Subject: Multicast On-Demand

From: Doron Rajwan <[email-masked]>
Date: Sat, 29 Jul 2000 00:46:09 +0200

The second issue that we want to raise is multicast on-demand.

Any comment will be appreciated.

Doron Rajwan.

=====================
 Multicast On-Demand
=====================

Multicast On-Demand:
====================
In the following there is a suggestion for "multicast on-demand", 
which will allow a server to announce a multicast group that receivers 
can join, yet not to send any data until a user actually joining the 
group.

Currently, if a server plans to send multicast data, but there are no 
users that want to receive the content, the data still comes out of 
the server and is blocked at the nearest router. Thus, supporting a 
large number of streams causes a computational load and usage of 
networking resources even when the number of request streams is small. 

We propose, then, that the server will not send anything until it 
receives the first join message to that stream. This requires the 
first join message sends towards the server, should reach the server 
itself, and not just the nearest router. This gives the server the 
ability to advertise itself as if it sends information for a lot of 
multicast groups, when actually it sends it on-demand.

This limitation exists because the root of the delivery tree is the 
designated router (DR) near the source, not the source itself. In the 
[SSM] model there is no need for the first hop to be different from 
any other hop in the delivery tree. We suggest that the root of the 
delivery tree will be the source itself.

Another method to support multicast on-demand is to add a query 
protocol, allowing the source to ask the DR about the registration 
status. It can be the query mechanism in IGMP, exchanging role between 
the host and the DR. The server can query information about all the 
groups using limited broadcast on the LAN, and get response from the 
routers.

Acknowledgement:
================
This document benefited a lot from discussions and specific comments 
of Radia Perlman and Jon Crowcroft.

References:
===========
[SSM] H. Holbrook, B. Cain, "Source-Specific Multicast for IP", Work 
in Progress.
http://www.ietf.org/internet-drafts/draft-holbrook-ssm-00.txt 

[IGMPv3] B. Cain, S. Deering, I. Kouvelas, A. Thyagarajan, "Internet 
Group Management Protocol, Version 3", Work in Progress.
http://www.ietf.org/internet-drafts/draft-ietf-idmr-igmp-v3-04.txt 

Author's Address:
=================
Doron Rajwan
BandWiz Inc.
[email-masked]

Meir Feder
BandWiz Inc.
[email-masked]