# Enhanced Flow Control for Low Latency in QUIC
## 1 Introduction  
With the recent emergence of the extended reality (XR) and tactile Internet technology, the demand for low-latency transmission and throughput increment has increased. Moreover, network bandwidth is expanding with the emergence of communication technologies, such as Gigabit Ethernet, 5G, and WiFi 6. Wired networks such as Gigabit Ethernet support throughput of tens of Gbps, while data rates of up to several Gbps are available in wireless networks such as 5G and WiFi 6 [1; 2; 3]. With increasing network capability, the total bytes in a web page is also increasing, as shown in Figure 1[4]. The web data have increased by more than 20% in desktop and mobile environments, from 1772 and 1569 KB in 2018 to 2174 and 1957 KB in 2022, respectively. However, the reduction in transmission latency is negligible, despite the median size of a web page being roughly 2.2 MB, which is trivial in comparison to network capacity. We utilized Pingdom to analyze the web data size and page load time of the Amazon, Netflix, YouTube, Naver, and Daum sites [5], and found that the average web page sizes are 4.2 MB, 2.1 MB, 2.6 MB, 2.9 MB, and 1.2 MB, and the average loading times are 1.547, 2.798, 2.219, 0.824, and 1.102 seconds, respectively, as shown in Figure 2. The result indicates that the transmission delay is significant in comparison to the network capacity of Gbps. Furthermore, 5G and WiFi 6 have been prepared to support low-latency communication [6; 7; 8; 9]. In 5G, the 3 GPP establishes three visions: enhanced mobile broadband (EMB), massive machine-type communications (MMTC), and ultra-reliable and low-latency communication (URLLC). Among them, URLLC aims for a delay time of less than 1 ms and transmission reliability of 99.99 % in radio access networks to meet the demand for low-latency communication [6; 7]. In WiFi 6, several methods such as orthogonal frequency division multiple access (OFDMA) and spatial reuse (SR) have been proposed to improve user performance metrics such as reduced latency in heavily crowded areas [8; 9]. Studies on the low-latency transmission of web content in transport and application layers have proposed using SPDY and Quick UDP Internet Connection (QUIC) protocols [10]. SPDY, an application layer protocol, uses multiplexing, header compression, and server push to minimize the web page load time [11]. The proposed features were included in the HTTP/2 standard, but SPDY support was phased out in 2016. The QUICprotocol is a novel transport layer protocol recently standardized by the Internet engineering task force (IETF) [12; 13; 14]. For reduced transmission latency, QUIC employs 0-RTT to reduce the connection establishment time, multiple streams within a connection to avoid head of line (LOL) blocking from sequential TCP delivery, and a new packet number to eliminate retransmission ambiguity. Hence, for network-based services demanding an immediate response, the QUIC protocol is often used to provide low-latency service. Flow control is disabled in TCP, because the buffer size is sufficiently large to accommodate the network-based service; moreover, packet errors caused by flow control are rare. However, the QUIC protocol allows for flow control to avoid buffer overflow, and employs static or autotuning allowances for flow control [15]. The static allowance employs a fixed-size maximum receive window [16]. The sender delivers data within the maximum receive window to avoid buffer overflow. The receiver notifies the sender of a receive window update by sending the MAX_STREAM_DATA frame when the upper layer protocol has read more than half of the maximum receive window, as shown in Figure 3. Because the amount of remaining data in the receive window is not half, the receive window is not updated until new data arrive. After one RTT, the receiver receives new data, and half of the maximum receive window is then updated. This indicates that the sender can only transmit half of the receive window at each RTT [17]. Google proposed autotuning allowance [16].