Session Initiation Protocol and me

mzinz

I've been playing a lot with Session Initiation Protocol (SIP).

When I would sniff a packet, I would see the information from each layer, starting with layer 1 of the OSI model:

Data
Ethernet
IP
UDP
SIP

I had been assuming the SIP operated at layer 5, session. After all, we all know that the Session layer is the layer which "creates and breaks down internet connection sessions".

I was very suprised to find out that SIP opporates at layer 7. Why would a protocol like this operate at level 7? All it does is organize sessions..

mikej412

SIP itself has multiple layers.... so you can consider part of it living at layer 5 sometimes (if you want to). Once I hit the SIP URLs.... I was willing to accept it as an application layer protocol like HTTP.

RFC 3261 says:
This document describes Session Initiation Protocol (SIP), an
application-layer control (signaling) protocol for creating,
modifying, and terminating sessions with one or more participants.
These sessions include Internet telephone calls, multimedia
distribution, and multimedia conferences."

If you ignore the OSI "model" and just look at the TCP/IP "model" ....