Endless Cookies: Exploring TCP Fast Open on the Internet

Abstract

TCP Fast Open (TFO) is a TCP option designed to reduce connection establishment overhead. In this paper, we conduct the first large-scale investigation of TFO adoption on the Internet across both IPv4 and IPv6. Contrary to prior belief, we find a non-negligible rate of server-side adoption, particularly among large content provider networks. We observe similar enablement rates for IPv4 and IPv6, reaching 24% of our target addresses on port 443 (HTTPS). Further, we investigate top domains and find TFO to be more commonly enabled over IPv6 for dual-stack domains than in our broader Internet census. Our results reveal high TFO adoption among hypergiants, including Cloudflare (99%), Akamai (59%), and Google (40%). Additionally, we investigate middlebox-induced path impairments and find that interference with SYN packets affects few networks, suggesting that middleboxes pose only a limited obstacle to TFO operation. Lastly, we explore lightweight yet effective measurement opportunities that leverage server TFO cookies to measure the prevalence of aliased and anycast prefixes.