Internet Engineering Task Force SIP WG
Internet Draft J. Loughney
Nokia
G. Camarillo
Ericsson
draft-ietf-sipping-aaa-req-04.txt
December 14, 2003
Expires: May, 2004
Authentication, Authorization and Accounting
Requirements for the Session Initiation Protocol
STATUS OF THIS MEMO
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Abstract
As SIP services are deployed on the Internet, there is a need for
authentication, authorization and accounting of SIP sessions. This
document sets out the basic requirements for this work.
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Table of Contents
1 Introduction ........................................ 4
1.1 RADIUS .............................................. 4
1.2 Terminology and Acronyms ............................ 5
1.3 Requirements Language ............................... 5
2 Requirements ........................................ 6
2.1 Common Requirements ................................. 6
2.1.1 Communication within the Same Domain ................ 6
2.1.2 Communication between Different Domains ............. 6
2.1.3 Discovery ........................................... 6
2.1.4 Ability to Integrate Different Networks, Services
and Users ........................................... 6
2.1.5 Updating SIP Server Entries ......................... 6
2.1.6 SIP Session Changes ................................. 7
2.1.7 Reliable Transfer of Protocol Messages .............. 7
2.1.8 Call Setup Times .................................... 7
2.1.9 Security ............................................ 7
2.2 Authentication Requirements ......................... 7
2.2.1 Authentication Based on SIP Requests ................ 7
2.2.2 Flexible Authentication of SIP Requests ............. 8
2.3 Authorization Requirements .......................... 8
2.3.1 Ability to Authorize SIP Requests ................... 8
2.3.2 Information Transfer ................................ 8
2.3.3 User De-authorization ............................... 8
2.3.4 User Re-authorization ............................... 9
2.3.5 Support for Credit Control .......................... 9
2.4 Accounting Requirements ............................. 9
2.4.1 Separation of Accounting Information ................ 9
2.4.2 Accounting Information Related to Session
Progression ......................................... 10
2.4.3 Accounting Information Not Related to Session
Progression ......................................... 10
2.4.4 Support for One-Time and Session-based Accounting
Records ............................................. 10
2.4.5 Support for Accounting on Different Media
Components .......................................... 10
2.4.6 Configuration of Accounting Generation Parameters ... 10
2.4.7 Support for Arbitrary Correlations .................. 10
3 Scenarios ........................................... 11
3.1 WLAN Roaming Using Third Party Service Providers .... 12
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3.2 Conditional Authorization ........................... 13
4 Security Considerations ............................. 13
5 Acknowledgements .................................... 13
6 Authors' Addresses .................................. 14
7 Normative References ................................ 14
8 Informative References .............................. 14
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1 Introduction
The AAA working group is chartered to work on authentication,
authorization and accounting solutions for the Internet. This work
consists of a base protocol, applications, end-to-end security
application and a general architecture for providing these services
[3]. The AAA working group has specified applicability of AAA-based
solutions for a number of protocols (e.g., AAA requirements for
Mobile IP [4]).
SIP is a signalling protocol for creating, modifying and terminating
different types sessions such as Internet phone calls, multimedia
distribution and multimedia conferences [1]. SIP sessions have needs
for session authentication, authorization and accounting (AAA.)
In order to authenticate and authorize users, it is typically more
convenient for SIP entities to communicate with a AAA sever than to
attempt to store user credentials and profiles locally. SIP entities
use the SIP-AAA interface to access the AAA server.
This document provides requirements on for the interface between SIP
entities and AAA servers. While accounting requirements are
discussed, this document does not cover SIP charging or billing
mechanisms.
One possible use of this document would be to create a AAA
application for SIP. Any protocol meeting the requirements outlined
by this document could be used. Possible candidates, among others,
are Diameter [3] and XML-based protocols following the web-services
model.
1.1 RADIUS
The main purpose of this document is to provide input to designers
working on AAA applications using new protocols such as Diameter and
XML-based protocols. Nevertheless, a few, limited RADIUS [5]
extensions may meet some of the requirements in this document (for
instance, some of the authentication requirements). We expect that
while RADIUS with these limited extensions will meet particular
functional requirements, it will not meet other requirements that are
important. The following are some requirements that are not expected
to be met by RADIUS:
1. Section 2.1.3: RADIUS does not support a discovery feature.
2. Section 2.1.7: RADIUS does not support reliable message
delivery.
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The following list contains the requirements that can be met by
RADIUS or RADIUS extensions.
1. Section 2.1.2: Communication between domains does not scale
well in RADIUS. As a result, inter-domain communications
are typically handled using a proxy architecture [6].
2. Section 2.1.5: RADIUS clients would need to support Dynamic
Authorization [7].
3. Section 2.1.9: RADIUS clients would need to rely on a
lower-layer security protocol, such as IPSec, to perform
mutual authentication.
4. Section 2.3.3: RADIUS clients would need to support Dynamic
Authorization [7].
5. Section 2.3.4: RADIUS clients would need to support Dynamic
Authorization [7].
1.2 Terminology and Acronyms
AAA: Authentication, Authorization and Accounting
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 [8].
Accounting with credit control: The application checks the end
user's account for coverage for the requested service event
charge prior to execution of that service event.
Home AAA Server: Server where user with which the user maintains
an account relationship.
SIP: Session Initiation Protocol
SIP proxies: SIP proxies are nodes which forward SIP requests
and responses as well as make policy decisions.
UAC: User Agent Client
UAS: User Agent Server
1.3 Requirements Language
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In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [2].
2 Requirements
In this section, we list the requirements. Protocol solutions are not
required to fulfill requirements for services that they do not
support. For example, a solution that provides authentication
services but not accounting services does not need to fulfill the
accounting requirements. It is expected that solutions do fulfill the
general requirements plus the requirements for the specific services
they are providing.
Section 2.1 lists general requirements, Section 2.2 lists
requirements related to authentication, Section 2.3 lists
requirements related to authorization and Section 2.4 lists
requirements related to accounting.
2.1 Common Requirements
This section outlines general requirements on the SIP-AAA interface.
2.1.1 Communication within the Same Domain
The SIP-AAA interface MUST support communications between a SIP
entity and a AAA server that belong to the same domain.
2.1.2 Communication between Different Domains
The SIP-AAA interface MUST support communications between a SIP
entity in one domain and a AAA server in another domain. This MAY
involve a proxy or a redirect server architecture between both
entities.
2.1.3 Discovery
With the information contained in the SIP messages, the SIP-AAA
interface SHOULD be able to deduce the particular AAA server that has
to be queried.
2.1.4 Ability to Integrate Different Networks, Services and Users
The basic AAA architecture MUST be access independent. Service
providers have to be able to provide AAA services for SIP,
irrespective of access method or technology.
2.1.5 Updating SIP Server Entries
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When required, the SIP-AAA interface MUST allow the AAA server to
update the information that a SIP entity has about a user.
2.1.6 SIP Session Changes
The SIP-AAA interface MUST allow a SIP entity to inform the AAA
server about changes in the SIP session that may affect the
authorization, authentication or accounting for that SIP session.
2.1.7 Reliable Transfer of Protocol Messages
The SIP-AAA interface SHOULD provide a reliable transfer of AAA
protocol messages between the SIP entity and the AAA server.
2.1.8 Call Setup Times
AAA SHOULD NOT unduly burden call setup times where appropriate. It
may be reasonable to support some delay during registration, but
delay during on-going sessions (especially real-time) are
problematic.
2.1.9 Security
The SIP-AAA interface is a potential target of an attack. An
eavesdropper may attempt to obtain confidential data by sniffing
messages. Additionally, an active attacker may attempt to modify,
insert, or replay messages between the SIP entity and the AAA server.
Attackers may also attempt to impersonate legitimate SIP entities or
AAA servers.
To address these threats, the SIP-AAA interface MUST support
confidentiality, data origin authentication, integrity, and replay
protection. In addition to this, bi-directional authentication
between the SIP entity and the AAA server MUST be supported as well.
2.2 Authentication Requirements
This section outlines requirements on the SIP-AAA interface related
to authentication.
2.2.1 Authentication Based on SIP Requests
The home AAA server MUST be able to authenticate a user based on any
SIP request, except CANCELs and ACKs for non-2xx final responses.
CANCELs and ACKs for non-2xx final response are hop-by-hop
requests that can be generated by proxies that do not have
the user's credentials.
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2.2.2 Flexible Authentication of SIP Requests
The SIP-AAA interface MUST be flexible enough to accommodate a
variety of authentication mechanisms used to authenticate SIP
requests. In particular, the SIP-AAA interface MUST be able to
accommodate all the authentication mechanisms mandated by the SIP
specs (e.g., Digest authentication.)
2.3 Authorization Requirements
This section outlines requirements on the SIP-AAA interface related
to authorization.
2.3.1 Ability to Authorize SIP Requests
The SIP-AAA interface MUST allow AAA servers to authorize any SIP
request, except CANCELs and ACKs for non-2xx final responses.
CANCELs and ACKs for non-2xx final responses are hop-by-hop
requests that can be generated by proxies. SIP servers
receiving a CANCEL or a ACK for a non-2xx final response do
not challenge them, as they would do with an end-to-end
request. Instead, they check at the transport or network
layer that the entity sending the CANCEL or the ACK is the
same as the one that generated the request being canceled
or acked.
2.3.2 Information Transfer
The SIP-AAA interface MUST allow transferring a wide range or set of
information to be used to make an authorization decision. In
particular, the SIP-AAA interface MUST allow a AAA server that is
making an authorization decision to deliver the user profile to the
SIP entity. Such a user profile may provide further information about
the authorization decision to the SIP entity.
For instance, a SIP proxy receives an INVITE from user A addressed to
user B. The SIP proxy queries a AAA server and gets the following
answer: user A is authorized to call user B as long as the requests
are routed through a particular SIP proxy server C. In this case, the
SIP proxy needs to use SIP loose routing techniques to forward the
INVITE so that it traverses SIP proxy C before reaching user B.
2.3.3 User De-authorization
The SIP-AAA interface MUST allow the AAA server to inform a SIP
entity when a particular user is no longer authorized to perform a
particular task, even if it is an ongoing task.
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2.3.4 User Re-authorization
The SIP-AAA interface MUST allow the AAA server to inform a SIP
entity that a particular authorization has been refreshed, and
therefore, the user is still authorized to perform a particular task.
2.3.5 Support for Credit Control
The SIP-AAA interface MUST support credit control. That is, the AAA
server has to be able to check the end user's account for coverage
for the requested service event charge before authorizing execution
of that service event. Note that this requirement is related to
accounting as well.
Credit control is useful to implement prepaid services where all
chargeable events related to a specific account are prevented from
the end user when the credit of that account is exhausted or expired.
2.4 Accounting Requirements
This section outlines requirements on the SIP-AAA interface related
to accounting. Accounting is more than simple charging. Accounting
may be a simple list of services accessed, servers accessed, duration
of session, etc. Charging for SIP sessions can be extremely complex
and requires some additional study. It is not the intent of this
section to focus on charging.
The information available to be accounted is different at
SIP proxies and at SIP UAs. When end-to-end encryption is
used, proxies do not have access to some parts of the SIP
messages while UAs have access to the whole messages. In
addition to this, UAs typically have information about the
session itself (e.g., number of audio packets exchanged
during an audio session). Therefore, even if the SIP-AAA
interface provides a means to transfer a wide range of
data, some SIP nodes may not have access to it. In order to
design a network, it is important to analyze which SIP
nodes will be able to generate the desired account records.
2.4.1 Separation of Accounting Information
AAA accounting messages MUST be able to provide granular information
based on different parameters.
For example, it should be possible to separate "session duration"
information from other information generated via additional services
(e.g., 3-way calling). Separating accounting information makes it
possible to provide accounting information to different parties based
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upon different aspects of the session.
2.4.2 Accounting Information Related to Session Progression
There MUST be support in the SIP-AAA interface for accounting
transfers where the information contained in the accounting data has
a direct bearing on the establishment, progression and termination of
a session (e.g., reception of a BYE request).
2.4.3 Accounting Information Not Related to Session Progression
There MUST be support in the SIP-AAA interface for accounting
transfers where the information contained in the accounting data does
NOT have a direct bearing on the establishment, progression and
termination of a session (e.g., an instant MESSAGE that is not
related to any session).
2.4.4 Support for One-Time and Session-based Accounting Records
The SIP-AAA interface MUST allow SIP servers to provide relevant
accounting information for billing and inter-network settlement
purpose to the AAA servers. Both one-time event accounting records
and session based (START, INTERIM, STOP records) accounting MUST be
supported.
2.4.5 Support for Accounting on Different Media Components
The SIP-AAA interface MUST support accounting per media component
(e.g., voice and video). That is, the SIP-AAA interface MUST be able
to provide the AAA server with the types (e.g., voice and video) of
the media streams of a given session.
Note, however, that some SIP entities do not have access to this
information, which is typically carried in session descriptions. An
example of a SIP entity with access to this information is a SIP UA
(e.g., a gateway towards the PSTN.)
The SIP-AAA interface MUST enable different parties to be charged per
media component.
2.4.6 Configuration of Accounting Generation Parameters
The SIP-AAA interface MUST allow AAA servers to communicate
parameters for accounting generation.
2.4.7 Support for Arbitrary Correlations
Some networks need to be able to relate accounting information to
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some aspect of the SIP messages involved. So, the SIP-AAA interface
MUST allow the AAA server to correlate a particular AAA session with
any aspect of the SIP messages. For example, a AAA server that
receives accounting information about a SIP dialog may be interested
in knowing the Call-ID of the SIP dialog.
3 Scenarios
This section outlines some possible scenarios for SIP and AAA
interaction. These are purely illustrative examples, and do not
impose any requirements.
Figure 1 shows the typical call flow between a SIP proxy that
communicates to a AAA server that performs authentication and
authorization. All the examples are based on this flow.
SIP SIP AAA
UAC Proxy Server
| | |
|---METHOD---->| |
| |--Is it OK?-->|
| | |
| |<-----OK------|
| | |
| | |
Figure 1: Call flow over the SIP-AAA interface
The SIP proxy receives a request with certain credentials. The SIP
UAC that generated the request may have included the credentials
after having been challenged by the proxy using a 407 (Proxy
Authentication Required) response. The SIP proxy sends a request to
the AAA server asking if it is OK to provide a particular service for
this request. The service may be simply routing forward the request
or may consist of a more complex service. The AAA server checks that
the credentials are correct (authentication), and checks the user
profile. The user profile indicates that it is OK to provide the
service, and responds to the SIP proxy. The SIP proxy provides the
service requested by the SIP UAC.
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3.1 WLAN Roaming Using Third Party Service Providers
User A wants to establish a voice session over the Internet with user
B. User A wants its SIP signalling to be routed through SIP proxy C,
because it provides a call log service (i.e., SIP proxy C sends an
email to user A once a month with the duration of all the calls made
during the month.)
SIP AAA
User A Proxy C Server User B
| | | |
|----INVITE----->| | |
| | | |
|<-----407-------| | |
| | | |
|------ACK------>| | |
| | | |
|----INVITE----->| | |
| |---Is this OK?-->| |
| | | |
| |<------OK--------| |
| | | |
| |---------INVITE------------------>|
| | | |
| |-Accounting msg->| |
| | | |
Figure 2: WLAN roaming user
User A accesses the Internet using a WLAN access outside his home
domain. User A, user B, SIP proxy C and the home AAA server of user A
are all in different domains.
SIP proxy C challenges the initial INVITE from user A with a 407
(Proxy Authentication Required) response, and user A reissues the
INVITE including his credentials. SIP proxy C consults user's A home
AAA server, which confirms that the credentials belong to user A and
that SIP proxy C can go ahead and provide its service for that call.
SIP proxy C routes the INVITE forward towards user B and sends an
accounting message to the AAA server, which will be used later to
charge user A for the service provided by SIP proxy C.
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3.2 Conditional Authorization
User A is not in his home domain, but he still uses SIP proxy C
(which is in user's A home domain) as the outbound proxy for an
INVITE. SIP proxy C consults the home AAA server, which indicates
that requests from user A have to be routed through SIP proxy D. SIP
proxy C uses SIP loose routing so that the INVITE traverses D before
reaching its destination. SIP proxy D will provide call log service
for user A.
SIP AAA SIP
User A Proxy C Server Proxy D
| | | |
|----INVITE----->| | |
| | | |
|<-----407-------| | |
| | | |
|------ACK------>| | |
| | | |
|----INVITE----->| | |
| |------Is this OK?---->| |
| | | |
| |<-OK if routed thru D-| |
| | | |
| |---------INVITE------------------>|
| | | |
Figure 3: Conditional Authorization
4 Security Considerations
This document is informational in nature, so it does not directly
affect the security of the Internet. However, security is a basic
requirement of this work. Section 2.1.9 contains security
requirements related to the SIP-AAA interface.
5 Acknowledgements
The authors would like to thank the participants of the SIP interim
meeting, May 2002 for their comments. The authors would also thank
Harri Hakala, Mary Barns, Pete McCann, Jari Arkko, Aki Niemi, Juha
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Heinanen, Henry Sinnreich, Allison Mankin and Bernard Aboba for their
comments.
The authors would like to thank the authors of the "AAA Requirements
for IP Telephony/Multimedia" draft, which some of the information in
this document is based on.
6 Authors' Addresses
John Loughney
Nokia
Itamerenkatu 11-13
00180 Helsinki
Finland
electronic mail: John.Loughney@nokia.com
Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland
electronic mail: Gonzalo.Camarillo@ericsson.com
7 Normative References
[1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.
[2] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
8 Informative References
[3] P. Calhoun, J. Loughney, E. Guttman, G. Zorn, and J. Arkko,
"Diameter base protocol," RFC 3588, Internet Engineering Task Force,
Sept. 2003.
[4] S. Glass, T. Hiller, S. Jacobs, and C. E. Perkins, "Mobile IP
authentication, authorization, and accounting requirements," RFC
2977, Internet Engineering Task Force, Oct. 2000.
[5] C. Rigney, S. Willens, A. Rubens, and W. Simpson, "Remote
authentication dial in user service (RADIUS)," RFC 2865, Internet
Engineering Task Force, June 2000.
[6] B. Aboba and J. Vollbrecht, "Proxy chaining and policy
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implementation in roaming," RFC 2607, Internet Engineering Task
Force, June 1999.
[7] M. Chiba, G. Dommety, M. Eklund, D. Mitton, and B. Aboba,
"Dynamic authorization extensions to remote authentication dial in
user service (RADIUS)," RFC 3576, Internet Engineering Task Force,
July 2003.
[8] B. Aboba, J. Arkko, and D. Harrington, "Introduction to
accounting management," RFC 2975, Internet Engineering Task Force,
Oct. 2000.
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