Subject: Re: Retrieving large DNS objects

From: "Anders Rundgren" <[email-masked]>
Date: Sun, 23 Jan 2005 18:10:03 +0100

Paul Vixie wrote:

>> THE QUESTION:
>> Does this (fat-object) paradigm fit DNS?

>no.

Not even 4K seems OK?

>> BTW, I have "ordered" an XML RR from IANA.  Putting the
>> famous process to test.... :-)

>i hope they turn you down.

Well, then we can dismiss iab-dns-choices right now.
From DOA to EIP :-)

>XML would be as bad as TXT.  without subclassing,
>every consumer would have to retrieve an arbitrarily large rrset

What does this really mean?  You get ALL TXT records and it is
the clients who do the actual select?

If that is NOT the case (which I doubt as I cannot find any support for
that in RFCs) the "structured prefix/suffix" is an easier solution.
Avtually you HAVE to use this for NAPTR etc. as
"all NAPTRs are not equal".

So if we anyway must prefix/suffx why not use that all the way?

Anders R

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From: Robert Elz <[email-masked]>
Date: Wed, 26 Jan 2005 10:56:58 +0700

Date:        Sun, 23 Jan 2005 16:29:24 +0000
    From:        Paul Vixie <[email-masked]>
    Message-ID:  <[email-masked]>

  | since the guaranteed
  | minimum maximum MTU has not changed (576 for V4)

Paul, you really know better I'm sure - that number isn't 576, it is 68.
576 is the "all hosts must be able to receive, reassembling if necessary,
datagrams this big" number.

The DNS packet size was never set to avoid fragmentation - it was set because
to send larger packets may mean that some hosts would be unable to receive
them, given that nothing requires that their IP stacks be able to handle that.

  | and is still small (1280 for V6),

Yes, though the v6 equivalent to 576 is 1500 I believe.

Two issues are important here - first though lots of things have grown
a lot in the past 20 years, packet sizes have not, they've barely changed
at all (factor of 3 increase probably).

  | any udp app for the wide area should be trying to avoid fragmentation.

Yes, fragmentation is not good.   It is indeed fortunate that many
people mistook the minimum required MTU as being 576, and so built
almost all links to at least that (in the past - these days nothing
that small gets deployed except on some - very few - end-links).
That had the effect that DNS packets all avoid fragmentation, which
is a very nice outcome, and something to keep encouraging, but it
neither is, nor ever was, fundamental - repeating a DNS query, when
a fragment of a reply is lost, is still cheaper than doing the whole
thing over TCP.

kre

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From: Alex Bligh <[email-masked]>
Date: Wed, 26 Jan 2005 10:17:37 +0000

--On 26 January 2005 10:56 +0700 Robert Elz <[email-masked]> wrote:

> It is indeed fortunate that many

Lower MTU would of course be helpful in order to support DNS in
IP tunneled over DNS. (ducks...)

Alex