How do we meter IPSEC data separate from other data for Remote Access VPN

[Chandan Purushothama]

I referred to the feature presented at 
https://issues.apache.org/jira/browse/CLOUDSTACK-759 . May I know how is the 
metering of IPSEC data done on CloudStack. May I know where can I find the 
Functional Specification for this feature to understand how this feature works,

Thank you,
Chandan.