Internet Engineering Task Force                                MMUSIC WG
Internet Draft                                                 Y. Nomura
                                                           Fujitsu Labs.
                                                                R. Walsh
                                                              J-P. Luoma
                                                               H. Asaeda
                                                          H. Schulzrinne
                                                     Columbia University
December 2, 2003
Expires: March 2004

           A Framework for the Usage of Internet Media Guides


   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
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   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at

   To view the list Internet-Draft Shadow Directories, see


   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 SDP, SDPng or some similar
   session description format. This document describes several use case
   scenarios requirering the IMG framework, a generalized model for IMG
   delivery mechanisms, and the use of existing protocol to create an
   IMG delivery infrastructure.

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Table of Contents
   1          Introduction ........................................    3
   1.1        Background and Motivation ...........................    3
   1.2        Scope of this Document ..............................    4
   2          Terminology .........................................    4
   3          Use Cases Requiring IMG Framework ...................    5
   3.1        Connectivity-based Use Cases ........................    5
   3.1.1      IP Datacast to a Wireless Receiver ..................    5
   3.1.2      Regular Fixed Dial-up Internet Connection ...........    6
   3.1.3      Broadband Always-on Fixed Internet Connection .......    6
   3.2        Content-orientated Use Cases ........................    6
   3.2.1      File Distribution ...................................    7
   3.2.2      TV and Radio Program Delivery .......................    7
   3.2.3      Media Coverage of a Live Event ......................    8
   3.2.4      Distance Learning ...................................    8
   3.2.5      Multiplayer Gaming ..................................    8
   4          IMG Common Framework Model ..........................    8
   4.1        IMG Data Types ......................................    9
   4.1.1      Complete Description ................................    9
   4.1.2      Delta Description ...................................    9
   4.1.3      Pointer .............................................   10
   4.2        Operation Set for IMG Delivery ......................   10
   4.2.1      IMG ANNOUNCE ........................................   10
   4.2.2      IMG QUERY ...........................................   10
   4.2.3      IMG RESOLVE .........................................   11
   4.2.4      IMG SUBSCRIBE .......................................   11
   4.2.5      IMG NOTIFY ..........................................   11
   4.3        IMG Entities ........................................   12
   4.4        Overview of Protocol Operations .....................   12
   5          Deployment Scenarios for IMG Entities ...............   13
   5.1        Intermediary Cases ..................................   13
   5.2        One-to-many Unidirectional Multicast ................   15
   5.3        One-to-one Bi-directional Unicast ...................   15
   5.4        Combined Operations with Common Metadata ............   16
   6          Applicability of Existing Protocols to the
   Proposed Framework Model .......................................   17
   6.1        Summary of Limitations of Existing Protocols ........   17
   6.2        Existing Protocol Fit to the IMG Framework Model
   6.3        Outstanding IMG Mechanism Needs .....................   19
   6.3.1      A Multicast Transport Protocol ......................   19
   6.3.2      Usage of Unicast Transport Protocols ................   20
   6.3.3      The Metadata Envelope ...............................   20
   6.3.4      Baseline (Meta)Data Model Specification .............   21
   6.4        IMG Needs Fitting the IETF's Scope ..................   22
   7          Security Considerations .............................   23
   8          Normative References ................................   24
   9          Informative References ..............................   25
   10         Acknowledgements ....................................   26

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   11         Authors' Addresses ..................................   26

1 Introduction

1.1 Background and Motivation

   Internet Media Guide (IMG) provide structured collections of
   multimedia descriptions expressed using SDP, SDPng or some similar
   description format. It is used to describe sets of multimedia
   sessions (e.g. television program schedules, content delivery
   schedules etc.) 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.

   Firstly, this document explains several use case scenarios requiring
   the IMG framework. IMGs are inherently required to be independent of
   any particular access network, and link in general.  Therefore, they
   are suitable in many Internet access scenarios including fixed and
   mobile devices, wired and satellite and terrestrial radio, always-on
   Internet and intermittent connectivity, and so on. Furthermore, IMGs
   provides essential functions that facilitate better distribution of
   content. Section 3 describes how IMGs and IMG delivery mechanisms
   contribute for the scenarios.

   Then, this document defines a generalized model for IMG delivery
   mechanisms and their deployment in network entities regarding the use
   case scenarios. IMG metadata must be delivered to a potentially large
   audience, who use it to join a subset of the sessions described, and
   who 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, too.

   Finally, this document outlines the use of existing protocols to
   create an IMG delivery infrastructure. It aims to organize existing
   protocols into common model and show their capabilities and
   limitations from the view point of IMG delivery functions. One of the
   multicast-enabling IMG requirements is scaling well to a large number
   of hosts and IMG senders in a network. Another issue is the need for
   flexibility and diversity in delivery methods, whereas existing
   protocols tend to be bound to a specific application.

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1.2 Scope of this Document

   This document defines a common framework model for the delivery of
   Internet Media Guide (IMG) metadata. The framework describes existing
   mechanisms and the level to which they support and enable the
   framework. The requirements for IMG delivery mechanisms and
   descriptions can be found in [1].

   A brief run through the usage and use cases of media guide is
   provided to illustrate the relevance of IMGs before the framework
   model is presented. The framework model defines the data types,
   operations and entities which are needed. These are then shown in a
   number of simplified deployment scenarios.

   Existing protocols are organized and referenced against the framework
   model to show the degree to which they fulfill IMG framework and
   requirements. This also makes it straightforward to identify gaps so
   that new protocols needs are made apparent.

2 Terminology

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

        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.

        IMG Metadata: This is a set of metadata describing the features
             of multimedia content used to enable selection of and
             access to media sessions containing content. For example,
             metadata may consist of the URI, title, airtime, bandwidth
             needed, file size, text summary, genre, and access

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

        IMG Sender: An IMG sender is a logical entity that sends IMGs to
             one or more IMG receivers.

        IMG Receiver: An IMG receiver is a logical entity that receives
             IMGs from an IMG sender.

        IMG Transceiver: An IMG transceiver combines an IMG receiver and
             sender. It may modify original IMGs or merge several IMGs
             from a different IMG sender.

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        IMG Operation: An atomic process for the IMG transport protocol
             to deliver IMG metadata or control IMG sender(s) or IMG

        IMG Transport Protocol: A protocol that transports IMG metadata
             from IMG sender to IMG receiver(s)

3 Use Cases Requiring IMG Framework

3.1 Connectivity-based Use Cases

3.1.1 IP Datacast to a Wireless Receiver

   IP Datacast is the delivery of IP-based services over broadcast
   radio. Internet content delivery is therefore unidirectional in this
   case. However, there can be significant benefits from being able to
   provide rich media one-to-many services to such receivers.

   There are two main classes of receiver in this use case: fixed
   mains-powered; and mobile battery-powered. Both of these are affected
   by radio phenomena and so robust, or error-resilient, delivery is
   important. Carouselled metadata transfer provides a base level of
   robustness for an IP datacast based announcement system, although the
   design of carouselled transfer should enable battery-powered
   receivers to go through periods of sleep to extend their operational
   time between charges. Insertion of Forward Error Correction (FEC)
   data into metadata announcements improves error resilience, and
   reordering (interleaving) data blocks further increases tolerance to
   burst errors.

   To enable receivers to more accurately specify the metadata they are
   interested in, the unidirectional delivery is distributed between
   several logical channels. This is so that a receiver need only access
   the channels of interest and thus can reduce the amount of time and
   processing of IP data (and storage). Also, hierarchical channels
   enable receivers to subscribe to a root, possibly well known,
   multicast channel/group and progressively access only those
   additional channels based on metadata in parent channels.

   In some cases the receiver may be multi-access, such that it is
   capable of bi-directional communications. This enables a multitude of
   options, but most importantly it enables NACK based reliability and
   the individual retrieval of missed or not-multicasted sets of

   Thus, essential IMG features in this case include: robust
   unidirectional delivery (with optional levels of reliability
   including "plug-in FEC") which implies easily identifiable

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   segmentation of delivery data to enable FEC, carousel, interleaving
   and other schemes possible; effective identification of metadata sets
   (probably uniquely) to enable more efficient use of multicast and
   unicast retrieval over multiple access systems regardless of the
   parts of metadata and application specific extensions in use;
   prioritization of metadata, which can (for instance) be achieved by
   spreading it between channels and allocating/distributing bandwidth

   Furthermore, some cases require IMG metadata authentication and some
   group security/encryption and supporting security message exchanges
   (out of band from the IMG multicast sessions).

3.1.2 Regular Fixed Dial-up Internet Connection

   Dial-up connections tend to be reasonably slow (<56kbps in any case)
   and thus large data transfers are less feasible, especially during an
   active application session (such as a file transfer described by IMG
   metadata). They can also be intermittent, especially if a user is
   paying for the connected time, or connected through a less reliable
   exchange. Thus this favors locally stored IMG metadata over web-based
   browsing, especially where parts of the metadata change infrequently.
   There may be no service provider preference over unicast and
   multicast transport for small and medium numbers of users as the
   last-mile dial-up connection limits per-user congestion, and a user
   may prefer the more reliable option (unicast unless reliable
   multicast is provided).

3.1.3 Broadband Always-on Fixed Internet Connection

   Typically bandwidth is less of an issue to a broadband user and
   unicast transport, such as using IMG QUERY, may be typical for a PC
   user. If a system were only used in this context, with content
   providers, ISPs and users having no other requirements, then web-
   based browsing may be equally suitable. However, broadband users
   sharing a local area network, especially wireless, may benefit more
   from local storage features than on-line browsing, especially if they
   have intermittent Internet access.

   Broadband enables rich media services, which are increasingly
   bandwidth hungry. Thus backbone operators may prefer multicast
   communications to reduce overall congestion, if they have the
   equipment and configuration to support this. Thus, broadband users
   may be forced to retrieve IMG metadata over multicast if the
   respective operators require this to keep system-wide bandwidth usage

3.2 Content-orientated Use Cases

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   IMGs will be able to support a very wide range of use cases for
   content/media delivery. The following few sections just touch the
   surface of what is possible and are intended to provide an
   understanding of the scope and type of IMG usage. Many more examples
   may be relevant, for instance those detailed in[3].  There are
   several unique features of IMGs that set them apart from related
   application areas such as SLP based service location discovery, LDAP
   based indexing services and search engines such as Google. Features
   unique to IMGs include:

        o IMG metadata is generally time-related

        o There are timeliness requirements in the delivery of IMG

        o IMG metadata may be updated as time elapses or when an event

3.2.1 File Distribution

   IMGs support the communication of file delivery session properties,
   enabling the scheduled delivery or synchronization of files between a
   number of hosts. For example, the metadata that IMGs provide could be
   subsequently used by a different application (outside the scope of
   IMGs) to download a file with a software update. An IMG metadata can
   provide a description to each file in a file delivery session,
   assisting users or receiving software in selecting individual files
   for reception.

   For example, when a content provider wants to distribute a large
   amount of data in file format to thousands clients, the content
   provider can use IMG metadata to schedule the delivery effectively.
   Since IMG metadata can describe time-related data for each receiver,
   the content provider can schedule delivery time for each receiver.
   This can save network bandwidth and capacity of delivery senders. In
   addition, IMG metadata can be used to synchronize a set of files
   between a sender host and receiver host, when those files change as
   time elapses.

3.2.2 TV and Radio Program Delivery

   A sender of audio/video streaming content can use the IMG metadata to
   describe the scheduling and other properties of audio/video sessions
   and events within those sessions, such as individual TV and radio
   programs and segments within those programs. IMG metadata describing
   audio/video streaming content could be represented in a format
   similar to that of a TV guide in newspapers, or an Electronic Program
   Guide available on digital TV receivers.

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   Similarly to file reception, TV and radio programs can be selected
   for reception either manually by the end-user or automatically based
   on the content descriptions and the preferences of the user. The
   received TV and radio content can be either presented in real time or
   recorded for consuming later. There may be changes in the scheduling
   of a TV or a radio program, possibly affecting the transmission times
   of subsequent programs. IMG metadata can be used to notify receivers
   of such changes, enabling users to be prompted or recording times to
   be adjusted.

3.2.3 Media Coverage of a Live Event

   The media coverage of a live event such as a rock concert or a sports
   event is a special case of regular TV/radio programming. There may be
   unexpected changes in the scheduling of live event, or the event may
   be unscheduled to start with (such as breaking news). In addition to
   audio/video streams, textual information relevant to the event (e.g.
   statistics of the players during a football match) may be sent to
   user terminals. Different transport modes or even different access
   technologies could be used for the different media: for example, a
   unidirectional datacast transport could be used for the audio/video
   streams and an interactive cellular connection for the textual data.
   IMG metadata should enable terminals to discover the availability of
   different media used to cover a live event.

3.2.4 Distance Learning

   IMG metadata could describe compound sessions or services enabling
   several alternative interaction modes between the participants. For
   example, the combination of one-to-many media streaming, unicast
   messaging and download of presentation material could be useful for
   distance learning.

3.2.5 Multiplayer Gaming

   Multiplayer games are an example of real-time multiparty
   communication sessions that could be advertised using IMGs. A gaming
   session could be advertised either by a dedicated server, or by the
   terminals of individual users. A user could use IMGs to learn of
   active multiplayer gaming sessions, or advertise the users interest
   in establishing such a session.

4 IMG Common Framework Model

   Two common elements are found in all of existing IMG candidate cases:
   the need to describe the services; the need to deliver the
   descriptions. In some cases, the descriptions are multicast enablers
   (such as the session parameters of SDP) and are thus intrinsically

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   part of the delivery aspects, and in other cases descriptions are
   application-specific (both machine and human readable). Thus, the
   technologies can be roughly divided into three areas:

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

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

        o Delivery Protocols -- the methods and protocols to exchange
          descriptions between the senders and the receivers.  An IMG
          delivery 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 metadata to reduce
          the amount of the messaging.

4.1 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 description, delta description and pointer.

4.1.1 Complete Description

   A Complete Description provides a complete syntax and semantics to
   describe a set of metadata, which does not need any additional
   information from other IMG entity.

   Note, this is not to be confused with "complete IMG metadata", which,
   although vaguely defined here, represents the complete database of a
   sender (or related group of senders -- potentially the complete
   Internet IMG knowledge). A sender will generally deliver only subsets
   of metadata from its complete database of metadata in a particular
   data exchange.

4.1.2 Delta Description

   A Delta Description provides only part of a Complete Description,

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   defining the difference from a previous version of the Complete
   Description in question. This descriptor may be used to reduce
   network resource usage (it may be more bandwidth and congestion
   friendly), for instance, data consistency is essential and, small and
   frequent changes occur to the Complete Description. Thus, this
   descriptor itself cannot represent complete metadata set until it is
   combined with existing, or future, description knowledge.

4.1.3 Pointer

   A pointer provided a simple identifier or locator, such as a URL,
   that the receiver is able to reference (or reference and locate)
   specific metadata with. This may be used to separately obtain
   metadata (complete or delta descriptions) or perform another IMG
   management function such as data expire (and erasure). The pointer
   may be used to reference IMG metadata elements within the IMG
   transport session and across IMG transport sessions. The pointer does
   not include metadata par se (although it may also appear as a data
   field in complete or delta descriptors).

4.2 Operation Set for IMG Delivery

   A finite set of operations both meets the IMG requirements [2] 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 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 which can maintain the distribution (this includes
   scenarios with multiple and co-operative senders). This operation is
   useful when there are considerably large number IMG receivers or IMG
   receiver(s) do not have a guaranteed uplink connection to the IMG
   sender(s). The sender may also include authentication data in the
   announce operation so that receivers may check the authenticity of
   the metadata. This operation is able to carry any of the IMG data-

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


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   If an IMG receiver needs to obtain IMG metadata, an IMG receiver can
   send an IMG QUERY message and initiate a receiver-driven IMG delivery
   session. The IMG receiver expects a synchronous response to the
   subsequent request from the IMG sender. This operation can be used
   where a bi-directional transport network is available between the IMG
   sender and receiver. Some IMG receivers may want to obtain IMG
   metadata when a resource 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
   (Extent indicates the number and grouping of metadata descriptions).
   In some cases requesting a sender's complete IMG metadata may be
   feasible, in others it may not.


   An IMG sender synchronously responds and sends IMG metadata to an IMG
   QUERY which has been sent by an IMG receiver. This operation can be
   used where a bi-directional 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 the IMG sender has the
   subset, the IMG sender can resolve this, otherwise it should indicate
   that it is not able to resolve the query. The IMG sender may
   authenticate the IMG receiver to investigate the IMG QUERY operation
   in order to determine whether the IMG receiver is authorized to be
   sent that metadata. The sender may also include authentication data
   in the resolve operation so that 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 metadata which the IMG
   receiver holds is stale, the IMG receiver can start the IMG SUBSCRIBE
   operation prior in order to solicit notify messages. Since the IMG
   receiver doesn't know when metadata will be updated and the notify
   message will arrive, this operations does not synchronize with the
   notify message. The IMG receiver may wait for the notify message for
   a long time. The IMG sender may authenticate the IMG receiver to
   investigate whether an IMG SUBSCRIBE operation is from an authorized


   An IMG receiver needs the response to an earlier IMG SUBSCRIBE and
   the IMG NOTIFY 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 sender may
   also include authentication data in the notify operation so that

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   receivers may check the authenticity of the messages.

4.3 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:

   IMG Announcer : send IMG ANNOUNCE
   IMG Listener  : receive IMG ANNOUNCE
   IMG Querier   : send IMG QUERY to receive IMG RESOLVE
   IMG Resolver  : receive IMG QUERY then send IMG RESOLVE
   IMG Subscriber: send IMG SUBSCRIBE then receive IMG NOTIFY
   IMG Notifier  : receive IMG SUBSCRIBE then send IMG NOTIFY

   Finally, the figure 1 shows a 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 with IMG Entities

4.4 Overview of Protocol Operations

   The figure 2 gives an overview of the relationship between transport
   cases, IMG Operations and IMG data types (note, it is not a protocol

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

   Figure 2: IMG Operations and IMG Data type

5 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 sender to receiver as shown in figure 3

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

   Figure 3: A Simple IMG Sender to IMG Receiver Relationship

5.1 Intermediary Cases

   Some IMG metadata may be distributed to a large number of receivers.
   If, for example, some IMG metadata is public information and the
   sender provides the same information for all receivers. This kind of

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   IMG metadata may be distributed from one sender to multiple receivers
   (Figure 4) and/or or may be relayed across a set of IMG transceivers
   that receive the IMG metadata, possibly filter or modify its content,
   and then forward it. The relayed network architecture is similar to
   content distribution network architecture [3] (CDNs). Existing CDNs
   may carry IMG metadata. Satellite and peer-to-peer networks may also
   carry IMG metadata.

    +----------+                                    +----------+
    | 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 receivers. An IMG
   receiver may be allowed to receive IMG metadata from any number of

   The IMG metadata is used to find, obtain, manage and play content.
   The IMG metadata distribution may be modified as they are
   distributed. 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 senders. For example, a rating service may add its own
   content ratings or recommendations to existing meta-data. An
   implication of changing (or aggregating) IMG metadata from one or
   more senders is that the original authenticity is lost. Thus for
   deployments requiring these kind of features, the original metadata

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   should be reasonably fragmented already (allowing the intermediary to
   replace a small fragment without changing the authenticity of the
   remainder). It may be beneficial to use smaller fragments for more
   volatile parts, and larger one for more stable parts.

5.2 One-to-many Unidirectional Multicast

   This case implies many receivers and one or more 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

5.3 One-to-one Bi-directional Unicast

          +----------+                +----------+
          |   IMG    |<------(1)------|   IMG    |
          | Resolver |-------(2)----->| Querier  |
          +----------+                +----------+

   Figure 6: Query/Resolve Deployment Example

   Both query/resolve (figure 6) and subscribe/notify (figure 7) message

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   exchange operations are feasible.

         +----------+                   +------------+
         |          |<-------(1)--------|            |
         |   IMG    |--------(2)------->|    IMG     |
         | Notifier |   (time passes)   | Subscriber |
         |          |--------(3)------->|            |
         +----------+                   +------------+

   Figure 7: Subscribe/Notify Deployment Example

5.4 Combined Operations with Common Metadata

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

    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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6 Applicability of Existing Protocols to the Proposed Framework Model

6.1 Summary of Limitations of Existing Protocols

   SDP [4] has a text-encoded syntax to specify multimedia sessions for
   announcements and invitations. Although SDP is extensible, it has
   limitations such as structured extensibility and capability to carry
   another multimedia session descriptors.

   These are mostly overcome by the XML-based SDPng [5] , which is
   intended for both two-way negotiation and also unidirectional
   delivery. Since SDPng addresses multiparty multimedia conferences, it
   is necessary to extend the XML schema in order to describe general
   multimedia content.

   MPEG-7 [6] is a collection of XML-based description tools for general
   multimedia content including structured multimedia sessions. TV-
   Anytime Forum [7] provides descriptions based on MPEG-7 for TV
   specific program guides. These are likely to be limited to describe
   pictures, music and movies, thus it may be necessary to extend these
   descriptions in order to define a variety of documents, game
   programs, binary files, live event and so on.

   HTTP [8] is a well known information retrieval protocol using bi-
   directional transport and widely used to deliver web-based content
   descriptions to many hosts. However, it has well recognized
   limitations of scalability in the number of HTTP clients since it
   relies on the polling mechanism to keep information consistent
   between the server and client.

   SAP [9] is an announcement protocol that distributes session
   descriptions via multicast. It does not support fine-grained meta
   data selection and update notifications, as it always sends the whole
   meta data. Given the lack of a wide-area multicast infrastructure, it
   is likely only deployable within a local area network.

   SIP [10] and SIP-specific event notification [11] can be used to
   notify subscribers of the update of IMG metadata for bi-directional
   transport. It is necessary for SIP Event to define an event package
   for each specific application such as [12].

6.2 Existing Protocol Fit to the IMG Framework Model

   SDP: The SDP format could be used to describe session-level
   parameters (e.g. scheduling, addressing and the use of media codecs)
   to be included in Complete Descriptors. Although there are extension
   points in SDP allowing the format to be extended, there are
   limitations in the flexibility of this extension mechanism. However,

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   SDP syntax can not provide with Partial Descriptors and Pointers
   without significant unused overhead. Because it is expected that the
   information conveyed by SDP is just a small subset of IMG metadata
   the use of SDP for other than session-level IMG parameters may not be

   SDPng: 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 is much more flexible with regards
   to extensions and combining with other description formats.

   MPEG-7: Descriptions based on the MPEG-7 standard 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: this is well in line with the IMG
   usage scenarios of IMGs introduced in 3.2. Because MPEG-7 is based on
   XML, it is well suited for combining with other XML-based formats
   such as SDPng.

   HTTP: The HTTP protocol can be used as a bi-directional/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. So alone, HTTP is not sufficient to fulfill IMG
   requirements in a unicast deployment.

   SAP: The announcement mechanism provided by SAP 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 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, the limitations of SAP as a generic IMG transport protocol

   - Lack of reliability (through forward error correction / retransmissions)
   - Lack of payload segmentation
   - Limited payload size
   - Only one description allowed per SAP message
   - Lack of congestion control
   - Lack of Internet standard authentication / encryption mechanisms
   - It is an Experimental RFC with no support for progressing further

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   In principle, the current SAP protocol could be extended to get
   around its limitations (e.g. the use of a multipart MIME type in the
   SAP payload has been proposed, enabling multiple descriptions to be
   carried in a single SAP message). 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 recommended.

   SIP: The SIP-specific event mechanism described in [11] provides a
   way to implement IMG SUBSCRIBE and IMG NOTIFY operations via a bi-
   directional unicast connection. However, there are scalability
   problems with this approach, as RFC 3265 currently does not consider

6.3 Outstanding IMG Mechanism Needs

   Several outstanding 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 which
   are: (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 which is common to, and independent of, the
   application metadata syntax(es) used; agreement on basic metadata
   models to enable interoperability testing of the above. The following
   sections describe each of these.

6.3.1 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) [13] protocol from the IETF
   Reliable Multicast Transport (RMT) working group is very interesting
   as it fulfils 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 RMT has a work-in-progress fully
   specified protocol using ALC called FLUTE (File Delivery over
   Unidirectional Transport) [14]. FLUTE seems to be the only fully
   specified transport and open specification on which a new IMG

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   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.
   Thus, we recommend only to attempt this if ALC-based protocols are
   later found to be insufficient.

   In particular, any announcement protocol must feature sufficient
   scalability, flexibility and reliability to meet IMG needs. Also, the
   ANNOUNCE operation must be supported and also NOTIFY capability could
   be investigated for both hybrid unicast-multicast and unidirectional
   unicast systems.

6.3.2 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 and point-to-point
   environment. Such a specification does not currently exist, although
   several usable unicast transport protocols and specifications can be
   harnessed for this (SIP, SIP events, HTTP, etc). In particular, both
   SUBSCRIBE-NOTIFY and QUERY-RESOLVE operation pairs must be enabled.
   We anticipate that the FETCH operation will be a trivial usage of
   HTTP, although other transport options may be beneficial to consider

6.3.3 The Metadata Envelope

   To be able to handle multiple metadata syntaxes, a common minimal set
   of information is needed to handle discrete blocks of metadata. The
   IMG framework model data types defined in this document. This minimal
   set of information field will be named a `metadata envelope' and

        1.   Uniquely identify the block of metadata, regardless of
             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. Scope/domain specific identifications should not
             `leak' outside of the scope, and always using globally
             unique identification (e.g. based on URIs) should avoid
             this error.

        2.   Version the block so that changes can be easily handled and
             stale data identified.

        3.   Give the temporal validity of the block. It must expire the

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             block (expiry time), and may optionally provide a time
             (presumably in the future) from when the block becomes
             valid. Temporal validity may be changeable for a block, and
             even a specific version of a block.

        4.   Be independent of the metadata syntax(es) used for the
             metadata block, in the sense that no useful syntax should
             be excluded.

   For blocks with multiple descriptors, it is assumed that any
   descriptors inherit the parameters of the (super)blocks. Thus the
   above information will implicitly describe the individual

   Four options for metadata envelope transport are feasible:

        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 may hinder interoperability.

        2.   A separate envelope object (a form of metadata itself)
             delivered in-band with the metadata. This would 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 RTSP).

        3.   A metadata wrapper which points to and/or embeds the
             service metadata into its `super-syntax'. For example, XML
             enables referencing (pointing to) other resources as well
             as embedding generic text objects.

        4.   Embedding in the metadata itself. However, this requires
             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.

6.3.4 Baseline (Meta)Data Model Specification

   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 and so
   should be an official IETF work item.

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   Relevant work-in-progress ensures that support of one, or very few,
   metadata syntaxes is not sufficient. Whereas the transport protocol
   should not restrict the metadata format, the metadata envelope may
   influence the choice metadata. However, metadata in binary format
   should not be prevented, in addition to the more abundant text and
   XML syntaxes currently available.

   In most cases the actual content of metadata will be application, or
   service domain, specific. For instance, digital cinema distribution
   and television channels will have many different requirements. The
   task of specifying the bulk of the world's metadata is well beyond
   the scope of this document: a framework for the delivery of IMG
   metadata. We do anticipate that existing and future metadata
   specifications, including those of several working groups and
   standardization bodies, shall be able to use the services of the IMG
   framework. However, it is not essential to the current IMG work to
   specify standards with application-specific metadata.

   It is essential that the three IMG data types are enabled, but it may
   not be necessary to achieve this for every metadata syntax available,
   nor may it be important to the IETF to cover every possibility for
   this. Generally, Complete Descriptions will be correct for existing
   syntaxes (e.g. for XML may be validated according to existing
   schema). Pointer data types may be served by a new syntax (extremely
   minimal), although it is feasible that the proposed metadata envelope
   specification will contain enough information to implement the
   Pointer data type. Partial Descriptions may need new or modified
   syntaxes and semantics (e.g. XML schema) as mandatory fields for a
   Complete Description may be redundant for a Partial Description.
   During and following the specification of the metadata envelope,
   enabling the delivery of Partial Descriptions should be considered.

   To gain implementation experience, it is essential to agree the basic
   of some metadata in interoperability tests and subsequently in
   widespread deployments. So we anticipate that a minimal IMG data
   model would be useful to the Internet community.

6.4 IMG Needs Fitting the IETF's Scope

   A Multicast Transport Protocol is essential to IMG delivery for
   unidirectional and multicast deployments and no alternative exists
   which fulfils the IMG requirements. We recommend that the
   specification of this be taken on as an official work item in the

   Specification of the usage of unicast transport protocols is
   essential for IMG delivery and control involving unicast
   communications, and will relate to existing IETF standard transport

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   protocols. Thus, we recommend that the specification of this be taken
   on as an official work item in the IETF.

   The metadata envelope functionality is essential for the IMG delivery
   fulfilling the IMG requirements. It is a required feature for IMG
   metadata transport and maintenance. Thus, we recommend that the
   metadata envelope specification be taken on as an official work item
   in the IETF.

   (Meta)data model specification and application specific metadata do
   not easily fit into the IETF scope and several other standardization
   bodies are well placed to do this work. We recommend that aspect
   shall not be an official IETF work item.

   Note, we acknowledge the need to exchange and agree on a baseline
   metadata model and application specific metadata for the purposes of
   interoperability testing between different implementations of IMG
   related IETF protocols. However, we feel that the IETF standards
   process is not required for this.

7 Security Considerations

   The IMG framework is developed from the IMG Requirements document [2]
   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, although this is outside the scope of this

   Protocol instantiations which are used to provide IMG operations will
   have very different security considerations depending on their scope
   and purpose. However, there are several general issues which are
   valuable to consider and, in some cases, provide technical solutions
   to deal with. 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 though the information
   itself is public. Capturing and protecting this IMG metadata requires
   the same actions and measures as for other point-to-point and
   multicast Internet communications. Naturally, the risk depends on the
   amount of individual or group personalization the snooped sessions
   contain. Further consideration is valuable at both transport and
   metadata levels.

   IMG Authenticity: In some cases, the receiver needs to be assured of

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   the origin of IMG metadata or its modification history. This can
   prevent denial of service or hijacking attempts which give an 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.
   Action upon detection of unauthorized data insertion is out of scope
   of this document.

   Receiver Authorization: Some or all of any IMG sender's metadata may
   be private or valuable enough to allow access to only certain
   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. Encryption and the required security parameters
   exchange are outside the scope of this document.

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

   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 FLUTE delivery service does not have write
   access to the system or any other critical areas.

   Protocol Specific Attacks: It is recommended that developers of any
   IMG protocol take account of the above risks in addition to any
   protocol design and deployment environment risks that may be
   reasonably identified. Currently this framework document does not
   recommend or mandate the use of any specific protocols, however the
   deployment of IMGs using specific protocol instantiations will
   naturally be subject to the security considerations of those

   Security Improvement Opportunity: The security properties of one
   channel and protocol can be improved through the use of another
   channel and protocol. For example, a secure unicast channel can be
   used to deliver the keys and initialization vectors for an encryption
   algorithm used on a multicast channel. The exploitation of this
   opportunity is specific to the protocols used and is outside the
   scope of this document.

8 Normative References

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   [1] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.

   [2] Y. Nomura et al., "Protocol requirements for Internet media
   guides," Internet Draft draft-ietf-mmusic-img-req-00, Internet
   Engineering Task Force, Sept. 2003.  Work in progress.

   [3] M. Day, B. Cain, G. Tomlinson, and P. Rzewski, "A model for
   content internetworking (CDI)," RFC 3466, Internet Engineering Task
   Force, Feb.  2003.

   [4] M. Handley and V. Jacobson, "SDP: session description protocol,"
   RFC 2327, Internet Engineering Task Force, Apr. 1998.

   [5] D. Kutscher, J. Ott, and C. Bormann, "Session description and
   capability negotiation," Internet Draft draft-ietf-mmusic-sdpng-07,
   Internet Engineering Task Force, Oct. 2003.  Work in progress.

   [6] ISO (International Organization for Standardization), "Overview
   of the MPEG-7 standard," ISO Standard ISO/IEC JTC1/SC29/WG11 N4509,
   International Organization for Standardization, Geneva, Switzerland,
   Dec.  2001.

   [7] T.-A. Forum, "Metadata specification S-3," TV-Anytime Forum
   Specification SP003v1.2 Part A, TV, TV-Anytime Forum, June 2002.

   [8] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, and T. Berners-
   Lee, "Hypertext transfer protocol -- HTTP/1.1," RFC 2068, Internet
   Engineering Task Force, Jan. 1997.

   [9] M. Handley, C. E. Perkins, and E. Whelan, "Session announcement
   protocol," RFC 2974, Internet Engineering Task Force, Oct. 2000.

   [10] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
   Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
   initiation protocol," RFC 3261, Internet Engineering Task Force, June

   [11] A. B. Roach, "Session initiation protocol (sip)-specific event
   notification," RFC 3265, Internet Engineering Task Force, June 2002.

   [13] M. Luby, J. Gemmell, L. Vicisano, L. Rizzo, and J. Crowcroft,
   "Asynchronous layered coding (ALC) protocol instantiation," RFC 3450,
   Internet Engineering Task Force, Dec. 2002.

9 Informative References

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   [12] J. Rosenberg, "A presence event package for the session
   initiation protocol (SIP)," internet draft, Internet Engineering Task
   Force, Jan. 2003.  Work in progress.

   [14] T. Paila, "FLUTE - file delivery over unidirectional transport,"
   Internet Draft draft-ietf-rmt-flute-06, Internet Engineering Task
   Force, Nov. 2003.  Work in progress.

10 Acknowledgements

   The authors would like to thank Joerg Ott, Colin Perkins, Toni Paila
   and Petri Koskelainen on for their ideas and input to this document.

11 Authors' Addresses

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

   Rod Walsh
   Nokia Corporation
   Nokia Research Center
   P.O. Box 100, FIN-33721 Tampere
   Email: rod,

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

   Hitoshi Asaeda
   Project PLANETE
   2004, Route des Lucioles, BP93,
   06902 Sophia Antipolis,

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University

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   1214 Amsterdam Avenue
   New York, NY 10027

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