SIP services

The Authorization header can be used to indicate a customer identity that associates a SIP request with a billable entity.

Examples of possibly chargeable SIP services include:

• Directory services such as SIP proxy/redirect lookups;
• Customer profile management;

SIP server operations can be charged based on server logs or, for real-time billing, via AAA.

Media services

Media services include retrieving and storing voice mail, as well as transcoding of media streams. They are not initiated by SIP, but, for example, via RTSP.

Gateway services

Similar to SIP services. Care has to be taken to stop billing when (say) RTP voice data is no longer flowing through the gateway. The gateway will generate call detail records (CDRs) either directly or through RADIUS.

Transport (network services)

It seems unlikely that voice calls carried over a best-effort service will generate per-minute charges. When reserving bandwidth or guaranteeing other quality-of-service parameters, the resource reservation protocol or differentiated services are the appropriate mechanism for including charging. These reservation protocols will likely be used in applications that are not initiated by SIP, for example, audio/video on demand or VPNs. Actual accounting records may be generated by AAA protocols (e.g., by policy enforcement points (PEP) or policy decision points (PDP)) or log files.

Under some circumstances, a SIP proxy server may be useful to initiate such reservations or differentiated services treatment on behalf of a call, since it may be easier to authenticate the SIP request than the lower-layer reservation request or the end system may not be capable of making reservations or marking packets. In those cases, the SIP proxy would initiate a resource reservation and "charge back" the caller identified by the SIP request.

Dean Willis wrote with regards to billing for SIP services:

Why can't service providers make a living providing (at a fixed cost) access to "free services"? Do carriers do per HTTP-transfer billing now? How much should they charge for an email? For a call, what parameters might be used? Bandwidth, duration, distance -- the Big Factors of the POTS bill -- are not issues that SIP is concerned with.