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ianswett authored Nov 10, 2024
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106 changes: 60 additions & 46 deletions draft-ietf-moq-transport.md
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Expand Up @@ -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
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