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Message-ID: <1459195293.6473.118.camel@edumazet-glaptop3.roam.corp.google.com>
Date: Mon, 28 Mar 2016 13:01:33 -0700
From: Eric Dumazet <eric.dumazet@...il.com>
To: Rick Jones <rick.jones2@....com>
Cc: Eric Dumazet <edumazet@...gle.com>,
"David S . Miller" <davem@...emloft.net>,
netdev <netdev@...r.kernel.org>,
Tom Herbert <tom@...bertland.com>
Subject: Re: [RFC net-next 2/2] udp: No longer use SLAB_DESTROY_BY_RCU
On Mon, 2016-03-28 at 12:11 -0700, Rick Jones wrote:
> I was under the impression that individual DNS queries were supposed to
> have not only random DNS query IDs but also originate from random UDP
> source ports. https://tools.ietf.org/html/rfc5452 4.5 at least touches
> on the topic but I don't see it making it hard and fast. By section 10
> though it is more explicit:
>
> This document recommends the use of UDP source port number
> randomization to extend the effective DNS transaction ID beyond the
> available 16 bits.
>
> That being the case, if indeed there were to be 30000-odd concurrent
> requests outstanding "upstream" from that location there'd have to be
> 30000 ephemeral ports in play.
Sure, so these heavy duty programs have to consider the randomness
implications already, when picking a port in their pool.
Using the kernel and force socket()/bind()/close() in the hope to get
random values is safe, but never had been optimal for performance.
Note : file structures got RCU freeing back in 2.6.14, and I do not
think named users ever complained about added cost ;)
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