SIP is supposed to be an "application layer control protocol" in the RFC. What exactly does the phrase mean? What is a control protocol and what is an application layer control protocol? Could someone give examples?
Secondly, the RFC says SIP is not a vertically integrated communications system. I am interpreting this statement to mean that SIP does not provide details of every layer of the protocol stack, and different solutions for these layers can be mixed and matched to provide a functioning communications infrastructure. Am I right?