Linux TCP implementation
 

Hello:

I am working with 2.4.21 kernel for iSCSI performance study. I have a question
regarding the congestion avoidance algorithm used in the linux TCP implementation.
After dumping the value of the snd_cwnd, snd_ssthresh and snd_cwnd_clamp when
the tcp_cong_avoid function is called, I observed that the value of snd_ssthresh
was always set to a value of 2147483647 and the value of snd_cwnd_clamp was set to 65535.
If the snd_cwnd is not be more than the snd_cwnd_clamp, the else part of the
function is never executed, as snd_cwnd is never going to be more than snd_ssthresh.
Also, if the snd_cwnd is maintained in terms of packets and snd_ssthresh and
snd_cwnd_clamp is maintained in terms of bytes, how come the comparison between them.


static __inline__ void tcp_cong_avoid(struct tcp_opt *tp)
{
if (tp->snd_cwnd <= tp->snd_ssthresh) {
/* In "safe" area, increase. */
if (tp->snd_cwnd < tp->snd_cwnd_clamp)
tp->snd_cwnd++;
} else {
/* In dangerous area, increase slowly.
* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd
*/
if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
if (tp->snd_cwnd < tp->snd_cwnd_clamp)
tp->snd_cwnd++;
tp->snd_cwnd_cnt=0;
} else
tp->snd_cwnd_cnt++;
}
tp->snd_cwnd_stamp = tcp_time_stamp;
}

var/log/messages:
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 2, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 3, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 4, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 5, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 6, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 7, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 8, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 9, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 10, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 11, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 12, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 13, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 14, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 15, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 16, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 17, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 18, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 19, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 20, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 21, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535
Mar 22 15:08:20 jupiter kernel: snd_cwnd: 22, snd_ssthresh: 2147483647, snd_cwnd_clamp: 65535


Any insight on this topic would be appreciated.

Thanks,
k