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<div id="preamble"><h1>Apache Module mod_http2</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/mod/mod_http2.html" title="English">&nbsp;en&nbsp;</a></p>
</div>
<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Support for the HTTP/2 transport layer</td></tr>
<tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>http2_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_http2.c</td></tr>
<tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.17 and later</td></tr></table>
<h3>Summary</h3>

        <p>This module provides HTTP/2 (RFC 7540) support for the Apache
            HTTP Server.</p>
        
        <p>This module relies on <a href="http://nghttp2.org/">libnghttp2</a>
            to provide the core http/2 engine.</p>
        
        <div class="warning"><h3>Warning</h3>
          <p>This module is experimental. Its behaviors, directives, and 
          defaults are subject to more change from release to 
          release relative to other standard modules. Users are encouraged to 
          consult the "CHANGES" file for potential updates.</p>
        </div>

        <p>You must enable HTTP/2 via <code class="directive"><a href="../mod/core.html#protocols">Protocols</a></code> in order to use the
        functionality described in this document:</p>

        <pre class="prettyprint lang-config">Protocols h2 http/1.1</pre>


    </div>
<div id="quickview"><h3 class="directives">Directives</h3>
<ul id="toc">
<li><img alt="" src="../images/down.gif" /> <a href="#h2direct">H2Direct</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2maxsessionstreams">H2MaxSessionStreams</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2maxworkeridleseconds">H2MaxWorkerIdleSeconds</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2maxworkers">H2MaxWorkers</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2minworkers">H2MinWorkers</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2moderntlsonly">H2ModernTLSOnly</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2push">H2Push</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2pushpriority">H2PushPriority</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2serializeheaders">H2SerializeHeaders</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2sessionextrafiles">H2SessionExtraFiles</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2streammaxmemsize">H2StreamMaxMemSize</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2tlscooldownsecs">H2TLSCoolDownSecs</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2tlswarmupsize">H2TLSWarmUpSize</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2upgrade">H2Upgrade</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#h2windowsize">H2WindowSize</a></li>
</ul>
<ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>

<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2Direct" id="H2Direct">H2Direct</a> <a name="h2direct" id="h2direct">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Direct Protocol Switch</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2Direct on|off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2Direct on for h2c, off for h2 protocol</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
</table>
            <p>
                This directive toggles the usage of the HTTP/2 Direct Mode. This
                should be used inside a 
                <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code> 
                section to enable direct HTTP/2 communication for that virtual host. 
            </p>
            <p>
                Direct communication means that if the first bytes received by the 
                server on a connection match the HTTP/2 preamble, the HTTP/2
                protocol is switched to immediately without further negotiation.
                This mode is defined in RFC 7540 for the cleartext (h2c) case. Its
                use on TLS connections not mandated by the standard.
            </p>
            <p>
                When a server/vhost does not have h2 or h2c enabled via
                <code class="directive"><a href="../mod/core.html#protocols">&lt;Protocols&gt;</a></code>,
                the connection is never inspected for a HTTP/2 preamble. H2Direct
                does not matter then. This is important for connections that
                use protocols where an initial read might hang indefinitely, such
                as NNTP.
            </p>
            <p>
                For clients that have out-of-band knowledge about a server
                supporting h2c, direct HTTP/2 saves the client from having to
                perform an HTTP/1.1 upgrade, resulting in better performance
                and avoiding the Upgrade restrictions on request bodies.
            </p>
            <p>
                This makes direct h2c attractive for server to server communication
                as well, when the connection can be trusted or is secured by other means.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2Direct on</pre>
</div>
        
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2MaxSessionStreams" id="H2MaxSessionStreams">H2MaxSessionStreams</a> <a name="h2maxsessionstreams" id="h2maxsessionstreams">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum number of active streams per HTTP/2 session.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2MaxSessionStreams <em>n</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2MaxSessionStreams 100</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
</table>
            <p>
                This directive sets the maximum number of active streams per HTTP/2 session (e.g. connection)
                that the server allows. A stream is active if it is not <code>idle</code> or 
                <code>closed</code> according to RFC 7540.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2MaxSessionStreams 20</pre>
</div>
        
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2MaxWorkerIdleSeconds" id="H2MaxWorkerIdleSeconds">H2MaxWorkerIdleSeconds</a> <a name="h2maxworkeridleseconds" id="h2maxworkeridleseconds">Directive</a></h2>
<table class="directive">
<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>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2MaxWorkerIdleSeconds <em>n</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2MaxWorkerIdleSeconds 600</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
</table>
            <p>
                This directive sets the maximum number of seconds a h2 worker may 
                idle until it shuts itself down. This only happens while the number of
                h2 workers exceeds <code>H2MinWorkers</code>.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2MaxWorkerIdleSeconds 20</pre>
</div>
        
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2MaxWorkers" id="H2MaxWorkers">H2MaxWorkers</a> <a name="h2maxworkers" id="h2maxworkers">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum number of worker threads to use per child process.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2MaxWorkers <em>n</em></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
</table>
            <p>
                This directive sets the maximum number of worker threads to spawn
                per child process for HTTP/2 processing. If this directive is not used,
                <code>mod_http2</code> will chose a value suitable for the <code>mpm</code>
                module loaded.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2MaxWorkers 20</pre>
</div>
        
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2MinWorkers" id="H2MinWorkers">H2MinWorkers</a> <a name="h2minworkers" id="h2minworkers">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Minimal number of worker threads to use per child process.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2MinWorkers <em>n</em></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
</table>
            <p>
                This directive sets the minimum number of worker threads to spawn
                per child process for HTTP/2 processing. If this directive is not used,
                <code>mod_http2</code> will chose a value suitable for the <code>mpm</code>
                module loaded.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2MinWorkers 10</pre>
</div>
        
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2ModernTLSOnly" id="H2ModernTLSOnly">H2ModernTLSOnly</a> <a name="h2moderntlsonly" id="h2moderntlsonly">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Require HTTP/2 connections to be "modern TLS" only</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2ModernTLSOnly on|off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2ModernTLSOnly on</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later.</td></tr>
</table>
            <p>
                This directive toggles the security checks on HTTP/2 connections
                in TLS mode (https:). This can be used server wide or for specific
                <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>s. 
            </p>
            <p>
                The security checks require that the TSL protocol is at least
                TLSv1.2 and that none of the ciphers listed in RFC 7540, Appendix A
                is used. These checks will be extended once new security requirements
                come into place.
            </p>
            <p>
                The name stems from the 
                <a href="https://wiki.mozilla.org/Security/Server_Side_TLS">Security/Server Side TLS</a>
                definitions at mozilla where "modern compatibility" is defined. Mozilla Firefox and
                other browsers require modern compatibility for HTTP/2 connections. As everything
                in OpSec, this is a moving target and can be expected to evolve in the future.
            </p>
            <p>
                One purpose of having these checks in mod_http2 is to enforce this
                security level for all connections, not only those from browsers. The other
                purpose is to prevent the negotiation of HTTP/2 as a protocol should
                the requirements not be met.
            </p>
            <p>
                Ultimately, the security of the TLS connection is determined by the
                server configuration directives for mod_ssl.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2ModernTLSOnly off</pre>
</div>
        
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2Push" id="H2Push">H2Push</a> <a name="h2push" id="h2push">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Server Push Switch</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2Push on|off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2Push on</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later.</td></tr>
</table>
            <p>
                This directive toggles the usage of the HTTP/2 server push 
                protocol feature. This should be used inside a 
                <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code> 
                section to enable direct HTTP/2 communication for that virtual host. 
            </p>
            <p>
                The HTTP/2 protocol allows the server to push other resources to
                a client when it asked for a particular one. This is helpful
                if those resources are connected in some way and the client can
                be expected to ask for it anyway. The pushing then saves the
                time it takes the client to ask for the resources itself. On the
                other hand, pushing resources the client never needs or already
                has is a waste of bandwidth.
            </p>
            <p>
                Server pushes are detected by inspecting the <code>Link</code> headers of
                responses (see https://tools.ietf.org/html/rfc5988 for the 
                specification). When a link thus specified has the <code>rel=preload</code>
                attribute, it is treated as a resource to be pushed.
            </p>
            <p> 
                Link headers in responses are either set by the application or
                can be configured via <code class="module"><a href="../mod/mod_headers.html">mod_headers</a></code> as:
            </p>
            <div class="example"><h3>mod_headers example</h3><pre class="prettyprint lang-config">&lt;Location /index.html&gt;
    Header add Link "&lt;/css/site.css&gt;;rel=preload"
    Header add Link "&lt;/images/logo.jpg&gt;;rel=preload"
&lt;/Location&gt;</pre>
</div>
            <p>
                As the example shows, there can be several link headers added
                to a response, resulting in several pushes being triggered. There
                are no checks in the module to avoid pushing the same resource
                twice or more to one client. Use with care.
            </p>
            <p> 
                HTTP/2 server pushes are enabled by default. This directive 
                allows it to be switch off on all resources of this server/virtual
                host.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2Push off</pre>
</div>
            <p>
                Last but not least, pushes happen only when the client signals
                its willingness to accept those. Most browsers do, some, like Safari 9,
                do not. Also, pushes also only happen for resources from the same
                <em>authority</em> as the original response is for.
            </p>
        
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2PushPriority" id="H2PushPriority">H2PushPriority</a> <a name="h2pushpriority" id="h2pushpriority">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Server Push Priority</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2PushPriority mime-type [after|before|interleaved] [weight]</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2PushPriority * After 16</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later. For having an
        effect, a nghttp2 library version 1.5.0 or newer is necessary.</td></tr>
</table>
            <p>
                This directive defines the priority handling of pushed responses
                based on the content-type of the response. This is usually defined
                per server config, but may also appear in a virtual host. 
            </p>
            <p>
                HTTP/2 server pushes are always related to a client request. Each
                such request/response pairs, or <em>streams</em> have a dependency
                and a weight, together defining the <em>priority</em> of a stream. 
            </p>
            <p>
                When a stream <em>depends</em> on another, say X depends on Y,
                then Y gets all bandwidth before X gets any. Note that this
                does not men that Y will block X. If Y has no data to send, 
                all bandwidth allocated to Y can be used by X.
            </p>
            <p>
                When a stream has more than one dependant, say X1 and X2 both
                depend on Y, the <em>weight</em> determines the bandwidth
                allocation. If X1 and X2 have the same weight, they both get
                half of the available bandwdith. If the weight of X1 is twice
                as large as that for X2, X1 gets twice the bandwidth of X2.
            </p>
            <p> 
                Ultimately, every stream depends on the <em>root</em> stream which
                gets all the bandwidht available, but never sends anything. So all
                its bandwidth is distributed by weight among its children. Which
                either have data to send or distribute the bandwidth to their
                own children. And so on. If none of the children have data
                to send, that bandwidth get distributed somewhere else according
                to the same rules.
            </p>
            <p> 
                The purpose of this priority system is to always make use of
                available bandwidth while allowing precedence and weight
                to be given to specific streams. Since, normally, all streams
                are initiated by the client, it is also the one that sets
                these priorities.
            </p>
            <p>
                Only when such a stream results in a PUSH, gets the server to
                decide what the <em>initial</em> priority of such a pushed
                stream is. In the examples below, X is the client stream. It
                depends on Y and the server decides to PUSH streams P1 and P2
                onto X.
            </p>
            <p>
                The default priority rule is:
            </p>
            <div class="example"><h3>Default Priority Rule</h3><pre class="prettyprint lang-config">H2PushPriority * After 16</pre>
</div>
            <p>
                which reads as 'Send a pushed stream of any content-type
                depending on the client stream with weight 16'. And so P1
                and P2 will be send after X and, as they have equal weight,
                share bandwidth equally among themselves.
            </p>
            <div class="example"><h3>Interleaved Priority Rule</h3><pre class="prettyprint lang-config">H2PushPriority text/css Interleaved 256</pre>
</div>
            <p>
                which reads as 'Send any CSS resource on the same dependency and
                weight as the client stream'. If P1 has content-type 'text/css',
                it will depend on Y (as does X) and its effective weight will be
                calculated as <code>P1ew = Xw * (P1w / 256)</code>. With P1w being 
                256, this will make the effective weight the same as the weight
                of X. If both X and P1 have data to send, bandwidth will be allocated
                to both equally.
            </p>
            <p>
                With Pw specified as 512, a pushed, interleaved stream would
                get double the weight of X. With 128 only half as much. Note that
                effective weights are always capped at 256.
            </p>
            <div class="example"><h3>Before Priority Rule</h3><pre class="prettyprint lang-config">H2PushPriority application/json Before</pre>
</div>
            <p>
                This says that any pushed stream of content type 'application/json'
                should be send out <em>before</em> X. This makes P1 dependent
                on Y and X dependent on P1. So, X will be stalled as long as
                P1 has data to send. The effective weight is inherited from the
                client stream. Specifying a weight is not allowed.
            </p>
            <p>
                Be aware that the effect of priority specifications is limited
                by the available server resources. If a server does not have
                workers available for pushed streams, the data for the stream
                may only ever arrive when other streams have been finished.
            </p>
            <p>
                Last, but not least, there are some specifics of the syntax
                to be used in this directive:
            </p>
            <ol>
                <li>'*' is the only special content-type that matches all others. 
                 'image/*' will not work.</li>
                <li>The default dependency is 'After'. </li>
                <li>There are also default weights: for 'After' it is 16, 'interleaved' is 256. 
                </li>
            </ol>
            <div class="example"><h3>Shorter Priority Rules</h3><pre class="prettyprint lang-config">H2PushPriority application/json 32         # an After rule
H2PushPriority image/jpeg before           # weight inherited
H2PushPriority text/css   interleaved      # weight 256 default</pre>
</div>
        
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2SerializeHeaders" id="H2SerializeHeaders">H2SerializeHeaders</a> <a name="h2serializeheaders" id="h2serializeheaders">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Serialize Request/Response Processing Switch</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2SerializeHeaders on|off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2SerializeHeaders off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
</table>
            <p>
                This directive toggles if HTTP/2 requests shall be serialized in
                HTTP/1.1 format for processing by <code>httpd</code> core or if
                received binary data shall be passed into the <code>request_rec</code>s
                directly.
            </p>
            <p>
                Serialization will lower performance, but gives more backward
                compatibility in case custom filters/hooks need it.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2SerializeHeaders on</pre>
</div>
        
</div>
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<div class="directive-section"><h2><a name="H2SessionExtraFiles" id="H2SessionExtraFiles">H2SessionExtraFiles</a> <a name="h2sessionextrafiles" id="h2sessionextrafiles">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Number of Extra File Handles</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2SessionExtraFiles <em>n</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2SessionExtraFiles 5</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
</table>
            <p>
                This directive sets maximum number of <em>extra</em> file handles
                a HTTP/2 session is allowed to use. A file handle is counted as
                <em>extra</em> when it is transferred from a h2 worker thread to
                the main HTTP/2 connection handling. This commonly happens when
                serving static files.
            </p><p>
                Depending on the processing model configured on the server, the
                number of connections times number of active streams may exceed
                the number of file handles for the process. On the other hand,
                converting every file into memory bytes early results in too 
                many buffer writes. This option helps to mitigate that.
            </p><p>
                The number of file handles used by a server process is then in
                the order of:
            </p>
            <pre>(h2_connections * extra_files) + (h2_max_worker)</pre>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2SessionExtraFiles 10</pre>
</div>
        
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2StreamMaxMemSize" id="H2StreamMaxMemSize">H2StreamMaxMemSize</a> <a name="h2streammaxmemsize" id="h2streammaxmemsize">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Maximum amount of output data buffered per stream.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2StreamMaxMemSize <em>bytes</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2StreamMaxMemSize 65536</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
</table>
            <p>
                This directive sets the maximum number of outgoing data bytes buffered in memory
                for an active streams. This memory is not allocated per stream as such. Allocations
                are counted against this limit when they are about to be done. Stream processing
                freezes when the limit has been reached and will only continue when buffered data
                has been sent out to the client.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2StreamMaxMemSize 128000</pre>
</div>
        
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2TLSCoolDownSecs" id="H2TLSCoolDownSecs">H2TLSCoolDownSecs</a> <a name="h2tlscooldownsecs" id="h2tlscooldownsecs">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td /></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2TLSCoolDownSecs seconds</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2TLSCoolDownSecs 1</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later.</td></tr>
</table>
            <p>
                This directive sets the number of seconds of idle time on a TLS
                connection before the TLS write size falls back to small (~1300 bytes)
                length.
                This can be used server wide or for specific
                <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>s. 
            </p>
            <p>
                See <code class="directive">&lt;H2TLSWarmUpSize&gt;</code> for a
                description of TLS warmup. H2TLSCoolDownSecs reflects the fact
                that connections may deteriorate over time (and TCP flow adjusts)
                for idle connections as well. It is beneficial to overall performance
                to fall back to the pre-warmup phase after a number of seconds that
                no data has been sent. 
            </p>
            <p>
                In deployments where connections can be considered reliable, this
                timer can be disabled by setting it to 0. 
            </p>
            <p>
                The following example sets the seconds to zero, effectively disabling
                any cool down. Warmed up TLS connections stay on maximum record
                size.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2TLSCoolDownSecs 0</pre>
</div>
        
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2TLSWarmUpSize" id="H2TLSWarmUpSize">H2TLSWarmUpSize</a> <a name="h2tlswarmupsize" id="h2tlswarmupsize">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td /></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2TLSWarmUpSize amount</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2TLSWarmUpSize 1048576</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.4.18 and later.</td></tr>
</table>
            <p>
                This directive sets the number of bytes to be sent in small
                TLS records (~1300 bytes) until doing maximum sized writes (16k)
                on https: HTTP/2 connections.
                This can be used server wide or for specific
                <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>s. 
            </p>
            <p>
                Measurements by <a href="https://www.igvita.com">google performance
                labs</a> show that best performance on TLS connections is reached,
                if initial record sizes stay below the MTU level, to allow a
                complete record to fit into an IP packet.
            </p>
            <p>
                While TCP adjust its flow-control and window sizes, longer TLS
                records can get stuck in queues or get lost and need retransmission.
                This is of course true for all packets. TLS however needs the 
                whole record in order to decrypt it. Any missing bytes at the end
                will stall usage of the received ones.
            </p>
            <p>
                After a sufficient number of bytes have been send successfully,
                the TCP state of the connection is stable and maximum TLS record
                sizes (16 KB) can be used for optimal performance.
            </p>
            <p>
                In deployments where servers are reached locally or over reliable
                connections only, the value might be decreased with 0 disabling
                any warmup phase altogether.
            </p>
            <p>
                The following example sets the size to zero, effectively disabling
                any warmup phase.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2TLSWarmUpSize 0</pre>
</div>
        
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="H2Upgrade" id="H2Upgrade">H2Upgrade</a> <a name="h2upgrade" id="h2upgrade">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>H2 Upgrade Protocol Switch</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2Upgrade on|off</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2Upgrade on for h2c, off for h2 protocol</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
</table>
            <p>
                This directive toggles the usage of the HTTP/1.1 Upgrade method 
                for switching to HTTP/2. This
                should be used inside a 
                <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code> 
                section to enable Upgrades to HTTP/2 for that virtual host. 
            </p>
            <p>
                This method of switching protocols is defined in HTTP/1.1 and
                uses the "Upgrade" header (thus the name) to announce willingness
                to use another protocol. This may happen on any request of a
                HTTP/1.1 connection.
            </p>
            <p>
                This method of protocol switching is enabled by default on cleartext
                (potential h2c) connections and disabled on TLS (potential h2), 
                as mandated by RFC 7540. 
            </p>
            <p>
                Please be aware that Upgrades are only accepted for requests
                that carry no body. POSTs and PUTs with content will never
                trigger an upgrade to HTTP/2. 
                See <code class="directive">&lt;H2Direct&gt;</code> for an 
                alternative to Upgrade.
            </p>
            <p>
                This mode only has an effect when h2 or h2c is enabled via
                the <code class="directive"><a href="../mod/core.html#protocols">&lt;Protocols&gt;</a></code>.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2Upgrade on</pre>
</div>
        
</div>
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<div class="directive-section"><h2><a name="H2WindowSize" id="H2WindowSize">H2WindowSize</a> <a name="h2windowsize" id="h2windowsize">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Size of Stream Window for upstream data.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>H2WindowSize <em>bytes</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>H2WindowSize 65536</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_http2</td></tr>
</table>
            <p>
                This directive sets the size of the window that is used for flow control
                from client to server and limits the amount of data the server has to buffer.
                The client will stop sending on a stream once the limit has been reached until
                the server announces more available space (as it has processed some of the data).
            </p><p>
                This limit affects only request bodies, not its meta data such as headers. Also,
                it has no effect on response bodies as the window size for those are managed
                by the clients.
            </p>
            <div class="example"><h3>Example</h3><pre class="prettyprint lang-config">H2WindowSize 128000</pre>
</div>
        
</div>
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