SIP signaling protocol relies on centralized SIP servers deployed on the infrastructure of the network to route SIP messages in order to enable user endpoints to discover each other. However, the lack of the infrastructure in ad hoc networks requires that the network nodes support the tasks of participants discovery and SIP messages routing. In this paper, we propose AdSIP protocol which is a completely distributed architecture for SIP that implements SIP servers in selected nodes. The SIP servers are selected using a distributed algorithm constructing a connected minimal global offensive alliance. AdSIP is implemented and compared under NS-2 with TCA protocol which uses a cluster based approach. The simulation results show the advantages of AdSIP and confirm that it is adapted to mobile ad hoc networks by giving low session establishment time, low control overhead and high service availability.