From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 57FCFC433EF for ; Thu, 16 Jun 2022 16:42:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236505AbiFPQmV (ORCPT ); Thu, 16 Jun 2022 12:42:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43098 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233404AbiFPQmU (ORCPT ); Thu, 16 Jun 2022 12:42:20 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 612D331DD4 for ; Thu, 16 Jun 2022 09:42:19 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id B5E89614B9 for ; Thu, 16 Jun 2022 16:42:18 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C67AAC34114; Thu, 16 Jun 2022 16:42:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1655397738; bh=5ZgX5XSLx4LI17piqGeL2ETC/NI8C2a9qc9/VELwp70=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=IKZsedNXR3zzpjp8WBazltOSiDczefhFVVaEf8IJGcvTmLRjuXZZ2kBAWhOgAMrtZ +WnxprLamTGWUb+pHbXe8W+ANgNiWut+9tasYIdVH1RcqzozZx7Yh8je17GdHrlgUG fcIfWZytHCDPSMYkUCGJIzdqzuzHfppmbhHTNesmtA+vln/bXQnSpEJgsGZk+5/4qp HGbr71psILEk+1sh1ag9Injzs4vmgufn6lDZ0xKroPDYOk3CZWjHrV3fbaKVX48JMO 0Sxwnjn5txsqLS9abLkIK8Nl1TAaqrHGk1xf7yKkib5ws9BkiOlvcitNAAGwnT617Y V1XObLjtDApsw== Date: Thu, 16 Jun 2022 09:42:16 -0700 From: Jakub Kicinski To: Maciej =?UTF-8?B?xbtlbmN6eWtvd3NraQ==?= Cc: "Subash Abhinov Kasiviswanathan (KS)" , "David S. Miller" , David Ahern , Hideaki YOSHIFUJI , Linux NetDev , Stefano Brivio , Kaustubh Pandey , Sean Tranchetti Subject: Re: [PATCH net v2 1/2] ipv6: Honor route mtu if it is within limit of dev mtu Message-ID: <20220616094216.3bc9aef2@kernel.org> In-Reply-To: References: <1655182915-12897-1-git-send-email-quic_subashab@quicinc.com> <1655182915-12897-2-git-send-email-quic_subashab@quicinc.com> <20220615173516.29c80c96@kernel.org> <132e514e-bad8-9f73-8f08-1bd5ac8aecd4@quicinc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Thu, 16 Jun 2022 00:33:02 -0700 Maciej =C5=BBenczykowski wrote: > On Wed, Jun 15, 2022 at 10:36 PM Subash Abhinov Kasiviswanathan (KS) wrote: > > >> CC maze, please add him if there is v3 > > >> > > >> I feel like the problem is with the fact that link mtu resets protoc= ol > > >> MTUs. Nothing we can do about that, so why not set link MTU to 9k (or > > >> whatever other quantification of infinity there is) so you don't have > > >> to touch it as you discover the MTU for v4 and v6? =20 > > > > That's a good point. =20 >=20 > Because link mtu affects rx mtu which affects nic buffer allocations. > Somewhere in the vicinity of mtu 1500..2048 your packets stop fitting > in 2kB of memory and need 4kB (or more) I was afraid someone would point that out :) Luckily the values Subash mentioned were both under 2k, and hope fully the device can do scatter?=20 =F0=9F=A4=9E=F0=9F=98=9F (Don't modems do LRO or some other form of aggrega= tion usually?) > > >> My worry is that the tweaking of the route MTU update heuristic will > > >> have no end. > > >> > > >> Stefano, does that makes sense or you think the change is good? =20 > > > > The only concern is that current behavior causes the initial packets > > after interface MTU increase to get dropped as part of PMTUD if the IPv6 > > PMTU itself didn't increase. I am not sure if that was the intended > > behavior as part of the original change. Stefano, could you please conf= irm? > > =20 > > > I vaguely recall that if you don't want device mtu changes to affect > > > ipv6 route mtu, then you should set 'mtu lock' on the routes. > > > (this meaning of 'lock' for v6 is different than for ipv4, where > > > 'lock' means transmit IPv4/TCP with Don't Frag bit unset) =20 > > > > I assume 'mtu lock' here refers to setting the PMTU on the IPv6 routes > > statically. The issue with that approach is that router advertisements > > can no longer update PMTU once a static route is configured. =20 >=20 > yeah. Hmm should RA generated routes use locked mtu too? > I think the only reason an RA generated route would have mtu information > is for it to stick... If link MTU is lower than RA MTU do we do min() or ignore the RA MTU?