I came across the terms Protocol and Service interface while studying the OSI model and as far as I can tell they seem to be doing the same thing.
So what's the difference between the protocol and service interface
I suggest you to do a tiny research before posting here. ;)
Protocols have two kinds of interfaces:
Take IP for example. In a network or otherwisely host, it's service interface is what it provides for higher leveled protocols. But also IP can send data to the IPs in another host. So it uses its peer to peer interface to communicate.
In other word, all protocols communicate with the layer (in OSI model) below and above it - which it uses service interface for, and also with the same protocol in another host - which it uses peer to peer interface for. ;)
Because of service interface being "in" the protocol, you can not differentiate them. the protocols consist of interfaces.