Rewrite the Priority section to improve clarity

draft-ietf-moq-transport.md

@@ -617,56 +617,70 @@ MoQ priorities allow a subscriber and original publisher to influence
 the transmission order of Objects within a session in the presence of
 congestion.
 
+## Definitions
+
+MoQT maintains priorities between different _schedulable objects_.
+A schedulable object in MoQT is either:
+
+1. An object that belongs to a peep where that object would be the next object
+   to be sent in that peep.
+1. An object that belongs to a track with delivery preference Datagram.
+
+Since a single peep or datagram has a single publisher priority, it can be
+useful to conceptualize this process as scheduling peeps or datagrams
+instead of individual objects on them.
+
+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
+subscription.  It is specified in the SUBSCRIBE message, and can be updated via
+SUBSCRIBE_UPDATE message.  The subscriber priority of an individual schedulable
+object is the subscriber priority of the subscription that caused that object
+to be sent. When subscriber priority is changed, a best effort SHOULD be made
+to change the apply that 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 indiviaul
+schedulable object.  It is specified in the header of the respective peep or
+datagram, and is the same for all objects in a single peep.
+
+_Group Order_ is a property of an invidual 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 publisher communicates its
+group order in the SUBSCRIBE_OK message; the subscriber can override it in its
+SUBSCRIBE message.  The group order of an existing subscription cannot be
+changed.
+
+## Scheduling Algorithm
+
+When an MoQT publisher has multiple schedulable objects it can choose between,
+the objects SHOULD be selected as follows:
+
+1. If two objects have a different subscriber priority associated with them,
+   the one with **the highest subscriber priority** is sent first.
+1. If two objects have the same subscriber priority, but a different publisher
+   priority, the one with **the highest publisher priority** is sent first.
+1. If two objects have the same subscriber and publisher priority, but belong
+   to two different groups of the same track received through the same
+   subscription, **the group order** of the associated subscription is used to
+   decide the one that is sent first.
+1. If two objects belong to the same group of the same track received through
+   the same subscription, the one with **the lowest Peep ID** (for tracks with
+   delivery preference Peep), or **the lowest Object ID** (for tracks with
+   delivery preference Datagram) is sent first.
+
+This algorithm does not provide a well-defined ordering for objects that belong
+to different subscriptions, but have the same subscriber and publisher
+priority.  The ordering in those cases is implementation-defined, though the
+expectation is that all subscriptions will be able to send some data.
+
 Given the critical nature of control messages and their relatively
 small size, the control stream SHOULD be prioritized higher than all
 subscribed Objects.
 
-Both SUBSCRIBE and FETCH messages indicate a subscriber priority and
-group order, so the following text applies equally to both types of
-subscriptions.
-
-The subscriber indicates the priority of a subscription via the
-Subscriber Priority field and the original publisher indicates priority
-in every stream or datagram header.  As such, the subscriber's priority is a
-property of the subscription and the original publisher's priority is a
-property of the Track and the Objects it contains. In both cases, a lower
-value indicates a higher priority, with 0 being the highest priority.
-
-When Objects are contained in Subgroups, all Objects in the Subgroup have the same
-priority.
-
-The Subscriber Priority is considered first when selecting a subscription
-to send data on within a given session. When two or more subscriptions
-have equal subscriber priority, the original publisher priority is considered
-next and can change within the track, so subscriptions are prioritized based
-on the highest priority data available to send. For example, if the subscription
-had data at priority 6 and priority 10 to send, the subscription priority would
-be 6. When both the subscriber and original publisher priorities for a
-subscription are equal, how much data to send from each subscription is
-implementation-dependent, but the expectation is that all subscriptions will
-be able to send some data.
-
-The subscriber's priority can be changed via a SUBSCRIBE_UPDATE message.
-This updates the priority of all unsent data within the subscription,
-though the details of the reprioritization are implementation-specific.
-
-Subscriptions have a Group Order of either 'Ascending' or 'Descending',
-which indicates whether the lowest or highest Group Id SHOULD be sent first
-when multiple Groups are available to send.  A subscriber can specify either
-'Ascending' or 'Descending' in the SUBSCRIBE message or they can specify they
-want to use the Original Publisher's Group Order, which is indicated in
-the corresponding SUBSCRIBE_OK.
-
-Within the same Group, and the same priority level,
-Objects with a lower Object Id are always sent before objects with a
-higher Object Id, regardless of the specified Group Order. If the group
-contains more than one Subgroup and the priority varies between these Subgroups,
-higher priority Subgroups are sent before lower priority Subgroups. If the specified
-priority of two Subgroups in a Group are equal, the lower Subgroup ID has priority.
-Within a Subgroup, Objects MUST be sent in increasing Object ID order.
-
-The Group Order cannot be changed via a SUBSCRIBE_UPDATE message, and
-instead an UNSUBSCRIBE and SUBSCRIBE can be used.
+## Considerations for Setting Priorities
 
 Relays SHOULD respect the subscriber and original publisher's priorities.
 Relays SHOULD NOT directly use Subscriber Priority or Group Order