SIP and accounting

1
SIP and accounting

• Henning Schulzrinne
• Columbia University
• SIP 2003 January 2003
• Paris, France

2
Overview

• Why SIP accounting?
• not just billing
• Whats different about SIP accounting?
• accounting for SIP as a protocol
• separation of data and signaling
• Architecture and protocol choices
• Open issues

3
Accounting terminology (RFC 2975)

• Accounting The collection of resource
  consumption data for the purposes of capacity and
  trend analysis, cost allocation, auditing, and
  billing. Accounting management requires that
  resource consumption be measured, rated,
  assigned, and communicated between appropriate
  parties.
• Rating The act of determining the price to be
  charged for use of a resource.
• Billing The act of preparing an invoice.
• Auditing The act of verifying the correctness of
  a procedure.
• Real-time accounting Real-time accounting
  involves the processing of information on
  resource usage within a defined time window. Time
  constraints are typically imposed in order to
  limit financial risk.
• Logging (Not in 2975) Keeping track of
  activities for debugging, abuse prevention or
  legal reasons such as lawful intercept.
  Typically, does not involve money.

4
Why SIP accounting?

• Usage of (relatively) expensive resources
• wireless bandwidth
• PSTN gateways, with international calls at
  3/minute
• translation and storage services
• Real-time for fraud detection and prepaid calling
  cards
• Off-line for traditional monthly billing or
  traffic analysis

5
Billing
utility

• PSTN evolution from distance/time-sensitive
  per-minute billing
• bucket of minutes
• flat-rate plans (all you can eat) Canada, ATT
• Per-minute billing doesnt fit well
• SIP sessions can remain open for months, without
  sending a single packet
• voice silence suppression ? unfair to charge for
  both directions for large conferences
• incremental value is non-linear
• thus, video unlikely

bit rate
6
Billing and charging

• What are we billing for?
• infrastructure
• services
• unlikely to be able to charge for call forwarding
  for corporate users
• but Yahoo might for residential users
• traffic
• but network cost depends on peak usage, not
  average usage
• treat all traffic the same?
• 3G charge more for data traffic than voice
  traffic?
• escalation of traffic cloaking and detection
• A simple billing model
• bill per-minute for calls gatewayed into the PSTN
• bill for services on a subscription basis (e.g.,
  as part of ISP service)
• bill for traffic
• independent of traffic type
• by volume, 95th percentile, congestion pricing

7
AAA Authentication, Authorization, Accounting

• separate SIP protocol elements from making
  authentication/authorization decisions
• allow visited proxy to ask home proxy of visitor
  whether visitor is legit
• accounting
• resource dimensioning
• apportionment of charges
• commercial billing
• three primary protocols
• RADIUS used for dial-up servers, popular with
  ISPs
• can lose data (UDP)
• DIAMETER successor of RADIUS
• will be used in 3G for AAA, not widely implemented

8
Accounting architecture for roaming
home.net
visited.net
RADIUS DIAMETER
accounting data authentication request challenge/r
esponse
SIP
alice_at_home.net
End systems never see AAA
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Accounting in SIP

• Components
• network resources used by SIP itself
• network resource consumption initiated by SIP
• sessions set up
• gateway resources
• services initiated and controlled by SIP
• voicemail storage
• media translation services
• Does not necessarily involve metering or
  logging SIP protocol requests!
• But useful to correlate byte metering to SIP
  sessions for usability

10
Resources consumed by SIP

• SIP is mainly signaling, but also carries user
  data
• most directly, as MESSAGE
• INFO messages
• headers such as Subject, User-Agent, From/To
  display field
• user-created header fields
• MIME body parts, possibly encrypted
• all of these are opaque to proxies
• can use unsuccessful call attempts to send data
• ISDN precedence UU data
• unless all non-protocol-state fields and
  extensibility are removed from SIP, cannot
  prevent data carriage
• even with removal SIP covert channel
• space SIP messages long and short pauses ? Morse
  code
• minor protocol variations upper case header 1,
  lower case 0
• if signaling doesnt cost, efficiency is not a
  concern (for the (ab)user)

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Resources consumed by SIP

• Thus, sane approach is to bill for call setup and
  other SIP messages
• GPRS 4/MB ?
• signaling without compression 4c/call
• with compression XX c/call
• maybe provide monthly allowance for user

12
Accounting in SIP

• Call Detail Records (CDR) just record time/date
  of call start/end and source/destination
• cost may depend on
• peak bandwidth (typical 95th percentile billing)
• average bandwidth
• compressibility (e.g., for text messages)
• QOS parameters,
• IPDR.org more generic format for IP services
• XML-based and Sun XDR-based (binary)
• SIP requests should only be used for session
  accounting
• if access to the right data
• if no incentive or possibility for bypass (e.g.,
  in gateway, not in proxies)
• e.g., send BYE with Max-Forward reaching to proxy
  only

13
Open Settlement Protocol (OSP)

• clearing-house model

• mostly off-line settlement between different
  providers
• CDR style

14
Conclusion

• Accounting easy for gateway services ? session
  model fits traditional CDR model
• Signaling services require accounting to prevent
  RTP-over-SIP cloaking
• Open issues
• RTP accounting?
• detail messages? dialog?
• how to indicate charging preferences
• "I pay for audio, you pay for video"
• Dutch everybody pays for bytes sent
  (mother-in-law mode)
• Preliminary requirements discussions in IETF
• Have done local RADIUS implementation

15
References

• H. Basilier, P. Calhoun, et al., AAA
  Requirements for IP Telephony/Multimedia,
  draft-calhoun-sip-aaa-reqs-04
• B. Aboba, J. Arkko, D. Harrington, Introduction
  to Accounting Management, RFC 2975
• J. Loughney, G. Camarillo, "Authentication,
  Authorization and Accounting Requirements for the
  Session Initiation Protocol", draft-ietf-sipping-a
  aa-req-01
• S. Nagarayan, et al., "A Diameter accounting
  application for the Session Initiation Protocol",
  draft-narayanan-sipping-aaa-diameter-00