Network Working Group                                          M. Barnes
Internet-Draft                                                   Polycom
Obsoletes: 4244 (if approved)                                   F. Audet
Intended status: Standards Track                                   Skype
Expires: April 29, August 18, 2011                                     S. Schubert
                                                                     NTT
                                                           J. van Elburg
                                              Detecon International Gmbh
                                                             C. Holmberg
                                                                Ericsson
                                                        October 26, 2010
                                                       February 14, 2011

   An Extension to the Session Initiation Protocol (SIP) for Request
                          History Information
                  draft-ietf-sipcore-rfc4244bis-02.txt
                  draft-ietf-sipcore-rfc4244bis-03.txt

Abstract

   This document defines a standard mechanism for capturing the history
   information associated with a Session Initiation Protocol (SIP)
   request.  This capability enables many enhanced services by providing
   the information as to how and why a call SIP request arrives at a specific
   application or user.  This document defines an optional SIP header, header
   field, History-Info, for capturing the history information in
   requests.  The document also defines SIP header field parameters are defined for
   the History-Info and Contact header fields to tag the method by which
   the target of a request is determined.  In addition, this document
   defines a value for the Privacy header field specific to the History-Info
   header. History-
   Info header field.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 29, August 18, 2011.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
   3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Overview of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  General User Agent Behavior  History-Info Header Field Protocol Structure . . . . . . . . .  7
     5.1.  History-Info Header Field Example Scenario . . . . . . . . 10
     5.1.  9
   6.  User Agent Handling of the History-Info Header Field . . . . . 12
     6.1.  User Agent Client (UAC) Behavior . . . . . . . . . . . . . 10
     5.2. 12
     6.2.  User Agent Server (UAS) Behavior . . . . . . . . . . . . . 10
       5.2.1. 12
       6.2.1.  Processing of Requests with History-Info . . . . . . . 10
       5.2.2. 12
       6.2.2.  Generation of Responses with History-Info  . . . . . . 11
     5.3.  Redirect Server Behavior 13
   7.  Proxy/Intermediary Handling of History-Info Header Fields  . . 13
     7.1.  Adding the History-Info Header Field to Requests . . . . . 13
       7.1.1.  Forwarding a Request . . . . . . . . . . 11
   6.  Proxy Behavior . . . . . . . 14
       7.1.2.  Retargeting based on failure or 3xx response . . . . . 14
     7.2.  Sending History-Info in Responses  . . . . . . . . . . . . 12
     6.1.  Adding the 15
   8.  Redirect Server Handling of History-Info Header to Requests . Fields . . . . 16
   9.  Processing the History-Info Header Field Parameters  . . . 12
       6.1.1.  Initial Request . . 16
     9.1.  Privacy in the History-Info Header Field . . . . . . . . . 16
       9.1.1.  Indicating Privacy . . . . . . . . 12
       6.1.2.  Re-sending based on failure response . . . . . . . . . 13
       6.1.3.  Re-sending based on redirection response . 16
       9.1.2.  Applying Privacy . . . . . . 14
     6.2.  Sending History-Info in Responses . . . . . . . . . . . . 14
   7.  The History-Info header field . 17
     9.2.  Reason in the History-info Header Field  . . . . . . . . . 18
     9.3.  Indexing in the History-Info Header Field  . . . . . . 14
     7.1.  Definition . . 19
     9.4.  Mechanism for Target Determination in the History-Info
           Header Field . . . . . . . . . . . . . . . . . . . . . . 14
     7.2.  Examples . 20
   10. Application Considerations . . . . . . . . . . . . . . . . . . 21
   11. Security Considerations  . . . . . . 17
     7.3.  Procedures . . . . . . . . . . . . . 23
   12. IANA Considerations  . . . . . . . . . . . 17
       7.3.1.  Privacy in the History-Info Header . . . . . . . . . . 17
       7.3.2.  Reason in the 23
     12.1. Registration of New SIP History-Info Header Field  . . . . 24
     12.2. Registration of "history" for SIP Privacy Header Field . . . . . . 18
       7.3.3.  Indexing in the History-Info 24
     12.3. Registration of Header Field Parameters  . . . . . . . . . 18
       7.3.4.  Request Target in the History-Info Header  . . . . . . 20
   8.  Application Considerations . . . . . . . . . . . . . . . . . . 21
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
     10.1. Registration of New SIP History-Info Header  . . . . . . . 23
     10.2. Registration of "history" for SIP Privacy Header . . . . . 24
     10.3. Registration of Header Field Parameters  . . . . . . . . . 24
   11. Acknowledgements 25
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
   12.
   14. Changes from RFC 4244  . . . . . . . . . . . . . . . . . . . . 25
     12.1.
     14.1. Backwards compatibility  . . . . . . . . . . . . . . . . . 26
   13. 27
   15. Changes since last Version . . . . . . . . . . . . . . . . . . 27
   14.
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     14.1. 33
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 32
     14.2. 33
     16.2. Informative References . . . . . . . . . . . . . . . . . . 32 34
   Appendix A.  Request History Requirements  . . . . . . . . . . . . 33 34
     A.1.  Security Requirements  . . . . . . . . . . . . . . . . . . 34 35
     A.2.  Privacy Requirements . . . . . . . . . . . . . . . . . . . 35 36
   Appendix B.  Example call flows  . . . . . . . . . . . . . . . . . 35 37
     B.1.  Sequentially Forking (History-Info in Response)  . . . . . 35 37
     B.2.  History-Info with Privacy Header Field . . . . . . . . . . . . . 43 44
     B.3.  Privacy Header for a Specific History-Info Entry  . . . . . 45 . . . 46
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 46 47

1.  Introduction

   Many services that SIP is anticipated to support require the ability
   to determine why and how the call a SIP requests arrived at a specific
   application.  Examples of such services include (but are not limited
   to) sessions initiated to call centers via "click to talk" SIP
   Uniform Resource Locators (URLs) on a web page, "call history/logging" history/
   logging" style services within intelligent "call management" software
   for SIP User Agents (UAs), and calls to voicemail servers.  Although
   SIP implicitly provides the retarget capabilities that enable calls SIP
   requests to be routed to chosen applications, there is a need for a
   standard mechanism within SIP for communicating the retargeting
   history of such a request. the requests.  This "request history" information allows
   the receiving application to determine hints about how and why the call
   SIP request arrived at the application/user.

   This document defines a SIP header, header field, History-Info, to provide a
   standard mechanism for capturing the request history information to
   enable a wide variety of services for networks and end-users.  SIP
   header field parameters are defined for the History-Info and Contact
   header fields to tag the method by which the target of a request is
   determined.  In addition, this document defines a value for the
   Privacy header field specific to the History-Info header.

   The History-Info History-info header field provides a building block for
   development of SIP based applications and services.  The requirements
   for the solution described in this document are included in
   Appendix A.  Example scenarios using the History-Info History-info header field
   are included in Appendix B.

2.  Conventions and 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].

   The term "retarget" is used in this document to refer to the process
   of a SIP entity changing a Uniform Resource Identifier (URI) in a
   request based on the rules for determining request targets as
   described in Section 16.5 of [RFC3261] and of the subsequent
   forwarding of that request as described in step 2 in section 16.6 of
   [RFC3261].  This includes changing the Request-URI due to a location
   service lookup and redirect processing.  This also includes internal
   (to a proxy/SIP intermediary) changes of the URI prior to forwarding
   of the request.

   The terms "location service", "forward", "redirect" and "AOR" are
   used consistent with the terminology in [RFC3261].

   The references to "domain for which the SIP entity/Proxy/Intermediary
   is responsible" are consistent with and intended to convey the same
   context as the usage of that terminology in [RFC3261].  The
   applicability of History-Info to architectures or models outside the
   context of [RFC3261] is outside the scope of this specification.

3.  Background

   SIP implicitly provides retargeting capabilities that enable calls SIP
   requests to be routed to specific applications as defined in
   [RFC3261].  The motivation for capturing the request history is that
   in the process of retargeting a request, old routing information can
   be forever lost.  This lost information may be important history that
   allows elements to which the call request is retargeted to process the call
   request in a locally defined, application-specific manner.  This
   document defines a mechanism for transporting the request history.  Application-
   specific
   Application-specific behavior is outside the scope of this
   specification.

   Current network applications provide the ability for elements
   involved with the call request to exchange obtain additional information relating
   to how and why the call request was routed to a particular destination.
   The following are examples of such applications:

   1.  Web "referral" applications, whereby an application residing
       within a web server determines that a visitor to a website has
       arrived at the site via an "associate" site that will receive
       some "referral" commission for generating this traffic

   2.  Email forwarding whereby the forwarded-to user obtains a
       "history" of who sent the email to whom and at what time

   3.  Traditional telephony services such as voicemail, call-center
       "automatic call distribution", and "follow-me" style services

   Several of the aforementioned applications currently define
   application-specific mechanisms through which it is possible to
   obtain the necessary history information.

   In addition, request history information could be used to enhance
   basic SIP functionality by providing the following:

   o  Some diagnostic information for debugging SIP requests.

   o  Capturing aliases and Globally Routable User Agent URIs (GRUUs)
      [RFC5627], which can be overwritten by a home proxy upon receipt
      of the initial request.

   o  Facilitating the use of limited use addresses (minted on demand)
      and sub-addressing.

   o  Preserving service specific URIs that can be overwritten by a
      downstream proxy, such as those defined in [RFC3087], and control
      of network announcements and IVR with SIP URI [RFC4240].

4.  Overview of Operation

   The fundamental functionality provided by the request history
   information is the ability to inform proxies and UAs involved in
   processing a request about the history or progress of that request
   (CAPABILITY-req, see Appendix A). request.
   The solution is to capture the Request-URIs as a request is
   retargeted, in a SIP header: History-
   Info (CONTENT-req, see Appendix A). header field: History-Info.  This allows for the
   capturing of the history of a request that would be lost with the
   normal SIP processing involved in the subsequent retargeting of the
   request.

   The History-Info History-info header can appear in any field is added to a Request when a new
   request not associated with
   an early is created by a UAC or established dialog (e.g., INVITE, REGISTER, MESSAGE,
   REFER and OPTIONS, PUBLISH and SUBSCRIBE, etc.)  (REQUEST-VALIDITY-
   req, see Appendix A) and any provisional or final responses to these
   requests (ISSUER-req, see Appendix A).

   The following information is carried in the History-Info header as
   detailed in Section 7.1:

   o  Targeted-to-URI: The targeted-to-URI entry captures the Request-
      URI for the specific request as it is forwarded.

   o  Index: The index reflects the chronological order of the
      information, indexed to also reflect the forking and nesting of
      requests.

   o  Reason: Reason describes why an entry was retargeted.

   o  Privacy: Privacy is used to request that an entry be anonymized if
      the request is retargeted to a domain for which the retargeting
      entity is not responsible.

   o  Target: A parameter indicating whether the Targeted-to-URI is a
      registered contact ("rc") for another user mapped ("mp") from the
      Request-URI in the incoming request that was retargeted.  The
      index of the History-Info entry for the URI that was retargeted is
      included in each of these parameters.  Note that there may be
      other reasons a request is retargeted such as normal routing and
      forwarding, strict routing, etc., thus not all history-info
      entries have a target header field parameter.  The "rc" and "mp"
      scenarios are what is anticipated to be most useful to end
      applications/users.

   In addition, this specification defines a value for the Privacy
   header, "history", that applies to all the History-Info header
   entries in a Request or to a specific History-Info header entry as
   described above.  Further detailed is provided in Section 7.3.1.

   The History-Info header is added to a Request when a new request is
   created by a UAC or forwarded by a Proxy, forwarded by a Proxy, or when the
   target of a request is changed.  It is possible for the target of a
   request to be changed by the same proxy/SIP Intermediary multiple
   times (referred to as 'internal retargeting').  A SIP entity changing
   the target of a request in response to a redirect or REFER also propagates any
   History-Info
   History-info header field from the initial Request request in the new
   request.  The following is an illustrative example of usage ABNF and detailed description of History-Info.

   In this example, Alice (sip:alice@atlanta.example.com) calls Bob
   (sip:bob@biloxi.example.com).  Alice's home the History-Info
   header field parameters, along with examples is provided in
   Section 5.  Section 6, Section 7 and Section 8 provide the detailed
   handling of the History-Info header field by SIP User Agents, Proxies
   and Redirect Servers respectively.

   This specification also defines two new SIP header field parameters,
   "rc" and "mp", for the History-Info and Contact header fields, to tag
   the method by which the target of a request is determined.  Further
   detail on the use of these header field parameters is provided in
   Section 9.4.

   In addition, this specification defines a priv-value for the Privacy
   header, "history", that applies to all the History-info header field
   entries in a Request or to a specific History-info header field hi-
   entry as described above.  Further detail is provided in Section 9.1.

5.  History-Info Header Field Protocol Structure

   The History-info header field can appear in any request not
   associated with an early or established dialog (e.g., INVITE,
   REGISTER, MESSAGE, REFER and OPTIONS, PUBLISH and SUBSCRIBE, etc.)
   and any non-100 provisional or final responses to these requests
   (ISSUER-req, see Appendix A).

   The following provides details for the information that is captured
   in the History-Info header field entries for each target used for
   forwarding a request:

   o  hi-targeted-to-uri: A mandatory parameter for capturing the
      Request-URI for the specific request as it is forwarded.

   o  hi-index: A mandatory parameter for History-Info reflecting the
      chronological order of the information, indexed to also reflect
      the forking and nesting of requests.  The format for this
      parameter is a string of digits, separated by dots to indicate the
      number of forward hops and retargets.  This results in a tree
      representation of the history of the request, with the lowest-
      level index reflecting a branch of the tree.  By adding the new
      entries in order (i.e., following existing entries per the details
      in Section 9.3), including the index and securing the header, the
      ordering of the History-info header fields in the request is
      assured.  In addition, applications may extract a variety of
      metrics (total number of retargets, total number of retargets from
      a specific branch, etc.) based upon the index values.

   o  hi-target-param: An optional parameter reflecting the mechanism by
      which the Request URI captured in the hi-targeted-to-uri in the
      hi-entry was determined.  This parameter contains either an "rc"
      or "mp" header field parameter, which is interpreted as follows:

         "rc": The hi-targeted-to-URI is a contact for the Request-URI,
         in the incoming request, that is bound to an AOR in an abstract
         location service.  The AOR-to-contact binding has been placed
         into the location service by a SIP Registrar that received a
         SIP REGISTER request.  The "rc" header field parameter contains
         the value of the hi-index in the hi-entry with an
         hi-targeted-to- uri that reflects the Request-URI that was
         retargeted

         "mp": The hi-targeted-to-URI represents a user other than the
         user associated with the Request-URI in the incoming request
         that was retargeted.  This occurs when a request is to
         statically or dynamically retargeted to another user.  The
         value of the index in the "mp" header field parameter
         represents the value of the hi-index in the hi-entry with an
         hi-targeted-to- uri that reflects the Request-URI that was
         retargeted, thus identifying the "mapped from" target.

   o  Extension (hi-extension): A parameter to allow for future optional
      extensions.  As per [RFC3261], any implementation not
      understanding an extension MUST ignore it.

   The ABNF syntax for the History-info header field and header field
   parameters is as follows:

   History-Info = "History-Info" HCOLON hi-entry *(COMMA hi-entry)

   hi-entry = hi-targeted-to-uri *(SEMI hi-param)

   hi-targeted-to-uri = name-addr

   hi-param = hi-index / hi-target / hi-extension

   index-val =  1*DIGIT *("." 1*DIGIT)

   hi-index = "index" EQUAL index-val

   hi-target-param = rc-param / mp-param

   rc-param = "rc" EQUAL index-val

   mp-param = "mp" EQUAL index-val

   hi-extension = generic-param

   The ABNF definitions for "generic-param" and "name-addr" are from
   [RFC3261].

   This document also extends the "contact-params" for the Contact
   header field as defined in [RFC3261] with the "rc" and "mp" header
   field parameters defined above.

   In addition to the parameters defined by the ABNF, an hi-entry may
   also include a Reason header field and a Privacy header field, which
   are both escaped in the hi-targeted-to-uri as described below:

   o  Reason: An optional parameter for History-Info, reflected in the
      History-info header field by including the Reason header field
      [RFC3326] escaped in the hi-targeted-to-uri.  A reason is included
      for the hi-targeted-to-uri that was retargeted as opposed to the
      hi-targeted-to-uri to which it was retargeted.

   o  Privacy: An optional parameter for History-Info, reflected in the
      History-Info header field values by including the Privacy Header
      [RFC3323] escaped in the hi- targeted-to-uri or by adding the
      Privacy header field to the request.  The latter case indicates
      that the History-Info entries for the domain MUST be anonymized
      prior to forwarding, whereas the use of the Privacy header field
      escaped in the hi-targeted-to-uri means that a specific hi-entry
      MUST be anonymized.

   Note that since both the Reason and Privacy parameters are escaped in
   the hi-targeted-to-uri, these fields will not be available in the
   case that the hi-targeted-to-uri is a Tel-URI [RFC3966].  In such
   cases, the Tel-URI SHOULD be transformed into a SIP URI per section
   19.1.6 of [RFC3261].

   The following provides examples of the format for the History-info
   header field.  Note that the backslash and CRLF between the fields in
   the examples below are for readability purposes only.

   History-Info: <sip:UserA@ims.example.com>;index=1;foo=bar

   History-Info: <sip:UserA@ims.example.com?Reason=SIP%3B\
                 cause%3D302>;index=1.1,\
                 <sip:UserB@example.com?Privacy=history&Reason=SIP%3B\
                 cause%3D486>;index=1.2;mp=1.1,\
                 <sip:45432@192.168.0.3>;index=1.3;rc=1.2

5.1.  History-Info Header Field Example Scenario

   The following is an illustrative example of usage of History-Info.

   In this example, Alice (sip:alice@atlanta.example.com) calls Bob
   (sip:bob@biloxi.example.com).  Alice's proxy in her home domain (sip:
   atlanta.example.com) forwards the request to Bob's proxy (sip:
   biloxi.example.com).  When the request arrives at sip:
   biloxi.example.com, it does a location service lookup for
   bob@biloxi.example.com and changes the target of the request to Bob's
   Contact URIs provided as part of normal SIP registration.  In this
   example, Bob is simultaneously contacted on a PC client and on a
   phone, and Bob answers on the PC client.

   One important thing illustrated by this call flow is that without
   History-Info, Bob would "lose" the target information, including any
   parameters in the request URI.  Bob can recover that information by
   locating the last hi-entry with an "rc" header field parameter.  This
   "rc" parameter contains the index of the hi-entry containing the lost
   target information - i.e., the sip:bob@biloxi.example.com entry hi-entry
   with index=1.1.  Note that an hi-entry is not included for the fork
   to sip:bob@192.0.2.7 since there was no response at the time the 200
   OK is sent to Alice.

   The formatting in this scenario is for visual purposes; thus,
   backslash and CRLF are used between the fields for readability and
   the headers in the URI are not shown properly formatted for escaping.
   Refer to Section 7.2 5.1 for the proper formatting.  Additional detailed
   scenarios are available in the Appendix B.

      Note: This example uses loose routing procedures.

   Alice   atlanta.example.com  biloxi.example.com   Bob@pc  Bob@phone
   |                |                |                |          |
   |   INVITE sip:bob@biloxi.example.com;p=x          |          |
   |--------------->|                |                |          |
   | Supported: histinfo             |                |          |
   |                |                |                |          |
   |                |   INVITE sip:bob@biloxi.example.com;p=x    |
   |                |--------------->|                |          |
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1    |
   |                |                |                |          |
   |                |                |   INVITE sip:bob@192.0.2.3|
   |                |                |--------------->|          |
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
   | History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc=1.1
   |                |                |                |          |
   |                |                |   INVITE sip:bob@192.0.2.7|
   |                |                |-------------------------->|
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
   | History-Info: <sip:bob@192.0.2.7>;index=1.1.2;rc=1.1
   |                |                |     200        |          |
   |                |                |<---------------|          |
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
   | History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc=1.1
   |                |                |                |          |
   |                |     200        |                |          |
   |                |<---------------|                |          |
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
   | History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc=1.1
   |                |                |                |          |
   |                |                | Proxy Cancels INVITE      |
   |                |                |<=========================>|
   |                |                |                |          |
   |     200        |                |                |          |
   |<---------------|                |                |          |
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
   | History-Info: <sip:bob@192.0.2.3;index=1.1.1;rc=1.1
   |                |                |                |          |
   |     ACK        |                |                |          |
   |--------------->|    ACK         |                |          |
   |                |--------------->|     ACK        |          |
   |                |                |--------------->|          |
                           Figure 1: Basic Call

5.  General

6.  User Agent Behavior Handling of the History-Info Header Field

   This section describes the processing specific to UAs for the
   History-Info header.

5.1.
   History-info header field.

6.1.  User Agent Client (UAC) Behavior

   The UAC MUST include the "histinfo" option tag in the Supported
   header in any new or out-of-dialog request for which the UAC would
   like the History-Info History-info header field in the response.  In addition, the
   UAC
   SHOULD MUST add a History-Info header, History-info header field, using the Request-URI of
   the request as the hi-targeted-to-uri, in which case the index hi-targeted-to-uri.  The hi-index MUST be set to a
   value of 1 in the hi-entry.  As a result,  This allows intermediaries and the UAS
   to at least know the original Request-URI, and if the Request-
   URI was modified by a previous hop. Request-URI.  In the case of a B2BUA
   implementation, B2BUA, a
   UAC MAY add the hi-entries received in the incoming request at the
   UAS to the subsequent outgoing request.

   A UAC that does not want an hi-entry added due to privacy
   considerations MUST include a Privacy header with a priv-value(s) of
   "header" or "history".  A UAC that wants to ensure that privacy not
   be applied to its identity MUST include a Privacy header with a priv-
   value of "none".

   In the case where a UAC receives a 3xx response with a Contact header
   field and sends a new request in response to it, the UAC MAY include
   in the outgoing request the previous hi-entry(s) received in the
   response.  In this case, the reason header UAC MUST be associated with escape the hi-
   targeted-to-uri Reason header field
   in the previous (last) hi-entry, as described in Section 7.3.2.  A new hi-entry 9.2.  The
   UAC MUST then be added for add an hi-entry using the URI from Request-URI of the Contact header (which becomes outgoing
   request as the new Request-URI). hi-targeted-to-uri.  An index hi-index MUST be added to the
   hi-entry.  The value for the index is determined following the rules
   for "Retargeting based upon a Response" as prescribed in Section 7.3.3. 9.3.
   If the Contact header contained any of
   the field contains an "rc" or "mp" header parameter fields defined in this specification, field
   parameter, the UAC MUST include add the header parameter field as a header parameter field
   associated with to the current hi-entry new hi-
   entry as described in Section 7.3.4.

5.2. 9.4.  The procedures defined in
   Section 9.1 are followed to indicate any privacy that the UAC wants
   applied to the hi-targeted-to-uri.

6.2.  User Agent Server (UAS) Behavior

5.2.1.

6.2.1.  Processing of Requests with History-Info

   Once the request terminates at the UAS, the UAS evaluates the
   History-Info header.
   History-info header field.  The last hi-entry reflects the most
   recent target and MUST contain contains the Request-URI for the received request,
   unless the previous entity that forwarded the request does not
   support the History-Info header. History-info header field.  If the Request-URI of the
   incoming request does not match the last hi-entry (e.g., the last proxy previous
   entity does not support History-Info), the UAS MUST insert an hi-entry. hi-
   entry.  The UAS MUST set the hi-targeted-to-uri based to the value of
   Request-URI in the incoming request.  If privacy is required, required for the
   hi-entry in the response, the UAS MUST escape a privacy Privacy header field
   with a value of "history" MUST be added to in the hi-entry.  The UAS MUST include an
   hi-index attribute parameter as described in Section 7.3.3. 9.3.  The UAS MUST NOT
   include a hi-target attribute, since the UAS has no way to know the
   mechanism by which the Request-URI was determined.  The addition of
   the missing hi-entry ensures that the most complete information can
   be provided in the response and provides consistency in the
   information presented to applications.  The information can also be
   useful for implementations with B2BUAs that include the History-Info,
   received in the incoming request, in the outgoing request.

   Prior to any application usage of the information, the validity MUST
   be ascertained.  If gaps are detected, this MUST NOT be treated as an
   error since gaps are possible if the request is forwarded through
   intermediate entities that do not support UAS ascertains
   the History-Info header.
   The interpretation validity of the information as described in the History-Info header by a
   UAS in a request depends upon the specific applications supported by
   the UAS; an application might need to provide special handling in
   some cases where there are gaps.  Application considerations and
   guidelines are provided in section 7.

5.2.2. Section 10.

6.2.2.  Generation of Responses with History-Info

   If the "histinfo" option tag is received in a request, the UAS MUST
   include any History-Info received in the request in the subsequent
   response.  If privacy is required, entries MUST be anonymized as
   described in Section 7.3.1.  The UAS MUST follow the rules for a
   redirect server per Section 5.3 in generating a 3xx response.

   The processing of History-Info in responses follows and any hi-entries
   added by the methodology
   described UAS.  In addition, in Section 16.7 of [RFC3261], with the processing case of
   History-Info headers adding an additional step, just before Step 9,
   "Forwarding a B2BUA, the Response".

5.3.  Redirect Server Behavior

   A redirect server MUST UAS MAY
   include the History-Info headers received in
   the request in the 3XX response that it sends.  A redirect server
   MUST add any new History-Info entries in cases of retargeting (both
   internal and to other SIP entities) in the same manner as prescribed
   for a proxy Section 6.  In generating hi-entries received by the Contact header UAC in the 3xx
   response, the redirect server MUST add the appropriate target header
   field parameter to each Contact header subsequent
   response.  If privacy is required, entries MUST be anonymized as
   described in Section 7.3.4.

6.  Proxy Behavior 9.1.  The UAS MUST follow the rules for a
   redirect server per Section 8 in generating a 3xx response.

7.  Proxy/Intermediary Handling of History-Info Header Fields

   This section describes the procedures for proxies and other SIP
   intermediaries for adding History-Info headers when requests are
   retargeted.

6.1.

7.1.  Adding the History-Info Header Field to Requests

   This section describes the process of adding the History-Info header
   to requests for in the following cases:

   o  Forwarding case of retargeting due to normal forwarding of
   requests (Section 7.1.1) and in the case of initial request (see Section 6.1.1)

   o  Resending based on failure response (see Section 6.1.2)

   o  Resending based on or redirection response (see Section 6.1.3)
   responses (Section 7.1.2).  Retargeting is an iterative process,
   i.e., a proxy an intermediary may redirect retarget "internally " more than one time.
   A typical example would be a proxy that retargets a request first to
   a different user (i.e., it maps to a different AOR), and then
   forwards to a registered contact bound to that new AOR.  In these cases, a proxy cases of
   internal retargeting, an intermediary MUST add multiple hi-entry
   fields. hi-entries.
   For example, a proxy an intermediary that retargets bob@example.com to
   office@example.com and then to office@192.0.2.5 adds hi-entries for
   both office@example.com and office@192.0.2.5, in order to provide a
   logical description of the retargeting process internal to the proxy.
   A Reason
   intermediary.  Thus, the intermediary MAY be associated escape a Reason header
   field in the hi-entry with the hi-targeted-to-uri that has been
   retargeted as shown in the INVITE (F6) in the example in
   Appendix B.1.

6.1.1.  Initial

7.1.1.  Forwarding a Request

   Upon receipt of an initial a request for a dialog, or a standalone request, a
   proxy or intermediary forwarding the request MUST perform performs the following
   steps.  Note that those steps below do not apply if the request is
   being re-sent retargeted as a result of failure (i.e., timeout, reception of
   an error response).

   Step 1:  Adding Entries on Behalf of Previous Hops

      If an incoming request does not already have a History-Info header
      field (e.g., (i.e., the UAC does not include any History-Info History-info header
      field and
      no proxies in between none of the previous SIP intermediaries support History-Info), History-
      Info) or if the Request-URI of the incoming request does not match
      the hi-targeted-to-uri in the last hi-entry (e.g., (i.e., the last proxy previous
      intermediary does not support History-Info), the proxy MUST
      insert History-Info) an hi-entry. hi-entry MUST be
      inserted on behalf of the UAC or intermediary.  The proxy SIP
      intermediary MUST set the hi-targeted-to-uri
      based to the value of the
      Request-URI in the incoming request, unless
      privacy is required. request.  If privacy is required, the
      procedures of Section 7.3.1 MUST be used. 9.1 are followed.  The proxy SIP intermediary MUST
      NOT include a hi-
      target attribute.  The proxy MUST include an "rc" or "mp" header field parameter.  The hi-index attribute
      with
      parameter MUST be set to a value of "1", as described in
      Section 7.3.3. 9.3.

   Step 2:  Generating New Entries for Each Outgoing Request

      The proxy SIP intermediary then proceeds to request forwarding as per 16.6/
      [RFC3261].  The proxy MUST add a separate hi-entry in
      16.6/[RFC3261].  For each
      separate outgoing request for each of the current (outgoing)
      targets relating to a target in
      the target set. set, the intermediary MUST add an hi-entry for the
      specific target.  The proxy intermediary MUST set the hi-targeted-to-
      uri hi-targeted-to-uri
      in those separate hi-entry(s) the hi-entry to the value of the Request-URI of the current
      (outgoing) request, unless privacy is required. request.  If privacy is required, the procedures of
      Section 7.3.1 MUST be used. 9.1 are followed.  The proxy MUST include an hi-index for each of
      the separate hi-
      entry(s) hi-entry as described in Section 7.3.3. 9.3.  The proxy MUST include
      the appropriate hi-target then includes
      an "rc" or "mp" header field parameter for each of in the
      separate entry(s) hi-entry, if
      applicable, as described in Section 7.3.4, if applicable.

6.1.2.  Re-sending 9.4.

7.1.2.  Retargeting based on failure or 3xx response

   When re-sending retargeting a request as a result of retargeting because of a failure (i.e., either
   reception of error responses or a timeout which is considered to be
   an implicit 408 error response), response) or receipt of a 3XX response, the proxy MUST
   perform SIP
   entity performs the following steps:

   Step 1:  Including the Entries from Error Responses & Timeouts

      The proxy MUST build

      Along with the History-Info header field(s) fields sent in the
      outgoing original
      request using the aggregate information associated with
      the received error responses(s) and timeout(s) for all the
      branches that are generating failures, including the header
      entries in the order indicated by the indexing (see
      Section 7.3.3).  If the received an error or 3xx response did not include
      any History-Info header fields, or timed out, the proxy
      SIP entity MUST use the same include any new History-Info header fields that were sent field(s)
      contained in all responses received thus far (including any new
      hi-entries in the outgoing request
      that failed to build error or 3xx response) in the new outgoing
      request.

   Step 2:  Tagging the Last Entries  The proxy then examines the last hi-entry of the History-Info that
      was just generated in Step 1 for each one of the branches that
      generated failures or timeouts and hi-entries MUST add a Reason header for
      each one of those entries as per the procedures of Section 7.3.2.

   Step 3:  Generating New Entries for Each Outgoing Requests

      Same as per Step 2 above for be added to the normal forwarding case
      Section 6.1.1.

6.1.3.  Re-sending based on redirection response

   When re-sending a new outgoing request as a result of retargeting because of
   redirection (i.e., receipt of a 3XX response),
      in the following steps
   apply:

   Step 1:  Including Previous Entries

      The proxy MUST include order indicated by the History-Info header fields that were
      sent values in the outgoing request that is being redirected. hi-index parameters of
      the hi-entries.

   Step 2:  Tagging the Last Entry Entries

      The proxy SIP entity then examines the last hi-entry of the History-Info that
      was just header fields
      generated in Step 1 and 1.  For each of the branches that generated
      failures or timeouts or received a 3xx response, the SIP entity
      MUST add a Reason header this
      entry as field to the hi-entry for each of those
      branches per the procedures of Section 7.3.2. 9.2.

   Step 3:  Generating New Entries for Each Outgoing Requests

      Same as per Step 2 above for the normal forwarding case
      Section 6.1.1.

6.2. 7.1.1.

7.2.  Sending History-Info in Responses

   A proxy SIP entity that receives a request with the "histinfo" option tag
   in the Supported header, MUST forward captured History-Info in
   subsequent, provisional, and final responses to the request sent by
   the ultimate UAS (see Section 5.2).  Any 6.2), applying the privacy procedures
   as described in Section 9.1.2.  The captured History-Info includes
   all hi-entries received in the request, as well as the hi-entries
   that are generated by a SIP entity as a request is retargeted as
   described in Section 7.1, which also includes hi-entries that are
   received in other responses.  Note that in the case of parallel
   forking where one branch is successful, only the branches for which
   responses have been received at the time the proxy sends the
   successful response are included in that response.  For example, an
   intermediary receives a request with the last hi-entry having an hi-
   index of 1.1.  The intermediary forks three requests in parallel with
   each request containing a Privacy header unique hi-entry with
   a the hi-targeted-to-
   uris set to the value of "history" MUST be anonymized prior to the Request URI for the request.  The hi-
   entries each have unique hi-index values of 1.1.1, 1.1.2 and 1.1.3
   respectively.  If the processing entity receives a proxy sending failure response
   for the branch reflected by the hi-entry with the index of 1.1.2 and
   a successful response for the branch reflected by the hi-entry with
   the index of 1.1.3, but has not yet received a response for the
   branch reflected by the hi-entry with an index of 1.1.1, it would
   return only the 1.1.2 and 1.1.3 entries with indices of 1.1.2 and
   1.1.3 to the entity that hi-entry. generated the hi-entry with index of 1.1.
   See Appendix B.1 for an example.

   The processing the addition of the History-Info in header fields to responses
   described above follows the methodology described in Section 16.7 of [RFC3261],
   [RFC3261] for the addition of header fields, with the processing inclusion of
   History-Info headers
   History-info header fields adding an additional step, just before
   Step 9, "Forwarding the Response".

7.  The History-Info header field

7.1.  Definition

8.  Redirect Server Handling of History-Info is a Header Fields

   A redirect server MUST include the History-info header field as defined by [RFC3261].  It may
   appear fields
   received in any initial request for a dialog, standalone the request or
   responses associated with these requests.  For example, History-Info
   may appear in INVITE, REGISTER, MESSAGE, REFER, OPTIONS, SUBSCRIBE,
   and PUBLISH and any valid responses, plus NOTIFY requests the 3XX response that
   initiate a dialog.

   The it sends.  A
   redirect server MUST add any new History-Info header carries entries in cases of
   retargeting (both internal and to other SIP entities) in the following information when same
   manner as prescribed for SIP intermediaries Section 7.  In generating
   the Contact header is included field in a request or response:

   o  Targeted-to-URI (hi-targeted-to-uri): A mandatory parameter for
      capturing the Request-URI for the specific request as it is
      forwarded.

   o  Index (hi-index): A mandatory parameter for History-Info
      reflecting 3xx response, the chronological order of redirect server
   adds the information, indexed appropriate target header field parameter to
      also reflect each Contact
   header field as described in Section 9.4.

9.  Processing the forking and nesting of requests. History-Info Header Field Parameters

   The format for
      this parameter is a string of digits, separated by dots to
      indicate following sections describe the number of forward hops procedures for processing
   History-Info header field parameters and retargets.  This results the header fields escaped in
   the History-info header field.  These procedures are applicable to
   SIP entities such as Proxies/Intermediaries, Redirect Servers or User
   Agents.

9.1.  Privacy in a tree representation of the history of the request, with History-Info Header Field

   The privacy requirements for this document are described in
   Appendix A.2.  Section 9.1.1 describes the
      lowest-level index reflecting a branch use of the tree.  By adding the
      new entries Privacy header
   field defined in order (i.e., following existing entries per [RFC3323] to indicate the
      details in privacy to be applied to
   the History-Info header field entries.  Section 6.1), including 9.1.2 describes the index and securing
   processing of the
      header, priv-values in the ordering of Privacy header field to privacy
   protect the History-Info headers header field entries in the request or
   response that is
      assured (SEC-req-2, see Appendix A.1).  In addition, applications
      may extract a variety of metrics (total number of retargets, total
      number of retargets from a specific branch, etc.) based upon being forwarded.

9.1.1.  Indicating Privacy

   As with other SIP headers described in [RFC3323], the
      index values.

   o  Reason: An optional parameter for History-Info, reflected hi-targeted-to-
   uris in the
      History-Info History-info header by including field can inadvertently reveal
   information about the Reason Header [RFC3326]
      escaped in initiator of the hi-targeted-to-uri.  A reason is included for request.  Thus, the
      hi-targeted-to-uri that was retargeted as opposed UAC needs
   a mechanism to indicate that the hi-
      targeted-to-uri to which it was retargeted.

   o  Privacy: An optional parameter for History-Info, reflected hi-targeted-to-uris in the
      History-Info hi-
   entries need to be privacy protected.  The Privacy header field values by including the Privacy Header
      [RFC3323] escaped in the hi- targeted-to-uri or is
   used by adding the
      Privacy header UAC to indicate the request.  The latter case indicates that privacy to be applied to all the
      History-Info hi-
   entries for in the domain MUST be anonymized prior to
      forwarding, whereas request as follows:

   o  If the UAC is including a Privacy header field with a priv-value
      of "header" in the use request, then the UAC SHOULD NOT include a
      priv-value of "history" in the the Privacy header escaped field in the
      hi-targeted-to-uri means that a specific hi-entry MUST be
      anonymized.
      Request.

   o  Target (hi-target): An optional parameter for  If the History-Info and
      Contact headers.  The hi-target is added for a hi-entry when it UAC is
      first added including any priv-values other than "header" in a History-Info the
      Privacy header field, and only one value is
      permitted.  Upon receipt of a request or response containing then the
      History-Info header, UAC MUST also include a UA can determine the mechanism by which priv-value
      of "history" in the
      target was determined.  The following Privacy header field parameters are
      defined for this parameter:

         "rc": The hi-targeted-to-URI is a contact for the Request-URI, in the incoming request, that Request.

   o  If the UAC is bound to an AOR not including any priv-values in an abstract
         location service.  The AOR-to-contact binding has been placed
         into the location service by a SIP Registrar that received a
         SIP REGISTER request.  The "rc" Privacy header
      field parameter contains in the index of request, then the hi-entry associated UAC MUST add a Privacy header
      field, with a priv-value of "history", to the URI in the
         incoming request.

         "mp":  The hi-targeted-to-URI represents UAC
      MUST NOT include a user other than priv-value of "critical" in the
         user associated with Privacy header
      field in the Request-URI Request in this case.

   In addition, the incoming
         requesting.  This occurs when a request is History-info header field can reveal general routing
   and diverting information within an intermediary, which the
   intermediary wants to be statically or
         dynamically retargeted privacy protect.  In this case, the
   intermediary MUST set a Privacy header field to another user.  The value a priv-value of
   "history" and escape the "mp" Privacy header field parameter is the index parameter in the hi-targeted-to-
   uri, for each hi-entry added by intermediary, as the hi-
         targeted-to- uri that was retargeted, thus identifying request is
   retargeted within the
         "mapped from" target.

   o  Extension (hi-extension): A parameter to allow for future optional
      extensions.  As per [RFC3261], any implementation not
      understanding an extension MUST ignore it.

   The ABNF syntax domain for which the History-Info header SIP entity is defined as follows:

   History-Info = "History-Info" HCOLON hi-entry *(COMMA hi-entry)

   hi-entry = hi-targeted-to-uri *(SEMI hi-param)

   hi-targeted-to-uri = name-addr

   hi-param = hi-index / hi-target / hi-extension

   index-val =  1*DIGIT *("." 1*DIGIT)

   hi-index = "index" EQUAL index-val

   hi-target = rc-param / mp-param

   rc-param = "rc" EQUAL index-val

   mp-param = "mp" EQUAL index-val

   hi-extension = generic-param responsible.
   The ABNF definitions for "generic-param" and "name-addr" are from
   [RFC3261]. intermediary MUST NOT include any other priv-values in this
   Privacy header field.  Note that since both the Reason and Privacy parameters are escaped priv-value in the hi-targeted-to-uri, these fields will Privacy header
   for the incoming request does not be available necessarily influence whether the
   intermediary includes a Privacy header field in the
   case that hi-entries.  For
   example, even if the hi-targeted-to-uri is Privacy header for the incoming request
   contained a Tel-URI [RFC3966].

7.2.  Examples

   The following provides some examples priv-value of "none", the History-Info header.
   Note that the backslash and CRLF between Proxy can still set a priv-
   value of "history" in the fields Privacy header field escaped in the examples
   below are for readability purposes only.

   History-Info: <sip:UserA@ims.example.com>;index=1;foo=bar

   History-Info: <sip:UserA@ims.example.com?Reason=SIP%3B\
                 cause%3D302>;index=1.1,\
                 <sip:UserB@example.com?Privacy=history&Reason=SIP%3B\
                 cause%3D486>;index=1.2;mp=1.1,\
                 <sip:45432@192.168.0.3>;index=1.3;rc=1.2

7.3.  Procedures

   The following sections define procedures for processing of hi-
   targeted-to-uri.

   Finally, the
   parameters (and headers) associated with terminator of the History-Info header.
   These procedures request may be applicable not want to processing entities such as
   Proxies, Redirect Servers or User Agents.

7.3.1. reveal the
   final reached target to the originator.  In this case, the terminator
   MUST include a Privacy header field with a priv-value of "history",
   escaped in the History-Info Header

   The privacy requirements for this document are described hi-targeted-to-uri in
   Appendix A.2.

   As with other SIP headers described the last hi-entry, in [RFC3323], the History-Info
   header can inadvertently reveal information about
   response.  As noted above, the originator.  A
   value terminator of "header" the request MUST NOT use
   any other priv-values in the Privacy header is used to indicate that such
   headers field escaped in the hi-
   entry.

9.1.2.  Applying Privacy

   When a request be privacy protected when is retargeted to a URI associated with a domain for
   which the request SIP intermediary is
   forwarded by not responsible or a response is
   forwarded, a Privacy Service at the boundary of the domain applies
   the appropriate privacy based on the intermediary.  A value of "history" in the Privacy header indicates that
   field in the History-Info header should be privacy
   protected. request and in the individual hi-entries.

   If there is a Privacy header field in the request with a priv-
   value priv-value
   of "header" or "history", then the initial hi-entry MUST be
   anonymized and hi-targeted-to-uris in the header removed when hi-
   entries, associated with the request leaves a domain for which the a SIP entity intermediary is responsible.

   In addition, the History-Info header can reveal general routing and
   diverting information within an intermediary, which
   responsible, are anonymized.  The Privacy Service MUST change any hi-
   targeted-to-uris in the intermediary
   may want hi-entries that have not been anonymized to privacy protect.  In this case,
   anonymous URIs containing a value domain of "history" in anonymous.invalid (e.g.,
   anonymous@anonymous.invalid).  If the hi-entry has an escaped Privacy
   header is used to indicate which History-Info field value, then the Privacy header
   entries added by a SIP entity are to field value MUST be anonymized.  The
   removed from the hi-entry.  Once all the appropriate hi-entries have
   been anonymized, the priv-value of "history" MUST be in removed from the privacy
   Privacy header escaped field.  If there are no remaining priv-values in the hi-
   targeted-to-uri for each hi-entry, added by
   Privacy header field, the SIP entity, as Privacy header field MUST be removed from
   the request is retargeted within the domain for which the SIP entity is
   responsible. per [RFC3323].

   If there is not a Privacy header field in the request or response
   that is being retargeted to a URI forwarded, the Privacy Service MUST anonymize any hi-
   entries, associated with a the domain for which the a SIP entity intermediary is not
   responsible, the
   processing entity MUST anonymize the hi-entries with that contain a priv-value of
   "history" and MUST remove the Privacy header from the hi-entries
   prior to forwarding, unless the processing entity knows a priori that
   it can rely on field with a downstream processing entity to apply the requested
   privacy.  The mechanism for the latter functionality is outside the
   scope of this specification.

   Finally, the terminator priv-value of
   "history".  The Privacy Service MUST populate the request may not want to reveal the
   final reached target to the originator.  In this case, the terminator
   uses the hi-targeted-to-uri
   with an anonymous URI with a value domain of "history" anonymous.invalid (e.g.,
   anonymous@anonymous.invalid).  Any other priv-values in the Privacy
   header field in the last hi-
   entry in the response.  The SIP entity that forwards hi-entries MUST be ignored.  In any case, the response
   Privacy Service MUST anonymize that hi-entry and remove the Privacy header.

7.3.2. header field from the hi-
   entries prior to forwarding.

9.2.  Reason in the History-Info History-info Header

   For retargets that are Field

   If the result retargeting is due to receipt of an explicit SIP response, a
   Reason MUST be associated with the hi-targeted-to-uri.  If response and
   the SIP response does not include a contains any Reason header fields (see [RFC3326]), then
   the SIP
   Response Code that triggered the retargeting entity MUST be included as escape the Reason associated header fields in the hi-entry
   with the hi-targeted-to-uri containing the URI of the request that has been
   was retargeted.  If the SIP response contains does not contain a Reason header for a protocol
   that is not
   field, the SIP (e.g., Q.850), it entity MUST be captured as include an additional escaped Reason associated with the hi-targeted-to-uri that has been
   retargeted, along with header field,
   containing the SIP Response Code.  If Code that triggered the Reason header
   is a SIP reason, then it MUST be used as retargeting, in
   the Reason associated hi-entry with the hi-targeted-to-uri rather than containing the SIP response code. URI of the
   request that was retargeted.

   If a request has timed out (instead of being explicitly rejected), it
   the SIP entity MUST be treated as if escape a 408 "Request Terminated" Reason header field, containing a SIP
   error response code of 408 "Request Timeout" in in the hi-entry with
   the hi-targeted-to-uri containing the URI of the request that was
   retargeted.  The SIP entity MAY also be escape a Reason header field
   in the hi-entry with the hi-targeted-to-uri containing the URI of the
   request that was received.

7.3.3. retargeted as a result of internal retargeting.

   If additional Reason headers are defined in the future per [RFC3326],
   the use of these Reason headers for the History-Info header field
   MUST follow the same rules as described above.

9.3.  Indexing in the History-Info Header Field

   In order to maintain ordering and accurately reflect the nesting and retargeting
   of the request, an index MUST be included along with the
   Targeted-to-URI being captured. SIP entity MUST add an hi-index to each hi-entry.
   Per the syntax in Section 7, 5, the
   index hi-index consists of a dot-delimited series of
   digits separated by dots (e.g., 1.1.2).  Each dot reflects a hop or level of nesting; thus, SIP
   forwarding hop.  The digit following each dot reflects the number of hops
   is determined by order in
   which a request was retargeted at the total number of dots.  Within hop.  The highest digit at each level, the
   integer
   hop reflects the number of peer entities to which the request has been routed.
   retargeted at the specific hop (i.e., the number of branches).  Thus,
   the indexing results in a logical tree representation for the history
   of the request.

   The first index in a series of History-Info entries MUST be set to 1.
   In the case that a proxy SIP entity (intermediary or UAS) adds an entry hi-entry
   on behalf of the previous hop, the index hi-index MUST be set to 1.  For
   each forward hop (i.e., each new level of indexing, indexing), the index hi-index
   MUST start at 1.  An increment of 1 MUST be used for advancing to a
   new branch.

   The basic rules for adding the index hi-index are summarized as follows:

   1.  Basic Forwarding: In the case of a request that is being
       forwarded, the index is determined by adding another sub-level of
       indexing since hi-index reflects the depth/length increasing length of the branch is increasing.  To
       accomplish this,
       branch.  In this case, the processing SIP entity MUST read the value from
       the History-Info History-info header field in the received request and MUST
       add another level of indexing by appending the dot delimiter
       followed by an initial index hi-index for the new level of 1.  For
       example, if the
       index hi-index in the last History-Info History-info header field in
       the received request is 1.1, this a proxy would initialize its index add an hi-entry with
       an hi-index to 1.1.1 and forward the request.

   2.  Retargeting within a processing entity - 1st instance: For the
       first instance of retargeting within a processing entity, the
       calculation of SIP
       entity MUST calculate the index follows that hi-index as prescribed for basic
       forwarding.

   3.  Retargeting within a processing entity - subsequent instance: For
       each subsequent retargeting of a request by the same processing SIP entity, another branch
       the SIP entity MUST be added. add another branch.  The index SIP entity MUST
       calculate the hi-index for each new branch MUST be calculated by incrementing the last/lowest digit
       value from the hi-index in the last hi-entry at the current level,
       level.  Per the index example above, the hi-index in the next request
       forwarded by this same processing entity, following the example above, SIP entity would be 1.1.2.

   4.  Retargeting based upon a Response: In the case of retargeting due
       to a specific response (e.g., 302), the index SIP entity MUST be calculate
       the hi-index calculated per rule 3.  That is, the lowest/last
       digit of the index hi-index MUST be incremented (i.e., a new branch is
       created), with the increment of 1.  For example, if the index hi-index
       in the History-Info header of the sent request is 1.2 and the
       response to the request is a 302, then the index hi-index in the
       History-Info header field for the new hi-
       targeted- hi-targeted- to-URI would
       be 1.3.

   5.  Forking requests: If the request forwarding is done in multiple
       forks (sequentially or in parallel), the index SIP entity MUST be captured set the
       hi-index for each hi-entry for each forked request per the rules
       above, with each new request having a unique index.  Each index
       MUST be sequentially assigned.  For example, if the index in the
       last History-Info header field in the received request is 1.1,
       this processing entity would initialize its index to 1.1.1 for
       the first fork, 1.1.2 for the second, and so forth (see Figure 1
       for an example).  Note that for each individual fork, only the entry
       hi-entry corresponding to that fork is included (e.g., the hi-
       entry for fork 1.1.1 is not
       included in the request sent to fork 1.1.2, and vice-versa).

   6.  When a response is built and it represents the aggregate of
       responses to multiple forks (e.g., multiple forks that fail), the
       processing entity MUST build the subsequent response using the
       aggregated information associated with each of the forks for
       which there have been responses and MUST include the header
       entries in the order indicated by the indexing.  For example, if
       a procesing entity received failure responses for forks 1.1.1 and
       1.1.2, it would return both the 1.1.1 and 1.1.2 entries to the
       entity that generated the hi-entry with index of 1.  See
       Appendix B.1 for an example.  Note that in the case of parallel
       forking where one fork is successful, only the forks for which
       responses have been received at the time the proxy sends the
       successful response are for fork 1.1.1 is not included in that response.  Responses are
       processed as described in Section 16.7 of [RFC3261] with the
       aggregated History-Info entries processed similar request sent to Step 7
       "Aggregate Authentication Header Field Values".

7.3.4.  Request fork
       1.1.2, and vice-versa).

9.4.  Mechanism for Target Determination in the History-Info Header
      Field

   This specification defines two header field parameters, "rc" and
   "mp", indicating two non-inclusive mechanisms by which a new target
   for a request is determined.  Both parameters contain an index whose
   value is the hi-index of the hi-entry with an hi-targeted-to-uri that
   represents the Request-URI that was retargeted.

   The SIP entity MUST determine the specific parameter field to be
   included in the History-Info History-info header is determined field as the targets are added to
   the target set per the procedures of in section 16.5 of [RFC3261] or when per
   section 8.1.3.4 [RFC3261] in the case of 3xx responses.  In the
   latter case, the specific header parameter field in the Contact
   header becomes the header field parameter that is used in a 3xx response the hi-
   entry when the request is populated.  Both parameters retargeted.  If the Contact header field
   does not contain an index whose value corresponds to "rc" or "mp" header field parameter, then the index SIP
   entity MUST NOT include an "rc" or "mp" in the hi-entry
   containing when the Request-URI that was
   request is retargeted.

   The SIP entity (intermediary or redirect server) determines the
   specific parameter header field parameter to be included in used based on the History-Info
   header is determined as follows: following
   criteria:

   o  "rc": If the hi-targeted-to-URI The target was determined based on a contact
      for the Request-URI being retargeted that is bound
      to an AOR in an abstract location service, then the "rc" header field parameter
      MUST be included in the hi-entry.  The index of the "rc" parameter
      MSUT be set to the hi-index containing service for the Request-URI
      being retargeted.

   o  "mp": If the hi-targeted-to-URI The target was determined based on a mapping to a user other
      than the user associated with the Request-URI being retargeted, then the "mp" header field parameter MUST be
      included in the hi-entry.  The index of the "mp" parameter MUST be
      set to the hi-index containing the Request-URI being retargeted.

   Note that there are two scenarios by which the "mp" parameter can be
   derived.

   o  The mapping was done by the receiving entity on its own authority,
      in which case the mp-value is the parent index of the hi-entry's
      index.

   o  The mapping was done due to receiving a 3xx response, in which
      case the mp-value is an earlier sibling of the hi-entry's index,
      that of the downstream downstream request which received the 3xx response.

   The SIP entity MUST add the "rc" or "mp" header field parameter to
   the hi-entry when the request which received is forwarded to the 3xx response.

8. target per step 2
   in Section 7.1.1.

10.  Application Considerations

   History-Info provides a very flexible building block that can be used
   by intermediaries and UAs for a variety of services.  Prior to any
   application usage of the information History-Info header field parameters, the entries
   SIP entity that processes the hi-entries MUST evaluate the hi-
   entries.  The SIP entity MUST be evaluated to determine if there are gaps in the
   indices.  If gaps are detected, this MUST NOT be
   treated as an error since gaps  Gaps are possible if the request is forwarded through intermediate entities
   intermediaries that do not support the History-info header field and
   are reflected by the existence of multiple hi-entries with an index
   of "1".  Gaps are also possible in the case of parallel forking if
   there is an outstanding request at the time the SIP entity sends a
   response as described in Section 7.2.  Thus, if gaps are detected,
   the SIP entity MUST NOT treat this as an error, but rather indicate
   to any applications that there are gaps.  The most complete
   information available to the application is the History-Info header. entries
   starting with the last hi-entry with an index of "1".  The
   interpretation of the information in the
   History-Info History-info header field
   depends upon the specific application; an application might need to
   provide special handling in some cases where there are gaps.

   The following summarizes the categories of information that
   applications may can use:

   1.  Complete history information - e.g., for debug or other
       operational and management aspects, optimization of determining
       targets to avoid retargeting to the same URI, etc.  This
       information is relevant to proxies, UACs and UASs.

   2.  Entry  Hi-entry with the index that matches the value of the last hi-
       entry with a "rc" header parameter in the Request received by a
       UAS - i.e., the Request URI associated with the destination of
       the request was determined based on an AOR-to-contact binding in
       an abstract location service.

   3.  Entry  Hi-entry with the index that matches the value of the last hi-
       entry with a "mp" header parameter in the Request received by a
       UAS - i.e., the last Request URI that was mapped to reach the
       destination.

   4.  Entry  Hi-entry with the index that matches the value of the first hi-
       entry with a "rc" header parameter in the Request received by a
       UAS.  Note, this would be the original AoR if all entries the entities
       involved support the
       History-Info History-info header field and there is
       absence of a "mp" header parameter prior to the "rc" header
       parameter in the History-Info
       header. History-info header field.  However, there is no
       guarantee that all entities will support History-Info, thus the
       first entry hi-entry with an "rc" header parameter within the domain
       associated with the target URI at the destination is more likely
       to be useful.

   5.  Entry  Hi-entry with the index that matches the value of the first hi-
       entry with a "mp" header parameter in the Request received by a
       UAS.  Note, this would be the original mapped URI if all entities
       supported the History-Info header. History-info header field.  However, there is no
       guarantee that all entities will support History-Info, thus the
       first entry hi-entry with an "mp" header parameter within the domain
       associated with the target URI at the destination is more likely
       to be useful.

   In many cases, applications are most interested in the information
   within a particular domain(s), thus only a subset of the information
   is required.

   Some applications may use multiple types of information.  For
   example, an Automatic Call Distribution (ACD)/Call center application
   that utilizes the entry hi-entry who index matches the index of the first
   History-Info entry with an hi-target value of "mp", may also display
   other agents, reflected by other History-Info entries prior to
   entries with hi-target values of "rc", to whom the call was targeted
   prior to its arrival at the current agent.  This could allow the
   agent the ability to decide how they might forward or reroute the
   call if necessary (avoiding agents that were not previously available
   for whatever reason, etc.).

   Since support for History-Info History-info header field is optional, a service
   MUST define default behavior for requests and responses not
   containing History-Info headers.  For example, an entity may receive
   only partial History-Info entries or entries which are not tagged
   appropriately with an hi-target parameter.  This may not impact some
   applications (e.g., debug), however, it could require some
   applications to make some default assumptions in this case.  For
   example, in an ACD scenario, the application could select the oldest
   hi-entry with the domain associated with the ACD system and display
   that as the original called party.  Depending upon how and where the
   request may have been retargeted, the complete list of agents to whom
   the call was targeted may not be available.

9.

11.  Security Considerations

   The security requirements for this document are specified in
   Appendix A.1.

   This document defines a header for SIP.  The use of the Transport
   Layer Security (TLS) protocol [RFC5246] as a mechanism to ensure the
   overall confidentiality of the History-Info headers (SEC-req-4) is
   strongly RECOMMENDED.  This results in History-Info having at least
   the same level of security as other headers in SIP that are inserted
   by intermediaries.  With TLS, History-Info headers are no less, nor
   no more, secure than other SIP headers, which generally have even
   more impact on the subsequent processing of SIP sessions than the
   History-Info header.

   With the level of security provided by TLS (SEC-req-3), the
   information in the History-Info
   History-info header can thus be evaluated to
   determine if information has been removed by evaluating the indices
   for gaps (SEC-req-1, SEC-req-2).  It would be up to the application
   to define whether it can make use of the information in the case of
   missing entries. field.

   Note that while using the SIPS scheme (as per [RFC5630]) protects
   History-Info from tampering by arbitrary parties outside the SIP
   message path, all the intermediaries on the path are trusted
   implicitly.  A malicious intermediary could arbitrarily delete,
   rewrite, or modify History-Info.  This specification does not attempt
   to prevent or detect attacks by malicious intermediaries.

10.

12.  IANA Considerations

   This document requires several IANA registrations detailed in the
   following sections.

   This document updates [RFC4244] but uses the same SIP header field
   name and option tag.  The IANA registry needs to update the
   references to [RFC4244] with [RFCXXXX].

10.1.

12.1.  Registration of New SIP History-Info Header Field

   This document defines a SIP header field name: History-Info and an
   option tag: histinfo.  The following changes have been made to
   http:///www.iana.org/assignments/sip-parameters The following row has
   been added to the header field section:.

   The following row has been added to the header field section:

   Header Name             Compact Form               Reference
   -----------             ------------               ---------
   History-Info               none                    [RFCXXXX]

   The following has been added to the Options Tags section:

   Name          Description                          Reference
   ----          -----------                          ---------
   histinfo      When used with the Supported header, [RFCXXXX]
                 this option tag indicates the UAC
                 supports the History Information to be
                 captured for requests and returned in
                 subsequent responses.  This tag is not
                 used in a Proxy-Require or Require
                 header field since support of
                 History-Info is optional.

   Note to RFC Editor: Please replace RFC XXXX with the RFC number of
   this specification.

10.2.

12.2.  Registration of "history" for SIP Privacy Header Field

   This document defines a priv-value for the SIP Privacy header: header field:
   history The following changes have been made to
   http://www.iana.org/assignments/sip-priv-values The following has
   been added to the registration for the SIP Privacy header: header field:

   Name      Description               Registrant   Reference
   ----      -----------               ----------   ---------
   history   Privacy requested for     Mary Barnes  [RFCXXXX]
             History-Info header(s)
             History-info header       mary.barnes@polycom.com
             fields(s)

   Note to RFC Editor: Please replace RFC XXXX with the RFC number of
   this specification.

10.3.

12.3.  Registration of Header Field Parameters

   This specification defines the following new SIP header field
   parameters in the SIP Header Field parameter sub-registry in the SIP
   Parameter Registry, http:/www.iana.org/assignments/sip-parameters.

   Header Field                  Parameter Name   Predefined  Reference
                                                    Values
   _____________________________________________________________________
   History-Info                  mp                   No     [RFC xxxx]
   History-Info                  rc                   No     [RFC xxxx]
   Contact                       mp                   No     [RFC xxxx]
   Contact                       rc                   No     [RFC xxxx]

   Note to RFC Editor: Please replace RFC XXXX with the RFC number of
   this specification.

11.

13.  Acknowledgements

   Jonathan Rosenberg et al produced the document that provided
   additional use cases precipitating the requirement for the new header
   parameters to capture the method by which a Request URI is
   determined.  The authors would like to acknowledge the constructive
   feedback provided by Ian Elz, Paul Kyzivat, John Elwell, Hadriel
   Kaplan and Dale Worley.

   Mark Watson, Cullen Jennings and Jon Peterson provided significant
   input into the initial work that resulted in the development of of
   [RFC4244].  The editor would like to acknowledge the constructive
   feedback provided by Robert Sparks, Paul Kyzivat, Scott Orton, John
   Elwell, Nir Chen, Palash Jain, Brian Stucker, Norma Ng, Anthony
   Brown, Jayshree Bharatia, Jonathan Rosenberg, Eric Burger, Martin
   Dolly, Roland Jesske, Takuya Sawada, Sebastien Prouvost, and
   Sebastien Garcin in the development of [RFC4244].

   The editor would like to acknowledge the significant input from Rohan
   Mahy on some of the normative aspects of the ABNF for [RFC4244],
   particularly around the need for and format of the index and around
   the security aspects.

12.

14.  Changes from RFC 4244

   This RFC replaces [RFC4244].

   Deployment experience with [RFC4244] over the years has shown a
   number of issues, warranting an update:

   o  In order to make [RFC4244] work in "real life", one needs to make
      "assumptions" on how History-Info is used.  For example, many
      implementations filter out many entries, and only leave specific
      entries corresponding, for example, to first and last redirection.
      Since vendors uses different rules, it causes significant
      interoperability isssues.

   o  [RFC4244] is overly permissive and evasive about recording
      entries, causing interoperability issues.

   o  The examples in the call flows had errors, and confusing because
      they often assume "loose routing".

   o  [RFC4244] has lots of repetitive and unclear text due to the
      combination of requirements with solution.

   o  [RFC4244] gratuitously mandates the use of TLS on every hop.  No
      existing implementation enforces this rule, and instead, the use
      of TLS or not is a general SIP issue, not an [RFC4244] issue per
      se.

   o  [RFC4244] does not include clear procedures on how to deliver
      current target URI information to the UAS when the Request-URI is
      replaced with a contact.

   o  [RFC4244] does not allow for marking History-Info entries for easy
      processing by User Agents.

   The following summarizes the functional changes between this
   specification and [RFC4244]:

   1.  Added header field parameters to capture the specific method by
       which a target is determined to facilitate processing by users of
       the
       History-Info History-info header field entries.  A specific header field
       parameter is captured for each of the target URIs as the target
       set is determined (per section 16.5 of [RFC3261]).  The header
       field parameter is used in both the History-Info and the Contact headers.
       header fields.

   2.  Rather than recommending that entries be removed in the case of
       certain values of the privacy header, recommend that Privacy header field, the entries are
       anonymized.

   3.  Updated the security section to be equivalent to the security
       recommendations for other SIP headers inserted by intermediaries.

   The first 2 changes are intended to facilitate application usage of
   the History-Info History-info header field and eliminate the need to make
   assumptions based upon the order of the entries and ensure that the
   most complete set of information is available to the applications.

   In addition, editorial changes were done to both condense and clarify
   the text, moving the requirements to an appendix. appendix and removing the
   inline references to the requirements.  The examples were simplified
   and updated to reflect the protocol changes.  Several of the call
   flows in the appendix were removed and put into a separate document
   that includes additional use cases that require the new header
   parameters.

12.1.

14.1.  Backwards compatibility

   This specification is backwards compatible since [RFC4244] allows for
   the addition of new optional parameters.  This specification adds an
   optional SIP header field parameter to the History-Info and Contact
   headers.  Entities that have not implemented this specification MUST
   ignore these parameters, however, per [RFC4244] an entity MUST NOT
   remove this parameter from an hi-entry.

13.

15.  Changes since last Version

   NOTE TO THE RFC-Editor: Please remove this section prior to
   publication as an RFC.

   Changes from 02 to 03:

   1.  Lots of editorial:

       A.  Reorganized sections similar to the RFC 4244 order - i.e.,
           introduce header field parameters and syntax first, then
           describe how the functional entities use the header.  This
           removes redundant (and often inconsistent) text describing
           the parameters.

       B.  Expanded use of "header" to "header field"

       C.  More precision in terms of "escaping" of the Privacy and
           Reason headers in the hi-targeted-to-uri (versus
           "adding"/"setting"/etc. them to the hi-entry).

       D.  Consistent use of parameter names (i.e., hi-entry versus
           entry, hi-target versus target, etc.)

       E.  Moved item 6 in the Index section to the section on Response
           handling

       F.  Removed last remaining vestiges of inline references to
           requirements.

   2.  Clarifications of functionality/applicability including:

       A.  which messages may contain History-Info

       B.  removing security text with regards to being able to figure
           out if there are missing entries when using TLS (issue #44)

       C.  More complete information on the new header field parameters
           as they relate to the hi-target parameter.

       D.  Changed wording from passive to active for normative
           statements in many cases and removed superfluous normative
           language.

   3.  Rewrite of the Privacy section to address issues and splitting
       into the setting of the Privacy header fields and the processing/
       application of the privacy header field priv-values.

   4.  Rewrite of the Reason header field section - simplifying the text
       and adding back the RFC 4244 text with regards to the use of the
       Reason header field in cases of internal retargeting.

   Changes from 01 to 02:

   1.   Editorial nits/clarifications.  [Issues: 1,6,17,18,21-
        23,25,26,30-33,35-37,39,40]

   2.   Removing extraneous 4244 text - e.g., errors in flows,
        "stronger" security, "session" privacy.  [Issues: 3,5,7,11 ]

   3.   Updated definition of "retarget" to be all encompassing - i.e.,
        also includes internal changes of target URI.  Clarified text
        for "internal retarging" in proxy section.  [Issues: 2,8,9]

   4.   Clarified that the processing for Proxies is equally applicable
        to other SIP intermediaries.  [Issue: 9].

   5.   Changed more SHOULDs to MUSTs.  [Issue: 10]

   6.   Fixes to Application considerations section.  [Issues: 12-15]
   7.   Changed language in the procedure for Indexing to normative
        language.

   8.   Clarifications for UAC processing:

        *  MUST add hi-entry.  [Issue: 28]

        *  Clarify applicability to B2BUA.  [Issue: 29]

        *  Fixed text for indexing for UAC in case of 3xx.

   9.   Changed "hit" URI parameter to header parameters: [Issues:4,40]

        *  Added index to all target header parameters.  [Issues: 41]

        *  Updated all the relevant sections documenting setting and use
           of new header parameters.  [Issue: 40]

   10.  Updated/clarified privacy handling.  [Issue: 16]

   11.  Updated Redirect Server section to allow adding History-Info
        headers. History-info
        header fields.  [Issue: 24 ]

   12.  Added text around restrictions for Tel-URIs - i.e., no privacy
        or reason.  [Issues: 4, 12]

   13.  Updated text for forking - what goes in response.  [Issues:
        19,20]

   Changes from 00 to 01:

   1.  Moved examples (except first) in appendix to a new
       (informational) document.

   2.  Updated UAS and UAC sections to clarify and expand on the
       handling of the History-Info header. History-info header field.

   3.  Updated the Application considerations section:

       *  Included more detail with regards to how applications can make
          use of the information, in particular based on the new tags.

       *  Removed privacy consideration (2nd bullet) since privacy is
          now accomplished by anonymizing rather than removal of
          entries.

   Changes from (individual) barnes-sipcore-4244bis-03 to (WG) ietf-
   sipcore-4244bis-00:

   1.  Added a new SIP/SIPS URI parameter to tag the URIs as they are
       added to the target list and those returned in the contact header
       in a 3xx response.

   2.  Updated description of "target" parameter to use the new URI
       parameter value in setting the value for the parameter.

   3.  Clarified privacy.

   4.  Changed handling at redirect server to include the use of the new
       URI parameter and to remove the functionality of adding the
       History-Info entries (basically reverting to core 4244
       processing).

   5.  Additional text to clarify that a service such as voicemail can
       be done in multiple ways.

   6.  Editorial changes including removal of some vestiges of tagging
       all entries (including the "aor" tag).

   Changes from barnes-sipcore-4244bis-02 to 03:

   1.  Fixed problem with indices in example in voicemail example.

   2.  Removed oc and rt from the Hi-target parameter.

   3.  Removed aor tag

   4.  Added index parameter to "mp"

   5.  Added use-cases and call-flows from target-uri into appendix.

   Changes from barnes-sipcore-4244bis-01 to 02:

   1.  Added hi-aor parameter that gets marked on the "incoming" hi-
       entry.

   2.  Hi-target parameter defined to be either rc, oc, mp, rt, and now
       gets included when adding an entry. hi-entry.

   3.  Added section on backwards compatibility, as well as added the
       recognition and handling of requests that do not support this
       specification in the appropriate sections.

   4.  Updated redirect server/3xx handling to support the new
       parameters - i.e., the redirecting entity must add the new hi-
       entry since the proxy does not have access to the information as
       to how the Contact was determined.

   5.  Added section on normative differences between this document and
       RFC 4244.

   6.  Restructuring of document to be more in line with current IETF
       practices.

   7.  Moved Requirements section into an Appendix.

   8.  Fixed ABNF to remove unintended ordering requirement on hi-index
       that was introduced in attempting to illustrate it was a
       mandatory parameter.

   Changes from barnes-sipcore-4244bis-00 to 01 :

   1.  Clarified "retarget" definition.

   2.  Removed privacy discussion from optionality section - just refer
       to privacy section.

   3.  Removed extraneous text from target-parameter (leftover from sip-
       4244bis).  Changed the terminology from the "reason" to the
       "mechanism" to avoid ambiguity with parameter.

   4.  Various changes to clarify some of the text around privacy.

   5.  Reverted proxy response handling text to previous form - just
       changing the privacy aspects to anonymize, rather than remove.

   6.  Other editorial changes to condense and simplify.

   7.  Moved Privacy examples to Appendix.

   8.  Added forking to Basic call example.

   Changes from barnes-sipcore-4244bis-00 to 01 :

   1.  Clarified "retarget" definition.

   2.  Removed privacy discussion from optionality section - just refer
       to privacy section.

   3.  Removed extraneous text from target-parameter (leftover from sip-
       4244bis).  Changed the terminology from the "reason" to the
       "mechanism" to avoid ambiguity with parameter.

   4.  Various changes to clarify some of the text around privacy.

   5.  Reverted proxy response handling text to previous form - just
       changing the privacy aspects to anonymize, rather than remove.

   6.  Other editorial changes to condense and simplify.

   7.  Moved Privacy examples to Appendix.

   8.  Added forking to Basic call example.

   Changes from barnes-sip-4244bis-00 to barnes-sipcore-4244bis-00:

   1.   Added tags for each type of retargeting including proxy hops,
        etc. - i.e., a tag is defined for each specific mechanism by
        which the new Request-URI is determined.  Note, this is
        extremely helpful in terms of backwards compatibility.

   2.   Fixed all the examples.  Made sure loose routing was used in all
        of them.

   3.   Removed example where a proxy using strict routing is using
        History-Info for avoiding trying same route twice.

   4.   Remove redundant Redirect Server example.

   5.   Index is now mandated to start at "1" instead of recommended.

   6.   Updated 3xx behavior as the entity sending the 3XX response MUST
        add the hi-target attribute to the previous hi-entry to ensure
        that it is appropriately tagged (i.e., it's the only one that
        knows how the contact in the 3xx was determined.)

   7.   Removed lots of ambiguity by making many "MAYs" into "SHOULDs"
        and some "SHOULDs" into "MUSTs".

   8.   Privacy is now recommended to be done by anonymizing entries as
        per RFC 3323 instead of by removing or omitting hi-entry(s).

   9.   Requirement for TLS is now same level as per RFC 3261.

   10.  Clarified behavior for "Privacy" (i.e., that Privacy is for Hi-
        entries, not headers).

   11.  Removed "OPTIONALITY" as specific requirements, since it's
        rather superflous.

   12.  Other editorial changes to remove redundant text/sections.

   Changes from RFC4244 to barnes-sip-4244bis-00:

   1.  Clarified that HI captures both retargeting as well as cases of
       just forwarding a request.

   2.  Added descriptions of the usage of the terms "retarget",
       "forward" and "redirect" to the terminology section.

   3.  Added additional examples for the functionality provided by HI
       for core SIP.

   4.  Added hi-target parameter values to HI header to ABNF and
       protocol description, as well as defining proxy, UAC and UAS
       behavior for the parameter.

   5.  Simplified example call flow in section 4.5.  Moved previous call
       flow to appendix.

   6.  Fixed ABNF per RFC4244 errata "dot" -> "." and added new
       parameter.

14.

16.  References

14.1.

16.1.  Normative References

   [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.

   [RFC3326]  Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
              Header Field for the Session Initiation Protocol (SIP)",
              RFC 3326, December 2002.

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

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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

14.2.

16.2.  Informative References

   [RFC5627]  Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent URIs (GRUUs) in the Session Initiation Protocol
              (SIP)", RFC 5627, October 2009.

   [RFC5630]  Audet, F., "The Use of the SIPS URI Scheme in the Session
              Initiation Protocol (SIP)", RFC 5630, October 2009.

   [RFC3087]  Campbell, B. and R. Sparks, "Control of Service Context
              using SIP Request-URI", RFC 3087, April 2001.

   [RFC4240]  Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network
              Media Services with SIP", RFC 4240, December 2005.

   [RFC3969]  Camarillo, G., "The Internet Assigned Number Authority
              (IANA) Uniform Resource Identifier (URI) Parameter
              Registry for the Session Initiation Protocol (SIP)",
              BCP 99, RFC 3969, December 2004.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, December 2004.

Appendix A.  Request History Requirements

   The following list constitutes a set of requirements for a "Request
   History" capability.

   1.  CAPABILITY-req: The "Request History" capability provides a
       capability to inform proxies and UAs involved in processing a
       request about the history/progress of that request.  Although
       this is inherently provided when the retarget is in response to a
       SIP redirect, it is deemed useful for non-redirect retargeting
       scenarios, as well.

   2.  GENERATION-req: "Request History" information is generated when
       the request is retargeted.

       A.  In some scenarios, it might be possible for more than one
           instance of retargeting to occur within the same Proxy.  A
           proxy MUST also generate Request History information for the
           'internal retargeting'.

       B.  An entity (UA or proxy) retargeting in response to a redirect
           or REFER MUST include any Request History information from
           the redirect/REFER in the new request.

   3.  ISSUER-req: "Request History" information can be generated by a
       UA or proxy.  It can be passed in both requests and responses.

   4.  CONTENT-req: The "Request History" information for each
       occurrence of retargeting shall include the following:

       A.  The new URI or address to which the request is in the process
           of being retargeted,

       B.  The URI or address from which the request was retargeted, and
           wether the retarget URI was an AOR

       C.  The mechanism by which the new URI or address was determined,

       D.  The reason for the Request-URI or address modification,

       E.  Chronological ordering of the Request History information.

   5.  REQUEST-VALIDITY-req: Request History is applicable to requests
       not sent within an early or established dialog (e.g., INVITE,
       REGISTER, MESSAGE, and OPTIONS).

   6.  BACKWARDS-req: Request History information may be passed from the
       generating entity backwards towards the UAC.  This is needed to
       enable services that inform the calling party about the dialog
       establishment attempts.

   7.  FORWARDS-req: Request History information may also be included by
       the generating entity in the request, if it is forwarded onwards.

A.1.  Security Requirements

   The Request History information is being inserted by a network
   element retargeting a Request, resulting in a slightly different
   problem than the basic SIP header problem, thus requiring specific
   consideration.  It is recognized that these security requirements can
   be generalized to a basic requirement of being able to secure
   information that is inserted by proxies.

   The potential security problems include the following:

   1.  A rogue application could insert a bogus Request History History-Info
       entry either by adding an additional entry hi-entry as a result of
       retargeting or entering invalid information.

   2.  A rogue application could re-arrange the Request History
       information to change the nature of the end application or to
       mislead the receiver of the information.

   3.  A rogue application could delete some or all of the Request
       History information.

   Thus, a security solution for "Request History" must meet the
   following requirements:

   1.  SEC-req-1: The entity receiving the Request History must be able
       to determine whether any of the previously added Request History
       content has been altered.

   2.  SEC-req-2: The ordering of the Request History information must
       be preserved at each instance of retargeting.

   3.  SEC-req-3: The entity receiving the information conveyed by the
       Request History must be able to authenticate the entity providing
       the request.

   4.  SEC-req-4: To ensure the confidentiality of the Request History
       information, only entities that process the request SHOULD have
       visibility to the information.

   It should be noted that these security requirements apply to any
   entity making use of the Request History information.

A.2.  Privacy Requirements

   Since the Request-URI that is captured could inadvertently reveal
   information about the originator, there are general privacy
   requirements that MUST be met:

   1.  PRIV-req-1: The entity retargeting the Request must ensure that
       it maintains the network-provided privacy (as described in
       [RFC3323]) associated with the Request as it is retargeted.

   2.  PRIV-req-2: The entity receiving the Request History must
       maintain the privacy associated with the information.  In
       addition, local policy at a proxy may identify privacy
       requirements associated with the Request-URI being captured in
       the Request History information.

   3.  PRIV-req-3: Request History information subject to privacy shall
       not be included in ougoing messages unless it is protected as
       described in [RFC3323].

Appendix B.  Example call flows

   The scenarios in this section provide sample use cases for the
   History-Info
   History-info header field for informational purposes only.  They are
   not intended to be normative.  A basic forking use case is included,
   along with two use cases illustrating the use of the privacy.

B.1.  Sequentially Forking (History-Info in Response)

   This scenario highlights an example where the History-Info in the
   response is useful to an application or user that originated the
   request.

   Alice sends a call to Bob via sip:example.com.  The proxy sip:
   example.com sequentially tries Bob on a SIP UA that has bound a
   contact with the sip:bob@example.com AOR, and then several alternate
   addresses (Office and Home) unsuccessfully before sending a response
   to Alice.  The hi-entry containing the initial contact is the hi-
   entry just prior to the firt entry first hi-entry tagged with an hi-target value
   of "rc".  In this example, the Office and Home are not the same AOR
   as sip:bob@example.com, but rather different AORs that have been
   configured as alternate addresses for Bob in the proxy.  In other
   words, Office and Bob are not bound through SIP Registration with
   Bob's AOR.  This type of arrangement is common for example when a
   "routing" rule to a PSTN number is manually configured in a Proxy.
   These hi-entries are identified by the index contained in the hi-
   target "mp" parameter in the hi-entries.

   This scenario illustrates that by providing the History-Info to
   Alice, the end-user or an application at Alice could make a decision
   on how best to attempt finding Bob without sending multiple requests
   to the same destination.  Upon receipt of the response containing the
   History-Info entries, the Request URIs for the History-Info entries
   tagged with "mp" are extracted.  Those Request-URIs can be compared
   to other URIs (if any) that might be attempted in order to establish
   the session with Bob. Thus, avoiding another INVITE to Bob's home
   phone.  Without this mechanism, Alice might well attempt to reach Bob
   at his office phone, which would then retarget the request to Bob's
   home phone.  When that attempt failed, then Alice might attempt to
   reach Bob directly at his home phone, unknowingly for a third time.

   Alice   example.com            Bob     Office    Home
   |            |                  |        |        |
   | INVITE F1  |                  |        |        |
   |----------->|    INVITE F2     |        |        |
   |            |----------------->|        |        |
   | 100 Trying F3                 |        |        |
   |<-----------|  302 Move Temporarily F4  |        |
   |            |<-----------------|        |        |
   |            |   ACK F5         |        |        |
   |            |----------------->|        |        |
   |            |       INVITE F6           |        |
   |            |-------------------------->|        |
   |            |      180 Ringing F7       |        |
   |            |<--------------------------|        |
   |  180 Ringing F8                        |        |
   |<-----------|   retransmit INVITE       |        |
   |            |-------------------------->|        |
   |            |      ( timeout )          |        |
   |            |             INVITE F9              |
   |            |----------------------------------->|
   |            |           100 Trying F10           |
   |            |<-----------------------------------|
   |            |           486 Busy Here F11        |
   |            |<-----------------------------------|
   |  486 Busy Here F12                              |
   |<-----------|             ACK F13                |
   |            |----------------------------------->|
   |  ACK F14   |                                    |
   |----------->|                                    |

   Message Details

   F1 INVITE alice -> example.com

   INVITE sip:alice@example.com SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->
   F2 INVITE  example.com -> Bob

   INVITE sip:bob@192.0.2.4 SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   Record-Route: <sip:proxy.example.com;lr>
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.4>;index=1.1;rc=1
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->

   F3 100 Trying example.com -> alice

   SIP/2.0 100 Trying
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   Content-Length: 0

   F4 302 Moved Temporarily Bob -> example.com

   SIP/2.0 302 Moved Temporarily
   Via: SIP/2.0/TCP proxy.example.com:5060
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>;tag=3
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   Record-Route: <sip:proxy.example.com;lr>
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.4>;index=1.1;rc=1
   Contact: <sip:office@example.com>;mp=1
   Content-Length: 0
   F5 ACK 192.0.2.4 -> Bob

   ACK sip:home@example.com SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Call-Id: 12345600@example.com
   CSeq: 1 ACK
   Content-Length: 0

   F6 INVITE example.com -> office

   INVITE sip:office@192.0.2.3.com SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=2
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   Record-Route: <sip:proxy.example.com;lr>
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.4?Reason=SIP;cause=302>;\ <sip:bob@192.0.2.4?Reason=SIP%3Bcause%3D302>;\
                 index=1.1;rc=1
   History-Info: <sip:office@example.com>;index=1.2;mp=1
   History-Info: <sip:office@192.0.2.5>;index=1.2.1
   CSeq: 1 INVITE
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->
   F7 180 Ringing office -> example.com

   SIP/2.0 180 Ringing
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=2
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>;tag=5
   Supported: histinfo
   Call-ID: 12345600@example.com
   Record-Route: <sip:proxy.example.com;lr>
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.4?Reason=SIP;cause=302>;\ <sip:bob@192.0.2.4?Reason=SIP%3Bcause%3D302>;\
                 index=1.1;rc=1
   History-Info: <sip:office@example.com>;index=1.2;mp=1
   History-Info: <sip:office@192.0.2.5>;index=1.2.1
   CSeq: 1 INVITE
   Content-Length: 0

   F8 180 Ringing example.com -> alice

   SIP/2.0 180 Ringing
   Via: SIP/2.0/TCP example.com:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.4?Reason=SIP;cause=302>;\ <sip:bob@192.0.2.4?Reason=SIP%3Bcause%3D302>;\
                 index=1.1;rc=1
   History-Info: <sip:office@example.com>;index=1.2;mp=1
   History-Info: <sip:office@192.0.2.5>;index=1.2.1
   CSeq: 1 INVITE
   Content-Length: 0
   F9 INVITE example.com -> home

   INVITE sip:home@192.0.2.6 SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=3
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   Record-Route: <sip:proxy.example.com;lr>
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.4?Reason=SIP;cause=302>;\ <sip:bob@192.0.2.4?Reason=SIP%3Bcause%3D302>;\
                 index=1.1;rc=1
   History-Info: <sip:office@example.com>;index=1.2;mp=1
   History-Info: <sip:office@192.0.2.5?Reason=SIP;cause=480>;\ <sip:office@192.0.2.5?Reason=SIP%3Bcause%3D408>;\
                 index=1.2.1>;index=1.2.1
   History-Info: <sip:home@example.com>;index=1.3;mp=1
   History-Info: <sip:home@192.0.2.6>;index=1.3.1
   CSeq: 1 INVITE
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->

   F10 100 Trying home -> example.com

   SIP/2.0 100 Trying
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=3
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   Content-Length: 0
   F11 486 Busy Here home -> example.com

   SIP/2.0  486 Busy Here
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=3
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Call-Id: 12345600@example.com
   Record-Route: <sip:proxy.example.com;lr>
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.4?Reason=SIP;cause=302>;\ <sip:bob@192.0.2.4?Reason=SIP%3Bcause%3D302>;\
                 index=1.1;rc=1
   History-Info: <sip:office@example.com>;index=1.2;mp=1
   History-Info: <sip:office@192.0.2.5?Reason=SIP;cause=480>;\ <sip:office@192.0.2.5?Reason=SIP%3Bcause%3D408>;\
                 index=1.2.1>;index=1.2.1
   History-Info: <sip:home@example.com>;index=1.3;mp=1
   History-Info: <sip:home@192.0.2.6>;index=1.3.1
   CSeq: 1 INVITE
   Content-Length: 0

   F12 486 Busy Here example.com -> alice

   SIP/2.0  486 Busy Here
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Call-Id: 12345600@example.com
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.4?Reason=SIP;cause=302>;\ <sip:bob@192.0.2.4?Reason=SIP%3Bcause%3D302>;\
                 index=1.1;rc=1
   History-Info: <sip:office@example.com>;index=1.2;mp=1
   History-Info: <sip:office@192.0.2.5?Reason=SIP;cause=480>;\ <sip:office@192.0.2.5?Reason=SIP%3Bcause%3D408>;\
                 index=1.2.1>;index=1.2.1
   History-Info: <sip:home@example.com>;index=1.3;mp=1
   History-Info: <sip:home@192.0.2.6>;index=1.3.1
   CSeq: 1 INVITE
   Content-Length: 0
   F13 ACK example.com -> home

   ACK sip:home@example.com SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Call-Id: 12345600@example.com
   CSeq: 1 ACK
   Content-Length: 0

   F14 ACK alice -> example.com

   ACK sip:bob@example.com SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Call-Id: 12345600@example.com
   Route: <sip:proxy.example.com;lr>
   CSeq: 1 ACK
   Content-Length: 0

B.2.  History-Info with Privacy Header Field

   This example provides a basic call scenario without forking.  Alice
   has indicated that she wants Privacy associated with her the History-Info
   entry and
   header field entries.  In addition, sip:biloxi.example.com adds
   Privacy headers header fields indicating that the History-Info History-info header field
   information is anonymized outside the biloxi.example.com domain.
   Note, that if the atlanta.example.com proxy had added privacy headers header
   fields to all its hi-entries, then all the hi-entries in the response
   would be anonymous.

  Alice   atlanta.example.com  biloxi.example.com   Bob
  |                |                |                |
  |   INVITE sip:bob@biloxi.example.com;p=x          |
  |--------------->|                |                |
  | Supported: histinfo             |                |
  | Privacy: History                |                |
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
  |                |                |                |
  |                |   INVITE sip:bob@biloxi.example.com;p=x
  |                |--------------->|                |
  | History-Info: <sip:anonymous@anonymous.invalid>;index=1
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
  |                |                |                |
  |                |                | INVITE sip:bob@192.0.2.3
  |                |                |--------------->|
  | History-Info: <sip:anonymous@anonymous.invalid>;index=1
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
  | History-Info: <sip:bob@192.0.2.3?Privacy=history>;index=1.1.1;rc=1.1
  |                |                |                |
  |                |                |     200        |
  |                |                |<---------------|
  | History-Info: <sip:anonymous@anonymous.invalid>;index=1
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
  | History-Info: <sip:bob@192.0.2.3?Privacy=history>;index=1.1.1;rc=1.1
  |                |                |                |
  |                |     200        |                |
  |                |<---------------|                |
  | History-Info: <sip:anonymous@anonymous.invalid>;index=1
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
  | History-Info: <sip:anonymous@anonymous.invalid>;index=1.1.1;rc=1.1
  |                |                |                |
  |     200        |                |                |
  |<---------------|                |                |
  | History-Info: <sip:anonymous@anonymous.invalid>;index=1
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
  | History-Info: <sip:anonymous@anonymous.invalid>;index=1.1.1;rc=1.1
  |                |                |                |
  |     ACK        |                |                |
  |--------------->|    ACK         |                |
  |                |--------------->|     ACK        |
  |                |                |--------------->|

               Figure 2: Example with Privacy Header Fields

B.3.  Privacy Header for a Specific History-Info Entry

   This example provides a basic call scenario similar to Appendix B.2,
   however, due to local policy at sip:biloxi.example.com, only the
   final hi-entry in the History-Info, which is Bob's local URI,
   contains a privacy header field with a priv-value of "history", thus
   providing Alice with some information about the history of the
   request, but anonymizing Bob's local URI.

  Alice   atlanta.example.com  biloxi.example.com   Bob
  |                |                |                |
  |   INVITE sip:bob@biloxi.example.com;p=x          |
  |--------------->|                |                |
  | Supported: histinfo             |                |
  |                |                |                |
  |                |   INVITE sip:bob@biloxi.example.com;p=x
  |                |--------------->|                |
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
  |                |                |                |
  |                |                |   INVITE sip:bob@192.0.2.3
  |                |                |--------------->|
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
  | History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc=1.1
  |                |                |                |
  |                |                |     200        |
  |                |                |<---------------|
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
  | History-Info: <sip:bob@192.0.2.3?Privacy=history>;index=1.1.1;rc=1.1
  |                |                |                |
  |                |     200        |                |
  |                |<---------------|                |
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
  | History-Info: <sip:anonymous@anonymous.invalid>;index=1.1.1;rc=1.1
  |                |                |                |
  |     200        |                |                |
  |<---------------|                |                |
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
  | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
  | History-Info: <sip:anonymous@anynymous.invalid>;index=1.1.1;rc=1.1
  |                |                |                |
  |     ACK        |                |                |
  |--------------->|    ACK         |                |
  |                |--------------->|     ACK        |
  |                |                |--------------->|

       Figure 3: Example with Privacy Header Field for Specific URI

Authors' Addresses

   Mary Barnes
   Polycom
   TX
   US

   Email: mary.ietf.barnes@gmail.com

   Francois Audet
   Skype

   Email: francois.audet@skype.net

   Shida Schubert
   NTT

   Email: shida@agnada.com

   Hans Erik van Elburg
   Detecon International Gmbh
   Oberkasseler str. 2
   Bonn,
   Germany

   Email: ietf.hanserik@gmail.com

   Christer Holmberg
   Ericsson
   Hirsalantie 11, Jorvas
   Finland

   Email: christer.holmberg@ericsson.com