7d7a304b__Storm_Center_Diary_2012_10_06__Purpose
[ID] CIP-005-4a [Description] Cyber Security - Electronic Security Perimeter [Purpose] 
It requires the identification and protection of the Electronic Security Perimeter inside which all Critical Cyber Assets reside. This means placing controls like Firewalls that have specific support for the SCADA protocols being used, Application Whitelisting, IPS among many others. All those controls cannot induce or modify the protocol flow between all the SCADA entities in place.
InfoSec Handlers Diary Blog - Internet Storm Center Diary 2012-10-06
Purpose