[dns-operations] dns-operations Digest, Vol 92, Issue 13

Paul Vixie paul at redbarn.org
Wed Sep 11 05:30:00 UTC 2013

excuse me, i left an edit out of my earlier proposal.

Paul Vixie wrote:
> i do not think that the definition of mtu is wrong. if i were going to
> update 6891-bis (which is itself 2671-bis) the logic i would draft is:
> ---

"A DNS UDP responder shall, when transmitting a message which does not
include embedded cryptographic marks such as TSIG or DNSSEC signatures,
use an effective DNS message payload size which is calculated as
the EDNS BUFSIZE received from the initiator, and DISCOVERED is the path
MTU if known or else the outbound interface MTU, and ESTIMATED is chosen
as 576 for IPv4 or 1280 for IPv6 as the minimum guaranteed size of an IP
datagram, and OVERHEAD is chosen as 64 for IPv4 or 48 for IPv6 as the
maximum likely size of the IP and UDP headers.

This specification does not define a maximum for any future IP transport
protocol, and so both initiators and responders should be prepared to
receive messages as large as the 9 kilobyte ethernet jumbogram size in
preparation for future transport protocol development."

> ---
> i'm trying to decide whether that "should" ought to be a "must". and,
> i know the number is 9K not 64K, because of buffer memory
> considerations on today's embedded servers.
> vixie

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