About replay attck to RADIUS.

Hello all. I have a question about RADIUS, including portmaster, especially authentication system. How RADIUS system protects his system and user's information from sniffers when the NAS sends some information including user's password which has eccrypted with MD5. I want to know the system against "replay attack" from attackers. If you vave any infomation about my question, please tell me. Thank you. Masafumi Tsuruta (from Tokyo) tsuruta@insi.co.jp