SIP                                                          M. Munakata
Internet-Draft                                               S. Schubert
Intended status: Standards Track                                 T. Ohba
Expires: January 10, 2008                                            NTT
                                                            July 9, 2007

                  UA-Driven Privacy Mechanism for SIP

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   To withhold a user's identity and related information, RFC 3323
   defines a Privacy mechanism for SIP, which requires the use of an
   privacy service.  This document proposes a new privacy mechanism that
   a user agent can facilitate to conceal privacy-sensitive information
   without the need for aid from a privacy service.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Concept of Privacy . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  Treatment of User Privacy Related Information  . . . . . . . .  5
     6.1.  Anonymous URI  . . . . . . . . . . . . . . . . . . . . . .  6
     6.2.  Anonymous IP Address . . . . . . . . . . . . . . . . . . .  6
   7.  User Agent Behavior  . . . . . . . . . . . . . . . . . . . . .  7
     7.1.  Generating Anonymous Message . . . . . . . . . . . . . . .  7
     7.2.  Indication to maintain Privacy . . . . . . . . . . . . . .  7
   8.  Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . .  8
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     11.1. Normative References . . . . . . . . . . . . . . . . . . .  8
     11.2. Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 10

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1.  Introduction

   Privacy is defined in this document as the withholding of the
   identity of a person (and related personal information) from
   destination(s) of messages and/or intermediaries handling these
   messages in a SIP (Session Initiation Protocol) [RFC3261] dialog.

   In SIP, identity is most commonly carried in the form of a SIP URI
   and an optional display name, which commonly appear in the To, From
   and other header fields of SIP requests and responses.

   There are numerous other places in SIP messages in which identity-
   related information can be revealed.  For example, the Contact header
   field contains a SIP URI.  Moreover, information in the Record-Route
   and Via headers could inadvertently reveal something about the
   originator of a message.

   To provide privacy, [RFC3323] defines a privacy mechanism for SIP,
   which was then the best current practice to maintain privacy.  Since
   then, numerous SIP extensions have been proposed and standardized.
   Some of those seem to enable a user agent to withhold its user's
   identity and related information without dependency on privacy
   services, which was not possible when RFC3323 was defined.

   Some aspect of RFC 3323, especially its dependency on a privacy
   service to provide privacy, seems to cause some issues, which we hope
   that we can resolve with this specification.

   Some of the issues identified with the RFC 3323 are shown below.

   1.  There is no assurance that a privacy service exists in the
       signaling path.

   2.  There is no way that the user requesting the privacy can figure
       out that the privacy function was properly executed.

   3.  A privacy service that modifies a Call-ID in the establishment of
       the original dialog must be in the signaling path of the
       subsequent request such as REFER.  If a privacy service
       anonymizes a Call-ID and the anonymized Call-ID is referenced in
       a subsequent SIP message for the purpose of a call-back or a call
       replacement, the privacy service needs to be in a signaling path
       to replace the anonymized Call-ID with the original Call-ID
       appropriately, regardless of being inside/outside the dialog.

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   4.  To map the referenced dialog to a dialog attempt invoked by
       REFER, for example, the privacy service needs to retain the
       correspondence relation between original information and modified
       information beyond the actual dialog duration of the referenced

   To solve the problems, this document proposes a new privacy mechanism
   in which a user agent executes all the privacy functions on its own
   utilizing SIP extensions such as GRUU (Globally Routable User Agent
   URIs)[I-D.ietf-sip-gruu] and TURN (Traversal Using Relay

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Concept of Privacy

   The concept of privacy in this document means the concealing of
   information that relates to a user, identifies a user, and belongs to
   a user, as well as the supplementary information that can be used to
   guess the user's identity.  The scope of this document is to withhold
   the identity of a user and supplementary information from other users
   and intermediaries handling the message.  The protection of network
   privacy (e.g., topology hiding) is outside the scope of this

   User-privacy-related information includes display name and URI in a
   From header that can reveal the user's name and affiliation (e.g.,
   company name), contact information in a Contact header that is used
   to communicate with the user, an IP address in an SDP (Session
   Description Protocol)[RFC4566] that tells the location of a user's
   terminal and can be used to establish a connection.  A host name in
   Call-ID is also regarded as user-privacy-related information because
   it may reveal the user's domain name.

   Privacy-sensitive information is divided into two types, user-
   inserted information and network-inserted information.  A user agent
   can maintain privacy of the user-inserted information by itself.  On
   the other hand, regarding the network-inserted information, a user
   agent can insert a privacy flag and request intermediaries not to add
   the user-privacy-related information.

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4.  Use Cases

   The following are the use cases from the viewpoint of privacy.

   Case 1:  User privacy is required and a user agent can anonymize all
            of the user-inserted privacy-related information by itself.

   Case 2:  User privacy is required but a user agent cannot anonymize
            the user-inserted privacy-related information all by itself.

   Case 3:  User privacy is not required.  The user does not want
            privacy at all and would like to reveal his/her identity.

   Note that Case 2 is based on the premise that the user agent has
   limited capabilities and it cannot find a GRUU or TURN server.  Case
   2 is outside the scope of this document.

5.  Requirements

   The following are requirements to cover the use cases in the previous

   Req 1:  A user agent MUST be able to send a SIP request that is fully
           anonymized.  This is, any headers and body inserted by the UA
           does not jeopardize user privacy.

   Req 2:  It MUST be possible for a user agent to indicate to
           downstream entities that a user is requesting privacy.

   Req 3:  When privacy is requested, a proxy SHOULD honor the request
           and only add information necessary to route the call while
           withholding any sensitive information that may reveal
           anything about the user if possible.

   Req 4:  Mechanism defined here MUST be backward compatible with the
           pre-existing privacy mechanism already in place.

6.  Treatment of User Privacy Related Information

   RFC 3323 does not provide means to obscure two important pieces of
   information about the user agent, which are a URI used to exchange
   signaling (Contact, From, for example), and an IP address used to
   exchange media.

   With the use of GRUU [I-D.ietf-sip-gruu] and TURN
   [I-D.rosenberg-midcom-turn], UA can now obtain URI and IP address

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   that are functional, which are usable to exchange either signaling or
   media while providing privacy.

6.1.  Anonymous URI

   A user agent wanting to obtain functional anonymous URI SHOULD
   support and SHOULD utilize the Global Routable User Agent URI (GRUU)
   mechanism.  By sending a REGISTER request requesting GRUU, the UA can
   obtain an anonymous URI, which can later be used for From, Contact
   and other headers where the URI of the originator is needed.

   The detailed process on how a user agent obtains a GRUU is described
   in [I-D.ietf-sip-gruu].  If the Registrar supports GRUU and returns a
   REGISTER response, the user agent SHOULD search within the REGISTER
   response for a "temp-gruu" URI parameter, which provides the desired
   privacy property.

   If the "temp-gruu" URI parameter and value exist within the REGISTER
   response, the user agent SHOULD use the value of the "temp-gruu" as
   an anonymous URI representing the originator.  This URI SHOULD be
   used for Contact and From, for example, wherever the originator of
   the URI is required.

   The user agent setting the "temp-gruu" as a GRUU SHOULD set
   "Anonymous" as a display name in any header where the display name of
   the originator is set.  That indicates the anonymity of the request
   to intermediaries that may invoke some services based on the
   anonymity of the call.  The temp-gruu alone is not sufficient to
   invoke such service because GRUU is merely a URI that is a sequence
   of strings and digits with no explicit semantics to indicate that it
   is an anonymous URI.

   If there is no "temp-gruu" URI parameter in the 200 response to the
   REGISTER request, a user agent SHOULD NOT proceed with its
   anonymization process, unless something equivalent to "temp-gruu" is
   provided through some administrative means.

   It is RECOMMENDED that user agent consult the user before sending a
   request without a functional anonymous URI when privacy is request
   from the user.

6.2.  Anonymous IP Address

   It is assumed that a user agent is either manually or automatically
   configured through means such as a configuration framework with one
   or more STUN relay servers.

   Two IP addresses are needed to maintain privacy, one to be used in

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   signaling such as in a Via header, another to be used in SDP for

   A user agent that is not provided with a functional anonymous IP
   address through some administrative means, SHOULD obtain a relayed
   address (IP address of the media relay) for use in SDP, derived from
   a STUN relay server using the STUN Relay
   Usage[I-D.rosenberg-midcom-turn], which allows a STUN server to act
   as a media relay.

   Note: A relayed IP address may be used for a Via header, but some
         commented that is not an appropriate to be used for signaling.
         There was a comment about the IP address in Via being stripped
         by the proxy, but that would require that a proxy compliant to
         this specification is in the signaling path.

7.  User Agent Behavior

   A user agent fully compliant with this document SHOULD obscure or
   conceal all the user-inserted privacy-related information in SIP
   requests and responses when user privacy is requested.  Section 7.1
   describes how to generate an anonymous message at a user agent.

   When a user agent generates an anonymous message based on this
   specification, it SHOULD set an indication to tell intermediaries not
   to add or modify user-privacy-related information.  Section 7.2
   describes more about this.

7.1.  Generating Anonymous Message

   The two pieces of information that a user agent needs to obscure
   while sustaining its purpose and functionality are the URI and IP
   address used for establishing a media/signaling session.
   Instructions on how to obtain an functional anonymous URI and IP
   address are given in Section 6.1 and 6.2, respectively.

   For anonymizing any headers and information in a SIP message, the
   user agent SHOULD follow the instructions in this document.

   Note: Instructions to treat each SIP header/parameter in generating
         an anonymous SIP message SIP message will be given in a future.

7.2.  Indication to maintain Privacy

   This document defines a privacy flag, which indicates that the user
   requires privacy for the SIP message.  Without a privacy flag,
   intermediaries might add some user-privacy-related information in the

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   message, even if a user agent had anonymized the message as perfectly
   as possible.

   When a user agent generates an anonymous message by itself according
   to the guidelines in Section 7.1, it SHOULD set a flag to request
   intermediaries not to add user-privacy-related information.

   Note: The mechanism of the flag is FFS.

8.  Proxy Behavior

   When a proxy receives a SIP message containing a privacy flag, the
   proxy compliant with this specification MUST NOT add any information
   that may reveal something about the sender that is irrelevant to
   routing unless the proxy knows that such information will be deleted
   before it leaves the boundary of the Trust Domain[RFC3324].

   A proxy MUST NOT modify the privacy flag, if present.

9.  Security Considerations


10.  IANA Considerations


11.  References

11.1.  Normative References

              Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent (UA) URIs (GRUU) in the  Session Initiation Protocol
              (SIP)", draft-ietf-sip-gruu-14 (work in progress),
              June 2007.

              Rosenberg, J., "Traversal Using Relay NAT (TURN)",
              draft-rosenberg-midcom-turn-08 (work in progress),
              September 2005.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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   [RFC3261]  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.

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323, November 2002.

11.2.  Informative References

   [RFC3324]  Watson, M., "Short Term Requirements for Network Asserted
              Identity", RFC 3324, November 2002.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

Authors' Addresses

   Mayumi Munakata
   NTT Corporation

   Phone: +81 422 36 7565
   Email: munakata.mayumi at

   Shida Schubert
   NTT Corporation

   Phone: +1 604 762 5606
   Email: shida at

   Takumi Ohba
   NTT Corporation
   9-11, Midori-cho 3-Chome
   Musashino-shi, Tokyo  180-8585

   Phone: +81 422 59 7748
   Email: ohba.takumi at

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