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TCP/IP Sockets and Socket Pairs: Process and Connection Identification
(Page 2 of 2)
Socket Pairs: Connection Identification
So, the exchange of data between
a pair of devices consists of a series of messages sent from a socket
on one device to a socket on the other. Each device will normally have
multiple such simultaneous conversations going on. In the case of TCP,
a connection is established for each pair of devices for the duration
of the communication session. These connections must be managed, and
this requires that they be uniquely identified. This is done using the
pair of socket identifiers for each of the two devices that are connected.
 Key Concept: Each device may have multiple TCP connections active at any given time. Each connection is uniquely identified using the combination of the client socket and server socket, which in turn contains four elements: the client IP address and port, and the server IP address and port.
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Let's return to the example we used
in the previous topic (Figure 199).
We are sending an HTTP request from our client at 177.41.72.6 to the
Web site at 41.199.222.3. The server for that Web site will use well-known
port number 80, so its socket is 41.199.222.3:80, as we saw before.
We have been ephemeral port number 3,022 for our Web browser, so the
client socket is 177.41.72.6:3022. The overall connection between these
devices can be described using this socket pair:
(41.199.222.3:80, 177.41.72.6:3022)
For much more on how TCP identifies
connections, see the
topic on TCP ports and connection identification
in the
section on TCP fundamentals.
Unlike TCP, UDP
is a connectionless protocol, so it obviously doesn't use connections.
The pair of sockets on the sending and receiving devices can still be
used to identify the two processes exchanging data, but since there
are no connections the socket pair doesn't have the significance that
it does in TCP.
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