Re: multicast: same port, different IP address?

From: Matt Garman
Date: Wed Apr 08 2009 - 14:28:25 EST

> Just my humble opinion here, but to the extent that IP addresses are used to identify a host (or in this case a set of hosts) that should receive messages and transport layer port numbers are used to identify applications (or services), if it was me I'd probably encapsulate the application layer messages in UDP and use UDP port numbers to delineate message types or groups of message types. UDP adds little overhead to the protocol (8 byte header, if I am not mistaken) and the standard sockets API provides a straightforward way to demux the incoming traffic to the interested applications. How that would be best done obviously depends upon the details of the application.

The first part of your comment isn't really what we want to do: each
host+application pair gets its own address.

But anyway, your suggestion about using raw UDP makes me wonder where
the distinction between multicast and UDP in the (i.e.
multicast IPs) lies...

Maybe I don't understand you correctly, but I think UDP would require
quite a paradigm shift for us. E.g., say we have HostA as a producer
of messages. Right now, HostA's programs just push messages out via
multicast; any number of hosts can become a consumer for this data.
If we switched to UDP, wouldn't HostA need to explicitly send() or
sendto() each and every receiver?

Or were you suggesting that the consumers all pull the data from
HostA, and HostA acts like e.g. a webserver?

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