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

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Message-ID: <egIQQeDF4RIP_614tU-XIK-01DgThdPOU3KA253uNHvZv76zymXyBFhey1G7wj5mA2nwHpdT7VBoGuFnATSO-PS0hUb0Qrt0AyN5JNbpATg=@protonmail.com>
Date: Mon, 09 Feb 2026 13:53:01 +0000
From: longxie86@...tonmail.com
To: Fernando Fernandez Mancera <fmancera@...e.de>
Cc: Eric Dumazet <edumazet@...gle.com>, 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 Monday, February 9th, 2026 at 12:57 PM, Fernando Fernandez Mancera <fmancera@...e.de> wrote:

> 
> 
> On 2/6/26 6:09 PM, Eric Dumazet wrote:
> 
> > 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.
> 
> 
> Right. I will reach out to him and get his opinion. I have plenty of
> time before net-next open again. I am also collecting some metrics
> regarding the 4-tuple collision frequency.
> 

We have had this problem in AWS for a long time. The patch works on our system. What is needed for it to be included in the next Linux release?

Please bring this to the stable versions.

> > > 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
> 
>