[c-nsp] ip tcp adjust-mss

Phil Mayers p.mayers at imperial.ac.uk
Thu Feb 14 05:23:37 EST 2013

On 02/12/2013 04:02 PM, Alexander Arseniev wrote:

> and DNS over UDP would not be experiencing issues due to too small
> MTU because own DNS payload limit is smaller than smallest real MTUs
> out there

Not so. The EDNS buffer size option allows clients to request UDP DNS 
replies anything up to 4096 bytes, which are typically sent as IP 
fragmented UDP packets.

Such DNS request/replies are now quite common, particularly between a 
recursive resolver and authoritative service. DNS RR types such as 
DNSKEY, RRSIG/NSEC and TXT/SPF are all capable of padding DNS payloads 
to sizes where this is necessary.

So, DNS is a quite-valid example for MTU-sized frames.

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