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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: April 29, 2011                                      S. Schubert
                                                                     NTT
                                                           J. van Elburg
                                              Detecon International Gmbh
                                                             C. Holmberg
                                                                Ericsson
                                                        October 26, 2010


   An Extension to the Session Initiation Protocol (SIP) for Request
                          History Information
                  draft-ietf-sipcore-rfc4244bis-02.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.  SIP
   header field parameters are defined to tag the method by which the
   target of a request is determined.  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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

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

Copyright Notice



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   Copyright (c) 2010 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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   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 of Operation  . . . . . . . . . . . . . . . . . . . .  6
   5.  General User Agent Behavior  . . . . . . . . . . . . . . . . . 10
     5.1.  User Agent Client (UAC) Behavior . . . . . . . . . . . . . 10
     5.2.  User Agent Server (UAS) Behavior . . . . . . . . . . . . . 10
       5.2.1.  Processing of Requests with History-Info . . . . . . . 10
       5.2.2.  Generation of Responses with History-Info  . . . . . . 11
     5.3.  Redirect Server Behavior . . . . . . . . . . . . . . . . . 11
   6.  Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  Adding the History-Info Header to Requests . . . . . . . . 12
       6.1.1.  Initial Request  . . . . . . . . . . . . . . . . . . . 12
       6.1.2.  Re-sending based on failure response . . . . . . . . . 13
       6.1.3.  Re-sending based on redirection response . . . . . . . 14
     6.2.  Sending History-Info in Responses  . . . . . . . . . . . . 14
   7.  The History-Info header field  . . . . . . . . . . . . . . . . 14
     7.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . . 14
     7.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 17
     7.3.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . 17
       7.3.1.  Privacy in the History-Info Header . . . . . . . . . . 17
       7.3.2.  Reason in the History-Info Header  . . . . . . . . . . 18
       7.3.3.  Indexing in the History-Info Header  . . . . . . . . . 18
       7.3.4.  Request Target in the History-Info Header  . . . . . . 20
   8.  Application Considerations . . . . . . . . . . . . . . . . . . 21
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
     10.1. Registration of New SIP History-Info Header  . . . . . . . 23
     10.2. Registration of "history" for SIP Privacy Header . . . . . 24
     10.3. Registration of Header Field Parameters  . . . . . . . . . 24
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
   12. Changes from RFC 4244  . . . . . . . . . . . . . . . . . . . . 25
     12.1. Backwards compatibility  . . . . . . . . . . . . . . . . . 26
   13. Changes since last Version . . . . . . . . . . . . . . . . . . 27
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 32
     14.2. Informative References . . . . . . . . . . . . . . . . . . 32
   Appendix A.  Request History Requirements  . . . . . . . . . . . . 33
     A.1.  Security Requirements  . . . . . . . . . . . . . . . . . . 34
     A.2.  Privacy Requirements . . . . . . . . . . . . . . . . . . . 35
   Appendix B.  Example call flows  . . . . . . . . . . . . . . . . . 35
     B.1.  Sequentially Forking (History-Info in Response)  . . . . . 35
     B.2.  History-Info with Privacy Header . . . . . . . . . . . . . 43
     B.3.  Privacy Header for a Specific History-Info Entry . . . . . 45
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 46




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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 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.  SIP
   header field parameters are defined to tag the method by which the
   target of a request is determined.  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 to the process
   of a SIP entity changing a Uniform Resource Identifier (URI) in a
   request based on the rules for determining request targets as
   described in Section 16.5 of [RFC3261] and of the subsequent
   forwarding of that request as described in step 2 in section 16.6 of
   [RFC3261].  This includes changing the Request-URI due to a location
   service lookup and redirect processing.  This also includes internal
   (to a proxy/SIP intermediary) changes of the URI prior to forwarding
   of the request.

   The terms "location service", "forward", "redirect" and "AOR" are



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   used consistent with the terminology in [RFC3261].

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


3.  Background

   SIP implicitly provides retargeting capabilities that enable calls 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



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      of the initial request.

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

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


4.  Overview of Operation

   The fundamental functionality provided by the request history
   information is the ability to inform proxies and UAs involved in
   processing a request about the history or progress of that request
   (CAPABILITY-req, see Appendix A).  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 early or established dialog (e.g., INVITE, REGISTER, MESSAGE,
   REFER and OPTIONS, PUBLISH and SUBSCRIBE, etc.)  (REQUEST-VALIDITY-
   req, see Appendix A) and any provisional or final responses to these
   requests (ISSUER-req, see Appendix A).

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

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

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

   o  Reason: Reason describes why an entry was retargeted.

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

   o  Target: A parameter indicating whether the Targeted-to-URI is a
      registered contact ("rc") for another user mapped ("mp") from the
      Request-URI in the incoming request that was retargeted.  The
      index of the History-Info entry for the URI that was retargeted is
      included in each of these parameters.  Note that there may be



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      other reasons a request is retargeted such as normal routing and
      forwarding, strict routing, etc., thus not all history-info
      entries have a target header field parameter.  The "rc" and "mp"
      scenarios are what is anticipated to be most useful to end
      applications/users.

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

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

   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 recover that information by
   locating the last hi-entry with an "rc" header field parameter.  This
   "rc" parameter contains the index of the hi-entry containing the lost
   target information - i.e., the sip:bob@biloxi.example.com entry with
   index=1.1.  Note that an hi-entry is not included for the fork to
   sip:bob@192.0.2.7 since there was no response at the time the 200 OK
   is sent to Alice.

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





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      Note: This example uses loose routing procedures.


















































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




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                           Figure 1: Basic Call


5.  General User Agent Behavior

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

5.1.  User Agent Client (UAC) Behavior

   The UAC MUST include the "histinfo" option tag in the Supported
   header in any new or out-of-dialog request for which the UAC would
   like the History-Info header in the response.  In addition, the UAC
   SHOULD add a History-Info header, using the Request-URI of the
   request as the hi-targeted-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.  In the case of a B2BUA
   implementation, a UAC MAY add the hi-entries received in the incoming
   request at the UAS to the subsequent outgoing request.

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

   In the case where a UAC receives a 3xx response with a Contact header
   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 7.3.2.  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
   following the rules for "Retargeting based upon a Response" as
   prescribed in Section 7.3.3.  If the Contact header contained any of
   the header parameter fields defined in this specification, the UAC
   MUST include the header parameter field as a header parameter field
   associated with the current hi-entry as described in Section 7.3.4.

5.2.  User Agent Server (UAS) Behavior

5.2.1.  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 MUST contain the Request-URI for the received request,



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   unless the previous entity that forwarded the request does not
   support the History-Info header.  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.  If privacy is required, a privacy header with
   a value of "history" MUST be added to the hi-entry.  The UAS MUST
   include an hi-index attribute as described in Section 7.3.3.  The UAS
   MUST NOT include a hi-target attribute, since the UAS has no way to
   know the mechanism by which the Request-URI was determined.  The
   addition of the missing hi-entry ensures that the most complete
   information can be provided in the response and provides consistency
   in the information presented to applications.  The information can
   also be useful for implementations with B2BUAs that include the
   History-Info, received in the incoming request, in the outgoing
   request.

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

5.2.2.  Generation of Responses with History-Info

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

   The processing of History-Info in responses follows 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".

5.3.  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 add any new History-Info entries in cases of retargeting (both
   internal and to other SIP entities) in the same manner as prescribed
   for a proxy Section 6.  In generating the Contact header in the 3xx
   response, the redirect server MUST add the appropriate target header



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   field parameter to each Contact header as described in Section 7.3.4.


6.  Proxy Behavior

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

6.1.  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 6.1.1)

   o  Resending based on failure response (see Section 6.1.2)

   o  Resending based on redirection response (see Section 6.1.3)

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

6.1.1.  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).

   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



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      based to the value of Request-URI in the incoming request, unless
      privacy is required.  If privacy is required, the procedures of
      Section 7.3.1 MUST be used.  The proxy MUST NOT include a hi-
      target attribute.  The proxy MUST include an hi-index attribute
      with a value of "1", as described in Section 7.3.3.

   Step 2:  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 7.3.1 MUST be used.
      The proxy MUST include an hi-index for each of the separate hi-
      entry(s) as described in Section 7.3.3.  The proxy MUST include
      the appropriate hi-target header field parameter for each of the
      separate entry(s) as described in Section 7.3.4, if applicable.

6.1.2.  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 408 error response), the proxy MUST
   perform the following steps:

   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 7.3.3).  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 7.3.2.






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   Step 3:  Generating New Entries for Each Outgoing Requests

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

6.1.3.  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 7.3.2.

   Step 3:  Generating New Entries for Each Outgoing Requests

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

6.2.  Sending History-Info in Responses

   A proxy that receives a request with the "histinfo" option tag in the
   Supported header, MUST forward captured History-Info in subsequent,
   provisional, and final responses to the request sent by the ultimate
   UAS (see Section 5.2).  Any hi-entry containing a Privacy header with
   a value of "history" MUST be anonymized prior to a proxy sending a
   response with that hi-entry.

   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".


7.  The History-Info header field

7.1.  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



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   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 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 6.1), 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
      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 and
      Contact headers.  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 header field parameters are
      defined for this parameter:



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

         "mp": The hi-targeted-to-URI represents a user other than the
         user associated with the Request-URI in the incoming
         requesting.  This occurs when a request is to be statically or
         dynamically retargeted to another user.  The value of the "mp"
         header field parameter is 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 MUST ignore it.

   The ABNF syntax for the History-Info header is defined as follows:

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

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

   hi-targeted-to-uri = name-addr

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

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

   hi-index = "index" EQUAL index-val

   hi-target = rc-param / mp-param

   rc-param = "rc" EQUAL index-val

   mp-param = "mp" EQUAL index-val

   hi-extension = generic-param


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

   Note that since both the Reason and Privacy parameters are escaped in
   the hi-targeted-to-uri, these fields will not be available in the



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   case that the hi-targeted-to-uri is a Tel-URI [RFC3966].

7.2.  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=1.2

7.3.  Procedures

   The following sections define procedures for processing of the
   parameters (and headers) associated with the History-Info header.
   These procedures may be applicable to processing entities such as
   Proxies, Redirect Servers or User Agents.

7.3.1.  Privacy in the History-Info Header

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

   As with other SIP headers described in [RFC3323], the History-Info
   header can inadvertently reveal information about the originator.  A
   value of "header" in the Privacy header is used to indicate that such
   headers in the request be privacy protected when the request is
   forwarded by the intermediary.  A value of "history" in the Privacy
   header indicates that the History-Info header should be privacy
   protected.  If there is a Privacy header in the request with a priv-
   value of "header" or "history", then the initial hi-entry MUST be
   anonymized and the header removed when the request leaves a domain
   for which the SIP entity is responsible.

   In addition, the History-Info header can reveal general routing and
   diverting information within an intermediary, which the intermediary
   may want to privacy protect.  In this case, a value of "history" in
   the Privacy header is used to indicate which History-Info header
   entries added by a SIP entity are to be anonymized.  The priv-value
   of "history" MUST be in the privacy header escaped in the hi-
   targeted-to-uri for each hi-entry, added by the SIP entity, as the
   request is retargeted within the domain for which the SIP entity is
   responsible.  If a request is being retargeted to a URI associated



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   with a domain for which the SIP entity is not responsible, the
   processing entity MUST anonymize the hi-entries with a priv-value of
   "history" and MUST remove the Privacy header from the hi-entries
   prior to forwarding, unless the processing entity knows a priori that
   it can rely on a downstream processing entity to apply the requested
   privacy.  The mechanism for the latter functionality is outside the
   scope of this specification.

   Finally, the terminator of the request may not want to reveal the
   final reached target to the originator.  In this case, the terminator
   uses the a value of "history" in the Privacy header in the last hi-
   entry in the response.  The SIP entity that forwards the response
   MUST anonymize that hi-entry and remove the Privacy header.

7.3.2.  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
   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
   MUST be treated as if a 408 "Request Terminated" error response code
   was received.

7.3.3.  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 7, 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.

   The first index in a series of History-Info entries MUST be set to 1.
   In the case that a proxy adds an entry on behalf of the previous hop,
   the index MUST be set to 1.  For each level of indexing, the index
   MUST start at 1.  An increment of 1 MUST be used for advancing to a



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   new branch.

   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 MUST read the value from
       the History-Info header in the received request and MUST add
       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 MUST be added.  The index for each new
       branch MUST be 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.

   4.  Retargeting based upon a Response: In the case of retargeting due
       to a specific response (e.g., 302), the index MUST be calculated
       per rule 3.  That is, the lowest/last digit of the index MUST be
       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 sent request is 1.2 and the response to the request is a 302,
       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 MUST be sequentially
       assigned.  For example, if the index in the last History-Info
       header field in the received request is 1.1, this processing
       entity would initialize its index to 1.1.1 for the first fork,
       1.1.2 for the second, and so forth (see Figure 1 for an example).
       Note that for each individual fork, only the entry corresponding
       to 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).



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   6.  When a response is built and it represents the aggregate of
       responses to multiple forks (e.g., multiple forks that fail), the
       processing entity MUST build the subsequent response using the
       aggregated information associated with each of the forks for
       which there have been responses and MUST include the header
       entries in the order indicated by the indexing.  For example, if
       a procesing entity received failure responses for forks 1.1.1 and
       1.1.2, it would return both the 1.1.1 and 1.1.2 entries to the
       entity that generated the hi-entry with index of 1.  See
       Appendix B.1 for an example.  Note that in the case of parallel
       forking where one fork is successful, only the forks for which
       responses have been received at the time the proxy sends the
       successful response are included in that response.  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".

7.3.4.  Request Target in the History-Info Header

   This specification defines two header field parameters, "rc" and
   "mp", indicating two non-inclusive mechanisms by which a new target
   for a request is determined.  The specific parameter field to be
   included in the History-Info header is determined as the targets are
   added to the target set per the procedures of 16.5 or when the
   Contact header field in a 3xx response is populated.  Both parameters
   contain an index whose value corresponds to the index in the hi-entry
   containing the Request-URI that was retargeted.

   The specific parameter field to be included in the History-Info
   header is determined as follows:

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

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

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





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

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


8.  Application Considerations

   History-Info provides a very flexible building block that can be used
   by intermediaries and UAs for a variety of services.  Prior to any
   application usage of the information the entries MUST be evaluated to
   determine gaps in indices.  If gaps are detected, this MUST NOT be
   treated as an error since gaps are possible if the request is
   forwarded through intermediate entities that do not support the
   History-Info header.  The interpretation of the information in the
   History-Info header depends upon the specific application; an
   application might need to provide special handling in some cases
   where there are gaps.

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

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

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

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

   4.  Entry with the index that matches the value of the first entry
       with a "rc" header parameter in the Request received by a UAS.
       Note, this would be the original AoR if all entries support the
       History-Info header and there is absence of a "mp" header
       parameter prior to the "rc" header parameter in the History-Info
       header.  However, there is no guarantee that all entities will
       support History-Info, thus the first entry with an "rc" header



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       parameter within the domain associated with the target URI at the
       destination is more likely to be useful.

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

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

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

   Since support for History-Info 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
   the call was targeted may not be available.


9.  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



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   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.


10.  IANA Considerations

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

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

10.1.  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:.

   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:




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


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

10.2.  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@polycom.com


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

10.3.  Registration of Header Field Parameters

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


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


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



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   this specification.


11.  Acknowledgements

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

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

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


12.  Changes from RFC 4244

   This RFC replaces [RFC4244].

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

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

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

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





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

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

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

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

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

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

   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
   header parameters.

12.1.  Backwards compatibility

   This specification is backwards compatible since [RFC4244] allows for
   the addition of new optional parameters.  This specification adds an



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   optional SIP header field parameter to the History-Info and Contact
   headers.  Entities that have not implemented this specification MUST
   ignore these parameters, however, per [RFC4244] an entity MUST NOT
   remove this parameter from an hi-entry.


13.  Changes since last Version

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

   Changes from 01 to 02:

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

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

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

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

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

   6.   Fixes to Application considerations section.  [Issues: 12-15]

   7.   Changed language in the procedure for Indexing to normative
        language.

   8.   Clarifications for UAC processing:

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

        *  Clarify applicability to B2BUA.  [Issue: 29]

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

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

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

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




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

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

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

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

   Changes from 00 to 01:

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

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

   3.  Updated the Application considerations section:

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

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

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

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

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

   3.  Clarified privacy.

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

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




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   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.

   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.




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

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

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

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

   6.  Other editorial changes to condense and simplify.

   7.  Moved Privacy examples to Appendix.

   8.  Added forking to Basic call example.

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

   1.  Clarified "retarget" definition.

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

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

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

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

   6.  Other editorial changes to condense and simplify.

   7.  Moved Privacy examples to Appendix.

   8.  Added forking to Basic call example.

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

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





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

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

   4.   Remove redundant Redirect Server example.

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

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

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

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

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

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

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

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

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

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

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

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

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

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



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


14.  References

14.1.  Normative References

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

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

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

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

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

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

14.2.  Informative References

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

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

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

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

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



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              BCP 99, RFC 3969, December 2004.

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


Appendix A.  Request History Requirements

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

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

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

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

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

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

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

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

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

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

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

       E.  Chronological ordering of the Request History information.




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

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

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

A.1.  Security Requirements

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

   The potential security problems include the following:

   1.  A rogue application could insert a bogus Request History 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.




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

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

A.2.  Privacy Requirements

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

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

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

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


Appendix B.  Example call flows

   The scenarios in this section provide sample use cases for the
   History-Info header 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



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   sip:bob@example.com, but rather different AORs that have been
   configured as alternate addresses for Bob in the proxy.  In other
   words, Office and Bob are not bound through SIP Registration with
   Bob's AOR.  This type of arrangement is common for example when a
   "routing" rule to a PSTN number is manually configured in a Proxy.
   These hi-entries are identified by the index contained in the hi-
   target "mp" parameter in the hi-entries.

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































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



   Message Details

   F1 INVITE alice -> example.com

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



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   F2 INVITE  example.com -> Bob

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



   F3 100 Trying example.com -> alice

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




   F4 302 Moved Temporarily Bob -> example.com

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




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   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=1
   History-Info: <sip:office@example.com>;index=1.2;mp=1
   History-Info: <sip:office@192.0.2.5>;index=1.2.1
   CSeq: 1 INVITE
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>
   <!-- SDP Not Shown -->



















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



   F8 180 Ringing example.com -> alice

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
















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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=1
   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
   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=1
   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
   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=1
   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
   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.  Alice
   has indicated that she wants Privacy associated with her History-Info
   entry and sip:biloxi.example.com adds Privacy headers indicating that
   the History-Info header information is anonymized outside the
   biloxi.example.com domain.  Note, that if the atlanta.example.com
   proxy had added privacy headers to all its hi-entries, then all the
   hi-entries in the response would be anonymous.


















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

                   Figure 2: Example with Privacy Header








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











































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


          Figure 3: Example with Privacy Header 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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