Week 3 – CST– 311 Intro to Computer Networks

Module 3 –Transport Layer

This week we studied the transport layer in more detail. While the Network Layer offers logical communication between hosts, the Transport Layer offers logical communication between processes. The two options made available to the application layer for transport-layer protocols are the User Datagram Protocol (UDP) or the Transmission Control Protocol (TCP). The UDP is an unreliable connectionless transport that does not guarantee data delivery or, if delivered, it does not guarantee that the data will be in order. TCP on the other hand, provides reliable in order delivery as well as congestion control, flow control and connection setup.

This chapter also covered multiplexing and demultiplexing. Delivering the data in a transport layer segment to the correct socket is called demultiplexing. Multiplexing is the gathering of data chunks at the source host from different sockets and encapsulating each data chunk with a header to create segments that are passed on to the network layer.

There are many interesting things in this week’s material, but the checksum was especially interesting. The receiving host uses the checksum to make sure that no errors have been introduced in the segment. Sometimes while a segment travels to its destination, it can be altered by noise in the links or while stored in a router. The checksum does this by performing the 1s complement of the sum of all the 16-bit words in the segment. Although the checksum provides some level of error checking, it may still be possible that an error in the checksum can go unnoticed. Sometimes altered bits in the 16-bit words can still produce the same checksum.

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