a) What is the main difference between the communication service provided by the transport layer and the network layer?

b) Does Figure 6-1 illustrate a connectionless transport layer demultiplexing or connection-oriented transport layer demultiplexing? Explain your answer.

c) In order to provide reliable transport layer services, the following mechanisms are used. Explain the purpose of these mechanisms:
(1) Checksum + ACK/NACK
(2)sequence number;
(3) timeout + retransmission

d) Congestion control mechanisms were developed to avoid network system degradation caused by congestion. For some congestion control mechanisms, routers are involved.
(i) Do routers get involved in the congestion control mechanism provided by TCP? Briefly explain your answer.
(ii) Brief describe how the routers are involved in the following two congestion control mechanisms: (1)Traffic-awareRouting;(2)TrafficThrottling