Section: 4.2.2.1 Initial Messages
Content: The IP Destination Address of a GTPv2 Initial message shall be an IP address of the destination GTPv2 entity. During the establishment of the GTP tunnel, the GTPv2 entity selects and communicates to the peer GTPv2 entity the IP Destination Address at which it expects to receive subsequent control plane Initial messages related to that GTP tunnel via the: - "Sender F-TEID for Control Plane" IE, - "PGW S5/S8/S2a/S2b F-TEID for PMIP based interface or for GTP based Control Plane interface" IE, - "MSC Server Sv Address for Control Plane" IE, - "S3/S16/S10 Address and TEID for Control Plane" IE, or - "MME/SGSN Sv Address for Control Plane" IE. A Create Session Request shall only include in the Sender F-TEID the same IP address type as the destination address used in the IP header. An IPv4/IPv6 capable SGW and PGW may advertize an IPv4 address and/or an IPv6 address in the F-TEID of the above IEs. Upon a change of MME, SGSN or SGW, the new MME, SGSN or SGW may switch to a different IP address type (e.g. IPv6 address) in the IP header if a different IP address type was advertized by the SGW or PGW earlier. A Modify Bearer Request shall only include in the Sender F-TEID the same IP address type as the destination address used in the IP header. NOTE 1: Advertizing a single IP address type in a Create Session Request or a Modify Bearer Request ensures that both GTP-C peers know without ambiguity the IP address type to be used in subsequent control plane Initial messages in the reverse direction related to that GTP-C tunnel, and it avoids intempestive IP address switching during the establishment of the GTP-C tunnel or during an established communication between two GTP-C peers. NOTE 2: IP switching between IPv4 and IPv6 can occur upon a change of MME/SGSN or SGW in deployments with MME/SGSNs or SGWs with different IPv6 capabilities. EXAMPLE 1: If an MME gets IPv4 addresses from the DNS for the SGW, the MME only includes an IPv4 address in the Sender F-TEID IE of the Create Session Request. In the response, the SGW advertises an IPv4 address and optionally an IPv6 address, and the SGW uses IPv4 addressing in subsequent control plane Initial messages it sends to the MME related to that GTP-C tunnel. EXAMPLE 2: As a continuation of EXAMPLE 1, upon a subsequent change of MMEs, assuming the source MME only supports IPv4 and the target MME supports IPv4 and IPv6, the target MME can switch to IPv6 addressing by sending a Modify Bearer Request to the SGW using the SGW S11 IPv6 address in the IP header and including a Sender F-TEID with an MME S11 IPv6 address only. During the network triggered service restoration procedure (see 3GPP TS 23.007[ Restoration procedures ]  [17]), if an MME/S4-SGSN sends a Downlink Data Notification Failure Indication message to the SGW, then the destination address for this message shall be the SGW IP address signalled via the Sender F-TEID for Control Plane IE in the Downlink Data Notification message (if present in the message), otherwise the source IP address of the Downlink Data Notification message received earlier. The IP Source Address of a GTPv2 Initial message shall be an IP address of the source GTPv2 entity from which the Initial message is originating.