Section: 5.5.2 Conditions for sending TEID=0 in GTPv2-C header
Content: If a peer's TEID is not available, the TEID field still shall be present in the header and its value shall be set to "0" in the following messages: - Create Session Request message on S2a/S2b/S5/S8 - Create Session Request message on S4/S11, if for a given UE, the SGSN/MME has not yet obtained the Control TEID of the SGW. - Create Indirect Data Forwarding Tunnel Request message on S4/S11, if the SGW selected by the MME/S4-SGSN for indirect data forwarding is different from the SGW used as anchor. - Identification Request/Response messages. - Forward Relocation Request message over the S10, S16 and N26 interfaces, and over the S3 interface during I-RAT handover when ISR is not active. - Forward Relocation Request message over the S3 interface during I-RAT handover between ISR associated nodes, when ISR is active for the UE, and if the node decides to allocate new S3 TEID-C. - Context Request message over the S10, S16, S3 and N26 interfaces. - Relocation Cancel Request message over the S10, S16, S3 and N26 interfaces, except for the case where the old SGSN/MME or AMF has already been assigned the Tunnel Endpoint Identifier Control Plane of the new SGSN/MME or AMF. - Relocation Cancel Response message over the S10, S16, S3 and N26 interfaces if the new SGSN/MME or AMF does not have the Tunnel Endpoint Identifier Control Plane of the old SGSN/MME or AMF. - Delete PDN Connection Set Request/Response messages. - Configuration Transfer Tunnel message. - RAN Information Relay message. - If a node receives a message and the TEID-C in the GTPv2 header of the received message is not known, it shall respond with "Context not found" Cause in the corresponding response message to the sender, the TEID used in the GTPv2-C header in the response message shall be then set to zero. - If a node receives a request message containing protocol error, e.g. Mandatory IE missing, which requires the receiver to reject the message as specified in clause 7.7, it shall reject the request message. For the response message, the node should look up the remote peer's TEID and accordingly set the GTPv2-C header TEID and the message cause code. As an implementation option, the node may not look up the remote peer's TEID and set the GTPv2-C header TEID to zero in the response message. However in this case, the cause code shall not be set to "Context not found". - MBMS Session Start Request message. - PGW Restart Notification / PGW Restart Notification Acknowledge messages. - Downlink Data Notification message sent on S11/S4 as part of the Network Triggered Service Restoration procedure (see 3GPP TS 23.007[ Restoration procedures ]  [17]), and corresponding Downlink Data Notification Acknowledge and Downlink Data Notification Failure Indication if the SGW did not include the Sender F-TEID for Control Plane IE in the Downlink Data Notification message. - Stop Paging Indication message is sent to the the restarted CN node (or another node in the same pool) as part of the Network Triggered Service Restoration procedure with ISR (see 3GPP TS 23.007[ Restoration procedures ]  [17]). - Suspend Notification and Suspend Acknowledge messages: over S16 interface; over S3 interface when ISR is not active. - PGW Downlink Triggering Notification message on S5 and S11/S4, PGW Downlink Triggering Acknowledge message on S11/S4, and PGW Downlink Triggering Acknowledge message on S5 if the PGW did not include the Sender F-TEID for Control Plane IE in the PGW Downlink Triggering Notification message. - UE Registration Query Request and UE Registration Query Response messages over S3 interface. NOTE: Legacy implementation conforming to earlier versions of this specification can send the Change Notification Request/Response messages on the TEID zero in spite of the peer's node TEID being available.