Reachability-Based Packet Scheduler of Multipath QUIC for Heterogeneous Mobile Networks

In recent years, we need more bandwidth to enjoy entertainment contents such as video streaming, music and online gaming. To gain enough bandwidth, technologies that combine bandwidth by using multiple interfaces at same time are desired. Multipath transport protocols which combine multiple paths for packet delivery at the transport layer are a promising technology. Such protocols have a mechanism, called “packet scheduler”, to select the interface to send a packet. However, existing studies of the packet scheduler have not explicitly considered the compatibility of mobility with bonding of bandwidth. Therefore, when smartphone users move out of coverage of communication networks such as wireless Local Area Network (LAN) and Long Term Evolution (LTE) by vehicle, packet loss occurs, leading to a decrease of throughput. In this study, we propose a packet scheduler that selects an appropriate communication path so that packets can reach the peer before it moves out of the coverage. Based on routes of a vehicle and the position and communication range of the access point, the time at which a communication path will be lost is predicted. In addition, we employ MPQUIC (Multipath QUIC (Quick UDP Internet Connections)), which is a multipath transport protocol proposed as the extension of QUIC protocol, to reduce the ACK packet loss in multipath communication, and to reduce the time until the starts of retransmission. We evaluated the number of packet losses, the throughput and the time until starts of retransmission using a simulator and show the superiority of proposed method.


Abstract
In recent years, we need more bandwidth to enjoy entertainment contents such as video streaming, music and online gaming. To gain enough bandwidth, technologies that combine bandwidth by using multiple interfaces at same time are desired. Multipath transport protocols which combine multiple paths for packet delivery at the transport layer are a promising technology. Such protocols have a mechanism, called "packet scheduler", to select the interface to send a packet. However, existing studies of the packet scheduler have not explicitly considered the compatibility of mobility with bonding of bandwidth. Therefore, when smartphone users move out of coverage of communication networks such as wireless Local Area Network (LAN) and Long Term Evolution (LTE) by vehicle, packet loss occurs, leading to a decrease of throughput. In this study, we propose a packet scheduler that selects an appropriate communication path so that packets can reach the peer before it moves out of the coverage. Based on routes of a vehicle and the position and communication range of the access point, the time at which a communication path will be lost is predicted. In addition, we employ MPQUIC (Multipath QUIC (Quick UDP Internet Connections)), which is a multipath transport protocol proposed as the extension of QUIC protocol, to reduce the ACK packet loss in multipath communication, and to reduce the time until the starts of retransmission. We evaluated the number of packet losses, the throughput and the time until starts of retransmission using a simulator and show the superiority of proposed method.

Introduction
The advances of smartphones support the services of video streaming, music, game, allowing user to enjoy the entertainment contents [1]. Smartphone enables these applications by connecting to heterogenous networks involving Long Term Evolution (LTE), 5G, and Wi-Fi [2]. Concurrent utilization of WLAN and LTE will increase the available bandwidth, and it is called bandwidth aggregation.
Multipath transport protocol is a technology that supports bandwidth aggregation.
An example application is to shift communication ways to a low-cost Wi-Fi from LTE when a vehicle enters the zone of Wi-Fi services. Multi-path communication is a method to increase the bandwidth of wireless communication. By the combination of multiple ways of communications such as LTE and WLAN, the performance of throughput and delay are expected to be improved.
Multipath transport protocol (MP-TCP) enables the multiple TCP connections in parallel [3], as shown in Figure 1. MP-TCP has multiple modes. The handover mode allows multiple communication path to be seamlessly switched while avoiding the temporary disconnection due to the handover. Furthermore, the full mesh mode allows multiple communication methods to be applied concurrently to improve the bandwidth.
MP-TCP has a mechanism called "packet scheduler" to select an interface to send a packet to. On the other hand, multiple-path TCP cannot support ACK packet trans-mission on a different route of the associated data packets. This problem is caused by the fact that the middle box such as firewall may discard ACK as the unsafe packet. This problem is difficult to solve in TCP and will cause the retransmission due to the loss of ACK packet, leading to the degradation of throughput.
Reference [4] proposed a notification method for MP-TCP based on frame retransmission and the number of successful packet delivery, using mesh mode to improve the throughput. However, this study has not considered the packet Reference [5] examines the congestion window and the utilization of Wi-Fi SNR variation in mobility for MP-TCP. The proposed method alleviates the degradation of throughput and packet loss in path handoff. However, this method considers the seamless handoff instead of bandwidth aggregation.
Reference [2] has basically discussed the issue of packet loss in path disconnection in the utilization of bandwidth aggregation. However, the paper has not included the detail examination of the problem and method.
This study aims to reduce the packet losses and avoid throughput degradation in case the communication paths are broken down during the movement of vehicles that utilize the multipath transport protocol for the bandwidth aggregation in mobile environment. In this paper, we introduce and explicitly examine a method of multipath-QUIC based method to reduce the ACK packet loss in multipath communication. The basic idea is to enable the delivery of ACK packet in a different path with the transmission to overcome the "on-light" problem that causes the losses of ACK packets.
We evaluate the number of packet losses and throughput by comparing the proposed method with the conventional methods. The evaluation results reveal that the proposed method increases the throughput and decreases the packet loses.

Overview of the Proposed Method
In the bandwidth aggregation of multipath transport protocol, mobility incurs the packet loss when a packet is on the road to the destination and the route is disconnected. We call such packets inflight packets. Furthermore, ACK packet loss incurs the delay of retransmission and degradation of throughput.
In this paper, we proposed a method that predicts the route disconnection and stops data delivery in the disconnected route in order to reduce packet losses caused by inflight packets. Moreover, the ACK packet is sent via a different route, avoiding the delay of retransmission.
The overview of proposed method is shown in Figure 2.

The Time of Path Disconnection
The proposed method decides the route-disconnection time by utilizing the travelling information, AP position and the communication range. The travelling information is obtained from the vehicle, while the AP information is got from server. As shown in Figure 3, smartphone estimates the position of disconnection   The time of path disconnection is calculated based on Friss model. Given RX power threshold, we obtain the distance between smartphone and AP as shown by Equations (1) and (2). In the equations, L 0 is the reference path loss at reference distance of d 0 = 1 [m], Pt is the transmission power (dBm), T is the energy threshold (dBm) of the receiving signal, and n is the propagation loss factor.

Downlink Delay Measurement
To measure the downlink delay from server to the client (smartphone), server generates a time stamp when sending packet to the client, which returns the RTT, called down RTT here, to the server. The server and client smooth the RTT based on Equation [3], same with that stated in [6]. And α is set to 0.125 in this study.

Delivery of ACK Using MPQUIC
The proposed method utilizes the MPQUIC, which is a multipath transport pro-  [7]. In QUIC, the header part is encrypted, avoiding being affected by the mid-box. For this reason, MPQUIC is different with MP-TCP, and supports the ACK packet delivery in different paths, as shown in Figure 4 [8].
In the proposed method, on the time to stop packet delivery via WLAN, the smart phone sends ACK packets through another route, so as to avoid the delay of retransmission caused by the loss of ACK packets.

The Process of the Proposed Method
We introduce the procedure of the proposed method as follows based on Figure   5.
1) Vehicle provides the smartphone its travelling path information.
2) Smartphone obtains the information of AP and the communication range of AP from the management server based on it's travelling path.
3) Smartphone computes the WLAN disconnection time TL from the travelling path, AP information, and shares TL information with the server. 4) Smartphone sets path connection with WLAN when entering the scope of WLAN. Then both LTE and WLAN are used for smartphone to communicate with the server. 5) On time Tp, which is derived from TL based on delay time, the scheduler of the smartphone makes the following actions. a) Server stops the packet delivery on WLAN, and selects LTE for packet delivery; b) Smartphone also selects LTE to delivery data packet. This allows ACK frame of receiving packets to be sent via LTE.

Performance Evaluation
In this section, we introduce the system environment and metrics that are employed in the simulation. Table 1 shows the simulation environment. The simulation is carried out based    Figure 6. The client connects to the server using Wi-Fi, which has a delay of 25 ms, and 5 Mbps bandwidth.

Simulation Environment
Since Mininet-WiFi does not support LTE in the simulation, we minimize the  In addition, logfile is outputted involving packet transmission and receiving process since it is difficult to analyze the packet MPQUIC internal states from the packet capture.

The Evaluation Metrics
Based on the above scenario, the proposed method is compared with the method that employs scheduler in terms of low RTT. Low RTT is the min RTT in the shortest path for which the scheduler has the highest priority in packet transmission. Furthermore, for comparison, the ACK frame will not be delivered in a different path with that the data is sent from.

Evaluation Results
The results of packet losses of LTE are shown in Table 2, and Figure 7 shows the average throughput per path. Figure 8 shows the cumulative distribution of packet arriving time.

1) Packet loss
The results in Table 2 show that the proposed method effectively reduces the packet loss by 90 percent in any bandwidth compared with ECF and low RTT. In every case of LTE bandwidth, the proposed method enables a much smaller number of packet losses. We consider the reason is that the proposed method reduces the number of inflight packets. On the other hand, the proposed method still has packet losses. We consider that the fluctuation of signal strength causes the difference with path loss position calculated.

2) Throughput
As shown in Figure 7, compared with low RTT, ECF, the proposed method for Wi-Fi allows 77% increase of the throughput in either case of bandwidth setup. As for the combination of LTE and Wi-Fi, the throughput has the largest   improvement when the bandwidth is set to 7.5 mbps.

Conclusion
The loss of inflight packets causes the loss of data packets. The throughput decreases due to the delay of packet retransmission when the ACK packets cannot be successfully delivered. This research proposed a packet scheduler method that