Dynamic DNS & IXFR

[Kathy Lankford]

Does anyone have detail on how dynamic DNS and IXFR are/will be 
implemented in BIND?  I'm trying to find out:

If dynamic DNS is configured without IXFR, then it seems like
every client's DDNS update would trigger a notify from master to
slaves and all the slaves would call for an AXFR.  On a busy network
I'd guess that would bury narrow WAN or busy LAN lines, and possibly
beat up the master's CPU.  Is there a solution to this (or is it just
"don't use DDNS without IXFR")?

If dynamic DNS is used with IXFR, and every DDNS update triggers a 
notify from master to slaves, then all the slaves might be expected to
come back and say "I'm a version behind, give me an IXFR."  Does the
master compare the slave version to its current version to find the 
incremental update to send for EVERY slave, or does it do that for the first
slave to ask, and cache the answer for all the rest?  If it caches, how
many comparisons does it save?

Or am I wrong that every dynamic update raises the master's serial number
and triggers notify messages?  Are updates batched?  How?  Is batch size/time
configurable?