Rewrite the Priority section to improve clarity (moq-wg#518)

This change provides a step-by-step algorithm for how to pick the next
object to send, and which priorities take precedence over another, all
in one place.

This PR may or may not change the normative behavior of priorities
depending on one's interpretation of the current text (which, as we
learned in today's call, appears to vary between people).

Fixes moq-wg#517

Closes moq-wg#585

Author: ianswett

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