netdev - Re: SKB Reference Question

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Message-ID: <CABGNecySKtpPb2VjqugXYLX4wif=f07nf=F3WZvfc4M0k4CJGA@mail.gmail.com>
Date:   Wed, 1 Nov 2017 12:22:21 -0700
From:   Joe Smith <codesoldier1@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: SKB Reference Question

On Wed, Nov 1, 2017 at 10:33 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2017-11-01 at 10:27 -0700, Joe Smith wrote:
>> How strictly are references on the SKB enforced. For example,
>> tcp_transmit_skb() clones the SKB and adds a TCP header. Can I assume
>> that in case of re-transmission the header added will be there and can
>> be reused instead of creating a new one from scratch. Some fields like
>> time stamp would need to be updated but they should be unmodified.
>
>
> Not sure what you are trying to do, but this seems messy ;)

As an example, consider reusing options that might be expensive to
calculate. Assuming no coalescing.

>
> At rtx time, there is no guarantee that the master skb has not been
> changed, so the content of TCP header might be wrong anyway.

I would have thought that each layer below TCP would only add itś own
header and would not touch anything else. Is that not the guarantee
that SKB references provideś, or else the data could be changed.

Can you give an example why the TCP header could change.

Thanks a lot for your time.




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-- 
JS