netdev - Re: [PATCH v2] tcp: Add TCP_INFO counter for packets received out-of-order

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Date:   Tue, 17 Sep 2019 10:21:48 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jason Baron <jbaron@...mai.com>
Cc:     Thomas Higdon <tph@...com>, netdev <netdev@...r.kernel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Dave Jones <dsj@...com>
Subject: Re: [PATCH v2] tcp: Add TCP_INFO counter for packets received out-of-order

 Tue, Sep 17, 2019 at 10:13 AM Jason Baron <jbaron@...mai.com> wrote:
>
>
> Hi,
>
> I was interested in adding a field to tcp_info around the TFO state of a
> socket. So for the server side it would indicate if TFO was used to
> create the socket and on the client side it would report whether TFO
> worked and if not that it failed with maybe some additional states
> around why it failed. I'm thinking it would be maybe 3 bits.
>
> My question is whether its reasonable to use the unused bits of
> __u8    tcpi_delivery_rate_app_limited:1;. Or is this not good because
> the size hasn't changed? What if I avoided using 0 for the new field to
> avoid the possibility of not knowing if 0 because its the old kernel or
> 0 because that's now its a TFO state? IE the new field could always be >
> 0 for the new kernel.
>

I guess that storing the 'why it has failed' would need more bits.

I suggest maybe using an event for this, instead of TCP_INFO ?

As of using the bits, maybe the monitoring application does not really care
if running on an old kernel where the bits would be zero.

Commit eb8329e0a04db0061f714f033b4454326ba147f4 reserved a single
bit and did not bother about making sure the monitoring would detect if this
runs on an old kernel.