Network Working Group                                         B. Mahoney
Internet-Draft                                                       MIT
Expires: January 16, 2002                                      G. Babics
                                                                A. Taler
                                                           July 18, 2001

                     Guide to Internet Calendaring

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
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on January 16, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.


   This document describes the various Internet calendaring and
   scheduling standards and works in progress progress, and the relationships
   between them.  It's intention  Its intent is to provide a context for these
   documents, assist in their understanding, and potentially help
   implementers in the
   design of their standards based standards-based calendaring and scheduling systems.  The
   standards addressed are RFC 2445 (iCalendar), RFC 2446 (iTIP), and
   RFC 2447 (iMIP).  The work in progress addressed is "Calendar Access
   Protocol" (CAP).  This document also describes issues and problems
   that are not solved by these protocols, and that could be targets for
   future work.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2   Concepts and Relationships . . . . . . . . . . . . . . . . .  5
   2.    Requirements . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.1   Fundamental Needs  . . . . . . . . . . . . . . . . . . . . .  6
   2.2   Protocol Requirements  . . . . . . . . . . . . . . . . . . .  6
   3.    Solutions  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.1   Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.2   Systems  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.2.1 Standalone single-user system Single-user System  . . . . . . . . . . . . . . .  9
   3.2.2 Single-user systems communicating Systems Communicating  . . . . . . . . . . . . .  9
   3.2.3 Single-user with multiple CUA Multiple CUAs . . . . . . . . . . . . . . . 10
   3.2.4 Single-user with multiple calendars Multiple Calendars  . . . . . . . . . . . . 10
   3.2.5 Users communicating Communicating on a multi-user system Multi-user System . . . . . . . . . 11
   3.2.6 Users communicating Communicating through different multi-user systems Different Multi-user Systems . . 11
   4.    Important Aspects  . . . . . . . . . . . . . . . . . . . . . 12
   4.1   Timezones  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.2   Choice of Transport  . . . . . . . . . . . . . . . . . . . . 12
   4.3   Security . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.4   Amount of data . . . . . . . . . . . . . . . . . . . . . . . 12
   4.5   Recurring Components . . . . . . . . . . . . . . . . . . . . 12
   5.    Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . 14
   5.1   Scheduling people, People, not calendars Calendars . . . . . . . . . . . . . . 14
   5.2   Administration . . . . . . . . . . . . . . . . . . . . . . . 14
   5.3   Notification . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.    Security considerations  . . . . . . . . . . . . . . . . . . 15
   6.1   Access Control . . . . . . . . . . . . . . . . . . . . . . . 15
   6.2   Authentication . . . . . . . . . . . . . . . . . . . . . . . 15
   6.3   Using email E-mail . . . . . . . . . . . . . . . . . . . . . . . . 15
   6.4   Other issues Issues . . . . . . . . . . . . . . . . . . . . . . . . 16
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
   A.    Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 17
   B.    Bibliography . . . . . . . . . . . . . . . . . . . . . . . . 18
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 19

1. Introduction

   The calendaring

   Calendaring and scheduling protocols are intended to provide for
   the needs of aid individuals attempting to obtain
   in obtaining calendaring information and schedule scheduling meetings across
   the Internet, organizations attempting to provide aid organizations in providing calendaring
   information on the Internet, as well as and to provide for organizations looking
   for a calendaring and scheduling solution to deploy internally.

   It is the intent of this document to provide a context for the
   calendar standards and works in progress, these
   documents, assist in their understanding, and potentially help implementers in the
   design of
   their Internet standards-based calendaring and scheduling systems.

   Problems not solved by these protocols, as well as security issues to
   be kept in mind, are discussed at the end of the document.

1.1 Terminology

   This memo uses much of the same terminology as iCalendar [RFC-2445],
   iTIP [RFC-2446], iMIP [RFC-2447], and [CAP].  The following
   definitions are provided as introductory, an introducttion; the definitions in the
   protocol specifications are the canonical ones. themselves should be considered canonical.


      A collection of events, to-dos, journal entries, etc.  A calendar
      could be the content of a person or a resource's agenda; it could
      also be a collection of data serving a more specialized need.
      Calendars are the basic storage containers for calendaring

   Calendar Access Rights

      A set of rules for a calendar describing defining who may perform which
      operations on that calendar, what operations, such as
      reading and or writing
      information. information, on a given calendar.

   Calendar Service

      A running server application which that provides access to a collection number of

   Calendar Store (CS)

      A data store of a calendar service.  A calendar service may have
      several calendar stores, and each store may contain several
      calendars, as well as properties and components outside of the those

   Calendar User (CU)
      An entity (often a human) that accesses calendar information.

   Calendar User Agent (CUA)

      Software used by with which the calendar user that communicates with a calendar
      service or local calendar store to provide the user access to calendar information.


      A piece of calendar data such as an event, a to-do or an alarm.
      Information about components is stored as properties of those


      Is a

      A calendar user (sometimes called the delegatee) who has assigned
      his or her participation in a scheduled calendar component (e.g., (e.g.
      a VEVENT) to another calendar user (sometimes called the delegate
      or delegatee).


      Is  An example of a delegator is a busy executive
      sending an employee to a meeting in his or her place.


      A calendar user (sometimes called the delegatee) who has been
      assigned participation to participate in a scheduled calendar component (e.g., (e.g.  a
      VEVENT) by one of the attendees in the scheduled calendar that component (sometimes
      called the delegator).  An example of a delegate is a team member told to go
      sent to a particular meeting.


      Is a

      A calendar user who is authorized to act on behalf of another calendar
      user.  An example of a designate is an assistant. assistant scheduling
      meetings for his or her superior.

   Local Store

      A CS which that is on the same platform device as the CUA.


      A property description of some element of a component, such as a description or a start time.
      time, title or location.

   Remote Store

      A CS which that is not on the same platform device as the CUA.

1.2 Concepts and Relationships

   iCalendar is the language used to describe calendar objects.  iTIP is
   describes a way to use the iCalendar language to do scheduling.  iMIP is
   describes how to do iTIP
   with email. scheduling via e-mail.  CAP is describes a way
   to use the language, iCalendar language to access a calendar store in real-time. real-

   The relationship between the calendaring protocols is similar to that
   between the email e-mail protocols.  In those terms terms, iCalendar is like analogous to
   RFC 822, iTIP and iMIP are like SMTP, analogous to the Simple Mail Transfer
   Protocol (SMTP), and CAP is like POP analogous to the Post Office Protocol
   (POP) or IMAP. Internet Message Access Protocol (IMAP).

2. Requirements

2.1 Fundamental Needs

   The following examples scenarios illustrate people's people and organizations' basic
   calendaring and scheduling needs:

      a] A doctor wishes to keep track of all his her appointments.

      Need: Read To read and manipulate one's own calendar with only one CUA.

      b] A busy musician wants to maintain her schedule with different multiple
      devices, such as with through an Internet-based agenda or and with a PDA.

      Need: Read To read and manipulate one's own calendar, possibly with
      solutions from different vendors.

      c] A software development team wishes to share agenda information
      by using a group scheduling product in order to more effectively schedule
      their time. time through viewing each other's calendar information.

      Need: Share To share calendar information with between users using of the same
      calendar service.

      d] A teacher wants his students to be able to schedule calendar
      entries appointments during
      his office hours.

      Need: Schedule To schedule calendar events, to-dos and journals with other
      using of the same calendar service.

      e] A movie theater wants to publish its schedule so that for prospective customers can easily access it.

      Need: Share To share calendar information with users using of other calendar
      services, possibly from multiple different vendors.

      f] A social club wants to be able to schedule calendar entries effectively
      with its members.

      Need: Schedule To schedule calendar events and to-dos with users using of other
      calendar services, possibly from multiple different vendors.

2.2 Protocol Requirements

   Some of the these needs can be met with by proprietary solutions (a, c, d),
   but others can not (b, e, f).  From these needs we can establish  These latter scenarios show that
   standard protocols are required for accessing information in a
   calendar store, store and for scheduling calendar entries.  In addition addition, these
   protocols require a common data format for representing calendar

   These roles requirements are filled met by the following protocol specifications.

      - Data format: iCalendar [RFC-2445] is the data format

      iCalendar [RFC-2445] provides a data format for representing
      calendar information which the information, to be used and exchanged by other protocols can use. protocols.
      iCalendar [RFC-2445] can also be used in other contexts contexts, such as a drag and
      drop format
      drag-and-drop interface, or an export/import format. feature.  All the
      other calendaring protocols depend on iCalendar [RFC-2445], so all
      elements of a standards-
      based standards-based calendaring and scheduling systems
      will have to be able interpret iCalendar [RFC-2445].

      - Scheduling protocol: iTIP [RFC-2446] is the scheduling protocol

      iTIP [RFC-2446] describes the messages used to schedule calendar
      events.  These messages  Within iTIP messages, events are represented in iCalendar [RFC-2445],
      [RFC-2445] format, and have semantics that include such things identify the message as
      being an invitation to a meeting, an acceptance of an invitation invitation,
      or the assignment of a task.

      iTIP [RFC-2446] messages are used in the scheduling workflow,
      where users exchange messages in order to organize things such as
      events and to-dos.  CUAs generate and interpret iTIP [RFC-2446]
      messages at the direction of the calendar user.  With iTIP [RFC-
      2446] one users can create, modify, delete, reply to, counter, and
      decline counters to the various iCalendar [RFC-2445] components.
      Furthermore, one users can also request the free/busy time of other

      iTIP [RFC-2446] is transport-independent, and has one specified
      transport bindings: binding: iMIP [RFC-2447] is a binding binds iTIP to email. e-mail.  In
      addition [CAP] will provide a real-time binding of iTIP [RFC-
      2446], allowing CUAs to perform calendar management as well as and scheduling
      over a single connection.

      - [CAP] is the calendar Calendar management protocol protocol: [CAP]

      [CAP] describes the messages used to manage calendars on a
      calendar store.  These messages use iCalendar [RFC-2445] to
      describe various components such as events and to-dos.  With these  These
      messages one can do the operations in make it possible to perform iTIP [RFC-2446] and operations,
      as well as other operations relating to a calendar store, store such as search,
      searching, creating calendars, specifying calendar properties, and being able to
      specifying calendar access rights to one's calendars. rights.

3. Solutions

3.1 Examples

   Returning to the examples of scenarios presented in section 2.1, they can be solved using the calendaring
   protocols can be used in the following ways:

      a] The doctor can use a proprietary CUA with a local store, and
      perhaps use iCalendar [RFC-2445] as a storage mechanism.  This
      would allow the doctor her to easily import his her data store into another
      application that supports iCalendar [RFC-2445].

      b] The musician who wishes to access her agenda from anywhere can
      use a [CAP] enabled [CAP]-enabled calendar service accessible through over the Internet.
      She can then use whichever any available [CAP] clients are available to access the data.

      A proprietary system could also be employed which that provides access through a web-based interface, Web-based
      interface could also be employed, but the use of [CAP] would be
      superior in that it would allow the use of third party tools,
      applications such as PDA synchronization tools.

      c] The development team can use a calendar service which supports
      [CAP], and then each member can use a [CAP]-enabled CUA of their

      Alternatively, each member could use an iMIP [RFC-2447]-enabled
      CUA, and they could book meetings over email. e-mail.  This solution has
      the drawback that it is difficult to examine the other users' agendas,
      making organizing meetings more difficult.

      Proprietary solutions are also available, but they require that
      all people members use clients by the same vendor, and disallow the use
      of third party applications.

      d] The teacher can set up a calendar service, and have students
      book time through any of the iTIP [RFC-2446] bindings.  [CAP]
      provides real-time access, but could require additional
      configuration.  iMIP [RFC-2447] would be the easiest to configure,
      but may require more email e-mail processing.

      If [CAP] access is provided then determining the state of the
      teacher's schedule is straightforward.  If not, this can be
      determined through iTIP [RFC-2446] free/busy requests.  Non-
      standard methods could also be employed, such as serving up ICAL, iCAL,
      HTML, and XML over HTTP.

      A proprietary system could also be used, but would require that
      all students be able to use software from a specific vendor.

      e] For [CAP] would be preferred for publishing a movie theater's schedule [CAP]
      schedule, since it provides the
      most advanced access and search
      capabilities.  It also allows easy integration with its customer's customers'
      calendar systems.

      Non-standard methods such as serving data over HTTP could also be
      employed, but would be harder to integrate with customer's customers'

      Using a completely proprietary solutions solution would be very difficult difficult,
      if not impossible, since it would require every user to install
      and use the proprietary software.

      f] The social club could distribute meeting information in the
      form of iTIP [RFC-2446] messages.  This could be done over email messages, sent via e-mail using iMIP [RFC-2447].  Meeting [RFC-
      2447].  The club could distribute meeting invitations, as well as
      a full published agenda could be distributed. agenda.

      Alternatively, the social club could provide access to a [CAP]
      enabled [CAP]-enabled
      calendar service, however service.  However, this solution would be more expensive
      since it requires the maintenance of a server.

3.2 Systems

   The following diagrams illustrate possible example systems and their usage of
   the various protocols.

3.2.1 Standalone single-user system Single-user System

   A single user system that does not communicate with other systems
   need not employ any of the protocols.  However, it may use iCalendar
   [RFC-2445] as a data format in some places.

          -----------       O
         | CUA w/    |     -+- user
         |local store|      A
          -----------      / \

3.2.2 Single-user systems communicating Systems Communicating

   Users with single-user systems may schedule meetings with each other
   using iTIP [RFC-2446].  The easiest binding of iTIP [RFC-2446] to use
   would be iMIP [RFC-2447], since since the messages can be held in their the users'
   mail queue, queues, which we assume to already exist.  [CAP] could also be

          O   -----------                    -----------   O
         -+- | CUA w/    | -----[IMIP]----- -----[iMIP]----- | CUA w/    | -+- user
          A  |local store|     Internet     |local store|  A
         / \  -----------                    -----------  / \

3.2.3 Single-user with multiple CUA Multiple CUAs

   A single user may use more than one CUA to access his or her
   calendar.  The user may use a PDA, a web Web client, a PC, or some other
   device, depending an on accessibility.  Some of these clients may have
   local stores and others may not.  If they do, then they  Those with local stores need to
   ensure that
   synchronize the data on the CUA is synchronized with the data on the CS.

                   |   CUA w   | -----[CAP]----------+
                   |local store|                     |
              O     -----------                    ----------
             -+-                                  |   CS     |
              A                                   |          |
             / \                                   ----------
                    -----------                      |
                   |  CUA w/o  | -----[CAP]----------+
                   |local store|

3.2.4 Single-user with multiple calendars Multiple Calendars

   A single user may have many independent calendars.  One calendars; for example, one
   may be work
   related, contain work-related information and another for personal use.
   information.  The CUA may or may not have a local store.  If it does,
   then it needs to ensure that synchronize the data on of the CUA is synchronized with the data on
   both of the CS.

                         +------------[CAP]------ |   CS     |
                         |                        |          |
              O     -----------                    ----------
             -+-   |  CUA      |
              A    |           |
             / \    -----------
                         |                         ----------
                         +------------[CAP]------ |   CS     |
                                                  |          |

3.2.5 Users communicating Communicating on a multi-user system Multi-user System

   Users on a multi-user system may schedule meetings with each other
   using [CAP]-enabled CUA CUAs and service. services.  The CUA CUAs may or may not have a
   local store.  If they do, then they stores.  Those with local stores need to ensure that synchronize the data
   on the CUA is synchronized CUAs with the data on the CS.

              O     -----------
             -+-   |   CUA w   | -----[CAP]----------+
              A    |local store|                     |
             / \    -----------                    ----------
                                                  |   CS     |
                                                  |          |
              O     -----------                      |
             -+-   |  CUA w/o  | -----[CAP]----------+
              A    |local store|
             / \    -----------

3.2.6 Users communicating Communicating through different multi-user systems Different Multi-user Systems

   Users on a multi-user system may need to schedule meetings with user users
   on a different multi user multi-user system.  The services can communicate using
   [CAP] or iMIP [RFC-2447].

             O     -----------                    ----------
            -+-   |   CUA w   | -----[CAP]-------|   CS     |
             A    |local store|                  |          |
            / \    -----------                    ----------
                                                [CAP] or [iMIP]
             O     -----------                    ----------
            -+-   |  CUA w/o  | -----[CAP]-------|   CS     |
             A    |local store|                  |          |
            / \    -----------                    ----------

4. Important Aspects

   There are a number of important aspects of these calendaring
   standards of which people, especially implementers, should be aware.

4.1 Timezones

   The dates and times in components can refer to a specific time zones.  These
   time zone.
   Time zones can be defined in some a central store, or they may be defined
   by a user to fit his or her needs.  Any user  All users and application applications should
   be aware of time zones and time zone differences.  New time zones may
   need to be added, and others removed.  Two different vendors may
   describe the same time zone differently (such as by using a different

4.2 Choice of Transport

   There are issues to be aware of in choosing a transport mechanism.
   The choices are between a network protocol,
   protocol such as CAP, [CAP], or a store and forward (email) (e-mail) solution.

   The use of a network ("on-the-wire") mechanism may require some
   organizations to make provisions to allow calendaring traffic to
   traverse a corporate firewall on the required ports.  Depending on
   the organizational culture, this may be a challenging social

   The use of an email-based mechanism exposes innately time sensitive time-sensitive data to
   unbounded latency.  Large or heavily utilized mail systems may
   experience an unacceptable delay in message receipt.

4.3 Security

   See the "Security Considerations" (Section 6) section below.

4.4 Amount of data

   In some cases cases, a component may be very large.  For large, for instance, some
   attachments may be a
   component with a very large. large attachment.  Some applications may be
   low- bandwidth or may be limited in the amount of data they can
   store.  The  Maximum component size of the data may be set in [CAP].  It can also be
   controlled in [CAP], by specifying maximums.
   In iMIP [RFC-2447] it can be controlled, by restricting the maximum size of the email
   e-mail that the application can download.

4.5 Recurring Components

   In iCAL [RFC-2445] one can specify complex recurrence rules for
   VEVENTs, VTODOs, and VJOURNALs.  There is the danger that
   applications interpret these rules differently.  Thus, one  One must make
   sure that one is be careful with to correctly
   interpret these recurrence rules. rules and pay extra attention to being
   able to interoperate using them.

5. Open Issues

   Many issues are not currently resolved by these protocols, and many
   desirable features are not yet provided.  Some of the more prominent
   ones follow. are outlined below.

5.1 Scheduling people, People, not calendars Calendars

   Meetings are scheduled with people, however people; however, people may have many
   calendars, and may store these calendars in many places.  There may
   also be many routes to contact them.  These  The calendaring protocols do
   not attempt to provide unique access for contacting a single given person.
   Instead, 'calendar addresses' are booked, which may be email e-mail
   addresses or individual calendars.  It is up to the users themselves
   to orchestrate mechanisms to ensure that the bookings go to the right

5.2 Administration


   The calendaring protocols do not address the issues of administering
   users and calendars on a calendar service.  This must be handled by
   proprietary mechanisms for each implementation.

5.3 Notification

   People often wish to be notified of upcoming events, new events, or
   changes to existing events.  These  The calendaring protocols do not attempt
   to address these needs in a real-time fashion. system.  Instead, the ability
   to store alarm information on events is provided, which can be used
   to provide client-side notification of upcoming events.  To organize
   notification of new or changed events events, clients will have to poll the data

6. Security considerations

6.1 Access Control

   There has to be reasonable granularity in the configuration options
   for access to data through [CAP], so that what should be released to
   requesters is released, and what shouldn't isn't. is not.  Details of
   handling this are described in [CAP].

6.2 Authentication

   Access control must be coupled with a good authentication system, so
   that the right people get the right information.  For [CAP] this
   means requiring authentication before any database access can be
   performed, and checking access rights and authentication credentials
   before releasing information.  [CAP] uses SASL the Simple Authentication
   Security Layer (SASL) for this authentication.  In iMIP [RFC-2447],
   this may present some challenges, as authentication is often not a
   consideration in store-
   and-forward store-and-forward protocols.

   Authentication is also important for scheduling, in that receivers of
   scheduling messages should be able to validate the apparent sender.
   Since scheduling messages are wrapped in MIME [RFC-2045], signing and
   encryption is available for free. are freely available.  For messages transmitted over mail
   this is the only available alternative.  It is suggested that
   developers take care in implementing the security features in iMIP
   [RFC-2447], bearing in mind that the concept and need may be foreign
   or non-obvious to users, yet essential for the system to function as
   they might expect.

   The real-time protocols provide for the authentication of users, and
   the preservation of that authentication information, allowing for
   validation by the receiving end-user or server.

6.3 Using email E-mail

   Because scheduling information can be transmitted over mail without
   any authentication information, email e-mail spoofing is extremely easy if
   the receiver is not checking for authentication.  It is suggested
   that implementers consider requiring authentication as a default,
   using mechanisms such as are described in Section 3 of iMIP [RFC-
   2447].  The use of email, e-mail, and the potential for anonymous
   connections, means that 'calendar spam' is possible.  Developers
   should consider this threat when designing systems, particularly
   those that allow for automated request processing.

6.4 Other issues Issues

   The current security context should be obvious to users.  Because the
   underlying mechanisms may not be clear to users, efforts to make
   clear the current state in the UI should be made.  One example of
   this is the 'lock' icon used in some web Web browsers during secure
   connections.  With both iMIP [RFC-2447] and [CAP], the possibilities
   of Denial of Service attacks must be considered.  The ability to
   flood a calendar system with bogus requests is likely to be exploited
   once these systems become widely deployed, and detection and recovery
   methods will need to be considered.

Authors' Addresses

   Bob Mahoney
   77 Massachusetts Avenue
   Cambridge, MA  02139

   Phone: (617) 253-0774

   George Babics
   2000 Peel Street
   Montreal, Quebec  H3A 2W5

   Phone: (514) 733-8500 x4201

   Alex Taler


Appendix A. Acknowledgments

   Thanks to the following following, who have participated in the development of
   this document:

      Eric Busboom, Pat Egen, David Madeo, Shawn Packwood, Bruce Kahn. Kahn,
      Alan Davies, Robb Surridge.

Appendix B. Bibliography

      [RFC-2445] Dawson, F.  and D.  Stenerson, "Internet Calendaring
      and Scheduling Core Object Specification - iCalendar", RFC 2445,
      November 1998.

      [RFC-2446] Silverberg, S., Mansour, S., Dawson, F.  and R.
      Hopson, "iCalendar Transport-Independent Interoperability Protocol
      (iTIP): Scheduling Events, Busy Time, To-dos and Journal Entries",
      RFC 2446, November 1998.

      [RFC-2447] Dawson, F., Mansour, S.  and S.  Silverberg, "iCalendar
      Message-Based Interoperability Protocol - iMIP", RFC 2447,
      November 1998.

      [RFC-2045] Freed, N.  and N.  Borenstein, "Multipurpose Internet
      Mail Extensions (MIME) - Part One: Format of Internet Message
      Bodies", RFC 2045, November 1996.

      [CAP] Mansour, S., Royer, D., Babics, G., and Hill, P.  "Calendar
      Access Protocol (CAP)" draft-ietf-calsch-cap-04.txt

Full Copyright Statement

   Copyright (C) The Internet Society (2001).  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
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