Handley, Mark James; (1997) On Scalable Internet Multimedia Conferencing Systems. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis I examine scaling aspects of IP-multicast based multimedia conferencing systems. The thesis is that application level semantics must be used in protocol design to cope with various forms of failure, and that these systems should be designed to permit inconsistencies in order to scale. I present an examination of the network conditions that such applications must face as motivation, and examine application designs for two aspects of multimedia conferencing as validation. These applications are a distributed shared editor and a distributed session directory. The general design principle of Application Level Framing (ALF) was applied during the design of these applications. I show that ALF can and should be applied in a wider context than that stated in the original ALF paper from Clark and Tennenhouse, and that it results in applications that perform well and are very robust to a wide variety of conditions. However it can also lead to designs that are difficult to generalise from. The design methodology of lightweight sessions as proposed by Van Jacobson and based on IP multicast presents a large design space in which very few points have been mapped. This thesis explores some of this design space. In the chapter on shared editors, I examine the effects of designing for robustness and redundancy in shared tools, and conclude that solutions resulting from such design perform well but are very specific to the design task and cannot easily be abstracted. In the chapter on session directories I examine the scaling limits of lightweight sessions through the example of a session directory which must scale at least as well as the sessions it describes, and examine the scaling limits of multicast address allocation schemes. In conclusion I reflect on the contradictory design goals in conferencing applications; those of producing abstraction layers to allow reuse of code and simplify the design task, and of designing for good performance in distributed over unreliable networks and attempt to draw some general guidelines for the design of such systems.

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