Thanks, Stephen. But I think there is still a problem with the AIMD
parameter update in HighSpeed TCP code.
Line 125~138 of the code (net/ipv4/tcp_highspeed.c):
/* Update AIMD parameters */
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--;
In fact, the second part (decreasing ca->ai) never decreases since the
while loop's inequality is in the reverse direction. This leads to
unfairness with multiple flows (once a flow happens to enjoy a higher
ca->ai, it keeps enjoying that even its cwnd decreases)
Here is a tentative fix (I also added a comment, trying to keep the
change clear):
--- /home/weixl/linux-2.6.16.18/net/ipv4/tcp_highspeed.c 2006-05-22
11:04:35.000000000 -0700
+++ /home/weixl/linux-2.6.16.18/net/ipv4/tcp_highspeed.c.new 2006-06-12
09:56:40.000000000 -0700
@@ -123,13 +123,13 @@
tcp_slow_start(tp);
else {
/* Update AIMD parameters */
+ /* We want to guarantee that hstcp_aimd_vals[ca->ai-1].cwnd <
snd_cwnd <= hstcp_aimd_vals[ca->ai].cwnd */
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)
+ } else if (ca->ai && tp->snd_cwnd <=
hstcp_aimd_vals[ca->ai-1].cwnd) {
+ while (ca->ai && tp->snd_cwnd <=
hstcp_aimd_vals[ca->ai-1].cwnd)
ca->ai--;
}
Thanks.
-David
On 6/2/06, Stephen Hemminger <[EMAIL PROTECTED]> wrote:
Went backed and looked at the RFC. The problem was just a simple
translation of table to C array (0 based). Added this to the TCP testing
repository.
Subject: [PATCH] Problem observed by Xiaoliang (David) Wei:
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.
The additive increase was confused because C array's are 0 based.
Signed-off-by: Stephen Hemminger <[EMAIL PROTECTED]>
---
net/ipv4/tcp_highspeed.c | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)
121685b7b61c8faeb87e6c6f0c346b0fe1c46fd2
diff --git a/net/ipv4/tcp_highspeed.c b/net/ipv4/tcp_highspeed.c
index b72fa55..ba7c63c 100644
--- a/net/ipv4/tcp_highspeed.c
+++ b/net/ipv4/tcp_highspeed.c
@@ -135,7 +135,8 @@ static void hstcp_cong_avoid(struct sock
/* Do additive increase */
if (tp->snd_cwnd < tp->snd_cwnd_clamp) {
- tp->snd_cwnd_cnt += ca->ai;
+ /* cwnd = cwnd + a(w) / cwnd */
+ tp->snd_cwnd_cnt += ca->ai + 1;
if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
tp->snd_cwnd_cnt -= tp->snd_cwnd;
tp->snd_cwnd++;
--
1.3.3
--
Xiaoliang (David) Wei Graduate Student, [EMAIL PROTECTED]
http://davidwei.org
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