Towards Multicast Session Directory Services

Alexandre Santos, Joaquim Macedo and Vasco Freitas

Universidade do Minho
Departamento de Informática
P-4700-320 Braga, Portugal

Email: {alex,macedo,vf} (at)


The current increase in interest in Internet Multicast and Multimedia Applications is self-evident and has profited from such infrastructures as the Multicast Backbone (Mbone) and its applications and tools which are several years old now. However, an ad-hoc usage of some of this technology's features, namely, session announcements and multicast addresses, do not scale easily.

The Session Description Protocol (SDP), a proposed standard, has been used and sometimes misused as a session directory tool to advertise multimedia sessions, without due attention to scalability, i.e., without taking into consideration the effect of the increasing number of simultaneous sessions being announced. A flat presentation of session announcements, with alphabeticaly ordered names, neither scale nor convey enough structured information to enable simple and efficient directory searches to pick up all the interesting sessions. This is what is really needed while keeping the directory manageable.

SDP and its companion Session Announcement Protocol (SAPv1) have enough scaling potential, if not misused. The quality of information presented to the user by popular tools, such as sdr, may be enhanced to a level similar to that obtained via the familiar Internet search engines and browsing and filtering services. Instead of burdening the user with the task of browsing and filtering the flat directory space, that task could be assigned to a server, should the information made available in the session descriptions be conveniently organised.

We propose that session directory information be structured and then disclosed by a Multicast Session Directory Service (MSDS). Session description fields in SDP, such as the value attributes category (a=cat:) and type (a=type:) and selected description fields, such as media, repeat times, time active, and so on, may be used to aggregate sessions in classes and even in a pre-arranged classification hierarchy. The user would then be able to query or browse MSDS servers.

The service itself would be announced either in multiple thematic or single well known multicast channels, with the directory structure being either port or time multiplexed, or made available as a server database.

Keywords: Session Directories, IP Multicasting, Multimedia Announcements, Session Description Protocol, Structured Browsing.

Terena NORDUnet Networking Conference 1999, Lund, Sweden, Jun 7-10, 1999