o establish a multimedia session' [13]. (b) Instant message SPIT. This is defined as an unsolicited set of bulk instant messages. (c) Presence SPIT is defined as an unsolicited set of bulk presence requests made with the objective of the initiator to gain membership of the 'address book of a user or potentially of multiple users' [13]. From these definitions SPIT threats are more potent that spam threats because three sources of SPIT attack can be mounted compared with just a single direct spam attack through bulk e-mails. 22.3.1 Threats from Vulnerability of SIP SIP is not an easy protocol to secure. Its use of intermediaries, its multi- faceted trust relationships, its expected usage between elements with no trust at all and its user-to-user operation make security far from trivial. Security solutions are needed that are deployable, without extensive coordination, in a wide variety of environments and usages. In order to meet these diverse needs, several distinct mechanisms applicable to different aspects and usages of SIP are required. Although security and privacy should be mandatory for IP telephony architecture, most of the attention during the initial design of the IETF IP telephony architecture and its SIP has been focused on the possibility of providing new dynamic and powerful services with simplicity. Less attention was paid to security features. IP networks using SIP can replace PSTN provided the same basic telephone service with comparable level of QoS and network security are part and parcel C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security of the IP network. The following security characteristics should therefore be guaranteed [24]: 1. High service availability. 2. Stable and error-free operation. 3. Protection of the user-to-network and user-to-user network traffic. SIP security should cover the overall communication chain. This includes the source, destination, links, signaling, packet headers and messages. In addition, protocols which interface with SIP sho