CSCFs in order to scale the network or to provide redundancy through a fail over mechanism. The Call Session Control Functions (CSCFs) are SIP servers that perform a variety of tasks. The PCSCF is the functional node that handles the initial and subsequent communication with the end user. It acts as a SIP Proxy, forwarding messages from the end user to the other functional elements. It has some security functions such as ensuring that the SIP message is compliant with the relevant protocols, as well as optimization functions such as the compression and decompression of SIP messages that travel over slow access networks. The SIP messages are small compared to the media traffic that is sent, thus this compression serves to reduce the end-to-end delay or total round trip time rather than the amount of bandwidth consumed. As the PCSCF is the point of entry to the IMS network for the end user, it is included in the signaling path and plays a role in every message that the end user sends (even if that role is to forward messages to the other nodes). The SCSCF is of particular interest to the discussion contained within this chapter. It handles the registration of the end user, and contains detail on any sessions currently underway. It communicates with the HSS to load information about the end user, which includes the initial Filter Criteria (iFC) which are used Service Orchestration in the IP Multimedia Subsystem to trigger services. These iFCs will be discussed in further detail in the next section, but for now it is suffice to say that the SCSCF decides where to send the end user's SIP message, that is, the SCSCF decides which services get triggered for a particular request. The ICSCF acts as a point of contact for other IMS net- works, and forwards messages to the local end user on behalf of a foreign net- work. It does not play a crucial role in service triggering, and as such will not be discussed further. The HSS contains information about the end user, including the necessary information to authenticate the use