Re: [tcpm] 793bis: variable MTU

Gorry Fairhurst <gorry@erg.abdn.ac.uk> Thu, 19 December 2019 17:24 UTC

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Date: Thu, 19 Dec 2019 17:24:02 +0000
From: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
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Subject: Re: [tcpm] 793bis: variable MTU
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On 19/12/2019, 16:53, Wesley Eddy wrote:
> I'm not sure the comment & suggested NEW text below is quite correct.  
> I tracked the text in question down to being from RFC 6691 (and it 
> first appeared in -03 of draft-ietf-tcpm-tcpmss-03, and it looks like 
> it came about from a thread here: 
> https://mailarchive.ietf.org/arch/msg/tcpm/LjfmysjQAKmTudpBgBa2Jp20O3M ).
>
> On 8/28/2019 11:39 AM, Gorry Fairhurst wrote:
>> OLD:
>>    As a result, when the effective MTU of an interface varies, TCP
>>    SHOULD use the smallest effective MTU of the interface to calculate
>>    the value to advertise in the MSS option (SHLD-6).
>> - what does "varies" mean here? The MSS is computed when the SYN is 
>> exchanged
>> is this actually just:
>> NEW:
>>    When multiple IP interfaces have different effective MTUs, TCP
>>    SHOULD use the smallest effective MTU of the interface to calculate
>>    the value to advertise in the MSS option (SHLD-6). 
>
You are indeed correct!

This makes muchmore sense if we refer back to that thread, which was 
about the same interface having maximum packet sizes that vary from 
packet-to-packet, e.g. from  variable-length headers/encapsulations are 
used by the local a link.

An interface SHOULD use the smallest effective MTU that can be used in 
all packets to calculate the value to advertise in the MSS option (SHLD-6).

Gorry

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