The introduction of persistent connections and pipelining into HTTP has resulted in potential performance benefits, but has also exposed the problem of head-of-line blocking. A simple, compatible, and optional extension to HTTP to allow a server to issue responses out of order could significantly reduce HOL blocking. In this extension, clients add short ID fields to their requests, and servers echo these IDs back in their responses. This extension is defined as a hop-by-hop rather than end-to-end mechanism, so it avoids much of the complexity of the end-to-end approach.