) systems, focusing specifically on the U-Plane components that are relevant from a central processing unit (CPU)/accelerator/memory footprint and performance perspective and therefore particularly of interest to the systems architect. GTP Protocol Two versions of the GPRS Tunneling Protocol (GTP) exist, GTPvl and GTPv2. GTPvl was used in pre-LTE communication interfacing between base station and General Packet Radio Service (GPRS) Support Nodes within a Public Land Mobile Network. GTPv2-C (defined in 3GPP TS 29.274) is used on Evolved Packet Core (EPC) interfaces and used User Datagram Protocol (UDP) port number 2123. GTP version 2 is different to version 1 only in GTP-C, because of required enhancements for bearer handling. GTP-U (defined in 3GPP TS 29.281) encapsulates user plane for Long Term Evolution (LTE) and 5G using UDP port number 2152. A GTP-U tunnel is identified by a Tunnel Endpoint Identifier (TEID), an IP address, and a UDP port number as shown in Fig. 6-1. A short summary of the header fields is: Version-defines the version of the GTP-U protocol and set to 1 for GTP. Protocol Type is set to 1 for GTP. Note that the 0 version is reserved for GTP' (GTP prime) that is used for carrying charging data in GSM/UMTS protocols Extension Header Flag indicates the presence of a meaningful value of the Next Extension Header field. Next Extension Header Type (last byte in the regular GTP header) indicates the type of extension header that follows this field in the Payload Data Unit (PDU). In 5G networks, the 6-bit QoS Flow Identifier (QFI) field is carried inside the GTP-U extension header that we copied (from 3GPPP 38.415) below in Fig. 6-2. We explained the use of TEID and QFI mapping to Quality of Service (QoS) flows earlier in this book. Sequence Number Flag indicates the presence of a meaningful value of the Sequence Number field. When present (as indicated by this flag), the Sequence Number field is incremented for each transmitted packet for those flows whose transmission order needs to be preserved. N-