The GTP layer for the user plane (GTP-U) provides services for carrying user data packets between the GSNs in the GPRS backbone network. Packets from the MS or external data packet network are encapsulated in a packet GTP-U PDU (G-PDU) that consists of a GTP header plus a T-PDU within the GPRS backbone network. A T-PDU corresponds to an IP datagram and is the payload tunneled in the user plane GTP tunnel associated with the concerned PDP context. A GTP tunnel in the user plane is called a GTP-U tunnel and is created during PDP context activation or during PDP context modification procedures.
A GTP-U tunnel is identified by a TEID, an IP address, and a UDP port number. The IP address and the UDP port number define a UDP/IP path, a connectionless path between two endpoints (i.e., SGSN or GGSN). The TEID identifies the tunnel endpoint in the receiving GTP-U protocol entity; it allows for the multiplexing and demultiplexing of GTP tunnels on a UDP/IP path between a given GSN-GSN pair.
When the sequence delivery option is requested by the SGSN or the GGSN during the tunnel management procedures, the sending GTP-U protocol entity sends T-PDUs in sequence. A sequence number is associated with each in-sequence T-PDU and is present in the GTP header. The receiving GTP-U protocol entity checks the sequence delivery of T-PDUs and must reorder out-of-sequence T-PDUs if necessary.