Page 77

Global Protocol Classifications

The originating system first sends a connection request to the destination

system. This request contains information that the two systems need to agree
upon before the connection can be established. Some of the information
includes the common protocol, protocol parameters, and required resources.

Protocol parameters

are the window sizes and other possible parameters.

The

window size

is the amount of data that a station can transmit before

needing an acknowledgment from the destination system that all the data
was received without error, or that errors existed and part of the data will
need to be retransmitted.

Required resources

can include necessary band-

width, specific port numbers, or other network resources.

The destination system receives this request; if it can accommodate the

common protocol, protocol attributes, and required resources, it replies with
a connection accept. If, for some reason, the destination system cannot
accommodate any of the requirements sent by the originating system, it
responds with a connection deny. A denied connection may result from a
blocked port on the destination system, insufficient bandwidth between the
systems, or other unavailable requested resources.

Assuming that a connection is established between the two systems, data

and control information are exchanged during the life of the connection.
This data exchange can be considered as a dialogue. First, the originating
system sends data until the window size is reached. It then waits for a
response from the destination system. The destination system sends control
information that informs the originating system what needs to happen next.
The transmission can be an acknowledgment that all data in the transmis-
sion was received without error and that the originating system can send the
next batch of data. In addition, the destination system can also send a mes-
sage informing the originating system that some of the data was missing, cor-
rupted, or had other errors that require the data to be retransmitted.

This procedure can be summarized with the description of three processes:

Sequenced data transfer

Each packet of a session is assigned a sequence

number

Flow control

Acknowledgments are required after a specified amount

of data has been sent.

Error control

Verification of contiguous and non-erroneous packets.

You will learn more about each of these three processes in the following
sections.

www.sybex.com