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[happy] Re: [ietf-wg-happy/draft-happy-eyeballs-v3] Section-10.1 : Why is there a specific IPv6 MTU problem? (Issue #124)

Gorry Fairhurst <gorry@erg.abdn.ac.uk> Fri, 03 July 2026 07:28 UTC

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Date: Fri, 03 Jul 2026 08:28:01 +0100
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To: Brian E Carpenter <brian.e.carpenter@gmail.com>, Dan Wing <danwing@gmail.com>
References: <ietf-wg-happy/draft-happy-eyeballs-v3/issues/124@github.com> <ietf-wg-happy/draft-happy-eyeballs-v3/issues/124/4787016440@github.com> <24deafcc-11c7-47df-893e-4ecfd436f82f@erg.abdn.ac.uk> <4E1D1C9A-B618-4D98-9668-8AEBC9E17161@gmail.com> <ae5d5aac-5e0c-4032-a067-de6d26bf4668@gmail.com>
From: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
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CC: ietf-wg-happy/draft-happy-eyeballs-v3 <reply+ABYLLEXPYYO7AWJ3AWSO4JLKWTKUREVBNHHPH3P7H4@reply.github.com>, Nidhi Jaju <notifications@github.com>, happy@ietf.org
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Subject: [happy] Re: [ietf-wg-happy/draft-happy-eyeballs-v3] Section-10.1 : Why is there a specific IPv6 MTU problem? (Issue #124)
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On 03/07/2026 00:55, Brian E Carpenter wrote:
> On 03-Jul-26 11:01, Dan Wing wrote:
>> On Jun 24, 2026, at 9:33 AM, Gorry Fairhurst <gorry@erg.abdn.ac.uk> 
>> wrote:
>>>
>>> On 24/06/2026 09:46, Nidhi Jaju wrote:
>>>> nidhijaju left a comment (ietf-wg-happy/draft-happy-eyeballs-v3#124)
>>>> Note: This text was added in #120.
>>>> —
>>>> Reply to this email directly, view it on GitHub, or unsubscribe.
>>>> You are receiving this because you authored the thread.Message ID: 
>>>> <ietf-wg-happy/draft-happy-eyeballs-v3/issues/124/4787016440@github.com>
>>> I still don't understand the origins or generallity of the statement 
>>> "Such issues might be specific to IPv6".
>>> Maybe I am missing something, so what is it that makes a TCP session 
>>> over IPv6 fail to recieve packets of the received MSS?
>>
>> Spit-balling: Decades of deployed routers with "ip tcp adjust-mss", 
>> but without corresponding "ipv6 tcp adjust-mss". Also due to header 
>> size differences the two values usually don't match and counting 
>> bytes is harder now than decades ago.
Indeed.
>
> And wouldn't variable length extension headers make it potentially 
> even worse?
>    Brian

If this is a source, ought the TCP sender to take EH and other headers 
into account when constructing a TCP Segment so that the payload is 
less, rather than the packet being discarded?

Were you thinking of transit routers that are configured to add EH 
(e.g., for OAM) and ought to have configured a sufficient MTU.

Gorry

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