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426</style><link rel="Contents" href="#rfc.toc">
427      <link rel="Author" href="#rfc.authors">
428      <link rel="Copyright" href="#rfc.copyrightnotice">
429      <link rel="Index" href="#rfc.index">
430      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
431      <link rel="Chapter" title="2 Starting HTTP/2.0" href="#rfc.section.2">
432      <link rel="Chapter" title="3 HTTP/2.0 Framing Layer" href="#rfc.section.3">
433      <link rel="Chapter" title="4 HTTP Layering over HTTP/2.0" href="#rfc.section.4">
434      <link rel="Chapter" title="5 Design Rationale and Notes" href="#rfc.section.5">
435      <link rel="Chapter" title="6 Security Considerations" href="#rfc.section.6">
436      <link rel="Chapter" title="7 Privacy Considerations" href="#rfc.section.7">
437      <link rel="Chapter" title="8 Requirements Notation" href="#rfc.section.8">
438      <link rel="Chapter" title="9 Acknowledgements" href="#rfc.section.9">
439      <link rel="Chapter" href="#rfc.section.10" title="10 Normative References">
440      <link rel="Appendix" title="A Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.A">
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442      <meta name="keywords" content="HTTP">
443      <link rel="schema.dct" href="">
444      <meta name="dct.creator" content="Belshe, M.">
445      <meta name="dct.creator" content="Peon, R.">
446      <meta name="dct.creator" content="Thomson, M.">
447      <meta name="dct.creator" content="Melnikov, A.">
448      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-http2-01">
449      <meta name="dct.issued" scheme="ISO8601" content="2013-01-22">
450      <meta name="dct.abstract" content="This document describes an optimised expression of the semantics of the HTTP protocol. The HTTP/2.0 encapsulation enables more efficient transfer of resources over HTTP by providing compressed headers, simultaneous requests, and unsolicited push of resources from server to client. This document is an alternative to, but does not obsolete RFC{http-p1}. The HTTP protocol semantics described in RFC{http-p2..p7} are unmodified.">
451      <meta name="description" content="This document describes an optimised expression of the semantics of the HTTP protocol. The HTTP/2.0 encapsulation enables more efficient transfer of resources over HTTP by providing compressed headers, simultaneous requests, and unsolicited push of resources from server to client. This document is an alternative to, but does not obsolete RFC{http-p1}. The HTTP protocol semantics described in RFC{http-p2..p7} are unmodified.">
452   </head>
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454      <table class="header">
455         <tbody>
456            <tr>
457               <td class="left">HTTPbis Working Group</td>
458               <td class="right">M. Belshe</td>
459            </tr>
460            <tr>
461               <td class="left">Internet-Draft</td>
462               <td class="right">Twist</td>
463            </tr>
464            <tr>
465               <td class="left">Intended status: Informational</td>
466               <td class="right">R. Peon</td>
467            </tr>
468            <tr>
469               <td class="left">Expires: July 26, 2013</td>
470               <td class="right">Google, Inc</td>
471            </tr>
472            <tr>
473               <td class="left"></td>
474               <td class="right">M. Thomson, Editor</td>
475            </tr>
476            <tr>
477               <td class="left"></td>
478               <td class="right">Microsoft</td>
479            </tr>
480            <tr>
481               <td class="left"></td>
482               <td class="right">A. Melnikov, Editor</td>
483            </tr>
484            <tr>
485               <td class="left"></td>
486               <td class="right">Isode Ltd</td>
487            </tr>
488            <tr>
489               <td class="left"></td>
490               <td class="right">January 22, 2013</td>
491            </tr>
492         </tbody>
493      </table>
494      <p class="title">Hypertext Transfer Protocol version 2.0<br><span class="filename">draft-ietf-httpbis-http2-01</span></p>
495      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
496      <p>This document describes an optimised expression of the semantics of the HTTP protocol. The HTTP/2.0 encapsulation enables
497         more efficient transfer of resources over HTTP by providing compressed headers, simultaneous requests, and unsolicited push
498         of resources from server to client.
499      </p> 
500      <p>This document is an alternative to, but does not obsolete RFC{http-p1}. The HTTP protocol semantics described in RFC{http-p2..p7}
501         are unmodified.
502      </p>
503      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
504      <p>This draft is a work-in-progress, and does not yet reflect Working Group consensus.</p> 
505      <p>This draft contains features from the SPDY Protocol as a starting point, as per the Working Group's charter. Future drafts
506         will add, remove and change text, based upon the Working Group's decisions.
507      </p> 
508      <p>Discussion of this draft takes place on the HTTPBIS working group mailing list (, which is archived at &lt;<a href=""></a>&gt;.
509      </p> 
510      <p>The current issues list is at &lt;<a href=""></a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href=""></a>&gt;.
511      </p> 
512      <p>The changes in this draft are summarized in <a href="#changes.since.draft-ietf-httpbis-http2-00" title="Since draft-ietf-httpbis-http2-00">Appendix&nbsp;A.1</a>.
513      </p>
514      <h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1>
515      <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
516      <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
517         working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href=""></a>.
518      </p>
519      <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
520         documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
521         in progress”.
522      </p>
523      <p>This Internet-Draft will expire on July 26, 2013.</p>
524      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
525      <p>Copyright © 2013 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
526      <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href=""></a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights
527         and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
528         text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
529         BSD License.
530      </p>
531      <hr class="noprint">
532      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
533      <ul class="toc">
534         <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#intro">Introduction</a><ul>
535               <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1.1">Document Organization</a></li>
536               <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1.2">Definitions</a></li>
537            </ul>
538         </li>
539         <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#starting">Starting HTTP/2.0</a><ul>
540               <li><a href="#rfc.section.2.1">2.1</a>&nbsp;&nbsp;&nbsp;<a href="#versioning">HTTP/2.0 Version Identification</a></li>
541               <li><a href="#rfc.section.2.2">2.2</a>&nbsp;&nbsp;&nbsp;<a href="#discover-http">Starting HTTP/2.0 for "http:" URIs</a></li>
542               <li><a href="#rfc.section.2.3">2.3</a>&nbsp;&nbsp;&nbsp;<a href="#discover-https">Starting HTTP/2.0 for "https:" URIs</a></li>
543            </ul>
544         </li>
545         <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#FramingLayer">HTTP/2.0 Framing Layer</a><ul>
546               <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1">Session (Connections)</a></li>
547               <li><a href="#rfc.section.3.2">3.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2">Framing</a><ul>
548                     <li><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#ControlFrames">Control frames</a></li>
549                     <li><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#DataFrames">Data frames</a></li>
550                  </ul>
551               </li>
552               <li><a href="#rfc.section.3.3">3.3</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.3">Streams</a><ul>
553                     <li><a href="#rfc.section.3.3.1">3.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#StreamFrames">Stream frames</a></li>
554                     <li><a href="#rfc.section.3.3.2">3.3.2</a>&nbsp;&nbsp;&nbsp;<a href="#StreamCreation">Stream creation</a><ul>
555                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.">Unidirectional streams</a></li>
556                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.">Bidirectional streams</a></li>
557                        </ul>
558                     </li>
559                     <li><a href="#rfc.section.3.3.3">3.3.3</a>&nbsp;&nbsp;&nbsp;<a href="#StreamPriority">Stream priority</a></li>
560                     <li><a href="#rfc.section.3.3.4">3.3.4</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.3.4">Stream headers</a></li>
561                     <li><a href="#rfc.section.3.3.5">3.3.5</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.3.5">Stream data exchange</a></li>
562                     <li><a href="#rfc.section.3.3.6">3.3.6</a>&nbsp;&nbsp;&nbsp;<a href="#StreamHalfClose">Stream half-close</a></li>
563                     <li><a href="#rfc.section.3.3.7">3.3.7</a>&nbsp;&nbsp;&nbsp;<a href="#StreamClose">Stream close</a></li>
564                  </ul>
565               </li>
566               <li><a href="#rfc.section.3.4">3.4</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.4">Error Handling</a><ul>
567                     <li><a href="#rfc.section.3.4.1">3.4.1</a>&nbsp;&nbsp;&nbsp;<a href="#SessionErrorHandler">Session Error Handling</a></li>
568                     <li><a href="#rfc.section.3.4.2">3.4.2</a>&nbsp;&nbsp;&nbsp;<a href="#StreamErrorHandler">Stream Error Handling</a></li>
569                  </ul>
570               </li>
571               <li><a href="#rfc.section.3.5">3.5</a>&nbsp;&nbsp;&nbsp;<a href="#flowcontrol">Stream Flow Control</a><ul>
572                     <li><a href="#rfc.section.3.5.1">3.5.1</a>&nbsp;&nbsp;&nbsp;<a href="#fc-principles">Flow Control Principles</a></li>
573                     <li><a href="#rfc.section.3.5.2">3.5.2</a>&nbsp;&nbsp;&nbsp;<a href="#fc-basic">Basic Flow Control Algorithm</a></li>
574                  </ul>
575               </li>
576               <li><a href="#rfc.section.3.6">3.6</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.6">Control frame types</a><ul>
577                     <li><a href="#rfc.section.3.6.1">3.6.1</a>&nbsp;&nbsp;&nbsp;<a href="#SYN_STREAM">SYN_STREAM</a></li>
578                     <li><a href="#rfc.section.3.6.2">3.6.2</a>&nbsp;&nbsp;&nbsp;<a href="#SYN_REPLY">SYN_REPLY</a></li>
579                     <li><a href="#rfc.section.3.6.3">3.6.3</a>&nbsp;&nbsp;&nbsp;<a href="#RST_STREAM">RST_STREAM</a></li>
580                     <li><a href="#rfc.section.3.6.4">3.6.4</a>&nbsp;&nbsp;&nbsp;<a href="#SETTINGS">SETTINGS</a></li>
581                     <li><a href="#rfc.section.3.6.5">3.6.5</a>&nbsp;&nbsp;&nbsp;<a href="#PING">PING</a></li>
582                     <li><a href="#rfc.section.3.6.6">3.6.6</a>&nbsp;&nbsp;&nbsp;<a href="#GOAWAY">GOAWAY</a></li>
583                     <li><a href="#rfc.section.3.6.7">3.6.7</a>&nbsp;&nbsp;&nbsp;<a href="#HEADERS">HEADERS</a></li>
584                     <li><a href="#rfc.section.3.6.8">3.6.8</a>&nbsp;&nbsp;&nbsp;<a href="#WINDOW_UPDATE">WINDOW_UPDATE</a></li>
585                     <li><a href="#rfc.section.3.6.9">3.6.9</a>&nbsp;&nbsp;&nbsp;<a href="#CREDENTIAL">CREDENTIAL</a></li>
586                     <li><a href="#rfc.section.3.6.10">3.6.10</a>&nbsp;&nbsp;&nbsp;<a href="#HeaderBlock">Name/Value Header Block</a><ul>
587                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#Compression">Compression</a></li>
588                        </ul>
589                     </li>
590                  </ul>
591               </li>
592            </ul>
593         </li>
594         <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#HTTPLayer">HTTP Layering over HTTP/2.0</a><ul>
595               <li><a href="#rfc.section.4.1">4.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.1">Connection Management</a><ul>
596                     <li><a href="#rfc.section.4.1.1">4.1.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.1.1">Use of GOAWAY</a></li>
597                  </ul>
598               </li>
599               <li><a href="#rfc.section.4.2">4.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.2">HTTP Request/Response</a><ul>
600                     <li><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.2.1">Request</a></li>
601                     <li><a href="#rfc.section.4.2.2">4.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.2.2">Response</a></li>
602                     <li><a href="#rfc.section.4.2.3">4.2.3</a>&nbsp;&nbsp;&nbsp;<a href="#Authentication">Authentication</a><ul>
603                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.">Stateless Authentication</a></li>
604                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.">Stateful Authentication</a></li>
605                        </ul>
606                     </li>
607                  </ul>
608               </li>
609               <li><a href="#rfc.section.4.3">4.3</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.3">Server Push Transactions</a><ul>
610                     <li><a href="#rfc.section.4.3.1">4.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.3.1">Server implementation</a></li>
611                     <li><a href="#rfc.section.4.3.2">4.3.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.3.2">Client implementation</a></li>
612                  </ul>
613               </li>
614            </ul>
615         </li>
616         <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5">Design Rationale and Notes</a><ul>
617               <li><a href="#rfc.section.5.1">5.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5.1">Separation of Framing Layer and Application Layer</a></li>
618               <li><a href="#rfc.section.5.2">5.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5.2">Error handling - Framing Layer</a></li>
619               <li><a href="#rfc.section.5.3">5.3</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5.3">One Connection Per Domain</a></li>
620               <li><a href="#rfc.section.5.4">5.4</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5.4">Fixed vs Variable Length Fields</a></li>
621               <li><a href="#rfc.section.5.5">5.5</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5.5">Compression Context(s)</a></li>
622               <li><a href="#rfc.section.5.6">5.6</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5.6">Unidirectional streams</a></li>
623               <li><a href="#rfc.section.5.7">5.7</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5.7">Data Compression</a></li>
624               <li><a href="#rfc.section.5.8">5.8</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5.8">Server Push</a></li>
625            </ul>
626         </li>
627         <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.6">Security Considerations</a><ul>
628               <li><a href="#rfc.section.6.1">6.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.6.1">Use of Same-origin constraints</a></li>
629               <li><a href="#rfc.section.6.2">6.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.6.2">HTTP Headers and HTTP/2.0 Headers</a></li>
630               <li><a href="#rfc.section.6.3">6.3</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.6.3">Cross-Protocol Attacks</a></li>
631               <li><a href="#rfc.section.6.4">6.4</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.6.4">Server Push Implicit Headers</a></li>
632            </ul>
633         </li>
634         <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.7">Privacy Considerations</a><ul>
635               <li><a href="#rfc.section.7.1">7.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.7.1">Long Lived Connections</a></li>
636               <li><a href="#rfc.section.7.2">7.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.7.2">SETTINGS frame</a></li>
637            </ul>
638         </li>
639         <li><a href="#rfc.section.8">8.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.8">Requirements Notation</a></li>
640         <li><a href="#rfc.section.9">9.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.9">Acknowledgements</a></li>
641         <li><a href="#rfc.section.10">10.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">Normative References</a></li>
642         <li><a href="#rfc.authors">Authors' Addresses</a></li>
643         <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul>
644               <li><a href="#rfc.section.A.1">A.1</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.draft-ietf-httpbis-http2-00">Since draft-ietf-httpbis-http2-00</a></li>
645               <li><a href="#rfc.section.A.2">A.2</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.draft-mbelshe-httpbis-spdy-00">Since draft-mbelshe-httpbis-spdy-00</a></li>
646            </ul>
647         </li>
648         <li><a href="#rfc.index">Index</a></li>
649      </ul>
650      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="intro" href="#intro">Introduction</a></h1>
651      <p id="rfc.section.1.p.1">HTTP is a wildly successful protocol. <a href="#HTTP-p1">HTTP/1.1 message encapsulation</a> <cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing" id="rfc.xref.HTTP-p1.1">[HTTP-p1]</cite> is optimized for implementation simplicity and accessibility, not application performance. As such it has several characteristics
652         that have a negative overall effect on application performance.
653      </p>
654      <p id="rfc.section.1.p.2">The HTTP/1.1 encapsulation ensures that only one request can be delivered at a time on a given connection. HTTP/1.1 pipelining,
655         which is not widely deployed, only partially addresses these concerns. Clients that need to make multiple requests therefore
656         use commonly multiple connections to a server or servers in order to reduce the overall latency of those requests.
657      </p>
658      <p id="rfc.section.1.p.3">Furthermore, HTTP/1.1 headers are represented in an inefficient fashion, which, in addition to generating more or larger network
659         packets, can cause the small initial TCP window to fill more quickly than is ideal. This results in excessive latency where
660         multiple requests are made on a new TCP connection.
661      </p>
662      <p id="rfc.section.1.p.4">This document defines an optimized mapping of the HTTP semantics to a TCP connection. This optimization reduces the latency
663         costs of HTTP by allowing parallel requests on the same connection and by using an efficient coding for HTTP headers. Prioritization
664         of requests lets more important requests complete faster, further improving application performance.
665      </p>
666      <p id="rfc.section.1.p.5">HTTP/2.0 applications have an improved impact on network congestion due to the use of fewer TCP connections to achieve the
667         same effect. Fewer TCP connections compete more fairly with other flows. Long-lived connections are also more able to take
668         better advantage of the available network capacity, rather than operating in the slow start phase of TCP.
669      </p>
670      <p id="rfc.section.1.p.6">The HTTP/2.0 encapsulation also enables more efficient processing of messages by providing efficient message framing. Processing
671         of headers in HTTP/2.0 messages is more efficient (for entities that process many messages).
672      </p>
673      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;Document Organization
674      </h2>
675      <p id="rfc.section.1.1.p.1">The HTTP/2.0 Specification is split into three parts: starting HTTP/2.0 (<a href="#starting" title="Starting HTTP/2.0">Section&nbsp;2</a>), which covers how a HTTP/2.0 is started; a framing layer (<a href="#FramingLayer" title="HTTP/2.0 Framing Layer">Section&nbsp;3</a>), which multiplexes a TCP connection into independent, length-prefixed frames; and an HTTP layer (<a href="#HTTPLayer" title="HTTP Layering over HTTP/2.0">Section&nbsp;4</a>), which specifies the mechanism for overlaying HTTP request/response pairs on top of the framing layer. While some of the
676         framing layer concepts are isolated from the HTTP layer, building a generic framing layer has not been a goal. The framing
677         layer is tailored to the needs of the HTTP protocol and server push.
678      </p>
679      <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;Definitions
680      </h2>
681      <p id="rfc.section.1.2.p.1"> </p>
682      <ul class="empty">
683         <li>client: The endpoint initiating the HTTP/2.0 session.</li>
684         <li>connection: A transport-level connection between two endpoints.</li>
685         <li>endpoint: Either the client or server of a connection.</li>
686         <li>frame: A header-prefixed sequence of bytes sent over a HTTP/2.0 session.</li>
687         <li>server: The endpoint which did not initiate the HTTP/2.0 session.</li>
688         <li>session: A synonym for a connection.</li>
689         <li>session error: An error on the HTTP/2.0 session.</li>
690         <li>stream: A bi-directional flow of bytes across a virtual channel within a HTTP/2.0 session.</li>
691         <li>stream error: An error on an individual HTTP/2.0 stream.</li>
692      </ul>
693      <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="starting" href="#starting">Starting HTTP/2.0</a></h1>
694      <p id="rfc.section.2.p.1">Just as HTTP/1.1 does, HTTP/2.0 uses the "http:" and "https:" URI schemes. An HTTP/2.0-capable client is therefore required
695         to discover whether a server (or intermediary) supports HTTP/2.0.
696      </p>
697      <p id="rfc.section.2.p.2">Different discovery mechanisms are defined for "http:" and "https:" URIs. Discovery for "http:" URIs is described in <a href="#discover-http" title="Starting HTTP/2.0 for &#34;http:&#34; URIs">Section&nbsp;2.2</a>; discovery for "https:" URIs is described in <a href="#discover-https" title="Starting HTTP/2.0 for &#34;https:&#34; URIs">Section&nbsp;2.3</a>.
698      </p>
699      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="versioning" href="#versioning">HTTP/2.0 Version Identification</a></h2>
700      <p id="rfc.section.2.1.p.1">HTTP/2.0 is identified in using the string "HTTP/2.0". This identification is used in the HTTP/1.1 Upgrade header, in the <a href="#TLSNPN">TLS-NPN</a> <cite title="TLS Next Protocol Negotiation" id="rfc.xref.TLSNPN.1">[TLSNPN]</cite> [[TBD]] field and other places where protocol identification is required.
701      </p>
702      <p id="rfc.section.2.1.p.2">[[Editor's Note: please remove the following text prior to the publication of a final version of this document.]]</p>
703      <p id="rfc.section.2.1.p.3">Only implementations of the final, published RFC can identify themselves as "HTTP/2.0". Until such an RFC exists, implementations
704         MUST NOT identify themselves using "HTTP/2.0".
705      </p>
706      <p id="rfc.section.2.1.p.4">Examples and text throughout the rest of this document use "HTTP/2.0" as a matter of editorial convenience only. Implementations
707         of draft versions MUST NOT identify using this string.
708      </p>
709      <p id="rfc.section.2.1.p.5">Implementations of draft versions of the protocol MUST add the corresponding draft number to the identifier before the separator
710         ('/'). For example, draft-ietf-httpbis-http2-03 is identified using the string "HTTP-03/2.0".
711      </p>
712      <p id="rfc.section.2.1.p.6">Non-compatible experiments that are based on these draft versions MUST include a further identifier. For example, an experimental
713         implementation of packet mood-based encoding based on draft-ietf-httpbis-http2-07 might identify itself as "HTTP-07-emo/2.0".
714         Note that any label MUST conform with the "token" syntax defined in Section 3.2.4 of <a href="#HTTP-p1" id="rfc.xref.HTTP-p1.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[HTTP-p1]</cite></a>. Experimenters are encouraged to coordinate their experiments on the mailing list.
715      </p>
716      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="discover-http" href="#discover-http">Starting HTTP/2.0 for "http:" URIs</a></h2>
717      <p id="rfc.section.2.2.p.1">A client that makes a request to an "http:" URI without prior knowledge about support for HTTP/2.0 uses the HTTP Upgrade mechanism <a href="#HTTP-p2" id="rfc.xref.HTTP-p2.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[HTTP-p2]</cite></a>. The client makes an HTTP/1.1 request that includes an Upgrade header field identifying HTTP/2.0.
718      </p>
719      <p id="rfc.section.2.2.p.2">For example:</p>
720      <div id="rfc.figure.u.1"></div><pre>   GET /default.htm HTTP/1.1
721   Host:
722   Connection: Upgrade
723   Upgrade: HTTP/2.0
724</pre><p id="rfc.section.2.2.p.4">A server that does not support HTTP/2.0 can respond to the request as though the Upgrade header field were absent:</p>
725      <div id="rfc.figure.u.2"></div><pre>   HTTP/1.1 200 OK
726   Content-length: 243
727   Content-type: text/html
728      ...
729</pre><p id="rfc.section.2.2.p.6">A server that supports HTTP/2.0 can accept the upgrade with a 101 (Switching Protocols) status code. After the empty line
730         that terminates the 101 response, the server can begin sending HTTP/2.0 frames. These frames MUST include a response to the
731         request that initiated the Upgrade.
732      </p>
733      <div id="rfc.figure.u.3"></div><pre>   HTTP/1.1 101 Switching Protocols
734   Connection: Upgrade
735   Upgrade: HTTP/2.0
737   [ HTTP/2.0 frames ...
738</pre><p id="rfc.section.2.2.p.8">A client can learn that a particular server supports HTTP/2.0 by other means. A client MAY immediately send HTTP/2.0 frames
739         to a server that is known to support HTTP/2.0. [[Open Issue: This is not definite. We may yet choose to perform negotiation
740         for every connection. Reasons include intermediaries; phased upgrade of load-balanced server farms; etc...]] [[Open Issue:
741         We need to enumerate the ways that clients can learn of HTTP/2.0 support.]]
742      </p>
743      <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a id="discover-https" href="#discover-https">Starting HTTP/2.0 for "https:" URIs</a></h2>
744      <p id="rfc.section.2.3.p.1">[[TBD, maybe NPN]]</p>
745      <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="FramingLayer" href="#FramingLayer">HTTP/2.0 Framing Layer</a></h1>
746      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;Session (Connections)
747      </h2>
748      <p id="rfc.section.3.1.p.1">The HTTP/2.0 framing layer (or "session") runs atop a reliable transport layer such as <a href="#RFC0793">TCP</a> <cite title="Transmission Control Protocol" id="rfc.xref.RFC0793.1">[RFC0793]</cite>. The client is the TCP connection initiator. HTTP/2.0 connections are persistent connections.
749      </p>
750      <p id="rfc.section.3.1.p.2">For best performance, it is expected that clients will not close open connections until the user navigates away from all web
751         pages referencing a connection, or until the server closes the connection. Servers are encouraged to leave connections open
752         for as long as possible, but can terminate idle connections if necessary. When either endpoint closes the transport-level
753         connection, it MUST first send a GOAWAY (<a href="#GOAWAY" title="GOAWAY">Section&nbsp;3.6.6</a>) frame so that the endpoints can reliably determine if requests finished before the close.
754      </p>
755      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;Framing
756      </h2>
757      <p id="rfc.section.3.2.p.1">Once the connection is established, clients and servers exchange framed messages. There are two types of frames: control frames (<a href="#ControlFrames" title="Control frames">Section&nbsp;3.2.1</a>) and data frames (<a href="#DataFrames" title="Data frames">Section&nbsp;3.2.2</a>). Frames always have a common header which is 8 bytes in length.
758      </p>
759      <p id="rfc.section.3.2.p.2">The first bit is a control bit indicating whether a frame is a control frame or data frame. Control frames carry a version
760         number, a frame type, flags, and a length. Data frames contain the stream ID, flags, and the length for the payload carried
761         after the common header. The simple header is designed to make reading and writing of frames easy.
762      </p>
763      <p id="rfc.section.3.2.p.3">All integer values, including length, version, and type, are in network byte order. HTTP/2.0 does not enforce alignment of
764         types in dynamically sized frames.
765      </p>
766      <h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a id="ControlFrames" href="#ControlFrames">Control frames</a></h3>
767      <div id="rfc.figure.u.4"></div> <pre>+----------------------------------+
768|C| Version(15bits) | Type(16bits) |
770| Flags (8)  |  Length (24 bits)   |
772|               Data               |
774  </pre> <p id="rfc.section.3.2.1.p.2">Control bit: The 'C' bit is a single bit indicating if this is a control message. For control frames this value is always
775         1.
776      </p>
777      <p id="rfc.section.3.2.1.p.3">Version: The version number of the HTTP/2.0 protocol. This document describes HTTP/2.0 version 3.</p>
778      <p id="rfc.section.3.2.1.p.4">Type: The type of control frame. See Control Frames for the complete list of control frames.</p>
779      <p id="rfc.section.3.2.1.p.5">Flags: Flags related to this frame. Flags for control frames and data frames are different.</p>
780      <p id="rfc.section.3.2.1.p.6">Length: An unsigned 24-bit value representing the number of bytes after the length field.</p>
781      <p id="rfc.section.3.2.1.p.7">Data: data associated with this control frame. The format and length of this data is controlled by the control frame type.</p>
782      <p id="rfc.section.3.2.1.p.8">Control frame processing requirements: </p>
783      <ul class="empty">
784         <li>Note that full length control frames (16MB) can be large for implementations running on resource-limited hardware. In such
785            cases, implementations MAY limit the maximum length frame supported. However, all implementations MUST be able to receive
786            control frames of at least 8192 octets in length.
787         </li>
788      </ul>
789      <h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a id="DataFrames" href="#DataFrames">Data frames</a></h3>
790      <div id="rfc.figure.u.5"></div> <pre>+----------------------------------+
791|C|       Stream-ID (31bits)       |
793| Flags (8)  |  Length (24 bits)   |
795|               Data               |
797  </pre> <p id="rfc.section.3.2.2.p.2">Control bit: For data frames this value is always 0.</p>
798      <p id="rfc.section.3.2.2.p.3">Stream-ID: A 31-bit value identifying the stream.</p>
799      <p id="rfc.section.3.2.2.p.4">Flags: Flags related to this frame. Valid flags are: </p>
800      <ul class="empty">
801         <li>0x01 = FLAG_FIN - signifies that this frame represents the last frame to be transmitted on this stream. See Stream Close (<a href="#StreamClose" title="Stream close">Section&nbsp;3.3.7</a>) below.
802         </li>
803         <li>0x02 = FLAG_COMPRESS - indicates that the data in this frame has been compressed.</li>
804      </ul>
805      <p id="rfc.section.3.2.2.p.5">Length: An unsigned 24-bit value representing the number of bytes after the length field. The total size of a data frame is
806         8 bytes + length. It is valid to have a zero-length data frame.
807      </p>
808      <p id="rfc.section.3.2.2.p.6">Data: The variable-length data payload; the length was defined in the length field.</p>
809      <p id="rfc.section.3.2.2.p.7">Data frame processing requirements: </p>
810      <ul class="empty">
811         <li>If an endpoint receives a data frame for a stream-id which is not open and the endpoint has not sent a GOAWAY (<a href="#GOAWAY" title="GOAWAY">Section&nbsp;3.6.6</a>) frame, it MUST send issue a stream error (<a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>) with the error code INVALID_STREAM for the stream-id.
812         </li>
813         <li>If the endpoint which created the stream receives a data frame before receiving a SYN_REPLY on that stream, it is a protocol
814            error, and the recipient MUST issue a stream error (<a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>) with the status code PROTOCOL_ERROR for the stream-id.
815         </li>
816         <li>Implementors note: If an endpoint receives multiple data frames for invalid stream-ids, it MAY close the session.</li>
817         <li>All HTTP/2.0 endpoints MUST accept compressed data frames. Compression of data frames is always done using zlib compression.
818            Each stream initializes and uses its own compression context dedicated to use within that stream. Endpoints are encouraged
819            to use application level compression rather than HTTP/2.0 stream level compression.
820         </li>
821         <li>Each HTTP/2.0 stream sending compressed frames creates its own zlib context for that stream, and these compression contexts
822            MUST be distinct from the compression contexts used with SYN_STREAM/SYN_REPLY/HEADER compression. (Thus, if both endpoints
823            of a stream are compressing data on the stream, there will be two zlib contexts, one for sending and one for receiving).
824         </li>
825      </ul>
826      <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;Streams
827      </h2>
828      <p id="rfc.section.3.3.p.1">Streams are independent sequences of bi-directional data divided into frames with several properties: </p>
829      <ul class="empty">
830         <li>Streams may be created by either the client or server.</li>
831         <li>Streams optionally carry a set of name/value header pairs.</li>
832         <li>Streams can concurrently send data interleaved with other streams.</li>
833         <li>Streams may be cancelled.</li>
834      </ul>
835      <h3 id="rfc.section.3.3.1"><a href="#rfc.section.3.3.1">3.3.1</a>&nbsp;<a id="StreamFrames" href="#StreamFrames">Stream frames</a></h3>
836      <p id="rfc.section.3.3.1.p.1">HTTP/2.0 defines 3 control frames to manage the lifecycle of a stream: </p>
837      <ul class="empty">
838         <li>SYN_STREAM - Open a new stream</li>
839         <li>SYN_REPLY - Remote acknowledgement of a new, open stream</li>
840         <li>RST_STREAM - Close a stream</li>
841      </ul>
842      <h3 id="rfc.section.3.3.2"><a href="#rfc.section.3.3.2">3.3.2</a>&nbsp;<a id="StreamCreation" href="#StreamCreation">Stream creation</a></h3>
843      <p id="rfc.section.3.3.2.p.1">A stream is created by sending a control frame with the type set to SYN_STREAM (<a href="#SYN_STREAM" title="SYN_STREAM">Section&nbsp;3.6.1</a>). If the server is initiating the stream, the Stream-ID must be even. If the client is initiating the stream, the Stream-ID
844         must be odd. 0 is not a valid Stream-ID. Stream-IDs from each side of the connection must increase monotonically as new streams
845         are created. E.g. Stream 2 may be created after stream 3, but stream 7 must not be created after stream 9. Stream IDs do not
846         wrap: when a client or server cannot create a new stream id without exceeding a 31 bit value, it MUST NOT create a new stream.
847      </p>
848      <p id="rfc.section.3.3.2.p.2">The stream-id MUST increase with each new stream. If an endpoint receives a SYN_STREAM with a stream id which is less than
849         any previously received SYN_STREAM, it MUST issue a session error (<a href="#SessionErrorHandler" title="Session Error Handling">Section&nbsp;3.4.1</a>) with the status PROTOCOL_ERROR.
850      </p>
851      <p id="rfc.section.3.3.2.p.3">It is a protocol error to send two SYN_STREAMs with the same stream-id. If a recipient receives a second SYN_STREAM for the
852         same stream, it MUST issue a stream error (<a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>) with the status code PROTOCOL_ERROR.
853      </p>
854      <p id="rfc.section.3.3.2.p.4">Upon receipt of a SYN_STREAM, the recipient can reject the stream by sending a stream error (<a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>) with the error code REFUSED_STREAM. Note, however, that the creating endpoint may have already sent additional frames for
855         that stream which cannot be immediately stopped.
856      </p>
857      <p id="rfc.section.3.3.2.p.5">Once the stream is created, the creator may immediately send HEADERS or DATA frames for that stream, without needing to wait
858         for the recipient to acknowledge.
859      </p>
860      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;Unidirectional streams
861      </h4>
862      <p id="rfc.section.">When an endpoint creates a stream with the FLAG_UNIDIRECTIONAL flag set, it creates a unidirectional stream which the creating
863         endpoint can use to send frames, but the receiving endpoint cannot. The receiving endpoint is implicitly already in the half-closed (<a href="#StreamHalfClose" title="Stream half-close">Section&nbsp;3.3.6</a>) state.
864      </p>
865      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;Bidirectional streams
866      </h4>
867      <p id="rfc.section.">SYN_STREAM frames which do not use the FLAG_UNIDIRECTIONAL flag are bidirectional streams. Both endpoints can send data on
868         a bi-directional stream.
869      </p>
870      <h3 id="rfc.section.3.3.3"><a href="#rfc.section.3.3.3">3.3.3</a>&nbsp;<a id="StreamPriority" href="#StreamPriority">Stream priority</a></h3>
871      <p id="rfc.section.3.3.3.p.1">The creator of a stream assigns a priority for that stream. Priority is represented as an integer from 0 to 7. 0 represents
872         the highest priority and 7 represents the lowest priority.
873      </p>
874      <p id="rfc.section.3.3.3.p.2">The sender and recipient SHOULD use best-effort to process streams in the order of highest priority to lowest priority.</p>
875      <h3 id="rfc.section.3.3.4"><a href="#rfc.section.3.3.4">3.3.4</a>&nbsp;Stream headers
876      </h3>
877      <p id="rfc.section.3.3.4.p.1">Streams carry optional sets of name/value pair headers which carry metadata about the stream. After the stream has been created,
878         and as long as the sender is not closed (<a href="#StreamClose" title="Stream close">Section&nbsp;3.3.7</a>) or half-closed (<a href="#StreamHalfClose" title="Stream half-close">Section&nbsp;3.3.6</a>), each side may send HEADERS frame(s) containing the header data. Header data can be sent in multiple HEADERS frames, and
879         HEADERS frames may be interleaved with data frames.
880      </p>
881      <h3 id="rfc.section.3.3.5"><a href="#rfc.section.3.3.5">3.3.5</a>&nbsp;Stream data exchange
882      </h3>
883      <p id="rfc.section.3.3.5.p.1">Once a stream is created, it can be used to send arbitrary amounts of data. Generally this means that a series of data frames
884         will be sent on the stream until a frame containing the FLAG_FIN flag is set. The FLAG_FIN can be set on a SYN_STREAM (<a href="#SYN_STREAM" title="SYN_STREAM">Section&nbsp;3.6.1</a>), SYN_REPLY (<a href="#SYN_REPLY" title="SYN_REPLY">Section&nbsp;3.6.2</a>), HEADERS (<a href="#HEADERS" title="HEADERS">Section&nbsp;3.6.7</a>) or a DATA (<a href="#DataFrames" title="Data frames">Section&nbsp;3.2.2</a>) frame. Once the FLAG_FIN has been sent, the stream is considered to be half-closed.
885      </p>
886      <h3 id="rfc.section.3.3.6"><a href="#rfc.section.3.3.6">3.3.6</a>&nbsp;<a id="StreamHalfClose" href="#StreamHalfClose">Stream half-close</a></h3>
887      <p id="rfc.section.3.3.6.p.1">When one side of the stream sends a frame with the FLAG_FIN flag set, the stream is half-closed from that endpoint. The sender
888         of the FLAG_FIN MUST NOT send further frames on that stream. When both sides have half-closed, the stream is closed.
889      </p>
890      <p id="rfc.section.3.3.6.p.2">If an endpoint receives a data frame after the stream is half-closed from the sender (e.g. the endpoint has already received
891         a prior frame for the stream with the FIN flag set), it MUST send a RST_STREAM to the sender with the status STREAM_ALREADY_CLOSED.
892      </p>
893      <h3 id="rfc.section.3.3.7"><a href="#rfc.section.3.3.7">3.3.7</a>&nbsp;<a id="StreamClose" href="#StreamClose">Stream close</a></h3>
894      <p id="rfc.section.3.3.7.p.1">There are 3 ways that streams can be terminated: </p>
895      <ul class="empty">
896         <li>Normal termination: Normal stream termination occurs when both sender and recipient have half-closed the stream by sending
897            a FLAG_FIN.
898         </li>
899         <li>Abrupt termination: Either the client or server can send a RST_STREAM control frame at any time. A RST_STREAM contains an
900            error code to indicate the reason for failure. When a RST_STREAM is sent from the stream originator, it indicates a failure
901            to complete the stream and that no further data will be sent on the stream. When a RST_STREAM is sent from the stream recipient,
902            the sender, upon receipt, should stop sending any data on the stream. The stream recipient should be aware that there is a
903            race between data already in transit from the sender and the time the RST_STREAM is received. See Stream Error Handling (<a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>)
904         </li>
905         <li>TCP connection teardown: If the TCP connection is torn down while un-closed streams exist, then the endpoint must assume that
906            the stream was abnormally interrupted and may be incomplete.
907         </li>
908      </ul>
909      <p id="rfc.section.3.3.7.p.2">If an endpoint receives a data frame after the stream is closed, it must send a RST_STREAM to the sender with the status PROTOCOL_ERROR.</p>
910      <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;Error Handling
911      </h2>
912      <p id="rfc.section.3.4.p.1">The HTTP/2.0 framing layer has only two types of errors, and they are always handled consistently. Any reference in this specification
913         to "issue a session error" refers to <a href="#SessionErrorHandler" title="Session Error Handling">Section&nbsp;3.4.1</a>. Any reference to "issue a stream error" refers to <a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>.
914      </p>
915      <h3 id="rfc.section.3.4.1"><a href="#rfc.section.3.4.1">3.4.1</a>&nbsp;<a id="SessionErrorHandler" href="#SessionErrorHandler">Session Error Handling</a></h3>
916      <p id="rfc.section.3.4.1.p.1">A session error is any error which prevents further processing of the framing layer or which corrupts the session compression
917         state. When a session error occurs, the endpoint encountering the error MUST first send a GOAWAY (<a href="#GOAWAY" title="GOAWAY">Section&nbsp;3.6.6</a>) frame with the stream id of most recently received stream from the remote endpoint, and the error code for why the session
918         is terminating. After sending the GOAWAY frame, the endpoint MUST close the TCP connection.
919      </p>
920      <p id="rfc.section.3.4.1.p.2">Note that the session compression state is dependent upon both endpoints always processing all compressed data. If an endpoint
921         partially processes a frame containing compressed data without updating compression state properly, future control frames
922         which use compression will be always be errored. Implementations SHOULD always try to process compressed data so that errors
923         which could be handled as stream errors do not become session errors.
924      </p>
925      <p id="rfc.section.3.4.1.p.3">Note that because this GOAWAY is sent during a session error case, it is possible that the GOAWAY will not be reliably received
926         by the receiving endpoint. It is a best-effort attempt to communicate with the remote about why the session is going down.
927      </p>
928      <h3 id="rfc.section.3.4.2"><a href="#rfc.section.3.4.2">3.4.2</a>&nbsp;<a id="StreamErrorHandler" href="#StreamErrorHandler">Stream Error Handling</a></h3>
929      <p id="rfc.section.3.4.2.p.1">A stream error is an error related to a specific stream-id which does not affect processing of other streams at the framing
930         layer. Upon a stream error, the endpoint MUST send a RST_STREAM (<a href="#RST_STREAM" title="RST_STREAM">Section&nbsp;3.6.3</a>) frame which contains the stream id of the stream where the error occurred and the error status which caused the error. After
931         sending the RST_STREAM, the stream is closed to the sending endpoint. After sending the RST_STREAM, if the sender receives
932         any frames other than a RST_STREAM for that stream id, it will result in sending additional RST_STREAM frames. An endpoint
933         MUST NOT send a RST_STREAM in response to an RST_STREAM, as doing so would lead to RST_STREAM loops. Sending a RST_STREAM
934         does not cause the HTTP/2.0 session to be closed.
935      </p>
936      <p id="rfc.section.3.4.2.p.2">If an endpoint has multiple RST_STREAM frames to send in succession for the same stream-id and the same error code, it MAY
937         coalesce them into a single RST_STREAM frame. (This can happen if a stream is closed, but the remote sends multiple data frames.
938         There is no reason to send a RST_STREAM for each frame in succession).
939      </p>
940      <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="flowcontrol" href="#flowcontrol">Stream Flow Control</a></h2>
941      <p id="rfc.section.3.5.p.1">Multiplexing streams introduces contention for access to the shared TCP connection. Stream contention can result in streams
942         being blocked by other streams. A flow control scheme ensures that streams do not destructively interfere with other streams
943         on the same TCP connection.
944      </p>
945      <h3 id="rfc.section.3.5.1"><a href="#rfc.section.3.5.1">3.5.1</a>&nbsp;<a id="fc-principles" href="#fc-principles">Flow Control Principles</a></h3>
946      <p id="rfc.section.3.5.1.p.1">Experience with TCP congestion control has shown that algorithms can evolve over time to become more sophisticated without
947         requiring protocol changes. TCP congestion control and its evolution is clearly different from HTTP/2.0 flow control, though
948         the evolution of TCP congestion control algorithms shows that a similar approach could be feasible for HTTP/2.0 flow control.
949      </p>
950      <p id="rfc.section.3.5.1.p.2">HTTP/2.0 stream flow control aims to allow for future improvements to flow control algorithms without requiring protocol changes.
951         The following principles guide the HTTP/2.0 design:
952      </p>
953      <ol>
954         <li>Flow control is hop-by-hop, not end-to-end.</li>
955         <li>Flow control is based on window update messages. Receivers advertise how many octets they are prepared to receive on a stream.
956            This is a credit-based scheme.
957         </li>
958         <li>Flow control is directional with overall control provided by the receiver. A receiver MAY choose to set any window size that
959            it desires for each stream [[TBD: ... and for the overall connection]]. A sender MUST respect flow control limits imposed
960            by a receiver. Clients, servers and intermediaries all independently advertise their flow control preferences as a receiver
961            and abide by the flow control limits set by their peer when sending.
962         </li>
963         <li>Flow control can be disabled by a receiver. A receiver can choose to either disable flow control, or to declare an infinite
964            flow control limit. [[TBD: determine whether just one mechanism is sufficient, and then which alternative]]
965         </li>
966         <li>HTTP/2.0 standardizes only the format of the window update message (<a href="#WINDOW_UPDATE" title="WINDOW_UPDATE">Section&nbsp;3.6.8</a>). This does not stipulate how a receiver decides when to send this message or the value that it sends. Nor does it specify
967            how a sender chooses to send packets. Implementations are able to select any algorithm that suits their needs. An example
968            flow control algorithm is described in <a href="#fc-basic" title="Basic Flow Control Algorithm">Section&nbsp;3.5.2</a>.
969         </li>
970      </ol>
971      <p id="rfc.section.3.5.1.p.3">Implementations are also responsible for managing how requests and responses are sent based on priority; choosing how to avoid
972         head of line blocking for requests; and managing the creation of new streams. Algorithm choices for these could interact with
973         any flow control algorithm.
974      </p>
975      <h3 id="rfc.section.3.5.2"><a href="#rfc.section.3.5.2">3.5.2</a>&nbsp;<a id="fc-basic" href="#fc-basic">Basic Flow Control Algorithm</a></h3>
976      <p id="rfc.section.3.5.2.p.1">This section describes a basic flow control algorithm. This algorithm is provided as an example, implementations can use any
977         algorithm that complies with flow control requirements.
978      </p>
979      <p id="rfc.section.3.5.2.p.2">[[Algorithm TBD]]</p>
980      <h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;Control frame types
981      </h2>
982      <h3 id="rfc.section.3.6.1"><a href="#rfc.section.3.6.1">3.6.1</a>&nbsp;<a id="SYN_STREAM" href="#SYN_STREAM">SYN_STREAM</a></h3>
983      <p id="rfc.section.3.6.1.p.1">The SYN_STREAM control frame allows the sender to asynchronously create a stream between the endpoints. See Stream Creation (<a href="#StreamCreation" title="Stream creation">Section&nbsp;3.3.2</a>)
984      </p>
985      <div id="rfc.figure.u.6"></div> <pre>+------------------------------------+
986|1|    version    |         1        |
988|  Flags (8)  |  Length (24 bits)    |
990|X|           Stream-ID (31bits)     |
992|X| Associated-To-Stream-ID (31bits) |
994| Pri|Unused | Slot |                |
995+-------------------+                |
996| Number of Name/Value pairs (int32) |   &lt;+
997+------------------------------------+    |
998|     Length of name (int32)         |    | This section is the
999+------------------------------------+    | "Name/Value Header
1000|           Name (string)            |    | Block", and is
1001+------------------------------------+    | compressed.
1002|     Length of value  (int32)       |    |
1003+------------------------------------+    |
1004|          Value   (string)          |    |
1005+------------------------------------+    |
1006|           (repeats)                |   &lt;+
1007            </pre> <p id="rfc.section.3.6.1.p.3">Flags: Flags related to this frame. Valid flags are: </p>
1008      <ul class="empty">
1009         <li>0x01 = FLAG_FIN - marks this frame as the last frame to be transmitted on this stream and puts the sender in the half-closed (<a href="#StreamHalfClose" title="Stream half-close">Section&nbsp;3.3.6</a>) state.
1010         </li>
1011         <li>0x02 = FLAG_UNIDIRECTIONAL - a stream created with this flag puts the recipient in the half-closed (<a href="#StreamHalfClose" title="Stream half-close">Section&nbsp;3.3.6</a>) state.
1012         </li>
1013      </ul>
1014      <p id="rfc.section.3.6.1.p.4">Length: The length is the number of bytes which follow the length field in the frame. For SYN_STREAM frames, this is 10 bytes
1015         plus the length of the compressed Name/Value block.
1016      </p>
1017      <p id="rfc.section.3.6.1.p.5">Stream-ID: The 31-bit identifier for this stream. This stream-id will be used in frames which are part of this stream.</p>
1018      <p id="rfc.section.3.6.1.p.6">Associated-To-Stream-ID: The 31-bit identifier for a stream which this stream is associated to. If this stream is independent
1019         of all other streams, it should be 0.
1020      </p>
1021      <p id="rfc.section.3.6.1.p.7">Priority: A 3-bit priority (<a href="#StreamPriority" title="Stream priority">Section&nbsp;3.3.3</a>) field.
1022      </p>
1023      <p id="rfc.section.3.6.1.p.8">Unused: 5 bits of unused space, reserved for future use.</p>
1024      <p id="rfc.section.3.6.1.p.9">Slot: An 8 bit unsigned integer specifying the index in the server's CREDENTIAL vector of the client certificate to be used
1025         for this request. see CREDENTIAL frame (<a href="#CREDENTIAL" title="CREDENTIAL">Section&nbsp;3.6.9</a>). The value 0 means no client certificate should be associated with this stream.
1026      </p>
1027      <p id="rfc.section.3.6.1.p.10">Name/Value Header Block: A set of name/value pairs carried as part of the SYN_STREAM. see Name/Value Header Block (<a href="#HeaderBlock" title="Name/Value Header Block">Section&nbsp;3.6.10</a>).
1028      </p>
1029      <p id="rfc.section.3.6.1.p.11">If an endpoint receives a SYN_STREAM which is larger than the implementation supports, it MAY send a RST_STREAM with error
1030         code FRAME_TOO_LARGE. All implementations MUST support the minimum size limits defined in the Control Frames section (<a href="#ControlFrames" title="Control frames">Section&nbsp;3.2.1</a>).
1031      </p>
1032      <h3 id="rfc.section.3.6.2"><a href="#rfc.section.3.6.2">3.6.2</a>&nbsp;<a id="SYN_REPLY" href="#SYN_REPLY">SYN_REPLY</a></h3>
1033      <p id="rfc.section.3.6.2.p.1">SYN_REPLY indicates the acceptance of a stream creation by the recipient of a SYN_STREAM frame.</p>
1034      <div id="rfc.figure.u.7"></div> <pre>+------------------------------------+
1035|1|    version    |         2        |
1037|  Flags (8)  |  Length (24 bits)    |
1039|X|           Stream-ID (31bits)     |
1041| Number of Name/Value pairs (int32) |   &lt;+
1042+------------------------------------+    |
1043|     Length of name (int32)         |    | This section is the
1044+------------------------------------+    | "Name/Value Header
1045|           Name (string)            |    | Block", and is
1046+------------------------------------+    | compressed.
1047|     Length of value  (int32)       |    |
1048+------------------------------------+    |
1049|          Value   (string)          |    |
1050+------------------------------------+    |
1051|           (repeats)                |   &lt;+
1052            </pre> <p id="rfc.section.3.6.2.p.3">Flags: Flags related to this frame. Valid flags are: </p>
1053      <ul class="empty">
1054         <li>0x01 = FLAG_FIN - marks this frame as the last frame to be transmitted on this stream and puts the sender in the half-closed (<a href="#StreamHalfClose" title="Stream half-close">Section&nbsp;3.3.6</a>) state.
1055         </li>
1056      </ul>
1057      <p id="rfc.section.3.6.2.p.4">Length: The length is the number of bytes which follow the length field in the frame. For SYN_REPLY frames, this is 4 bytes
1058         plus the length of the compressed Name/Value block.
1059      </p>
1060      <p id="rfc.section.3.6.2.p.5">Stream-ID: The 31-bit identifier for this stream.</p>
1061      <p id="rfc.section.3.6.2.p.6">If an endpoint receives multiple SYN_REPLY frames for the same active stream ID, it MUST issue a stream error (<a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>) with the error code STREAM_IN_USE.
1062      </p>
1063      <p id="rfc.section.3.6.2.p.7">Name/Value Header Block: A set of name/value pairs carried as part of the SYN_STREAM. see Name/Value Header Block (<a href="#HeaderBlock" title="Name/Value Header Block">Section&nbsp;3.6.10</a>).
1064      </p>
1065      <p id="rfc.section.3.6.2.p.8">If an endpoint receives a SYN_REPLY which is larger than the implementation supports, it MAY send a RST_STREAM with error
1066         code FRAME_TOO_LARGE. All implementations MUST support the minimum size limits defined in the Control Frames section (<a href="#ControlFrames" title="Control frames">Section&nbsp;3.2.1</a>).
1067      </p>
1068      <h3 id="rfc.section.3.6.3"><a href="#rfc.section.3.6.3">3.6.3</a>&nbsp;<a id="RST_STREAM" href="#RST_STREAM">RST_STREAM</a></h3>
1069      <p id="rfc.section.3.6.3.p.1">The RST_STREAM frame allows for abnormal termination of a stream. When sent by the creator of a stream, it indicates the creator
1070         wishes to cancel the stream. When sent by the recipient of a stream, it indicates an error or that the recipient did not want
1071         to accept the stream, so the stream should be closed.
1072      </p>
1073      <div id="rfc.figure.u.8"></div> <pre>+----------------------------------+
1074|1|   version    |         3       |
1076| Flags (8)  |         8           |
1078|X|          Stream-ID (31bits)    |
1080|          Status code             |
1082            </pre> <p id="rfc.section.3.6.3.p.3">Flags: Flags related to this frame. RST_STREAM does not define any flags. This value must be 0.</p>
1083      <p id="rfc.section.3.6.3.p.4">Length: An unsigned 24-bit value representing the number of bytes after the length field. For RST_STREAM control frames, this
1084         value is always 8.
1085      </p>
1086      <p id="rfc.section.3.6.3.p.5">Stream-ID: The 31-bit identifier for this stream.</p>
1087      <p id="rfc.section.3.6.3.p.6">Status code: (32 bits) An indicator for why the stream is being terminated.The following status codes are defined: </p>
1088      <ul class="empty">
1089         <li>1 - PROTOCOL_ERROR. This is a generic error, and should only be used if a more specific error is not available.</li>
1090         <li>2 - INVALID_STREAM. This is returned when a frame is received for a stream which is not active.</li>
1091         <li>3 - REFUSED_STREAM. Indicates that the stream was refused before any processing has been done on the stream.</li>
1092         <li>4 - UNSUPPORTED_VERSION. Indicates that the recipient of a stream does not support the HTTP/2.0 version requested.</li>
1093         <li>5 - CANCEL. Used by the creator of a stream to indicate that the stream is no longer needed.</li>
1094         <li>6 - INTERNAL_ERROR. This is a generic error which can be used when the implementation has internally failed, not due to anything
1095            in the protocol.
1096         </li>
1097         <li>7 - FLOW_CONTROL_ERROR. The endpoint detected that its peer violated the flow control protocol.</li>
1098         <li>8 - STREAM_IN_USE. The endpoint received a SYN_REPLY for a stream already open.</li>
1099         <li>9 - STREAM_ALREADY_CLOSED. The endpoint received a data or SYN_REPLY frame for a stream which is half closed.</li>
1100         <li>10 - INVALID_CREDENTIALS. The server received a request for a resource whose origin does not have valid credentials in the
1101            client certificate vector.
1102         </li>
1103         <li>11 - FRAME_TOO_LARGE. The endpoint received a frame which this implementation could not support. If FRAME_TOO_LARGE is sent
1104            for a SYN_STREAM, HEADERS, or SYN_REPLY frame without fully processing the compressed portion of those frames, then the compression
1105            state will be out-of-sync with the other endpoint. In this case, senders of FRAME_TOO_LARGE MUST close the session.
1106         </li>
1107         <li>Note: 0 is not a valid status code for a RST_STREAM.</li>
1108      </ul>
1109      <p id="rfc.section.3.6.3.p.7">After receiving a RST_STREAM on a stream, the recipient must not send additional frames for that stream, and the stream moves
1110         into the closed state.
1111      </p>
1112      <h3 id="rfc.section.3.6.4"><a href="#rfc.section.3.6.4">3.6.4</a>&nbsp;<a id="SETTINGS" href="#SETTINGS">SETTINGS</a></h3>
1113      <p id="rfc.section.3.6.4.p.1">A SETTINGS frame contains a set of id/value pairs for communicating configuration data about how the two endpoints may communicate.
1114         SETTINGS frames can be sent at any time by either endpoint, are optionally sent, and are fully asynchronous. When the server
1115         is the sender, the sender can request that configuration data be persisted by the client across HTTP/2.0 sessions and returned
1116         to the server in future communications.
1117      </p>
1118      <p id="rfc.section.3.6.4.p.2">Persistence of SETTINGS ID/Value pairs is done on a per origin/IP pair (the "origin" is the set of scheme, host, and port
1119         from the URI. See <a href="#RFC6454" id="rfc.xref.RFC6454.1"><cite title="The Web Origin Concept">[RFC6454]</cite></a>). That is, when a client connects to a server, and the server persists settings within the client, the client SHOULD return
1120         the persisted settings on future connections to the same origin AND IP address and TCP port. Clients MUST NOT request servers
1121         to use the persistence features of the SETTINGS frames, and servers MUST ignore persistence related flags sent by a client.
1122      </p>
1123      <div id="rfc.figure.u.9"></div> <pre>+----------------------------------+
1124|1|   version    |         4       |
1126| Flags (8)  |  Length (24 bits)   |
1128|         Number of entries        |
1130|          ID/Value Pairs          |
1131|             ...                  |
1132            </pre> <p id="rfc.section.3.6.4.p.4">Control bit: The control bit is always 1 for this message.</p>
1133      <p id="rfc.section.3.6.4.p.5">Version: The HTTP/2.0 version number.</p>
1134      <p id="rfc.section.3.6.4.p.6">Type: The message type for a SETTINGS message is 4.</p>
1135      <p id="rfc.section.3.6.4.p.7">Flags: FLAG_SETTINGS_CLEAR_SETTINGS (0x1): When set, the client should clear any previously persisted SETTINGS ID/Value pairs.
1136         If this frame contains ID/Value pairs with the FLAG_SETTINGS_PERSIST_VALUE set, then the client will first clear its existing,
1137         persisted settings, and then persist the values with the flag set which are contained within this frame. Because persistence
1138         is only implemented on the client, this flag can only be used when the sender is the server.
1139      </p>
1140      <p id="rfc.section.3.6.4.p.8">Length: An unsigned 24-bit value representing the number of bytes after the length field. The total size of a SETTINGS frame
1141         is 8 bytes + length.
1142      </p>
1143      <p id="rfc.section.3.6.4.p.9">Number of entries: A 32-bit value representing the number of ID/value pairs in this message.</p>
1144      <p id="rfc.section.3.6.4.p.10">ID: A 32-bit ID number, comprised of 8 bits of flags and 24 bits of unique ID. </p>
1145      <ul class="empty">
1146         <li>ID.flags:
1147            <ul class="empty">
1148               <li>FLAG_SETTINGS_PERSIST_VALUE (0x1): When set, the sender of this SETTINGS frame is requesting that the recipient persist the
1149                  ID/Value and return it in future SETTINGS frames sent from the sender to this recipient. Because persistence is only implemented
1150                  on the client, this flag is only sent by the server.
1151               </li>
1152               <li>FLAG_SETTINGS_PERSISTED (0x2): When set, the sender is notifying the recipient that this ID/Value pair was previously sent
1153                  to the sender by the recipient with the FLAG_SETTINGS_PERSIST_VALUE, and the sender is returning it. Because persistence is
1154                  only implemented on the client, this flag is only sent by the client.
1155               </li>
1156            </ul>
1157         </li>
1158         <li>Defined IDs:
1159            <ul class="empty">
1160               <li>1 - SETTINGS_UPLOAD_BANDWIDTH allows the sender to send its expected upload bandwidth on this channel. This number is an estimate.
1161                  The value should be the integral number of kilobytes per second that the sender predicts as an expected maximum upload channel
1162                  capacity.
1163               </li>
1164               <li>2 - SETTINGS_DOWNLOAD_BANDWIDTH allows the sender to send its expected download bandwidth on this channel. This number is
1165                  an estimate. The value should be the integral number of kilobytes per second that the sender predicts as an expected maximum
1166                  download channel capacity.
1167               </li>
1168               <li>3 - SETTINGS_ROUND_TRIP_TIME allows the sender to send its expected round-trip-time on this channel. The round trip time is
1169                  defined as the minimum amount of time to send a control frame from this client to the remote and receive a response. The value
1170                  is represented in milliseconds.
1171               </li>
1172               <li>4 - SETTINGS_MAX_CONCURRENT_STREAMS allows the sender to inform the remote endpoint the maximum number of concurrent streams
1173                  which it will allow. By default there is no limit. For implementors it is recommended that this value be no smaller than 100.
1174               </li>
1175               <li>5 - SETTINGS_CURRENT_CWND allows the sender to inform the remote endpoint of the current TCP CWND value.</li>
1176               <li>6 - SETTINGS_DOWNLOAD_RETRANS_RATE allows the sender to inform the remote endpoint the retransmission rate (bytes retransmitted
1177                  / total bytes transmitted).
1178               </li>
1179               <li>7 - SETTINGS_INITIAL_WINDOW_SIZE allows the sender to inform the remote endpoint the initial window size (in bytes) for new
1180                  streams.
1181               </li>
1182               <li>8 - SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE allows the server to inform the client of the new size of the client certificate
1183                  vector.
1184               </li>
1185            </ul>
1186         </li>
1187      </ul>
1188      <p id="rfc.section.3.6.4.p.11">Value: A 32-bit value.</p>
1189      <p id="rfc.section.3.6.4.p.12">The message is intentionally extensible for future information which may improve client-server communications. The sender
1190         does not need to send every type of ID/value. It must only send those for which it has accurate values to convey. When multiple
1191         ID/value pairs are sent, they should be sent in order of lowest id to highest id. A single SETTINGS frame MUST not contain
1192         multiple values for the same ID. If the recipient of a SETTINGS frame discovers multiple values for the same ID, it MUST ignore
1193         all values except the first one.
1194      </p>
1195      <p id="rfc.section.3.6.4.p.13">A server may send multiple SETTINGS frames containing different ID/Value pairs. When the same ID/Value is sent twice, the
1196         most recent value overrides any previously sent values. If the server sends IDs 1, 2, and 3 with the FLAG_SETTINGS_PERSIST_VALUE
1197         in a first SETTINGS frame, and then sends IDs 4 and 5 with the FLAG_SETTINGS_PERSIST_VALUE, when the client returns the persisted
1198         state on its next SETTINGS frame, it SHOULD send all 5 settings (1, 2, 3, 4, and 5 in this example) to the server.
1199      </p>
1200      <h3 id="rfc.section.3.6.5"><a href="#rfc.section.3.6.5">3.6.5</a>&nbsp;<a id="PING" href="#PING">PING</a></h3>
1201      <p id="rfc.section.3.6.5.p.1">The PING control frame is a mechanism for measuring a minimal round-trip time from the sender. It can be sent from the client
1202         or the server. Recipients of a PING frame should send an identical frame to the sender as soon as possible (if there is other
1203         pending data waiting to be sent, PING should take highest priority). Each ping sent by a sender should use a unique ID.
1204      </p>
1205      <div id="rfc.figure.u.10"></div> <pre>+----------------------------------+
1206|1|   version    |         6       |
1208| 0 (flags) |     4 (length)       |
1210|            32-bit ID             |
1212            </pre> <p id="rfc.section.3.6.5.p.3">Control bit: The control bit is always 1 for this message.</p>
1213      <p id="rfc.section.3.6.5.p.4">Version: The HTTP/2.0 version number.</p>
1214      <p id="rfc.section.3.6.5.p.5">Type: The message type for a PING message is 6.</p>
1215      <p id="rfc.section.3.6.5.p.6">Length: This frame is always 4 bytes long.</p>
1216      <p id="rfc.section.3.6.5.p.7">ID: A unique ID for this ping, represented as an unsigned 32 bit value. When the client initiates a ping, it must use an odd
1217         numbered ID. When the server initiates a ping, it must use an even numbered ping. Use of odd/even IDs is required in order
1218         to avoid accidental looping on PINGs (where each side initiates an identical PING at the same time).
1219      </p>
1220      <p id="rfc.section.3.6.5.p.8">Note: If a sender uses all possible PING ids (e.g. has sent all 2^31 possible IDs), it can wrap and start re-using IDs.</p>
1221      <p id="rfc.section.3.6.5.p.9">If a server receives an even numbered PING which it did not initiate, it must ignore the PING. If a client receives an odd
1222         numbered PING which it did not initiate, it must ignore the PING.
1223      </p>
1224      <h3 id="rfc.section.3.6.6"><a href="#rfc.section.3.6.6">3.6.6</a>&nbsp;<a id="GOAWAY" href="#GOAWAY">GOAWAY</a></h3>
1225      <p id="rfc.section.3.6.6.p.1">The GOAWAY control frame is a mechanism to tell the remote side of the connection to stop creating streams on this session.
1226         It can be sent from the client or the server. Once sent, the sender will not respond to any new SYN_STREAMs on this session.
1227         Recipients of a GOAWAY frame must not send additional streams on this session, although a new session can be established for
1228         new streams. The purpose of this message is to allow an endpoint to gracefully stop accepting new streams (perhaps for a reboot
1229         or maintenance), while still finishing processing of previously established streams.
1230      </p>
1231      <p id="rfc.section.3.6.6.p.2">There is an inherent race condition between an endpoint sending SYN_STREAMs and the remote sending a GOAWAY message. To deal
1232         with this case, the GOAWAY contains a last-stream-id indicating the stream-id of the last stream which was created on the
1233         sending endpoint in this session. If the receiver of the GOAWAY sent new SYN_STREAMs for sessions after this last-stream-id,
1234         they were not processed by the server and the receiver may treat the stream as though it had never been created at all (hence
1235         the receiver may want to re-create the stream later on a new session).
1236      </p>
1237      <p id="rfc.section.3.6.6.p.3">Endpoints should always send a GOAWAY message before closing a connection so that the remote can know whether a stream has
1238         been partially processed or not. (For example, if an HTTP client sends a POST at the same time that a server closes a connection,
1239         the client cannot know if the server started to process that POST request if the server does not send a GOAWAY frame to indicate
1240         where it stopped working).
1241      </p>
1242      <p id="rfc.section.3.6.6.p.4">After sending a GOAWAY message, the sender must ignore all SYN_STREAM frames for new streams.</p>
1243      <div id="rfc.figure.u.11"></div> <pre>+----------------------------------+
1244|1|   version    |         7       |
1246| 0 (flags) |     8 (length)       |
1248|X|  Last-good-stream-ID (31 bits) |
1250|          Status code             |
1252            </pre> <p id="rfc.section.3.6.6.p.6">Control bit: The control bit is always 1 for this message.</p>
1253      <p id="rfc.section.3.6.6.p.7">Version: The HTTP/2.0 version number.</p>
1254      <p id="rfc.section.3.6.6.p.8">Type: The message type for a GOAWAY message is 7.</p>
1255      <p id="rfc.section.3.6.6.p.9">Length: This frame is always 8 bytes long.</p>
1256      <p id="rfc.section.3.6.6.p.10">Last-good-stream-Id: The last stream id which was replied to (with either a SYN_REPLY or RST_STREAM) by the sender of the
1257         GOAWAY message. If no streams were replied to, this value MUST be 0.
1258      </p>
1259      <p id="rfc.section.3.6.6.p.11">Status: The reason for closing the session. </p>
1260      <ul class="empty">
1261         <li>0 - OK. This is a normal session teardown.</li>
1262         <li>1 - PROTOCOL_ERROR. This is a generic error, and should only be used if a more specific error is not available.</li>
1263         <li>2 - INTERNAL_ERROR. This is a generic error which can be used when the implementation has internally failed, not due to anything
1264            in the protocol.
1265         </li>
1266      </ul>
1267      <h3 id="rfc.section.3.6.7"><a href="#rfc.section.3.6.7">3.6.7</a>&nbsp;<a id="HEADERS" href="#HEADERS">HEADERS</a></h3>
1268      <p id="rfc.section.3.6.7.p.1">The HEADERS frame augments a stream with additional headers. It may be optionally sent on an existing stream at any time.
1269         Specific application of the headers in this frame is application-dependent. The name/value header block within this frame
1270         is compressed.
1271      </p>
1272      <div id="rfc.figure.u.12"></div> <pre>+------------------------------------+
1273|1|   version     |          8       |
1275| Flags (8)  |   Length (24 bits)    |
1277|X|          Stream-ID (31bits)      |
1279| Number of Name/Value pairs (int32) |   &lt;+
1280+------------------------------------+    |
1281|     Length of name (int32)         |    | This section is the
1282+------------------------------------+    | "Name/Value Header
1283|           Name (string)            |    | Block", and is
1284+------------------------------------+    | compressed.
1285|     Length of value  (int32)       |    |
1286+------------------------------------+    |
1287|          Value   (string)          |    |
1288+------------------------------------+    |
1289|           (repeats)                |   &lt;+
1290            </pre> <p id="rfc.section.3.6.7.p.3">Flags: Flags related to this frame. Valid flags are: </p>
1291      <ul class="empty">
1292         <li>0x01 = FLAG_FIN - marks this frame as the last frame to be transmitted on this stream and puts the sender in the half-closed (<a href="#StreamHalfClose" title="Stream half-close">Section&nbsp;3.3.6</a>) state.
1293         </li>
1294      </ul>
1295      <p id="rfc.section.3.6.7.p.4">Length: An unsigned 24 bit value representing the number of bytes after the length field. The minimum length of the length
1296         field is 4 (when the number of name value pairs is 0).
1297      </p>
1298      <p id="rfc.section.3.6.7.p.5">Stream-ID: The stream this HEADERS block is associated with.</p>
1299      <p id="rfc.section.3.6.7.p.6">Name/Value Header Block: A set of name/value pairs carried as part of the SYN_STREAM. see Name/Value Header Block (<a href="#HeaderBlock" title="Name/Value Header Block">Section&nbsp;3.6.10</a>).
1300      </p>
1301      <h3 id="rfc.section.3.6.8"><a href="#rfc.section.3.6.8">3.6.8</a>&nbsp;<a id="WINDOW_UPDATE" href="#WINDOW_UPDATE">WINDOW_UPDATE</a></h3>
1302      <p id="rfc.section.3.6.8.p.1">The WINDOW_UPDATE control frame is used to implement per stream flow control in HTTP/2.0. Flow control in HTTP/2.0 is per
1303         hop, that is, only between the two endpoints of a HTTP/2.0 connection. If there are one or more intermediaries between the
1304         client and the origin server, flow control signals are not explicitly forwarded by the intermediaries. (However, throttling
1305         of data transfer by any recipient may have the effect of indirectly propagating flow control information upstream back to
1306         the original sender.) Flow control only applies to the data portion of data frames. Recipients must buffer all control frames.
1307         If a recipient fails to buffer an entire control frame, it MUST issue a stream error (<a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>) with the status code FLOW_CONTROL_ERROR for the stream.
1308      </p>
1309      <p id="rfc.section.3.6.8.p.2">Flow control in HTTP/2.0 is implemented by a data transfer window kept by the sender of each stream. The data transfer window
1310         is a simple uint32 that indicates how many bytes of data the sender can transmit. After a stream is created, but before any
1311         data frames have been transmitted, the sender begins with the initial window size. This window size is a measure of the buffering
1312         capability of the recipient. The sender must not send a data frame with data length greater than the transfer window size.
1313         After sending each data frame, the sender decrements its transfer window size by the amount of data transmitted. When the
1314         window size becomes less than or equal to 0, the sender must pause transmitting data frames. At the other end of the stream,
1315         the recipient sends a WINDOW_UPDATE control back to notify the sender that it has consumed some data and freed up buffer space
1316         to receive more data.
1317      </p>
1318      <div id="rfc.figure.u.13"></div> <pre>+----------------------------------+
1319|1|   version    |         9       |
1321| 0 (flags) |     8 (length)       |
1323|X|     Stream-ID (31-bits)        |
1325|X|  Delta-Window-Size (31-bits)   |
1327            </pre> <p id="rfc.section.3.6.8.p.4">Control bit: The control bit is always 1 for this message.</p>
1328      <p id="rfc.section.3.6.8.p.5">Version: The HTTP/2.0 version number.</p>
1329      <p id="rfc.section.3.6.8.p.6">Type: The message type for a WINDOW_UPDATE message is 9.</p>
1330      <p id="rfc.section.3.6.8.p.7">Length: The length field is always 8 for this frame (there are 8 bytes after the length field).</p>
1331      <p id="rfc.section.3.6.8.p.8">Stream-ID: The stream ID that this WINDOW_UPDATE control frame is for.</p>
1332      <p id="rfc.section.3.6.8.p.9">Delta-Window-Size: The additional number of bytes that the sender can transmit in addition to existing remaining window size.
1333         The legal range for this field is 1 to 2^31 - 1 (0x7fffffff) bytes.
1334      </p>
1335      <p id="rfc.section.3.6.8.p.10">The window size as kept by the sender must never exceed 2^31 (although it can become negative in one special case). If a sender
1336         receives a WINDOW_UPDATE that causes the its window size to exceed this limit, it must send RST_STREAM with status code FLOW_CONTROL_ERROR
1337         to terminate the stream.
1338      </p>
1339      <p id="rfc.section.3.6.8.p.11">When a HTTP/2.0 connection is first established, the default initial window size for all streams is 64KB. An endpoint can
1340         use the SETTINGS control frame to adjust the initial window size for the connection. That is, its peer can start out using
1341         the 64KB default initial window size when sending data frames before receiving the SETTINGS. Because SETTINGS is asynchronous,
1342         there may be a race condition if the recipient wants to decrease the initial window size, but its peer immediately sends 64KB
1343         on the creation of a new connection, before waiting for the SETTINGS to arrive. This is one case where the window size kept
1344         by the sender will become negative. Once the sender detects this condition, it must stop sending data frames and wait for
1345         the recipient to catch up. The recipient has two choices:
1346      </p>
1347      <ul class="empty">
1348         <li>immediately send RST_STREAM with FLOW_CONTROL_ERROR status code.</li>
1349         <li>allow the head of line blocking (as there is only one stream for the session and the amount of data in flight is bounded by
1350            the default initial window size), and send WINDOW_UPDATE as it consumes data.
1351         </li>
1352      </ul>
1353      <p id="rfc.section.3.6.8.p.12">In the case of option 2, both sides must compute the window size based on the initial window size in the SETTINGS. For example,
1354         if the recipient sets the initial window size to be 16KB, and the sender sends 64KB immediately on connection establishment,
1355         the sender will discover its window size is -48KB on receipt of the SETTINGS. As the recipient consumes the first 16KB, it
1356         must send a WINDOW_UPDATE of 16KB back to the sender. This interaction continues until the sender's window size becomes positive
1357         again, and it can resume transmitting data frames.
1358      </p>
1359      <p id="rfc.section.3.6.8.p.13">After the recipient reads in a data frame with FLAG_FIN that marks the end of the data stream, it should not send WINDOW_UPDATE
1360         frames as it consumes the last data frame. A sender should ignore all the WINDOW_UPDATE frames associated with the stream
1361         after it send the last frame for the stream.
1362      </p>
1363      <p id="rfc.section.3.6.8.p.14">The data frames from the sender and the WINDOW_UPDATE frames from the recipient are completely asynchronous with respect to
1364         each other. This property allows a recipient to aggressively update the window size kept by the sender to prevent the stream
1365         from stalling.
1366      </p>
1367      <h3 id="rfc.section.3.6.9"><a href="#rfc.section.3.6.9">3.6.9</a>&nbsp;<a id="CREDENTIAL" href="#CREDENTIAL">CREDENTIAL</a></h3>
1368      <p id="rfc.section.3.6.9.p.1">The CREDENTIAL control frame is used by the client to send additional client certificates to the server. A HTTP/2.0 client
1369         may decide to send requests for resources from different origins on the same HTTP/2.0 session if it decides that that server
1370         handles both origins. For example if the IP address associated with both hostnames matches and the SSL server certificate
1371         presented in the initial handshake is valid for both hostnames. However, because the SSL connection can contain at most one
1372         client certificate, the client needs a mechanism to send additional client certificates to the server.
1373      </p>
1374      <p id="rfc.section.3.6.9.p.2">The server is required to maintain a vector of client certificates associated with a HTTP/2.0 session. When the client needs
1375         to send a client certificate to the server, it will send a CREDENTIAL frame that specifies the index of the slot in which
1376         to store the certificate as well as proof that the client posesses the corresponding private key. The initial size of this
1377         vector must be 8. If the client provides a client certificate during the first TLS handshake, the contents of this certificate
1378         must be copied into the first slot (index 1) in the CREDENTIAL vector, though it may be overwritten by subsequent CREDENTIAL
1379         frames. The server must exclusively use the CREDENTIAL vector when evaluating the client certificates associated with an origin.
1380         The server may change the size of this vector by sending a SETTINGS frame with the setting SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE
1381         value specified. In the event that the new size is smaller than the current size, truncation occurs preserving lower-index
1382         slots as possible.
1383      </p>
1384      <p id="rfc.section.3.6.9.p.3">TLS renegotiation with client authentication is incompatible with HTTP/2.0 given the multiplexed nature of HTTP/2.0. Specifically,
1385         imagine that the client has 2 requests outstanding to the server for two different pages (in different tabs). When the renegotiation
1386         + client certificate request comes in, the browser is unable to determine which resource triggered the client certificate
1387         request, in order to prompt the user accordingly.
1388      </p>
1389      <div id="rfc.figure.u.14"></div> <pre>+----------------------------------+
1392| flags (8)  |  Length (24 bits)   |
1394|  Slot (16 bits) |                |
1395+-----------------+                |
1396|      Proof Length (32 bits)      |
1398|               Proof              |
1399+----------------------------------+ &lt;+
1400|   Certificate Length (32 bits)   |  |
1401+----------------------------------+  | Repeated until end of frame
1402|            Certificate           |  |
1403+----------------------------------+ &lt;+
1404            </pre> <p id="rfc.section.3.6.9.p.5">Slot: The index in the server's client certificate vector where this certificate should be stored. If there is already a certificate
1405         stored at this index, it will be overwritten. The index is one based, not zero based; zero is an invalid slot index.
1406      </p>
1407      <p id="rfc.section.3.6.9.p.6">Proof: Cryptographic proof that the client has possession of the private key associated with the certificate. The format is
1408         a TLS digitally-signed element (<a href="#RFC5246" id="rfc.xref.RFC5246.1"><cite title="The Transport Layer Security (TLS) Protocol Version 1.2">[RFC5246]</cite></a>, <a href="">Section 4.7</a>). The signature algorithm must be the same as that used in the CertificateVerify message. However, since the MD5+SHA1 signature
1409         type used in TLS 1.0 connections can not be correctly encoded in a digitally-signed element, SHA1 must be used when MD5+SHA1
1410         was used in the SSL connection. The signature is calculated over a 32 byte TLS extractor value (
1411         with a label of "EXPORTER HTTP/2.0 certificate proof" using the empty string as context. ForRSA certificates the signature
1412         would be a PKCS#1 v1.5 signature. For ECDSA, it would be an ECDSA-Sig-Value (
1413         For a 1024-bit RSA key, the CREDENTIAL message would be ~500 bytes.
1414      </p>
1415      <p id="rfc.section.3.6.9.p.7">Certificate: The certificate chain, starting with the leaf certificate. Each certificate must be encoded as a 32 bit length,
1416         followed by a DER encoded certificate. The certificate must be of the same type (RSA, ECDSA, etc) as the client certificate
1417         associated with the SSL connection.
1418      </p>
1419      <p id="rfc.section.3.6.9.p.8">If the server receives a request for a resource with unacceptable credential (either missing or invalid), it must reply with
1420         a RST_STREAM frame with the status code INVALID_CREDENTIALS. Upon receipt of a RST_STREAM frame with INVALID_CREDENTIALS,
1421         the client should initiate a new stream directly to the requested origin and resend the request. Note, HTTP/2.0 does not allow
1422         the server to request different client authentication for different resources in the same origin.
1423      </p>
1424      <p id="rfc.section.3.6.9.p.9">If the server receives an invalid CREDENTIAL frame, it MUST respond with a GOAWAY frame and shutdown the session.</p>
1425      <h3 id="rfc.section.3.6.10"><a href="#rfc.section.3.6.10">3.6.10</a>&nbsp;<a id="HeaderBlock" href="#HeaderBlock">Name/Value Header Block</a></h3>
1426      <p id="rfc.section.3.6.10.p.1">The Name/Value Header Block is found in the SYN_STREAM, SYN_REPLY and HEADERS control frames, and shares a common format:</p>
1427      <div id="rfc.figure.u.15"></div> <pre>+------------------------------------+
1428| Number of Name/Value pairs (int32) |
1430|     Length of name (int32)         |
1432|           Name (string)            |
1434|     Length of value  (int32)       |
1436|          Value   (string)          |
1438|           (repeats)                |
1439            </pre> <p id="rfc.section.3.6.10.p.3">Number of Name/Value pairs: The number of repeating name/value pairs following this field.</p>
1440      <p id="rfc.section.3.6.10.p.4">List of Name/Value pairs: </p>
1441      <ul class="empty">
1442         <li>Length of Name: a 32-bit value containing the number of octets in the name field. Note that in practice, this length must
1443            not exceed 2^24, as that is the maximum size of a HTTP/2.0 frame.
1444         </li>
1445         <li>Name: 0 or more octets, 8-bit sequences of data, excluding 0.</li>
1446         <li>Length of Value: a 32-bit value containing the number of octets in the value field. Note that in practice, this length must
1447            not exceed 2^24, as that is the maximum size of a HTTP/2.0 frame.
1448         </li>
1449         <li>Value: 0 or more octets, 8-bit sequences of data, excluding 0.</li>
1450      </ul>
1451      <p id="rfc.section.3.6.10.p.5">Each header name must have at least one value. Header names are encoded using the <a href="#ASCII">US-ASCII character set</a> <cite title="US-ASCII. Coded Character Set - 7-Bit American Standard Code for Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986." id="rfc.xref.ASCII.1">[ASCII]</cite> and must be all lower case. The length of each name must be greater than zero. A recipient of a zero-length name MUST issue
1452         a stream error (<a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>) with the status code PROTOCOL_ERROR for the stream-id.
1453      </p>
1454      <p id="rfc.section.3.6.10.p.6">Duplicate header names are not allowed. To send two identically named headers, send a header with two values, where the values
1455         are separated by a single NUL (0) byte. A header value can either be empty (e.g. the length is zero) or it can contain multiple,
1456         NUL-separated values, each with length greater than zero. The value never starts nor ends with a NUL character. Recipients
1457         of illegal value fields MUST issue a stream error (<a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>) with the status code PROTOCOL_ERROR for the stream-id.
1458      </p>
1459      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="Compression" href="#Compression">Compression</a></h4>
1460      <p id="rfc.section.">The Name/Value Header Block is a section of the SYN_STREAM, SYN_REPLY, and HEADERS frames used to carry header meta-data.
1461         This block is always compressed using zlib compression. Within this specification, any reference to 'zlib' is referring to
1462         the <a href="#RFC1950">ZLIB Compressed Data Format Specification Version 3.3 as part of RFC1950.</a> <cite title="ZLIB Compressed Data Format Specification version 3.3" id="rfc.xref.RFC1950.1">[RFC1950]</cite></p>
1463      <p id="rfc.section.">For each HEADERS compression instance, the initial state is initialized using the following <a href="#UDELCOMPRESSION">dictionary</a> <cite title="A Methodology to Derive SPDY's Initial Dictionary for Zlib Compression" id="rfc.xref.UDELCOMPRESSION.1">[UDELCOMPRESSION]</cite>:
1464      </p>
1465      <div id="rfc.figure.u.16"></div> <pre class="ccmarker cct"><span>&lt;CODE BEGINS&gt;</span></pre><pre class="text">const unsigned char http2_dictionary_txt[] = {
1466  0x00, 0x00, 0x00, 0x07, 0x6f, 0x70, 0x74, 0x69,  \\ - - - - o p t i
1467  0x6f, 0x6e, 0x73, 0x00, 0x00, 0x00, 0x04, 0x68,  \\ o n s - - - - h
1468  0x65, 0x61, 0x64, 0x00, 0x00, 0x00, 0x04, 0x70,  \\ e a d - - - - p
1469  0x6f, 0x73, 0x74, 0x00, 0x00, 0x00, 0x03, 0x70,  \\ o s t - - - - p
1470  0x75, 0x74, 0x00, 0x00, 0x00, 0x06, 0x64, 0x65,  \\ u t - - - - d e
1471  0x6c, 0x65, 0x74, 0x65, 0x00, 0x00, 0x00, 0x05,  \\ l e t e - - - -
1472  0x74, 0x72, 0x61, 0x63, 0x65, 0x00, 0x00, 0x00,  \\ t r a c e - - -
1473  0x06, 0x61, 0x63, 0x63, 0x65, 0x70, 0x74, 0x00,  \\ - a c c e p t -
1474  0x00, 0x00, 0x0e, 0x61, 0x63, 0x63, 0x65, 0x70,  \\ - - - a c c e p
1475  0x74, 0x2d, 0x63, 0x68, 0x61, 0x72, 0x73, 0x65,  \\ t - c h a r s e
1476  0x74, 0x00, 0x00, 0x00, 0x0f, 0x61, 0x63, 0x63,  \\ t - - - - a c c
1477  0x65, 0x70, 0x74, 0x2d, 0x65, 0x6e, 0x63, 0x6f,  \\ e p t - e n c o
1478  0x64, 0x69, 0x6e, 0x67, 0x00, 0x00, 0x00, 0x0f,  \\ d i n g - - - -
1479  0x61, 0x63, 0x63, 0x65, 0x70, 0x74, 0x2d, 0x6c,  \\ a c c e p t - l
1480  0x61, 0x6e, 0x67, 0x75, 0x61, 0x67, 0x65, 0x00,  \\ a n g u a g e -
1481  0x00, 0x00, 0x0d, 0x61, 0x63, 0x63, 0x65, 0x70,  \\ - - - a c c e p
1482  0x74, 0x2d, 0x72, 0x61, 0x6e, 0x67, 0x65, 0x73,  \\ t - r a n g e s
1483  0x00, 0x00, 0x00, 0x03, 0x61, 0x67, 0x65, 0x00,  \\ - - - - a g e -
1484  0x00, 0x00, 0x05, 0x61, 0x6c, 0x6c, 0x6f, 0x77,  \\ - - - a l l o w
1485  0x00, 0x00, 0x00, 0x0d, 0x61, 0x75, 0x74, 0x68,  \\ - - - - a u t h
1486  0x6f, 0x72, 0x69, 0x7a, 0x61, 0x74, 0x69, 0x6f,  \\ o r i z a t i o
1487  0x6e, 0x00, 0x00, 0x00, 0x0d, 0x63, 0x61, 0x63,  \\ n - - - - c a c
1488  0x68, 0x65, 0x2d, 0x63, 0x6f, 0x6e, 0x74, 0x72,  \\ h e - c o n t r
1489  0x6f, 0x6c, 0x00, 0x00, 0x00, 0x0a, 0x63, 0x6f,  \\ o l - - - - c o
1490  0x6e, 0x6e, 0x65, 0x63, 0x74, 0x69, 0x6f, 0x6e,  \\ n n e c t i o n
1491  0x00, 0x00, 0x00, 0x0c, 0x63, 0x6f, 0x6e, 0x74,  \\ - - - - c o n t
1492  0x65, 0x6e, 0x74, 0x2d, 0x62, 0x61, 0x73, 0x65,  \\ e n t - b a s e
1493  0x00, 0x00, 0x00, 0x10, 0x63, 0x6f, 0x6e, 0x74,  \\ - - - - c o n t
1494  0x65, 0x6e, 0x74, 0x2d, 0x65, 0x6e, 0x63, 0x6f,  \\ e n t - e n c o
1495  0x64, 0x69, 0x6e, 0x67, 0x00, 0x00, 0x00, 0x10,  \\ d i n g - - - -
1496  0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, 0x74, 0x2d,  \\ c o n t e n t -
1497  0x6c, 0x61, 0x6e, 0x67, 0x75, 0x61, 0x67, 0x65,  \\ l a n g u a g e
1498  0x00, 0x00, 0x00, 0x0e, 0x63, 0x6f, 0x6e, 0x74,  \\ - - - - c o n t
1499  0x65, 0x6e, 0x74, 0x2d, 0x6c, 0x65, 0x6e, 0x67,  \\ e n t - l e n g
1500  0x74, 0x68, 0x00, 0x00, 0x00, 0x10, 0x63, 0x6f,  \\ t h - - - - c o
1501  0x6e, 0x74, 0x65, 0x6e, 0x74, 0x2d, 0x6c, 0x6f,  \\ n t e n t - l o
1502  0x63, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x00, 0x00,  \\ c a t i o n - -
1503  0x00, 0x0b, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e,  \\ - - c o n t e n
1504  0x74, 0x2d, 0x6d, 0x64, 0x35, 0x00, 0x00, 0x00,  \\ t - m d 5 - - -
1505  0x0d, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, 0x74,  \\ - c o n t e n t
1506  0x2d, 0x72, 0x61, 0x6e, 0x67, 0x65, 0x00, 0x00,  \\ - r a n g e - -
1507  0x00, 0x0c, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e,  \\ - - c o n t e n
1508  0x74, 0x2d, 0x74, 0x79, 0x70, 0x65, 0x00, 0x00,  \\ t - t y p e - -
1509  0x00, 0x04, 0x64, 0x61, 0x74, 0x65, 0x00, 0x00,  \\ - - d a t e - -
1510  0x00, 0x04, 0x65, 0x74, 0x61, 0x67, 0x00, 0x00,  \\ - - e t a g - -
1511  0x00, 0x06, 0x65, 0x78, 0x70, 0x65, 0x63, 0x74,  \\ - - e x p e c t
1512  0x00, 0x00, 0x00, 0x07, 0x65, 0x78, 0x70, 0x69,  \\ - - - - e x p i
1513  0x72, 0x65, 0x73, 0x00, 0x00, 0x00, 0x04, 0x66,  \\ r e s - - - - f
1514  0x72, 0x6f, 0x6d, 0x00, 0x00, 0x00, 0x04, 0x68,  \\ r o m - - - - h
1515  0x6f, 0x73, 0x74, 0x00, 0x00, 0x00, 0x08, 0x69,  \\ o s t - - - - i
1516  0x66, 0x2d, 0x6d, 0x61, 0x74, 0x63, 0x68, 0x00,  \\ f - m a t c h -
1517  0x00, 0x00, 0x11, 0x69, 0x66, 0x2d, 0x6d, 0x6f,  \\ - - - i f - m o
1518  0x64, 0x69, 0x66, 0x69, 0x65, 0x64, 0x2d, 0x73,  \\ d i f i e d - s
1519  0x69, 0x6e, 0x63, 0x65, 0x00, 0x00, 0x00, 0x0d,  \\ i n c e - - - -
1520  0x69, 0x66, 0x2d, 0x6e, 0x6f, 0x6e, 0x65, 0x2d,  \\ i f - n o n e -
1521  0x6d, 0x61, 0x74, 0x63, 0x68, 0x00, 0x00, 0x00,  \\ m a t c h - - -
1522  0x08, 0x69, 0x66, 0x2d, 0x72, 0x61, 0x6e, 0x67,  \\ - i f - r a n g
1523  0x65, 0x00, 0x00, 0x00, 0x13, 0x69, 0x66, 0x2d,  \\ e - - - - i f -
1524  0x75, 0x6e, 0x6d, 0x6f, 0x64, 0x69, 0x66, 0x69,  \\ u n m o d i f i
1525  0x65, 0x64, 0x2d, 0x73, 0x69, 0x6e, 0x63, 0x65,  \\ e d - s i n c e
1526  0x00, 0x00, 0x00, 0x0d, 0x6c, 0x61, 0x73, 0x74,  \\ - - - - l a s t
1527  0x2d, 0x6d, 0x6f, 0x64, 0x69, 0x66, 0x69, 0x65,  \\ - m o d i f i e
1528  0x64, 0x00, 0x00, 0x00, 0x08, 0x6c, 0x6f, 0x63,  \\ d - - - - l o c
1529  0x61, 0x74, 0x69, 0x6f, 0x6e, 0x00, 0x00, 0x00,  \\ a t i o n - - -
1530  0x0c, 0x6d, 0x61, 0x78, 0x2d, 0x66, 0x6f, 0x72,  \\ - m a x - f o r
1531  0x77, 0x61, 0x72, 0x64, 0x73, 0x00, 0x00, 0x00,  \\ w a r d s - - -
1532  0x06, 0x70, 0x72, 0x61, 0x67, 0x6d, 0x61, 0x00,  \\ - p r a g m a -
1533  0x00, 0x00, 0x12, 0x70, 0x72, 0x6f, 0x78, 0x79,  \\ - - - p r o x y
1534  0x2d, 0x61, 0x75, 0x74, 0x68, 0x65, 0x6e, 0x74,  \\ - a u t h e n t
1535  0x69, 0x63, 0x61, 0x74, 0x65, 0x00, 0x00, 0x00,  \\ i c a t e - - -
1536  0x13, 0x70, 0x72, 0x6f, 0x78, 0x79, 0x2d, 0x61,  \\ - p r o x y - a
1537  0x75, 0x74, 0x68, 0x6f, 0x72, 0x69, 0x7a, 0x61,  \\ u t h o r i z a
1538  0x74, 0x69, 0x6f, 0x6e, 0x00, 0x00, 0x00, 0x05,  \\ t i o n - - - -
1539  0x72, 0x61, 0x6e, 0x67, 0x65, 0x00, 0x00, 0x00,  \\ r a n g e - - -
1540  0x07, 0x72, 0x65, 0x66, 0x65, 0x72, 0x65, 0x72,  \\ - r e f e r e r
1541  0x00, 0x00, 0x00, 0x0b, 0x72, 0x65, 0x74, 0x72,  \\ - - - - r e t r
1542  0x79, 0x2d, 0x61, 0x66, 0x74, 0x65, 0x72, 0x00,  \\ y - a f t e r -
1543  0x00, 0x00, 0x06, 0x73, 0x65, 0x72, 0x76, 0x65,  \\ - - - s e r v e
1544  0x72, 0x00, 0x00, 0x00, 0x02, 0x74, 0x65, 0x00,  \\ r - - - - t e -
1545  0x00, 0x00, 0x07, 0x74, 0x72, 0x61, 0x69, 0x6c,  \\ - - - t r a i l
1546  0x65, 0x72, 0x00, 0x00, 0x00, 0x11, 0x74, 0x72,  \\ e r - - - - t r
1547  0x61, 0x6e, 0x73, 0x66, 0x65, 0x72, 0x2d, 0x65,  \\ a n s f e r - e
1548  0x6e, 0x63, 0x6f, 0x64, 0x69, 0x6e, 0x67, 0x00,  \\ n c o d i n g -
1549  0x00, 0x00, 0x07, 0x75, 0x70, 0x67, 0x72, 0x61,  \\ - - - u p g r a
1550  0x64, 0x65, 0x00, 0x00, 0x00, 0x0a, 0x75, 0x73,  \\ d e - - - - u s
1551  0x65, 0x72, 0x2d, 0x61, 0x67, 0x65, 0x6e, 0x74,  \\ e r - a g e n t
1552  0x00, 0x00, 0x00, 0x04, 0x76, 0x61, 0x72, 0x79,  \\ - - - - v a r y
1553  0x00, 0x00, 0x00, 0x03, 0x76, 0x69, 0x61, 0x00,  \\ - - - - v i a -
1554  0x00, 0x00, 0x07, 0x77, 0x61, 0x72, 0x6e, 0x69,  \\ - - - w a r n i
1555  0x6e, 0x67, 0x00, 0x00, 0x00, 0x10, 0x77, 0x77,  \\ n g - - - - w w
1556  0x77, 0x2d, 0x61, 0x75, 0x74, 0x68, 0x65, 0x6e,  \\ w - a u t h e n
1557  0x74, 0x69, 0x63, 0x61, 0x74, 0x65, 0x00, 0x00,  \\ t i c a t e - -
1558  0x00, 0x06, 0x6d, 0x65, 0x74, 0x68, 0x6f, 0x64,  \\ - - m e t h o d
1559  0x00, 0x00, 0x00, 0x03, 0x67, 0x65, 0x74, 0x00,  \\ - - - - g e t -
1560  0x00, 0x00, 0x06, 0x73, 0x74, 0x61, 0x74, 0x75,  \\ - - - s t a t u
1561  0x73, 0x00, 0x00, 0x00, 0x06, 0x32, 0x30, 0x30,  \\ s - - - - 2 0 0
1562  0x20, 0x4f, 0x4b, 0x00, 0x00, 0x00, 0x07, 0x76,  \\ - O K - - - - v
1563  0x65, 0x72, 0x73, 0x69, 0x6f, 0x6e, 0x00, 0x00,  \\ e r s i o n - -
1564  0x00, 0x08, 0x48, 0x54, 0x54, 0x50, 0x2f, 0x31,  \\ - - H T T P - 1
1565  0x2e, 0x31, 0x00, 0x00, 0x00, 0x03, 0x75, 0x72,  \\ - 1 - - - - u r
1566  0x6c, 0x00, 0x00, 0x00, 0x06, 0x70, 0x75, 0x62,  \\ l - - - - p u b
1567  0x6c, 0x69, 0x63, 0x00, 0x00, 0x00, 0x0a, 0x73,  \\ l i c - - - - s
1568  0x65, 0x74, 0x2d, 0x63, 0x6f, 0x6f, 0x6b, 0x69,  \\ e t - c o o k i
1569  0x65, 0x00, 0x00, 0x00, 0x0a, 0x6b, 0x65, 0x65,  \\ e - - - - k e e
1570  0x70, 0x2d, 0x61, 0x6c, 0x69, 0x76, 0x65, 0x00,  \\ p - a l i v e -
1571  0x00, 0x00, 0x06, 0x6f, 0x72, 0x69, 0x67, 0x69,  \\ - - - o r i g i
1572  0x6e, 0x31, 0x30, 0x30, 0x31, 0x30, 0x31, 0x32,  \\ n 1 0 0 1 0 1 2
1573  0x30, 0x31, 0x32, 0x30, 0x32, 0x32, 0x30, 0x35,  \\ 0 1 2 0 2 2 0 5
1574  0x32, 0x30, 0x36, 0x33, 0x30, 0x30, 0x33, 0x30,  \\ 2 0 6 3 0 0 3 0
1575  0x32, 0x33, 0x30, 0x33, 0x33, 0x30, 0x34, 0x33,  \\ 2 3 0 3 3 0 4 3
1576  0x30, 0x35, 0x33, 0x30, 0x36, 0x33, 0x30, 0x37,  \\ 0 5 3 0 6 3 0 7
1577  0x34, 0x30, 0x32, 0x34, 0x30, 0x35, 0x34, 0x30,  \\ 4 0 2 4 0 5 4 0
1578  0x36, 0x34, 0x30, 0x37, 0x34, 0x30, 0x38, 0x34,  \\ 6 4 0 7 4 0 8 4
1579  0x30, 0x39, 0x34, 0x31, 0x30, 0x34, 0x31, 0x31,  \\ 0 9 4 1 0 4 1 1
1580  0x34, 0x31, 0x32, 0x34, 0x31, 0x33, 0x34, 0x31,  \\ 4 1 2 4 1 3 4 1
1581  0x34, 0x34, 0x31, 0x35, 0x34, 0x31, 0x36, 0x34,  \\ 4 4 1 5 4 1 6 4
1582  0x31, 0x37, 0x35, 0x30, 0x32, 0x35, 0x30, 0x34,  \\ 1 7 5 0 2 5 0 4
1583  0x35, 0x30, 0x35, 0x32, 0x30, 0x33, 0x20, 0x4e,  \\ 5 0 5 2 0 3 - N
1584  0x6f, 0x6e, 0x2d, 0x41, 0x75, 0x74, 0x68, 0x6f,  \\ o n - A u t h o
1585  0x72, 0x69, 0x74, 0x61, 0x74, 0x69, 0x76, 0x65,  \\ r i t a t i v e
1586  0x20, 0x49, 0x6e, 0x66, 0x6f, 0x72, 0x6d, 0x61,  \\ - I n f o r m a
1587  0x74, 0x69, 0x6f, 0x6e, 0x32, 0x30, 0x34, 0x20,  \\ t i o n 2 0 4 -
1588  0x4e, 0x6f, 0x20, 0x43, 0x6f, 0x6e, 0x74, 0x65,  \\ N o - C o n t e
1589  0x6e, 0x74, 0x33, 0x30, 0x31, 0x20, 0x4d, 0x6f,  \\ n t 3 0 1 - M o
1590  0x76, 0x65, 0x64, 0x20, 0x50, 0x65, 0x72, 0x6d,  \\ v e d - P e r m
1591  0x61, 0x6e, 0x65, 0x6e, 0x74, 0x6c, 0x79, 0x34,  \\ a n e n t l y 4
1592  0x30, 0x30, 0x20, 0x42, 0x61, 0x64, 0x20, 0x52,  \\ 0 0 - B a d - R
1593  0x65, 0x71, 0x75, 0x65, 0x73, 0x74, 0x34, 0x30,  \\ e q u e s t 4 0
1594  0x31, 0x20, 0x55, 0x6e, 0x61, 0x75, 0x74, 0x68,  \\ 1 - U n a u t h
1595  0x6f, 0x72, 0x69, 0x7a, 0x65, 0x64, 0x34, 0x30,  \\ o r i z e d 4 0
1596  0x33, 0x20, 0x46, 0x6f, 0x72, 0x62, 0x69, 0x64,  \\ 3 - F o r b i d
1597  0x64, 0x65, 0x6e, 0x34, 0x30, 0x34, 0x20, 0x4e,  \\ d e n 4 0 4 - N
1598  0x6f, 0x74, 0x20, 0x46, 0x6f, 0x75, 0x6e, 0x64,  \\ o t - F o u n d
1599  0x35, 0x30, 0x30, 0x20, 0x49, 0x6e, 0x74, 0x65,  \\ 5 0 0 - I n t e
1600  0x72, 0x6e, 0x61, 0x6c, 0x20, 0x53, 0x65, 0x72,  \\ r n a l - S e r
1601  0x76, 0x65, 0x72, 0x20, 0x45, 0x72, 0x72, 0x6f,  \\ v e r - E r r o
1602  0x72, 0x35, 0x30, 0x31, 0x20, 0x4e, 0x6f, 0x74,  \\ r 5 0 1 - N o t
1603  0x20, 0x49, 0x6d, 0x70, 0x6c, 0x65, 0x6d, 0x65,  \\ - I m p l e m e
1604  0x6e, 0x74, 0x65, 0x64, 0x35, 0x30, 0x33, 0x20,  \\ n t e d 5 0 3 -
1605  0x53, 0x65, 0x72, 0x76, 0x69, 0x63, 0x65, 0x20,  \\ S e r v i c e -
1606  0x55, 0x6e, 0x61, 0x76, 0x61, 0x69, 0x6c, 0x61,  \\ U n a v a i l a
1607  0x62, 0x6c, 0x65, 0x4a, 0x61, 0x6e, 0x20, 0x46,  \\ b l e J a n - F
1608  0x65, 0x62, 0x20, 0x4d, 0x61, 0x72, 0x20, 0x41,  \\ e b - M a r - A
1609  0x70, 0x72, 0x20, 0x4d, 0x61, 0x79, 0x20, 0x4a,  \\ p r - M a y - J
1610  0x75, 0x6e, 0x20, 0x4a, 0x75, 0x6c, 0x20, 0x41,  \\ u n - J u l - A
1611  0x75, 0x67, 0x20, 0x53, 0x65, 0x70, 0x74, 0x20,  \\ u g - S e p t -
1612  0x4f, 0x63, 0x74, 0x20, 0x4e, 0x6f, 0x76, 0x20,  \\ O c t - N o v -
1613  0x44, 0x65, 0x63, 0x20, 0x30, 0x30, 0x3a, 0x30,  \\ D e c - 0 0 - 0
1614  0x30, 0x3a, 0x30, 0x30, 0x20, 0x4d, 0x6f, 0x6e,  \\ 0 - 0 0 - M o n
1615  0x2c, 0x20, 0x54, 0x75, 0x65, 0x2c, 0x20, 0x57,  \\ - - T u e - - W
1616  0x65, 0x64, 0x2c, 0x20, 0x54, 0x68, 0x75, 0x2c,  \\ e d - - T h u -
1617  0x20, 0x46, 0x72, 0x69, 0x2c, 0x20, 0x53, 0x61,  \\ - F r i - - S a
1618  0x74, 0x2c, 0x20, 0x53, 0x75, 0x6e, 0x2c, 0x20,  \\ t - - S u n - -
1619  0x47, 0x4d, 0x54, 0x63, 0x68, 0x75, 0x6e, 0x6b,  \\ G M T c h u n k
1620  0x65, 0x64, 0x2c, 0x74, 0x65, 0x78, 0x74, 0x2f,  \\ e d - t e x t -
1621  0x68, 0x74, 0x6d, 0x6c, 0x2c, 0x69, 0x6d, 0x61,  \\ h t m l - i m a
1622  0x67, 0x65, 0x2f, 0x70, 0x6e, 0x67, 0x2c, 0x69,  \\ g e - p n g - i
1623  0x6d, 0x61, 0x67, 0x65, 0x2f, 0x6a, 0x70, 0x67,  \\ m a g e - j p g
1624  0x2c, 0x69, 0x6d, 0x61, 0x67, 0x65, 0x2f, 0x67,  \\ - i m a g e - g
1625  0x69, 0x66, 0x2c, 0x61, 0x70, 0x70, 0x6c, 0x69,  \\ i f - a p p l i
1626  0x63, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x2f, 0x78,  \\ c a t i o n - x
1627  0x6d, 0x6c, 0x2c, 0x61, 0x70, 0x70, 0x6c, 0x69,  \\ m l - a p p l i
1628  0x63, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x2f, 0x78,  \\ c a t i o n - x
1629  0x68, 0x74, 0x6d, 0x6c, 0x2b, 0x78, 0x6d, 0x6c,  \\ h t m l - x m l
1630  0x2c, 0x74, 0x65, 0x78, 0x74, 0x2f, 0x70, 0x6c,  \\ - t e x t - p l
1631  0x61, 0x69, 0x6e, 0x2c, 0x74, 0x65, 0x78, 0x74,  \\ a i n - t e x t
1632  0x2f, 0x6a, 0x61, 0x76, 0x61, 0x73, 0x63, 0x72,  \\ - j a v a s c r
1633  0x69, 0x70, 0x74, 0x2c, 0x70, 0x75, 0x62, 0x6c,  \\ i p t - p u b l
1634  0x69, 0x63, 0x70, 0x72, 0x69, 0x76, 0x61, 0x74,  \\ i c p r i v a t
1635  0x65, 0x6d, 0x61, 0x78, 0x2d, 0x61, 0x67, 0x65,  \\ e m a x - a g e
1636  0x3d, 0x67, 0x7a, 0x69, 0x70, 0x2c, 0x64, 0x65,  \\ - g z i p - d e
1637  0x66, 0x6c, 0x61, 0x74, 0x65, 0x2c, 0x73, 0x64,  \\ f l a t e - s d
1638  0x63, 0x68, 0x63, 0x68, 0x61, 0x72, 0x73, 0x65,  \\ c h c h a r s e
1639  0x74, 0x3d, 0x75, 0x74, 0x66, 0x2d, 0x38, 0x63,  \\ t - u t f - 8 c
1640  0x68, 0x61, 0x72, 0x73, 0x65, 0x74, 0x3d, 0x69,  \\ h a r s e t - i
1641  0x73, 0x6f, 0x2d, 0x38, 0x38, 0x35, 0x39, 0x2d,  \\ s o - 8 8 5 9 -
1642  0x31, 0x2c, 0x75, 0x74, 0x66, 0x2d, 0x2c, 0x2a,  \\ 1 - u t f - - -
1643  0x2c, 0x65, 0x6e, 0x71, 0x3d, 0x30, 0x2e         \\ - e n q - 0 -
1645</pre><pre class="ccmarker ccb"><span>&lt;CODE ENDS&gt;</span></pre> <p id="rfc.section.">The entire contents of the name/value header block is compressed using zlib. There is a single zlib stream for all name value
1646         pairs in one direction on a connection. HTTP/2.0 uses a SYNC_FLUSH between each compressed frame.
1647      </p>
1648      <p id="rfc.section.">Implementation notes: the compression engine can be tuned to favor speed or size. Optimizing for size increases memory use
1649         and CPU consumption. Because header blocks are generally small, implementors may want to reduce the window-size of the compression
1650         engine from the default 15bits (a 32KB window) to more like 11bits (a 2KB window). The exact setting is chosen by the compressor,
1651         the decompressor will work with any setting.
1652      </p>
1653      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="HTTPLayer" href="#HTTPLayer">HTTP Layering over HTTP/2.0</a></h1>
1654      <p id="rfc.section.4.p.1">HTTP/2.0 is intended to be as compatible as possible with current web-based applications. This means that, from the perspective
1655         of the server business logic or application API, the features of HTTP are unchanged. To achieve this, all of the application
1656         request and response header semantics are preserved, although the syntax of conveying those semantics has changed. Thus, the
1657         rules from the <a href="#RFC2616">HTTP/1.1 specification in RFC2616</a> <cite title="Hypertext Transfer Protocol -- HTTP/1.1" id="rfc.xref.RFC2616.1">[RFC2616]</cite> apply with the changes in the sections below.
1658      </p>
1659      <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;Connection Management
1660      </h2>
1661      <p id="rfc.section.4.1.p.1">Clients SHOULD NOT open more than one HTTP/2.0 session to a given <a href="#RFC6454">origin</a> <cite title="The Web Origin Concept" id="rfc.xref.RFC6454.2">[RFC6454]</cite> concurrently.
1662      </p>
1663      <p id="rfc.section.4.1.p.2">Note that it is possible for one HTTP/2.0 session to be finishing (e.g. a GOAWAY message has been sent, but not all streams
1664         have finished), while another HTTP/2.0 session is starting.
1665      </p>
1666      <h3 id="rfc.section.4.1.1"><a href="#rfc.section.4.1.1">4.1.1</a>&nbsp;Use of GOAWAY
1667      </h3>
1668      <p id="rfc.section.4.1.1.p.1">HTTP/2.0 provides a GOAWAY message which can be used when closing a connection from either the client or server. Without a
1669         server GOAWAY message, HTTP has a race condition where the client sends a request (a new SYN_STREAM) just as the server is
1670         closing the connection, and the client cannot know if the server received the stream or not. By using the last-stream-id in
1671         the GOAWAY, servers can indicate to the client if a request was processed or not.
1672      </p>
1673      <p id="rfc.section.4.1.1.p.2">Note that some servers will choose to send the GOAWAY and immediately terminate the connection without waiting for active
1674         streams to finish. The client will be able to determine this because HTTP/2.0 streams are determinstically closed. This abrupt
1675         termination will force the client to heuristically decide whether to retry the pending requests. Clients always need to be
1676         capable of dealing with this case because they must deal with accidental connection termination cases, which are the same
1677         as the server never having sent a GOAWAY.
1678      </p>
1679      <p id="rfc.section.4.1.1.p.3">More sophisticated servers will use GOAWAY to implement a graceful teardown. They will send the GOAWAY and provide some time
1680         for the active streams to finish before terminating the connection.
1681      </p>
1682      <p id="rfc.section.4.1.1.p.4">If a HTTP/2.0 client closes the connection, it should also send a GOAWAY message. This allows the server to know if any server-push
1683         streams were received by the client.
1684      </p>
1685      <p id="rfc.section.4.1.1.p.5">If the endpoint closing the connection has not received any SYN_STREAMs from the remote, the GOAWAY will contain a last-stream-id
1686         of 0.
1687      </p>
1688      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;HTTP Request/Response
1689      </h2>
1690      <h3 id="rfc.section.4.2.1"><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;Request
1691      </h3>
1692      <p id="rfc.section.4.2.1.p.1">The client initiates a request by sending a SYN_STREAM frame. For requests which do not contain a body, the SYN_STREAM frame
1693         MUST set the FLAG_FIN, indicating that the client intends to send no further data on this stream. For requests which do contain
1694         a body, the SYN_STREAM will not contain the FLAG_FIN, and the body will follow the SYN_STREAM in a series of DATA frames.
1695         The last DATA frame will set the FLAG_FIN to indicate the end of the body.
1696      </p>
1697      <p id="rfc.section.4.2.1.p.2">The SYN_STREAM Name/Value section will contain all of the HTTP headers which are associated with an HTTP request. The header
1698         block in HTTP/2.0 is mostly unchanged from today's HTTP header block, with the following differences:
1699      </p>
1700      <ul class="empty">
1701         <li>The first line of the request is unfolded into name/value pairs like other HTTP headers and MUST be present:
1702            <ul class="empty">
1703               <li>":method" - the HTTP method for this request (e.g. "GET", "POST", "HEAD", etc)</li>
1704               <li>":path" - the url-path for this url with "/" prefixed. (See <a href="#RFC1738">RFC1738</a> <cite title="Uniform Resource Locators (URL)" id="rfc.xref.RFC1738.1">[RFC1738]</cite>). For example, for "" the path would be "/search?q=dogs".
1705               </li>
1706               <li>":version" - the HTTP version of this request (e.g. "HTTP/1.1")</li>
1707            </ul>
1708         </li>
1709         <li>In addition, the following two name/value pairs must also be present in every request:
1710            <ul class="empty">
1711               <li>":host" - the hostport (See <a href="#RFC1738">RFC1738</a> <cite title="Uniform Resource Locators (URL)" id="rfc.xref.RFC1738.2">[RFC1738]</cite>) portion of the URL for this request (e.g. ""). This header is the same as the HTTP 'Host' header.
1712               </li>
1713               <li>":scheme" - the scheme portion of the URL for this request (e.g. "https"))</li>
1714            </ul>
1715         </li>
1716         <li>Header names are all lowercase.</li>
1717         <li>The Connection, Host, Keep-Alive, Proxy-Connection, and Transfer-Encoding headers are not valid and MUST not be sent.</li>
1718         <li>User-agents MUST support gzip compression. Regardless of the Accept-Encoding sent by the user-agent, the server may always
1719            send content encoded with gzip or deflate encoding.
1720         </li>
1721         <li>If a server receives a request where the sum of the data frame payload lengths does not equal the size of the Content-Length
1722            header, the server MUST return a 400 (Bad Request) error.
1723         </li>
1724         <li>POST-specific changes:
1725            <ul class="empty">
1726               <li>Although POSTs are inherently chunked, POST requests SHOULD also be accompanied by a Content-Length header. There are two
1727                  reasons for this: First, it assists with upload progress meters for an improved user experience. But second, we know from
1728                  early versions of HTTP/2.0 that failure to send a content length header is incompatible with many existing HTTP server implementations.
1729                  Existing user-agents do not omit the Content-Length header, and server implementations have come to depend upon this.
1730               </li>
1731            </ul>
1732         </li>
1733      </ul>
1734      <p id="rfc.section.4.2.1.p.3">The user-agent is free to prioritize requests as it sees fit. If the user-agent cannot make progress without receiving a resource,
1735         it should attempt to raise the priority of that resource. Resources such as images, SHOULD generally use the lowest priority.
1736      </p>
1737      <p id="rfc.section.4.2.1.p.4">If a client sends a SYN_STREAM without all of the method, host, path, scheme, and version headers, the server MUST reply with
1738         a HTTP 400 Bad Request reply.
1739      </p>
1740      <h3 id="rfc.section.4.2.2"><a href="#rfc.section.4.2.2">4.2.2</a>&nbsp;Response
1741      </h3>
1742      <p id="rfc.section.4.2.2.p.1">The server responds to a client request with a SYN_REPLY frame. Symmetric to the client's upload stream, server will send
1743         data after the SYN_REPLY frame via a series of DATA frames, and the last data frame will contain the FLAG_FIN to indicate
1744         successful end-of-stream. If a response (like a 202 or 204 response) contains no body, the SYN_REPLY frame may contain the
1745         FLAG_FIN flag to indicate no further data will be sent on the stream.
1746      </p>
1747      <p id="rfc.section.4.2.2.p.2"> </p>
1748      <ul class="empty">
1749         <li>The response status line is unfolded into name/value pairs like other HTTP headers and must be present:
1750            <ul class="empty">
1751               <li>":status" - The HTTP response status code (e.g. "200" or "200 OK")</li>
1752               <li>":version" - The HTTP response version (e.g. "HTTP/1.1")</li>
1753            </ul>
1754         </li>
1755         <li>All header names must be lowercase.</li>
1756         <li>The Connection, Keep-Alive, Proxy-Connection, and Transfer-Encoding headers are not valid and MUST not be sent.</li>
1757         <li>Responses MAY be accompanied by a Content-Length header for advisory purposes. (e.g. for UI progress meters)</li>
1758         <li>If a client receives a response where the sum of the data frame payload lengths does not equal the size of the Content-Length
1759            header, the client MUST ignore the content length header.
1760         </li>
1761      </ul>
1762      <p id="rfc.section.4.2.2.p.3">If a client receives a SYN_REPLY without a status or without a version header, the client must reply with a RST_STREAM frame
1763         indicating a PROTOCOL ERROR.
1764      </p>
1765      <h3 id="rfc.section.4.2.3"><a href="#rfc.section.4.2.3">4.2.3</a>&nbsp;<a id="Authentication" href="#Authentication">Authentication</a></h3>
1766      <p id="rfc.section.4.2.3.p.1">When a client sends a request to an origin server that requires authentication, the server can reply with a "401 Unauthorized"
1767         response, and include a WWW-Authenticate challenge header that defines the authentication scheme to be used. The client then
1768         retries the request with an Authorization header appropriate to the specified authentication scheme.
1769      </p>
1770      <p id="rfc.section.4.2.3.p.2">There are four options for proxy authentication, Basic, Digest, NTLM and Negotiate (SPNEGO). The first two options were defined
1771         in <a href="#RFC2617">RFC2617</a> <cite title="HTTP Authentication: Basic and Digest Access Authentication" id="rfc.xref.RFC2617.1">[RFC2617]</cite>, and are stateless. The second two options were developed by Microsoft and specified in <a href="#RFC4559">RFC4559</a> <cite title="SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows" id="rfc.xref.RFC4559.1">[RFC4559]</cite>, and are stateful; otherwise known as multi-round authentication, or connection authentication.
1772      </p>
1773      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;Stateless Authentication
1774      </h4>
1775      <p id="rfc.section.">Stateless Authentication over HTTP/2.0 is identical to how it is performed over HTTP. If multiple HTTP/2.0 streams are concurrently
1776         sent to a single server, each will authenticate independently, similar to how two HTTP connections would independently authenticate
1777         to a proxy server.
1778      </p>
1779      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;Stateful Authentication
1780      </h4>
1781      <p id="rfc.section.">Unfortunately, the stateful authentication mechanisms were implemented and defined in a such a way that directly violates
1782         RFC2617 - they do not include a "realm" as part of the request. This is problematic in HTTP/2.0 because it makes it impossible
1783         for a client to disambiguate two concurrent server authentication challenges.
1784      </p>
1785      <p id="rfc.section.">To deal with this case, HTTP/2.0 servers using Stateful Authentication MUST implement one of two changes: </p>
1786      <ul class="empty">
1787         <li>Servers can add a "realm=&lt;desired realm&gt;" header so that the two authentication requests can be disambiguated and run concurrently.
1788            Unfortunately, given how these mechanisms work, this is probably not practical.
1789         </li>
1790         <li>Upon sending the first stateful challenge response, the server MUST buffer and defer all further frames which are not part
1791            of completing the challenge until the challenge has completed. Completing the authentication challenge may take multiple round
1792            trips. Once the client receives a "401 Authenticate" response for a stateful authentication type, it MUST stop sending new
1793            requests to the server until the authentication has completed by receiving a non-401 response on at least one stream.
1794         </li>
1795      </ul>
1796      <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;Server Push Transactions
1797      </h2>
1798      <p id="rfc.section.4.3.p.1">HTTP/2.0 enables a server to send multiple replies to a client for a single request. The rationale for this feature is that
1799         sometimes a server knows that it will need to send multiple resources in response to a single request. Without server push
1800         features, the client must first download the primary resource, then discover the secondary resource(s), and request them.
1801         Pushing of resources avoids the round-trip delay, but also creates a potential race where a server can be pushing content
1802         which a user-agent is in the process of requesting. The following mechanics attempt to prevent the race condition while enabling
1803         the performance benefit.
1804      </p>
1805      <p id="rfc.section.4.3.p.2">Browsers receiving a pushed response MUST validate that the server is authorized to push the URL using the <a href="#RFC6454">browser same-origin</a> <cite title="The Web Origin Concept" id="rfc.xref.RFC6454.3">[RFC6454]</cite> policy. For example, a HTTP/2.0 connection to is generally not permitted to push a response for
1806      </p>
1807      <p id="rfc.section.4.3.p.3">If the browser accepts a pushed response (e.g. it does not send a RST_STREAM), the browser MUST attempt to cache the pushed
1808         response in same way that it would cache any other response. This means validating the response headers and inserting into
1809         the disk cache.
1810      </p>
1811      <p id="rfc.section.4.3.p.4">Because pushed responses have no request, they have no request headers associated with them. At the framing layer, HTTP/2.0
1812         pushed streams contain an "associated-stream-id" which indicates the requested stream for which the pushed stream is related.
1813         The pushed stream inherits all of the headers from the associated-stream-id with the exception of ":host", ":scheme", and
1814         ":path", which are provided as part of the pushed response stream headers. The browser MUST store these inherited and implied
1815         request headers with the cached resource.
1816      </p>
1817      <p id="rfc.section.4.3.p.5">Implementation note: With server push, it is theoretically possible for servers to push unreasonable amounts of content or
1818         resources to the user-agent. Browsers MUST implement throttles to protect against unreasonable push attacks.
1819      </p>
1820      <h3 id="rfc.section.4.3.1"><a href="#rfc.section.4.3.1">4.3.1</a>&nbsp;Server implementation
1821      </h3>
1822      <p id="rfc.section.4.3.1.p.1">When the server intends to push a resource to the user-agent, it opens a new stream by sending a unidirectional SYN_STREAM.
1823         The SYN_STREAM MUST include an Associated-To-Stream-ID, and MUST set the FLAG_UNIDIRECTIONAL flag. The SYN_STREAM MUST include
1824         headers for ":scheme", ":host", ":path", which represent the URL for the resource being pushed. Subsequent headers may follow
1825         in HEADERS frames. The purpose of the association is so that the user-agent can differentiate which request induced the pushed
1826         stream; without it, if the user-agent had two tabs open to the same page, each pushing unique content under a fixed URL, the
1827         user-agent would not be able to differentiate the requests.
1828      </p>
1829      <p id="rfc.section.4.3.1.p.2">The Associated-To-Stream-ID must be the ID of an existing, open stream. The reason for this restriction is to have a clear
1830         endpoint for pushed content. If the user-agent requested a resource on stream 11, the server replies on stream 11. It can
1831         push any number of additional streams to the client before sending a FLAG_FIN on stream 11. However, once the originating
1832         stream is closed no further push streams may be associated with it. The pushed streams do not need to be closed (FIN set)
1833         before the originating stream is closed, they only need to be created before the originating stream closes.
1834      </p>
1835      <p id="rfc.section.4.3.1.p.3">It is illegal for a server to push a resource with the Associated-To-Stream-ID of 0.</p>
1836      <p id="rfc.section.4.3.1.p.4">To minimize race conditions with the client, the SYN_STREAM for the pushed resources MUST be sent prior to sending any content
1837         which could allow the client to discover the pushed resource and request it.
1838      </p>
1839      <p id="rfc.section.4.3.1.p.5">The server MUST only push resources which would have been returned from a GET request.</p>
1840      <p id="rfc.section.4.3.1.p.6">Note: If the server does not have all of the Name/Value Response headers available at the time it issues the HEADERS frame
1841         for the pushed resource, it may later use an additional HEADERS frame to augment the name/value pairs to be associated with
1842         the pushed stream. The subsequent HEADERS frame(s) must not contain a header for ':host', ':scheme', or ':path' (e.g. the
1843         server can't change the identity of the resource to be pushed). The HEADERS frame must not contain duplicate headers with
1844         a previously sent HEADERS frame. The server must send a HEADERS frame including the scheme/host/port headers before sending
1845         any data frames on the stream.
1846      </p>
1847      <h3 id="rfc.section.4.3.2"><a href="#rfc.section.4.3.2">4.3.2</a>&nbsp;Client implementation
1848      </h3>
1849      <p id="rfc.section.4.3.2.p.1">When fetching a resource the client has 3 possibilities: </p>
1850      <ul class="empty">
1851         <li>the resource is not being pushed</li>
1852         <li>the resource is being pushed, but the data has not yet arrived</li>
1853         <li>the resource is being pushed, and the data has started to arrive</li>
1854      </ul>
1855      <p id="rfc.section.4.3.2.p.2">When a SYN_STREAM and HEADERS frame which contains an Associated-To-Stream-ID is received, the client must not issue GET requests
1856         for the resource in the pushed stream, and instead wait for the pushed stream to arrive.
1857      </p>
1858      <p id="rfc.section.4.3.2.p.3">If a client receives a server push stream with stream-id 0, it MUST issue a session error (<a href="#SessionErrorHandler" title="Session Error Handling">Section&nbsp;3.4.1</a>) with the status code PROTOCOL_ERROR.
1859      </p>
1860      <p id="rfc.section.4.3.2.p.4">When a client receives a SYN_STREAM from the server without a the ':host', ':scheme', and ':path' headers in the Name/Value
1861         section, it MUST reply with a RST_STREAM with error code HTTP_PROTOCOL_ERROR.
1862      </p>
1863      <p id="rfc.section.4.3.2.p.5">To cancel individual server push streams, the client can issue a stream error (<a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>) with error code CANCEL. Upon receipt, the server MUST stop sending on this stream immediately (this is an Abrupt termination).
1864      </p>
1865      <p id="rfc.section.4.3.2.p.6">To cancel all server push streams related to a request, the client may issue a stream error (<a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>) with error code CANCEL on the associated-stream-id. By cancelling that stream, the server MUST immediately stop sending frames
1866         for any streams with in-association-to for the original stream.
1867      </p>
1868      <p id="rfc.section.4.3.2.p.7">If the server sends a HEADER frame containing duplicate headers with a previous HEADERS frame for the same stream, the client
1869         must issue a stream error (<a href="#StreamErrorHandler" title="Stream Error Handling">Section&nbsp;3.4.2</a>) with error code PROTOCOL ERROR.
1870      </p>
1871      <p id="rfc.section.4.3.2.p.8">If the server sends a HEADERS frame after sending a data frame for the same stream, the client MAY ignore the HEADERS frame.
1872         Ignoring the HEADERS frame after a data frame prevents handling of HTTP's trailing headers (
1873      </p>
1874      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;Design Rationale and Notes
1875      </h1>
1876      <p id="rfc.section.5.p.1">Authors' notes: The notes in this section have no bearing on the HTTP/2.0 protocol as specified within this document, and
1877         none of these notes should be considered authoritative about how the protocol works. However, these notes may prove useful
1878         in future debates about how to resolve protocol ambiguities or how to evolve the protocol going forward. They may be removed
1879         before the final draft.
1880      </p>
1881      <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;Separation of Framing Layer and Application Layer
1882      </h2>
1883      <p id="rfc.section.5.1.p.1">Readers may note that this specification sometimes blends the framing layer (<a href="#FramingLayer" title="HTTP/2.0 Framing Layer">Section&nbsp;3</a>) with requirements of a specific application - HTTP (<a href="#HTTPLayer" title="HTTP Layering over HTTP/2.0">Section&nbsp;4</a>). This is reflected in the request/response nature of the streams, the definition of the HEADERS and compression contexts
1884         which are very similar to HTTP, and other areas as well.
1885      </p>
1886      <p id="rfc.section.5.1.p.2">This blending is intentional - the primary goal of this protocol is to create a low-latency protocol for use with HTTP. Isolating
1887         the two layers is convenient for description of the protocol and how it relates to existing HTTP implementations. However,
1888         the ability to reuse the HTTP/2.0 framing layer is a non goal.
1889      </p>
1890      <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;Error handling - Framing Layer
1891      </h2>
1892      <p id="rfc.section.5.2.p.1">Error handling at the HTTP/2.0 layer splits errors into two groups: Those that affect an individual HTTP/2.0 stream, and those
1893         that do not.
1894      </p>
1895      <p id="rfc.section.5.2.p.2">When an error is confined to a single stream, but general framing is in tact, HTTP/2.0 attempts to use the RST_STREAM as a
1896         mechanism to invalidate the stream but move forward without aborting the connection altogether.
1897      </p>
1898      <p id="rfc.section.5.2.p.3">For errors occuring outside of a single stream context, HTTP/2.0 assumes the entire session is hosed. In this case, the endpoint
1899         detecting the error should initiate a connection close.
1900      </p>
1901      <h2 id="rfc.section.5.3"><a href="#rfc.section.5.3">5.3</a>&nbsp;One Connection Per Domain
1902      </h2>
1903      <p id="rfc.section.5.3.p.1">HTTP/2.0 attempts to use fewer connections than other protocols have traditionally used. The rationale for this behavior is
1904         because it is very difficult to provide a consistent level of service (e.g. TCP slow-start), prioritization, or optimal compression
1905         when the client is connecting to the server through multiple channels.
1906      </p>
1907      <p id="rfc.section.5.3.p.2">Through lab measurements, we have seen consistent latency benefits by using fewer connections from the client. The overall
1908         number of packets sent by HTTP/2.0 can be as much as 40% less than HTTP. Handling large numbers of concurrent connections
1909         on the server also does become a scalability problem, and HTTP/2.0 reduces this load.
1910      </p>
1911      <p id="rfc.section.5.3.p.3">The use of multiple connections is not without benefit, however. Because HTTP/2.0 multiplexes multiple, independent streams
1912         onto a single stream, it creates a potential for head-of-line blocking problems at the transport level. In tests so far, the
1913         negative effects of head-of-line blocking (especially in the presence of packet loss) is outweighed by the benefits of compression
1914         and prioritization.
1915      </p>
1916      <h2 id="rfc.section.5.4"><a href="#rfc.section.5.4">5.4</a>&nbsp;Fixed vs Variable Length Fields
1917      </h2>
1918      <p id="rfc.section.5.4.p.1">HTTP/2.0 favors use of fixed length 32bit fields in cases where smaller, variable length encodings could have been used. To
1919         some, this seems like a tragic waste of bandwidth. HTTP/2.0 choses the simple encoding for speed and simplicity.
1920      </p>
1921      <p id="rfc.section.5.4.p.2">The goal of HTTP/2.0 is to reduce latency on the network. The overhead of HTTP/2.0 frames is generally quite low. Each data
1922         frame is only an 8 byte overhead for a 1452 byte payload (~0.6%). At the time of this writing, bandwidth is already plentiful,
1923         and there is a strong trend indicating that bandwidth will continue to increase. With an average worldwide bandwidth of 1Mbps,
1924         and assuming that a variable length encoding could reduce the overhead by 50%, the latency saved by using a variable length
1925         encoding would be less than 100 nanoseconds. More interesting are the effects when the larger encodings force a packet boundary,
1926         in which case a round-trip could be induced. However, by addressing other aspects of HTTP/2.0 and TCP interactions, we believe
1927         this is completely mitigated.
1928      </p>
1929      <h2 id="rfc.section.5.5"><a href="#rfc.section.5.5">5.5</a>&nbsp;Compression Context(s)
1930      </h2>
1931      <p id="rfc.section.5.5.p.1">When isolating the compression contexts used for communicating with multiple origins, we had a few choices to make. We could
1932         have maintained a map (or list) of compression contexts usable for each origin. The basic case is easy - each HEADERS frame
1933         would need to identify the context to use for that frame. However, compression contexts are not cheap, so the lifecycle of
1934         each context would need to be bounded. For proxy servers, where we could churn through many contexts, this would be a concern.
1935         We considered using a static set of contexts, say 16 of them, which would bound the memory use. We also considered dynamic
1936         contexts, which could be created on the fly, and would need to be subsequently destroyed. All of these are complicated, and
1937         ultimately we decided that such a mechanism creates too many problems to solve.
1938      </p>
1939      <p id="rfc.section.5.5.p.2">Alternatively, we've chosen the simple approach, which is to simply provide a flag for resetting the compression context.
1940         For the common case (no proxy), this fine because most requests are to the same origin and we never need to reset the context.
1941         For cases where we are using two different origins over a single HTTP/2.0 session, we simply reset the compression state between
1942         each transition.
1943      </p>
1944      <h2 id="rfc.section.5.6"><a href="#rfc.section.5.6">5.6</a>&nbsp;Unidirectional streams
1945      </h2>
1946      <p id="rfc.section.5.6.p.1">Many readers notice that unidirectional streams are both a bit confusing in concept and also somewhat redundant. If the recipient
1947         of a stream doesn't wish to send data on a stream, it could simply send a SYN_REPLY with the FLAG_FIN bit set. The FLAG_UNIDIRECTIONAL
1948         is, therefore, not necessary.
1949      </p>
1950      <p id="rfc.section.5.6.p.2">It is true that we don't need the UNIDIRECTIONAL markings. It is added because it avoids the recipient of pushed streams from
1951         needing to send a set of empty frames (e.g. the SYN_STREAM w/ FLAG_FIN) which otherwise serve no purpose.
1952      </p>
1953      <h2 id="rfc.section.5.7"><a href="#rfc.section.5.7">5.7</a>&nbsp;Data Compression
1954      </h2>
1955      <p id="rfc.section.5.7.p.1">Generic compression of data portion of the streams (as opposed to compression of the headers) without knowing the content
1956         of the stream is redundant. There is no value in compressing a stream which is already compressed. Because of this, HTTP/2.0
1957         does allow data compression to be optional. We included it because study of existing websites shows that many sites are not
1958         using compression as they should, and users suffer because of it. We wanted a mechanism where, at the HTTP/2.0 layer, site
1959         administrators could simply force compression - it is better to compress twice than to not compress.
1960      </p>
1961      <p id="rfc.section.5.7.p.2">Overall, however, with this feature being optional and sometimes redundant, it is unclear if it is useful at all. We will
1962         likely remove it from the specification.
1963      </p>
1964      <h2 id="rfc.section.5.8"><a href="#rfc.section.5.8">5.8</a>&nbsp;Server Push
1965      </h2>
1966      <p id="rfc.section.5.8.p.1">A subtle but important point is that server push streams must be declared before the associated stream is closed. The reason
1967         for this is so that proxies have a lifetime for which they can discard information about previous streams. If a pushed stream
1968         could associate itself with an already-closed stream, then endpoints would not have a specific lifecycle for when they could
1969         disavow knowledge of the streams which went before.
1970      </p>
1971      <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;Security Considerations
1972      </h1>
1973      <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;Use of Same-origin constraints
1974      </h2>
1975      <p id="rfc.section.6.1.p.1">This specification uses the <a href="#RFC6454">same-origin policy</a> <cite title="The Web Origin Concept" id="rfc.xref.RFC6454.4">[RFC6454]</cite> in all cases where verification of content is required.
1976      </p>
1977      <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;HTTP Headers and HTTP/2.0 Headers
1978      </h2>
1979      <p id="rfc.section.6.2.p.1">At the application level, HTTP uses name/value pairs in its headers. Because HTTP/2.0 merges the existing HTTP headers with
1980         HTTP/2.0 headers, there is a possibility that some HTTP applications already use a particular header name. To avoid any conflicts,
1981         all headers introduced for layering HTTP over HTTP/2.0 are prefixed with ":". ":" is not a valid sequence in HTTP header naming,
1982         preventing any possible conflict.
1983      </p>
1984      <h2 id="rfc.section.6.3"><a href="#rfc.section.6.3">6.3</a>&nbsp;Cross-Protocol Attacks
1985      </h2>
1986      <p id="rfc.section.6.3.p.1">By utilizing TLS, we believe that HTTP/2.0 introduces no new cross-protocol attacks. TLS encrypts the contents of all transmission
1987         (except the handshake itself), making it difficult for attackers to control the data which could be used in a cross-protocol
1988         attack.
1989      </p>
1990      <h2 id="rfc.section.6.4"><a href="#rfc.section.6.4">6.4</a>&nbsp;Server Push Implicit Headers
1991      </h2>
1992      <p id="rfc.section.6.4.p.1">Pushed resources do not have an associated request. In order for existing HTTP cache control validations (such as the Vary
1993         header) to work, however, all cached resources must have a set of request headers. For this reason, browsers MUST be careful
1994         to inherit request headers from the associated stream for the push. This includes the 'Cookie' header.
1995      </p>
1996      <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;Privacy Considerations
1997      </h1>
1998      <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;Long Lived Connections
1999      </h2>
2000      <p id="rfc.section.7.1.p.1">HTTP/2.0 aims to keep connections open longer between clients and servers in order to reduce the latency when a user makes
2001         a request. The maintenance of these connections over time could be used to expose private information. For example, a user
2002         using a browser hours after the previous user stopped using that browser may be able to learn about what the previous user
2003         was doing. This is a problem with HTTP in its current form as well, however the short lived connections make it less of a
2004         risk.
2005      </p>
2006      <h2 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a>&nbsp;SETTINGS frame
2007      </h2>
2008      <p id="rfc.section.7.2.p.1">The HTTP/2.0 SETTINGS frame allows servers to store out-of-band transmitted information about the communication between client
2009         and server on the client. Although this is intended only to be used to reduce latency, renegade servers could use it as a
2010         mechanism to store identifying information about the client in future requests.
2011      </p>
2012      <p id="rfc.section.7.2.p.2">Clients implementing privacy modes, such as Google Chrome's "incognito mode", may wish to disable client-persisted SETTINGS
2013         storage.
2014      </p>
2015      <p id="rfc.section.7.2.p.3">Clients MUST clear persisted SETTINGS information when clearing the cookies.</p>
2016      <p id="rfc.section.7.2.p.4">TODO: Put range maximums on each type of setting to limit inappropriate uses.</p>
2017      <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;Requirements Notation
2018      </h1>
2019      <p id="rfc.section.8.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
2020         in this document are to be interpreted as described in <a href="#RFC2119">RFC 2119</a> <cite title="Key words for use in RFCs to Indicate Requirement Levels" id="rfc.xref.RFC2119.1">[RFC2119]</cite>.
2021      </p>
2022      <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;Acknowledgements
2023      </h1>
2024      <p id="rfc.section.9.p.1">This document includes substantial input from the following individuals: </p>
2025      <ul>
2026         <li>Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa Wilk, Costin Manolache, William Chan, Vitaliy Lvin, Joe
2027            Chan, Adam Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, Kevin Lindsay, Paul Amer, Fan Yang, Jonathan Leighton (SPDY contributors).
2028         </li>
2029         <li>Gabriel Montenegro and Willy Tarreau (Upgrade mechanism)</li>
2030         <li>William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro, Jitu Padhye, Roberto Peon, Rob Trace (Flow control principles)</li>
2031         <li>Mark Nottingham and Julian Reschke</li>
2032      </ul>
2033      <h1 id="rfc.references"><a href="#rfc.section.10" id="rfc.section.10">10.</a> Normative References
2034      </h1>
2035      <table>                           
2036         <tr>
2037            <td class="reference"><b id="ASCII">[ASCII]</b></td>
2038            <td class="top">“US-ASCII. Coded Character Set - 7-Bit American Standard Code for Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.”.</td>
2039         </tr>
2040         <tr>
2041            <td class="reference"><b id="HTTP-p1">[HTTP-p1]</b></td>
2042            <td class="top">Fielding, R. and J. Reschke, “<a href="">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-21 (work in progress), October&nbsp;2012.
2043            </td>
2044         </tr>
2045         <tr>
2046            <td class="reference"><b id="HTTP-p2">[HTTP-p2]</b></td>
2047            <td class="top">Fielding, R. and J. Reschke, “<a href="">Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-21 (work in progress), October&nbsp;2012.
2048            </td>
2049         </tr>
2050         <tr>
2051            <td class="reference"><b id="RFC0793">[RFC0793]</b></td>
2052            <td class="top">Postel, J., “<a href="">Transmission Control Protocol</a>”, STD&nbsp;7, RFC&nbsp;793, September&nbsp;1981.
2053            </td>
2054         </tr>
2055         <tr>
2056            <td class="reference"><b id="RFC1738">[RFC1738]</b></td>
2057            <td class="top">Berners-Lee, T., Masinter, L., and M. McCahill, “<a href="">Uniform Resource Locators (URL)</a>”, RFC&nbsp;1738, December&nbsp;1994.
2058            </td>
2059         </tr>
2060         <tr>
2061            <td class="reference"><b id="RFC1950">[RFC1950]</b></td>
2062            <td class="top"><a href="" title="Aladdin Enterprises">Deutsch, L.</a> and J. Gailly, “<a href="">ZLIB Compressed Data Format Specification version 3.3</a>”, RFC&nbsp;1950, May&nbsp;1996.
2063            </td>
2064         </tr>
2065         <tr>
2066            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
2067            <td class="top"><a href="" title="Harvard University">Bradner, S.</a>, “<a href="">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP&nbsp;14, RFC&nbsp;2119, March&nbsp;1997.
2068            </td>
2069         </tr>
2070         <tr>
2071            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
2072            <td class="top"><a href="" title="University of California, Irvine">Fielding, R.</a>, <a href="" title="W3C">Gettys, J.</a>, <a href="" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="" title="Xerox Corporation">Masinter, L.</a>, <a href="" title="Microsoft Corporation">Leach, P.</a>, and <a href="" title="W3C">T. Berners-Lee</a>, “<a href="">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
2073            </td>
2074         </tr>
2075         <tr>
2076            <td class="reference"><b id="RFC2617">[RFC2617]</b></td>
2077            <td class="top"><a href="" title="Northwestern University, Department of Mathematics">Franks, J.</a>, <a href="" title="Verisign Inc.">Hallam-Baker, P.</a>, <a href="" title="AbiSource, Inc.">Hostetler, J.</a>, <a href="" title="Agranat Systems, Inc.">Lawrence, S.</a>, <a href="" title="Microsoft Corporation">Leach, P.</a>, Luotonen, A., and <a href="" title="Open Market, Inc.">L. Stewart</a>, “<a href="">HTTP Authentication: Basic and Digest Access Authentication</a>”, RFC&nbsp;2617, June&nbsp;1999.
2078            </td>
2079         </tr>
2080         <tr>
2081            <td class="reference"><b id="RFC4559">[RFC4559]</b></td>
2082            <td class="top">Jaganathan, K., Zhu, L., and J. Brezak, “<a href="">SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows</a>”, RFC&nbsp;4559, June&nbsp;2006.
2083            </td>
2084         </tr>
2085         <tr>
2086            <td class="reference"><b id="RFC5246">[RFC5246]</b></td>
2087            <td class="top">Dierks, T. and E. Rescorla, “<a href="">The Transport Layer Security (TLS) Protocol Version 1.2</a>”, RFC&nbsp;5246, August&nbsp;2008.
2088            </td>
2089         </tr>
2090         <tr>
2091            <td class="reference"><b id="RFC6454">[RFC6454]</b></td>
2092            <td class="top">Barth, A., “<a href="">The Web Origin Concept</a>”, RFC&nbsp;6454, December&nbsp;2011.
2093            </td>
2094         </tr>
2095         <tr>
2096            <td class="reference"><b id="TLSNPN">[TLSNPN]</b></td>
2097            <td class="top">Langley, A., “<a href="">TLS Next Protocol Negotiation</a>”, Internet-Draft&nbsp;draft-agl-tls-nextprotoneg-01 (work in progress), August&nbsp;2010.
2098            </td>
2099         </tr>
2100         <tr>
2101            <td class="reference"><b id="UDELCOMPRESSION">[UDELCOMPRESSION]</b></td>
2102            <td class="top">Yang, F., Amer, P., and J. Leighton, “<a href="">A Methodology to Derive SPDY's Initial Dictionary for Zlib Compression</a>”, &lt;<a href=""></a>&gt;.
2103            </td>
2104         </tr>
2105      </table>
2106      <div class="avoidbreak">
2107         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
2108         <address class="vcard"><span class="vcardline"><span class="fn">Mike Belshe</span><span class="n hidden"><span class="family-name">Belshe</span><span class="given-name">Mike</span></span></span><span class="org vcardline">Twist</span><span class="vcardline">Email: <a href=""><span class="email"></span></a></span></address>
2109         <address class="vcard"><span class="vcardline"><span class="fn">Roberto Peon</span><span class="n hidden"><span class="family-name">Peon</span><span class="given-name">Roberto</span></span></span><span class="org vcardline">Google, Inc</span><span class="vcardline">Email: <a href=""><span class="email"></span></a></span></address>
2110         <address class="vcard"><span class="vcardline"><span class="fn">Martin Thomson</span>
2111               (editor)
2112               <span class="n hidden"><span class="family-name">Thomson</span><span class="given-name">Martin</span></span></span><span class="org vcardline">Microsoft</span><span class="adr"><span class="street-address vcardline">3210 Porter Drive</span><span class="vcardline"><span class="locality">Palo Alto</span>, <span class="postal-code">94043</span></span><span class="country-name vcardline">US</span></span><span class="vcardline">Email: <a href=""><span class="email"></span></a></span></address>
2113         <address class="vcard"><span class="vcardline"><span class="fn">Alexey Melnikov</span>
2114               (editor)
2115               <span class="n hidden"><span class="family-name">Melnikov</span><span class="given-name">Alexey</span></span></span><span class="org vcardline">Isode Ltd</span><span class="adr"><span class="street-address vcardline">5 Castle Business Village</span><span class="street-address vcardline">36 Station Road</span><span class="vcardline"><span class="locality">Hampton</span>, <span class="region">Middlesex</span>&nbsp;<span class="postal-code">TW12 2BX</span></span><span class="country-name vcardline">UK</span></span><span class="vcardline">Email: <a href=""><span class="email"></span></a></span></address>
2116      </div>
2117      <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a id="change.log" href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
2118      <h2 id="rfc.section.A.1"><a href="#rfc.section.A.1">A.1</a>&nbsp;<a id="changes.since.draft-ietf-httpbis-http2-00" href="#changes.since.draft-ietf-httpbis-http2-00">Since draft-ietf-httpbis-http2-00</a></h2>
2119      <p id="rfc.section.A.1.p.1">Changed title throughout.</p>
2120      <p id="rfc.section.A.1.p.2">Removed section on Incompatibilities with SPDY draft#2.</p>
2121      <p id="rfc.section.A.1.p.3">Changed INTERNAL_ERROR on GOAWAY to have a value of 2 &lt;<a href="!topic/spdy-dev/cfUef2gL3iU">!topic/spdy-dev/cfUef2gL3iU</a>&gt;.
2122      </p>
2123      <p id="rfc.section.A.1.p.4">Replaced abstract and introduction.</p>
2124      <p id="rfc.section.A.1.p.5">Added section on starting HTTP/2.0, including upgrade mechanism.</p>
2125      <p id="rfc.section.A.1.p.6">Removed unused references.</p>
2126      <p id="rfc.section.A.1.p.7">Added flow control principles (<a href="#fc-principles" title="Flow Control Principles">Section&nbsp;3.5.1</a>) based on &lt;<a href=""></a>&gt;.
2127      </p>
2128      <h2 id="rfc.section.A.2"><a href="#rfc.section.A.2">A.2</a>&nbsp;<a id="changes.since.draft-mbelshe-httpbis-spdy-00" href="#changes.since.draft-mbelshe-httpbis-spdy-00">Since draft-mbelshe-httpbis-spdy-00</a></h2>
2129      <p id="rfc.section.A.2.p.1">Adopted as base for draft-ietf-httpbis-http2.</p>
2130      <p id="rfc.section.A.2.p.2">Updated authors/editors list.</p>
2131      <p id="rfc.section.A.2.p.3">Added status note.</p>
2132      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
2133      <p class="noprint"><a href="#rfc.index.A">A</a> <a href="#rfc.index.H">H</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.T">T</a> <a href="#rfc.index.U">U</a>
2134      </p>
2135      <div class="print2col">
2136         <ul class="ind">
2137            <li><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul>
2138                  <li><em>ASCII</em>&nbsp;&nbsp;<a href="#rfc.xref.ASCII.1">3.6.10</a>, <a href="#ASCII"><b>10</b></a></li>
2139               </ul>
2140            </li>
2141            <li><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul>
2142                  <li><em>HTTP-p1</em>&nbsp;&nbsp;<a href="#rfc.xref.HTTP-p1.1">1</a>, <a href="#rfc.xref.HTTP-p1.2">2.1</a>, <a href="#HTTP-p1"><b>10</b></a></li>
2143                  <li><em>HTTP-p2</em>&nbsp;&nbsp;<a href="#rfc.xref.HTTP-p2.1">2.2</a>, <a href="#HTTP-p2"><b>10</b></a></li>
2144               </ul>
2145            </li>
2146            <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
2147                  <li><em>RFC0793</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC0793.1">3.1</a>, <a href="#RFC0793"><b>10</b></a></li>
2148                  <li><em>RFC1738</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC1738.1">4.2.1</a>, <a href="#rfc.xref.RFC1738.2">4.2.1</a>, <a href="#RFC1738"><b>10</b></a></li>
2149                  <li><em>RFC1950</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC1950.1"></a>, <a href="#RFC1950"><b>10</b></a></li>
2150                  <li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">8</a>, <a href="#RFC2119"><b>10</b></a></li>
2151                  <li><em>RFC2616</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">4</a>, <a href="#RFC2616"><b>10</b></a></li>
2152                  <li><em>RFC2617</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2617.1">4.2.3</a>, <a href="#RFC2617"><b>10</b></a></li>
2153                  <li><em>RFC4559</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC4559.1">4.2.3</a>, <a href="#RFC4559"><b>10</b></a></li>
2154                  <li><em>RFC5246</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5246.1">3.6.9</a>, <a href="#RFC5246"><b>10</b></a><ul>
2155                        <li><em>Section 4.7</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5246.1">3.6.9</a></li>
2156                     </ul>
2157                  </li>
2158                  <li><em>RFC6454</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC6454.1">3.6.4</a>, <a href="#rfc.xref.RFC6454.2">4.1</a>, <a href="#rfc.xref.RFC6454.3">4.3</a>, <a href="#rfc.xref.RFC6454.4">6.1</a>, <a href="#RFC6454"><b>10</b></a></li>
2159               </ul>
2160            </li>
2161            <li><a id="rfc.index.T" href="#rfc.index.T"><b>T</b></a><ul>
2162                  <li><em>TLSNPN</em>&nbsp;&nbsp;<a href="#rfc.xref.TLSNPN.1">2.1</a>, <a href="#TLSNPN"><b>10</b></a></li>
2163               </ul>
2164            </li>
2165            <li><a id="rfc.index.U" href="#rfc.index.U"><b>U</b></a><ul>
2166                  <li><em>UDELCOMPRESSION</em>&nbsp;&nbsp;<a href="#rfc.xref.UDELCOMPRESSION.1"></a>, <a href="#UDELCOMPRESSION"><b>10</b></a></li>
2167               </ul>
2168            </li>
2169         </ul>
2170      </div>
2171   </body>
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