Making Things Talk

COMMUNICATING IN (NEAR) REAL TIME 149

Transmission Control Protocol: Sockets & Sessions

For example, think about the exchanges between a web

client and server that you saw in the last two chapters. The

pipe is opened when the server acknowledges the client’s

contact, and remains open until the server has finished

sending the data.

There’s a lot going on behind the scenes of a socket con-

nection. The exchange of data over a TCP connection can

range in size anywhere from a few bytes to a few terabytes

or more. All that data is sent in discrete packets, and the

packets are sent by the best route from one end to the

other.

NOTE: “Best” is a deliberately vague term: the optimal route is

calculated differently by different network hardware, and involves

a variety of metrics (such as the number of hops between two

points as well as the available bandwidth and reliability of a

given path).

The period between the opening of a socket and the suc-

cessful close of the socket is called a session. During the

session, the program that maintains the socket tracks the

status of the connection (open or closed) and the port

number; counts the number of packets sent and received;

notes the order of the packets and sees to it that packets

are presented in the right order, even if the later packets

arrive first; and accounts for any missing packets by

requesting that they be re-sent. All of that is taken ...