Re: [Int-area] draft-learmonth-intarea-rfc1226-bis-00

[Joseph Touch <[e-mail]>]

Hi, all,

> On May 21, 2020, at 12:30 PM, Alexandre Petrescu <[e-mail]> wrote:
> 
>> For AX.25 version 2.0, the
>>   maximum frame size expected is 330 bytes and implementations MUST be
>>   prepared to handle frames of this size.  Higher frame sizes can be
>>   negotiated by AX.25 version 2.2 and so this is a minimum requirement
>>   and not a limit.
> 
> It is ok.   For IPv6, the minimum MTU (minimum Maximum Transmission Unit) is 1280 bytes.

There are several maximums that are relevant - path MTU is only one of them, and not necessarily the one that is relevant here.

For this protocol, all you really care about is EMTU_S (as defined in RFC1122 - see draft-ietf-intarea-tunnels, which admittedly needs to be refreshed in ample spare time).

For IPv6, this is 1500; for IPv4 it is 576. Those are more than sufficient if you aren’t significantly affected by packet loss per se, which *can* be amplified by source fragmentation, but that might not be relevant in this case.

If you want to talk about performance under certain losses where fragmentation causes a known problem, you should add an additional suggestion to do path MTU discovery (PLPMTUD - which you SHOULD really build in if it isn’t there yet), because otherwise you’re stuck with the path mins of 1280 for IPv6 and (sorry, still) 64 for IPv4.

> For IPv4, it is not worth mentioning.  One, including myself, would go as far as suggesting to remove the IPv4 keyword from the draft altogether.

Although I agree all new drafts need to address IPv6, it’s naive to ignore IPv4 - especially for a protocol such as this, which is, if anything, more likely to operate using legacy equipment.

Joe