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

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Message-ID: <641a1a66-705e-46ee-9612-db35d92729cf@suse.de>
Date: Mon, 9 Feb 2026 16:25:54 +0100
From: Fernando Fernandez Mancera <fmancera@...e.de>
To: longxie86@...tonmail.com
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 2/9/26 2:53 PM, longxie86@...tonmail.com wrote:
> 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?
> 

This just an RFC, I discourage using it in production yet. An official 
v1 will be sent once net-next is open and there it needs to be reviewed 
and approved by the maintainers.

> Please bring this to the stable versions.
> 

I don't think that will happen. This is an improvement not a "fix" per 
definition. Anyway, you could ask for a backport to your 
vendor/distribution.

Thanks,
Fernando.

>>>> 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
>>
>>