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Date: Sat, 26 Dec 2009 14:45:19 -0800
From: Julian Elischer <julian@elischer.org>
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To: Hideki Yamamoto <hyama99@gmail.com>
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Cc: freebsd-net@freebsd.org
Subject: Re: GIF MTU parmeter is needed
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Hideki Yamamoto wrote:
> Hi,
> 
> I often use FreeBSD for developing the gateway. For example, I use gif for the
> tunnel protocol when using IPv6 over IPv4 and use an application for changing
> packet address for special purpose.  When we were using old FreeBSD, such as
> FreeBSD 4.11, the MTU of the tunnel packets or forwarded packets
> was not limited into 1280. However, FreeBSD 6,7, and 8 fragments packets
> by 1280 when using tunnel.

?? huh?
it is defined by the MTU of the gif interface as set with ifconfig

> 
> I know that this behavior is based on the RFC specification.  However
> it is not useful when using AV application that use around 1400B RTP packet.
> AV packets will be fragmented into long packets (1280) and short
> packets (1400-1280)
> when using tunnel, and short packet will sometimes be lost by network.
> 
> I hope new parameter by sysctl to control MTU of tunnel will be implemented.
> The following is an example of new paramter to control MTU size.
> 
>      net.inet6.use_mmtu :1 --- is the same as current versions, it means
>                                                      minimum MTU 1280
> will be used when gateway node.
>                                            :0--- is the same as the
> old versions. It means
>                                                                OS uses
> as long MTU size as possible
>             ( I hope "1" will be default)
> 
> Are there any comment on this matter?
> 
> Best regards,
> 
> Hideki Yamamoto
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