Misunderstanding in pkg/pacing

[Krage56]

Hi there!

In the pkg/pacing/interceptor.go in the burst function on line 179 (tap here) perhaps there is an error: $\frac{rate}{f}$ means bits per sending (i suppose rate in bits per second because it was so in the pkg/gcc - see comment in pkg/gcc/send_side_bwe.go). And the burst function returns $bits/sending \times 8$ for why? What the meaning of the eight? Maybe it is a typo and author intended to get bytes per sending?

[mengelbart]

I think you are correct, but I should probably also test this with pion/bwe. Would you like to open a PR and add a testcase?

[Krage56]

I think you are correct, but I should probably also test this with pion/bwe. Would you like to open a PR and add a testcase?

I apologize, but I am unsure how to proceed. If you propose to make PR with $bits / sending / 8$ instead of $bits / sending \times 8$ there is a problem: i don't sure if the burst function must return bytes instead of bits because of calculating the size of an queue item in the loop function (tap here). Therefore, I would prefer not to make any PR at this time and believe that someone with a full understanding of the CC implementation in PION should negotiate units across the all related repos

[Krage56]

Btw also I don't fully understand the functionality of pacing interceptor. There is no API to get size of current queue from external packages for example (pacer interface is private and rateLimitPacer too). And I have no idea how it should be integrated with BWE on a way like current GCC or how I should it use with custom BWE interceptors on the major PION version.

[mengelbart]

No worries. The pacer is supposed to be used as an interceptor configured with pion/webrtc. You can use SetRate on the InterceptorFactory to set the rate of the interceptor. The interceptor queues packets and sends them out when the rate limiter allows. No need to access the internal queue from outside.

It is not supposed to be integrated tightly with BWE. The current BWE implementation in this repository tightly couples pacing and BWE, but that is not necessary and will be changed in the future. In the future, you can use the pacer independently of BWE and set the pacing rate using the rate obtained from the BWE.

The rate is expressed in bits per second, but when sending packets, we need to compare it to the packet size, which is given in bytes. But as you rightly pointed out, the burst size is passed to the rate limiter, and is calculated using the target rate, which already uses bits, so no need to convert there.