The deployment of IP Multicast has fostered the development of a suite of applications, collectively known as the MBone tools, for real-time multimedia conferencingover the Internet. Two of these tools --- nv from Xerox PARC and ivs from INRIA --- provide video transmission using softwarebased codecs. We describe a new video tool, vic, that extends the groundbreaking work of nv and ivs with a more flexible system architecture. This flexibility is characterized by network layer independence, support for hardware-based codecs, a conference coordination model, an extensible user interface, and support for diverse compression algorithms. We also propose a novel compression scheme called "IntraH. 261". Created as a hybrid of the nv and ivs codecs, IntraH. 261 provides a factor of 2-3 improvement in compression gain over the nv encoder (6 dB of PSNR) as well as a substantial improvement in run-time performance over the ivs H.261 coder. KEYWORDS Conferencing protocols; digital video; image ...

Several recent proposals for an "active networks" architecture advocate the placement of user-defined computation within the network as a key mechanism to enable a wide range of new applications and protocols, including reliable multicast transports, mechanisms to foil denial of service attacks, intra-network real-time signal transcoding, and so forth. This laudable goal, however, creates a number of very difficult research problems, and although a number of pioneering research efforts in active networks have solved some of the preliminary small-scale problems, a large number of wide open problems remain. In this paper, we propose an alternative to active networks that addresses a restricted and more tractable subset of the active-networks design space. Our approach, which we (and others) call "active services", advocates the placement of user-defined computation within the network as with active networks, but unlike active networks preserves all of the routing and forwarding semantics o...

In recent years, the "Internet Multicast Backbone," or MBone, has risen from a small, research curiosity to a largescale and widely used communications infrastructure. A driving force behind this growth was the development of multipoint audio, video, and shared whiteboard conferencing applications. Because these real-time media are transmitted at a uniform rate to all of the receivers in the network, a source must either run at the bottleneck rate or overload portions of its multicast distribution tree. We overcome this limitation by moving the burden of rate adaptation from the source to the receivers with a scheme we call receiver-driven layered multicast, or RLM. In RLM, a source distributes a hierarchical signal by striping the different layers across multiple multicast groups, and receivers adjust their reception rate by simply joining and leaving multicast groups. In this paper, we describe a layered video compression algorithm which, when combined with RLM, provides a comprehensive solution for scalable multicast video transmission in heterogeneous networks. In addition to a layered representation, our coder has low complexity (admitting an efficient software implementation) and high loss resilience (admitting robust operation in loosely controlled environments like the Internet) . Even with these constraints, our hybrid DCT/wavelet-based coder exhibits good compression performance. It outperforms all publicly available Internet video codecs while maintaining comparable run-time performance. We have implemented our coder in a "real" application---the UCB/LBL videoconferencing tool vic. Unlike previous work on layered video compression and transmission, we have built a fully operational system that is currently being deployed on a very large scale over the MBone.

\Soft state " is an often cited yet vague concept in network protocol design in which two or more network entities intercommunicate in a loosely coupled, often anonymous fashion. Researchers often de ne this concept operationally (if at all) rather than analytically: a source of soft state transmits periodic \refresh messages " over a (lossy) communication channel to one or more receivers that maintain a copy of that state, which in turn \expires " if the periodic updates cease. Though a number of crucial Internet protocol building blocks are rooted in soft state-based designs | e.g., RSVP refresh messages, PIM membership updates, various routing protocol updates, RTCP control messages, directory services like SAP, and so forth | controversy is building as to whether the performance overhead of soft state refresh messages justify their qualitative bene t of enhanced system \robustness". We believe that this controversy has risen not from fundamental performance tradeo s but rather from our lack of a comprehensive understanding of soft state. To better understand these tradeo s, we propose herein a formal model for soft state communication based on a probabilistic delivery model with relaxed reliability. Using this model, we conduct queueing analysis and simulation to characterize the data consistency and performance tradeo s under a range of workloads and network loss rates. We then extend our model with feedback and show, through simulation, that adding feedback dramatically improves data consistency (by up to 55%) without increasing network resource consumption. Our model not only provides a foundation for understanding soft state, but also induces a new fundamental transport protocol based on probabilistic delivery. Toward this end, we sketch our design of the \Soft State Transport Protocol " (SSTP), which enjoys the robustness of soft state while retaining the performance bene t of hard state protocols like TCP through its judicious use of feedback. This research was supported by DARPA contract N66001-96-C-8508, by the State of California under the MICRO program, and by

Real-time multimedia streams like audio and video are now integral data types in modern programming environments. Although a great deal of research has investigated effective and efficient programming support for manipulating such streams and although the design of digital media "middleware" is fairly well understood, no widely available or commonly accepted programming model exists within the research community. We believe this lack of common practice impedes our collective progress because it prevents disparate research groups from easily leveraging each other's work. In this paper, we propose a solution to this problem that combines the best features of a number of existing multimedia toolkits --- Berkeley's Continuous Media Toolkit, MIT's VuSystem, and the LBL/UCB MBone tools --- into a fine-grained, extensible, and highperformance toolkit. We describe the convergence of these three toolkits into a common programming infrastructure and argue that the availability and acceptance of ...

Current Internet multicast conferencing tools treat all sources with equal importance in that they either statically allocate a fixed bandwidth to each source in a session, or they automatically adapt each source's transmission rate independently of all other sources. But not all sources are of equal interest to all receivers. We believe that to effectively support human to human communication, this disparity in receiver interest should be reflected in the rate-adaptation process. To this end, we propose a protocol called "SCUBA" that enables media sources to intelligently account for receiver interest in their rate-adjustment algorithms. SCUBA is orthogonal to and complements existing rate-adaptation schemes and can interoperate with either sender- or receiver-directed control systems. To scale the SCUBA protocol with multicast session size, we decouple the receiver-feedback process from the session size through sampling. This approach introduces a "tunable" tradeoff between convergen...

In this survey article we describe the roots of IP Multicast in the Internet, the evolution of the Internet Multicast Backbone or “MBone,” and the technologies that have risen around the MBone to support large-scale Internet-based multimedia conferencing. We develop the technical rationale for the design decisions that underly the MBone tools, describe the evolution of this work from early prototypes into Internet standards, and outline the open challenges that remain and must be overcome to realize a ubiquitous multicast infrastructure. We and others in the MBone research community have implemented our protocols and methods in “real” applications and have deployed a fully operational system on a very large scale over the MBone. This infrastructure — including our audio, video, shared whiteboard tools and protocols — is now in daily use by the large and growing MBone user and research communities and the success and utility of this approach has resulted in commercialization of many of the underlying technologies.

The recent advent of the Internet Multicast service has enabled a number of successful real-time multimedia applications, yet the scalability of these applications remains challenged by the inherent heterogeneity of the underlying Internet. One promising approach for taming this heterogeneity is to encode each media flow as a layered signal that is striped across multiple multicast groups, thereby allowing a receiver to tune its individual reception rate by modulating its subscription to multicast groups. Though significant progress had been made on media transport protocols and congestion control strategies for adjusting multicast groups in this fashion, comparatively little work has been devoted to extending the session directory service and address allocation architecture to meet the needs and requirements of layered media. Moreover, the large-scale deployment of layered media formats is hindered by the lack of support for layered formats in existing session directory tools. To over...

Most conferencing systems are focused on facilitating one of two types of meetings: those in a single room, consisting entirely of colocated participants, or those with isolated individuals at different physical locations. Our experiences are of a third style: hybrid meetings consisting of both colocated groups and isolated participants. We illustrate the limitations of using an existing desktop-based tools in the shared meeting room portion of this hybrid meeting style, and propose adding a software control substrate matched to the specifics of the application to address the inadequacies. We derive requirements for the in-room applications, and, as a concrete example from the domain, describe the design and implementation of an application for manipulation of in-room shared video display. Our design employs a user interface split across multiple physical devices paired with a control protocol managing communication between them. The client portion runs on wirelessly-connected portable...

. IP Multicast has enabled a variety of large-scale applications on the Internet which would otherwise bombard the network and the content servers if unicast communication was used. However, the efficiency of multicast is often constrained by preference heterogeneity, where receivers range in their preferences for application data. We examine an approach in which approximately similar preferences are clustered together and transmitted on a limited number of multicast addresses, while consuming bounded total session bandwidth. We present a protocol called Matchmaker that coordinates sources and receivers to perform clustering. The protocol is designed to be scalable, fault tolerant and reliable through the use of decentralized design, soft-state operations and sampling techniques. Our simulation results show that clustering can reduce the amount of superfluous data at the receivers for certain preference distributions. By factoring in application-level semantics into the protocol, it ca...