Chapter 23- Conferencing on the Internet

Conferencing involves communication among several users. Multimedia conferencing, including audio, video, instant messaging, whiteboard sharing, and file transfer, is a popular service on the Internet and in enterprises. Chat rooms where users exchange instant messages are an example of a conference service on the Internet. The collaboration tools used in most enterprises are also examples of conferences.
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Chapter 23- Conferencing on the Internet Chapter 23Conferencing on the InternetConferencing involves communication among several users. Multimedia conferencing,including audio, video, instant messaging, whiteboard sharing, and file transfer, is a popularservice on the Internet and in enterprises. Chat rooms where users exchange instant messagesare an example of a conference service on the Internet. The collaboration tools used in mostenterprises are also examples of conferences. Thus, conferences are not limited to traditional unmoderated audio or video conferences.They can include all types of media and can be moderated by using floor control mechanisms. Conferencing is an important area for enterprises with employees working in differentcountries. A conference system including collaboration tools can save much money and timeby reducing the need for face-to-face meetings where attendees need to travel great distances. However, we are still far from having conference systems that can replace face-to-face meetings completely. That is why there is much ongoing research in areas such astelepresence and virtual reality. The goal is to make virtual interactions as close to real onesas possible.23.1 Conferencing Standardization at the IETFIn the past, working groups such as MMUSIC did some work on conferencing (e.g., SDPwas designed with multiparty sessions in mind). Lately, the working groups that have beenactive in this area have been SIPPING and XCON. In fact, implementers sometimes find itconfusing to have similar specifications in the same area coming from two different workinggroups. Knowing the history behind conferencing standardization at the IETF will helpreaders understand how the specifications coming from both working groups relate amongthem. Initially, the SIPPING working group developed a set of specifications that describedhow to provide conferencing services using SIP. Coming from the SIPPING working group,these specifications were, unsurprisingly, very much focused on SIP. Pieces needed to build acomplete conference service such as floor control and conference management mechanisms(beyond the simple ones SIP provides) were out of the scope of this work. The XCON working group was chartered to work on generalizing the work done inSIPPING so that different signaling protocols (not only SIP) could be used and to specifythose missing pieces needed to build a complete conference system. The charter was limitedto centralized conferences where clients connect to a central server following a star topology.The 3G IP Multimedia Subsystem (IMS): Merging the Internet and the Cellular Worlds Third EditionGonzalo Camarillo and Miguel A . Garc ıa-Mart´n ´ ı© 2008 John Wiley & Sons, Ltd. ISBN: 978-0-470-51662-1 CHAPTER 23. CONFERENCING ON THE INTERNET484Conferences using different topologies such as full-meshed and cascaded conferences wereleft out of scope. The results of the work of these two working groups include two conferencing frame-works: the SIPPING conferencing framework and the XCON conferencing framework. Wediscuss both of them, their differences, and how they relate to each other.23.2 The SIPPING Conferencing FrameworkThe SIPPING conferencing framework (specified in RFC 4353 [272]) describes threeconferencing models: loosely coupled, fully distributed, and tightly coupled. In the loosely-coupled conferencing model, shown in Figure 23.1, media streams are multicast. Conferenceparticipants join the multicast group of the conference using, for example, IGMP (InternetGroup Management Protocol, specified in RFC 3376 [95]) in order to receive media.Conference participants do not typically have any signaling relationship between them. Still,they can use SIP to invite new participants into the conference. A SIP INVITE request sentto a new participant would contain (in its body) all information needed to join the multicastgroup. Figure 23.1: The loosely-coupled conference model In the fully-distributed conferencing model, shown in Figure 23.2, each participant hasa signaling relationship with all of the other participants in the conference. Each participantsends media to all of the other participants. In the tightly-coupled conferencing model, shown in Figure 23.3, each participant has asignaling relationship with a central conference server. The central conference server mixesthe media received from different participants and distributes it to all of them. Of course, the three conferencing models just described are not the only models that canbe implemented with SIP. Many other variants are possible. For example, when the centralconference server in a tightly-coupled conference is distributed among several SIP nodes, theresulting model is typically referred to as the cascaded conferencing model. In any case, theSIPPING conferencing framework focuses on the tightly-coupled conferencing model; therest of the models are considered to be out of scope of our work.23.2. THE SIPPING CONFERENCING FRAMEWORK 485 Figure 23.2: The fully-distributed conference model Figure 23.3: The tightly-coupled conference model23.2.1 Signaling ArchitectureFigure 23.4 shows the signaling architecture proposed by the SIPPING conferencingframework. The conference server consists of several logical functions: the conferencepolicy, the conference policy server, and the focus, which includes the conference notificationservice. The conference policy is the set of rules that define a conference. The conference policyincludes information about the participants of the conference, the time and date when theconference will take place, the media streams the conference has, etc. Participants ma ...