Communications of the ACM
Taking a Long Look at QUIC: An Approach for Rigorous Evaluation of Rapidly Evolving Transport Protocols
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Google's Quick UDP Internet Connections (QUIC) protocol, which implements TCP-like properties at the application layer atop a UDP transport, is now used by the vast majority of Chrome clients accessing Google properties but has no formal state machine specification, limited analysis, and ad-hoc evaluations based on snapshots of the protocol implementation in a small number of environ-merits. Further frustrating attempts to evaluate QUIC is the fact that the protocol is under rapid development, with extensive rewriting of the protocol occurring over the scale of months, making individual studies of the protocol obsolete before publication.
Given this unique scenario, there is a need for alternative techniques for understanding and evaluating QUIC when compared with previous transport-layer protocols. First, we develop an approach that allows us to conduct analysis across multiple versions of QUIC to understand how code changes impact protocol effectiveness. Next, we instrument the source code to infer QUIC's state machine from execution traces. With this model, we run QUIC in a large number of environments that include desktop and mobile, wired and wireless environments and use the state machine to understand differences in transport-and application-layer performance across multiple versions of QUIC and in different environments. QUIC generally outperforms TCP, but we also identified performance issues related to window sizes, re-ordered packets, and multiplexing large number of small objects; further, we identify that QUIC's performance diminishes on mobile devices and over cellular networks.
1. Introduction
Transport-layer congestion control is one of the most important elements for enabling both fair and high utilization of Internet links shared by multiple flows. As such, new transport-layer protocols typically undergo rigorous design, analysis, and evaluation—producing public and repeatable results demonstrating a candidate protocol's correctness and fairness to existing protocols—before deployment in the Operating System (S) kernel at scale.
Because this process takes time, years can pass between development of a new transport-layer protocol and its wide deployment in operating systems. In contrast, developing an application-layer transport (i.e., one not requiring OS kernel support) can enable rapid evolution and innovation by requiring only changes to application code, with the potential cost due to performance issues arising from processing packets in userspace instead of in the kernel.
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