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 of sessions, such as Internet phone calls, multimedia
distribution, and multimedia conferences [1]. SIP sessions have
needs for session authentication, authorization, and accounting
(AAA).
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In order to authenticate and authorize users, it is typically more
convenient for SIP entities to communicate with an 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 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 an 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.
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 important
requirements. 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.
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.
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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].
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
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 BCP 14, RFC 2119
[2].
In this section, we list the requirements. Protocol solutions are
not required to satisfy 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 will fulfill
the general requirements, plus the requirements for the specific
services they are providing.
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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.
The SIP-AAA interface MUST support communications between a SIP
entity in one domain and an AAA server in another domain. This MAY
involve a proxy or a redirect server architecture between both
entities.
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.
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.
The SIP-AAA interface SHOULD provide a reliable transfer of AAA
protocol messages between the SIP entity and the AAA server.
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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) is problematic.
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.
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 responses are hop-by-hop
requests that can be generated by proxies that do not have the
user's credentials.
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
specifications (e.g., Digest authentication).
This section outlines requirements on the SIP-AAA interface related
to authorization.
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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.
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 an 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 an 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.
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.
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.
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.
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Credit control is useful to implement prepaid services where all
chargeable events related to a specific account are withheld from the
end user when the credit of that account is exhausted or expired.
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.
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
upon different aspects of the session.
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).
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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).
The SIP-AAA interface MUST allow SIP servers to provide relevant
accounting information for billing and inter-network settlement
purposes to the AAA servers. Both one-time event accounting records
and session based (START, INTERIM, STOP records) accounting MUST be
supported.
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.
Some networks need to be able to relate accounting information to
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, an AAA server that
receives accounting information about a SIP dialog may be interested
in knowing the Call-ID of the SIP dialog.
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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 an 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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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 A's 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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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 a 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
Security is a critical requirement of the SIP-AAA Interface. Section
2.1.9 describes the threats and security requirements. Sections 2.2
and 2.3 elaborate on the authentication and authorization
requirements.
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
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" document, as it provided a basis for
some of the information contained in this document.
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[1] 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.
[2] Bradner, S., "Key words for use in RFCs to indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J. Arkko,
"Diameter Base Protocol", RFC 3588, September 2003.
[4] Glass, S., Hiller, T., Jacobs, S. and C. Perkins, "Mobile IP
Authentication, Authorization, and Accounting Requirements", RFC
2977, October 2000.
[5] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial in User Service (RADIUS)", RFC 2865, June
2000.
[6] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[7] Chiba, M., Dommety, G., Eklund, M., Mitton, D. and B. Aboba,
"Dynamic Authorization Extensions to Remote Authentication Dial
in User Service (RADIUS)", RFC 3576, July 2003.
[8] Aboba, B., Arkko, J. and D. Harrington, "Introduction to
Accounting Management", RFC 2975, October 2000.
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John Loughney
Nokia
Itamerenkatu 11-13
00180 Helsinki
Finland
EMail: John.Loughney@nokia.com
Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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RFC 3702 AAA Requirements for SIP February 2004
Copyright (C) The Internet Society (2004). This document is subject
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