Fast UDP I/O for Firefox in Rust

Summary

Motivation# Around 20% of Firefox’s HTTP traffic today uses HTTP/3, which runs over QUIC, which in turn runs over UDP. This translates to substantial UDP I/O activity. Firefox uses NSPR for most of its network I/O. When it comes to UDP I/O, NSPR only offers a limited set of dated APIs, most relevant here PR_SendTo and PR_RecvFrom, wrappers around POSIX’s sendto and recvfrom. The N in NSPR stands for Netscape, giving you a hint of its age. Operating systems have evolved since. Many offer multi-message APIs like sendmmsg and recvmmsg. Some offer segmentation offloading like GSO (Generic Segmentation Offload) and GRO (Generic Receive Offload). Each of these promise significant performance improvements for UDP I/O. Can Firefox benefit from replacing its aging UDP I/O stack with modern system calls? Overview# This project began in mid-2024 with the goal of rewriting Firefox’s QUIC UDP I/O stack using modern system calls across all supported operating systems. Beyond performance improvements, we wanted to increase security by using a memory-safe language to do UDP I/O. Firefox’s QUIC state machine itself is implemented in Rust already. We thereby chose Rust for this project as well, giving us both increased security and easy integration with the existing QUIC codebase. Instead of starting from scratch, we built on top of quinn-udp, the UDP I/O library of the Quinn project, a QUIC implementation in Rust. This sped up our development efforts significantly. Big thank you to the Quinn project. Operating system calls are complex, with a myriad of idiosyncrasies, especially across versions. Firefox is multi-platform, focusing on Windows, Android, MacOS and Linux as tier 1. The main complexity though stems from Firefox supporting ancient versions of each of them, e.g. Android 5. One year later, i.e., mid 2025, this project is now rolling out to the majority of Firefox users. Performance benchmark results are promising. In extreme cases, on purely CPU bound benchmarks, we’re seei...