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Internet Engineering Task Force                                MMUSIC WG
Internet Draft                                                 Y. Nomura
                                                           Fujitsu Labs.
                                                                R. Walsh
                                                              J-P. Luoma
                                                                   Nokia
                                                                  J. Ott
                                                     Universitaet Bremen
                                                          H. Schulzrinne
                                                     Columbia University
draft-ietf-mmusic-img-req-02.txt
December 22, 2003
Expires: June 2004


            Protocol Requirements for Internet Media Guides

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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
   http://www.ietf.org/ietf/1id-abstracts.txt

   To view the list Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.


Abstract

   This memo specifies requirements for a protocol for accessing and
   updating Internet Media Guide (IMG) information for media-on-demand
   and multicast applications. These requirements are designed to guide
   development of an IMG protocol for efficient and scalable delivery.






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Table of Contents

   1          Introduction ........................................    2
   1.1        Background and Motivation ...........................    3
   1.2        Scope of This Document ..............................    4
   2          Terminology .........................................    4
   3          Problem Statement ...................................    5
   4          Use Cases Requiring IMGs ............................    6
   4.1        Connectivity-based Use Cases ........................    6
   4.1.1      IP Datacast to a Wireless Receiver ..................    7
   4.1.2      Regular Fixed Dial-up Internet Connection ...........    8
   4.1.3      Broadband Always-on Fixed Internet Connection .......    8
   4.2        Content-orientated Use Cases ........................    8
   4.2.1      File Distribution ...................................    9
   4.2.2      TV and Radio Program Delivery .......................    9
   4.2.3      Media Coverage of a Live Event ......................   10
   4.2.4      Distance Learning ...................................   10
   4.2.5      Multiplayer Gaming ..................................   10
   5          Requirements ........................................   10
   5.1        General Requirements ................................   10
   5.1.1      Independence of IMG Operations from IMG Metadata
   5.1.2      Multiple IMG Senders ................................   11
   5.1.3      Modularity ..........................................   11
   5.2        Delivery Properties .................................   11
   5.2.1      Scalability .........................................   11
   5.2.2      Support for Intermittent Connectivity ...............   12
   5.2.3      Congestion Control ..................................   12
   5.2.4      Sender and Receiver Driven Delivery .................   12
   5.3        Customized IMGs .....................................   13
   5.4        Reliability .........................................   13
   5.4.1      Managing consistency ................................   14
   5.4.2      Reliable Message Exchange ...........................   14
   5.5        IMG Descriptions ....................................   15
   6          Security Considerations .............................   16
   6.1        IMG Authentication and Integrity ....................   16
   6.2        Privacy .............................................   17
   6.3        Access Control for IMGs .............................   17
   6.4        Denial-of-Service attacks ...........................   18
   6.5        Replay Attacks ......................................   18
   7          Acknowledgements ....................................   19
   8          Normative References ................................   19
   9          Informative References ..............................   19



1 Introduction

1.1 Background and Motivation



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   For some ten years, multicast-based (multimedia) conferences
   (including IETF WG sessions) as well as broadcasts of
   lectures/seminars, concerts, and other events have been used in the
   Internet, more precisely, on the MBONE. Schedules and descriptions
   for such multimedia sessions as well as the transport addresses,
   codecs, and their parameters have been described using SDP [1] as a
   rudimentary (but as of then largely sufficient) means. Dissemination
   of the descriptions has been performed using the Session Announcement
   Protocol (SAP) [2] and tools such as SD [3] or SDR [4]; descriptions
   have also been put up on web pages, sent by electronic mail, etc.

   Recently, interest has grown to expand -- or better: to generalize --
   the applicability of these kinds of session descriptions.
   Descriptions are becoming more elaborate in terms of included
   metadata; more generic regarding the types of media sessions; and
   possibly also support other transports than just IP (e.g. legacy TV
   channel addresses). This peers well with the DVB Organization's
   increased activities towards IP-based communications over satellite,
   cable, and terrestrial radio networks, also considering IP as the
   basis for TV broadcasts and further services. The program/content
   descriptions are referred to as Internet Media Guides (IMGs) and can
   be viewed as a generalization of Electronic Program Guides (EPGs) and
   multimedia session descriptions.

   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.

   The 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. 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.  Furthermore, IMG metadata may
   need to be retrieved interactively, similar to web pages (e.g. after
   rebooting a system or when a user is browsing after network
   connectivity has been re-established).  Finally, IMG metadata may be
   updated as time elapses because content described in the guide may be
   changed: for example, the airtime of an event such as a concert or
   sports event may change, possibly affecting the airtime of subsequent



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   media. This may be done by polling the IMG as well as asynchronous
   change notifications.

   Furthermore, we assume that any Internet host can be a sender of
   content and thus an IMG. Some of the content sources and sinks may
   only be connected to the Internet sporadically. Also, a single human
   user may use many different devices to access metadata. Thus, we
   envision that IMG metadata can be sent and received by, among others,
   by cellular phones, PDA (Personal Digital Assistant), personal
   computer, streaming video server, set-top box, video camera, and PVR
   (Personal Video Recorder) and that they be carried across arbitrary
   types of link layers, including bandwidth-constrained mobile
   networks.

   Finally, with many potential senders and sinks, different types of
   networks, and presumably numerous service providers, IMG metadata may
   need to be combined, split, filtered, augmented, modified, etc. on
   their way from the sender(s) to the receiver(s) to provide the
   ultimate user with a suitable selection of multimedia programs
   according to her preferences, subscriptions, location, context (e.g.
   devices, access networks), etc.

1.2 Scope of This Document

   This document defines requirements that Internet Media Guide (IMG)
   mechanisms must satisfy in order to deliver IMG to a potentially
   large audience. Since the IMG can describe many kinds of multimedia
   content, IMG methods are generally applicable to several scenarios.

   In considering wide applicability, this document provides the problem
   statement and existing mechanisms in this area. Then several use case
   scenarios for IMGs are explained including descriptions of how IMG
   metadata and IMG delivery mechanisms contribute to these scenarios.
   Following this, this document provides general requirements that are
   independent of any transport layer mechanism and application, such as
   delivery properties, reliability and IMG descriptions.

   This document reflects investigating work on delivery mechanisms for
   IMGs and generalizing work on session announcement and initiation
   protocols, especially in the field of the MMUSIC working group (SAP,
   SIP [5], SDP).

2 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [6].




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

        IMG Metadata:  A set of metadata consisting of one or more IMG
             elements. IMG metadata describes the features of multimedia
             content used to enable selection of and access to media
             sessions containing content. For example, metadata may
             consist of the URI, title, airtime, 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 IMG
             metadata to one or more IMG receivers.

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

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

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

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

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

3 Problem Statement

   The MMUSIC working group has long been investigating content, media
   and service information delivery mechanisms and protocols, and has
   itself produces Session Announcement Protocol (SAP), Session



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   Description Protocol (SDP), and the Session Initiation Protocol
   (SIP). SDP is capable of describing multimedia sessions (i.e.
   content in a wider sense) by means of limited descriptive information
   intended for human perception plus transport, scheduling information,
   and codecs and addresses for setting up media sessions. SIP and SAP
   are protocols to distribute these session descriptions.

   HTTP is a well known data retrieval protocol using bi-directional
   transport and widely used to deliver content descriptions to many
   hosts.

   However, we perceive a lack of standard solution for scalable IMG
   delivery mechanism in the number of receivers with consistency of IMG
   metadata between an IMG sender and IMG receiver for both bi-
   directional and unidirectional transport. With increased service
   dynamics and complexity, there is an increased requirement for
   updates to these content descriptions.

   Whenever an HTTP client requires updated content descriptions, the
   client has to reload those using the same URL. For mass media, the
   large number of users polling a server causes scalability and
   congestion concerns and so the technique is feasible only if the
   period between reloading is long and the amount of content
   descriptions or the number of users is small. A well-behaved
   implementation limits the timeliness of receiver-side updates for
   mass audiences.

   The unicast equivalent of this is to maintain a unicast
   connection/session between sender and receiver for the whole time a
   receiver is interested in a service. This may be feasible in many
   wire line systems for servers with only a few receivers, but both of
   these become less attractive for both wireless links and large
   numbers of sender-receiver connections, especially as both of these
   share a resource (the radio bandwidth or the server resources) and
   thus limit the number of receivers that can be served, without
   additional infrastructure investment.

   We also perceive a lack of standard solution for flexible content
   descriptions to support a multitude of application-specific data
   models with differing amount of detail and different target
   audiences.

4 Use Cases Requiring IMGs

4.1 Connectivity-based Use Cases

4.1.1 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. This is so that a receiver need only access
   the channels of interest and thus can reduce the amount of time,
   storage and CPU resources needed for processing the IP data.  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
   metadata.

   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 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
   accordingly.

   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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4.1.2 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).

4.1.3 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
   feasible.

4.2 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 and type of IMG usage. Many more examples
   may be relevant, for instance those detailed in [7].  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




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        o There are timeliness requirements in the delivery of IMG
          metadata

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

4.2.1 File Distribution

   IMGs support the communication of file delivery session properties,
   enabling the scheduled delivery or synchronization of files between a
   number of hosts. The received IMG metadata could be subsequently used
   by any application (also outside the scope of IMGs), for example to
   download a file with a software update.  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 delivery capacity of 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.

4.2.2 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.

   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.

4.2.3 Media Coverage of a Live Event




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   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 can 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.

4.2.4 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.

4.2.5 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.

5 Requirements

5.1 General Requirements

5.1.1 Independence of IMG Operations from IMG Metadata

   REQ GEN-1: Carrying different kinds of IMG metadata format in the IMG
   message body MUST be allowed.

   REQ GEN-2: Delivery mechanisms SHOULD support many different
   applications specific metadata formats to keep the system
   interoperable with existing applications.

   This provides flexibility in selecting/designing IMG transport
   protocol suited to various scenarios.

5.1.2 Multiple IMG Senders




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   REQ GEN-3: IMG receivers MUST be allowed to communicate with any
   number of IMG senders simultaneously.

   This might lead to receiving redundant IMG metadata describing the
   same items, however it enables the IMG receiver access to more IMG
   metadata than may be available from a single IMG sender. This also
   provides flexibility for the IMG transport protocols and does not
   preclude a mechanism to solve inconsistency among IMG metadata due to
   multiple IMG senders. This document assumes a typical IMG environment
   will involve many more IMG receivers than IMG senders and that IMG
   senders are continually connected for the duration of interest
   (rather than intermittently connected).

5.1.3 Modularity

   REQ GEN-4: The IMG delivery mechanisms MUST allow the combination of
   several IMG Operations.

   This is for the purpose of extending functionality (e.g. several or
   one protocol(s) to provide all the needed operations).  Applications
   can select an appropriate operation set to fulfill their purpose.

5.2 Delivery Properties

   This section describes general performance requirements based on the
   assumption that the range of IMG usage shall be important. However,
   it may be noted that requirements of delivery properties may vary
   based on the usage scenario, and thus some limited use
   implementations place less importance on some requirements.

   For example, it is clear that a multicast transport may provide more
   scalable delivery than a unicast transport, however scalability
   requirements do not preclude the unicast transport mechanisms. In
   this sense, scalability is always important for the protocols
   irrespective of transport mechanisms.

5.2.1 Scalability

   REQ DEL-1: The system MUST be scalable in that it does not fail to
   deliver up-to-date information under huge numbers of transactions and
   massive quantities of IMG metadata.

   REQ DEL-2: IMGs SHOULD provide a method to prevent an IMG sender from
   sending verbose IMG metadata that have been stored or deleted in IMG
   receivers. Note, 'verbose' data is unneeded or unused detail or
   repetitions.

   REQ DEL-3: The protocol MUST be scalable to very large audience sizes



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   requiring IMG delivery.

5.2.2 Support for Intermittent Connectivity

   REQ DEL-4: The system MUST enable IMG receivers with intermittent
   access to network resources (connectivity) to receive and adequately
   maintain sufficient IMG metadata.

   This allows intermittent access to save power where there is no need
   to keep communications links powered-up while they are sitting idle.
   For instance, in this situation periodic bursts of notifies, or a
   fast cycling update carousel, allows hosts to wake up for short
   periods of time and still be kept up-to-date. This can be beneficial
   for IMG receivers with sporadic connects to the fixed Internet, but
   is critical in the battery-powered wireless Internet.

5.2.3 Congestion Control

   REQ DEL-5: Internet-friendly congestion control MUST be provided for
   use on the public Internet.

   For instance, notifications of updates (containing only minimal
   change related data) can reduce congestion, especially for very large
   groups, while allowing individual "congestion free" parts of the
   Internet to do things "their way". Where some hosts are on
   unidirectional links, and other have bi-directional links (or both),
   this is sensible "diversity".

   REQ DEL-6: An IMG entity SHOULD invalidate the IMG metadata item when
   an IMG metadata item has lifetime information and its lifetime is
   over.

   This mechanism can reduce notifications of updates from the IMG
   sender to the IMG receiver to invalidate the item. It may be
   beneficial for congestion control.

5.2.4 Sender and Receiver Driven Delivery

   REQ DEL-7: The system MUST be flexible in choosing sender-driven,
   receiver-driven or both delivery schemes.

   Sender-driven delivery achieves high scalability without interaction
   between the IMG sender and receiver. This avoids keeping track of a
   delivery state of every IMG receiver.

   In contrast, the receiver-driven delivery provides on-demand delivery
   for IMG receivers. Since an IMG sender's complete IMG metadata may be
   a very large amount of data, the IMG receiver needs to be able to



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   access the guide when convenient (e.g., when sufficient network
   bandwidth is available to the IMG receiver).

5.3 Customized IMGs

   REQ CUS-1: The system MUST allow delivery of customized IMG metadata.

   The IMG receiver may require a subset of all the IMG metadata
   available according to their preferences (type of content, media
   description, appropriate age group, etc.) and configuration.

   The IMG receiver might send its preferences in the IMG operations
   which can specify user specific IMG metadata to be delivered. These
   preferences could consist of filtering rules. When receiving these
   messages, the IMG sender might respond appropriate messages carrying
   a subset of IMG metadata which matches the IMG receiver's
   preferences.

   This mechanism can reduce the amount of IMG metadata delivered from
   the IMG sender to IMG receiver, and consequently it can save the
   resource consumption on the IMG entities and networks. It is
   typically useful in unicast case and also beneficial in multicast
   case where IMG sender distributes the same IMG metadata to interested
   IMG receivers at the same time.

   For multicast and unicast cases where the IMG sender does not provide
   customized IMG metadata, the IMG receiver could receive all IMG
   metadata transmitted (on its joined channels). However, it may select
   and filter the IMG metadata to get customized IMG metadata by its
   preferences, and thus drop unwanted metadata immediately upon
   reception.

   Customized metadata might be achieved by changing the IMG
   descriptions sent and IMG receivers and/or changing the delivery
   properties (channels used).

   Note, customization and scalability are only somewhat exclusive.
   Systems providing an IMG receiver to an IMG sender request-based
   customization, will be generally less scalable to massive IMG
   receiver populations than those without this return signaling
   technique. Thus, customization, as with any feature which affects
   scalability, should be carefully designed for the intended
   application, and it may not be possible that a one-size-fits-all
   solution for customization would meet the scalability requirements
   for all applications and deployment cases.

5.4 Reliability




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5.4.1 Managing consistency

   IMG metadata tends to change as time elapses, as new content is
   added, the old IMG metadata stored in the IMG receiver becomes
   unavailable and the parameters of the existing IMG metadata are
   changed.

   REQ REL-1: The system MUST manage IMG metadata consistency.

   The IMG sender can either simply make updates available
   (unsynchronized) or IMG sender and receiver can interact to keep
   their copies of the IMG metadata synchronized.

   In the unsynchronized model, the IMG sender does not know whether a
   particular IMG receiver has an up-to-date copy of the IMG metadata.

   In the synchronized model, updating cached copy of the IMG metadata
   is necessary to control consistency when the IMG sender or receiver
   could not communicate for a while. In this case, the IMG sender or
   receiver may need to confirm its consistency by IMG operations.

   REQ REL-2: Since IMG metadata can change at any time, IMG receivers
   SHOULD be notified of such changes.

   Depending on the size of the guide, the interested party may want to
   defer retrieving the actual information. The change notification
   should be addressed to a logical user (or user group), not a host,
   since users may change devices.

   Note that depending on the deployment environment and application
   specifics, the level of acceptable inconsistency varies. Thus, this
   document does not define inconsistency as specific time and state
   differences between IMG metadata stored in an IMG sender and IMG
   metadata stored in an IMG receiver.

   In general, the consistency of metadata for a content and media is
   more important immediately prior to and during the media's
   session(s). Hosts which forward (or otherwise resend) metadata may be
   less tolerant to inconsistencies as delivering out of date data is
   both misleading and bandwidth inefficient.

   By contrast, intermittent connectivity make immediate distribution of
   changes infeasible and so managing data consistency should be focused
   on the timely delivery of data.

5.4.2 Reliable Message Exchange

   REQ REL-3: An IMG transport protocol MUST support reliable message



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   exchange.

   The extent to which this could result in 100% error free delivery to
   100% of IMG receivers is a statistical characteristic of the
   protocols used. Usage of reliable IMG delivery mechanisms is expected
   to depend on the extent to which underlying networks provide
   reliability and, conversely, introduce errors. Note, some deployments
   of IMG transport protocols may not aim to provide perfect reception
   to all IMG receivers in all possible cases.

5.5 IMG Descriptions

   REQ DES-1: IMG metadata MUST be interoperable over any IMG transport
   protocol, such that an application receiving the same metadata over
   any one (or more) of several network connections and/or IMG transport
   protocols will interpret the metadata in exactly the same way. (This
   also relates to the 'Independence of IMG Operations from IMG
   Metadata' requirement).

   REQ DES-2: IMG delivery MUST enable the carriage of any format of
   application-specific metadata.

   Thus, the system will support the description of many kinds of
   multimedia content, without the need for a single homogenous metadata
   syntax for all uses (which would be infeasible anyway). This is
   essential for environments using IMG systems to support many kinds of
   multimedia content and to achieve wide applicability.

   REQ DES-3: Whereas specific applications relying on IMGs will need to
   select one or more specific application-specific metadata formats
   (standard, syntax, etc.), the IMG system MUST be independent of this
   (it may be aware, but it will operate in the same way for all).

   Thus, a transfer envelope format, that is uniform across all
   different application-specific IMG metadata formats, is needed. The
   payload of this transfer envelope would be some application-specific
   metadata.

   REQ DES-4: IMG metadata MUST be structured such that it is possible
   to deliver only part of an IMG sender's (and the global) complete IMG
   metadata knowledge.

   REQ DES-5: A transfer envelope MUST be defined to include essential
   parameters.

   Examples of essential parameters are those that allow the metadata in
   question to be uniquely identified and updated by new versions of the
   same metadata.



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   REQ DES-6: It SHALL be possible to deduce the metadata format from
   the transfer envelope.

   REQ DES-7: IMG senders SHALL use the transfer envelope for each IMG
   metadata transfer.

   Thus, it will even be possible to describe relationships between
   syntactically dissimilar application-specific formats within the same
   body of IMG metadata knowledge.

   REQ DES-8: IMG metadata SHOULD support to describe differences
   between update version and old version of IMG metadata when IMG
   delivery mechanism carries updated IMG metadata and those differences
   are considerably little. (This also relates the delivery property
   requirements for congestion control in Section 5.2.3).

6 Security Considerations

   Internet Media Guides are used to convey information about multimedia
   resources from one or more IMG senders across one or intermediaries
   to one or more IMG receivers. IMG metadata may be pushed to the IMG
   receivers or interactively retrieved by them. IMGs provide metadata
   as well as scheduling and rendezvous information about multimedia
   resources, etc. and requests for IMG metadata may contain information
   about the requesting users.

   The information contained in IMG metadata as well as the operations
   related to IMGs should be secured to avoid forging information,
   misdirecting users, spoofing IMG senders, etc. and to protect user
   privacy.

   The remainder of section addresses the security requirements for
   IMGs.

6.1 IMG Authentication and Integrity

   IMG metadata and its parts need to be protected against unauthorized
   altering/adding/deletion on the way. Their originator needs to be
   authenticated.

   REQ AUT-1: It MUST be possible to authenticate the originator of a
   set of IMG metadata.

   REQ AUT-2: It MUST be possible to authenticate the originator of a
   subpart of IMG metadata (e.g. a delta or a subset of the
   information).

   REQ AUT-3: It MUST be possible to validate the integrity of IMG



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   metadata.

   REQ AUT-4: It MUST be possible to validate the integrity of a subpart
   of IMG metadata (e.g. a delta or a subset of the information).

   REQ AUT-5: It MUST be possible to separate or combine individually
   authenticated pieces of IMG metadata (e.g. in an IMG transceiver)
   without invalidating the authentication.

   REQ AUT-6: It MUST be possible to validate the integrity of an
   individually authenticated piece of IMG metadata even after this
   piece had been separated from other pieces of IMG metadata and
   combined with other pieces to form new IMG metadata.

   REQ AUT-7: It MUST be possible to authenticate the originator of an
   IMG operation.

   REQ AUT-8: It MUST be possible to validate the integrity of any
   contents of an IMG operation (e.g. the subscription or inquiry
   information).

6.2 Privacy

   Customized IMG metadata and IMG metadata delivered by notification to
   individual users may reveal information about the habits and
   preferences of a user and may thus deserve confidentiality
   protection, even though the information itself is public.

   REQ PRI-1: It MUST be possible to keep user requests to subscribe to
   or retrieve certain (parts of) IMG metadata confidential.

   REQ PRI-2: It MUST be possible to keep IMG metadata, pieces of IMG
   metadata, or pointers to IMG metadata delivered to individual users
   or groups of users confidential.

   REQ PRI-3: It SHOULD be possible to ensure this confidentiality end-
   to-end, that is, to prevent intermediaries (such as IMG transceivers)
   from accessing the contained information.

6.3 Access Control for IMGs

   Some IMG metadata may be freely available, while access to other may
   be restricted to closed user groups (e.g. paying subscribers). Also,
   different parts of IMG metadata may be protected at different levels:
   e.g. metadata describing a media session may be freely accessible
   while rendezvous information to actually access the media session may
   require authorization.




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   REQ ACC-1: It MUST be possible to authorize user access to IMG
   metadata.

   REQ ACC-2: It MUST be possible to authorize access of users to pieces
   of IMG metadata (delta information, subparts, pointers).

   REQ ACC-3: It MUST be possible to require different authorization for
   different parts of the same IMG metadata.

   REQ ACC-4: It MUST be possible to access selected IMG metadata
   anonymously.

   REQ ACC-5: It MUST be possible for an IMG receiver to choose not to
   receive (parts of) IMG metadata in order to avoid being identified by
   the IMG sender.

   REQ ACC-6: It SHOULD be possible for IMG transceiver to impose
   different authorization requirements.

   REQ ACC-7: It MAY be possible for IMG senders to require certain
   authorization that cannot be overridden by intermediaries.

6.4 Denial-of-Service attacks

   Retrieving or distributing IMG metadata may require state in the IMG
   senders, transceivers, and/or receivers for the respective IMG
   transport sessions. Attackers may create large numbers of sessions
   with any of the above IMG entities to disrupt regular operation.

   REQ DOS-1: IMG operations SHOULD be authenticated.

   REQ DOS-2: It SHOULD be possible to prevent DoS attacks that build up
   session state in IMG entities to exhaust their resources.

   REQ DOS-3: It SHOULD be possible to avoid DoS attacks that exhaust
   resources of IMG entities by flooding them with IMG metadata.

6.5 Replay Attacks

   IMG metadata disseminated by an IMG sender or an IMG transceiver may
   be updated, deleted, or lose validity over time for some other
   reasons. Replaying outdated IMG metadata needs to be prevented.

   Furthermore, replay attacks may also apply to IMG operations (rather
   than just their payload). Replaying operations needs also be
   prevented.

   REQ REP-1: IMG metadata MUST be protected against partial or full



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   replacement of newer ("current") versions by older ones.

   REQ REP-2: Mechanisms MUST be provided to mitigate replay attacks on
   the IMG operations.

7 Acknowledgements

   The authors would like to thank Hitoshi Asaeda, Petri Koskelainen,
   Toni Paila and Dirk Kutscher for their comments and ideas on this
   work.

8 Normative References



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

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

   [5] 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
   2002.

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



9 Informative References



   [3] Session Directory, ftp://ftp.ee.lbl.gov/conferencing/sd/

   [4] Session Directory Tool, http://www-
   mice.cs.ucl.ac.uk/multimedia/software/sdr/



10 Authors' Addresses

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



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   Email: nom@flab.fujitsu.co.jp

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

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

   Joerg Ott
   Universitaet Bremen
   MZH 5180
   Bibliothekstr. 1
   D-28359 Bremen
   Germany
   Email: jo@tzi.uni-bremen.de

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


   Full Copyright Statement

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   and distributed, in whole or in part, without restriction of any
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   The limited permissions granted above are perpetual and will not be
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