QUIC Steps: Evaluating Pacing Strategies in QUIC Implementations

TL;DR

This paper evaluates different pacing strategies in QUIC implementations, analyzing their effectiveness and impact on network performance, and proposes kernel enhancements for improved pacing precision in user-space protocols.

Contribution

It systematically compares pacing implementations across QUIC stacks and introduces kernel modifications to enhance pacing accuracy and efficiency.

Findings

User-space pacing can be effective with proper system support.

FQ qdisc provides good pacing accuracy for QUIC traffic.

Kernel patch enables precise pacing within GSO buffers.

Abstract

Pacing is a key mechanism in modern transport protocols, used to regulate packet transmission timing to minimize traffic burstiness, lower latency, and reduce packet loss. Standardized in 2021, QUIC is a UDP-based protocol designed to improve upon the TCP / TLS stack. While the QUIC protocol recommends pacing, and congestion control algorithms like BBR rely on it, the user-space nature of QUIC introduces unique challenges. These challenges include coarse-grained timers, system call overhead, and OS scheduling delays, all of which complicate precise packet pacing. This paper investigates how pacing is implemented differently across QUIC stacks, including quiche, picoquic, and ngtcp2, and evaluates the impact of system-level features like GSO and Linux qdiscs on pacing. Using a custom measurement framework and a passive optical fiber tap, we establish a baseline with default settings and systematically explore the effects of qdiscs, hardware offloading using the ETF qdisc, and GSO on pacing precision and network performance. We also extend and evaluate a kernel patch to enable pacing of individual packets within GSO buffers, combining batching efficiency with precise pacing. Kernel-assisted and purely user-space pacing approaches are compared. We show that pacing with only user-space timers can work well, as demonstrated by picoquic with BBR. With quiche, we identify FQ as a qdisc well-suited for pacing QUIC traffic, as it is relatively easy to use and offers precise pacing based on packet timestamps. Our findings provide new insights into the trade-offs involved in implementing pacing in QUIC and highlight potential optimizations for real-world applications like video streaming and video calls.