The GPRS Tunneling Protocol is one of the last common bits of signaling seen in 5G networks, having existed since GPRS was standardized in 1998, and 23 years later, it’s still in use on the user plane.
But networks evolve, and 5G Networks required some extensions to GTP to support these on the N9 and N3 reference points. (UPF to UPF and UPF to gNodeB / Access Network).
3GPP TS 38.415 outlines the PDU session user plane protocol used in 5GC.
The Need for GTP Header Extensions
As increasingly complex QoS capabilities are introduced into 5GC, there is a need to signal certain information on a per-packet basis.
In previous generations of mobile network, traffic could be differentiated with different Tunnel Endpoint Identifiers (TEIDs) but not on a per-packet basis,
The expansion of QoS in 5GC means the UPF of gNodeB may need to set the QoS Flow Identifier per-packet, include delay measurements or signal that Reflective QoS is being used per packet, for this, you need to extend GTP.
Fortunately GTP has support for Extension Headers and this has been leveraged to add the PDU Session Container in the Extension Header of a GTP packet.
In here you can set on a per packet basis:
- QoS Flow Identifier (QFI) – Used to identify the QoS flow to be used (Pretty self explanatory)
- Reflective QoS Indicator (RQI) – To indicate reflective QoS is supported for the encapsulated packet
- Paging Policy Presence (PPP) – To indicate support for Paging Policy Indicator (PPI)
- Paging Policy Indicator (PPI) – Sets parameters of paging policy differentiation to be applied
- QoS Monitoring Packet – Indicates packet is used for QoS Monitoring and DL & UL Timestamps to come
- UL/DL Sending Time Stamps – 64 bit timestamp generated at the time the UPF or UE encodes the packet
- UL/DL Received Time Stamps – 64 bit timestamp generated at the time the UPF or UE received the packet
- UL/DL Delay Indicators – Indicates Delay Results to come
- UL/DL Delay Results – Delay measurement results
- Sequence Number Presence – Indicates if QFI sequence number to come
- UL/DL QFI Sequence Number – Sequence number as assigned by the UPF or gNodeB