Safari on IPv6 ?

Shane Kerr shane at
Tue Feb 2 15:20:27 CET 2010


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.


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.

> 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. ;)


More information about the ipv6-ops mailing list