QUIC Steps: Evaluating Pacing Strategies in QUIC Implementations
Journal:
arXiv
Published Date:
May 14, 2025
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.
