Subject: [PATCH 1/6] tcp: fix RTT for quick packets in congestion control

From: Stephen Hemminger <[email-masked]>
Date: Thu, 10 Mar 2011 08:51:23 -0800

In the congestion control interface, the callback for each ACK
includes an estimated round trip time in microseconds.
Some algorithms need high resolution (Vegas style) but most only
need jiffie resolution.  If RTT is not accurate (like a retransmission)
-1 is used as a flag value.

When doing coarse resolution if RTT is less than a a jiffie
then 0 should be returned rather than no estimate. Otherwise algorithms
that expect good ack's to trigger slow start (like CUBIC Hystart)
will be confused.

Signed-off-by: Stephen Hemminger <[email-masked]>

--- a/net/ipv4/tcp_input.c	2011-03-08 11:11:26.093183654 -0800
+++ b/net/ipv4/tcp_input.c	2011-03-08 11:11:46.641404939 -0800
@@ -3350,7 +3350,7 @@ static int tcp_clean_rtx_queue(struct so
 						 net_invalid_timestamp()))
 					rtt_us = ktime_us_delta(ktime_get_real(),
 								last_ackt);
-				else if (ca_seq_rtt > 0)
+				else if (ca_seq_rtt >= 0)
 					rtt_us = jiffies_to_usecs(ca_seq_rtt);
 			}