Subject: Re: [PATCH net-next-2.6] ipv6: make fragment identifications less predictable

From: Fernando Gont <[email-masked]>
Date: Thu, 21 Jul 2011 22:18:09 -0300

On 07/21/2011 09:34 PM, Rick Jones wrote:
>> That scenario assumes packet reordering and/or packet loss.
> 
> Isn't that a given?  

I mean that if these "collisions" of Identification numbers are of
concern, then, then, at such bandwidth rates, fragmentation itself would
be a concern.

Chances of collisions are proportional to reordering and losses. That
means that at such bandwidths, you'd need to be able to queue a huge
number of packets (which you might not be able to queue, becacuse of
lack of resources), etc.

> And indeed, fragmentation is considered bad, and was considered bad
> enough that the "revenge of the router guys" that is IPv6 punted it to
> the end systems, and yes, one should use PMTUD. Which is all well and
> good when 999 times out of 1 traffic is flowing over a transport that
> does its own segmentation and reassembly.  

And provided that there's no ICMPv6 filtering out there (which there is)
-- at which point you need to implement some for of blackhole detection
a la PLMPTUD.

> And when IPv6 got spec'ed it
> looked to all the world that UDP was on the way out - NFS was migrating
> over to TCP, and DNS was "never" more than 512 byte messages. No problem
> right?  But since then we've gotten things like EDNS which will be
> sending DNS messages in UDP datagrams that will have to be fragmented,
> PMTUD notwithstanding.

Hopefully you won't have the aforementioned 40GB traffic rate between
two DNS servers ;-)

Thanks,
-- 
Fernando Gont
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