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After doing some search for this, I got to know a few points

  1. We cannot multiplex a port for TCP.
  2. If two connections use the same protocol and have the same destination ports, they must have the same connection.
  3. I am quite confused about how some sites say that TCP can only have one application listening on the same port at one time while others say multiple listening TCP sockets, all bound to the same port, can co-exist, provided they are all bound to different local IP addresses.

Reading the above stuff has left me more confused than ever. Can a destination port be associated with more than one TCP connection?

Kriti Singh
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  • There is already an answer to this here: https://stackoverflow.com/questions/2997754/does-the-port-change-when-a-server-accepts-a-tcp-connection – Necklondon May 15 '22 at 20:06
  • @Necklondon it doesn't answer my question. I am quite confused about how some sites say that TCP can only have one application listening on the same port at one time while others say multiple listening TCP sockets, all bound to the same port, can co-exist, provided they are all bound to different local IP addresses. – Kriti Singh May 15 '22 at 20:13
  • As you say, this is an application problem. Not a TCP constraint. The answer depends on the network layer level. Your question must be stated more precisely. – Necklondon May 15 '22 at 20:25

1 Answers1

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We cannot multiplex a port for TCP.

This is wrong. You can run multiple TCP connections on the same port, as long as they are unique connections. And it is not very difficult to write code that multiplexes I/O on multiple TCP sockets within the same process.

If two connections use the same protocol and have the same destination ports, they must have the same connection.

This is wrong. A TCP connection is uniquely identified by a combination of protocol + local IP/port + and remote IP/port.

Two connections that use the same protocol and same destination IP/port are still unique if they use different source IP/port to connect from. For instance, multiple clients running on the same machine can connect to the same server if they use a different local port to connect from. Which is typically the case, as most clients use a random available local port, selected by the OS, for the outbound connection.

Likewise, two connections that use the same protocol and the same source IP/port are still unique if they connect to different destination IP/port. For instance, multiple clients running on the same machine can use the same local IP/port to connect from if they connect to different servers.

some sites say that TCP can only have one application listening on the same port at one time

This is correct, but only if all of the listeners are trying to use the same local IP/port at the same time. Only 1 listener is allowed on it.

others say multiple listening TCP sockets, all bound to the same port, can co-exist, provided they are all bound to different local IP addresses.

This is correct.

Can a destination port be associated with more than one TCP connection?

Yes. Even if there is only 1 listener on that port, every connection it accepts will be using that same local port on the server side, but a different source IP/port from the client side. This allows multiple clients from different remote machines to connect to the same server at the same time.

Remy Lebeau
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  • May I ask something? I tried on my Macbook that make two TCP clients have same local port (both bind explicitly) and then 1st client successfully connected to server A, but 2nd client failed to connect to server B with error 48 (Address already in use). Since TCP sockets are identified by src IP/port, dest IP/port, I thought that 2nd client also should have successfully connected to server B. Would you mind to explain what I mistake? – obanadingyo Apr 19 '23 at 01:03
  • @obanadingyo hard to say without seeing your actual code (feel free to post a new question about this). It could be (probably likely) a bug in your code, not binding/connecting the sockets correctly, or it could be (unlikely) a bug in Apple's socket stack (in which case, you would have to contact Apple for help). – Remy Lebeau Apr 19 '23 at 01:37
  • Thank you for your reply :) I posted it on new question : https://stackoverflow.com/questions/76050339/two-tcp-clients-with-same-port-connecting-to-different-servers-not-working – obanadingyo Apr 19 '23 at 02:29