Question

i have a few basic questions:

1.A socket is represented by a protocol, a local ip, local port, remote ip and remote port. Suppose such a connection exists between a client and a server. Now when i bind another client to same local port and ip, it got bound(i used SO_REUSEADDR) but connect operation by second client to the same remote ip and port failed.So, is there no way a third process can share the same socket?

2.When we call listen() on a socket bound to a local port and ip, it listens for connections. When a client connects, it creates a socket (say A). It completes 3 way handshake and then starts a different socket(say B) and also deletes the socket A (Source) .The new client is taken care of by the new socket B. So, what kind of a socket represents a listening socket i.e. what is the remote ip and port and is socket A different than that socket or just addition of remote ip and port to listening socket forms A?

3.I read that SO_REUSEADDR can establish a listening socket on a port if there is no socket listening on that port and ip and all sockets on that port and ip have SO_REUSEADDR option set.But then i also came across a text which said if a client is bound to a port and ip, another client can’t bind to it(even if SO_REUSEADDR is used) unless the first client successfully calls connect(). There was no listening socket(it is a client so we there is no call to connect()) on that port and ip in this example. So, why isn’t another client allowed?

Thanks in advance.

Answer 1

1. Correct: there is no way to create two different sockets with the same protocol, local port, local address, remote port, and remote address. There would be nothing to tell which packets belonged to which socket!

2. A listening socket does not have a remote address and remote port. That’s OK, because there are no packets on the wire associated with this socket (yet). Actually, all sockets start out with neither a local nor remote address or port. These properties are only assigned later when bind() (for local) and connect()/accept() (for remote) are called.

3. Until you call connect() or listen() on a socket, there isn’t any different between a server (listening) or client socket. They’re the same thing. So it would be more correct here to say that no two sockets are allowed to share the same protocol, local address, and local port if neither has a remote address or port.

   This isn’t a problem in practice though, because you usually don’t call bind() on a client socket, which means there is an implicit bind() to an ephemeral port at connect() time. These typical client sockets can’t conflict with a listening socket because they go from having no addresses associated with them to having both local and remote addresses associated with them, skipping the state where they have only a local one.