Date: Sun, 15 Jul 2012 22:40:42 -0400 From: Jason Hellenthal <jhellenthal@dataix.net> To: George Neville-Neil <gnn@freebsd.org> Cc: Navdeep Parhar <np@freebsd.org>, net@freebsd.org Subject: Re: Interface MTU question... Message-ID: <20120716024042.GA90389@DataIX.net> In-Reply-To: <096E1D9F-6F88-4063-B59C-34E94E17138D@freebsd.org> References: <86liiqrnnq.wl%gnn@neville-neil.com> <4FFDF6C7.3030301@FreeBSD.org> <C06D346A-97BE-4498-B4E5-0ED85731A8BD@freebsd.org> <20120712165502.GA61341@DataIX.net> <096E1D9F-6F88-4063-B59C-34E94E17138D@freebsd.org>
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--9amGYk9869ThD9tj Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Filed as, http://www.freebsd.org/cgi/query-pr.cgi?pr=3D169898 Thanks for looking into this when you get time. On Thu, Jul 12, 2012 at 04:49:23PM -0400, George Neville-Neil wrote: >=20 > On Jul 12, 2012, at 12:55 , Jason Hellenthal wrote: >=20 > >=20 > >=20 > > On Thu, Jul 12, 2012 at 10:55:16AM -0400, George Neville-Neil wrote: > >>=20 > >> On Jul 11, 2012, at 17:57 , Navdeep Parhar wrote: > >>=20 > >>> On 07/11/12 14:30, gnn@freebsd.org wrote: > >>>> Howdy, > >>>>=20 > >>>> Does anyone know the reason for this particular check in > >>>> ip_output.c? > >>>>=20 > >>>> if (rte !=3D NULL && (rte->rt_flags & (RTF_UP|RTF_HOST))) { > >>>> /* > >>>> * This case can happen if the user changed the MTU > >>>> * of an interface after enabling IP on it. Because > >>>> * most netifs don't keep track of routes pointing to > >>>> * them, there is no way for one to update all its > >>>> * routes when the MTU is changed. > >>>> */ > >>>> if (rte->rt_rmx.rmx_mtu > ifp->if_mtu) > >>>> rte->rt_rmx.rmx_mtu =3D ifp->if_mtu; > >>>> mtu =3D rte->rt_rmx.rmx_mtu; > >>>> } else { > >>>> mtu =3D ifp->if_mtu; > >>>> } > >>>>=20 > >>>> To my mind the > ought to be !=3D so that any change, up or down, of= the > >>>> interface MTU is eventually reflected in the route. Also, this code > >>>> does not check if it is both a HOST route and UP, but only if it is > >>>> one other the other, so don't be fooled by that, this check happens > >>>> for any route we have if it's up. > >>>=20 > >>> I believe rmx_mtu could be low due to some intermediate node between = this host and the final destination. An increase in the MTU of the local i= nterface should not increase the path MTU if the limit was due to someone e= lse along the route. > >>=20 > >> Yes, it turns out to be complex. We have several places that store th= e MTU. There is the interface, > >> which knows the MTU of the directly connected link, a route, and the h= ost cache. All three of these > >> are used to determine the maximum segment size (MSS) of a TCP packet. = The route and the interface > >> determine the maximum MTU that the MSS can have, but, if there is an e= ntry in the host cache > >> then it is preferred over either of the first two. See tcp_update_mss= () in tcp_input.c to > >> see what I'm talking about. > >>=20 > >> I believe that the quoted code above has been wrong from the day it wa= s written, in that what it > >> really says is "if the route is up" and not "if the route is up and is= a host route" which is > >> what I believe people to read that as. If the belief is that this cod= e is really only there for > >> hosts routes, then the proper fix is to make the sense of the first if= match that belief > >> and, again, to change the > to !=3D so that when the administrator of = the box bumps the MTU in > >> either direction that the route reflects this. It is not possible for= PMTU on a single link > >> to a host route to bump the number down if the interface says it's not= to be bumped. And, > >> even so, any host cache entry will override and avoid this code. > >>=20 > >=20 > > Something else to look into ...=20 > >=20 > > # ifconfig lagg0 mtu 1492 > > ifconfig: ioctl (set mtu): Invalid argument > >=20 > > This is on stable/8 r238264 when the interface was up/up and down/down > >=20 > > Also attempted on the member interfaces dc0 and dc1 > >=20 >=20 > Can you file a bug on that one? >=20 > Best, > George >=20 --=20 - (2^(N-1)) --9amGYk9869ThD9tj Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- iQEcBAEBAgAGBQJQA38pAAoJEBSh2Dr1DU7WvJQH/35b7E7xtWAHmUB3HtalxeMc W4JLmWeIXey0uiBprk7dtRnqzFdUq6JVg/yU7XzL9k8HwNsnwuThpHEKI2ZbTpYq 1Jk8d8EcSlzF7QOU08JQuyy88rX0t81uBCgEd7XmdOo5cYmoqIOEMl5RMPY3+xRQ qJsO6zkDpDmrfDhBxX4DmzK5ixQ2ANzhz6PmrMgvrpuxSkCLz7dS/gfnnhwvEjad yOzva9XUPLoAz4VIJRyhHs4YS6lhPhdoI/igGguye6RiCVgl2CnA4hl6ISBcqX1p 6mRUeuUNzStaEKNd2RqOR6t64IGXDqXHP+QBvuJe4MlZSN17VtfjEXd6r3hep9Q= =XcGA -----END PGP SIGNATURE----- --9amGYk9869ThD9tj--
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