Abstract:
A multicast scalable video communication server (MSVCS) is disposed in a multi-endpoint video conferencing system having multicast capabilities and in which audiovisual signals are scalably coded. The MVCVS additionally has unicast links to endpoints. The MSVCS caches audiovisual signal data received from endpoints over multicast communication channels, and retransmits the data over either unicast or multicast communication channels to an endpoint that requests the cached data.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional patent application Ser. No. 60/827,469, filed Sep. 29, 2006. Further, this application is related to International patent application Nos. PCT/US06/28365, PCT/US06/028366, PCT/US06/061815, PCT/US06/62569, PCT/US07/062357, PCT/US07/65554, PCT/US07/065003, PCT/US06/028367, and PCT/US07/63335. All of the aforementioned applications, which are commonly assigned, are hereby incorporated by reference herein in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to multimedia data communication systems. In particular, the invention, relates to multipoint video conferencing functionality enabled by using network supported multicast and scalable video conferencing servers together. 
     BACKGROUND OF THE INVENTION 
     In a multipoint video conference, it is desirable that compressed video data generated by each participant should be made available to every other participant who wants to receive it. Over a unicast network, this requires repeated transmissions of the same data to several participants/endpoints, either by each endpoint source or by a multipoint control unit (MCU). An MCU receives data generated by all the participants/endpoints and sends the data, usually as a mix, to the other participants/endpoints who want to receive it. Clearly, the bandwidth requirements for a video conference on the unicast network system increases in proportion to the number of conference participants. 
     When a network-supported multicast is available for a video conference, each endpoint can send its video data only once into a multicast group. Other endpoints that want to receive the data can join the multicast group. The network can establish optimized distribution trees to transport the multicast video data to the members of the multicast group using well-established techniques (e.g., the DVMRP protocol as described in RFC 1075). 
     In a receiver-driven layered multicast technique (see e.g., Receiver-driven layered multicast, Steven McCanne, Van Jacobson, Martin Vetterli, ACM SIGCOMM Computer Communication Review, Volume 26, Issue 4, Pages 117-130, October 1996, ISBN:0-89791-790-1), an MCU is not required and video data are encoded using a layered or scalable coding technique where each additional layer received increases the quality of the received video. Each layer is then sent to a separate multicast group by each endpoint, allowing each receiver to choose the bandwidth and the reception quality it receives from other participants by selecting the particular multicast groups it joins. 
     Although use of multicast techniques can result in efficient use of the network bandwidth for multipoint conferencing, a conferencing system architecture that depends solely on network supported multicast, i.e., without a specialized MCU, has several shortcomings:
         1. Network supported multicast is not available on the global Internet. Thus, a purely multicast-based solution can not be used for global multipoint conferencing.   2. When multicast groups are managed locally on different networks that are not multicast connected to each other, a mapping between these multicast groups must be established.   3. Multicast group address management for two or more simultaneous conferences must be carried out jointly to eliminate potential address conflicts.   4. When packet data is lost by a receiving endpoint, re-multicasting the lost data from the source to the entire group of participants/endpoints is not efficient because other member endpoints will receive redundant information.   5. When a new participant joins a multicast group, the compressed video may not be decodable for the new participant because predictive encoding may be used for the compressed video.       

     Therefore, for efficient multipoint videoconferencing over partially or fully multicast supported networks, consideration is now being given to the design of a new multicast and scalable coding aware MCU or server. 
     SUMMARY OF THE INVENTION 
     The present invention provides systems and methods for performing multipoint videoconferencing that uses scalable digital video coding as well as multicast transmission capability of the underlying communication network. The combination of scalability and multicasting operates synergistically to overcome several limitations of current videoconferencing systems. 
     Designs for a new multicast and scalable coding aware MCU or server (hereinafter “Multicast Aware Scalable Video Coding Server (MSVCS) are provided. 
     The inventive methods may include localized unicast transmissions. The inventive methods exploit multicast transmissions for efficient use of bandwidth in the network, and at the same time exploit the use of localized unicast transmissions to minimize the bandwidth overhead associated with ancillary operations such as error recovery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, the nature, and various advantages of the invention will be more apparent from the following detailed description of the preferred embodiments and the accompanying drawings in which: 
         FIGS. 1   a - 1   d  are schematic illustrations of exemplary system configurations for combining the SVCS architecture with multicast, in accordance with the principles of the present invention; 
         FIG. 2  is a schematic illustration of exemplary MSVCS operation on a multicast-enabled enterprise network, in accordance with the principles of the present invention; 
         FIG. 3  is a schematic illustration of exemplary MSVCS operation on a multicast-enabled enterprise network with multiple locations, in accordance with the principles of the present invention; 
         FIG. 4  is a schematic illustration of exemplary MSVCS operation on a multi-site enterprise network with local multicast, in accordance with the principles of the present invention; 
         FIG. 5  is a schematic illustration of exemplary MSVCS operation on a carrier-managed multicast network, in accordance with the principles of the present invention; 
         FIG. 6  is a schematic illustration of exemplary MSVCS operation on a multicast backbone with unicast enterprise networks, in accordance with the principles of the present invention; 
         FIG. 7  is a schematic illustration of exemplary multicast connectivity discovery processes; and 
         FIG. 8  is an exemplary connectivity table, in accordance with the principles of the present invention. 
     
    
    
     Throughout the figures the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the present invention will now be described in detail with reference to the Figures, it is done so in connection with the illustrative embodiments. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention combines the use of network-supported multicast functionality and the use of Scalable Video Conferencing Servers (SVCS) that enable video communications and multipoint conferencing based on Scalable Video Coding (SVC) techniques. 
     SVC is an encoding process in which a video stream is represented by a plurality of bitstreams that provide for the reconstruction of the original video stream at multiple spatial resolutions, frame rates, and picture qualities (SNR). The various reconstructions can be selected to account for differing CPU capabilities, display sizes, user preferences and bit rates in a computer network environment. SVC bitstreams are typically constructed in a pyramidal fashion, with a base layer stream and one or more enhancement layer streams. Decoding of an enhancement layer stream by itself cannot produce a faithful rendition of the original video stream. Instead, for proper decoding at a desired level of fidelity (e.g., in the spatial, temporal, or quality dimensions), access to at least the base layer stream and possibly other enhancement layer streams is necessary. Amendment 3 of the ITU-T H.264 International Standard, Annex G, provides an example of SVC. Further, International Patent Application. No. PCT/US06/28365 describes scalable video coding techniques specifically designed for video conferencing applications. Scalable coding techniques are also applicable to audio (see e.g., the ITU-T G.729.1 (G.729EV) standard). In the following, for convenience in description, a set of bitstreams, which collectively represent a given video source using SVC, may be referred to in the singular as an ‘SVC stream.’ It will be understood that an SVC stream will contain at least one bitstream (the base layer), and possibly one or more enhancement layer bitstreams. It will be further understood that, while the invention is described herein in the context of video transmissions, the inventive techniques are also applicable in the context of audio transmissions, which use scalable audio coding. 
     In a video conference, an SVC stream is transmitted from each endpoint to the SVCS. The transmission may be over two or more virtual or physical channels. Typically, but not necessarily, these different channels may offer different Quality of Service (QoS) levels. QoS support may be provided by the underlying communication network, or may be implemented at the application layer using well-known transport layer techniques (e.g., FEC, ARQ). Further, International Patent Application No. PCT/US06/061815 describes specific techniques for error-resilient transmission of scalable video, by which the lowest temporal level is guaranteed delivery via retransmissions without increasing the end-to-end delay of the system. 
     When different QoS levels are provided, assignment of SVC layers to different channels can take advantage of an increased reliability level by using it to transport the base layer and, if more bandwidth is available on that channel, also some of the enhancement layers. Assuming, without loss of generality, two virtual or physical channels are used, then a High Reliability Channel (HRC) includes basic picture quality information (base layer) and a Low Reliability Channel (LRC) includes enhancements to the picture (better quality, resolution or frame rate). The information is structured such that information loss on the LRC will result in only unsubstantial degradation of the picture quality. When no QoS is available from the underlying network or the application layer, assignment of SVC layers to different channels can be accomplished based on other considerations, as will be described hereinbelow. In such case, no particular channel is required to be identified as an HRC or LRC. In the following description, the notation ‘CN’, where N=0, 1, 2 . . . , indicates the various channels through which an SVC stream is transmitted or carried. The base layer of the coded video signal is always carried only on channel C 0 , which however may also carry other layers. When QoS is available, it will be further understood that C 0  corresponds to the HRC. 
     In a video conference on a network using an SVCS, upon receiving the SVC stream from each participant&#39;s endpoint, the SVCS selects and transmits the necessary parts of each scalable video stream to each endpoint. An SVCS may appear as an endpoint to another SVCS, allowing cascading of servers if needed. Detailed information on methods and systems for the design of SVCS and SVCS-based communication systems is available, for example, in International Patent Application Nos. PCT/US06/028366, PCT/US06/061815, PCT/US06/62569, PCT/US07/63335, PCT/US07/062,357, PCT/US07/65554, PCT/US07/065,003, and PCT/US06/028367. It is noted that in the SVCS system described in PCT/US06/028366, the endpoint-to-SVCS, SVCS-to-SVCS, and SVCS-to-endpoint communications are all realized using point-to-point unicast transmission. 
     The present invention includes mechanisms by which the multicast capability of the underlying network can be utilized in the design of improved videoconferencing and other video communication systems. The present invention also includes mechanisms for applying the techniques described in International Patent Application Nos. PCT/US07/63335, PCT/US07/062,357, PCT/US07/65554, PCT/US07/065,003, and PCT/US06/028367 directly in a multicast-enabled system. These mechanisms are described one after another herein. 
     With respect to the mechanism for utilizing network multicast capability, it is noted that network-supported multicast has been considered since the beginning of the internet protocol (IP) based networks. However, its deployment on the Internet has been recent. The multicast IP addresses play an important role in establishing multicast applications over IP networks. In IPv4, a special group of addresses (i.e., class D addresses) are reserved as multicast IP addresses. Any endpoint can send packets to any one of these addresses. Other endpoints that want to receive these packets must explicitly indicate to their (multicast-enabled) routers that they want to receive the packets that are sent to this particular multicast address (commonly referred to as “joining” a multicast group). Similarly, the endpoints must also indicate to their routers when the want to stop receiving packets sent to this particular multicast address (commonly referred to as “leaving” a multicast group). Such indication may be accomplished by means of standard protocols such as the Internet Group Membership Protocol (IGMP). 
     In the IP networks, there is no central control that is responsible for the allocation of specific multicast addresses to be used by endpoints at any given time over the Internet. Therefore, this allocation task must be handled by local network management or by the endpoints themselves. S. Pejhan, A. Eleftheriadis, and D. Anastassiou, in “Distributed Multicast Address Management in the Global Internet,” IEEE Journal on Selected Areas in Communications, Special Issue on the Global Internet, Vol. 13, Nr. 8, October 1995, pp. 1445-1456), provide a detailed analysis of the problem and also describe possible protocols for performing multicast address management. The present invention also includes techniques through which an MSVCS may “discover” the multicast structure of the network through which it is connected with its endpoints and other MSVCSs. These techniques are described herein with reference to  FIG. 7 . 
     At present, network supported multicast deployments are found in one of two forms: (1) a private enterprise multicast solution; and (2) a managed enterprise multicast solution, which is managed by an IP service provider. In either solution, an enterprise has multiple sites that are connected via either a unicast or multicast carrier backbone. In the private enterprise solution, multicast address allocations are made by the private enterprise network management. The private enterprise solutions are susceptible to multicast IP address conflict across several private (unmanaged) enterprise multicast networks. In contrast, in the managed enterprise multicast solution, the IP service provider manages the multicast address block and can provide a unique division of the multicast IP address space across subscribing multiple enterprise networks, thus avoiding the possibility of multicast IP address conflict. 
     The mechanisms of the present invention are described herein with reference to  FIGS. 1-7 .  FIG. 1  shows the operation of the MSVCS within the context of an intranet with full or partial multicast support. The other figures show extensions to cover the more complex cases associated with private or managed enterprise solutions. It is assumed that MSVCS has available to it a set of multicast addresses (either through administrative assignment, interaction with a multicast address management entity, or discovery). 
       FIG. 1(   a ) shows a simple SVCS system  100   a  in which two or more endpoints (e.g., endpoints E 1 -E 3 ) are connected to an SVCS  102  in a star configuration. (See e.g., PCT/US06/28365 and PCT/US06/028366). All endpoints E 1 -E 3  transmit their video and audio data to SVCS  102 , which in turn selects the appropriate information from each sender and forwards it to the intended recipients. In this configuration, the bit rate load on the SVCS is significant. With N participants each transmitting B Kbps, the total incoming rate is N-B Kbps. Further, assuming that every participant receives the entire streams from all other participants, the outgoing rate is N(N−1)B Kbps (i.e., a quadratic function of the number of participants N). 
       FIG. 1(   b ) shows an MSVCS system  100   b , which is similar to SVCS system  100   a  except in that SVCS  102  is replaced by Multicast aware Scalable Video Coding Server (MSVCS)  104 . In system  10   b , endpoints E 1 -E 3  communicate with each other directly using multicast. As shown in the figure, system  100   b  may be configured so that MSVCS  104  can also optionally receive the transmissions of the endpoints (‘listening’ on their multicast addresses) over listening path LP, with significant benefits for localizing error recovery as explained below. In the configuration of system  100   b  shown in  FIG. 1(   b ), the main flows of video and audio data do not pass through MSVCS  104 , and as a result the bit rate requirements on the server are minimized. Ignoring the listening path LP, there is no video or audio data flow on MSVCS  104 . If flows over the listening path LP are considered, then the load on the server is only the N·B Kbps in the receive direction. There is no outgoing data traffic (except as described below). Hence, the quadratic (on the number of participants) dependency of the outgoing bandwidth is eliminated. 
     In the operation of system  100   b , in which multicast transmission is used between the endpoints, each endpoint transmits its information to a set of multicast addresses that are particularly associated with it (i.e., the endpoint). MSVCS  104  informs each endpoint which multicast addresses to use through the unicast control paths CP that it establishes with each endpoint (see  FIG. 1(   b )). These same addresses are communicated to all other endpoints, so that if a receiving participant/endpoint desires reception from a particular transmitting endpoint, the receiving endpoint can subscribe or join (in IGMP terminology) the multicast group associated with the addresses of the transmitting endpoint. In order to add a particular SVC layer to its incoming video stream, the receiving endpoint simply joins the associated multicast group of the transmitting endpoint. Conversely, the receiving endpoint leaves the associated multicast group to drop the particular SVC layer from its incoming video stream. 
     It is possible to configure system  100   b  so that all endpoints E 1 -E 3  transmit to a single set of addresses (i.e., a single multicast group). A limitation of this configuration is that each receiving endpoint will now have to receive corresponding video (or audio) layer data from either all or none of the other participants. 
     Either the transmitting endpoint itself or MSVCS  104  can decide how various SVC layers in the transmissions of the endpoint should be assigned to individual multicast groups. For example, if MSVCS  104  knows the minimum bit rate available to each endpoint, it can instruct endpoints to include in their C 0  channel a sufficient number of temporal or quality layers so that the channel is fully utilized but not overloaded. Conversely, MSVCS  104  can instruct an endpoint to create an additional lower quality layer (e.g., with a lower frame rate), so that at least the base layer can be transmitted to all endpoints through C 0 . 
     It is noted that in some SVC system implementations, it is also possible to achieve points in quality that are in between the layers that are explicitly coded in the video bitstreams. (See e.g., International Patent Application Nos. PCT/US06/061815 and PCT/US07/63335). By receiving a subset of the enhancement layers and performing appropriate concealment, it is possible to obtain a ‘fractional’ quality level (from a PSNR point of view) that is between that of the quality of the base layer alone, or the base layer with the full enhancement layers. International Patent application No. PCT/US07/63335 describes an example of the transmission of L 0 , L 1 , L 2 , and S 0  frames, with concealment-based recovery of the missing S 1  and S 0  frames. In a system  102   b , which is configured for making use of such concealment-based recovery, MSVCS  104  can instruct an endpoint (e.g., through its unicast control channel CP) to create an additional “fractional” or intermediate quality multicast group to which it (the endpoint) will transmit only the appropriate partial portions of the enhancement layer(s). An endpoint that wishes to receive this intermediate quality can subscribe to the base layer channel and this partial enhancement multicast group. In such a system  100   b , there will be a bit rate overhead if the full enhancement layer is simultaneously transmitted in its own multicast group, but such overhead will be small. This bit rate overhead may be eliminated by transmitting instead of the full enhancement layer only the difference between the full enhancement layer and the subset that is used to achieve the fractional quality to a separate multicast group. 
     A known drawback of multicast transmission systems is that if errors occur in transmission of a particular packet on a channel, a potential error-correcting or compensating retransmission from the source has to be transmitted again to all participants/endpoints, and not only to the endpoint(s) that did not receive the particular packet. While it is possible for the source endpoint to retransmit the missing packet using a unicast connection to a particular receiving endpoint that experienced the packet loss, in practice the distance (in network terms) between the source and the receiving endpoints can be significant. As a result, the retransmitted packet could arrive too late for display. The present invention includes a technique for error recovery services which overcomes this drawback. The technique involves delegating the responsibility of retransmission to the MSVCS instead of to the sending endpoints. For example, MSVCS  104  may be configured to listen to the transmissions of all endpoints, and cache all or some of the packets (e.g., ‘R’ packets, in the terminology of International Patent application No. PCT/US06/061815). When a receiving endpoint detects an error, it may then instruct MSVCS  104  to retransmit the missing/error packet over the receiving endpoint&#39;s unicast link to the MSVCS. The MSVCS then transmits the packet through the unicast link. MSVCS  104  does not normally transmit any outgoing video or audio data traffic, but instead only transmits when errors occur. Furthermore, the technique relieves the endpoints of the task of performing caching and retransmission. 
     This technique for error recovery services has excellent error localization properties, since MSVCS  104  is typically much closer (in network terms) to the receiving endpoint that experienced the error than the transmitting endpoint. Further, this technique has a lower bandwidth demand than the conventional retransmission-by-endpoint techniques. As an illustrative example, assume that each endpoint in system  100   b  has a probability of packet loss p (0&lt;p&lt;1). Then the average outgoing rate from MSVCS  104  to provide error recovery services will be N·B/(1−p) Kbps, which is the expected value for a Bernoulli random process. Even for values of p as high as 20%, the average outgoing bit rate from MSVCS  104  will be only 25% of the incoming bit rate. In contrast, in a traditional star configuration (e.g., with an SVCS), if errors are present, this same outgoing bit rate would still have to be added to the total. 
     An alternative configuration of system  100   b  for implementing a variation of this error-recovery scheme is shown in  FIG. 1(   c ) as system  100   c . In system  100   c , the transmissions from the endpoints are unicast to MSVCS  104 , followed by multicast transmission from MSVCS  104  to the endpoints. In this configuration, the total incoming bit rate to MSVCS  104  is identical to that in system  100   b  described above. The outgoing bit rate is identical to the incoming bit rate (ignoring possible retransmissions), and is still much lower than that of a star configuration ( FIG. 1   a ) if the number of participants is greater than 2. 
     The operation of MSVCS  104  in system  100   c  is similar to SVCS  102  operation in system  100   a . In system  100   c , MSVCS  104  itself establishes a set of multicast groups through which the various SVC layers of the incoming video (and audio) will be transmitted. The similarity of operation of MSVCS  104  in the two systems is apparent upon considering each group of endpoints that join a particular multicast group as an ‘aggregate’ endpoint equivalent to the traditional single endpoints of an SVCS system (e.g., system  10   a ). MSVCS  104  in system  100   c , like in system  10   b , can perform rate matching by ensuring that the C 0  group meets the minimum bit rate requirements of all recipients. Whereas an SVCS (e.g. SVCS  102 ) performs rate matching for each individual endpoint, the MSVCS performs rate matching for the ‘aggregate’ endpoint. An additional benefit of the system  100   c  configuration is that the MSVCS  104  can more easily perform statistical multiplexing, since all streams are combined at the MSVCS prior to their transmission to the endpoints. Further, like system  100   b , ‘fractional quality’ multicast groups can be established in system  100   c  for improved rate matching and rate control ability, with similarly low bandwidth overhead as described above. 
     For example, assume that in system  100   c  endpoint E 1  transmits to endpoint E 3 . The number of layers of endpoint E 1  included in each multicast channel from MSVCS  104  will change depending on the available bit rate to E 1 -E 3 . The endpoints E 1 -E 3 , in other words, become an ‘aggregate user,’ which is treated by MSVCS  104  the same way that an individual endpoint would be treated by an SVCS. As in system  100   b , MSVCS  104  can cache incoming packets and make them available for retransmission if errors are detected by the receiving endpoints. These retransmissions can occur on unicast channels between MSVCS  104  and the endpoints so that the bandwidth overhead associated with retransmissions is completely localized. Further, more complicated SVCS-based techniques for error resilience and rate control can be applied in system  100   c  in the same manner as they are applied in system  100   b.    
     In general, all the error resilience, compositing, thinning and other techniques that are described in International Patent applications PCT/US06/061815, PCT/US07/63335, PCT/US07/65554, and PCT/US07/062,357 can be directly applied in the operation of the MSVCS. The only difference is that the MSVCS cases require consideration as to whether data transmission should be on the multicast channel or unicast. Advantageously, error recovery measures that address losses in a particular endpoint should be transmitted unicast. In a Compositing SVCS (see e.g., PCT/US06/62569), the transmission of the composed stream can be multicast. Similarly, in an SVCS that performs thinning (PCT/US07/062,357), a choice of multicast or unicast transmission of the thinned stream can be decided upon based on whether all receiving endpoints or only one endpoint has the appropriate channel for reception (high quality, with no or very small packet loss). In either case, the non-thinned stream can simultaneously exist alongside the thinned stream, in a separate multicast group. When transcoding is used at the SVCS (see e.g., PCT/US07/65554), it is typically done to accommodate a non-SVC endpoint (e.g., an H.264 AVC endpoint), and hence unicast transmission of the transcoded stream would be appropriate. If two or more such legacy endpoints are present in a system and are receiving a particular participant/endpoint transmission, then a separate multicast group can be established at the MSVCS for such legacy endpoints alone. In such a system, only a server-driven multicast configuration will apply here, as legacy endpoints—by definition—are not multicast aware. 
     It is noted that the system server and endpoint configurations shown in  FIGS. 1   a - 1   c  can be merged or combined in any suitable manner.  FIG. 1(   d ) shows an exemplary system  100   d  in which endpoint E 1  uses unicast to communicate with the MSVCS  104  as in system  100   a , endpoints E 2  and E 3  transmit unicast and receive multicast from MSVCS  104  as in system  100   c , and endpoints E 4 -E 6  all lie in a multicast cloud in which the MSVCS listens as in system  100   b . In system  100   d , in order to bridge the various transmission types or endpoints, MSVCS  104  also transmits to the multicast cloud of E 4 -E 5  signals originating from E 1  and E 2 -E 3 . In all other respects, the operation of each subset of endpoints (E 1 , E 2 -E 3 , E 4 -E 5 ) in cooperation with MSVCS  104  is the same or similar to that in systems  100   a - c  as described above. Hereinafter, the terms “unicast,” “endpoint-driven multicast” and “server-driven multicast” may be used to refer to systems of the types (e.g., systems  100   a - 100   c ) shown in  FIGS. 1   a - 1   c , respectively. 
       FIGS. 2-7  show systems with more complicated MSVCS configurations that are likely to be encountered in enterprise environments where multiple SVCSs (both multicast aware and non-multicast aware) may be employed. 
     For example,  FIG. 2  shows a configuration  200  for a private or managed enterprise multicast solution  220 , in which MSVCS  210 , which is located at the enterprise intranet, is configured for the following services and features: 
     a) MSVCS  210  will use a set of multicast addresses, in either of the server-driven or endpoint-driven multicast configurations (or combinations thereof). In order to accomplish this, MSVCS  210  must be capable of cooperating with the intranet&#39;s existing IP multicast group address management system in order to properly reserve a multicast address for a conference session. The necessary communications between the address management system and MSVCS  210  can be carried out by means of site-specific protocols. Alternatively, MSVCS  210  can handle the multicast address allocations itself. 
     b) MSVCS  210  sends video data it receives from outside the enterprise network endpoints, SVCSs or other MSVCSs (e.g., SVCS  230 ) to the chosen multicast address to distribute the data internally via multicast. This operation is the same regardless of whether the originating MSVCS (e.g., SVCS  230 ) is performing server-driven or endpoint-driven multicast. In the latter case, the server MSVCS  210  is effectively bridging the two networks. 
     c. MSVCS  210  sends video data received from intranet clients to the external endpoints, SVCSs, or MSVCSs (e.g., SVCS  230 ). 
     d. MSVCS  210  on a multicast enabled network can act as a proxy in repairing lost packets by resending them to the endpoints or providing intraframes to start newly joined participants. (See e.g., error recovery services provided by MSVCS as described above). 
       FIG. 3  shows a configuration  300  for a private enterprise solution having multiple locations or sites  320  that are interconnected, e.g., via a virtual private network (VPN), and where the chosen multicast addresses run across the multiple sites in a tunneled fashion across the provider&#39;s backbone. In configuration  300 , implementation of multipoint conferencing requires only a single MSVCS  310  at one of the locations that is capable of sending and receiving data using that multicast address. 
       FIGS. 4 and 5  show configurations  400  and  500 , respectively, for managed enterprise solutions having multiple locations, which are not interconnected via a virtual private network (VPN), but are connected instead by a carrier&#39;s multicast-supported backbone network. In such cases, there will be a unique multicast address allocated by the carrier to each managed enterprise customer. The multicast addresses are reserved for multipoint video conferencing and multicasting within the enterprise&#39;s multiple sites. In such cases, implementation of multipoint conferencing will require either an MSVCS  410  located at each intranet site as shown in  FIG. 4  or an MSVCS  510  located at the carrier&#39;s backbone network as shown in  FIG. 5 . 
     In configuration  400 , the two MSVCS  410   s  at the different sites communicate with each other using unicast. The two MSVCS  410   s  can exchange configuration information, so that each of them can make appropriate decisions about rate control, statistical multiplexing, etc. Effectively, when converting data from multicast to unicast, MSVCS  410  acts as a proxy of the remote network and presents all the sources combined. MSVCS  410  also applies all intelligent rate control/shaping on the aggregate source by taking into account the link constraints. MSVCS  410  may perform error recovery functions on that link as well, in the same manner as is seen in standard SVCS procedures. 
     In configuration  500 , MSVCS  510  can orchestrate multipoint video conferencing sessions set up across many managed enterprises  520 , since each multicast conference session uses a unique set of multicast addresses across a managed enterprise&#39;s sites assigned by the carrier. If, as shown in  FIG. 5 , a multipoint video conference session is set up across several managed enterprise customers  520 , the carrier may use a separate multicast address block for the inter-enterprise conferences, in which case, having MSVCS  510  hosted by the carrier may be advantageous. 
     Lastly,  FIG. 6  shows configuration  600  for a scenario in which multicast is available in backbone network  620 , but all enterprise networks  630  have no multicast support available. In this configuration, MSVCS  610  disposed in carrier network  620  can communicate with other MSVCS  610  using multicast. In doing so, various MSVCS  610  can act as proxies for enterprise  630  SVCS that are directly connected to them. For example, MSVCS  610 ′ can multicast transmit the information it receives from SVCS  630 ′. MSVCS  610 ′ also informs peer MSVCS  610   s  about the requirements of the endpoints connected to its associated SVCS  630 ′ so that peer MSVCS  630   s  can make the correct decisions about which layers to transmit and to which multicast addresses. 
     In general, the MSVCS (or MSVCSs when there is more than one) should be positioned within the network in such a way so that a fast response to retransmission requests is possible to all participating endpoints. While unicast and multicast routes between two endpoints are not guaranteed to have significant common components, for a reasonably well-connected network with sufficient multicast router density, it is reasonable to assume that the computed multicast routes will traverse network points that offer a good balance (network distance) between the endpoints. It may therefore be advantageous if the MSVCS is co-hosted (i.e., operates on the same equipment), co-located, or very near (in network terms) to a multicast router. An additional benefit of such placement is that packets traveling from one endpoint to the others over a multicast communication channel do not have to be copied off the multicast router to reach the listening MSVCS across a secondary path that is not along the given or other multicast communication channel routes. 
     The configurations  200 - 600  shown in  FIGS. 2-6  assume that the MSVCSs cooperate with existing IP multicast address management systems. However, there may be cases in which such address management systems do not exist or such cooperation is not feasible or practical. For such cases, the present invention provides the MSVCS with a mechanism for multicast address and connectivity discovery. The MSVCS may be configured to fully or partially discover (1) the available multicast address(es), and (2) the multicast topology (i.e., which clients are in a multicast IP address group). 
     In a discovery procedure, the configured MSVCS chooses a set of multicast IP addresses and sends them over unicast connections to all clients willing to participate in a multipoint conference. Each client is also assigned as the sender for only one of the multicast addresses in the set. The clients, then, send test packages to their assigned multicast address while listening for test packages coming from other clients on the entire set of multicast addresses. Each client, then, reports to the MSVCS its reception record, i.e., the list of multicast addresses from which they received test packets. This test procedure enables the MSVCS to generate a multicast connectivity diagram. The test procedure may be executed at the time of conference session establishment, allowing MSVCS the determine which clients can use multicast with each other. 
     The operation of multicast connectivity discovery procedure can be understood with reference to system  700  shown in  FIG. 7 . System  700  includes a network  720  with unknown multicast support. For example, four endpoints Ep 1 -Ep 4  may want to participate in a multipoint conference over network  720 . Initially, MSVCS  710  is not aware of the multicast connectivity between these endpoints Ep 1 -Ep 4 . To start the discovery procedure, MSVCS  710  sends to each endpoint Ep 1 -Ep 4  a suitable set of multicast addresses that it chooses. The choice of the addresses may be such that there is no overlap with other conferences MSVCS  710  is handling. MSVCS  710  also assigns a single endpoint as the sender for each address. One more multicast address is added to allow the MSVCS to participate in the discovery process. Hence the total number of addresses, in this example, is five addresses (e.g., “A”, “B”, “C”, “D”, and “E”). Without loss of generality, it may be assumed that the first endpoint Ep 1  is chosen as the sender for the first multicast address A, the second endpoint Ep 2  for B, etc., and the fifth address E is used by MSVCS  710 . After receiving these address assignments, all endpoints and MSVCS  710  start to send predetermined test packets to their respective assigned address A, B, C, D, or E. Concurrently, all endpoints and MSVCS  710  listen to the other four addresses (i.e. they join and become members of the four multicast groups corresponding to the other four addresses). The test packets sent under the discovery procedure may carry a pre-determined signature so that they can be discriminated from other packets that may be sent to these multicast addresses by other machines or endpoints. After a pre-determined time period, which may be suitably selected to account for the time delays between the end-points participating in the session, each end-point EP 1 -Ep 4  sends a list of addresses from which it has received test packets. Using this data, MSVCS  710  can, for example, prepare a reference table (e.g., table  800 ,  FIG. 8 ). 
     In table  800 , the notation “x” in a cell indicates that the end-point whose number is shown on the corresponding row header reported that it has received test packets at the multicast address shown on the column header. Usually, the multicast reception may be expected to be symmetric (i.e., if endpoint  1  hears endpoint  2 , then endpoint  2  should also hear endpoint  1 ), and transitive (i.e., if endpoint  2  hears endpoint  1  and endpoint  3  hears endpoint  2 , then endpoint  3  should also hear endpoint  1 ). However, these properties may not hold in all cases depending on the implementation of the multicast infrastructure. For the example presented in the table  800 , MSVCS  710  can report that end-points  1 ,  2  and  4  can use bidirectional multicast for transmitting data to each other. 
     While there have been described what are believed to be the preferred embodiments of the present invention, those skilled in the art will recognize that other and further changes and modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the true scope of the invention. For example, the functionalities of an MSVCS describe herein can all be implemented in an SVC client as well. In which case, deployment of a separate MSVCS can be avoided. 
     It will be understood that in accordance with the present invention, the techniques described herein may be implemented using any suitable combination of hardware and software. The software (i.e., instructions) for implementing and operating the aforementioned rate estimation and control techniques can be provided on computer-readable media, which can include without limitation, firmware, memory, storage devices, microcontrollers, microprocessors, integrated circuits, ASICs, on-line downloadable media, and other available media.