inflight low question

[Mihai Mazilu]

Hello, i have a question in regards to inflight low. The way I understand that this lower bound works is that it is arrived at through "inflight_latest" which trends down, in turn lowering inflight low over time until it is reset when refilling provided constant loss occurs(if i am understanding correctly). However, in adapt_lower_bounds, it is arrived at though this formula "inflight_lo = max( inflight_latest, BBRBeta * inflight_lo )". My question is what is the rationale behind BBRBeta in this instance. Why is the maximum between inflight latest andBBRBeta * inflight_l  necessary here ?

Thanks.

[Neal Cardwell]

> Why is the maximum between inflight latest andBBRBeta * inflight_l  necessary here ?

The max() is there for two main reasons:

(a) to avoid being overly sensitive to random loss in application-limited scenarios, and

(b) to avoid being more sensitive to loss than CUBIC

Consider a simple case:

+ A TCP flow has a BDP of 1 million packets, and cwnd of 1 million packets. All is well.

+ The application sends 2 packets, and 1 of the 2 is lost

In this case: 

+ CUBIC would set its cwnd to:

      0.7*1M = 700,000 packets

   ... achieving reasonable performance

+ BBRv2 or BBRv3 *without* the max() would set its cwnd to:

      inflight_latest = 1 packet

   ...crushing its own performance

+ BBRv2 or BBRv3 *with* the max() would set its cwnd to:

      max(inflight_latest, BBRBeta * inflight_lo) 

      = max(1, 0.7 * 1M) packets = 700,000 packets

   ... matching CUBIC and achieving reasonable performance

best regards,

neal

On Mon, Feb 26, 2024 at 9:32 PM Mihai Mazilu <mihaif...@gmail.com> wrote:

Hello, i have a question in regards to inflight low. The way I understand that this lower bound works is that it is arrived at through "inflight_latest" which trends down, in turn lowering inflight low over time until it is reset when refilling provided constant loss occurs(if i am understanding correctly). However, in adapt_lower_bounds, it is arrived at though this formula "inflight_lo = max( inflight_latest, BBRBeta * inflight_lo )". My question is what is the rationale behind BBRBeta in this instance. Why is the maximum between inflight latest andBBRBeta * inflight_l  necessary here ?

Thanks.

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[Mihai Mazilu]

thank you for the detailed response.

[Muhammad Ahsan]

 

Neal and Dave are always helpful.

I have learnt a lot from Neal, specially.

 

Rgds,

Ahsan