netdev - Re: Jumbo frame question...

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Date:	Fri, 24 Jul 2009 14:21:55 -0400
From:	Robin Getz <rgetz@...ckfin.uclinux.org>
To:	"Rick Jones" <rick.jones2@...com>
CC:	"David Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: Jumbo frame question...

On Fri 24 Jul 2009 12:39, Rick Jones pondered:
> David Miller wrote:
> > From: Robin Getz <rgetz@...ckfin.uclinux.org>
> > Date: Fri, 24 Jul 2009 11:41:55 -0400
> > 
> >>Should a gigabit card, configured as 100, be sending jumbo UDP frames?
> >>
> >>My understanding, is no - this is a spec violation..
> 
> In so far as there is no de jure spec for Jumbo Frames, it is rather
> difficult to have a spec violation :).

The spec I was talking about was the MTU...

rgetz@...ky:~> /sbin/ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:11:11:B0:A5:D4
          inet addr:192.168.0.10  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::211:11ff:feb0:a5d4/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:45978 errors:5 dropped:0 overruns:0 frame:0
          TX packets:44536 errors:0 dropped:0 overruns:0 carrier:0
          collisions:3193 txqueuelen:1000
          RX bytes:11583575 (11.0 Mb)  TX bytes:20025122 (19.0 Mb)
          Interrupt:16

My MTU is 1500, but when tftp requests a block size of over that - the host 
does not fragment it (like I thought it should).

> > There is nothing wrong with supporting jumbo frames
> > when the speed is lower than 1GB.

I would agree - if you had the MTU set up that big.

> > If you configure the MTU to be jumbo size, it should
> > be no surprise to you that this is what gets used.

Which it is not.

> Not a case of too much rope?  Given that (IIRC) Jumbo Frame was not
> introduced in Ethernet NICs until Gigabit came along (eg Alteon), the
> chances a (legacy) 100 Mbit/s network would have JF-capable NICs is epsilon.

Yeah - I think that this is the issue - my old hub (which is what I normally 
use for ethernet testing is only transferring it's MTU (1500 bytes), and 
dropping the rest...

Isn't there a MTU max size discovery that should be done somewhere before the 
host sends jumbo packets?

http://tools.ietf.org/html/rfc1191

And -- in the UDP/TFTP case - isn't the server responsible for determining 
this? (since it need to determine if fragmentation needs to happen or not?)

-Robin
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