Message-ID: <44DAF559.8010705@comcast.net>
Date: Thu, 10 Aug 2006 04:59:05 -0400
From: John Richard Moser <nigelenki@comcast.net>
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To: David Miller <davem@davemloft.net>
CC: linux-kernel@vger.kernel.org
Subject: Re: How does Linux do RTTM?
References: <44DACA22.6090701@comcast.net> <20060809.231244.35509467.davem@davemloft.net>
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David Miller wrote:
> From: John Richard Moser <nigelenki@comcast.net>
> Date: Thu, 10 Aug 2006 01:54:42 -0400
> 
>> Sorry for the dumb questions but Google is being massively bad at "tell
>> me about an obscure feature of the Linux kernel that nobody cares about"
>> today :)
> 
> When I type "Linux RTT measurement" to google, the following
> very authoritative paper on Linux's TCP congestion control
> shows up on the very first page:
> 
> http://www.cs.helsinki.fi/research/iwtcp/papers/linuxtcp.pdf
> 

This says Linux does use the RTTM "Timestamp" option for TCP; it does
not however tell me HOW it uses it.

So does it stamp the current time in UTC in milliseconds in the field?
Does it stamp in the number of jiffies since boot?  Does it use a
pseudo-randomly generated per-connection initial virtual clock value
with a microsecond-per-tick skew-factor to hide information about the
internal state of the system to avoid attacks on the PRNG to guess TCP
ISNs or attack ASLR?

> You should try a little bit harder with google next time, and
> also ask your question on a more appropriate list such as
> netdev@vger.kernel.org which is where the networking developers
> are subscribed.
> 

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