POSIX sockets: Accepting connections with different transport protocols?

Question

I'm wondering what is the minimum required overhead in terms of the number of listening ports / server sockets required for accepting say N different connections, each using a different transport protocol that runs on top of IP, e.g. TCP, SCTP, DCCP, UDP etc.

Of course, a straghtforward approach would be to have N independent server sockets (each created via a call to socket() with the appropriate protocol parameter), each listening on a unique port. However, implementing this approach in an application that uses multiple protocols simultaneously would be extremely inconvenient since a client would need to know multiple server ports. In addition, in a peer-to-peer application that peers only once for each protocol (with the same client), the fact that each of the N server sockets only accepts a single (client) connection looks like a huge overhead (N additional sockets are introduced purely for handling N "real" connections to a single peering client).

Is it possible to do better than that, e.g. by reducing the number of listening server sockets and/or listening on the same port?

(For simplicity, you can assume N=2, one connection is TCP and the other one is DCCP or UDP (please don't make assumptions on connectionless communication since DCCP is connection-oriented).)

EDIT: I'm not interested in N (client) connections whose file descriptors are returned by N calls to accept. The question is about the additional overhead to make those N connections possible (i.e. there has to be at least one additional server socket that listens for incoming connections).

Answer 1

To sum up what's said in the comments above: Since in the

int socket(int domain, int type, int protocol)

call we must specify the protocol, we cannot use one socket for multiple protocols. We also cannot portably write

socket(AF_INET, SOCK_RAW, 0);

see SOCK_RAW Demystified.

Regarding using the same port with different sockets: the possibility for this varies by system; e. g. see HP-UX (man 7f inet):

  The local port address is selected from independent domains for TCP
  and UDP sockets.  This means that creating a TCP socket and binding it
  to local port number 10000, for example, does not interfere with
  creating a UDP socket and also binding it to local port number 10000
  at the same time.

vs. Linux (man ip):

  Only one IP socket may be bound to any given local (address, port) pair.