mWeb: a framework for distributed presentations using the WWW and the MBone

P. Parnes, M. Mattsson, K. Synnes, D. Schefstrom (Lulea), 1996

Summary. mWeb is a webcast application which uses Scalable Reliable RTP (SRRTP) for synchronization and Scalable Reliable File Distribution Protocol (SRFDP) for document distribution. Both protocols use SRM-like mechanism to achieve reliability.

More Detail

The authors nicely break down the challenges in architecting a webcast application into synchronization and distribution.

On related work:

The authors cite a study on Ed Burns' Webcast saying, "Tests conducted during the sprint of 1995 which showed thet the multicast distribution protocol used by that version of RMP wasn't suitable for wide-area-networks, because only the original sender of the data could do a repair of lost packets." I'm not sure if the study is done on RMP or Webcast. A footnote states that in recent versions of RMP the distribution problems have been solved.

The authors say of mMosaic that "initial tests show that mMosaid works well with HTML-pages and smaller images, but the distribution delay gets too large with bigger images." There is no cite specific to this statement; they do cite mMosaic.

RTP. Three points about RTP:

Best-effort service.
Functions (?) include loss detection for quality estimation and rate adaptin, data sequencind, intra- and inter-media synchronization (?)
Consists of two protocols operating on two different channels: RTP for data and RTCP for control packets.

I'm unclear what extending RTP gives you over using the SRM library. Perhaps the SRM library wasn't available at that time.

SRFDP sends headers and data. The headers contain the last-modification time used for invalidating cached copies. Indices of headers are multicast; applications view the header and send requests for files of interest.

Web documents can be "collected" in three ways:

Batch mode.
Some CCI interface I don't understand.
Set the HTML proxy to point to the mWeb WWW-proxy whcih passes URLs to the mWeb app.

The latter two methods can also run in cache-mode whereby recipients caches are primed with documents before a presentation.

Differences between MASHCast and mWeb:

mWeb doesn't dampen requests to the server.
mWeb has a scheme for invalidating old cache copies.
Although they claim to be browser independent, I think they have to port to each browser. In Section 5, Implementation and Status, they state, "Although the Java-language itself is platform independent there doesn't exist any platform independent way of interfacing WWW browsers." I'm not sure why this is since we (and Webcanal) can control the browser via the Remote Controller applet.

The authors frame webcast data as real-time data and use RTP as the underlying protocol. While webcast data does need synchronization, I disagree with their characterization as real-time because:

Traditional real-time data (video, audeo) is updated at a rate so rapid that reliability has limited use. Webcast data is not and not only benefits from reliability, but requires reliability for browsers to faithfully render documents and their embedded images.