SIPPING J. Rosenberg
Internet-Draft dynamicsoft
Expires: June 26, 2003 December 26, 2002
Requirements for Session Policy for the Session Initiation Protocol
(SIP)
draft-rosenberg-sipping-session-policy-req-00
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on June 26, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). 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.
Rosenberg Expires June 26, 2003 [Page 1]
Internet-Draft Session Policy Requirements December 2002
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 Consent Requirements . . . . . . . . . . . . . . . . . . . . . 8
3.4 Security Requirements . . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
Informative References . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . 14
Rosenberg Expires June 26, 2003 [Page 2]
Internet-Draft Session Policy Requirements December 2002
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
Rosenberg Expires June 26, 2003 [Page 3]
Internet-Draft Session Policy Requirements December 2002
into the bodies of SIP messages, and modify them in order to impose
their policies. However, this SIP editing technique has many
drawbacks.
Rosenberg Expires June 26, 2003 [Page 4]
Internet-Draft Session Policy Requirements December 2002
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 [9], 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
Rosenberg Expires June 26, 2003 [Page 5]
Internet-Draft Session Policy Requirements December 2002
will need to have state in some form in order to properly
manipulate the SDP. This means that, should the proxy fail, the
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.
Rosenberg Expires June 26, 2003 [Page 6]
Internet-Draft Session Policy Requirements December 2002
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.
Rosenberg Expires June 26, 2003 [Page 7]
Internet-Draft Session Policy Requirements December 2002
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.
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 [10].
3.3 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.
Rosenberg Expires June 26, 2003 [Page 8]
Internet-Draft Session Policy Requirements December 2002
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.
3.4 Security Requirements
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
Rosenberg Expires June 26, 2003 [Page 9]
Internet-Draft Session Policy Requirements December 2002
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.]]
Rosenberg Expires June 26, 2003 [Page 10]
Internet-Draft Session Policy Requirements December 2002
4. Security Considerations
Requirements related to security are considered in Section 3.4.
Rosenberg Expires June 26, 2003 [Page 11]
Internet-Draft Session Policy Requirements December 2002
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): (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] Ott, J., Bormann, C. and D. Kutscher, "Session Description and
Capability Negotiation", draft-ietf-mmusic-sdpng-05 (work in
progress), July 2002.
[10] Evans, D., Marshall, W. and B. Marshall, "SIP Extensions for
Media Authorization", draft-ietf-sip-call-auth-06 (work in
progress), May 2002.
[11] Baugher, M., "The Secure Real-time Transport Protocol",
draft-ietf-avt-srtp-05 (work in progress), June 2002.
Rosenberg Expires June 26, 2003 [Page 12]
Internet-Draft Session Policy Requirements December 2002
Author's Address
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
East Hanover, NJ 07936
US
Phone: +1 973 952-5000
EMail: jdrosen@dynamicsoft.com
URI: http://www.jdrosen.net
Rosenberg Expires June 26, 2003 [Page 13]
Internet-Draft Session Policy Requirements December 2002
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
Rosenberg Expires June 26, 2003 [Page 14]
Internet-Draft Session Policy Requirements December 2002
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Rosenberg Expires June 26, 2003 [Page 15]