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Versions: 00 01 02 03 04 05 06 RFC 4244

Internet Draft                                              M. Barnes
Document: draft-ietf-sip-history-info-02.txt                   Editor
Category: Standards Track                             Nortel Networks

Expires: August, 2004                                February,  2004

    An Extension to the Session Initiation Protocol for Request History
                                Information

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This draft defines a standard mechanism for capturing the history
   information associated with a SIP request.  This capability enables
   many enhanced services by providing the information as to how and why
   a call arrives at a specific application or user.  This draft defines
   a new optional SIP header, History-Info, for capturing the history
   information in requests. A new option tag, Histinfo, to be included
   in the Supported header, is defined to allow UAs to indicate whether
   the History-Info should be returned in responses to a request which
   has captured the history information.

Table of Contents

   1.Background:  Why define a Generic "Request History" capability?.3
   2. "Request History" Requirements.................................4
   2.1 Security Requirements.........................................6


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      2.2 Privacy Requirements.......................................6
   3. Request History Information Description........................7
      3.1 Optionality of History-Info................................8
      3.2 Securing History-Info......................................8
      3.3 Ensuring the Privacy of History-Info.......................9
   4 Request History Information Protocol Details....................9
      4.1 Protocol Structure of History-Info.........................9
      4.2 Protocol Examples.........................................11
      4.3 Protocol usage............................................11
      4.4 Security for History-Info.................................15
      4.5 Example Applications using History-Info...................16
   5. Application Considerations....................................17
   6. Security Considerations.......................................18
   7. IANA Considerations...........................................18
   Normative References.............................................21
   Informational References.........................................22
   Appendix A  Forking Scenarios....................................23
      A.1 Sequentially forking (History-Info in Response)...........23
      A.2 Sequential Forking (with Success).........................24
   Appendix B  Voicemail............................................25
   Appendix C  Automatic Call Distribution Example..................30
   Full Copyright Statement.........................................32

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 URLs on a web page,
   "call history/logging" style services within intelligent "call
   management" software for SIP UAs and calls to voicemail servers and
   call centers.  While SIP implicitly provides the redirect/retarget
   capabilities that enable calls to be routed to chosen applications,
   there is currently no standard mechanism within SIP for communicating
   the history of such a request. This "request history" information
   allows the receiving application to determine hints about how and why
   the call arrived at the application/user. This draft 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.

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

   Section 4 provides the details of the additions to the SIP protocol.
   An example use of the new header is included in Section 4.5, with


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   additional scenarios included in the Appendix. It is anticipated that
   these would be moved and progressed in a general Service examples
   draft such as [SIPSVCEX] or individual informational drafts
   describing these specific services, since the History-Info header is
   just one of the building blocks for implementing these services.
   Individual drafts would be particularly useful for documenting
   services for which there are multiple solutions, as it is not the
   intent, nor is it within the scope, of this draft to prescribe a
   complete solution for any of these applications.

   Section 5 summarizes the application considerations identified in the
   previous sections. Section 6 summarizes the security solution as
   described in section 4.4.

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].

   In order to provide a cross reference of the solution description to
   the requirements without reiterating the entirety of the requirements
   inline, the requirements are referenced as [REQNAME-req] following
   the text or paragraph which explicitly satisfies the requirement.


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

   SIP implicitly provides redirect/retarget capabilities that enable
   calls to be routed to specific applications as defined in [RFC3261].
   The term retarget will be used henceforth in this draft to refer to
   the process of a Proxy Server/UAC changing a 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 draft.

   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 draft 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.


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

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

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

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


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

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

  4. Some diagnostic information for debugging SIP requests.

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


2. "Request History" Requirements

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

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

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





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

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



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

2.1 Security Requirements

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

   The potential security problems include the following:
   1) A rogue application could insert a bogus Request History entry
   either by adding an additional entry as a result of retargeting or
   entering invalid information.

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

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

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

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

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

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

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

2.2 Privacy Requirements





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   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 [4])
   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, which includes INVITE, REGISTER, MESSAGE,
   REFER and OPTIONS [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 Proxy, or when the target of a request is
   changed. The term 'retarget' is introduced to refer to this changing
   of the target of a request and the subsequent forwarding of that
   request. It should be noted that retargeting only occurs when the
   Request-URI indicates a domain for which the processing entity is
   responsible.  In terms of the SIP protocol, the processing associated
   with retargeting is described in sections 16.5, and 16.6 of
   [RFC3261].  As described in section 16.5 of [RFC3261], it is possible


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   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 which make use of the Request History Information to be
   limited to retargeting within domain(s) controlled by the same local
   policy, or between domain(s) which negotiate policies with other
   domains to ensure support of the given policy, or services for which
   "complete" History Information isn't required to provide the service.
   [OPTIONALITY-req]  All applications making use of the History-info
   header MUST clearly define the impact of the information not being
   available and specify the processing of such a request.

3.2 Securing History-Info

   This draft defines a new header for SIP. The draft does RECOMMEND the
   use of a secure transport mechanism such as TLS to ensure the overall
   confidentiality of the History-Info headers[SEC-req-4]. However, the
   problem is slightly different than the hop by hop security problem
   solved by TLS, as each hop is not required to add the History-Info
   header.  Since the History-Info header is being inserted by an entity
   as it targets and forwards a Request, the resulting security
   requirements also introduce a slightly different problem than the
   basic SIP header or Identity [SIPATHID] problems, which are focused
   on securing the information in the initial request end to end.
   However, the requirements for the security solution are similar to
   the Via and Record-Route headers.   For the History-Info header, the
   general requirement is to secure a header that is inserted by an
   intermediary and then subsequently referenced, by other
   intermediaries to build the next header entry, or by an end
   application using the information to provide a service. Thus, the
   general requirement takes the form of a middle to middle and middle


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   to end security solution, which is addressed in a separate document
   [SIPIISEC]. The use of the middle-to-end security solution discussed
   in [SIPIISEC] allows the integrity of the History-Info to be
   ascertained as it traverses the intermediaries.  Thus, including the
   History-Info header in SIP Requests and securing in this manner adds
   an additional level of security end to end, assuring the initiator of
   a Request that it has indeed reached the intended recipient.  Further
   discussion of the security mechanism for History-Info is provided in
   section 2.4.


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]
   [Issue-1: It has been proposed on the mailing list that there is a
   protocol requirement to support this functionality. It has been
   suggested that adding an additional field to the History-Info header
   (or extending the priv-values defined in RFC 3323) would facilitate
   the implementation of this functionality.]

   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.

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 and provides some examples.

4.1 Protocol Structure of History-Info




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


   The History-Info header carries the following information:

     o Targeted-to-URI: the Request URI captured as the Request is
        forwarded.

     o 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 including the
        index and securing the header, the ordering of the History-info
        headers in the request is assured.[SEC-req-2]

     o Reason: An optional parameter for History-info. The reason for
        the retargeting is captured by including the Reason Header
        [RFC3326] associated with the Request URI being retargeted.
        Thus, a reason is not included for a Request URI when it is
        first added in a History-info header, but rather is added when
        that particular Request-URI is retargeted.  Note, that this does
        appear to complicate the security problem, however, retargeting
        only occurs when the Request-URI indicates a domain for which
        the processing entity is responsible, thus it would be the same
        processing entity that initially added the Request-URI to the
        header that would be updating it with the Reason.


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

          History-Info = "History-Info" HCOLON

                            hist-info *(COMMA hist-info)

          hist-info = 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)


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          hi-extension = generic-param


4.2 Protocol Examples

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


      History-Info:<sip:UserA@ims.nortelnetworks.com?Reason=SIP;\
        cause=302;text="Moved Temporarily">; index=1; foo=bar

      History-Info: <sip:UserA@ims.nortelnetworks.com?Reason=SIP;\
         cause=302; text="Moved Temporarily">; index=1.1,
         <sip:UserB@nortelnetworks.com? Reason=SIP;cause=486;\
         text="Busy Here">;index=1.2,
         <sip:45432@vm.nortelnetworks.com> ; index=1.3

   [Editor's note: need to insert row for Table 2].

4.3 Protocol usage

   This section describes the processing specific to UAs and Proxies for
   the History-Info header and the Histinfo option tag. As discussed in
   section 1, the fundamental objective is to capture the target
   Request-URIs as a request is forwarded.  This allows for the
   capturing of the history of a request that would be lost due to
   subsequent (re)targeting and forwarding.  To accomplish this for the
   entire history of a request, either the UAC must capture the Request-
   URI in the initial request or a proxy must add History-Info headers
   for both the Request-URI in the initial request and the target
   Request-URI as the request is forwarded.  The basic processing is for
   each entity forwarding a request to add a History-Info header for the
   target Request-URI, updating the index and adding the Reason as
   appropriate for any retargeted Request-URI.

   [Editor's note: Once the Security solution is fully fleshed out, it
   may be reasonable to move this section 4.3 after section 4.4 and
   provide the detailed security related processing prior to this
   section, so that security aspects can be detailed in this section, as
   well.]

   4.3.1 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 in the Response.  In


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

   The processing of the History-Info header received in the Response is
   application specific and outside the scope of this draft. However,
   the validity of the information SHOULD be ensured prior to any
   application usage. [Editor's note: Further detail to be provided once
   the security solution is available.]


   4.3.2 UAS Behavior

   The processing of the History-Info header by a UAS in a Request
   depends upon local policy and specific applications at the UAS which
   might make use of the information.  Prior to any application usage of
   the information, the validity SHOULD be ascertained.  [Editor's note:
   Further detail to be provided once the security solution is
   available.]

   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
   the History-Info header as it forwards a Request depends upon local
   policy, with the following being considerations in the definition of
   that policy:
       o Whether the Request contains the Histinfo option tag in the
          Supported header.
       o Whether the proxy supports the History-Info header.
       o 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.) prohibit the exposure of that
          information outside that domain.  An example of such an
          application is provided in Appendix C.
       o Whether the History-Info header is added for a specific
          Request URI due to local privacy policy considerations.
       o Within a given domain, whether there is a limit on the number
          of History-Info entries and the mechanism for applying the
          limit. [Issue-2: It has been highlighted that messages


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          carrying History-Info entries can become quite large in cases
          where there is a lot of retargeting. It seems that a
          reasonable recommendation could be provided for pruning the
          entries (albeit only entries added by that intermediary MAY
          be removed)].

   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.

   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 History-Info headers, 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 is described in detail in the
   following sections.

   4.3.3.1 Adding the History-Info header to Requests

   If the proxy supports the History-Info header, the proxy SHOULD add a
   History-Info header as it forwards a Request. Section 16.6 of [4]
   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 History-Info
   header SHOULD be added following any History-Info header received in
   the request being forwarded.  Additionally, if a request is received
   that doesn't include a History-Info header, the proxy MAY add an
   additional History-Info header preceding the one being added for the
   current request being forwarded.  The index for this entry is
   RECOMMENDED to start at 1.

   For retargets that are the result of an explicit SIP response, the
   SIP Response Code that triggered the retargeting MUST be included in


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   the Reason header field of the Request URI that has been retargeted.
   For retargets as a result of timeouts or internal events, a Reason
   MAY be included in the Reason header field of the Request URI that
   has been retargeted.

   In order to maintain ordering and accurately reflect the nesting and
   retargeting of the request, an index MUST be included along with the
   Targeted-to-URI being captured. Per the ABNF in section 4.1, the
   index consists of a dot delimited series of digits (e.g. 1.1.2), with
   each dot reflecting the number of hops or level of nesting of the
   request.  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.  For retargets within a proxy, the
   proxy MUST maintain the current level of nesting by incrementing the
   lowest/last digit of the index for each instance of retargeting, thus
   reflecting the number of retargets within the proxy.

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

     1. If the Request-URI in the original request indicates a resource
     for which this proxy is responsible, then the proxy reads the value
     from the History-Info header in the received request, if available,
     and adds another level of indexing by appending the DOT delimiter
     followed by an initial index for the new level of 1. For example,
     if the index in the last History-Info header field in the received
     request is 1.1, this proxy would initialize its index to 1.1.1.
     For each subsequent target that is forwarded by the same proxy,
     theindex is calculated by incrementing the last/lowest digit at the
     current level.

     2. If the Request-URI indicates a resource that this proxy is not
     responsible for, then the lowest/last digit of the index is
     incremented (i.e. a new level is not created).  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 added by this proxy
     would be 1.3.

   If the request forwarding is done in parallel, the proxy MUST capture
   each of the Request-URIs to which the Request is forwarded in the
   manner previously described per rule 1 above. The index MUST be
   captured for each forked request per the rules above, with each new
   Request having a unique index. The proxy builds the subsequent
   requests and responses using the amalgamated information associated
   with each of those requests and including the header entries in the
   order indicated by the indexing.  Section 4.5 provides an example of
   a parallel request scenario, highlighting this indexing mechanism.

   4.3.3.2 Processing History-Info in Responses



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   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.

   It should be noted that local policy considerations, for network and
   intermediary privacy, MAY restrict the sending of the History-Info
   headers added by the intermediary in subsequent responses.  Thus, in
   such cases, the proxy MAY remove from these responses the History-
   Info headers which it inserted in the original forwarded request.

   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 1, the security requirements are partially
   met by recommending the use of TLS (a basic SIP requirement per
   [RFC3261]) for hop by hop security.   In addition, the use of the
   middle-to-end security solution discussed in [SIPIISEC] allows the
   integrity of the History-Info to be ascertained as it traverses the
   intermediaries.
   For the History-Info header, the general requirement is to secure a
   header that is inserted by an intermediary and then subsequently
   referenced, by other intermediaries to build the next header entry or
   by an end application using the information to provide a service. In
   terms of exactly what is being secured, it is primarily the captured
   Request-URIs that are the security concern, since they can reflect
   some aspect of a user's identity and service routing. However, the
   indices are also important in that they can be used to determine if
   specific Request-URIs have been removed from the header. Thus, the
   primary objective of the security solution is to ensure that the
   entire History-Info header is protected from being accessed or
   manipulated by non-authorized entities, with the fundamental
   assumption that retargeting entities are implicitly authorized.

   The security associated with the Request History Information is
   optional and depends upon local policy and the impact on specific
   applications of having the information compromised.  Since, the
   Request History Information itself is also optional and it has been
   recommended that applications document the impact of the information
   not being available, it is also suggested that the impact of not
   supporting the security recommendations also be documented by the
   application to ensure that the impacts have been sufficiently
   addressed by the application.


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   4.4.1 Security examples

   [Editor's Note: Need to add some protocol details for protecting
   History-Info once [SIPIISEC] is further along].

4.5 Example Applications using History-Info

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

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



   UA1        Proxy1  Proxy2     UA2      UA3      UA4      UA5

   |            |         |        |        |        |        |
   |--INVITE -->|         |        |        |        |        |
   |            |-INVITE->|        |        |        |        |
                 Supported: Histinfo
                 History-Info: <sip:Bob@P1>;index=1,
                               <sip:Bob@P2>; index=2
   |            |         |        |        |        |        |
   |            |         |-INVITE>|        |        |        |
                  History-Info: <sip:Bob@P1>;index=1,
                                <sip:Bob@P2>; index=2,
                                <sip:User2@UA2>; index=2.1
   |            |         |        |        |        |        |
   |            |         |-----INVITE ---->|        |        |
                  History-Info: <sip:Bob@P1>;index=1,
                                <sip:Bob@P2 >; index=2,
                                <sip:User3@UA3>; index=2.2
   |            |         |        |        |        |        |
   |            |         |-------INVITE------------>|        |
                  History-Info: <sip:Bob@P1>;index=1,
                                <sip:Bob@P2 >; index=2,


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                                <sip:User4@UA4 >; index=2.3

   /* All Responses from the INVITEs indicate non-success/non-
   availability*/
  |            |         |        |        |        |        |
  |            |<-480 ---|        |        |        |        |
               History-Info: <sip:Bob@P1>;index=1,
                  <sip:Bob@P2>; index=2,
                  <sip:User2@UA2?Reason:SIP;\
                   cause=408;text="RequestTimeout">;index=2.1,
                  <sip:User3@UA3?Reason:SIP; \
                   cause=487;text="Request Terminated">; index=2.2,
                  <sip:User4@UA4?Reason:SIP;\
                   cause=603;text="Decline">; index=2.3


  |            |         |        |        |        |        |
 /* Upon receipt of the response, P1 determines another route for the
  INVITE, but finds that it matches some routes already attempted
 (e.g. UA2 and UA3, thus the INVITE is only forwarded to UA5, where
  the session is successfully established  */
  |            |         |        |        |        |        |
   |            |----------------INVITE --------------------->|
                History-Info: <sip:Bob@P1>;index=1,
                   <sip:Bob@P2>; index=2,
                   <sip:User2@UA2?Reason:SIP;cause=408;\
                    text="RequestTimeout">;index=2.1,
                   <sip:User3@UA3?Reason:SIP;cause=487;\
                    text="Request Terminated">; index=2.2,
                   <sip:User4@UA4?Reason:SIP;cause=603;\
                    text="Decline">; index=2.3
                  <sip:User5@UA5>;index=1.1
  |            |         |        |        |        |        |
   |            |<-----200 OK---------------------------------|
  |<--200 OK---|         |        |        |        |        |
  |            |         |        |        |        |        |
  |--ACK --------------------------------------------------->|


   Additional detailed scenarios are available in the appendix.

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:



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   1) History-Info is optional, thus a service should 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 or Session 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 Request History Information is
      optional. Whether there is security applied to the entries
      depends upon local policy. The impact of lack of having the
      information compromised 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.  For the display example, a visual indicator
      could be applied highlighting that the information has not been,
      or could not be, validated.



6. Security Considerations

   This draft provides a proposal in sections 3.2 and 4.4 for addressing
   the Security requirements identified in section 2.1 by proposing the
   use of TLS between entities, and by securing the History-Info headers
   added by proxies as described in [SIPIISEC].

7. IANA Considerations



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

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

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

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

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

  The following should be added to the Options Tags section:

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

Open Issues

   The following summarizes the current open issues in this document:

      o Issue-1: Privacy indication for specific History-Info entries.
        It has been proposed on the mailing list that there is a
        requirement beyond the basic Header or Session privacy provided
        by RFC 3323 for History-Info entries in terms of supporting
        local policy based privacy requirements. It has been suggested
        that adding an additional field to the History-Info header (or
        extending the priv-values defined in RFC 3323) would facilitate
        the implementation of this functionality. Adding such
        information to the HI entries would impact the protocol
        structure in section 4.1 and processing in 4.3.3 (and 4.3.3.1
        and 4.3.3.2)

      o Issue-2: Bounding the History-Info entries and a mechanism for
        applying the limit. It has been highlighted by developers that
        messages carrying History-Info entries can become quite large
        in cases where there is a lot of retargeting. It seems that a
        reasonable recommendation could be provided for pruning the
        entries (albeit only entries added by that intermediary should
        be removed).  For example:


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               . Keeping only the first and last entries
               . Keeping only the last leaf of each of the branches.
               . Restricting the breadth and depth of the History-Info
                  tree.
          Such bounding would require normative processing guidelines
          in section 4.3.3 and introduce an additional application
          consideration in section 5.

Changes since last version

   Changes from the û01 to the û02 version:

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



   Changes from the û00 to the û01 version:

     o Attempted to be more explicit about the fundamental processing
        associated with the header.  Removed definitions of new terms,
        only referencing the terms from the requirements in the context
        of the fundamental SIP processing implied by the terms.
     o Attempted to clarify the Index and the related processing.
     o Added more detail addressing the privacy requirements.
     o Added a bit more detail on security. The security solution
        remains in a separate document and this document will need
        updating once that is completed.


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     o Updated the examples (in section 2.5 and appendix) and clarified
        the definition and the maintenance of the Index in sections 2.1
        and 2.3.3.1.
     o Clarified the Reason description in section 2.1.  There had been
        an error in the description of the processing that was a remnant
        of the change to include only a single URI for each History-Info
        header.
     o Miscellaneous editorial changes (i.e. HistInfo -> Histinfo,
        etc.)

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

Normative References

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

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

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

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

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

   [SIPIISEC] M. Barnes, "A Mechanism to Secure SIP Headers Inserted by
   Intermediaries", draft-barnes-sipping-inserted-info-01.txt, October,
   2003.




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Informational References

   [SIPSVCEX] A. Johnson, "SIP Service Examples", draft-ietf-sipping-
   service-examples-05.txt, November, 2002.

   [SIPATHID] J. Peterson, "Enhancements for Authenticated Identity
   Management in the Session Initiation Protocol (SIP)", draft-ietf-sip-
   identity-01.txt, February, 2003.



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 and Eric Burger.

   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 enhanced
   SIP security aspects enabled by this draft.

Contributors' Addresses

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

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

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

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

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

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



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

   Tel: +44 (0)1628-434456
   Email:  mwatson@nortelnetworks.com


Author's Address

   Mary Barnes
   Nortel Networks
   2380 Performance Drive
   Richardson, TX USA

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

Appendix A  Forking Scenarios

A.1 Sequentially forking (History-Info in Response)

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

   UA 1 sends a call to "Bob" via proxy 1. Proxy 1 sequentially tries
   several places (UA2, UA3 and UA4) unsuccessfully before sending a
   response to UA1.

   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 draft to
   prescribe detailed applications.




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   UA1        Proxy1              UA2      UA3      UA4
   |            |                  |        |        |
   |--INVITE -->|                  |        |        |
   |            |                  |        |        |
   |            |--INVITE -------->|        |        |
   |<--100 -----|                  |        |        |
   |            |<-302 ------------|        |        |
   |            |                  |        |        |
   |            |-------INVITE ------------>|        |
   |            |                  |        |        |
   |            |<-------180 ---------------|        |
   |<---180 ----|                  |        |        |
   |  . .       |-------INVITE------------->|        |
   |            |       timeout    |        |        |
   |            |                  |        |        |
   |            |------INVITE ---------------------->|
  |<--100 -----|                  |        |        |
   |            |                  |        |        |
   |            |<-486 ------------------------------|
   |            |                  |        |        |
   |            |-- ACK ---------------------------->|
  |<--486------|                  |        |        |
  |            |                  |        |        |
  |--ACK ----->|                  |        |        |
  |            |                  |        |        |


   [Editor's Note: Need to detail the message flow.]



A.2 Sequential Forking (with Success)

   This scenario highlights an example where the History-Info in the
   request 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 like be a local proxy or even user
   specific policy.

   UA 1 sends a call to "Bob" to proxy 1. Proxy 1 sequentially tries
   several places (UA2, UA3 and UA4) before retargeting the call to
   Proxy 2.  Proxy 2, without the History-Info, would try several of the
   same places (UA3 and UA4)based upon registered contacts for "Bob",
   before completing at UA5. However, with the History-Info, Proxy 2
   determines that UA3 and UA4 have already received the invite, thus
   the INVITE goes directly to UA5.




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   UA1        Proxy1  Proxy2     UA2      UA3      UA4      UA5

   |            |         |        |        |        |        |
   |--INVITE -->|         |        |        |        |        |
   |            |         |        |        |        |        |
   |            |--INVITE -------->|        |        |        |
   |<--100 -----|         |        |        |        |        |
   |            |<-302 ------------|        |        |        |
   |            |         |        |        |        |        |
   |            |-------INVITE ------------>|        |        |
   |            |         |        |        |        |        |
   |            |<-------180 ---------------|        |        |
   |<---180 ----|         |        |        |        |        |
   |  . .       |-------INVITE------------->|        |        |
   |            |       timeout    |        |        |        |
   |            |         |        |        |        |        |
   |            |------INVITE ---------------------->|        |
  |<--100 -----|         |        |        |        |        |
   |            |<-302 ------------------------------|        |
   |            |         |        |        |        |        |
   |            |-INVITE->|        |        |        |        |
   |            |         |        |        |        |        |
   |            |         |        |        |        |        |
   |            |         |------INVITE --------------------->|
  |            |         |        |        |        |        |
   |            |         |<-----200 OK---------------------->|
  |<--200 OK-------------|        |        |        |        |
  |            |         |        |        |        |        |
  |--ACK --------------------------------------------------->|

 [Editor's Note: Need to add the details of the messages here.]


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

   UA 1 called UA A which had been forwarded to UA B which forwarded to
   a UA VM (voicemail server).  Based upon the retargeted URIs and
   Reasons (and other information) in the INVITE, the VM server makes a
   policy decision about what mailbox to use, which greeting to play
   etc.



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   UA1          Proxy           UA-A         UA-B        UA-VM

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

   Message Details

 INVITE F1   UA1->Proxy

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

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   /*Client for UA1 prepares to receive data on port 49170


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   from the network. */

   INVITE F2 Proxy->UA-A

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

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   100 Trying F3 Proxy->UA1

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


   302 Moved Temporarily F4  UserA->Proxy
   SIP/2.0 302 Moved Temporarily
   Via: SIP/2.0/UDP ims.nortelnetworks.com:5060;branch=1
   Via: SIP/2.0/UDP here.com:5060
   From: BigGuy <sip:User1@here.com>
   To: LittleGuy <sip:UserA@nortelnetworks.com>;tag=3
   Call-Id: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@nortelnetworks.com>
   Content-Length: 0





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   INVITE F5 Proxy-> UA-B

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

   v=0
   o=User1 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   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 there.com:5060
   From: BigGuy <sip:User1@here.com>
   To: LittleGuy <sip:UserA@nortelnetworks.com>;tag=5
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0

   180 Ringing F7  Proxy-> UA1

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


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   INVITE F8  Proxy-> UA-VM

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

   v=0
   o=User1 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   200 OK F9

   SIP/2.0 200 OK UA-VM->Proxy

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

   v=0
   o=UserA 2890844527 2890844527 IN IP4 vm.nortelnetworks.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=0 0
   m=audio 3456 RTP/AVP 0


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   a=rtpmap:0 PCMU/8000


   200 OK F10  Proxy->UA1

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

   v=0
   o=UserA 2890844527 2890844527 IN IP4 vm.nortelnetworks.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   ACK F11 UA1-> UA-VM

   ACK sip:VM@nortelnetworks.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060
   From: BigGuy <sip:User1@here.com>
   To: LittleGuy<sip:UserA@nortelnetworks.com>;tag=3
   Call-Id: 12345600@here.com
   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


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   Gold call, from how many groups it might have overflowed before
   reaching the agent, etc. and thus can be handled appropriately by the
   agent.

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

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



   UA1          Proxy        ACDGRP1 Svr   ACDGRP2 Svr UA2-ACDGRP2

   |              |              |             |          |
   |--INVITE F1-->|              |             |          |
    Supported:Histinfo
   |              |              |             |          |
   |              |--INVITE F2-->|             |          |
                    Supported:Histinfo
                    History-Info: <sip:Gold@ACD.com>; index=1
                    History-Info: <sip:ACDGRP1@ACD.com>; index=1.1
  |              |              |             |          |
   |              |<-302 F3------|             |          |
                    Contact: <sip:ACDGRP2@ACD.com>
  |              |              |             |          |
   |              |--------INVITE F4---------->|          |
                    History-Info: <sip:Gold@ACD.com>; index=1
                    History-Info: <sip:ACDGRP1@ACD.com>; index=1.1
                    History-Info: <sip:ACDGRP2@ACD.com>; index=1.2
   |              |              |             |          |
  |              |              |             |          |
   |              |              |             |INVITE F5>|
                    History-Info: <sip:Gold@ACD.com>; index=1
                    History-Info: <sip:ACDGRP1@ACD.com>; index=1.1
                    History-Info: <sip:ACDGRP2@ACD.com>; index=1.2
   |              |              |             |          |
   |              |              |             |<-200 F6--|
   |              |              |             |          |
   |              |<-200 F7--------------------|          |
                    History-Info: <sip:Gold@ACD.com>; index=1
                    History-Info: <sip:ACDGRP1@ACD.com>; index=1.1


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                    History-Info: <sip:ACDGRP2@ACD.com>; index=1.2
   |<-200 F8------|              |             |          |
 No History-Info included in the response due to Local Policy>
   |              |              |             |          |
   |--ACK F9--------------------------------------------->|

   Message Details

   [To be completed]


Full Copyright Statement

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.  The limited permissions granted above are perpetual and
   will not be revoked by the Internet Society or its successors or
   assigns.  This document and the information contained herein is
   provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE
   INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
















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