Question about TCP/IP port binding

Question

Hello, I would like to know whether a server application will always answer on the same port? I have read somewhere that in order to keep listening on the "listening" port, the server will use different port. Could anybody provide me with more details? Thanks!

Answer 1

As a part of the TCP handshaking process, a communication channel is established on another port. The source and destination 'port' numbers correspond to 16 bit fields in the TCP packet header. Once the channel is established, the communication takes place on that port, with the appropriate source and destination port values in the fields on the header.

The initial connection is made to the listening port on the server; after this, the process establishes a TCP connection on the new ports assigned and further communication takes place on those ports.

Answer 2

If you make a TCP connection to a server on a particular port, that TCP connection will continue to use the same port on the server side.

A TCP connection is established between an (address1, port1) pair on one side and an (address2, port2) pair on the other side - and all four of these values are fixed over the life of the TCP connection.

This does not stop the server from listening on the port after it has accepted a connection - because it can differentiate between any packets for the established connection(s) and new connections based on the (address, port) pair used by the client.

For example, this netstat output from an Oracle server shows it listening on port 1521, and several established connections also using that port:

Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address               Foreign Address             State
tcp        0      0 0.0.0.0:1521                0.0.0.0:*                   LISTEN
tcp        0      0 127.0.0.1:1521              127.0.0.1:32776             ESTABLISHED
tcp        0      0 192.168.9.126:1521          192.168.9.124:32798         ESTABLISHED
tcp        0      0 192.168.9.126:1521          192.168.9.124:32823         ESTABLISHED
tcp        0      0 192.168.9.126:1521          192.168.9.124:32822         ESTABLISHED
tcp        0      0 192.168.9.126:1521          192.168.9.124:32821         ESTABLISHED
tcp        0      0 192.168.9.126:1521          192.168.9.124:32820         ESTABLISHED
tcp        0      0 192.168.9.126:1521          192.168.9.124:32819         ESTABLISHED
tcp        0      0 192.168.9.126:1521          192.168.9.124:32818         ESTABLISHED
tcp        0      0 127.0.0.1:1521              127.0.0.1:30536             ESTABLISHED
tcp        0      0 192.168.9.126:1521          192.168.9.124:36969         ESTABLISHED
tcp        0      0 127.0.0.1:32776             127.0.0.1:1521              ESTABLISHED
tcp        0      0 127.0.0.1:30536             127.0.0.1:1521              ESTABLISHED
tcp        0      0 192.168.9.126:1521          192.168.9.124:37786         ESTABLISHED
tcp        0      0 192.168.9.126:1521          192.168.9.124:37035         ESTABLISHED
tcp        0      0 192.168.9.126:1521          192.168.9.124:37034         ESTABLISHED
tcp        0      0 192.168.9.126:1521          192.168.9.124:33018         ESTABLISHED