Implement ICE-TCP

[Sean-Der]

TODO

• Convert length checks to assert
• Simplify agent<->pfd lookup
• conn_poll_connect and a callback to notify Agent that candidate is ready
• Generate active TCP candidate when enabled
• Ignore remote active TCP candidates

[paullouisageneau]

OK, this is complicated because the whole library is designed with the assumption that candidates can only be UDP. It would be way simpler to implement TURN TCP.

I think you can't realistically implement something on the side running in its own thread. It seems to me the best way to implement this is to add optional TCP sockets for each agent to the thread and poll connectivity backends (I don't think mux mode can have TCP candidates as you would have to create new sockets).

TCP sockets would be polled and read by the backend. A lot of functions assume addresses are UDP only, like conn_get_addrs() and agent_conn_recv(), you will probably need to introduce a socket type in addr_record_t to distinguish between UDP and TCP.

You would then need to modify the logic to match candidates pairs properly. You would probably need to introduce a function to order the backend to actively connect to a remote TCP address.

Again, this is complicated and requires fundamental changes, contrary to TURN TCP (as candidates are still UDP in that case).

[paullouisageneau]

Here you could check if pair->remote->resolved.addr.ss_family == remote->resolved.addr.ss_family to allow one IPv4 and one IPv6 TCP candidate.

[paullouisageneau]

You also need to handle POLLHUP indicating connection closed and you should also treat POLLERR like connection reset. (They can be set at the same time as POLLIN so it's not an else)

[Sean-Der]

Logic changed to the following. POLLHUP and POLLERR are now fatal.

      if (pfd->revents & POLLHUP || pfd->revents & POLLERR) {                                                                                                                                                                                        
          agent_conn_fail(agent);                                                                                                                                                                                                                    
          conn_impl->state = CONN_STATE_FINISHED;                                                                                                                                                                                                    
          return;                                                                                                                                                                                                                                    
      }

[paullouisageneau]

The issue with this logic is that when receiving POLLHUP you discard any data sitting in the buffer. While not a big problem in this context, it should be avoided. I'll fix it after merging.

[paullouisageneau]

OK, it looks good. There are a couple things I'll fix later but they don't seem blocking to me. Thank you for the work on this!