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Connections Revisited: Transport Service
The "connection" used in transferring data is normally implemented
in a software module in the operating system of each of the
computers involved in the transfer.
This module provides a (so-called) transport
service to processes. The transport service module is
responsible for breaking an incoming stream of data into chunks
(more formally: Transport Protocol Data Units or in Internet
terminology, segments) for transfer across the
network Each segment is prefixed with a transport
header, to indicate to the remote transport service module
what data it contains, and to carry other communications-related
information.
As each segment is received from the network, an
acknowledgement (or ACK)is sent
back, so that damaged or lost segments can be resent, ensuring
reliable communications. The transport protocol
defines the rules by which this is achieved.
Note that the need for reliable interprocess communications is
independent of the nature of the application (in
our example, the WWW), hence this service can be shared by many
different applications which need reliable data transfer across the
network.
In the Internet, the reliable transport service module implements
the TCP protocol.
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