Hi gurus,
I am not sure if there is a problem in the additive increment (AI)
parameter of tcp_highspeed.c:
When snd_cwnd is smaller than 38 and the connection is in
congestion avoidance phase (snd_cwnd > snd_ssthresh), the snd_cwnd
seems to stop growing. I guess the problem is in the function of
hstcp_cong_avoid (Ln 126~Ln 138 of tcp_highspeed.c):
--------------------------------
if (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd) {
while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&
ca->ai < HSTCP_AIMD_MAX - 1)
ca->ai++;
} else if (tp->snd_cwnd < hstcp_aimd_vals[ca->ai].cwnd) {
while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&
ca->ai > 0)
ca->ai--;
}
/* Do additive increase */
if (tp->snd_cwnd < tp->snd_cwnd_clamp) {
tp->snd_cwnd_cnt += ca->ai;
--------------------------------
When snd_cwnd < snd_ssthresh, this part of code is effective. And
since snd_cwnd is smaller than 38, which is hstcp_aimd_vals[0].cwnd,
ca->ai will be equal to 0.
Hence, snd_cwnd_cnt will be unchanged for the rest of the connection.
As a result, snd_cwnd also freezes.
I guess we can add change the Line 138 to make the algorithm more reliable?
- tp->snd_cwnd_cnt += ca->ai;
+ tp->snd_cwnd_cnt += (ca->ai+1);
Thanks.
-David
---------------------------------------------------------
Xiaoliang (David) Wei
http://davidwei.org Graduate Student, Netlab, Caltech
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