SIP problem poin-to-point

hi dear developers,
i'm a newbie on android development, and i should create an app that communicate with a device which send an audio/video stream. The connection is point-to-point (without a server SIP in the middle, so easy using just the IP address for the SIP profile)
My problem is begin the communication with the device with the SIP protocol. I need to manage the single SIP message (INVITE, RINGING, ACK, OK, BYE, etc), but i've found just examples with a server registration, or high-level functions that create a SIP session (like android SIP stack) but without providing the basic SIP function that I need (because the fact we insert in the SIP message some variables like video and audio settings).
Somebody knows if exists a library that provides the single basic SIP functions? or an easy management of them?
thanks a lot,
Mike