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RFC 5379 - Guidelines for Using the Privacy Mechanism for SIP


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Independent Submission                                       M. Munakata
Request for Comments: 5379                                   S. Schubert
Category: Informational                                          T. Ohba
ISSN: 2070-1721                                                      NTT
                                                           February 2010

           Guidelines for Using the Privacy Mechanism for SIP

Abstract

   This is an informational document that provides guidelines for using
   the privacy mechanism for the Session Initiation Protocol (SIP) that
   is specified in RFC 3323 and subsequently extended in RFCs 3325 and
   4244.  It is intended to clarify the handling of the target SIP
   headers/parameters and the Session Description Protocol (SDP)
   parameters for each of the privacy header values (priv-values).

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5379.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. Semantics of Existing priv-values ...............................4
   4. Target for Each priv-value ......................................5
      4.1. Target SIP Headers for Each priv-value .....................5
      4.2. Target SDP Parameters for Each priv-value ..................6
      4.3. Treatment of priv-value Not Supported by the
           Privacy Service ............................................7
   5. Recommended Treatment of User-Privacy-Sensitive Information .....7
      5.1. Target SIP Headers .........................................7
           5.1.1. Call-ID .............................................7
           5.1.2. Call-Info ...........................................8
           5.1.3. Contact .............................................8
           5.1.4. From ................................................9
           5.1.5. History-Info .......................................10
           5.1.6. In-Reply-To ........................................10
           5.1.7. Organization .......................................11
           5.1.8. P-Asserted-Identity ................................11
           5.1.9. Record-Route .......................................12
           5.1.10. Referred-By .......................................13
           5.1.11. Reply-To ..........................................14
           5.1.12. Server ............................................14
           5.1.13. Subject ...........................................15
           5.1.14. User-Agent ........................................15
           5.1.15. Via ...............................................15
           5.1.16. Warning ...........................................16
      5.2. Target SDP Parameters .....................................16
           5.2.1. c/m Lines ..........................................16
           5.2.2. o Line .............................................17
           5.2.3. i/u/e/p Lines ......................................17
      5.3. Considerations for Non-Target SIP Headers/Parameters ......17
           5.3.1. Identity/Identity-Info .............................17
           5.3.2. Path ...............................................18
           5.3.3. Replaces Header/Parameter ..........................18
           5.3.4. Route ..............................................21
           5.3.5. Service-Route ......................................21
           5.3.6. Target-Dialog ......................................21
   6. Security Considerations ........................................21
   7. Acknowledgements ...............................................22
   8. References .....................................................22
      8.1. Normative References ......................................22
      8.2. Informative References ....................................22

1.  Introduction

   This document clarifies the privacy mechanism for the Session
   Initiation Protocol (SIP) [RFC3261] defined in [RFC3323], which was
   later extended in [RFC3325] and [RFC4244].  This document describes
   the practical manner of operations of the privacy mechanism as a
   guideline and does not change the existing privacy mechanism.

   In RFC 3323, the semantics of the basic set of priv-values (header,
   session, user, none, and critical) is defined, but there are some
   ambiguities in regards to the target information to be obscured per
   priv-value, which are not explicitly specified.  An ambiguity such as
   this could result in different interpretations of privacy handling
   for each of the priv-values defined, both at an entity setting a
   Privacy header and at an entity processing a Privacy header, which
   could have an adverse impact on interoperability.

   Additional priv-values "id" and "history" are defined in RFCs 3325
   and 4244, respectively.

   In RFC 4244, the priv-value "history" is defined in order to request
   privacy for History-Info headers, and the target to be obscured for
   "history" priv-value is specified as only the History-Info headers.
   In addition, the RFC clearly describes that History-Info headers are
   also the target when "header"- and "session"-level privacy are
   requested.

   On the other hand, RFC 3325 defines the P-Asserted-Identity header
   and a priv-value "id", which is used to request privacy for only the
   P-Asserted-Identity header, but it does not specify how other priv-
   values may impact the privacy handling of the P-Asserted-Identity
   header.  Because of this lack of specification, it has been observed
   that some implementations are suffering from the inability to achieve
   the intended privacy due to discrepancies in interpretations.

   This document tries to clarify the SIP headers and SDP parameters to
   be obscured for each of the priv-values to alleviate the potential
   interoperability issues already seen due to a lack of explicit text.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   Note: This document is informational; therefore, it does not specify
         any new normative behaviors of privacy mechanism.  All the RFC
         2119 language in this document is derived from the normative
         text in the existing RFCs, such as RFC 3323.

   priv-value:
         Values registered with IANA to be used in the Privacy header.
         Registered priv-values are "header", "session", "user", "none",
         and "critical" defined in [RFC3323]; "id" defined in [RFC3325];
         and "history" defined in [RFC4244].

   privacy service:
         A network entity that executes privacy functions before
         forwarding messages to the next hop.  It is sometimes
         abbreviated to PS in this document.

   user-level privacy:
         Privacy for user-inserted information that can be anonymized by
         the user agent itself.

3.  Semantics of Existing priv-values

   This section provides the semantics of each priv-value defined in
   RFCs 3323, 3325, and 4244.  The descriptions are taken from the IANA
   registration.

   Privacy Type  Description                             Reference
   ------------- ----------------------------------      ----------
   user          Request that privacy services           [RFC3323]
                 provide a user-level privacy function

   header        Request that privacy services modify    [RFC3323]
                 headers that cannot be set arbitrarily
                 by the user (Contact/Via).

   session       Request that privacy services provide   [RFC3323]
                 privacy for session media

   none          Privacy services must not perform any   [RFC3323]
                 privacy function

   critical      Privacy service must perform the        [RFC3323]
                 specified services or fail the request

   id            Privacy requested for Third-Party       [RFC3325]
                 Asserted Identity

   history       Privacy requested for                   [RFC4244]
                 History-Info header(s)

4.  Target for Each priv-value

   Tables in this section show the recommended treatment of SIP headers
   and SDP parameters per priv-value.  SIP headers and SDP parameters
   not shown in the tables are regarded as non-targets of these priv-
   values.  Some non-target SIP headers/SDP parameters may carry
   privacy-sensitive information that may need privacy treatment
   regardless of the privacy level requested.  This is further described
   in 5.3.

   The way in which SIP headers and SDP parameters listed here are
   obscured may depend on the implementation and network policy.  This
   document does not prevent different variations that may exist based
   on local policy but tries to provide recommendations for how a
   privacy service treats SIP headers and SDP parameters.

   Note: The scope of these tables is SIP headers and related parameters
         specified in RFCs.

4.1.  Target SIP Headers for Each priv-value

   Table 1 below shows a recommended treatment of each SIP header for
   each priv-value.  Detailed descriptions of the recommended treatment
   per SIP header are covered in Section 5.

   The "where" column describes the request and response types in which
   the header needs the treatment to maintain privacy.  Values in this
   column are:

      R: The header needs the treatment when it appears in a request.

      r: The header needs the treatment when it appears in a response.

   The next five columns show the recommended treatment for each priv-
   value:

      delete: The header is recommended to be deleted at a privacy
         service.

      not add: The header is recommended not to be added at a privacy
         service.

      anonymize: The header is recommended to be anonymized at a privacy
         service.  How to anonymize the header depends on the header.
         Details are given in Section 5.

      anonymize*: An asterisk indicates that the involvement of a
         privacy service and treatment of the relevant header depend on
         the circumstance.  Details are given in Section 5.

   Target headers    where   user     header    session   id   history
   --------------------------------------------------------------------
   Call-ID (Note)      R   anonymize    -         -       -      -
   Call-Info           Rr   delete    not add     -       -      -
   Contact             R      -      anonymize    -       -      -
   From                R   anonymize    -         -       -      -
   History-Info        Rr     -       delete    delete    -    delete
   In-Reply-To         R    delete      -         -       -      -
   Organization        Rr   delete    not add     -       -      -
   P-Asserted-Identity Rr     -       delete      -     delete   -
   Record-Route        Rr     -      anonymize    -       -      -
   Referred-By         R   anonymize*   -         -       -      -
   Reply-To            Rr   delete      -         -       -      -
   Server              r    delete    not add     -       -      -
   Subject             R    delete      -         -       -      -
   User-Agent          R    delete      -         -       -      -
   Via                 R      -      anonymize    -       -      -
   Warning             r   anonymize    -         -       -      -

           Table 1: Recommended PS behavior for each SIP header

   Note: Any time a privacy service modifies a Call-ID, it MUST retain
         the former and modified values as indicated in Section 5.3 in
         RFC 3323.  It MUST then restore the former value in a Call-ID
         header and other corresponding headers and parameters (such as
         In-Reply-To, Replaces, and Target-Dialog) in any messages that
         are sent using the modified Call-ID to the originating user
         agent.  It should also modify a Call-ID header and other
         corresponding headers/parameters (such as Target-Dialog and
         "replaces" parameter) in any further relevant messages that are
         sent by the originating user agent.  Refer to Section 5.1.1
         (Call-ID) for the detailed behavior.

   Identity/Identity-Info, Path, Replaces, Route, Service-Route, and
   Target-Dialog headers are not targets of these priv-values (and
   should not be anonymized or modified by a privacy service based on a
   priv-value in a Privacy header).  Refer to Section 5.3 for details.

4.2.  Target SDP Parameters for Each priv-value

   The recommended PS behaviors for each SDP parameters are simple.  The
   c, m, o, i, u, e, and p lines in SIP request/response are recommended
   to be anonymized when user privacy is requested with Privacy:session.

4.3.  Treatment of priv-value Not Supported by the Privacy Service

   As specified in RFC 3323, if the priv-value of "critical" is present
   in the Privacy header of a request, and if the privacy service is
   incapable of performing all of the levels of privacy specified in the
   Privacy header, it MUST fail the request with a 500 (Server Error)
   response code as indicated in Section 5 in RFC 3323.

   Since the protection of privacy is important, even if the priv-value
   "critical" is not present in the Privacy header, the privacy service
   should fail the request with a 500 response code when it is incapable
   of performing all of the levels of privacy specified in the Privacy
   header.

5.  Recommended Treatment of User-Privacy-Sensitive Information

   The following SIP headers and related parameters may concern privacy.
   This section describes what kind of user-privacy-sensitive
   information may be included in each SIP header/parameter, and how to
   maintain privacy for such information at a user agent or a privacy
   service when the information is indeed privacy-sensitive.

5.1.  Target SIP Headers

   This section describes privacy considerations and recommended
   treatment for each SIP header that may reveal user-privacy-sensitive
   information.  This section goes into details about how each header
   affects privacy, the desired treatment of the value by the user agent
   and privacy service, and other instructions/additional notes
   necessary to provide privacy.

5.1.1.  Call-ID

   This field frequently contains an IP address or hostname of a UAC
   (User Agent Client) appended to the Call-ID value.

   A user agent executing a user-level privacy function on its own
   SHOULD substitute for the IP address or hostname that is frequently
   appended to the Call-ID value a suitably long random value (the value
   used as the 'tag' for the From header of the request might even be
   reused) as indicated in Section 4.1 in RFC 3323.

   A privacy service MAY anonymize the Call-ID header when the request
   contains Privacy:user by substituting for the IP address or hostname
   in the Call-ID a suitably long random value (such as a From tag
   value) so that it is sufficiently unique as indicated in Section 5.3
   in RFC 3323.

   Call-ID is essential to dialog matching, so any time a privacy
   service modifies this field, it MUST retain the former value and
   restore it in a Call-ID header in any messages that are sent to/by
   the originating user agent inside the dialog as indicated in Section
   5.3 in RFC 3323.  A privacy service should be prepared to receive a
   request outside the dialog containing the value of the Call-ID set by
   the PS in other SIP headers (e.g., In-Reply-To/Replaces/
   Target-Dialog), at least while the dialog state is active for the
   dialog whose Call-ID was modified by that PS.  When such a request is
   received, the Call-ID value contained in the relevant headers
   indicated above should be replaced by the retained value.

   Note: This is possible only if the privacy service maintains the
         state and retains all the information it modified to provide
         privacy.  Some PSs are known to encrypt information prior to
         obfuscation in the Via header, etc.  In this case, the PS
         cannot correlate the modified Call-ID value with the original
         Call-ID.  Further challenges are imposed when the PS needs to
         stay on a signaling path to ensure that it receives all the
         messages targeted towards the caller for which a PS provides
         privacy, especially when the request is out-of-dialog.

   Refer to the corresponding sections, 5.1.6 (In-Reply-To), 5.3.3
   (Replaces Header/Parameter), and 5.3.6 (Target-Dialog), for detailed
   discussion.

5.1.2.  Call-Info

   This field contains additional information about the user.

   A user agent executing a user-level privacy function on its own
   SHOULD NOT add a Call-Info header as indicated in Section 4.1 in RFC
   3323.

   A privacy service MUST delete a Call-Info header if one exists when
   user privacy is requested with Privacy:user as indicated in Section
   5.3 in RFC 3323.  A privacy service SHOULD NOT add a Call-Info header
   when user privacy is requested with Privacy:header as indicated in
   Section 5.1 in RFC 3323.

5.1.3.  Contact

   This field contains a URI used to reach the user agent for mid-dialog
   requests and possibly out-of-dialog requests, such as REFER
   [RFC3315].  Since the Contact header is essential for routing further
   requests to the user agent, it must include a functional URI even
   when it is anonymized.

   A user agent MUST NOT anonymize a Contact header, unless it can
   obtain an IP address or contact address that is functional yet has a
   characteristic of anonymity as indicated in Section 4.1.1.3 in RFC
   3323.

   Since RFC 3323 was published, there have been proposals that allow
   UAs to obtain an IP address or contact address with a characteristic
   of anonymity.

   The mechanisms that are discussed at the time of this writing are
   Globally Routable User Agent URIs (GRUU) [SIPGRUU], which provides a
   functional Contact address with a short life span, making it ideal
   for privacy sensitive calls, and Traversal Using Relays around NAT
   (TURN) [TURN], through which an IP address of a relay can be obtained
   for use in a Contact header.

   A privacy service SHOULD anonymize a Contact header by replacing the
   existing Contact header field value with the URI that dereferences to
   the privacy service when user privacy is requested with
   Privacy:header, as indicated in Section 5.1 in RFC 3323.  This is
   generally done by replacing the IP address or hostname with that of
   the privacy service.

5.1.4.  From

   This field contains the identity of the user, such as display-name
   and URI.

   A user agent executing a user-level privacy function on its own
   SHOULD anonymize a From header using an anonymous display-name and an
   anonymous URI as indicated in Section 4.1 in RFC 3323.

   A privacy service should anonymize a From header when user privacy is
   requested with Privacy:user.

   Note: This does not prevent a privacy service from anonymizing the
         From header based on local policy.

   The anonymous display-name and anonymous URI mentioned in this
   section use display-name "Anonymous", a URI with "anonymous" in the
   user portion of the From header, and the hostname value
   "anonymous.invalid" as indicated in Section 4.1.1.3 in RFC 3323.

   The recommended form of the From header for anonymity is:

   From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=1928301774

   The tag value varies from dialog to dialog, but the rest of this
   header form is recommended as shown.

5.1.5.  History-Info

   History-Info [RFC4244] header URIs to which the request was forwarded
   or retargeted can reveal general routing information.

   A user agent executing a user-level privacy function on its own
   SHOULD NOT add a History-Info header as indicated in Section 3.3 in
   RFC 4244.

   A privacy service SHOULD delete the History-Info headers when user
   privacy is requested with Privacy:header, Privacy:session, or
   Privacy:history as indicated in Section 3.3 in RFC 4244.

   The privacy could be also expressed for a specific History-Info entry
   by inserting "privacy=history" in the History-Info header.  In such a
   case, a privacy service SHOULD delete the History-Info entry as
   indicated in Section 4.3.3.1.1 in RFC 4244.

   Refer to [RFC4244] for detailed behavior for dealing with History-
   Info headers.

5.1.6.  In-Reply-To

   The In-Reply-To header contains a Call-ID of the referenced dialog.
   The replying user may be identified by the Call-ID in an In-Reply-To
   header.

   Alice > INV(Call-ID:C1) > Bob
   Bob   > INV(In-Reply-To:C1) > Alice

   In this case, unless the In-Reply-To header is deleted, Alice might
   notice that the replying user is Bob because Alice's UA knows that
   the Call-ID relates to Bob.

   A user agent executing a user-level privacy function on its own
   should not add an In-Reply-To header as implied in Section 4.1 in RFC
   3323.

   A privacy service MUST delete the In-Reply-To header when user
   privacy is requested with Privacy:user as indicated in Section 5.3 in
   RFC 3323.

   In addition, since an In-Reply-To header contains the Call-ID of the
   dialog to which it is replying, special attention is required, as
   described in Section 5.1.1 (Call-ID), regardless of the priv-value or

   presence of a Privacy header.  Once a privacy service modifies a
   Call-ID in the request, a privacy service should restore the former
   value in an In-Reply-To header, if present in the INVITE request
   replying to the original request, as long as the privacy service
   maintains the dialog state.

   Example:
   Alice > INV(Call-ID:C1, Privacy:user) > PS > INV(Call-ID:C2) > Bob
   Bob   > INV(In-Reply-To:C2, Privacy:none) > PS >
           INV(In-Reply-To:C1) > Alice

   Note: This is possible only if the privacy service maintains the
         state and retains all the information that it modified to
         provide privacy even after the dialog has been terminated,
         which is unlikely.  Call-back is difficult to achieve when a
         privacy service is involved in forming the dialog to be
         referenced.

5.1.7.  Organization

   This field contains additional information about the user.

   A user agent executing a user-level privacy function on its own
   should not add an Organization header as implied in Section 4.1 in
   RFC 3323.

   A privacy service MUST delete the Organization header if one exists
   when user privacy is requested with Privacy:user as indicated in
   Section 5.3 in RFC 3323.  A privacy service SHOULD NOT add an
   Organization header when user privacy is requested with Privacy:
   header as indicated in Section 5.1 in RFC 3323.

5.1.8.  P-Asserted-Identity

   This header contains a network-verified and network-asserted identity
   of the user sending a SIP message.

   A privacy service MUST delete the P-Asserted-Identity headers when
   user privacy is requested with Privacy:id as indicated in Section 7
   in RFC 3325 and should delete the P-Asserted-Identity headers when
   user privacy is requested with Privacy:header before it forwards the
   message to an entity that is not trusted.

   It is recommended for a privacy service to remove the P-Asserted-
   Identity header if user privacy is requested with Privacy:id or
   Privacy:header even when forwarding to a trusted entity, unless it
   can be confident that the message will not be routed to an untrusted
   entity without going through another privacy service.

5.1.9.  Record-Route

   This field may reveal information about the administrative domain of
   the user.

   In order to hide Record-Route headers while keeping routability to
   the sender, privacy services can execute a practice referred to as
   "stripping".  Stripping means removing all the Record-Route headers
   that have been added to the request prior to its arrival at the
   privacy service and then adding a single Record-Route header
   representing itself.  In this case, the privacy service needs to
   retain the removed headers and restore them in a response.

   Alternatively, privacy services can remove the Record-Route headers
   and encrypt them into a single Record-Route header field.  In this
   case, the privacy service needs to decrypt the header and restore the
   former values in a response.

   A privacy service SHOULD strip or encrypt any Record-Route headers
   that have been added to a message before it reaches the privacy
   service when user privacy is requested with Privacy:header as
   indicated in Section 5.1 in RFC 3323.

   As in the case of a Call-ID, if a privacy service modifies the
   Record-Route headers, it MUST be able to restore Route headers with
   retained values as indicated in Section 5.1 in RFC 3323.  Some
   examples where the restoration of the Route headers is necessary and
   unnecessary are given below.

   When a UAC (Alice) requires privacy for a request, a privacy service
   does not have to restore the Route headers in the subsequent request
   (see Example 1).

   On the other hand, when a UAS (User Agent Server) (Bob) requires
   privacy for a response, a privacy service has to restore the Route
   headers in the subsequent request (see Example 2).

   Example 1:
   Restoration of Route header is UNNECESSARY when UAC requires privacy
   Alice > INV(Privacy:header) > P1 >
           INV(Record-Route:P1, Privacy:header) > PS >
           INV(Record-Route:PS) > P2 >
           INV(Record-Route:P2,PS) > Bob
   Bob   > 200(Record-Route:P2,PS) > P2 > PS >
           200(Record-Route:P2,PS,P1) > P1 > Alice
   Alice > re-INV(Route:P2,PS,P1, Privacy:header) > P1 >
           re-INV(Route:P2,PS, Privacy:header) > PS >
           re-INV(Route:P2) > P2 > re-INV > Bob

 Alice             P1                PS                P2            Bob
 |                 |                 |                 |               |
 | INV Priv        |INV Priv RR:P1   | INV RR:PS       | INV RR:P2,PS  |
 |---------------->|---------------->|---------------->|-------------->|
 |                 |                 |                 |               |
 | 200 RR:P2,PS,P1 | 200 RR:P2,PS,P1 | 200 RR:P2,PS    | 200 RR:P2,PS  |
 |<----------------|<----------------|<----------------|<--------------|
 |                 |                 |                 |               |
 | INV R:P2,PS,P1  | INV R:P2,PS     | INV R:P2        | INV           |
 |---------------->|---------------->|---------------->|-------------->|
 |                 |                 |                 |               |

     Figure 1: Example when restoration of Route header is UNNECESSARY

   Example 2:
   Restoration of Route header is NECESSARY when UAS requires privacy
   Alice > INV > P1 > INV(Record-Route:P1) > P2 >
           INV(Record-Route:P2,P1) > Bob
   Bob   > 200(Record-Route:P2,P1, Privacy:header) > P2 > PS' >
           200(Record-Route:PS',P1) > P1 > Alice
   Alice > re-INV(Route:PS',P1) > P1 > re-INV(Route:PS') > PS' >
           re-INV(Route:P2) > P2 > Bob

 Alice           P1                PS'               P2              Bob
 |               |                 |                 |                 |
 | INV           |INV RR:P1        |                 | INV RR:P2,P1    |
 |-------------->|---------------------------------->|---------------->|
 |               |                 |                 |                 |
 | 200 RR:PS',P1 | 200 RR:PS',P1   |200 Priv RR:P2,P1|200 Priv RR:P2,P1|
 |<--------------|<----------------|<----------------|<----------------|
 |               |                 |                 |                 |
 | INV R:PS',P1  | INV R:PS'       | INV R:P2        | INV             |
 |-------------->|---------------->|---------------->|---------------->|
 |               |                 | (Restored)      |                 |

     Figure 2: Example when restoration of Route header is NECESSARY

   Note: In Figures 1 and 2, Priv means Privacy:header, RR means Record-
         Route header, and R means Route header.

5.1.10.  Referred-By

   The Referred-By [RFC3892] header carries a SIP URI representing the
   identity of the referrer.

   The Referred-By header is an anonymization target when the REFER
   request with the Referred-By header is sent by the user (referrer)
   whose privacy is requested to be processed in the privacy service.

   A user agent that constructs REFER requests executing a user-level
   privacy function on its own should anonymize a Referred-By header by
   using an anonymous URI.

   A privacy service should anonymize a Referred-By header in a REFER
   request by using an anonymous URI when user privacy is requested with
   Privacy:user.

   On the other hand, the Referred-By header is not an anonymization
   target when it appears in a request other than REFER (e.g., INVITE)
   because the URI in the Referred-By header does not represent the
   sender of the request.

   Example 1:
   Referrer requests no privacy and referee requests privacy
   Alice > REF(Referred-By:Alice) > Bob
   Bob   > INV(Referred-By:Alice, Privacy:user) > PS >
           INV(Referred-By:Alice) > Carol

   Example 2:
   Referrer requests privacy and referee requests privacy
   Alice > REF(Referred-By:Alice, Privacy:user) > PS >
           REF(Referred-By:X) > Bob
   Bob   > INV(Referred-By:X, Privacy:user) > PS >
           INV(Referred-By:X) > Carol

5.1.11.  Reply-To

   This field contains a URI that can be used to reach the user on
   subsequent call-backs.

   A user agent executing a user-level privacy function on its own
   should not add a Reply-To header in the message as implied in Section
   4.1 in RFC 3323.

   A privacy service MUST delete a Reply-To header when user privacy is
   requested with Privacy:user as indicated in Section 5.3 in RFC 3323.

5.1.12.  Server

   This field contains information about the software used by the UAS to
   handle the request.

   A user agent executing a user-level privacy function on its own
   should not add a Server header in the response as implied in Section
   4.1 in RFC 3323.

   A privacy service must delete a Server header in a response when user
   privacy is requested with Privacy:user.  A privacy service SHOULD NOT
   add a Server header in a response when user privacy is requested with
   Privacy:header as indicated in Section 5.1 in RFC 3323.

5.1.13.  Subject

   This field contains free-form text about the subject of the call.  It
   may include text describing something about the user.

   A user agent executing a user-level privacy function on its own
   should not include any information identifying the caller in a
   Subject header.

   A privacy service MUST delete a Subject header when user privacy is
   requested with Privacy:user as indicated in Section 5.3 in RFC 3323.

5.1.14.  User-Agent

   This field contains the UAC's information.

   A user agent executing a user-level privacy function on its own
   should not add a User-Agent header as implied in Section 4.1 in RFC
   3323.

   A privacy service MUST delete a User-Agent header when user privacy
   is requested with Privacy:user as indicated in Section 5.3 in RFC
   3323.

5.1.15.  Via

   The bottommost Via header added by a user agent contains the IP
   address and port or hostname that are used to reach the user agent
   for responses.  Via headers added by proxies may reveal information
   about the administrative domain of the user.

   A user agent MUST NOT anonymize a Via header as indicated in Section
   4.1.1.3 in RFC 3323, unless it can obtain an IP address that is
   functional yet has a characteristic of anonymity.  This may be
   possible by obtaining an IP address specifically for this purpose
   either from the service provider or through features such as TURN.

   A privacy service SHOULD strip or encrypt any Via headers that have
   been added prior to reaching the privacy service when user privacy is
   requested with Privacy:header as indicated in Section 5.1 in RFC
   3323.  Refer to Section 5.1.9 (Record-Route) for details of stripping
   and encryption.

   A privacy service MUST restore the original values of Via headers
   when handling a response in order to route the response to the
   originator as indicated in Section 5.1 in RFC 3323.

   No Via stripping is required when handling responses.

5.1.16.  Warning

   This field may contain the hostname of the UAS.

   A user agent executing a user-level privacy function on its own
   should not include the hostname representing its identity in a
   Warning header.

   A privacy service should anonymize a Warning header by deleting the
   hostname portion (if it represents a UAS's identity) from the header
   when user privacy is requested with Privacy:user.

5.2.  Target SDP Parameters

   This section describes privacy considerations for each SDP [RFC4566]
   parameter that may reveal information about the user.

   When privacy functions for user-inserted information are requested to
   be executed at a privacy service, user agents MUST NOT encrypt SDP
   bodies in messages as indicated in Section 4.2 in RFC 3323.

5.2.1.  c/m Lines

   The c and m lines in the SDP body convey the IP address and port for
   receiving media.

   A user agent must not anonymize the IP address and port in the c and
   m lines, unless it can obtain an IP address that is functional yet
   has a characteristic of anonymity as implied in Section 4.1.1.3 in
   RFC 3323.  This may be possible by obtaining an IP address
   specifically for this purpose either from the service provider or
   through features such as TURN.

   A privacy service must anonymize the IP address and port in c and m
   lines using a functional anonymous IP address and port when user
   privacy is requested with Privacy:session.  This is generally done by
   replacing the IP address and port present in the SDP with that of a
   relay server.

5.2.2.  o Line

   The username and IP address in this parameter may reveal information
   about the user.

   A user agent may anonymize the username in an o line by setting
   username to "-" and anonymize the IP address in the o line by
   replacing it with a value so that it is sufficiently unique.

   A privacy service must anonymize the username and IP address in the o
   line by setting the username to "-" and replacing the IP address with
   a value so that it is sufficiently unique when user privacy is
   requested with Privacy:session.

5.2.3.  i/u/e/p Lines

   These lines may contain information about the user.

   A user agent executing a session-level privacy function on its own
   should not include user's information in the i, u, e, and p lines.

   A privacy service should modify the i, u, e, and p lines to delete
   the user's identity information when user privacy is requested with
   Privacy:session.

5.3.  Considerations for Non-Target SIP Headers/Parameters

5.3.1.  Identity/Identity-Info

   The Identity [RFC4474] header field contains a signature used for
   validating the identity.  The Identity-Info header field contains a
   reference to the certificate of the signer of Identity headers.  An
   Identity-Info header may reveal information about the administrative
   domain of the user.

   The signature in an Identity header provides integrity protection
   over the From, To, Call-ID, Cseq, Date, and Contact headers and over
   the message body.  The integrity protection is violated if a privacy
   service modifies these headers and/or the message body for the
   purpose of user privacy protection.

   Once those integrity-protected headers (such as From and Call-ID) are
   modified, the Identity/Identity-Info header fields are not valid any
   more.  Thus, a privacy service acting on a request for Privacy:user,
   Privacy:header, or Privacy:session can invalidate integrity
   protection provided by an upstream authentication service that has
   inserted Identity/Identity-Info header fields.  The use of such a
   privacy service should be avoided if integrity protect needs to be

   retained.  Otherwise, if the privacy service invalidates the
   integrity protection, it should remove the Identity/Identity-Info
   header fields.

   An authentication service downstream of the privacy service may add
   Identity/Identity-Info header fields if the domain name of the From
   header field URI has not been anonymized (e.g.,
   'sip:anonymous@example.com'), which makes it possible for the service
   to authenticate the UAC.  This authenticated yet anonymous From
   header means "this is a known user in my domain that I have
   authenticated, but I am keeping its identity private" as indicated in
   Section 12 in RFC 4474.

   The desired deployment will have a privacy service located before or
   co-located with the identity service; thus, integrity and privacy can
   both be provided seamlessly.

5.3.2.  Path

   This field may contain information about the administrative domain
   and/or the visited domain of the user agent.  However, the Path
   header is not the target of any priv-values.

   Given that the Path header [RFC3327] only appears in REGISTER
   requests/responses and is essential for a call to reach the
   registered UA in the visited domain, it serves no purpose to withhold
   or hide the information contained in the Path header; rather, it is
   harmful.

   The only reason privacy may be considered desirable is if the visited
   domain wants to withhold its topology from the home domain of the
   user.  In doing so, the domain withholding the topology needs to
   ensure that it provides sufficient information so that the home
   domain can route the call to the visited domain, thus reaching the
   UA.

   However, anonymization of network-privacy-sensitive information is
   out of scope.

5.3.3.  Replaces Header/Parameter

   The Replaces [RFC3891] header and the "replaces" parameter contain
   identifiers of a dialog to be replaced, which are composed of Call-
   ID, local tag, and remote tag.

   The sender of the INVITE with a Replaces header is usually not the
   originating user agent or terminating user agent of the target dialog
   to be replaced.  Therefore, the Call-ID within the Replaces header is
   unlikely to be generated by the sender, and thus this header is
   outside the anonymization target per priv-value.

   The "replaces" parameter, which appears in a Refer-To header in a
   REFER request, is not the target of any particular priv-values
   either.  As described in Section 5.1.1 (Call-ID), regardless of the
   priv-value or the presence of a Privacy header, once a privacy
   service modifies a Call-ID in the request, it should monitor headers
   that may contain Call-ID and restore the portion of the value
   representing the modified Call-ID to the original Call-ID value in a
   Replaces header received.

   The main challenge for this to function properly is that a privacy
   service has to be on a signaling path to the originator for every
   dialog.  This is generally not possible and results in REFER requests
   not functioning at all times.  This is a trade-off that is
   anticipated when privacy is imposed.

   The privacy requirements mentioned in Section 5.1.1 will cause the
   Replaces header and "replaces" parameter to contain values that will
   fail the resulting dialog establishment in some situations.  This
   loss of functionality is allowed and/or intended as illustrated above
   (i.e., it is not the responsibility of a privacy service to ensure
   that these features always work).

   The functionality of the Replaces header/parameter when anonymized
   depends on the circumstances in which it is used.  REFER may work or
   may not work depending on the following three criteria.

   1. Who generated the Call-ID.
   2. Where the privacy service is on the signaling path.
   3. Who initiates the REFER with the "replaces" parameter.

   A few examples that explore when the Replaces header/parameter works
   or fails are given below.

   Example 1:
   Transfer initiated by the originator, PS added for first INV and REF
   Alice > INV(Call-ID:C1, Privacy:user) > PS > INV(Call-ID:C2) > Bob
   Alice > REF(Refer-To:Bob?Replaces=C1, Privacy:user) > PS >
           REF(Refer-To:Bob?Replaces=C2) > Carol
   Carol > INV(Replaces:C2) > Bob (SUCCEED)

   Example 2:
   Transfer initiated by the originator, PS added only for first INV
   Alice > INV(Call-ID:C1, Privacy:user) > PS > INV(Call-ID:C2) > Bob
   Alice > REF(Refer-To:Bob?Replaces=C1) > Carol
   Carol > INV(Replaces:C1) > Bob (FAIL)

   Note: Example 2 would succeed if the same PS (that modifies the Call-
         ID in the INVITE from Alice) is also added for REFER and
         modifies the value in the "replaces" parameter from C1 to C2
         even if there is no Privacy header in the REFER.

   Example 3:
   Transfer initiated by the originator, PS added only for REF
   Alice > INV(Call-ID:C1) > INV(Call-ID:C1) > Bob
   Alice > REF(Refer-To:Bob?Replaces=C1, Privacy:user) > PS >
           REF(Refer-To:Bob?Replaces=C1) > Carol
   Carol > INV(Replaces:C1, Privacy:user) > PS' >
           INV(Replaces:C1) > Bob (SUCCEED)

   Example 4:
   Transfer initiated by the terminating party, PS added for both INV
   Alice > INV(Call-ID:C1, Privacy:user) > PS > INV(Call-ID:C2) > Bob
   Bob   > REF(Refer-To:Alice?Replaces=C2) > Carol
   Carol > INV(Replaces:C2) > PS > INV(Replaces:C1) > Alice (SUCCEED)

   Note: Example 4 succeeds because the same PS (that modifies the Call-
         ID in the INVITE from Alice) checks the incoming requests and
         modifies the value in a Replaces header in the INVITE from
         Carol to the former value of Call-ID (C1).

   Example 5:
   Hold, PS added only for first INV
   Alice > INV(Call-ID:C1, Privacy:user) > PS > INV(Call-ID:C2) > Bob
   Alice > REF(Refer-To:Bob?Replaces=C1) > Music-Server
   Music-Server > INV(Replaces:C1) > Bob (FAIL)

   Note: Example 5 would succeed if the same PS (that modifies the Call-
         ID in the INVITE from Alice) is added for the INVITE from the
         Music-Server and modifies the value in a Replaces header from
         C1 to C2.

   As the above examples show, in some scenarios, information carried in
   the Replaces header/parameter would result in failure of the REFER.
   This will not happen if the Call-ID is not modified at a privacy
   service.

5.3.4.  Route

   This field may contain information about the administrative domain of
   the user agent, but the Route header is not the target of any priv-
   values.

   Route headers appear only in SIP requests to force routing through
   the listed set of proxies.  If a privacy service anonymizes the Route
   header, the routing does not function.  Furthermore, there is no risk
   in revealing the information in the Route headers to further network
   entities, including the terminating user agent, because a proxy
   removes the value from the Route header when it replaces the value in
   the Request-URI as defined in RFC 3261.

   A privacy service that modifies Record-Route headers may need to
   restore the values in Route headers as necessary.  As indicated in
   Section 5.1 in RFC 3323, if a privacy service modifies the Record-
   Route headers, it MUST be able to restore Route headers with retained
   values.  Please refer to Section 5.1.9 (Record-Route) for further
   detail and examples.

5.3.5.  Service-Route

   Service-Route headers [RFC3608] appear only in 200 OK responses to
   REGISTER requests and contain information about the registrar.  The
   purpose of the privacy mechanism defined in RFC 3323 is to secure the
   user's privacy, so the case where a registrar sets a Privacy header
   is not considered here.  Therefore, the Service-Route header is not
   the target of any priv-values.

5.3.6.  Target-Dialog

   The Target-Dialog [RFC4538] header faces exactly the same issues as
   seen for the Replaces header.  Please refer to Section 5.3.3
   (Replaces Header/Parameter) for why this is not a target for any
   particular priv-values and how a privacy service still needs to
   evaluate and modify the value contained, even if no privacy is
   requested.

6.  Security Considerations

   This guideline document adds no new security considerations to those
   discussed in [RFC3323], [RFC3325], and [RFC4244].

7.  Acknowledgements

   The authors would like to thank John Elwell, Jon Peterson, Jonathan
   Rosenberg, Mary Barnes, Paul Kyzivat, and Roland Jesske for their
   reviews and comments.

8.  References

8.1.  Normative References

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

   [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.

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

   [RFC4244]  Barnes, M., Ed., "An Extension to the Session Initiation
              Protocol (SIP) for Request History Information", RFC 4244,
              November 2005.

8.2.  Informative References

   [TURN]     Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", Work in Progress,
              July 2008.

   [SIPGRUU]  Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent URIs (GRUUs) in the Session Initiation Protocol
              (SIP)", RFC 5627, October 2009.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3327]  Willis, D. and B. Hoeneisen, "Session Initiation Protocol
              (SIP) Extension Header Field for Registering Non-Adjacent
              Contacts", RFC 3327, December 2002.

   [RFC3608]  Willis, D. and B. Hoeneisen, "Session Initiation Protocol
              (SIP) Extension Header Field for Service Route Discovery
              During Registration", RFC 3608, October 2003.

   [RFC3891]  Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
              Protocol (SIP) "Replaces" Header", RFC 3891, September
              2004.

   [RFC3892]  Sparks, R., "The Session Initiation Protocol (SIP)
              Referred-By Mechanism", RFC 3892, September 2004.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4538]  Rosenberg, J., "Request Authorization through Dialog
              Identification in the Session Initiation Protocol (SIP)",
              RFC 4538, June 2006.

   [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 7502
   EMail: munakata.mayumi@lab.ntt.co.jp

   Shida Schubert
   NTT Corporation

   EMail: shida@ntt-at.com

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

   Phone: +81 422 59 7748
   EMail: ohba.takumi@lab.ntt.co.jp

 

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