Layer 4: The Transport Layer
The transport layer segments data from the sending host??™s system and reassembles the data
into a data stream on the receiving host??™s system. For example, business users in large
corporations often transfer large ?¬?les from ?¬?eld locations to a corporate site. Reliable
delivery of the ?¬?les is important, so the transport layer breaks down large ?¬?les into smaller
segments that are less likely to incur transmission problems.
The boundary between the transport layer and the session layer can be thought of as the
boundary between application protocols and data-?¬‚ow protocols. Whereas the application,
presentation, and session layers are concerned with application issues, the lower four layers
are concerned with data-transport issues.
The transport layer attempts to provide a data-transport service that shields the upper layers
from transport implementation details. Speci?¬?cally, issues such as reliability of transport
between two hosts are the concern of the transport layer. In providing communication
service, the transport layer establishes, maintains, and properly terminates virtual circuits.
Transport error detection and recovery and information ?¬‚ow control provide reliable
service.
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