ge that encloses all available IP addresses. For IPv4-only and IPv6-only SIP user agents, only one single IP address is carried in SIP REGISTER messages. For dual-stack SIP user agents, two IP addresses are enclosed. Upon receipt of this message, the Registrar Server stores these addresses. Two registration databases may be maintained, one for IPv4 and another one for IPv6. A second alternative is to send several REGISTER requests using available IP connectivity types. Thus, a dual-stack SIP user agent must send two REGISTER messages. The first one is sent using its IPv4 connectivity and the second one using its IPv6 one(s). Two databases are maintained by the conversational service platform. Consequently, upon reception of an INVITE message, the SIP proxy server may question its registration databases to retrieve the types of the involved parties. A SIP ALG is invoked accordingly. The drawback of this procedure is that it depends on the behavior of the terminal user agents. Instead, we propose to modify the SIP protocol in order to have a standardized behavior. This behavior is described in the next sub-section. 3.4.1.2 New SIP Attribute: "atypes" The purpose of this procedure is to put at the disposal of SIP proxy servers means to discover the connectivity types of involved SIP user agents in order C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security to ease the call routing and to optimize the assignment of temporary addresses through SIP ALGs. This procedure uses only information related to the Appli- cation Layer and not ones related to the Transport Layer to execute its routing logic. This procedure consists in modifying SIP messages in order to carry a new indicator to notify SIP proxy server about the supported IP interfaces (logical or physical ones) and therefore connectivity types. This new SIP indicator is called "atypes" and is enclosed in CONTACT header of SIP requests, mainly REGISTER and INVITE. Below the modified Augmented Backus-Naur Form (ABNF) syntax of CONT