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rfc:rfc4244

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

Barnes Standards Track [Page 1] RFC 4244 SIP Request History Information November 2005

         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.

Barnes Standards Track [Page 2] RFC 4244 SIP Request History Information November 2005

 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:

Barnes Standards Track [Page 3] RFC 4244 SIP Request History Information November 2005

 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.

Barnes Standards Track [Page 4] RFC 4244 SIP Request History Information November 2005

    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.

Barnes Standards Track [Page 5] RFC 4244 SIP Request History Information November 2005

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.

Barnes Standards Track [Page 6] RFC 4244 SIP Request History Information November 2005

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

Barnes Standards Track [Page 7] RFC 4244 SIP Request History Information November 2005

 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

Barnes Standards Track [Page 8] RFC 4244 SIP Request History Information November 2005

 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.

Barnes Standards Track [Page 9] RFC 4244 SIP Request History Information November 2005

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

Barnes Standards Track [Page 10] RFC 4244 SIP Request History Information November 2005

     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.

Barnes Standards Track [Page 12] RFC 4244 SIP Request History Information November 2005

 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

Barnes Standards Track [Page 14] RFC 4244 SIP Request History Information November 2005

 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

Barnes Standards Track [Page 15] RFC 4244 SIP Request History Information November 2005

 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

Barnes Standards Track [Page 16] RFC 4244 SIP Request History Information November 2005

      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.

Barnes Standards Track [Page 17] RFC 4244 SIP Request History Information November 2005

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:

Barnes Standards Track [Page 18] RFC 4244 SIP Request History Information November 2005

   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.

Barnes Standards Track [Page 19] RFC 4244 SIP Request History Information November 2005

 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;\

Barnes Standards Track [Page 20] RFC 4244 SIP Request History Information November 2005

                  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

Barnes Standards Track [Page 21] RFC 4244 SIP Request History Information November 2005

 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
 |            |         |        |        |        |        |

Barnes Standards Track [Page 22] RFC 4244 SIP Request History Information November 2005

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

Barnes Standards Track [Page 23] RFC 4244 SIP Request History Information November 2005

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.

Barnes Standards Track [Page 24] RFC 4244 SIP Request History Information November 2005

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.

Barnes Standards Track [Page 25] 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,

Barnes Standards Track [Page 26] 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

Barnes Standards Track [Page 27] 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.

Barnes Standards Track [Page 28] RFC 4244 SIP Request History Information November 2005

 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

Barnes Standards Track [Page 29] RFC 4244 SIP Request History Information November 2005

 /*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

Barnes Standards Track [Page 30] RFC 4244 SIP Request History Information November 2005

    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. */

Barnes Standards Track [Page 31] 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

Barnes Standards Track [Page 32] 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

Barnes Standards Track [Page 33] 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-------------------------------------------->|

Barnes Standards Track [Page 34] 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

Barnes Standards Track [Page 35] 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>

Barnes Standards Track [Page 36] 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

Barnes Standards Track [Page 37] 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

Barnes Standards Track [Page 38] RFC 4244 SIP Request History Information November 2005

 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.

Barnes Standards Track [Page 39] RFC 4244 SIP Request History Information November 2005

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

Barnes Standards Track [Page 40] RFC 4244 SIP Request History Information November 2005

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

Barnes Standards Track [Page 41] RFC 4244 SIP Request History Information November 2005

 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>

Barnes Standards Track [Page 42] RFC 4244 SIP Request History Information November 2005

 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

Barnes Standards Track [Page 43] RFC 4244 SIP Request History Information November 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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Acknowledgement

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Barnes Standards Track [Page 44]

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