These questions relate to networking and telecommunications. What are the requirements of a reliable service and how is each requirement enforced? Also another question, a protocol concerns with communications between: (a) a layer on the two ends (e.g., a client and a server) or (b) a layer and its underneath layer which provides it a service through an interface. For this last question is it a or b? Another question, On Receiver, when Layer 3 receives the message from Layer 2, the number of headers, including encapsulated headers in the message is [3]. What are layers of those headers, starting from the outermost header to the innermost header? There are 5 layers in this network model.