Re: backwards compatible introduction of NEW attribute such as CU I

[Jari Arkko <jari.arkko@piuha.net>]

Bernard Aboba wrote:

> With a VSA, if the NAS doesn't understand it, it can ignore it.  Is that
> true of CUI as well?  If this really is about the NAS, then I think the
> answer is yes.  The RADIUS Server can send CUI along with Class, and if
> the NAS doesn't support it, it can ignore it and the home server will get
> all the billing info it needs from the Class attribute.  This is backward
> compatible on the NAS (since it doesn't need to be upgraded to support
> CUI) as well as on the RADIUS server (who doesn't see a CUI attribute it
> didn't ask for).

I believe there are two "layers" that we need to distinguish here.
The most obvious layer is what happens at RADIUS. For this layer
the question "can we ignore the CUI" depends on whether the
CUI affects some of the subsequent RADIUS messages. Is there
a requirement for the CUI to be copied back in Accounting-Requests?
If yes, then the CUI needs to be understood at least in that
sense. But I forget what the base RFCs say about copying unknown
attributes to Accounting-Requests. If there is no requirement
for CUI to be copied back, then we are assuming that the home
network can associate the access and accounting requests by
other means.

But we also have a second layer, which is the billing and policy
processes. For the CUI to be useful, we are already assuming that
there is a need for the organizations to correlate some billing
information based on the CUI, or do some policing based on the
CUI (such as # of concurrent sessions per CUI). Here we can
see a few different cases:

o  Home network can't complete its billing/reconciliation
   process without getting a CUI from the access network.
   Here we MUST have support for the CUI in the access
   network, or otherwise the two networks can't work together.
   Fortunately this can be checked at the time the roaming
   contract is signed.

o  Home network offers CUI for everyone, but does not
   need it by itself. Support for CUI is optional,
   and if the access network needs it they will upgrade
   their equipment to support it.

o  Home network does not offer CUI to anyone. If the
   access networks do not need it, no problem. If they
   do, they can not work together with this home network.
   Given that the usage of CUI is expected to be in
   the billing and reconciliation process, we should
   never get into the latter situation. However, the
   access network may be unable to perform policy
   operations that it wants to, if those operations
   require knowledge of the CUI.

--Jari

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