dupack counter processing (was: Re: The trouble with sack..)

[hiren panchasara]

On 10/07/15 at 12:17P, Randall Stewart via freebsd-transport wrote:
> 
> 2) When we recognize a dup-ack we *will not* recognize it if for example if the rwnd changes even
>     if new SACK information is reported in the sack blocks. This is due to the fact that in non-SACK you don?t
>     (on purpose) recognize ACK?s where the window changed (since you can?t really tell if its a
>      plain window update or a dup-ack).. This means we occasionally miss out
>     on stroking the dup-ack counter and getting out of recovery....

Just learned that linux triggers fast recovery right away when it
receives a dupack with SACK info that covers at least 3 packets
(essentially indicating that 3 packets made it through successfully
after a loss/drop event).
Here, 3 is tcprexmtthresh for us.

This also follows rfc6675. (IsLost()).

Cheers,
Hiren
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