Automatic Defragmentation of Message/Partial Messages

Keywords: defragment, nodefragment

The MIME standard 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, or traverse unreliable networks where message fragmentation can provide a form of “checkpointing,” allowing for less subsequent duplication of effort when network failures occur during message transfer. Information is included in each part so that the message can be automatically reassembled after it arrives at its destination.

The defragment channel keyword and the defragmentation channel provide the means to reassemble messages in the MTA. When a channel is marked defragment, any partial messages queued to the channel are placed in the defragmentation channel queue instead. After all the parts have arrived, the message is rebuilt and sent on its way. The nodefragment disables this special processing. The keyword nodefragment is the default.

Defragmentation Channel Retention Time

Messages are retained in the defragment channel queue only for a limited time. When one half of the time before the first nondelivery notice is sent has elapsed, the various parts of a message will be sent on without being reassembled. This choice of time value eliminates the possibility of a nondelivery notification being sent about a message in the defragment channel queue.

The notices channel keyword controls the amount of time that can elapse before nondelivery notifications are sent, and hence also controls the amount of time messages are retained before being sent on in pieces. Set the notices keyword value to twice the amount of time you wish to retain messages for possible defragmentation. For example, a notices value of 4 would cause retention of message fragments for two days:

defragment notices 4 
DEFRAGMENT-DAEMON