DISCUSS comments on MTU setting from Lars Eggert:

> >  DISCUSS: For anything sent over UDP, i.e., D- and Q-mode,
> it must be
> >  made clear that messages must have payloads smaller than the PMTU
> >  (minus headers) or 512 bytes if the PMTU is unknown, to avoid
> >  fragmentation. (Section 5.4.1 and 5.8.1.2 do so, but other
> >sections do
> >  not.) Because some messages MUST be transmitted in D- or 
> Q-mode, are
> >  such messages guaranteed to be less than this limit in all cases?

on the MTU front: fair point. Probably the note in 5.8.1.2 should really be in
5.3 (where it applies to all cases of D mode). See below on actual message sizes.

and later:

> >>>  Additionally, because some
> >>>  messages MUST be carried in D- or Q-mode - is it guaranteed  that 
> >>> their payloads are always less than 512 bytes?
> >
> > In theory no, because things like the cookies and peer identity are 
> > variable length strings. But I just did a quick count, and with a 
> > worst case for the other fields (e.g. using v6 addresses, and a 
> > couple of possible stack proposals) adds up to about 150 bytes. So I 
> > think we are safe.
> 
> Since you were going to add MTU-related text anyway, it may be good
> to point out that if payloads grow past the MTU, C-mode SHOULD be  
> used if possible.

OK.