Layer 4 - Transport layer

The Transport layers main job is to provide error free end to end delivery of data. The file is broken up into smaller sized data units called segments. The segments are numbered and sent off to the destination. The destination acknowledges receipt of the each segment by replying with an acknowledgement.

Figure 1

Networks are dynamic, meaning that the path to the destination may change while the information is being sent. This can cause the segments to arrive in an out of order sequence. Since there is no guarantee that the segments will arrive in the correct order, the Transport layer will correct out of order segments and put them in the correct order.

If a segment that has been sent to the destination has not been acknowledged within a certain time period by the source. The transport layer will time out and resend the segment. If the destination does not receive or misses a segment in a sequence with in a certain period of time, it will request that the missing segment be resent.

The Transport layer also provides error checking to make sure that data received hasn't been corrupted during transmission. The Transport layer guarantees an error-free host to host connection, it is not concerned with the path between machines.

The Transport layer has several subsections:

TCP Protocol - The Transmission Control Protocol (TCP) is responsible for reliable end to end delivery of segments of information. Segments is the term that is used to describe the data that is transmitted and received at the Transport level of the OSI model where TCP resides. TCP also redirects the data to the appropriate which port (upper level service) that is required.

UDP Protocol - The User Datagram Protocol (UDP) is a connectionless host to host service that operates at the Transport layer of the OSI model. UDP relies on the upper layer protocol for error correction and reliable service. The protocol is transaction oriented, and delivery and duplicate protection are not guaranteed.

Firewalls - A firewall is a transport level device that makes decisions on whether to allow traffic to pass through based on the port numbers. It is a basic security device used to secure the network.

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TelecomWorld 101

Copyright July 2013 Eugene Blanchard