MOQT maintains priorities between different schedulable objects. A schedulable object in MOQT is either:
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The first or next Object in a Subgroup that is in response to a subscription.
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An Object with forwarding preference Datagram.
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An Object in response to a FETCH where that Object is the next Object in the response.
An Object is not schedulable if it is known that no part of it can be written due to underlying transport flow control limits.
A single subgroup or datagram has a single publisher priority. Within a response to SUBSCRIBE, it can be useful to conceptualize this process as scheduling subgroups or datagrams instead of individual objects on them. FETCH responses however can contain objects with different publisher priorities.
A priority numberis an unsigned integer with a value between 0 and 255.
A lower priority number indicates higher priority; the highest priority is 0.
Subscriber Priority is a priority number associated with an individual
request. It is carried in the SUBSCRIBER_PRIORITY parameter
(Section 10.2.7), and can be updated. The subscriber priority of an
individual schedulable object is the subscriber priority of the request that
caused that object to be sent. When subscriber priority is changed, a best
effort SHOULD be
made to apply the change to all objects that have not been scheduled, but it is
implementation dependent what happens to objects that have already been
scheduled.
Publisher Priority is a priority number associated with an individual
schedulable object. A default for the subscription is specified in the
DEFAULT_PUBLISHER_PRIORITY Track Property (Section 12.4). Publisher
priority can also be set per subgroup or datagram in the subgroup header or
datagram (see Section 11), which overrides the default.
Group Order is a property of an individual subscription. It can be either
'Ascending' (groups with lower group ID are sent first), or 'Descending'
(groups with higher group ID are sent first). The subscriber optionally
communicates its group order preference in the SUBSCRIBE or SUBSCRIBE_TRACKS
message; the publisher's preference, carried in the
DEFAULT_PUBLISHER_GROUP_ORDER Track Property (Section 12.5), is used if
the subscriber did not express one (by omitting the Group Order parameter). The
group order of an existing subscription cannot be changed.