Date: Tue, 20 Oct 2015 18:32:01 -0700 From: hiren panchasara <hiren@strugglingcoder.info> To: transport@FreeBSD.org Cc: rrs@FreeBSD.org Subject: Help me understand this SACK code in tcp_output() Message-ID: <20151021013201.GD28288@strugglingcoder.info>
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tcp_output() starts out with following two vars set to zero:
sack_rxmit = 0;
sack_bytes_rxmt = 0;
And if we are in recovery and trying to send from holes, i.e.
if ((tp->t_flags & TF_SACK_PERMIT) && IN_FASTRECOVERY(tp->t_flags) &&
(p = tcp_sack_output(tp, &sack_bytes_rxmt))) {
And if we end up with if (len > 0), we set sack_rxmit = 1;
A little below is following code which I do not understand:
/* If sack_rxmit is true we are retransmitting from the scoreboard
* in which case len is already set.
*/
if (sack_rxmit == 0) {
if (sack_bytes_rxmt == 0)
......
else {
}
}
What I do not understand is how can we get into the 'else' part.
AFAICT,
sack_rxmit: when TRUE, we are retransmitting from the scoreboard
sack_bytes_rxmt: number of retransmitted bytes from the scoreboard.
Now, if sack_rxmit is false, how can sack_bytes_rxmt be > 0?
OR is there something I am missing in the code OR how SACK works?
Cheers,
Hiren
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