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Obsoleted by: 7044 PROPOSED STANDARD
Errata Exist
Network Working Group                                     M. Barnes, Ed.
Request for Comments: 4244                                        Nortel
Category: Standards Track                                  November 2005


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

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document defines a standard mechanism for capturing the history
   information associated with a Session Initiation Protocol (SIP)
   request.  This capability enables many enhanced services by providing
   the information as to how and why a call arrives at a specific
   application or user.  This document defines a new optional SIP
   header, History-Info, for capturing the history information in
   requests.

Table of Contents

   1. Introduction ....................................................2
      1.1. Overview ...................................................2
      1.2. Conventions Used in This Document ..........................3
      1.3. Background:  Why define a Generic "Request History"
           capability? ................................................3
   2. "Request History" Requirements ..................................4
      2.1. Security Requirements ......................................6
      2.2. Privacy Requirements .......................................7
   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 .......................9
   4. Request History Information Protocol Details ....................9
      4.1. Protocol Structure of History-Info ........................10
      4.2. Protocol Examples .........................................11
      4.3. Protocol Usage ............................................12



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           4.3.1. User Agent Client (UAC) Behavior ...................12
           4.3.2. User Agent Server (UAS) Behavior ...................13
           4.3.3. Proxy Behavior .....................................13
           4.3.4. Redirect Server Behavior ...........................18
      4.4. Security for History-Info .................................18
      4.5. Example Applications Using History-Info ...................19
           4.5.1. Example with Privacy Header for Entire
                  Request at Proxy2 ..................................21
           4.5.2. Example with Privacy Header for Specific
                  URI (UA4) at Proxy2 ................................22
   5. Application Considerations .....................................24
   6. Security Considerations ........................................25
   7. IANA Considerations ............................................25
      7.1. Registration of New SIP History-Info Header ...............25
      7.2. Registration of "history" for SIP Privacy Header ..........26
   8. Normative References ...........................................26
   9. Informative References .........................................26
   10. Acknowledgements ..............................................26
   11. Contributors' Addresses .......................................27
   Appendix. Example Scenarios........................................28
      Appendix A. Sequentially forking (History-Info in Response).....28
      Appendix B. Voicemail...........................................34
      Appendix C. Automatic Call Distribution Example.................39
      Appendix D. Session via Redirect and Proxy Servers..............41

1.  Introduction

1.1.  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.  Although 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 1.3 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.

1.2.  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].

1.3.  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 that will receive some
      "referral" commission for generating this traffic

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

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

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

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

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

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

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.  Although this
      is inherently provided when the retarget is in response to a SIP
      redirect, it is deemed useful for non-redirect retargeting
      scenarios, as well.

   2) 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.





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      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 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.






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2.1.  Security Requirements

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

   The potential security problems include the following:

   1) A rogue application could insert a bogus Request History entry
      either by adding an additional entry as a result of retargeting or
      entering invalid information.

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

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

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

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

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

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

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

   It should be noted that these security requirements apply to any
   entity making use of the Request History information, either by
   retargeting and capturing the information, or as an application
   making use of the information received in either a Request or
   Response.






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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.

   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



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   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)
   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 that 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 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) is strongly RECOMMENDED.  This results in History-Info having
   at least the same level of security as other headers in SIP that are
   inserted by intermediaries.  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



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   for gaps (SEC-req-1, SEC-req-2).  It would be up to the application
   to define whether it can make use of the information in the case of
   missing entries.

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

3.3.  Ensuring the Privacy of History-Info

   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 [RFC3323] 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.







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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
   that 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

                                      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



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       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.

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

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

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

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

          hi-targeted-to-uri= name-addr

          hi-param = hi-index / hi-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,



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         <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 1.3, 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 the Request-URI in a History-Info header in the
   initial request or a proxy must add a History-Info header with both a
   hi-entry for the Request-URI in the initial request and a hi-entry
   for the target Request-URI as the request is forwarded.  The basic
   processing is for each entity forwarding a request to add a 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 MAY improve the diagnostic utility of its request
   by adding a History-Info header, using the Request-URI of the request
   as the hi-target-to-uri and initializing the index to the RECOMMENDED
   value of 1 in the hi-entry.  As a result, intermediaries and the UAS
   will know at least the original Request-URI, and if the Request-URI
   was modified by a previous hop.

   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.



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   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 that
   might make use of the information.  Prior to any application usage of
   the information, the validity SHOULD be ascertained.  For example,
   the entries MAY be evaluated to determine gaps in indices, which
   could indicate that an entry has been maliciously removed or removed
   for privacy reasons.  Either way, an application MAY want to be aware
   of potentially missing information.

   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.




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      5. Whether a 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.,
   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 a 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



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   add a History-Info header with a 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", a 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
   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 a 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




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   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.

   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 syntax 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



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        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, the
        index in the next request forwarded by this same proxy,
        following the example above, would be 1.1.2.

     4. Retargeting based upon a Response:  In the case of retargeting
        due to a specific response (e.g., 302), the index would be
        calculated per rule 3.  That is, the lowest/last digit of the
        index is incremented (i.e., a new branch is created), with the
        increment 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.









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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
       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:




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     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.

   UA1 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 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.

   Section 4.5.1 provides this same scenario using one of the privacy
   mechanisms, with Proxy2 (P2) 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.

   The formatting in these scenarios is for visual purposes; thus,
   backslash and CRLF are used between the fields for readability and
   the headers in the URI are not shown properly formatted for escaping.
   Refer to Section 4.2 examples for the proper formatting.  Additional
   detailed scenarios are available in the appendix.







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   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;\
                    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;\



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                    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,
                               <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



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   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

   |            |         |        |        |        |        |
   |--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
   |            |         |        |        |        |        |



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















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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 Sections 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) that 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

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

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

   Header Name             Compact Form               Reference
   -----------             ------------               ---------
   History-Info               none                    [RFC4244]

   The following has been added to the Options Tags section:

   Name          Description                          Reference
   ----          -----------                          ---------
   histinfo      When used with the Supported header, [RFC4244]
                 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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RFC 4244            SIP Request History Information        November 2005


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 have been made to
   http://www.iana.org/assignments/sip-priv-values

   The following has been added to the registration for the SIP Privacy
   header:

   Name      Description               Registrant   Reference
   ----      -----------               ----------   ---------
   history   Privacy requested for     Mary Barnes  [RFC4244]
             History-Info header(s)    mary.barnes@nortel.com

8.  Normative References

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

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

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

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

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

9.  Informative References

   [RFC3665]  Johnston, A., Donovan, S., Sparks, R., Cunningham, C., and
              K. Summers, "Session Initiation Protocol (SIP) Basic Call
              Flow Examples", BCP 75, RFC 3665, December 2003.

10.  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,



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RFC 4244            SIP Request History Information        November 2005


   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.

11.  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

   Phone: +1 408 421 9990
   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
   Digital Fountain
   39141 Civic Center Drive Suite 300
   Fremont, CA 94538
   U.S.A.

   EMail: mark@digitalfountain.com








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RFC 4244            SIP Request History Information        November 2005


Appendix.  Example Scenarios

   The scenarios in Appendices 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 UA1 sends a call to Bob via Proxy1.  Proxy1 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.

   In this scenario, since UA1 has not included the original Request-URI
   in the INVITE, the proxy adds a hi-entry to capture the original
   Request-URI to provide the complete set of information, as discussed
   in Section 4.3.3.1.















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



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

      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@example.com>; index=1,
       <sip:UserA@ims.example.com>; index=1.1
      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



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      F5 INVITE Proxy1 -> UA3

      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@example.com>; index=1,
       <sip:UserA@ims.example.com?Reason=SIP;\
       cause=302; text="Moved Temporarily">; index=1.1,
       <sip:UserB@example.com>;index=1.2
      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. */




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RFC 4244            SIP Request History Information        November 2005


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

      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@example.com>; index=1,
       <sip:UserA@ims.example.com?Reason=SIP;\
       cause=302; text="Moved Temporarily">;index=1.1,
       <sip:UserB@example.com?Reason=SIP;cause=480;\
       text="Temporarily Unavailable" >;index=1.2,
       <sip:UserC@example.com>;index=1.3
      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



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RFC 4244            SIP Request History Information        November 2005


      CSeq: 1 INVITE
      Content-Length: 0

      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@example.com>; index=1,
       <sip:UserA@ims.example.com?Reason=SIP;\
       cause=302; text="Moved Temporarily">;index=1.1,
       <sip:UserB@example.com?Reason=SIP;cause=480;\
       text="Temporarily Unavailable" >;index=1.2,
       <sip:UserC@example.com>;index=1.3
      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









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RFC 4244            SIP Request History Information        November 2005


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

   UA1 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-------------------------------------------->|










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RFC 4244            SIP Request History Information        November 2005


   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>
   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



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RFC 4244            SIP Request History Information        November 2005


   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
   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>



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RFC 4244            SIP Request History Information        November 2005


   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
   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



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RFC 4244            SIP Request History Information        November 2005


   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
   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



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   CSeq: 1 ACK
   Content-Length: 0

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

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.


















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   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
                    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--------------------------------------------->|
















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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             |--------------->|

   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



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   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>
   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>



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   Content-Length: 0

   Detailed Call Flow continues per section 6.3 in [RFC3665].

Editor's Address

   Mary Barnes
   Nortel
   2201 Lakeside Blvd
   Richardson, TX USA

   Phone:  1-972-684-5432
   EMail:  mary.barnes@nortel.com






































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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
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Acknowledgement

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