Calculation of SendAllowance in BBR is Wrong

[lrfeng1]

Describe the bug

In the BbrCongestionControlGetSendAllowance function,we expect a result in BYTES to indicate that how many bytes can be sent next time. And in this function,
"SendAllowance = (uint32_t)(BandwidthEst * Bbr->PacingGain * TimeSinceLastSend / GAIN_UNIT);"
is used to calculate the value of SendAllowance.

However, from the codes, we know that,BandwidthEst is in bps (bit per second), TimeSinceLastSend is in us(micro second),PacingGain/GAIN_UNIT will be 1 or 1.25 or 0.75,we can ignore its unit.

Obiviously, when pacing is executing, we cannot get a value in BYTES in this function, we have to use a formula "bit per micro sec * micro sec / 8" to get a value in bytes.

Affected OS

• Windows
• Linux
• macOS
• Other (specify below)

Additional OS information

No response

MsQuic version

main

Steps taken to reproduce bug

it can easily be seen in the codes.
I found it in the following case.When I manually set the BandwidthEst to 6Gbps, i found that the actual throughput is too low and it is limited by the SendAllowance value.
So I found that the SendAllowance calculation is wrong.
this is because that 32-bit SendAllowance is not enough is this case (6G bps * 1 * several micro seconds), so when the 64-bit result is cut to 32-bit, the SendAllowacne result turns too small.

Expected behavior

a right value of SendAllowance

Actual outcome

a wrong value of SendAllowance

Additional details

No response

[lrfeng1]

@anrossi Hi, May I ask if this is indeed a Bug?

[anrossi]

It sounds like a bug from your description, but I haven't had time to dig in and investigate it further

[nibanks]

Feel free to suggest a PR, @lrfeng1.