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.
Yes, a good point.
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