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From:   Eric Dumazet <edumazet@google.com>
Date:   Fri, 16 Apr 2021 13:52:10 +0200
Message-ID: <CANn89iJ5-4u98sGXt6oH5ZbWGFcYCy3T-B+qktvm3-cMkFQXKA@mail.gmail.com>
Subject: Re: [PATCH] tcp: fix silent loss when syncookie is trigered
To:     zhaoya.gaius@bytedance.com
Cc:     David Miller <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Florian Westphal <fw@strlen.de>
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On Fri, Apr 16, 2021 at 12:52 PM zhaoya <zhaoya.gaius@bytedance.com> wrote:
>
> When syncookie is triggered, since $MSSID is spliced into cookie and
> the legal index of msstab  is 0,1,2,3, this gives client 3 bytes
> of freedom, resulting in at most 3 bytes of silent loss.
>
> C ------------seq=12345-------------> S
> C <------seq=cookie/ack=12346-------- S S generated the cookie
>                                         [RFC4987 Appendix A]
> C ---seq=123456/ack=cookie+1-->X      S The first byte was loss.
> C -----seq=123457/ack=cookie+1------> S The second byte was received and
>                                         cookie-check was still okay and
>                                         handshake was finished.
> C <--------seq=.../ack=12348--------- S acknowledge the second byte.


I think this has been discussed in the past :
https://kognitio.com/blog/syn-cookies-ate-my-dog-breaking-tcp-on-linux/

If I remember well, this can not be fixed "easily"

I suspect you are trading one minor issue with another (which is
considered more practical these days)
Have you tried what happens if the server receives an out-of-order
packet after the SYN & SYN-ACK ?
The answer is : RST packet is sent, killing the session.

That is the reason why sseq is not part of the hash key.

In practice, secure connexions are using a setup phase where more than
3 bytes are sent in the first packet.
We recommend using secure protocols over TCP. (prefer HTTPS over HTTP,
SSL over plaintext)

Your change would severely impair servers under DDOS ability to really
establish flows.

Now, if your patch is protected by a sysctl so that admins can choose
the preferred behavior, then why not...