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.
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
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:
The latter two methods can also run in cache-mode whereby
recipients caches are primed with documents before a presentation.
- 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.
Differences between MASHCast and mWeb:
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:
- 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.
- 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.