Re: [IPsec] I-D Action: draft-ietf-ipsecme-ikev2-fragmentation-03.txt

[Yaron Sheffer <[e-mail]>]

>>> I also think that PMTU discovery isn't very useful for IKE.
>>> That's why it is MAY.
>>
>> That does not help implementors who still have to implement the MAY's.
>> if even you as a document author does not think it is veru usefil,
>> then I think it should just not be in the document.
>
> Sorry, I wasn't very clear. By "isn't very useful" I meant that it is
> not useful
> for the usual PMTU discovery goal in TCP - to find _maximum_ IP datagram
> size that is not fragmented by IP level. In IKE its the goal is different -
> to find _some_reasonable_ IP datagram size that is not fragmented by IP.
>
> If we have the size that is guaranteed to not be fragmented,
> no PMTU discovery will be needed. As far as I understand, for IPv6
> it is 1280 bytes. But as far as I know, there's no such value for IPv4.
> If we mandate (or recommend) using really small value e.g. 128 bytes,
> than the perfomance will suffer badly, so it it not a good option.
> I'm especially worring about network I'm not familiar with -
> mobile networks or other constrained environments.
> It would be great if some experts in such networks could clarify this.
>
I'm even more worried that if we use small fragments, reliability will 
deteriorate. Because we do not have per-packet acknowledgement, and so 
if any fragment is dropped, the whole message must be resent. This is 
probably a greater risk in mobile networks.