netdev - Re: veth regression with "don’t modify ip_summed; doing so treats packets with bad checksums as good."

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Date:	Fri, 25 Mar 2016 07:35:35 -0700
From:	Ben Greear <greearb@...delatech.com>
To:	Vijay Pandurangan <vijayp@...ayp.ca>
CC:	Cong Wang <xiyou.wangcong@...il.com>,
	netdev <netdev@...r.kernel.org>, Evan Jones <ej@...njones.ca>,
	Cong Wang <cwang@...pensource.com>
Subject: Re: veth regression with "don’t modify ip_summed; doing so treats packets with bad checksums as good."



On 03/24/2016 10:24 PM, Vijay Pandurangan wrote:
> On Fri, Mar 25, 2016 at 1:07 AM, Ben Greear <greearb@...delatech.com> wrote:
>> On 03/24/2016 09:45 PM, Vijay Pandurangan wrote:
>>>
>>> Actually, maybe they should be set to CHECKSUM_PARTIAL if we want veth
>>> to drop the packets if they have bad checksums before they hit the
>>> application level.
>>
>>
>> VETH is pretty special in that when you transmit a frame on one
>> device, it's pair receives it, and unless there is RAM corruption
>> or bugs in the kernel, then it cannot be corrupted.
>
> Yeah, you're right that that's an optimization. However, I think that
> we should first ensure that
>
> a->veth->b
>
> operates exactly like:
>
> a->physical eth 1 -> physical eth 2->b
>
> in all cases.  Once we have that working everywhere we could think
> about optimizations.
>
>
> If we're willing to refactor, we could implement the optimization by
> allowing veth devices to know whether their immediate peer is. If a
> veth knows it's talking to another veth, it could under some
> circumstances elide checksum calculation and verification.  I'm not
> sure what abstractions that would break, though. What do you guys
> think?

veth ALWAYS transmits to another VETH.  The problem is that when veth is
given a packet to transmit, it is difficult to know where that packet
came from.

And, adding software checksumming to veth for every frame would be a huge
performance hit.

Thanks,
Ben

-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com