Re: [v6ops] Happy eyeballs suggestions, was: Re: Apple and IPv6, a few clarifications

james woodyatt <> Mon, 22 June 2015 21:26 UTC

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From: james woodyatt <>
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Date: Mon, 22 Jun 2015 14:26:51 -0700
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To: Iljitsch van Beijnum <>, "" <>
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Subject: Re: [v6ops] Happy eyeballs suggestions, was: Re: Apple and IPv6, a few clarifications
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On Jun 22, 2015, at 13:51, Iljitsch van Beijnum <> wrote:
> On 22 Jun 2015, at 18:53, Erik Nygren <> wrote:
>> Having the NAT64/DNS64 environment is great for IPv6-only testing, but if it doesn't provide the ability to get through to real IPv6 content/resources when the MacOS host has IPv6 connectivity (either natively or via a tunnel) then it could be counter-productive in some circumstances and could discourage app writers from developing their apps to work well in scenarios where true end-to-end IPv6 connectivity is possible.
> Hm, what kind of stuff would work through NAT64 that wouldn't work with real IPv6 connectivity?
> (Thinking of stuff that _would_ work with real IPv6 but not NAT64 is easier.)

An obvious example would be a UDP application that depends on a 1500 octet path MTU to all the IPv6 destinations it cares about. The NAT64 will shave a 1500 octet packet down to 1480, which might make it everywhere you want to go over IPv4 without needing to handle ICMPv6 Packet Too Big Errors, leaving you to discover later that native IPv6 destinations require you to deal with paths that are 1492 octets and not 1500 octets.

I’m sure if I think for a few minutes, I might be able to come up with another one. There are probably TCP cases like this too given the way ICMP6 filters seem to be so terribly popular despite the guidance our humble working group tries to provide.