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[v6ops] Re: [DNSOP] Re: draft-hinden-v6ops-dns

Brian E Carpenter <brian.e.carpenter@gmail.com> Sun, 23 June 2024 20:09 UTC

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Date: Mon, 24 Jun 2024 08:09:13 +1200
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To: David Farmer <farmer@umn.edu>
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Subject: [v6ops] Re: [DNSOP] Re: draft-hinden-v6ops-dns
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On 24-Jun-24 04:40, David Farmer wrote:
> 
> 
> On Thu, Jun 20, 2024 at 3:23 PM Brian E Carpenter <brian.e.carpenter@gmail.com <mailto:brian.e.carpenter@gmail.com>> wrote:
> 
>       That doesn't mean that recommending 1400
>     or 1500 is wrong, if there is *certainty* that IPv6-in-IPv6 is absent
>     in a particular deployment scenario.
> 
>      > this number should be stamped out everywhere. 1232
>      > likewise. the safe-to-use effective MTU (1500)
> 
>     In the general case, it still isn't safe.
> 
> 
> Shouldn't anywhere that 1500 bytes is known to be unsafe have an appropriate MTU advertised in the RA announced to that network?
> 
> Also, 1400 leaves 48 bytes for a tunnel or optional headers. It's not enough for nested tunnels but seems enough for a basic IPv6-in-IPv6 tunnel. If that is not the case, how many more bytes are needed for a basic tunnel?
> 
> IPv6 DNS servers don't need to assume the worst possible scenario for MTU when sending UDP DNS packets. I believe they can be optimistic and use 1400 unless they have direct knowledge otherwise, and IPv6 provides a very usable mechanism to distribute such knowledge: the MTU option in the RA.

I don't understand how that helps if there is a 1280-byte bottleneck in the middle of the path from the DNS responder to its client, unknown to either end. I fully agree that this is an unusual situation.

     Brian

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