ins the values of the TEID parameter, which will enable the source SGW entity to redirect the traffic to the target SGW entity during handover. Upon receiving this message, the source MME entity sends the source eNB entity the command to initiate handover. The source MME entity sends the target MME entity the FORWARD ACCESS CONTEXT NOTIFICATION message to provide it with the elements of the context of the E-RAB bearer, such as the PDCP sequence number. The target MME entity sends the source MME entity the FORWARD RELOCATION NOTIFICATION message to indicate that the handover procedure is complete. Signaling Protocols The new MME entity sends the CONTEXT REQUEST message to the former one in the procedure of TAI updating, to retrieve information about the context of the mobile. The former MME entity provides this information in the CONTEXT RESPONSE message, which may contain a positive or negative response. The new MME entity acknowledges this previous message with the message CONTEXT ACKNOWLEDGE. 2.6. SIP protocol Session initiation protocol (SIP) is a control protocol which can establish, modify and terminate multimedia sessions. Media can be added to or removed from an existing session. SIP is based on a request/response pair such as hypertext transfer protocol (HTTP). Each transaction consists of a request, which uses a particular method and one or more responses. 2.6.1. Requests The request begins with a line containing the method, a uniform resource iidentifier (URI) and the version of the protocol. INVITE sip : bob@homeB. net SIP/2.0 2.6.1.1. REGISTER method The REGISTER method is used by an user agent (UA) to notify the REGISTRAR entity of the correspondence between the IP address of the UA entity and URI concerned. This correspondence is necessary for incoming calls. The use of the URI of the REGISTER request, and of the headers To and From, is slightly different to that of other requests. The URI contains only the domain of the REGISTRAR entity, with no part relating to the user. VoLTE and ViLTE The header