I'm using 2.2 beta 1 btw.
Multicast is not required. *If* you enable multicast detection, it just means the agent will detect the server coming up and down faster than if you just use polling. By default the agent will also poll the server every 60 seconds (using multicast detection, it can detect the server going up and down within a few seconds, as opposed to up to a minute).
The communications is pretty easy - by default, the agents will listen to port 16163. So when the server needs to talk to the agent, it goes over 16163.
By default, the server will listen on port 7080 over http and 7443 over https. So when the agent needs to talk to the server, it goes to port 7080 (over HTTP, it would be port 7443 if its going over HTTPS).
Those are really the only ports you need to worry about in the default installation for server->agent and agent->server comm.
The 2.1 documentation is pretty close to the current stuff (even though you are using 2.2 beta):
Here's the full suite of docs:
I also recommend you read the comments at the top and throughout the agent-configuration.xml file: