ARP
Allocation and Retention Priority: a QoS parameter defining the priority level, pre-emption capability, and pre-emption vulnerability of a QoS flow or bearer.
ARP is the parameter that decides who wins when resources run short. It isn't about day-to-day scheduling — packet priority and delay budget handle that — it's about what happens during congestion or admission. ARP carries three things: a priority level (1 to 15, lower is more important), a pre-emption capability flag, and a pre-emption vulnerability flag.
The two flags are what make it interesting. Capability says "this flow is allowed to bump others out to get admitted"; vulnerability says "this flow may itself be torn down to make room for something more important." So an emergency voice call might be set to pre-empt and not be vulnerable, while bulk background data is the opposite. When a new GBR flow needs resources the cell can't currently spare, the admission logic walks ARP to decide whether to reject the newcomer or evict an existing, vulnerable flow.
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