Hi!
What is User-Password attribute in this DHCP Radius Request?
NAS-Identifier MikroTik
User-Name 00:0C:29:06:B1:87
User-Password WмFЕKо♦а*▄$yП@Уb
NAS-IP-Address 10.10.1.100
NAS-Port 2198863873
NAS-Port-Type Ethernet
Called-Station-Id dhcp1
Calling-Station-Id 1:0:c:29:6:b1:87
What attributes should RADIUS server return in the reply to this query?
User-Password is like md5 hash of ‘something’ - so it’s useful
all return attributes you may find at http://www.mikrotik.com/testdocs/ros/3.0/pnp/dhcp_content.php#.3.2
from my investigations: if you send in reply ‘Session-Timeout - max lease time (lease-time)’, then DHCP server will ask RADIUS every time DHCP client sends DHCP-request. if you do not send Session-Timeout, then DHCP server will ask RADIUS only if it do not have an active lease for that client (else it will silently reply with information, received from the RADIUS last time)
You are welcome! 
p.s. OMG! I typed ‘useful’? Of cource I meant ‘useless’! =)