Date: Wed, 20 Dec 2023 13:27:06 +0100 From: tuexen@freebsd.org To: "Scheffenegger, Richard" <rscheff@freebsd.org> Cc: FreeBSD Net <freebsd-net@freebsd.org> Subject: Re: TSO + ECN Message-ID: <9A07A31E-F5FF-4769-8A3A-3987F67AEBA5@freebsd.org> In-Reply-To: <1ad59f9a-1f8b-4e4b-8a78-5293d9e77776@freebsd.org> References: <1ad59f9a-1f8b-4e4b-8a78-5293d9e77776@freebsd.org>
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> On Dec 20, 2023, at 12:15, Scheffenegger, Richard = <rscheff@freebsd.org> wrote: >=20 > Hi, >=20 > I am curious if anyone here has expirience with the handling of ECN in = TSO-enabled drivers/hardware... Some data pointer if I read the specification correctly. Have a look at the specification of the 10GBit/sec card ix: https://cdrdv2-public.intel.com/331520/82599-datasheet-v3-4.pdf According to section 7.2.4 and 8.2.3.9.3 and 8.2.3.9.4 the * first segment gets all flags except PSH and FIN. * middle segments get all flags except PSH and FIN. * last segment gets all flags except the CWR. I think you should be able to change the masks. Best regards Michael >=20 > The other day I found that the virtio driver would bail out with = ENOTSUP when encountering the TCP CWR header bit on a TSO-enabled flow, = when the host does not also claim ECN-support for TSO. >=20 > But this made me wonder, how the expected behavior is. >=20 > Presumably, this means that the hardware (or driver) would clear the = CWR bit after the first packet is sent, correct? >=20 > However, in light of the upcoming AccECN signalling protocol, that is = not what TSO should be doing (with AccECN, all segments should retain = the exact same header flags, maybe expect PSH). >=20 > Probably "non-ECN" capable TSO offload would actually work better with = AccECN - and if the above behavior is what ECN-aware TSO is doing, = AccECN sessions would need to somehow work around that (e.g. = spoon-feeding any segment with CWR set individually - e.g. bypassing the = TSO capabilities in tcp_output)? >=20 >=20 > Would appreciate any feedback around this... >=20 > Best regards, > Richard > <OpenPGP_0x17BE5899E0B1439B.asc>
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