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One of the bottlenecks of current HTTP is that it relies solely on multiple connections for concurrency. This causes several problems, including additional round trips for connection setup, slow-start delays, and a constant rationing by the client where it tries to avoid opening too many connections to a single server. HTTP "pipelining" doesn't help, as each connection may be blocked on the request at the head of the line; in addition, many proxies apparently have poor support for pipelining. Applications, in their desire to create many connections, create many sub-domains to work around browser per-domain connection throttling.