Internet Engineering Task Force
Internet Draft                                                 Y. Nomura
                                                           Fujitsu Labs.
                                                                R. Walsh
                                                                  J. Ott
                                                     Universitaet Bremen
                                                          H. Schulzrinne
                                                     Columbia University
September 10, 2003
Expires: March 2004

            Protocol Requirements for Internet Media Guides


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

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

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

   The list of current Internet-Drafts can be accessed at

   To view the list Internet-Draft Shadow Directories, see


   This 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 ...........................    2
   1.2        Scope of this Document ..............................    4
   2          Terminology .........................................    4
   3          Problem Statement ...................................    5
   4          Requirements ........................................    6
   4.1        Independence of IMG Operations from IMG Metadata ....    6
   4.2        Multiple IMG Senders ................................    6
   4.3        Modularity ..........................................    6
   4.4        Delivery Properties .................................    6
   4.4.1      Scalability .........................................    7
   4.4.2      Support for Intermittent Connectivity ...............    7
   4.4.3      Congestion Control ..................................    7
   4.5        Flexibility .........................................    8
   4.5.1      Customized IMGs .....................................    8
   4.5.2      Many Kinds of Multimedia Content ....................    8
   4.5.3      Sender and Receiver Driven ..........................    8
   4.6        Reliability .........................................    9
   4.6.1      Managing consistency ................................    9
   4.6.2      Reliable Message Exchange ...........................   10
   4.7        IMG Descriptions ....................................   10
   5          Security Considerations .............................   10
   5.1        IMG Authentication and Integrity ....................   11
   5.2        Privacy .............................................   11
   5.3        Access Control for IMG ..............................   12
   5.4        Denial-of-Service attacks ...........................   12
   5.5        Replay Attacks ......................................   13
   6          Acknowledgements ....................................   13
   7          Normative References ................................   13
   8          Informative References ..............................   13
   9          Authors' Addresses ..................................   14

1 Introduction

1.1 Background and Motivation

   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

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   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 must be delivered to a potentially large audience, who use it
   to join a subset of the sessions described, and who may need to be
   notified of changes to the IMG. Hence, a framework for distributing
   IMGs in various different ways is needed to accommodate the needs of
   different audiences: For traditional broadcast-style scenarios,
   multicast-based (push) distribution of IMGs needs to be supported.
   Where no multicast is available, unicast-based push is required, too.
   Furthermore, IMGs 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
   data 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 as well as
   asynchronous change notifications.

   Furthermore, we assume that any Internet host can be a source 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 IMGs can be sent and received by, among others, by
   cellular phones, PDA (Personal Digital Assistant), personal computer,

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   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 sources and sinks, different types of
   networks, and presumably numerous service providers, IMGs 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 an analysis
   of the problem space and existing mechanisms in this area. Then gives
   general requirements that are independent of any transport layer
   mechanism, existing protocol and application, such as performance,
   flexibility and reliability.

   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

   SHOULD NOT, RECOMMENDED, MAY, and "OPTIONAL" in this document are to
   be interpreted as described in RFC 2119 [6].

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

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

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

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

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

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

3 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
   Description Protocol (SDP), and the Session Initiation Protocol
   (SIP).  SDP is capable of describing multimedia sessions (i.e.
   content in a wider sence) 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.

   Also in the IETF, HTTP is a well known information 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
   between an IMG sender and IMG receiver for both bi- 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
   wireline 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

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   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 preceive 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

4 Requirements

4.1 Independence of IMG Operations from IMG Metadata

   Carrying different kinds of IMG metadata format in the IMG message
   body MUST be allowed. Delivery mechanisms SHOULD be agnostic to
   applications specific metadata to keep the system interoperable with
   existing applications. This provides flexibility in
   selecting/designing delivery protocol suited to various scenarios.

4.2 Multiple IMG Senders

   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 receiver
   access to more IMG metadata than may be available from a single IMG
   sender.  This also provides flexibility for the delivery protocols
   and does not Preclude a mechanism to solve inconsistency among IMG
   metadata due to multiple IMG senders.

4.3 Modularity

   The IMG delivery mechanisms MUST allow the combination of several IMG
   operations for the purpose of extending functionality (e.g. several
   or one protocol(s) to provide all the needed operations).
   Applications may select an appropriate operation set to fulfill their

4.4 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.

   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

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   this sense, scalability is always important for the protocols
   irrespective of transport mechanisms.

4.4.1 Scalability

   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.

   An IMG system SHOULD provide a method to prevent an IMG sender from
   sending verbose IMGs that have been stored or deleted in IMG
   receivers. Note, 'verbose' data is unneeded or unused detail or

   The protocol MUST be scalable to very large audience sizes requiring
   IMG delivery.

4.4.2 Support for Intermittent Connectivity

   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 may be beneficial
   in the fixed-Internet, but is critical in the battery-powered
   wireless Internet.

   In addition, some of the IMG senders and receivers may only be
   connected to the Internet sporadically.  As an example, consider a
   storage device requires the up-to-date video file from an IP-
   reachable video camera but the camera is connected manually within a
   limited period. When the camera is connected on the network and has a
   new video object, the storage device must be notified of the
   availability of the video file immediately.

4.4.3 Congestion Control

   Internet-friendly congestion control MUST be provided. 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

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   When an IMG item has lifetime information, the IMG entity SHOULD
   invalidate the IMG item when its lifetime is over without any IMG
   operations. This mechanism can reduce notifications of updates from
   the IMG sender to receiver to invalidate the item. It may be
   beneficial for congestion control.

4.5 Flexibility

4.5.1 Customized IMGs

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

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

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

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

4.5.2 Many Kinds of Multimedia Content

   The system MUST be able to deliver a variety of media descriptions,
   which represents multimedia items available (e.g. by download,
   streaming or multicast distribution.) This is essential for the
   system to support many kinds of multimedia content and to achieve
   wide applicability.

4.5.3 Sender and Receiver Driven

   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 receiver.

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   In contrast, the receiver-driven delivery provides on-demand delivery
   for IMG receivers. Since an IMG may contain a 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

4.6 Reliability

4.6.1 Managing consistency

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

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

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

   In the synchronized model, updating cached copy of the IMG 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.

   Since IMGs 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 two IMG metadata stored in the IMG receiver and
   IMG sender.

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

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4.6.2 Reliable Message Exchange

   IMG transport MUST support reliable message exchange. The extent to
   which this will result in 100 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.

4.7 IMG Descriptions

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

   IMG delivery MUST enable the carriage of any format of application-
   specific metadata. Whereas specific applications relying on IMG shall
   need to select one or more specific application-specific metadata
   formats (standard, syntax, etc.), the IMG system shall be agnostic to
   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

   IMG metadata MUST be structured such that it is possible to deliver
   only part of a sender's (and the global) complete IMG knowledge MUST
   be defined to include parameters, from the data model, that allow its
   payload to be uniquely identified and updated by new versions of the
   same payload. It SHALL be possible to deduce the payload format from
   the transfer envelope. 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.

   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".

5 Security Considerations

   Internet Media Guides are used to convey information about multimedia
   resources from one or more senders across one or intermediaries to
   one or more receivers. IMGs may be pushed to the receivers or

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   interactively retrieved by them. IMGs contain metadata as well as
   scheduling and rendezvous information about multimedia resources,
   etc. and requests for IMGs may contain information about the
   requesting users.

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

   This section addresses the security requirements for IMGs.

5.1 IMG Authentication and Integrity

   IMGs and their constituents need to be protected against unauthorized
   altering/adding/deletion on the way. Their originator needs to be

   R: It MUST be possible to authenticate the originator of an IMG.

   R: It MUST be possible to authenticate the originator of a subpart of
   an IMG (e.g. a delta or a subset of the information).

   R: It MUST be possible to validate the integrity of an IMG.

   R: It MUST be possible to validate the integrity of a subpart of an
   IMG (e.g. a delta or a subset of the information).

   R: It SHOULD be possible to separate or combine individually
   authenticated pieces of an IMG (e.g. in an IMG transceiver) without
   invalidating the authentication.

   R: It SHOULD be possible to validate the integrity of a piece of an
   IMG even after this piece had been separated from other pieces of an
   IMG and combined with other pieces to form a new IMG.

   R: It MUST be possible to authenticate the originator of an IMG
   related primitive.

   R: It MUST be possible to validate the integrity of any contents of
   an IMG related primitive (e.g. the subscription or inquiry

5.2 Privacy

   Customized IMGs and IMGs 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.

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   R: It MUST be possible to keep user requests to subscribe to or
   retrieve certain (parts of) IMGs confidential.

   R: It MUST be possible to keep IMGs, pieces of IMGs, or pointers to
   IMGs delivered to individual users or groups of users confidential.

   R: 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.

5.3 Access Control for IMG

   Some IMGs may be freely available, while access to other may be
   restricted to closed user groups (e.g. paying subscribers). Also,
   different parts of an IMG 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.

   R: It MUST be possible to authorize user access to IMGs.

   R: It MUST be possible to authorize access of users to pieces of IMGs
   (delta information, subparts, pointers).

   R: It MUST be possible to require different authorization for
   different parts of the same IMG.

   R: It MUST be possible to access selected IMGs anonymously.

   R: It MUST be possbile for an IMG receiver to choose not to receive
   (parts of) an IMG in order to avoid authentication by the source.

   R: It SHOULD be possible for IMG transceiver to impose different
   authorization requirements.

   R: It MAY be possible for IMG originators to require certain
   authorization that cannot be overridden by intermediaries.

5.4 Denial-of-Service attacks

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

   R: IMG operations SHOULD be authenticated.

   R: It SHOULD be possible to prevent DoS attacks that build up session

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   state in IMG components to exhaust their resources.

   R: It SHOULD be possible to avoid DoS attachs that exhaust resources
   of IMG components by flooding them with IMG content.

5.5 Replay Attacks

   IMGs dissiminated by the source or a transceiver may be updated,
   deleted, or lose validity over time for some other reasons.
   Replaying outdated IMGs needs to be prevented.

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

   R: IMGs MUST be protected against partial or full replacement of
   newer ("current") versions by older ones.

   R: Mechanisms MUST be provided to mitigate replay attacks on the IMG

6 Acknowledgements

   The authors would like to thank Hitoshi Asaeda, Juka-Pekka Luoma ,
   Petri Koskelainen, Toni Paila and Dirk Kutscher for thier comments on
   the draft.

7 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

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

8 Informative References

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   [3] Session Directory,

   [4] Session Directory Tool, http://www-

9 Authors' Addresses

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

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

   Joerg Ott <>
   Universitaet Bremen
   MZH 5180
   Bibliothekstr. 1
   D-28359 Bremen

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

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