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SIP and accounting
Henning Schulzrinne
Columbia University
SIP 2003 – January 2003
Paris, France
Overview
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Why SIP accounting?
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What’s different about SIP accounting?
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not just billing
accounting for SIP as a protocol
separation of data and signaling
Architecture and protocol choices
Open issues
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.
Why SIP accounting?
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Usage of (relatively) expensive resources
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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
Billing
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PSTN: evolution from
distance/time-sensitive perminute billing
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bucket of minutes
flat-rate plans (“all you can
eat”): Canada, AT&T
Per-minute billing doesn’t fit
well:
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utility
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
Billing and charging
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What are we billing for?
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infrastructure
services
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unlikely to be able to charge for call forwarding for corporate users
but Yahoo might for residential users
traffic
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but network cost depends on peak usage, not average usage
treat all traffic the same?
3G: charge more for data traffic than voice traffic?
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escalation of traffic cloaking and detection
A simple billing model
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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
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independent of traffic type
by volume, 95th percentile, congestion pricing
AAA = Authentication,
Authorization, Accounting
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separate SIP protocol elements from making
authentication/authorization decisions
allow visited proxy to ask home proxy of visitor
whether visitor is legit
accounting:
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resource dimensioning
apportionment of charges
commercial billing
three primary protocols:
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RADIUS – used for dial-up servers, popular with ISPs
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can lose data (UDP)
DIAMETER – successor of RADIUS
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will be used in 3G for AAA, not widely implemented
Accounting architecture for
roaming
visited.net
RADIUS
DIAMETER
home.net
accounting data
authentication request
challenge/response
SIP
[email protected]
End systems never see AAA
Accounting in SIP
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Components:
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network resources used by SIP itself
network resource consumption initiated by SIP
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services initiated and controlled by SIP
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sessions set up
gateway resources
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
Resources consumed by SIP
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SIP is mainly signaling, but also carries user data
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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
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space SIP messages: long and short pauses  Morse code
minor protocol variations: upper case header = 1, lower case = 0
if signaling doesn’t cost, efficiency is not a concern (for the
(ab)user…)
Resources consumed by SIP
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Thus, sane approach is to bill for call
setup and other SIP messages
GPRS: $4/MB 
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signaling without compression ~ 4c/call
with compression ~ XX c/call
maybe provide monthly allowance for user
Accounting in SIP
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Call Detail Records (CDR) just record time/date of call start/end
and source/destination
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cost may depend on
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IPDR.org: more generic format for IP services
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peak bandwidth (typical 95th percentile billing)
average bandwidth
compressibility (e.g., for text messages)
QOS parameters, …
XML-based and Sun XDR-based (binary)
SIP requests should only be used for session accounting:
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if access to the right data
if no incentive or possibility for bypass (e.g., in gateway, not in
proxies)
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e.g., send BYE with Max-Forward reaching to proxy only
Open Settlement Protocol
(OSP)
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clearing-house model
 mostly off-line
settlement
between different
providers
 CDR style
Conclusion
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Accounting easy for gateway services  session
model fits traditional CDR model
Signaling services require accounting to prevent RTPover-SIP cloaking
Open issues:
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RTP accounting?
detail – messages? dialog?
how to indicate charging preferences
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"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
References
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H. Basilier, P. Calhoun, et al., “AAA Requirements for
IP Telephony/Multimedia”, draft-calhoun-sip-aaareqs-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-aaareq-01
S. Nagarayan, et al., "A Diameter accounting
application for the Session Initiation Protocol", draftnarayanan-sipping-aaa-diameter-00