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Versions: (draft-barnes-sipcore-rfc4244bis) 00 01 02 03 04 05 06 07 08 RFC 7044

Network Working Group                                          M. Barnes
Internet-Draft                                                   Polycom
Obsoletes: 4244 (if approved)                                   F. Audet
Intended status: Standards Track                                   Skype
Expires: April 3, 2012                                       S. Schubert
                                                                     NTT
                                                           J. van Elburg
                                              Detecon International Gmbh
                                                             C. Holmberg
                                                                Ericsson
                                                                Oct 2011


   An Extension to the Session Initiation Protocol (SIP) for Request
                          History Information
                  draft-ietf-sipcore-rfc4244bis-06.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 SIP request arrives at a specific
   application or user.  This document defines an optional SIP header
   field, History-Info, for capturing the history information in
   requests.  The document also defines SIP header field parameters for
   the History-Info and Contact header fields to tag the method by which
   the target of a request is determined.  In addition, this
   specification defines a value for the Privacy header field specific
   to the History-Info header field.  This document obsoletes RFC 4244.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 3, 2012.




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Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  5
   3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  History-Info Header Field Protocol Structure . . . . . . . . .  8
     5.1.  History-Info Header Field Example Scenario . . . . . . . . 10
   6.  User Agent Handling of the History-Info Header Field . . . . . 13
     6.1.  User Agent Client (UAC) Behavior . . . . . . . . . . . . . 13
     6.2.  User Agent Server (UAS) Behavior . . . . . . . . . . . . . 13
   7.  Proxy/Intermediary Handling of History-Info Header Fields  . . 13
   8.  Redirect Server Handling of History-Info Header Fields . . . . 14
   9.  Handling of History-Info Header Fields in Requests and
       Responses  . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Receiving a Request  . . . . . . . . . . . . . . . . . . . 14
     9.2.  Sending a Request with History-Info  . . . . . . . . . . . 15
     9.3.  Receiving a Response with History-Info or Requet
           Timeouts . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.4.  Sending History-Info in Responses  . . . . . . . . . . . . 16
   10. Processing the History-Info Header Field . . . . . . . . . . . 16
     10.1. Privacy in the History-Info Header Field . . . . . . . . . 17
       10.1.1.  Indicating Privacy  . . . . . . . . . . . . . . . . . 17
       10.1.2.  Applying Privacy  . . . . . . . . . . . . . . . . . . 18
     10.2. Reason in the History-info Header Field  . . . . . . . . . 19
     10.3. Indexing in the History-Info Header Field  . . . . . . . . 19
     10.4. Mechanism for Target Determination in the History-Info
           Header Field . . . . . . . . . . . . . . . . . . . . . . . 21
   11. Application Considerations . . . . . . . . . . . . . . . . . . 22
   12. Application Specific Usage . . . . . . . . . . . . . . . . . . 24
     12.1. PBX Voicemail  . . . . . . . . . . . . . . . . . . . . . . 24
     12.2. Consumer Voicemail . . . . . . . . . . . . . . . . . . . . 24
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25
     14.1. Registration of New SIP History-Info Header Field  . . . . 26
     14.2. Registration of "history" for SIP Privacy Header Field . . 26
     14.3. Registration of Header Field Parameters  . . . . . . . . . 27
   15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
   16. Changes from RFC 4244  . . . . . . . . . . . . . . . . . . . . 28
     16.1. Backwards compatibility  . . . . . . . . . . . . . . . . . 29
   17. Changes since last Version . . . . . . . . . . . . . . . . . . 30
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     18.1. Normative References . . . . . . . . . . . . . . . . . . . 37
     18.2. Informative References . . . . . . . . . . . . . . . . . . 37
   Appendix A.  Request History Requirements  . . . . . . . . . . . . 38
     A.1.  Security Requirements  . . . . . . . . . . . . . . . . . . 39
     A.2.  Privacy Requirements . . . . . . . . . . . . . . . . . . . 40
   Appendix B.  Example call flows  . . . . . . . . . . . . . . . . . 40



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     B.1.  PBX Voicemail call floww . . . . . . . . . . . . . . . . . 40
     B.2.  Consumer Voicemail example call flow . . . . . . . . . . . 45
     B.3.  Sequentially Forking (History-Info in Response)  . . . . . 49
     B.4.  History-Info with Privacy Header Field . . . . . . . . . . 56
     B.5.  Privacy for a Specific History-Info Entry  . . . . . . . . 60
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 64













































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1.  Introduction

   Many services that SIP is anticipated to support require the ability
   to determine why and how a SIP request 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" 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 SIP
   requests to be routed to chosen applications, there is a need for a
   standard mechanism within SIP for communicating the retargeting
   history of the requests.  This "request history" information allows
   the receiving application to determine hints about how and why the
   SIP request arrived at the application/user.

   This document defines a SIP 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 specification defines a value for the
   Privacy header field specific to the History-Info header.

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



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   used consistent with the terminology in [RFC3261].

   The terms "terget user" is used in this specification as the human
   user associated with particular AoR or AoRs (in case the human user
   has multiple alias).

   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 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 request is retargeted to process the
   request in a locally defined, application-specific manner.  This
   document defines a mechanism for transporting the request history.
   Application-specific behavior is outside the scope of this
   specification.

   Current network applications provide the ability for elements
   involved with the request to obtain additional information relating
   to how and why the 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:



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   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 REGISTRAR or a "home
      proxy" (a proxy serving as the terminal point for routing an
      address-of-record) 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

   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.
   The solution is to capture the Request-URIs as a request is
   retargeted, in a SIP 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 header field is added to a Request when a new
   request is created by a UAC or 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 also propagates any
   History-info header field from the initial request in the new
   request.  The ABNF and detailed description of 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 three new SIP header field
   parameters, "rc", "mp" and "np", 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 10.4.

   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-



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   entry as described above.  Further detail is provided in
   Section 10.1.


5.  History-Info Header Field Protocol Structure

   The History-info header field defined in this specification defines
   the usage in out-of-dialog requests or initial requests for a 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 10.3), including the index and sending the messages
      using a secure transport, 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
      History-info header field value (hi-entry) was determined.  This
      parameter contains either an "rc", "mp" or "np" header field
      parameter, which is interpreted as follows:

         "rc": The hi-targeted-to-URI represents a change in Request-URI
         while the target user remains the same.  This occurs for
         example when user has multiple AoRs as an alias.  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





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         "mp": The hi-targeted-to-URI represents a user other than the
         target user associated with the Request-URI in the incoming
         request that was retargeted.  This occurs when a request is
         statically or dynamically retargeted to another user
         represented by an AoR unassociated with the AoR of the original
         target user.  The "mp" header field parameter contains the
         value which 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.

         "np": The hi-targeted-to-URI represents that there was no
         change in Request-URI.  This would apply for example when a
         proxy merely forwards a request to a next hop proxy and loose
         routing is used.

   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 = addr-spec / name-addr

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

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

   hi-index = "index" EQUAL index-val

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

   rc-param = "rc" EQUAL index-val

   mp-param = "mp" EQUAL index-val

   np-param = "np" EQUAL index-val

   hi-extension = generic-param


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




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   This document also extends the "contact-params" for the Contact
   header field as defined in [RFC3261] with the "rc", "mp" and "np"
   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/or a Privacy header field,
   which are both included in the "headers" component of 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] included in the hi-targeted-to-uri.  A reason is
      included in the hi-targeted-to-uri of an hi-entry to reflect
      information received in a response to the request sent to that
      URI.

   o  Privacy: An optional parameter for History-Info, reflected in the
      History-Info header field values by including the Privacy Header
      [RFC3323] included 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
      included 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 included
   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].

   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:



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   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" header field parameter contains the index of the hi-entry
   containing the lost target information - i.e., the
   sip:bob@biloxi.example.com 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.
   Refer to Section 5.1 for the proper formatting.  Additional detailed
   scenarios are available in Appendix B.

      Note: This example uses loose routing procedures.



























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   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>;np=1;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>;np=1;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>;np=1;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>;np=1;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>;np=1;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>;np=1;index=1.1
   | History-Info: <sip:bob@192.0.2.3;index=1.1.1;rc=1.1
   |                |                |                |          |
   |     ACK        |                |                |          |
   |--------------->|    ACK         |                |          |
   |                |--------------->|     ACK        |          |
   |                |                |--------------->|          |




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                           Figure 1: Basic Call


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

   A B2BUA MAY follow the behavior of a SIP intermediary as an
   alternative to following the behavior of a UAS per Section 6.2 and a
   UAC per Section 6.1.  In behaving as an intermediary, a B2BUA carries
   forward hi-entries received in requests at the UAS to requests being
   forwarded by the UAC, as well as carrying forward hi-entries in
   responses received at the UAC to the responses forwarded by the UAS,
   subject to privacy considerations per Section 10.1.

6.1.  User Agent Client (UAC) Behavior

   The UAC MUST include the "histinfo" option tag in the Supported
   header field in any out-of-dialog requests or initial requests for a
   dialog for which the UAC would like the History-info header field in
   the response.  When issuing a request, the UAC MUST follow the
   procedures in Section 9.2.  In the case of an initial request, except
   where the UAC is part of a B2BUA, there is no cache of hi- entries
   with which to populate the History-info header field and the hi-index
   is set to 1 per Section 10.3.  When receiving a response the UAC MUST
   follow the procedures in Section 9.3.

6.2.  User Agent Server (UAS) Behavior

   When receiving a request, a UAS MUST follow the procedures defined in
   Section 9.2.  When sending a response other than a 3xx response, a
   UAS MUST follows the procedures as defined in Section 9.4.  When
   sending a 3xx response, the UAS MUST follow the procedures defined
   for a redirect server per Section 8.  An application at the UAS can
   make use of the cached hi-entries as described in Section 11.


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

   This section describes the procedures for proxies and other SIP
   intermediaries for the handling of the History-Info header fields for
   each of the following scenarios:

   Receiving a Request:  An intermediary MUST follow the procedures in
      Section 9.1 for the handling of hi-entries in incoming SIP
      requests.







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   Sending a Request:  For each outgoing request relating to a target in
      the target set, the intermediary MUST follow the procedures of
      Section 9.2.

   Receiving a Response ot Timeout:  An intermediary MUST follow the
      procedures of Section 9.3 when a SIP response is received or a
      request times out.

   Sending a Response:  An intermediary MUST follow the procedures of
      Section 9.4 for the handling of the hi-entries when sending a SIP
      response.

   In some cases, an intermediary may retarget a request more than once
   before forwarding - i.e., a request is retargeted to a SIP entity
   that is "internal" to the intermediary before the same intermediary
   retargets the request to an external target .  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 the same AOR.  In this case, the intermediary MUST
   add a hi-entry for (each of) the internal target(s) per the
   procedures in Section 9.2.  The intermediary MAY include 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.3.  Figure 1 provides an example of internal retargeting.


8.  Redirect Server Handling of History-Info Header Fields

   A redirect server MUST follow the procedures in Section 9.1 when it
   receives a SIP Request.  A redirect server MUST follow the procedures
   in Section 9.4 when it sends a SIP Response.  When generating the
   Contact header field in a 3xx response, the redirect server MUST add
   the appropriate "mp", "np" or "rc" header field parameter to each
   Contact header field as described in Section 10.4, if applicable.


9.  Handling of History-Info Header Fields in Requests and Responses

   This section describes the procedures for SIP entities for the
   handling of the History-Info header field in SIP requests and
   responses.

9.1.  Receiving a Request

   When receiving a request, a SIP entity MUST create a cache containing
   the hi-entries associated with the request.  The hi-entries MUST be
   added to the cache in the order in which they were received in the
   request.



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   If the Request-URI of the incoming request does not match the hi-
   targeted-to-uri in the last hi-entry (i.e., the previous SIP entity
   that sent the request did not include a History-Info header field),
   the SIP entity MUST add a hi-entry to end of the cache, on behalf of
   the previous SIP entity, as follows:

      The SIP entity MUST set the hi-targeted-to-uri to the value of the
      Request-URI in the incoming request.  If the Request-URI is a Tel-
      URI, it SHOULD be transformed into a SIP URI per section 19.1.6 of
      [RFC3261] before being added as a hi-targted-to-uri.

      If privacy is required, the SIP entity MUST follow the procedures
      of Section 10.1.

      The SIP entity MUST set the hi-index parameter as described in
      Section 10.3.

      The SIP entity MUST NOT include an "rc", "mp" or "np" header field
      parameter.

9.2.  Sending a Request with History-Info

   When sending a request, a SIP entity MUST include all cached hi-
   entries in the request.  In addition, the SIP entity MUST add a new
   hi-entry to the outgoing request, but the SIP entity MUST NOT add the
   hi-entry to the outgoing request(but not the cache) populating the
   new hi-entry to the cache at this time.  The new hi-entry is
   populated as follows:

   hi-targeted-to-uri:  The hi-targeted-to-uri MUST be set to the value
      of the Request-URI of the current (outgoing) request.

   privacy:  If privacy is required, the procedures of Section 10.1 MUST
      be followed.

   hi-index:  The SIP entity MUST include an hi-index for the hi-entry
      as described in Section 10.3.

   rc/mp:  The SIP entity MUST include an "rc", "mp" or "np" header
      field parameter in the hi-entry, if applicable, per the procedures
      in Section 10.4.

9.3.  Receiving a Response with History-Info or Requet Timeouts

   When a SIP entity receives a non-100 response or a request times out,
   the SIP entity performs the following steps:





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   Step 1:  Add hi-entry to cache

      The SIP entity MUST add the hi-entry that was added to the request
      that received the non-100 response or timed out to the cache, if
      it was not already cached.  The hi-entry MUST be added to the
      cache in ascending order as indicated by the values in the hi-
      index parameters of the hi-entries (e.g., 1.2.1 comes after 1.2
      but before 1.2.2 or 1.3).

   Step 2:  Add Reason header field

      The SIP entity adds one or more Reason header fields to the hi-
      targeted-to-uri in the (newly) cached hi-entry reflecting the SIP
      response code in the non-100 response, per the procedures of
      Section 10.2.

   Step 3:  Add additional hi-entries

      The SIP entity MUST also add to the cache any hi-entries received
      in the response that are not already in the cache.  This situation
      can occur when the entity that generated the non-100 response
      retargeted the request before generating the response.  As per
      Step 1, the hi-entries MUST be added to the cache in ascending
      order as indicated by the values in the hi-index parameters of the
      hi-entries

   It is important to note that the cache does not contain hi-entries
   for requests that have not yet received a non-100 response, so there
   can be gaps in indices (e.g., 1.2 and 1.4 could but present but not
   1.3).

9.4.  Sending History-Info in Responses

   When sending a response other than a 100, a SIP entity MUST include
   all the cached hi-entries in the response, subject to the privacy
   consideration in Section 10.1.2 with the following exception: If the
   received request contained no hi-entries and there is no "histinfo"
   option tag in the Supported header field, the SIP entity MUST NOT
   include hi-entries in the response.


10.  Processing the History-Info Header Field

   The following sections describe the procedures for processing the
   History-Info header field.  These procedures are applicable to SIP
   entities such as Proxies/Intermediaries, Redirect Servers or User
   Agents.




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10.1.  Privacy in the History-Info Header Field

   The privacy requirements for this document are described in
   Appendix A.2.  Section 10.1.1 describes the insertion of the Privacy
   header field defined in [RFC3323] to indicate the privacy to be
   applied to the History-Info header field entries.  Section 10.1.2
   describes how to apply privacy to a request or response that is being
   forwarded, based on the presence of the Privacy header field.

10.1.1.  Indicating Privacy

   As with other SIP headers described in [RFC3323], the hi-targeted-to-
   uris in the History-info header field can inadvertently reveal
   information about the initiator of the request.  Thus, the UAC needs
   a mechanism to indicate that the hi-targeted-to-uris in the hi-
   entries need to be privacy protected.  The Privacy header field is
   used by the UAC to indicate that privacy is to be applied to all the
   hi-entries in the request as follows:

   o  If the UAC is including a Privacy header field with a priv-value
      of "header" in the request, then the UAC SHOULD NOT include a
      priv-value of "history" in the the Privacy header field in the
      Request.

   o  If the UAC is including any priv-values other than "header" in the
      Privacy header field, then the UAC MUST also include a priv-value
      of "history" in the Privacy header field in the Request.

   o  If the UAC is not including any priv-values in the Privacy header
      field in the request, then the UAC MUST add a Privacy header
      field, with a priv-value of "history", to the request.  The UAC
      MUST NOT include a priv-value of "critical" in the Privacy header
      field in the Request in this case.

   In addition, the History-info header field can reveal general routing
   and diverting information within an intermediary, which the
   intermediary wants to privacy protect.  In this case, the
   intermediary MUST set a Privacy header field to a priv-value of
   "history" and include the Privacy header field in the hi-targeted-to-
   uri, for each new hi-entry added by the intermediary, as the request
   is retargeted within the domain for which the SIP entity is
   responsible.  Note, that the hi-entries that are added to a request
   from the cache are excluded in this case since the appropriate
   privacy was set for those hi-entries when they were originally added
   to a request.  The intermediary MUST NOT include any other priv-
   values in this Privacy header field.  Note that the priv-value in the
   Privacy header for the incoming request does not necessarily
   influence whether the intermediary includes a Privacy header field in



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   the hi-entries.  For example, even if the Privacy header for the
   incoming request contained a priv-value of "none", the Proxy can
   still set a priv-value of "history" in the Privacy header field
   included in the hi-targeted-to-uri.

   Finally, the terminator of the request may not want to 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" in
   the hi-targeted-to-uri in the last hi-entry, in the response.  As
   noted above, the terminator of the request MUST NOT use any other
   priv-values in the Privacy header field included in the hi-entry.

10.1.2.  Applying Privacy

   When a request is retargeted to a URI associated with a domain for
   which the SIP intermediary is not responsible or a response is
   forwarded to a domain for which the SIP intermediary is not
   responsible, a Privacy Service at the boundary of the domain applies
   the appropriate privacy based on the value of the Privacy header
   field in the message header or in the "headers" component of the hi-
   targeted-to-uri in the individual hi-entries.

   If there is a Privacy header field in the message header of a request
   or response, with a priv-value of "header" or "history", then all the
   hi-targeted-to-uris in the hi-entries, associated with the domain for
   which a SIP intermediary is responsible, are anonymized by the
   Privacy Service.  The Privacy Service MUST change any hi-targeted-to-
   uris in the hi-entries that have not been anonymized to anonymous
   URIs containing a domain of anonymous.invalid (e.g.,
   anonymous@anonymous.invalid).  If there is a Privacy header field in
   the "headers" component of the hi-targeted-to-uri in the hi-entry,
   then the Privacy header field value MUST be removed from the hi-
   entry.  Once all the appropriate hi-entries have been anonymized, the
   Privacy Service MUST remove the priv-value of "history" from the
   Privacy header field in the message header of the request or
   response.  If there are no remaining priv-values in the Privacy
   header field, the Privacy Service MUST remove the Privacy header
   field from the request or response per [RFC3323].

   If there is not a Privacy header field in the message header of the
   request or response that is being forwarded, but there is a Privacy
   header field with a priv-value of "history" in the "headers"
   component in any of the hi-targeted-uris in the hi-entries associated
   with the domain for which a SIP intermediary is responsible, then the
   Privacy Service MUST anonymize those hi-targeted-to-uris.  The
   Privacy Service MUST populate each of the hi-targeted-to-uris with an
   anonymous URI with a domain of anonymous.invalid (e.g.,
   anonymous@anonymous.invalid).  Any other priv-values in the Privacy



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   header field in the "headers" component of the hi-targeted-to-uris in
   the hi-entries MUST be ignored.  In any case, the Privacy Service
   MUST remove the Privacy header field from the "headers" compenent of
   the hi-targeted-to-uris in the hi-entries prior to forwarding.

10.2.  Reason in the History-info Header Field

   A Reason header field is added to the "headers" component in an hi-
   targeted-to-uri when the hi-entry is added to the cache based upon
   the receipt of a non-100 or non-2xx SIP response, as described in
   Section 9.3.  If the Reason header field is being added due to
   receipt of an explicit SIP response and the response contains any
   Reason header fields (see [RFC3326]), then the SIP entity MUST
   include the Reason header fields in the "headers" component of the
   hi-targeted-to-uri in the last hi-entry added to the cache, unless
   the hi-targeted-to-uri is a Tel-URI.  If the SIP response does not
   contain a Reason header field, the SIP entity MUST include a Reason
   header field, containing the SIP Response Code, in the "headers"
   component of the hi-targeted-to-uri in the last hi-entry added to the
   cache, unless the hi-targeted-to-uri is a Tel-URI.

   If a request has timed out (instead of being explicitly rejected),
   the SIP entity MUST include a Reason header field, containing a SIP
   error response code of 408 "Request Timeout".

   A SIP entity MAY also include a Reason header field in the "headers"
   component of an hi-targeted-to-uri containing the URI of a request
   that was retargeted as a result of internal retargeting.

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

10.3.  Indexing in the History-Info Header Field

   In order to maintain ordering and accurately reflect the retargeting
   of the request, the SIP entity MUST add a hi-index to each hi-entry.
   Per the syntax in Section 5, the hi-index consists of a series of
   digits separated by dots (e.g., 1.1.2).  Each dot reflects a SIP
   forwarding hop.  The digit following each dot reflects the order in
   which a request was retargeted at the hop.  The highest digit at each
   hop reflects the number of entities to which the request has been
   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.



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   In the case that a SIP entity (intermediary or UAS) adds an hi-entry
   on behalf of the previous hop, the hi-index MUST be set to 1.  For
   each forward hop (i.e., each new level of indexing), the 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 hi-index are summarized as follows:

   1.  Forwarding a request without changing the target: In the case of
       a request that is being forwarded without changing the target,
       the hi-index reflects the increasing length of the branch.  In
       this case, the SIP entity MUST read the value from the History-
       info header field in the received request and MUST add another
       level of indexing by appending the dot delimiter followed by an
       initial value of 1 for the new level.  For example, if the hi-
       index in the last History-info header field in the received
       request is 1.1, a proxy would add a hi-entry with an hi-index of
       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 SIP
       entity MUST calculate the 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 SIP entity,
       the SIP entity MUST calculate and add the hi-index for each new
       branch by incrementing the rightmost value from the hi-index in
       the last hi-entry.  Per the example above, the hi-index in the
       next request forwarded by this same 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 SIP entity MUST calculate
       the hi-index calculated per rule 3.  That is, the rightmost value
       of the hi-index MUST be incremented (i.e., a new branch is
       created).  For example, if the hi-index in the History-Info
       header field of the sent request is 1.2 and the response to the
       request is a 302, then the hi-index in the History-Info header
       field for the new 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 SIP entity MUST 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



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       for an example).  Note, that in the case of parallel forking,
       only the hi-entry corresponding to the fork is included in the
       request.

   6.  Missing entity: If the request clearly has a gap in the hi-entry
       (e.g. by evaluating via header to the existing request history to
       see if it traversed a domain which doesn't exist in History-Info
       header field.), the entity adding an hi-entry MUST add an index a
       digit of "0" to the current index prior to adding appropriate
       index for the action to be taken.  If the index of the last hi-
       entry in the request received was 1.1.2 and there was a missing
       hi-entry and the request was being forwarded to the next hop, the
       resulting index will be 1.1.2.0.1.

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

   This specification defines two header field parameters, "rc", "mp"
   and "np", indicating two 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 hi-target-param, in the History-info header field, as
   the targets are added to the target set per the procedures in section
   16.5 of [RFC3261] or per section 8.1.3.4 [RFC3261] in the case of
   retargeting to a contact URI received in a 3xx response.  In the
   latter case, the specific header field parameter in the Contact
   header field becomes the header field parameter that is used in the
   hi-entry when the request is retargeted.  If the Contact header field
   does not contain an "rc", "mp" or "np" header field parameter, then
   the SIP entity MUST NOT include an "rc", "mp" or "np" header field
   parameter in the hi-target-param in the hi-entry when the request is
   retargeted to a contact URI received in a 3xx response..

   The SIP entity (intermediary or redirect server) determines the
   specific header field parameter ("rc", "mp" or "np") to be used based
   on the following criteria:

   o  "rc": The Request-URI has changed while retaining the target user
      associated with the original Request-URI prior to retargeting.

   o  "mp": The target was determined based on a mapping to a user other
      than the target user associated with the Request-URI being
      retargeted.





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   o  "np": The target hasn't changed and associated Request-URI
      remained the same.

   Note that there are two scenarios by which the "mp" header field
   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 or descendant of an
      earlier sibling of the hi-entry's index, that of the downstream
      request which received the 3xx response.


11.  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 History-Info header field parameters, the
   SIP entity that processes the hi-entries MUST evaluate the hi-
   entries.  The SIP entity MUST determine if there are gaps in the
   indices.  Gaps are possible if the request is forwarded through
   intermediaries that do not support the History-info header field and
   are reflected by the existence of multiple hi-entries with an digit
   of "0" e.g. "1.1.0.1".  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 9.4, in which
   case the gap will not be visible as the branch responsible for the
   gap wasn't on the path of the request received.  Thus, if gaps are
   detected, the SIP entity MUST NOT treat this as an error, but SHOULD
   indicate to any applications that there are gaps.  The interpretation
   of the information in the 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 describes some categories of information that
   applications 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.  Hi-entry with the index that matches the value of the "rc" header
       field parameter in the last hi-entry with a "rc" header field
       parameter in the Request received by a UAS - i.e., the last AOR



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       that was retargeted to a contact based on an AOR-to-contact
       binding.

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

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

   5.  Hi-entry with the index that matches the value of "mp" header
       field parameter in the first hi-entry with an "mp" header field
       parameter in the Request received by a UAS.  Note, this would be
       the original mapped URI if all entities supported the History-
       info header field.  However, there is no guarantee that all
       entities will support History-Info, thus the hi-entry that
       matches the value of the "mp" header field parameter of the first
       hi-entry with an "mp" header field 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 hi-entry which index matches the value of the "mp"
   header field parameter of the first hi-entry with an "mp" header
   field parameter, may also display other agents, reflected by other
   hi-entries prior to entries with hi-target value of "rc" header field
   parameter, 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.).



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   Since support for History-info header field is optional, a service
   MUST define default behavior for requests and responses not
   containing History-Info header fields.  For example, an entity may
   receive an incomplete set of hi-entries or hi-entries which are not
   tagged appropriately with an hi-target-param.  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.


12.  Application Specific Usage

12.1.  PBX Voicemail

   A voicemail system typically requires the original called party
   information to determine the appropriate mailbox so an appropriate
   greeting can be provided and the appropriate party notified of the
   message.

   The original target is determined by finding the first hi-entry
   tagged with "rc" and using the hi-entry referenced by the index of
   "rc" header field parameter as the target for determining the
   appropriate mailbox.  This hi-entry is used to populate the "target"
   URI parameter as defined in [RFC5246] The VMS can look at the last
   hi-entry and find the target of the mailbox by looking at the URI
   entry in the "target" URI parameter in the hi-entry.  For example
   call flow please refer to the Appendix B.1.

   This example usage does not work properly in the presence of
   forwarding that takes place before the call reaches the company in
   that case not the first hi-entry with an rc value, but the first hi-
   entry with an rc value following an mp entry needs to be picked.

12.2.  Consumer Voicemail

   The voicemail system in these environment typically requires the last
   called party information to determine the appropriate mailbox so an
   appropriate greeting can be provided and the appropriate party
   notified of the message.

   The last target is determined by finding the hi-entry referenced by
   the index of last hi-entry tagged with "rc" for determining the
   appropriate mailbox.  This hi-entry is used to populate the "target"
   URI parameter as defined in [RFC5246].  The VMS can look at the last



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   hi-entry and find the target of the mailbox by looking for the
   "target" URI parameter in the hi-entry.  For example call flow please
   refer to the Appendix B.2.


13.  Security Considerations

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

   This document defines a header field for SIP.  The use of the
   Transport Layer Security (TLS) protocol [RFC5246] as a mechanism to
   ensure the overall confidentiality of the History-Info header fields
   (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 header
   fields are no less, nor no more, secure than other SIP header fields,
   which generally have even more impact on the subsequent processing of
   SIP sessions than the History-info header 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.

   In terms of ensuring the privacy of hi-entries, the same security
   considerations as those described in [RFC3323] apply.  Namely if the
   entity requesting privacy wants to ensure privacy is applied to the
   hi-entries, a Privacy Service that supports both [RFC3323] and this
   specification is REQUIRED in the entity's domain, so that the privacy
   can be applied, as described in Section 10.1.2, when a request or
   response leaves the domain.


14.  IANA Considerations

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

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







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14.1.  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                    [RFC XXXX]


   The following has been added to the Options Tags section:

   Name          Description                          Reference
   ----          -----------                          ---------
   histinfo      When used with the Supported header field, [RFC XXXX]
                 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.

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

   This document defines a priv-value for the SIP Privacy 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 field:

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


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




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14.3.  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]
   History-Info                  np                   No     [RFC xxxx]
   Contact                       mp                   No     [RFC xxxx]
   Contact                       rc                   No     [RFC xxxx]
   Contact                       np                   No     [RFC xxxx]


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


15.  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.  John Elwell provided excellent suggestions
   in terms of document structure.

   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.






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16.  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 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
       header fields.

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



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       anonymized.

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

   The first 2 changes are intended to facilitate application usage of
   the 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 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 field
   parameters.

16.1.  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
   header fields.  Entities that have not implemented this specification
   will ignore these parameters, however, per [RFC4244] an entity will
   not remove this parameter from an hi-entry.

   As for the behavior of the entity followings have changed since the
   [RFC4244].

   UAC behavior

   1.  Inclusion of option tag by UAC has changed from SHOULD to MUST.

   2.  Inclusion of hi-target-entry along with hi-index has changed rom
       MAY/RECOMMEND to MUST/MUST.

   3.  Behavior surrounding the adddition of hi-target-entry based on
       3xx response has changed from MAY/SHOULD to MUST.

   None of the behavior changes would cause any backward compatibility
   issues.

   UAS behavior






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   1.  Inclusion of hi-entry in response has changed from SHOULD to
       MUST.

   As the entity receiving response with hi-entry expected it with
   SHOULD, this change will not cause any backward compatibility issues.

   Proxy/Redirect Server behavior

   1.  Inclusion of H-I as forwarding the request has changed from
       SHOULD to MUST.

   2.  Association of Reason with time-out/internal reason has changed
       from MAY to MUST.

   3.  Inclusion of hi-index has changed from RECOMMENDED to MUST.

   4.  Inclusion of hi-entries in response has changed from SHOULD to
       MUST.

   None of the behavior changes will cause any backward compatibility
   issues as entity interacting with the updated code, expects the
   values set by the revised behavior anyway.


17.  Changes since last Version

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

   Changes from 04 to 05:

   1.  Lots of editorial corrections/clarifications per John Elwell's
       comment.

   2.  Updated Reason header section 10.2 to be consistent (i.e.,
       removed references to retargeting) with section 9.3 (Receiving a
       response) where the hi-entries and reason header are added to the
       cache.

   3.  Updated section 9.3 (receiving responses) to also include
       timeouts and updated to reflect that we don't add the Reason
       header in the case of 2xx responses.

   4.  Added text in Security considerations with regards to needing a
       Privacy Service per RFC 3323 to ensure that the privacy is
       applied.

   Changes from 03 to 04:



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   1.  Reorganization of sections per John Elwell's comments - i.e., a
       common section for building HI referenced by the UA, Intermediary
       and Redirect server sections.

   2.  Removing the use of "escape" when describing the handling of the
       Privacy and Reason header fields.

   3.  Clarification of TEL URIs in terms of not having a Privacy or
       Reason header field in the hi-targeted-to-uri.

   Changes from 02 to 03:

   1.  Lots of editorial:

   2.

       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 field.
           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.

   3.  Clarifications of functionality/applicability including:

   4.

       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.



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

   5.  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.

   6.  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 field parameters: [Issues:
        4,40]

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





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        *  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
        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 field.

   3.  Updated the Application considerations section:

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

   o  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).





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   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 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 specification
       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 :



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   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.



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   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.



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


18.  References

18.1.  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.

18.2.  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)",



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              BCP 99, RFC 3969, December 2004.

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

   [RFC4458]  Jennings, C., Audet, F., and J. Elwell, "Session
              Initiation Protocol (SIP) URIs for Applications such as
              Voicemail and Interactive Voice Response (IVR)", RFC 4458,
              April 2006.


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





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       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-Info
       entry either by adding an additional 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.





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   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 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.  PBX Voicemail call floww

   In this example, Alice calls Bob, whose SIP client is forwarded to
   Carol.  Carol does not answer the call, thus it is forwarded to a VM
   (voicemail) server (VMS).  In order to determine the appropriate
   mailbox to use for this call, the VMS needs the original target for



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   the request.  The original target is determined by finding the first
   hi-entry tagged with "rc" and using the hi-entry referenced by the
   index of "rc" header field parameter as the target for determining
   the appropriate mailbox.  This hi-entry is used to populate the
   "target" URI parameter as defined in [RFC4458].  The reason
   associated with the first hi-entry tagged with "rc" (i.e., 302) could
   be used to provide a customized voicemail greeting and is used to
   populate the "cause" URI parameter as defined in [RFC4458].  Note
   that some VMSs may also (or instead) use the information available in
   the History-Info headers for custom handling of the VM in terms of
   how and why the call arrived at the VMS.

   Furthermore it is the proxy forwarding the call to VMS that
   determines the target of the voicemail, it is the proxy that sets the
   target of voicemail which is also the entity that utilizes RFC4244bis
   to find the target which is usually based on local policy installed
   by the user or an administrator.

   Alice      example.com       Bob          Carol        VM

   | INVITE F1    |              |             |          |
   |------------->|              |             |          |
   |              | INVITE  F2   |             |          |
   |              |------------->|             |          |
   |              |              |             |          |
   |  100 Trying  |              |             |          |
   |<-------------| 302 Moved Temporarily F3    |          |
   |              |<-------------|             |          |
   |              |              |             |          |
   |              | INVITE F4    |             |          |
   |              |--------------------------->|          |
   |              |              |             |          |
   |              |         180 Ringing  F5    |          |
   |              |<---------------------------|          |
   |              |              |             |          |
   | 180 Ringing  |              |             |          |
   |<-------------|              |             |          |
   |              |              |             |          |
   |              |       (timeout)            |          |
   |              |              |             |          |
   |              | INVITE  F6   |             |          |
   |              |-------------------------------------->|
   |              |              |             |          |
   |              |               200 OK  F7              |
   |              |<--------------------------------------|
   |   200 OK     |              |             |          |
   |<-------------|              |             |          |
   |              |              |             |          |



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   |     ACK      |              |             |          |
   |------------->|                    ACK                |
   |              |-------------------------------------->|


   F1 INVITE Alice -> Example.com

   INVITE sip:bob@example.com
   Via: SIP/2.0/TCP  192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Length: <appropriate value>

   [SDP Not Shown]


   F2 INVITE Example.com -> Bob

   INVITE sip:bob@192.0.2.5 SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=as2334se
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.5>;index=1.1;rc=1
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate vlue>

   [SDP Not Shown]

   F3 302 Moved Temporarily Bob -> Example.com

   SIP/2.0 302 Moved Temporarily
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1



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   History-Info: <sip:bob@192.0.2.5>; index=1.1;rc=1
   Contact: <sip:carol@example.com>
   Content-Type: application/sdp
   Content-Length: <appropriate vlue>

   [SDP Not Shown]


   F4 INVITE Example.com -> Carol

   INVITE sip:carol@192.0.2.4 SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=as2334se
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.5?Reason%3BSIP%3Dcause%3B302>;\
                      index=1.1;rc=1
   History-Info: <sip:carol@example.com>;index=1.2;mp=1
   History-Info: <sip:carol@192.0.2.4>;index=1.2.1;rc=1.2
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate vlue>

   [SDP Not Shown]

   F5 180 Ringing Carol -> Example.com

   SIP/2.0 180 Ringing
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>;tag=setss3x
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.5?Reason%3BSIP%3Dcause%3B302>;\
                      index=1.1;rc=1
   History-Info: <sip:carol@example.com>;index=1.2;mp=1
   History-Info: <sip:carol@192.0.2.4>;index=1.2.1;rc=1.2
   Contact: <sip:carol@example.com>
   Content-Type: application/sdp
   Content-Length: <appropriate vlue>

   [SDP Not Shown]



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   F6 INVITE Example.com -> VM

   INVITE sip:vm0192.0.2.6;target=sip:bob@example.com;cause=408
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=as2334se
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.5?Reason%3BSIP%3Dcause%3B302>;\
                      index=1.1;rc=1
   History-Info: <sip:carol@example.com>;index=1.2;mp=1
   History-Info: <sip:carol@192.0.2.4?Reason%3BSIP%3Dcause%3B408>;\
                      index=1.2.1;rc=1.2
   History-Info: <sip:vm@example.com;\
                      target=sip:bob@example.com;cause=408>\
                      index=1.3;mp=1.2
   History-Info: <sip:vm@192.0.2.6;\
                      target=sip:bob@example.com;cause=408>\
                      index=1.3.1;rc=1.3
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate vlue>

   [SDP Not Shown]

   F7 200 OK VM -> Example.com

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>;tag=3dweggs
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.5?Reason%3BSIP%3Dcause%3B302>;\
                      index=1.1;rc=1
   History-Info: <sip:carol@example.com>;index=1.2;mp=1
   History-Info: <sip:carol@192.0.2.4?Reason%3BSIP%3Dcause%3B408>;\
                      index=1.2.1;rc=1.2
   History-Info: <sip:vm@example.com;\
                      target=sip:bob@example.com;cause=408>\
                      index=1.3;mp=1.2
   History-Info: <sip:vm@192.0.2.6;\
                      target=sip:bob@example.com;cause=408>\



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                      index=1.3.1;rc=1.3
   Contact: <sip:carol@example.com>
   Content-Type: application/sdp
   Content-Length: <appropriate vlue>

   [SDP Not Shown]


B.2.  Consumer Voicemail example call flow

   In this example, Alice calls the Bob but Bob has temporarily
   forwarded his phone to Carol because she is his wife.  Carol does not
   answer the call, thus it is forwarded to a VM (voicemail) server
   (VMS).  In order to determine the appropriate mailbox to use for this
   call, the VMS needs the appropriate target for the request.  The last
   target is determined by finding the hi-entry referenced by the index
   of last hi-entry tagged with "rc" for determining the appropriate
   mailbox.  This hi-entry is used to populate the "target" URI
   parameter as defined in [RFC4458].  Note that some VMSs may also (or
   instead) use the information available in the History-Info headers
   for custom handling of the VM in terms of how and why the called
   arrived at the VMS.



   Alice      example.com       Bob          Carol        VM

   | INVITE F1    |              |             |          |
   |------------->|              |             |          |
   |              | INVITE  F2   |             |          |
   |              |------------->|             |          |
   |              |              |             |          |
   |  100 Trying  |              |             |          |
   |<-------------| 302 Moved Temporarily F3    |          |
   |              |<-------------|             |          |
   |              |              |             |          |
   |              | INVITE F4    |             |          |
   |              |--------------------------->|          |
   |              |              |             |          |
   |              |         180 Ringing  F5    |          |
   |              |<---------------------------|          |
   |              |              |             |          |
   | 180 Ringing  |              |             |          |
   |<-------------|              |             |          |
   |              |              |             |          |
   |              |       (timeout)            |          |
   |              |              |             |          |
   |              | INVITE  F6   |             |          |



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   |              |-------------------------------------->|
   |              |              |             |          |
   |              |               200 OK  F7              |
   |              |<--------------------------------------|
   |   200 OK     |              |             |          |
   |<-------------|              |             |          |
   |              |              |             |          |
   |     ACK      |              |             |          |
   |------------->|                    ACK                |
   |              |-------------------------------------->|

   F1 INVITE Alice -> Example.com

   INVITE sip:bob@example.com
   Via: SIP/2.0/TCP  192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Length: <appropriate value>

   [SDP Not Shown]


   F2 INVITE Example.com -> Bob

   INVITE sip:bob@192.0.2.5 SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=as2334se
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.5>;index=1.1;rc=1
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate vlue>

   [SDP Not Shown]

   F3 302 Moved Temporarily Bob -> Example.com

   SIP/2.0 302 Moved Temporarily
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg



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   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.5>; index=1.1;rc=1
   Contact: <sip:carol@example.com>
   Content-Type: application/sdp
   Content-Length: <appropriate vlue>

   [SDP Not Shown]


   F4 INVITE Example.com -> Carol

   INVITE sip:carol@192.0.2.4 SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=as2334se
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.5?Reason%3BSIP%3Dcause%3B302>;\
                      index=1.1;rc=1
   History-Info: <sip:carol@example.com>;index=1.2;mp=1
   History-Info: <sip:carol@192.0.2.4>;index=1.2.1;rc=1.2
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate vlue>

   [SDP Not Shown]

   F5 180 Ringing Carol -> Example.com

   SIP/2.0 180 Ringing
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>;tag=setss3x
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.5?Reason%3BSIP%3Dcause%3B302>;\
                      index=1.1;rc=1
   History-Info: <sip:carol@example.com>;index=1.2;mp=1



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   History-Info: <sip:carol@192.0.2.4>;index=1.2.1;rc=1.2
   Contact: <sip:carol@example.com>
   Content-Type: application/sdp
   Content-Length: <appropriate vlue>

   [SDP Not Shown]

   F6 INVITE Example.com -> VM

   INVITE sip:vm0192.0.2.6;target=sip:carol@example.com
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=as2334se
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.5?Reason%3BSIP%3Dcause%3B302>;\
                      index=1.1;rc
   History-Info: <sip:carol@example.com?Reason%3BSIP%3Dcause%3B408>;\
                      index=1.2;mp=1
   History-Info: <sip:carol@192.0.2.4>;index=1.2.1;rc=1.2
   History-Info: <sip:vm@example.com;target=sip:carol@example.com>;\
                       index=1.3;mp=1.2
   History-Info: <sip:vm@192.0.2.5;\target=sip:carol@example.com>;\
                       index=1.3.1
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate vlue>

   [SDP Not Shown]

   F7 200 OK VM -> Example.com

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: Bob <sip:bob@example.com>;tag=3dweggs
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.5?Reason%3BSIP%3Dcause%3B302>;\
                      index=1.1;rc
   History-Info: <sip:carol@example.com?Reason%3BSIP%3Dcause%3B408>;\
                      index=1.2;mp=1
   History-Info: <sip:carol@192.0.2.4>;index=1.2.1;rc=1.2



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   History-Info: <sip:vm@example.com;target=sip:carol@example.com>;\
                       index=1.3;mp=1.2
   History-Info: <sip:vm@192.0.2.5;\target=sip:carol@example.com>;\
                       index=1.3.1
   Contact: <sip:carol@example.com>
   Content-Type: application/sdp
   Content-Length: <appropriate vlue>

   [SDP Not Shown]



   The VMS can look at the last hi-entry and find the target of the
   mailbox by looking for the "target" URI parameter in the hi-entry.

B.3.  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 first hi-entry tagged with an "rc" header
   field parameter.  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-param "mp" header field 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" header field parameter 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,



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   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   |                                    |
   |----------->|                                    |




















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   Message Details

   F1 INVITE alice -> example.com

   INVITE 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>
   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




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   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:bob@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@192.0.2.5 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%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;rc=1.2
   CSeq: 1 INVITE
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>



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   <!-- 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%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;rc=1.2
   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%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;rc=1.2
   CSeq: 1 INVITE
   Content-Length: 0












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   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%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%3Bcause%3D408>;\
                 index=1.2.1>;index=1.2.1;rc=1.2
   History-Info: <sip:home@example.com>;index=1.3;mp=1
   History-Info: <sip:home@192.0.2.6>;index=1.3.1;rc=1.3
   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















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   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%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%3Bcause%3D408>;\
                 index=1.2.1>;index=1.2.1;rc=1.2
   History-Info: <sip:home@example.com>;index=1.3;mp=1
   History-Info: <sip:home@192.0.2.6>;index=1.3.1;rc=1.3
   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%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%3Bcause%3D408>;\
                 index=1.2.1>;index=1.2.1;rc=1.2
   History-Info: <sip:home@example.com>;index=1.3;mp=1
   History-Info: <sip:home@192.0.2.6>;index=1.3.1;rc=1.3
   CSeq: 1 INVITE
   Content-Length: 0












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   F13 ACK example.com -> home

   ACK sip:home@192.0.2.6 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.4.  History-Info with Privacy Header Field

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

















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   Alice   atlanta.example.com  biloxi.example.com   Bob
   |                |                |                |
   |   INVITE F1    |                |                |
   |--------------->|                |                |
   |                |                |                |
   |                |   INVITE F2    |                |
   |                |--------------->|                |
   |                |                |                |
   |                |                | INVITE F3      |
   |                |                |--------------->|
   |                |                |                |
   |                |                |     200 F4     |
   |                |                |<---------------|
   |                |                |                |
   |                |     200 F5     |                |
   |                |<---------------|                |
   |                |                |                |
   |     200 F6     |                |                |
   |<---------------|                |                |
   |                |                |                |
   |     ACK        |                |                |
   |--------------->|    ACK         |                |
   |                |--------------->|     ACK        |
   |                |                |--------------->|

               Figure 2: Example with Privacy Header Fields


   Message Details

   F1 INVITE alice -> atlanta.example.com

   INVITE sip:bob@biloxi.example.com;p=x SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=1
   From: Alice <sip:alice@atlanta.example.com>;tag=22
   To: Bob <sip:bob@biloxi.example.com>
   Supported: histinfo
   Privacy: History
   Call-Id: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->






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   F2 INVITE  atlanta.example.com -> biloxi.example.com

   INVITE sip:bob@biloxi.example.com;p=x SIP/2.0
   Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=3
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=1
   From: Alice <sip:alice@atlanta.example.com>;tag=22
   To: Bob <sip:bob@biloxi.example.com>
   Supported: histinfo
   Call-Id: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:anonymous@anonymous.invalid>;index=1
   History-Info: <sip:bob@biloxi.example.com;p=x>;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 INVITE  biloxi.example.com -> Bob

   INVITE sip:bob@192.0.1.11 SIP/2.0
   Via: SIP/2.0/TCP proxy.biloxi.example.com:5060;branch=5
   Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=3
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=1
   From: Alice <sip:alice@atlanta.example.com>;tag=22
   To: Bob <sip:bob@biloxi.example.com>
   Supported: histinfo
   Call-Id: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:anonymous@anonymous.invalid>;index=1
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;rc=1
   History-Info: <sip:bob@192.0.1.11?Privacy=history>;index=1.1.1;rc=1.1
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->













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   F4 200 OK  Bob -> biloxi.example.com

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP proxy.biloxi.example.com:5060;branch=5
   Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=3
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=1
   From: Alice <sip:alice@atlanta.example.com>;tag=22
   To: Bob <sip:bob@biloxi.example.com>;tag=33
   Supported: histinfo
   Call-Id: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:anonymous@anonymous.invalid>;index=1
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;rc=1
   History-Info: <sip:bob@192.0.1.11?Privacy=history>;index=1.1.1;rc=1.1
   Contact: Bob <sip:bob@192.0.1.11>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->




   F5 200 OK  biloxi.example.com -> atlanta.example.com

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=3
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=1
   From: Alice <sip:alice@atlanta.example.com>;tag=22
   To: Bob <sip:bob@biloxi.example.com>;tag=33
   Supported: histinfo
   Call-Id: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:anonymous@anonymous.invalid>;index=1
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;rc=1
   History-Info: <sip:anonymous@anonymous.invalid>>;index=1.1.1;rc=1.1
   Contact: Bob <sip:bob@192.0.1.11>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->












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   F6 200 OK  atlanta.example.com -> Alice

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=1
   From: Alice <sip:alice@atlanta.example.com>;tag=22
   To: Bob <sip:bob@biloxi.example.com>;tag=33
   Supported: histinfo
   Call-Id: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:anonymous@anonymous.invalid>;index=1
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;rc=1
   History-Info: <sip:anonymous@anonymous.invalid>>;index=1.1.1;rc=1.1
   Contact: Bob <sip:bob@192.0.1.11>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->


B.5.  Privacy for a Specific History-Info Entry

   This example provides a basic call scenario similar to Appendix B.4,
   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.

























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   Alice   atlanta.example.com  biloxi.example.com   Bob
   |                |                |                |
   |   INVITE F1    |                |                |
   |--------------->|                |                |
   |                |                |                |
   |                |   INVITE F2    |                |
   |                |--------------->|                |
   |                |                |                |
   |                |                | INVITE F3      |
   |                |                |--------------->|
   |                |                |                |
   |                |                |     200 F4     |
   |                |                |<---------------|
   |                |                |                |
   |                |     200 F5     |                |
   |                |<---------------|                |
   |                |                |                |
   |     200 F6     |                |                |
   |<---------------|                |                |
   |                |                |                |
   |     ACK        |                |                |
   |--------------->|    ACK         |                |
   |                |--------------->|     ACK        |
   |                |                |--------------->|

       Figure 3: Example with Privacy Header Field for Specific URI


   Message Details

   F1 INVITE alice -> atlanta.example.com

   INVITE sip:bob@biloxi.example.com;p=x SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=1
   From: Alice <sip:alice@atlanta.example.com>;tag=22
   To: Bob <sip:bob@biloxi.example.com>
   Supported: histinfo
   Call-Id: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->







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   F2 INVITE  atlanta.example.com -> biloxi.example.com

   INVITE sip:bob@biloxi.example.com;p=x SIP/2.0
   Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=3
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=1
   From: Alice <sip:alice@atlanta.example.com>;tag=22
   To: Bob <sip:bob@biloxi.example.com>
   Supported: histinfo
   Call-Id: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   History-Info: <sip:bob@biloxi.example.com;p=x>;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 INVITE  biloxi.example.com -> Bob

   INVITE sip:bob@192.0.1.11 SIP/2.0
   Via: SIP/2.0/TCP proxy.biloxi.example.com:5060;branch=5
   Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=3
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=1
   From: Alice <sip:alice@atlanta.example.com>;tag=22
   To: Bob <sip:bob@biloxi.example.com>
   Supported: histinfo
   Call-Id: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;rc=1
   History-Info: <sip:bob@192.0.1.11?Privacy=history>;index=1.1.1;rc=1.1
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->













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   F4 200 OK  Bob -> biloxi.example.com

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP proxy.biloxi.example.com:5060;branch=5
   Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=3
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=1
   From: Alice <sip:alice@atlanta.example.com>;tag=22
   To: Bob <sip:bob@biloxi.example.com>;tag=33
   Supported: histinfo
   Call-Id: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;rc=1
   History-Info: <sip:bob@192.0.1.11?Privacy=history>;index=1.1.1;rc=1.1
   Contact: Bob <sip:bob@192.0.1.11>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->




   F5 200 OK  biloxi.example.com -> atlanta.example.com

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP proxy.atlanta.example.com:5060;branch=3
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=1
   From: Alice <sip:alice@atlanta.example.com>;tag=22
   To: Bob <sip:bob@biloxi.example.com>;tag=33
   Supported: histinfo
   Call-Id: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;rc=1
   History-Info: <sip:anonymous@anonymous.invalid>>;index=1.1.1;rc=1.1
   Contact: Bob <sip:bob@192.0.1.11>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->












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   F6 200 OK  atlanta.example.com -> Alice

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=1
   From: Alice <sip:alice@atlanta.example.com>;tag=22
   To: Bob <sip:bob@biloxi.example.com>;tag=33
   Supported: histinfo
   Call-Id: 12345600@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1;rc=1
   History-Info: <sip:anonymous@anonymous.invalid>>;index=1.1.1;rc=1.1
   Contact: Bob <sip:bob@192.0.1.11>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->



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@ntt-at.com










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   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




































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