SIPPING C. Jennings
Internet-Draft Cisco Systems
Expires: April 26, 2006 G. Jun
Bitpass, Inc.
J. Fischl
CounterPath Solutions, Inc.
H. Tschofenig
Siemens
October 23, 2005
Payment for Services in Session Initiation Protocol (SIP)
draft-jennings-sipping-pay-03.txt
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Copyright (C) The Internet Society (2005).
Abstract
Service usage might require some form of compensation and this is
also true for many communication systems where an entity receiving a
call should be able to charge the caller. This is necessary for
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allowing fair communication between two communicating parties and is
a major strategy for reducing the viability of SPAM. This draft
proposes an approach for doing this in SIP using the Security
Assertion Markup Language (SAML). It relies on a third party to act
as a payment provider and is designed for low value transactions. It
does not aim to provide the same capability as other authentication,
authorization and accounting systems.
This draft is in a fairly early state and has many details that are
missing. Earlier versions of this document did not use SAML. This
version offers a sketch of what the SAML based solution would look
like but still lacks many details that would be needed for an actual
implementation.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Requirements, Assumptions and Goals . . . . . . . . . . . . . 7
4. Non-Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . 9
5.3. Computing costs . . . . . . . . . . . . . . . . . . . . . 10
5.4. Transition Scenarios . . . . . . . . . . . . . . . . . . . 10
5.4.1. Merchant Proxy . . . . . . . . . . . . . . . . . . . . 11
5.4.2. Customer Proxy . . . . . . . . . . . . . . . . . . . . 13
5.5. Payment Provider Behavior . . . . . . . . . . . . . . . . 14
5.6. Account Names . . . . . . . . . . . . . . . . . . . . . . 15
5.7. Merchant Fetching Public Key . . . . . . . . . . . . . . . 15
6. SIP Extensions . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Update response code 402 . . . . . . . . . . . . . . . . . 16
7. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1. Charge . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.2. Request for Payment . . . . . . . . . . . . . . . . . . . 17
7.3. Receipt . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.4. Refunds . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.5. Verifying the Receipt . . . . . . . . . . . . . . . . . . 21
8. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . . 21
8.1. SPAM . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.2. Micro Billing . . . . . . . . . . . . . . . . . . . . . . 22
9. XML Schemas . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Payment Request and Payment Receipt . . . . . . . . . . . 22
9.2. Charge . . . . . . . . . . . . . . . . . . . . . . . . . . 24
10. Security Considerations . . . . . . . . . . . . . . . . . . . 26
10.1. Stolen Assertion . . . . . . . . . . . . . . . . . . . . . 27
10.2. MitM Attack . . . . . . . . . . . . . . . . . . . . . . . 27
10.3. Forged Assertion . . . . . . . . . . . . . . . . . . . . . 27
10.4. Replay Attack . . . . . . . . . . . . . . . . . . . . . . 28
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
11.1. Payment-Receipt Header . . . . . . . . . . . . . . . . . . 28
11.2. 402 Response . . . . . . . . . . . . . . . . . . . . . . . 29
11.3. charge+xml . . . . . . . . . . . . . . . . . . . . . . . . 29
12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 30
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
13.1. Normative References . . . . . . . . . . . . . . . . . . . 30
13.2. Informational References . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32
Intellectual Property and Copyright Statements . . . . . . . . . . 33
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1. Introduction
Figure 1 shows protocol exchange that implements payment capabilities
using SIP and SAML. It involves three parties: a Customer who is the
caller, a Merchant who is the party being called, and a third party
to broker the transaction, which is called the Payment Provider. In
SAML terminology this entity is called identity provider.
Payment Provider (P) Customer (C) Merchant (M)
| | 1) Call |
| |--------------------------->|
| | |
| | 2) 402 + Charge |
| 3) Request for |<---------------------------|
| a Payment | |
|<-------------------------| |
| | |
| 4) SAML Assertion | |
| (=Receipt) | |
|------------------------->| |
| | 5) Call + Receipt |
| |--------------------------->|
| | |
| | 6) 200 OK |
| |<---------------------------|
| | |
Figure 1: Communication Overview
The Customer and the Merchant interact with each other using SIP and
the Customer uses HTTP to exchange messages with the Payment Provider
Payment Provider.
1. Initially, Customer makes a call to Merchant
2. Merchant determines that a payment is required and includes
information about the payment in an Charge body of a 402 (Payment
Required) response to Customer
3. Customer looks at this Charge and decides to make a payment. The
Customer therefore instructs its Payment Provider to make a
transfer from the Customer's account to the Merchant's account
using a request for a SAML assertion with the extensions defined
in this document.
4. The Payment Provider returns a receipt for this transfer. This
receipt is a SAML Assertion (or a SAML Artifact, if the exchange
is triggered by a proxy or if desired by the Customer).
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5. Customer resubmits the call to Merchant but this time provides
the Receipt for the transaction. Merchant determines whether the
Receipt is valid (by checking the digital signature and the
content of the assertion) and continues with the call processing,
if the authorization was successful.
The Charge contains information about the three parties:
o Payment Providers that are acceptable to the Merchant
o cost(s) of the transaction
o account identifier for the Merchant at each supplied Payment
Provider
o cookie to make it easier for the Merchant to avoid replay attacks.
The Customer includes this information when making the Request for
Payment to a Payment Provider; adds its own account information and
authorization password; and sends this to Payment Provider, which
produces a Receipt for the transaction if it is successful. This
transfer from Customer to Payment Provider is made across an
encrypted, integrity protected channel. The Receipt includes a
timestamp when the Payment Provider made the transaction and protects
the Receipt with a digital signature.
The Customer resubmits the call to the Merchant with the Receipt from
Payment Provider. The Merchant can check for replay attacks using
the timestamp and the cookie initially provided with the Charge. The
Merchant can then check the signature is valid using Payment
Provider's public key.
Figure 1 does not show the the interaction between the Merchant and
the Customer if refunding is necessary. Additionally, the
interaction between the Merchant and the Payment Provider to
reconcile is not shown.
The proposal described in this document does not aim to provide
functionality equivalent to AAA protocols. For example, there is no
guarantee or recourse if Merchant does not provide the service after
Customer transfers money into Merchant's account. The system is
designed for low value transactions in which, if Merchant cheats,
Customer can choose to never deal with Merchant again but the value
of the transaction is lost. This scheme is designed for systems
whereby the communication between Merchant and Customer and the
communication between Customer and Payment Provider can be executed
in real time. While it is possible to develop schemes that deal with
some of these problems, Payment Providers deploying them have not
been willing to provide services for transaction fees on the order of
one US cent. The authors believe that the simplified scheme
presented here will make it easier to reach these low value
transactions.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
This work adopts the terminology from the framework in RFC 2801 [12].
Additionally, the following terms are used:
Customer:
The entity that is paying for the call, typically the SIP UAC or a
proxy in the same administrative domain as the SIP UAC.
Merchant:
The entity wishing to be paid for the call, typically the SIP UAS
or a proxy in the same administrative domain as the UAS.
Payment Provider:
The third party that handles the transfer of currency from
Customer to Merchant. The Internet Open Trading Protocol (IOTP)
[12] refers to this as the Brand.
Charge:
The information sent from the Merchant to the Customer describing
what payment is needed.
Request for Payment:
The information sent from the Customer to the Payment Provider
describing the transfer of currency needed. This request is
implemented as a request for a SAML assertion.
Receipt:
The information sent from the Payment Service Provider to the
Customer and passed on to the Merchant. The Receipt is either a
SAML assertion or a SAML artifact. The Receipt tells the Customer
that the payment transaction was successfully completed (if either
a SAML assertion or a SAML artifact is returned). The Merchant
receives an assurance that the transaction was successful after
receiving the Receipt and verifying it successfully.
Currency:
Could be a classical currency such as the Euro or US Dollar or
could be a pseudo currency such as airline mileage points.
Assertion:
An assertion is an XML document that contains authentication
statements, attribute statements and authorization decision
statements. In an authentication statement, an issuing authority
asserts that a certain subject was authenticated by certain means
at a certain time. In an attribute statement, an issuing
authority asserts that a certain subject is associated with
certain attributes which has certain values. In an authorization
decision statement, a certain subject with a certain access type
to a certain resource has given certain evidence that the identity
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is correct. The assertion is digitally signed to protect it
against unwanted modifications by third parties.
Artifact:
An artifact is a base-64 encoded string that is 40 bytes long that
serves as a pointer to a server that temporarily stores an
assertion. The first 20 bytes provide an indication from which
server the assertion needs to be retrieved. The remaining 20
bytes identify a unique assertion.
For further terminology related to SAML the reader is referred to
[2].
3. Requirements, Assumptions and Goals
This section lists some basic requirements and goals for the protocol
presented in this document:
o Provide a system for callers to pay the person they are calling
using a 3rd party clearing house. A trust relationship between
the Merchant and the Payment Provider and between the Customer and
the Payment Provider must exist. The Customer must be able to
authenticate itself to the Payment Provider and to instruct the
Payment Provider to transfer money from its account to another
account (i.e., the account of the Merchant). It is not necessary
to for the Customer and the Merchant to have an account at the
same Payment Provider.
o The protocol must support multiple currencies and must offer the
ability for the Customer to learn the price before initiating a
transaction.
o Support various billing models including: flat rate, per unit
time, per unit data
o The solution must be cost effective for low cost transactions.
o The solution must allow Customers to remain anonymous towards the
Merchants.
o Support for a simple refund mechanism must be provided.
4. Non-Goals
[Editor's Note: A future version of this document will provide a list
of non-goals in addition to goals.]
There needs to be a way for Customers and Merchants to enroll and
transfer money in and out from their accounts. The mechanisms to
accomplish this functionality are outside the scope of this document.
They can be provided by using web forms to enroll, get an account
number, and provide the typical credit card mechanism to transfer
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money into the account. Transfers out of the account could be done
with the typical mechanism for transfers to bank accounts.
5. Protocol
5.1. UAC Behavior
The UAC SHOULD indicate that it can accept the application/charge
MIME type in SIP requests it sends.
In the case where the UAC receives a 402 response containing an
application/charge body, it MUST check that this Charge is acceptable
to the user of the UAC. This could be done using a policy that was
previously entered by the user. If the Charge contains a Payment
Provider with which the user has an account and the Charge is
acceptable, then the UAC sends a Request for Payment to the Payment
Provider.
The UAC needs to look at the available Payment Providers, cost, and
currency and select an appropriate one. The UAC MUST copy the
PaymentServiceProviderData fields from the Charge into the Request
for Payment parameters. The UAC must look at the cost elements in
the Charge to decide how large a payment the user wishes to make and
set the amount and currency parameters appropriately. Finally, it
needs to fill the CustomerData parameters. It is critical that the
UAC check the certificate of the HTTPS TLS connection as specified in
RFC 2818 [3] and RFC 2246 [4].
After selecting and constructing the SAML request message it is sent
over the HTTPS secured connection. The detailed format of the
request is shown in Section 9.
The response will be an error or a SAML artifact/assertion based on
the Request for Payment. The user needs to be informed if an error
is received and the transaction SHOULD NOT be retried unchanged.
When a valid response is received, the UAC SHOULD resubmit the SIP
request that caused the 402 but this time by including the Receipt
(i.e., the artifact or the assertion).
The UAC needs to compute the amount of payment it wishes to make by
looking at the cost information provided as part of the Charge. The
UAC is also responsible for determine when a new payment has to be
made and refreshing the call with additional payments before this
happens. For example, the UAC could initially decide to provide
enough payment for 3 minutes. After 2.5 minutes the UAC might decide
to pay for an additional 3 minutes. It would do this by issuing a
new Payment Request to the Payment Provider for an additional 3
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minutes' worth of resources and then sending the new receipt with a
SIP Re-INVITE or UPDATE transaction to the UAS.
5.2. UAS Behavior
When the UAS receives a request it wishes to charge for, it should
check whether the UAC has set the Accept header to include
application/charge. If it has, it MAY reject the request with a 402
and attach an application/charge to the response. Note that the
application/charge document can be attached on any failure response.
For example, this could allow a UAS to combine the Charge with a
request for authorization in a 401 response. It needs to include
lists of all the Payment Providers that are acceptable to the UAS and
include the identity of the Merchant. It also needs to form a list
of currencies that are acceptable and what the cost in each one is.
The merchant may also specify a minimum cost and/or a maximum cost in
the Charge. The costs are described in Section 5.3.
When the UAS receives a request that contains a Receipt (as a SAML
assertion) as defined in [5], it MUST verify that the assertion/
artifact is valid using the following steps.
1. The UAS needs to make sure that the amount of the payment is
appropriate and if this receipt matches a previous Charge to
prevent replay attacks.
2. Check that the digital signature of the Receipt is valid. This
includes path validation and to check that the Payment Provider's
certificate is still valid.
3. Check that the time between the payment and now is acceptably
low. This MUST be a configurable parameter and should default to
30 seconds. The UAS SHOULD support NTP RFC 1305 [13]. [Editor's
Note: I think that this mechanism is not needed.]
4. The UAS MUST check that this receipt has not been previous used.
The limited time window limits the amount of state the UAS must
keep to make this check. If several UASs are using the same
merchant-id at the , this replay detection needs to be done
across all the UASs. The ChargeData can be used with opaque
encrypted data to help do this. [Editor's Note: This aspect has
to do with the first item.]
If the payment is accepted, it is the Merchant's responsibility to
end the call after the amount paid becomes inadequate to cover the
session. The UAS SHOULD use a mechanism such as SIP session timer
[14]. The UAC MAY send a Re-INVITE or an UPDATE message with a new
receipt for a payment to prolong the session.
There are certain cases where the Merchant may wish to offer a
refund. This could occur if the Customer has prepaid for a 10 minute
session and the call terminates after 1 minute. In this case, the
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Merchant may wish to provide a refund for the 9 minutes that were not
used. Alternatively, the Customer could provide a receipt to place a
call but the destination is busy. In this case, the Merchant would
likely want to provide a full refund.
The refund mechanism is simple and identical to the Payment Request
procedure described in Section 5.1. In this case, the Merchant posts
a Payment Request to the Payment Provider specified in the Payment
Receipt.
If the call ends early or can not be completed, it may still be
possible that the Customer has provided a receipt of payment where no
service has been delivered. This may have occurred due to an
upstream proxy error or a network connectivity problem between the
UAC and the UAS. Since the receipt of payment was never delivered to
the UAS, there is no immediate mechanism of delivering a refund to
the Customer.
5.3. Computing costs
There are three types of costs:
o initial setup costs,
o costs per second, and
o cost per unit data.
All three costs are added together to form the total cost and are
assumed to be zero if not specified. The cost of the first time unit
block size worth of time and the first data unit block size of data
are considered to be included in the initial connection or setup
costs.
If a call costs 30 cents to connect and then 12 cents per minute and
is billed in 15 second increments (rounded down), the cost would be
set so that the currency was USD and the currency divisor (power of
10) was 1000 making the initial cost 300, the cost per unit time is
40, and the time unit size is 15000. If the time is to be rounded
up, then some extra to cover the price of the first increment would
be added to the connect cost.
Note that the additional specification of a currency divisor allows
all currency amounts to be specified in fixed point. In the above
example, a currency divisor of 1000 means that all currency amounts
are in tenths of a cent (USD).
5.4. Transition Scenarios
The deployment of the mechanisms described in this document might
take place incrementally. This section provides some information
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about two likely transition scenarios: Merchant Proxy and Customer
Proxy.
5.4.1. Merchant Proxy
In this scenario, the Customer places a call to a Merchant where the
Merchant's UAS does not implement this mechanism. The Merchant's
Proxy acts on behalf of the Merchant. The key difference here is
that the Merchant's proxy must use SAML Artifacts instead of SAML
Assertions and an extra step must be taken by the Proxy to validate
the Artifact with the Payment Provider.
Note: Many of the normal steps in a SIP call flow have been left out
of this example to focus on the pertinent items.
Customer Merchant Proxy Payment Provider Merchant
| | | |
| INVITE F1 | | |
|--------------->| | |
| 402+Charge F2 | | |
|<---------------| | |
| | | |
| Request for Payment F3 | |
|-------------------------------->| |
| Receipt (Artifact) F4 | |
|<--------------------------------| |
| | | |
| INVITE F5 | | |
| + Receipt | | |
|----------------> | |
| 180 F6 | | |
|<---------------| | |
| | Artifact F7 | |
| |--------------->| |
| | Assertion F8 | |
| |<---------------| |
| | INVITE F9 |
| |-------------------------------->|
| | 200 F10 |
| 200 F11 |<--------------------------------|
|<---------------| | |
F1 INVITE: Customer -> Merchant Proxy
The Customer sends a SIP INVITE which arrives at the Merchant's
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Proxy.
F2 402 + Charge: Merchant Proxy -> Customer
The Merchant's Proxy, acting on behalf of the Merchant, sends a 402
response with a Charge Request back to the Customer. The Charge
Request must indicate that the Payment Provider must provide a SAML
Artifact rather than a SAML Assertion as a Receipt.
F3 Request for Payment: Customer -> Payment Provider
Based on the contents of the Charge Request, the Customer makes a
Request for Payment to one of the specified Payment Providers. The
Request for Payment specifies that the Payment Provider should return
an Artifact.
F4 Receipt (Artifact): Payment Provider -> Customer
The Payment Provider transfers the appropriate funds to the
Merchant's account and returns a SAML Artifact to the Customer.
F5 Invite + Receipt: Customer -> Merchant Proxy
The Customer inserts a new header containing the SAML Artifact into
the INVITE request.
F6 180: Merchant Proxy -> Customer
The Merchant Proxy immediately sends a 180 response to the Customer
indicating that the Receipt is being validated with the Payment
Provider.
F7 Artifact: Merchant Proxy -> Payment Provider
The Merchant Proxy, acting on behalf of the Merchant, requests the
SAML Assertion from the Payment Provider by providing the SAML
Artifact.
F8 Assertion: Payment Provider -> Merchant Proxy
The SAML Assertion is returned to the Merchant Proxy.
F9 INVITE: Merchant Proxy -> Merchant
Since the SAML Assertion was valid, and was not used before by this
Merchant, the Merchant Proxy forwards the INVITE request to the
Merchant.
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5.4.2. Customer Proxy
In this example, the Customer endpoint does not implement this
payment mechanism so the Customer's Edge Proxy acts on behalf of the
Customer. Note that the Merchant's Proxy has been left out of the
example.
Customer Customer Proxy Payment Provider Merchant
| | | |
| INVITE F1 | | |
|--------------->| | |
| 100 F2 | | |
|<---------------| INVITE F3 |
| |------------------------------------->|
| | 402+charge F4 |
| |<-------------------------------------|
| | | |
| | Request Payment F5 | |
| |-------------------->| |
| |Receipt(Artifact) F6 | |
| |<--------------------| |
| | | |
| | INVITE+Receipt F7 |
| |------------------------------------->|
| | | 180 F8 |
| |<-------------------------------------|
| 180 F9 | | |
|<---------------| | |
| | | Artifact F10 |
| | |<---------------|
| | | Assertion F11 |
| | |--------------->|
| | | |
| | |200 F12 |
| |<-------------------------------------|
| 200 F13 | | |
|<---------------| | |
F1 INVITE: Customer -> Customer Proxy
The Customer sends an INVITE to the Customer's Edge Proxy (in the
same Administrative domain as the Customer).
F2 100: Customer Proxy -> Customer
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The Customer's Proxy sends a 100 response to the Customer in order to
quench retransmissions.
F3 INVITE: Customer Proxy -> Merchant
The Customer's Proxy forwards the request to the Merchant. In most
cases, the Merchant will also have a Proxy but this has not been
indicated in this example.
F4 402 + charge: Merchant -> Customer Proxy
The Merchant sends back a 402 response with a Charge Request to the
Customer's Proxy.
F5 Request Payment: Customer Proxy -> Payment Provider
The Customer Proxy, acting on behalf of the Customer, makes a Request
for Payment to the selected Payment Provider specifying that a SAML
Artifact is to be returned. It is assumed that the Customer's Proxy
would have the payment credentials and Payment Provider preferences
for the Customer.
F6 Receipt (Artifact): Payment Provider -> Customer Proxy
The Receipt is returned as a SAML Artifact.
F7 INVITE + Receipt: Customer Proxy -> Merchant
The Customer Proxy now inserts the SAML Artifact as a SIP header in
the INVITE request to be sent to the Merchant.
F10 Artifact: Merchant -> Payment Provider
The Merchant pulls the SAML Artifact from the INVITE and sends a
request to the specified Payment Provider to return the SAML
Assertion.
F11 Assertion: Payment Provider -> Merchant
When the SAML Assertion is verified, the Merchant completes the call
and sends a 200 OK response.
5.5. Payment Provider Behavior
The primary function of the Payment Provider is to receive Requests
for Payments over HTTPS, to transfer the currency from one account to
another one, and to return a Receipt over HTTPS.
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A Payment Provider MUST support TLS to offer channel security (with
integrity, replay and confidentiality protection).
When a Payment Provider receives a Request for Payment, it performs
the following steps:
1. Verify that the customer-id corresponds to a valid account and
that the presented credentials are correct for the given account.
If either the account is invalid or the authentication of the
account holder was not successful then an error message MUST be
returned. The procedure for responding to error responses is
described in the respective SAML specifications and the Payment
Provider MUST respect the SAML specific message processing
handling.
2. MUST validate that the currency is acceptable by the Merchant
3. MUST ensure that the Customer has sufficient funds, and
4. MUST verify that the account identifier of the Merchant
corresponds to a valid account.
5. MUST create a Receipt as a SAML assertion (or an artifact) by
setting the corresponding fields in the Assertion.
6. MUST set the Date to the current time.
7. MUST digitally sign the Assertion.
8. MUST transfer the money from the customer's account to the
merchant's account.
9. MUST return either the Assertion or the Artifact back to the
payment requesting entity (depending on the request).
5.6. Account Names
Note: Need to define they syntax for valid account id. Email style
account names (where the host part is not the same as the PSP domain)
need to be allowed.
5.7. Merchant Fetching Public Key
The Merchant needs to be able to fetch the Payment Provider's public
key. This MAY be done by an HTTPS request to a URI provided by the
Payment Service Provider. The Merchant and should use some standard
online certificate revocation mechanism to protect against the
private key being compromised. Note that the s may wish to use
signing certificates that are only valid for a short period of time.
The duration should be determined by the PSP based on the amount of
money at risk.
6. SIP Extensions
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6.1. Update response code 402
This document updates the 402 response code in RFC 3261 [6] to mean
that "A Payment is Required". Other mechanisms are used to indicate
what type of payment is required. In the case of this draft, a
particular MIME body type indicates the type of payment required. A
single 402 may indicate that more than one type of payment is
required. Both proxies and UASs can issue a 402 response code.
7. Syntax
7.1. Charge
The Charge contains a list of costs and a list of Payment Providers.
The Customer can choose to pay any one of the provided costs and can
choose any one of the Payment Providers to use, as long as the
Payment Provider supports the currency for the chosen cost. A
Merchant can also specify a currency namespace with a particular
cost. This allows the Merchant to create non-standard currencies,
e.g., airline mileage/points. A cost can also specify an optional
minimum and/or maximum cost.
The currency is specified as 3 uppercase letters using the ISO
currency code from ISO.4217. All cost attributes (initialCost,
costPerUnitTime, costPerUnitData, minCost, maxCost) are specified in
currency units where the actual cost is to be divided by
currencyDivisor. All amounts are base 10 integers with no leading
zeros and zero is not a valid entry. The timeUnitSize is in
milliseconds. The dataUnitSize is in octets. merchantBits and
pspBits are base 64 encoded data
Each PaymentServiceProvider provided in a Charge has a serviceUrl
attribute which is where the PaymentRequest will be sent to. There
is also an optional url attribute which is a general HTTP URL that
can provide information about the specific Payment Provider.
A simple example is provided with a single charge for a single
Payment Provider where the initial cost is $0.25 and the charge is
0.06/minute charged in 6 second increments.
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The body is defined with XML Schema. The syntax is specified in
Section 9.
7.2. Request for Payment
In order to make the generation of the Request for Payment as simple
as possible for both the Customer and the Payment Provider, the
RequestForPayment is constructed as a SAML Request. The Request for
Payment consists of three main components:
o The ChargeData is copied from the Charge from the Merchant.
o The PaymentServiceProviderData is selected from the possible
Payment Providers by the Customer.
o The amount that needs to be payed.
o Additionally, the Customer might provide authentication
credentials as part of the SAML request.
Note that it is up to the Customer to select a currency.
The ChargeData consists of the following fields from the Payment
Request: ChargeExpiry and merchantBits.
The PaymentServiceProviderData is copied from the chosen Payment
Provider/Cost in the Payment Request consisting of the following
fields: merchantId, serviceUrl, pspBits, currencyNamespace,
currencyDivisor, and currency.
The CustomerData is provided by the Customer and consists of the
following fields: customerId, customerAuth, customerBillingCode and
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an amount. The customerBillingCode is optional and could be stored
by the PaymentServiceProvider and included in the transaction history
for the convenience of the Customer. The amount divided by
currencyDivisor indicates the amount of funds being requested to be
transferred from the Customer to the Merchant in currency units. The
customerId is a token identifying the Customer's account with the and
the customerAuth is the authentication credentials.
The SAML Request MUST be TLS protected. Authentication of the client
MUST be provided, either as part of the TLS handshake or using SAML
specific mechanisms.
The XML schema of the Payment Request is shown in Section 9.
Below is an example 'Payment Request' based on the previous 'Charge'
example with a payment of $0.30.
The example below should give the reader a first impression about the
layout of the Payment request. A future version of this document
will provide examples that can be validated.
joe@example.com
2005-02-01T13:40:26.52Z
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MDE1Mw==
15
https://psp.example.com/paymentService
1000
USD
joe
300
7.3. Receipt
A SAML assertion or artifact is returned, as a Receipt, in response
to the Request for Payment. The assertion or the artifact is
included in the SIP message, as described in [5], in the reINVITE
from the UAC.
The following example receipt is returned from the Payment Provider
and corresponds to a Receipt for a payment of $0.30.
The example below should give the reader a first impression about the
layout of the Payment response. A future version of this document
will provide examples that can be validated.
Success
www.payment-provider.com
joe@example.com
urn:oasis:names:tc:SAML:2.0:ac:\
classes:PasswordProtectedTransport
MDE1Mw==
15
https://psp.example.com/paymentService
1000
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USD
300
7.4. Refunds
Refunds share the same syntax as a Request for Payment. The refund
fields populated from the Receipt are: ChargeId, ChargeExpiry,
merchantBits, merchantId, serviceUrl, pspBits, currencyNamespace,
currencyDivisor, and currency. The customerId and customerAuth refer
to the Merchant's account information with the referenced by the
serviceUrl. The amount to refund is determined by the Merchant at
the time of the refund.
7.5. Verifying the Receipt
The Merchant MUST verify the signature in the Receipt by applying the
following steps:
1. Check ReceiptData.Date. If too old, reject.
2. Check whether receipt-id has been accepted in a previous payment
since the TTL used by the UAS. If so, reject. (See section 7.6
on replay prevention)
3. Check whether Charge comes from this UAS. If not, reject.
4. Perform RSA signature verification. UAS chooses the public key
based on PaymentServiceProvider-id.
8. Usage Scenarios
This section shows the applicability of the proposed protocol for two
example scenarios.
8.1. SPAM
Payment at risk has been suggested as part of a possible solution to
SPAM in VoIP systems [15]. The idea is that A would call B. If A was
not on B's white list, B could ask A to pay 5 cents for the privilege
of ringing B's phone. A could pay, and if B wished, B could refund
the 5 cents by simply doing a second payment from B to A. The payment
service provider would collect two transaction fees in this scenario.
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Another possible scenario is that B simply requests that A donate 5
cents to one of B's favorite charities and show B the receipt for
this transaction.
8.2. Micro Billing
In this scenario, a merchant running a PSTN GW may charge a customer
5 cents to connect and operate for the first 90 seconds and then may
charge 5 more cents for each additional minute. The customer would
initially transfer 5 cents and then, before the 90 seconds ran out,
would transfer another 5 cents and keep on doing this until the call
ended.
9. XML Schemas
This section defines the XML schema for the protocol introduced in
this document.
9.1. Payment Request and Payment Receipt
This schema is a strawman proposal and will change as soon as an
agreement for the required attributes can be found.
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9.2. Charge
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10. Security Considerations
The security properties of the proposed protocol depends on the
security of the communication between the three parties. The
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following threats and countermeasures have been considered:
10.1. Stolen Assertion
Threat:
An adversary can eavesdrop on the communication between the
Customer and the Payment Provider and between the Customer and the
Merchant and thereby learn the SAML assertion (Receipt). The
adversary could use this Assertion to request a service from the
Merchant on behalf of the legitimate Customer.
Countermeasures:
Two countermeasures are provided to deal with this treat. The
communication between the Customer and the Payment Provider must
be confidentiality, integrity and replay protected. The
communication between the Customer and the Merchant MAY experience
either hop-by-hop (e.g., TLS in a hop-by-hop fashion between the
Customer, SIP proxies and the Merchant) or end-to-end by using
S/MIME protection. Furthermore, the content of the requested SAML
assertion contains statements that prevent reusage of the SAML
assertion in other contexts. The Charge, for example, provides a
Cookie value that allows the Merchant to match the Charge to the
Receipt. The identities of the Customer and the Merchant are
included in the Receipt and the lifetime might be limited.
10.2. MitM Attack
Threat:
Since the SAML assertion is carried within a SIP message sent by
the Customer towards the Merchant an intermedidate SIP proxy could
use the assertion in order to impersonate the user towards the
Merchant in future protocol sessions.
Countermeasures:
This document does not assume that the Assertion is bound to a
symmetric or an asymmetric key. Hence, only the content of the
assertion can be used to prevent replay attacks and man-in-the-
middle attacks. Similarly to the threat described in Section 10.1
the content of the assertion limits its usage to particular
endpoints, potentially for a particular duration, for a particular
service and for certain amount of time.
10.3. Forged Assertion
Threat:
A malicious Customer or a malicious intermediate SIP proxy could
forge or alter a SAML assertion in order to communicate with the
Merchant to lead to unexpected behavior or even to refunding to a
preselected account.
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Countermeasures:
To avoid this kind of attack, the Payment Provider must assure
that proper mechanisms for protecting the SAML assertion is
provided. It is recommended to protect the assertion using a
digital signature.
10.4. Replay Attack
Threat:
An adversary might eavesdrop an assertion in order to later replay
it in order to gain access to resources (e.g., to make a phone
call).
Countermeasures:
When the Customer transmits a SIP message that requires payment
then the Merchant creates an Charge that contains a Cookie value.
The Charge will be used by the Customer to obtain an assertion by
the Payment Provider and will again be presented to the Merchant.
The Merchant is therefore able to determine whether the Cookie is
still valid and that it can be associated with an Charge. The
Charge is only valid for a certain time. Timestamp information
carried inside the Assertion might also indicate a validity
timeframe.
11. IANA Considerations
This specification registers a new header and a new response code.
IANA is requested to make the following updates in the registry at:
http:///www.iana.org/assignments/sip-parameters. It also defines a
new mime type that IANA is requested to add to the registry at
http://www.iana.org/assignments/media-types/application.
This specification registers a new header and a new response code.
IANA is requested to make the following updates in the registry at:
http:///www.iana.org/assignments/sip-parameters. It also defines a
new mime type that IANA is requested to add to the registry at
http://www.iana.org/assignments/media-types/application.
11.1. Payment-Receipt Header
Add the following entry to the header sub-registry.
Header Name compact Reference
----------------- ------- ---------
Payment-Receipt [RFCXXXX]
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11.2. 402 Response
Add the following entry to the response code sub-registry under the
"Request Failure 4xx" heading.
402 Payment Required [RFCXXXX]
11.3. charge+xml
To: ietf-types@iana.org
Subject: Registration of MIME media type application/charge+xml
MIME media type name: application
MIME subtype name: charge+xml
Required parameters: None
Optional parameters: charset
Same as charset parameter of application/xml [RFC3023]
Encoding considerations:
Same as for application/xml [RFC3023]
Security considerations: TBD
Interoperability considerations: TBD
Published specification: None
Applications which use this media type: Any MIME-compliant transport
Additional information:
Magic number(s): None
File extension(s): None
Macintosh File Type Code(s): None
Person & email address to contact for further information:
Cullen Jennings
Intended usage: COMMON
Author/Change controller:
the IESG
[Editor's Note: This section also needs to discuss the extensions to
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the SAML request and the SAML assertion.]
12. Open Issues
Would like to unify the body syntax so that it can also be used for
Advice Of Charge information.
What does a SAML Artifact look like and how does it get associated
with the Charge and Dialog ID.
How can a Proxy protect from a SAML Artifact from being used in a
Replay Attack?
13. References
13.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Maler, E. and J. Hughes, "Technical Overview of the OASIS
Security Assertion Markup Language (SAML) V1.1", March 2004.
[3] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[4] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[5] Tschofenig, H., "Using SAML for SIP",
draft-tschofenig-sip-saml-04 (work in progress), July 2005.
[6] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[7] International Organization for Standardization, "Codes for the
representation of currencies and funds", ISO Standard 4217,
2001.
[8] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
[9] Maler, E. and R. Philpott, "Security and Privacy Considerations
for the OASIS Security Markup Language (SAML) V1.1",
September 2003.
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[10] Maler, E., Philpott, R., and P. Mishra, "Bindings and Profiles
for the OASIS Security Assertion Markup Language (SAML) V1.1",
September 2003.
[11] Maler, E., Philpott, R., and P. Mishra, "Assertions and
Protocol for the OASIS Security Assertion Markup Language
(SAML) V1.1", September 2003.
13.2. Informational References
[12] Burdett, D., "Internet Open Trading Protocol - IOTP Version
1.0", RFC 2801, April 2000.
[13] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation", RFC 1305, March 1992.
[14] Donovan, S. and J. Rosenberg, "Session Timers in the Session
Initiation Protocol (SIP)", draft-ietf-sip-session-timer-15
(work in progress), August 2004.
[15] Rosenberg, J., "The Session Initiation Protocol (SIP) and
Spam", draft-ietf-sipping-spam-01 (work in progress),
July 2005.
[16] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
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Authors' Addresses
Cullen Jennings
Cisco Systems
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 902-3341
Email: fluffy@cisco.com
Gyuchang Jun
Bitpass, Inc.
3300 Hillview Avenue, Suite 165
Palo Alto, CA 94304
USA
Phone: 650 354-1882
Jason Fischl
CounterPath Solutions, Inc.
8th Floor, 100 West Pender Street
Vancouver, BC V6B 1R8
Canada
Phone: +1 604 320-3340
Email: jason@counterpath.com
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@siemens.com
URI: http://www.tschofenig.com
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