Van Jacobson. SIGCOMM '94 Tutorial: Multimedia Conferencing on the Internet, August 1994.  Home/Search   Document Not in Database   Summary   Related Articles   Check

....systems, the system must be scalable, fault tolerant, TCP friendly, and able to support heterogeneity in participants and network conditions. These goals have been achieved in a number of routing and application layer protocols through the design principles of light weight sessions [McC98] Jac94] Lightweight sessions use shared multicast control rather than centralized control and soft state rather than hard state. Soft state [Cla88,RM99a] describes a protocol style which uses unreliable transmission of periodic state messages to achieve eventual consistency. State which is not ....

Van Jacobson. SIGCOMM '94 Tutorial: Multimedia Conferencing on the Internet, August 1994.

....related work on soft ARQ, we briefly review these alternative techniques. One such technique for interactive media is to adjust the playback point at the receiver to compensate for variations in network delay (jitter) Many algorithms have been developed to automatically adjust the playback point [15, 8, 24, 14, 19]. The common goal of these algorithms is to minimize the playback delay small without causing signal dropouts stemming from frames that arrive past their scheduled playback times. Because they were developed for interactive multimedia, these algorithms do not try to increase the playback buffer ....

Van Jacobson. SIGCOMM '94 Tutorial: Multimedia conferencing on the Internet, August 1994.

....virtual synchrony among members, the control overhead and latency involved in accomplishing it is costly if not unacceptable with large member populations. Thus, we allow inconsistency among members, and elect only one to execute the clustering algorithm. This follows the Lightweight Session model [16, 24], which advocates loosely coupled and lightweight communication for multi party applications for enhanced scalability to large session sizes. 2.1 The Clustering Algorithm We briefly describe a clustering algorithm used by Matchmaker. For details and performance results of the algorithm, please ....

Jacobson, V. SIGCOMM '94 Tutorial: Multimedia conferencing on the Internet, Aug. 1994.

....addresses to transmit to and subscribe for data, resulting in coordination failure. Distributed programming toolkits such as ISIS [6] and HORUS [35] provide robust group communication primitives to achieve this virtual synchrony among members. However, this violates the Lightweight Session concept [16], the cornerstone of a new model for multipoint communication, which advocates loosely coupled, light weight communication for enhanced scalability in multicast applications. Also, the control overhead and latency involved in accomplishing virtual synchrony is costly if not unacceptable with large ....

Jacobson, V. SIGCOMM '94 Tutorial: Multimedia conferencing on the Internet, Aug. 1994.

....to operate in the face of adversity. For example, a multimedia conferencing system that relies upon a centralized controller to track group membership or perform multiplexing would fail catastrophically if that controller goes down [49] In contrast, the light weight sessions conferencing model [28], which is based on a loosely coupled, distributed group membership disseminated through announce listen, gracefully accommodates end system failures and network partitions. When a network partition occurs, for instance, the partitioned sub sessions continue to operate and group membership ....

Jacobson, V. SIGCOMM '94 Tutorial: Multimedia conferencing on the Internet, Aug. 1994.

....within the current Internet multimedia conferencing architecture. 1 Introduction The Internet Multicast service and its realization in the public Internet the Multicast Backbone or MBone form the cornerstone of a new model for multipoint communication called light weight sessions [15]. In light weight sessions, a multimedia application disseminates its media flow simply by framing the flow as a sequence of packets and multicasting those packets to a multicast group address. Receivers interested in a certain flow simply join the multicast group corresponding to the flow in ....

Jacobson, V. Sigcomm '94 tutorial: Multimedia conferencing on the internet, August 1994.

....within the current Internet multimedia conferencing architecture. 1 Introduction The Internet Multicast service and its realization in the public Internet the Multicast Backbone or MBone form the cornerstone of a new model for multipoint communication called light weight sessions [14]. In light weight sessions, a multimedia application disseminates its media flow simply by framing the flow as a sequence of packets and multicasting those packets to a multicast group address. Receivers interested in a certain flow simply join the multicast group corresponding to the flow in ....

Jacobson, V. Sigcomm '94 tutorial: Multimedia conferencing on the internet, August 1994.

....a graphical user interface that displays all of the participants in the audio conference and highlights the active speaker. Additionally, in joint work with Jacobson [90] we enhanced vt s algorithms for counteracting network packet jitter through receiver buffering and playback point estimation [88]. Our new application 4 became the LBLVisual Audio Tool, vat 1 , and the DARTNet community gradually began using our new tool upon its release in the fall of 1991 2 . The success and utility of the weekly DARTNet meetings generated interest in extending the multicast infrastructure beyond ....

....UCB LBL video conferencing application, vic, which became a core research vehicle for the work contained herein. We originally conceived vic as an application to demonstrate the Tenet real time networking protocols [53] and to simultaneously support the evolving Lightweight Sessions architecture [88] that implicitly underlies all of the tools discussed. Work on vic 4 has since driven the evolution of the Real time Transport Protocol (RTP) 153] As RTP evolved, we tracked and implemented protocol changes, and fed back implementation experience to the design process. Moreover, our experience ....

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Van Jacobson. SIGCOMM '94 Tutorial: Multimedia conferencing on the Internet, August 1994. 3 ftp://cs.ucl.ac.uk/mice/sdr/ 4 http://www.cs.ucl.ac.uk/mice/codec manual/doc.html 5 ftp://ftp.ee.lbl.gov/conferencing/sd 165

....or network failures. 2. 4 Design Principles We have developed an archive control protocol that addresses maintenance of shared state, group formation and management, and fault detection and recovery through the use of the multicast protocol design principles of lightweight sessions (LWS) McC98, Jac94] Light weight sessions refers to the fault tolerant, scalable, decentralized communications model that has been used successfully in many MBone conferencing protocols such as RTP RTCP [SCFJ96] for audio and video and SAP [HJ97] for session announcements. Instead of instantiating hard server ....

Van Jacobson. SIGCOMM '94 Tutorial: Multimedia conferencing on the Internet, August 1994.

....to nv and substantial improvement in both run time performance and packet loss tolerance compared to ivs. Vic was originally conceived as an application to demonstrate the Tenet real time networking protocols [14] and to simultaneously support the evolving Lightweight Sessions architecture [24] in the MBone. It has since driven the evolution of the Real time Transport Protocol (RTP) 40] As RTP evolved, we tracked and implemented protocol changes, and fed back implementation experience to the design process. Moreover, our experience implementing the RTP payload specification for H.261 ....

....cause problems because it will be muted by all participants that follow the protocol. Synchronization. Cross media synchronization can also be carried out over the Conference Bus. Each real time application induces a bufferingdelay, called the playback point, to adapt to packet delay variations [24]. This playback point can be adjusted to synchronize across media. By broadcasting synchronize messages across the Conference Bus, the different media can compute the maximum of all advertised playout delays. This maximum is then used in the delay adaptation algorithm. In order to assure ....

JACOBSON, V. SIGCOMM '94 Tutorial: Multimedia conferencing on the Internet, Aug. 1994.

....communication model where parties in the collaboration session simply tune in to or tune out of the multicast group without any explicit group notification operation. This loosely coupled, light weight, real time multimedia communicationmodel is known as the light weight sessions architecture [21]. Announce listen communication serves as the primary protocol building block for light weight session applications. The model is characterized by several properties: a shared (multicast) communication channel over which all parties communicate; periodic, self descriptive (i.e. temporally ....

JACOBSON, V. SIGCOMM '94 Tutorial: Multimedia conferencing on the Internet, Aug. 1994.

....by simply sending them to a multicast group address. Receivers interested in a particular transmission simply tune in by subscribing to the multicast group in question. This loosely coupled, light weight, real time multimedia communication model is known as the Light weight Sessions architecture [11]. Although the MBone tools can be used for a variety of communication styles (e.g. point to point internet phone calls, small group design collaborations or research meetings, or seminar distributionto large groups with tightly coupled feedback from the remote audience) a non interactive, ....

JACOBSON, V. SIGCOMM '94 Tutorial: Multimedia conferencing on the Internet, Aug. 1994. 3 ftp://ftp.parc.xerox.com/net-research

.... at hand e.g. shared whiteboards [15] webcast tools [11] or floor control applications [14] ffl primitives for archive and playback of collaborative sessions, e.g. modules from the MBone VCR [7] ffl multicast address allocation modules for dynamically creating lightweight sessions [8]; ffl building blocks for session advertisement and explicit session invitation protocols [5] ffl building blocks for special effects processing of video, audio signal processing algorithms like echo cancelation; and so forth. Unfortunately, all of the above systems are currently implemented in ....

JACOBSON, V. SIGCOMM '94 Tutorial: Multimedia conferencing on the Internet, Aug. 1994.

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