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

Mark Andrews <marka@isc.org> Tue, 23 June 2015 00:59 UTC

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To: Iljitsch van Beijnum <iljitsch@muada.com>
From: Mark Andrews <marka@isc.org>
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In-reply-to: Your message of "Tue, 23 Jun 2015 02:29:52 +0200." <52696645-3449-4AD4-8C80-1D17DFB52F2D@muada.com>
Date: Tue, 23 Jun 2015 10:58:56 +1000
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Cc: "v6ops@ietf.org" <v6ops@ietf.org>
Subject: Re: [v6ops] Happy eyeballs suggestions, was: Re: Apple and IPv6, a few clarifications
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In message <52696645-3449-4AD4-8C80-1D17DFB52F2D@muada.com>om>, Iljitsch van Beijnum writes:
> On 23 Jun 2015, at 2:17, Owen DeLong <owen@delong.com> wrote:
> 
> >> But IPv6 UDP applications shouldn't concern themselves with testing 
> for PMTUD black holes. This breaks TCP anyway, giving the user much 
> bigger fish to fry. And IPv4 UDP applications should leave the DF bit 
> alone because setting it to one can only end in tears.
> 
> > And once again, Iljitsch lets religion get in the way of understanding 
> the way the real world works
> 
> That would be true if I were talking about IPv4 UDP applications not 
> testing for PMTUD black holes. Which I also stand behind; the internet 
> needs larger packets, so any action to artificially limit packet sizes is 
> counterproductive.
> 
> But my point about IPv6 UDP applications is not religious, it's just 
> simple common sense: if the protocol that occupies 85% of the internet's 
> packets is going to fail anyway, why bother doing extra work to keep one 
> particular application that occupies a small fraction of the remaining 
> 15% running? (I.e., with unlike with IPv4, with IPv6, TCP and UDP have 
> the same failure mode in the presence of PMTUD black holes. And if 
> depending on PMTUD is too deemed dangerous, then 1280 is the answer, with 
> no need for any testing.)

Actually the don't have the same failure modes.

For UDP you send DNS request to a anycast cluster and you hits
server A.  It gets the PTB response which the kernel records but
the DNS server can't do anything with it as the transaction state
is gone.

The client times out and sends a second UDP DNS request.  This hits
server B.

The client times out and sends a third UDP DNS request.  This hits
server C.

The client times out and sends a fourth UDP DNS request.  This hits
server A which may or may not still remember the PMTU from the PTB.

For TCP the PTB should go to server A and TCP transaction should
resume.  If the PTB gets lost / misdirected then by the time server
A notices it is too late.  The client has given up.

Add multiple drops in MTU along the path and it gets worse.

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-- 
Mark Andrews, ISC
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