I would like a socket option that let's me share a source port as long as the 4-tuples are unique in a UDP socket *and* specify which source port that is. It would be similar to IP_BIND_ADDRESS_NO_PORT. It would do exactly the same when you bind to port 0, but when you bind to a different port it would say "don't do anything with this port yet, but when I connect() please use the port as source port instead of a random ephemeral port".
The reason is the following. I would like to have a server receiving and sending UDP datagrams from multiple clients using a single port. I would like to use multiple sockets for sending, so I can use different SO_MAX_PACING_RATEs for each one. One socket per client/session. So there is a single "main socket" bind to the relevant port (equivalent to the one that accept()s in TCP), and when it receives a new session request it creates a new socket specific for that season (as accept() does). When creating that new socket I first need to bind() it to select the port, and then connect() it to make it receive only datagrams from the relevant session. The problem is that between the bind() and the connect() there is a small time in which the new socket can receive data that should go to the "main socket". It's not an unsolvable problem but makes the server code more complicated than it needs to be.
