Where is priority for a particular frame set?

Question

The Medium Access Control sublayer appends 3 bits for the priority of the frame. The possible choices for the priority are

Traffic Type	Priority
Background	0 (lowest)
Best effort	1 (default)
Excellent effort	2
Critical applications	3
Video (< 100 ms latency and jitter)	4
Voice (< 10 ms latency and jitter)	5
Internetwork control	6
Network control	7 (highest)

These can be found in the table here. This leads to several related questions:

1. Where is the priority level for a particular frame set? (whose code decides and in what device)
2. Since the MAC layer does not inspect the contents of a packet, how does the MAC layer know what priority to assign to a packet? (video vs. best effort)
3. What prevents abuse of the priority system? If I want my application to provide lightning-fast speeds to users, what prevents me from requesting the highest-priority delivery?

Answer 1

From the Catalyst 4000 documentation:

ISL or 802.1Q frames are not classified or marked at the ingress port; the existing CoS value is honored.

When an 802.1Q frame enters the switch through a supported ingress port, QoS accepts the User Priority bits as the CoS value.

QoS classifies and marks all other frame types that enter the switch with the default CoS value that is configured for the entire switch. You cannot mark traffic on a per-port basis.

According to other answers, this information is only used on the local network and can only be "abused" on this network.

Answer 2

Edited:

Where is the priority level for a particular frame set? (whose code decides and in what device) Since the MAC layer does not inspect the contents of a packet, how does the MAC layer know what priority to assign to a packet? (video vs. best effort)

Typically, the access switch sets the frame priority. Switches can use higher layer information (such as IP address or port) to set the priority. In some cases, the transmitting host, if trusted, can set its own priority.

What prevents abuse of the priority system? If I want my application to provide lightning-fast speeds to users, what prevents me from requesting the highest-priority delivery?

Switches typically set the priority, and they will overwrite any priority setting from the host. Remember that the frame priority only affects the local layer 2 link. The frame information is discarded when the data passes through a router.