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+<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN"
+ "http://www.w3.org/TR/html4/loose.dtd">
+
+<html>
+
+<head>
+
+<title>Postfix Connection Cache </title>
+
+<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
+
+</head>
+
+<body>
+
+<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix Connection Cache </h1>
+
+<hr>
+
+<h2>Introduction</h2>
+
+<p> This document describes the Postfix connection cache implementation,
+which is available with Postfix version 2.2 and later. </p>
+
+<p> See <a href="TLS_README.html#client_tls_reuse">Client-side TLS
+connection reuse</a> for how this connection cache is used to
+implement multiple deliveries per TLS-encrypted connection. </p>
+
+<p> Topics covered in this document: </p>
+
+<ul>
+
+<li><a href="#summary"> What SMTP connection caching can do for you</a>
+
+<li><a href="#implementation"> Connection cache implementation</a>
+
+<li><a href="#configuration"> Connection cache configuration</a>
+
+<li><a href="#safety">Connection cache safety mechanisms </a>
+
+<li><a href="#limitations">Connection cache limitations</a>
+
+<li><a href="#statistics">Connection cache statistics</a>
+
+</ul>
+
+<h2><a name="summary">What SMTP connection caching can do for
+you</a></h2>
+
+<p> With SMTP connection caching, Postfix can deliver multiple
+messages over the same SMTP connection. By default, Postfix 2.2
+reuses an SMTP connection automatically when a destination has
+high volume of mail in the active queue. </p>
+
+<p> SMTP Connection caching is a performance feature. Whether or not
+it actually improves performance depends on the conditions: </p>
+
+<ul>
+
+<li> <p> SMTP Connection caching can greatly improve performance
+when delivering mail to a destination with multiple mail servers,
+because it can help Postfix to skip over a non-responding server.
+</p>
+
+<li> <p> Otherwise, the benefits of SMTP connection caching are
+minor: it eliminates the latency of the TCP handshake (SYN, SYN+ACK,
+ACK), plus the latency of the SMTP initial handshake (220 greeting,
+EHLO command, EHLO response).
+
+<li> <p> SMTP Connection caching gives no gains with respect to
+SMTP session tear-down. The Postfix smtp(8) client normally does
+not wait for the server's reply to the QUIT command, and it never
+waits for the TCP final handshake to complete. </p>
+
+<li> <p> SMTP Connection caching introduces some overhead: the
+client needs to send an RSET command to find out if a connection
+is still usable, before it can send the next MAIL FROM command.
+This introduces one additional round-trip delay. </p>
+
+</ul>
+
+<p> For other potential issues with SMTP connection caching, see
+the discussion of <a href="#limitations">limitations</a> at the end
+of this document. </p>
+
+<h2><a name="implementation">Connection cache implementation</a></h2>
+
+<p> For an overview of how Postfix delivers mail, see the Postfix
+architecture OVERVIEW document. </p>
+
+<p> The Postfix connection cache is shared among Postfix mail
+delivering processes. This maximizes the opportunity to reuse an
+open connection. Other MTAs such as Sendmail or exim have a
+non-shared connection cache. Here, a connection can be reused only
+by the mail delivering process that creates the connection. To get
+the same performance improvement as with a shared connection cache,
+non-shared connections need to be kept open for a longer time. </p>
+
+<p> The scache(8) server, introduced with Postfix version 2.2,
+maintains the shared connection cache. With Postfix version 2.2,
+only the smtp(8) client has support to access this cache. </p>
+
+<blockquote>
+
+<table>
+
+<tr> <td> </td> <td> <tt> /-- </tt> </td> <td align="center"
+colspan="3" bgcolor="#f0f0ff"> smtp(8) </td> <td colspan="2"> <tt>
+--&gt; </tt> Internet </td> </tr>
+
+<tr> <td align="center" bgcolor="#f0f0ff"> qmgr(8) </td> <td> </td>
+<td align="center" rowspan="3"> &nbsp; </td> <td align="center"
+rowspan="3"><tt>|<br>|<br>|<br>|<br>v</tt></td> <td> &nbsp; </td>
+</tr>
+
+<tr> <td> </td> <td> <tt> \-- </tt> </td> <td align="center"
+colspan="2" bgcolor="#f0f0ff"> smtp(8) </td> <td align="left"> <tt>
+--&gt; </tt> Internet </td> </tr>
+
+<tr> <td colspan="3"> </td> <td align="center"><tt>^<br>|</tt></td>
+<td> &nbsp; </td> </tr>
+
+<tr> <td colspan="3"> </td> <td align="center" colspan="3"
+bgcolor="#f0f0ff"> scache(8) </td> </tr>
+
+</table>
+
+</blockquote>
+
+<p> When SMTP connection caching is enabled (see next section), the
+smtp(8) client does not disconnect after a mail transaction, but
+gives the connection to the scache(8) server which keeps the
+connection open for a limited amount of time. </p>
+
+<p> After handing over the open connection to the scache(8) server,
+the smtp(8) client continues with some other mail delivery request.
+Meanwhile, any smtp(8) client process can ask the scache(8) server
+for that cached connection and reuse it for mail delivery. </p>
+
+<p> The connection cache can be searched by destination domain name
+(the right-hand side of the recipient address) and by the IP address
+of the host at the other end of the connection. This allows Postfix
+to reuse a connection even when the remote host is mail server for
+domains with different names. </p>
+
+<h2><a name="configuration">Connection cache configuration </a></h2>
+
+<p> The Postfix smtp(8) client supports two connection caching
+strategies: </p>
+
+<ul>
+
+<li> <p> On-demand connection caching. This is enabled by default,
+and is controlled with the smtp_connection_cache_on_demand configuration
+parameter. When this feature is enabled, the Postfix smtp(8) client
+automatically saves a connection to the connection cache when a
+destination has a high volume of mail in the active queue. </p>
+
+<p> Example: </p>
+
+<blockquote>
+
+<pre>
+/etc/postfix/main.cf:
+ smtp_connection_cache_on_demand = yes
+</pre>
+
+</blockquote>
+
+<li> <p> Per-destination connection caching. This is enabled by
+explicitly listing specific destinations with the
+smtp_connection_cache_destinations configuration parameter. After
+completing delivery to a selected destination, the Postfix smtp(8)
+client <i>always</i> saves the connection to the connection cache.
+</p>
+
+<p> Specify a comma or white space separated list of destinations
+or pseudo-destinations: </p>
+
+<ul>
+
+<li> <p> if mail is sent without a relay host: a domain name (the
+right-hand side of an email address, without the [] around a numeric
+IP address), </p>
+
+<li> <p> if mail is sent via a relay host: a relay host name (without
+the [] or non-default TCP port), as specified in main.cf or in the
+transport map, </p>
+
+<li> <p> a /file/name with domain names and/or relay host names as
+defined above, </p>
+
+<li> <p> a "type:table" with domain names and/or relay host names
+on the left-hand side. The right-hand side result from "type:table"
+lookups is ignored. </p>
+
+</ul>
+
+<p> Examples: </p>
+
+<blockquote>
+
+<pre>
+/etc/postfix/main.cf:
+ smtp_connection_cache_destinations = $relayhost
+ smtp_connection_cache_destinations = hotmail.com, ...
+ smtp_connection_cache_destinations = static:all (<i>not recommended</i>)
+</pre>
+
+</blockquote>
+
+</ul>
+
+<h2><a name="safety">Connection cache safety mechanisms </a></h2>
+
+<p> Connection caching must be used wisely. It is anti-social to
+keep an unused SMTP connection open for a significant amount of
+time, and it is unwise to send huge numbers of messages through
+the same connection. In order to avoid problems with SMTP connection
+caching, Postfix implements the following safety mechanisms: </p>
+
+<ul>
+
+<li> <p> The Postfix scache(8) server keeps a connection open for
+only a limited time. The time limit is specified with the
+smtp_connection_cache_time_limit and with the connection_cache_ttl_limit
+configuration parameters. This prevents anti-social behavior. </p>
+
+<li> <p> The Postfix smtp(8) client reuses a session for only a
+limited number of times. This avoids triggering bugs in implementations
+that do not correctly handle multiple deliveries per session. </p>
+
+<p> As of Postfix 2.3 connection reuse is preferably limited with
+the smtp_connection_reuse_time_limit parameter. In addition, Postfix
+2.11 provides smtp_connection_reuse_count_limit to limit how many
+times a connection may be reused, but this feature is unsafe as it
+introduces a "fatal attractor" failure mode (when a destination has
+multiple inbound MTAs, the slowest inbound MTA will attract most
+connections from Postfix to that destination). </p>
+
+<p> Postfix 2.3 logs the use count of multiply-used connections,
+as shown in the following example: </p>
+
+<blockquote>
+<pre>
+Nov 3 16:04:31 myname postfix/smtp[30840]: 19B6B2900FE:
+to=&lt;wietse@test.example.com&gt;, orig_to=&lt;wietse@test&gt;,
+relay=mail.example.com[1.2.3.4], <b>conn_use=2</b>, delay=0.22,
+delays=0.04/0.01/0.05/0.1, dsn=2.0.0, status=sent (250 2.0.0 Ok)
+</pre>
+</blockquote>
+
+<li> <p> The connection cache explicitly labels each cached connection
+with destination domain and IP address information. A connection
+cache lookup succeeds only when the correct information is specified.
+This prevents mis-delivery of mail. </p>
+
+</ul>
+
+<h2><a name="limitations">Connection cache limitations</a></h2>
+
+<p> Postfix SMTP connection caching conflicts with certain applications:
+</p>
+
+<ul>
+
+<li> <p> With Postfix versions &lt; 3.4, the Postfix shared connection
+cache cannot be used with TLS, because an open TLS connection can
+be reused only in the process that creates it. For this reason,
+the Postfix smtp(8) client historically always closed the connection
+after completing an attempt to deliver mail over TLS.</p>
+
+<li> <p> Postfix connection caching currently does not support
+multiple SASL accounts per mail server. Specifically, Postfix
+connection caching assumes that a SASL credential is valid for all
+hostnames or domain names that deliver via the same mail server IP
+address and TCP port, and assumes that the SASL credential does not
+depend on the message originator. </p>
+
+</ul>
+
+
+<h2><a name="statistics">Connection cache statistics </a></h2>
+
+<p> The scache(8) connection cache server logs statistics about the
+peak cache size and the cache hit rates. This information is logged
+every connection_cache_status_update_time seconds, when the process
+terminates after the maximal idle time is exceeded, or when Postfix
+is reloaded. </p>
+
+<ul>
+
+<li> <p> Hit rates for connection cache lookups by domain will tell
+you how useful connection caching is. </p>
+
+<li> <p> Connection cache lookups by network address will always
+fail, unless you're sending mail to different domains that share
+the same MX hosts. </p>
+
+<li> <p> No statistics are logged when no attempts are made to
+access the connection cache. </p>
+
+</ul>
+
+
+</body>
+
+</html>