From owner-freebsd-net@FreeBSD.ORG  Fri Mar 13 23:53:59 2009
Return-Path: <owner-freebsd-net@FreeBSD.ORG>
Delivered-To: freebsd-net@FreeBSD.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id 674B31065670;
	Fri, 13 Mar 2009 23:53:59 +0000 (UTC) (envelope-from scf@FreeBSD.org)
Received: from mail.farley.org (mail.farley.org [IPv6:2001:470:1f0f:20:2::11])
	by mx1.freebsd.org (Postfix) with ESMTP id 2E9778FC15;
	Fri, 13 Mar 2009 23:53:59 +0000 (UTC) (envelope-from scf@FreeBSD.org)
Received: from thor.farley.org (HPooka@thor.farley.org
	[IPv6:2001:470:1f0f:20:1::5])
	by mail.farley.org (8.14.3/8.14.3) with ESMTP id n2DNrv4b032692;
	Fri, 13 Mar 2009 18:53:57 -0500 (CDT) (envelope-from scf@FreeBSD.org)
Date: Fri, 13 Mar 2009 18:53:57 -0500 (CDT)
From: "Sean C. Farley" <scf@FreeBSD.org>
To: Bruce Simpson <bms@incunabulum.net>
In-Reply-To: <49BAE99B.2030508@incunabulum.net>
Message-ID: <alpine.BSF.2.00.0903131837040.7444@thor.farley.org>
References: <alpine.BSF.2.00.0903122048230.1704@thor.farley.org>
	<188BDECC-1064-4421-8687-F9759E25FD1F@freebsd.org>
	<alpine.BSF.2.00.0903130918400.4522@thor.farley.org>
	<49BAE99B.2030508@incunabulum.net>
User-Agent: Alpine 2.00 (BSF 1167 2008-08-23)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
X-Spam-Status: No, score=-3.0 required=3.0 tests=AWL,BAYES_00,NO_RELAYS
	autolearn=ham version=3.2.5
X-Spam-Checker-Version: SpamAssassin 3.2.5 (2008-06-10) on mail.farley.org
Cc: freebsd-net@FreeBSD.org, Rui Paulo <rpaulo@FreeBSD.org>
Subject: Re: tap(4) SIOCSIFMTU patch
X-BeenThere: freebsd-net@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Networking and TCP/IP with FreeBSD <freebsd-net.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-net>,
	<mailto:freebsd-net-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-net>
List-Post: <mailto:freebsd-net@freebsd.org>
List-Help: <mailto:freebsd-net-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-net>,
	<mailto:freebsd-net-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Fri, 13 Mar 2009 23:53:59 -0000

On Fri, 13 Mar 2009, Bruce Simpson wrote:

> Sean C. Farley wrote:
>> 
>> Yes, this fixes my issue with bridging a tap device to an interface 
>> with an MTU higher than 1500.  I will probably commit this patch this 
>> weekend.
>
> I can't think of any reason why not, other than you might want to 
> ensure that tap's MTU is bounded within reasonable limits, 'cause yoi 
> don't want to exhaust the jumbo cluster pool if say mtu is more than 
> 9000.

I was letting ifhwioctl() perform the MTU limit check.  It insures: 
IF_MINMTU <= ifr->ifr_mtu <= IF_MAXMTU

I admitted to being new.  See!  :)

Exhausting the jumbo cluster pool refers to kern.ipc.nmbjumbo[p|9|16], 
yes?  em(4) has an upper limit of 16114 for MTU.  I could limit the MTU 
to TAPMRU (16384) which is the limit for a write to the driver anyway.

> I think ifconfig already performs such a check but you might want to double 
> check.

I noticed that ifconfig can report JUMBO_MTU, but few drivers actually 
flag it.  Should I set this for tap?

Sean
-- 
scf@FreeBSD.org