MOQT maintains priorities between different schedulable objects. A schedulable object in MOQT is either:
-
An object in response to a SUBSCRIBE that belongs to a subgroup where that object is the next object in that subgroup.
-
An object in response to a SUBSCRIBE that belongs to a track with delivery preference Datagram.
-
An object in response to a FETCH where that object is the next object in the response.
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 number is 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 specified in the SUBSCRIBE or FETCH message, and can be updated via SUBSCRIBE_UPDATE message. 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 sent, but it is implementation dependent what happens to objects that have already been received and possibly scheduled.
Publisher Priority is a priority number associated with an individual schedulable object. It is specified in the header of the respective subgroup or datagram, or in each object in a FETCH response.
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 message; the publisher's preference is used if the subscriber did not express one (by setting Group Order field to value 0x0). The group order of an existing subscription cannot be changed.