SIPPING J. Rosenberg
Internet-Draft dynamicsoft
Expires: August 16, 2004 February 16, 2004
Requirements for Session Policy for the Session Initiation Protocol
(SIP)
draft-ietf-sipping-session-policy-req-01
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
The proxy server plays a central role as an intermediary in the
establishment of sessions in the Session Initiation Protocol (SIP).
In that role, they can define and impact policies on call routing,
rendezvous, and other call features. However, there is no standard
means by which proxies can have any influence on session policies,
such as the codecs that are to be used. As such, ad-hoc and
non-conformant techniques have been deployed to allow for such policy
mechanisms. There is a need for a standards-based and complete
mechanism for session policies. This document defines a set of
requirements for such a mechanism.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problems with Existing Situation . . . . . . . . . . . . . . . 5
3. Requirements for a Solution . . . . . . . . . . . . . . . . . 7
3.1 General Requirements . . . . . . . . . . . . . . . . . . . . . 7
3.2 Policy Requirements . . . . . . . . . . . . . . . . . . . . . 7
3.3 Policy Types . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4 Consent Requirements . . . . . . . . . . . . . . . . . . . . . 9
3.5 Security Requirements . . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
Informative References . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15
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1. Introduction
The Session Initiation Protocol [2] enables the setup and management
of interactive multimedia sessions on IP networks. A central element
in SIP is the proxy server. Proxies are responsible for request
routing, rendezvous, authentication and authorization, mobility, and
other signaling services. However, proxies are divorced from the
actual sessions - audio, video, and messaging - that SIP establishes.
Details of the sessions are carried in the payload of SIP messages,
and are usually described with the Session Description Protocol (SDP)
[1]. Indeed, SIP provides end-to-end encryption features using S/
MIME, so that all information about the sessions can be hidden from
eavesdroppers and proxies alike.
However, experience has shown that there is a need for SIP
intermediaries to impact aspects of the session. One aspect is the
path that the media streams will take. Frequently, a SIP provider
will need or want the media to traverse some kind of intermediary,
such as a NAT. Indeed, the central concept of the midcom framework
[4] is to define a model of how this can be done. In this model, a
midcom agent, typically a proxy server, interacts with the middlebox
to open and close media pinholes, obtain NAT bindings, and so on. In
this role as a midcom agent, the proxy will need to examine and
possibly modify the session description in the body of the SIP
message. This modification is to achieve a specific policy objective:
to force the media to route through an intermediary.
In another application, SIP is used in a wireless network. The
network provider has limited resources for media traffic. During
periods of high activity, the provider would like to restrict codec
usage on the network to lower rate codecs.
In yet a third application, SIP is used in a network that has
gateways which support a single codec type (say, G.729). When
communicating with a partner network that uses gateways with a
different codec (say, G.723), the network modifies the SDP to route
the session through a converter that changes the G.729 to G.723.
The desire to impact aspects of the session inevitably occurs in
domains where the administrator of the SIP domain is also the owner
and administrator of an IP network over which it is known that the
sessions will traverse. This includes enterprises, Internet access
providers, and in some cases, backbone providers.
Since SIP is the protocol by which the details of these sessions are
negotiated, it is natural for providers to wish to impose their
session policies through some kind of SIP means. To date, this has
been accomplished through SDP editing, a process where proxies dig
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into the bodies of SIP messages, and modify them in order to impose
their policies. However, this SIP editing technique has many
drawbacks.
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2. Problems with Existing Situation
RFC 3261 explicitly disallows proxy servers from manipulating the
content of bodies. This is at odds with the common industry practice
of extensive manipulation of bodies by proxies. Although a common
practice, it is at odds with the SIP specification for many reasons:
End-to-End Encryption: SIP uses S/MIME to support end-to-end
security security features. Authentication, message integrity, and
encryption are provided. The encryption capabilities are important
for end-to-end privacy services, for example. The end-to-end
message integrity and authentication are important for preventing
numerous attacks, including theft of calls, eavesdropping attacks,
and so on. If end-to-end authentication is used, any manipulation
of the body will cause the message integrity check to fail. If
end-to-end encryption is used, the proxy won't even be able to
look at the SDP to modify it. In this case, media may not
function, and the call will fail.
Require Processing: A UA may require that an extension be applied
to the SDP body. This is accomplished by including a Require
header in the SIP message. Proxies do not look at such headers. If
the proxy processes the SDP without understanding the extension,
it may improperly modify the SDP, resulting in a call failure.
Consent: Ultimately, end users need to be in control of the media
they send. If a user makes a call through a SIP network, they have
the expectation that their media is delivered to the recipient. By
having proxies modify the SDP in some way, they act in ways
outside of expected behavior of the system.
Future Proofing: One of the benefits of the SIP architecture is
that only the endpoints need to understand sessions, session
descriptions, bodies, and so on. This facilitates the use of proxy
networks to provide communications services for future session
types, such as games and messaging. However, if proxies require an
understanding of session types and session descriptions, the SIP
network becomes locked in to providing features for a particular
set of session types. If a new session description protocol, such
as SDPng [10], were introduced, calls would not function even
though the endpoints support SDPng. Furthermore, it would be hard
to determine why it did not function, since the failure would
occur transparently in some proxy in the middle of the network.
Robustness: Having a proxy manipulate the body introduces a host
of new failure modes into the network. Firstly, the proxy itself
will need to have state in some form in order to properly
manipulate the SDP. This means that, should the proxy fail, the
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call may not be able to continue. Secondly, proxies typically
won't enforce the media policy. Rather, they leave that to some
media middlebox somewhere on the media path. This media middlebox
may fail as well. Since the user does not know of its existence,
they may not be able to detect this failure or retry the media
path around it.
Scalability: One of the reasons SIP scales so well is that proxies
don't have to be aware of the details of the sessions being
established through them. If a proxy needs to examine and/or
manipulate session descriptions, this could require many
additional processing steps. The proxy may need to traverse a
multi-part body to find the SDP, in the case of SIP-T [5]. The
proxy will need to parse, modify, and possibly re-serialize the
session description. All of this requires additional processing
that worsens the performance of the proxies.
We note that many of these problems are similar to those pointed out
by the IAB regarding Open Pluggable Exchange Services (OPES) [6].
Indeed, the problems are similar. Both have to do with the
involvement of intermediaries in manipulation of end-to-end content.
Here, the content is not in the body itself, but is a session
described by the body.
We believe a better solution is needed.
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3. Requirements for a Solution
In order to prevent the continuing usage of SDP editing to achieve
session policies, we believe explicit protocol support is needed to
provide a mechanism that can overcome the limitations above. As per
the IETF SIP change process [7], the first step in any such activity
is to specify requirements for the solution. This section is an
enumeration of those requirements.
3.1 General Requirements
REQ-GEN-1: The solution should work even with SIP end-to-end
encryption and end-to-end authentication enabled.
REQ-GEN-2: The solution should not force a proxy to violate the SIP
specification or any defined extensions.
REQ-GEN-3: The solution should not require substantial processing
burden on the proxies.
REQ-GEN-4: The solution should not require proxies to understand a
specific type of session description (i.e., SDP or SDPng).
REQ-GEN-5: The solution should have a minimal impact on call setup
delays, and ideally, have no impact on call setup delays.
REQ-GEN-6: The solution should require minimal overhead, since it is
anticipated to receive wide use in wireless networks.
REQ-GEN-7: The solution should be extensible, supporting new session
policy types in the future.
REQ-GEN-8: The solution must not require that the proxies be in the
same administrative domain as the media intermediaries.
3.2 Policy Requirements
REQ-POL-1: The solution should allow specification of independent
policies by each proxy along the call setup path, without any
coordination between proxies.
REQ-POL-2: The solution should allow a proxy to specify media
policies on a stream-by-stream basis.
REQ-POL-3: When used in conjunction with the offer/answer model [3],
the solution should allow a proxy to specify independent policies
for the media streams in each direction.
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REQ-POL-4: The mechanism must provide the ability to inform the UA
about the set of session-independent session policies when the
device starts up. These are session policies that do not depend on
a particular session.
REQ-POL-5: The mechanism must allow the provider to change the
session-independent policies at least a few times a day.
REQ-POL-6: The mechanism must allow the session independent policies
to vary on a user by user basis.
REQ-POL-7 The mechanism must provide a way to inform the client about
changes in session independent session policies when they occur.
3.3 Policy Types
REQ-POL-4: The solution should allow a proxy to request media
sessions to traverse through one or more intermediaries.
REQ-POL-5: The solution should allow a proxy to request a specific
source routing mechanism to be used (when applicable) in order to
traverse those intermediaries. The source routing technique may be
media-specific, or a generic technique, such as IP-in-IP [8]
REQ-POL-6: Intermediaries must be identifiable using either an IP
address or an FQDN, in order to support DNS-based load balancing
and failover techniques.
REQ-POL-7: The solution should allow a proxy to inspect the addresses
for the media sessions, so that it can set policies in intervening
firewalls.
REQ-POL-8: The solution should allow proxies to request that a
particular media stream not be used (video, for example).
REQ-POL-9: The solution should allow proxies to request that a
particular codec not be used.
REQ-POL-10: The solution should allow proxies to express preferences
for the use of particular codecs.
REQ-POL-11: The solution should allow proxies to request that Quality
of Service (QoS) should be requested for a stream.
REQ-POL-12: The solution should allow proxies to ask endpoints to use
specific parameters in their QoS reservations.
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REQ-POL-13: The solution should allow proxies to ask endpoints to
provide a specific credential in their QoS requests. This
requirement covers the functionality currently described in [9].
3.4 Consent Requirements
Consent plays a critical role for this problem. End users must be
allowed control over how they communicate with each other. Indeed,
with end-to-end IP connectivity, there is frequently little the
provider can do to force users to communicate one way or another.
Ultimately, any means a provider comes up with can be circumvented by
some creative engineering in the clients. As such, policy requests by
proxies are just that - requests, and are ultimately honored at the
discretion of the end users. The mechanism needs to recognize this,
and be engineered to work within this model, rather than try to work
around it.
REQ-CON-1: The mechanism should allow the UAC to know the set of
policies requested by the proxies along the call path. [[OPEN
ISSUE: Is it more important for the UAC to know about changes
requested for media in one direction or the other?]]
REQ-CON-2: The mechanism should allow the UAS to know the set of
policies requested by the proxies along the call path.
REQ-CON-3: The mechanism should allow the UAC to reject any policy
requests made by proxies.
REQ-CON-4: The mechanism should allow the UAS to reject any policy
requests made by proxies.
REQ-CON-5: The mechanism should allow the proxies to know whether or
not the UAC has accepted its policy requests.
REQ-CON-6: The mechanism should allow the proxies to know whether or
not the UAS has accepted its policy requests.
REQ-CON-7: The mechanism should allow the proxies to inform the UAC
and UAS of the consequences of non-compliance to the policies.
Potential consequences include call rejection, degraded media
quality, lack of connectivity for a media stream, and so on.
3.5 Security Requirements
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REQ-SEC-1: The mechanism should allow user agents to verify the
identity of the providers requesting the session policies.
REQ-SEC-2: The mechanism should allow user agents to verify the
integrity of the session policies.
REQ-SEC-3: The mechanism must provide assurances to the UAC and UAS
that only proxies on the actual SIP signaling path have requested
session policies.
REQ-SEC-4: The mechanism should allow proxies to ensure the
confidentiality of the session policies, so that no one but the
UAC or UAS can observe them. [[OPEN ISSUE: Is this really a
requirement?]]
REQ-SEC-5: The mechanism must not enable any new denial-of-service
attacks to be launched. [[OPEN ISSUE: This is motherhood and apple
pie - does it need to be here?]]
REQ-SEC-6: The mechanism shall still allow for media security through
Secure RTP [11]. In the case of intermediaries which process the
RTP in some way that would invalidate any signatures, the UAs must
be aware of the presence of the intermediary, and perform key
exchanges with it. [[OPEN ISSUE: This may be an impossible
requirement to meet without using a B2BUA.]]
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4. Security Considerations
Requirements related to security are considered in Section 3.5.
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5. Acknowledgements
I would like to thank Volker Hilt, Gonzalo Camarillo, Miguel Garcia
and Kumiko Ono for their input.
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Informative References
[1] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[2] 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.
[3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[4] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A. and A.
Rayhan, "Middlebox communication architecture and framework",
RFC 3303, August 2002.
[5] Vemuri, A. and J. Peterson, "Session Initiation Protocol for
Telephones (SIP-T): Context and Architectures", BCP 63, RFC
3372, September 2002.
[6] Floyd, S. and L. Daigle, "IAB Architectural and Policy
Considerations for Open Pluggable Edge Services", RFC 3238,
January 2002.
[7] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J. and B.
Rosen, "Change Process for the Session Initiation Protocol
(SIP)", BCP 67, RFC 3427, December 2002.
[8] Perkins, C., "IP Encapsulation within IP", RFC 2003, October
1996.
[9] Marshall, W., "Private Session Initiation Protocol (SIP)
Extensions for Media Authorization", RFC 3313, January 2003.
[10] Kutscher, D., Ott, J. and C. Bormann, "Session Description and
Capability Negotiation", draft-ietf-mmusic-sdpng-07 (work in
progress), October 2003.
[11] Baugher, M., "The Secure Real-time Transport Protocol",
draft-ietf-avt-srtp-09 (work in progress), July 2003.
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Author's Address
Jonathan Rosenberg
dynamicsoft
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
EMail: jdrosen@dynamicsoft.com
URI: http://www.jdrosen.net
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