A subgroup is a sequence of one or more objects from the same group (Section 2.3) in ascending order by Object ID. Objects in a subgroup have a dependency and priority relationship consistent with sharing a stream and are sent on a single stream whenever possible. A Group is delivered using at least as many streams as there are Subgroups, typically with a one-to-one mapping between Subgroups and streams.
When a Track's forwarding preference (see Section 9.2.1) is "Datagram", Objects are not sent in Subgroups and the description in the remainder of this section does not apply.
Streams offer in-order reliable delivery and the ability to cancel sending and retransmission of data. Furthermore, many implementations offer the ability to control the relative priority of streams, which allows control over the scheduling of sending data on active streams.
Every object within a Group belongs to exactly one Subgroup.
Objects from two subgroups cannot be sent on the same stream. Objects from the same Subgroup MUST NOT be sent on different streams, unless one of the streams was reset prematurely, or upstream conditions have forced objects from a Subgroup to be sent out of Object ID order.
Original publishers assign each Subgroup a Subgroup ID, and do so as they see fit. The scope of a Subgroup ID is a Group, so Subgroups from different Groups MAY share a Subgroup ID without implying any relationship between them. In general, publishers assign objects to subgroups in order to leverage the features of streams as described above.
An example strategy for using stream properties follows. If object B is dependent on object A, then delivery of B can follow A, i.e. A and B can be usefully delivered over a single stream. Furthermore, in this example:
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If an object is dependent on all previous objects in a Subgroup, it is added to that Subgroup.
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If an object is not dependent on all of the objects in a Subgroup, it goes in a different Subgroup.
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There are often many ways to compose Subgroups that meet these criteria. Where possible, choose the composition that results in the fewest Subgroups in a group to minimize the number of streams used.