DVMRP uses a concept called soft-state in its messaging protocol. This implies that a join message (for example) is sent periodically by the downstream routers to the upstream router to refresh the join state. An alternative would be to deploy a hard-state messaging scheme, in which an acknowledgement is sent for each message, such that a join and a join-acknowledgement (2 messages) only need to be sent between an upstream and a downstream router on a link. This so-called "hard state" remains until the multicast topology re-configures, for example, due to receivers leaving the group, or because an upstream failure has occurred.

a) Which protocol incurs less overhead on the network?

b) There are concerns regarding the scalability of protocols that use the soft-state approach. What can be done to improve this situation? Suggest an approach to alleviate this problem in soft state approach and discuss its advantages.