netdev - [PATCH] TCP: H-TCP maxRTT estimation at startup

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Date:	Fri, 3 Aug 2007 10:57:56 +0100
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	"David S. Miller" <davem@...emloft.net>
Cc:	Douglas Leith <doug.leith@...m.ie>, netdev@...r.kernel.org
Subject: [PATCH] TCP: H-TCP maxRTT estimation at startup

Small patch to H-TCP from Douglas Leith. 

Fix estimation of maxRTT.  The original code ignores rtt measurements
during slow start (via the check tp->snd_ssthresh < 0xFFFF) yet this
is probably a good time to try to estimate max rtt as delayed acking
is disabled and slow start will only exit on a loss which presumably
corresponds to a maxrtt measurement.  Second, the original code (via
the check htcp_ccount(ca) > 3) ignores rtt data during what it
estimates to be the first 3 round-trip times.  This seems like an
unnecessary check now that the RCV timestamp are no longer used
for rtt estimation.

Signed-off-by: Stephen Hemminger <shemminger@...ux-foundation.org>

--- a/net/ipv4/tcp_htcp.c	2007-08-03 10:51:51.000000000 +0100
+++ b/net/ipv4/tcp_htcp.c	2007-08-03 10:51:53.000000000 +0100
@@ -79,7 +79,6 @@ static u32 htcp_cwnd_undo(struct sock *s
 static inline void measure_rtt(struct sock *sk, u32 srtt)
 {
 	const struct inet_connection_sock *icsk = inet_csk(sk);
-	const struct tcp_sock *tp = tcp_sk(sk);
 	struct htcp *ca = inet_csk_ca(sk);

 	/* keep track of minimum RTT seen so far, minRTT is zero at first */
@@ -87,8 +86,7 @@ static inline void measure_rtt(struct so
 		ca->minRTT = srtt;

 	/* max RTT */
-	if (icsk->icsk_ca_state == TCP_CA_Open
-	    && tp->snd_ssthresh < 0xFFFF && htcp_ccount(ca) > 3) {
+	if (icsk->icsk_ca_state == TCP_CA_Open) {
 		if (ca->maxRTT < ca->minRTT)
 			ca->maxRTT = ca->minRTT;
 		if (ca->maxRTT < srtt
-
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