Defragmentation channel

The MIME standard (RFC 2046) provides the message/partial content type for breaking up messages into smaller parts. This is useful when messages have to traverse networks with size limits. It can also be useful when sending over networks subject to connection drops, as a  form of "check-pointing" of the sending of a message, since a  connection drop during transmission of a fragment of a message will  require resending only of that fragment, rather than of the entire  message. Information is included in each part so that the message can be automatically reassembled once it arrives at its destination.

The  channel option (used on channels other than the defragmentation channel) and the defragmentation  channel itself provide the means to reassemble messages in the MTA. When a channel is marked , any message/partial messages  queued to that channel will be placed in the defragmentation channel  queue instead. The defragmentation channel maintains a database which is used to match the parts of each message up with each other. Once all the parts have arrived, the message is rebuilt and sent on its way.

The defragment database can optionally be stored on a filesystem accessible to multiple hosts (for instance, over NFS), and  then shared by multiple hosts. Such sharing of the defragment database can be particularly useful for achieving appropriate and efficient message defragmentation in multi-tiered, multi-plexed deployments. See Multi-host defragmentation channel operation for further details.

All channels that perform local delivery or send messages on to hosts (e.g., LMTP back end Message Store hosts) or networks that  cannot deal with fragmented messages should be marked with the    channel option. In particular all   and    channels should  be marked with the    channel option. The  channel option will have  no effect unless a defragmentation channel is also defined.

A defragmentation channel is produced automatically by  initial configuration.

See also:
 * defragment Option
 * Multi-host defragmentation channel operation
 * Defragmentation-channel-specific option
 * Defragmentation channel configuration
 * Defragmentation channel message retention time
 * Channels
 * Available channels
 * ims-ms channels
 * LMTP client TCPIP channels
 * Other channels