netdev - Re: route metrics in jiffies??

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Message-Id: <20080521.133154.173559889.davem@davemloft.net>
Date:	Wed, 21 May 2008 13:31:54 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...tta.com
Cc:	Stephane_Chazelas@...oo.fr, netdev@...r.kernel.org
Subject: Re: route metrics in jiffies??

From: Stephen Hemminger <shemminger@...tta.com>
Date: Wed, 21 May 2008 11:43:54 -0700

> There is an even bigger mess up.  The API for route metrics has several
> values encoded in jiffies.  This is a problem because there is no good
> way to find the internal kernel value of HZ. So all kernel/user ABI's
> are supposed to use an absolute value (like milliseconds) or clock_t
> which user USER_HZ.
> 
> The problem is that these values are now hardcoded into people's systems
> so anyone using the 'ip route' options: rttvar, rtomin, or rtt are broken.
> They might be lucky now (but I doubt it).
> 
> I propose doing the right thing and fixing kernel and iproute to always
> use milliseconds for these values. To maintain compatibility, the new metric
> values will be renumbered.  So old kernels don't misinterpret the new values.

That is one way to solve the problem.  But we could be adding
surprises on a source level for people with this approach.

Just use new names and leave the old ones alone, with a _MS or similar
postfix to them.

This is how we've handled this kind of situation in the past.
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