Section: 4.2.5 Messages with GTPv2 defined replies: Classification of Initial and Triggered Messages
Content: An Initial message is a GTPv2 message that is not triggered as a response to another GTPv2 message across the given interface. The expected reply to a Request message is a Triggered message and the reply has the same message name as the Request but with "Response" replacing "Request". NOTE 1: If the SGW receives a "Create Session Request" on S11/S4, this can trigger either of the following GTPv2 messages across S5/S8: "Create Session Request" or "Modify Bearer Request". However, neither of these messages across S5/S8 is considered to be a Triggered message. If a Request message is a reply to a Command message, then the Request message is a Triggered message; otherwise the Request message is an Initial message. Responses do not have replies except when a "Context Acknowledge" is required as a reply to "Context Response" message as specified in relevant Stage 2 procedures. Context Acknowledge is always triggered message and does not have a reply. NOTE 2: The "Context Acknowledge" message is sent only if the "Context Response" message is received with the acceptance cause. A message whose name ends in "Command" is always an initial message. If a "Command" message fails, the name of the reply message is constructed by replacing "Command" with "Failure Indication". Apart from "Downlink Data Notification Failure Indication" message, a "Failure Indication" is a Triggered message. The "Failure Indication" message does not have a reply. If a "Command" message is successful, its reply will be a Request as specified in relevant Stage 2 procedures. A message whose name ends in "Notification" is always an Initial message, The expected Triggered message in reply has the same message name but with "Acknowledge" replacing "Notification", except for the case of the message "Downlink Data Notification" which has the reply "Downlink Data Notification Acknowledge" and "PGW Resart Notification" which has the reply "PGW Restart Notification Acknowledge". An "Acknowledge" message does not have a reply. CS Paging Indication, Stop Paging Indication, RAN Information Relay, Configuration Transfer Tunnel, Trace Session Activation, Trace Session Deactivation, ISR Status Indication and Downlink Data Notification Failure Indication messages are Initial messages that do not have a reply. A Version Not Supported Indication message is a Triggered message.