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diff --git a/README_FILES/CONNECTION_CACHE_README b/README_FILES/CONNECTION_CACHE_README new file mode 100644 index 0000000..bd2fd03 --- /dev/null +++ b/README_FILES/CONNECTION_CACHE_README @@ -0,0 +1,206 @@ +PPoossttffiixx CCoonnnneeccttiioonn CCaacchhee + +------------------------------------------------------------------------------- + +IInnttrroodduuccttiioonn + +This document describes the Postfix connection cache implementation, which is +available with Postfix version 2.2 and later. + +See Client-side TLS connection reuse for how this connection cache is used to +implement multiple deliveries per TLS-encrypted connection. + +Topics covered in this document: + + * What SMTP connection caching can do for you + * Connection cache implementation + * Connection cache configuration + * Connection cache safety mechanisms + * Connection cache limitations + * Connection cache statistics + +WWhhaatt SSMMTTPP ccoonnnneeccttiioonn ccaacchhiinngg ccaann ddoo ffoorr yyoouu + +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. + +SMTP Connection caching is a performance feature. Whether or not it actually +improves performance depends on the conditions: + + * 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. + + * 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). + + * 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. + + * 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. + +For other potential issues with SMTP connection caching, see the discussion of +limitations at the end of this document. + +CCoonnnneeccttiioonn ccaacchhee iimmpplleemmeennttaattiioonn + +For an overview of how Postfix delivers mail, see the Postfix architecture +OVERVIEW document. + +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. + +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. + + /-- smtp(8) --> Internet + + qmgr(8) + | + \-- | smtp(8) --> Internet + | + ^ + | + + scache(8) + +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. + +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. + +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. + +CCoonnnneeccttiioonn ccaacchhee ccoonnffiigguurraattiioonn + +The Postfix smtp(8) client supports two connection caching strategies: + + * 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. + + Example: + + /etc/postfix/main.cf: + smtp_connection_cache_on_demand = yes + + * 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 always saves the connection to the + connection cache. + + Specify a comma or white space separated list of destinations or pseudo- + destinations: + + o 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), + + o 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, + + o a /file/name with domain names and/or relay host names as defined + above, + + o 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. + + Examples: + + /etc/postfix/main.cf: + smtp_connection_cache_destinations = $relayhost + smtp_connection_cache_destinations = hotmail.com, ... + smtp_connection_cache_destinations = static:all (not recommended) + +CCoonnnneeccttiioonn ccaacchhee ssaaffeettyy mmeecchhaanniissmmss + +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: + + * 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. + + * 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. + + 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). + + Postfix 2.3 logs the use count of multiply-used connections, as shown in + the following example: + + Nov 3 16:04:31 myname postfix/smtp[30840]: 19B6B2900FE: + to=<wietse@test.example.com>, orig_to=<wietse@test>, + relay=mail.example.com[1.2.3.4], ccoonnnn__uussee==22, delay=0.22, + delays=0.04/0.01/0.05/0.1, dsn=2.0.0, status=sent (250 2.0.0 Ok) + + * 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. + +CCoonnnneeccttiioonn ccaacchhee lliimmiittaattiioonnss + +Postfix SMTP connection caching conflicts with certain applications: + + * With Postfix versions < 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. + + * 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. + +CCoonnnneeccttiioonn ccaacchhee ssttaattiissttiiccss + +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. + + * Hit rates for connection cache lookups by domain will tell you how useful + connection caching is. + + * Connection cache lookups by network address will always fail, unless you're + sending mail to different domains that share the same MX hosts. + + * No statistics are logged when no attempts are made to access the connection + cache. + |