  The Session Initiation Protocol (SIP) has become the most predominant
protocol for Voice over Internet Protocol (VoIP) signaling. Security of SIP is
an important consideration for VoIP communication as the traffic is transmitted
over the insecure IP network. And the authentication process in SIP ranges from
pre-shared secret based solutions to Public Key Infrastructure (PKI) based
solution. However, due to the limitations in PKI based solutions, some PKI less
authentications mechanisms are proposed. This paper aims to present an overview
of different authentication methods used in or together with SIP. We start by
highlighting the security issues in SIP in the context of VoIP communication.
Then we illustrate the current activities regarding the SIP authentication
mechanisms including the recent developments in the research community and
standardization efforts within the Internet Engineering Task Force (IETF).
Finally we analyze the security aspects of these approaches.
