I know one socket connection are established by both Server Socket and Client Socket.
And I read some documents said one Server Socket could serve many Client Sockets, means one Server Port could server multi Client Ports.
1.But I wonder that does Server use random ports to server different Clients after connection under hood, or Server just uses the same port listening and serving many client's connections ?
2.If so, when I implement a Server and Client Socket Connection, could I random a new port to establish a new Server Socket and tell Client to reconnect to new Server Socket, and the listening Server Socket just keep listening other clients ? it means use different port to server different clients ?
3.And what is the advantage of using one Server Socket(port) to server many Client? and advantage of using multi Server Sockets(ports) to server different Clients?
Thank you
The two value that idenify each end point, ip address and port number often called socket.
A server socket listens on a single port. All established client connections on that server are associated with that same listening port on the server side of the connection.Multiple connections on the same server can share the same server-side IP/Port pair as long as they are associated with different client-side IP/Port pairs, and the server would be able to handle as many clients as available system resources allow it to.
var express = require('express');
var app = express();
var server = require('http').createServer(app);
var io = require('socket.io')(server);
server.listen(4200);
Here u can attach your http port with socket.io.
by using a random client-side port, in which case it is possible to run out of available ports if you make a lot of connections in a short amount of time.
for more detail visit this site
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I am trying to create a client-server model using socket programming in Java. I have multiple clients connecting to a server socket, but once the connection is lost, I need to reconnect to the server but using the same port number for the client. I have data stored on the server with respect to the port number through which it came. Is it possible to get the same port number for a socket again?
The server has no control over which port a client connects from.
On the client side, however, a socket can be bind()'ed to a specific local IP/Port before it is then connect()'ed to the server. Just note that it may take some time for the OS to release the port from the previous connection before it can be reused again. And also, if the client has to connect through a proxy/router to reach the server, the IP the server sees will be the proxy/router's IP, not the client's IP, and there is no guarantee that the port which the server sees will be the same port which the client is using.
The real question is, why are you relying on something unreliable like a client ip/port to store your data? I would suggest using a unique ID to identify the data, like say a user login, or a server-generated ID that is given to the client. If the client disconnects and reconnect, it can just login/send back the same ID.
I have a Server-Client program using java, I tried to create a ServerSocket with a port and Client Socket with different port and they cannot connect to each other. Client throw ConnectException. When I change the socket on Client to the same as the one I use for ServerSocket, they worked.
As I understand from the aswer from this thread Java Networking: Explain InputStream and OutputStream in Socket if a machine create a socket with a port then that socket is bind to that machine, so why do client and server need to use same port to connect to each other?
Also, two application can't use same port on a machine so what happen when two difference Server having same port and a machine need to connect to both of them through 2 different application?
You need some basic understanding of TCP communication. Just Google TCP tutorials.
In a nutshell; the server will listen on a specific port. When a server is listening on a port it is bound to it. Only one server (or process) on a machine can be listening on a certain port.
The client will connect to a machine and specify the port to communicate on. If the server is listening on the port the client asked, then comms happens. Otherwise the connection cannot continue.
So the port that the server is bound to (or listening on) must be the same as the port the client specified.
The client and server don't need to use the same port. As you pointed out, a port can only be allocated to a single process at a time on a machine. To be more correct, a port and IP address pair is the allocation unit. So if your machine has two addresses or more one can bind the port to different processes per IP.
The standard setup is for the server process to listen for connections on a port, say 10000 using a server socket. The client process tries to connect to that port using a client socket. It will use a OS allocated port. Once the connection is setup, the server will allocate another client socket, on its side, in order to manage communication with the client process, and this will also have a OS allocated port.
Answer is NO, server will listen on a specific port but when the client start connecting to server
For example: Server is listening on port 80
When client connect to server, it will connect to serverIP address on port 80.
Client socket is live on another port, it is allocated by OS
I've seen this post
http://docs.oracle.com/javase/tutorial/networking/sockets/definition.html
since it wrote:
If everything goes well, the server accepts the connection. Upon
acceptance, the server gets a new socket bound to the same local port
and also has its remote endpoint set to the address and port of the
client. It needs a new socket so that it can continue to listen to the
original socket for connection requests while tending to the needs of
the connected client.
So are there multiple server sockets which has the same port in the server side?
There is one ServerSocket. It accepts incoming connections through the accept() method. This returns a Socket which you use on the server side to handle the connection to a particular client.
In the server side, i use this code :
ServerSocket server = new ServerSocket(1234);
Socket server_socket = server.accept();
I found the server is listening on port 1234.
When one or more client sockets are connected, they are all using the same port 1234 !
That is really confusing :
I remember that multi sockets can't use the same port, isn't it right ? Thanks.
A TCP connection is identified by four numbers:
client (or peer 1) IP
server (or peer 2) IP
client port
server port
A typical TCP connection is open as follows:
The client IP is given by the client's ISP or NAT.
The server IP is given by the user or looked up in a DNS.
The client chooses a port arbitrarily from the unassigned range (while avoiding duplicate quadruples)
The server port is given by the protocol or explicitly.
The port that you specify in the ServerSocket is the one the clients connect to. It's nothing more than a port number that the OS knows that belongs to your application and an object that passes the events from the OS to your application.
The ServerSocket#accept method returns a Socket. A Socket is an object that wraps a single TCP connection. That is, the client IP, the server IP, the client TCP port and the server TCP port (and some methods to pass the associated data around)
The first TCP packet that the client sends must contain the server port that your app listens on, otherwise the operating system wouldn't know what application the connection belongs to.
Further on, there is no incentive to switch the server TCP port to another number. It doesn't help the server machine OR the client machine, it needs some overhead to perform (you need to send the new and the old TCP port together), and there's additional overhead, since the server OS can no longer identify the application by a single port - it needs to associate the application with all server ports it uses (the clients still needs to do it, but a typical client has less connections than a typical server)
What you see is
two inbound connections, belonging to the server (local port:1234). Each has its own Socket in the server application.
two outbound connections, belonging to the client (remote port:1234). Each has its own Socket in the client application.
one listening connection, belonging to the server. This corresponds to the single ServerSocket that accepts connections.
Since they are loopback connections, you can see both endpoints mixed together on a single machine. You can also see two distinct client ports (52506 and 52511), both on the local side and on the remote side.
I am trying to implement a minimal chat server in java over regular TCP protocol. The chat server will listen on a specific port. The question I have is if there are multiple clients sending messages to the same port, can the server distinguish between the clients and respond to each individually if the messages do not contain the IP address or destination name of the client?
to make my question a bit more clear, suppose the server gets a packet that contains only
"user: abc to-user:efg message:"Hello""
Can I find out in java the address of the client who sent the packet and respond back to the same address or will I need to include some identifier in the message itself like "sender-ip = 1.1.1.1"
Multiple clients will never send data over the same port. The only time your clients will talk over the same port is when they will connect to the server. In the server, whenever the ServerSocket receives a connection it returns a new Socket. This socket is a combination of the following : Server IP+ServerPort and Client IP+Client Port. The Server IP and the Server Port will be same for each socket; what differs is the client IP and Port. Usually this socket is passed to a new thread for further communication while the ServerSocket goes back to listen to incoming connections. Once you have a reference to the socket you can call socket..getInetAddress().getHostAddress() to get the remote IP and socket.getPort() to get the port of the respective client.
Yes, each connection will be separate - you'll have a different stream to read from for each connection. It's up to you to associate the relevant user information with the connection though.