User-perceived latency is recognized as the central performance problem in the Web. We systematically measure factors contributing to this latency, across several locations. Our study reveals that DNS query times, TCP connection establishment, and start-of-session delays at HTTP servers, more so than transmission time, are major causes of long waits. Wait due to these factors also afflicts high-bandwidth users and has detrimental effect on perceived performance. We propose simple techniques that address these factors: (i) pre-resolving host-names (pre-performing DNS lookup); (ii) pre-connecting (prefetching TCP connections prior to issuance of HTTP request); and (iii) pre-warming (sending a "dummy" HTTP HEAD request to Web servers). Trace-based simulations demonstrate a potential to reduce perceived latency dramatically. Our techniques surpass document prefetching in performance improvement per bandwidth used and can be used with non-prefetchable URL. Deployment of these techniques at Web browsers or proxies does not require protocol modifications or the cooperation of other entities. Applicable servers can be identified, for example, by analyzing hyperlinks. Bandwidth overhead is minimal, and so is processing overhead at the user's browser. We propose scalable deployment solutions to control the potential overhead to proxies and particularly to Web servers.
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