How TCP/UDP demultiplexing works?

Question

I have the following statement.

"In TCP, the receiver host uses all of source IP, source port, destination IP and destination port to direct datagram to appropriate socket. While in UDP, the receiver only checks destination port number to direct the datagram. "

Is the above statement true?

If yes, does it mean that in TCP the same port can be used for multiple socket in one process, while in UDP only one socket can use on a port in one process? What about sockets in different processes? Can multiple processes use the same port in TCP/UDP? (in programming language: C/C++/Java)

If not, why?

Answer 1

"In TCP, the receiver host uses all of source IP, source port, destination IP and destination port to direct datagram to appropriate socket. While in UDP, the receiver only checks destination port number to direct the datagram. "

Is the above statement true?

Yes.

If yes, does it mean that in TCP the same port can be used for multiple socket in one process,

Yes, under some circumstances.

while in UDP only one socket can use on a port in one process?

No, see below.

What about sockets in different processes? Can multiple processes use the same port in TCP/UDP? (in programming language: C/C++/Java)

Under some circumstances, yes. A UDP port has to be designated as reusable by all processes that want to share it. A TCP port can only be reused by sockets bound to different interfaces: there is no sharing.

Answer 2

What that means is, in TCP, a unique communication "channel" can be described as the four-tuple: (src-ip, src-port, dst-ip, dst-port).

In UDP, all packets destined to a certain port are delivered to the only UDP socket listening on that port, regardless of the source address and port of said packet. I like to think of it as a funnel.