netdev - Re: dhcp-based netboot over jumbo-frame network?

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Date:	Wed, 08 Apr 2009 16:10:34 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Rick Jones <rick.jones2@...com>
CC:	linux-kernel@...r.kernel.org,
	Linux Network Development list <netdev@...r.kernel.org>
Subject: Re: dhcp-based netboot over jumbo-frame network?

Rick Jones wrote:
> Chris Friesen wrote:

>> We have an existing system that loads a bare kernel using tftp, then 
>> mounts a rootfs via NFS.  The wrinkle is that this is on a jumbo-frame 
>> enabled network, so somehow the MTU needs to be set before mounting the 
>> rootfs.
> 
> Why?  Is the NFS mount not TCP?  If it is TCP, then the TCP MSS exchange will 
> provide you cover as it will (should) cause TCP in the NFS server to segment to 
> the MSS option your system with the then-smaller MTU sends.
> 
> I would think that only if the NFS mount is UDP that you would have a problem 
> with the system initially booting with a small MTU.

Currently it uses the default, which is udp.  I might be able to get 
them to try TCP.  Thanks for the suggestion.

>> In a previous version we patched ipconfig.c to support the DHCP option 
>> to specify the MTU value, but I'm wondering about the "proper" way to do 
>> this on current kernels.
> 
> Was the change submitted to upstream and rejected?

The change was submitted (not by us) back in the 2.6.10 timeframe, and 
was rejected with the suggestion to use initramfs.

Chris
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