Forward Queuing for Time-Sensitive Streams (FQTSS)

Milan utilizes FQTSS to shape traffic. The approach is explained in detail below.

Switch ports are described based on the traffic direction for the current observation.

• An ingress port describes traffic flowing into a switch.
• An egress port describes traffic flowing out of a switch.

Strict priority forwarding

Consider the example shown in the figure below.

This figure illustrates an example of three ingress port with packets that are all sent to the same egress port. The prioritization of packets is as follows:

• Time-critical: packets have the highest priority.
• Less critical: packets have medium priority.
• Legacy: packets have low priority.

In strict priority forwarding, time-critical packets are always forwarded first, preventing lower-priority packets from being transmitted as long as high-priority packets remain in the transmission queue. This results in a blocking effect on lower-priority packets whenever high-priority packets are present at an ingress port that needs to forward packets to an egress port.

Credit-based forwarding

The blocking effect of strict prioirity forwarding as shown in Fig. 1 can be mitigated using a credit-based shaper. To keep this example straightforward, we will focus only on time-critical and less critical packets, leaving out legacy packet behavior for simplicity.

The core idea behind the credit-based shaper is simple:
Each priority level has a credit counter.

• This counter builds up when packets are waiting to be sent.
• It decreases when packets are being transmitted.

As soon as there is an opportunity to send data and the credit for a particular priority level is above zero, packets of that priority can be transmitted.

This approach spreads packet transmissions more evenly over time, reducing burstiness and smoothing traffic flow. In this example, we assume that when a packet is being transmitted, the credit decreases twice as fast as it builds up when waiting for a transmission opportunity. In practice, the rate at which credit decreases (sendSlope) and increases (idleSlope) is calculated based on the amount of bandwidth that is reserved.