How the clients (client sockets) are identified?

Question

To my understanding by serverSocket = new ServerSocket(portNumber) we create an object which potentially can "listen" to the indicated port. By clientSocket = serverSocket.accept() we force the server socket to "listen" to its port and to "accept" a connection from any client which tries to connect to the server through the port associated with the server. When I say "client tries to connect to the server" I mean that client program executes "nameSocket = new Socket(serverIP,serverPort)".

If client is trying to connect to the server, the server "accepts" this client (i.e. creates a "client socket" associated with this client).

If a new client tries to connect to the server, the server creates another client socket (associated with the new client). But how the server knows if it is a "new" client or an "old" one which has already its socket? Or, in other words, how the clients are identified? By their IP? By their IP and port? By some "signatures"?

What happens if an "old" client tries to use Socket(serverIP,serverIP) again? Will server create the second socket associated with this client?

Answer 1

The server listens on an address and port. For example, your server's IP address is 10.0.0.1, and it is listening on port 8000.

Your client IP address is 10.0.0.2, and the client "connects" to the server at 10.0.0.1 port 8000. In the TCP connect, you are giving the port of the server that you want to connect to. Your client will actually get its own port number, but you don't control this, and it will be different on each connection. The client chooses the server port that it wants to connect to and not the client port that it is connecting from.

For example, on the first connection, your client may get client-side port 12345. It is connecting from 10.0.0.2 port 12345 to the server 10.0.0.1 port 8000. Your server can see what port the client is connecting from by calling getpeername on its side of the connection.

When the client connects a second time, the port number is going to be different, say port 12377. The server can see this by calling getpeername on the second connection -- it will see a different port number on the client side. (getpeername also shows the client's IP address.)

Also, each time you call accept on the server, you are getting a new socket. You still have the original socket listening, and on each accept you get a new socket. Call getpeername on the accepted socket to see which client port the connection is coming from. If two clients connect to your server, you now have three sockets -- the original listening socket, and the sockets of each of the two clients.

You can have many clients connected to the same server port 8000 at the same time. And, many clients can be connected from the same client port (e.g. port 12345), only not from the same IP address. From the same client IP address, e.g. 10.0.0.2, each client connection to the server port 8000 will be from a unique client port, e.g. 12345, 12377, etc. You can tell the clients apart by their combination of IP address and port.

The same client can also have multiple connections to the server at the same time, e.g. one connection from client port 12345 and another from 12377 at the same time. By client I mean the originating IP address, and not a particular software object. You'll just see two active connections having the same client IP address.

Also, eventually over time, the combination of client-address and client-port can be reused. That is, eventually, you may see a new client come in from 10.0.0.2 port 12345, long after the first client at 10.0.0.2 port 12345 has disconnected.

Answer 2

Every TCP connection has as identifier the quadruple (src port, src address, dest port, dest address).

Whenever your server accepts a new client, a new Socket is created and it's indipendent from every other socket created so far. The identification of clients is not implictly handled somehow..

You don't have to think sockets as associated to "clients", they are associated with an ip and a port, but there is not direct correlation between these two.

If the same client tries to open another socket by creating a new one you'll have two unrelated sockets (because ports will be different for sure). This because the client cannot use the same port to open the new connection so the quadruple will be different, same client ip, same server ip, same server port but different client port.

EDIT for your questions:

1. clients don't specify a port because it's randomly choosen from the free ones (> 1024 if I'm not wrong) from the underlying operating system
2. a connection cannot be opened from a client using the same port, the operating system won't let you do that (actually you don't specify any port at all) and in any case it would tell you that port is already bound to a socket so this issue cannot happen.
3. whenever the server receives a new connection request it's is considered new, because also if ip is the same port will be different for sure (in case of old packet resend or similar caveats I think that the request will be discarded)

By the way all these situations are clearly explained in TCP RFC here.

Answer 3

I think the question here is why do you care if the client is new or old. What is new and old? For example, a web browser could connect to a web server to request a web page. This will create a connection so serverSocket.accept() will return a new Socket. Then the connection is closed by the web browser.

Afer a couple of minutes, the end used click on a link in the web page and the browser request a new page to the server. This will create a connection so serverSocket.accept() will return a new Socket.

Now, the web server do not care if this is a new or old client. It just need to server the requested page. If the server do care if the "client" already requested a page in the past, it should do so using some information in the protocol used on the socket. Check out http://en.wikipedia.org/wiki/OSI_model In this case, the ServerSocket and Socket ack on the transport level. The question "does this client already requested a page on the server" should be answered by information on the session or even application layer.

In the web browser/server example, the http protocol (which is an application) protocol hold information about who is this browser in the parameters of the request (the browser transmit cookie informations with every request). The http server can then set/read cookie information to known if the browser connected before and eventually maintain a server side session for that browser.

So back to your question: why do you care if it's a new or old client?

Answer 4

A socket is identified by:

(Local IP,Local Port, Remote IP, Remote Port,IP Protocol(UDP/TCP/SCTP/etc.)

And that's the information the OS uses to map the packets/data to the right handle/file descriptor of your program. For some kinds of sockets,(e.g. an non-connected UDP socket)the remote port/remote IP might be wildcards.

Answer 5

By definition, this is not a Java related question, but about networking in general, since Sockets and SeverSockets apply to any networking-enabled programming language.

A Socket is bounded to a local-port. The client will open a connection to the server (by the Operating System/drivers/adapters/hardware/line/.../line/hardware/adapters/drivers/Server OS). This "connection" is done by a protocol, called the IP (Internet Protocol) when you are connected to the Internet. When you use "Sockets", it will use another protocol, which is the TCP/IP-protocol.

The Internet Protocol will identify nodes on a network by two things: their IP-address and their port. The TCP/IP-protocol will send messages using the IP, and making sure messages are correctly received.

Now; to answer your question: it all depends! It depends on your drivers, your adapters, your hardware, your line. When you connect to your localhost machine, you will not get further than the adapter. The hardware isn't necessairy, since no data is actually sent over the line. (Though often you need hardware before you can have an adapter.)

By definition, the Internet Protocol defines a connection as pair of nodes (thus four things: two IP-adresses and two ports). Also, the Internet Protocol defines that one node can only use one port at a time to initiate a connection with another node (note: this only applies for the client, not the server).

To answer your second question: if there are two Sockets: the "new" and the "old". Since, by the Internet Protocol, a connection is a pair of nodes, and nodes can only use one port at a time for a connection, the ports of "new" and "old" must be different. And because this is different, the "new" client can be discriminated from the "old", since the port-number is differently.