Three relative priority classes have been defined to date. These priority classes typically imply some relative scheduling prioritization among bundles in queue at a sender.
- Bulk.Bulk bundles are shipped on a “least effort” basis.No bundles of this class will be shipped until all bundles of other classes bound for the same destination and originating from the same source have been shipped.
- Normal.Normal-class bundles are shipped prior to any bulk-class bundles and are otherwise the same as bulk bundles.
- Expedited.Expedited bundles,in general, are shipped prior to bundles of other classes and are otherwise the same.
Applications specify their requested priority class and data lifetime for each ADU they send.This information, coupled with policy applied at DTN nodes that select how messages are forwarded and which routing algorithms are in use, affects the overall likehood and timeliness of ADU delivery.
The priority class of a bundle is only required to relate to other bundles from the same source.
Depending on a particular DTN nodes forwarding/scheduling policy, priority may or may not be enforced across different sources.