Note that 'i' may be re-used from one session to the next, whereas 'n'
is always fresh.
+The protocol version selection stuff is not yet implemented: I'm not
+yet convinced it's a good idea. Instead, the initiator could try
+using its preferred protocol (which starts with a different magic
+number) and fall back if there's no reply.
+
Messages:
1) A->B: *,iA,msg1,A,B,protorange-A,nA