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  10 <title>mod_http2 - Apache HTTP Server Version 2.5</title>
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  25 <a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="../">Version 2.5</a> &gt; <a href="./">Modules</a></div>
  26 <div id="page-content">
  27 <div id="preamble"><h1>Apache Module mod_http2</h1>
  28 <div class="toplang">
  29 <p><span>Available Languages: </span><a href="../en/mod/mod_http2.html" title="English">&nbsp;en&nbsp;</a></p>
  30 </div>
  31 <table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Support for the HTTP/2 transport layer</td></tr>
  32 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
  33 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>http2_module</td></tr>
  34 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_http2.c</td></tr>
  35 <tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.17 and later</td></tr></table>
  36 <h3>Summary</h3>
  37
  38         <p>This module provides HTTP/2 (RFC 7540) support for the Apache
  39             HTTP Server.</p>
  40
  41         <p>This module relies on <a href="http://nghttp2.org/">libnghttp2</a>
  42             to provide the core http/2 engine.</p>
  43
  44         <div class="warning"><h3>Warning</h3>
  45           <p>This module is experimental. Its behaviors, directives, and
  46           defaults are subject to more change from release to
  47           release relative to other standard modules. Users are encouraged to
  48           consult the "CHANGES" file for potential updates.</p>
  49         </div>
  50
  51         <p>You must enable HTTP/2 via <code class="directive"><a href="../mod/core.html#protocols">Protocols</a></code> in order to use the
  52         functionality described in this document:</p>
  53
  54         <pre class="prettyprint lang-config">Protocols h2 http/1.1</pre>
  55
  56
  57     </div>
  58 <div id="quickview"><h3>Topics</h3>
  59 <ul id="topics">
  60 <li><img alt="" src="../images/down.gif" /> <a href="#envvars">Environment Variables</a></li>
  61 </ul><h3 class="directives">Directives</h3>
  62 <ul id="toc">
  63 <li><img alt="" src="../images/down.gif" /> <a href="#h2direct">H2Direct</a></li>
  64 <li><img alt="" src="../images/down.gif" /> <a href="#h2keepalivetimeout">H2KeepAliveTimeout</a></li>
  65 <li><img alt="" src="../images/down.gif" /> <a href="#h2maxsessionstreams">H2MaxSessionStreams</a></li>
  66 <li><img alt="" src="../images/down.gif" /> <a href="#h2maxworkeridleseconds">H2MaxWorkerIdleSeconds</a></li>
  67 <li><img alt="" src="../images/down.gif" /> <a href="#h2maxworkers">H2MaxWorkers</a></li>
  68 <li><img alt="" src="../images/down.gif" /> <a href="#h2minworkers">H2MinWorkers</a></li>
  69 <li><img alt="" src="../images/down.gif" /> <a href="#h2moderntlsonly">H2ModernTLSOnly</a></li>
  70 <li><img alt="" src="../images/down.gif" /> <a href="#h2push">H2Push</a></li>
  71 <li><img alt="" src="../images/down.gif" /> <a href="#h2pushdiarysize">H2PushDiarySize</a></li>
  72 <li><img alt="" src="../images/down.gif" /> <a href="#h2pushpriority">H2PushPriority</a></li>
  73 <li><img alt="" src="../images/down.gif" /> <a href="#h2serializeheaders">H2SerializeHeaders</a></li>
  74 <li><img alt="" src="../images/down.gif" /> <a href="#h2sessionextrafiles">H2SessionExtraFiles</a></li>
  75 <li><img alt="" src="../images/down.gif" /> <a href="#h2streammaxmemsize">H2StreamMaxMemSize</a></li>
  76 <li><img alt="" src="../images/down.gif" /> <a href="#h2streamtimeout">H2StreamTimeout</a></li>
  77 <li><img alt="" src="../images/down.gif" /> <a href="#h2timeout">H2Timeout</a></li>
  78 <li><img alt="" src="../images/down.gif" /> <a href="#h2tlscooldownsecs">H2TLSCoolDownSecs</a></li>
  79 <li><img alt="" src="../images/down.gif" /> <a href="#h2tlswarmupsize">H2TLSWarmUpSize</a></li>
  80 <li><img alt="" src="../images/down.gif" /> <a href="#h2upgrade">H2Upgrade</a></li>
  81 <li><img alt="" src="../images/down.gif" /> <a href="#h2windowsize">H2WindowSize</a></li>
  82 </ul>
  83 <ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
  84 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
  85 <div class="section">
  86 <h2><a name="envvars" id="envvars">Environment Variables</a></h2>
  87
  88         <p>This module can be configured to provide HTTP/2 related information
  89             as additional environment variables to the SSI and CGI namespace.
  90         </p>
  91
  92         <table class="bordered">
  93
  94             <tr>
  95                 <th><a name="table3">Variable Name:</a></th>
  96                 <th>Value Type:</th>
  97                 <th>Description:</th>
  98             </tr>
  99             <tr><td><code>HTTPe</code></td>                         <td>flag</td>      <td>HTTP/2 is being used.</td></tr>
 100             <tr><td><code>H2PUSH</code></td>                        <td>flag</td>      <td>HTTP/2 Server Push is enabled for this request and also supported by the client.</td></tr>
 101         </table>
 102
 103     </div>
 104 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 105 <div class="directive-section"><h2><a name="H2Direct" id="H2Direct">H2Direct</a> <a name="h2direct" id="h2direct">Directive</a></h2>
 106 <table class="directive">
 107 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Direct Protocol Switch</td></tr>
 108 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2Direct on|off</code></td></tr>
 109 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2Direct on for h2c, off for h2 protocol</code></td></tr>
 110 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 111 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 112 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 113 </table>
 114             <p>
 115                 This directive toggles the usage of the HTTP/2 Direct Mode. This
 116                 should be used inside a
 117                 <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>
 118                 section to enable direct HTTP/2 communication for that virtual host.
 119             </p>
 120             <p>
 121                 Direct communication means that if the first bytes received by the
 122                 server on a connection match the HTTP/2 preamble, the HTTP/2
 123                 protocol is switched to immediately without further negotiation.
 124                 This mode is defined in RFC 7540 for the cleartext (h2c) case. Its
 125                 use on TLS connections not mandated by the standard.
 126             </p>
 127             <p>
 128                 When a server/vhost does not have h2 or h2c enabled via
 129                 <code class="directive"><a href="../mod/core.html#protocols">&lt;Protocols&gt;</a></code>,
 130                 the connection is never inspected for a HTTP/2 preamble. H2Direct
 131                 does not matter then. This is important for connections that
 132                 use protocols where an initial read might hang indefinitely, such
 133                 as NNTP.
 134             </p>
 135             <p>
 136                 For clients that have out-of-band knowledge about a server
 137                 supporting h2c, direct HTTP/2 saves the client from having to
 138                 perform an HTTP/1.1 upgrade, resulting in better performance
 139                 and avoiding the Upgrade restrictions on request bodies.
 140             </p>
 141             <p>
 142                 This makes direct h2c attractive for server to server communication
 143                 as well, when the connection can be trusted or is secured by other means.
 144             </p>
 145             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2Direct on</pre>
 146 </div>
 147
 148 </div>
 149 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 150 <div class="directive-section"><h2><a name="H2KeepAliveTimeout" id="H2KeepAliveTimeout">H2KeepAliveTimeout</a> <a name="h2keepalivetimeout" id="h2keepalivetimeout">Directive</a></h2>
 151 <table class="directive">
 152 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Timeout (in seconds) for idle HTTP/2 connections</td></tr>
 153 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2KeepAliveTimeout seconds</code></td></tr>
 154 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 155 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 156 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 157 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.19 and later.</td></tr>
 158 </table>
 159             <p>
 160                 This directive sets the timeout for read/write operations on
 161                 idle connections where HTTP/2 is negotiated. This can be used server wide or for specific
 162                 <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>s.
 163             </p>
 164             <p>
 165                 This directive is similar to the
 166                 <code class="directive"><a href="../mod/core.html#keepalivetimeout">&lt;KeepAliveTimeout&gt;</a></code>, but
 167                 applies only to HTTP/2 connections. A HTTP/2 connection is considered
 168                 idle when no streams are open, e.g. no requests are ongoing.
 169             </p>
 170             <p>
 171                 By default, for non-async MPMs (prefork, worker) the keepalive timeout
 172                 will be the same as H2Timeout. For async MPMs, the keepalive handling for
 173                 HTTP/1 connections applies as no special action is taken.
 174             </p>
 175
 176 </div>
 177 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 178 <div class="directive-section"><h2><a name="H2MaxSessionStreams" id="H2MaxSessionStreams">H2MaxSessionStreams</a> <a name="h2maxsessionstreams" id="h2maxsessionstreams">Directive</a></h2>
 179 <table class="directive">
 180 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum number of active streams per HTTP/2 session.</td></tr>
 181 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2MaxSessionStreams <em>n</em></code></td></tr>
 182 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2MaxSessionStreams 100</code></td></tr>
 183 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 184 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 185 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 186 </table>
 187             <p>
 188                 This directive sets the maximum number of active streams per HTTP/2 session (e.g. connection)
 189                 that the server allows. A stream is active if it is not <code>idle</code> or
 190                 <code>closed</code> according to RFC 7540.
 191             </p>
 192             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2MaxSessionStreams 20</pre>
 193 </div>
 194
 195 </div>
 196 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 197 <div class="directive-section"><h2><a name="H2MaxWorkerIdleSeconds" id="H2MaxWorkerIdleSeconds">H2MaxWorkerIdleSeconds</a> <a name="h2maxworkeridleseconds" id="h2maxworkeridleseconds">Directive</a></h2>
 198 <table class="directive">
 199 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum number of seconds h2 workers remain idle until shut down.</td></tr>
 200 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2MaxWorkerIdleSeconds <em>n</em></code></td></tr>
 201 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2MaxWorkerIdleSeconds 600</code></td></tr>
 202 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
 203 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 204 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 205 </table>
 206             <p>
 207                 This directive sets the maximum number of seconds a h2 worker may
 208                 idle until it shuts itself down. This only happens while the number of
 209                 h2 workers exceeds <code>H2MinWorkers</code>.
 210             </p>
 211             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2MaxWorkerIdleSeconds 20</pre>
 212 </div>
 213
 214 </div>
 215 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 216 <div class="directive-section"><h2><a name="H2MaxWorkers" id="H2MaxWorkers">H2MaxWorkers</a> <a name="h2maxworkers" id="h2maxworkers">Directive</a></h2>
 217 <table class="directive">
 218 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum number of worker threads to use per child process.</td></tr>
 219 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2MaxWorkers <em>n</em></code></td></tr>
 220 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
 221 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 222 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 223 </table>
 224             <p>
 225                 This directive sets the maximum number of worker threads to spawn
 226                 per child process for HTTP/2 processing. If this directive is not used,
 227                 <code>mod_http2</code> will chose a value suitable for the <code>mpm</code>
 228                 module loaded.
 229             </p>
 230             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2MaxWorkers 20</pre>
 231 </div>
 232
 233 </div>
 234 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 235 <div class="directive-section"><h2><a name="H2MinWorkers" id="H2MinWorkers">H2MinWorkers</a> <a name="h2minworkers" id="h2minworkers">Directive</a></h2>
 236 <table class="directive">
 237 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Minimal number of worker threads to use per child process.</td></tr>
 238 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2MinWorkers <em>n</em></code></td></tr>
 239 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
 240 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 241 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 242 </table>
 243             <p>
 244                 This directive sets the minimum number of worker threads to spawn
 245                 per child process for HTTP/2 processing. If this directive is not used,
 246                 <code>mod_http2</code> will chose a value suitable for the <code>mpm</code>
 247                 module loaded.
 248             </p>
 249             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2MinWorkers 10</pre>
 250 </div>
 251
 252 </div>
 253 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 254 <div class="directive-section"><h2><a name="H2ModernTLSOnly" id="H2ModernTLSOnly">H2ModernTLSOnly</a> <a name="h2moderntlsonly" id="h2moderntlsonly">Directive</a></h2>
 255 <table class="directive">
 256 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Require HTTP/2 connections to be "modern TLS" only</td></tr>
 257 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2ModernTLSOnly on|off</code></td></tr>
 258 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2ModernTLSOnly on</code></td></tr>
 259 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 260 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 261 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 262 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later.</td></tr>
 263 </table>
 264             <p>
 265                 This directive toggles the security checks on HTTP/2 connections
 266                 in TLS mode (https:). This can be used server wide or for specific
 267                 <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>s.
 268             </p>
 269             <p>
 270                 The security checks require that the TSL protocol is at least
 271                 TLSv1.2 and that none of the ciphers listed in RFC 7540, Appendix A
 272                 is used. These checks will be extended once new security requirements
 273                 come into place.
 274             </p>
 275             <p>
 276                 The name stems from the
 277                 <a href="https://wiki.mozilla.org/Security/Server_Side_TLS">Security/Server Side TLS</a>
 278                 definitions at mozilla where "modern compatibility" is defined. Mozilla Firefox and
 279                 other browsers require modern compatibility for HTTP/2 connections. As everything
 280                 in OpSec, this is a moving target and can be expected to evolve in the future.
 281             </p>
 282             <p>
 283                 One purpose of having these checks in mod_http2 is to enforce this
 284                 security level for all connections, not only those from browsers. The other
 285                 purpose is to prevent the negotiation of HTTP/2 as a protocol should
 286                 the requirements not be met.
 287             </p>
 288             <p>
 289                 Ultimately, the security of the TLS connection is determined by the
 290                 server configuration directives for mod_ssl.
 291             </p>
 292             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2ModernTLSOnly off</pre>
 293 </div>
 294
 295 </div>
 296 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 297 <div class="directive-section"><h2><a name="H2Push" id="H2Push">H2Push</a> <a name="h2push" id="h2push">Directive</a></h2>
 298 <table class="directive">
 299 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Server Push Switch</td></tr>
 300 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2Push on|off</code></td></tr>
 301 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2Push on</code></td></tr>
 302 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 303 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 304 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 305 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later.</td></tr>
 306 </table>
 307             <p>
 308                 This directive toggles the usage of the HTTP/2 server push
 309                 protocol feature. This should be used inside a
 310                 <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>
 311                 section to enable direct HTTP/2 communication for that virtual host.
 312             </p>
 313             <p>
 314                 The HTTP/2 protocol allows the server to push other resources to
 315                 a client when it asked for a particular one. This is helpful
 316                 if those resources are connected in some way and the client can
 317                 be expected to ask for it anyway. The pushing then saves the
 318                 time it takes the client to ask for the resources itself. On the
 319                 other hand, pushing resources the client never needs or already
 320                 has is a waste of bandwidth.
 321             </p>
 322             <p>
 323                 Server pushes are detected by inspecting the <code>Link</code> headers of
 324                 responses (see https://tools.ietf.org/html/rfc5988 for the
 325                 specification). When a link thus specified has the <code>rel=preload</code>
 326                 attribute, it is treated as a resource to be pushed.
 327             </p>
 328             <p>
 329                 Link headers in responses are either set by the application or
 330                 can be configured via <code class="module"><a href="../mod/mod_headers.html">mod_headers</a></code> as:
 331             </p>
 332             <div class="example"><h3>mod_headers example</h3><pre class="prettyprint lang-config">&lt;Location /index.html&gt;
 333     Header add Link "&lt;/css/site.css&gt;;rel=preload"
 334     Header add Link "&lt;/images/logo.jpg&gt;;rel=preload"
 335 &lt;/Location&gt;</pre>
 336 </div>
 337             <p>
 338                 As the example shows, there can be several link headers added
 339                 to a response, resulting in several pushes being triggered. There
 340                 are no checks in the module to avoid pushing the same resource
 341                 twice or more to one client. Use with care.
 342             </p>
 343             <p>
 344                 HTTP/2 server pushes are enabled by default. This directive
 345                 allows it to be switch off on all resources of this server/virtual
 346                 host.
 347             </p>
 348             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2Push off</pre>
 349 </div>
 350             <p>
 351                 Last but not least, pushes happen only when the client signals
 352                 its willingness to accept those. Most browsers do, some, like Safari 9,
 353                 do not. Also, pushes also only happen for resources from the same
 354                 <em>authority</em> as the original response is for.
 355             </p>
 356
 357 </div>
 358 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 359 <div class="directive-section"><h2><a name="H2PushDiarySize" id="H2PushDiarySize">H2PushDiarySize</a> <a name="h2pushdiarysize" id="h2pushdiarysize">Directive</a></h2>
 360 <table class="directive">
 361 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Server Push Diary Size</td></tr>
 362 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2PushDiarySize n</code></td></tr>
 363 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2PushDiarySize 128</code></td></tr>
 364 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 365 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 366 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 367 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.19 and later.</td></tr>
 368 </table>
 369             <p>
 370                 This directive toggles the maximum number of HTTP/2 server pushes
 371                 that are remembered per HTTP/2 connection. This can be used inside the
 372                 <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>
 373                 section to influence the number for all connections to that virtual host.
 374             </p>
 375             <p>
 376                 The push diary records a digest (currently using SHA256) of pushed
 377                 resources (their URL) to avoid duplicate pushes on the same connection.
 378                 These value are not persisted, so clients openeing a new connection
 379                 will experience known pushes again. There is ongoing work to enable
 380                 a client to disclose a digest of the resources it already has, so
 381                 the diary maybe initialized by the client on each connection setup.
 382             </p>
 383             <p>
 384                 If the maximum size is reached, newer entries replace the oldest
 385                 ones. Using SHA256, each diary entry uses 32 bytes, letting a
 386                 default diary with 128 entries consume around 4 KB of memory.
 387             </p>
 388             <p>
 389                 A size of 0 will effectively disable the push diary.
 390             </p>
 391
 392 </div>
 393 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 394 <div class="directive-section"><h2><a name="H2PushPriority" id="H2PushPriority">H2PushPriority</a> <a name="h2pushpriority" id="h2pushpriority">Directive</a></h2>
 395 <table class="directive">
 396 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Server Push Priority</td></tr>
 397 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2PushPriority mime-type [after|before|interleaved] [weight]</code></td></tr>
 398 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2PushPriority * After 16</code></td></tr>
 399 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 400 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 401 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 402 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later. For having an
 403         effect, a nghttp2 library version 1.5.0 or newer is necessary.</td></tr>
 404 </table>
 405             <p>
 406                 This directive defines the priority handling of pushed responses
 407                 based on the content-type of the response. This is usually defined
 408                 per server config, but may also appear in a virtual host.
 409             </p>
 410             <p>
 411                 HTTP/2 server pushes are always related to a client request. Each
 412                 such request/response pairs, or <em>streams</em> have a dependency
 413                 and a weight, together defining the <em>priority</em> of a stream.
 414             </p>
 415             <p>
 416                 When a stream <em>depends</em> on another, say X depends on Y,
 417                 then Y gets all bandwidth before X gets any. Note that this
 418                 does not men that Y will block X. If Y has no data to send,
 419                 all bandwidth allocated to Y can be used by X.
 420             </p>
 421             <p>
 422                 When a stream has more than one dependant, say X1 and X2 both
 423                 depend on Y, the <em>weight</em> determines the bandwidth
 424                 allocation. If X1 and X2 have the same weight, they both get
 425                 half of the available bandwdith. If the weight of X1 is twice
 426                 as large as that for X2, X1 gets twice the bandwidth of X2.
 427             </p>
 428             <p>
 429                 Ultimately, every stream depends on the <em>root</em> stream which
 430                 gets all the bandwidht available, but never sends anything. So all
 431                 its bandwidth is distributed by weight among its children. Which
 432                 either have data to send or distribute the bandwidth to their
 433                 own children. And so on. If none of the children have data
 434                 to send, that bandwidth get distributed somewhere else according
 435                 to the same rules.
 436             </p>
 437             <p>
 438                 The purpose of this priority system is to always make use of
 439                 available bandwidth while allowing precedence and weight
 440                 to be given to specific streams. Since, normally, all streams
 441                 are initiated by the client, it is also the one that sets
 442                 these priorities.
 443             </p>
 444             <p>
 445                 Only when such a stream results in a PUSH, gets the server to
 446                 decide what the <em>initial</em> priority of such a pushed
 447                 stream is. In the examples below, X is the client stream. It
 448                 depends on Y and the server decides to PUSH streams P1 and P2
 449                 onto X.
 450             </p>
 451             <p>
 452                 The default priority rule is:
 453             </p>
 454             <div class="example"><h3>Default Priority Rule</h3><pre class="prettyprint lang-config">H2PushPriority * After 16</pre>
 455 </div>
 456             <p>
 457                 which reads as 'Send a pushed stream of any content-type
 458                 depending on the client stream with weight 16'. And so P1
 459                 and P2 will be send after X and, as they have equal weight,
 460                 share bandwidth equally among themselves.
 461             </p>
 462             <div class="example"><h3>Interleaved Priority Rule</h3><pre class="prettyprint lang-config">H2PushPriority text/css Interleaved 256</pre>
 463 </div>
 464             <p>
 465                 which reads as 'Send any CSS resource on the same dependency and
 466                 weight as the client stream'. If P1 has content-type 'text/css',
 467                 it will depend on Y (as does X) and its effective weight will be
 468                 calculated as <code>P1ew = Xw * (P1w / 256)</code>. With P1w being
 469                 256, this will make the effective weight the same as the weight
 470                 of X. If both X and P1 have data to send, bandwidth will be allocated
 471                 to both equally.
 472             </p>
 473             <p>
 474                 With Pw specified as 512, a pushed, interleaved stream would
 475                 get double the weight of X. With 128 only half as much. Note that
 476                 effective weights are always capped at 256.
 477             </p>
 478             <div class="example"><h3>Before Priority Rule</h3><pre class="prettyprint lang-config">H2PushPriority application/json Before</pre>
 479 </div>
 480             <p>
 481                 This says that any pushed stream of content type 'application/json'
 482                 should be send out <em>before</em> X. This makes P1 dependent
 483                 on Y and X dependent on P1. So, X will be stalled as long as
 484                 P1 has data to send. The effective weight is inherited from the
 485                 client stream. Specifying a weight is not allowed.
 486             </p>
 487             <p>
 488                 Be aware that the effect of priority specifications is limited
 489                 by the available server resources. If a server does not have
 490                 workers available for pushed streams, the data for the stream
 491                 may only ever arrive when other streams have been finished.
 492             </p>
 493             <p>
 494                 Last, but not least, there are some specifics of the syntax
 495                 to be used in this directive:
 496             </p>
 497             <ol>
 498                 <li>'*' is the only special content-type that matches all others.
 499                  'image/*' will not work.</li>
 500                 <li>The default dependency is 'After'. </li>
 501                 <li>There are also default weights: for 'After' it is 16, 'interleaved' is 256.
 502                 </li>
 503             </ol>
 504             <div class="example"><h3>Shorter Priority Rules</h3><pre class="prettyprint lang-config">H2PushPriority application/json 32         # an After rule
 505 H2PushPriority image/jpeg before           # weight inherited
 506 H2PushPriority text/css   interleaved      # weight 256 default</pre>
 507 </div>
 508
 509 </div>
 510 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 511 <div class="directive-section"><h2><a name="H2SerializeHeaders" id="H2SerializeHeaders">H2SerializeHeaders</a> <a name="h2serializeheaders" id="h2serializeheaders">Directive</a></h2>
 512 <table class="directive">
 513 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Serialize Request/Response Processing Switch</td></tr>
 514 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2SerializeHeaders on|off</code></td></tr>
 515 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2SerializeHeaders off</code></td></tr>
 516 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 517 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 518 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 519 </table>
 520             <p>
 521                 This directive toggles if HTTP/2 requests shall be serialized in
 522                 HTTP/1.1 format for processing by <code>httpd</code> core or if
 523                 received binary data shall be passed into the <code>request_rec</code>s
 524                 directly.
 525             </p>
 526             <p>
 527                 Serialization will lower performance, but gives more backward
 528                 compatibility in case custom filters/hooks need it.
 529             </p>
 530             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2SerializeHeaders on</pre>
 531 </div>
 532
 533 </div>
 534 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 535 <div class="directive-section"><h2><a name="H2SessionExtraFiles" id="H2SessionExtraFiles">H2SessionExtraFiles</a> <a name="h2sessionextrafiles" id="h2sessionextrafiles">Directive</a></h2>
 536 <table class="directive">
 537 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Number of Extra File Handles</td></tr>
 538 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2SessionExtraFiles <em>n</em></code></td></tr>
 539 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 540 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 541 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 542 </table>
 543             <p>
 544                 This directive sets maximum number of <em>extra</em> file handles
 545                 a HTTP/2 session is allowed to use. A file handle is counted as
 546                 <em>extra</em> when it is transferred from a h2 worker thread to
 547                 the main HTTP/2 connection handling. This commonly happens when
 548                 serving static files.
 549             </p><p>
 550                 Depending on the processing model configured on the server, the
 551                 number of connections times number of active streams may exceed
 552                 the number of file handles for the process. On the other hand,
 553                 converting every file into memory bytes early results in too
 554                 many buffer writes. This option helps to mitigate that.
 555             </p><p>
 556                 The number of file handles used by a server process is then in
 557                 the order of:
 558             </p>
 559             <pre>(h2_connections * extra_files) + (h2_max_worker)</pre>
 560             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2SessionExtraFiles 10</pre>
 561 </div>
 562             <p>
 563                 If nothing is configured, the module tries to make a conservative
 564                 guess how many files are safe to use. This depends largely on the
 565                 MPM chosen.
 566             </p>
 567
 568 </div>
 569 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 570 <div class="directive-section"><h2><a name="H2StreamMaxMemSize" id="H2StreamMaxMemSize">H2StreamMaxMemSize</a> <a name="h2streammaxmemsize" id="h2streammaxmemsize">Directive</a></h2>
 571 <table class="directive">
 572 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum amount of output data buffered per stream.</td></tr>
 573 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2StreamMaxMemSize <em>bytes</em></code></td></tr>
 574 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2StreamMaxMemSize 65536</code></td></tr>
 575 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 576 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 577 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 578 </table>
 579             <p>
 580                 This directive sets the maximum number of outgoing data bytes buffered in memory
 581                 for an active streams. This memory is not allocated per stream as such. Allocations
 582                 are counted against this limit when they are about to be done. Stream processing
 583                 freezes when the limit has been reached and will only continue when buffered data
 584                 has been sent out to the client.
 585             </p>
 586             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2StreamMaxMemSize 128000</pre>
 587 </div>
 588
 589 </div>
 590 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 591 <div class="directive-section"><h2><a name="H2StreamTimeout" id="H2StreamTimeout">H2StreamTimeout</a> <a name="h2streamtimeout" id="h2streamtimeout">Directive</a></h2>
 592 <table class="directive">
 593 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Timeout (in seconds) for idle HTTP/2 connections</td></tr>
 594 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2StreamTimeout seconds</code></td></tr>
 595 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2StreamTimeout 0</code></td></tr>
 596 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 597 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 598 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 599 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.19 and later.</td></tr>
 600 </table>
 601             <p>
 602                 This directive sets the timeout for read/write operations on
 603                 HTTP/2 streams, e.g. individual requests. This can be used server wide or for specific
 604                 <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>s.
 605             </p>
 606             <p>
 607                 Due to the nature of HTTP/2, which sends multiple requests over a single
 608                 connection and has priority scheduling, individual streams might not
 609                 see input for much longer times than HTTP/1.1 requests would.
 610             </p>
 611             <p>
 612                 A value of 0 enforces no timeout, so could wait on chances to receive
 613                 input or write data indefinitely. This expose a server to
 614                 risks of thread exhaustion.
 615             </p>
 616             <p>
 617                 Depending on your handling of pushed streams,
 618                 priorities and general responsiveness, a site might need to increase
 619                 this value. For example, if you PUSH a large resource <em>before</em>
 620                 the requested one, the initial stream will not write until the
 621                 pushed resource is fully sent.
 622             </p>
 623
 624 </div>
 625 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 626 <div class="directive-section"><h2><a name="H2Timeout" id="H2Timeout">H2Timeout</a> <a name="h2timeout" id="h2timeout">Directive</a></h2>
 627 <table class="directive">
 628 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Timeout (in seconds) for HTTP/2 connections</td></tr>
 629 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2Timeout seconds</code></td></tr>
 630 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2Timeout 5</code></td></tr>
 631 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 632 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 633 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 634 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.19 and later.</td></tr>
 635 </table>
 636             <p>
 637                 This directive sets the timeout for read/write operations on
 638                 connections where HTTP/2 is negotiated. This can be used server wide or for specific
 639                 <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>s.
 640             </p>
 641             <p>
 642                 This directive is similar to the
 643                 <code class="directive"><a href="../mod/core.html#timeout">&lt;Timeout&gt;</a></code>, but
 644                 applies only to HTTP/2 connections.
 645             </p>
 646             <p>
 647                 A value of 0 enforces no timeout.
 648             </p>
 649
 650 </div>
 651 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 652 <div class="directive-section"><h2><a name="H2TLSCoolDownSecs" id="H2TLSCoolDownSecs">H2TLSCoolDownSecs</a> <a name="h2tlscooldownsecs" id="h2tlscooldownsecs">Directive</a></h2>
 653 <table class="directive">
 654 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td /></tr>
 655 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2TLSCoolDownSecs seconds</code></td></tr>
 656 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2TLSCoolDownSecs 1</code></td></tr>
 657 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 658 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 659 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 660 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later.</td></tr>
 661 </table>
 662             <p>
 663                 This directive sets the number of seconds of idle time on a TLS
 664                 connection before the TLS write size falls back to small (~1300 bytes)
 665                 length.
 666                 This can be used server wide or for specific
 667                 <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>s.
 668             </p>
 669             <p>
 670                 See <code class="directive">&lt;H2TLSWarmUpSize&gt;</code> for a
 671                 description of TLS warmup. H2TLSCoolDownSecs reflects the fact
 672                 that connections may deteriorate over time (and TCP flow adjusts)
 673                 for idle connections as well. It is beneficial to overall performance
 674                 to fall back to the pre-warmup phase after a number of seconds that
 675                 no data has been sent.
 676             </p>
 677             <p>
 678                 In deployments where connections can be considered reliable, this
 679                 timer can be disabled by setting it to 0.
 680             </p>
 681             <p>
 682                 The following example sets the seconds to zero, effectively disabling
 683                 any cool down. Warmed up TLS connections stay on maximum record
 684                 size.
 685             </p>
 686             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2TLSCoolDownSecs 0</pre>
 687 </div>
 688
 689 </div>
 690 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 691 <div class="directive-section"><h2><a name="H2TLSWarmUpSize" id="H2TLSWarmUpSize">H2TLSWarmUpSize</a> <a name="h2tlswarmupsize" id="h2tlswarmupsize">Directive</a></h2>
 692 <table class="directive">
 693 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td /></tr>
 694 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2TLSWarmUpSize amount</code></td></tr>
 695 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2TLSWarmUpSize 1048576</code></td></tr>
 696 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 697 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 698 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 699 <tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later.</td></tr>
 700 </table>
 701             <p>
 702                 This directive sets the number of bytes to be sent in small
 703                 TLS records (~1300 bytes) until doing maximum sized writes (16k)
 704                 on https: HTTP/2 connections.
 705                 This can be used server wide or for specific
 706                 <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>s.
 707             </p>
 708             <p>
 709                 Measurements by <a href="https://www.igvita.com">google performance
 710                 labs</a> show that best performance on TLS connections is reached,
 711                 if initial record sizes stay below the MTU level, to allow a
 712                 complete record to fit into an IP packet.
 713             </p>
 714             <p>
 715                 While TCP adjust its flow-control and window sizes, longer TLS
 716                 records can get stuck in queues or get lost and need retransmission.
 717                 This is of course true for all packets. TLS however needs the
 718                 whole record in order to decrypt it. Any missing bytes at the end
 719                 will stall usage of the received ones.
 720             </p>
 721             <p>
 722                 After a sufficient number of bytes have been send successfully,
 723                 the TCP state of the connection is stable and maximum TLS record
 724                 sizes (16 KB) can be used for optimal performance.
 725             </p>
 726             <p>
 727                 In deployments where servers are reached locally or over reliable
 728                 connections only, the value might be decreased with 0 disabling
 729                 any warmup phase altogether.
 730             </p>
 731             <p>
 732                 The following example sets the size to zero, effectively disabling
 733                 any warmup phase.
 734             </p>
 735             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2TLSWarmUpSize 0</pre>
 736 </div>
 737
 738 </div>
 739 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 740 <div class="directive-section"><h2><a name="H2Upgrade" id="H2Upgrade">H2Upgrade</a> <a name="h2upgrade" id="h2upgrade">Directive</a></h2>
 741 <table class="directive">
 742 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Upgrade Protocol Switch</td></tr>
 743 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2Upgrade on|off</code></td></tr>
 744 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2Upgrade on for h2c, off for h2 protocol</code></td></tr>
 745 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 746 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 747 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 748 </table>
 749             <p>
 750                 This directive toggles the usage of the HTTP/1.1 Upgrade method
 751                 for switching to HTTP/2. This
 752                 should be used inside a
 753                 <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>
 754                 section to enable Upgrades to HTTP/2 for that virtual host.
 755             </p>
 756             <p>
 757                 This method of switching protocols is defined in HTTP/1.1 and
 758                 uses the "Upgrade" header (thus the name) to announce willingness
 759                 to use another protocol. This may happen on any request of a
 760                 HTTP/1.1 connection.
 761             </p>
 762             <p>
 763                 This method of protocol switching is enabled by default on cleartext
 764                 (potential h2c) connections and disabled on TLS (potential h2),
 765                 as mandated by RFC 7540.
 766             </p>
 767             <p>
 768                 Please be aware that Upgrades are only accepted for requests
 769                 that carry no body. POSTs and PUTs with content will never
 770                 trigger an upgrade to HTTP/2.
 771                 See <code class="directive">&lt;H2Direct&gt;</code> for an
 772                 alternative to Upgrade.
 773             </p>
 774             <p>
 775                 This mode only has an effect when h2 or h2c is enabled via
 776                 the <code class="directive"><a href="../mod/core.html#protocols">&lt;Protocols&gt;</a></code>.
 777             </p>
 778             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2Upgrade on</pre>
 779 </div>
 780
 781 </div>
 782 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 783 <div class="directive-section"><h2><a name="H2WindowSize" id="H2WindowSize">H2WindowSize</a> <a name="h2windowsize" id="h2windowsize">Directive</a></h2>
 784 <table class="directive">
 785 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Size of Stream Window for upstream data.</td></tr>
 786 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2WindowSize <em>bytes</em></code></td></tr>
 787 <tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2WindowSize 65535</code></td></tr>
 788 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 789 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 790 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
 791 </table>
 792             <p>
 793                 This directive sets the size of the window that is used for flow control
 794                 from client to server and limits the amount of data the server has to buffer.
 795                 The client will stop sending on a stream once the limit has been reached until
 796                 the server announces more available space (as it has processed some of the data).
 797             </p><p>
 798                 This limit affects only request bodies, not its meta data such as headers. Also,
 799                 it has no effect on response bodies as the window size for those are managed
 800                 by the clients.
 801             </p>
 802             <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2WindowSize 128000</pre>
 803 </div>
 804
 805 </div>
 806 </div>
 807 <div class="bottomlang">
 808 <p><span>Available Languages: </span><a href="../en/mod/mod_http2.html" title="English">&nbsp;en&nbsp;</a></p>
 809 </div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&amp;A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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