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Network Working Group J. Peterson
Request for Comments: 4484 NeuStar
Category: Informational J. Polk
Cisco
D. Sicker
CU Boulder
H. Tschofenig
Siemens
August 2006
Trait-Based Authorization Requirements
for the Session Initiation Protocol (SIP)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document lays out a set of requirements related to trait-based
authorization for the Session Initiation Protocol (SIP). While some
authentication mechanisms are described in the base SIP
specification, trait-based authorization provides information used to
make policy decisions based on the attributes of a participant in a
session. This approach provides a richer framework for
authorization, as well as allows greater privacy for users of an
identity system.
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Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................4
3. Trait-Based Authorization Framework .............................4
4. Example Use Cases ...............................................7
4.1. Settlement for Services ....................................7
4.2. Associating Gateways with Providers ........................7
4.3. Permissions on Constrained Resources .......................8
4.4. Managing Priority and Precedence ...........................9
4.5. Linking Different Protocols ...............................10
5. Trait-Based Authorization Requirements .........................11
6. Security Considerations ........................................13
7. Acknowledgements ...............................................13
8. References .....................................................13
8.1. Normative References ......................................13
8.2. Informative References ....................................13
1. Introduction
This document explores requirements of the Session Initiation
Protocol (SIP) [1] for enabling trait-based authorization. This
effort stems from the recognition that when SIP requests are received
by a User Agent Server (UAS), there are authorization requirements
that are orthogonal to ascertaining of the identity of the User Agent
Client (UAC). Supplemental authorization information might allow the
UAS to implement non-identity-based policies that depend on further
attributes of the principal that originated a SIP request.
For example, in traditional SIP authorization architectures, the mere
fact that a UAC has been authenticated by a UAS doesn't mean that the
UAS will grant the UAC full access to its services or capabilities --
in most instances, a UAS will compare the authenticated identity of
the UAC to some set of users that are permitted to make particular
requests (as a way of making an authorization decision). However, in
large communities of users with few preexisting relationships (such
as federations of discrete service providers), it is unlikely that
the authenticated identity of a UAC alone will give a UAS sufficient
information to decide how to handle a given request.
Trait-based authorization entails an assertion by an authorization
service of attributes associated with an identity. An assertion is a
sort of document consisting of a set of these attributes that are
wrapped within a digital signature provided by the party that
generates the assertion (the operator of the authorization service).
These attributes describe the 'trait' or 'traits' of the identity in
question -- facts about the principal corresponding to that identity.
For example, a given principal might be a faculty member at a
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university. An assertion for that principal's identity might state
that they have the 'trait' of 'is a faculty member', and the
assertion would be issued (and signed) by a university. When a UAS
receives a request with this trait assertion, if it trusts the
signing university, it can make an authorization decision based on
whether or not faculty members are permitted to make the request in
question, rather than just looking at the identity of the UAC and
trying to discern whether or not they are a faculty member through
some external means. Thus, these assertions allow a UAS to authorize
a SIP request without having to store or access attributes associated
with the identity of the UAC itself. Even complex authorization
decisions based the presence of multiple disjointed attributes are
feasible; for example, a 'faculty' member could be part of the
'chemistry' department, and both of these traits could be used to
make authorization decisions in a given federation.
It is easy to see how traits can be used in a single administrative
domain, for example, a single university, where all users are managed
under the same administration. In order for traits to have a broader
usage for services like SIP, which commonly are not bounded by
administrative domains, domains that participate in a common
authorization scheme must federate with one another. The concept of
federation is integral to any trait-based authorization scheme.
Domains that federate with one another agree on the syntax and
semantics of traits -- without this consensus, trait-based
authorization schemes would only be useful in an intradomain context.
A federation is defined as a set of administrative domains that
implement common policies regarding the use and applicability of
traits for authorization decisions. Federation necessarily implies a
trust relationship, and usual implies some sort of pre-shared keys or
other means of cryptographic assurance that a particular assertion
was generated by an authorization service that participates in the
federation.
In fact, when trait-based authorization is used, an assertion of
attributes can be presented to a UAS instead of the identity of user
of the UAC. In many cases, a UAS has no need to know who, exactly,
has made a request -- knowing the identity is only a means to the end
of matching that identity to policies that actually depend on traits
independent of identity. This fact allows trait-based authorization
to offer a very compelling privacy and anonymity solution. Identity
becomes one more attribute of an assertion that may or may not be
disclosed to various destinations.
Trait-based authorization for SIP depends on authorization services
that are trusted by both the UAC and the UAS that wish to share a
session. For that reason, the authorization services described in
this document are most applicable to clients either in a single
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domain or in federated domains that have agreed to trust one
another's authorization services. This could be common in academic
environments, or business partnerships that wish to share attributes
of principals with one another. Some trait-based authorization
architectures have been proposed to provide single sign-on services
across multiple providers.
Although trait-based identity offers an alternative to traditional
identity architectures, this effort should be considered
complementary to the end-to-end cryptographic SIP identity effort
[3]. An authentication service might also act as an authorization
service, generating some sort of trait assertion token instead of an
authenticated identity body.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC 2119 [2] and indicate requirement levels for
compliant SIP implementations.
3. Trait-Based Authorization Framework
A trait-based authorization architecture entails the existence of an
authorization service. Devices must send requests to an
authorization service in order to receive an assertion that can be
used in the context of a given network request. Different network
request types will often necessitate different or additional
attributes in assertions from the authorization service.
For the purposes of SIP, SIP requests might be supplied to an
authorization service to provide the basis for an assertion. It
could be the case that a user agent will take a particular SIP
request, such as an INVITE, for which it wishes to acquire an
assertion and forward this to the authorization service (in a manner
similar to the way that an authenticated identity body is requested
in [3]). User agents might also use a separate protocol to request
an assertion. In either case, the client will need to authenticate
itself to an authorization service before it receives an assertion.
This authentication could use any of the standard mechanisms
described in RFC 3261 [1], or use some other means of authentication.
Once a SIP UA has an assertion, it will need some way to carry an
assertion within in a SIP request. It's possible that this assertion
could be provided by reference or by value. For example, a SIP UA
could include a MIME body within a SIP request that contains the
assertion; this would be inclusion by value. Alternatively, content
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indirection [4], or some new header, could be used to provide a URI
(perhaps an HTTP URL) where interested parties could acquire the
assertion; this is inclusion by reference.
The basic model is as follows:
+----------------+ | |
| +------------+ | Request | +------------+ |
| | Entity | |------------------------>| | Assertion | |
| | requesting | | | | Granting | |
| | authz | |<------------------------| | Entity | |
| | assertions | | Assertion | +------------+ |
| +------------+ | | ^ |
| | | | . Trust |
| | | | . Rel. |
| | | | v |
| | | | +------------+ |
| Transfer | | | Assertion | |
| | | | | Verifying | |
| | | | | Entity | |
| | | | +------------+ |
| | | | |
| v | +----------------+
| +------------+ | Service Request + ^ |
| | Entity | | Assertion | |
| | using authz| | -----------------------------+ |
| | assertion | | |
| +------------+ | <-------------------------------+
+----------------+ Response/Error
The entity requesting authorization assertions (or the entity that
gets some assertions granted) and the entity using these
authorization assertions might be co-located in the same host or
domain, or they might be entities in different domains that share a
federate with one another. The same is true for the entity that
grants these assertions to a particular entity and the entity that
verifies these assertions.
From a protocol point of view, it is worth noting that the process of
obtaining some assertions might happen some time before the usage of
these assertions. Furthermore, different protocols might be used and
the assertions may have a lifetime that might allow that these
assertions are presented to the verifying entity multiple times
(during the lifetime of the assertion).
Some important design decisions are associated with carrying
assertions in a SIP request. If an assertion is carried by value, or
uses a MIME-based content indirection system, then proxy servers will
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be unable to inspect the assertion themselves. If the assertion were
referenced in a header, however, it might be possible for the proxy
to acquire and inspect the assertion itself. There are certainly
architectures in which it would be meaningful for proxy servers to
apply admissions controls based on assertions.
It is also the case that carrying assertions by reference allows
versatile access controls to be applied to the assertion itself. For
instance, an HTTP URL where an assertion could be acquired could
indicate a web server that challenged requests, and only allowed
certain authorized sources to inspect the assertion, or that provided
different versions of the assertion depending on who is asking. When
a SIP UA initiates a request with privacy controls [5], a web server
might provide only trait information ('faculty', 'student', or
'staff') to most queries, but provide more detailed information,
including the identity of the originator of the SIP request, to
certain privileged askers. The end-users that make requests should
have some way to inform authorization services of the attributes that
should be shared with particular destinations.
Assertions themselves might be scoped to a particular SIP transaction
or SIP dialog, or they might have a longer lifetime. The recipient
of an assertion associated with a SIP request needs to have some way
to verify that the authorization service intended that this assertion
could be used for the request in question. However, the format of
assertions is not specified by these requirements.
Trait assertions for responses to SIP requests are outside the scope
of these requirements; it is not clear if there is any need for the
recipient of a request to provide authorization data to the
requestor.
Trait-based authorization has significant applicability to SIP.
There are numerous instances in which it is valuable to assert
particular facts about a principal other than the principal's
identity to aid the recipient of a request in making an authorization
policy decision. For example, a telephony service provider might
assert that a particular user is a 'customer' as a trait. An
emergency services network might indicate that a particular user has
a privileged status as a caller.
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4. Example Use Cases
The following use cases are by no means exhaustive, but provide a few
high-level examples of the sorts of services that trait-based
authorization might provide. All of the cases below consider
interdomain usage of authorization assertions.
4.1. Settlement for Services
When endpoints in two domains share real-time communications
services, sometimes there is a need for the domains to exchange
accounting and settlement information in real-time. The operators of
valuable resources (for example, Public Switched Telephone Network
(PSTN) trunking, conference bridges, or the like) in the called
domain may wish to settle with the calling domain (either with the
operators of the domain or a particular user), and some accounting
operations might need to complete before a call is terminated. For
example, a caller in one domain might want to access a conference
bridge in another domain, and the called domain might wish to settle
for the usage of the bridge with the calling domain. Or in a
wireless context, a roaming user might want to use services in a
visited network, and the visited network might need to understand how
to settle with the user's home network for these services.
Assuming that the calling domain constitutes some sort of commercial
service capable of exchanging accounting information, the called
domain may want to verify that the remote user has a billable account
in good standing before allowing a remote user access to valuable
resources. Moreover, the called domain may need to discover the
network address of an accounting server and some basic information
about how to settle with it.
An authorization assertion created by the calling domain could
provide the called domain with an assurance that a user's account can
settle for a particular service. In some cases, no further
information may be required to process a transaction, but if more
specific accounting data is needed, traits could also communicate the
network address of an accounting server, the settlement protocol that
should be used, and so on.
4.2. Associating Gateways with Providers
Imagine a case where a particular telephone service provider has
deployed numerous PSTN-SIP gateways. When calls come in from the
PSTN, they are eventually proxied to various SIP user agents. Each
SIP user agent server is interested to know the identity of the PSTN
caller, of course, which could be given within SIP messages in any
number of ways (in SIP headers, bodies, or what have you). However,
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in order for the recipient to be able to trust the identity (in this
instance, the calling party's telephone number) stated in the call,
they must first trust that the call originated from the gateway and
that the gateway is operated by a known (and trusted) provider.
There are a number of ways that a service provider might try to
address this problem. One possibility would be routing all calls
from gateways through a recognizable 'edge' proxy server (say,
'sip.example.com'). Accordingly, any SIP entity that received a
request via the edge proxy server (assuming the use of hop-by-hop
mutual cryptographic authentication) would know the service provider
from whom the call originated. However, it is possible that requests
from the originating service provider's edge proxy might be proxied
again before reaching the destination user agent server, and thus in
many cases the originating service provider's identity would be known
only transitively. Moreover, in many architectures requests that did
not originate from PSTN gateways could be sent through the edge proxy
server. In the end analysis, the recipient of the request is less
interested in knowing which carrier the request came from than in
knowing that the request came from a gateway.
Another possible solution is to issue certificates to every gateway
corresponding to the hostname of the gateway
('gateway1.example.com'). Gateways could therefore sign SIP requests
directly, and this property could be preserved end-to-end. But
depending on the public key infrastructure, this could, however,
become costly for large numbers of gateways, and moreover a user
agent server that receives the request has no direct assurance from a
typical certificate that the host is in fact a gateway just because
it happens to be named 'gateway1'.
Trait-based authorization would enable the trait 'is a gateway' to be
associated with an assertion that is generated by the service
provider (i.e., signed by 'example.com'). Since these assertions
would travel end-to-end from the originating service provider to the
destination user agent server, SIP requests that carry them can pass
through any number of intermediaries without discarding cryptographic
authentication information. This mechanism also does not rely on
hostname conventions to identify what constitutes a gateway and what
does not -- it relies on an explicit and unambiguous attribute in an
assertion.
4.3. Permissions on Constrained Resources
Consider a scenario wherein two universities are making use of a
videoconferencing service over a constrained-bandwidth resource.
Both universities would like to enforce policies that determine how
this constrained bandwidth will be allocated to members of their
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respective communities. For example, faculty members might have
privileges to establish videoconferences during the day, while
students might not. Faculty might also be able to add students to a
particular videoconference dynamically, or otherwise moderate the
content or attendance of the conference, whereas students might
participate only more passively.
Trait-based authorization is ideal for managing authorization
decisions that are predicated on membership in a group. Rather than
basing access on individual users, levels (or roles) could be
assigned that would be honored by both universities, since they both
participate in the same federation.
If the federation honored the traits "faculty", "staff", and
"student", they could be leveraged to ensure appropriate use of the
network resource between universities participating in the
federation. An assertion would then be attached to every request to
establish a session that indicated the role of the requestor. Only
if the requestor has the appropriate trait would the session request
be granted. Ideally, these policies would be enforced by
intermediaries (SIP proxy servers) that are capable of inspecting and
verifying the assertions.
4.4. Managing Priority and Precedence
There is a significant amount of interest in the Internet telephony
community in assigning certain calls a 'priority' based on the
identity of the user, with the presumption that prioritized calls
will be granted preferential treatment when network resources are
scarce. Different domains might have different criteria for
assigning priority, and it is unlikely that a domain would correlate
the identity of a non-local user with the need for priority, even in
situations where domains would like to respect one another's
prioritization policies.
Existing proposals have focused largely on adding a new header field
to SIP that might carry a priority indicator. This use case does not
challenge this strategy, but merely shows by way of example how this
requirement might be met with a trait-based authorization system. As
such, the limitations of the header field approach will not be
contrasted here with a hypothetical trait-based system.
An assertion created by a domain for a particular request might have
an associated 'priority' attribute. Recipients of the request could
inspect and verify the signature associated with the assertion to
determine which domain had authenticated the user and made the
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priority assessment. If the assertion's creator is trusted by the
evaluator, the given priority could be factored into any relevant
request processing.
4.5. Linking Different Protocols
Cryptographic computations are expensive and computing authorization
decisions might require a lot of time and multiple messages between
the entity enforcing the decisions and the entity computing the
authorization decision. Particularly in a mobile environment these
entities are physically separated -- or not even in the same
administrative domain. Accordingly, the notion of "single sign-on"
is another potential application of authorization assertions and
trait-based authorization -- a user is authenticated and authorized
through one protocol, and can reuse the resulting authorization
assertion in other, potential unrelated protocol exchanges.
For example, in some environments it is useful to make the
authorization decision for a "high-level" service (such as a voice
call). The authorization for the "voice call" itself might include
authorization for SIP signaling and also for lower-level network
functions, for example, a quality-of-service (QoS) reservation to
improve the performance of real-time media sessions established by
SIP. Since the SIP signaling protocol and the QoS reservation
protocol are totally separate, it is necessary to link the
authorization decisions of the two protocols. The authorization
decision might be valid for a number of different protocol exchanges,
for different protocols and for a certain duration or some other
attributes.
To enable this mechanism as part of the initial authorization step,
an authorization assertion is returned to the end host of the SIP UAC
(cryptographically protected). If QoS is necessary, the end host
might reuse the returned assertion in the QoS signaling protocol.
Any domains in the federation that would honor the assertion
generated to authorize the SIP signaling would similarly honor the
use of the assertion in the context of QoS. Upon the initial
generation of the assertion by an authorization server, traits could
be added that specify the desired level of quality that should be
granted to the media associated with a SIP session.
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5. Trait-Based Authorization Requirements
The following are the constraints and requirements for trait-based
authorization in SIP:
1. The mechanism MUST support a way for SIP user agents to embed an
authorization assertion in SIP requests. Assertions can be
carried either by reference or by value.
2. The mechanism MUST allow SIP UACs to deliver to an authorization
service those SIP requests that need to carry an assertion. The
mechanism SHOULD also provide a way for SIP intermediaries to
recognize that an assertion will be needed, and either forward
requests to an authorization service themselves or notify the UAC
of the need to do so.
3. Authorization services MUST be capable of delivering an assertion
to a SIP UAC, either by reference or by value. It MAY also be
possible for an authorization service to add assertions to
requests itself, if the user profile permits this (for example,
through the use of content-indirection as described in [4]).
4. Authorization services MUST have a way to authenticate a SIP UAC.
5. The assertions generated by authorization services MUST be
capable of providing a set of values for a particular trait that
a principal is entitled to claim.
6. The mechanism MUST provide a way for authorized SIP
intermediaries (e.g., authorized proxy servers) to inspect
assertions.
7. The mechanism MUST have a single baseline mandatory-to-implement
authorization assertion scheme. The mechanism MUST also allow
support of other assertion schemes, which would be optional to
implement. One example of an assertion scheme is Security
Assertion Markup Language (SAML) [6] and another is RFC 3281
X.509 Attribute Certificates [7].
8. The mechanism MUST ensure reference integrity between a SIP
request and assertion. Reference integrity refers to the
relationship between a SIP message and the assertion authorizing
the message. For example, a reference integrity check would
compare the sender of the message (as expressed in the SIP
request, for example, in the "From" header field value) with the
identity provided by the assertion. Reference integrity is
necessary to prevent various sorts of relay and impersonation
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attacks. Note that reference integrity MAY apply on a per-
message, per-transaction, or per-dialog basis.
9. Assertion schemes used for this mechanism MUST be capable of
asserting attributes and/or traits associated with the identity
of the principal originating a SIP request. No specific traits
or attributes are required by this specification.
10. The mechanism MUST support a means for end-users to specify
policies to an authorization service for the distribution of
their traits and/or attributes to various destinations.
11. The mechanism MUST provide a way of preventing unauthorized
parties (either intermediaries or endpoints) from viewing the
contents of assertions.
12. Assertion schemes MUST provide a way of selectively sharing the
traits and/or attributes of the principal in question. In other
words, it must be possible to show only some of the attributes of
a given principal to particular recipients, based on the
cryptographically- assured identity of the recipient.
13. It MUST be possible to provide an assertion that contains no
identity -- that is, to present only attributes or traits of the
principal making a request, rather than the identity of the
principal.
14. The manner in which an assertion is distributed MUST permit
cryptographic authentication and integrity properties to be
applied to the assertion by the authorization service.
15. It MUST be possible for a UAS or proxy server to reject a request
that lacks a present and valid authorization assertion, and to
inform the sending UAC that it must acquire such an assertion in
order to complete the request.
16. The recipient of a request containing an assertion MUST be able
to ascertain which authorization service generated the assertion.
17. It MUST be possible for a UAS or proxy server to reject a request
containing an assertion that does not provide any attributes or
traits that are known to the recipient or that are relevant to
the request in question.
18. It SHOULD be possible for a UAC to attach multiple assertions to
a single SIP request, in cases where multiple authorization
services must provide assertions in order for a request to
complete.
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6. Security Considerations
The subject of this document is an authorization system for SIP that
is not predicated on the distribution of end-users' identities, but
rather shares traits of the users. As such, the bulk of this
document discusses security.
The distribution of authorization assertions requires numerous
security properties. An authorization service must be able to sign
assertions, or provide some similar cryptographic assurance that can
provide non-repudiation for assertions. These requirements are
further detailed in Section 3.
7. Acknowledgements
The authors thank Christopher Eagan and Mary Barnes for their
valuable input.
8. References
8.1. Normative References
[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.
8.2. Informative References
[3] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
RFC 4474, August 2006.
[4] Burger, E., Ed., "A Mechanism for Content Indirection in Session
Initiation Protocol (SIP) Messages", RFC 4483, May 2006.
[5] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.
[6] Organization for the Advancement of Structured Industry
Standards, "Security Assertion Markup Language v1.0", November
2002, <http://www.oasis-open.org>.
[7] Farrell, S. and R. Housley, "An Internet Attribute Certificate
Profile for Authorization", RFC 3281, April 2002.
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Authors' Addresses
Jon Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
US
Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz
URI: http://www.neustar.biz/
James M. Polk
Cisco Systems
2200 East President George Bush Turnpike
Suite 570
Richardson, TX 75802
US
EMail: jmpolk@cisco.com
Douglas C. Sicker
University of Colorado at Boulder
ECOT 531
Boulder, CO 80309
US
EMail: douglas.sicker@colorado.edu
Hannes Tschofenig
Siemens AG
Otto-Hahn-Ring 6
Munich 81739
Germany
EMail: Hannes.Tschofenig@siemens.com
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