[PATCH] [net-next] tcp: avoid sending zero TSval

[Octavian Purdila <opurdila@ixiacom.com>]

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Per RFC1323, zero TSecr is considered invalid. Thus we must avoid when
possible sending a zero TSval.

Currently, we use the least significant 32 bits of jiffies to fill in
TSval. But that can wrap around to zero (in 5 minutes after reboot,
and every 49 days after that in the worst case).

This patch approximate a wrap-around zero TSval to 1. This is better
then emitting a value which will be ignored.

Signed-off-by: Octavian Purdila <opurdila@ixiacom.com>
---
 net/ipv4/tcp_output.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

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diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 4e00442..607d675 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -415,7 +415,10 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
 				       (TCPOPT_TIMESTAMP << 8) |
 				       TCPOLEN_TIMESTAMP);
 		}
-		*ptr++ = htonl(opts->tsval);
+		/* Currently, opts->tsval always comes from tcp_time_stamp.
+		 * Thus, if zero, its caused by jiffie wrap-around, and we can
+		 * approximate it to one to avoid a later TSecr drop */
+		*ptr++ = htonl(opts->tsval?:1);
 		*ptr++ = htonl(opts->tsecr);
 	}