There is no intrinsic fragmentation problem, but IPsec does add a
header and thus possibly fragment.  But I have turned down the maximum
piggybacked validations, and most ops use 1024 bytes of data.
So this might cause timeouts/disconnection, but that's not my real
complaint - it is the unresolvable conflicts.

I was having fragmentation trouble, but that was on wireless links
where back-to-back packets often got dropped.  The current trouble is
with two machines on the same 100 Mb/s wired ethernet.

  I'm not sure I'm seeing disconnection, when I get back I'm connected
  (these heavy updates take place at 4am).  However, IIRC there are
  several different trees being cvs updated, and the early one warns
  about a time out, the later ones say "can't create" all the CVS
  Entries.backup files -- could this be a disconnect / lose token /
  reconnect kind of thing?  What do I look for in the Coda logs?

I'm not sure - you could leave a 'codacon' running onto a file, which
seems to get more than /usr/coda/etc/console.