> Is there a simple way to get statistics out of IP about the percentage
> of retransmitted TCP packets over a period of time? 

use netstat -s -p tcp (and postprocess with your favorite scripting
language):

%; netstat -s -p tcp
tcp:
        219276 packets sent
                67351 data packets (2423448 bytes)
                14 data packets (11812 bytes) retransmitted	<<=====
		...
        338080 packets received
                67269 acks (for 2427819 bytes)
                74 duplicate acks
                0 acks for unsent data
                319555 packets (26081432 bytes) received in-sequence
                44 completely duplicate packets (108 bytes)	<<=====
		...
                83 out-of-order packets (15068 bytes)		<<=====
		...
        26 connection requests
	...
        10 retransmit timeouts
                0 connections dropped by rexmit timeout		<<=====
	...
        0 SYN,ACKs retransmitted				<<====
        0 duplicate SYNs received for entries already in the cache <<===
	...

						- Bill