Design for Elimination of Clipping Effects and Ghost Ringing in SIP Initiated IPSec based VoIP

Abstract

Security is an important parameter in Voice over Internet Protocol (VoIP) which faces the inherited loopholes of IP networks and few of its own. Security in VoIP has two main aspects. First, securing the call signaling, i.e. securing the IP traffic used for the establishment of the call using Session Initiation Protocol (SIP), which is provided by Secure Multimedia Internet Mail Extension (S/MIME) or Transport layer Security (TLS). Second, securing the call itself, i.e. securing the Real Time transport Protocol (RTP) media session over the User Datagram Protocol. Security for RTP Media is provided at two different layers. Secure Real Time transport Protocol (SRTP) is a security profile of RTP which provide confidentiality service at application layer and Internet Protocol Security (IPSec) at the network layer. IPSec, when initiated by utilizing Offer Answer model of SIP, i.e. SIP initiate IPSec, uses less number of roundtrips with the help of Multimedia Internet Keying (MIKEY) protocol. Call establishment in SIP initiated IPSec deals with cryptographic processing at the caller and the recipient end. Parameter negotiation for IPSec is done through the SIP INVITE message in calling phase and 200OK response in the answering phase when the responder picks up the phone. This IPSec cryptographic processing at caller end makes it unable to send and receive the RTP media packets at the beginning of the RTP session. This situation is called as call clipping. The caller faces media transmit clipping and media reception clipping at the start of the call due to which the caller can neither send nor receive the RTP media packets for first few milliseconds. Ghost ringing is the byproduct of the solution of clipping effect which is also dealt with the use of another provisional response at the cost of ringing delay. An Algorithm for eliminating clipping effect (Caller’s transmission clipping, Caller’s Reception Clipping) is described. The SIP’s extension of PRACK has been used as an essential aid in this solution to provide reliability to unreliable response messages that are used for transporting IPSec cryptographic parameters. The Algorithm describes the process for shifting the IPSec cryptographic processing from the answering phase to the calling phase and a way to eliminate ghost ringing.