This specification defines HPACK, a compression format for efficiently representing HTTP header fields in the context of HTTP/2. Editorial Note (To be removed by RFC Editor) Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at <https:// lists.w3.org/Archives/Public/ietf-http-wg/>. Working Group information can be found at <http://tools.ietf.org/wg/ httpbis/>; that specific to HTTP/2 are at <http://http2.github.io/>. The changes in this draft are summarized in Appendix A.1.