From owner-freebsd-net@FreeBSD.ORG  Tue Apr 20 21:44:51 2010
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Date: Tue, 20 Apr 2010 23:55:27 +0200
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: net@freebsd.org
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Subject: Re: max_linkhdr defaults to 16, too short ?
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On Tue, Apr 20, 2010 at 07:28:45PM +0200, Luigi Rizzo wrote:
> just noticed that sys/kern/uipc_domain.c still sets max_linkhdr=16
> as a default.
> The value is often used to reserve head space in mbufs for
> the MAC header. As such, 16 is too short for systems that make
> use of vlans, and the effect might be that we would need
> additional mbuf entries or at least move stuff down
> as the vlan tag is added.
> 
> Any objection to bumping the default to 20 ?

forgot to mention:

max_linkhdr is available as a sysctl, kern.max_linkhdr , but other
than that, there is no code in sys/ that sets the value, so systems
are stuck at 16 unless users override the default.

cheers
luigi