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Versions: 00 01 02                                                      
SIPPING                                                     J. Rosenberg
Internet-Draft                                               dynamicsoft
Expires: January 17, 2005                                  July 19, 2004

  Requirements for Session Policy for the Session Initiation Protocol

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   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

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   requirements for such a mechanism.

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 . . . . . . . . . . . . . . . . . . .  8
     3.5   Security Requirements  . . . . . . . . . . . . . . . . . .  9
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Informative References . . . . . . . . . . . . . . . . . . . . 11
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12
       Intellectual Property and Copyright Statements . . . . . . . . 13

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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

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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
      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
      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

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      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.
   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.

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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
   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 [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.

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   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
   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
   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.

6  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

   [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
   600 Lanidex Plaza
   Parsippany, NJ  07054

   Phone: +1 973 952-5000
   EMail: jdrosen@dynamicsoft.com
   URI:   http://www.jdrosen.net

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