CST311 - week 3

 

CST311 – Intro to Computer Networks 

We are reviewing the second layer of the top-down approach this week. The transport layer is responsible for communication between end systems. The layer is implemented entirely on the end systems, not the network routers. Messages of an application are sent in segments called packets before being passed to the network layer for encapsulation in the datagram. Application developers choose between distinct types of transport-layer protocols based on use cases. The UDP protocol provides unreliable, connectionless service, while the TCP protocol offers a reliable connection-oriented service. The transport-layer multiplexing and demultiplexing extend the end-to-end service delivery to running processes on the application TCP or UDP sockets. In UDP multiplexing and demultiplexing, the destination port determines which socket receives the segments, and the source port is used as a return address in an inversion manner. On the other hand, the connection-oriented TCP is identified by four tuples: source IP address, source port number, destination IP address, and destination port number.

We also reviewed the UDP checksum mechanism, which determines if the segment was modified during transmission. A UDP segment structure consists of the source port, destination port, segment length (header included), checksum, and application message. Additionally, we examined the TCP segment structure, which provides a reliable connection between hosts.

The sequence number and acknowledgment number are the fields in the TCP header used between sender and receiver to ensure service reliability. For example, the receiver host will drop segments that have already been transmitted once if the sequence number matches a previous one. Another scenario is dropping one of two retransmitted segments if an acknowledgment for the second segment in the original phase is received. Finally, we have looked into TCP connection management and congestion control.