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

[Fernando Fernandez Mancera]

On 2/6/26 6:09 PM, Eric Dumazet wrote:
> On Fri, Feb 6, 2026 at 5:28 PM Fernando Fernandez Mancera
> <fmancera@xxxxxxx> 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