Re: proper SACK behavior?

[yamt%mwd.biglobe.ne.jp@localhost (YAMAMOTO Takashi)]

hi,

>   [long ago, I questioned what happens with sack enabled]
> 
> Bev Schwartz, Laura Ma and I (all of BBN) have found and fixed a SACK
> problem.  I will send a patch as soon as I have a chance but I wanted to
> outline the problem quickly.
> 
> When newreno (no sack) gets a triple duplicate ack, it halves cwnd,
> retransmits, and continues.  When running in sack mode, the triple
> dupack causes cwnd to go to 1 instead.  This is allowed by the specs,
> but the preferred option is to halve cwnd.  The code is confusing
> because it sets cwnd to 1 to call tcp_output to retransmit a segment,
> but then it doesn't restore it.
> 
> With this fixed, our TCP is competive with OS X; without it, we send
> less than OS X.

sure, our sack code is too confusing.  can you share your fix?

YAMAMOTO Takashi