If the billing is based on actual bytes delivered to the consumer,
there must be a business relationship between the billing stage
servers and adaptation servers. The relationship will usually allow
the adaptation servers to cache the content and report to the billing
servers about the number of bytes delivered. However, the reporting
is a reverse flow in the client-server model, and that is why it seems
problematical. By convention, the adaptation server might make a
request back to the provider with the number of bytes encoded in the
request, or it might, on occasion, upload a report. The rules
governing this behavior would most naturally be encoded on the
adaptation server, not on the OPES callout server. So, this seems to
me to be an issue for the rules language.
An alternative model would have this byte reporting functionality
handled on the callout server, and it would directly contact
the provider's billing service with reports.