>Okay, so, I'm trying to understand what you're saying here -- and failing
>miserably.
>What, exactly, does changing these two defaults have to do with "backward
>compatibility"?

	or, least surprise.

	there are multiple ways in doing this.
	- change kernel default value.  some may surprise, but kernel upgrade
	  alone will take care of everything.
	- put new default value in sysctl.conf, with some documentation.

		# path MTU discovery - kernel default value is 0
		net.inet.ip.mtudisc=1
		# redirect route timeouts - kernel default value is 0
		net.inet.icmp.redirtimeout=30

	- put new default value in sysctl.conf as comment, with some
	  documentation.

		# path MTU discovery - kernel default value is 0,
		# suggested value these days is 1
		#net.inet.ip.mtudisc=1
		# redirect route timeouts - kernel default value is 0,
		# suggested value these days is 30
		#net.inet.icmp.redirtimeout=30

	- put current default value in sysctl.conf, with some documentation.
	  no surprise, but peopl won't start using new values only after
	  checking sysctl.conf

		# path MTU discovery - off by default, for most of the systems, on (1) is suggested
		net.inet.ip.mtudisc=0	
		# redirect route timeouts - for most of the systems, 30 is suggested
		net.inet.icmp.redirtimeout=0

	openbsd uses 2nd approach.

itojun