netdev - Re: Bad TCP timestamps on non-PC platforms

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Message-ID: <1294301356.2723.73.camel@edumazet-laptop>
Date:	Thu, 06 Jan 2011 09:09:16 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alex Dubov <oakad@...oo.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Bad TCP timestamps on non-PC platforms

Le mercredi 05 janvier 2011 à 22:55 -0800, Alex Dubov a écrit :
> Greetings.
> I'm dealing with 2.6.37-rc7 kernel on MPC8548 platform.
> 
> It so appears, that recent kernels have sysctl_tcp_timestamps set to "1"
> by default.
> 
> On embedded platforms, where real time clock is initialized lately or
> absent outright, this causes TSval field of outgoing TCP packets to be
> set to some garbage value, in my case in the vicinity of 0xffffffff. As a
> result, other Linux machines silently drop such packets, preventing normal
> completion of network boot or any other TCP dependent operation.
> 
> Therefore, I feel, two changes are necessary (I can send in a patch):
> 1. Make sysctl_tcp_timestamps value config-time selectable (it must be
> disabled by default on machines without RTC).
> 2. When re-enabling tcp_timestamps through sysctl, reset the timestamp
> counter to the current system clock value.
> 
> And an optional, tricky one:
> 3. Postpone TCP timestamp counter initialization until RTC is actually
> available (if RTC is connected to i2c bus, TCP is initialized well ahead
> of it).
> 

I am trying to understand the problem, but I guess you hit something not
related to RTC at all.

#define tcp_time_stamp		((__u32)(jiffies))
#define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))

jiffies can be ZERO 300 seconds after boot.

It should not matter with RFC1323. Some TCP session could in theory be
done without TCP timestamps being activated (because SYN paquet would
carry a ZERO  tsval/tsecr), but this should not prevents normal network
operation.

Are you sure the "other linux machines" dont have
strange /etc/sysctl.conf settings ?



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