DL Data between CU-UP and DU-UP flows via NRUP protocol layer. NRUP is used for flow control between CU-UP and DU-UP.
And it is used for DL data only. NRUP sits at both CU-UP and DU-UP.
You may refer to 3GPP Spec 38.425 for more information.
You can think of the call flow as:
CU: EGTP-Upper=>SDAP=>PDCP=>NRUP=>EGTP-Lower => DU
DU: EGTP => NRUP => RLC => MAC => L1
DU: L1 => MAC => RLC => EGTP => CU
CU: EGTP-Lower =>PDCP => SDAP=> EGTP-Upper => UPF