Multicast: a. Define what is a multicast transmission b. What is the purpose of IGMP protocol? Which entities use the protocol? c. Can TCP be used for multicast? Explain your answer d. In PIM-SM, when will we switch to a source-based shortest-path-tree? e. Why a minimum-cost tree is not used in the multicast protocols we have learnt? f. List the main difference(s) between PIM-SM and PIM-DM, and briefly state when each protocol is preferred