Safari on IPv6 ?

Sam Wilson Sam.Wilson at
Tue Feb 2 18:25:57 CET 2010

On 2 Feb 2010, at 14:20, Shane Kerr wrote:

> Martin,
> On 2010-02-02 14:13, Martin Millnert wrote:
>> On Tue, 2010-02-02 at 13:57 +0100, Shane Kerr wrote:
>>> As I understand it, Safari does the equivalent of:
>>> 1. DNS lookup
>>> 2. TCP connection
>>> 3. HTTP request
>>> In both IPv4 and IPv6 at the same time. Whichever gets to step #3
>>> first goes to completion "wins", and the other is canceled.
>>> Note that once the A and AAAA record are in the DNS cache, then you
>>> are really just looking at connecting on whichever TCP session opens
>>> first.
>> Shane,
>> this might very well be their goal, but it doesn't seem to be what's
>> going on, since Ron showed us on Monday that the mDNSResponder just
>> shuts down the remaining queries (one lingering AAAA in the example),
>> after having received its first reply.
> Right.
> Even when (if?) they fix mDNSResponder not to be horribly broken, you
> will still have non-deterministic behavior, depending on whether your
> IPv4 or IPv6 connection happens to be faster. FWIW, I think this is
> actually the right thing to do, much as it sucks for network engineers

I've been pondering whether it would make sense to cache some kind of
preference probe occasionally, something like BIND does to check
alternative name servers.  It may be that web traffic doesn't lend  
to that approach - too many different servers to make it worth while.

>> In other words, the actual implementation, intended or not, is  
>> right now
>> that whoever accomplishes #1 first, may continue.
>> I would be more inclined to agree with that whoever performs #3 first
>> wins, than #1.  DNS lookup speed says nothing definite about IPv4 vs
>> IPv6 network/routing topologies and round trip times.
> Well, #2. You really don't want to actually ask for the web page  
> twice,
> because this can mean a lot of work for the web server if you've got a
> dynamic page, and is a bit unfriendly. Even Apple has limits as to how
> much they are willing to abuse the network to improve the user
> experience. ;)

Is there a way to extract information from web server logs or the like  
see whether there's a significant number of dual-stacked Macs making
double connections?  (Or perhaps that's masked by the mDNSResponder  

Sam Wilson
Network Team, IT Infrastructure
Information Services, The University of Edinburgh
Edinburgh, Scotland, UK

The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.

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