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Versions: (draft-dcsgroup-sip-privacy) 00 01 02 03 04

SIP Working Group                                           W. Marshall
Internet Draft                                                     AT&T
Document: <draft-ietf-sip-privacy-04.txt>
Category: Standards Track                               K. Ramakrishnan
                                                     TeraOptic Networks

                                                              E. Miller
                                                                Terayon

                                                             G. Russell
                                                              CableLabs

                                                               B. Beser
                                                       Juniper Networks

                                                            M. Mannette
                                                        K. Steinbrenner
                                                                   3Com

                                                                D. Oran
                                                           F. Andreasen
                                                                  Cisco

                                                             J. Pickens
                                                                  Com21

                                                            P. Lalwaney
                                                                  Nokia

                                                             J. Fellows
                                               Copper Mountain Networks

                                                               D. Evans
                                                 D. R. Evans Consulting

                                                               K. Kelly
                                                               NetSpeak

                                                              M. Watson
                                                        Nortel Networks

                                                      February 27, 2002


          SIP Extensions for Network-Asserted Caller Identity
                  and Privacy within Trusted Networks


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].


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             SIP Extensions for Caller Identity and Privacy    Feb. 2002

   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.

1. Abstract

   This document describes extensions to SIP that enable a network of
   trusted SIP servers to assert the identity of end users or end
   systems, and to convey indications of end-user requested privacy.
   The use of these extensions are only applicable inside an
   administrative domain, or among federations of administrative
   domains with previously agreed-upon policies for usage of such
   information. This document does NOT offer a general privacy or
   identity model suitable for inter-domain use or use in the Internet
   at large.

2. Scope of Applicability

   This document describes extensions to SIP that enable a network of
   trusted SIP servers to assert the identity of end users or end
   systems, and to convey indications of end-user requested privacy.
   The use of these extensions are only applicable inside an
   administrative domain, or among federations of administrative
   domains with previously agreed-upon policies for usage of such
   information. Such a "network" is explicitly trusted by its users and
   end-systems to either publicly assert the identity of each party, or
   be responsible for withholding that identity outside of the trusted
   domain or federation of domains if privacy is requested. The means
   by which the network determines the identity to assert is outside
   the scope of this document.

   This document does NOT offer a general privacy or identity model
   suitable for inter-domain use or use in the Internet at large. Its
   assumptions about the trust relationship between the user and the
   network may not apply in many applications. For example, these
   extensions do not accommodate a model whereby end users can
   independently assert their identity by use of the extensions defined
   here. Furthermore, since the asserted identities are not
   cryptographically certified, they are subject to forgery, replay,
   and falsification in any architecture that does not provide full
   transitive trust. The asserted identities also lack an indication of
   who is asserting the identity, and therefore the assertions are not
   useful outside of the federation of domains, where such information

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   would be crucial in order to determine the validity or value of the
   assertion.

   Despite these limitations, there are sufficiently useful specialized
   deployments that meet the assumptions described above, and can
   accept the limitations that result, to warrant publication of this
   mechanism. An example deployment would be a closed network which
   emulates a traditional circuit switched telephone network.

   It should be noted, that the mechanisms described in this draft are
   not intended to be used for user-asserted identity. As described
   above, the mechanisms are merely intended to enable trusted
   intermediaries to assert an identity for users.

3. Conventions used in this document

   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 RFC-2119 [2].

4. Introduction

   Various providers which attempt to offer a telephony service over IP
   networks have selected SIP as a base protocol. These environments
   require a way for trusted network elements (for example SIP proxy
   servers) to communicate the identity of users using such a service,
   yet also need to withhold this information from untrusted entities
   under certain circumstances. Such networks typically assume some
   level of transitive trust.

   These networks must support certain popular telephony services as
   well as some regulatory and public safety requirements. These
   include Calling Identity Delivery services, Calling Identity
   Delivery Blocking, and the ability to trace the originator of a
   call. While baseline SIP can support each of these services
   independently, certain combinations cannot be supported. For
   example, a caller that wants to maintain privacy and consequently
   provides unintelligible information in the SIP From header field
   will not be identifiable by intermediaries. However, since SIP does
   not allow the contents of the From header field to be modified by
   intermediaries, this will prevent certain services, e.g., call
   trace, from being performed by intermediaries which do not directly
   perform SIP authentication. Furthermore, the issue of privacy in an
   IP environment is more complicated than in the PSTN. The caller and
   callee will normally exchange IP traffic directly, and IP address
   information itself may reveal some privacy. The issue of IP address
   privacy for both the caller and callee consequently needs to be
   addressed as well. Although we recognize and discuss the IP address
   privacy problem, we do not provide a solution to it in this
   document.

   In order to solve the network asserted caller identity and privacy
   problem we assume an architecture where the caller initiates a

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   session to the callee via a trusted entity in its network. The
   callee in turn receives the session initiation via a trusted entity
   in its network. A trusted entity is here defined as a SIP proxy or
   SIP UA, that belongs to and is controlled by the "network". The
   trusted entities provide the services of determining the identity of
   the calling or called party, and furthermore add identity
   information for that party to the SIP messages. Trusted entities in
   the network thus serve as intermediaries that provide the caller and
   callee with network asserted identity information about the remote
   party. It should be noted, that the means by which the trusted
   entities determine these asserted identities are outside the scope
   of this document.

   The trusted entities do not trust the end users or end systems they
   serve. Furthermore, trusted entities may not trust their next or
   previous hop, e.g., if that hop represents a different untrusted
   domain. This leads our architecture to have the concept of trust
   boundaries. Trust and trust boundaries are in the eye of the
   beholder. For example, if A trusts B, but B does not trust A, then A
   does not see a trust boundary between the two, but B does. When an
   entity receives a message across a trust boundary, it does not trust
   it. From a network asserted caller identity point of view, this
   means the identity information cannot be trusted. Similarly, if a
   message is to be sent across a trust boundary, the sending entity
   does not trust the next hop to handle the message as desired. This
   implies, that the entity must ensure, that any privacy needed is
   provided before the message is forwarded across a trust boundary,
   while still being able to trace the originating party if needed.

   The architecture is illustrated in the following figures.


                 +---------+       .        +---------+
                 | Proxy-o |     B).        | Proxy-t |
         +------>|         |------{.}------>|         |------+
         |       +---------+       .        +---------+      |
         |                         .                         |
      A) |                         .                         |
    .....|...................................................|.....
         |                         .                         | C)
         |                         .                         |
         |                         .                         v
     +------+                      .D)                    +------+
     | UA-o |<---------------------.--------------------->| UA-t |
     +------+                      .                      +------+

          Figure 1 - Basic Architecture with Trust Boundaries (1)


   In Figure 1, we show the basic architecture which includes two user
   agents, two proxies (trusted entities), and four trust boundaries.
   The trust relationships associated with these are:


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     A) UA-o trusts Proxy-o, however Proxy-o does not trust UA-o.
     B) Proxy-o may or may not trust Proxy-t, and Proxy-t may or may
        not trust Proxy-o.
     C) Proxy-t does not trust UA-t, however UA-t trusts Proxy-t.
     D) UA-o may or may not trust UA-t, and UA-t may or may not trust
        UA-o.

   In the above, Proxy-o serves as the trusted intermediary for UA-o,
   whereas Proxy-t serves as the trusted intermediary for UA-t. Proxy-o
   determines and asserts the identity information for UA-o, whereas
   Proxy-t determines and asserts the identity information for UA-t.
   Both UA-o and UA-t are referred to as untrusted user agents in the
   above.

   In Figure 2, we consider another example, this time introducing the
   concept of a trusted user agent. A trusted user agent is a UA
   operated by the network, for example a PSTN gateway or a voicemail
   system, as opposed to an end user or customer:


                 +---------+       .        +---------+
                 | trusted |     B).        | Proxy-t |
                 |  UA-o   |------{.}------>|         |------+
                 +---------+       .        +---------+      |
                      ^                                      |
      A)              |            .                         |
    .........................................................|.....
                      |            .                         | C)
                      |            .                         |
                      |            .                         v
                      |            .D)                    +------+
                      +------------.--------------------->| UA-t |
                                   .                      +------+

         Figure 2 - Basic Architecture with Trust Boundaries (2)


   The trust relationships associated with the trust boundaries are:

     A) UA-o does not trust UA-t. UA-t may or may not trust UA-o.
     B) UA-o may or may not trust Proxy-t, and Proxy-t may or may not
        trust UA-o.
     C) Proxy-t does not trust UA-t, however UA-t trusts Proxy-t.
     D) Same as A.

   In this case, UA-o is a trusted user agent and hence does not need a
   trusted intermediary; UA-o simply provides the asserted identity
   information itself. However, UA-t is an untrusted user agent, and
   hence Proxy-t, which serves as the trusted intermediary for UA-t,
   determines and asserts the identity information for UA-t.

   Finally, in Figure 3 we consider the case of two trusted user
   agents:

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                 +---------+       .        +---------+
                 | trusted |     B).        | trusted |
                 |  UA-o   |------{.}------>|   UA-t  |
                 +---------+       .        +---------+
      A)                           .
    ...............................................................
                                   .                           C)
                                   .
                                   .
                                   .D)
                                   .
                                   .

           Figure 3 - Basic Architecture with Trust Boundaries (3)


   The trust relationships associated with the trust boundaries are:

     A) N/A.
     B) UA-o may or may not trust UA-t, and UA-t may or may not trust
        UA-o.
     C) N/A.
     D) N/A.

   In this case, both UA-o and UA-t are trusted user agent and hence do
   not need a trusted intermediary; the user agents simply provide the
   asserted identity information themselves.

   In this document we define two extensions to SIP that allow the
   calling and called parties to have their identity asserted by a
   trusted intermediary while still being able to maintain their
   identity privacy with respect to one another.

   The first extension is a new general header, Remote-Party-ID, which
   identifies a party and is added by the trusted network entities.
   Different types of party information can be provided, e.g., calling,
   or called party, and for each type of party, different types of
   identity information, e.g. subscriber, or terminal, can be provided.
   Since a party may not wish to reveal some or all of this information
   to an untrusted entity, the party can request a specific level of
   privacy for each. The intermediary also has the ability to specify a
   required level of privacy.

   The second extension is a new general header, RPID-Privacy, which
   specifies the privacy handling desired by the user for any Remote-
   Party-ID headers added by intermediaries. This enables an entity to
   control the desired level of privacy when intermediaries add Remote-
   Party-ID headers that assert the identity of the entity.

   When a trusted intermediary receives a message from an untrusted
   entity, the trusted intermediary attempts to determine the identity

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   of the originator, by means outside the scope of this document. When
   the identity has been determined, the trusted intermediary ensures
   that corresponding Remote-Party-ID information is included in the
   message sent. Also, the trusted intermediary ensures that any
   privacy requested (with respect to the other party) is provided
   prior to forwarding a message across a trust boundary to an
   untrusted entity. Any Remote-Party-ID information received from an
   untrusted entity is either verified successfully using an
   authentication mechanism or an explicit policy of trust, or tagged
   with an indication that it could not be verified, so the receiver
   knows that it should not necessarily trust the information.

   This document defines a set of party types and identity information.
   New types of party and identity information as well as other
   attributes may be introduced, thereby allowing new services to make
   use of the generic network asserted identity information and privacy
   handling defined here.

5. Protocol Overview

   When an untrusted UAC sends an INVITE, OPTIONS, REGISTER or
   extension method request through a trusted intermediary, i.e.,
   proxy, the proxy MAY be adding one or more Remote-Party-ID headers
   that identify the calling party. The UAC can indicate the level of
   privacy that should be afforded to such Remote-Party-ID headers by
   including one or more RPID-Privacy headers with the request. The
   RPID-Privacy header allows the UAC to control the privacy down to
   the party-type and identity-type level.

   When a trusted UAC sends an INVITE, OPTIONS, REGISTER or extension
   method request, the trusted UAC includes a calling subscriber
   Remote-Party-ID header field in the request in order to identify the
   originator of the call. The calling subscriber Remote-Party-ID MUST
   contain an addr-spec identifying the caller and MAY contain a
   "display-name" for the caller as well. Other types of network
   asserted Remote-Party-ID MAY be included as well. If privacy is
   desired for a given Remote-Party-ID header, the UAC MUST include a
   privacy token set to one or more of "uri", "name" or "full".
   Furthermore, if the UAC wants to control privacy for any Remote-
   Party-ID headers added by downstream proxies, the UAC MUST include
   one or more RPID-Privacy headers specifying the desired privacy.

   When a proxy supporting this extension receives an INVITE, OPTIONS,
   REGISTER or extension method request from an untrusted entity (UA or
   proxy), the proxy first examines the request for the presence of any
   Remote-Party-ID headers. The value of these headers cannot be
   trusted and hence the proxy will either have to validate them (by
   means outside the scope of this document) or make sure they are not
   marked as trusted. If the proxy wants to ensure, that the calling
   party can be identified by the called party, the proxy MUST
   authenticate the calling party (by means outside the scope of this
   document) and insert a calling party Remote-Party-ID header that is
   marked as being trusted. If the proxy is unable to authenticate the

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   calling party, it MAY reject the request, e.g., with a 403 or 407.
   If the proxy can determine other types of identity information for
   the calling party, it MAY insert those as trusted Remote-Party-ID
   headers as well. If the request contained one or more RPID-Privacy
   header fields, any Remote-Party-ID header fields added by the proxy
   MUST have their privacy indication set accordingly.

   Prior to a trusted entity (UA or proxy) forwarding the INVITE,
   OPTIONS, REGISTER or extension method request to an untrusted
   entity, the trusted entity MUST look for the presence of a privacy
   request indication in each Remote-Party-ID header field. If one is
   found, the privacy requested MUST be provided for that Remote-Party-
   ID header field prior to forwarding the request to the untrusted
   entity. For "uri" and "name" privacy, this typically involves
   encrypting and possibly removing information provided in the Remote-
   Party-ID.

   Once a UAS supporting this extension receives the INVITE, OPTIONS,
   REGISTER or extension method request via a trusted entity, the UAS
   can use the calling subscriber Remote-Party-ID information provided
   to identify the originator of the call, unless the originator had
   requested privacy. Note that if the UAS did not receive the call via
   a trusted entity, any Remote-Party-ID information provided may be
   false.

   The subsequent behavior now depends on whether the UAS is trusted or
   not:

   * If the UAS is trusted, it SHOULD include a called subscriber
     Remote-Party-ID identifying the called party in the first non-100
     response. The party information SHOULD be set to "called" and the
     identity information SHOULD be set to "subscriber". Additional
     Remote-Party-ID header fields MAY be provided as well. If the UAS
     desires privacy for a Remote-Party-ID, it MUST include a privacy
     request indication in that Remote-Party-ID header. Note that the
     privacy request is only guaranteed to be satisfied if the previous
     hop is trusted and it furthermore supports the extensions defined
     here. If the UAS can not guarantee both, then any privacy desired
     MUST be provided before the response is forwarded upstream.

   * If the UAS instead is untrusted, the session simply continues as
     described below.

   If the UAS (trusted or untrusted) wants to control the level of
   privacy afforded to any Remote-Party-ID headers that may be inserted
   by upstream proxies in its response, the UAS MUST include one or
   more RPID-Privacy header fields with the relevant privacy
   indication(s) in that response.

   A trusted UAS MAY also include Remote-Party-ID headers in subsequent
   provisional and final responses to the request. The trusted UAS
   SHOULD include a called party Remote-Party-ID header if the contents
   are different than sent in a previous response. The party

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   information SHOULD be set to "called" and the identity information
   SHOULD be set to "subscriber". Additional Remote-Party-ID header
   fields may be provided as well.

   When a proxy supporting this extension receives a non-100 response
   to an INVITE, OPTIONS, REGISTER or extension method request from an
   untrusted entity, the proxy first examines the response for the
   presence of any Remote-Party-ID headers. By definition, the value of
   these headers cannot be trusted and hence the proxy will either have
   to validate them (by means outside the scope of this document) or
   make sure they are not marked as trusted. If the proxy wants to make
   sure that the called party can be identified, the proxy MUST
   authenticate the called party (by means outside the scope of this
   document) and insert a called party Remote-Party-ID header that is
   marked as being trusted. If the proxy is unable to identify the
   called party, the proxy SHOULD simply mark any Remote-Party-ID
   headers in the message as untrusted. If the proxy can determine
   other types of identity information for the calling party, the proxy
   MAY insert those as trusted Remote-Party-ID headers as well. If the
   response contained one or more RPID-Privacy parameters, any Remote-
   Party-ID header fields added by the proxy MUST have their privacy
   indication set accordingly.

   The proxy MUST also ensure that any privacy requested in the
   response is provided prior to forwarding it to an untrusted entity.

   Finally, when a UAC (trusted or not) receives the first non-100
   response to an INVITE, OPTIONS, or REGISTER request from a trusted
   entity, the UAC can use the called subscriber Remote-Party-ID
   information (if present) to identify the called party, unless the
   terminator had requested privacy. Subsequent non-100 responses MAY
   contain Remote-Party-ID information as well. When the UAC receives
   the final 200 response, it MAY contain a called subscriber Remote-
   Party-ID header identifying the party the UAC was connected to.
   Again, this information SHOULD NOT be trusted if it was not received
   via a trusted entity.

6. Header Field Definitions

   Table 1 below is an extension of tables 2 and 3 in [3] for the new
   header fields defined here:


                      where  proxy  ACK  BYE  CAN  INV  OPT  REG
     Remote-Party-ID          amd    -    -    -    o    o    o
     RPID-Privacy              r     -    -    -    o    o    o

                   Table 1: Summary of header fields.


   The Remote-Party-ID and the RPID-Privacy headers can be used with
   the INVITE, OPTIONS, and REGISTER methods as well as any response to
   these. Extension methods MAY utilize these headers to achieve remote

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   party identification and privacy. The procedures at User Agents MAY
   be specific to particular new methods, however, the generic handling
   at proxies MUST be as specified in this document.

   Note that any privacy requested may not be honored unless the
   request or response is sent through a trusted entity that supports
   the extensions defined here. Similarly, Remote-Party-ID information
   may not be trustworthy if it was received in a request or response
   from a non-trusted entity or an entity that does not support the
   extensions defined here.

6.1 Remote-Party-ID Header Field Definitions

   The Remote-Party-ID header field provides information about the
   remote party. Different types of party information can be provided,
   e.g. calling and called, and for each, different types of identity
   information can be provided as well. A request or response MAY
   contain more than one Remote-Party-ID header field, with privacy
   requested independently for each. Remote-Party-ID is defined by the
   following ABNF [4] (productions for the undefined nonterminals can
   be found in [3]):

     Remote-Party-ID    = "Remote-Party-ID" HCOLON rpid *(COMMA rpid)

     rpid               =  [display-name] LAQUOT addr-spec RAQUOT
                                                     *(SEMI rpi-token)

     rpi-token          = rpi-screen / rpi-pty-type /
                           rpi-id-type / rpi-privacy / other-rpi-token

     rpi-screen         = "screen" EQUAL ("no" / "yes")

     rpi-pty-type       = "party" EQUAL ("calling" / "called" / token)

     rpi-id-type        = "id-type" EQUAL ("subscriber" / "user" /
                                                      "term"  / token)

     rpi-privacy        = "privacy" EQUAL
                             (rpi-priv-element
                              /(LDQUOT rpi-priv-element
                                 *(COMMA rpi-priv-element) RDQUOT)
                              )

     rpi-priv-element   = ("full" / "name" / "uri" / "off" / token)
                                          ["-" ( "network" / token )]

     other-rpi-token    = ["-"] token [EQUAL (token / quoted-string)]

   Furthermore, we define the value "private" for "other-user" in an
   "addr-spec", to indicate that the userinfo part of an "addr-spec" is
   in a non-intelligible form. The syntax for "other-user" is therefore
   refined to:


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     other-user         = token  / "private"

   Comparisons follow the case-sensitivity rules defined by SIP [3].

   The "display-name" in Remote-Party-ID is a text string that
   identifies the name of the party. The "addr-spec" contains
   information identifying the party either in clear-text or encrypted
   form. In the latter case, the addr-spec contains a SIP-URI or a
   SIPS-URI where the "userinfo" part of the "addr-spec" typically
   contains the encrypted party information, whereas the "hostport"
   identifies the entity that can decrypt the information. Furthermore,
   an "other-user" value of "private" will then be present to indicate
   that the "addr-spec" is non-intelligible. Depending on the rpi-pty-
   type, the "addr-spec" can be used as the Request-URI by the UA to
   initiate certain call control functions or subsequent calls that are
   required to reference the party.

   The rpi-screen parameter describes what level of trust network
   associates with the Remote-Party-ID information. The value "yes"
   indicates the Remote-Party-ID was asserted by the previous hop or
   the previous hop received the message from a trusted entity with
   this indication. The value "no" (assumed by default) indicates the
   Remote-Party-ID was either not asserted by the previous hop or the
   previous hop received the message from an untrusted entity. Multiple
   rpi-screen parameters MAY be present in a Remote-Party-ID - if both
   "yes" and "no" are present, "no" will take precedence. It should be
   noted, that the rpi-screen parameter provides a somewhat weak form
   of trustworthiness. In particular, it depends on transitive trust as
   well as correct implementation, configuration and support for the
   extensions defined here in the associated chain of trust. Should any
   of these dependencies not hold, the value "yes" may actually not be
   trustworthy. Future extensions to SIP may define a general and more
   robust mechanism that can be used here.

   The rpi-pty-type describes the type of party to which this header
   refers. There MUST NOT be more than one rpi-pty-type present in a
   Remote-Party-ID. If the rpi-pty-type parameter is absent, the
   "display-name" and "addr-spec" describe the party from which the
   request or response was received, i.e., "calling" party in the case
   of requests and "called" in the case of responses. Additional values
   MAY be defined as extensions. Such extensions SHALL be documented in
   an RFC and registered with IANA subject to the considerations given
   in Section 10.2.

   The rpi-id-type describes the nature of the identity provided.
   Several types of identity can be provided for each party, however
   there MUST NOT be more than one rpi-id-type present in a given
   Remote-Party-ID. If the rpi-id-type parameter is absent, the Remote-
   Party-ID contains the subscriber identity.

   This document defines three identity types that can be asserted by a
   trusted intermediary - additional values MAY be defined as
   extensions in which case they SHALL be documented in an RFC and

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   registered with IANA subject to the considerations given in Section
   10.2. The types are defined based on the nature of the information
   they represent, as opposed to a particular application. Individual
   applications requiring network asserted identity information will
   specify which identities should be used for that application:

   Subscriber identity (rpi-id-type="subscriber"):

     This identifies the owner of the subscription which is being used
     for the session.

   User identity (rpi-id-type="user"):

     This identifies the individual participating in the session. This
     can be used when multiple individuals are able to originate
     sessions under the same subscription. If absent, this is assumed
     to be the same as the subscriber identity.

   Terminal identity (rpi-id-type="term")

     This identifies the terminal being used for the session. For
     example several users may be able to 'log in' to a single
     terminal, in which case the identity of the terminal will differ
     from that of the user, subscriber, etc. If absent, this is assumed
     to be the same as the subscriber identity.

   Entities SHOULD NOT include multiple Remote-Party-ID headers
   containing the same identity information marked with different rpi-
   id-types.

   The rpi-privacy parameter describes whether the identity information
   must be hidden from untrusted entities. There MAY be multiple rpi-
   privacy parameters in a Remote-Party-ID. If privacy is requested, it
   MUST be one or more of "full", "uri", or "name". The value "full"
   means that both the "display-name" and the "addr-spec" MUST be
   hidden. The values "name" and "uri" mean that the "display-name" or
   the "addr-spec" MUST be hidden respectively. The value "off"
   indicates that lack of privacy is explicitly requested, and MUST be
   the only value if present. The values may be postfixed with a string
   indicating that the privacy request was made by an entity other than
   the party itself. Postfixing with the value "-network" indicates
   that intermediaries ("the network") have requested that the
   information be hidden, despite the user not making such a request.
   Additional values MAY be defined as extensions. Such extensions
   SHALL be documented in an RFC and registered with IANA subject to
   the considerations given in Section 10.2.

   It should be noted, that an entity requesting only Remote-Party-ID
   privacy will not receive complete privacy. The values "uri" and
   "name" merely affect information that may be displayed as opposed to
   truly hiding the identity of the requesting entity, since the
   identity of the host, e.g., IP address, is not hidden. For full


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   privacy, the entity will need IP address privacy as well - this is
   discussed further in Section 8.2.

   Finally, the "other-rpi-token" parameter allows the Remote-Party-ID
   header field to be extended with other types of parameters which
   SHALL be documented in an RFC and registered with IANA subject to
   the considerations given in Section 10.2. By default, such
   extensions will be assumed to contain network asserted identity
   information, and hence MUST be supported for identity verification
   to pass successfully. The prefix "-" is used to indicate that a
   parameter extension does not need to be supported by a given entity
   in order for that Remote-Party-ID to be verified successfully.
   Consequently, such extensions MUST NOT begin with the character "-".

6.2 RPID-Privacy Header Field Definition

   The RPID-Privacy header field allows an entity (typically an
   untrusted user agent) to indicate a desired level of privacy for any
   Remote-Party-ID header that may be added by subsequent entities,
   e.g., a downstream proxy. Any Remote-Party-ID header added of the
   party-type and identity-type indicated, shall have the privacy
   specified applied to it. If the party-type is omitted, the privacy
   specified applies to all party-types. If the identity-type is
   omitted, the privacy specified applies to all identity-types. A
   request or response MAY contain zero, one or more RPID-Privacy
   header fields. The RPID-Privacy header field is defined by the
   following ABNF [4]:

     RPID-Privacy       = "RPID-Privacy" HCOLON rpid-priv
                                             *(COMMA rpid-priv)

     rpid-priv          = rpid-privacy-token *(SEMI rpid-privacy-token)
                                        ; rpi-privacy MUST be present

     rpid-privacy-token = rpi-pty-type / rpi-id-type / rpi-privacy

   Comparisons follow the case-sensitivity rules defined by SIP [3].

   When multiple RPID-Privacy headers are present, the following
   precedence rules MUST be used:

   * RPID-Privacy with both rpi-id-type and rpi-pty-type takes
     precedence over
   * RPID-Privacy with only rpi-id-type, which takes precedence over
   * RPID-Privacy with only rpi-pty-type, which takes precedence over
   * RPID-Privacy with neither rpi-id-type nor rpi-pty-type.

   Any remaining overlaps or conflicts are resolved by order: a later
   RPID-Privacy indication in a message will take precedence over an
   earlier RPID-Privacy indication in that message. The following
   example illustrates the above:

     RPID-Privacy: privacy=full;party=calling;id-type=subscriber

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     RPID-Privacy: party=calling;rpi-privacy=off
     RPID-Privacy: party=calling;rpi-privacy=uri

   Per the rules above, a new calling subscriber Remote-Party-ID will
   get full privacy, and any other calling party Remote-Party-ID will
   get uri privacy.

7. Protocol Semantics

   Below, we provide the protocol semantics for an untrusted UAC, a
   trusted UAC, an untrusted UAS, a trusted UAS, and a proxy.

7.1 Untrusted UAC Behavior

   When an untrusted UAC supporting this extension sends an INVITE,
   OPTIONS, REGISTER or extension method request, and the UAC wants to
   control the privacy for any Remote-Party-ID header that might be
   added by a downstream proxy, the UAC MUST include one or more RPID-
   Privacy headers. Each of these RPID-Privacy headers MUST include an
   rpi-privacy parameter specifying the desired level of privacy, e.g.
   "uri", to maintain privacy of the "addr-spec".

   If the UAC desires "name" or "full" privacy, the UAC MUST NOT reveal
   the originating subscriber's name in the "display-name" portion of
   any header. This can be achieved by either not providing a "display-
   name" or by setting the "display-name" to "Anonymous" in such
   fields, e.g., From and Contact.

   If the UAC desires "uri" or "full" privacy, the UAC MUST NOT reveal
   the subscriber's identity in any header field. In particular, the
   contents of header fields need to be considered as described below:

   * From:      The UAC SHOULD supply a cryptographically random
     identifier for the userinfo, and a non-identifying hostname, e.g.,
     "localhost", in the host name. The cryptographically random
     identifier ensures a globally unique dialog identification
     (despite the use of "localhost") while still providing privacy.

   * To:        If a telephone number is used in the addr-spec, the
     telephone number SHOULD be a full E.164 number (including the
     country code) that is different from the From header field. If a
     host name is included, it SHOULD be a fully qualified domain name.

   * Contact:   The same cryptographically random identifier used in
     the From header field SHOULD be supplied for the userinfo, and an
     IP-address SHOULD be used in the host name.

   * All other headers that may contain either an IP address or a
     domain name, e.g., Call-ID, and Via, SHOULD use the IP-address
     form. It should however be noted, that this simple privacy step
     may be overcome fairly easily in many cases.



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   The UAC may also explicitly request that privacy is not to be
   provided by setting the rpi-privacy parameter in the corresponding
   RPID-Privacy header to "off". This is also the default value, unless
   provisioned otherwise.

   The first non-100 response to the INVITE, OPTIONS, REGISTER or
   extension method request received by the UAC through its trusted
   proxy MAY contain one or more Remote-Party-ID header fields. A
   Remote-Party-ID with party type "called" will identify the called
   party. If such a Remote-Party-ID header field either does not
   contain an rpi-screen parameter, or it contains an rpi-screen
   parameter with the value "no" (this includes the case where both
   "yes" and "no" is provided), the UAC SHOULD NOT trust the identity
   information provided. An end-to-end encrypted Remote-Party-ID header
   field can of course also not be trusted, regardless of the value of
   the rpi-screen parameter. It should be noted, that the rpi-screen
   parameter provides a somewhat weak form of trustworthiness. In
   particular, it depends on transitive trust as well as correct
   implementation, configuration and support for the extensions defined
   here in the associated chain of trust. Should any of these
   dependencies not hold, the value "yes" may actually not be
   trustworthy. Future extensions to SIP may define a general and more
   robust mechanism that can be used here.

   Subsequent responses to the requests MAY also contain Remote-Party-
   ID header fields. Such Remote-Party-ID header fields with party type
   "called" identify other parties to which the session has been
   directed, for whatever reason.

   Remote-Party-ID headers contained in the final response, with rpi-
   pty-type set to "called" identify the party which provided the final
   answer. In the case of an INVITE response, this identifies the
   answering party. Again, end-to-end encrypted Remote-Party-ID header
   fields can not be trusted.

7.2 Trusted UAC Behavior

   When a trusted UAC supporting this extension sends an INVITE,
   OPTIONS, REGISTER or extension method request, and it knows the
   identity of the calling party, the UAC SHOULD include a calling
   subscriber Remote-Party-ID header in the request in order to
   identify the originator of the call. However, if the request is part
   of an existing dialog, and the request is sent directly to the UAS,
   then the UAC MAY omit the calling subscriber Remote-Party-ID header.
   The Remote-Party-ID header MUST at a minimum contain an "addr-spec"
   to uniquely identify the calling subscriber. The "addr-spec" SHOULD
   be the same string as appears in the Request-URI for incoming call
   attempts. The Remote-Party-ID SHOULD include an rpi-pty-type set to
   "calling" and an rpi-id-type set to "subscriber" - we refer to this
   as a calling subscriber Remote-Party-ID. The rpi-screen parameter
   SHOULD be set to "yes". The Remote-Party-ID MAY optionally include a
   "display-name" which SHOULD be set to a name that the trusted UAC
   has associated with the calling subscriber, e.g. the subscriber's

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   full name. The UAC MAY include other Remote-Party-ID information as
   well.

   If the UAC desires privacy for the Remote-Party-ID header fields it
   added, it MUST include an rpi-privacy parameter with each relevant
   Remote-Party-ID. The rpi-privacy parameter MUST specify the desired
   level of privacy, e.g. "uri", to maintain privacy of the "addr-
   spec".

   If the UAC wants to control the privacy for any Remote-Party-ID
   header that might be added by a downstream proxy, the UAC MUST
   furthermore include one or more RPID-Privacy headers indicating the
   desired level of privacy. Each such RPID-Privacy header MUST include
   an rpi-privacy parameter specifying the desired level of privacy,
   e.g. "uri", to maintain privacy of the "addr-spec".

   If the UAC indicates "name" or "full" privacy (in either Remote-
   Party-ID or RPID-Privacy), the UAC MUST NOT reveal the originating
   subscriber's name in the "display-name" portion of any other header
   than Remote-Party-ID. This can be achieved by either not providing a
   "display-name" or setting the "display-name" to "Anonymous" in such
   fields, e.g. From and Contact.

   If the UAC desires "uri" or "full" privacy, the UAC MUST NOT reveal
   the subscriber's identity in any other header field than Remote-
   Party-ID. In particular, the contents of header fields needs to be
   considered as described for untrusted UACs (Section 7.1).

   The UAC may also explicitly request that privacy is not to be
   provided for a Remote-Party-ID by setting the rpi-privacy parameter
   to "off". This is also the default value, unless provisioned
   otherwise.

   When privacy is requested for one or more Remote-Party-ID headers,
   the UAC MUST ensure that such privacy is provided prior to
   forwarding the message to an untrusted entity. Two different options
   for achieving this are defined here:

   1) Do not provide the privacy until the message is forwarded to an
      untrusted entity.
   2) Provide the privacy before forwarding the message, irrespective
      of whether the next hop is trusted or not.

   We first describe option 1, which has the benefit of leaving the
   Remote-Party-ID in clear as long as possible at the expense of
   introducing a Proxy-Require "privacy":

   If privacy was requested, and the next hop is trusted, the UA MUST
   ensure that a Proxy-Require header with an option-tag of "privacy"
   is present. This will ensure that a downstream proxy will apply the
   necessary privacy prior to forwarding the message to an untrusted
   entity. Should a 420 response listing "privacy" as an unsupported
   option be returned, then privacy can not be provided for this call.

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   The UA MUST then either initiate a new session without requiring
   privacy, or the session initiation attempt MUST be abandoned.
   Furthermore, the UA MUST take precautions to protect the identity
   information from eavesdropping and interception, e.g., by use of
   IPSec.

   If the UA forwards the request to an untrusted entity, and privacy
   was requested, the UA MUST ensure the privacy requested will be
   honored. For each Remote-Party-ID requesting privacy, the UAC MUST
   do the following:

   * If rpi-privacy contains the value "full" or "uri", the UA MUST
     replace the "addr-spec" in that Remote-Party-ID header field with
     a private "addr-spec" containing a SIP-URI or a SIPS-URI. The
     private "addr-spec" MUST list the UA itself in the hostport and
     include a "user=private" user-param.

   * If rpi-privacy contains the value "full" or "name", the UA MUST
     delete the "display-name" in that Remote-Party-ID header field.

   Generation of the userinfo part of a private "addr-spec" is a UA
   internal issue, as long as the requested privacy is honored and the
   ability to trace the originator is preserved. However, it is
   RECOMMENDED to construct the user part by including:

   * the initial "addr-spec",

   * the value of rpi-privacy, and

   * sufficient checksum information to prevent tampering by an
     untrusted entity.

   All of this information MUST then be encoded or encrypted such that
   the next hop is unable to discern the original Remote-Party-ID. It
   is RECOMMENDED that the string be encrypted with a symmetric private
   key, and converted to a printable string using Base64 encoding. The
   UA MAY include other information in the userinfo part as well.

   Prior to forwarding the request to an untrusted entity, the UA
   SHOULD remove any "privacy" option tag present in a Proxy-Require
   header field to prevent unnecessary failure of the request if
   downstream proxies do not support this extension.

   We now describe the second option for providing Remote-Party-ID
   privacy. With this option, the UA applies the same processing for
   each Remote-Party-ID as in option 1, however it does so regardless
   of whether the next hop is trusted or not. Since the privacy has now
   been applied, there is no need to insert a Proxy-Require "privacy".
   However, there is also no well-defined way for a downstream
   (trusted) entity to determine the identity of the calling party,
   without that entity knowing both the details of how the private
   "addr-spec" was constructed (crypto algorithm, MAC, encoding, etc.)
   as well as which key to use for decrypting the information. The

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   solutions to these problems are left as an exercise to the reader,
   and hence interoperability should not be expected.

   The first non-100 response received by the UAC MAY contain one or
   more Remote-Party-ID header fields. A Remote-Party-ID with party
   type "called" will identify the called party. If the response was
   received via a trusted entity, and the Remote-Party-ID header field
   either does not contain an rpi-screen parameter, or it contains an
   rpi-screen parameter with the value "no" (this includes the case
   where both "yes" and "no" is provided), the UAC SHOULD NOT trust the
   identity information provided. An end-to-end encrypted Remote-Party-
   ID header field can of course also not be trusted, regardless of the
   value of the rpi-screen parameter.

   Subsequent responses received by the UAC MAY also contain Remote-
   Party-ID header fields. Such Remote-Party-ID header fields with
   party type "called" identify other parties to which the request has
   been directed, for whatever reason.

   Remote-Party-ID headers contained in the final response, with rpi-
   pty-type set to "called" identify the party which provided the final
   answer. In the case of an INVITE response, this identifies the
   answering party. Again, end-to-end encrypted Remote-Party-ID header
   fields can not be trusted.

7.3 Untrusted UAS Behavior

   An untrusted UAS supporting this extension and receiving an INVITE,
   OPTIONS, REGISTER or extension method request via its trusted proxy
   looks for a Remote-Party-ID header field with rpi-pty-type "calling"
   and rpi-id-type "subscriber", i.e., a calling subscriber Remote-
   Party-ID, to identify the originator of the request. If rpi-pty-type
   is omitted from a Remote-Party-ID in the request, "calling" is
   assumed, and if rpi-id-type is omitted, "subscriber" is assumed. If
   a calling subscriber Remote-Party-ID either does not contain an rpi-
   screen parameter or it contains an rpi-screen parameter with a value
   of "no" (this includes the case where both "yes" and "no" is
   provided), the UAS SHOULD NOT trust the identity information
   provided. An end-to-end encrypted Remote-Party-ID header field can
   of course also not be trusted, regardless of the value of the rpi-
   screen parameter. Otherwise, the UAS SHOULD use the information
   provided to identify the calling party rather than any information
   provided in the From or any other header field. Note that the
   request MAY contain other Remote-Party-ID header fields.

   If the UAS wants to control the privacy for any Remote-Party-ID
   header that might be added to its response by an upstream proxy, the
   UAS MUST include one or more RPID-Privacy headers indicating the
   desired level of privacy. Each such RPID-Privacy header MUST include
   an rpi-privacy parameter specifying the desired level of privacy,
   e.g., "uri" to maintain privacy of the "addr-spec".



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7.4 Trusted UAS Behavior

   A trusted UAS supporting this extension and receiving an INVITE,
   OPTIONS, REGISTER or extension method request from a trusted entity
   looks for a Remote-Party-ID header field with rpi-pty-type "calling"
   and rpi-id-type "subscriber", i.e. a calling subscriber Remote-
   Party-ID, to identify the originator of the request. If rpi-pty-type
   is omitted from a Remote-Party-ID in the request, "calling" is
   assumed, and if rpi-id-type is omitted, "subscriber" is assumed. If
   a calling subscriber Remote-Party-ID either does not contain an rpi-
   screen parameter or it contains an rpi-screen parameter with a value
   of "no" (this includes the case where both "yes" and "no" is
   provided), the UAS SHOULD NOT trust the identity information
   provided. An end-to-end encrypted Remote-Party-ID header field can
   of course also not be trusted, regardless of the value of the rpi-
   screen parameter. Otherwise, the UAS SHOULD use the information
   provided to identify the calling party rather than any information
   provided in the From or any other header field. Note that the
   request MAY contain other Remote-Party-ID header fields.

   If the trusted UAS knows the identity of the party that was reached,
   it SHOULD include a called subscriber Remote-Party-ID identifying
   the called party in the first non-100 response. However, if the
   request was part of an existing dialog, and the request was sent
   directly to the UAS, then the UAS MAY omit the called subscriber
   Remote-Party-ID header from the response. In addition, the UAS MAY
   insert Remote-Party-ID headers in any further non-100 responses. The
   UAS SHOULD insert a new called subscriber Remote-Party-ID header if
   the called party information changed from the called party
   information sent in the previous response. For each of these, the
   party information SHOULD be set to "called" and the identity
   information SHOULD be set to "subscriber". Otherwise, the rules for
   the Remote-Party-ID are similar to those for the INVITE, OPTIONS,
   REGISTER or extension method request sent by a trusted UAC.
   Additional Remote-Party-ID header fields MAY be provided as well.

   If the UAS desires privacy for a Remote-Party-ID header field it
   added, it MUST include a privacy request indication in that Remote-
   Party-ID header. Note that the privacy request is only guaranteed to
   be satisfied if the previous hop is trusted and it furthermore
   supports the extensions defined here. If the UAS cannot guarantee
   both, then any privacy desired MUST be provided before the response
   is forwarded upstream. Alternatively, the UAS MAY simply omit
   Remote-Party-ID's requiring privacy from the response.










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7.5 Proxy Behavior

   When a proxy supporting this extension receives an INVITE, OPTIONS,
   REGISTER or extension method request from a trusted entity, it does
   not apply any special processing until the message is forwarded to
   the next hop. If the message instead came from an untrusted entity,
   the proxy MUST do the following:

   First, the proxy MUST examine the message for the presence of any
   Remote-Party-ID headers. Since the request was received from an
   untrusted entity, each of these MUST either be verified by the proxy
   or have their rpi-screen parameter set to "no". If the proxy is able
   to successfully verify the information in a Remote-Party-ID header
   field (by means outside the scope of this document), the proxy MUST
   add an rpi-screen parameter set to "yes" for that Remote-Party-ID.
   Furthermore, this MUST be the only rpi-screen parameter for that
   Remote-Party-ID. If verification fails however, further processing
   depends on the reason for the failure. Two different failure reasons
   are defined here:

   * The information provided could not be verified because the proxy
     does not support verification of the identity information for this
     particular Remote-Party-ID.

   * The proxy supports verification of this particular Remote-Party-
     ID, however the identity information provided is incorrect and the
     proxy detected that, or the identity information could not be
     verified.

   In the first case, the proxy MUST add an rpi-screen parameter set to
   "no". The proxy SHOULD furthermore ensure this is the only rpi-
   screen parameter. In the second case, the proxy MUST by default add
   an rpi-screen parameter set to "no" and ensure this is the only rpi-
   screen parameter, however individual extensions and local procedures
   MAY specify a different behavior, for example rewrite or removal of
   the offending Remote-Party-ID header field.

   Second, if the proxy knows the identity of the calling party (by
   means outside the scope of this document), and there is no
   corresponding calling subscriber Remote-Party-ID header field
   present in the request, the proxy SHOULD include a calling
   subscriber Remote-Party-ID with the request in order to identify the
   originator of the request. The Remote-Party-ID header MUST at a
   minimum contain an "addr-spec" to uniquely identify the calling
   subscriber. The "addr-spec" SHOULD be the same as appears in the
   Request-URI for incoming call attempts to that party. The Remote-
   Party-ID SHOULD include an rpi-pty-type set to "calling" and an rpi-
   id-type set to "subscriber". The rpi-screen parameter SHOULD be set
   to "yes". The Remote-Party-ID MAY optionally include a "display-
   name" which SHOULD be set to a name that the proxy has associated
   with the calling subscriber, e.g., the subscriber's full name. The
   proxy MAY include other Remote-Party-ID information as well.


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   If the proxy is unable to determine the identity of the calling
   party, it MAY alternatively reject the request, e.g. with a 403 or
   407. The details of this is outside the scope of this document.

   If the proxy added one or more Remote-Party-ID headers to the
   request, the proxy MUST look for the presence of any RPID-Privacy
   header fields and set the rpi-privacy parameter on the Remote-Party-
   ID headers the proxy added accordingly (see Section 6.2). If there
   were no RPID-Privacy headers present, but the From header field
   contained the value "Anonymous" as the display-name, the proxy MUST
   apply "full" privacy to all Remote-Party-ID headers it added - this
   ensures backwards compatibility with current SIP. Note however, that
   the proxy does not check the validity of a display-name provided in
   the From header field.

   The proxy is now ready to forward the message. If there are no
   Remote-Party-ID headers requesting privacy, the message is simply
   forwarded. However, if there is a request for some kind of privacy,
   the proxy MUST apply the same processing as a trusted UAC would (see
   Section 7.2). In particular, the proxy MUST ensure that any privacy
   requested is provided prior to forwarding the message to an
   untrusted entity - refer to Section 7.2 for details.

   When the proxy receives a response to the INVITE, OPTIONS, REGISTER
   or extension method request from a trusted entity, it does not apply
   any special processing until the message is forwarded to the next
   hop. If the response instead came from an untrusted entity, and it
   was a non-100 response, the proxy MUST do the following:

   First, the proxy examines the response for the presence of any
   Remote-Party-ID headers and applies similar processing as it did for
   the request.

   Second, if the proxy knows the identity of the party that was
   reached (by means outside the scope of this document), and there is
   no corresponding called subscriber Remote-Party-ID header field
   present in the response, the proxy SHOULD add a called subscriber
   Remote-Party-ID to the response in order to identify the party
   reached. The Remote-Party-ID header MUST at a minimum contain an
   "addr-spec" to uniquely identify the subscriber reached. The "addr-
   spec" SHOULD be the same string as appears in the Request-URI for
   incoming call attempts to that party. The Remote-Party-ID SHOULD
   include an rpi-pty-type set to "called" and an rpi-id-type set to
   "subscriber". The rpi-screen parameter SHOULD be set to "yes". The
   Remote-Party-ID MAY optionally include a "display-name" which SHOULD
   be set to a name that the proxy has associated with the called
   subscriber, e.g. the subscriber's full name. The proxy MAY include
   other Remote-Party-ID information as well.

   If the proxy is unable to determine the identity of the party
   reached, it SHOULD continue normal processing, and simply omit
   adding a called party Remote-Party-ID to the response.


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   If the proxy added one or more Remote-Party-ID header fields to the
   response, the proxy MUST look for the presence of any RPID-Privacy
   header fields in the response and set the rpi-privacy parameter on
   the Remote-Party-ID headers the proxy added accordingly (see Section
   6.2).

   The proxy is now ready to forward the response. If there are no
   Remote-Party-ID headers requesting privacy, the response is simply
   forwarded upstream. However, if there is a request for some kind of
   privacy, the proxy MUST apply the same processing as a trusted UAS
   would (see Section 7.4). In particular, the proxy MUST ensure that
   any privacy requested is provided prior to forwarding the response
   to an untrusted entity - refer to Section 7.4 for details. Again, it
   should be noted, that either type of privacy request is only
   guaranteed to be satisfied if the previous hop is trusted and it
   furthermore supports the extensions defined here. If the proxy
   cannot guarantee both, then any privacy desired MUST be provided
   before the response is forwarded upstream. Alternatively, the proxy
   MAY simply omit Remote-Party-ID's requiring privacy from the
   response.

7.6 Additional Proxy and Trusted User Agent Behavior

   A proxy or trusted UA supporting this extension SHOULD be prepared
   to receive a request containing a SIP-URI or SIPS-URI with a user-
   param set to "private". If the "hostport" part of the URI identifies
   the entity handling the request, the entity MUST recover the private
   information. For entities that use the encryption recommendation
   provided earlier, this implies decrypting the "userinfo" portion of
   the URI and replacing it with the decrypted addr-spec that was
   contained in the "userinfo" portion as well as any other SIP
   information included. Note that the decrypted addr-spec may itself
   contain a "private" URI.

   If the entity is unable to recover a "private" SIP-URI or SIPS-URI,
   it MUST fail the request with a 4xx error code.

8. Examples of Use

   In this Section, we illustrate how the request for privacy may work
   in practice. It should be noted that the privacy service described
   can be implemented in a number of ways; we merely describe one
   possible solution in this section.

8.1 Basic Privacy Example

   The Figure below illustrates a basic privacy example scenario:







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                  +---------+             +--------+
       1: INVITE  | Proxy-o | 2: INVITE   | Proxy-t| 3: INVITE
         +------->|         |------------>|        |---------+
         |        +---------+             +--------+         |
         |                                                   |
         |                  trust boundary                   |
     . . |. . . . . . . . . . . . . . . . . . . . . . .. . . | . . .
         |                                                   |
         |                                                   \/
     +------+                  RTP/RTCP                   +------+
     | UA-o |<------------------------------------------->| UA-t |
     +------+                                             +------+

                     Figure 4 - Basic Privacy Example


   The originating user agent (UA-o) sends an INVITE (1) to Proxy-o
   where it requests uri and name, i.e. full, privacy for any Remote-
   Party-ID headers that might be added. Since the From header field
   contains calling identity information, UA-o supplies a
   cryptographically random identifier for the userinfo, and the non-
   identifying hostname "localhost" rather than its true identity:

        INVITE
        From:            sip:xyz@localhost
        RPID-Privacy:    full

   Proxy-o determines the calling subscriber identity, and adds a
   corresponding Remote-Party-ID header to the request. The privacy
   setting on this header is derived from the RPID-Privacy header
   present in the INVITE (1) received from the UA. Since proxy-o knows
   that the identity information in the Remote-Party-ID is correct,
   Proxy-o also includes an rpi-screen parameter set to "yes". Proxy-o
   trusts Proxy-t, and hence the Remote-Party-ID can be passed in
   clear. However, to ensure proper privacy processing, Proxy-o adds a
   Proxy-Require "privacy" to the request before it sends INVITE(2) to
   Proxy-t:

        INVITE
        From:            sip:xyz@localhost
        Remote-Party-ID: "John Doe" <sip:jdoe@foo.com>;party=calling;
                            id-type=subscriber;privacy=full;screen=yes
        Proxy-Require:   privacy

   When Proxy-t receives the INVITE, it examines the privacy request
   included in the INVITE and sees that uri and name privacy is
   requested. Since the next hop is untrusted, Proxy-t therefore
   removes the "display-name" from the calling subscriber Remote-Party-
   ID, encrypts the "addr-spec" and rpi-privacy, puts the result in the
   "userinfo" part, inserts itself as the "hostport" and adds a
   "user=private" user-param. Also, Proxy-t removes the
   Proxy-Require "privacy" before sending the INVITE(3) to UA-t:


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        INVITE
        From:            sip:xyz@localhost
        Remote-Party-ID: <sip:e(<sip:jdoe@foo.com>;privacy=full
                                )@proxy-t.foo.com;user=private
                           >;party=calling;id-type=subscriber;
                                         privacy=full;screen=yes

   UA-t notes the presence of the Remote-Party-ID, but since it
   indicates full privacy, UA-t can only identify the calling
   subscriber as private, however it knows that the subscriber's
   identity has been asserted by somebody its proxy trusts, since the
   rpi-screen parameter is set to "yes". UA-t decides to accept the
   call setup, and responds with a 180 Ringing. In this case, there is
   no request for privacy for any Remote-Party-ID headers by upstream
   proxies, so a normal 180 response is sent back.

   Proxy-t determines the identity of UA-t and adds a corresponding
   Remote-Party-ID as well as an rpi-screen parameter set to "yes".
   Since no privacy was requested, proxy-t can provide the Remote-
   Party-ID information to proxy-o in clear:

        180
        Remote-Party-ID: "Mary Doe" <sip:mdoe@foo.com>;party=called;
                                        id-type=subscriber;screen=yes

   Proxy-o forwards the response to UA-o as is.

   While this illustrates the basic operation of the service, there are
   additional issues that need to be considered. In SIP, there are
   several fields that can reveal the identity of the calling party,
   either in part or completely. Other protocols used, e.g. SDP and RTP
   may reveal identity information as well. A user agent wishing to not
   reveal its identity should consider each of these. The next example
   looks more closely at this.

8.2 Complete Privacy Example

   The second example we look at is one where IP-address privacy is
   requested. Note that we do not actually define an IP address privacy
   mechanism in this document, however the example below illustrates
   how IP address privacy could be provided and the underlying
   considerations behind providing it.

   The Figure below illustrates how IP address privacy can be achieved
   by inserting a trusted intermediary, an anonymizer, for the
   signaling and the media streams between UA-o and UA-t. The interface
   between the proxies and the media anonymizer is purposely not
   defined here:






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                  +---------+             +--------+
       1: INVITE  | Proxy-o | 2: INVITE   | Proxy-t| 3: INVITE
         +------->|         |------------>|        |----------+
         |        +---------+             +--------+          |
         |                  \            /                    |
         |                   \          /                     |
         |      SIP           +--------+           SIP        |
         | +----------------->| anony- |-------------------+  |
         | |          +------>|  mizer |--------+          |  |
         | |          |       +--------+        |          |  |
         | |          |                         |          |  |
         | |          |                         |          |  |
         | |          |     trust boundary      |          |  |
     . . |.|. . . . . | . . . . . . . . . . . . | . . .. . |..| . . .
         | |          |                         |          |  |
         | |          |                         |          \/ \/
     +------+ RTP/RTCP|                         |RTP/RTCP +------+
     | UA-o |<--------+                         +-------->| UA-t |
     +------+                                             +------+

                      Figure 5 - Full Privacy Example


   For all signaling and media exchange purposes, the anonymizer adds a
   level of indirection thereby hiding the IP address(es) of UA-o from
   UA-t. This indirection is used both for the media streams and SIP
   signaling, beyond the initial INVITE, exchanged directly between
   UA-o and UA-t. A further refinement into IP address privacy just for
   signaling and IP address privacy just for media streams could be
   provided as well.

   In order to provide IP address privacy for the SIP signaling, we
   need to consider the header fields which may reveal IP address
   information. These header fields and their IP address privacy
   considerations are:

   * The From header field must use the non-identifying host name
     "localhost".

   * The Call-ID UAC must not be based on the originator's IP address.

   * A Contact header field must be set to point to the anonymizer to
     prevent any direct signaling between UA-o and UA-t.

   * An Alert-Info, Call-Info, Error-Info, or Reply-To header field
     should either be omitted or not point to any location that can
     help identify UA-o.

   * Via, Record-Route, Route, and any other header fields identifying
     either UA-o or Proxy-o must be hidden, e.g., by encryption or
     simple stateful removal and re-insertion by Proxy-t and/or the
     anonymizer.


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   Note that in addition to the above SIP header fields, extensions may
   define additional header fields which compromise IP address privacy.

   An alternative to the media anonymizer function shown above is to
   implement the anonymizer as a back to back User Agent thereby
   trivially hiding IP address information in the SIP signaling itself.

   Furthermore, when SDP or another session description protocol is
   used to describe the media in the session, the session descriptions
   exchanged by the user agents need to be modified to direct the media
   streams to the anonymizer. The use of SDP fields revealing calling
   identity information needs to be considered as well. Similar
   concerns apply to the use of RTCP.

9. Security Considerations

   The mechanism provided in this document is a partial consideration
   of the problem of identity and privacy in SIP. For example, these
   mechanisms provide no means by which end users can conceal their
   identities from the network. Additionally, information which the
   user designates as 'private' can be inspected by any intermediaries
   participating in the trusted network.

   Also, this document does not include means by which a network will
   authenticate a user, nor means by which intermediaries can
   authenticate one another, although both of these functions are
   required for a complete implementation of this system. If the means
   chosen for this are inherently insecure (for example, blindly using
   an originating IP address as an assertion that a request came from a
   certain entity or user), then it will significantly degrade the
   services provided by this extension.

   When a trusted entity has determined the identity information for a
   given party that wishes to have its identity remain private, and the
   trusted entity then sends a message to any destination with that
   party's identity in a Remote-Party-ID header, the entity MUST take
   precautions to protect the identity information from eavesdropping
   and interception to protect the confidentiality and integrity of
   that identity information. The use of transport or network layer
   hop-by-hop security mechanisms, such as TLS or IPSec, can satisfy
   this requirement.

   As noted above, Remote-Party-ID information that is received can
   only be trusted if it is received from a trusted entity and it was
   not encrypted end-to-end. The reason is, that end-to-end encryption
   of a Remote-Party-Id will prevent the trusted intermediaries from
   setting the rpi-screen parameter correctly.

   Finally, a user agent or proxy can only assume that a privacy
   request will be honored, if it is sent to a trusted entity. Thus, if
   a user agent or proxy does not know if its SIP message (request or
   response) is sent to a trusted entity (proxy or UA), it should
   assume that a privacy request for that message  will not be honored.

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10. IANA Considerations

10.1 New SIP Header Fields and Option Tag

   This document defines two new SIP header fields (without the double-
   quotes):

   * "Remote-Party-ID", and

   * "RPID-Privacy"

   This document also defines a new option-tag "privacy".

10.2 Remote-Party-ID IANA Registry

   Upon publication of this document as a standards track RFC, IANA is
   instructed to establish a new sub-registry under
   http://www.iana.org/assignments/sip-parameters and enter the Remote-
   Party-ID extensions defined below in it. Before any such extension
   can be registered, the extension MUST be documented in an RFC and be
   reviewed by a designated expert [RFC2434] for applicability to SIP
   and the Remote-Party-ID as defined in this document. As part of this
   review, the expert will verify that the extension complies with the
   applicability defined for this document (see Section 2). The expert
   reviewer will send email to the IESG on the overall review
   determination.

   Any reviewed and approved Remote-Party-ID extensions defined SHALL
   be registered with IANA as follows:

   * rpi-id-type:     A literal name MUST be provided.

   * rpi-pty-type:    A literal name MUST be provided.

   * rpi-privacy:     A new privacy indication or a new privacy postfix
     can be defined - each of these have a separate name space:

     * privacy indication:     A literal name MUST be provided.

     * privacy postfix:        A literal name MUST be provided.

   * other-rpi-token:   A literal name, which MUST NOT start with the
     dash character ("-"), MUST be provided. If the extension is on the
     form "type = value", then a description of the permissible values
     SHOULD furthermore be provided.

   * privacy-tag:       A literal name MUST be provided.






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10.3 New Remote-Party-ID "other-rpi-token"

   This document defines a new Remote-Party-ID "other-rpi-token"
   (without the double-quotes):

   * "np" for Nature of Party.

   The definition of this "other-rpi-token" is provided in Appendix A.

11. Notice Regarding Intellectual Property Rights

   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.

   The IETF has been notified of intellectual property rights claimed
   in regard to some or all of the specification contained in this
   document. For more information consult the online list of claimed
   rights.

12. References

   1  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.

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

   3  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
      Peterson, J., Sparks, R., Handley, M., E. Schooler, "SIP: Session
      Initiation Protocol", Work in Progress, February 21, 2002.

   4  Crocker, D. and Overell, P.(Editors), "Augmented BNF for Syntax
      Specifications: ABNF", RFC 2234, Internet Mail Consortium and
      Demon Internet Ltd., November 1997.



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13. Acknowledgments

   The basis of this document is the Distributed Call Signaling work in
   the PacketCable project, which is the work of a large number of
   people, representing many different companies.  The authors would
   like to recognize and thank the following for their assistance: John
   Wheeler, Motorola; David Boardman, Daniel Paul, Arris Interactive;
   Bill Blum, Jon Fellows, Jay Strater, Jeff Ollis, Clive Holborow,
   Motorola; Doug Newlin, Guido Schuster, Ikhlaq Sidhu, 3Com; Jiri
   Matousek, Bay Networks; Farzi Khazai, Nortel; John Chapman, Bill
   Guckel, Michael Ramalho, Cisco; Chuck Kalmanek, Doug Nortz, John
   Lawser, James Cheng, Tung-Hai Hsiao, Partho Mishra, AT&T; Telcordia
   Technologies; and Lucent Cable Communications. Additionally, the
   authors would like to thank the SIP working group, and in particular
   the following individuals who all made significant contributions to
   this document: Jonathan Rosenberg, Igor Slepchin, Michael Thomas,
   Dean Willis, and Rohan Mahy. Alan Johnston provided the "nature of
   party" extension in Appendix A.

14. Authors' Addresses

   Bill Marshall
   AT&T
   Florham Park, NJ  07932
   Email: wtm@research.att.com

   K. K. Ramakrishnan
   TeraOptic Networks
   Sunnyvale, CA
   Email: kk@teraoptic.com

   Ed Miller
   Terayon
   Louisville, CO  80027
   Email: E.Miller@terayon.com

   Glenn Russell
   CableLabs
   Louisville, CO  80027
   Email: G.Russell@Cablelabs.com

   Burcak Beser
   Juniper Networks
   Sunnyvale, CA
   Email: burcak@juniper.net

   Mike Mannette
   3Com
   Rolling Meadows, IL  60008
   Email: Michael-Mannette@3com.com




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   Kurt Steinbrenner
   3Com
   Rolling Meadows, IL  60008
   Email: Kurt-Steinbrenner@3com.com

   Dave Oran
   Cisco
   Acton, MA  01720
   Email: oran@cisco.com

   Flemming Andreasen
   Cisco
   Edison, NJ
   Email: fandreas@cisco.com

   John Pickens
   Com21
   San Jose, CA
   Email: jpickens@com21.com

   Poornima Lalwaney
   Nokia
   San Diego, CA  92121
   Email: poornima.lalwaney@nokia.com

   Jon Fellows
   Motorola
   San Diego, CA  92121
   Email: jfellows@gi.com

   Doc Evans
   D. R. Evans Consulting
   Boulder, CO  80303
   Email: n7dr@arrl.net

   Keith Kelly
   NetSpeak
   Boca Raton, FL  33587
   Email: keith@netspeak.com

   Mark Watson
   Nortel Networks
   Maidenhead, UK
   Email: mwatson@nortelnetworks.com










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Appendix A: Nature of Party

   This document defines a new "other-rpi-token" to identity the nature
   of the party in the Remote-Party-id. The Remote-Party-ID Nature of
   Party information (rpi-np) is supplied on a "token = value" form as
   defined by the following grammar:

   rpi-np = "np" "=" ("ordinary" | "residential" | "business" |
                    "priority" | "hotel" | "failure" | "hospital" |
                    "prison" | "police" | "test" | "payphone" |
                    "coin" | "payphone-public" | "payphone-private" |
                    "coinless" | "restrict" | "coin-restrict" |
                    "coinless-restrict" | "reserved" | "operator" |
                    "trans-freephone" | "isdn-res" | "isdn-bus" |
                    "unknown" | "emergency" | "not-applicable" |
                    "cellular-ordinary" | "cellular-roaming" | token )

   The rpi-np describes the nature of the party identified - additional
   values can be defined as extensions. Typically, this information
   will come from ANI information digits (II) which are used in the
   Public switched network in the US. Information digits are two digit
   codes that precede the Called Party Number and provide information
   to the Exchange carriers and IECs about the type of line that
   originated the call or any special characteristics of the Billing
   number. In the non-US environments, a parameter called the Calling
   Party Category (CPC) usually plays the role of information digits in
   the US and provides similar information. The rpi-np is meant to be
   primarily used in the PSTN to SIP direction. It is not intended to
   be used in the SIP to PSTN direction. Mapping to information digits
   towards the PSTN might invoke unintended results in the PSTN.

   The following example illustrates the use of the Nature of Party:

        Remote-Party-ID: "Mary Doe" <sip:mdoe@foo.com>;party=called;
                        id-type=subscriber;np=ordinary;screen=yes


   Below is the recommended mapping from II to rpi-np:

        00      ordinary
        01      not-applicable
        02      failure
        06      not-applicable
        07      not-applicable
        20      not-applicable
        23      not-applicable
        24      trans-freephone
        25      trans-freephone and payphone
        27      payphone
        29      prison
        30-32   not-applicable
        34      operator
        40-49   depends on the carrier's implementation

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        52      not-applicable
        60      not-applicable
        61      cellular-ordinary
        62      cellular-ordinary
        63      cellular-roaming
        66      not-applicable
        67      not-applicable
        70      payphone
        80-89   reserved
        93      not-applicable

   Note that a particular II value could map to two different values of
   the rpi-np. For example, II value of 25 can map to rpi-np=trans-
   freephone and rpi-np=payphone.








































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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
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   The limited permissions granted above are perpetual and will
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   This document and the information contained herein is provided on an
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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