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

Aaron Campbell aaron at arbor.net
Thu Sep 12 11:07:29 UTC 2013

On 2013-09-11, at 11:50 PM, Paul Vixie <paul at redbarn.org> wrote:

>>> "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 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.
>> Shouldn't OVERHEAD actually be 68 for IPv4?
> i am counting 40 for the ipv6 header and 8 for the udp header. where are you getting your 68 number from?

48 is correct for IPv6.  But max IPv4 header size is 60 bytes, plus 8 for UDP is 68.


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