## 1 Introduction
### Challenges and Contributions
#### 1.2.1 Multipath TCP over IEEE 802.11_ay_  
Discovering a prevailing, efficient, and high-performing multipath TCP scheduling mechanism over 802.11_ay_ mmWave and MIMO beamforming links is undoubtedly a non-trivial task. Substantial research works are being conducted on multipath scheduling to overcome the impeding limitations [9]. To the best of our knowledge, this paper is a first step towards developing an intelligent cross-Layer framework for the analysis and collaborative design of efficient multipath scheduling with optimal bonding and beamforming over IEEE 802.11_ay_ networks.  
Multipath data transport protocols have lately been promoted for network convergence. The potentiality of MPTCP is advocated by 3 GPP [10], where the proposed ATSSS architecture envisions 5 GC-based support for transport layer multipath connectivity across 3 GPP and non-3 GPP networks. As a result, MPTCP schedulers that operate efficaciously in convergent network situations are in very high demand [10].  
Future 6G (sixth generation) and WiFi networks will present new issues for MPTCP scheduling because their links (e.g., mmWave, MIMO, and beamforming advances) impose substantially more transient variation and dynamicity than that of the existing lower frequency networks. Furthermore, future use cases (recall Figure 1) necessitate significant gains in targeted performance requirements and QoE. As a result, multipath scheduler investigation can help illuminate existing and potential unanticipated performance restraints of the emerging applications, ultimately aiding in the invention of a more efficient TRA-Layer approach for 802.11_ay_.  
To this end, we identify the following key bottleneck challenges, being gaps in our knowledge:  
* _Uncertain mmWave links and delays_. Future applications such as trajectory control and haptic communications require extremely low delay control information and high-speed video delivery guarantees. However, as a result of queueing, beamforming, channel bonding, MIMO, mobility-induced handoffs, other connections sharing the link, retransmissions, etc., the time taken to travel a packet from end-to-end is inherently stochastic and time-varying. Therefore, packets can easily arrive at the devices reordered, in which case they need to be buffered until they can be delivered in the correct order to the TRA-Layer [11], creating head-of-line blocking (HoL). The resulting buffering delay can be substantial, to the point where it essentially undermines the speed/latency gain from the use of several 802.11_ay_ links.
* _Unknown delays in learning PHY/MAC dynamics from the TRA layer_. When a connection is first established, the end hosts are unaware of the link's properties (e.g., queuing/retransmissions) and PHY/MAC techniques (e.g., MIMO/bonding, backoffs, etc.). Typically, information is limited to obtained during the first connection handshake and perhaps stored past historical data. As the connection proceeds, feedback is received from packet transmissions, but this feedback is delayed (round trip time (RTT) can correspond to the transmission of hundreds of packets over high-speed 802.11_ay_). Small packets (with short control information from the server) thus have limited information to link characteristics, and bulk connections (8K video streaming uplink to the server) need to learn the link characteristics on the fly (e.g., simultaneous tracking, estimation, and optimization) as packets are being transmitted.
* _Low delay forward error correction (FEC)_. The most efficient way to use coded packets or packet replication (e.g., cloning) over modern WiFi is currently unclear, and the design of low-delay cloning/coding for multipath links remains poorly understood in the literature.