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

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Message-ID: <d76cd8c3-deb0-4f39-b5b9-a711d155cd5f@suse.de>
Date: Mon, 9 Feb 2026 12:56:57 +0100
From: Fernando Fernandez Mancera <fmancera@...e.de>
To: Eric Dumazet <edumazet@...gle.com>
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 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.

>>
>> 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
>