What is Downlink Data Delivery Status (DDDS) in 5G NR

Downlink Data Delivery Status (DDDS) is the F1⁃U message sent by DU during the inter-DU-intra-CU handover phase to inform the gNB⁃CU about the highest PDCP PDU Sequence Number (SN) successfully delivered in sequence to UE and the highest NR PDCP PDU SN transmitted to the lower layers of PDCP (i.e. RLC layer).

DDDS enables fast packet retransmission in case of intra-DU handover.

Note: Such a handover can have several reasons which are beyond the scope of this article.