I have a somewhat simple, but specific, question about implementing the transport layer for a SIP UAC.

Do I expect the response to a request on the same socket that I sent the request on, or do I let the UDP or TCP listener pick up the response and then route it to the correct transaction from there? The RFC does not seem to say anything on the matter.

It seems that especially using UDP, which is connection-less, that I should just let the listeners pick up the response, but that seems sort of counter intuitive. Particularly, I have seen plenty of UAC implementations which do not depend on having a Listener in the transport layer.

Also, most implementations I have looked at do not have the UAS receiving loop responding on the socket at all. This would tend to indicate that the client should not be expecting a reply on the socket that it sent the request on.

For clarification:

Suppose my transport layer consists of the following elements:

TCPClient (Sends Requests for a UAC via TCP)
UDPClient (Sends Requests for a UAC vid UDP)
TCPSever (Loop receiving Requests and dispatching to transaction layer via TCP)
UDPServer (Loop receiving Requests and dispatching to transaction layer via UDP)

Obviously, the *Client sends my Requests. The question is, what receives the Response? The *Client waiting on a recv or recvfrom call on the socket it used to send the request, or the *Server?

Conversely, the *Server receives my requests, What sends the Response? The *Client? doesn't this break the roles of each member a bit?