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Ok so I've been trying to teach myself some socket programming. I wrote myself a little C# application with an async server and I understand most of it, except for the following:

So the server has a port it listens on for connections then when it receives a connection it creates a different socket to do the communication on. This it what I dont understand... How does the communication happen between the client and the server when in theory the client has no idea what port has been elected for this new connection?

Thanks for all your answers

Edit: As far as I understand the listening thread listens on the default port, but all messages are then handled on a different socket for each client?

Edit Again: Some how you guys are misunderstanding my question. I understand normal socket communication. My problem is with an async server where the listening socket is different from the connecting socket. Ie.

  1. Server listens on default port
  2. Client attrmpts to connect.
  3. Server receiver request.
  4. Server then creates a communication socket between client and server and continues listening on the default port.

My problem is at the last step. How does the client now know how to communicate on the new socket? Here is some sample code http://msdn.microsoft.com/en-us/library/5w7b7x5f.aspx

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  • The listening thread listens on a port you specify, and should always listen on that port. Each time it receives connection request from client then it will using a new socket to handle that connection. One socket for each connection. Commented Jul 8, 2011 at 7:11
  • To your "Edit Again": asyn server is not quite different with syn server in term of socket (which you called normal socket communication). For asyn server: when server accepts connection from client then it will give away that socket to another thread to continue processing. After that it will use another socket to listen on the same port. Nothing to be confusing here. Commented Jul 8, 2011 at 7:41
  • I just check your link and see your point: Asyn server here only means that it does not block the current thread when call to Accept function. Note that when call Socket handler = listener.EndAccept(ar); it will return a new socket to handle that connection. And that socket is still on that default port. Commented Jul 8, 2011 at 7:50
  • Hi longbkit. Thanks for your comments. You are more on the point to what I want, but not exactly answering my question(I may be asking it badly so I apologise). I understand everything you said. The part I dont understand is in the technical details of what happens when "it will using a new socket to handle that connection". I mean the client does not know the details of the new socket yet since it is created on the server. I does it now know it needs to communicate on a different socket/port? Commented Jul 8, 2011 at 8:21
  • Got your point. Actually each connection includes two sockets: 1 client socket and 1 server socket. Client uses a socket to connect to server, When server accept the connection, then client can start communicate with server through its client socket, and on the server side, a new socket will be created to communicate with that client. So no need any new socket for client. Commented Jul 8, 2011 at 8:34

4 Answers 4

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Although this question was posted over a year ago, I belive it is worth trying to clarify (or confuse?) it a bit more.

"How does the client now know how to communicate on the new socket?" - The client is not aware that a new socket was created. It simply keeps on sending data (packets) to the same port.

However, this gives rise to another question: How does the server know which data comes from which client? - Thanks to the TCP and IP protocols the server knows both the address of the client and the source port from which the packets were sent. With this information the server can receive packets from multiple clients and multiple (client) ports and route them to the correct socket. For this question, think of server sockets as filters: when packets are received from client X - port Y then route them to socket Z.

"...does it now know it needs to communicate on a different socket/port?" - This is a frequent source of confusion. When a new socket is created on the server to receive packets (after the connection has been established) it does not use a new port, it keeps on using the original port number. The entire socket creation process on the server side is transparent to the client. The client never knows (nor does it need to know) that a new socket was created to handle its packets.

Google TCP header for more information.

Hope this helps someone.

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When the client connects to the server, it selects the port to connect to. The client also includes a port that it will receive responses on. This is typically a randomly selected port, but it's possible for the client to override that.

Think of it like a phone call. When you call someone, there is the phone number you call, and you also have a phone number. even though you both talk to each other, both phone numbers are in use.

That's not a perfect analogy, since phone numbers are more like IP addresses and trunk lines need not have an originating phone number in all cases, but the same concept applies.

Simply put, the TCP protocol requires an originating port and destination port as well as originating ip address and destination IP. When packets are sent in either direction, the apropriate IP/Port is used either way.

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Actually the new connection use the same port. A server listen on a specific port for incoming connection, anytime it receives connection request from client, the server accept it and create a new thread to process the request. And then continue to listen on that port.

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That's not his point. He's confused that there are seperate destination and source ports, and those ports can be different.
No, that's what he want. He is " trying to teach myself some socket programming" so he is a beginner in socket programming. And this is very basic in socket programming.
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Definitions

  • Client: The socket at the remote machine that is connecting to the server
  • Server: The socket that is waiting for connections on the server
  • ServerClient: The socket which is communicating with the Client

Answer

I couldn't find any details about how the ServerClient port is transfered to the Client after the Server has accepted it. But it's most likely transfered in the handshake. Feel free to read RFC793 if you want to know more.

I wont go through the details, but you can read about passive connects to get more information about how listener sockets work at lower levels. But basically the purpose of listener sockets (Server) is only to accept sockets (ServerClient).

The port that the ServerClient uses is assigned by the socket implementation in the operating system and is nothing that you can control. All you need to know is that the each connected ServerClient will get it's own port and it's transfered to the Client during the (threeway) handskake (I think ;))

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