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Versions: 00 01 02 03 04 05 06 RFC 4244

 INTERNET-DRAFT                                              M. Barnes
 Document: draft-ietf-sip-history-info-06.txt                   Editor
 Category: Standards Track                             Nortel Networks

 Expires: July 17th, 2005                              Jan 17th, 2005

    An Extension to the Session Initiation Protocol for Request History
                                Information

 Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

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   This Internet-Draft will expire on July 17th, 2005.

 Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.

 Abstract

   This document defines a standard mechanism for capturing the history
   information associated with a 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 a new optional SIP header, History-Info, for capturing the
   history information in requests.






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 Table of Contents

   1.Background:  Why define a Generic "Request History" capability?.3
   2. "Request History" Requirements.................................4
      2.1 Security Requirements......................................5
      2.2 Privacy Requirements.......................................6
   3. Request History Information Description........................7
      3.1 Optionality of History-Info................................8
      3.2 Securing History-Info......................................8
      3.3 Ensuring the Privacy of History-Info.......................8
   4 Request History Information Protocol Details....................9
      4.1 Protocol Structure of History-Info.........................9
      4.2 Protocol Examples.........................................11
      4.3 Protocol usage............................................11
      4.4 Security for History-Info.................................18
      4.5 Example Applications using History-Info...................18
   5. Application Considerations....................................23
   6. Security Considerations.......................................24
   7. IANA Considerations...........................................24
   Normative References.............................................25
   Informational References.........................................25
   Appendix. Example Scenarios......................................27
   Appendix A. Sequentially forking (History-Info in Response)......27
   Appendix B.  Voicemail...........................................32
   Appendix C.  Automatic Call Distribution Example.................38
   Appendix D. Session via Redirect and Proxy Servers...............39

 Overview

   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.  While SIP implicitly provides
   the redirect/retarget capabilities that enable calls to be routed to
   chosen applications, there is currently no standard mechanism within
   SIP for communicating the 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 new 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.




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   Section 1 provides additional background motivation for the Request
   History capability.  Section 2 identifies the requirements for a
   solution, with Section 3 providing an overall description of the
   solution.

   Section 4 provides the details of the additions to the SIP protocol.
   Example uses of the new header are included in Section 4.5, with
   additional scenarios included in the Appendix.

   Section 5 summarizes the application considerations identified in the
   previous sections. Section 6 summarizes the security solution.

 Conventions used in this document

   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 RFC 2119 [RFC2119].

 1.Background:  Why define a Generic "Request History" capability?

   SIP implicitly provides redirect/retarget capabilities that enable
   calls to be routed to specific applications as defined in [RFC3261].
   The term 'retarget' will be used henceforth in this document to refer
   to the process of a Proxy Server/User Agent Client (UAC) changing a
   Uniform Resource Identifier(URI) in a request and thus changing the
   target of the request.  This term is chosen to avoid associating this
   request history only with the specific SIP Redirect Server capability
   that provides for a response to be sent back to a UAC requesting that
   the UAC should retarget the original request to an alternate URI.
   The rules for determining request targets as described in section
   16.5 of [RFC3261] are consistent with the use of the retarget term in
   this document.

   The motivation for the request history is that in the process of
   retargeting 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. The proposal in this document is to
   provide a mechanism for transporting the request history.  It is not
   proposing any application specific behavior for a Proxy or UA upon
   receipt of the information. Indeed, such behavior should be a local
   decision for the recipient application.

   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:




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  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 which will receive
     some "referral" commission for generating this traffic,

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

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


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

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

   o Some diagnostic information for debugging SIP requests.

   o A stronger security solution for SIP. A side effect is that each
     proxy which 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.


 2. "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. While 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) OPTIONALITY-req: The "Request History" information is optional.

   2.1) In many cases, it is anticipated that whether the history is
   added to the Request would be a local policy decision enforced by the
   specific application, thus no specific protocol element is needed.

   2.2) Due to the capability being "optional" from the SIP protocol
   perspective, the impact to an application of not having the "Request
   History" must be described. Applicability guidelines to be addressed



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   by applications using this capability must be provided as part of the
   solution to these requirements.


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

   3.1) 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'.

   3.2) 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.


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


   5) CONTENT-req:  The "Request History" information for each
   occurrence of retargeting, shall include the following:

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

     5.2) The URI or address from which the request was retargeted,

     5.3) The reason for the Request-URI or address modification,

     5.4) Chronological ordering of the Request History information.

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

   7) 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.

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

 2.1 Security Requirements

   The Request History information is being inserted by a network
   element retargeting a Request, resulting in a slightly different


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   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.

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

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

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

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

   4) SEC-req-4: To ensure the confidentiality of the Request History
   information, only entities which 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, either by
   retargeting and capturing the information, or as an application
   making use of the information received in either a Request or
   Response.

 2.2 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.




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   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
   requirements shall not be included in outgoing messages unless it is
   protected as described in [RFC3323].



 3. Request History Information Description

   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).  The solution is to capture the Request-URIs as a
   request is forwarded in a new header for SIP messages: History-Info
   (CONTENT-req).  This allows for the capturing of the history of a
   request that would be lost with the normal SIP processing involved in
   the subsequent forwarding 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) and any
   valid response to these requests. (ISSUER-req)

   The History-Info header is added to a Request when a new request is
   created by a UAC or forwarded by a Proxy, or when the target of a
   request is changed. The term 'retarget' is introduced to refer to
   this changing of the target of a request and the subsequent
   forwarding of that request. It should be noted that retargeting only
   occurs when the Request-URI indicates a domain for which the
   processing entity is responsible.  In terms of the SIP protocol, the
   processing associated with retargeting is described in sections 16.5,
   and 16.6 of [RFC3261].  As described in section 16.5 of [RFC3261], it
   is possible for the target of a request to be changed by the same
   proxy multiple times (referred to as 'internal retargeting' in
   section 2), as the proxy MAY add targets to the target set after
   beginning Request Forwarding. Section 16.6 of [RFC3261] describes
   Request Forwarding. It is during this process of Request Forwarding,
   that the History Information is captured as an optional, additional
   header field. Thus, the addition of the History-Info header does not
   impact fundamental SIP Request Forwarding. An entity (UA or proxy)


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   changing the target of a request in response to a redirect or REFER
   SHOULD also propagate any History-Info header from the initial
   Request in the new request (GENERATION-req, FORWARDS-req).

 3.1 Optionality of History-Info

   The History-Info header is optional in that neither UAs nor Proxies
   are required to support it.  A new Supported header, "histinfo", is
   included in the Request to indicate whether the History-Info header
   is returned in Responses (BACKWARDS-req). In addition to the
   "histinfo" Supported header, local policy determines whether or not
   the header is added to any request, or for a specific Request-URI,
   being retargeted. It is possible that this could restrict the
   applicability of services which make use of the Request History
   Information to be limited to retargeting within domain(s) controlled
   by the same local policy, or between domain(s) which negotiate
   policies with other domains to ensure support of the given policy, or
   services for which complete History Information isn't required to
   provide the service. (OPTIONALITY-req)  All applications making use
   of the History-info header MUST clearly define the impact of the
   information not being available and specify the processing of such a
   request.

 3.2 Securing History-Info

   This document defines a new header for SIP. The document strongly
   RECOMMENDs the use of the Transport Layer Security (TLS) protocol
   [RFC2246] as a mandatory mechanism to ensure the overall
   confidentiality of the History-Info headers (SEC-req-4). This results
   in History-Info having at least the same level of security as other
   headers in SIP which are inserted by intermediaries. If TLS is not
   available for the connection over which the request is being
   forwarded, then the request MUST not include the History-Info header
   or the request MUST be redirected to the client, including the
   History-Info header, so that the request can be retargeted by the
   client.

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



 3.3 Ensuring the Privacy of History-Info




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   Since the History-Info header can inadvertently reveal information
   about the requestor as described in [RFC3323], the Privacy header
   SHOULD be used to determine whether an intermediary can include the
   History-Info header in a Request that it receives and forwards (PRIV-
   req-2) or that it retargets (PRIV-req-1). Thus, the History-Info
   header SHOULD not be included in Requests where the requestor has
   indicated a priv-value of Session or Header level privacy.

   In addition, the History-Info header can reveal general routing
   information, which may be viewed by a specific intermediary or
   network, to be subject to privacy restrictions.  Thus, local policy
   MAY also be used to determine whether to include the History-Info
   header at all, whether to capture a specific Request-URI in the
   header, or whether it be included only in the Request as it is
   retargeted within a specific domain (PRIV-req-3).  In the latter
   case, this is accomplished by adding a new priv-value, history, to
   the Privacy header [RFC 3323] indicating whether any or a specific
   History-Info header(s) SHOULD be forwarded.

   It is recognized that satisfying the privacy requirements can impact
   the functionality of this solution by overriding the request to
   generate the information. As with the optionality and security
   requirements, applications making use of History-Info SHOULD address
   any impact this may have or MUST explain why it does not impact the
   application.

 4 Request History Information Protocol Details

   This section contains the details and usage of the proposed new SIP
   protocol elements.  It also discusses the security aspects of the
   solution.

 4.1 Protocol Structure of History-Info

   History-Info is a header field as defined by [RFC3261].  It is an
   optional header field and MAY appear in any request or response not
   associated with a dialog or which starts a dialog. For example,
   History-Info MAY appear in INVITE, REGISTER, MESSAGE, REFER, OPTIONS,
   SUBSCRIBE and PUBLISH and any valid responses, plus NOTIFY requests
   which initiate a dialog.

   This document adds the following entry to Table 2 of [RFC3261]. The
   additions to this table are also provided for extension methods at
   the time of publication of this document.  This is provided as a
   courtesy to the reader and is not normative in any way.

      Header field    where   proxy   ACK  BYE  CAN  INV  OPT  REG  MSG
      ------------    -----   -----   ---  ---  ---  ---  ---  ---  ---
      History-Info            amdr     -    -    -    o    o    o    o


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                                      SUB  NOT  REF  INF  UPD  PRA  PUB
                                      ---  ---  ---  ---  ---  ---  ---
      History-Info            amdr     o    o    o    -    -    -    o


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

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

     o Index (hi-index): A mandatory parameter for History-Info
        reflecting the chronological order of the information, indexed
        to also reflect the forking and nesting of requests. The format
        for this parameter is a string of digits, separated by dots to
        indicate the number of forward hops and retargets. This results
        in a tree representation of the history of the request, with the
        lowest level index reflecting a branch of the tree. By adding
        the new entries in order (i.e. following existing entries per
        the details in section 4.3.3.1), including the index and
        securing the header, the ordering of the History-info headers in
        the request is assured (SEC-req-2).  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 not included for
        a hi-targeted-to-uri when it is first added in a History-info
        header, but rather is added when the retargeting actually
        occurs.  Note, that this does appear to complicate the security
        problem, however, retargeting only occurs when the hi-targeted-
        to-uri indicates a domain for which the processing entity is
        responsible, thus it would be the same processing entity that
        initially added the hi-targeted-to-URI to the header that would
        be updating it with the Reason.

     o Privacy: An optional parameter for History-info, reflected in
        the History-Info header field values by including the Privacy
        Header [RFC3323] with a priv-value of "history" escaped in the
        hi-targeted-to-uri or by adding the Privacy header with a priv-
        value of "history" to the Request.  The use of the Privacy
        Header with a priv-value of "history" indicates whether a
        specific or all History-Info headers should not be forwarded.


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     o Extension (hi-extension): An optional parameter to allow for
       future optional extensions.  As per the [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-extension

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

          hi-extension = generic-param



 4.2 Protocol Examples

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


      History-Info:<sip:UserA@ims.example.com?Reason=SIP%3B\
        cause%3D302>;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,
         <sip:45432@vm.example.com>;index=1.3

 4.3 Protocol usage

   This section describes the processing specific to UAs and Proxies for
   the History-Info header, the "histinfo" option tag and the priv-value
   of "history". As discussed in section 1, the fundamental objective is
   to capture the target Request-URIs as a request is forwarded.  This
   allows for the capturing of the history of a request that would be
   lost due to subsequent (re)targeting and forwarding.  To accomplish
   this for the entire history of a request, either the UAC must capture


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   the Request-URI in a History-Info header in the initial request or a
   proxy must add a History-Info header with both an hi-entry for the
   Request-URI in the initial request and an hi-entry for the target
   Request-URI as the request is forwarded.  The basic processing is for
   each entity forwarding a request to add an hi-entry for the target
   Request-URI, updating the index and adding the Reason as appropriate
   for any retargeted Request-URI.

   4.3.1 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 SHOULD initiate the capturing of the History
   Information by adding a History-Info header, using the Request-URI of
   the request as the hi-targeted-to-uri and initializing the index to
   the RECOMMENDED value of 1 in the hi-entry.

   In the case where the request is routed to a redirect server and the
   UAC receives a 3xx response with a Contact header, the UAC MAY
   maintain the previous hi-entry(s) in the request. In this case, the
   reason header SHOULD be associated with the hi-targeted-to-uri in the
   previous (last) hi-entry, as described in section 4.3.3.1.2. A new
   hi-entry MAY then be added for the URI from the Contact header (which
   becomes the new Request-URI). In this case, the index is 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 4.3.3.1.3. An example of this
   scenario can be found in Appendix D.

   A UAC that does not want the History-Info header added due to privacy
   considerations SHOULD include a Privacy header with a priv-value(s)
   of "session", "header" or "history" in the request.

   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. However, the validity of the information SHOULD be ensured
   prior to any application usage.  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.


   4.3.2 User Agent Server (UAS) Behavior

   The processing of the History-Info header by a UAS in a Request
   depends upon local policy and specific applications at the UAS which


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   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.

   If the "histinfo" option tag is received in a request, the UAS SHOULD
   include any History-Info received in the request in the subsequent
   response.


   4.3.3 Proxy Behavior

   The inclusion of the History-Info header in a Request does not alter
   the fundamental processing of proxies for determining request targets
   as defined in section 16.5 of [RFC3261].  Whether a proxy adds the
   History-Info header or a new hi-entry as it forwards a Request
   depends upon the following considerations:
       1. Whether the Request contains the "histinfo" option tag in the
          Supported header.
       2. Whether the proxy supports the History-Info header.
       3. Whether the Request contains a Privacy header with a priv-
          value of "session", "header" or "history".
       4. Whether any History-Info header added for a proxy/domain
          should go outside that domain.  An example being the use of
          the History-Info header within the specific domain in which
          it is retargeted, however, policies (for privacy, user and
          network security, etc.) would prohibit the exposure of that
          information outside that domain.  To accommodate such a
          scenario, a proxy MAY insert the Privacy header with a priv-
          value of "history" when the request is being forwarded within
          the same domain. An example of such an application is
          provided in Appendix C.
       5. Whether an hi-entry is added for a specific Request URI due
          to local privacy policy considerations.  A proxy MAY add the
          Privacy header with a priv-value of "history" associated with
          the specific hi-targeted-to-uri.

   An example policy would be a proxy that only adds the History-Info
   header if the "histinfo" option tag is in the Supported header.
   Other proxies may have a policy that they always add the header, but
   never forward it outside a particular domain, accomplishing this by
   adding a Privacy header with a priv-value of "history" to each hi-
   entry to allow the information to be collected for internal
   retargeting only.

   Each application making use of the History-Info header SHOULD address
   the impacts of the local policies on the specific application (e.g.


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   what specification of local policy is optimally required for a
   specific application and any potential limitations imposed by local
   policy decisions).

   Consistent with basic SIP processing of optional headers, proxies
   SHOULD maintain the History-Info header(s), received in messages
   being forwarded, independent of whether local policy supports
   History-Info.

   The specific processing by proxies for adding the History-Info
   headers in Requests and Responses, to accommodate the considerations
   outlined above, is described in detail in the following sections.

   4.3.3.1 Adding the History-Info header to Requests

   Upon evaluation of the considerations under which the History-Info
   header is to be included in requests (e.g. no Privacy header
   overriding inclusion, local policy supports, etc.), detailed in
   section 4.3.3, a proxy SHOULD add an hi-entry as it forwards a
   Request. Section 16.6 of [RFC3261] defines the steps to be followed
   as the proxy forwards a Request.  Step 5 prescribes the addition of
   optional headers.  Although, this would seem the appropriate step for
   adding the History-info header, the interaction with Step 6
   "Postprocess routing information" and the impact of a strict route in
   the Route header could result in the Request-URI being changed, thus
   adding the History-info header between steps 8 (adding Via header)
   and 9 (adding Content-Length) is RECOMMENDED. Note, that in the case
   of loose routing, the Request-URI does not change during the
   forwarding of a Request, thus the capturing of History-Info for such
   a request would result in duplicate Request-URIs with different
   indices. The hi-entry MUST be added following any hi-entry received
   in the request being forwarded.  Additionally, if a request is
   received that doesn't include a History-Info header, the proxy MAY
   add a History-Info header with an hi-entry preceding the one being
   added for the current request being forwarded.  The index for this
   hi-entry is RECOMMENDED to start at 1. The following subsections
   define the details of creating the information associated with each
   hi-entry.

   4.3.3.1.1 Privacy in the History-Info header

   If there is a Privacy header in the request with a priv-value of
   "session", "header" or "history", an hi-entry MAY be added, if the
   request is being forwarded to a Request URI associated with a domain
   for which the processing entity is responsible (and provided local
   policy supports the History-Info header, etc.).  If a request is
   being forwarded to a Request URI associated with a domain for which
   the proxy is not responsible and there is a Privacy header in the
   request with a priv-value of "session", "header" or "history", the


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   proxy SHOULD remove any hi-entry(s) prior to forwarding, depending
   upon local policy and whether the proxy might know a priori that it
   can rely on a downstream privacy service to apply the requested
   privacy.

   For the scenario where there is no Privacy header in the request and
   the request is being forwarded to a Request URI associated with the
   domain(s) for which this entity is responsible, there are several
   additional considerations:
     o If there is no local policy associated with privacy, then an hi-
       entry MAY be added to the Request.

     o If the proxy's local policies, per consideration 4 in section
       4.3.3, indicate that the History-Info header should not 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 that
       proxy in this scenario.

     o If the proxy's policy per consideration 5 in section 4.3.3,
       indicates that History-Info for a specific Request URI should
       not 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 the specific hi-entry, for
       that specific hi-targeted-to-uri, added by that proxy in this
       scenario.

   If a request is being forwarded to a Request URI associated with a
   domain for which the proxy is not responsible and local policy
   requires privacy associated with any, or with specific hi-entries it
   has added, any hi-entry with a priv-value of "history" SHOULD be
   removed prior to forwarding.

   4.3.3.1.2 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, 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 non-SIP Reason header (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.





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   For retargets as a result of timeouts or internal events, a Reason
   MAY be associated with the hi-targeted-to-uri that has been
   retargeted.

   The addition of the Reason should occur prior to the forwarding of
   the request (which may add a new hi-entry with a new hi-targeted-to-
   uri) as it is associated with the hi-targeted-to-uri that has been
   retargeted, since it reflects the reason why the Request to that
   specific URI was not successful.


   4.3.3.1.3 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 ABNF in section 4.1, the
   index consists of a dot delimited series of digits (e.g. 1.1.2). Each
   dot reflects a hop or level of nesting, thus the number of hops is
   determined by the total number of dots. Within each level, the
   integer reflects the number of peer entities to which the request has
   been routed.  Thus, the indexing results in a logical tree
   representation for the history of the Request. It is recommended that
   for each level of indexing, the index start at 1.  It is recommended
   that an increment of 1 is used for advancing to a new branch.

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

     1. Basic Forwarding:  In the case of a Request that is being
     forwarded, the index is determined by adding another level of
     indexing since the depth/length of the branch is increasing. To
     accomplish this, the proxy 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 RECOMMENDED to be 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 Proxy - 1st instance:  For the first
     instance of retargeting within a Proxy, the calculation of the
     index follows that prescribed for basic forwarding.

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



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     4. Retargeting based upon a Response:  In the case of retargeting
     due to a specific response (e.g. 302), the index would be
     calculated per rule 3.  That is, the lowest/last digit of the index
     is incremented (i.e. a new branch is created), with the increment
     RECOMMENDED to be 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. Retargeting the request in parallel (forking): If the request
     forwarding is done in parallel, the index MUST be captured for each
     forked request per the rules above, with each new Request having a
     unique index. The only difference in the messaging for this
     scenario and the messaging produced per basic proxy retargeting in
     rules 2 and 3 is these forwarded requests do not have History-Info
     entries associated with their peers.  The proxy builds the
     subsequent response (or request) using the aggregated information
     associated with each of those requests and including the header
     entries in the order indicated by the indexing.  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". Section 4.5
     provides an example of a parallel request scenario, highlighting
     this indexing mechanism.

   4.3.3.2 Processing History-Info in Responses

   A proxy that receives a Request with the "histinfo" option tag in the
   Supported header, and depending upon a local policy supporting the
   capture of History-Info, SHOULD return captured History-Info in
   subsequent, provisional and final responses to the Request, subject
   to the following considerations for privacy:

     o If the response is being forwarded to a Request URI associated
       with a domain for which the proxy is not responsible and there
       was a Privacy header, in the request received by the proxy, with
       a priv-value of "session", "header" or "history", the proxy MUST
       remove the History-Info header (i.e. all hi-entries) prior to
       forwarding.

     o If a request is being forwarded to a Request URI associated with
       a domain for which the proxy is not responsible and local policy
       requires privacy associated with any or all hi-entry(s) it has
       added, any hi-entry with a priv-value of "history" MUST be
       removed prior to forwarding.

     o If a proxy receives a response, from another intermediary
       associated with a domain for which it is responsible, including
       hi-entry(s) with privacy headers and that response is to be


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       forwarded to a domain for which it is not responsible, then
       those hi-entry(s) MUST be removed.

   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".


   4.3.4 Redirect Server Behavior

   A redirect server SHOULD NOT add any new History-Info, as that would
   be done by the entity receiving the 3xx response. However, a redirect
   server MAY include History-Info in responses by adding any History-
   Info headers received in a request to a subsequent response.

   4.4 Security for History-Info

   As discussed in Section 3, the security requirements are met by
   recommending the use of TLS (a basic SIP requirement per [RFC3261])
   for hop by hop security.  If TLS is not available on the connection
   over which a request, containing a History-Info header, is being
   forwarded, then either of the following two options MUST be
   implemented:
     o The History-Info header MUST be removed prior to forwarding the
       request, or
     o The request MUST be redirected, including the History-Info header
       in the response, to allow the UAC to securely issue the request,
       including the History-Info header.

  4.5 Example Applications using History-Info

   This scenario highlights an example where the History-Info in the
   response is primarily of use in not retrying routes that have already
   been tried by another proxy. Note, that this is just an example and
   that there may be valid reasons why a Proxy would want to retry the
   routes and thus, this would likely be a local proxy or even user
   specific policy.

   UA 1 sends a call to "Bob" to proxy 1. Proxy 1 forwards the request
   to Proxy 2.  Proxy 2 sends the requests in parallel and tries several
   places (UA2, UA3 and UA4) before sending a response to Proxy 1 that
   all the places are busy.   Proxy 1, without the History-Info, would
   try several some of the same places (e.g. UA3) based upon registered
   contacts for "Bob", before completing at UA5. However, with the
   History-Info, Proxy 1 determines that UA3 has already received the
   invite, thus the INVITE goes directly to UA5.




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   Section 4.5.1 provides this same scenario using one of the privacy
   mechanisms, with Proxy2 adding the Privacy header indicating that the
   History-Info header is not to be propagated outside P2's 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.

   Section 4.5.2 also provides the same scenario using one of the
   privacy mechanisms, however, due to local policy at Proxy2, only one
   of the Request-URIs (UA4) in the History-Info contains a priv-value
   of "history", thus allowing some optimized functionality in the
   routing of the request, but still maintaining privacy for specific
   URIs.

   Additional detailed scenarios are available in the appendix.


   UA1        Proxy1  Proxy2     UA2      UA3      UA4      UA5

   |            |         |        |        |        |        |
   |--INVITE -->|         |        |        |        |        |
   |            |-INVITE->|        |        |        |        |
                 Supported: histinfo
                 History-Info: <sip:Bob@P1.example.com>;index=1,
                               <sip:Bob@P2.example.com>; index=1.1
   |            |         |        |        |        |        |
   |            |         |-INVITE>|        |        |        |
                 History-Info: <sip:Bob@P1.example.com>;index=1,
                               <sip:Bob@P2.example.com>;index=1.1,
                               <sip:User2@UA2.example.com>;index=1.1.1
   |            |         |        |        |        |        |
   |            |         |-----INVITE ---->|        |        |
                  History-Info:<sip:Bob@P1.example.com>;index=1,
                               <sip:Bob@P2.example.com>; index=1.1,
                               <sip:User3@UA3.example.com>;index=1.1.2
   |            |         |        |        |        |        |
   |            |         |-------INVITE------------>|        |
                  History-Info:<sip:Bob@P1.example.com>;index=1,
                               <sip:Bob@P2.example.com>;index=1.1,
                               <sip:User4@UA4.example.com>;index=1.1.3

   /* All Responses from the INVITEs indicate non-success/non-
   availability*/
   |            |         |        |        |        |        |
   |            |<-480 ---|        |        |        |        |
                History-Info: <sip:Bob@P1.example.com>;index=1,
                   <sip:Bob@P2.example.com>; index=1.1,
                   <sip:User2@UA2.example.com?Reason=SIP;\


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                    cause=408;text="RequestTimeout">;index=1.1.1,
                   <sip:User3@UA3.example.com?Reason=SIP; \
                    cause=487;text="Request Terminated">; index=1.1.2,
                   <sip:User4@UA4.example.com?Reason=SIP;\
                    cause=603;text="Decline">; index=1.1.3
   |            |         |        |        |        |        |
  /* Upon receipt of the response, P1 determines another route for the
   INVITE, but finds that it matches a route already attempted
  (e.g. UA3, thus the INVITE is only forwarded to UA5, where
   the session is successfully established  */
   |            |         |        |        |        |        |
   |            |----------------INVITE --------------------->|
                History-Info: <sip:Bob@P1.example.com>;index=1,
                   <sip:Bob@P2.example.com>; index=1.1,
                   <sip:User2@UA2.example.com?Reason=SIP;cause=408;\
                    text="RequestTimeout">;index=1.1.1,
                   <sip:User3@UA3.example.com?Reason=SIP;cause=487;\
                    text="Request Terminated">; index=1.1.2,
                   <sip:User4@UA4.example.com?Reason=SIP;cause=603;\
                    text="Decline">; index=1.1.3
                   <sip:User5@UA5.example.com>;index=1.2
   |            |         |        |        |        |        |
   |            |<-----200 OK---------------------------------|
   |<--200 OK---|         |        |        |        |        |
   |            |         |        |        |        |        |
   |--ACK --------------------------------------------------->|


   4.5.1 Example with Privacy header for entire request at Proxy2

   UA1        Proxy1  Proxy2     UA2      UA3      UA4      UA5

   |            |         |        |        |        |        |
   |--INVITE -->|         |        |        |        |        |
   |            |-INVITE->|        |        |        |        |
                 Supported: histinfo
                 History-Info: <sip:Bob@P1.example.com>;index=1,
                               <sip:Bob@P2.example.com>;index=1.1
   |            |         |        |        |        |        |
   |            |         |-INVITE>|        |        |        |
                 Privacy: history
                 History-Info:<sip:Bob@P1.example.com>;index=1,
                              <sip:Bob@P2.example.com>;index=1.1,
                              <sip:User2@UA2.example.com>;index=1.1.1
   |            |         |        |        |        |        |
   |            |         |-----INVITE ---->|        |        |
                  Privacy: history
                  History-Info:<sip:Bob@P1.example.com>;index=1,
                               <sip:Bob@P2.example.com>; index=1.1,


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                               <sip:User3@UA3.example.com>;index=1.1.2
   |            |         |        |        |        |        |
   |            |         |-------INVITE------------>|        |
                  Privacy: history
                  History-Info:<sip:Bob@P1.example.com>;index=1,
                               <sip:Bob@P2.example.com>;index=1.1,
                               <sip:User4@UA4.example.com>;index=1.1.3

   /* All Responses from the INVITEs indicate non-success/non-
   availability and only the initial, received History-Info entries
   are NOT returned to P1 due to the Privacy header value.*/
   |            |         |        |        |        |        |
   |            |<-480 ---|        |        |        |        |
                History-Info: <sip:Bob@P1.example.com>;index=1,
                   <sip:Bob@P2.example.com>; index=1.1
   |            |         |        |        |        |        |
   /* Upon receipt of the response, P1 determines another route for the
   INVITE, including UA3 which was attempted by P2, but due to
   Privacy P1 is not aware of this, so UA3 is re-attempted prior to
   forwarding the INVITE to UA5, where the session is successfully
   established  */
   |            |         |        |        |        |        |
   |            |--------------INVITE ----->|        |        |
                  History-Info: <sip:Bob@P1.example.com>;index=1,
                                <sip:Bob@P2.example.com>; index=1.1,
                                <sip:User3@UA3.example.com>; index=1.2
   |            |         |        |        |        |        |
   |            |<-- 486 -------------------|        |        |
                  History-Info: <sip:Bob@P1.example.com>;index=1,
                                <sip:Bob@P2.example.com>; index=1.1,
                                <sip:User3@UA3.example.com>; index=1.2
   |            |         |        |        |        |        |
   |            |----------------INVITE --------------------->|
                History-Info: <sip:Bob@P1.example.com>;index=1,
                   <sip:Bob@P2.example.com>; index=1.1,
                   <sip:User3@UA3.example.com?Reason=SIP;cause=486;\
                    text="Busy Here">;index=1.2,
                   <sip:User5@UA5.example.com>;index=1.3
   |            |         |        |        |        |        |
   |            |<-----200 OK---------------------------------|
   |<--200 OK---|         |        |        |        |        |
   |            |         |        |        |        |        |
   |--ACK --------------------------------------------------->|


   4.5.2 Example with Privacy header for specific URI (UA4) at Proxy2

   UA1        Proxy1  Proxy2     UA2      UA3      UA4      UA5



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   |            |         |        |        |        |        |
   |--INVITE -->|         |        |        |        |        |
   |            |-INVITE->|        |        |        |        |
                 Supported: histinfo
                 History-Info: <sip:Bob@P1.example.com>;index=1,
                               <sip:Bob@P2.example.com>; index=1.1
   |            |         |        |        |        |        |
   |            |         |-INVITE>|        |        |        |
                 History-Info:<sip:Bob@P1.example.com>;index=1,
                              <sip:Bob@P2.example.com>;index=1.1,
                              <sip:User2@UA2.example.com>;index=1.1.1
   |            |         |        |        |        |        |
   |            |         |-----INVITE ---->|        |        |
                  History-Info:<sip:Bob@P1.example.com>;index=1,
                               <sip:Bob@P2.example.com>;index=1.1,
                               <sip:User3@UA3.example.com>;index=1.1.2
   |            |         |        |        |        |        |
   |            |         |-------INVITE------------>|        |
                  History-Info: <sip:Bob@P1.example.com>;index=1,
                                <sip:Bob@P2.example.com>;index=1.1,
                                <sip:User4@UA4.example.com?\
                                 Privacy=history>; index=1.1.3

   /* All Responses from the INVITEs indicate non-success/non-
   availability.  The History-Info associated with UA4 is not returned
   in the response due to the privacy header associated with that URI */
   |            |         |        |        |        |        |
   |            |<-480 ---|        |        |        |        |
                History-Info: <sip:Bob@P1.example.com>;index=1,
                   <sip:Bob@P2.example.com>; index=1.1,
                   <sip:User2@UA2.example.com?Reason=SIP;\
                    cause=408;text="RequestTimeout">;index=1.1.1,
                   <sip:User3@UA3.example.com?Reason=SIP; \
                    cause=487;text="Request Terminated">; index=1.1.2,
   |            |         |        |        |        |        |
  /* Upon receipt of the response, P1 determines another route for the
   INVITE, but finds that it matches a route already attempted
  (e.g. UA3), thus the INVITE is only forwarded to UA5, where
   the session is successfully established  */
   |            |         |        |        |        |        |
   |            |----------------INVITE --------------------->|
                History-Info: <sip:Bob@P1.example.com>;index=1,
                   <sip:Bob@P2.example.com>; index=1.1,
                   <sip:User2@UA2.example.com?Reason=SIP;cause=408;\
                    text="RequestTimeout">;index=1.1.1,
                   <sip:User3@UA3.example.com?Reason=SIP;cause=487;\
                    text="Request Terminated">; index=1.1.2,
                   <sip:User5@UA5.example.com>;index=1.2
   |            |         |        |        |        |        |


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


 5. Application Considerations

   As seen by the example scenarios in the appendix, 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:
   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.
   4) The security associated with the History-Info header requires the
      use of TLS. In the case of TLS not being available for a
      connection over which a request is being forwarded, the History-
      Info header may be removed from a request. The impact of lack of
      having the information depends upon the nature of the specific
      application (e.g. is the information something that appears on a
      display or is it processed by automata which could have negative
      impacts on the subsequent processing of a request?).   It is
      suggested that the impact of an intermediary not supporting the
      security recommendations should be evaluated by the application
      to ensure that the impacts have been sufficiently addressed by
      the application.


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 6. Security Considerations

   The threat model and related security and privacy requirements for
   the History-Info header are described in section 2.1 and 2.2 of this
   document.  Sections 3.2, 3.3 and 4.4 provide normative
   recommendations related to security and privacy fulfilling these
   requirements.  The use of TLS is mandated between the entities (i.e.
   UAC to Proxy, Proxy to Proxy, and Proxy to UAS) which use the
   History-Info header.  The appropriate handling of a request in the
   case that TLS is not available for a specific connection is described
   in section 5.

   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.

7. IANA Considerations

   (Note to RFC Editor: Please fill in all occurrences of XXXX in this
   section with the RFC number of this specification).

   7.1 Registration of new SIP History-Info header

   This document defines a new SIP header field name: History-Info and a
   new option tag: histinfo.

   The following changes should be made to
   http:///www.iana.org/assignments/sip-parameters

   The following row should be added to the header field section:

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

   The following should be added to the Options Tags section:

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



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   7.2 Registration of "history" for SIP Privacy header

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

   The following changes should be made to
   http://www.iana.org/assignments/sip-priv-values


   The following should be 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@nortelnetworks.com


 Normative References

   [RFC3261] J. Rosenberg et al, "SIP: Session initiation protocol," RFC
   3261, June, 2002.

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

   [RFC3323] J. Peterson, "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", RFC 2119, March 1997.

   [RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
   Specifications: ABNF", RFC 2234, November 1997.

   [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
   RFC 2246, January 1999.


 Informational References

   [SIPSVCEX] A. Johnson, "SIP Service Examples", draft-ietf-sipping-
   service-examples-07.txt, July, 2004.

   [RFC3665] A. Johnson et al, "SIP Basic Call Flow Examples", RFC 3665,
   BCP 75, December, 2003.




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 Acknowledgements

   The editor would like to acknowledge the constructive feedback
   provided by Robert Sparks, Paul Kyzivat, Scott Orton, John Elwell,
   Nir Chen, Francois Audet, 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.

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

 Contributors' Addresses

   Cullen, Mark and Jon contributed to the development of the initial
   requirements.

   Cullen and Mark provided substantial input in the form of email
   discussion in the development of the initial version of the
   individual solution document.

   Cullen Jennings
   Cisco Systems
   170 West Tasman Dr
   MS: SJC-21/3

   Tel: +1 408 527 9132
   Email: fluffy@cisco.com

   Jon Peterson
   NeuStar, Inc.
   1800 Sutter Street, Suite 570
   Concord, CA  94520
   USA

   Phone: +1 925-363-8720
   EMail: Jon.Peterson@NeuStar.biz

   Mark Watson
   Nortel Networks (UK)
   Maidenhead Office Park (Bray House)
   Westacott Way
   Maidenhead,
   Berkshire
   England

   Tel: +44 (0)1628-434456


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   Email:  mwatson@nortelnetworks.com


 Author's Address

   Mary Barnes
   Nortel Networks
   2380 Performance Drive
   Richardson, TX USA

   Phone:  1-972-684-5432
   Email:  mary.barnes@nortelnetworks.com


 Appendix. Example Scenarios

   The scenarios in Appendix A-D provide sample use cases for the
   History-Info header for informational purposes only.  They are not
   intended to be normative and the formatting is for visual purposes,
   thus the headers in the URI are not shown properly formatted for
   escaping. Refer to section 4.2 examples with the proper formatting.

 Appendix A. Sequentially forking (History-Info in Response)

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

   "Alice" at UA 1sends a call to "Bob" via proxy 1. Proxy 1
   sequentially tries several places (UA2, UA3 and UA4) unsuccessfully
   before sending a response to "Alice".

   This scenario is provided to show that by providing the History-Info
   to UA1, the end user or an application at UA1 could make a decision
   on how best to attempt finding "Bob".  Without this mechanism UA1
   might well attempt UA3 (and thus UA4) and then re-attempt UA4 on a
   third manual attempt at reaching "Bob". With this mechanism, either
   the end user or application could know that "Bob" is busy on his home
   phone and is physically not in the office. 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.

   UA1      Proxy1                UA2      UA3      UA4
   |            |                  |        |        |
   |-INVITE F1->|                  |        |        |


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   |            |                  |        |        |
   |            |--INVITE F2------>|        |        |
   |<--100 F3---|                  |        |        |
   |            |<-302 F4----------|        |        |
   |            |                  |        |        |
   |            |-------INVITE F5 --------->|        |
   |            |                  |        |        |
   |            |<-------180 F6 ------------|        |
   |<---180 F7--|                  |        |        |
   |  . .       |---retransmit INVITE ----->|        |
   |            |                  |        |        |
   |            |      ( timeout ) |        |        |
   |            |                  |        |        |
   |            |------INVITE F8 ------------------->|
   |<--100 F9 --|                  |        |        |
   |            |                  |        |        |
   |            |<-486 F10 --------------------------|
   |            |                  |        |        |
   |            |-- ACK F11------------------------->|
   |<--486 F12--|                  |        |        |
   |            |                  |        |        |
   |--ACK F13-->|                  |        |        |
   |            |                  |        |        |


   Message Details

      F1 INVITE UA1 ->Proxy1

      INVITE sip:UserA@example.com SIP/2.0
      Via: SIP/2.0/UDP example.net:5060
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>
      Call-Id: 12345600@example.net
      CSeq: 1 INVITE
      Contact: Alice <sip:User1@example.net>
      Content-Type: application/sdp
      Content-Length: <appropriate value>

      v=0
      o=UserA 2890844526 2890844526 IN IP4 client.example.net
      s=Session SDP
      c=IN IP4 192.0.2.3
      t=0 0
      m=audio 49170 RTP/AVP 0
      a=rtpmap:0 PCMU/8000

      /*Client for UA1 prepares to receive data on port 49170
      from the network. */


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      F2 INVITE  Proxy1 ->UA2

      INVITE sip:UserA@ims.example.com SIP/2.0
      Via: SIP/2.0/UDP ims.example.com:5060;branch=1
        Via: SIP/2.0/UDP example.net:5060
      Record-Route: <sip:UserA@example.com>
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>
      Call-Id: 12345600@example.net
      CSeq: 1 INVITE
      History-Info: <sip:UserA@ims.example.com>; index=1
      Contact: Alice <sip:User1@example.net>
      Content-Type: application/sdp
      Content-Length: <appropriate value>

      v=0
      o=UserA 2890844526 2890844526 IN IP4 client.example.net
      s=Session SDP
      c=IN IP4 192.0.2.3
      t=0 0
      m=audio 49170 RTP/AVP 0
      a=rtpmap:0 PCMU/8000

      F3 100 Trying Proxy1 ->UA1

      SIP/2.0 100 Trying
      Via: SIP/2.0/UDP example.net:5060
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>
      Call-Id: 12345600@example.net
      CSeq: 1 INVITE
      Content-Length: 0


      F4 302 Moved Temporarily UA2 ->Proxy1

      SIP/2.0 302 Moved Temporarily
      Via: SIP/2.0/UDP ims.example.com:5060;branch=1
      Via: SIP/2.0/UDP example.net:5060
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>;tag=3
      Call-Id: 12345600@example.net
      CSeq: 1 INVITE
      Contact: <sip:UserB@example.com>
      Content-Length: 0


      F5 INVITE Proxy1 -> UA3


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      INVITE sip:UserB@example.com SIP/2.0
      Via: SIP/2.0/UDP ims.example.com:5060;branch=2
      Via: SIP/2.0/UDP example.net:5060
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>
      Call-Id: 12345600@example.net
      History-Info: <sip:UserA@ims.example.com?Reason=SIP;\
      cause=302; text="Moved Temporarily">; index=1,
      <sip:UserB@example.com>;index=2
      CSeq: 1 INVITE
      Contact: Alice <sip:User1@example.net>
      Content-Type: application/sdp
      Content-Length: <appropriate value>

      v=0
      o=User1 2890844526 2890844526 IN IP4 client.example.net
      s=Session SDP
      c=IN IP4 192.0.2.3
      t=0 0
      m=audio 49170 RTP/AVP 0
      a=rtpmap:0 PCMU/8000

      F6 180 Ringing UA3 ->Proxy1

      SIP/2.0 180 Ringing
      Via: SIP/2.0/UDP example.net:5060
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>;tag=5
      Call-ID: 12345600@example.net
      CSeq: 1 INVITE
      Content-Length: 0

      F7 180 Ringing Proxy1 -> UA1

      SIP/2.0 180 Ringing
      SIP/2.0/UDP example.net:5060
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>
      Call-Id: 12345600@example.net
      CSeq: 1 INVITE
      Content-Length: 0

      /* User B is not available. INVITE is sent multiple
      times until it times out. */

        /* The proxy forwards the INVITE to UA4 after adding the
      additional History Information entry. */



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      F8 INVITE Proxy1 -> UA4

      INVITE sip:UserC@example.com SIP/2.0
      Via: SIP/2.0/UDP ims.example.com:5060;branch=3
      Via: SIP/2.0/UDP example.net:5060
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>
      Call-Id: 12345600@example.net
      History-Info:<sip:UserA@ims.example.com?Reason=SIP;\
      cause=302; text="Moved Temporarily">;index=1,
      <sip:UserB@example.com?Reason=SIP;cause=480;\
      text="Temporarily Unavailable" >;index=2,
      <sip:UserC@example.com>;index=3
      CSeq: 1 INVITE
      Contact: Alice <sip:User1@example.net>
      Content-Type: application/sdp
      Content-Length: <appropriate value>

      v=0
      o=User1 2890844526 2890844526 IN IP4 client.example.net
      s=Session SDP
      c=IN IP4 192.0.2.3
      t=0 0
      m=audio 49170 RTP/AVP 0
      a=rtpmap:0 PCMU/8000


      F9 100 Trying Proxy1 ->UA1

      SIP/2.0 100 Trying
      Via: SIP/2.0/UDP example.net:5060
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>
      Call-Id: 12345600@example.net
      CSeq: 1 INVITE
      Content-Length: 0


      F10 486 Busy Here UA4 -> Proxy1

      SIP/2.0  486 Busy Here
      Via: SIP/2.0/UDP ims.example.com:5060;branch=3
      Via: SIP/2.0/UDP example.net:5060
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>
      Call-Id: 12345600@example.net
      CSeq: 1 INVITE
      Content-Length: 0


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      F11 ACK Proxy1 -> UA4

      ACK sip:UserC@example.com SIP/2.0
      Via: SIP/2.0/UDP example.net:5060
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>
      Call-Id: 12345600@example.net
      CSeq: 1 ACK
      Content-Length: 0


       /* The proxy forwards the 486 to Alice after adding the
          associated History Information entries from the series of
          INVITES */

      F12 486 Busy Here Proxy1 -> UA1

      SIP/2.0  486 Busy Here
      Via: SIP/2.0/UDP example.net:5060
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>
      Call-Id: 12345600@example.net
      History-Info:<sip:UserA@ims.example.com?Reason=SIP;\
      cause=302; text="Moved Temporarily">;index=1,
      <sip:UserB@example.com?Reason=SIP;cause=480;\
      text="Temporarily Unavailable" >;index=2,
      <sip:UserC@example.com>;index=3
      CSeq: 1 INVITE
      Content-Length: 0

      F13 ACK Alice -> Proxy 1

      ACK sip:UserA@example.com SIP/2.0
      Via: SIP/2.0/UDP example.net:5060
      From: Alice <sip:User1@example.net>
      To: Bob <sip:UserA@example.com>
      Call-Id: 12345600@example.net
      CSeq: 1 ACK
      Content-Length: 0


 Appendix B.  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 isn't intended to be a complete
   specification for this specific edge service as it is quite likely


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   that additional information is needed by the edge service. History-
   Info is just one building block that this service makes use of.

   UA 1 called UA A which had been forwarded to UA B which forwarded to
   a UA 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.

   UA1          Proxy           UA-A         UA-B        UA-VM

   |              |              |             |          |
   |--INVITE F1-->|              |             |          |
   |              |              |             |          |
   |              |--INVITE F2-->|             |          |
   |<--100 F3-----|              |             |          |
   |              |<-302 F4------|             |          |
   |              |              |             |          |
   |              |--------INVITE F5---------->|          |
   |              |              |             |          |
   |              |<--------180 F6-------------|          |
   |<---180 F7----|              |             |          |
   |  . . .       |              |             |          |
   |              |------retransmit INVITE---->|          |
   |  . . .       |              |             |          |
   |              |       (timeout)            |          |
   |              |              |             |          |
   |              |-------INVITE F8---------------------->|
   |              |              |             |          |
   |              |<-200 F9-------------------------------|
   |              |              |             |          |
   |<-200 F10-----|              |             |          |
   |              |              |             |          |
   |--ACK F11-------------------------------------------->|

   Message Details

  INVITE F1   UA1->Proxy

  INVITE sip:UserA@example.com SIP/2.0
  Via: SIP/2.0/UDP example.net:5060
  From: BigGuy <sip:User1@example.net>
  To: LittleGuy <sip:UserA@example.com>
  Call-Id: 12345600@example.net
  CSeq: 1 INVITE
  Contact: BigGuy <sip:User1@example.net>
  Content-Type: application/sdp
  Content-Length: <appropriate value>



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   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.example.net
   s=Session SDP
   c=IN IP4 192.0.2.3
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   /*Client for UA1 prepares to receive data on port 49170
   from the network. */

   INVITE F2 Proxy->UA-A

   INVITE sip:UserA@ims.example.com SIP/2.0
   Via: SIP/2.0/UDPims.example.com:5060;branch=1
     Via: SIP/2.0/UDP example.net:5060
   Record-Route: <sip:UserA@example.com>
   From: BigGuy <sip:User1@example.net>
   To: LittleGuy <sip:UserA@example.com>
   Call-Id: 12345600@example.net
   CSeq: 1 INVITE
   History-Info: <sip:UserA@ims.example.com>; index=1
   Contact: BigGuy <sip:User1@example.net>
   Content-Type: application/sdp
   Content-Length: <appropriate value>

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.example.net
   s=Session SDP
   c=IN IP4 192.0.2.3
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   100 Trying F3 Proxy->UA1

   SIP/2.0 100 Trying
   Via: SIP/2.0/UDP example.net:5060
   From: BigGuy <sip:User1@example.net>
   To: LittleGuy <sip:UserA@example.com>
   Call-Id: 12345600@example.net
   CSeq: 1 INVITE
   Content-Length: 0


   302 Moved Temporarily F4  UserA->Proxy
   SIP/2.0 302 Moved Temporarily
   Via: SIP/2.0/UDP ims.example.com:5060;branch=1
   Via: SIP/2.0/UDP example.net:5060


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   From: BigGuy <sip:User1@example.net>
   To: LittleGuy<sip:UserA@example.com>;tag=3
   Call-Id: 12345600@example.net
   CSeq: 1 INVITE
   Contact: <sip:UserB@example.com>
   Content-Length: 0


   INVITE F5 Proxy-> UA-B

   INVITE sip:UserB@example.com SIP/2.0
   Via: SIP/2.0/UDP ims.example.com:5060;branch=2
   Via: SIP/2.0/UDP example.net:5060
   From: BigGuy <sip:User1@example.net>
   To: LittleGuy <sip:UserA@example.com>
   Call-Id: 12345600@example.net
   History-Info: <sip:UserA@ims.example.com?Reason=SIP;\
   cause=302; text="Moved Temporarily">; index=1,
   <sip:UserB@example.com>;index=2
   CSeq: 1 INVITE
   Contact: BigGuy <sip:User1@example.net>
   Content-Type: application/sdp
   Content-Length: <appropriate value>

   v=0
   o=User1 2890844526 2890844526 IN IP4 client.example.net
   s=Session SDP
   c=IN IP4 192.0.2.3
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   180 Ringing F6  UA-B ->Proxy

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP example.net:5060
   From: BigGuy <sip:User1@example.net>
   To: LittleGuy <sip:UserA@example.com>;tag=5
   Call-ID: 12345600@example.net
   CSeq: 1 INVITE
   Content-Length: 0

   180 Ringing F7  Proxy-> UA1

   SIP/2.0 180 Ringing
   SIP/2.0/UDP example.net:5060
   From: BigGuy <sip:User1@example.net>
   To: LittleGuy <sip:UserA@example.com>
   Call-Id: 12345600@example.net


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   CSeq: 1 INVITE
   Content-Length: 0

   /* User B is not available. INVITE is sent multiple
   times until it times out. */

     /* The proxy forwards the INVITE to UA-VM after adding the
   additional History Information entry. */


   INVITE F8  Proxy-> UA-VM

   INVITE sip:VM@example.com SIP/2.0
   Via: SIP/2.0/UDP ims.example.com:5060;branch=3
   Via: SIP/2.0/UDP example.net:5060
   From: BigGuy <sip:User1@example.net>
      To: LittleGuy <sip:UserA@example.com>
   Call-Id: 12345600@example.net
   History-Info:<sip:UserA@ims.example.com?Reason=SIP;\
   cause=302; text="Moved Temporarily">;index=1,
   <sip:UserB@example.com?Reason=SIP;cause=480;\
   text="Temporarily Unavailable" >;index=2,
   <sip:VM@example.com>;index=3
   CSeq: 1 INVITE
   Contact: BigGuy <sip:User1@example.net>
   Content-Type: application/sdp
   Content-Length: <appropriate value>

   v=0
   o=User1 2890844526 2890844526 IN IP4 client.example.net
   s=Session SDP
   c=IN IP4 192.0.2.3
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   200 OK F9

   SIP/2.0 200 OK UA-VM->Proxy

   Via: SIP/2.0/UDP ims.example.com:5060;branch=3
   Via: SIP/2.0/UDP example.net:5060
   From: BigGuy <sip:User1@example.net>
   To: LittleGuy <sip:UserA@example.com>;tag=3
   Call-Id: 12345600@example.net
   CSeq: 1 INVITE
   Contact: TheVoiceMail <sip:VM@example.com>
   Content-Type: application/sdp


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   Content-Length: <appropriate value>

   v=0
   o=UserA 2890844527 2890844527 IN IP4 vm.example.com
   s=Session SDP
   c=IN IP4 192.0.2.4
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   200 OK F10  Proxy->UA1

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ims.example.com:5060;branch=3
   Via: SIP/2.0/UDP example.net:5060
   From: BigGuy <sip:User1@example.net>
   To: LittleGuy <sip:UserA@example.com>;tag=3
   Call-Id: 12345600@example.net
   CSeq: 1 INVITE
   Contact: TheVoiceMail <sip:VM@example.com>
   Content-Type: application/sdp
   Content-Length: <appropriate value>

   v=0
   o=UserA 2890844527 2890844527 IN IP4 vm.example.com
   s=Session SDP
   c=IN IP4 192.0.2.4
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   ACK F11 UA1-> UA-VM

   ACK sip:VM@example.com SIP/2.0
   Via: SIP/2.0/UDP example.net:5060
   From: BigGuy <sip:User1@example.net>
   To: LittleGuy<sip:UserA@example.com>;tag=3
   Call-Id: 12345600@example.net
   CSeq: 1 ACK
   Content-Length: 0

   /* RTP streams are established between UA1 and
   UA-VM. UA-VM starts announcement for UA1 */







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 Appendix C.  Automatic Call Distribution Example

   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 (ACDGRP1)
   were busy, by retargeting the request to the Silver Group.  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
   the calling UA.

   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.



   UA1          Proxy        ACDGRP1 Svr   ACDGRP2 Svr UA2-ACDGRP2

   |              |              |             |          |
   |--INVITE F1-->|              |             |          |
    Supported:histinfo
   |              |              |             |          |
   |              |--INVITE F2-->|             |          |
                    Supported:histinfo
                    History-Info: <sip:Gold@example.com>; index=1
                    History-Info: <sip:ACDGRP1@example.com>; index=1.1
   |              |              |             |          |
   |              |<-302 F3------|             |          |
                    Contact: <sip:ACDGRP2@ACD.com>
   |              |              |             |          |
   |              |--------INVITE F4---------->|          |
                    History-Info: <sip:Gold@example.com>; index=1
                    History-Info: <sip:ACDGRP1@example.com>; index=1.1


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                    History-Info: <sip:ACDGRP2@example.com>; index=1.2
   |              |              |             |          |
   |              |              |             |          |
   |              |              |             |INVITE F5>|
                    History-Info: <sip:Gold@example.com>; index=1
                    History-Info: <sip:ACDGRP1@example.com>; index=1.1
                    History-Info: <sip:ACDGRP2@example.com>; index=1.2
   |              |              |             |          |
   |              |              |             |<-200 F6--|
   |              |              |             |          |
   |              |<-200 F7--------------------|          |
                    History-Info: <sip:Gold@example.com>; index=1
                    History-Info: <sip:ACDGRP1@example.com>; index=1.1
                    History-Info: <sip:ACDGRP2@example.com>; index=1.2
   |<-200 F8------|              |             |          |
   < No History-Info included in the response due to Local Policy>
   |              |              |             |          |
   |--ACK F9--------------------------------------------->|



 Appendix D. Session via Redirect and Proxy Servers

   In this scenario, Alice places a call to Bob using first a Redirect
   server then a Proxy Server.  The INVITE message is first sent to the
   Redirect Server.  The Server returns a 302 Moved Temporarily response
   (F2) containing a Contact header with Bob's current SIP address.
   Alice then generates a new INVITE with Bob's current SIP address
   included in another History-Info entry.  The INVITE is then sent to
   Bob via the Proxy Server, with Bob receiving the complete History
   information; the call then proceeds normally.  The complete call flow
   for this scenario, without the use of History-Info is described in
   section 3.6 of the SIP Basic Call Flow Examples [RFC3665].


   Alice        Redirect Server     Proxy 3             Bob
     |                |                |                |
     |   INVITE F1    |                |                |
     |--------------->|                |                |
     |     302 F2     |                |                |
     |<---------------|                |                |
     |     ACK F3     |                |                |
     |--------------->|                |                |
     |     INVITE F4                   |                |
     |-------------------------------->|    INVITE F5   |
     |             100  F6             |--------------->|





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   Message Details

   F1 INVITE Alice -> Redirect Server

   INVITE sip:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKbf9f44
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>
   Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@biloxi.example.com>; index=1
   Contact: <sip:alice@client.atlanta.example.com>
   Content-Length: 0


   F2 302 Moved Temporarily Redirect Proxy -> Alice

   SIP/2.0 302 Moved Temporarily
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKbf9f44
    ;received=192.0.2.1
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=53fHlqlQ2
   Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
   CSeq: 1 INVITE
   History-Info: <sip:bob@biloxi.example.com>; index=1
   Contact: <sip:bob@chicago.example.com;transport=tcp>
   Content-Length: 0


   F3 ACK Alice -> Redirect Server

   ACK sip:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKbf9f44
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>;tag=53fHlqlQ2
   Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
   CSeq: 1 ACK
   Content-Length: 0


   F4 INVITE Alice -> Proxy 3

   INVITE sip:bob@chicago.example.com SIP/2.0
   Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>


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   Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
   CSeq: 2 INVITE
   History-Info: <sip:bob@biloxi.example.com?Reason=SIP;cause=302>\
                  text="Moved Temporarily">; index=1,
                 <sip:bob@chicago.example.com>; index=2
   Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
   Content-Length: 0


   F5 INVITE Proxy 3 -> Bob

   INVITE sip:bob@client.chicago.example.com SIP/2.0
   Via: SIP/2.0/TCP ss3.chicago.example.com:5060;branch=z9hG4bK721e.1
   Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
    ;received=192.0.2.1
   Max-Forwards: 69
   Record-Route: <sip:ss3.chicago.example.com;lr>
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>
   Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
   CSeq: 2 INVITE
   History-Info: <sip:bob@biloxi.example.com?Reason=SIP;cause=302>\
                  text="Moved Temporarily">; index=1,
                 <sip:bob@chicago.example.com>; index=2,
                 <sip:bob@client.chicago.example.com>; index=2.1
   Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
   Content-Length: 0

   Detailed Call Flow continues per section 6.3 in [RFC 3665].


Appendix E. Changelog

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

   Changes from the 05 to 06 version:

     o General changes to tidy document:
          o  Change the term "this draft" to "this document".
          o  Spell out first use of acronyms (ex: UA, UAC, UAS, URI,
             URL, TLS)
          o  Change Histinfo to "histinfo"

     o Abstract: delete last 2 sentences in Abstract

     o Overview:
          o Insert paragraph break just before "This draft defined a new
             SIP header..."


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          o 2nd to last paragraph of Overview, delete all but first
             sentence." (the other stuff on where examples would be
             further documented isn't really relevant to what this draft
             does and had been there for purposes of WG discussion
             during development of document).

     o Background: Put "retarget" in single quotes when the term is
       introduced in the second sentence (consistent with reference in
       section 3.

     o Section 3, 2nd para, add SUBSCRIBE and PUBLISH as appropriate
       methods (consistent with section 4.1)

     o Section 3.2:  Changed "RECOMMENDs the use of TLS" to "strongly
       RECOMMENDS the use of TLS".

     o Section 4.1, Privacy bullet item, clarified the first reference
        of "History-Info header" as "History-Info header field values".

     o Section 4.2:
        oFor clarity in the examples, removed the text phrases escaped
          into the URI.
        oChange capitalized "Cause" to LC "cause".

      o Section 6 (Security): deleted the last two paragraphs (on AIB,
        Body additions, Security of Inserted Info since those are not
        relevant to the security solution required for History-Info),
        and restated what security mechanisms are mandatory to
        implement, including references to the sections of the document
        dealing with those mechanisms.

      o References: Updated references: Added TLS and removed body-add,
        AIB, and sec-inserted.

   Changes from the 04 to 05 version:

     o Section 3, 3rd paragraph:  Clarified that the Proxy does not
       create the requests, but rather forwards. (SP - individual email
       Nov. 18)

     o Section 4.3.1:
          - 2nd paragraph: Added text for handling the reason, referring
             to section 4.3.3.1.2 for details(JRE-individual email Nov.
             15)
          - last paragraph: Clarified that with the exception of 3xx
             responses, handling of responses is application specific.

     o Section 4.3.3.1.1, lst paragraph, last statment:  changed "...the
       proxy MUST remove any hi-entry(s) prior to forwarding."


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       to:
       "...the proxy SHOULD remove any hi-entry(s) prior to forwarding,
       depending upon local policy and whether the proxy might know
       apriori that it can rely on a downstream privacy service to
       apply the requested privacy." (WG mailing list - conclusion
       posted on Nov. 17)

     o Section 4.3.3.1.2, last paragraph : a "is" has been added
        between "it" and "associated" in the phrase "as it associated
        with the hi-targeted-to-uri ..."  (SP - individual email Nov.
        18)

     o Section 4.5 examples:
          o  Fixed indexing (2 should have been 1.1, which affects the
             whole series). (WG mailing list response to TS posted on
             Nov. 3rd)
          o  Fixed the 480 "timeouts" which should be 408s (note this
             error was introduced with changes made in this section in
             the 03 version. (NC-individual email Oct 31)
          o  Fixed missing " on the "Busy Here" on the INVITE to UA5 in
             4.5.1. (NC-individual email Oct 31)


   Changes from the 03 to the 04 version:
     o Editorial nits:
          o Removed second reference to "call center" in Overview
            section. (EB)
          o Changed square brackets on references to requirements to
            parenthesis so they wouldn't appear to be external
            references. (EB)
          o Moved the updates to table 2 in section 4.1, so that it
            appears right after the paragraph discussing in which
            messages the header can appear. (RM)
     o Section 3.2:
          o Moved discussion of new security solution proposals per
            updated identity draft and rohan's body addition to section
            6 as they're not relevant to the solution in this document
            (per (JRE-1)).
          o Per IETF-60 discussion and Rohan's input, added a statement
            that if TLS isn't available on the connection over which
            the History-info is being forwarded, either a redirection
            (per identity document) is required or the History-info is
            not forwarded.
     o Section 3.3:
          o 2nd paragraph: adding clarification text "In the latter
            case,.." to the last sentence. (JRE-2)
          o Last paragraph: Clarified in the last sentence that if there
            is no impact on the application due to privacy



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            considerations, why that is so MUST also be explained by
            that application. (EB)
     o Section 4.1:
          o Added SUBSCRIBE and PUBLISH to the list of messages in which
            History-Info header may appear. (JRE-3/10)
          o Changed the wording in the text descriptions of the fields
            to be non-normative (i.e. not caps for the reserved words
            in RFC 2119). (KD)
          o In the text description for the Index, clarified that the
            entries are added in a specific order, with the indexing to
            ensure the proper ordering. (JRE-4)
          o In the text descriptions for the Reason and Privacy
            parameters, changed the references of "Request URI" to "hi-
            targeted-to-uri" to explicitly refer to the field in the
            header entry. (JRE-5)
          o In the ABNF syntax, changed the "hist-info" field name to
            "hi-entry".  This is then used throughout the remainder of
            the document to refer to a "History-Info header entry".
            (Note, this impacted the text primarily in section 4.3 -
            specifically )(JRE-8/9)
     o Section 4.2: Added appropriate hex characters for the escaped
       headers in the example (e.g. for ", = and ;). (LIST 10/15)
     o Section 4.3.1: changed "notified" to "aware" in terms of
       application interface to be less specific about interface to
       application  (consistent with 4.3.2)
     o Section 4.3.2: same change as in 4.3.1.  Capitalized "should" in
       last sentence. (EB)
     o Section 4.3.3: Clarified item 4 to be specific that the privacy
       header may be used when the request is being forwarded within
       the same domain (accomodating the scenario which allows
       information to only be forwarded within the domain in which it
       was retargeted). (JRE-11)
     o Section 4.3.3.1: rewording to include explicit references to hi-
       entry [JRE-8/9] and changed the "header should be added
       following..." to "the hi-entry MUST be added following..." (JRE-
       4)
     o Section 4.3.3.1.1: Reworded privacy section for clarity.
       Basically, need to tag each of the entires with
       "privacy=history" for retargeting within the domain and strip
       out the entries when leaving the domain IF the request has a
       privacy header of Session, Header or History or local policy
       requires.  (JRE-13)
     o Section 4.3.3.1.2: Added the use of a Reason header in a
       response, as the Reason field associated with a retargeted URI.
       (LIST-10/5)
     o Section 4.3.3.1.3:
          o Clarified that the number of hops is reflected by the total
            number of dots (and not the value of the digits) (JRE-14.1)



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          o Deleted last sentence of 1st paragraph as that was a
            holdover from a previous version. (JRE-14.2)
          o Item 5: clarified that the referenced scenario is forking
            and that the response consists of the aggregated (rather
            than the word "amalgamated") information. (JRE-15)
     o Section 4.3.3.2:
          o Clarified that response processing for History-Info follows
            the general processing described in section 16.7 of
            RFC3261.  (related to JRE-15)
          o More detail added on the processing of responses with
            Privacy header. (LIST-8/18)
     o Section 4.4: Added text addressing the security when TLS is not
       available, per Rohan's comment above.
     o Section 5:
          o Changed the "should" to a "MUST" in the 1st application
            consideration in terms of the requirement to define default
            behavior should the information not be available, due to
            History-Info being an optional header. (EB)
          o Updated the 5th consideration for security to reflect the
            lack of information due to potential TLS inavailability for
            a connection, thus the potential for no History-Info header
            (per Rohan's comment).
     oSection 6:
          o Updated security considerations per TLS issue (Rohan) and to
            reference the new security solution proposals.
          o Added discussion of new security solution proposals per
            updated identity document and rohan's body addition.
     oAppendix:
          o Added overview clarifying that flows are informational and
            not normative.
          o Changed domains to appropriate example.com and example.net
            ones.
          o A.1 Added message details
          o A.2 Removed as this is redundant since this example is what
            is now in section 4.2.

   Changes from the 02 to the 03 version:
      o Editorial changes: Updating to the new template to reflect new
        IPR guidelines, ensuring that the normative text is complete
        and accurate in section 4.1, removing "Editor's Notes", etc.
      o Section 4.5: Fixed error in cause (408 -> 480).
      o Examples: changed the domain to "example.com", IP addresses to
        the 192.0.2.0/24 range, changed  occurrences of "Reason:" to
        "Reason=", added use of Privacy header to examples.
      o Added text to reflect WG consensus on Issue-1: Privacy
        indication for History-Info entries.  Proposed an extension to
        the priv-values defined in RFC 3323 in abstract and section
        3.3, impacting the protocol structure in section 4.1 and
        processing in 4.3.3 (and 4.3.3.1 and 4.3.3.2).  In addition,


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        the new priv-value needs to be registered with IANA, per
        section 7.
      o Removed Open Issues section. For Issue-2, there was not WG
        consensus to define an algorithm for bounding the number of
        History-Info entries, but rather that is left as an
        implementation decision.
      o Updated Security discussions to reflect WG consensus that TLS
        is mandatory and sufficient for general History-Info
        implementation. The e2m and m2m security solutions can be
        applied to History-Info when they become available to provide a
        more robust SIP solution.
      o Section 4.1: Added additional text to ensure that all the
        information in the History-Info header is appropriately and
        normatively described (in text).
      o Added text in section 4.3.1 and an example to the appendices to
        address the UAC having added multiple History-Info headers for
        the case where the 3xx response goes back to the UAC and it's
        the UAC that retargets the INVITE request.
      o Clarified the addition of the Reason header in section
        4.3.3.1.2.
      o Further delineated the basic rules in section 4.3.3.1.3 for
        calculating the index for various scenarios, as this was still
        causing some confusion.

   Changes from the 01 to the 02 version:

      o Merged the SIPPING WG requirements draft into this document.
        Note that this increments the section references in the
        remainder of the document by 2 (and by 3 for Security and IANA
        considerations due to new section added). Also, removed
        redirect server from ISSUER-req since the solution identified
        this as not being required (or desirable).
      o Added an explicit privacy requirement (PRIV-req-3) for the
        proxy's role in recognizing and maintaining privacy associated
        with a Request-URI being captured in History-Info due to local
        policy. (Note, that the text was already there, it just wasn't
        highlighted as an explicit requirement).
      o Clarified the use of CRLF and spacing in the example headers in
        section 4.2.
      o Removed the compact form for the header since unknown headers
        with multiple entries would not be recognized (i.e. this may
        cause parsing problems).
      o Added a summary of Application Considerations to address
        concerns about the optional usage of History-Info.
      o Converted the references from numbers to labels to avoid the
        continual problem of renumbering.
      o Minor editorial changes (per NITS highlighted by Rohan and Eric
        and some minor rewording for clarity).



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   Changes from the 00 to the 01 version:

     o Attempted to be more explicit about the fundamental processing
        associated with the header.  Removed definitions of new terms,
        only referencing the terms from the requirements in the context
        of the fundamental SIP processing implied by the terms.
     o Attempted to clarify the Index and the related processing.
     o Added more detail addressing the privacy requirements.
     o Added a bit more detail on security. The security solution
        remains in a separate document and this document will need
        updating once that is completed.
     o Updated the examples (in section 2.5 and appendix) and clarified
        the definition and the maintenance of the Index in sections 2.1
        and 2.3.3.1.
     o Clarified the Reason description in section 2.1.  There had been
        an error in the description of the processing that was a remnant
        of the change to include only a single URI for each History-Info
        header.
     o Miscellaneous editorial changes (i.e. HistInfo -> Histinfo,
        etc.)

   Changes from individual draft-barnes-sipping-history-info-02 to the
   00 WG version:
      o Updated references and added reference to Security solution
        draft.
      o Removed appendix D which included background on analysis of
        solution options.
      o Cleaned up the document format per rfc2223bis.
      o Strengthened the inclusion of the INDEX as a MUST (per
        discussion at IETF-56).
      o Added text around the capturing of the Reason (SHOULD be
        captured for SIP responses and MAY be captured for other things
        such as timeouts).
      o Clarified the response processing 2.3.3.2 to include
        provisional responses and the sending of a 183 to convey
        History-Info.
     Added section 2.3.4 to address Redirect Server behavior.

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