In LTE-WiFi aggregation user data IP packets flow through both LTE and Wi-Fi AP. Two new protocols are used for to achieve LTE-WiFi aggregation. Those new protocols are LWAAP and Xw. LWAAP works between UE and eNodeB while Xw protocol is used between WiFi AP and eNodeB.
eNodeB encapsulates user data in the form of PDCP PDUs within LWA-AP packets. After forming LWA-AP packets eNodeB uses GTP-U tunnel protocol to send these packets to Wi-Fi AP. WiFi AP removes GTP-U header and encapsulatesLWA-AP packets within the 802.11 radio frame.
This is the way to send packets from eNodeB to UE in the downlink direction.