[dns-operations] Unexpected truncation

Douglas Otis dotis at mail-abuse.org
Thu Apr 7 16:18:03 UTC 2011

On 4/3/11 2:22 PM, Unbitrium wrote:
> On 3 April 2011 18:41, George Barwood<george.barwood at blueyonder.co.uk>  wrote:
>> I think it's odd to truncate below 1460 bytes, which is where IP fragmentation starts, IIRC.
>> I'd be interested in the reasoning here
> Semi-agreed, don't forget IPv6 :)
> IPv6 specifies a minimum MTU that must be supported of 1280 (with a 40
> byte IP header). I doubt there are any IPv4 networks which have
> problems with packets this size, so this can probably be taken as a
> baseline. (unfortunately there are people who have configured 1280 MTU
> links "to make sure they don't have MTU problems")
> Assuming their max packet size is 1169, if they are planning for a
> worst case IPv6 network then that's still a lot of wiggle room they've
> left themselves.

To be assured of continued operation for critical services that do not 
use path MTU discovery, the minimum becomes 1280 - 40 - 8 = 1232 or less 
as indicated in RFC2460 to handle possible IPv4 router fragmentation. 
There might be another 18 octet header used, like L2TP, where a safely 
assumed minimum MTU becomes 1214 instead of 1280.


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