Allowing an unencrypted channel for legacy Firewall clients
Firewall Client for ISA Server 2004 (and later versions) provides encryption of Firewall client credentials sent transparently over a TCP control channel with each client request to Forefront TMG. This default setting prevents legacy clients from connecting. You can select to allow non-encrypted communications to support the following clients:
Firewall Client versions earlier than Firewall Client for ISA Server 2004.
Client computers running Windows NT 4.0, Windows ME, or Windows 98.
For more information about supported operating systems for Firewall Client software, see About Firewall clients.