Policy control — PCC rules and URSP — the rules that decide a session's shape

Into the network — PCC rules (TS 23.503 §6.3). A PCC rule pairs a service data flow template (how to recognise traffic) with the treatment to apply: which 5QI, what priority, what bit rates, whether to gate it, redirect it, or charge for it. The PCF gives PCC rules to the SMF, which turns them into packet detection and enforcement rules on the UPF over N4.

Into the device — URSP rules (TS 23.503 §6.6.2). UE Route Selection Policy tells the device how to sort its own traffic: which application, destination or connection capability matches which route selection descriptor — and therefore which S-NSSAI, which DNN, which session type. Delivered over NAS.

URSP is the direction people miss, and it is the one that makes slicing work. Policy is normally imagined as something done to traffic after it arrives. URSP is the operator instructing the device before anything is sent — because the decision which slice does this app use can only be made where the app is, and the network cannot classify traffic it may not be able to read.

Without URSP, a device would have no way to know which of its PDU sessions to put a given application on, and network slicing would be a promise the network could not keep.

TS 23.501 §5.6.1 assumes URSP is current: "The expectation is that the URSP in the UE is always up to date" — and then spends a paragraph on what to do when it is not, which is DNN replacement. The specification is aware that a rule living in the device is a rule that can go stale.

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