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| author |   hongbotian <hongbo.tianhongbo@huawei.com> | 2015-11-30 01:35:09 -0500 |
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| committer | hongbotian <hongbo.tianhongbo@huawei.com> | 2015-11-30 01:36:35 -0500 |
| commit | cc40af334e619bb549038238507407866f774f8f (patch) | |
| tree | 43ddc1974f72997a57173151eafb23e6025a13c9 /rubbos/app/apache2/manual/misc/fin_wait_2.html.en | |
| parent | 68e74fd78b2485e5914ce34a5b30f4485029e021 (diff) | |
upload apache
JIRA: BOTTLENECK-10
Change-Id: I67eae31de6dc824097dfa56ab454ba36fdd23a2c
Signed-off-by: hongbotian <hongbo.tianhongbo@huawei.com>
Diffstat (limited to 'rubbos/app/apache2/manual/misc/fin_wait_2.html.en')
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diff --git a/rubbos/app/apache2/manual/misc/fin_wait_2.html.en b/rubbos/app/apache2/manual/misc/fin_wait_2.html.en new file mode 100644 index 00000000..60da4b0d --- /dev/null +++ b/rubbos/app/apache2/manual/misc/fin_wait_2.html.en @@ -0,0 +1,422 @@ +<?xml version="1.0" encoding="ISO-8859-1"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!-- + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + This file is generated from xml source: DO NOT EDIT + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX + --> +<title>Connections in the FIN_WAIT_2 state and Apache - Apache HTTP Server</title> +<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /> +<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /> +<link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /> +<link href="../images/favicon.ico" rel="shortcut icon" /></head> +<body id="manual-page"><div id="page-header"> +<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p> +<p class="apache">Apache HTTP Server Version 2.0</p> +<img alt="" src="../images/feather.gif" /></div> +<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div> +<div id="path"> +<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.0</a> > <a href="./">Miscellaneous Documentation</a></div><div id="page-content"><div id="preamble"><h1>Connections in the FIN_WAIT_2 state and Apache</h1> +<div class="toplang"> +<p><span>Available Languages: </span><a href="../en/misc/fin_wait_2.html" title="English"> en </a></p> +</div> + + + <div class="warning"><h3>Warning:</h3> + <p>This document has not been fully updated + to take into account changes made in the 2.0 version of the + Apache HTTP Server. Some of the information may still be + relevant, but please use it with care.</p> + </div> + + <p>Starting with the Apache 1.2 betas, people are reporting + many more connections in the FIN_WAIT_2 state (as reported + by <code>netstat</code>) than they saw using older + versions. When the server closes a TCP connection, it sends + a packet with the FIN bit set to the client, which then + responds with a packet with the ACK bit set. The client + then sends a packet with the FIN bit set to the server, + which responds with an ACK and the connection is closed. + The state that the connection is in during the period + between when the server gets the ACK from the client and + the server gets the FIN from the client is known as + FIN_WAIT_2. See the <a href="ftp://ds.internic.net/rfc/rfc793.txt">TCP RFC</a> for + the technical details of the state transitions.</p> + + <p>The FIN_WAIT_2 state is somewhat unusual in that there + is no timeout defined in the standard for it. This means + that on many operating systems, a connection in the + FIN_WAIT_2 state will stay around until the system is + rebooted. If the system does not have a timeout and too + many FIN_WAIT_2 connections build up, it can fill up the + space allocated for storing information about the + connections and crash the kernel. The connections in + FIN_WAIT_2 do not tie up an httpd process.</p> + + </div> +<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#why">Why Does It Happen?</a></li> +<li><img alt="" src="../images/down.gif" /> <a href="#what">What Can I Do About it?</a></li> +<li><img alt="" src="../images/down.gif" /> <a href="#appendix">Appendix</a></li> +</ul></div> +<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> +<div class="section"> +<h2><a name="why" id="why">Why Does It Happen?</a></h2> + + <p>There are numerous reasons for it happening, some of them + may not yet be fully clear. What is known follows.</p> + + <h3><a name="buggy" id="buggy">Buggy Clients and Persistent + Connections</a></h3> + + <p>Several clients have a bug which pops up when dealing with + persistent connections (aka + keepalives). When the connection is idle and the server + closes the connection (based on the <code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code>), + the client is programmed so that the client does not send + back a FIN and ACK to the server. This means that the + connection stays in the FIN_WAIT_2 state until one of the + following happens:</p> + + <ul> + <li>The client opens a new connection to the same or a + different site, which causes it to fully close the older + connection on that socket.</li> + + <li>The user exits the client, which on some (most?) + clients causes the OS to fully shutdown the + connection.</li> + + <li>The FIN_WAIT_2 times out, on servers that have a + timeout for this state.</li> + </ul> + + <p>If you are lucky, this means that the buggy client will + fully close the connection and release the resources on + your server. However, there are some cases where the socket + is never fully closed, such as a dialup client + disconnecting from their provider before closing the + client. In addition, a client might sit idle for days + without making another connection, and thus may hold its + end of the socket open for days even though it has no + further use for it. <strong>This is a bug in the browser or + in its operating system's TCP implementation.</strong></p> + + <p>The clients on which this problem has been verified to + exist:</p> + + <ul> + <li>Mozilla/3.01 (X11; I; FreeBSD 2.1.5-RELEASE + i386)</li> + + <li>Mozilla/2.02 (X11; I; FreeBSD 2.1.5-RELEASE + i386)</li> + + <li>Mozilla/3.01Gold (X11; I; SunOS 5.5 sun4m)</li> + + <li>MSIE 3.01 on the Macintosh</li> + + <li>MSIE 3.01 on Windows 95</li> + </ul> + + <p>This does not appear to be a problem on:</p> + + <ul> + <li>Mozilla/3.01 (Win95; I)</li> + </ul> + + <p>It is expected that many other clients have the same + problem. What a client <strong>should do</strong> is + periodically check its open socket(s) to see if they have + been closed by the server, and close their side of the + connection if the server has closed. This check need only + occur once every few seconds, and may even be detected by a + OS signal on some systems (<em>e.g.</em>, Win95 and NT + clients have this capability, but they seem to be ignoring + it).</p> + + <p>Apache <strong>cannot</strong> avoid these FIN_WAIT_2 + states unless it disables persistent connections for the + buggy clients, just like we recommend doing for Navigator + 2.x clients due to other bugs. However, non-persistent + connections increase the total number of connections needed + per client and slow retrieval of an image-laden web page. + Since non-persistent connections have their own resource + consumptions and a short waiting period after each closure, + a busy server may need persistence in order to best serve + its clients.</p> + + <p>As far as we know, the client-caused FIN_WAIT_2 problem + is present for all servers that support persistent + connections, including Apache 1.1.x and 1.2.</p> + + + + <h3><a name="code" id="code">A necessary bit of code + introduced in 1.2</a></h3> + + <p>While the above bug is a problem, it is not the whole + problem. Some users have observed no FIN_WAIT_2 problems + with Apache 1.1.x, but with 1.2b enough connections build + up in the FIN_WAIT_2 state to crash their server. The most + likely source for additional FIN_WAIT_2 states is a + function called <code>lingering_close()</code> which was + added between 1.1 and 1.2. This function is necessary for + the proper handling of persistent connections and any + request which includes content in the message body + (<em>e.g.</em>, PUTs and POSTs). What it does is read any + data sent by the client for a certain time after the server + closes the connection. The exact reasons for doing this are + somewhat complicated, but involve what happens if the + client is making a request at the same time the server + sends a response and closes the connection. Without + lingering, the client might be forced to reset its TCP + input buffer before it has a chance to read the server's + response, and thus understand why the connection has + closed. See the <a href="#appendix">appendix</a> for more + details.</p> + + <p>The code in <code>lingering_close()</code> appears to + cause problems for a number of factors, including the + change in traffic patterns that it causes. The code has + been thoroughly reviewed and we are not aware of any bugs + in it. It is possible that there is some problem in the BSD + TCP stack, aside from the lack of a timeout for the + FIN_WAIT_2 state, exposed by the + <code>lingering_close</code> code that causes the observed + problems.</p> + + + </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> +<div class="section"> +<h2><a name="what" id="what">What Can I Do About it?</a></h2> + + <p>There are several possible workarounds to the problem, some + of which work better than others.</p> + + <h3><a name="add_timeout" id="add_timeout">Add a timeout for FIN_WAIT_2</a></h3> + + <p>The obvious workaround is to simply have a timeout for the + FIN_WAIT_2 state. This is not specified by the RFC, and + could be claimed to be a violation of the RFC, but it is + widely recognized as being necessary. The following systems + are known to have a timeout:</p> + + <ul> + <li><a href="http://www.freebsd.org/">FreeBSD</a> + versions starting at 2.0 or possibly earlier.</li> + + <li><a href="http://www.netbsd.org/">NetBSD</a> version + 1.2(?)</li> + + <li><a href="http://www.openbsd.org/">OpenBSD</a> all + versions(?)</li> + + <li><a href="http://www.bsdi.com/">BSD/OS</a> 2.1, with + the <a href="ftp://ftp.bsdi.com/bsdi/patches/patches-2.1/K210-027"> + K210-027</a> patch installed.</li> + + <li><a href="http://www.sun.com/">Solaris</a> as of + around version 2.2. The timeout can be tuned by using + <code>ndd</code> to modify + <code>tcp_fin_wait_2_flush_interval</code>, but the + default should be appropriate for most servers and + improper tuning can have negative impacts.</li> + + <li><a href="http://www.linux.org/">Linux</a> 2.0.x and + earlier(?)</li> + + <li><a href="http://www.hp.com/">HP-UX</a> 10.x defaults + to terminating connections in the FIN_WAIT_2 state after + the normal keepalive timeouts. This does not refer to the + persistent connection or HTTP keepalive timeouts, but the + <code>SO_LINGER</code> socket option which is enabled by + Apache. This parameter can be adjusted by using + <code>nettune</code> to modify parameters such as + <code>tcp_keepstart</code> and <code>tcp_keepstop</code>. + In later revisions, there is an explicit timer for + connections in FIN_WAIT_2 that can be modified; contact + HP support for details.</li> + + <li><a href="http://www.sgi.com/">SGI IRIX</a> can be + patched to support a timeout. For IRIX 5.3, 6.2, and 6.3, + use patches 1654, 1703 and 1778 respectively. If you have + trouble locating these patches, please contact your SGI + support channel for help.</li> + + <li><a href="http://www.ncr.com/">NCR's MP RAS Unix</a> + 2.xx and 3.xx both have FIN_WAIT_2 timeouts. In 2.xx it + is non-tunable at 600 seconds, while in 3.xx it defaults + to 600 seconds and is calculated based on the tunable + "max keep alive probes" (default of 8) multiplied by the + "keep alive interval" (default 75 seconds).</li> + + <li><a href="http://www.sequent.com">Sequent's ptx/TCP/IP + for DYNIX/ptx</a> has had a FIN_WAIT_2 timeout since + around release 4.1 in mid-1994.</li> + </ul> + + <p>The following systems are known to not have a + timeout:</p> + + <ul> + <li><a href="http://www.sun.com/">SunOS 4.x</a> does not + and almost certainly never will have one because it as at + the very end of its development cycle for Sun. If you + have kernel source should be easy to patch.</li> + </ul> + + <p>There is a <a href="http://www.apache.org/dist/httpd/contrib/patches/1.2/fin_wait_2.patch"> + patch available</a> for adding a timeout to the FIN_WAIT_2 + state; it was originally intended for BSD/OS, but should be + adaptable to most systems using BSD networking code. You + need kernel source code to be able to use it.</p> + + + + <h3><a name="no_lingering" id="no_lingering">Compile without using + <code>lingering_close()</code></a></h3> + + <p>It is possible to compile Apache 1.2 without using the + <code>lingering_close()</code> function. This will result + in that section of code being similar to that which was in + 1.1. If you do this, be aware that it can cause problems + with PUTs, POSTs and persistent connections, especially if + the client uses pipelining. That said, it is no worse than + on 1.1, and we understand that keeping your server running + is quite important.</p> + + <p>To compile without the <code>lingering_close()</code> + function, add <code>-DNO_LINGCLOSE</code> to the end of the + <code>EXTRA_CFLAGS</code> line in your + <code>Configuration</code> file, rerun + <code class="program"><a href="../programs/Configure.html">Configure</a></code> and rebuild the server.</p> + + + + <h3><a name="so_linger" id="so_linger">Use <code>SO_LINGER</code> as + an alternative to <code>lingering_close()</code></a></h3> + + <p>On most systems, there is an option called + <code>SO_LINGER</code> that can be set with + <code>setsockopt(2)</code>. It does something very similar + to <code>lingering_close()</code>, except that it is broken + on many systems so that it causes far more problems than + <code>lingering_close</code>. On some systems, it could + possibly work better so it may be worth a try if you have + no other alternatives.</p> + + <p>To try it, add <code>-DUSE_SO_LINGER + -DNO_LINGCLOSE</code> to the end of the + <code>EXTRA_CFLAGS</code> line in your + <code>Configuration</code> file, rerun + <code class="program"><a href="../programs/Configure.html">Configure</a></code> and rebuild the server.</p> + + <div class="note"><h3>NOTE</h3>Attempting to use + <code>SO_LINGER</code> and <code>lingering_close()</code> + at the same time is very likely to do very bad things, so + don't.</div> + + + + <h3><a name="increase_mem" id="increase_mem">Increase the amount of memory + used for storing connection state</a></h3> + + <dl> + <dt>BSD based networking code:</dt> + + <dd> + BSD stores network data, such as connection states, in + something called an mbuf. When you get so many + connections that the kernel does not have enough mbufs + to put them all in, your kernel will likely crash. You + can reduce the effects of the problem by increasing the + number of mbufs that are available; this will not + prevent the problem, it will just make the server go + longer before crashing. + + <p>The exact way to increase them may depend on your + OS; look for some reference to the number of "mbufs" or + "mbuf clusters". On many systems, this can be done by + adding the line <code>NMBCLUSTERS="n"</code>, where + <code>n</code> is the number of mbuf clusters you want + to your kernel config file and rebuilding your + kernel.</p> + </dd> + </dl> + + + + <h3><a name="disable" id="disable">Disable KeepAlive</a></h3> + + <p>If you are unable to do any of the above then you + should, as a last resort, disable KeepAlive. Edit your + httpd.conf and change "KeepAlive On" to "KeepAlive + Off".</p> + + + </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> +<div class="section"> +<h2><a name="appendix" id="appendix">Appendix</a></h2> + + <p>Below is a message from Roy Fielding, one of the authors + of HTTP/1.1.</p> + + <h3><a name="message" id="message">Why the lingering close + functionality is necessary with HTTP</a></h3> + + <p>The need for a server to linger on a socket after a close + is noted a couple times in the HTTP specs, but not + explained. This explanation is based on discussions between + myself, Henrik Frystyk, Robert S. Thau, Dave Raggett, and + John C. Mallery in the hallways of MIT while I was at W3C.</p> + + <p>If a server closes the input side of the connection + while the client is sending data (or is planning to send + data), then the server's TCP stack will signal an RST + (reset) back to the client. Upon receipt of the RST, the + client will flush its own incoming TCP buffer back to the + un-ACKed packet indicated by the RST packet argument. If + the server has sent a message, usually an error response, + to the client just before the close, and the client + receives the RST packet before its application code has + read the error message from its incoming TCP buffer and + before the server has received the ACK sent by the client + upon receipt of that buffer, then the RST will flush the + error message before the client application has a chance to + see it. The result is that the client is left thinking that + the connection failed for no apparent reason.</p> + + <p>There are two conditions under which this is likely to + occur:</p> + + <ol> + <li>sending POST or PUT data without proper + authorization</li> + + <li>sending multiple requests before each response + (pipelining) and one of the middle requests resulting in + an error or other break-the-connection result.</li> + </ol> + + <p>The solution in all cases is to send the response, close + only the write half of the connection (what shutdown is + supposed to do), and continue reading on the socket until + it is either closed by the client (signifying it has + finally read the response) or a timeout occurs. That is + what the kernel is supposed to do if SO_LINGER is set. + Unfortunately, SO_LINGER has no effect on some systems; on + some other systems, it does not have its own timeout and + thus the TCP memory segments just pile-up until the next + reboot (planned or not).</p> + + <p>Please note that simply removing the linger code will + not solve the problem -- it only moves it to a different + and much harder one to detect.</p> + + </div></div> +<div class="bottomlang"> +<p><span>Available Languages: </span><a href="../en/misc/fin_wait_2.html" title="English"> en </a></p> +</div><div id="footer"> +<p class="apache">Copyright 2009 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p> +<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div> +</body></html>
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