CODEWITHSUNDEEP
csockets
BillTheKid
BillTheKid

Reputation: 447

How does a client know which port to send data after accept()

Let's say we have a server that can accept multiple clients. First, it has to create a socket, then bind it with a port and an IP and finally listen to requests for connection from clients. After accept()ing a connection with a client, the server creates a new socket to communicate with the specific client. My question is whether or not the client is going to send its data to the same port it sent its initial request to, and if not how does it know where to send it?

Upvotes: 3

Views: 1565

Answers (3)

Remy Lebeau
Remy Lebeau

Reputation: 595319

A socket connection is uniquely identified by a tuple of [Protocol, Local IP, Local Port, Peer IP, Peer Port].

A TCP server creates a listening socket with a tuple of [TCP, Listen IP, Listen Port, 0, 0]. When a client requests to connect to a server, the network routes the request to the specified IP/Port. The receiving device then routes the request to a matching listening socket, performs a 3way handshake with the client, and puts it into a queue. Later, when accept() is called, it extracts the next pending client from the queue and returns a new socket identified with a tuple of [TCP, Listen IP, Listen Port, Client IP, Client Port]. Because of this, a single listening socket can accept multiple Clients from different Client IP/Port combinations.

A TCP client creates a connecting socket with a tuple of [TCP, Local IP, Local Port, 0, 0]. When the 3way handshake is complete, the socket's tuple is updated to [TCP, Local IP, Local Port, Server IP, Server Port]. Because of this, a Client can connect separate sockets to different Servers at differing Server IP/Port combinations.

All subsequent data exchanges use these tuples.

Data sent out from a Client's connecting socket will be sent to the associated Server IP/Port and stored in the buffer of the accepted Server socket whose tuple matches both the Client and Server.

Data sent out from a Server's listening socket will be ignored, since there is no associated Client.

Data sent out from an accepted Server socket will be sent to the associated Client IP/Port and stored in the buffer of the connected Client socket whose tuple matches the Client and Server.

Upvotes: 5

Adiitrack
Adiitrack

Reputation: 11

Generally There is always a default port allotted for each kind of communication.Operating System may kept it open or close ,it can be checked .

Let's say for FTP connection, There is a separate port allotted for handshake,It don't matter how many new FTP connection are being requested, all new connection will go to that same port , Once handshake is completed data exchange is done via another port, Even if we don't specify port. If Network manager has Port List entries earlier it will request to the same port.

Example for SSH if you request for

ssh -X <IP>

Even if you don't mention port , Your system know which port to request for and at server side there is always some port open who will be listening to your request and based on data you send while handshake it will continue listening or block you.

Bonus is you can open your custom port at server side who will be listening to your request. TCP implementation by default declare which port will be used for what kind of communication.

Upvotes: 1

Steffen Ullrich
Steffen Ullrich

Reputation: 123260

The client connects with a source IP and port to a server with a destination IP and port. After accept exactly the same IP and port on both sides are continued to be used for data exchange as for the establishment of the connection.

Upvotes: 0

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