[AusNOG] IPv6: Who's dual stacked? Why don't I look stacked?
gih at apnic.net
Fri Mar 8 10:18:59 EST 2013
On 08/03/2013, at 10:10 AM, Mark Andrews <marka at isc.org> wrote:
>> For that matter, really of what apps do and do not
>> support IPv6 transit and which do not, and even if an app (a mail client
>> for example) supports IPv6, whether the server (Internode's mail server for
>> example) supports IPv6. And finally, if everything supports IPv6, what
>> priority is given to IPv6 over IPv4 (or vice versa).
> IPv6 is given priority over IPv4 for most things.
Thats not always the case any more.
Various implementations of "happy eyeballs" out there today use algorithms that have different embedded heuristics as to which protocol to select when given a choice.
The old algorithm of "Try V6 and wait for the TCP connection to fail, and if it does then try V4" imposed either slow (20s, Windows), tediously slow (75s, FreeBSD) or catastrophically slow (108s, Linux) delays when the IPv6 connection was unable to complete. These days many browser/OS combinations do the dual protocol dance in parallel and end up using the fastest protocol rather than always just preferring to initiate a connection on IPv6 whenever possible.
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