GTP-U — The tunnel that carries a packet across N3

5G Systems Notes · Concept map · GTP-U Route · Hierarchy · Index · All concepts · Hub

Sessions, flows and slices

Where it sits

Sits atLevel 5 of the hierarchy · Sessions, flows and slices · explained
Learn firstUPF · QFI — 9 concepts in the full chain, see the paths
UnlocksNothing else depends on it directly.
Primary clauseTS 23.501 §8.3, §5.7.1.1
Used inref-core 5 · ref-session 5 · index 1 · ref-ue-state 1
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is

What it is. The tunnelling protocol that carries user packets between the base station and the UPF across N3 — and between UPFs across N9. TS 23.501 §8.3. The user's packet is wrapped in a GTP-U header, which is itself carried over UDP/IP across the operator's transport network.

Why a tunnel at all, rather than just routing. Three reasons, and each is load-bearing:

The device's address must not depend on where it is. A device's IP address belongs to its UPF anchor. If packets were routed natively, moving to another base station would mean the address became wrong. The tunnel makes the radio path an overlay: the outer header changes as the device moves, the inner header never does.

Per-session and per-flow identity has to survive the transport network. The tunnel header carries a TEID identifying the session's tunnel and a QFI identifying the flow. That is how the base station knows which device a packet is for and how to treat it, without inspecting the packet.

The user's packet must arrive unmodified. Encapsulation is what lets the network mark and steer traffic "without any changes to the e2e packet header" (§5.7.1.1) — including traffic it cannot read.

Two functions in the specification live at this layer, and they are worth noticing. The UPF's list (§6.2.3) includes "Packet duplication in downlink direction and elimination in uplink direction in GTP-U layer" — reliability by sending the same packet down two paths and discarding the loser, the core-network counterpart of PDCP duplication on the radio side. Both exist for the same URLLC reason and neither knows about the other.

The "end marker" is the other: when a session's path moves, the old UPF sends one down the old tunnel so the target knows nothing more is coming and can stop reordering. A one-packet solution to a race that would otherwise be a timeout.

It is not the service-based interface. The control plane of the 5G core is HTTP/2; the user plane is GTP-U, inherited essentially unchanged from GPRS onward. Whenever someone says "the 5G core is all REST", this is the counterexample: no user packet ever travels over HTTP.

Read on

This concept is read as part of one argument in ref-session, alongside the rest of its group.

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Before this concept, the hierarchy says to learn the following — the full chain, in order:

To understand GTP-U (level 5) you first need 9 other concepts. Read them in this order — everything on one line can be read in any order, but no line before the one above it:
Level Learn these
0 5G Core
1 DNN · Network function · Reference point
2 CP/UP split · SMF
3 PDU session · UPF
4 QFI
Immediately before GTP-U: QFI, UPF.
Keep going — where this sits on the route
◀NRFstep 56 · level 5
Step 57 of 118level 5 · Sessions, flows and slices
5G AKAstep 58 · level 5▶
Nothing depends on this one directly
It is a leaf of the graph — follow the route rather than the branch.
↳ all 11 concepts at level 6 · level 5 in full · the whole route
The route is every concept in the folder ordered by level, so nothing here needs anything after it. Computed at page load from terms.json; the same numbering as the route page.
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