policy modules that control automated system behaviour (e.g. based upon user preferences, administrative settings, call/conference processing scripts, etc.);
      • Applets that provide an abstraction from specific implementations of background/backend services (such as address resolution, certificate validation, user authentication, directory services, etc.); and
      • An Mbus controller that may combine any number of the aforementioned modules and add specific interpretation/processing to create a particular integrated system type.

      Gateways are designed based upon the Mbus concept. They consists of a set of components attaching to the Mbus with one dedicated Mbus controller defining the type of gateway and the others fulfilling well-defined pieces of the overall functionality the gateway provides. The building block approach and the types of components involved in the MECCANO gateways are described in the following subsection. Subsequently, the role of the Mbus as a transparent mechanism to control gateways is outlined as are the control Mbus components, followed by considerations on handling of media streams in gateways.

      1. Building Block Approach
      2. As all other complex systems developed in the MECCANO project, gateways combine a set of Mbus entities to provide the desired system functionality. Out of the aforementioned categories, the currently developed gateways:

        • use media engines for transcoding purposes but not for local capturing / replay of media streams;
        • use one or more control protocol engines to interpret and encode messages of the respective protocols;
        • optionally may make use of graphical user interface components to provide status information to an administrator and allow the administrator to configure the gateway, intervene with ongoing calls if necessary, etc. (however, none of the gateways currently do so);
        • do not make use of policy modules (at this point in time);
        • employ control applets for modular extensions to the core functions of the gateway; and
        • implement an Mbus controller that actually implements the signaling conversion, invokes transcoding as necessary, etc.
      3. Mbus Entities for control
      4. This subsection briefly outlines the status of the generic Mbus entities implemented so far, while the following sections on the respective gateways describe their Mbus controllers and how the various generic Mbus entities fit together.

        To meet the MECCANO aims, we plan to develop the following control protocol entities:

        • A fully Mbus capable SIP engine that implements endpoint as well as SIP proxy functionality.
        • An H.323 Mbus engine providing the same abstract call control interface as the SIP engine.
        • A SAP/SDP engine capable of receiving and interpreting SAP/SDP-based session announcements. It should include an abstract Mbus interface for querying/passing information about the session announcements. This engine will be incorporated in the gateways of Sections 8.3 and 8.4.
        • An ISDN call control engine – again with an Mbus interface.
        • Because the Real-Time Streaming Protocol (RTSP) is used for remote access to media servers, we may develop an Mbus which performs these functions

        Three conversion engines will be developed in MECCANO for use in gateways:

        • Media conversion engines between two different encoding and/or packetisation schemes on the IP side; they may also act as an RTP translator – for both IPv4 to IPv6 and unicast to multicast conversion. The UCL audio tool (RAT) already has some of this functionality for audio, and the LBL vgw for video, but both will need considerable updating.
        • A Communication engine that will provide an interface to an ISDN BRI thereby allowing conversion of audio streams from the line-switched to the packet-switched environment and vice versa.
        • A dedicated multicast to unicast packet reflector

        Only the first is really significant. The other two largely exist, and will be convenient to deploy early in the project.

        We expect more than pure audio communication between the packet and the line-switched environments, based upon IP-capable endpoints at both sides with a multicast-unicast gateway in the middle providing the necessary addressing conversion. An optional transcoding gateway can reduce audio/video quality to achieve a transmission rate suitable for the line-switched network.

        Full interoperability with H.320-based videoconferencing systems may easily be achieved by combining the MECCANO Mbone-H.323 gateway with an H.323-H.320 gateway. As the latter type of gateway is now commercially available from a variety of manufacturers, no specific efforts within MECCANO are addressing this particular issue.

    5. AudioGate
    6. The MECCANO Audio Mbone-Telephony Gateway AudioGate should provide users on an arbitrary telephone network (PSTN, ISDN and GSM) with access to the audio channel of Mbone conferences. Upon connection set-up, functions such as dynamic conference selection will be provided. As soon as a connection to an Mbone session is established, additional services such as user identification, muting, most recent speaker indication, etc. may be provided.

      AudioGate should provide a dial-in interface that allows users to call a phone number and automatically be transferred into a pre-selected Mbone session. AudioGate will use an ISDN BRI to connect to the phone network. If a Calling Line Identification Presentation (CLIP) service is supported by the caller and the ISDN network, the caller's phone number is provided in the SDES NAME item to identify the person on the phone to the other parties in the Mbone conference.


      Figure 8: Mbus architecture for AudioGate 1.0


      The target architecture splits the gateway process into several different logical components as shown in Fig. 8. These components are expected to be implemented as separate processes, mainly using engines discussed in Section 8.2. Only the utilisation of the same ISDN board for control and data exchange require the ISDN call controller and the RAT engine for a single ISDN B channel to reside within the same process, forming two independent Mbus entities nevertheless.

      The components depicted in Fig. 8 contribute to the AudioGate functionality as follows:

      • The RAT media engine acts as a line switching to packet-switching converter as outlined above.
      • The ISDN call controller provides a simple interface to allow setup and teardown of calls, detection of busy and call completion indications and provides an interface towards detection and generation of DTMF signals as well as generation of voice clips for the ISDN side.
      • The SAP/SDP module receives session descriptions from the announcement channel(s) and extracts the information relevant for identifying and joining Mbone conferences.
      • The conference selection module receives session descriptions from the SAP/SDP module and turns them into a choice list assigning each conference a numeric identifier by which the conference to be joined can be picked (via DTMF). The conference selection module also implements a filtering mechanism (to be configured e.g. via a simple resource file) to limit the access to a certain subset of conferences.
      • The Mbus entity marked additional control is intended to provide an extensible set of additional features, largely based upon DTMF selection by the telephone user. Such services may include muting/un-muting the telephony user, changing the volume, among others.
      • Finally, the Mbus controller for the AudioGate glues all these entities together by accepting input from the various other Mbus entities and forwarding them appropriately. For example, to direct DTMF received from the user to the conference selection module while in the set-up phase and directing them to the additional control module when already joined to a conference.

      AudioGate will be developed in several phases. The initial phase will focus on implementing, testing, and optionally enhancing the core components with less focus on the overall Mbus architecture.

    7. StarGate
    8. The MECCANO call signalling and media transcoding gateway StarGate is supposed to provide connectivity between different kinds of endpoints interconnected through different types of networks (hence the name *Gate). This is expected to include in particular:

      • Conversion between the three most important call signalling protocols (H.323, SIP, and ISDN) including media stream conversion if necessary;
      • Actively accessing Mbone sessions from H.323 endpoints; and
      • Inviting H.323 endpoints into Mbone sessions for audio and optional video communications.

      The architecture of StarGate also allows us to easily extend the number of supported call signaling protocols. In addition, if feasible from the standardisation point of view (i.e. the necessary specification are complete and stable), security aspects will be incorporated into the StarGate implementation.

      StarGate is conceptually built upon the same general Mbus architecture as AudioGate, with largely different Mbus entities and different interactions between them, of course. A conceptual outline of a possible StarGate implementation is shown in Fig. 9.



      Figure 9: Outline of the MECCANO StarGate


      The various components perform the following tasks:

      • The H.323, SIP/SDP, and ISDN modules implement call signalling and (as far as applicable) conference control functions for the respective protocol suite.
      • An Mbus RAT entity is instantiated whenever transcoding (e.g. for interconnection to the telephone network) is required.
      • The Call Routing module provides address and endpoint reachability resolution and, in particular, decides which protocol to route an incoming call across.
      • The access control module is used to verify that incoming calls are authorised to be completed according to the reachability decision taken by the Call Routing module (e.g. whether an IP-side caller is allowed to call a long-distance number via the telephone network).
      • Finally, the Mbus controller again provides the necessary glue between all the modules forwarding call messages back and forth, keeping per call and resource utilisation state, etc. In particular, it knows which control protocol entities are present and is optionally capable to translate non-standard Mbus call control messages between the various protocols and instantiate / configure the rat media engine(s) accordingly.
      • Further Mbus entities (control applets as well as policy modules) may be introduced to provide additional functionality such as value added services based upon DTMF tones or similar signalling from the IP side.

      All three of the aforementioned control protocol entities share a core set of Mbus messages to set up, tear down, and monitor progress of a call. In addition, each entity supports protocol-specific Mbus extensions that may not be (easily) mapped to other control protocols. The Mbus controller is expected to understand all these Mbus commands, route incoming messages, and optionally perform translation between different protocols.

      Call control messages are intended for interaction with call control and invitation protocols such as H.323 and SIP. They are designed to constitute the union of the call control messaging needed by endpoints, gateways, proxies, multi-point controllers, and gatekeepers. This allows the use of the Message Bus to act as gluing mechanism to create any type of system from roughly the same building blocks.

      Mbus call control messages are based on a common basic message set defined in the following that will be supported by any kind of call control protocol entity. The basic message set may be augmented by protocol-specific extensions required for protocol specific interactions between a local controller and/or local applications on one side and the respective protocol engine on the other. While the basic Call Control commands have been worked through, these will be described in futureWP6 Deliverables. They must be mapped to H.323, SIP, and ISDN-specific messages.

      A possible future extension for MECCANO is an Mbus command set for the Real-Time Streaming Protocol (RTSP) extensions. However, this is left for further study.

    9. Multicast-Unicast reflector
      1. Introduction
      2. In the current IP multicast model, it is often desirable, for several reasons, to transmit multicast traffic to selected destinations using unicast transport. One reason is that not all IP hosts are connected to the Mbone, for both technical and administrative reasons. Another is that, as the virtual private networks become more widespread, particular care must be taken to not shuffle the multicast traffic originating from them and from the regular Mbone; otherwise multicast routing may be affected (routing loops, undesired routes and traffic).

        Several multicast-to-unicast reflector solutions (called also replicators, transmitters or gateways) are implemented today. These are either based on specific, custom-designed protocols and software [livegate], or are far too difficult for an inexperienced user to install and use [rtptrans]. A more complex functionality is implemented in UTG (see Section 8.6).

        While implementing the reflector engine is rather simple, controlling it in a simple manner is far from trivial. We prefer to set only two requirements for the reflector control interface:

        1. using the reflector to stream media must be as simple as browsing the Web
        2. there should be no assumptions on the users’ capabilities but the ability to use a Web-browser and run an installer application.

        We have chosen to control the reflector functionality using Real-Time Streaming Protocol [schu98]. RTSP simplifies Web-integration and includes the necessary streaming control functionality for both unicast and multicast transport.

      3. Architecture
      4. The multicast-unicast reflector is build around its principal part, the reflector engine and control server (or simply Reflector, residing on Host 4 in Fig. 10). This entity joins the multicast groups and maps the traffic to/from selected unicast hosts.

        The reflector is controlled by an RTSP server. The RTSP server includes the reflector control module, which is used whenever a multicast session needs to be delivered using unicast transport (this task is easy to specify using the standard RTSP syntax).

        On the client side, a separate control application, StreamerApp, is used in addition to the standard media tools and Web client. The Web client is configured so that StreamerApp is the helper application for .rtsp file extension. StreamerApp has a simple control interface, and includes RTSP client functionality. It also starts (and terminates) media tools.

        The HTTP server shows the multicast session announcement information for active and future multicast sessions. The Announcement collector, listening to the well-known SDR announcement address/port, supplies this information. The same information is sent to the RTSP server.

        In Fig. 10, only Host 4 must reside on a multicast-enabled network. All other hosts may be on non-multicast networks. Nevertheless, Host 1 has full access to all multicast sessions known to Host 4.

        Figure 10: Reflector architecture


        Co-hosting the announcement collector with the reflector engine is desirable due to the multicast group scope (sessions seen on different LANs may vary). Otherwise, the various components can be co-hosted as needed (e.g. HTTP server, RTSP server and the reflector can be executed on a computer equipped with a DBS satellite receiver).

      5. Usage example
      6. The user reads the information about the current multicast sessions using his Web browser. Clicking at any of the links returns an RTSP-file (e.g. seminar.rtsp), containing only one line: URL for the session description (rtsp:// The Web client is configured so that it starts StreamerApp as the helper application for the .rtsp file extension. StreamerApp connects to the RTSP-server specified by the URL, and demands the description of seminar.sdp. This file describes a multicast session, but StreamerApp demands it delivered by unicast transport, using a standard RTSP set-up call. The RTSP server satisfies this demand by instructing the reflector engine to join the new multicast group(s) and to replicate the RTP traffic to specified unicast ports on Host 1. After the set-up request is acknowledged, StreamerApp starts the media tools.

    10. The UCL Transcoding Gateway
    11. The UCL Transcoding Gateway (UTG) is another approach to providing access to multicast conferences for hosts with only unicast connectivity. In addition, it provides limited transcoding and mixing functions, primarily for audio.

      The initial version of the UTG was developed as part of the MERCI project before the Mbus concept was fully developed. We delivered an enhanced version of this (UTG v1.2) in MECCANO Deliverable D4.1. As part of MECCANO we plan to extend the UTG to fully embrace the Mbus architecture in a similar manner to the StarGate system, although the components used are somewhat different. The functionality is more powerful than that of Section 8.5, though the control concepts may converge eventually. A conceptual outline of the UTG system is illustrated in Fig. 11.



      Figure 11: Conceptual outline of the UTG system


      The components in the UTG architecture are expected to perform the following tasks:

      • The RTSP controller module will provide the control interface to the unicast end-system.
      • The access control module is used to verify that requests for transcoding and gatewaying are from authorised users.
      • One or more media engines are instantiated to perform transcoding and gatewaying.
      • Finally, the Mbus controller provides the necessary glue between all the other modules.

      A number of the components necessary for the UTG already exist. In particular, the media engines are well developed although some need updating to match the current Mbus specification, rather than earlier ad-hoc control protocols. The access control module is expected to be somewhat similar in concept to that employed in the StarGate system.

      The RTSP controller is a new piece of the UTG architecture, and is intended as a replacement for the current control protocol. We expect to develop this in two stages:

      1. by integrating an Mbus interface into the current UTG control module, reusing as many of the call control commands defined in Section 8.2 of this deliverable as possible; and
      2. by converting the current control protocol to use RTSP whilst retaining the control interface.

    12. Firewall Gateways
    13. Most commercial organisations, and increasingly even universities, use firewalls to constrain Internet packets passing between the outside and their internal networks. In a firewall-less environment, network security relies totally on host security. A firewall approach provides numerous advantages to sites by helping to increase overall host security. It can filter inherently insecure services, enhance privacy of certain sites by blocking innocuous information that would be useful to an attacker, like IP address or user name. It also provides the ability to control accesses to site systems, and hence the means for implementing and enforcing a network access policy. The firewall can log accesses, provide valuable statistics about network usage and details on whether the firewall and network are being probed or attacked.

      Normally firewalls do not allow the free flow of the UDP packets, which are fundamental to the Mbone. For, blocking them is the only effective way to block access to dangerous RPC-based services. In MECCANO we are trying to develop mechanisms which will be considered sufficiently secure to allow tool deployment inside organisations protected by firewalls. A multicast security policy consists of specifying the set of allowed multicast groups and UDP ports that are candidates to be relayed across the firewall. There are two different ways to support such a policy: an explicit dynamic configuration or an implicit dynamic configuration.

      In the case of an implicit dynamic configuration, the set of candidate groups/ports could be determined implicitly, based upon the contents of SAP announcements or other SDP descriptions. A watcher process reads and interprets these announcements in order to update dynamically the filtering rules of the firewall. In the case of an explicit dynamic configuration, the set of candidate groups/ports could be set dynamically, based upon an explicit request from an internal trusted client. This solution is similar to a proxy architecture. An insider talks to a UDP proxy server and asks it to relay a multicast session. If this request is approved, the proxy server joins the specified multicast group and relay the data to and from the client.

      Of course, this architecture could be adapted to allow the use of multicast on the Intranet or to make easier the practical application of such a solution. The UDP proxy server must indeed be set on the firewall itself. But some organisations are reluctant to modify their firewall. In that case, a tunnelling approach could be considered. A multicast-unicast relay is situated outside the firewall to an organisation; only unicast connections can be initiated from the inside of the firewall through the protected area. Inside the protected area, another unicast-multicast relay is situated. If users inside the protected area wish to join a conference, they launch their applications internally. The relay inside the protected area then opens a unicast tunnel to the relay outside; it can then perform any necessary checks on traffic passing between the two areas.

      The implicit and explicit approaches enhance security by dynamically defining the set of allowed sessions. They can both provide additional services like user authentication and logging facilities. However, the proxy solution seems to be a better option since it has more efficient logging facilities and more control over the permitted port numbers. Moreover, at the moment, it is a fundamental tenet of current security thinking by most organisations, that Mbone traffic should be authorised only from inside the protected area. For this reason, we are exploring the proxy solution or its adaptations.

      Some important issues still remain with such a proxy mechanism. In particular, it does not scale well. We will explore how severe a drain on resources this mechanism is in reality, and whether it is satisfactory to all the partner organisations.

  12. Security
    1. Introduction
    2. 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 [deer88-2]. However, IP multicast does make it simple for a host to anonymously join a multicast group and receive traffic destined for that group without the knowledge of the other senders and receivers. 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 symmetric encryption schemes such as DES [des]. It also specifies a standard mechanism to manipulate plain text keys using MD5 [riv92], so that the resulting bit string can be used as an encryption key. Similar techniques can be used for encrypting the contents of the non-AV portions of the conferences. Most of our early work has been done with DES because of the prevalence of the implementations; later we will move to more secure encryption algorithms. The symmetric encryption algorithm used has only pragmatic, not architectural, significance. We realise that DES is now denigrated in the IETF community - being relegated to historical status. In this architectural note we will normally use the words DES for a fairly conventional symmetric encryption algorithm more as a short-hand.

      There are mechanisms defined in the IETF for standard secure operations [gup99]. These are not yet defined fully for multicast operation; in particular, the key-exchange mechanisms are not yet developed. There will be some discussion of mechanisms for multicast key distribution given below; however, we have not yet decided how IPSEC would be used in these applications, and it is not considered further in this report. The use of IPSEC is part of other research projects that the MECCANO participants (in particular UCL) are doing with other funding bodies. The results will not be reflected, however, into the MECCANO Deliverables during the remaining life of MECCANO.

      Because the use of plain text pass-phrases can be used to derive symmetric encryption keys, one can use simple out-of-band mechanisms such as any privacy-enhanced mail e.g. [pgp] or S-MIME for encryption key exchange. It is also possible to integrate the key-exchange mechanism at least partially into the session announcements and invitations. Each of these methods is considered below.

      There is currently considerable discussion of whether Session Announcements are an appropriate mechanism for announcing limited sessions - and thus whether there is a place for encrypted Session Announcements at all. Already, with most sessions still unencrypted, the bandwidth taken up with Session Announcements is a large proportion of the bandwidth assigned to announcements, which has been deliberately limited. Hence there is typically a ten minute gap between announcements of a session. Another concern is that one of the functions of Session Announcements is to avoid conflicts in the use of multicast addresses; this avoidance is impractical if the whole announcement is encrypted; if the multicast address and time is sent in the Clear, too much information is released. The first concern could be addressed by using Session Announcement cache proxies; the second by separating out the address allocation functionality from the rest of the announcement mechanism. Both questions are still being discussed in the IETF. In the meantime, the standardisation of encrypted session announcements is being hampered; some of the mechanisms considered here have not been ratified by the IETF.

    3. Encryption of Media Streams
    4. As mentioned in Section 6.2.1, if the media or shared application tools send their data in the clear, then it is easy for anybody knowing the time and multicast address of a session to participate. Moreover, since the RTCP responses could be suppressed, the participation may be unnoticed by the other participants. For this reason, private conferences will need to have the data streams encrypted. Because of the processing load arising from encryption, it is customary to always encrypt the streams with symmetric algorithms. Typically one uses DES [des], though it would be possible to use triple-DES [tdes] or IDEA [lai92] or any other such algorithm if desired. In view of the current move in the IETF to abandon DES, because of the ease of cracking it, we will presumably move over to triple-DES or IDEA here too - though this is a pragmatic detail, not something of architectural significance.

      A subset of the tools used in MECCANO have had the capability for encryption added; the subset is large enough that most of functionality for encrypted conferencing is available. These tools are currently VIC, RAT, VAT, WB, WBD and NTE.

    5. Encrypted and Authenticated Session Descriptions
    6. As mentioned in Section 6.2.1, the announcement and invitation to conference sessions is critically dependent on the passing the Session Description (SDP) [han98] to the authorised invitees. This information can be passed by many technologies - both in-band and out-of-band.

      There are three main reasons for providing authentication in announcements and invitations. One is that if one intends to provide billing depending on the announcement itself, then some form of authentication is essential. The second is that one may wish to be sure that the conference has indeed been called by someone who is authorised to do so. A third is that there are also mechanisms for modifying Session Announcements; a simple Denial-of-Service attack is to modify the announced time or location with unauthenticated announcements.

      It is essential that any information about the Session Description encryption be passed securely between the persons authorised to participate.

      1. Authentication and Key Distribution
      2. Key distribution is closely tied to authentication. Conference or session Description 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 set-up mechanism). However the security 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 [x509] are, however, not specific to conferencing, and in the conferencing portions of the IETF (the MMUSIC group), a strong preference for using PGP certificates [pgp] has been expressed.

        Session keys can be distributed using encrypted session descriptions carried in SIP session invitations, in encrypted session announcements, or stored in secured depositories with access control. None 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 most usage 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. These considerations are orthogonal to the announcement of sessions; they are relevant, however, to the mechanisms adopted on joining sessions.

      3. Encrypted Session Announcements using only Symmetric Encryption
      4. Private sessions can be announced in many ways; and we will be using several in the MECCANO project. All are based on using encrypted sessions, and providing the Session Description (SD), complete with its Session Encryption Key(s) (SEK), in a secure way to all authorised participants. While each media stream may use a different SEK, it is important that the same SD can be used irrespective of the manner it is transferred. This allows the facility that launches the encrypted media tools to be oblivious of how the SD came to the recipient.

        It is fundamental to our architecture that anybody participating in secure conferencing must have a Public Key Certificate. This is used to send some shared secret between the conference organiser and the conferees. For small-scale, ad-hoc conferences, it is possible merely to send the SD to all the conferees with PGP or S-MIME mail, merely encrypting the SD with the Public Key of each recipient. An alternative is to use SIP [sip], with its security features, to invite specific people to a conference.

        A third way of distributing the Session Directories securely, is to put them in a depository like a web page or an X.500 Directory. There can be access control to ensure that only persons authorised to participate can retrieve the Session Description. The access control problem then reduces to the management of the access control lists in the relevant depository. If the depository is a secured Web Server, for example, one can ensure that access is only possible in an encrypted and authenticated session; this ensures that eavesdroppers could not tap into the SD information while it is being retrieved,

        A fourth way of distributing the Session Directories securely is to encrypt them with a symmetric encryption mechanism such as DES [des], Triple DES [tdes] or IDEA [lai92], using a Session Announcement Encryption Key (SAEK), and to send them out in the encrypted form by SAP. There are then at least two mechanisms that can be used with SAP. One is distribute previously a number of symmetric SAEKs in advance to the authorised participants, for instance by secure e-mail. These keys could be distributed with an index number. The SAP announcement could then carry that index number and the relevant key could be used to decrypt the SAP announcement. Alternately, one could choose not to use an index number; the recipients would then need to try to decrypt each incoming session announcement using one after another of the SAEKs in his/her cache. This need be done only the first time the encrypted announcement is transmitted, since there is a unique hash associated with each announcement, which tells the recipient if they have seen the announcement before.

      5. Encrypted Session Announcements using Public Key Systems
      6. Yet another way of sending encrypted announcements uses Public Key Cryptography (PKC) in an unusual manner. The SAEK can then be encrypted with one of a PKC key pair (say the Public one, and this message is pre-pended to the transmitted announcement. The Private one of the PKC key pair is distributed only the authorised participants; thus only they are able to decrypt the pre-pended message – and hence derive the SAEK. Even here there are several variants. In one both the private and the public key pair are distributed to each authorised participant. In this case any one of them can announce private conferences. Another variant maintains more of the spirit of PKC, in that it allows only one person to announce conferences; thus only the Public key is distributed. This mechanism is particularly appropriate if one wishes to ensure that broadcast events are indeed announced only to authorised persons.

        The use of public-key or strong symmetric cryptography for this purpose has not yet been standardised, because of disagreements on which technology is most suitable. The standardisation of these mechanisms will clearly accelerate the use of secure conferencing or commercial broadcasting. The considerations include the frequency of change of the groups, the nature of the events to be announced, the amount of infrastructure one assumes amongst potential participants, and which is the easiest to implement - in view of the security toolkits now available. For this reason the IETF does not prescribe which method is to be used.

        All these mechanisms require that there be some mechanism for managing groups securely. Either shared secrets must be sent to groups, or access control lists must be maintained for groups. Specific infrastructures for group management are being studied in the ICECAR project. A companion architecture document for ICECAR [icesecarc], goes into further detail on this issue.

  13. Recording and Replay
    1. Introduction
    2. The ability to archive multimedia data from conferences and to introduce stored data into conferences are requirements in many multimedia conferencing applications.

      Recording a multimedia session enables anyone who could not originally participate to replay it and find out the content of the discussion or seminar. Additionally, a participant in an on-going conference may play back a pre-recorded clip in order to illustrate a point. Multimedia servers with multicast capabilities along with recording, playback and editing facilities must be an integral part of the emerging multimedia computing infrastructure.

      The five functions of recording, storage, editing, announcement and playback are quite separate. They may use quite different equipment and techniques - though there must be an integrating software system to ensure that the system is easily usable. It is both acceptable, and in the future probably normal, that there will be diverse systems for recording, with yet others for playback. There will often be economy of scale in the set-up of large play-back centres. Because of performance problems in communications subnets, it may not even be feasible to have all recording so centralised. This section describes some of the architectural considerations in such systems.

      The whole question of Recording and Playback of multimedia data is a large subject; its current usage dwarfs that of conferencing by several orders of magnitude. In this section we will consider only those aspects of the subject applicable directly to multicast, multimedia, conferencing.

    3. Recording Multimedia Conferences over the Mbone
      1. Recorder Requirements
      2. A multimedia conferencing session on the Mbone may consist of multi-way real-time audio and video, a presentation tool for figures/slides, and the so-called ‘shared workspace’ media (e.g. an interactive drawing tool and an interactive text editor). For the purpose of this section, it is important to consider the provision of both storage and retrieval of accurate representations of the conference. Because the multimedia conferencing that is the subject of this report is based on IP multicast, a source can send data to multiple receivers, without the need to address each receiver individually

        The requirements on a recorder are the following:

        • Recording from the Mbone The ability to record multicast data over the Mbone is vital. During a videoconference, the data can be transmitted based on a variety of protocols (e.g. SRM, RTP) which the record mechanism must be able to handle; it must be possible to record data from all conference participants unless the user prefers otherwise. It would be permissible to use recording caches.
        • Error Recovery If there are error recovery techniques used in the transmission (e.g. redundancy in the audio stream [redenc] as used in the ‘rat’ audio tool), the recorder, or recording caches, should utilise these prior to storage of the data.
        • Data Formatting It is best to store cache data in its original transmitted form, but the reconstructed data in a storage format, which may well be different from the transmission format. The storage format will normally include timing and stream information to allow faithful play-out of the stored data.
        • Flexible data storage The amount of data involved can be very large and thus require access to a large data repository. Thus, the recorder must have access to a large archive space where people can store data; this has the additional effect of keeping all recordings in a single location (which can be transparent to the users) making them readily accessible.

        • Confidential storage Notwithstanding the access control mentioned above, it may be desirable to store the data in encrypted form. Often the encryption algorithms used in transmission may be inappropriate for storage. In addition, we have already mentioned the need to correct and format data for storage. As a result, it will normally be desirable to make the recorder a trusted member of any encrypted conference. This allows the recorder to decrypt the received streams, apply the relevant data correction, re-format the data, and add the appropriate annotation. It is still possible to re-encrypt the data prior to storage, and to store the new encryption algorithm ID and key under a specially access-protected portion of the Server.
      3. The General Problem
      4. As stated in Section 4.1.1, applications transmitting continuous media data (i.e. real-time audio and video) use the RTP (real-time transport) protocol [schu96-2] on top of UDP. Use of RTP/RTCP does not guarantee the quality of service; neither is the delivery guaranteed nor may the delay be bounded. The interactive use of continuous media makes the application intolerant of long delays exceeding a few hundred milliseconds. A recording is not so intolerant; it can tolerate long delays - provided only that the time of the launching of the original packets is known. Some whiteboards also send their streams with RTP and may even permit synchronising them with the continuous media streams [bach]. The AOFwb described in Section 5 is an example of such a stream.

        Many multicast tools require reliable data delivery. One mechanism is to use a form of reliable multicast [floy95] (again on top of UDP) to guarantee that the data will eventually reach all participants. In some forms the sender is informed, and retransmits lost packets to individual receivers. In other forms of reliable multicast, recipients detecting loss send out a general request for repair; the nearest up-stream source retransmits the missing packets. Some tools are not real time; they ensure that data is sent eventually; examples include presentation material that is pre-sent to be cached locally. Other tools are near real-time, such as shared workspace. Here, both the speed of response and reliable delivery are required, but some speed of response is sacrificed for more reliable delivery. It is possible for the Recorder to wait rather longer than a real-time conference participant, but the problem of achieving an accurate record of data transactions is essentially the same.

      5. Recording Caches
      6. A record can be captured merely by making one or more recording workstations a member of the conference; each will then receive the data transmitted from each of the sources. If there is any loss in the network, recording workstations at different locations may gather different view of the conference. One way of improving the quality of recordings over real-time conference participants, is to provide recording caches at strategic parts of the network. These act as multiple recorders, and may participate in any of the error correction schemes used by real-time participants - e.g. FEC with some corrections by redundancy, or reliable multicast with error recovery from neighbours. In addition, the caches will number received packets, so each knows which ones are still missing.

        Any recorder may then augment its record by using a reliable mechanism (multicast or unicast) to obtain missing packets from other caches. This correction can occur during the conference, or in a subsequent repair phase. The advantage of this technique is that one can overcome losses, at least improve over temporary catastrophic performance losses in the network, or even recover from temporary network partition, at a later stage. There remains questions of how to optimise the number and location of such caches, and what mechanisms of repair to use - e.g. unicast or multicast methods. Further details on this technique are available in [lam99].

      7. Recording Encrypted Streams
      8. One important enhancement to multimedia conferencing is the use of data encryption mechanisms [kirstein99] to provide privacy. Only participants that are in possession of a valid decryption key can decipher the media streams. The complexity raised by security considerations is discussed in Section 9. If the encryption is applied to all the data, including sequence numbering, then the use of data recovery techniques may be impeded both during the production of any data caches, and in subsequent repair. If one deliberately limits the amount of packet header data that is encrypted, e.g. by keeping sequence numbers in the clear, then there will be some potential security compromise. The seriousness, and mechanisms for limiting, such compromise must be evaluated in specific instances - related to the relevant security policies.

      9. Storage Formats versus Transmission Formats
      10. The formats used for data transmission were designed with specific considerations in mind. For example the RTP/RTCP format allows the receivers to tell when data packets are missing. This can be used in several ways. For example, if there is redundancy in the real-time data (e.g. the low quality audio from an earlier audio packet as used in RAT [redenc]), an attempt can be made to patch up the stream. At the same time an RTCP control packet, informing of the loss can be generated; the sender may use this information to reduce the rate of traffic transmission. For reliable multicast traffic, requests for retransmission can be generated by the receiver. If an up-stream unit re-transmits the data, a repaired stream will be generated; if not, it may be possible, or even necessary to discard some of the data.

        By using the properties of the tools, it will often be possible to reconstruct a reasonable replica of the information originally transmitted. If the transmitted form of data is stored in caches and later re-transmitted other errors may be corrected. If the data is stored permanently in exactly the same way as it was received in a single cache, it may be difficult to reconstruct later the data (because of changes in the tools, for example), and the characteristics of the re-transmitted data may not be optimal. For example, the later request for transmission of the stored data may be over a network with different error characteristics or speeds, so that different transmission formats may be optimal.

        Most of the current Mbone-based recorders just store the RTP packets; this eases the problems of storage and replay - but may have both performance problems, and lead to sub-optimal solutions. There are few Standards yet for the storage formats; most of the companies providing commercial servers (e.g. Oracle [ovs], Cisco [iptv], Microsoft [nshow], Real Networks [realn]) have their own formats. There has been little activity on standardising these - except the ASF format of Microsoft [asf].

    4. Multimedia players
    5. Even in the simplest multimedia conferencing scenario, a record/playback tool may prove to be useful. Some example uses of a Mbone recorder/player are given below. Based on its potential uses and the characteristics of existing media tools, the requirements that a Mbone recorder/player must fulfil are then determined.

      1. Example applications
        1. Introducing a pre-recorded clip in an on-going conference in order to emphasise a point (Examples are pieces of operations in medical training, foreign scenes for language teaching, and the results of flight simulations for aerospace conferences).
        2. Replay of conferences recorded previously (Examples are because of an inconvenient time for people who missed, a permanent record of important seminars, and the complete background for a later conference where some event is being reviewed).
      2. Player requirements
        • Playing to the Mbone or a single user The playback mechanism must be capable of playing back the streams (providing the option to play all or just some of them) from an archived conference. This playback can be directly to the user requesting it (i.e. unicast) or to another multicast group on the Mbone.
        • Random access It is deemed necessary that ‘random access’, ‘fast forward’ and ‘rewind’ facilities be provided in the player. These are needed both for the media streams and for the reliable data.

        • Data access control The player must be able to require strong authentication to allow access to particular recordings. This requires the ability to handle encrypted data. Several mechanisms can be used for keeping data confidential. Clearly there should be access control with Access Control Lists and/or password control on access (with strong authentication to preclude replay attacks).

        • Data formatting The player must be able to reformat the steams in a format appropriate to the re-transmission medium. This may be either unicast or multicast, and use the appropriate packaging like RTP, Reliable Scaleable multicast, etc. Additionally, the facilities to play back media streams other than those of pre-recorded conferences (e.g. digitised MPEG [mpeg] or AVI [avi] streams) must also be provided.
        • Data browsing Most recording engines will quickly store a sizeable collection of recordings. There must be mechanisms for cataloguing the data. There must also be mehanisms to allow authorised parties to browse the catalogue, and to start up particular playbacks to specific multicast groups.

        • Remote Control The system should allow users to interact with the record/playback mechanism remotely; a locally controlled system is too inflexible. The remote control mechanism should allow interactive set-up and control of recordings and playbacks.
        • Navigation and editing The system should provide editing facilities to enable users to create their own material from already archived streams.

        • Annotation tracks For many purposes it is important to be able to navigate through recordings - e.g. for editing. An annotation track can be a very useful adjunct to aid such navigation and editing.
        • Large-scale access For some purposes, it may be desirable to have extremely large and highly available libraries of recordings.
      3. Subsequent Data Retrieval
      4. It is relatively easy to introduce the data from a multimedia recording of a whole conference. It is necessary only to make the recording workstation a member of the multicast conference, and it may distribute specific components of the stored data to all the conferees. During the retrieval phase, the relaxation on latency times no longer apply. The impact of network performance on the quality of data perceived by subsequent conference participants is the same for recorded as for real-time data. However the mechanisms used at the replay stage may be optimised to reflect the network topology and performance at that time rather than at the recording time. Thus, for example, the recorded data may be stored in a way that has removed redundancy; further redundancy may be added during the subsequent retrieval stage in a way that is optimised for the retrieval network and workstations.

    6. Multimedia Recording and Playback systems
    7. Several multimedia server systems have been developed previously. Some systems have the Mbone as their application area. Their facilities range from simple command line tools handling a single stream, to systems for recording and playback of a whole multimedia session. Others aim to work over the Internet or high speed intranets.

      1. Generic Systems
      2. Irrespective of their application area, all systems have a generally common architecture. The generic schematic of a Recorder/Player system is shown in Fig. 23.

        Figure 23 The generic architecture of a multimedia server system

        The different systems have different characteristics with respect to Fig. 23. Many of the commercial Video-on-Demand systems such as IP/TV [iptv] from Cisco, Real Networks [realn] and Net-Show [nshow] from Microsoft, and the Oracle Video Server [ovs], concentrate only on the Player section. They usually have proprietary data formats in the data archive. In most cases they play through an Intranet, without multicast, to single players. In some the Client and Server are based on WWW clients and servers, with specific plug-ins for the media tools. These can operate over multicast only with a multicast proxy, since HTTP is defined only for unicast. However, many of these systems are designed for high performance, multiple sessions. They pay great attention to optimal use of disc storage - often by striping the data across the discs in the archive.

        For the rest of this paper we will consider only those recorder/players which work with multicast over the Mbone. These normally use the generic architecture of Fig. 23. At present all of them use the available Mbone conferencing tools in the client role - though there are some variants.

      3. Standards for Recorders and Players
      4. There are considerable variations on the client/server dialogue. While there is an increasing tendency to have this modelled on the WWW, this is not universal. Many of the current systems have ad hoc control of the Server from the client, but there has now been a specification of the Real-Time Streaming Protocol (RTSP) to specify this control. RTSP has the more general aim of controlling all classes of recorders and players, so that it is more general than may be needed for this application. Many of the players and recorders currently being developed use RTSP. There is insufficient experience whether enough common features are used that the clients from one system can interwork with the Servers from another from the control viewpoint.

        Many of the server systems allow the clients to browse the archives using a WWW interface. Again, the way that the information on stored sessions is shown on the client is still somewhat different in the various implementations. This difference is not important for interoperability.

        Some of the existing standards used during multimedia conferencing (as described in Section 3.1) can be used during recording and playback of sessions. The information in a Session Description is important for setting up the recorder and in any case may need to be stored in with the recorded streams. The SDP information used for play-back may be different from that used on recording. For example different redundancy schemes may be used on play-back, and there may even have been different encoding between the scheme used for transmission prior to recording, and that used for playback. At present, many of the VoD players do not use SDP, but most of those used in conferencing systems do.

        There are implementations of SAP (e.g. sdr [sdr]) which allow for a Record option; this could start a specific recorder when the conference is started. Additionally, the Session Invitation Protocol can be used to invite a player to an on-going session. It is therefore possible to arrange for the introduction of recorded clips into the conference. SIP has not yet been used to invite players but we expect this to happen shortly.

        The recorders may operate in several ways. The simplest is to store the media packets as they arrive, usually together with time-stamps. This is a possible approach; in fact many of the current implementations use it. However, as discussed in Section 10.2.1, it is often better to store the media in a more suitable storage format. This requires an equivalent of the original media tool to be incorporated with the Recorder. In that case, it is also possible for the recorder to undertake the stream repair, so that the information stored on the Server has the same quality as that in real-time players located at the same site. The information from the SDP relevant to the conference is used to set the correct parameters in the media tools co-located with the Recorder.

        The players must operate in an analogous way to the recorder. If the native media packets are stored, then the player can play these back - using the stored time-stamps to pace the output. If QoS parameters can be set in the network, these can be used in the play-out. If a different storage format has been used for the media streams, then the player must also incorporate format converters and media tools.

    8. Existing Multimedia Recorders and Players using Multicast without Caches.
    9. The UCL development of MMCR [lam99] is typical of such recorder/players. MMCR is a system specifically designed for recording and playing back multicast multimedia conferences over the Mbone. It has a client - server architecture (as shown in Fig. 23) and consists of the client User Interface and the server (which incorporates the playback, recording and browsing mechanisms); logical component independence simplifies development and component replication.



      Figure 24 The overall architecture of the MMCR


      All server components have access to the database archive (see Fig. 24) to store/retrieve recordings and information about them. Much research has considered ways of providing efficient storage/access mechanisms for Video On Demand (VOD) systems, which require high bandwidth delivery. However, the simple disk model used here is adequate considering the current bandwidth limitations on the Mbone and a Redundant Array of Independent Disks (RAID) can be integrated in the system as an enhancement, if necessary.

      1. The Server
      2. The server acts as the single point of contact for recording, browsing and playback. Most of the existing implementations have a similar architecture. They consist of independent components; a server manager, the player, the recorder and the browser; some of these components may be missing in specific implementations.

        The server manager controls the whole service; it handles the establishment of connections with the clients. It has a separate, independent interface for each task and more interfaces can be added when required (e.g. an editing interface). Depending upon the type of service requested by the client, the server manager starts one of the recording, browsing or playback mechanisms. Once the mechanism required has started, the remote client communicates directly with that mechanism. Each mechanism has its own text-based control protocol.

      3. The Recorder
      4. To record the media streams the recorder need not be an active part of the conference; it ‘listens’ to the specified multicast groups and collects the data. Each stream is stored separately. In the case of RTP media, the RTCP messages transmitted are stored along with the data packets.

        Information about each recorded media (e.g. type, name) and each source (e.g. data location) is saved in header files. This information is either provided by the user or it is included in the Session Description of the conference. It is then possible to catalogue and index the descriptions for subsequent retrieval. This indexing may be in text form; some current research projects [sahouria] are attempting to use non-textual forms of indexing to allow more sophisticated retrieval.

      5. The Browser
      6. A listing of conferences a server has stored in its archive can be obtained through the browsing mechanism. A title keyword search facility is also available to help identify titles of interest.

        Further details about a particular conference can also be obtained to assist a user in deciding which conference to play back. These details include the conference’s title and description and the media that constitute the session. Additional information on each media includes the data type (i.e. RTP, wb etc) and the names of the users (where available) to help users select only the required data streams.

      7. The Player
      8. The real advantage of storing data on a per source basis is that users can playback only the streams they are actually interested in - ignoring the rest. This allows utilisation of all kinds of networks as users with bandwidth limitations may choose to play a subset of the available streams (e.g. just audio that requires much less bandwidth than video).

        The player schedules real-time packet transmission based on the timestamp in the index entry. RTP compliant media provide additional information in the RTP header that can be used for providing smoother playback (see Section 10.2). Other media (e.g. shared workspace) packets are sent on the network based on their received timestamp (i.e. with the same inter-packet gap as they originally arrived).

        The different media characteristics affect the fast forward and rewind operations. Audio and video are continuous media and therefore moving to a random point in the stream simply involves skipping intermediate parts and restarting at the new position. Additionally, the RTP header of the packets must be modified to maintain the continuity in timestamps and sequence numbers. For non-continuous media, such as shared workspace (wb/nte) fast-forward should involve the transmission of intermediate parts so that the data set is complete.

      9. The ACC Video Server
      10. The ACC Scalable Video Distributed Architecture (SVDA) is another player/browser/recorder system, which illustrates the mechanisms that can be provided for large video stores. It is shown schematically in Fig.25.

        This system is designed to be able to process a significant number of simultaneous requests from a large library of multimedia data. At the highest level of abstraction, SVDA can be considered as consisting of three parts:

        • The main server, responsible for serving user requests and managing the multimedia clips,
        • VFS (Video File Server) proxies, covering video servers,
        • AS (Archive Server) proxies, covering storage devices.


        Figure 25 Architecture of SVDA

        The system functionality for playing, browsing or recording multimedia streams is gathered together in one module, called the ‘main server’. It is the only part of the system that is visible to a client, actually. The main server:

        • makes available to a client a large storage space for saving video clips,
        • facilitates browsing by establishing a file system with hierarchical directory structure,
        • provides access control mechanisms such as ownership of multimedia data or access rights to it and, finally,
        • allows the delivery of the saved data to the specified destination point.

        Other modules mentioned above, i.e. the VFS and AS proxies, are relevant to another feature of SVDA system — the ‘virtual file system’. This virtual file system’ property distinguishes SVDA from other systems. The ‘virtual file system’ property means that the system offers clients a transparent, consistent view of a large storage space - possibly spread across many different file systems, without almost their being aware of their existence. The system assumptions are that the disk space of video servers is not sufficient to store all multimedia data, and that only tertiary storage device is large enough to keep them all. Such storage is not usually designed for real-time replay of real-time data. Thus when a user wants to play a clip, he normally makes a reservation stating the time at which the clip should be ready to play (i.e. copied onto a video server), and the system will do everything possible to actually copy it there.

        By introducing new objects, the VFS and AS proxies, and employing CORBA technology [cor], the system can be expanded easily by adding new video and archive servers. To enlarge the system, a programmer has merely to implement a well-defined CORBA interface. Provided another storage controller also supports CORBA, it is easy to substitute one specialised mass storage device with another. Thus SVDA is almost independent of hardware solutions. Moreover, in line with the CORBA concept of component reusability, any CORBA-compliant system can simply take advantage of proxies once they have been created. These considerations apply only to the data flow control; nothing has been stated yet about the multimedia data transport, and SVDA does not constrain the protocols that can be used for data transmission.

        The system uses CORBA technology only for controlling data flow; it leaves multimedia data transmission to the video server. Thus the way the data is sent over the network depend only on capabilities of the video server used by the system. If the server was able to provide data only with raw UDP datagrams, the system would be limited to this protocol. But if the video server is able to send data using RTP or multicast, the system seems to be more useful. The current implementation that uses Sun MediaCenter server can take advantage of multicast.

        An example of a user session and consecutive data flow is presented in Fig. 26.



        Figure 26 Example of user session with SVDA


        At first sight, it seems that this system addresses a very different set of applications from the rest of this section or indeed of this report. This is not, however, the case. Although the system control is essentially unicast client-server, so is SIP, H.323 etc., multicast is part of the data distribution. The present implementation of the SVDA server is unicast, because in real conferences media clips are always initiated by one person — so the client-server model is appropriate.

        There are other detailed differences, but these are not architecturally significant. The client-server control is via CORBA; this contrasts with the RTSP used for this purpose in the relevant IETF protocols, but has similar, though less targeted, functionality. The current browsing of the file structure and the request for, and launching of, media clips is somewhat different from the normal Mbone conference practice. But taking into consideration that SVDA is a VoD tool rather than video conferencing one, such solution seems to be suitable.

  14. Summary
  15. 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. It discusses also the different aspects that are being pursued in the MECCANO project.

    The lightweight session's 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 participant only adds state close to him/her 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 are still many parts of this architecture that are incomplete, being 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 standardisation 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 approach described in this report is not a complete answer since it does not use the IPSEC technology which most believe will be a key part of the future security infrastructure; it is the best that can be deployed now in the absence of large-scale IPSEC deployment.

    The report also gives some of the architectural considerations behind the gateways being deployed, the networks supported and some of the other technologies provided. The key feature of all of these is that they are capable of working in the Internet multicast environment.

    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.

  16. Acknowledgements
  17. Sections 2, 3, 4 and 6 of this Deliverable are based strongly on [han99-3], and the contributions of Mark Handley and Jon Crowcroft are gratefully acknowledged. In addition we acknowledge the many other members of the MECCANO project who have contributed to the document in addition to those acknowledged as authors. Finally, we acknowledge the very helpful comments from Peter Parnes, who acted as peer reviewer; these comments caused significant improvements in the deliverable.

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    Amir, E et al., "An Active Service Framework and its Application to Real-time Multimedia Transcoding", ACM SIGCOMM’98, August 1998, Vancouver, Canada.

    Advanced Streaming Format (ASF) is a streaming multimedia file format developed by Microsoft.

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    MarraTech AB’s MarratechPro desktop multimedia conferencing software (video, audio, slides, chat and whiteboard).

    The MASH research group at the University of California, Berkeley

    Multimedia Assisted distributed Tele-Engineering Services (MATES), ESPRIT 20598 project, Information Technology in the 4th Framework Program of the European Community.

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    Multimedia European Research Conferencing Integration (MERCI)

    Multimedia Integrated Conferencing for Europe (MICE)

    Moving Pictures Experts Group (MPEG), officially ISO/IEC JTC1 SC29 WG11. See

    Patrick, A, "User-Centred an Mbone Videoconference Polling Tool", March 11, 1998.
    MPoll application at

    Microsoft Corporation, "NetMeeting",

    Windows NT Server NetShow Services
    NetShow player with Internet Explorer, Windows 95, and Windows NT

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    Perkins, C et al., "RTP Payload for Redundant Audio Data", IETF RFC 2198, September 1997

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    Braden, R, Editor, "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification", IETF RFC 2205, September 1997.

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    Shenker, S et al., "Specification of Guaranteed Quality of Service", IETF RFC 2212, September 1997.

    Payload Formats RFC 2032, 2035, etc. for specific codecs.

    Sisalem, D & Casner, S, "RTP translator between unicast and multicast networks; also translates between VAT and RTP formats",

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    Schulzrinne, H et al., "RTP: A Transport Protocol for Real-Time Applications", IETF RFC 1889, January 1996.

    Schulzrinne, H, "RTP Profile for Audio and Video Conferences with Minimal Control", IETF RFC 1890, January 1996.

    Session Directory (SDR),

    Sun Microsystems, "SunForum 2.0 Workgroup Collaboration Tools",

    ITU Recommendation T.120, February 1996

    American National Standards Institute, "Triple Data Encryption Algorithm Modes of Operation," ANSI X9.52-1998, 1998.

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    See also

    Thaler, D et al., "Border Gateway Multicast Protocol (BGMP):Protocol Specification", Internet draft <draft-ietf-idmr-gum-04.txt>, November 1998.

    UniDirectional Link Routing home page,

    Furht, B, et al., "Motion Estimation Algorithms for Video Compression", ISBN 0-7923-9793-2, Kluwer Academic Publishers, 1997.
    Sullivan, GJ & Wiegand, T, "Rate-Distortion Optimization for Video Compression", in IEEE Signal Processing Magazine, November 1998.
    Bhaskaran, V & Konstantinides, K, "Image and Video Compression Standards: Algorithms and Architectures" ISBN 0-7923-9591-3, Kluwer Academic Publishers, 1995.
    Ghanberi, M, "Video coding: an introduction to standard codecs", IEE Telecommunications Series no. 42, the Institute of Electrical Engineering (IEE), UK, 1999.

    AT&T Laboratories, Cambridge, "Virtual Network Computing (VNC)",

    Wolf, H et al., "MaX (formerly QuiX) – Macintosh Quickdraw to X Window Converter",

    CCITT (Consultative Committee on International Telegraphy and Telephony). "Recommendation X.509: The Directory - Authentication Framework", 1988.