on I-CSCF Interrogating - Call Session Control Function S-CSCF Serving - Call Session Control Function User Equipment Media Gateway Control Function Media Gateway Application Server Media Resource Function Controller Breakout Gateway Control Function Home Subscriber Server SIP: Understanding the Session Initiation Protocol The 3GPP architecture relies on Mobile IP instead of some of the mobility aspects of SIP described in the previous section. The reasons for doing SO are pri- marily business related rather then technical in nature. Another requirement of mobility systems is a keep alive signal, which allows end points and proxies to know that a user agent still has network con- nectivity. On an end point to end point basis, this can be done using RTCP (see Section 7.2) reports sent periodically, even when the media is on hold or silence suppression is taking place. However, proxies do not have access to these direct end-to-end reports. Instead, the Session Timer extension [8] and re-INVITES can be used for this purpose. CSCF are SIP proxies that also sometimes behave as a B2BUA under cer- tain circumstances. For example, if a P-CSCF looses the radio link to the UE (user equipment) that contains the SIP UA, it can send a BYE on behalf of the UE to tear down the session. The motivation for doing this is for sessions that have a per-minute billing charge, which the out-of-contact UE would otherwise have to pay for but not have the ability to disconnect. To save bandwidth on the wireless connection, a P-CSCF removes Route, Record-Route, Path, Via, Service-Route and other header fields and reinserts them in the opposite direction. To prevent high bandwidth codecs from being used by a UE, a P-CSCF may edit the list of codecs in an SDP offer or answer, pre- venting the codec from being used. A P-CSCF may change the To and From headers to provide privacy, which is a B2BUA function. The Proxy CSCF provides emergency service, triggers for local services, and does telephone number normalization for the rest of the network