OpenBSD pfsync protocol change between 4.3 and 4.4

This doesn't appear to be well advertised, but there are some protocol changes in pfsync between OpenBSD 4.3 and 4.4 that make them not able to talk to each other.

https://kerneltrap.org/mailarchive/openbsd-misc/2008/12/4/4311914

If you're doing staggered upgrades of firewalls like I did, this might burn you.