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Versions: (draft-barnes-sipcore-rfc4244bis) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 7044

Network Working Group                                          M. Barnes
Internet-Draft                                                   Polycom
Obsoletes: 4244 (if approved)                                   F. Audet
Intended status: Standards Track                                   Skype
Expires: December 26, 2010                                   S. Schubert
                                                                     NTT
                                                           J. van Elburg
                                              Detecon International Gmbh
                                                             C. Holmberg
                                                                Ericsson
                                                           June 24, 2010


   An Extension to the Session Initiation Protocol (SIP) for Request
                          History Information
                  draft-ietf-sipcore-rfc4244bis-01.txt

Abstract

   This document defines a standard mechanism for capturing the history
   information associated with a Session Initiation Protocol (SIP)
   request.  This capability enables many enhanced services by providing
   the information as to how and why a call arrives at a specific
   application or user.  This document defines an optional SIP header,
   History-Info, for capturing the history information in requests.  A
   SIP/SIPS URI parameter is defined to tag information necessary to
   populate the History-Info header.  In addition, this document defines
   a value for the Privacy header specific to the History-Info header.

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
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   This Internet-Draft will expire on December 26, 2010.

Copyright Notice




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

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
   3.  Overview of Operations . . . . . . . . . . . . . . . . . . . .  5
   4.  General User Agent Behavior  . . . . . . . . . . . . . . . . .  9
     4.1.  User Agent Client (UAC) Behavior . . . . . . . . . . . . .  9
     4.2.  User Agent Server (UAS) Behavior . . . . . . . . . . . . . 10
       4.2.1.  Processing of Requests with History-Info . . . . . . . 10
       4.2.2.  Generation of Responses with History-Info  . . . . . . 10
     4.3.  Redirect Server Behavior . . . . . . . . . . . . . . . . . 11
   5.  Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.1.  Adding the History-Info Header to Requests . . . . . . . . 11
       5.1.1.  Initial Request  . . . . . . . . . . . . . . . . . . . 11
       5.1.2.  Re-sending based on failure response . . . . . . . . . 12
       5.1.3.  Re-sending based on redirection response . . . . . . . 13
     5.2.  Sending History-Info in Responses  . . . . . . . . . . . . 14
   6.  The History-Info header field  . . . . . . . . . . . . . . . . 14
     6.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 16
     6.3.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . 16
       6.3.1.  SIP/SIPS URI target parameter for History-Info
               Header . . . . . . . . . . . . . . . . . . . . . . . . 17
       6.3.2.  Privacy in the History-Info Header . . . . . . . . . . 17
       6.3.3.  Reason in the History-Info Header  . . . . . . . . . . 18
       6.3.4.  Indexing in the History-Info Header  . . . . . . . . . 19
       6.3.5.  Request Target in the History-Info Header  . . . . . . 20
   7.  Application Considerations . . . . . . . . . . . . . . . . . . 20
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
     9.1.  Registration of New SIP History-Info Header  . . . . . . . 22
     9.2.  Registration of "history" for SIP Privacy Header . . . . . 23
     9.3.  Registration of "hit" SIP/SIPS URI Parameter . . . . . . . 23
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
   11. Changes from RFC 4244  . . . . . . . . . . . . . . . . . . . . 24
     11.1. Backwards compatibility  . . . . . . . . . . . . . . . . . 26
   12. Changes since last Version . . . . . . . . . . . . . . . . . . 26
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 29
     13.2. Informative References . . . . . . . . . . . . . . . . . . 30
   Appendix A.  Request History Requirements  . . . . . . . . . . . . 30
     A.1.  Security Requirements  . . . . . . . . . . . . . . . . . . 32
     A.2.  Privacy Requirements . . . . . . . . . . . . . . . . . . . 32
   Appendix B.  Example call flows  . . . . . . . . . . . . . . . . . 33
     B.1.  Sequentially Forking (History-Info in Response)  . . . . . 33
     B.2.  History-Info with Privacy Header . . . . . . . . . . . . . 40
     B.3.  Privacy Header for a Specific History-Info Entry . . . . . 41
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 43



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1.  Introduction

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

   This document defines a SIP header, 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.  A SIP/
   SIPS URI parameter is defined to tag information necessary to
   populate the History-Info header.  In addition, this document defines
   a value for the Privacy header specific to the History-Info header.

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


2.  Conventions and Terminology

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

   The term "retarget" is used in this document to refer both to the
   process of a Proxy Server/User Agent Client (UAC) 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 the subsequent forwarding of that request as described in section
   16.6 of [RFC3261].

   The term "forward" is used consistent with the terminology in
   [RFC3261].  Noting that [RFC3261] uses the term "forwarding" to
   describe a proxy's handling of requests for domains for which is not
   responsible, as well as to describe the basic "forwarding" of a
   request (in section 16.6) once a target has been determined.



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   However, the context of the usage is sufficient to differentiate the
   slightly different meanings.

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


3.  Overview of Operations

   SIP implicitly provides retargeting capabilities that enable calls to
   be routed to specific applications as defined in [RFC3261].  The
   motivation for capturing the request history is that in the process
   of retargeting a request, old routing information can be forever
   lost.  This lost information may be important history that allows
   elements to which the call is retargeted to process the call 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 call to exchange additional information relating to
   how and why the call was routed to a particular destination.  The
   following are examples of such applications:

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

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

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

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

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

   o  Some diagnostic information for debugging SIP requests.

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




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   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].

   o  A stronger security solution for SIP.  A side effect is that each
      proxy that captures the "request history" information in a secure
      manner provides an additional means (without requiring signed
      keys) for the original requestor to be assured that the request
      was properly retargeted.

   The fundamental functionality provided by the request history
   information is the ability to inform proxies and UAs involved in
   processing a request about the history or progress of that request
   (CAPABILITY-req, see Appendix A).  The solution is to capture the
   Request-URIs as a request is retargeted, in a SIP header: History-
   Info (CONTENT-req, see Appendix A).  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 can appear in any request not associated with
   an established dialog (e.g., INVITE, REGISTER, MESSAGE, REFER and
   OPTIONS, PUBLISH and SUBSCRIBE, etc.)  (REQUEST-VALIDITY-req, see
   Appendix A) and any valid response to these requests (ISSUER-req, see
   Appendix A).

   This specification defines parameters detailed in Section 6.1 for
   carrying the following information in the History-Info header:

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

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

   o  Reason: Reason describes why an entry was retargeted.

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

   o  Target: The target parameter indicates whether the Targeted-to-URI
      is a registered contact ("rc") for or another user mapped ("mp")
      from the Request-URI in the incoming request that was retargeted.




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   This specification also defines the "hit" SIP/SIPS URI Parameter.
   This parameter is added to URIs as they are added to the target list
   for a request based on the same criteria as the target parameter
   described above.  This URI parameter is used to determine the value
   for the target parameter.  Further detail is provided in
   Section 6.3.1.

   In addition, this specification defines a value for the Privacy
   header, "history", that applies to all the History-Info header
   entries in a Request or to a specific History-Info header entry as
   described above.  Further detailed is provided in Section 6.3.2.

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

   One important thing illustrated by this call flow is that without
   History-Info, Bob would "lose" the target information, including any
   parameters in the request URI.  Bob can now recover that information
   by locating the first entry just prior to the last hi-entry marked as
   "rc", which is one level up in indexing from the last hi-entry -
   i.e., the sip:bob@biloxi.example.com entry with index=1.1.

   The formatting in this scenario is for visual purposes; thus,
   backslash and CRLF are used between the fields for readability and
   the headers in the URI are not shown properly formatted for escaping.
   Refer to Section 6.2 for the proper formatting.  Additional detailed
   scenarios are available in the Appendix B.

      Note: This example uses loose routing procedures.


   Alice   atlanta.example.com  biloxi.example.com   Bob@pc  Bob@phone
   |                |                |                |          |
   |   INVITE sip:bob@biloxi.example.com;p=x          |          |
   |--------------->|                |                |          |
   | Supported: histinfo             |                |          |
   |                |                |                |          |
   |                |   INVITE sip:bob@biloxi.example.com;p=x
   |                |--------------->|                |          |



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   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
   |                |                |                |          |
   |                |                |   INVITE sip:bob@192.0.2.3
   |                |                |--------------->|          |
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
   | History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc
   |                |                |                |          |
   |                |                |     INVITE sip:bob@192.0.2.7
   |                |                |-------------------------->|
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
   | History-Info: <sip:bob@192.0.2.7>;index=1.1.2;rc
   |                |                |     200        |          |
   |                |                |<---------------|          |
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
   | History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc
   |                |                |                |          |
   |                |                |<===Proxy cancels INVITE==>|
   |                |     200        |                |          |
   |                |<---------------|                |          |
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
   | History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc
   | History-Info: <sip:bob@192.0.2.7?Reason=SIP;cause=487>;\
   |               index=1.1.2;rc
   |                |                |                |          |
   |     200        |                |                |          |
   |<---------------|                |                |          |
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
   | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1.1
   | History-Info: <sip:bob@192.0.2.3;index=1.1.1;rc
   | History-Info: <sip:bob@192.0.2.7?Reason=SIP;cause=487>;\
   |               index=1.1.2;rc
   |                |                |                |          |
   |     ACK        |                |                |          |
   |--------------->|    ACK         |                |          |
   |                |--------------->|     ACK        |          |
   |                |                |--------------->|          |


                           Figure 1: Basic Call







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4.  General User Agent Behavior

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

4.1.  User Agent Client (UAC) Behavior

   The UAC SHOULD include the "histinfo" option tag in the Supported
   header in any request not associated with an established dialog for
   which the UAC would like the History-Info header in the response.  In
   addition, the UAC MAY add a History-Info header, using the Request-
   URI of the request as the hi-target-to-uri, in which case the index
   MUST be set to a value of 1 in the hi-entry.  As a result,
   intermediaries and the UAS at least know the original Request-URI,
   and if the Request-URI was modified by a previous hop.  Normally,
   UACs are not expected to include a History-Info header in an initial
   request as it is more of a Proxy function; the main reason it is
   allowed is for B2BUAs who are performing proxy-like functions such as
   routing.

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

   In the case where a UAC receives a 3xx response with a Contact header
   and sends a new request in response to it, the UAC MAY include in the
   outgoing request the previous hi-entry(s) received in the response.
   In this case, the reason header MUST be associated with the hi-
   targeted-to-uri in the previous (last) hi-entry, as described in
   Section 6.3.3.  A new hi-entry MUST then be added for the URI from
   the Contact header (which becomes the new Request-URI).  An index
   MUST be added to the hi-entry.  The value for the index is determined
   by reading and incrementing the value of the index from the previous
   hi-entry, thus following the same rules as those prescribed for a
   proxy in retargeting, described in Section 6.3.4.  If the URI in the
   Contact header contains a "hit" URI parameter, the UAC MUST add a
   target parameter to the hi-entry and MUST remove the URI parameter as
   described in Section 6.3.5.

   Prior to any application usage of the information by the UAC (e.g.,
   debug), the validity SHOULD be ascertained.  The entries SHOULD be
   evaluated to determine gaps in indices, which could indicate that an
   entry has been maliciously removed.  Either way, an application
   SHOULD be aware of potentially missing information.  The
   interpretation of the information in the History-Info header by a UAC
   in a request depends upon the specific applications supported by the



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   UAC.  Application considerations and guidelines are provided in
   section 7.

4.2.  User Agent Server (UAS) Behavior

4.2.1.  Processing of Requests with History-Info

   Once the request terminates at the UAS, the UAS evaluates the
   History-Info header.  The last hi-entry reflects the most recent
   target and SHOULD contain the Request-URI for the received request.
   If the Request-URI of the incoming request does not match the last
   hi-entry (e.g., the last proxy does not support History-Info), the
   UAS MUST insert an hi-entry.  The UAS MUST set the hi-targeted-to-uri
   based to the value of Request-URI in the incoming request, unless
   privacy is required.  If privacy is required, the procedures of
   Section 6.3.2 MUST be used.  The UAS MUST include an hi-index
   attribute as described in Section 6.3.4.  The UAS MUST NOT include a
   hi-target attribute, since the UAS has no way to know the mechanism
   by which the Request-URI was determined.  The addition of the missing
   hi-entry ensures that the most complete information can be provided
   in the response and provides consistency in the information presented
   to applications.  The information can also be useful for
   implementations with B2BUAs that include the History-Info, received
   in the incoming request, in the subsequent outgoing request.

   Prior to any application usage of the information, the validity
   SHOULD be ascertained.  The entries SHOULD be evaluated to determine
   gaps in indices, which could indicate that an entry has been
   maliciously removed.  Either way, an application SHOULD be aware of
   potentially missing information.  The interpretation of the
   information in the History-Info header by a UAS in a request depends
   upon the specific applications supported by the UAS.  Application
   considerations and guidelines are provided in section 7.

4.2.2.  Generation of Responses with History-Info

   If the "histinfo" option tag is received in a request, the UAS MUST
   include any History-Info received in the request in the subsequent
   response.  If privacy is required, entries MUST be anonymized using
   [RFC3323].  The UAS MUST follow the rules for a redirect server per
   Section 4.3 in generating a 3xx response.

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





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4.3.  Redirect Server Behavior

   A redirect server MUST include the History-Info headers received in
   the request in the 3XX response that it sends.  A redirect server
   MUST NOT add any new History-Info entries.  In order to provide the
   necessary information to populate the target parameter for the new
   History-Info entries as a request is redirected, the redirect server
   MUST add a "hit" URI parameter to the URIs in the contact header.
   The appropriate values for this parameter are described in
   Section 6.3.1.


5.  Proxy Behavior

   The specific processing by proxies for adding the History-Info
   headers in SIP requests and responses is described in this section.

5.1.  Adding the History-Info Header to Requests

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

   o  Forwarding of initial request (see Section 5.1.1)

   o  Resending based on failure response (see Section 5.1.2)

   o  Resending based on redirection response (see Section 5.1.3)

   Retargeting is an iterative process, i.e., a proxy may redirect
   "internally " more than one time.  A typical example would be a proxy
   that redirects a request first to a different user (i.e., it maps to
   a different AOR), and then forwards to a registered contact bound to
   that new AOR.  A proxy that uses such mechanism SHOULD add multiple
   hi-entry fields (e.g., bob@example.com to office@example.com to
   office@192.0.2.5) to provide a logical description of the retargeting
   process.  A Reason MAY be associated with the hi-targeted-to-uri that
   has been retargeted as shown in the example in Appendix B.1.

5.1.1.  Initial Request

   Upon receipt of an initial request for a dialog, or a standalone
   request, a proxy forwarding the request MUST perform the following
   steps.  Note that those steps below do not apply if the request is
   being re-sent as a result of failure (i.e., timeout, reception of an
   error response), or redirection caused by receipt of a 3XX message).






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   Step 1:  Adding Entries on Behalf of Previous Hops

      If an incoming request does not already have a History-Info header
      field (e.g., the UAC does not include any History-Info header and
      no proxies in between support History-Info), or if the Request-URI
      of the incoming request does not match the last hi-entry (e.g.,
      the last proxy does not support History-Info), the proxy MUST
      insert an hi-entry.  The proxy MUST set the hi-targeted-to-uri
      based to the value of Request-URI in the incoming request, unless
      privacy is required.  If privacy is required, the procedures of
      Section 6.3.2 MUST be used.  The proxy MUST NOT include a hi-
      target attribute.  The proxy MUST include an hi-index attribute as
      described in Section 6.3.4.

   Step 2:  Tagging URIs in Target Set with "hit" Parameter

      The proxy then proceeds to determining the request targets as per
      16.5/[RFC3261].  The proxy MUST add a "hit" SIP/SIPS URI parameter
      for the target URI that are determined based on either of the two
      mechanisms as described in Section 6.3.1.

   Step 3:  Generating New Entries for Each Outgoing Request

      The proxy then proceeds to request forwarding as per 16.6/
      [RFC3261].  The proxy MUST add a separate hi-entry in each
      separate outgoing request for each of the current (outgoing)
      targets in the target set.  The proxy MUST set the hi-targeted-to-
      uri in those separate hi-entry(s) to the value of the Request-URI
      of the current (outgoing) request, unless privacy is required.  If
      privacy is required, the procedures of Section 6.3.2 MUST be used.
      The proxy MUST include an hi-index for each of the separate hi-
      entry(s) as described in Section 6.3.4.  If the Request-URI
      contains a "hit" SIP/SIPS URI parameter, the proxy MUST include a
      hi-target attribute for each of the separate entry(s) as described
      in Section 6.3.5.  The proxy MUST remove the "hit" URI parameter
      from the Request-URI.

5.1.2.  Re-sending based on failure response

   When re-sending a request as a result of retargeting because of
   failure (i.e., either reception of error responses or a timeout which
   is considered to be an implicit 487 error response), the proxy MUST
   perform the following steps:








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   Step 1:  Including the Entries from Error Responses & Timeouts

      The proxy MUST build the History-Info header field(s) sent in the
      outgoing request using the aggregate information associated with
      the received error responses(s) and timeout(s) for all the
      branches that are generating failures, including the header
      entries in the order indicated by the indexing (see
      Section 6.3.4).  If the received error response did not include
      any History-Info header fields, the proxy MUST use the same
      History-Info header fields that were sent in the outgoing request
      that failed to build the outgoing request.

   Step 2:  Tagging the Last Entries

      The proxy then examines the last hi-entry of the History-Info that
      was just generated in Step 1 for each one of the branches that
      generated failures or timeouts and MUST add a Reason header for
      each one of those entries as per the procedures of Section 6.3.3.

   Step 3:  Generating New Entries for Each Outgoing Requests

      Same as per Step 3 above for the normal forwarding case
      Section 5.1.1.

5.1.3.  Re-sending based on redirection response

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

   Step 1:  Including Previous Entries

      The proxy MUST include the History-Info header fields that were
      sent in the outgoing request that is being redirected.

   Step 2:  Tagging the Last Entry

      The proxy then examines the last hi-entry of the History-Info that
      was just generated in Step 1 and MUST add a Reason header this
      entry as per the procedures of Section 6.3.3.

   Step 3:  Generating New Entries for Each Outgoing Requests

      Same as per Step 3 for the normal forwarding case Section 5.1.1.







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5.2.  Sending History-Info in Responses

   A proxy that receives a request with the "histinfo" option tag in the
   Supported header, SHOULD forward captured History-Info in subsequent,
   provisional, and final responses to the request sent by the ultimate
   UAS (see Section 4.2).

   A proxy MAY anonymize any hi-entry whose domain corresponds to a
   domain for which it is responsible (as per [RFC3323]).  For example,
   a proxy could require that such entries be anonymized when a response
   is forwarded to a domain for which the proxy is not responsible.

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


6.  The History-Info header field

6.1.  Definition

   History-Info is a header field as defined by [RFC3261].  "It may
   appear in any initial request for a dialog, standalone request or
   responses associated with these requests.  For example, History-Info
   may appear in INVITE, REGISTER, MESSAGE, REFER, OPTIONS, SUBSCRIBE,
   and PUBLISH and any valid responses, plus NOTIFY requests that
   initiate a dialog.

   The History-Info header carries the following information, with the
   mandatory parameters required when the header is included in a
   request or response:

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

   o  Index (hi-index): A mandatory parameter for History-Info
      reflecting 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 5.1), including the index and securing the
      header, the ordering of the History-Info headers in the request is
      assured (SEC-req-2, see Appendix A.1).  In addition, applications



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

   o  Privacy: An optional parameter for History-Info, reflected in the
      History-Info header field values by including the Privacy Header
      [RFC3323] escaped in the hi- targeted-to-uri or by adding the
      Privacy header 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 escaped in the
      hi-targeted-to-uri means that a specific hi-entry MUST be
      anonymized.

   o  Target (hi-target): An optional parameter for the History-Info
      header.  This parameter is added if the URI to which the outgoing
      request is targeted contains a "hit" URI parameter Section 6.3.1.
      The hi-target is added for a hi-entry when it is first added in a
      History-Info header field, and only one value is permitted.  Upon
      receipt of a request or response containing the History-Info
      header, a UA can determine the mechanism by which the target was
      determined.  The following attributes are defined for this
      parameter derived from the values of the "hit" URI parameter:

      *  "rc": The hi-target is set to "rc" if the "hit" URI parameter
         is set to a value of "rc".

      *  "mp": The hi-target is set to "mp" if the "hit" URI parameter
         is set to a value of "mp".The "mp" attribute in the hi-target
         parameter includes the index parameter for the hi-targeted-to-
         uri that was retargeted, thus identifying the "mapped from"
         target.

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

   The following summarizes the syntax of the History-Info header, based
   upon the standard SIP syntax [RFC3261]:







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

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

   hi-targeted-to-uri = name-addr

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

   hi-index = "index" EQUAL 1*DIGIT *("." 1*DIGIT)

   hi-target = "rc" / mp-param

   mp-param = "mp" EQUAL 1*DIGIT *("." 1*DIGIT)

   hi-extension = generic-param


   The following rules apply:

   o  There MUST be exactly one hi-index parameter per hi-entry.

   o  There MUST be no more than one hi-target parameter.

   o  There MAY be any number of hi-extension parameters.

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

6.2.  Examples

   The following provides some examples of the History-Info header.
   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

6.3.  Procedures

   The following sections define procedures for processing of the
   History-Info header and the "hit" SIP/SIPS URI parameter and Privacy
   header.  These procedures may be applicable to processing entities
   such as Proxies, Redirect Servers or User Agents.



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6.3.1.  SIP/SIPS URI target parameter for History-Info Header

   This specification defines a new SIP/SIPS URI Parameter indicating
   the mechanism by which a new target for a request is determined.
   This parameter is added to the SIP/SIPS URIs as they are added to the
   target set per the procedures of 16.5 [RFC3261]] or by a redirect
   server as it populates the Contact header for a 3xx response.  This
   parameter is used to populate the hi-target parameter Section 6.3.5
   when a Request-URI is first added in a hi-entry in the History-Info
   header field.  This parameter MUST be removed from the URI when the
   hi-entry is constructed.

   The following two values are defined for this URI parameter:

   o  "rc": The Request-URI is a contact that is bound to an AOR in an
      abstract location service.  The AOR-to-contact binding has been
      placed into the location service by a SIP Registrar that received
      a SIP REGISTER request.

   o  "mp": The Request-URI is a URI that represents another user.  This
      occurs when a request is to be statically or dynamically
      retargeted to another user.

   The following defines the ABNF for the "hit" URI parameter:


             hit-param      = "hit=" ("rc" / "mp")


6.3.2.  Privacy in the History-Info Header

   The privacy requirements for this document are described in
   Appendix A.2.

   The History-Info header can inadvertently reveal information about
   the requestor as described in [RFC3323].  The Privacy header is used
   to ensure the privacy requirements are satisfied for both the
   History-Info entries received by the intermediary in the incoming
   request (per PRIV-req-2 in Appendix A.2) and the History-Info entries
   that are added by the intermediary as the request is retargeted per
   PRIV-req-1 in Appendix A.2.  If the requestor has indicated a priv-
   value of "session" or "header" in the request, all History-Info
   entries MUST be anonymized when the request leaves a domain for which
   the intermediary is responsible.

   Privacy can also be associated with a specific History-Info entry,
   and any entry that corresponds to that same user rather than all
   History-Info entries in a request.  For example, if Alice sends a



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   request to Bob without any privacy, and Bob redirects to Carol with
   privacy setup for himself, Carol should receive a request where
   Alice's history information is present, but Bob's has been
   anonymized.  This is accomplished by adding a new priv-value,
   history, to the Privacy header [RFC3323] indicating that a specific
   History-Info header entry can not be forwarded outside the domain.

   In addition, the History-Info header can reveal general routing
   information which may be viewed by a specific intermediary or
   network.  Thus, a proxy can use local policy to determine whether the
   History-Info header entries for it's whole domain are private or not
   when exiting the domain through retargeting (PRIV-req-3).

   It is recognized that satisfying the privacy requirements can impact
   the functionality of this solution by overriding the request to
   generate the information.  Thus, the following recommendations are
   made to ensure optimal functionality without compromising privacy:

      If there is a Privacy header in the request with a priv-value of
      "session", "header", or "history", an hi-entry SHOULD be added if
      the request is being retargeted to a URI associated with a domain
      for which the processing entity is responsible.  If there is no
      Privacy header, but the processing entity's local policies
      indicate that the hi-entry(s) cannot be forwarded beyond the
      domain for which this intermediary is responsible, then a Privacy
      header with a priv-value of "history" SHOULD be associated with
      each hi-entry added by the proxy as the request is forwarded
      within the domain.

      If a request is being retargeted to a URI associated with a domain
      for which the processing identity is not responsible and there is
      a Privacy header in the request with a priv-value of "session",
      "header", or "history", the processing entity MUST anonymize hi-
      entry(s) as per [RFC3323] prior to forwarding, unless the
      processing entity knows a priori that it can rely on a downstream
      processing entity within its domain to apply the requested privacy
      or local policy allows the forwarding.

6.3.3.  Reason in the History-Info Header

   For retargets that are the result of an explicit SIP response, a
   Reason MUST be associated with the hi-targeted-to-uri.  If the SIP
   response does not include a Reason header (see [RFC3326]), the SIP
   Response Code that triggered the retargeting MUST be included as the
   Reason associated with the hi-targeted-to-uri that has been
   retargeted.  If the response contains a Reason header for a protocol
   that is not SIP (e.g., Q.850), it MUST be captured as an additional
   Reason associated with the hi-targeted-to-uri that has been



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   retargeted, along with the SIP Response Code.  If the Reason header
   is a SIP reason, then it MUST be used as the Reason associated with
   the hi-targeted-to-uri rather than the SIP response code.

   If a request has timed out (instead of being explicitly rejected), it
   SHOULD be treated as if a 487 "Request Terminated" error response
   code was received.

6.3.4.  Indexing in the History-Info Header

   In order to maintain ordering and accurately reflect the nesting and
   retargeting of the request, an index MUST be included along with the
   Targeted-to-URI being captured.  Per the syntax in Section 6, the
   index consists of a dot-delimited series of digits (e.g., 1.1.2).
   Each dot reflects a hop or level of nesting; thus, the number of hops
   is determined by the total number of dots.  Within each level, the
   integer reflects the number of peer entities to which the request has
   been routed.  Thus, the indexing results in a logical tree
   representation for the history of the request.  For each level of
   indexing, the index MUST start at 1.  An increment of 1 MUST be used
   for advancing to a new branch.  The first entry MUST be set to 1.

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

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

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

   3.  Retargeting within a processing entity - subsequent instance: For
       each subsequent retargeting of a request by the same processing
       entity, another branch is added.  With the index for each new
       branch calculated by incrementing the last/lowest digit at the
       current level, the index in the next request forwarded by this
       same processing entity, following the example above, would be
       1.1.2.




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   4.  Retargeting based upon a Response: In the case of retargeting due
       to a specific response (e.g., 302), the index would be calculated
       per rule 3.  That is, the lowest/last digit of the index is
       incremented (i.e., a new branch is created), with the increment
       of 1.  For example, if the index in the History-Info header of
       the received request was 1.2, then the 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 index MUST be captured
       for each forked request per the rules above, with each new
       request having a unique index.  Each index are sequentially
       assigned.  For example, if the index in the last History-Info
       header field in the received request is 1.1, this processing
       entity would initialize its index to 1.1.1 for the first fork,
       1.1.2 for the second, and so forth (see Figure 1 for an example).
       Note that for each individual fork, only the entry corresponding
       that that fork is included (e.g., the entry for fork 1.1.1 is not
       included in the request sent to fork 1.1.2, and vice-versa).

   6.  When a response is built and it represents the aggregate of
       multiple forks (e.g., multiple forks that fail), the processing
       entity builds the subsequent response using the aggregated
       information associated with each of those forks and including the
       header entries in the order indicated by the indexing.  For
       example, if a procesing entity received failure responses for
       forks 1.1.1 and 1.1.2, it would forward both the 1.1.1 and 1.1.2
       entries to 1.1.  See Appendix B.1 for an example.  Responses are
       processed as described in Section 16.7 of [RFC3261] with the
       aggregated History-Info entries processed similar to Step 7
       "Aggregate Authentication Header Field Values".

6.3.5.  Request Target in the History-Info Header

   The hi-target attribute MUST be added to the History-Info header if
   the target to which the request is being forwarded contains a "hit"
   URI parameter as defined in Section 6.3.1.

   If the value of the "hit" parameter is "mp", then the index of the
   entry corresponding to the original target (i.e., the "mapped-from"
   target) MUST be added as a parameter to "mp".


7.  Application Considerations

   History-Info provides a very flexible building block that can be used
   by intermediaries and UAs for a variety of services.  The following
   summarizes the categories of information that applications may use:



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

   2.  Entry prior to last entry with hi-target of "rc" in the Request
       received by a UAS - i.e., the Request URI associated with the
       destination of the request was determined based on a Registered
       Contact.

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

   4.  Entry prior to first entry with hi-target of "rc" in the Request
       received by a UAS - i.e., the first Registered Contact to which
       the request was targeted.

   5.  Entry with the index that matches the value of the first entry
       with a "mp" hi-target parameter - i.e., the original target of
       the request.

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

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

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



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   the call was targeted may not be available.


8.  Security Considerations

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

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

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

   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.


9.  IANA Considerations

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

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

9.1.  Registration of New SIP History-Info Header

   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:.



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

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


   The following has been added to the Options Tags section:

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


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

9.2.  Registration of "history" for SIP Privacy Header

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

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


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

9.3.  Registration of "hit" SIP/SIPS URI Parameter

   This specification adds a new value to the IANA registration in the
   "SIP/SIPS URI Parameters" registry, http:/www.iana.org/assignments/
   sip-parameters, as defined in [RFC3969].


         Parameter Name  Predefined Values  Reference



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         ____________________________________________
         hit             Yes ("rc"/"mp")    [RFCxxxx]


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


10.  Acknowledgements

   Jonathan Rosenberg et al produced the document that provided
   additional use cases precipitating the requirement for the new
   "target" parameter in the History-Info header and the new SIP/SIPS
   URI parameter.  Ian Elz provided feedback on the privacy aspects.

   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.


11.  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.





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   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] gratuitly 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 a parameter, "target", to the History-Info header and
       defined a SIP/SIPS URI parameter indicating targets that are
       determined by a registered contact or based on mapping the
       incoming Request-URI to another user to facilitate processing at
       the UAS.  The URI parameter is added to the target URIs as the
       target set is determined (per section 16.5 of [RFC3261]) and
       removed when the "target" parameter is populated, using the URI
       parameter value, in the History-Info header (per section 16.6 of
       [RFC3261]).

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

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

   The first 2 changes are intended to facilitate application usage of
   the History-Info header 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.  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



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   parameters.

11.1.  Backwards compatibility

   This specification is backwards compatible since [RFC4244] allows for
   the addition of new optional parameters.  This specification adds an
   optional parameter "target" to the History-Info header.  Entities
   that have not implemented this specification should ignore this
   parameter, however, per [RFC4244] an entity MUST NOT remove this
   parameter from an hi-entry.  This specification defines a SIP/SIPS
   URI parameter, "hit".  An entity that does not understand the "hit"
   URI parameter SHOULD ignore the parameter and MAY remove the
   parameter from the URI as it is used as the target for a request.


12.  Changes since last Version

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

   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.

   3.  Updated the Application considerations section:

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

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

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

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

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





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   3.  Clarified privacy.

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

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

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

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

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

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

   3.  Removed aor tag

   4.  Added index parameter to "mp"

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

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

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

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

   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 entry
       since the proxy does not have access to the information as to how
       the Contact was determined.

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

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




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   7.  Moved Requirements section into an Appendix.

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

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

   1.  Clarified "retarget" definition.

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

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

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

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

   6.  Other editorial changes to condense and simplify.

   7.  Moved Privacy examples to Appendix.

   8.  Added forking to Basic call example.

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

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

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

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

   4.   Remove redundant Redirect Server example.

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

   6.   Clarified 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



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        that knows how the contact in the 3xx was determined.)

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

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

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

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

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

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

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

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

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

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

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

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

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


13.  References

13.1.  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.



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   [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.

13.2.  Informative References

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

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

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

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

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


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



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       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 should also generate Request History information for
           the 'internal retargeting'.

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

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

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

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

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

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

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

       E.  Chronological ordering of the Request History information.

   5.  REQUEST-VALIDITY-req: Request History is applicable to requests
       not sent within an 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.








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

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

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

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

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

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

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

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

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

A.2.  Privacy Requirements

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



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   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 for informational purposes only.  They are not
   intended to be normative.  A basic forking use case is included,
   along with two use cases illustrating the use of the privacy.

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

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

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

   This scenario illustrates that by providing the History-Info to
   Alice, the end-user or an application at Alice could make a decision
   on how best to attempt finding Bob without sending multiple requests
   to the same destination.  Upon receipt of the response containing the
   History-Info entries, the Request URIs for the History-Info entries



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   tagged with "mp" are extracted.  Those Request-URIs can be compared
   to other URIs (if any) that might be attempted in order to establish
   the session with Bob. Thus, avoiding another INVITE to Bob's home
   phone.  Without this mechanism, Alice might well attempt to reach Bob
   at his office phone, which would then retarget the request to Bob's
   home phone.  When that attempt failed, then Alice might attempt to
   reach Bob directly at his home phone, unknowingly for a third time.

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














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   Message Details

   F1 INVITE alice -> example.com

   INVITE sip:alice@example.com SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   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
   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
   Contact: <sip:office@example.com;hit=mp>
   Content-Length: 0



   F5 ACK 192.0.2.4 -> Bob

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



   F6 INVITE example.com -> office

   INVITE sip:office@192.0.2.3.com SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=2
   Via: SIP/2.0/TCP 192.0.2.3:5060
   From: Alice <sip:alice@example.com>
   To: Bob <sip:bob@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   Record-Route: <sip:proxy.example.com;lr>
   History-Info: <sip:bob@example.com>;index=1
   History-Info: <sip:bob@192.0.2.4?Reason=SIP;cause=302>;\
                 index=1.1;rc
   History-Info: <sip:office@example.com>;index=1.2;mp=1
   History-Info: <sip:office@192.0.2.5>;index=1.2.1
   CSeq: 1 INVITE
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>



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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;cause=302>;\
                 index=1.1;rc
   History-Info: <sip:office@example.com>;index=1.2;mp=1
   History-Info: <sip:office@192.0.2.5>;index=1.2.1
   CSeq: 1 INVITE
   Content-Length: 0



   F8 180 Ringing example.com -> alice

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












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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;cause=302>;\
                 index=1.1;rc
   History-Info: <sip:office@example.com>;index=1.2;mp=1
   History-Info: <sip:office@192.0.2.5?Reason=SIP;cause=480>;\
                 index=1.2.1>;index=1.2.1
   History-Info: <sip:home@example.com>;index=1.3;mp=1.2
   History-Info: <sip:home@192.0.2.6>;index=1.3.1
   CSeq: 1 INVITE
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->



   F10 100 Trying home -> example.com

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















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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;cause=302>;\
                 index=1.1;rc
   History-Info: <sip:office@example.com>;index=1.2;mp=1
   History-Info: <sip:office@192.0.2.5?Reason=SIP;cause=480>;\
                 index=1.2.1>;index=1.2.1
   History-Info: <sip:home@example.com>;index=1.3;mp=1.2
   History-Info: <sip:home@192.0.2.6>;index=1.3.1
   CSeq: 1 INVITE
   Content-Length: 0



   F12 486 Busy Here example.com -> alice

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












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   F13 ACK example.com -> home

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



   F14 ACK alice -> example.com

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


B.2.  History-Info with Privacy Header

   This example provides a basic call scenario without forking, with
   sip:biloxi.example.com adding the Privacy header indicating that the
   History-Info header information is anonymized outside the
   biloxi.example.com domain.





















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

                   Figure 2: Example with Privacy Header

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

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



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   information about the history of the request, but anonymizing Bob's
   local URI.

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


          Figure 3: Example with Privacy Header for Specific URI






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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@agnada.com


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

   Email: ietf.hanserik@gmail.com


   Christer Holmberg
   Ericsson
   Hirsalantie 11, Jorvas
   Finland

   Email: christer.holmberg@ericsson.com












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