ECN with QUIC: Challenges in the Wild

TL;DR

This study investigates the real-world support and challenges of using Explicit Congestion Notification (ECN) with QUIC, revealing low adoption and validation rates across millions of websites and analyzing underlying causes.

Contribution

It provides the first large-scale measurement analysis of ECN support with QUIC in the wild, highlighting significant gaps and challenges in deployment and validation.

Findings

Only 20% of QUIC hosts mirror ECN codepoints.

Less than 2% of QUIC hosts pass ECN validation, affecting less than 0.3% of websites.

Content providers and network impairments hinder ECN support with QUIC.

Abstract

TCP and QUIC can both leverage ECN to avoid congestion loss and its retransmission overhead. However, both protocols require support of their remote endpoints and it took two decades since the initial standardization of ECN for TCP to reach 80% ECN support and more in the wild. In contrast, the QUIC standard mandates ECN support, but there are notable ambiguities that make it unclear if and how ECN can actually be used with QUIC on the Internet. Hence, in this paper, we analyze ECN support with QUIC in the wild: We conduct repeated measurements on more than 180M domains to identify HTTP/3 websites and analyze the underlying QUIC connections w.r.t. ECN support. We only find 20% of QUIC hosts, providing 6% of HTTP/3 websites, to mirror client ECN codepoints. Yet, mirroring ECN is only half of what is required for ECN with QUIC, as QUIC validates mirrored ECN codepoints to detect network impairments: We observe that less than 2% of QUIC hosts, providing less than 0.3% of HTTP/3 websites, pass this validation. We identify possible root causes in content providers not supporting ECN via QUIC and network impairments hindering ECN. We thus also characterize ECN with QUIC distributedly to traverse other paths and discuss our results w.r.t. QUIC and ECN innovations beyond QUIC.