What network overhead is involved?

The ToolTalk service does not use hardware broadcast or multicast. The message is sent directly to the ttsession process for the session (whether across the network or not). When a pattern is registered, it also is sent directly to the ttsession process. The ttsession process matches the message against all the patterns and sends the message directly to only the processes that registered patterns that match the message — if no process on another machine is interested in a message, that machine does not need to wake up and look at it.