FWIW:

On 5/12/2017 5:31 AM, Michael Welzl wrote:
>> -------------------
>> Get Interface MTU is missing from pass 2 and 3:
>>
>> ADD to pass 2:
>>
>>      GET_INTERFACE_MTU.UDP:
>>              Pass 1 primitive: GET_INTERFACE_MTU
>>              Returns: Maximum datagram size (bytes)
> But this doesn’t exist!  It’s strictly an IP function and I couldn’t find it 
> described for UDP anywhere. I think we agreed on how a TAPS system should 
> handle this, and this is reflected in
> https://tools.ietf.org/html/draft-gjessing-taps-minset-04#section-6.4.1
> … which may require a system to implement new local functionality, maybe 
> based on this MTU function - but to my understanding it’s just not something 
> that UDP offers.
IP is supposed to export MMS_S and MMS_R to the transport, which are
calculated from EMTU_S, EMTU_R, and the link MTU for the given source IP
address.. PMTUD exports PMTU to the transport. Both already take into
account the link MTU and other MTU information, where available.

In no cases does the transport actually ever need to see the link MTU.

Joe
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