n be executed by external security mechanisms to SIP (IPSEC or TLS). As we conclude, it is necessary to consider the efforts of standardization processing in the improvement of security mechanisms for SIP. The important issue is the problem in agreement on the chosen security mechanism between © 2009 by Taylor & Francis Group, LLC Secure Intelligent SIP Services two SIP entities (user agents and/or proxy) that want to communicate by applying a level of 'sufficient' security. For this reason, it is very important to define how an SIP entity can select an appropriate mechanism to communicate with a next proxy entity. One of the proposals for an agreement security mechanism that allows two agents to exchange their clean security aptitudes of preferences in order to select and apply a common mechanism is one where a client initiates a procedure, the SIP agent includes in the first request sent to the neighbor proxy entity, the list of its mechanisms of security sustained. The other element replies with a list of its clean security mechanisms and its parameters. The client then selects the preferred common security mechanism and then makes use this chosen mechanism (e.g., TLS), to contact the proxy by using the new mechanism of security [12]. With this technique, another problem is identified. It is the problem of the assertion and validation of the user identity by SIP server. The SIP protocol allows a user to assert its identity by several manners (e.g., in the letterhead); but the information of identity requested by the user is not verified in the fundamental SIP operation. On the other hand, an IP telephony client could have initially required the identity of a user in order to provide a specific service and/or to condition the type of service to the user identity. The model of SIP authentication could be a way to obtain such identity; however, the user agents do not always have the necessary key information to authenticate with all the other agents. A model is proposed by Profos [11] for 'confirmed identity' and