RFC 4435

Network Working Group                                          Y. Nomura
Request for Comments: 4435                                 Fujitsu Labs.
Category: Informational                                         R. Walsh
                                                              J-P. Luoma
                                                               H. Asaeda
                                                         Keio University
                                                          H. Schulzrinne
                                                     Columbia University
                                                              April 2006

       A Framework for the Usage of Internet Media Guides (IMGs)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document defines a framework for the delivery of Internet Media
   Guides (IMGs).  An IMG is a structured collection of multimedia
   session descriptions expressed using the Session Description Protocol
   (SDP), SDPng, or some similar session description format.  This
   document describes a generalized model for IMG delivery mechanisms,
   the use of existing protocols, and the need for additional work to
   create an IMG delivery infrastructure.

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Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
      2.1. New Terms ..................................................4
   3. IMG Common Framework Model ......................................5
      3.1. IMG Data Types .............................................5
           3.1.1. Complete IMG Description ............................5
           3.1.2. Delta IMG Description ...............................6
           3.1.3. IMG Pointer .........................................6
      3.2. IMG Entities ...............................................6
      3.3. Operation Set for IMG Delivery .............................7
           3.3.1. IMG ANNOUNCE ........................................7
           3.3.2. IMG QUERY ...........................................8
           3.3.3. IMG RESOLVE .........................................8
           3.3.4. IMG SUBSCRIBE .......................................8
           3.3.5. IMG NOTIFY ..........................................9
           3.3.6. Binding between IMG Operations and Data Types .......9
      3.4. Overview of Protocol Operations ...........................10
   4. Deployment Scenarios for IMG Entities ..........................10
      4.1. Intermediary Cases ........................................10
      4.2. One-to-Many Unidirectional Multicast ......................12
      4.3. One-to-One Bidirectional Unicast ..........................12
      4.4. Combined Operations with Common Metadata ..................13
   5. Applicability of Existing Protocols to the Proposed
      Framework Model ................................................14
      5.1. Existing Standard Fitting the IMG Framework Model .........14
      5.2. IMG Mechanism Needs Which Are Not Met by Existing
           Standards .................................................15
           5.2.1. A Multicast Transport Protocol .....................16
           5.2.2. Usage of Unicast Transport Protocols ...............16
           5.2.3. IMG Envelope .......................................17
           5.2.4. Metadata Data Model ................................18
   6. Security Considerations ........................................18
   7. Normative References ...........................................19
   8. Informative References .........................................19
   9. Acknowledgements ...............................................20

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1.  Introduction

   Internet Media Guides (IMGs) provide and deliver structured
   collections of multimedia descriptions expressed using SDP [2], SDPng
   [3], or other description formats.  They are used to describe sets of
   multimedia services (e.g., television program schedules, content
   delivery schedules) and refer to other networked resources including
   web pages.  IMGs provide an envelope for metadata formats and session
   descriptions defined elsewhere with the aim of facilitating
   structuring, versioning, referencing, distributing, and maintaining
   (caching, updating) such information.

   IMG metadata may be delivered to a potentially large audience, which
   uses it to join a subset of the sessions described and which may need
   to be notified of changes to the IMG metadata.  Hence, a framework
   for distributing IMG metadata in various different ways is needed to
   accommodate the needs of different audiences: For traditional
   broadcast-style scenarios, multicast-based (push) distribution of IMG
   metadata needs to be supported.  Where no multicast is available,
   unicast-based push is required.

   This document defines a common framework model for IMG delivery
   mechanisms and their deployment in network entities.  There are three
   fundamental components in the IMG framework model: data types,
   operation sets, and entities.  These components specify a set of
   framework guidelines for efficient delivery and description of IMG
   metadata.  The data types give generalized means to deliver and
   manage the consistency of application-specific IMG metadata.  IMG
   operations cover broadcast, multicast distribution, event
   notification upon change, unicast-based push, and interactive
   retrievals similar to web pages.

   Since we envision that any Internet host can be a sender and receiver
   of IMG metadata, a host involved in IMG operations performs one or
   more of the roles defined as the entities in the IMG framework model.
   The requirements for IMG delivery mechanisms and descriptions can be
   found in the IMG requirements document [4].

   This document outlines the use of existing protocols to create an IMG
   delivery infrastructure.  It aims to organize existing protocols into
   a common model and show their capabilities and limitations from the
   viewpoint of IMG delivery functions.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [1].

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2.1.  New Terms

   Internet Media Guide (IMG): IMG is a generic term to describe the
      formation, delivery, and use of IMG metadata.  The definition of
      the IMG is intentionally left imprecise [4].

   IMG Element: The smallest atomic element of metadata that can be
      transmitted separately by IMG operations and referenced
      individually from other IMG elements [4].

   IMG Metadata: A set of metadata consisting of one or more IMG
      elements.  IMG metadata describes the features of multimedia
      content used to enable selection of and access to media sessions
      containing content.  For example, metadata may consist of a media
      object's URI, title, airtime, bandwidth needed, file size, text
      summary, genre, and access restrictions [4].

   IMG Description:  A collection of IMG metadata with a data type
      indicating a self-contained set or a subset of IMG metadata, or a
      reference to IMG metadata.

   IMG Delivery: The process of exchanging IMG metadata both in terms of
      large-scale and atomic data transfers [4].

   IMG Sender: An IMG sender is a logical entity that sends IMG metadata
      to one or more IMG receivers [4].

   IMG Receiver: An IMG receiver is a logical entity that receives IMG
      metadata from an IMG sender [4].

   IMG Transceiver: An IMG transceiver combines an IMG receiver and
      sender.  It may modify received IMG metadata or merge IMG metadata
      received from a several different IMG senders [4].

   IMG Operation: An atomic operation of an IMG transport protocol, used
      between IMG sender(s) and IMG receiver(s) for the delivery of IMG
      metadata and for the control of IMG sender(s)/receiver(s) [4].

   IMG Transport Protocol: A protocol that transports IMG metadata from
      an IMG sender to IMG receiver(s) [4].

   IMG Transport Session: An association between an IMG sender and one
      or more IMG receivers within the scope of an IMG transport
      protocol.  An IMG transport session involves a time-bound series
      of IMG transport protocol interactions that provide delivery of
      IMG metadata from the IMG sender to the IMG receiver(s) [4].

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   IMG Transfer: A transfer of IMG metadata from an IMG sender to IMG

3.  IMG Common Framework Model

   Two common elements are found in all existing IMG candidate cases:
   the need to describe the services and the need to deliver the
   descriptions.  In some cases, the descriptions provide multicast
   addresses and thus are part of the transport configuration.  In other
   cases, descriptions are specific to the media application and may be
   meant for either human or machine consumption.  Thus, the
   technologies can be roughly divided into three areas:

      o Application-specific Metadata: data describing the content of
        services and media which are both specific to certain
        applications and generally human readable.

      o Delivery Descriptions: the descriptions (metadata) that are
        essential to enable (e.g., multicast) delivery.  These include
        framing (headers) for application-specific metadata, the
        metadata element identification and structure, and fundamental
        session data.

      o Delivery Protocols: the methods and protocols to exchange
        descriptions between the senders and the receivers.  An IMG
        transport protocol consists of two functions: carrying IMG
        metadata from an IMG sender to an IMG receiver and controlling
        an IMG transport protocol.  These functions are not always
        exclusive; therefore, some messages may combine control messages
        and metadata carriage functions to reduce the amount of the

3.1.  IMG Data Types

   A data model is needed to precisely define the terminology and
   relationships between sets, supersets, and subsets of metadata.  A
   precise data model is essential for the implementation of IMGs,
   although it is not within the scope of this framework and requires a
   separate specification.  However, there are three IMG data types in
   general: Complete IMG Descriptions, Delta IMG Descriptions, and IMG

3.1.1.  Complete IMG Description

   A Complete IMG Description provides a self-contained set of metadata
   for one media object or service, i.e., it does not need additional
   information from any other IMG element.  This is not to be confused
   with "complete IMG metadata", which, although vaguely defined here,

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   represents the complete IMG metadata database of an IMG sender (or
   related group of IMG senders -- potentially the complete Internet IMG
   knowledge).  An IMG sender will generally deliver only subsets of
   metadata from its complete database in a particular IMG transport

3.1.2.  Delta IMG Description

   A Delta IMG Description provides only part of a Complete IMG
   Description, defining the difference from a previous version of the
   Complete IMG Description.  Delta IMG Descriptions may be used to
   reduce network resource usage, for instance, when data consistency is
   essential and small and frequent changes occur to IMG elements.
   Thus, this description does not represent a complete set of metadata
   until it is combined with other metadata that may already exist or
   arrive in the future.

3.1.3.  IMG Pointer

   An IMG Pointer identifies or locates metadata.  This may be used to
   separately obtain metadata (Complete or Delta IMG Descriptions) or
   perform another IMG management function such as data expiry (and
   erasure).  The IMG Pointer may be used to reference IMG metadata
   elements within the IMG transport session and across IMG transport
   sessions.  This pointer type does not include IMG metadata per se
   (although it may also appear as a data field in Complete or Delta IMG

3.2.  IMG Entities

   There are several fundamental IMG entities that indicate the
   capability to perform certain roles.  An Internet host involved in
   IMG operations may adopt one or more of these roles, which are
   defined in more detail in Section 3.3.

   IMG Announcer:  sends IMG ANNOUNCE

   IMG Listener:   receives IMG ANNOUNCE

   IMG Querier:    sends IMG QUERY and receives IMG RESOLVE

   IMG Resolver:   receives IMG QUERY then sends IMG RESOLVE

   IMG Subscriber: sends IMG SUBSCRIBE then receives IMG NOTIFY

   IMG Notifier:   receives IMG SUBSCRIBE then sends IMG NOTIFY

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   Figure 1 shows the relationship between IMG entities and the IMG
   sender and receiver.

        |                      IMG Sender                        |
        |  IMG Announcer   |   IMG Notifier   |    IMG Resolver  |
                |                    ^                  ^
   message      |                    |                  |
   direction    v                    v                  v
        |   IMG Listener   |  IMG Subscriber  |    IMG Querier   |
        |                      IMG Receiver                      |

    Figure 1: Relationship between IMG Entities, Senders, and Receivers

3.3.  Operation Set for IMG Delivery

   A finite set of operations both meets the IMG requirements [4] and
   fits the roles of existing protocols.  These are crystallized in the
   next few sections.


   When an IMG receiver participates in unidirectional communications
   (e.g., over satellite, terrestrial radio, and wired multicast
   networks), an IMG receiver may not need to send any IMG message to an
   IMG sender prior to IMG metadata delivery.  In this case, an IMG
   sender can initiate unsolicited distribution for IMG metadata and an
   IMG sender is the only entity that can maintain the distribution
   (this includes scenarios with multiple and cooperative IMG senders).
   This operation is useful when there are large numbers of IMG
   receivers or the IMG receivers do not have a guaranteed uplink
   connection to the IMG sender.  The IMG sender may also include
   authentication data in the ANNOUNCE operation so that IMG receivers
   may check the authenticity of the metadata.  This operation can carry
   any of the IMG data types.

   There is no restriction to prevent IMG ANNOUNCE from being used in an
   asynchronous solicited manner, where a separate operation (possibly
   out of band) enables IMG receivers to subscribe/register to the IMG
   ANNOUNCE operation.

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3.3.2.  IMG QUERY

   If an IMG receiver needs to obtain IMG metadata, an IMG receiver can
   use an IMG QUERY operation and initiate a receiver-driven IMG
   transport session.  The IMG receiver expects a synchronous response
   to the subsequent request from the IMG sender.  This operation can be
   used where a bidirectional transport network is available between the
   IMG sender and receiver.  Some IMG receivers may want to obtain IMG
   metadata when network connectivity is available or just to avoid
   caching unsolicited IMG metadata.  The IMG receiver must indicate the
   extent and data type of metadata wanted in some message in the
   operation.  The extent indicates the number and grouping of metadata
   descriptions.  In some cases, it may be feasible to request an IMG
   sender's complete metadata collection.


   An IMG sender synchronously responds, and sends IMG metadata, to an
   IMG QUERY that has been sent by an IMG receiver.  This operation can
   be used where a bidirectional transport network is available between
   the IMG sender and receiver.  If the IMG QUERY specifies a subset of
   IMG metadata (extent and data type) that is available to the IMG
   sender, the IMG sender can resolve the query; otherwise, it should
   indicate that it is not able to resolve the query.  The IMG sender
   may authenticate the IMG receiver during the QUERY operation to
   determine if the IMG receiver is authorized to receive that metadata.
   The sender may also include authentication data in the RESOLVE
   operation so that IMG receivers may check the authenticity of the
   metadata.  This operation may carry any of the IMG data types.


   If an IMG receiver wants to be notified when the IMG metadata it
   holds becomes stale, the IMG receiver can use the IMG SUBSCRIBE
   operation in advance in order to solicit IMG NOTIFY messages from an
   IMG sender.

   This operation may provide the IMG sender with specific details of
   which metadata or notification services it is interested in the case
   where the IMG sender offers more than the simplest "all data"
   service.  This operation implicitly provides the functionality of
   unsubscribing to inform an IMG sender that an IMG receiver wishes to
   stop getting certain (or all) notifications.  It should be noted that
   unsubscription may be provided implicitly by the expiry (timeout) of
   a subscription before it is renewed.

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   Since the IMG receiver does not know when metadata will be updated
   and the notify message will arrive, this operation does not
   synchronize with the notify messages.  The IMG receiver may wait for
   notify messages for a long time.  The IMG sender may authenticate the
   IMG receiver to check whether an IMG SUBSCRIBE operation is from an
   authorized IMG receiver.

3.3.5.  IMG NOTIFY

   An IMG NOTIFY is used asynchronously in response to an earlier IMG
   SUBSCRIBE.  An IMG NOTIFY operation indicates that updated IMG
   metadata is available or part of the existing IMG metadata is stale.
   An IMG NOTIFY may be delivered more than once during the time an IMG
   SUBSCRIBE is active.  This operation may carry any of the IMG data
   types.  The IMG sender may also include authentication data in the
   IMG NOTIFY operation so that IMG receivers may check the authenticity
   of the messages.

3.3.6.  Binding between IMG Operations and Data Types

   There is a need to provide a binding between the various IMG
   operations and IMG data types to allow management of each discrete
   set of IMG metadata transferred using an IMG operation.  This binding
   must be independent of any particular metadata syntax used to
   represent a set of IMG metadata, as well as be compatible with any
   IMG transport protocol.  The binding must uniquely identify the set
   of IMG metadata delivered within an IMG transfer, regardless of the
   metadata syntax used.  The uniqueness may only be needed within the
   domains the metadata is used, but this must enable globally unique
   identification to support Internet usage.  Care should be taken that
   scope- and domain-specific identifiers do not leak outside the scope;
   using globally unique identifiers such as URLs avoids these problems.

   The binding must provide versioning to the transferred IMG metadata
   so that changes can be easily handled and stale data identified, and
   give temporal validity of the transferred IMG metadata.  It must
   invalidate the IMG metadata by indicating an expiry time, and may
   optionally provide a time (presumably in the future) from when the
   IMG metadata becomes valid.  The temporal validity of a metadata
   object may need to be updated later.  Furthermore, the binding must
   be independent of any specific metadata syntax used for the IMG
   metadata, in the sense that no useful syntax should be excluded.

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3.4.  Overview of Protocol Operations

   Figure 2 gives an overview of the relationship between transport
   cases, IMG operations, and IMG data types.  It is not a protocol
   stack.  Generalized multicast point-to-multipoint (P-to-M) and
   unicast point-to-point (P-to-P) transports are shown.

    IMG        |                                                  |
    Data Types |       Complete Desc., Delta Desc., Pointer       |
               |                                                  |
    Operations |                   |  IMG NOTIFY    | IMG RESOLVE |
    IMG        |              |                                   |
    Transport  |   P-to-M     |              P-to-P               |
               |              |                                   |

        Figure 2: IMG Transport, IMG Operations, and IMG Data Types

4.  Deployment Scenarios for IMG Entities

   This section provides some basic deployment scenarios for IMG
   entities that illustrate common threads from protocols to use cases.
   For the purposes of clarity, this document presents the simple
   dataflow from an IMG sender to an IMG receiver, as shown in Figure 3.

            +-------------+                +---------------+
            | IMG Sender  |                | IMG Receiver  |
            |             |--------------->|               |
            +-------------+                +---------------+

        Figure 3: A Simple IMG Sender to IMG Receiver Relationship

4.1.  Intermediary Cases

   Some IMG metadata may be distributed to a large number of IMG
   receivers, for example, when public metadata is distributed to all
   IMG receivers of a certain group.  This kind of IMG metadata may be
   distributed from one IMG sender to multiple IMG receivers (Figure 4)
   or may be relayed across a set of IMG transceivers that receive the
   IMG metadata, possibly filter or modify its content, and then forward

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    +----------+                                    +----------+
    | IMG      |                                    | IMG      |
    | Sender   |----                           ---->| Receiver |
    +----------+    \                         /     +----------+
                     \                       /
         .            \   +-----------+     /            .
         .             -->|IMG        |-----             .
         .             -->|Transceiver|     \            .
                      /   +-----------+      \
    +----------+     /                        \     +----------+
    | IMG      |    /                          ---->| IMG      |
    | Sender   |----                                | Receiver |
    +----------+                                    +----------+

             Figure 4: A Relay Network with an IMG Transceiver

   IMG senders and receivers are logical functions, and it is possible
   for some or all hosts in a system to perform both roles, as, for
   instance, in many-to-many communications or where a transceiver is
   used to combine or aggregate IMG metadata for some IMG receivers.  An
   IMG receiver may be allowed to receive IMG metadata from any number
   of IMG senders.

   IMG metadata is used to find, obtain, manage, and play media content.
   IMG metadata may be modified during IMG transfer.  For example, a
   server may use IMGs to retrieve media content via unicast and then
   make it available at scheduled times via multicast, thus requiring a
   change of the corresponding metadata.  IMG transceivers may add or
   delete information or aggregate IMG metadata from different IMG
   senders.  For example, a rating service may add its own content
   ratings or recommendations to existing metadata.  An implication of
   changing (or aggregating) IMG metadata from one or more IMG senders
   is that the original authenticity is lost.  Thus, it may be
   beneficial to sign fragments so that the intermediary can replace a
   fragment without changing the authenticity of the remainder.  For
   example, smaller fragments may be appropriate for more volatile
   parts, and larger ones may be appropriate for stable parts.

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4.2.  One-to-Many Unidirectional Multicast

   The one-to-many unidirectional multicast case implies many IMG
   receivers and one or more IMG senders implementing IMG announcer and
   IMG listener operations as shown in Figure 5.

                   Unidirectional            +----------+
                  --------------->           |   IMG    |
                      downlink               | Listener |
                               ------------->|    1     |
                              /              +----------+
        +-----------+        /                    .
        | IMG       |--------                     .
        | Announcer |        \                    .
        +-----------+         \              +----------+
                               ------------->|   IMG    |
                                             | Listener |
                                             |    #     |

        Figure 5: IMG Unidirectional Multicast Distribution Example

   Note, as defined in the IMG requirement REL-4 [4], an IMG transport
   protocol MUST support reliable message exchange.  This includes the
   one-to-many unidirectional multicast case; however, the mechanism to
   provide this is beyond the scope of this document.

4.3.  One-to-One Bidirectional Unicast

   In the one-to-one bidirectional unicast case, both query/resolve
   (Figure 6) and subscribe/notify (Figure 7) message exchange
   operations are feasible.

             +----------+                +----------+
             |   IMG    |                |   IMG    |
             | Resolver |                | Querier  |
             +----------+                +----------+
                 |                                |
                 |<----------IMG QUERY -----------|
                 |                                |
                 |----------IMG RESOLVE---------->|
                 |                                |

             Figure 6: Query/Resolve Sequence Example

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            +----------+                   +------------+
            |   IMG    |                   |    IMG     |
            | Notifier |                   | Subscriber |
            +----------+                   +------------+
                 |                                |
                 |<---------IMG SUBSCRIBE---------|
                 :                                :
                            (time passes)
                 :                                :
                 |-----------IMG NOTIFY---------->|
                 :                                :
                            (time passes)
                 :                                :
                 |-----------IMG NOTIFY---------->|
                 |                                |

                Figure 7: Subscribe/Notify Sequence Example

4.4.  Combined Operations with Common Metadata

   As shown in Figure 8, a common data model for multiple protocol
   operations allows a diverse range of IMG senders and receivers to
   provide consistent and interoperable sets of IMG metadata.

    IMG Metadata             IMG Senders             IMG Receivers

                             +-----------+      ---->| IMG Listener |
                             | IMG       |     /     +--------------+
                            /| Announcer |-----
    +-------------+        / +-----------+     \     +--------------+
    |    IMG      |-+     /                     ---->| IMG Listener |
    | description | |-+  /                           | - - - - - - -|
    | metadata  1 | | | /    +-----------+      /--->| IMG Querier  |
    +-------------+ | | -----| IMG       |<----/     +--------------+
      +-------------+ | \    | Resolver  |
        +-------------+  \   +-----------+<----\     +--------------+
                          \                     \--->| IMG Querier  |
                           \ +-----------+           | - - - - - - -|
                            \| IMG       |<--------->| IMG          |
                             | Notifier  |           | Subscriber   |
                             +-----------+           +--------------+

              Figure 8: Combined System with Common Metadata

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5.  Applicability of Existing Protocols to the Proposed Framework Model

5.1.  Existing Standards Fitting the IMG Framework Model

   SDP: The SDP format [2] could be used to describe session-level
   parameters (e.g., scheduling, addressing, and the use of media
   codecs) to be included in Complete IMG Descriptions.  Although there
   are extension points in SDP allowing the format to be extended, there
   are limitations in the flexibility of this extension mechanism.
   However, SDP syntax cannot provide IMG Descriptions and IMG Pointers
   without significant overhead.  It is expected that the information
   conveyed by SDP is just a small subset of IMG metadata; thus, the use
   of SDP for other than session parameters may not be reasonable.

   SDPng [3]: Similar to SDP, this format could also be used for
   representing session-level parameters of IMG metadata.  Compared to
   SDP, the XML-based format of SDPng should be much more flexible and
   allow extensions and integration with other description formats.

   MPEG-7: Descriptions based on the MPEG-7 standard [5] could provide
   application-specific metadata describing the properties of multimedia
   content beyond parameters carried in SDP or SDPng descriptions.
   MPEG-7 provides a machine-readable format of representing content
   categories and attributes, helping end-users or receiving software in
   choosing content for reception.  MPEG-7 is based on XML, so it is
   well suited to be combined with other XML-based formats such as

   TV-Anytime: The TV-Anytime Forum [6] provides descriptions based on
   XML schema for TV-specific program guides.  TV-Anytime uses the
   MPEG-7 User description profile to a limited extent, only for user
   preferences and usage history, and also a TV-Anytime-specific data
   model for other schema.  These are optimized to describe broadcast
   schedules, on-demand program guides and program events.

   HTTP: The HTTP protocol [7] can be used as a bidirectional unicast
   IMG transport protocol.  Being a request-reply-oriented protocol,
   HTTP is well suited for implementing synchronous operations such as
   QUERY, RESOLVE, and even SUBSCRIBE.  However, HTTP does not provide
   asynchronous operations such as ANNOUNCE and NOTIFY and to implement
   asynchronous operations using HTTP, IMG receivers should poll the IMG
   sender periodically.  Thus, by itself, HTTP is not sufficient to
   fulfill all of the IMG requirements [4] in a unicast deployment.

   Session Announcement Protocol (SAP): The announcement mechanism
   provided by SAP [8] provides unidirectional delivery of session
   discovery information.  Although SDP is the default payload format of
   SAP, the use of a MIME type identifier for the payload allows

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   arbitrary payload formats to be used in SAP messages.  Thus, SAP
   could be used to implement the multicast and unicast IMG ANNOUNCE and
   IMG NOTIFY operations.

   However, SAP lacks scalable and efficient reliability, extensibility
   for payload size, and congestion control, and only one description is
   allowed per SAP message due to lack of payload segmentation.

   In principle, SAP could be extended to get around its limitations.
   However, the amount of changes needed in SAP to address all of the
   above limitations would effectively result in a new protocol.  Due to
   these limitations, the use of SAP as an IMG transport protocol is not

   SIP: The SIP-specific event mechanism described in RFC 3265 [9]
   provides a way to implement IMG SUBSCRIBE and IMG NOTIFY operations
   via a bidirectional unicast connection.  However, there are
   scalability problems with this approach, as RFC 3265 currently does
   not consider multicast.

   Real Time Streaming Protocol (RTSP): The RTSP protocol [10] defines a
   retrieval-and-update notification mechanism, named DESCRIBE and
   ANNOUNCE, for the description of a presentation or media object in
   order to initialize a streaming session.  These methods are a subset
   of the entire streaming control operations in RTSP; thus, these could
   not be available for individual mechanisms.  However, the DESCRIBE
   method in RTSP could be used to instantiate IMG QUERY, IMG RESOLVE,
   and IMG SUBSCRIBE, and the RTSP ANNOUNCE could be used to instantiate
   an IMG NOTIFY for a streaming session controlled by RTSP.

5.2.  IMG Mechanism Needs Which Are Not Met by Existing Standards

   Several needs result from the IMG requirements, framework model, and
   existing relevant mechanisms as already shown in this document.  Four
   specific groupings of work are readily apparent: (a) specification of
   an adequate multicast- and unidirectional-capable announcement
   protocol; (b) specification of the use of existing unicast protocols
   to enable unicast subscribe and announcement/notification
   functionality; (c) specification of the metadata envelope that is
   common to, and independent of, the application metadata syntax(es)
   used; and (d) agreement on basic metadata models to enable
   interoperability testing of the above.  The following sections
   describe each of these.

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5.2.1.  A Multicast Transport Protocol

   SAP is currently the only open standard protocol suited to the
   unidirectional/multicast delivery of IMG metadata.  As discussed, it
   fails to meet the IMG requirements in many ways and, since it is not
   designed to be extensible, we recognize that a new multicast
   transport protocol for announcements needs to be specified to meet
   IMG needs.  This protocol will be essential to IMG delivery for
   unidirectional and multicast deployments.

   The Asynchronous Layered Coding (ALC) [11] protocol from the IETF
   Reliable Multicast Transport (RMT) working group is very interesting
   as it fulfills many of the requirements, is extensible, and has the
   ability to 'plug-in' both FEC (Forward Error Correction, for
   reliability) and CC (Congestion Control) functional blocks.  It is
   specifically designed for unidirectional multicast object transport.
   ALC is not fully specified, although the RMT working group had a
   fully specified protocol using ALC called FLUTE (File Delivery over
   Unidirectional Transport) [12].  FLUTE seems to be the only fully
   specified transport and open specification on which a new IMG
   announcement protocol could be designed.  Thus, we recommend that ALC
   and FLUTE be the starting points for this protocol's design.

   Developing a new protocol from scratch, or attempting to improve SAP,
   is also feasible, although it would involve repeating many of the
   design processes and decisions already made by the IETF for ALC.  In
   particular, any announcement protocol must feature sufficient
   scalability, flexibility, and reliability to meet IMG needs.  Also,
   the IMG ANNOUNCE operation must be supported and IMG NOTIFY
   capability could be investigated for both hybrid unicast-multicast
   and unidirectional unicast systems.

5.2.2.  Usage of Unicast Transport Protocols

   A thorough description of the use of existing unicast protocols is
   essential for the use of IMGs in a unicast point-to-point
   environment.  Such a specification has not been published, although
   several usable unicast transport protocols and specifications can be
   harnessed for this (SIP [13], SIP events [9], HTTP [7], etc.).  In
   particular, both IMG SUBSCRIBE-NOTIFY and IMG QUERY-RESOLVE operation
   pairs must be enabled.  We anticipate that the IMG QUERY-RESOLVE
   operation can be achieved using HTTP, although other transport
   protocol options may be beneficial to consider too.

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5.2.3.  IMG Envelope

   An IMG envelope provides the binding between IMG operations and data
   types.  Such a binding can be realized by defining a common minimal
   set of information needed to manage IMG metadata transfers, and by
   including this information with any set of IMG metadata delivered to
   IMG receivers.

   Four options for IMG metadata transfer envelope delivery are

      1.  Embedding in a transport protocol header.  This can be done
          with either header extensions of existing protocols, or newly
          defined header fields of a new (or new version of a) transport
          protocol.  However, multiple methods for the variety of
          transport protocols would hinder interoperability and
          transport protocol independence.

      2.  A separate envelope object, which points to the IMG metadata
          'object', delivered in-band with the metadata transport
          protocol session.  This might complicate delivery as the
          envelope and 'service' metadata objects would have to be
          bound, e.g., by pairing some kind of transport object numbers
          (analogous to port number pairs sometimes used for RTP and
          RTCP [14]).  This would also enable schemes that deliver
          envelope and metadata 'objects' by different media, also using
          more than a single transport protocol.

      3.  A metadata wrapper that points to and/or embeds the service
          metadata into its 'super-syntax'.  For example, XML would
          enable embedding generic text objects.

      4.  Embedding in the metadata itself.  However, this requires a
          new field in many metadata syntaxes and would not be feasible
          if a useful syntax were not capable of extensibility in this
          way.  It also introduces a larger 'implementation
          interpretation' variety, which would hinder interoperability.
          Thus, this option is not recommended.

   It is likely that more than one of these options will fulfill the
   needs of IMGs, so the selection, and possibly optimization, is left
   for subsequent specification and feedback from implementation
   experience.  Such a specification is essential for IMG delivery.

   When there are superset/subset relations between IMG Descriptions, it
   is assumed that the IMG Descriptions of the subset inherit the
   parameters of the superset.  Thus, an IMG metadata transfer envelope
   carrying the IMG Descriptions of a superset may implicitly define

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   parameters of IMG Descriptions belonging to its subset.  The
   relations between IMG Descriptions may span from one envelope to
   another according to a data model definition.

5.2.4.  Metadata Data Model

   A structured data model would allow reuse and extension of the set of
   metadata and may enable use of multiple syntaxes (SDP, MPEG-7, etc.)
   as part of the same body of IMG metadata.

   For the successful deployment of IMGs in various environments,
   further work may be needed to define metadata and data models for
   application-specific requirements.  Existing (and future) work on
   these would need to be mapped to the IMG data types and use of the
   IMG transfer envelope concept as described above.

   This document is a framework for the delivery of IMG metadata and
   thus further discussion on the definition data models for IMGs is
   beyond its scope.

6.  Security Considerations

   The IMG framework is developed from the IMG requirements document
   [4], and so the selection of specific protocols and mechanism for use
   with the IMG framework must also take into account the security
   considerations of the IMG requirements document.  This framework
   document does not mandate the use of specific protocols.  However, an
   IMG specification would inherit the security considerations of
   specific protocols used.

   Protocol instantiations that are used to provide IMG operations will
   have very different security considerations depending on their scope
   and purpose.  However, there are several general issues that are
   valuable to consider and, in some cases, provide technical solutions
   for.  These are described below.

   Individual and group privacy: Customized IMG metadata may reveal
   information about the habits and preferences of a user and may thus
   deserve confidentiality protection, even if the original information
   were public.  Protecting this metadata against snooping requires the
   same actions and measures as for other point-to-point and multicast
   Internet communications.  Naturally, the risk of snooping depends on
   the amount of individual or group personalization the IMG metadata

   IMG authenticity: In some cases, the IMG receiver needs to be assured
   of the sender or origin of IMG metadata or its modification history.
   This can prevent denial-of-service or hijacking attempts that give an

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   IMG receiver incorrect information in or about the metadata, thus
   preventing successful access of the media or directing the IMG
   receiver to the incorrect media possibly with tasteless material.

   IMG receiver authorization: Some or all of any IMG sender's metadata
   may be private or valuable enough to allow access to only certain IMG
   receivers and thus make it worth authenticating users.  Encrypting
   the data is also a reasonable step, especially where group
   communications methods results in unavoidable snooping opportunities
   for unauthorized nodes.

   Unidirectional specifics: A difficulty that is faced by
   unidirectional delivery operations is that many protocols providing
   application-level security are based on bidirectional communications.
   The application of these security protocols in case of strictly
   unidirectional links is not considered in the present document.

   Malicious code: Currently, IMGs are not envisaged to deliver
   executable code at any stage.  However, as some IMG transport
   protocols may be capable of delivering arbitrary files, it is
   RECOMMENDED that the IMG operations do not have write access to the
   system or any other critical areas.

7.  Normative References

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

8.  Informative References

   [2]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [3]  Kutscher, D., Ott, J., and C. Bormann, "Session description and
        capability negotiation", Work in Progress, October 2003.

   [4]  Nomura, Y., Walsh, R., Luoma, J-P., Ott, J., and H. Schulzrinne,
        "Requirements for Internet Media Guides", Work in Progress,
        December 2005.

   [5]  "Multimedia content description interface -- Part 1: Systems",
        ISO/IEC 15938-1, July 2002.

   [6]  TV-Anytime Forum, "Broadcast and On-line Services: Search,
        select, and rightful use of content on personal storage systems
        ("TV-Anytime Phase 1"); Part 2: System description," ETSI-TS 102
        822-2: System Description, V1.1.1, October 2003.

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   [7]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
        Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
        HTTP/1.1", RFC 2616, June 1999.

   [8]  Handley, M., Perkins, C., and E. Whelan, "Session Announcement
        Protocol", RFC 2974, October 2000.

   [9]  Roach, A., "Session Initiation Protocol (SIP)-Specific Event
        Notification", RFC 3265, June 2002.

   [10] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
        Protocol (RTSP)", RFC 2326, April 1998.

   [11] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., and J.
        Crowcroft, "Asynchronous Layered Coding (ALC) Protocol
        Instantiation", RFC 3450, December 2002.

   [12] Paila, T., Luby, M., Lehtonen, R., Roca, V., and R. Walsh,
        "FLUTE - File Delivery over Unidirectional Transport", RFC 3926,
        October 2004.

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

   [14] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications", STD 64,
        RFC 3550, July 2003.

9.  Acknowledgements

   The authors would like to thank Spencer Dawkins, Jean-Pierre Evain,
   Ted Hardie, Petri Koskelainen, Joerg Ott, Colin Perkins, Toni Paila,
   and Magnus Westerlund for their excellent ideas and input to this

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Authors' Addresses

   Yuji Nomura
   Fujitsu Laboratories Ltd.
   4-1-1 Kamikodanaka, Nakahara-ku, Kawasaki 211-8588

   EMail: nom@flab.fujitsu.co.jp

   Rod Walsh
   Nokia Research Center
   P.O. Box 100, FIN-33721 Tampere

   EMail: rod.walsh@nokia.com

   Juha-Pekka Luoma
   Nokia Research Center
   P.O. Box 100, FIN-33721 Tampere

   EMail: juha-pekka.luoma@nokia.com

   Hitoshi Asaeda
   Keio University
   Graduate School of Media and Governance
   5322 Endo, Fujisawa, 252-8520 Kanagawa,

   EMail: asaeda@wide.ad.jp

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027

   EMail: schulzrinne@cs.columbia.edu

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Full Copyright Statement

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