Signalling emanating from the home IMS (IP Multimedia Subsystem) may be protected through topology hiding. This increases the robustness of the IMS in terms of security. Since functions outside the trust domain of the home IMS have no view of the URI (Uniform Resource Identifiers) that are used to address functions such as the S-CSCF (Serving – Call Session Control Function) or MGCF (Media Gateway Control Functions). The basic process in which topology hiding is implemented is through the encryption of any URI that relates to functions within the home IMS, in SIP signalling messages leaving the network. These include: Via – used to route responses relating to a specific request; Route – indicating the path subsequent signalling messages should take; Record Route – a list of SIP agents that have proxied the specific message.