Yet another service of the TCP protocol is regulating the speed of senders. Data communications people
refer to this as flow control. With every message and acknowledgement, each computer informs the other of the
number of bytes it is ready to receive. This number of bytes is known as the data window. If a fast sender
starts to overrun a slower receiver, the acknowledgement for a packet will soon show a zero for the data window.
The sender will stop transmitting until it receives another packet from the receiver announcing that it has room
in its data window again.
To the application programs on both sides of the connection, TCP presents a byte stream of the data, very
much like a file. Applications can write to and read from a TCP connection very much as they would write and
read files.
APPLICATION LAYER
The application layer is everything above the transport, network, and data-link layers. Application programs
such as web browsers, file transfer programs, web servers, distributed data bases, and electronic mail use the
application program interface (API) of the network protocol stack, and libraries of common networking
routines, to access other computers over the network. The Java language also has many built-in classes that
make network programming much easier.
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