Internet Engineering Task Force                                SIMPLE WG
Internet Draft                                               J.Rosenberg
June 24, 2002
Expires: December 2002

                 A SIP Event Package for List Presence


   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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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at

   To view the list Internet-Draft Shadow Directories, see


   This document presents a SIP event package for subscribing to a list
   of presentities. Instead of the subscriber sending a SUBSCRIBE to
   each presentity individually, the subscriber can subscribe to their
   presence list as a whole, and then receive notifications when the
   state of any of the presentities on the list changes.

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                           Table of Contents

   1          Introduction ........................................    3
   2          Overview of Operation ...............................    4
   3          Event Package for "presencelist" ....................    5
   3.1        Event Package Name ..................................    6
   3.2        Event Package Parameters ............................    6
   3.3        SUBSCRIBE Bodies ....................................    6
   3.4        Subscription Duration ...............................    6
   3.5        NOTIFY Bodies .......................................    7
   3.6        Notifier Processing of SUBSCRIBE Requests ...........    7
   3.7        Notifier Generation of NOTIFY Requests ..............    8
   3.8        Subscriber Processing of NOTIFY Requests ............    8
   3.9        Handling of Forked Requests .........................    9
   3.10       Rate of Notifications ...............................    9
   3.11       State Agents ........................................    9
   4          Presence List Information Data Format ...............   10
   4.1        Constructing Coherent Presence State ................   11
   4.2        Example .............................................   12
   4.3        XML Schema ..........................................   12
   5          Security Considerations .............................   13
   6          IANA Considerations .................................   13
   6.1        application/cpim-plidf+xml MIME Registration ........   13
   6.2        URN Sub-Namespace Registration for
   urn:ietf:params:xml:ns:plidf ...................................   14
   7          Author's Addresses ..................................   15
   8          Normative References ................................   15
   9          Informative References ..............................   16

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1 Introduction

   The SIP for presence specification [1] allows a user (the subscriber)
   to request to be notified of changes in the presence state of a
   particular user (the presentity) [12]. This is accomplished by having
   the subscriber generate a SUBSCRIBE request for the presentity, which
   is processed at a presence agent in the domain of the presentity.
   Typically, a subscriber has a collection of presentities they are
   interested in. This collection is called a "presence list", and
   typically has anywhere from a few to even a hundred members.

   For environments where bandwidth is limited, such as a wireless
   network, subscribing to each presentity individually is problematic.
   The specific problems are:

        o It generates substantial message traffic, in the form of the
          initial SUBSCRIBE requests for each presentity, and the
          refreshes of each individual subscription.

        o The presence agent may insist on low refresh intervals, in
          order to avoid long lived subscription state. This means that
          the subscriber may need to generate subscriptions faster than
          it would like to, or has the capacity to.

        o The presence agent may generate NOTIFY requests more rapidly
          than the subscriber desires, causing NOTIFY traffic at a
          greater volume than is desired by the subscriber.

        o If a subscriber has only intermittent connectivity, and
          generally polls for presence rather than simply subscribing,
          the latency to obtain the presence state of the entire
          presence list can be large. The messaging required for each
          poll can also be substantial.

   To solve these problems, this specification defines a presence list
   event package. A presence list is identified by a SIP URI [2], and it
   represents a list of zero or more URIs. Each of those URIs, in turn,
   is either a presentity, or another presence list. The state of the
   presence list is the presence state of the list of presentities at
   the leaves of the tree defined by the URI. As a result, instead of
   subscribing to each presentity, a subscriber can subscribe to the
   presence list, and obtain the same information.

   The notifier for the presence list package is called a "presence list
   server", or PLS. In order to determine the state of the entire list,
   the PLS will typically generate a presence list or presence
   subscription to each element of the list.

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   The presence list may exist within the domain of the subscriber, but
   it can also exist within a third party domain.

   The first section provides more detail on the operation of the PLS,
   and the second section defines the event package for presence list

2 Overview of Operation

   This section provides an overview of the typical mode of operation of
   this event package. It is not normative.

   When a user wishes to subscribe to the presence of a list of
   presentities, they create a presence list. This presence list is
   represented by a SIP URI. The list contains a set of URIs, each of
   which is either another list or a presentity. The presence list can
   exist at any domain. Typically, the user who creates the list (and
   subsequently subscribes to it) will have a trust relationship with
   the domain that hosts the list. The specific means by which the list
   is created and maintained is outside of the scope of this
   specification. The list could be manipulated through a web page,
   through a voice response system, or through some protocol.

   To learn the presence state of the set of elements on the list, the
   user sends a single SUBSCRIBE request targeted to the URI of the
   list. This will be routed to a PLS for that URI. The PLS acts as a
   notifier, authenticates the subscriber, and accepts the subscription.
   In order to provide the subscriber with the presence state of the
   presentities on the list, the PLS itself will subscribe to each
   element on the list, using the presence list event package. If the
   subscription is rejected because that package is not supported, the
   list element is a presentity, and not another list, and it can
   therefore fall back to a regular presence subscription. Since the PLS
   is acting on behalf of the user, it will provide the identity of the
   user in the From field. If the presentities require credentials in
   order to accept the subscription, the user will have had to provide
   them to the PLS ahead of time. This requires a trust relationship
   between the user and PLS.

   As notifications arrive from individual presentities, the PLS accepts
   them, extracts the presence information, and generates a notification
   to the subscriber. The PLS can, at its discretion, buffer
   notifications that it receives, and send the presence information to
   the subscriber in batches, rather than individually. This allows the
   PLS to provide rate limiting for the subscriber.

   As an example, consider a presence list with two presentities,

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         Joe              PLS              User A            User B
          |                 |                 |                 |
          |(1) SUBSCRIBE    |                 |                 |
          | list            |                 |                 |
          |---------------->|                 |                 |
          |(2) 200 OK       |                 |                 |
          |<----------------|                 |                 |
          |(3) NOTIFY       |                 |                 |
          |<----------------|                 |                 |
          |(4) 200 OK       |                 |                 |
          |---------------->|                 |                 |
          |                 |(5) SUBSCRIBE a  |                 |
          |                 |---------------->|                 |
          |                 |(6) SUBSCRIBE b  |                 |
          |                 |---------------------------------->|
          |                 |(7) 200 OK       |                 |
          |                 |<----------------|                 |
          |                 |(8) 200 OK       |                 |
          |                 |<----------------------------------|
          |                 |(9) NOTIFY       |                 |
          |                 |<----------------|                 |
          |                 |(10) 200 OK      |                 |
          |                 |---------------->|                 |
          |(11) NOTIFY      |                 |                 |
          | a's state       |                 |                 |
          |<----------------|                 |                 |
          |(12) 200 OK      |                 |                 |
          |---------------->|                 |                 |
          |                 |(13) NOTIFY      |                 |
          |                 |<----------------------------------|
          |                 |(14) 200 OK      |                 |
          |                 |---------------------------------->|
          |(15) NOTIFY      |                 |                 |
          | b's state       |                 |                 |
          |<----------------|                 |                 |
          |(16) 200 OK      |                 |                 |
          |---------------->|                 |                 |

   Figure 1: Typical Package Usage and A typical flow for a
   subscription to this presence list is shown in Figure 1.

3 Event Package for "presencelist"

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   The following subsections formally define the presence list event
   package, following the requirements defined by the SIP events
   framework [3].

3.1 Event Package Name

   The name of this event package is "presencelist".

   The following is the information needed to register this event
   package with IANA:

        Package Name: presencelist

        Type: package

        Contact: Jonathan Rosenberg,

        Reference: RFC XXXX [[Note to RFC Editor: replace with the RFC
             number for this specification]]

        OPEN ISSUE: We could potentially make this a template
        package. In template form, it would represent a
        "collection" template, that allows you to subscribe to a
        list of -something-, where -something- has an event
        package. In the case of presence, the subscription would be
        for presence.collection.

3.2 Event Package Parameters

   This specification does not define any parameters in the Event header
   for this package.

3.3 SUBSCRIBE Bodies

   The SUBSCRIBE message MAY contain a body whose purpose is to define
   filters on the operation of the buddylist. These filters would
   include any rate limitation desired for the notifications, or any
   aggregation that is desired. There is no default or mandatory body
   type defined for this purpose.

3.4 Subscription Duration

   Since the primary benefit of the buddy list server is to reduce the
   overall messaging volume to a handset, it is RECOMMENDED that the
   subscription duration to a buddylist be reasonably long. The default,
   when no duration is specified, is two hours. That reduces the need to
   refresh too frequently. Of course, the standard techniques [3] can be

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   used to increase or reduce this amount.

3.5 NOTIFY Bodies

   There are two mandatory-to-implement body types for this package.

   The first mandatory-to-implement body type in notifications is
   application/cpim-plidf+xml. This type is defined in Section 4 of this
   specification. It is almost identical to the presence data format
   [4], but allows for lists of presenties, rather than a single one.
   All implementors of this package MUST support this type.

   The second mandatory-to-implement body type in notifications is
   application/cpim-pidf+xml [4]. The PIDF format only supports
   information for a single presentity. Therefore, its usage is limited
   to notifications that report a change in state for a single
   presentity. It is mandated in order to facilitate operation of the
   PLS. The PLS can simply pass on any presence documents it receives
   from the presentities in a notification, without modification.

   An implementation compliant to this specification MAY support the
   multipart/mixed type. As described in [4], this allows a notification
   to contain multiple presence documents. This type, like
   application/cpim-pidf+xml, can only be used in notifications that
   report changes in state, not full state. This is described in more
   detail in Section 4.1.

        OPEN ISSUE: It seems like there are two many choices here.
        Eliminating pidf and multipart/mixed will require PLS to
        generate their own documents, which would result in the
        loss of end-to-end signatures. The other alternative is to
        eliminate plidf, which is needed for partial state updates.

   The absence of an Accept header in the SUBSCRIBE indicates support
   for both application/cpim-pidf+xml and application/cpim-plidf+xml. If
   an Accept header is present, both these types MUST be included, in
   additional to any other types supported by the client.

3.6 Notifier Processing of SUBSCRIBE Requests

   All subscriptions for buddy lists SHOULD be authenticated. The use of
   the SIP HTTP Digest mechanism [2] over TLS is RECOMMENDED.

   Once authenticated, the subscription is authorized. Typically, each
   presence list is associated with a particular user (the one who
   created it and manages the set of elements in it), and only that user
   will be allowed to subscribe. Of course, there may be exceptions to

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   this rule, and a notifier MAY use any authorization policy it

3.7 Notifier Generation of NOTIFY Requests

   This specification leaves the choice about how and when to generate
   NOTIFY requests at the discretion of the implementor. One of the
   value propositions of the PLS is the means by which it aggregates,
   rate limits, or optimizes the way in which notifications are

   As a baseline behavior, if the PLS acts as a subscriber to determine
   the state of the presentities on the buddy list, it MAY generate a
   NOTIFY to the PLS subscriber whenever it receives a NOTIFY about a
   state change in one or more presentities. The body of the NOTIFY,
   assuming it's application/cpim-pidf+xml, would merely be copied from
   that NOTIFY into the NOTIFY sent by the PLS to the subscriber. If a
   subscription to a presentity is refused, the BLSS MAY generate a new
   presence document for that presentity, setting its status to
   "subscription refused", and then pass that NOTIFY to the subscriber.
   This would give the subscriber a visual clue that its subscription
   was refused, and that the presentity should probably be removed from
   the buddy list.

        OPEN ISSUE: This seems insufficient. There should probably
        be an explicit way to indicate that the subscription to a
        specific presentity has been refused. More generally, there
        should be an explicit way to pass information on the
        subscription states to particular presentities. Perhaps an
        additional element in application/cpim-plidf+xml.

   Immediate notifications triggered as a result of a SUBSCRIBE SHOULD
   result in the full state of all presentities to be present in the
   NOTIFY. This allows the subscriber to refresh their state, and to
   recover from lost notifications.

3.8 Subscriber Processing of NOTIFY Requests

   The SIP Events framework expects packages to specify how a subscriber
   processes NOTIFY requests in any package specific ways, and in
   particular, how it uses the NOTIFY requests to contruct a coherent
   view of the state of the subscribed resource.

   Notifications within this package can convey partial information;
   that is, they can indicate information about a subset of the state
   associated with the subscription. This means that an explicit
   algorithm needs to be defined in order to construct coherent and

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   consistent state. The details of this mechanism are specific to the
   particular document type. See Section 4.1 for information on
   constructing coherent information from an application/cpim-plidf+xml
   document. If the document received is an application/cpim-pidf+xml
   document, the procedures of Section 4.1 are followed, as if it was a
   plidf document, with a version one higher than the current version,
   and a state of partial.

        OPEN ISSUE: This implies that version numbers of the
        documents are shared across pidf and plidf. Is that what we

   If the document received is of type multipart/mixed, the procedures
   of Section 4.1 are followed, as if it was an plidf document, with a
   version one higher than the previous version, and a state of partial.

3.9 Handling of Forked Requests

   Forking makes little sense with this event package, since the whole
   idea is a centralization of the source of notifications. Therefore, a
   subscriber MUST create just a single dialog as a result of a single
   subscription request, using the techniques described in [3].

3.10 Rate of Notifications

   One potential role of the PLS is to perform rate limitations on
   behalf of the subscriber. As such, this specification does not
   mandate any particular rate limitation, and rather leaves that to the
   discretion of the implementation.

3.11 State Agents

   Effectively, a presence list server is nothing more than a state
   agent for the presence event type. A separate event package is needed
   because of the differing body types which can be used in NOTIFY, and
   the need to construct complete state from the partial notifications.
   Furthermore, there are differing values of the subscription interval,
   differing recommendations on rate limitation, and so on.

   The usage of the PLS does introduce some security considerations. The
   end user is no longer the direct subscriber to the presence state of
   the presentity. If the PA for the presentity demands end-to-end
   authentication, the PLS will need to be provided the shared secrets
   for those presentities (assuming Digest is used). This requires a
   certain level of trust between the user and their PLS. This
   specification does not describe any particular means of uploading the
   shared secret for a particular subscriber to the PLS. However, that

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   upload mechanism MUST ensure privacy of the key data; using HTTPS to
   fill out a form is a reasonable method.

   If the PA for the presentity is using a transitive trust to validate
   the subscriber, then this works well with the PLS concept. The PLS
   would authenticate the subscriber, and then MAY use the SIP
   extensions for network asserted identity [5] [6] to provide an
   authenticated identity to the PA.

4 Presence List Information Data Format

   Presence list information is an XML document [7] that MUST be well-
   formed and SHOULD be valid. Presence list documents MUST be based on
   XML 1.0 and MUST be encoded using UTF-8. This specification makes use
   of XML namespaces for identifying presence list documents and
   document fragments. The namespace URI for elements defined by this
   specification is a URN [8], using the namespace identifier 'ietf'
   defined by [9] and extended by [10]. This URN is:


   A presence list information document begins with the root element tag
   "presence-list". It consists of any number of "presence" sub-
   elements, each of which is describes a particular presentity. The
   "presence" element is defined in [4]. Elements from different
   namespaces MAY be present for the purposes of extensibility; elements
   or attributes from unknown namespaces MUST be ignored. There are
   three attributes associated with the "presence-list" element, all of
   which MUST be present:

        version: This attribute allows the recipient of presence list
             documents to properly order them. Versions start at 0, and
             increment by one for each new document sent to a
             subscriber. Versions are scoped within a subscription.
             Versions MUST be representable using a 32 bit integer.

        state: This attribute indicates whether the document contains
             the full presence list state, or whether it contains only
             information on those presentities which have changed since
             the previous document (partial).

        entity: This attribute contains a URI that identifies the
             presence list whose information is reported in the
             remainder of the document.

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4.1 Constructing Coherent Presence State

   The presence list subscriber maintains a table for each presence
   list. The table contains a row for each presentity in the presence
   list. Each row is indexed by the URI for that presentity. That URI is
   obtained from the entity attribute of the "presence" element. The
   contents of each row contain the state of that presentity as conveyed
   in the presence document. The table is also associated with a version
   number. The version number MUST be initialized with the value of the
   "version" attribute from the "presence-list" element in the first
   document received. Each time a new document is received, the value of
   the local version number, and the "version" attribute in the new
   document, are compared. If the value in the new document is one
   higher than the local version number, the local version number is
   increased by one, and the document is processed. If the value in the
   document is more than one higher than the local version number, the
   local version number is set to the value in the new document, the
   document is processed, and the watcherinfo subscriber SHOULD generate
   a refresh request to trigger a full state notification. If the value
   in the document is less than the local version, the document is
   discarded without processing.

   The processing of the presence list document depends on whether it
   contains full or partial state. If it contains full state, indicated
   by the value of the "state" attribute in the "presence-list" element,
   the contents of the presence-list table are flushed. They are
   repopulated from the document. A new row in the table is created for
   each "presence" element. If the presence list document contains
   partial state, as indicated by the value of the "state" attribute in
   the "presence-list" element, the document is used to update the
   table. For each "presence" element in the document, the subscriber
   checks to see whether a row exists for that presentity. This check is
   done by comparing the URI in the "entity" attribute of the "presence"
   element with the URI associated with the row. If the presentity
   doesn't exist in the table, a row is added, and its state is set to
   the information from that "presence" element. If the presentity does
   exist, its state is updated to be the information from that
   "presence" element. If a row is updated or created, such that its
   state is now terminated, that entry MAY be removed from the table at
   any time.

        OPEN ISSUE: There is currently nothing that would indicate
        that the state of a presentity is "terminated"; this
        doesn't mean the user has been terminated (!), but rather,
        that the subscription from the PLS to the presentity has
        been terminated.

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4.2 Example

   The following is an example presence list document:

   <?xml version="1.0"?>
   <list:presence-list entity=""
     <impp:presence entity="">
        <impp:tuple id="mobile-phone">
          <impp:contact priority="0.8">tel:09012345678</impp:contact>

4.3 XML Schema

   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema xmlns:xs=""
   <xs:import namespace="urn:ietf:params:xml:ns:cpim-pidf"
     <xs:element name="presence-list">
           <xs:element ref="cpim-pidf:presence" minOccurs="0"
           <xs:any namespace="##other" processContents="lax" minOccurs="0"
         <xs:attribute name="version" type="xs:nonNegativeInteger"
         <xs:attribute name="state" use="required">
             <xs:restriction base="xs:string">
               <xs:enumeration value="full"/>
               <xs:enumeration value="partial"/>

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         <xs:attribute name="entity" type="xs:anyURI" use="required" />

        OPEN ISSUE: In order to properly import the PIDF schema, it
        needs to have a well defined location. This presumably
        requires PIDF to have registered its schema with IANA, but
        it currently does not.

5 Security Considerations

   This package does introduce some security considerations, which are
   discussed in Section 3.11.

        OPEN ISSUE: Need to discuss the security issues with the
        choice of document formats; i.e., multipart/mixed vs.
        application/cpim-plidf+xml and their impact on end-to-end

6 IANA Considerations

   This document registers a new MIME type, application/cpim-plidf+xml
   and registers a new XML namespace.

6.1 application/cpim-plidf+xml MIME Registration

        MIME media type name: application

        MIME subtype name: cpim-plidf+xml

        Mandatory parameters: none

        Optional parameters: Same as charset parameter application/xml
             as specified in RFC 3023 [11].

        Encoding considerations: Same as encoding considerations of
             application/xml as specified in RFC 3023 [11].

        Security considerations: See Section 10 of RFC 3023 [11] and
             Section 5 of this specification.

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        Interoperability considerations: none.

        Published specification: This document.

        Applications which use this media type: This document type has
             been used to support subscriptions to lists of

        Additional Information:

             Magic Number: None

             File Extension: .plidf or .xml

             Macintosh file type code: "TEXT"

        Personal and email address for further information: Jonathan
             Rosenberg, <>

        Intended usage: COMMON

        Author/Change controller: The IETF.

6.2 URN Sub-Namespace Registration for urn:ietf:params:xml:ns:plidf

   This section registers a new XML namespace, as per the guidelines in

        URI: The URI for this namespace is urn:ietf:params:xml:ns:plidf

        Registrant Contact: IETF, SIMPLE working group,
             <>, Jonathan Rosenberg


             <?xml version="1.0"?>
             <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
             <html xmlns="">
               <meta http-equiv="content-type"
               <title>Presence List Information Namespace</title>
               <h1>Namespace for Presence List Information</h1>

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               <p>See <a href="[[[URL of published RFC]]]">RFCXXXX</a>.</p>

7 Author's Addresses

   Jonathan Rosenberg
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936

   Ben Campbell
   5100 Tennyson Parkway
   Suite 1200
   Plano, Texas 75024

8 Normative References

   [1] J. Rosenberg, "Session initiation protocol (SIP) extensions for
   presence," Internet Draft, Internet Engineering Task Force, May 2002.
   Work in progress.

   [2] J. Rosenberg, H. Schulzrinne, et al.  , "SIP: Session initiation
   protocol," Internet Draft, Internet Engineering Task Force, Feb.
   2002.  Work in progress.

   [3] A. Roach, "SIP-specific event notification," Internet Draft,
   Internet Engineering Task Force, Mar. 2002.  Work in progress.

   [4] H. Sugano, S. Fujimoto, et al.  , "Common presence and instant
   messaging (CPIM)presence information data format," Internet Draft,
   Internet Engineering Task Force, May 2002.  Work in progress.

   [5] C. Jennings, J. Peterson, and M. Watson, "Private extensions to
   the session initiation protocol (SIP) for asserted identity within
   trusted networks," Internet Draft, Internet Engineering Task Force,
   June 2002.  Work in progress.

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   [6] J. Peterson, "Enhancements for authenticated identity management
   in the session initiation protocol (SIP)," Internet Draft, Internet
   Engineering Task Force, Apr. 2002.  Work in progress.

   [7] W. W. W. C. (W3C), "Extensible markup language (xml) 1.0." The
   XML 1.0 spec can be found at

   [8] R. Moats, "URN syntax," RFC 2141, Internet Engineering Task
   Force, May 1997.

   [9] R. Moats, "A URN namespace for IETF documents," RFC 2648,
   Internet Engineering Task Force, Aug. 1999.

   [10] M. Mealling, "The IANA XML registry," Internet Draft, Internet
   Engineering Task Force, Nov. 2001.  Work in progress.

   [11] M. Murata, S. S. Laurent, and D. Kohn, "XML media types," RFC
   3023, Internet Engineering Task Force, Jan. 2001.

9 Informative References

   [12] M. Day, J. Rosenberg, and H. Sugano, "A model for presence and
   instant messaging," RFC 2778, Internet Engineering Task Force, Feb.

   Full Copyright Statement

   Copyright (c) The Internet Society (2002). All Rights Reserved.

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   The limited permissions granted above are perpetual and will not be
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