Question

5) Discuss how demultiplexing is carried out using BOTH TCP and UDP. [5 marks]

6) Can the receiver of UDP packet be absolutely sure that no errors have occurred? Explain your answer. [5 marks]

Please explain clearly and label each answer properly.

Answer 1

5.UDP and TCP perform the demultiplexing jobs by including two special fields in the segment headers: the source port number field and the destination port number field.

In TCP, the receiver host uses all of source IP, source port, destination IP and destination port to direct datagram to appropriate socket. While in UDP, the receiver only checks destination port number to direct the datagram.

6.No, the receiver cannot be absolutely certain that no errors have occurred. This is because of the manner in which the checksum for the packet is calculated.Since the data received is from the network layer to transport layer, and we know at network layer we have IP which uses checksum at header only.so there is lots of probability that data might be corrupted.

For example,If the corresponding bits (that would be added together) of two 16-bit words in the packet were 0 and 1 then even if these get flipped to 1 and 0 respectively, the sum still remains the same. Hence, the 1s complement the receiver calculates will also be the same. This means the checksum will verify even if there was transmission error.