From b7c15c31519dc44c1f691e0466badd556ffe9423 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 7 Apr 2024 18:18:56 +0200 Subject: Adding upstream version 3.7.10. Signed-off-by: Daniel Baumann --- proto/CONNECTION_CACHE_README.html | 350 +++++++++++++++++++++++++++++++++++++ 1 file changed, 350 insertions(+) create mode 100644 proto/CONNECTION_CACHE_README.html (limited to 'proto/CONNECTION_CACHE_README.html') diff --git a/proto/CONNECTION_CACHE_README.html b/proto/CONNECTION_CACHE_README.html new file mode 100644 index 0000000..bc2e34e --- /dev/null +++ b/proto/CONNECTION_CACHE_README.html @@ -0,0 +1,350 @@ + + + + + + +Postfix Connection Cache + + + + + + + +

Postfix Connection Cache

+ +
+ +

Introduction

+ +

This document describes the Postfix connection cache implementation, +which is available with Postfix version 2.2 and later.

+ +

Topics covered in this document:

+ + + +

What SMTP connection caching can do for +you

+ +

With SMTP connection caching, Postfix can deliver multiple +messages over the same SMTP connection. By default, Postfix 2.2 +reuses a plaintext 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:

+ + + +

For other potential issues with SMTP connection caching, see +the discussion of limitations at the end +of this document.

+ +

Connection cache implementation

+ +

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. Some MTAs such as Sendmail 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.

+ +

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.

+ +
+ + + + + + + + + + + + + + + +
/-- smtp(8) +--> Internet
qmgr(8) |
|
|
|
v
  \-- smtp(8)
  ^
|
scache(8)
+ +
+ +

With TLS connection reuse (Postfix 3.4 and later), the Postfix +smtp(8) client connects to a remote SMTP server and sends plaintext +EHLO and STARTTLS commands, then inserts a tlsproxy(8) process into +the connection as shown below.

+ +

After delivering mail, the smtp(8) client hands over the open +smtp(8)-to-tlsproxy(8) connection to the scache(8) server, and +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.

+ +
+ + + + + + + + + + + + + + + +
/-- smtp(8) +--> tlsproxy(8) + --> Internet
qmgr(8) |
|
|
|
v
  \-- smtp(8)
  ^
|
scache(8)
+ +
+ +

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 a mail server for +domains with different names.

+ +

Connection cache configuration

+ +

The Postfix smtp(8) client supports two connection caching +strategies:

+ + + +

Connection cache safety mechanisms

+ +

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:

+ + + +

Connection cache limitations

+ +

Postfix SMTP connection caching conflicts with certain applications: +

+ + + + +

Connection cache statistics

+ +

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.

+ + + + + + + -- cgit v1.2.3