Date: Tue, 11 Apr 2017 22:02:23 +0300 From: "Andrey V. Elsukov" <bu7cher@yandex.ru> To: rgrimes@freebsd.org Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r316699 - head/sys/net Message-ID: <7b4a8622-d7cc-a597-846e-0bdf5519a5d7@yandex.ru> In-Reply-To: <201704111849.v3BInuuB075892@pdx.rh.CN85.dnsmgr.net> References: <201704111849.v3BInuuB075892@pdx.rh.CN85.dnsmgr.net>
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From: "Andrey V. Elsukov" <bu7cher@yandex.ru>
To: rgrimes@freebsd.org
Cc: src-committers@freebsd.org, svn-src-all@freebsd.org,
svn-src-head@freebsd.org
Message-ID: <7b4a8622-d7cc-a597-846e-0bdf5519a5d7@yandex.ru>
Subject: Re: svn commit: r316699 - head/sys/net
References: <201704111849.v3BInuuB075892@pdx.rh.CN85.dnsmgr.net>
In-Reply-To: <201704111849.v3BInuuB075892@pdx.rh.CN85.dnsmgr.net>
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On 11.04.2017 21:49, Rodney W. Grimes wrote:
>>>> -#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;=20
>>> Or what ever the approprite sizeof(foo) is;
>>
>> I just returned back constants that were here :)
>=20
> Can we try to improve on this, or maybe just ignore it?
There are a lot of things that we need and can improve, but usually we
choose the most interested or important.
>=20
>> The man page says:
>> "The MTU of gre interfaces is set to 1476 by default, to match the
>> value used by Cisco routers."
>=20
> That might need some clean up too?
>=20
>>> 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 valu=
e
>> 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.
>=20
> I may some day want to #define ETHERMTU 9000 and just have all the righ=
t
> magic happen, wouldnt that be nice?
It would be nice.
>>>> static const char grename[] =3D "gre";
>>>> static MALLOC_DEFINE(M_GRE, grename, "Generic Routing Encapsulation=
");
>>>> static VNET_DEFINE(struct mtx, gre_mtx);
>>>> @@ -173,7 +173,7 @@ gre_clone_create(struct if_clone *ifc, i
>>>> GRE2IFP(sc)->if_softc =3D sc;
>>>> if_initname(GRE2IFP(sc), grename, unit);
>>>> =20
>>>> - GRE2IFP(sc)->if_mtu =3D sc->gre_mtu =3D GREMTU;
>>>> + GRE2IFP(sc)->if_mtu =3D GREMTU;
>>>> GRE2IFP(sc)->if_flags =3D IFF_POINTOPOINT|IFF_MULTICAST;
>>>> GRE2IFP(sc)->if_output =3D gre_output;
>>>> GRE2IFP(sc)->if_ioctl =3D gre_ioctl;
>>>> @@ -231,7 +231,8 @@ gre_ioctl(struct ifnet *ifp, u_long cmd,
>>>> /* XXX: */
>>>> if (ifr->ifr_mtu < 576)
>>>
>>> This is another magic constant that should be in a #define.
>>
>> It was here too. I think it is something from the past and can be remo=
ved.
>>
>> From RFC791: "It is recommended that hosts only send datagrams larger
>> than 576 octets".
>=20
> No, I doubt we want to remove that, it is also part of ipv6 documentati=
on,
> and iirc part of some BCP's. You really dont want to be running on the=
> internet with MTU < 576. It use to be 296 in the days when we had SLIP=
=2E
Why not? We have many things that allow foot shooting.
=3D=3D=3D=3D=3D=3D
>>>> --- head/sys/net/if_me.c Tue Apr 11 08:29:12 2017 (r316698)
>>>> +++ head/sys/net/if_me.c Tue Apr 11 08:56:18 2017 (r316699)
>>>> @@ -66,7 +66,7 @@ __FBSDID("$FreeBSD$");
>>>> #include <machine/in_cksum.h>
>>>> #include <security/mac/mac_framework.h>
>>>> =20
>>>> -#define MEMTU 1500
>>>> +#define MEMTU (1500 - sizeof(struct mobhdr))
>>>
>>> 1500 -> ETHERMTU?
>>
>> Feel free to fix this. Many pseudo interfaces has similar MTU limitati=
ons.
--=20
WBR, Andrey V. Elsukov
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