Weboard: a Web Conference Tool


October 18 1996

Table of contents
  1. Objectives
  2. Requirements
  3. Architecture and Protocols
  4. Light-weight Reliable Multicast Protocol
  5. Related Issues
  6. Current Status
  7. References

This is a compressed version. More detailed version is in progress.

1. Objectives

This tool is designed for exchange of Web documents, basically of the text/html MIME type, on the Internet, in multicast mode among a group of people, or in unicast between two users. It is also aimed as a testbed to experiment reliable multicast protocol on the "text" media.

This tool should avoid the problems of the existing Web multicast tools, i.e., with better loss control, portability and user interface.

It is intended to be used for:

2. Requirements

Different from other continuous-media applications on the MBONE, the multicast of HTML documents has very different requirements:

3. Architecture and Protocols

To avoid to reinvent the wheel, we intend to make use of the already available protocols and take the maximum benefits from the experience from various projects on reliable multicast protocol.

Since 1980's, a number of reliable multicast protocols have been emerged. Each of them has its own emphasis and application scope.

The complexity of this type protocol grows with the level of requirements on reliability, such as throughput, delay, ordering and loss repair, etc. It is rather difficult, if not impossible, to design one universal protocol which meets the needs of all applications. Reliable protocols should be tailored to the particular requirements of a particular application.

The protocols to be used are shown in the following table. At the top level, Web data are encoded in the MIME format. Then they are delivered to the layer of light-weight reliable multicast protocol which is built on top of RTP (Real Time Protocol).

MIME Encoded Data
Light-weight Reliable Multicast Protocol (LRMP)

The light-weight reliable multicast protocol is much like the SRM (Scalable Reliable Multicast) protocol, but with some simplifications. In addition, LRMP provides some functions for session control.

4. Light-weight Reliable Multicast Protocol

Not yet available.

4.1 Session control

4.2 Synchronization

4.3 Loss and Flow Control

5. Related Issues

5.1 Interface with Web Browsers

Weboard is intended to be interoperable with multiple Web browsers. It functions like a multicast proxy server to browsers. There are three ways to multicast documents: We have tested the last method, it works well when the HREF links in the document are relative path names.

5.2 Embedded Objects

Embedded objects and inline images are parts of an integral Web document. They should be sent together with the document where they are embedded. Weboard parses the HTML document to be sent to collect embedded objects. Embedded objects are maintained in a list related to the HTML document.

While multicast, Weboard sends first the HTML file and sequentially sends embedded objects in the list. At reception, each time an embedded object is received, the displayed HTML document will be refreshed.

There are problems when embedded objects reference other objects, which is the case for java objects.

5.3 Caching

Weboard caches all received documents.

While the currently received document is displayed automatically, users are allowed to view the list of received documents together with related information and have the possibility to display or delete them.

6. Current Status

The prototype is being implemented in java. Current version does not include the LRMP protocol. It runs well on a local network where almost no loss is encountered.

Work is in progress to implement the LRMP protocol and a multicast simulator that allows test on local networks.

At the time we consider that the code is stable, we will put the Weboard tool available on our ftp site.


  1. Henning Schulzrinne, Stephen Casner, Ron Frederick, Van Jacobson, RTP: A Transport Protocol for Real-Time Applications, RFC 1889, January 1996.
  2. H. Schulzrinne, RTP Profile for Audio and Video Conferences with Minimal Control, Audio-Video Transport WG, RFC 1890, January 1996.
  3. T. Turletti and C. Huitema, RTP payload format for H.261 video streams, Audio Video Transport WG, INTERNET-DRAFT, July 7, 1996.
  4. Sally Floyd, Van Jacobson, Steven McCanne, Ching-Guang Liu, Lixia Zhang, A Reliable Multicast Framework for Light-weight Sessions and Application Level Framing, ee.lbl.edu, Nov, 1995.
  5. Reliable Multicast Protocol, research.ivv.nasa.gov.