path: root/doc/rfc/rfc4473.txt

Network Working Group                                          Y. Nomura
Request for Comments: 4473                                  Fujitsu Labs
Category: Informational                                         R. Walsh
                                                              J-P. Luoma
                                                                   Nokia
                                                                  J. Ott
                                       Helsinki University of Technology
                                                          H. Schulzrinne
                                                     Columbia University
                                                                May 2006


             Requirements for Internet Media Guides (IMGs)

Status of This Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

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




















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Table of Contents

   1. Introduction ....................................................3
      1.1. Background and Motivation ..................................3
      1.2. Scope of This Document .....................................4
   2. Terminology .....................................................5
      2.1. New Terms ..................................................5
   3. Problem Statement ...............................................6
   4. Use Cases Requiring IMGs ........................................7
      4.1. Connectivity-based Use Cases ...............................7
           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 .......9
      4.2. Content-orientated Use Cases ...............................9
           4.2.1. TV and Radio Program Delivery .......................9
           4.2.2. Media Coverage of a Live Event .....................10
           4.2.3. Distance Learning ..................................10
           4.2.4. Multiplayer Gaming .................................10
           4.2.5. File Distribution ..................................11
           4.2.6. Coming-release and Pre-released Content ............11
   5. Requirements ...................................................11
      5.1. General Requirements ......................................11
           5.1.1. Independence of IMG Operations from IMG Metadata ...11
           5.1.2. Multiple IMG Senders ...............................12
           5.1.3. Modularity .........................................12
      5.2. Delivery Properties .......................................12
           5.2.1. Scalability ........................................13
           5.2.2. Support for Intermittent Connectivity ..............13
           5.2.3. Congestion Control .................................13
           5.2.4. Sender- and Receiver-Driven Delivery ...............13
      5.3. Customized IMGs ...........................................14
      5.4. Reliability ...............................................15
           5.4.1. Managing Consistency ...............................15
           5.4.2. Reliable Message Exchange ..........................16
      5.5. IMG Descriptions ..........................................16
   6. Security Considerations ........................................17
      6.1. IMG Authentication and Integrity ..........................18
      6.2. Privacy ...................................................19
      6.3. Access Control for IMGs ...................................19
      6.4. Denial-of-Service (DOS) Attacks ...........................20
      6.5. Replay Attacks ............................................20
   7. Normative References ...........................................21
   8. Informative References .........................................21
   9. Acknowledgements ...............................................22







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1.  Introduction

1.1.  Background and Motivation

   For some ten years, multicast-based (multimedia) conferences
   (including IETF working group 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 the Session
   Description Protocol (SDP) [2] as a rudimentary (but as of then
   largely sufficient) means.  Descriptions have been disseminated using
   the Session Announcement Protocol (SAP) [3] and Session Directory
   Tools such as SD [4] or SDR [5]; 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 (Digital Video
   Broadcasting) [6] 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) has a structured collection of
   multimedia session descriptions expressed using SDP, SDPng [7], or
   some similar session description format.  This is used to describe a
   set of multimedia services (e.g., television program schedules,
   content delivery schedules) but may also refer to other networked
   resources including web pages.  IMGs provide the 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 may be delivered to a potentially large audience,
   who uses it to join a subset of the sessions described, and who may
   need to be notified of changes to this information.  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.



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

   Furthermore, any Internet host can be a sender of content and thus an
   IMG sender.  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, cellular
   phones, Personal Digital Assistants (PDAs), personal computers,
   streaming video servers, set-top boxes, video cameras, and Digital
   Video Recorders (DVRs), and that the data be carried across arbitrary
   types of link layers, including bandwidth-constrained mobile
   networks.  However, generally we expect IMG senders to be well-
   connected hosts.

   Finally, with many potential senders and receivers, 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 services
   according to her preferences, subscriptions, location, and context
   (e.g., devices, access networks).

1.2.  Scope of This Document

   This document defines requirements that Internet Media Guide
   mechanisms must satisfy in order to deliver IMG metadata to a
   potentially large audience.  Since IMGs 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 discusses 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.





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   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 [8], 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 [1].

2.1.  New Terms

   Internet Media Guide (IMG): IMG is a generic term used 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 in terms of both
         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 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).




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

3.  Problem Statement

   As we enumerate the requirements for IMGs, it will become clear that
   they are not fully addressed by the existing protocols.  The
   "Framework for the Usage of Internet Media Guides" [9] discusses
   about these issues in more detail.

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

   However, we perceive a lack of a 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.

   HTTP [10] is a well-known information retrieval protocol using bi-
   directional transport and is 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 [3] is an announcement protocol that distributes session
   descriptions via multicast.  It does not support prioritization or
   fine-grained metadata selection and update notifications, as it
   places restrictions on metadata payload size and always sends the
   whole metadata.  It requires a wide-area multicast infrastructure for
   it to be deployable beyond a local area network.







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   SIP [8] and SIP-specific event notifications [11] can be used to
   notify subscribers of the update of IMG metadata for bi-directional
   transport.  However, it is necessary to define an event package for
   IMGs.

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

   SDP [2] has a text-encoded syntax to specify multimedia sessions for
   announcements and invitations.  It is primarily intended to describe
   client capability requirements and enable client application
   selection.  Although SDP is extensible, it has limitations such as
   structured extensibility and capability to reference properties of a
   multimedia session from the description of another session.

   These can mostly be overcome by the XML-based SDPng [7] -- which is
   intended for both two-way negotiation and unidirectional delivery --
   or similar content description mechanisms.  Since SDPng addresses
   multiparty multimedia conferences, it would be necessary to extend
   the XML schema in order to describe general multimedia content.

4.  Use Cases Requiring IMGs

4.1.  Connectivity-based Use Cases

4.1.1.  IP Datacast to a Wireless Receiver

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

   There are two main classes of receiver in this use case: fixed
   mains-powered and mobile battery-powered.  Both of these are affected
   by radio phenomena and so robust, or error-resilient, delivery is
   important.  Carouselled metadata transfer (cyclically repeated with a
   fixed bandwidth) 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.





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   To enable receivers to more accurately specify the metadata they are
   interested in, the unidirectional delivery may be distributed between
   several logical channels.  This is so that a receiver needs 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
   (possibly well-known) root multicast channel/group and progressively
   access only those additional channels based on metadata in parent
   channels.

   In some cases, the receiver may have multiple access interfaces
   adding bi-directional communications capability.  This enables a
   multitude of options, but most important, it enables NACK-based
   reliability and the individual retrieval of missed or not-multicast
   sets of metadata.

   Thus, essential IMG features in this case include the following:
   robust unidirectional delivery (with optional levels of reliability
   including "plug-in FEC" supported by a transport layer protocol),
   which implies easily identifiable segmentation of delivery data to
   make 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; and 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).

4.1.2.  Regular Fixed Dial-up Internet Connection

   Dial-up connections tend to be reasonably slow (<56 kbps 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).





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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 query-response methods, 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.

   Some services on broadband, such as live media broadcasting, benefit
   from multicast transport for streaming media because of scalability.
   In the cases where multicast transport is already available, it is
   convenient for a sender and receiver to retrieve IMG metadata over
   multicast transport.  Thus, broadband users may be forced to retrieve
   IMG metadata over multicast if backbone 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
   enabling 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 [12].  There are
   several unique features of IMGs that set them apart from related
   application areas such as Service Location Protocol (SLP) based
   service location discovery, Lightweight Directory Access Protocol
   (LDAP) based indexing services, and search engines such as Google.
   Features unique to IMGs include the following:

      o  IMG metadata is generally time-related

      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.  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




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   similar to that of a TV guide in newspapers, or an Electronic Program
   Guide available on digital TV receivers.

   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 later
   consumption.  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.

4.2.2.  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 a 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.

4.2.3.  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 downloading of presentation material could be useful
   for distance learning.

4.2.4.  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 user's interest
   in establishing such a session.







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4.2.5.  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 of 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 consistency check, and thus
   synchronize, a set of files between a sender host and receiver host,
   when those files change as time elapses.

4.2.6.  Coming-release and Pre-released Content

   IMG metadata can be used to describe items of content before the
   details of their final release are known.  A user may be interested
   in coming content (a new movie or software title where some aspects
   of the content description are known in advance) and so subscribe to
   an information service that notifies the user of changes to metadata
   describing that content.  Thus, as the coming release (or pre-
   releases, e.g., as movie trailers or software demos) become
   available, the IMG metadata changes and the user is notified of this
   change.  For example, the user could see an announcement of a movie
   that will be released sometime in the next few months, and configure
   the user's terminal to receive and record any trailers or promotional
   material as they become available.

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 and
   different IMG metadata formats in the IMG message body MUST be
   allowed.





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   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 an IMG transport
   protocol suited to various scenarios.

5.1.2.  Multiple IMG Senders

   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 that 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,
   note that requirements for 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.







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5.2.1.  Scalability

   REQ DEL-1: The IMG system MUST be scalable to large numbers of
   messages, so as to allow design and use of delivery mechanisms that
   will not fail in delivering 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 unnecessary IMG metadata that have been stored or deleted in
   IMG receivers.

   REQ DEL-3: The protocol MUST be scalable to very large audience sizes
   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 allow 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 connections to the fixed Internet, but is
   critical in the battery-powered wireless Internet.

   The implication of intermittent connectivity is that immediate
   distribution of changes becomes infeasible and so managing data
   consistency should be focused on the timely delivery of data.

5.2.3.  Congestion Control

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

   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 will lessen the need for notifications of updates from
   the IMG sender to the IMG receiver to invalidate the item and may
   help in reducing load.

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.




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   Sender-driven delivery achieves high scalability without interaction
   between the IMG sender and receiver.

   In contrast, 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
   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
   that 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 with appropriate messages
   carrying a subset of IMG metadata that 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 cases and also beneficial in multicast
   cases where an 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 the channels that the IMG receiver has
   joined.  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.

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

   Note that 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 that affects scalability,
   should be carefully designed for the intended application, and it may



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   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

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.

   Either the IMG sender can simply make updates available
   (unsynchronized), or the 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 a 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.

   Fulfilling this requirement needs to be compatible with the
   scalability requirements for the number of IMG receivers and the
   consistency of metadata.

   Depending on the size of the IMG metadata, the interested party may
   want to defer retrieving the actual information.  The change
   notification should be addressed to a logical user (or user group),
   rather than 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 content and media is more
   important immediately prior to and during the media's session(s).
   Hosts that forward (or otherwise resend) metadata may not tolerate



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   inconsistencies because delivering out-of-date data is both
   misleading and bandwidth inefficient.

5.4.2.  Reliable Message Exchange

   REQ REL-4: An IMG transport protocol MUST support reliable message
   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 that 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' requirements.)

   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 homogeneous
   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 metadata transfer envelope format that is uniform across all
   different application-specific IMG metadata formats is needed.  The
   envelope would reference (point to) or carry (as payload) some
   application-specific metadata, and the envelope would support the
   maintenance of the application-specific metadata, which may also
   serve the metadata relationships determined by the data model(s)
   used.  The envelope would not need to be aware of the data model(s)
   in use.




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   REQ DES-4: IMG metadata MUST be structured to enable fragmentation
   for efficient delivery.

   This is intended to ensure that an IMG sender with more than a
   trivial knowledge of metadata is able to deliver only part of its
   (and the global) complete IMG metadata knowledge.  (For instance, a
   trivial quantity of knowledge could be a single SDP description.)  In
   general, the resolution of this fragmentation will be very much
   dependent on the optimal delivery of a deployment, although some
   metadata syntaxes will inherently affect the sensible lower limit for
   a single element/fragment.

   REQ DES-5: A metadata 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.

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

   REQ DES-7: IMG senders SHALL use a metadata 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.  (For instance, a data model could be
   instantiated using both SDP and SDPng.)

   REQ DES-8: IMG metadata SHOULD support the description of differences
   between an updated version and an old version of IMG metadata when
   the IMG delivery mechanism carries updated IMG metadata and those
   differences are considerably little (e.g., by providing a 'delta' of
   the two versions; 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 more
   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, and so on, and requests for IMG metadata
   may contain information about the requesting users.





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   The information contained in IMG metadata as well as the operations
   related to IMGs should be secured to avoid forged information,
   misdirected users, and spoofed IMG senders, for example, and to
   protect user privacy.

   The remainder of this section addresses the security requirements for
   IMGs.

6.1.  IMG Authentication and Integrity

   IMG metadata and its parts need to be protected against unauthorized
   alteration/addition/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
   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 has 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).









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

   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 an IMG transceiver to select
   suitable authorization methods that are compatible between both IMG
   senders and IMG receivers it interacts with.





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

6.4.  Denial-of-Service (DOS) 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 avoid 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.

   As an example, two potential solutions that may be considered are
   running an IMG entity in stateless mode or identification and
   discarding of malicious packets by an IMG entity.

6.5.  Replay Attacks

   IMG metadata disseminated by an IMG sender or an IMG transceiver may
   be updated, be 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 also needs to be
   prevented.

   REQ REP-1: IMG metadata MUST be protected against partial or full
   replacement of newer ("current") versions by older ones.

   In a system with multiple senders, it may not be feasible to prevent
   some senders from delivering older versions of metadata than others -
   as a result of imperfect sender-sender data consistency.  Thus,
   replay attacks and delivery of inconsistent data require that an IMG
   receiver verifies that the IMG metadata is valid and reliable by
   using some security mechanism(s) (e.g., authorization,
   authentication, or integrity).

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






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   The level of threat from replay attacks varies very much depending on
   system scale and how well defined or open it is.  Thus, mitigating
   replay attacks may lead to different solutions for different systems,
   independent of the basic delivery method and metadata definitions.  A
   system with multiple senders presents a more challenging scenario for
   handling replay attacks.  As an example, bundling metadata with a
   security mechanism is one potential solution.

7.  Normative References

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

8.  Informative References

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

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

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

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

   [6]  Digital Video Broadcasting Project, http://www.dvb.org/

   [7]  Kutscher, D., Ott, J., and C. Bormann, "Session description and
        capability negotiation", Work in Progress, February 2005.

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

   [9]  Nomura, Y., Walsh, R., Luoma, J-P., Asaeda, H., and H.
        Schulzrinne, "Framework for the Usage of Internet Media Guides
        (IMG)", RFC 4435, April 2006.

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

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

   [12] Quinn, B. and K. Almeroth, "IP Multicast Applications:
        Challenges and Solutions", RFC 3170, September 2001.



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9.  Acknowledgements

   The authors would like to thank Hitoshi Asaeda, Gonzalo Camarillo,
   Jean-Pierre Evain, Dirk Kutscher, Petri Koskelainen, Colin Perkins,
   Toni Paila, and Magnus Westerlund for their excellent comments and
   ideas on this work.

Authors' Addresses

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

   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
   Helsinki University of Technology
   Networking Laboratory
   PO Box 3000
   FIN-02015 TKK
   Finland

   EMail: jo@netlab.tkk.fi

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

   EMail: schulzrinne@cs.columbia.edu




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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

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Acknowledgement

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