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Versions: 00 01 02 03 04 05 06 07 08 RFC 4473
MMUSIC Working Group Y. Nomura
Internet-Draft Fujitsu Labs.
Expires: June 23, 2006 R. Walsh
J-P. Luoma
Nokia
J. Ott
Universitaet Bremen
H. Schulzrinne
Columbia University
December 19, 2005
Requirements for Internet Media Guides
draft-ietf-mmusic-img-req-08
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Copyright Notice
Copyright (C) The Internet Society (2005).
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
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efficient and scalable delivery.
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 ....... 8
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 ................................... 10
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 ................................ 11
5.1.3 Modularity .......................................... 12
5.2 Delivery Properties ................................. 12
5.2.1 Scalability ......................................... 12
5.2.2 Support for Intermittent Connectivity ............... 12
5.2.3 Congestion Control .................................. 13
5.2.4 Sender and Receiver Driven Delivery ................. 13
5.3 Customized IMGs ..................................... 13
5.4 Reliability ......................................... 14
5.4.1 Managing Consistency ................................ 14
5.4.2 Reliable Message Exchange ........................... 15
5.5 IMG Descriptions .................................... 15
6 Security Considerations ............................. 17
6.1 IMG Authentication and Integrity .................... 17
6.2 Privacy ............................................. 18
6.3 Access Control for IMGs ............................. 18
6.4 Denial-of-Service attacks ........................... 19
6.5 Replay Attacks ...................................... 19
7 IANA Considerations ................................. 20
8 Normative References ................................ 20
9 Informative References .............................. 20
10 Acknowledgements .................................... 21
11 Authors' Addresses .................................. 21
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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 [2] as a
rudimentary (but as of then largely sufficient) means. Dissemination
of the descriptions has been performed using the Session Announcement
Protocol (SAP) [3] and 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 use 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.
Furthermore, IMG metadata may need to be retrieved interactively,
similar to web pages (e.g. after rebooting a system or when a user is
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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, by cellular
phones, PDA (Personal Digital Assistant), personal computer,
streaming video server, set-top box, video camera, and DVR (Digital
Video Recorder) and that they 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, context (e.g.
devices, access networks), etc.
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 existing mechanisms in this area. Then several use case
scenarios for IMGs are explained including descriptions of how IMG
metadata and IMG delivery mechanisms contribute to these scenarios.
Following this, this document provides general requirements that are
independent of any transport layer mechanism and application, such as
delivery properties, reliability and IMG descriptions.
This document reflects investigating work on delivery mechanisms for
IMGs and generalizing work on session announcement and initiation
protocols, especially in the field of the MMUSIC working group (SAP,
SIP [8], SDP).
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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 to describe
the formation, delivery and use of IMG metadata. The
definition of the IMG is intentionally left imprecise.
IMG Element: The smallest atomic element of metadata that can be
transmitted separately by IMG operations and referenced
individually from other IMG elements.
IMG Metadata: A set of metadata consisting of one or more IMG
elements. IMG metadata describes the features of multimedia
content used to enable selection of and access to media
sessions containing content. For example, metadata may
consist of the URI, title, airtime, bandwidth needed, file
size, text summary, genre and access restrictions.
IMG Delivery: The process of exchanging IMG metadata both in
terms of large scale and atomic data transfers.
IMG Sender: An IMG sender is a logical entity that sends IMG
metadata to one or more IMG receivers.
IMG Receiver: An IMG receiver is a logical entity that receives
IMG metadata from an IMG sender.
IMG Transceiver: An IMG transceiver combines an IMG receiver and
sender. It may modify received IMG metadata or merge IMG
metadata received from a several different IMG senders.
IMG Operation: An atomic operation of an IMG transport protocol,
used between IMG sender(s) and IMG receiver(s) for the
delivery of IMG metadata and for the control of IMG
sender(s)/receiver(s).
IMG Transport Protocol: A protocol that transports IMG metadata
from an IMG sender to IMG receiver(s).
IMG Transport Session: An association between an IMG sender and
one or more IMG receivers within the scope of an IMG
transport protocol. An IMG transport session involves a
time bound series of IMG transport protocol interactions
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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] talks 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 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 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 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.
SIP [8] and SIP-specific event notification [11] can be used to
notify subscribers of the update of IMG metadata for bi-directional
transport. However, it is necessary for SIP Event to define an event
package as a standard protocol for each specific application
including IMGs.
We also perceive a lack of standard solution for flexible content
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descriptions to support a multitude of application-specific metadata
and associated data models with differing 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 are mostly overcome by the XML-based SDPng [7], which is
intended for both two-way negotiation and also unidirectional
delivery. Since SDPng addresses multiparty multimedia conferences, it
is necessary to extend the XML schema in order to describe general
multimedia content.
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
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.
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
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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 importantly it enables NACK based reliability and
the individual retrieval of missed or not-multicasted sets of
metadata.
Thus, essential IMG features in this case include: robust
unidirectional delivery (with optional levels of reliability
including "plug-in FEC" supported by a transport layer protocol)
which implies easily identifiable segmentation of delivery data to
enable FEC, carousel, interleaving and other schemes possible;
effective identification of metadata sets (probably uniquely) to
enable more efficient use of multicast and unicast retrieval over
multiple access systems regardless of the parts of metadata and
application specific extensions in use; prioritization of metadata,
which can (for instance) be achieved by spreading it between channels
and allocating/distributing bandwidth accordingly.
Furthermore, some cases require IMG metadata authentication and some
group security/encryption and supporting security message exchanges
(out of band from the IMG multicast sessions).
4.1.2 Regular Fixed Dial-up Internet Connection
Dial-up connections tend to be reasonably slow (<56kbps in any case)
and thus large data transfers are less feasible, especially during an
active application session (such as a file transfer described by IMG
metadata). They can also be intermittent, especially if a user is
paying for the connected time, or connected through a less reliable
exchange. Thus this favors locally stored IMG metadata over web-based
browsing, especially where parts of the metadata change infrequently.
There may be no service provider preference over unicast and
multicast transport for small and medium numbers of users as the
last-mile dial-up connection limits per-user congestion, and a user
may prefer the more reliable option (unicast unless reliable
multicast is provided).
4.1.3 Broadband Always-on Fixed Internet Connection
Typically bandwidth is less of an issue to a broadband user and
unicast transport, such as using 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
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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 SLP based service location discovery, LDAP
based indexing services and search engines such as Google. Features
unique to IMGs include:
o IMG metadata is generally time-related
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
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
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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 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 download 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 users interest
in establishing such a session.
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 clients, the content
provider can use IMG metadata to schedule the delivery effectively.
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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 which notifies the user of changes to
metadata describing that content. Thus, as the coming release, or
pre-releases, (such 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 in the IMG
message body MUST be allowed.
REQ GEN-2: Delivery mechanisms SHOULD support many different
applications' specific metadata formats to keep the system
interoperable with existing applications.
This provides flexibility in selecting/designing IMG transport
protocol suited to various scenarios.
5.1.2 Multiple IMG Senders
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
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multiple IMG senders. This document assumes a typical IMG environment
will involve many more IMG receivers than IMG senders and that IMG
senders are continually connected for the duration of interest
(rather than intermittently connected).
5.1.3 Modularity
REQ GEN-4: The IMG delivery mechanisms MUST allow the combination of
several IMG Operations.
This is for the purpose of extending functionality (e.g. several or
one protocol(s) to provide all the needed operations). Applications
can select an appropriate operation set to fulfill their purpose.
5.2 Delivery Properties
This section describes general performance requirements based on the
assumption that the range of IMG usage shall be important. However,
it may be noted that requirements of delivery properties may vary
based on the usage scenario, and thus some limited use
implementations place less importance on some requirements.
For example, it is clear that a multicast transport may provide more
scalable delivery than a unicast transport, however scalability
requirements do not preclude the unicast transport mechanisms. In
this sense, scalability is always important for the protocols
irrespective of transport mechanisms.
5.2.1 Scalability
REQ DEL-1: The 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.
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This allows intermittent access to save power where there is no need
to keep communications links powered-up while they are sitting idle.
For instance, in this situation periodic bursts of notifies, or a
fast cycling update carousel, allows hosts to wake up for short
periods of time and still be kept up-to-date. This can be beneficial
for IMG receivers with sporadic 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 enable
lesser congestion.
5.2.4 Sender and Receiver Driven Delivery
REQ DEL-7: The system MUST be flexible in choosing sender-driven,
receiver-driven or both delivery schemes.
Sender-driven delivery achieves high scalability without interaction
between the IMG sender and receiver. This avoids keeping track of a
delivery state for every IMG receiver.
In contrast, the receiver-driven delivery provides on-demand delivery
for IMG receivers. Since an IMG sender's complete IMG metadata may be
a very large amount of data, the IMG receiver needs to be able to
access the guide when convenient (e.g. when sufficient network
bandwidth is available to the IMG receiver).
5.3 Customized IMGs
REQ CUS-1: The system MUST allow delivery of customized IMG metadata.
The IMG receiver may require a subset of all the IMG metadata
available according to their preferences (type of content, media
description, appropriate age group, etc.) and configuration.
The IMG receiver might send its preferences in the IMG operations
which can specify user specific IMG metadata to be delivered. These
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preferences could consist of filtering rules. When receiving these
messages, the IMG sender might respond with appropriate messages
carrying a subset of IMG metadata which matches the IMG receiver's
preferences.
This mechanism can reduce the amount of IMG metadata delivered from
the IMG sender to IMG receiver, and consequently it can save the
resource consumption on the IMG entities and networks. It is
typically useful in unicast 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 its joined channels). However, it may select
and filter the IMG metadata to get customized IMG metadata by its
preferences, and thus drop unwanted metadata immediately upon
reception.
Customized metadata might be achieved by changing the IMG
descriptions sent and IMG receivers and/or changing the delivery
properties (channels used).
Note, customization and scalability are only somewhat exclusive.
Systems providing an IMG receiver to an IMG sender request-based
customization, will be generally less scalable to massive IMG
receiver populations than those without this return signaling
technique. Thus, customization, as with any feature which affects
scalability, should be carefully designed for the intended
application, and it may not be possible that a one-size-fits-all
solution for customization would meet the scalability requirements
for all applications and deployment cases.
5.4 Reliability
5.4.1 Managing Consistency
IMG metadata tends to change as time elapses, as new content is
added, the old IMG metadata stored in the IMG receiver becomes
unavailable and the parameters of the existing IMG metadata are
changed.
REQ REL-1: The system MUST manage IMG metadata consistency.
The IMG sender can either simply make updates available
(unsynchronized) or IMG sender and receiver can interact to keep
their copies of the IMG metadata synchronized.
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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 requirements 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 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), 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 a content and media is
more important immediately prior to and during the media's
session(s). Hosts which forward (or otherwise resend) metadata may be
less tolerant to inconsistencies as delivering out of date data is
both misleading and bandwidth inefficient.
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, 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
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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 homogenous metadata
syntax for all uses (which would be infeasible anyway). This is
essential for environments using IMG systems to support many kinds of
multimedia content and to achieve wide applicability.
REQ DES-3: Whereas specific applications relying on IMGs will need to
select one or more specific application-specific metadata formats
(standard, syntax, etc.), the IMG system MUST be independent of this
(it may be aware, but it will operate in the same way for all).
Thus, a 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.
REQ DES-4: IMG metadata MUST be structured to enable fragmentation
for efficient delivery.
This is intended to ensure that and 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 effect 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.
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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. (E.g. a data model could be
instantiated using both SDP and SDPng.)
REQ DES-8: IMG metadata SHOULD support the description of differences
between update version and old version of IMG metadata when 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 intermediaries
to one or more IMG receivers. IMG metadata may be pushed to the IMG
receivers or interactively retrieved by them. IMGs provide metadata
as well as scheduling and rendezvous information about multimedia
resources, etc. and requests for IMG metadata may contain information
about the requesting users.
The information contained in IMG metadata as well as the operations
related to IMGs should be secured to avoid forged information,
misdirected users, spoofed IMG senders, etc. and to protect user
privacy.
The remainder of section addresses the security requirements for
IMGs.
6.1 IMG Authentication and Integrity
IMG metadata and its parts need to be protected against unauthorized
altering/adding/deletion on the way. Their originator needs to be
authenticated.
REQ AUT-1: It MUST be possible to authenticate the originator of a
set of IMG metadata.
REQ AUT-2: It MUST be possible to authenticate the originator of a
subpart of IMG metadata (e.g. a delta or a subset of the
information).
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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 had been separated from other pieces of IMG metadata and
combined with other pieces to form new IMG metadata.
REQ AUT-7: It MUST be possible to authenticate the originator of an
IMG operation.
REQ AUT-8: It MUST be possible to validate the integrity of any
contents of an IMG operation (e.g. the subscription or inquiry
information).
6.2 Privacy
Customized IMG metadata and IMG metadata delivered by notification to
individual users may reveal information about the habits and
preferences of a user and may thus deserve confidentiality
protection, even though the information itself is public.
REQ PRI-1: It MUST be possible to keep user requests to subscribe to
or retrieve certain (parts of) IMG metadata confidential.
REQ PRI-2: It MUST be possible to keep IMG metadata, pieces of IMG
metadata, or pointers to IMG metadata delivered to individual users
or groups of users confidential.
REQ PRI-3: It SHOULD be possible to ensure this confidentiality
end-to-end, that is, to prevent intermediaries (such as IMG
transceivers) from accessing the contained information.
6.3 Access Control for IMGs
Some IMG metadata may be freely available, while access to other 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: e.g. metadata describing a media session may be
freely accessible while rendezvous information to actually access the
media session may require authorization.
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Internet Draft IMG Requirements December 19, 2005
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 which are compatible between both
IMG senders and IMG receivers it interacts with.
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 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 which 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, deleted, or lose validity over time for some other
reasons. Replaying outdated IMG metadata needs to be prevented.
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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 delivering older versions of metadata than others - as a
result of imperfect sender-sender data consistency. Thus, replay
attacks and delivery of inconsistent data requires that an IMG
receiver veryfies 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.
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 IANA Considerations
This document does not request any IANA action.
8 Normative References
[1] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, March 1997.
9 Informative References
[2] M. Handley and V. Jacobson, "SDP: session description protocol,"
RFC 2327, Internet Engineering Task Force, April 1998.
[3] M. Handley, C. E. Perkins, and E. Whelan, "Session announcement
protocol," RFC 2974, Internet Engineering Task Force, 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/
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[6] Digital Video Broadcasting Project,
http://www.dvb.org/
[7] D. Kutscher, J. Ott, and C. Bormann, "Session description and
capability negotiation," Internet Draft draft-ietf-mmusic-sdpng-07,
Internet Engineering Task Force, October 2003. Work in progress.
[8] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.
[9] Y. Nomura, R. Walsh, J-P. Luoma, H. Asaeda, and H. Schulzrinne,
"A framework for the usage of Internet media guides," Internet Draft
draft-ietf-mmusic-img-framework-09, Internet Engineering Task Force,
December 2005. Work in progress.
[10] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, L. Masinter, P.
Leach and T. Berners-Lee, "Hypertext transfer protocol -- HTTP/1.1,"
RFC 2616, Internet Engineering Task Force, June 1999.
[11] A. B. Roach, "Session initiation protocol (sip)-specific event
notification," RFC 3265, Internet Engineering Task Force, June 2002.
[12] B. Quinn and K. Almeroth, "IP multicast applications: Challenges
and solutions," RFC 3170, Internet Engineering Task Force, September
2001.
10 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.
11 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
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Internet Draft IMG Requirements December 19, 2005
Juha-Pekka Luoma
Nokia Research Center
P.O. Box 100, FIN-33721 Tampere
Finland
Email: juha-pekka.luoma@nokia.com
Joerg Ott
Universitaet Bremen
MZH 5180
Bibliothekstr. 1
D-28359 Bremen
Germany
Email: jo@tzi.uni-bremen.de
Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA
Email: schulzrinne@cs.columbia.edu
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