Answer the following questions: 1) What is the different between OSI reference model and TCP/IP protocol stack? 2) What are the advantage of disadvantage of OSI reference model? 3) For a point-to-point link, whether network layer is necessary? 4) What is the different of the error controls performed at data linklayer and transport layer? 5) If each packet has been verified by data link layer in each hop, why does the whole message, consisting of multiple packets, need to be checked again by transport layer