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M-Switch Intervals, Schedules, Precedence & Permanent Associations

 

Between a pair of MTAs, M-Switch provides a number of related options to control associations and priorities. There are two top level mechanisms – Intervals and Schedules:

1. An interval specifies a period when messages may be sent. This is used for two basic tasks.
   • Where the peer MTA is available for a known period (e.g., an MTA on a submarine coming into scheduled contact).
   • In conjunction with priority control, to define a period where lower priority messages are not sent (e.g., in support of a military minimize condition)
2. A schedule is a regular interval, specified by day of week, hour and minute. This is used for:
   • Regularly scheduled communication to MTAs with specific availability (as in 1a above).
   • In conjunction with priority, to only send low priority messages at specific times (e.g., to not send lower priority messages in know busy periods)
   • A special “Always” schedule, which is used when there are no timing constraints, and when other features described below are needed for policy or performance reasons.

Intervals and Schedules have two basic modes:

1. On Demand. This type of interval/schedule is essentially a permission to open one or more connections.
2. Permanent. This type of interval/schedule requires a single permanent connection (association) to be held open. The reason for this is that it takes typically 2-3 seconds to open a connection, which is much longer than typical message transfer time. Use of permanent associations enables message switching time for small messages to be reduced from a few seconds to a few hundred milliseconds. Multiple permanent associations can be specified. If you need to open on demand connections in addition to the permanent connection, an on demand interval/schedule needs to be added.

Intervals and Schedules can specify a minimum priority (precedence) of message to which the interval/schedule applies. This control can be used for a number of reasons:

1. As a resource reservation mechanism, to ensure that specific connections for higher priority messages, are always available.
2. To give very low latency switching for higher priority messages, while allowing lower priority messages to open connections on demand.
3. To prevent transfer of lower priority messages, either in support of scheduling or in support of a minimize condition.

Where an interval or schedule specifies a permanent association, the priority relates to that association and will ensure that only messages of the specified priority or higher will make use of that association.

Where an interval or schedule specifies on demand connections, an associated priority specifies a policy that messages of lower priorities shall not be sent, unless permitted by another interval or schedule. This mechanism can be used in isolation or in conjunction with permanent associations to prevent transfer of lower priority messages.