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Versions: 00 01 02                                                      
Internet Engineering Task Force                                MMUSIC WG
Internet Draft                                                 Y. Nomura
                                                           Fujitsu Labs.
                                                                R. Walsh
                                                                J. Luoma
                                                               H. Asaeda
                                                          H. Schulzrinne
                                                     Columbia University
October 27, 2003
Expires: April 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
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

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

   The list of current Internet-Drafts can be accessed at

   To view the list Internet-Draft Shadow Directories, see


   This document 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 ..............................    3
   2          Terminology .........................................    4
   3          Use Cases Requiring IMG Framework ...................    4
   3.1        Connectivity-based Use Cases ........................    4
   3.1.1      IP Datacast to a Wireless Receiver ..................    4
   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 ......................    7
   3.2.4      Distance Learning ...................................    8
   3.2.5      Multiplayer Gaming ..................................    8
   4          IMG Common Framework Model ..........................    8
   4.1        IMG Data-Type .......................................    9
   4.1.1      Complete Description ................................    9
   4.1.2      Delta Description ...................................    9
   4.1.3      Pointer .............................................    9
   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 .........................................   10
   4.2.4      IMG SUBSCRIBE .......................................   11
   4.2.5      IMG NOTIFY ..........................................   11
   4.3        IMG Entities ........................................   11
   4.4        Overview of Protocol Operations .....................   12
   5          Deployment Scenarios for IMG Entities ...............   13
   5.1        One-to-many Unidirectional Multicast ................   14
   5.2        One-to-one Bi-directional Unicast ...................   15
   5.3        Combined Operations with Common Metadata ............   15
   6          Applicability of Existing Protocols to the
              Proposed Framework Model ............................   16
   6.1        Summary of Limitations of Existing Protocols ........   16
   6.2        Existing Protocol Fit to the IMG Framework Model
   6.3        Outstanding IMG Mechanism Needs .....................   18
   6.3.1      A Multicast Transport Protocol ......................   19
   6.3.2      Usage of Unicast Transport Protocols ................   19
   6.3.3      The Metadata Envelope ...............................   20
   6.3.4      Baseline (Meta)Data Model Specification .............   21
   7          Security Considerations .............................   22
   8          Normative References ................................   23
   9          Infomative References ...............................   25
   10         Acknowledgements ....................................   25
   11         Authors' Addresses ..................................   25
   12         Bibliography ........................................   26

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1 Introduction

1.1 Background and Motivation

   An Internet Media Guide (IMG) is a structured collection of
   multimedia session descriptions expressed using SDP, SDPng or some
   similar session description format. It is used to describe a set of
   multimedia sessions (e.g. television program schedules, content
   delivery schedules etc.) but may also refer to other networked
   resources including web pages. An IMG provides 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
   requirering 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. The IMG 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. Hence, a framework
   for distributing IMGs in various different ways is needed to
   accommodate the needs of different audiences: For traditional
   broadcast-style scenarios, multicast-based (push) distribution of
   IMGs needs to be supported. Where no multicast is available,
   unicast-based push is required, too.

   Finally, this document outlines the use of existing protocol to
   create an IMG delivery infrastructure. It aims to organize existing
   protocol 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.

1.2 Scope of this Document

   This document defines the a common framework model for the delivery
   of Internet Media Guides (IMG). The framework describes existing

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   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 fulfil IMG framework and
   requirements. This also makes it straightforward to identify gaps so
   that new protocols needs are made apparent.

2 Terminology

   SHOULD NOT, RECOMMENDED, MAY, and "OPTIONAL" in this document are to
   be interpreted as described in RFC 2119 [2].

        Internet Media Guide (IMG): An IMG is a set of meta-data
             describing the features of multimedia content. For example,
             meta-data may consist of the URI, title, air time,
             bandwidth needed, file size, text summary, genre, and
             access restrictions.

        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 source.

        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.

        IMG Operations: An atomic process for the IMG protocol to
             deliver IMG or control the IMG sender or IMG receiver.

3 Use Cases Requiring IMG Framework

3.1 Connectivity-based Use Cases

3.1.1 IP Datacast to a Wireless Receiver

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   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. Such that a receiver need only access the
   channels of interest and thus 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
   segmentation f 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).

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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 an
   IMG). 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 IMGs 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 IMGs over multicast if the respective
   operators require this to keep system-wide bandwidth usage feasible.

3.2 Content-orientated Use Cases

   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 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 information is generally time-related

        o The are timeliness requirements in the delivery of IMG

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

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. An IMG 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 to schedule the delivery effectively. Since IMG
   can describe time-related data for each client, the content provider
   can schedule delivery time for each client. This can save network
   bandwidth and capacity of delivery servers. In addition, IMG can be
   used to synchronize a set of files between a source host and
   destination host, when those files change as time elapses.

3.2.2 TV and Radio Program Delivery

   A source of audio/video streaming content can use the IMG 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 information
   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.

   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 information 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

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   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 information should enable terminals to discover the availability
   of different media used to cover a live event.

3.2.4 Distance Learning

   An IMG 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

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
   part of the delivery aspects, and in other cases descriptors 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 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, fundamental
          session descriptors.

        o Delivery Protocol -- the methods and protocols to exchange
          descriptions between the senders and the receivers.  An IMG

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          delivery protocol consists of two functions: carrying IMG
          metadata from an IMG sender to an IMG receiver and controlling
          an IMG 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-Type

   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

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 a complete IMG, 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,
   defining the difference from a previous version of the Compete
   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 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 does not include metadata par se
   (although it may also appear as a data field in complete or delta

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4.2 Operation Set for IMG Delivery

   A finite set of operations both meets the IMG requirements [1] 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 IMGs delivery. In this case, a IMG sender can initiate
   unsolicited distribution for IMGs 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.


   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 whole IMG 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

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   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 messagess. 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 an updated IMG is available or part of
   the existing IMG 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 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

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   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

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

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   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

   Some IMG may be distributed to a large number of receivers. If, for
   example, a particular IMG is public information and the sender
   provides the same information for all receivers. This kind of IMG 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, possibly filter or modify its content, and then
   forward it. The relayed network architecture is similar to content
   distribution network architecture[4] (CDNs). Existing CDNs may carry
   IMGs. Satellite and peer-to-peer networks may also carry IMGs.

   IMG sender and receiver 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 or
   proxy is used to combine or aggregate IMG data for some receivers. An
   IMG receiver may be allowed to receive IMG metadata from any number
   of senders.

   The IMG 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 an IMG 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 IMGs from
   different senders. For example, a rating service may add its own
   content ratings or recommendations to existing meta-data.

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

   Figure 4: A Relay Network with an IMG Transceiver

5.1 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

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5.2 One-to-one Bi-directional Unicast

   Both query/resolve (figure 6) and subscribe/notify (figure 7) message
   exchange operations are feasible.

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

   Figure 6: Query/Resolve Deployment Example

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

   Figure 7: Subscribe/Notify Deployment Example

5.3 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.

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    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

6 Applicability of Existing Protocols to the Proposed Framework Model

6.1 Summary of Limitations of Existing Protocols

   SDP[5] 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[6] , 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[7] is a collection of XML-based description tools for general
   multimedia content including structured multimedia sessions. TV-
   Anytime Forum[8] 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.

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   HTTP[9] 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[10] 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[11] and SIP-specific event notification[12] can be used to notify
   subscribers of the update of IMG for bi-directional transport. It is
   necessary for SIP Event to define an event package for each specific
   application such as [13].

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,
   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
   information, the use of SDP for other than session-level IMG
   parameters may not be reasonable.

   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 for combining with other XML-based formats
   such as SDPng.

   HTTP: The HTTP protocol can be used as a bi-directional/unicast IMG
   delivery protocol. Being a request-reply oriented protocol, HTTP is

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   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 fulfil IMG
   needs 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 mechanism

   - 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

   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 mechanism is not recommended.

   SIP: The SIP-specific event mechanism described in RFC 3265 [11]
   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.

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

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   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 data. 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 so we recommend that this be
   taken on as an official IETF work item.

   The Asynchronous Layered Coding (ALC)[14] 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)[15]. 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.
   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

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   too. Thus, it is important that this specification is taken on as an
   official work item in the IETF. It is also important that individual
   interest in specialist areas (such as peer-to-peer) is welcome too
   should a part of the community benefit from this.

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
             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.

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        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.

   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 worldfs metadata is well beyond
   the scope of this document: a framework for the delivery of IMGs. 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

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   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, although it should
   not be informational rather than mandatory. This may not need to be
   an official IETF work item.

7 Security Considerations

   The IMG framework is developed from the IMG Requirements document [1]
   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 system would inherit the security considerations of specific
   protocols used, although this is out side the scope of this document.

   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 IMGs 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 information 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

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

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   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

   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 delivery 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 IMG 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

   [1] Y. Nomura et al., "Protocol requirements for Internet media

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   guides," Internet Draft draft-ietf-mmusic-img-req-00, Internet
   Engineering Task Force, Sept. 2003.  Work in progress.

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

   [3] B. Quinn and K. Almeroth, "IP multicast applications: Challenges
   and solutions," RFC 3170, Internet Engineering Task Force, Sept.

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

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

   [6] D. Kutscher, J. Ott, and C. Bormann, "Session description and
   capability negotiation," internet draft, Internet Engineering Task
   Force, Mar. 2003.  Work in progress.

   [7] 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.

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

   [9] 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.

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

   [11] 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

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

   [14] 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.

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9 Infomative References

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

   [15] T. Paila et al., "FLUTE - file delivery over unidirectional
   transport," Internet Draft draft-ietf-rmt-flute-02, Internet
   Engineering Task Force, Sept. 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
   Email: nom@flab.fujitsu.co.jp

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

   Juha-Pekka Luoma
   Nokia Corporation
   Nokia Research Center
   P.O. Box 100, FIN-33721 Tampere
   Email: juha-pekka.luoma@nokia.com

   Hitoshi Asaeda
   Project PLANETE
   2004, Route des Lucioles, BP93,
   06902 Sophia Antipolis,
   Email: Hitoshi.Asaeda@sophia.inria.fr

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   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027
   Email: schulzrinne@cs.columbia.edu

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