SIP clarifications?

An "application" is typically a program running on a PC or in a phone that interacts with the user. In this case, the user would control the call somehow by instructing the application, for example by lifting a handset or clicking a mouse. SIP doesn't care how this is done, because that detail is "above" the application layer.

The particular format and timing that SIP uses for the control information is the "protocol". The messages would typically be carried over an IP connection, but SIP doesn't really care if they go by carrier pigeon or smoke signals, because this mechanism is "below" the application layer.

Also, SIP doesn't care how the voice information is carried once the call is set up. It would typically be in an RTP session but this detail is also outside the application layer. All SIP specifies is how to initiate a session, hence the name.

Right.

Thanks Phil.

Cheers, SH