At any time between two particular end hosts, only one TCP connection can be there between any two ports.
A TCP connection is uniquely identified by 5 pieces of information in the TCP and IP headers.
1. IP Source Address(IPSA)
2. IP Destination Address(IPDA)
3. Protocol ID field in IP Datagram
4. TCP Source Port number
5. TCP Destination Port number.
IPSA & IPDA uniquely identify the end hosts & the Protocol ID field in the IP datagram tell us the connection is TCP.
The TCP source and destination port numbers identify the application processes on the end hosts.
Together AT ANY INSTANT, all 5 fields uniquely identify the TCP connection internet wide.
Whenever a TCP connection initiates, the end host who initiates the connection, picks a unique source port ID.
But in a TCP header the source port ID can not be more than 16 bits, so at a time we can have 216 = 64k different connections, between a pair of end hosts, if the end host at the other end always listens at a specific port number.
When a TCP connection closes, the port number become available for reuse.
