netdev - Re: [PATCH RFC net-next] inet: add ip_retry_random_port sysctl to reduce sequential port retries

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Message-ID: <CANn89iK3Pu2NXxejTSLF-7MhBc03_ZJUjOtZcTC4nMsbsDpbSw@mail.gmail.com>
Date: Fri, 6 Feb 2026 18:09:32 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Fernando Fernandez Mancera <fmancera@...e.de>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com, horms@...nel.org, corbet@....net, ncardwell@...gle.com, 
	kuniyu@...gle.com, dsahern@...nel.org, idosch@...dia.com, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Thorsten Toepper <thorsten.toepper@....com>
Subject: Re: [PATCH RFC net-next] inet: add ip_retry_random_port sysctl to
 reduce sequential port retries

On Fri, Feb 6, 2026 at 5:28 PM Fernando Fernandez Mancera
<fmancera@...e.de> wrote:
>
>
>
> It makes sense. I have tested this approach and we got a more even
> distribution of source ports when having thousands of reserved ports. No
> difference at all when not using reserved ports.
>
> Please, you can find the distribution graph with the current algorithm
> [1] and with the random step algorithm [2].
>
> While I understand that this approach is introducing a call to
> get_random_u32_below() on every connect, I am wondering if it makes
> sense to replace the existing algorithm with this variant. What do you
> think?

I would ask RFC 6056 experts like Fernando Gont what they think.

Note that if we use random at each connect(), we defeat one of the principles
of ephemeral port selection : try very hard to avoid 4-tuple collision.

>
> Please, notice the implementation below. I plan to send an official v1
> once net-next is open. In addition, I am rewriting the commit message as
> I find the current one confusing.
>
> [1] https://0xffsoftware.com/port_graph_current_alg.html
>
> [2] https://0xffsoftware.com/port_graph_random_step_alg.html