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Versions: (RFC 4244) 00 01 02 03 draft-ietf-sipcore-rfc4244bis

Network Working Group                                          M. Barnes
Internet-Draft                                                    Nortel
Obsoletes: RFC4244                                              F. Audet
(if approved)                                                 Skype Labs
Intended status: Standards Track                             S. Schubert
Expires: April 29, 2010                                              NTT
                                                           J. van Elburg
                                              Detecon International Gmbh
                                                             C. Holmberg
                                                                Ericsson
                                                        October 26, 2009


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

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   This Internet-Draft will expire on April 29, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
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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.






























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  5
   3.  Overview of Operations . . . . . . . . . . . . . . . . . . . .  6
   4.  General User Agent Behavior  . . . . . . . . . . . . . . . . .  9
     4.1.  User Agent Client (UAC) Behavior . . . . . . . . . . . . .  9
     4.2.  User Agent Server (UAS) Behavior . . . . . . . . . . . . . 10
       4.2.1.  Redirect Server Behavior . . . . . . . . . . . . . . . 11
   5.  Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.1.  Adding the History-Info Header to Requests . . . . . . . . 12
       5.1.1.  Initial Request  . . . . . . . . . . . . . . . . . . . 12
       5.1.2.  Re-sending based on failure response . . . . . . . . . 13
       5.1.3.  Re-sending based on redirection response . . . . . . . 14
     5.2.  Sending History-Info in Responses  . . . . . . . . . . . . 15
   6.  The History-Info header field  . . . . . . . . . . . . . . . . 15
     6.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 17
     6.3.  Procedures . . . . . . . . . . . . . . . . . . . . . . . . 18
       6.3.1.  Privacy in the History-Info Header . . . . . . . . . . 18
       6.3.2.  Reason in the History-Info Header  . . . . . . . . . . 19
       6.3.3.  Indexing in the History-Info Header  . . . . . . . . . 19
       6.3.4.  Request Target in the History-Info Header  . . . . . . 21
   7.  Application Considerations . . . . . . . . . . . . . . . . . . 21
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
     9.1.  Registration of New SIP History-Info Header  . . . . . . . 23
     9.2.  Registration of "history" for SIP Privacy Header . . . . . 24
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 24
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
   12. Changes from RFC 4244  . . . . . . . . . . . . . . . . . . . . 24
     12.1. Backwards compatibility  . . . . . . . . . . . . . . . . . 26
   13. Changes since last Version . . . . . . . . . . . . . . . . . . 26
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 29
     14.2. Informative References . . . . . . . . . . . . . . . . . . 29
   Appendix A.  Request History Requirements  . . . . . . . . . . . . 30
     A.1.  Security Requirements  . . . . . . . . . . . . . . . . . . 31
     A.2.  Privacy Requirements . . . . . . . . . . . . . . . . . . . 32
   Appendix B.  Detailed call flows . . . . . . . . . . . . . . . . . 32
     B.1.  Sequentially Forking (History-Info in Response)  . . . . . 32
     B.2.  Voicemail  . . . . . . . . . . . . . . . . . . . . . . . . 40
     B.3.  Automatic Call Distribution  . . . . . . . . . . . . . . . 42
     B.4.  History-Info with Privacy Header . . . . . . . . . . . . . 44
     B.5.  Privacy Header for a Specific History-Info Entry . . . . . 45
     B.6.  Determining the Alias used.  . . . . . . . . . . . . . . . 47
     B.7.  GRUU . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
     B.8.  Limited Use Address  . . . . . . . . . . . . . . . . . . . 51



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     B.9.  Sub-Address  . . . . . . . . . . . . . . . . . . . . . . . 53
     B.10. Service Invocation . . . . . . . . . . . . . . . . . . . . 57
     B.11. Toll Free Number . . . . . . . . . . . . . . . . . . . . . 58
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 60















































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1.  Introduction

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

   This document defines a SIP header, History-Info, to provide a
   standard mechanism for capturing the request history information to
   enable a wide variety of services for networks and end-users.  The
   History-Info header provides a building block for development of new
   services.

   The requirements for this document are described in Appendix A.


2.  Conventions and Terminology

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

   The term "retarget" is used in this document to refer both to the
   process of a Proxy Server/User Agent Client (UAC) changing a Uniform
   Resource Identifier (URI) in a request based on the rules for
   determining request targets as described in Section 16.5 of [RFC3261]
   and the subsequent forwarding of that request as described in section
   16.6 of [RFC3261].

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

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



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3.  Overview of Operations

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

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

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

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

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

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

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

   o  Some diagnostic information for debugging SIP requests.  (Note
      that the diagnostic utility of this mechanism is limited by the
      fact that its use by entities that retarget is optional.)

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

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

   o  Preserving service specific URIs that can be overwritten by a
      downstream proxy, such as those defined in [RFC3087], and control



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      of network announcements and IVR with SIP URI [RFC4240].

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

   The fundamental functionality provided by the request history
   information is the ability to inform proxies and UAs involved in
   processing a request about the history or progress of that request
   (CAPABILITY-req, see Appendix A).  The solution is to capture the
   Request-URIs as a request is retargeted, in a new header for SIP
   messages: 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.  This solution proposes no changes in the fundamental
   determination of request targets or in the request forwarding as
   defined in Sections 16.5 and 16.6 of the SIP protocol specification
   [RFC3261].

   The History-Info header can appear in any request not associated with
   an established dialog (e.g., INVITE, REGISTER, MESSAGE, REFER and
   OPTIONS, PUBLISH and SUBSCRIBE, etc.)  (REQUEST-VALIDITY-req,
   seeAppendix A) and any valid response to these requests (ISSUER-req,
   seeAppendix A).

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

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

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

   o  Reason: Reason describes why an entry was retargeted.

   o  Privacy: Privacy is used to request that entries be anonymized.

   o  Target: The target parameter indicates the mechanism by which the
      new target is determined, i.e., a "registered contact", or a
      "mapped URI"

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

   In this example, Alice (sip:alice@atlanta.example.com) calls Bob



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

   One important thing illustrated by this call flow is that without
   History-Info, Bob would "lose" the target information, including any
   parameters in the request URI.  Bob can now recover that information
   by looking for the prior entry to the last hi-entry marked as "rc"

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

      Note: This example uses loose routing procedures.


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



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


                           Figure 1: Basic Call


4.  General User Agent Behavior

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

4.1.  User Agent Client (UAC) Behavior

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



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   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 MUST include in
   the outgoing request the previous hi-entry(s) received in the
   response.  The UAC MUST evaluate the last hi-entry in the 3xx
   response and verify that they are the same (as per the procedures in
   section Section 4.2.1); if the hi-entry is not the same as the value
   in contact, hi-entry MUST be added using the value of Contact.

   If the hi-entry for the redirection is not included in the 3xx
   response, then an hi-entry MUST be added to the outgoing request.  In
   this case, the index MUST be created by reading and incrementing the
   value of the index from the previous hi-entry, thus following the
   same rules as those prescribed for a proxy in retargeting, described
   in Section 6.3.3.  The reason MUST be added per Section 6.3.2.  The
   hi-target and hi-aor attributes MUST NOT be added to this hi-entry
   since there is no way to know the mechanism by which the redirecting
   entity determined the URI in the Contact header nor whether the
   previous hi-targeted-to-uri was an AOR.

   If no hi-entry for redirection were included at all in the 3xx
   response, and multiple redirection occurs, the UAC MAY attempt to
   synthetise the missing hi-entrie(s) before inserting the last one (as
   per the previous step).  At a minimum, the last entry (as per the
   previous step) MUST be included.

   With the exception of the processing of a 3xx response described
   above, the processing of the History-Info header received in the
   Response is application specific and outside the scope of this
   document.

4.2.  User Agent Server (UAS) Behavior

   Once the request terminates at the UAS, the processing of the
   information in the History-Info header by a UAS in a Request depends
   upon local policy and specific applications at the UAS that might
   make use of the information.  Prior to any application usage of the
   information, the validity SHOULD be ascertained.  For example, the
   entries MAY be evaluated to determine gaps in indices, which could
   indicate that an entry has been maliciously removed or removed for
   privacy reasons.  Either way, an application MAY want to be aware of
   potentially missing information.




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   If the "histinfo" option tag is received in a request, the UAS MUST
   include any History-Info received in the request in the subsequent
   response.  If privacy is required, entries MUST be anonymized using
   [RFC3323].  The UAS MUST follow the rules for a redirect server per
   Section 4.2.1 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".

4.2.1.  Redirect Server Behavior

   A redirect server MUST include the History-Info headers received in
   the request in the 3XX response that it sends, and it MUST perform
   the following steps:

   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 hop proxy does not support History-Info), the redirect
      server MUST insert an hi-entry.  The redirect server MUST set the
      hi-targeted-to-uri to the value of Request URI in the incoming
      request, unless privacy is required.  If privacy is required, the
      procedures of Section 6.3.1 MUST be used.  The proxy MUST NOT
      include a hi-target attribute.  The proxy MUST include an hi-index
      attribute as described in Section 6.3.3.

   Step 2:  Tagging the Last Incoming Entry

      The redirect server then examines the last hi-entry of the
      History-Info header resulting from the previous step.  If privacy
      is required for this entry, the procedures of Section 6.3.1 MUST
      be used for that entry.  The Reason header MUST be added to that
      entry as per the procedures of Section 6.3.2, and must be set to
      the proper SIP 3XX response.

   Step 3:  Generating New Entries for the Response

      The redirect servert MUST add a new hi-entry for each of the
      Contact header URIs, which becomes the new Request-URIs when the
      recipient forwards the new Request.  The index is created as
      described in Section 6.3.3.  If privacy is required, the
      procedures of Section 6.3.1 MUST be used.  A hi-target parameter
      MUST be included if the new Request-URI either represents another



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      user or registered contact as per the procedures of Section 6.3.4.

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


5.  Proxy Behavior

   The specific processing by proxies for adding the History-Info
   headers in Requests and Responses is described in this section for
   the following cases:

   o  Forwarding of initial request (see Section 5.1.1)

   o  Resending based on failure response (see Section 5.1.2)

   o  Resending based on redirection response (see Section 5.1.3)

5.1.  Adding the History-Info Header to Requests

   This section describes the process of adding the History-Info Header
   to Requests.

   Retargeting is an iterative process, i.e., a proxy may redirect
   "internally " more than one time.  A typical example would be a proxy
   that redirects a request first to a different user (i.e., it maps to
   a different AOR), and then forwards to a registered contact bound to
   that new AOR.  A proxy that uses such mechamism SHOULD add multiple
   hi-entry fields to provide a logical description of the retargeting
   process.

5.1.1.  Initial Request

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





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   Step 1:  Adding Entries on Behalf of Previous Hops

      If an incoming request does not already have a History-Info header
      field (e.g., the UAC does not include any History-Info header and
      no proxies in between support History-Info), or if the Request-URI
      of the incoming request does not match the last hi-entry (e.g.,
      the last proxy does not support History-Info), the proxy MUST
      insert an hi-entry.  The proxy MUST set the hi-targeted-to-uri
      based to the value of Request URI in the incoming request, unless
      privacy is required.  If privacy is required, the procedures of
      Section 6.3.1 MUST be used.  The proxy MUST NOT include a hi-
      target attribute.  The proxy MUST include an hi-index attribute as
      described in Section 6.3.3.

   Step 2:  Generating New Entries for Each Outgoing Requests

      The proxy then proceeds to determining the request targets as per
      16.5/[RFC3261] and request forwarding as per 16.6/[RFC3261].  The
      proxy MUST add a separate hi-entry in each separate outgoing
      request for each of the current (outgoing) targets in the target
      set.  The proxy MUST set the hi-targeted-to-uri in those separate
      hi-entry(s) to the value of the Request-URI of the current
      (outgoing) request, unless privacy is required.  If privacy is
      required, the procedures of Section 6.3.1 MUST be used.  The proxy
      MUST include a hi-target attribute for each of the separate
      entry(s) as described in Section 6.3.4.  The proxy MUST include an
      hi-index for each of the separate hi-entry(s) as described in
      Section 6.3.3.

5.1.2.  Re-sending based on failure response

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

   Step 1:  Including the Entries from Error Responses & Timeouts

      The proxy MUST build the History-Info header field(s) sent in the
      outgoing request using the aggregate information associated with
      the received error responses(s) and timeout(s) for all the
      branches that are generating failures, including the header
      entries in the order indicated by the indexing (see
      Section 6.3.3).  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.




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   Step 2:  Tagging the Last Entries

      The proxy then examines the last hi-entry of the History-Info that
      was just generated in Step 1 for each one of the branches that
      generated failures or timeouts and MUST add a Reason header for
      each one of those entries as per the procedures of Section 6.3.2.

   Step 3:  Generating New Entries for Each Outgoing Requests

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

5.1.3.  Re-sending based on redirection response

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

   Step 1:  Including Previous Entries

      If the received 3XX response does not include any History-Info
      header fields, the proxy MUST include the History-Info header
      fields that were sent in the outgoing request that is being
      redirected.  The proxy MUST then perform Steps 2 and 3.

      If the 3XX response contains a History-Info Header field, but the
      last entries does not correspond to the current target (i.e., they
      do not correspond to the Contact(s) in the 3XX), the proxy MUST
      include in the outgoing request the same History-Info header
      fields that were received in the 3XX response.  The proxy MUST
      then perform Steps 2 and 3.

      If the 3XX response contains a History-Info Header field and the
      last entries correspond to the current target (i.e., they
      correspond to the Contact(s) in the 3XX), the proxy MUST include
      in the outgoing request the same History-Info header fields that
      were received in the 3XX response.  No other entries need to be
      added as this is the complete set: the proxy MUST NOT perform
      Steps 2 and 3.

   Step 2:  Tagging the Last Entry

      The proxy then examines the last hi-entry of the History-Info that
      was just generated in Step 1 and MUST add a Reason header this
      entry as per the procedures of Section 6.3.2.







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   Step 3:  Generating New Entries for Each Outgoing Requests

      Same as per Step 3 above for the normal forwarding case, except
      that the hi-target parameter MUST NOT be set when the proxy
      receiving the 3xx does not know the mechanism by which this target
      was determined.  For example, the proxy can not determine the hi-
      target mechanism when the domain of the Contact is not under the
      control of the proxy.  However, if it under the control of the
      proxy, then it may be able to determine the mechanism (e.g., Bob
      can deflect a call to his SIP PC client to his cell phone).

5.2.  Sending History-Info in Responses

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

   A proxy MAY anonymize any hi-entry whose domain corresponds to a
   domain for which it is responsible (as per [RFC3323]).  For example,
   anonymity may be required when responses are forwarded to a domain
   for which it is not responsible.

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


6.  The History-Info header field

6.1.  Definition

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

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

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




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   o  Index (hi-index): A mandatory parameter for History-Info
      reflecting the chronological order of the information, indexed to
      also reflect the forking and nesting of requests.  The format for
      this parameter is a string of digits, separated by dots to
      indicate the number of forward hops and retargets.  This results
      in a tree representation of the history of the request, with the
      lowest-level index reflecting a branch of the tree.  By adding the
      new entries in order (i.e., following existing entries per the
      details in Section 5.1), including the index and securing the
      header, the ordering of the History-Info headers in the request is
      assured (SEC-req-2, see Appendix A.1).  In addition, applications
      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
      indicating the mechanism by which the new target is determined,
      based on the procedures of 16.5 [RFC3261].  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 nature of the target.  The following values are defined for
      this parameter:

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

      *  "mp": The entry is a URI that represents another user.  This
         occurs in cases where a request is statically or dynamically
         retargeted to another user.  The index entry of the target of
         the original target is added as a parameter to the "mp" (i.e.,



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         it represents the "mapped from" target).

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

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

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

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

   hi-targeted-to-uri = name-addr

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

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

   hi-target = "rc" / mp-param

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

   hi-extension = generic-param


   The following rules apply:

   o  There MUST exactly 1 hi-index parameter per hi-entry.

   o  There MUST be no more than 1 hi-target parameter.

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

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

6.2.  Examples

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









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

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

6.3.  Procedures

   The following sections defines procedures for different parameters in
   the History-Info header.  These procedures may be applicable to
   "processing entities" such as Proxies, Redirect Servers or User
   Agents.

6.3.1.  Privacy in the History-Info Header

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

   Since the History-Info header can inadvertently reveal information
   about the requestor as described in [RFC3323], the Privacy header can
   be used to determine whether an intermediary can include the Request-
   URI in a Request that it receives (PRIV-req-2, see Appendix A.2) or
   that it retargets (PRIV-req-1, see Appendix A.2) as an entry in a
   History-Info header.  Thus, the History- Info entry for that identity
   can be anonymized where the requestor has indicated a priv-value of
   Session- or Header-level privacy.

   Privacy is associated with a specific history information entry, and
   perhaps any entry that corresponds to that same user, and not the
   History-Info header itself.  This allows for anonymizing some
   entries, but not others, as required.  For example, if Alice sends a
   request to Bob without any privacy, and Bob redirects to Carol with
   privacy setup for himself, Carol should receive a request where
   Alice's history information is present, but Bob's has been
   anonymized.

   In addition, the History-Info header can reveal general routing
   information which may be viewed by a specific intermediary or
   network.  Thus, a proxy can use local policy to determine whether the
   History-Info header entries for it's whole domain are private or not
   when exiting the domain through retargeting (PRIV-req-3).  This is
   accomplished by adding a new priv-value, history, to the Privacy
   header [RFC3323] indicating that a specific History-Info header entry
   can not be forwarded outside the domain.

   It is recognized that satisfying the privacy requirements can impact



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   the functionality of this solution by overriding the request to
   generate the information.

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

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

6.3.2.  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
   SHOULD be treated as if a 487 "Request Terminated" error response
   code was received.

6.3.3.  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 6, the
   index consists of a dot-delimited series of digits (e.g., 1.1.2).
   Each dot reflects a hop or level of nesting; thus, the number of hops
   is determined by the total number of dots.  Within each level, the



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   integer reflects the number of peer entities to which the request has
   been routed.  Thus, the indexing results in a logical tree
   representation for the history of the Request.  For each level of
   indexing, the index MUST start at 1.  An increment of 1 MUST be used
   for advancing to a new branch.  The first entry MUST be set to 1.

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

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

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

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

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

   5.  Forking requests: If the request forwarding is done in multiple
       forks (sequentially or in parallel), the index MUST be captured
       for each forked request per the rules above, with each new
       Request having a unique index.  Each index are sequentially
       assigned.  For example, if the index in the last History-Info
       header field in the received request is 1.1, this processing
       entity would initialize its index to 1.1.1 for the first fork,
       1.1.2 for the second, and so forth (see Figure 1 for an example).



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       Note that for each individual fork, only the entry corresponding
       that that fork is included (e.g., the entry for fork 1.1.1 is not
       included in the request sent to fork 1.1.2, and vice-versa).

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

6.3.4.  Request Target in the History-Info Header

   The value for the hi-target attribute is based upon the mechanism by
   which the new target has been determined per the procedures described
   in 16.5/[RFC3261].  The following describes how the specific values
   for the hi-target attribute are determined:

   o  If the Request-URI is a contact that is bound to an AOR in an
      abstract location service for the domain for which the processing
      entity is responsible, and the AOR-to-contact binding has been
      placed into the location service by a SIP Registrar that received
      a REGISTER request, the hi-target attribute MUST be added to the
      hi-entry with a value of "rc."

   o  If the Request-URI is a URI that represents another user than the
      one indicated by the incoming Request-URI, as this would occur in
      cases where a request is statically or dynamically retargeted to
      another user, the hi-target attribute MUST be added to the hi-
      entry with a value of "mp."  The index of the entry corresponding
      to the original target (i.e., the "mapped-from" target) MUST be
      added as a parameter to "mp".


7.  Application Considerations

   As seen by the example scenarios in the Appendix B, History-Info
   provides a very flexible building block that can be used by
   intermediaries and UAs for a variety of services.  As such, any
   services making use of History-Info must be designed with the
   following considerations:





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   1.  History-Info is optional; thus, a service MUST define default
       behavior for requests and responses not containing History-Info
       headers.

   2.  History-Info may be impacted by privacy considerations.
       Applications requiring History-Info need to be aware that if
       Header-, Session-, or History-level privacy is requested by a UA
       (or imposed by an intermediary) that History-Info may not be
       available in a request or response.  This would be addressed by
       an application in the same manner as the previous consideration
       by ensuring there is reasonable default behavior should the
       information not be available.

   3.  History-Info may be impacted by local policy.  Each application
       making use of the History-Info header SHOULD address the impacts
       of the local policies on the specific application (e.g., what
       specification of local policy is optimally required for a
       specific application and any potential limitations imposed by
       local policy decisions).  Note that this is related to the
       optionality and privacy considerations identified in 1 and 2
       above, but goes beyond that.  For example, due to the optionality
       and privacy considerations, an entity may receive only partial
       History-Info entries; will this suffice?  Note that this would be
       a limitation for debugging purposes, but might be perfectly
       satisfactory for some models whereby only the information from a
       specific intermediary is required.


8.  Security Considerations

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

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

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



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   missing entries.

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


9.  IANA Considerations

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

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

9.1.  Registration of New SIP History-Info Header

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

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

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


   The following has been added to the Options Tags section:

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


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



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   this specification.

9.2.  Registration of "history" for SIP Privacy Header

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

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


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


10.  Contributors

   Cullen Jennings, Mark Watson, and Jon Peterson contributed to the
   development of the initial requirements for [RFC4244].

   Jonathan Rosenberg, Christer Holmberg, Hans Erik van Elburg and Shida
   Schubert produced the document that provided much of the content for
   this specification.


11.  Acknowledgements

   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.

   Thanks to Ian Elz for his feedback on privacy.


12.  Changes from RFC 4244

   This RFC replaces [RFC4244].




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

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

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

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

   o  [RFC4244] has lots of repetitive and unclear text

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

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

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

   This specification is backwards compatible with [RFC4244].  The
   following summarizes the functional changes:

   1.  Added a tag to indicate the mechanism by which the target for an
       outgoing request is determined.

   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 processing/handling for 3xx responses to ensure accuracy
       of the new tags - 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.

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




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   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 and examples were simplified and updated to reflect the
   protocol changes.

12.1.  Backwards compatibility

   Proxies conforming to this specification tag the hi-entry parameters
   with an hi-target parameter.  The hi-target parameter did not exist
   in [RFC4244]; therefore, [RFC4244] implementations do not tag the hi-
   entry parameters.  This tagging allows for distinguishing entries
   that were added by an [RFC4244] entity, versus one that was added by
   an entity conforming to this specification.


13.  Changes since last Version

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

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

   1.  Fixed problem with indices in example in Appendix B.2.

   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.




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   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.

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

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

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

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

   6.  Other editorial changes to condense and simplify.

   7.  Moved Privacy examples to Appendix.

   8.  Added forking to Basic call example.

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

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

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



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

   4.   Remove redundant Redirect Server example.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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



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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.

   [RFC5039]  Rosenberg, J. and C. Jennings, "The Session Initiation
              Protocol (SIP) and Spam", RFC 5039, January 2008.

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



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   [RFC3761]  Faltstrom, P. and M. Mealling, "The E.164 to Uniform
              Resource Identifiers (URI) Dynamic Delegation Discovery
              System (DDDS) Application (ENUM)", RFC 3761, April 2004.

   [RFC4769]  Livingood, J. and R. Shockey, "IANA Registration for an
              Enumservice Containing Public Switched Telephone Network
              (PSTN) Signaling Information", RFC 4769, November 2006.

   [I-D.ietf-enum-cnam]
              Shockey, R., "IANA Registration for an Enumservice Calling
              Name Delivery (CNAM)  Information and IANA Registration
              for URI type 'pstndata'", draft-ietf-enum-cnam-08 (work in
              progress), September 2008.


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

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

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

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

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




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

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

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

       E.  Chronological ordering of the Request History information.

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

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

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

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



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       content has been altered.

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

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

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

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

A.2.  Privacy Requirements

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

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

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

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


Appendix B.  Detailed 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.

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.



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   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.  In this example, note that Office and Home are not the
   same AOR as sip:bob@example.com, but rather different AORs that have
   been configured as alternate addresses for Bob in the proxy.  In
   other words, Office and Bob are not bound through SIP Registration
   with Bob's AOR.  This type of arrangement is common for example when
   a "routing" rule to a PSTN number is manually configured in a Proxy.

   This scenario is provided to show 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 this mechanism,
   Alice might well attempt Office (and thus Home) and then re-attempt
   Home on a third manual attempt at reaching Bob. With this mechanism,
   either the end-user or application could know that Bob is not
   answering in the Office, and his busy on his home phone.  If there
   were an alternative address for Bob known to this end-user or
   application, that hasn't been attempted, then either the application
   or the end- user could attempt that.  The intent here is to highlight
   an example of the flexibility of this mechanism that enables
   applications well beyond SIP as it is certainly well beyond the scope
   of this document to prescribe detailed applications.



























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



   F8 180 Ringing example.com -> alice

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
















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   F9 INVITE example.com -> home

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



   F10 100 Trying home -> example.com

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















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   F11 486 Busy Here home -> example.com

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



   F12 486 Busy Here example.com -> alice

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












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   F13 ACK example.com -> home

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



   F14 ACK alice -> example.com

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


B.2.  Voicemail

   This scenario highlights an example where the History-Info in the
   request is primarily of use by an edge service (e.g., voicemail
   server).  It should be noted that this is not intended to be a
   complete specification for this specific edge service as it is quite
   likely that additional information is needed by the edge service.
   History-Info is just one building block that this service can use.

   Alice called Bob, which had been forwarded to Carol, which forwarded
   to VM (voicemail server).  Based upon the retargeted URIs and Reasons
   (and other information) in the INVITE, the VM server makes a policy
   decision about what mailbox to use, which greeting to play, etc.

   Alice      example.com       Bob          Carol        VM

   | INVITE sip:bob@example.com  |             |          |
   |------------->|              |             |          |
   |              | INVITE sip:bob@192.0.2.3   |          |
   |              |------------->|             |          |
     History-Info: <sip:bob@example.com>;index=1
     History-Info: <sip:bob@192.0.2.3>;index=1.1;rc
   |              |              |             |          |
   |  100 Trying  |              |             |          |



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   |<-------------| 302 Moved Temporarily      |          |
   |              |<-------------|             |          |
     History-Info: <sip:bob@example.com>;index=1
     History-Info: <sip:bob@192.0.2.3?Reason=SIP;cause=302>;\
                   index=1.1;rc
     History-Info: <sip:carol@example.com>;index=1.2
   |              |              |             |          |
   |              | INVITE sip:Carol@192.0.2.4 |          |
   |              |--------------------------->|          |
     History-Info: <sip:bob@example.com>;index=1
     History-Info: <sip:bob@192.0.2.3?Reason=SIP;cause=302>;\
                   index=1.1;rc
     History-Info: <sip:carol@example.com>;index=1.2;mp=1
     History-Info: <sip:carol@192.0.2.4>;index=1.2.1;rc
   |              |              |             |          |
   |              |         180 Ringing        |          |
   |              |<---------------------------|          |
     History-Info: <sip:bob@example.com>;index=1
     History-Info: <sip:bob@192.0.2.3?Reason=SIP;cause=302>;\
                   index=1.1;rc
     History-Info: <sip:carol@example.com>;index=1.2;mp=1
     History-Info: <sip:carol@192.0.2.4>;index=1.2.1;rc
   |              |              |             |          |
   | 180 Ringing  |              |             |          |
   |<-------------|              |             |          |
   |              |              |             |          |
   |  . . .       |              |             |          |
   |              |       (timeout)            |          |
   |              |              |             |          |
   |              | INVITE sip:vm@192.0.2.5               |
   |              |-------------------------------------->|
     History-Info: <sip:bob@example.com>;index=1
     History-Info: <sip:bob@192.0.2.3?Reason=SIP;cause=302>;\
                   index=1.1;rc
     History-Info: <sip:carol@example.com>;index=1.2;mp=1
     History-Info: <sip:carol@192.0.2.4>;index=1.2.1;rc
     History-Info: <sip:vm@example.com>;index=1.3;mp=1.2
     History-Info: <sip:vm@192.0.2.5>;index=1.3.1
   |              |              |             |          |
   |              |               200 OK                  |
   |              |<--------------------------------------|
     History-Info: <sip:bob@example.com>;index=1
     History-Info: <sip:bob@192.0.2.3?Reason=SIP;cause=302>;\
                   index=1.1;rc
     History-Info: <sip:carol@example.com>;index=1.2;mp=1
     History-Info: <sip:carol@192.0.2.4>;index=1.2.1;rc
     History-Info: <sip:vm@example.com>;index=1.3;mp=1.2
     History-Info: <sip:vm@192.0.2.5>;index=1.3.1



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


B.3.  Automatic Call Distribution

   This scenario highlights an example of an Automatic Call Distribution
   service, where the agents are divided into groups based upon the type
   of customers they handle.  In this example, the Gold customers are
   given higher priority than Silver customers, so a Gold call would get
   serviced even if all the agents servicing the Gold group were busy,
   by retargeting the request to the Silver Group for delivery to an
   agent.  Upon receipt of the call at the agent assigned to handle the
   incoming call, based upon the History-Info header in the message, the
   application at the agent can provide an indication that this is a
   Gold call, from how many groups it might have overflowed before
   reaching the agent, etc. and thus can be handled appropriately by the
   agent.

   For scenarios whereby calls might overflow from the Silver to the
   Gold, clearly the alternate group identification, internal routing,
   or actual agent that handles the call should not be sent to UA1.
   Thus, for this scenario, one would expect that the Proxy would not
   support the sending of the History-Info in the response, even if
   requested by Alice.

   As with the other examples, this is not prescriptive of how one would
   do this type of service but an example of a subset of processing that
   might be associated with such a service.  In addition, this example
   is not addressing any aspects of Agent availability, which might also
   be done via a SIP interface.

   Alice       example.com     Gold          Silver       Agent

   |              |              |             |            |
   | INVITE sip:Gold@example.com |             |            |
   |------------->|              |             |            |
   | Supported: histinfo
   |              |              |             |            |
   |              |  INVITE sip:Gold@example.com            |
   |              |------------->|             |            |
     History-Info: <sip:Gold@example.com>;index=1
     History-Info: <sip:Gold@gold.example.com>;index=1.1
   |              |              |             |            |



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   |              |  302 Moved Temporarily     |            |
   |              |<-------------|             |            |
     History-Info: <sip:Gold@example.com>;index=1
     History-Info: <sip:Gold@gold.example.com?Reason=SIP;cause=302>;\
                   index=1.1
     Contact: <sip:Silver@example.com>
                  |              |             |            |
   |              |  INVITE sip:Silver@example.com          |
   |              |--------------------------->|            |
     History-Info: <sip:Gold@example.com>;index=1
     History-Info: <sip:Gold@gold.example.com?Reason=SIP;cause=302>;\
                   index=1.1
     History-Info: <sip:Silver@example.com>;index=2;mp=1
     History-Info: <sip:Silver@silver.example.com>;index=2.1
   |              |              |             |            |
   |              |              | INVITE sip:Silver@192.0.2.7
   |              |              |             |----------->|
     History-Info: <sip:Gold@example.com>;index=1
     History-Info: <sip:Gold@gold.example.com?Reason=SIP;cause=302>;\
                   index=1.1
     History-Info: <sip:Silver@example.com>;index=2;mp=1
     History-Info: <sip:Silver@silver.example.com>;index=2.1
     History-Info: <sip:Silver@192.0.2.7>;index=2.1.1;rc
   |              |              |             |            |
   |              |              |             |  200 OK    |
   |              |              |             |<-----------|
     History-Info: <sip:Gold@example.com>;index=1
     History-Info: <sip:Gold@gold.example.com?Reason=SIP;cause=302>;\
                   index=1.1
     History-Info: <sip:Silver@example.com>;index=2;mp=1
     History-Info: <sip:Silver@silver.example.com>;index=2.1
     History-Info: <sip:Silver@192.0.2.7>;index=2.1.1;rc
   |              |              |             |            |
   |              |         200 OK             |            |
   |              |<---------------------------|            |
     History-Info: <sip:Gold@example.com>;index=1
     History-Info: <sip:Gold@gold.example.com?Reason=SIP;cause=302>;\
                   index=1.1
     History-Info: <sip:Silver@example.com>;index=2;mp=1
     History-Info: <sip:Silver@silver.example.com>;index=2.1
     History-Info: <sip:Silver@192.0.2.7>;index=2.1.1;rc
   |              |              |             |            |
      200 OK      |              |             |            |
   |<-------------|              |             |            |
   |              |              |             |            |
   |    ACK       |              |             |            |
   |------------->|                  ACK                    |
   |              |---------------------------------------->|



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B.4.  History-Info with Privacy Header

   This example provides a basic call scenario such as the one in
   Figure 1 but without forking, with sip:biloxi.example.com adding the
   Privacy header indicating that the History-Info header information is
   anonymized outside the biloxi.example.com domain.  This scenario
   highlights the potential functionality lost with the use of "history"
   privacy in the Privacy header for the entire request and the need for
   careful consideration on the use of privacy for History-Info.










































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

                   Figure 2: Example with Privacy Header

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

   This example also provides a basic call scenario such as the one in
   Figure 1 but without forking, 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



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   providing Alice with some information about the history of the
   request, but anonymizing Bob's local URI.

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


          Figure 3: Example with Privacy Header for Specific URI






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B.6.  Determining the Alias used.

   SIP user agents are associated with an address-of-record (AOR).  It
   is possible for a single UA to actually have multiple AOR associated
   with it.  One common usage for this is aliases.  For example, a user
   might have an AOR of sip:john@example.com but also have the AORs
   sip:john.smith@example.com and sip:jsmith@example.com.  Rather than
   registering against each of these AORs individually, the user would
   register against just one of them, and the home proxy would
   automatically accept incoming calls for any of the aliases, treating
   them identically and ultimately forwarding them towards the UA.  This
   is common practice in the Internet Multimedia Subsystem (IMS), where
   it is called implicit registrations and each alias is called a public
   identity.

   It is a common requirement for a UAS, on receipt of a call, to know
   which of its aliases was used to reach it.  This knowledge can be
   used to choose ringtones to play, determine call treatment, and so
   on.  For example, a user might give out one alias to friends and
   family only, resulting in a special ring that alerts the user to the
   importance of the call.

   Following call-flow and example messages show how History-Info can be
   used to find out the alias used to reach the callee.

   UAS can see which alias was used in the call by looking at the hi-
   entry prior to the last hi-entry with the "rc" tag.

          Alice             Example.com             John
          |                     | REGISTER F1         |
          |                     |<--------------------|
          |                     | 200 OK F2           |
          |                     |-------------------->|
          | INVITE F3           |                     |
          |-------------------->|                     |
          |                     | INVITE F4           |
          |                     |-------------------->|
                       * Rest of flow not shown *

   F1 REGISTER John -> Example.com

   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: John <sip:john@example.com>;tag=a73kszlfl
   To: John <sip:john@example.com>
   Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
   CSeq: 1 REGISTER



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   Contact: <sip:john@192.0.2.1>
   Content-Length: 0


   F2 200 OK Example.com -> John

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   From: John <sip:john@example.com>;tag=a73kszlfl
   To: John <sip:john@example.com>
   Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
   CSeq: 1 REGISTER
   Contact: <sip:john@192.0.2.1>;expires=3600
   Content-Length: 0

   F3 INVITE Alice -> Example.com

   INVITE sip:john.smith@example.com SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>
   To: John <sip:john.smith@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:john.smith@example.com>;index=1;
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>

   [SDP Not Shown]

   F4 INVITE Example.com -> Bob

   INVITE sip:john@192.0.2.1 SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=as2334se
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>
   To: John <sip:john.smith@example.com>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   Record-Route: <sip:proxy.example.com;lr>
   History-Info: <sip:john.smith@example.com>;index=1;
   History-Info: <sip:john@192.0.2.1>;index=1.1;rc
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>




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   [SDP Not Shown]


                          Figure 4: Alias Example

B.7.  GRUU

   A variation on the problem in Appendix B.6 occurs with Globally
   Routable User Agent URI (GRUU) [RFC5627].  A GRUU is a URI assigned
   to a UA instance which has many of the same properties as the AOR,
   but causes requests to be routed only to that specific instance.  It
   is desirable for a UA to know whether it was reached because a
   correspondent sent a request to its GRUU or to its AOR.  This can be
   used to drive differing authorization policies on whether the request
   should be accepted or rejected, for example.  However, like the AOR
   itself, the GRUU is lost in translation at the home proxy.  Thus, the
   UAS cannot know whether it was contacted via the GRUU or its AOR.

   Following call-flow and example messages show how History-Info can be
   used to find out the GRUU used to reach the callee.

   GRUU is merely an AoR with a URI parameter that distinguishes the
   target instance, and as any URI parameters are preserved in history-
   info as Request-URI is trasnlated, UA can see if the request was
   addressed to a specific instance (gruu) by evaluating the presence of
   "gr" parameter in the hi-entry prior to the last hi-entry with the
   "rc" tag.

          Alice             Example.com             John
          |                     | REGISTER F1         |
          |                     |<--------------------|
          |                     | 200 OK F2           |
          |                     |-------------------->|
          | INVITE F3           |                     |
          |-------------------->|                     |
          |                     | INVITE F4           |
          |                     |-------------------->|
                       * Rest of flow not shown *

   F1 REGISTER John -> Example.com

   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: John <sip:John@example.com>;tag=a73kszlfl
   Supported: gruu
   To: John <sip:john@example.com>
   Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1



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   CSeq: 1 REGISTER
   Contact: <sip:john@192.0.2.1>
    ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
   Content-Length: 0

   F2 200 OK Example.com -> John

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   From: John <sip:john@example.com>;tag=a73kszlfl
   To: John <sip:john@example.com> ;tag=b88sn
   Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
   CSeq: 1 REGISTER
   Contact: <sip:john@192.0.2.1>
    ;pub-gruu="sip:john@example.com
    ;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
    ;temp-gruu=
     "sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com;gr"
    ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
    ;expires=3600
   Content-Length: 0

   Assuming Alice has a knowledge of a gruu either through
   prior communication or through other means such as presence
   places a call to John's gruu.

   F3 INVITE Alice -> Example.com

   INVITE sip:john@example.com
    ;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6 SIP/2.0
   Via: SIP/2.0/TCP  192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: <sip:john@example.com
    ;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>
   Supported: gruu, histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:john@example.com
    ;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>;index=1
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Length: <appropriate value>

   F4 INVITE Example.com -> John

   INVITE sip:john@192.0.2.1 SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=as2334se
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-



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   To: John <sip:john@example.com>
   Supported: gruu, histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   Record-Route: <sip:proxy.example.com;lr>
   History-Info: <sip:john@example.com
    ;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>;index=1
   History-Info: <sip:john@192.0.2.1>;index=1.1;rc
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>


                          Figure 5: GRUU Example

B.8.  Limited Use Address

   A limited use address is a SIP URI that is minted on-demand, and
   passed out to a small number (usually one) remote correspondent.
   Incoming calls targeted to that limited use address are accepted as
   long as the UA still desires communications from the remote target.
   Should they no longer wish to be bothered by that remote
   correspondent, the URI is invalidated so that future requests
   targeted to it are rejected.

   Limited use addresses are used in battling voice spam [RFC5039].  The
   easiest way to provide them would be for a UA to be able to take its
   AOR, and "mint" a limited use address by appending additional
   parameters to the URI.  It could then give out the URI to a
   particular correspondent, and remember that URI locally.  When an
   incoming call arrives, the UAS would examine the parameter in the URI
   and determine whether or not the call should be accepted.
   Alternatively, the UA could push authorization rules into the
   network, so that it need not even see incoming requests that are to
   be rejected.

   This approach, especially when executed on the UA, requires that
   parameters attached to the AOR, but not used by the home proxy in
   processing the request, will survive the translation at the home
   proxy and be presented to the UA.  This will not be the case with the
   logic in RFC 3261, since the Request-URI is replaced by the
   registered contact, and any such parameters are lost.

   Using the history-info John's UA can easily see if the call was
   addressed to its AoR, GRUU or a temp-gruu and treat the call
   accordingly by looking at the hi-entry prior to the last hi-entry
   with the "rc" tag.




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          Alice             Example.com             John
          |                     | REGISTER F1         |
          |                     |<--------------------|
          |                     | 200 OK F2           |
          |                     |-------------------->|
          | INVITE F3           |                     |
          |-------------------->|                     |
          |                     | INVITE F4           |
          |                     |-------------------->|
                       * Rest of flow not shown *


   F1 REGISTER John -> Example.com

   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: John <sip:John@example.com>;tag=a73kszlfl
   Supported: gruu
   To: John <sip:john@example.com>
   Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
   CSeq: 1 REGISTER
   Contact: <sip:john@192.0.2.1>
    ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
   Content-Length: 0

   F2 200 OK Example.com -> John

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   From: John <sip:john@example.com>;tag=a73kszlfl
   To: John <sip:john@example.com> ;tag=b88sn
   Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
   CSeq: 1 REGISTER
   Contact: <sip:john@192.0.2.1>
    ;pub-gruu="sip:john@example.com
    ;gr=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
    ;temp-gruu=
     "sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com;gr"
    ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
    ;expires=3600
   Content-Length: 0

    Assuming Alice has a knowledge of a temp-gruu, she places a
    call to the temp-gruu.

   F3 INVITE Alice -> Example.com




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   INVITE sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com
    ;gr SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: <sip:sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com
    ;gr>
   Supported: gruu, histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info:
    <sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com;gr>
    ;index=1
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Length: <appropriate value>

   F4 INVITE Example.com -> John

   INVITE sip:john@192.0.2.1 SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=as2334se
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>;tag=kkaz-
   To: John <sip:john@example.com>
   Supported: gruu, histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   Record-Route: <sip:proxy.example.com;lr>
   History-Info:
    <sip:tgruu.7hs==jd7vnzga5w7fajsc7-ajd6fabz0f8g5@example.com;gr>
    ;index=1
   History-Info: <sip:john@192.0.2.1>;index=1.1;rc
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>


                   Figure 6: Limited Use Address Example

B.9.  Sub-Address

   Sub-Addressing is very similar to limited use addresses.  Sub-
   addresses are addresses within a subdomain that are multiplexed into
   a single address within a parent domain.  The concept is best
   illustrated by example.  Consider a VoIP service provided to
   consumers.  A consumer obtains a single address from its provider,
   say sip:family@example.com.  However, Joe is the patriarch of a
   family with four members, and would like to be able to have a
   separate identifier for each member of his family.  One way to do
   that, without requiring Joe to purchase new addresses for each member



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   from the provider, is for Joe to mint additional URI by adding a
   parameter to the AOR.  For example, his wife Judy with have the URI
   sip:family@example.com;member=judy, and Joe himself would have the
   URI sip:family@example.com;member=joe.  The SIP server provider would
   receive requests to these URI, and ignoring the unknown parameters
   (as required by [RFC3261]) route the request to the registered
   contact, which corresponds to a SIP server in Joes home.  That
   server, in turn, can examine the URI parameters and determine which
   phone in the home to route the call to.

   This feature is not specific to VoIP, and has existing in Integrated
   Services Digital Networking (ISDN) for some time.  It is particularly
   useful for small enterprises, in addition to families.  It is also
   similar in spirit (though not mechanism) to the ubiquitous home
   routers used by consumers, which allow multiple computers in the home
   to "hide" behind the single IP address provided by the service
   provider, by using the TCP and UDP port as a sub-address.

   The sub-addressing feature is not currently feasible in SIP because
   of the fact that any SIP URI parameter used to convey the sub-address
   would be lost at the home proxy, due to the fact that the Request-URI
   is rewritten there.

   Call-flow and example messages below show the how History-Info can be
   used to deliver the sub-address.  UAS or Proxy can determine the sub-
   address by looking at the hi-entry prior to the last hi-entry with
   the "rc" tag.

     Alice    Example.com     John's Home      Judy         John
      |            | REGISTER F1  |              |              |
      |            |<-------------|              |              |
      |            | 200 OK F2    |              |              |
      |            |------------->|              |              |
      |            |              | REGISTER F3  |              |
      |            |              |<-------------|              |
      |            |              | 200 OK F4    |              |
      |            |              |------------->|              |
      |            |              |              | REGISTER F5  |
      |            |              |<----------------------------|
      |            |              |              | 200 OK F6    |
      |            |              |---------------------------->|
      | INVITE F7  |              |              |              |
      |----------->|              |              |              |
      |            | INVITE F8    |              |              |
      |            |------------->|              |              |
      |            |              | INVITE F9    |              |
      |            |              |------------->|              |
                       * Rest of flow not shown *



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   F1 REGISTER John's Home Server -> Example.com

   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: John <sip:johnhome@example.com>;tag=a73kszlfl
   To: John <sip:johnhome@example.com>
   Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
   CSeq: 1 REGISTER
   Contact: <sip:johnhome@192.0.2.1>
   Content-Length: 0

   F2 200 OK Example.com -> John's Home Server

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   From: John <sip:johnhome@example.com>;tag=a73kszlfl
   To: John <sip:johnhome@example.com> ;tag=b88sn
   Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
   CSeq: 1 REGISTER
   Contact: <sip:johnhome@192.0.2.1>;expires=3600
   Content-Length: 0

   We assume that John's server acts as a proxy allowing
   each of the device in the house to register.

   F3 REGISTER Judy's phone -> John's Home Server

   REGISTER sip:192.168.1.1 SIP/2.0
   Via: SIP/2.0/UDP 192.168.1.2;branch=z9hG4bKnasdds
   Max-Forwards: 70
   From: Judy <sip:judy@192.168.1.1>;tag=a73kszlfl
   To: Judy <sip:judy@192.168.1.1>
   Call-ID: 12345pLxk3uxtm8tn@192.168.1.2
   CSeq: 1 REGISTER
   Contact: <sip:judy@192.168.1.2>
   Content-Length: 0

   F4 200 OK John's Home Server -> Judy's phone

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.168.1.2;branch=z9hG4bKnashds7
   From: Judy <sip:judy@192.168.1.1>;tag=a73kszlfl
   To: Judy <sip:judy@192.168.1.1>tag=b88sn
   Call-ID: 12345pLxk3uxtm8tn@192.168.1.2
   CSeq: 1 REGISTER
   Contact: <sip:judy@192.168.1.2>;expires=3600
   Content-Length: 0



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   F5 REGISTER John's phone -> John's Home Server

   REGISTER sip:192.168.1.1 SIP/2.0
   Via: SIP/2.0/UDP 192.168.1.3;branch=z9hG4bKnasdds
   Max-Forwards: 70
   From: Judy <sip:john@192.168.1.1>;tag=a73kszlfl
   To: Judy <sip:john@192.168.1.1>
   Call-ID: 12346pLxk3uxtm8tn@192.168.1.3
   CSeq: 1 REGISTER
   Contact: <sip:john@192.168.1.3>
   Content-Length: 0

   F6 200 OK John's Home Server -> John's phone

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.168.1.3;branch=z9hG4bKnashds7
   From: John <sip:john@192.168.1.1>;tag=a73kszlfl
   To: John <sip:john@192.168.1.1> ;tag=b88sn
   Call-ID: 12346pLxk3uxtm8tn@192.168.1.3
   CSeq: 1 REGISTER
   Contact: <sip:john@192.168.1.3>;expires=3600
   Content-Length: 0

   F7 INVITE Alice -> Example.com

   INVITE sip:johnhome@example.com;member=judy SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>
   To: Judy <sip:johnhome@example.com;member=judy>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   History-Info: <sip:johnhome@example.com;member=judy>;index=1;
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>

   [SDP Not Shown]

   F8 INVITE Example.com -> John's Home

   INVITE sip:johnhome@192.0.2.1 SIP/2.0
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=as2334se
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>
   To: Judy <sip:johnhome@example.com;member=judy>
   Supported: histinfo
   Call-Id: 12345600@example.com



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   CSeq: 1 INVITE
   Record-Route: <sip:proxy.example.com;lr>
   History-Info: <sip:johnhome@example.com;member=judy>;index=1;
   History-Info: <sip:john@192.0.2.1>;index=1.1;rc
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>

   [SDP Not Shown]

   John's Home server can see that the call was addressed to
   Judy by evaluating the entry prior to the last entry with the
   "rc" tag and forwards the call accordingly.

   F9 INVITE John's Home -> Judy

   INVITE sip:judy@192.168.1.2 SIP/2.0
   Via: SIP/2.0/TCP 192.168.1.1:5060;branch=abc2334se
   Via: SIP/2.0/TCP proxy.example.com:5060;branch=as2334se
   Via: SIP/2.0/TCP 192.0.2.3:5060;branch=232sxxeserg
   From: Alice <sip:alice@example.com>
   To: Judy <sip:johnhome@example.com;member=judy>
   Supported: histinfo
   Call-Id: 12345600@example.com
   CSeq: 1 INVITE
   Record-Route: <sip:proxy.example.com;lr>
   History-Info: <sip:johnhome@example.com;member=judy>;index=1;
   History-Info: <sip:john@192.0.2.1>;index=1.1;rc
   History-Info: <sip:judy@192.168.1.1>;index=1.1.1;mp=1.1
   History-Info: <sip:judy@192.168.1.2>;index=1.1.1.1;rc
   Contact: Alice <sip:alice@192.0.2.3>
   Content-Type: application/sdp
   Content-Length: <appropriate value>

   [SDP Not Shown]


                       Figure 7: Sub-Address Example

B.10.  Service Invocation

   Several SIP specifications have been developed which make use of
   complex URIs to address services within the network rather than
   subscribers.  The URIs are complex because they contain numerous
   parameters that control the behavior of the service.  Examples of
   this include the specification which first introduced the concept,
   [RFC3087], control of network announcements and IVR with SIP URI
   [RFC4240], and control of voicemail access with SIP URI [RFC4458].



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   A common problem with all of these mechanisms is that once a proxy
   has decided to rewrite the Request-URI to point to the service, it
   cannot be sure that the Request-URI will not be destroyed by a
   downstream proxy which decides to forward the request in some way,
   and does so by rewriting the Request-URI.

   Section on voicemail (Appendix B.2) shows how History-Info can be
   used to invocate a service.

B.11.  Toll Free Number

   Toll free numbers, also known as 800 or 8xx numbers in the United
   States, are telephone numbers that are free for users to call.

   In the telephone network, toll free numbers are just aliases to
   actual numbers which are used for routing of the call.  In order to
   process the call in the PSTN, a switch will perform a query (using a
   protocol called TCAP), which will return either a phone number or the
   identity of a carrier which can handle the call.

   There has been recent work on allowing such PSTN translation services
   to be accessed by SIP proxy servers through IP querying mechanisms.
   ENUM, for example [RFC3761] has already been proposed as a mechanism
   for performing Local Number Portability (LNP) queries [RFC4769], and
   recently been proposed for performing calling name queries
   [I-D.ietf-enum-cnam].  Using it for 8xx number translations is a
   logical next-step.

   Once such a translation has been performed, the call needs to be
   routed towards the target of the request.  Normally, this would
   happen by selecting a PSTN gateway which is a good route towards the
   translated number.  However, one can imagine all-IP systems where the
   8xx numbers are SIP endpoints on an IP network, in which case the
   translation of the 8xx number would actually be a SIP URI and not a
   phone number.  Assuming for the moment it is a PSTN connected entity,
   the call would be routed towards a PSTN gateway.  Proper treatment of
   the call in the PSTN (and in particular, correct reconciliation of
   billing records) requires that the call be marked with both the
   original 8xx number AND the target number for the call.  However, in
   our example here, since the translation was performed by a SIP proxy
   upstream from the gateway, the original 8xx number would have been
   lost, and the call will not interwork properly with the PSTN.

   Furthermore, even if the translation of the 8xx number was a SIP URI,
   the enterprise or user who utilize the 8xx service would like to know
   whether the call came in via 8xx number in order to treat the call
   differently (for example to play a special announcement..) but if the
   original R-URI is lost through translation, there is no way to tell



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   if the call came in via 8xx number.

   Similar problems arise with other "special" numbers and services used
   in the PSTN, such as operator services, pay numbers (9xx numbers in
   the U.S), and short service codes such as 311.

   To find the service number, the UAS can look at the hi-entry prior to
   the first hi-entry with "mp" tag.  Technically call can be forwarded
   to these "special" numbers from non "special" numbers, but with the
   way these services authorize trasnlation, it is not common.

         Alice      Toll Free Service   Atlanta.com          John
          |                |              |                   |
          |    INVITE F1   |              |                   |
          |--------------->|   INVITE F2  |                   |
          |                |------------->|                   |
          |                |              |  INVITE F3        |
          |                |              |------------------>|

                       * Rest of flow not shown *

   F1: INVITE 192.0.2.1 -> proxy.example.com

   INVITE sip:+18005551002@example.com;user=phone  SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.1:5060;branch=z9hG4bK-74bf9
   From: Alice <sip:+15551001@example.com;user=phone>;tag=9fxced76sl
   To: sip:+18005551002@example.com;user=phone
   Call-ID: c3x842276298220188511
   CSeq: 1 INVITE
   Max-Forwards: 70
   Supported: histinfo
   History-Info: <sip:+18005551002@example.com;user=phone >;index=1
   Contact: <sip:alice@192.0.2.1>
   Content-Type: application/sdp
   Content-Length: <appropriate value>

   [SDP Not Shown]

   F2: INVITE proxy.example.com -> atlanta.com

   INVITE sip:+15555551002@atlanta.com SIP/2.0
   Via: SIP/2.0/TCP 192.0.2.4:5060;branch=z9hG4bK-ik80k7g-1
   Via: SIP/2.0/TCP 192.0.2.1:5060;branch=z9hG4bK-74bf9
   From: Alice <sip:+15551001@example.com;user=phone>;tag=9fxced76sl
   To: sip:+18005551002@example.com;user=phone
   Call-ID: c3x842276298220188511
   CSeq: 1 INVITE
   Max-Forwards: 70



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   Supported: histinfo
   History-Info: <sip:+18005551002@example.com;user=phone >;index=1,
                 <sip:+15555551002@atlanta.com>;index=1.1;mp=1
   Contact: <sip:alice@192.0.2.1>
   Content-Type: application/sdp
   Content-Length: <appropriate value>

   [SDP Not Shown]

   F3: INVITE atlanta.com -> Joe

   INVITE sip:joe@192.168.1.2 SIP/2.0
   Via: SIP/2.0/TCP 192.168.1.1:5060;branch=z9hG4bK-pxk7g-3
   Via: SIP/2.0/TCP 192.0.2.4:5060;branch=z9hG4bK-ik80k7g-1
   Via: SIP/2.0/TCP 192.0.2.1:5060;branch=z9hG4bK-74bf9
   From: Alice <sip:+15551001@example.com;user=phone>;tag=9fxced76sl
   To: sip:+18005551002@example.com;user=phone
   Call-ID: c3x842276298220188511
   CSeq: 1 INVITE
   Max-Forwards: 70
   Supported: histinfo
   History-Info: <sip:+18005551002@example.com;user=phone >;index=1,
                 <sip:+15555551002@atlanta.com>;index=1.1;mp=1,
                 <sip:joe@atlanta.com>;index=1.1.1;mp=1.1,
                 <sip:joe@192.168.1.2>;index=1.1.2;rc
   Contact: <sip:alice@192.0.2.1>
   Content-Type: application/sdp
   Content-Length: <appropriate value>

   [SDP Not Shown]


                     Figure 8: Service Number Example


Authors' Addresses

   Mary Barnes
   Nortel
   Richardson, TX

   Email: mary.barnes@nortel.com









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   Francois Audet
   Skype Labs


   Email: francois.audet@skypelabs.com


   Shida Schubert
   NTT


   Email: shida@ntt.com


   Hans Erik van Elburg
   Detecon International Gmbh
   Oberkasseler str. 2
   Bonn,   53227
   Germany

   Email: ietf.hanserik@gmail.com


   Christer Holmberg
   Ericsson
   Hirsalantie 11, Jorvas
   Finland

   Email: christer.holmberg@ericsson.com






















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