An application level gateway for traversal of SIP transaction through NATs

SIP has been used to develop many kinds of SIP service, such as instant messaging, conference and presence, and these various services are being bundled to one application. NAT traversal has long been identified as a large problem when considered in the context of the session initiation protocol (SIP). The problem is further confused by the variety of NATs that are available in the market place today and the large number of potential deployment scenarios. In this paper, we suggest an ALG which operates inside NAT for traversal of SIP transactions.