mechanism of the headers. FMS (First Missing): this field is coded on 12, 15 or 18 bits. It contains the first sequence number of the missing PDCP frames. Bitmap: this field is a collection of bits indicating whether the PDCP frame was received correctly (bit to ONE) or not (bit to ZERO). The most significant bit of the first byte represents the sequence number following the value of the FMS field. HRW (Highest Received PDCP SN on WLAN): this field is coded on 12, 15 or 18 bits. It contains the highest value of the PDCP SN parameter of the PDCP frame received on the Wi-Fi interface. NMP (Number of Missing PDCP SDUs): this field is coded on 12, 15 or 18 bits. It contains the number of missing frames from and including the value corresponding to the FMS parameter up to the value of the HRW parameter. MPTCP Aggregation 12.1. Functional architecture In the case of LWA/LWIP aggregation solutions, the LTE (Long-Term Evolution) and Wi-Fi access aggregation is done at the eNB entity level. In MPTCP-based (Multi-Path Transmission Control Protocol) aggregation solutions, aggregation occurs at the TCP layer. The target of MPTCP aggregation is to transmit data using multiple paths without causing modifications to existing infrastructures (the 4G mobile network, Wi-Fi access). The MPTCP connection is performed by the MPTCP client hosted in the mobile and the MPTCP server hosted in an MPTCP proxy. The MPTCP connection is built on TCP connections, each corresponding to an access, LTE or Wi-Fi (Figure 12.1). LTE access 4G network Internet Internet Wi-Fi access Internet MTCP proxy server Server Figure 12.1. Architecture for MPTCP aggregation Wi-Fi Integration to the 4G Mobile Network, First Edition. André Perez. C ISTE Ltd 2018. Published by ISTE Ltd and John Wiley & Sons, Inc. 218 Wi-Fi Integration to the 4G Mobile Network 12.2. TCP The TCP (Transmission Control Protocol) is a reliable protocol for the following reasons: - data transmission is conditional upon the establishment of a connection between the source and destination;