Multicast delivery of Web documents is an experimental concept
in which an alternative network delivery, or transport, mechanism
is used. Typically Web document delivery is accomplished through
the use of pointtopoint (unicast) network connections,
effectively creating a virtual wire from the source to the destination.
Multicast Web services are on the pointtomultipoint
model. That is an origin server (document source) will send one
copy to multiple recipients. Multicast allows one copy of the
message to be sent to multiple destinations, whereas multiple
copies are needed in a unicast service.
One idea behind multicast Web services is that documents will
be broadcast on a roundrobin schedule and that clients join
the multicast group and listen for the document that they are
interested in. The primarily benefit is that packet traffic is
reduced since multiple copies are not sent. However, the proposed
scheme has an increased latency problem due to the roundrobin
document broadcast. It also may result in wasted bandwidth if nobody
wants the document. An alternative idea is that the document is multicast
after a certain number of requests or elapsed time period. One general
problem to any multicast scheme is that multicast services
are not currently available to every user.
An experimental version of NCSA Mosaic that implements multicast
document delivery is available at
ftp://sig.enst.fr/pub/multicast/mMosaic-2_7b4m3_tar.gz.
25.3.2 Remote distribution mechanisms
The remote distribution concept is similar to a common file distribution
model used in FTP. Under this scheme, there are multiple server
sites that contain copies of the document. These sites are generally
closer to the user, which reduces the load and bandwidth requirements
for individual servers. Since the site is closer to the user,
response time is improved.
The primary difference between this method and caching is that
the user may know that there are multiple sites that contain the
document. The user chooses the site closest to him or her in
this model. In a cache hierarchy, the cache server will retrieve
the document from the closest copy in the caching hierarchy.
Copyright © 1996 David C. Lee, All Rights Reserved
David C. Lee
<dlee@vt.edu>
Last modified: Mon Nov 18 15:22:36 EST 1996