Usage quotas

In an attempt to distract me from OSPF, I have been playing with ways to handle usage quotas.

The idea is that on some services, some customers would like a way to pre-set a cap on usage, whether it is a data SIM or a broadband line.

The catch with such ideas is that the usage metering is by RADIUS accounting which we run every hour. Now, we could do this more often, but even short intervals can be vey large amounts of usage on some services (especially with things like 330Mb/s FTTP lines in the pipeline).

So the trick is to tell the LNS a quota for a connection and have it spot the usage has exceeded that in something like real time.

Now, RADIUS already has something like that, but it was designed for dialup and is a Session-Timeout or Idle-Timeout which is based on time not data usage. We already support Session-Timeout (though not Idle-Timeout as that makes little sense on broadband). I can't find a RADIUS AVP for data quota limit, so I am using the Filter settings (like most of the other special settings we use).

First snag is what you meter. For data SIMs the usage is Tx and Rx (total) but for ADSL it is only Tx that matters. So we have to support a choice of quota metering type.

Then we have the question of what to do when we reach a limit. Well, RADIUS has Terminate-Action AVP which is used with Session-Timeout and allows either ending the session or resending an Access-Request. Terminating the session is messy. Many routers reconnect within a few seconds but some take minutes. It would be neater not to drop the PPP session.

Sadly the idea of re-authentication is somewhat flawed. For a start, the way we work, we throw away all the PPP negotiation and authentication data once the connection is completed so we can't re-auth. We could try a new CHAP challenges, but many routers and systems barf at that (including mobile data links) even though the spec says that should work, and anyway this does not work with PAP. Even if we solved these self imposed issues, the RADIUS server will not have the up to date data to decide what to do as the accounting could be up to an hour before. The solution is not to re-authenticate, but to send an intermediate RADIUS accounting packet instead.

This fits well as the RADIUS accounting server then has the up to date information to decide if over quota, but it can also check the customer database to confirm if there are changes (billing errors, start of new month, top-up, etc), and decide if the line needs to be locked down or not, and what quota it now needs. Sending an accounting update as soon as we hit the quota will allow the accounting server to know we are over limit immediately. It can then use a RADIUS CoA (Change of Authorisation) to change the line in some way if needed.

The CoA can be used to disconnect the line, clamp it to a low speed, or force on to a special routing table to hit a captive portal prompting people to top-up. It can also un-do these effects, and all without ever dropping the PPP link. If can update the quota as we roll over to a new month. All handled in one place. As the saying goes, "simples!".

So, having changed the LNS to support Terminate-Action choice of hang-up or accounting update, and to handle filters for Tx limit or Tx+Rx limit, we can now make some new service options around that. Perhaps pre-pay data SIMs? Or usage capped broadband services... Watch this space.