ike2-v4: request or response

["jpickering@creeksidenet.com" <jpickering@creeksidenet.com>]

After multiple reads of ike2-v4 I still cant figure out how to determine 
if a received message is
a request or response. Clearly, I must be missing something. For 
example, if the original ike-sa
initiator receives msg sequence N and is waiting for a response with 
sequence N, how does it
know that it is actually a response if on the off chance the next 
expected request from the
original responder is always sequence N?

Any enlightenment would be greatly appreciated.

Regards,
Jeff