path: root/doc/rfc/rfc7044.txt

Internet Engineering Task Force (IETF)                         M. Barnes
Request for Comments: 7044                                       Polycom
Obsoletes: 4244                                                 F. Audet
Category: Standards Track                                          Skype
ISSN: 2070-1721                                              S. Schubert
                                                                     NTT
                                                           J. van Elburg
                                              Detecon International Gmbh
                                                             C. Holmberg
                                                                Ericsson
                                                           February 2014


       An Extension to the Session Initiation Protocol (SIP) for
                      Request History Information

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 that
   directs the anonymization of values in the History-Info header field.
   This document obsoletes RFC 4244.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7044.







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Copyright Notice

   Copyright (c) 2014 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
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   4
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  History-Info Header Field Protocol Structure  . . . . . . . .   7
     5.1.  History-Info Header Field Example Scenario  . . . . . . .  10
   6.  User Agent Handling of the History-Info Header Field  . . . .  12
     6.1.  User Agent Client (UAC) Behavior  . . . . . . . . . . . .  12
     6.2.  User Agent Server (UAS) Behavior  . . . . . . . . . . . .  12
     6.3.  Back-to-Back User Agent (B2BUA) Behavior  . . . . . . . .  12
   7.  Proxy/Intermediary Handling of History-Info Header Fields . .  13
   8.  Redirect Server Handling of History-Info Header Fields  . . .  13
   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 . . . . . . . . . . .  14
     9.3.  Receiving a Response with History-Info or Request
           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 . . . . . . . .  16
       10.1.1.  Indicating Privacy . . . . . . . . . . . . . . . . .  16
       10.1.2.  Applying Privacy . . . . . . . . . . . . . . . . . .  17
     10.2.  Reason in the History-Info Header Field  . . . . . . . .  18
     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 . . . . . . . . . . . . . . . . . . .  25
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  25
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
     14.1.  Registration of New SIP History-Info Header Field  . . .  26
     14.2.  Registration of "history" for SIP Privacy Header Field .  27
     14.3.  Registration of Header Field Parameters  . . . . . . . .  27
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
   16. Changes from RFC 4244 . . . . . . . . . . . . . . . . . . . .  28
     16.1.  Backwards Compatibility  . . . . . . . . . . . . . . . .  29
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     17.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Appendix A.  Request History Requirements . . . . . . . . . . . .  33
     A.1.  Security Requirements . . . . . . . . . . . . . . . . . .  34
     A.2.  Privacy Requirements  . . . . . . . . . . . . . . . . . .  35




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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 obtain information 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.  This specification also defines a value, "history", for
   the Privacy header field.  In addition, a SIP option tag, "histinfo",
   is defined.

   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 available in [CALLFLOWS].

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 the Request-URI (Section 7.1 of
   [RFC3261]) 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 the forwarding of the request.





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   The terms "location service", "forward", "redirect", and "AOR"
   (address-of-record) are used consistently with the terminology in
   [RFC3261].

   The term "target user" is used in this specification as the human
   user associated with one or more particular AORs (in case the human
   user has multiple aliases).

   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 for other protocols 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 relaying whereby the recipient obtains a detailed "trace of
       the path" of the message from originator to receiver, including
       the time of each relay.

   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.



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

   o  Some diagnostic information for debugging SIP requests.

   o  Capturing aliases and Globally Routable User Agent URIs (GRUUs)
      [RFC5627], which can be overwritten by a 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 Interactive Voice Response (IVR) with
      a SIP URI [RFC4240].

4.  Overview

   The fundamental functionality provided by the request history
   information is the ability to inform proxies and user agents (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 User Agent Client (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, are provided in Section 5.  Sections 6, 7, and 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 5.




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   This specification also defines a priv-value for the Privacy header,
   "history"; it requires anonymization of all the History-Info header
   field entries in a request or to a specific History-Info header field
   value (hi-entry) as described below.  Further detail is provided in
   Section 10.1.

   In addition, a SIP option tag, "histinfo", is defined.  The use of
   this option tag is described in Section 6.1.

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.

   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 reflect the
      forking and retargeting of requests.  The format for this
      parameter is a sequence of nonnegative integers, 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 leaf.  By adding the new
      entries in chronological 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 is either an "rc", "mp", or "np" header field parameter,
      which is interpreted as follows:







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         "rc": The hi-targeted-to-URI represents a change in
         Request-URI, while the target user remains the same.  This
         occurs, for example, when the 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.

         "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 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 the Request-URI.  This would apply, for example, when
         a proxy merely forwards a request to a next-hop proxy and loose
         routing is used.  The "np" 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
         copied unchanged into the request represented by this hi-entry.
         That value will usually be the hi-index of the parent hi-entry
         of this hi-entry.

   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 [RFC5234] for the History-Info header field and
   header field parameters is as follows:

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

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

   hi-targeted-to-uri = name-addr

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

   hi-index = "index" EQUAL index-val

   index-val =  number *("." number)

   number =  [ %x31-39 *DIGIT ] DIGIT




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   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", "name-addr", "HCOLON",
   "COMMA", "SEMI", and "EQUAL" are from [RFC3261].

   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] with a priv-value of "history", as defined in this
      document, included in the hi-targeted-to-uri or by adding the
      Privacy header field with a priv-value of "history" to the
      request.  The latter case indicates that the History-Info entries
      for all History-Info entries whose hi-targeted-to-uri has the same
      domain as the domain for which the SIP entity processing the
      message is responsible 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].






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   The following provides examples of the format for the History-Info
   header field.  Note that the backslash, CRLF, and whitespace between
   the lines in the examples below are inserted for readability purposes
   only.  Note, however, that History-Info can be broken into multiple
   lines due to the SWS (sep whitespace) that is part of HCOLON, COMMA,
   and SEMI, and there can be multiple History-Info header fields due to
   the rule of Section 7.3 of [RFC3261].  Additional detailed examples
   are available in [CALLFLOWS].

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

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

5.1.  History-Info Header Field Example Scenario

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

   In this example, Alice (sip:alice@atlanta.example.com) calls Bob
   (sip:bob@biloxi.example.com).  Alice's proxy in her home domain
   (sip:atlanta.example.com) forwards the request to Bob's proxy
   (sip:biloxi.example.com).  When the request arrives at
   sip:biloxi.example.com, it does a location service lookup for
   bob@biloxi.example.com and changes the target of the request to Bob's
   Contact URIs that were 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 original target information or the
   initial Request-URI, 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 in the 200 response to Alice, an hi-entry is
   not included for the fork to sip:bob@192.0.2.7 (index 1.1.1) since
   biloxi.example.com had not received a response from that fork at the
   time it sent the 200 OK that ultimately reached Alice.

   Additional detailed examples are available in [CALLFLOWS].

      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             |                |          |
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1      |
   |                |                |                |          |
   |                |   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        |          |
   |                |                |--------------->|          |

                          Figure 1: Basic Call



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

   This section describes the processing specific to UAs -- User Agent
   Clients (UACs), User Agent Servers (UASs), and Back-to-Back User
   Agents (B2BUAs) -- for the History-Info header.

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.

   If the UAC generates further forks of the initial request (either due
   to acting on a 3xx response or internally directed forking to
   multiple destinations), the successive requests will add hi-entries
   with hi-indexes of 2, 3, etc.

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 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.

6.3.  Back-to-Back User Agent (B2BUA) Behavior

   A B2BUA MAY follow the behavior of a SIP intermediary, per Section 7,
   as an alternative to following the behavior of a UAS per Section 6.2
   or 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.









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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.

   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 or 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 it to a registered
   contact bound to the same AOR.  In this case, the intermediary MUST
   add an 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.  Note that this is shown in the INVITE (F6) in the
   example entitled "Sequentially Forking (History-Info in Response)" in
   [CALLFLOWS].

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.







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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 keep a copy of the
   hi-entries from the incoming request.  This document describes this
   copy in terms of 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.

   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 an hi-entry to the end of the cache, on
   behalf of the previous SIP entity.  This is done as follows, before
   proceeding to Section 9.2.

      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 an
      hi-targeted-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 the hi-entries
   from the cache that was created per Section 9.1.  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 cache at this time.  The
   hi-entries in the outgoing request's History-Info header field
   represent the preorder of the tree of hi-entries, that is, by the
   lexicographic ordering of the hi-indexes.  The new hi-entry is
   populated as follows:





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

   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/np:  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 Request Timeouts

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

   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

      If the response is not a 100 or 2xx response, 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 or non-2xx 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.





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   It is important to note that the cache (and the request or response)
   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 be present but not 1.3).

   Note that in the case that a request has traversed one or more
   intermediaries that do not support RFC 4244 or this document, there
   can be duplicate indices (due to forking), which would be added to
   the appropriate position in the cache in the order in which they are
   received.

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, and 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 History-Info in the response.

10.  Processing the History-Info Header Field

   The following subsections 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.

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:






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   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 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 that is within an intermediary and that the
   intermediary wants to privacy protect.  In this case, the
   intermediary MUST construct a Privacy header field with the single
   priv-value of "history" and include the Privacy header field in the
   hi-targeted-to-uri, for each new hi-entry created by the intermediary
   whose hi-targeted-to-uri it wishes to privacy protect.  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 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 UAS may not want to reveal the final reached target to
   the originator.  In this case, the UAS 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 UAS 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 SIP message 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 the SIP intermediary is responsible) are anonymized by the



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   Privacy Service.  The Privacy Service MUST change any
   hi-targeted-to-uris in these hi-entries that have not been anonymized
   (evidenced by their domain not being "anonymous.invalid") to
   anonymous URIs containing a domain of anonymous.invalid as
   recommended in Section 4.1.1.3 of [RFC3323].  As defined in
   Section 4.1.1.2 of [RFC3323], the recommendations of [RFC3261] for
   anonymizing the URI Username SHOULD be followed (i.e., "anonymous" in
   the user portion of the URI).  If there is a Privacy header field in
   the "headers" component of the hi-targeted-to-uri in the hi-entries,
   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 update those hi-targeted-to-uris as described
   above.  Any other priv-values in the Privacy 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" component 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 when the hi-entry is added to the
   cache based upon the receipt of a SIP response that is neither a 100
   nor a 2xx response, as described in Section 9.3.  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.  In addition, if the response contains any Reason header
   fields (see [RFC3326]), then the SIP entity MUST also include the
   Reason header fields in the "headers" component of the
   hi-targeted-to-uri in the last hi-entry added to the cache.

   If a request has timed out (instead of being explicitly rejected),
   the SIP entity MUST update the cache as if the request received a SIP
   error response code of 408 "Request Timeout".






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   A request can receive multiple responses that are neither 100 nor 2xx
   responses and that carry or imply (for responses without Reason
   headers, and for timeouts) multiple, possibly duplicated,
   reason-values to be applied to an hi-targeted-to-uri.  In these
   situations, the SIP entity creating the History-Info header value
   would choose the appropriate Reason header field value.

   A SIP entity MAY also include a Reason header field (in the "headers"
   component of an hi-targeted-to-uri) that contains 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 an hi-index to each hi-entry.
   Per the syntax in Section 5, the hi-index consists of a series of
   nonnegative integers separated by dots (e.g., 1.1.2).  Each dot
   reflects a SIP forwarding hop.  The nonnegative integer following
   each dot reflects the order in which a request was retargeted at the
   hop.  The highest nonnegative integer 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) at the time that the request
   represented by this hi-entry was generated.  Thus, the indexing
   results in a logical tree representation for the history of the
   request and the hi-entries are given in the preorder of the tree.

   The first index in a series of History-Info entries MUST be set to 1.
   In the case that a SIP entity (intermediary or UAS) adds a first
   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 last
   integers of the hi-indexes of the new requests MUST be generated
   starting at 1 and incrementing by 1 for each additional request.

   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




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       hi-index in the last History-Info header field in the received
       request is 1.1, a proxy would add an hi-entry with an hi-index of
       1.1.1 and forward the request.

   2.  Retargeting within a processing entity - first 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 for an example.)  Note that, in the case of parallel
       forking, only the hi-entry corresponding to the fork is included
       in the request because no response can yet have been received for
       any of the parallel forked requests.

   6.  Missing entry: If the request clearly has a gap in the hi-entry
       (i.e., the last hi-entry and Request-URI differ), the entity
       adding an hi-entry MUST add a single index with a value of "0"
       (i.e., the nonnegative integer zero) prior to adding the
       appropriate index for the action to be taken.  For example, 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.
       In the case of requests that are forked by a proxy that does not
       support History-Info, it is possible for hi-entries generated by



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       different entities to have the same index, i.e., each entity
       supporting History-Info would receive a forked request with the
       same hi-index to which they would add the value of ".0" prior to
       adding the appropriate index.  Thus, in the previous example,
       each of the next-hop entities would generate an hi-index of
       1.1.2.0.1.

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

   This specification defines three header field parameters, "rc", "mp",
   and "np".  The header field parameters "rc" and "mp" indicate the
   mechanism by which a new target for a request is determined.  The
   header field "np" reflects that the target has not changed.  All
   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 of [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" or "mp" header field parameter, then the SIP
   entity MUST NOT include an "rc" or "mp" 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.  This is because the redirect
   server is the only element with any knowledge on how the target was
   determined.  Note that the "np" header field parameter is not
   applicable in the case of redirection.

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

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

   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.

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



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   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; the index is that of the
      downstream request that 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
   and determine if there are any gaps in the hi-entries.  The SIP
   entity MUST be prepared to process effectively messages whose
   hi-entries show evidence of "gaps", that is, situations that reveal
   that not all of the forks of the request have been recorded in the
   hi-entries.  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 hi-entries with a nonnegative
   integer 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 message.  In addition, gaps may introduce the
   possibility of duplicate values for the hi-index in the case that a
   proxy that does not support History-Info forks a request.  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 debugging 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.








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   2.  Hi-entry with the index that matches the value of the "rc" header
       field parameter in the last hi-entry with an "rc" header field
       parameter in the request received by a UAS, i.e., the last AOR
       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 an "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 an "rc" header field
       parameter in the request received by a UAS.  Note that this would
       be the original AOR if all the entities involved support the
       History-Info header field and there is an 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 the "mp" header
       field parameter in the first hi-entry with an "mp" header field
       parameter in the request received by a UAS.  Note that 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 one or more particular domains; 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 with an index that matches the
   value of the "mp" header field parameter in the first hi-entry with
   an "mp" header field parameter may also display other agents,
   reflected by hi-entries prior to hi-entries with an "rc" header field
   parameter, to whom the call was targeted prior to its arrival at the



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   current agent.  This could allow the agent the ability to decide how
   they might forward or reroute the call if necessary (avoiding agents
   that were not previously available for whatever reason, etc.).

   Since support for History-Info 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 that are not
   tagged appropriately with an hi-target-param in the case of entries
   added by entities that are only compliant to RFC 4244.  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

   The following are possible (non-normative) application-specific
   usages of History-Info.

12.1.  PBX Voicemail

   A voicemail system (VMS) 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
   the "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 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.

   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.  Further detail for this example can be found in the call
   flow entitled "PBX Voicemail Example" in [CALLFLOWS].

   Note that in the case where there is no entry tagged with "rc", a VMS
   can follow the procedures, as defined in [RFC4458], for the
   "Interaction with Request History Information".



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12.2.  Consumer Voicemail

   The voicemail system in this 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 the 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].  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.  Further detail for this
   example can be found in the call flow entitled "Consumer Voicemail
   Example" in [CALLFLOWS].

   In the case where there is no entry tagged with "rc", a VMS can
   follow the procedures, as defined in [RFC4458], for the "Interaction
   with Request History Information".

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.  If TLS is NOT used, the
   intermediary MUST ensure that the messages are only sent within an
   environment that is secured by other means or that the messages don't
   leave the intermediary's domain.  This results in History-Info's
   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.







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   In terms of ensuring the privacy of hi-entries, the same security
   considerations as those described in [RFC3323] apply.  The Privacy
   Service that's defined in [RFC3323] MUST also support the new Privacy
   header field priv-value of "history" and anonymize hi-entries in the
   case of a priv-value of "header" as described in Section 10.1.2.

14.  IANA Considerations

   IANA registrations have been implemented or updated as detailed in
   the following subsections.

   This document obsoletes [RFC4244] but uses the same SIP header field
   name, Privacy header field, and Option tag.  References to [RFC4244]
   in the IANA "Session Initiation Protocol (SIP) Parameters" registry
   (<http://www.iana.org/assignments/sip-parameters>) have been replaced
   with references to this document.

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 updates have been made to
   <http://www.iana.org/assignments/sip-parameters>.

   The following row has been updated in the "Header Fields" sub-
   registry:

   Header Name             Compact Form               Reference
   -----------             ------------               ---------
   History-Info               none                    [RFC7044]


   The following has been updated in the "Option Tags" sub-registry:

   Name        Description                                 Reference
   ----        -----------                                 ---------
   histinfo    When used with the Supported header field,  [RFC7044]
               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.









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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 updates have been made to the "SIP Privacy
   Header Field Values" sub-registry in <http://www.iana.org/assignments
   /sip-parameters> for the registration of the SIP Privacy header
   field:

   Privacy
   Type     Description             Registrant                 Reference
   ------   -----------             ----------                 ---------
   history  Privacy requested for   Mary Barnes                [RFC7044]
            History-Info header     mary.ietf.barnes@gmail.com
            field(s)

14.3.  Registration of Header Field Parameters

   This specification defines the following new SIP header field
   parameters in the "Header Field Parameters and Parameter Values" sub-
   registry in <http:/www.iana.org/assignments/sip-parameters>.

    Header Field     Parameter Name   Predefined Values  Reference
   -------------     --------------   -----------------  ---------
    History-Info           mp                 No         [RFC7044]
    History-Info           rc                 No         [RFC7044]
    History-Info           np                 No         [RFC7044]
    Contact                mp                 No         [RFC7044]
    Contact                rc                 No         [RFC7044]
    Contact                np                 No         [RFC7044]

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, Marianne Mohali, Brett Tate, and Dale Worley.  John Elwell
   also provided excellent suggestions in terms of document structure.
   Dan Romascanu performed the Gen-ART review.

   Mark Watson, Cullen Jennings, and Jon Peterson provided significant
   input into the initial work that resulted in the development of
   [RFC4244].  The authors 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




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   Dolly, Roland Jesske, Takuya Sawada, Sebastien Prouvost, and
   Sebastien Garcin in the development of [RFC4244].

   The authors would like to acknowledge the significant input from
   Rohan Mahy on some of the normative aspects of the ABNF for
   [RFC4244], particularly regarding security and the index (the need
   for it as well as its format).

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, numerous
      implementations filter out many entries and only leave specific
      entries corresponding, for example, to first and last redirection.
      Since vendors use different rules, this causes significant
      interoperability issues.

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

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

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

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









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   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.  Added a way to indicate a gap in History-Info by adding a
       nonnegative integer of "0".

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

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

   5.  Removed Appendix B ("Voicemail") since a separate call flow
       document is being published as a companion to this document.

   The first two 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 because [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 these parameters from an hi-entry.  While
   entities compliant to this document and [RFC4244] must be able to
   recognize gaps in the hi-entries, this document requires that an



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   index of "0" be used in this case.  In comparison, [RFC4244]
   recommended (but did not require) the use of "1".  However, since the
   ABNF in [RFC4244] defines the index as a DIGIT, "0" would be a valid
   value; thus, an [RFC4244] implementation should not have an issue if
   it receives hi-entries added by intermediaries compliant to this
   document.

   As for the behavior of the UACs, UASs, and intermediaries, the
   following additional normative changes have been made:

   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 from
       MAY/RECOMMEND to MUST/MUST.

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

   None of the behavior changes will cause any backward or forward
   compatibility issues.

   UAS behavior

   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 the History-Info header field when forwarding the
       request has changed from SHOULD to MUST.

   2.  Association of Reason with timeout/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 the response has changed from SHOULD
       to MUST.

   None of the above behavior changes impact backwards compatibility
   since they only strengthen normative behavior to improve
   interoperability.




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   In cases where an entity that is compliant to this document receives
   a request that contains hi-entries compliant only to RFC 4244 (i.e.,
   the hi-entries do not contain any of the new header field
   parameters), the entity MUST NOT add any of the new header field
   parameters to the hi-entries.  The hi-entries MUST be cached and
   forwarded as any other entries are, as specified in Section 9.1.  As
   with entities that are compliant to RFC 4244, applications must be
   able to function in cases of missing information, as specified in
   Section 11.

17.  References

17.1.  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.

   [RFC5234]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
               Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [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.

17.2.  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.




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   [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.

   [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.

   [CALLFLOWS] Barnes, M., Audet, F., Schubert, S., Elburg, H., and C.
               Holmberg, "Session Initiation Protocol (SIP) History-Info
               Header Call Flow Examples", Work in Progress, November
               2013.

































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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
           whether the retarget URI was an AOR,

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

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

       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).






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   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 by either adding an additional hi-entry as a result of
       retargeting or entering invalid information.

   2.  A rogue application could rearrange the "Request History"
       information to change the nature of the end application or to
       mislead the receiver of the information.

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

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

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

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

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

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



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   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 outgoing messages unless it is protected
       as described in [RFC3323].





























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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


   Hans Erik van Elburg
   Detecon International Gmbh
   Sternengasse 14-16
   Cologne
   Germany

   EMail: ietf.hanserik@gmail.com


   Christer Holmberg
   Ericsson
   Hirsalantie 11, Jorvas
   Finland

   EMail: christer.holmberg@ericsson.com














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