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Date: Tue, 11 Apr 2017 22:05:13 +0300
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: rgrimes@freebsd.org
Cc: "Andrey V. Elsukov" <bu7cher@yandex.ru>, svn-src-head@freebsd.org,
 svn-src-all@freebsd.org, src-committers@freebsd.org
Subject: Re: svn commit: r316699 - head/sys/net
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On Tue, Apr 11, 2017 at 11:49:56AM -0700, Rodney W. Grimes wrote:

> [ Charset UTF-8 unsupported, converting... ]
> > On 11.04.2017 19:51, Rodney W. Grimes wrote:
> > >> Modified: head/sys/net/if_gre.c
> > >> ==============================================================================
> > >> --- head/sys/net/if_gre.c	Tue Apr 11 08:29:12 2017	(r316698)
> > >> +++ head/sys/net/if_gre.c	Tue Apr 11 08:56:18 2017	(r316699)
> > >> @@ -88,7 +88,7 @@ __FBSDID("$FreeBSD$");
> > >>  #include <machine/in_cksum.h>
> > >>  #include <security/mac/mac_framework.h>
> > >>  
> > >> -#define	GREMTU			1500
> > >> +#define	GREMTU			1476
> > > 
> > > I would of thought that this was
> > > #define	GREMTU	ETHERMTU
> > > and now should probably be:
> > > #define	GREMTU	ETHERMTU - gre_hlen; 
> > > Or what ever the approprite sizeof(foo) is;
> > 
> > I just returned back constants that were here :)
> 
> Can we try to improve on this, or maybe just ignore it?
> 
> > The man page says:
> >  "The MTU of gre interfaces is set to 1476 by default, to match the
> >   value used by Cisco routers."
> 
> That might need some clean up too?
> 
> > > Isn't this arguably wrong in the face of JumboFrames?
> > >
> > 
> > I don't see why it may be wrong for jumbo frames. The default MTU value
> > used by any interfaces is ETHERMTU, if you know that your NIC supports
> > jumbo frames, you will configure specific MTU value.
> > If you want 9k MTU on gre(4), you should configure it.
> 
> I may some day want to #define ETHERMTU 9000 and just have all the right
> magic happen, wouldnt that be nice?

Not before 2038.