Network Working Group Y. Nomura

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: [email protected]

Rod Walsh
Nokia Research Center
P.O. Box 100, FIN-33721 Tampere
Finland

EMail: [email protected]

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

EMail: [email protected]

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

EMail: [email protected]

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

EMail: [email protected]

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Full Copyright Statement

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