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

Paul Vixie paul at redbarn.org
Wed Sep 11 03:15:28 UTC 2013



Yasuhiro Orange Morishita / 森下泰宏 wrote:
>> And I know the IP specification defines the minimal MTU size to 576.
>> So, we may need a very short RFC for updating the definition of MTU,
>                                                                      ^
>                                                                     to 1280
>
> -- Orange
>
>> in RFC 791.

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 signatures such as TSIG or DNSSEC
signatures, use an effective DNS message payload size which is
calculated as MIN(OFFERED, MIN(DISCOVERED, ESTIMATED) - OVERHEAD)) where
OFFERED is 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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