Apple snuggles closer to IPv6

iOS 9 and El Capitan will try to drive traffic to v6 hosts

With the latest public betas of iOS 9 and the already-patched “El Capitan” OS X 10.11, Apple is leaning further towards IPv6.

In a post to the IETF's v6ops mailing list, Apple CoreOS networking engineer David Schinazi says the new releases include a re-vamped version of the three-year-old Google-authored “Happy Eyeballs” RFC.

Apple's been a long-time supporter of IPv6, having first introduced support for the protocol in 2006, but we live in a dual-stack world. Happy Eyeballs was designed to let clients decide whether to use IPv4 or v6, depending on which path to a server offered the best performance (chiefly based on latency).

Tweaks to the implementation in the latest betas now favour IPv6 connections, Schinazi writes: “Based on our testing, this makes our Happy Eyeballs implementation go from roughly 50/50 IPv4/IPv6 in iOS 8 and Yosemite to ~99% IPv6 in iOS 9 and El Capitan betas.”

That's because “biasing towards IPv6 is now beneficial”, he writes. With the still-slow-but-accelerating mainstreaming of the new protocol, there aren't so many broken tunnels, and IPv6 performance “may even be better on average”.

Of course, it only works at all if the user has a connection that is somehow IPv6-capable.

Apple's implementation first uses DNS to check availability of a remote host over IPv6. Since the idea is to have a bias in favour of IPv6, the Happy Eyeballs algorithm no longer defaults to the first DNS response received.

Instead, if the first responder is IPv4, the algorithm sets a 25 ms timer to see if a response comes back from an IPv6 host.

The latency test is still there, but with a 25 ms window: “This algorithm uses historical RTT data to prefer addresses that have lower latency - but has a 25ms leeway: if the historical RTT of two compared address are within 25ms of each other, we use RFC3484 [IPv6 default address selection – Editor] to pick the best one”.

“If this behaviour proves successful during the beta period, you should expect more IPv6 traffic from Apple products in the future”, Schinazi writes. ®

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