Re: Speeding up tail loss detection (Re: Congestion control algorithm questions)

[Christian Huitema <[e-mail]>]

On 6/29/2018 11:26 PM, Praveen Balasubramanian wrote:
>
> All out of order packets elicit an immediate ACK, this applies to QUIC
> the same way it applies to TCP.
>
>  
>
> Section 3.4 of recovery spec:
>
>  
>
>    Out-of-order packets SHOULD be acknowledged more quickly, in order to
>
>    accelerate loss recovery.  The receiver SHOULD send an immediate ACK
>
>    when it receives a new packet which is not one greater than the
>
>    largest received packet number.
>

Yes, but I don't think this is what Ian is looking for. Ian is looking
for something mechanical, "if you receive this signal please send an ACK
right now."

The paragraph that you quote does not achieve that. In it, "SHOULD"
stands for something like "SHOULD unless you think you are smarter than
that." For example, a receiver that experiences lots of out of order
deliveries might replace the last sentence by "when it receives a new
packet which is not X greater than the largest received packet number",
where X is based on an estimate of current out of order deliveries.

Besides, the sender might wish for an immediate ACK for other reasons
than managing out of order deliveries. Precise time management or
checking for connectivity come to mind.

-- Christian Huitema