The Curious Case of Parallel Connections in HTTP/2

Abstract

Web pages and web-based services are becoming more and more complex. The average page size for the Alexa top 1000 websites in 2016 has reached 2.1 MB and fetching a page requires requests for 128 different objects. Although the bandwidth has been increasing exponentially in the last few years, the web experience is not improving at the same pace because of latency issues in HTTP/1. The HTTP/2 protocol aims to solve these issues by allowing clients and servers to multiplex HTTP requests and responses on a single TCP connection. If HTTP/2 is widely adopted, it can have enormous benefits not only for the user experience, but also for the servers and the network. Since clients do not have to open multiple parallel connections to avoid the problem of head-of-line blocking in HTTP/1.1, the number of concurrent TCP sessions can be significantly reduced. However, although multiplexing is one of the main features of HTTP/2, nothing actually prevents a client from opening multiple HTTP/2 connections to a server. In this paper we investigate the behavior of HTTP/2 traffic in the wild. We perform experiments to examine if web browsers use a single connection per domain over HTTP/2 in practice. Contrary to popular belief, our experiments on the traffic of a large university campus network and a residential network show that a significant number of HTTP/2 accesses are performed using parallel connections to a single domain on a server. We present two possible hypotheses for this behavior and discuss its implications for the future of the web.