11 Reliable Delivery of Signalling Messages

29.2813GPPGeneral Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U)Release 17TS

Each path maintains a queue with signalling messages to be sent to the peer. The message at the front of the queue, if it is a request for which a response has been defined, shall be sent with a Sequence Number, and shall be held in a path list until a response is received. Each path has its own list. The Sequence Number shall be unique for each outstanding request message sourced from the same IP/UDP endpoint.

The T3-RESPONSE timer shall be started when a signalling request message (for which a response has been defined) is sent. A signalling message request or response has probably been lost if a response has not been received before the T3-RESPONSE timer expires. At the expiry of the timer the request is retransmitted if the total number of request attempts is less than N3‑REQUESTS times.

All received request messages shall be responded to and all response messages associated with a certain request shall always include the same information. Duplicated response messages shall be discarded. A response message without a matching outstanding request should be considered as a duplicate.

If a GTP protocol entity is not successful with the transfer of a signalling message, it shall inform the upper layer of the unsuccessful transfer so that the controlling upper entity may take the necessary measures.