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Date:   Sat, 22 Oct 2022 12:51:26 -0700
From:   Cong Wang <>
To:     "J.J. Mars" <>
Subject: Re: Confused about ip_summed member in sk_buff

On Fri, Oct 21, 2022 at 02:29:26PM +0800, J.J. Mars wrote:
> Hi everyone, I'm new here and I hope this mail won't disturb you :)
> Recently I was working with something about ip_summed, and I'm really
> confused about the question what does ip_summed exactly mean?
> This member is defined with comment Driver fed us an IP checksum'. So
> I guess it's about IP/L3 checksum status.
> But the possible value of ip_summed like CHECKSUM_UNNECESSARY is about L4.
> What confused me a lot is ip_summed seems to tell us the checksum of
> IP/L3 layer is available from its name.
> But it seems to tell us the checksum status of L4 layer from its value.
> Besides, in ip_rcv() it seems the ip_summed is not used before
> calculating the checksum of IP header.
> So does ip_summed indicate the status of L3 checksum status or L4
> checksum status?
> If L4, why is it named like that?

The name itself is indeed confusing, however, there are some good
explanations in the code, at the beginning of include/linux/skbuff.h. I
think that could help you to clear your confusions here.


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