Internet Engineering Task Force MMUSIC WG
Internet Draft Y. Nomura
Fujitsu Labs.
R. Walsh
J-P. Luoma
Nokia
H. Asaeda
INRIA
H. Schulzrinne
Columbia University
draft-ietf-mmusic-img-framework-01.txt
December 2, 2003
Expires: March 2004
A Framework for the Usage of Internet Media Guides
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
Abstract
This document defines a framework for the delivery of Internet Media
Guides (IMGs). An IMG is a structured collection of multimedia
session descriptions expressed using SDP, SDPng or some similar
session description format. This document describes several use case
scenarios requirering the IMG framework, a generalized model for IMG
delivery mechanisms, and the use of existing protocol to create an
IMG delivery infrastructure.
Y. Nomura et. al. [Page 1]
Internet Draft IMG Framework December 2, 2003
Table of Contents
1 Introduction ........................................ 3
1.1 Background and Motivation ........................... 3
1.2 Scope of this Document .............................. 4
2 Terminology ......................................... 4
3 Use Cases Requiring IMG Framework ................... 5
3.1 Connectivity-based Use Cases ........................ 5
3.1.1 IP Datacast to a Wireless Receiver .................. 5
3.1.2 Regular Fixed Dial-up Internet Connection ........... 6
3.1.3 Broadband Always-on Fixed Internet Connection ....... 6
3.2 Content-orientated Use Cases ........................ 6
3.2.1 File Distribution ................................... 7
3.2.2 TV and Radio Program Delivery ....................... 7
3.2.3 Media Coverage of a Live Event ...................... 8
3.2.4 Distance Learning ................................... 8
3.2.5 Multiplayer Gaming .................................. 8
4 IMG Common Framework Model .......................... 8
4.1 IMG Data Types ...................................... 9
4.1.1 Complete Description ................................ 9
4.1.2 Delta Description ................................... 9
4.1.3 Pointer ............................................. 10
4.2 Operation Set for IMG Delivery ...................... 10
4.2.1 IMG ANNOUNCE ........................................ 10
4.2.2 IMG QUERY ........................................... 10
4.2.3 IMG RESOLVE ......................................... 11
4.2.4 IMG SUBSCRIBE ....................................... 11
4.2.5 IMG NOTIFY .......................................... 11
4.3 IMG Entities ........................................ 12
4.4 Overview of Protocol Operations ..................... 12
5 Deployment Scenarios for IMG Entities ............... 13
5.1 Intermediary Cases .................................. 13
5.2 One-to-many Unidirectional Multicast ................ 15
5.3 One-to-one Bi-directional Unicast ................... 15
5.4 Combined Operations with Common Metadata ............ 16
6 Applicability of Existing Protocols to the
Proposed Framework Model ....................................... 17
6.1 Summary of Limitations of Existing Protocols ........ 17
6.2 Existing Protocol Fit to the IMG Framework Model
6.3 Outstanding IMG Mechanism Needs ..................... 19
6.3.1 A Multicast Transport Protocol ...................... 19
6.3.2 Usage of Unicast Transport Protocols ................ 20
6.3.3 The Metadata Envelope ............................... 20
6.3.4 Baseline (Meta)Data Model Specification ............. 21
6.4 IMG Needs Fitting the IETF's Scope .................. 22
7 Security Considerations ............................. 23
8 Normative References ................................ 24
9 Informative References .............................. 25
10 Acknowledgements .................................... 26
Y. Nomura et. al. [Page 2]
Internet Draft IMG Framework December 2, 2003
11 Authors' Addresses .................................. 26
1 Introduction
1.1 Background and Motivation
Internet Media Guide (IMG) provide structured collections of
multimedia descriptions expressed using SDP, SDPng or some similar
description format. It is used to describe sets of multimedia
sessions (e.g. television program schedules, content delivery
schedules etc.) and refer to other networked resources including web
pages. IMGs provide an envelope for metadata formats and session
descriptions defined elsewhere with the aim of facilitating
structuring, versioning, referencing, distributing, and maintaining
(caching, updating) such information.
Firstly, this document explains several use case scenarios requiring
the IMG framework. IMGs are inherently required to be independent of
any particular access network, and link in general. Therefore, they
are suitable in many Internet access scenarios including fixed and
mobile devices, wired and satellite and terrestrial radio, always-on
Internet and intermittent connectivity, and so on. Furthermore, IMGs
provides essential functions that facilitate better distribution of
content. Section 3 describes how IMGs and IMG delivery mechanisms
contribute for the scenarios.
Then, this document defines a generalized model for IMG delivery
mechanisms and their deployment in network entities regarding the use
case scenarios. IMG metadata must be delivered to a potentially large
audience, who use it to join a subset of the sessions described, and
who may need to be notified of changes to the IMG metadata. 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.
Finally, this document outlines the use of existing protocols to
create an IMG delivery infrastructure. It aims to organize existing
protocols into common model and show their capabilities and
limitations from the view point of IMG delivery functions. One of the
multicast-enabling IMG requirements is scaling well to a large number
of hosts and IMG senders in a network. Another issue is the need for
flexibility and diversity in delivery methods, whereas existing
protocols tend to be bound to a specific application.
Y. Nomura et. al. [Page 3]
Internet Draft IMG Framework December 2, 2003
1.2 Scope of this Document
This document defines a common framework model for the delivery of
Internet Media Guide (IMG) metadata. The framework describes existing
mechanisms and the level to which they support and enable the
framework. The requirements for IMG delivery mechanisms and
descriptions can be found in [1].
A brief run through the usage and use cases of media guide is
provided to illustrate the relevance of IMGs before the framework
model is presented. The framework model defines the data types,
operations and entities which are needed. These are then shown in a
number of simplified deployment scenarios.
Existing protocols are organized and referenced against the framework
model to show the degree to which they fulfill IMG framework and
requirements. This also makes it straightforward to identify gaps so
that new protocols needs are made apparent.
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].
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 Metadata: This is a set of metadata describing 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 IMGs to
one or more IMG receivers.
IMG Receiver: An IMG receiver is a logical entity that receives
IMGs from an IMG sender.
IMG Transceiver: An IMG transceiver combines an IMG receiver and
sender. It may modify original IMGs or merge several IMGs
from a different IMG sender.
Y. Nomura et. al. [Page 4]
Internet Draft IMG Framework December 2, 2003
IMG Operation: An atomic process for the IMG transport protocol
to deliver IMG metadata or control IMG sender(s) or IMG
receiver(s).
IMG Transport Protocol: A protocol that transports IMG metadata
from IMG sender to IMG receiver(s)
3 Use Cases Requiring IMG Framework
3.1 Connectivity-based Use Cases
3.1.1 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 provides a base level of
robustness for an IP datacast based announcement system, although the
design of carouselled transfer should enable battery-powered
receivers to go through periods of sleep to extend their operational
time between charges. Insertion of Forward Error Correction (FEC)
data into metadata announcements improves error resilience, and
reordering (interleaving) data blocks further increases tolerance to
burst errors.
To enable receivers to more accurately specify the metadata they are
interested in, the unidirectional delivery is distributed between
several logical channels. This is so that a receiver need only access
the channels of interest and thus can reduce the amount of time and
processing of IP data (and storage). Also, hierarchical channels
enable receivers to subscribe to a root, possibly well known,
multicast channel/group and progressively access only those
additional channels based on metadata in parent channels.
In some cases the receiver may be multi-access, such that it is
capable of bi-directional communications. This enables a multitude of
options, but most importantly it enables NACK based reliability and
the individual retrieval of missed or not-multicasted sets of
metadata.
Thus, essential IMG features in this case include: robust
unidirectional delivery (with optional levels of reliability
including "plug-in FEC") which implies easily identifiable
Y. Nomura et. al. [Page 5]
Internet Draft IMG Framework December 2, 2003
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).
3.1.2 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).
3.1.3 Broadband Always-on Fixed Internet Connection
Typically bandwidth is less of an issue to a broadband user and
unicast transport, such as using IMG QUERY, may be typical for a PC
user. If a system were only used in this context, with content
providers, ISPs and users having no other requirements, then web-
based browsing may be equally suitable. However, broadband users
sharing a local area network, especially wireless, may benefit more
from local storage features than on-line browsing, especially if they
have intermittent Internet access.
Broadband enables rich media services, which are increasingly
bandwidth hungry. Thus backbone operators may prefer multicast
communications to reduce overall congestion, if they have the
equipment and configuration to support this. Thus, broadband users
may be forced to retrieve IMG metadata over multicast if the
respective operators require this to keep system-wide bandwidth usage
feasible.
3.2 Content-orientated Use Cases
Y. Nomura et. al. [Page 6]
Internet Draft IMG Framework December 2, 2003
IMGs will be able to support a very wide range of use cases for
content/media delivery. The following few sections just touch the
surface of what is possible and are intended to provide an
understanding of the scope and type of IMG usage. Many more examples
may be relevant, for instance those detailed in[3]. 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
3.2.1 File Distribution
IMGs support the communication of file delivery session properties,
enabling the scheduled delivery or synchronization of files between a
number of hosts. For example, the metadata that IMGs provide could be
subsequently used by a different application (outside the scope of
IMGs) to download a file with a software update. An IMG metadata can
provide a description to each file in a file delivery session,
assisting users or receiving software in selecting individual files
for reception.
For example, when a content provider wants to distribute a large
amount of data in file format to thousands clients, the content
provider can use IMG metadata to schedule the delivery effectively.
Since IMG metadata can describe time-related data for each receiver,
the content provider can schedule delivery time for each receiver.
This can save network bandwidth and capacity of delivery senders. In
addition, IMG metadata can be used to synchronize a set of files
between a sender host and receiver host, when those files change as
time elapses.
3.2.2 TV and Radio Program Delivery
A sender of audio/video streaming content can use the IMG metadata to
describe the scheduling and other properties of audio/video sessions
and events within those sessions, such as individual TV and radio
programs and segments within those programs. IMG metadata describing
audio/video streaming content could be represented in a format
similar to that of a TV guide in newspapers, or an Electronic Program
Guide available on digital TV receivers.
Y. Nomura et. al. [Page 7]
Internet Draft IMG Framework December 2, 2003
Similarly to file reception, TV and radio programs can be selected
for reception either manually by the end-user or automatically based
on the content descriptions and the preferences of the user. The
received TV and radio content can be either presented in real time or
recorded for consuming later. There may be changes in the scheduling
of a TV or a radio program, possibly affecting the transmission times
of subsequent programs. IMG metadata can be used to notify receivers
of such changes, enabling users to be prompted or recording times to
be adjusted.
3.2.3 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 could 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.
3.2.4 Distance Learning
IMG metadata could describe compound sessions or services enabling
several alternative interaction modes between the participants. For
example, the combination of one-to-many media streaming, unicast
messaging and download of presentation material could be useful for
distance learning.
3.2.5 Multiplayer Gaming
Multiplayer games are an example of real-time multiparty
communication sessions that could be advertised using IMGs. A gaming
session could be advertised either by a dedicated server, or by the
terminals of individual users. A user could use IMGs to learn of
active multiplayer gaming sessions, or advertise the users interest
in establishing such a session.
4 IMG Common Framework Model
Two common elements are found in all of existing IMG candidate cases:
the need to describe the services; the need to deliver the
descriptions. In some cases, the descriptions are multicast enablers
(such as the session parameters of SDP) and are thus intrinsically
Y. Nomura et. al. [Page 8]
Internet Draft IMG Framework December 2, 2003
part of the delivery aspects, and in other cases descriptions are
application-specific (both machine and human readable). Thus, the
technologies can be roughly divided into three areas:
o Application-specific Metadata -- data describing the services'
content and media which are both specific to certain
applications and generally human readable.
o Delivery Descriptors -- the descriptors (metadata) that are
essential to enable (e.g. multicast) delivery. These include
framing (headers) for application-specific metadata, the
metadata element identification and structure, fundamental
session descriptors.
o Delivery Protocols -- the methods and protocols to exchange
descriptions between the senders and the receivers. An IMG
delivery protocol consists of two functions: carrying IMG
metadata from an IMG sender to an IMG receiver and controlling
an IMG transport protocol. These functions are not always
exclusive, therefore some messages may combine control
messages and metadata carriage functions metadata to reduce
the amount of the messaging.
4.1 IMG Data Types
A data model is needed to precisely define the terminology and
relationships between sets, supersets and subsets of metadata. A
precise data model is essential for the implementation of IMGs
although it is not within the scope of this framework and requires a
separate specification. However there are three IMG data types in
general: Complete description, delta description and pointer.
4.1.1 Complete Description
A Complete Description provides a complete syntax and semantics to
describe a set of metadata, which does not need any additional
information from other IMG entity.
Note, this is not to be confused with "complete IMG metadata", which,
although vaguely defined here, represents the complete database of a
sender (or related group of senders -- potentially the complete
Internet IMG knowledge). A sender will generally deliver only subsets
of metadata from its complete database of metadata in a particular
data exchange.
4.1.2 Delta Description
A Delta Description provides only part of a Complete Description,
Y. Nomura et. al. [Page 9]
Internet Draft IMG Framework December 2, 2003
defining the difference from a previous version of the Complete
Description in question. This descriptor may be used to reduce
network resource usage (it may be more bandwidth and congestion
friendly), for instance, data consistency is essential and, small and
frequent changes occur to the Complete Description. Thus, this
descriptor itself cannot represent complete metadata set until it is
combined with existing, or future, description knowledge.
4.1.3 Pointer
A pointer provided a simple identifier or locator, such as a URL,
that the receiver is able to reference (or reference and locate)
specific metadata with. This may be used to separately obtain
metadata (complete or delta descriptions) or perform another IMG
management function such as data expire (and erasure). The pointer
may be used to reference IMG metadata elements within the IMG
transport session and across IMG transport sessions. The pointer does
not include metadata par se (although it may also appear as a data
field in complete or delta descriptors).
4.2 Operation Set for IMG Delivery
A finite set of operations both meets the IMG requirements [2] and
fits the roles of existing protocols. These are crystallized in the
next few sections.
4.2.1 IMG ANNOUNCE
When an IMG receiver participates in unidirectional communications
(e.g. over satellite, terrestrial radio and wired multicast networks)
an IMG receiver may not need to send any message to an IMG sender
prior to IMG metadata delivery. In this case, an IMG sender can
initiate unsolicited distribution for IMG metadata and an IMG sender
is the only entity which can maintain the distribution (this includes
scenarios with multiple and co-operative senders). This operation is
useful when there are considerably large number IMG receivers or IMG
receiver(s) do not have a guaranteed uplink connection to the IMG
sender(s). The sender may also include authentication data in the
announce operation so that receivers may check the authenticity of
the metadata. This operation is able to carry any of the IMG data-
types.
Note, there is no restriction to prevent IMG ANNOUNCE from being used
in an asynchronous solicited manner, where a separate operation
(possibly out of band) is able to subscribe/register receivers to the
IMG ANNOUNCE operation.
4.2.2 IMG QUERY
Y. Nomura et. al. [Page 10]
Internet Draft IMG Framework December 2, 2003
If an IMG receiver needs to obtain IMG metadata, an IMG receiver can
send an IMG QUERY message and initiate a receiver-driven IMG delivery
session. The IMG receiver expects a synchronous response to the
subsequent request from the IMG sender. This operation can be used
where a bi-directional transport network is available between the IMG
sender and receiver. Some IMG receivers may want to obtain IMG
metadata when a resource is available or just to avoid caching
unsolicited IMG metadata. The IMG receiver must indicate the extent
and data type of metadata wanted in some message in the operation
(Extent indicates the number and grouping of metadata descriptions).
In some cases requesting a sender's complete IMG metadata may be
feasible, in others it may not.
4.2.3 IMG RESOLVE
An IMG sender synchronously responds and sends IMG metadata to an IMG
QUERY which has been sent by an IMG receiver. This operation can be
used where a bi-directional transport network is available between
the IMG sender and receiver. If the IMG QUERY specifies a subset of
IMG metadata (extent and data type) that the IMG sender has the
subset, the IMG sender can resolve this, otherwise it should indicate
that it is not able to resolve the query. The IMG sender may
authenticate the IMG receiver to investigate the IMG QUERY operation
in order to determine whether the IMG receiver is authorized to be
sent that metadata. The sender may also include authentication data
in the resolve operation so that receivers may check the authenticity
of the metadata. This operation may carry any of the IMG data types.
4.2.4 IMG SUBSCRIBE
If an IMG receiver wants to be notified when metadata which the IMG
receiver holds is stale, the IMG receiver can start the IMG SUBSCRIBE
operation prior in order to solicit notify messages. Since the IMG
receiver doesn't know when metadata will be updated and the notify
message will arrive, this operations does not synchronize with the
notify message. The IMG receiver may wait for the notify message for
a long time. The IMG sender may authenticate the IMG receiver to
investigate whether an IMG SUBSCRIBE operation is from an authorized
receiver.
4.2.5 IMG NOTIFY
An IMG receiver needs the response to an earlier IMG SUBSCRIBE and
the IMG NOTIFY indicates that updated IMG metadata is available or
part of the existing IMG metadata is stale. An IMG NOTIFY may be
delivered more than once during the time an IMG SUBSCRIBE is active.
This operation may carry any of the IMG data types. The sender may
also include authentication data in the notify operation so that
Y. Nomura et. al. [Page 11]
Internet Draft IMG Framework December 2, 2003
receivers may check the authenticity of the messages.
4.3 IMG Entities
There are several fundamental IMG entities that indicate the
capability to perform certain roles. An Internet host involved in IMG
operations may adopt one or more of these roles:
IMG Announcer : send IMG ANNOUNCE
IMG Listener : receive IMG ANNOUNCE
IMG Querier : send IMG QUERY to receive IMG RESOLVE
IMG Resolver : receive IMG QUERY then send IMG RESOLVE
IMG Subscriber: send IMG SUBSCRIBE then receive IMG NOTIFY
IMG Notifier : receive IMG SUBSCRIBE then send IMG NOTIFY
Finally, the figure 1 shows a relationship between IMG entities and
the IMG sender and receiver.
+--------------------------------------------------------+
| IMG Sender |
+------------------+------------------+------------------+
| IMG Announcer | IMG Notifier | IMG Resolver |
+------------------+------------------+------------------+
| ^ ^
message | | |
direction v v v
+------------------+------------------+------------------+
| IMG Listener | IMG Subscriber | IMG Querier |
+------------------+------------------+------------------+
| IMG Receiver |
+------------------+------------------+------------------+
Figure 1: Relationship with IMG Entities
4.4 Overview of Protocol Operations
The figure 2 gives an overview of the relationship between transport
cases, IMG Operations and IMG data types (note, it is not a protocol
Y. Nomura et. al. [Page 12]
Internet Draft IMG Framework December 2, 2003
stack).
+--------------------------------------------------+
IMG | |
Data types | Complete Desc., Delta Desc., Pointer |
| |
+-------------------+----------------+-------------+
IMG | IMG ANNOUNCE | IMG SUBSCRIBE | IMG QUERY |
Operations | | IMG NOTIFY | IMG RESOLVE |
+--------------+----+----------------+-------------+
IMG | | |
Transport | P-to-M | P-to-P |
| | |
+--------------+-----------------------------------+
Figure 2: IMG Operations and IMG Data type
5 Deployment Scenarios for IMG Entities
This section provides some basic deployment scenarios for IMG
entities that illustrate common threads from protocols to use cases.
For the purposes of clarity, this document presents the simple
dataflow from sender to receiver as shown in figure 3
+-------------+ +---------------+
| IMG Sender | | IMG Receiver |
| |--------------->| |
+-------------+ +---------------+
Figure 3: A Simple IMG Sender to IMG Receiver Relationship
5.1 Intermediary Cases
Some IMG metadata may be distributed to a large number of receivers.
If, for example, some IMG metadata is public information and the
sender provides the same information for all receivers. This kind of
Y. Nomura et. al. [Page 13]
Internet Draft IMG Framework December 2, 2003
IMG metadata may be distributed from one sender to multiple receivers
(Figure 4) and/or or may be relayed across a set of IMG transceivers
that receive the IMG metadata, possibly filter or modify its content,
and then forward it. The relayed network architecture is similar to
content distribution network architecture [3] (CDNs). Existing CDNs
may carry IMG metadata. Satellite and peer-to-peer networks may also
carry IMG metadata.
+----------+ +----------+
| IMG | | IMG |
| Sender |---- ---->| Receiver |
+----------+ \ / +----------+
\ /
. \ +-----------+ / .
. -->|IMG |----- .
. -->|Transceiver| \ .
/ +-----------+ \
+----------+ / \ +----------+
| IMG | / ---->| IMG |
| Sender |---- | Receiver |
+----------+ +----------+
Figure 4: A Relay Network with an IMG Transceiver
IMG senders and receivers are logical functions and it is possible
for some or all hosts in a system to perform both roles, as, for
instance, in many-to-many communications or where a transceiver is
used to combine or aggregate IMG metadata for some receivers. An IMG
receiver may be allowed to receive IMG metadata from any number of
senders.
The IMG metadata is used to find, obtain, manage and play content.
The IMG metadata distribution may be modified as they are
distributed. For example, a server may use IMGs to retrieve media
content via unicast and then make it available at scheduled times via
multicast, thus requiring a change of the corresponding metadata. IMG
transceivers may add or delete information or aggregate IMG metadata
from different senders. For example, a rating service may add its own
content ratings or recommendations to existing meta-data. An
implication of changing (or aggregating) IMG metadata from one or
more senders is that the original authenticity is lost. Thus for
deployments requiring these kind of features, the original metadata
Y. Nomura et. al. [Page 14]
Internet Draft IMG Framework December 2, 2003
should be reasonably fragmented already (allowing the intermediary to
replace a small fragment without changing the authenticity of the
remainder). It may be beneficial to use smaller fragments for more
volatile parts, and larger one for more stable parts.
5.2 One-to-many Unidirectional Multicast
This case implies many receivers and one or more senders implementing
IMG ANNOUNCER and IMG LISTENER operations as shown in figure 5.
Unidirectional +----------+
---------------> | IMG |
downlink | Listener |
------------->| 1 |
/ +----------+
+-----------+ / .
| IMG |-------- .
| Announcer | \ .
+-----------+ \ +----------+
------------->| IMG |
| Listener |
| # |
+----------+
Figure 5: IMG Unidirectional Multicast Distribution Example
5.3 One-to-one Bi-directional Unicast
+----------+ +----------+
| IMG |<------(1)------| IMG |
| Resolver |-------(2)----->| Querier |
+----------+ +----------+
Figure 6: Query/Resolve Deployment Example
Both query/resolve (figure 6) and subscribe/notify (figure 7) message
Y. Nomura et. al. [Page 15]
Internet Draft IMG Framework December 2, 2003
exchange operations are feasible.
+----------+ +------------+
| |<-------(1)--------| |
| IMG |--------(2)------->| IMG |
| Notifier | (time passes) | Subscriber |
| |--------(3)------->| |
+----------+ +------------+
Figure 7: Subscribe/Notify Deployment Example
5.4 Combined Operations with Common Metadata
As shown in figure 8, a common data model for multiple protocol
operations allows a diverse range of senders and receivers to provide
consistent and interoperable IMGs.
IMG Metadata IMG Senders IMG Receivers
+--------------+
+-----------+ ---->| IMG Listener |
| IMG | / +--------------+
/| Announcer |-----
+-------------+ / +-----------+ \ +--------------+
| IMG |-+ / ---->| IMG Listener |
| Description | |-+ / | - - - - - - -|
| metadata 1 | | | / +-----------+ /--->| IMG Querier |
+-------------+ | | -----| IMG |<----/ +--------------+
+-------------+ | \ | Resolver |
+-------------+ \ +-----------+<----\ +--------------+
\ \--->| IMG Querier |
\ +-----------+ | - - - - - - -|
\| IMG |<--------->| IMG |
| Notifier | | Subscriber |
+-----------+ +--------------+
Figure 8: Combined System with Common Metadata
Y. Nomura et. al. [Page 16]
Internet Draft IMG Framework December 2, 2003
6 Applicability of Existing Protocols to the Proposed Framework Model
6.1 Summary of Limitations of Existing Protocols
SDP [4] has a text-encoded syntax to specify multimedia sessions for
announcements and invitations. Although SDP is extensible, it has
limitations such as structured extensibility and capability to carry
another multimedia session descriptors.
These are mostly overcome by the XML-based SDPng [5] , 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.
MPEG-7 [6] is a collection of XML-based description tools for general
multimedia content including structured multimedia sessions. TV-
Anytime Forum [7] provides descriptions based on MPEG-7 for TV
specific program guides. These are likely to be limited to describe
pictures, music and movies, thus it may be necessary to extend these
descriptions in order to define a variety of documents, game
programs, binary files, live event and so on.
HTTP [8] 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 [9] is an announcement protocol that distributes session
descriptions via multicast. It does not support fine-grained meta
data selection and update notifications, as it always sends the whole
meta data. Given the lack of a wide-area multicast infrastructure, it
is likely only deployable within a local area network.
SIP [10] and SIP-specific event notification [11] can be used to
notify subscribers of the update of IMG metadata for bi-directional
transport. It is necessary for SIP Event to define an event package
for each specific application such as [12].
6.2 Existing Protocol Fit to the IMG Framework Model
SDP: The SDP format could be used to describe session-level
parameters (e.g. scheduling, addressing and the use of media codecs)
to be included in Complete Descriptors. Although there are extension
points in SDP allowing the format to be extended, there are
limitations in the flexibility of this extension mechanism. However,
Y. Nomura et. al. [Page 17]
Internet Draft IMG Framework December 2, 2003
SDP syntax can not provide with Partial Descriptors and Pointers
without significant unused overhead. Because it is expected that the
information conveyed by SDP is just a small subset of IMG metadata
the use of SDP for other than session-level IMG parameters may not be
reasonable.
SDPng: Similar to SDP, this format could also be used for
representing session-level parameters of IMG metadata. Compared to
SDP, the XML-based format of SDPng is much more flexible with regards
to extensions and combining with other description formats.
MPEG-7: Descriptions based on the MPEG-7 standard could provide
application-specific metadata describing the properties of multimedia
content beyond parameters carried in SDP or SDPng descriptions.
MPEG-7 provides a machine-readable format of representing content
categories and attributes, helping end-users or receiving software in
choosing content for reception: this is well in line with the IMG
usage scenarios of IMGs introduced in 3.2. Because MPEG-7 is based on
XML, it is well suited for combining with other XML-based formats
such as SDPng.
HTTP: The HTTP protocol can be used as a bi-directional/unicast IMG
transport protocol. Being a request-reply oriented protocol, HTTP is
well suited for implementing synchronous operations such as QUERY,
RESOLVE and even SUBSCRIBE. However, HTTP does not provide
asynchronous operations such as ANNOUNCE and NOTIFY and to implement
asynchronous operations using HTTP, IMG receivers should poll the IMG
sender periodically. So alone, HTTP is not sufficient to fulfill IMG
requirements in a unicast deployment.
SAP: The announcement mechanism provided by SAP provides
unidirectional delivery of session discovery information. Although
SDP is the default payload format of SAP, the use of a MIME type
identifier for the payload allows arbitrary payload formats to be
used in SAP messages. Thus, SAP could be used to implement the
(multicast and unicast) IMG ANNOUNCE and IMG NOTIFY operations.
However, the limitations of SAP as a generic IMG transport protocol
include:
- Lack of reliability (through forward error correction / retransmissions)
- Lack of payload segmentation
- Limited payload size
- Only one description allowed per SAP message
- Lack of congestion control
- Lack of Internet standard authentication / encryption mechanisms
- It is an Experimental RFC with no support for progressing further
Y. Nomura et. al. [Page 18]
Internet Draft IMG Framework December 2, 2003
In principle, the current SAP protocol could be extended to get
around its limitations (e.g. the use of a multipart MIME type in the
SAP payload has been proposed, enabling multiple descriptions to be
carried in a single SAP message). However, the amount of changes
needed in SAP to address all of the above limitations would
effectively result in a new protocol. Due to these limitations, the
use of SAP as an IMG transport protocol is not recommended.
SIP: The SIP-specific event mechanism described in [11] provides a
way to implement IMG SUBSCRIBE and IMG NOTIFY operations via a bi-
directional unicast connection. However, there are scalability
problems with this approach, as RFC 3265 currently does not consider
multicast.
6.3 Outstanding IMG Mechanism Needs
Several outstanding needs result from the IMG requirements, framework
model and existing relevant mechanisms as already shown in this
document. Four specific groupings of work are readily apparent which
are: (a) specification of an adequate multicast and unidirectional
capable announcement protocol; (b) specification of the use of
existing unicast protocols to enable unicast subscribe and
announcement/notification functionality; (c) specification of the
metadata envelope which is common to, and independent of, the
application metadata syntax(es) used; agreement on basic metadata
models to enable interoperability testing of the above. The following
sections describe each of these.
6.3.1 A Multicast Transport Protocol
SAP is currently the only open standard protocol suited to the
unidirectional/multicast delivery of IMG metadata. As discussed, it
fails to meet the IMG requirements in many ways and, since it is not
designed to be extensible, we recognize that a new multicast
transport protocol for announcements needs to be specified to meet
IMG needs. This protocol will be essential to IMG delivery for
unidirectional and multicast deployments.
The Asynchronous Layered Coding (ALC) [13] protocol from the IETF
Reliable Multicast Transport (RMT) working group is very interesting
as it fulfils many of the requirements, is extensible and has the
ability to `plug-in' both FEC (Forward Error Correction -- for
reliability) and CC (Congestion Control) functional blocks -- it is
specifically designed for unidirectional multicast object transport.
ALC is not fully specified, although RMT has a work-in-progress fully
specified protocol using ALC called FLUTE (File Delivery over
Unidirectional Transport) [14]. FLUTE seems to be the only fully
specified transport and open specification on which a new IMG
Y. Nomura et. al. [Page 19]
Internet Draft IMG Framework December 2, 2003
announcement protocol could be designed. Thus we recommend that ALC
and FLUTE be the starting points for this protocol's design.
Developing a new protocol from scratch, or attempting to improve SAP,
is also feasible, although it would involve repeating many of the
design processes and decisions already made by the IETF for ALC.
Thus, we recommend only to attempt this if ALC-based protocols are
later found to be insufficient.
In particular, any announcement protocol must feature sufficient
scalability, flexibility and reliability to meet IMG needs. Also, the
ANNOUNCE operation must be supported and also NOTIFY capability could
be investigated for both hybrid unicast-multicast and unidirectional
unicast systems.
6.3.2 Usage of Unicast Transport Protocols
A thorough description of the use of existing unicast protocols is
essential for the use of IMGs in a unicast and point-to-point
environment. Such a specification does not currently exist, although
several usable unicast transport protocols and specifications can be
harnessed for this (SIP, SIP events, HTTP, etc). In particular, both
SUBSCRIBE-NOTIFY and QUERY-RESOLVE operation pairs must be enabled.
We anticipate that the FETCH operation will be a trivial usage of
HTTP, although other transport options may be beneficial to consider
too.
6.3.3 The Metadata Envelope
To be able to handle multiple metadata syntaxes, a common minimal set
of information is needed to handle discrete blocks of metadata. The
IMG framework model data types defined in this document. This minimal
set of information field will be named a `metadata envelope' and
must:
1. Uniquely identify the block of metadata, regardless of
metadata syntax used. The uniqueness may only be needed
within the domains the metadata is used but this must
enable globally unique identification to support Internet
usage. Scope/domain specific identifications should not
`leak' outside of the scope, and always using globally
unique identification (e.g. based on URIs) should avoid
this error.
2. Version the block so that changes can be easily handled and
stale data identified.
3. Give the temporal validity of the block. It must expire the
Y. Nomura et. al. [Page 20]
Internet Draft IMG Framework December 2, 2003
block (expiry time), and may optionally provide a time
(presumably in the future) from when the block becomes
valid. Temporal validity may be changeable for a block, and
even a specific version of a block.
4. Be independent of the metadata syntax(es) used for the
metadata block, in the sense that no useful syntax should
be excluded.
For blocks with multiple descriptors, it is assumed that any
descriptors inherit the parameters of the (super)blocks. Thus the
above information will implicitly describe the individual
descriptors.
Four options for metadata envelope transport are feasible:
1. Embedding in a transport protocol header. This can be done
with either header extensions of existing protocols, or
newly defined header fields of a new (or new version of a)
transport protocol. However, multiple methods for the
variety of transport protocols may hinder interoperability.
2. A separate envelope object (a form of metadata itself)
delivered in-band with the metadata. This would complicate
delivery as the envelope and `service' metadata objects
would have to be bound, e.g. by pairing some kind of
transport object numbers (analogous to port number pairs
sometimes used for RTP and RTSP).
3. A metadata wrapper which points to and/or embeds the
service metadata into its `super-syntax'. For example, XML
enables referencing (pointing to) other resources as well
as embedding generic text objects.
4. Embedding in the metadata itself. However, this requires
new field in many metadata syntaxes and would not be
feasible if a useful syntax were not capable of
extensibility in this way. It also introduces a larger
'implementation interpretation' variety which would hinder
interoperability. Thus this option is not recommended.
6.3.4 Baseline (Meta)Data Model Specification
It is likely that more than one of these options will fulfill the
needs of IMGs so the selection, and possibly optimization, is left
for subsequent specification and feedback from implementation
experience. Such a specification is essential for IMG delivery and so
should be an official IETF work item.
Y. Nomura et. al. [Page 21]
Internet Draft IMG Framework December 2, 2003
Relevant work-in-progress ensures that support of one, or very few,
metadata syntaxes is not sufficient. Whereas the transport protocol
should not restrict the metadata format, the metadata envelope may
influence the choice metadata. However, metadata in binary format
should not be prevented, in addition to the more abundant text and
XML syntaxes currently available.
In most cases the actual content of metadata will be application, or
service domain, specific. For instance, digital cinema distribution
and television channels will have many different requirements. The
task of specifying the bulk of the world's metadata is well beyond
the scope of this document: a framework for the delivery of IMG
metadata. We do anticipate that existing and future metadata
specifications, including those of several working groups and
standardization bodies, shall be able to use the services of the IMG
framework. However, it is not essential to the current IMG work to
specify standards with application-specific metadata.
It is essential that the three IMG data types are enabled, but it may
not be necessary to achieve this for every metadata syntax available,
nor may it be important to the IETF to cover every possibility for
this. Generally, Complete Descriptions will be correct for existing
syntaxes (e.g. for XML may be validated according to existing
schema). Pointer data types may be served by a new syntax (extremely
minimal), although it is feasible that the proposed metadata envelope
specification will contain enough information to implement the
Pointer data type. Partial Descriptions may need new or modified
syntaxes and semantics (e.g. XML schema) as mandatory fields for a
Complete Description may be redundant for a Partial Description.
During and following the specification of the metadata envelope,
enabling the delivery of Partial Descriptions should be considered.
To gain implementation experience, it is essential to agree the basic
of some metadata in interoperability tests and subsequently in
widespread deployments. So we anticipate that a minimal IMG data
model would be useful to the Internet community.
6.4 IMG Needs Fitting the IETF's Scope
A Multicast Transport Protocol is essential to IMG delivery for
unidirectional and multicast deployments and no alternative exists
which fulfils the IMG requirements. We recommend that the
specification of this be taken on as an official work item in the
IETF.
Specification of the usage of unicast transport protocols is
essential for IMG delivery and control involving unicast
communications, and will relate to existing IETF standard transport
Y. Nomura et. al. [Page 22]
Internet Draft IMG Framework December 2, 2003
protocols. Thus, we recommend that the specification of this be taken
on as an official work item in the IETF.
The metadata envelope functionality is essential for the IMG delivery
fulfilling the IMG requirements. It is a required feature for IMG
metadata transport and maintenance. Thus, we recommend that the
metadata envelope specification be taken on as an official work item
in the IETF.
(Meta)data model specification and application specific metadata do
not easily fit into the IETF scope and several other standardization
bodies are well placed to do this work. We recommend that aspect
shall not be an official IETF work item.
Note, we acknowledge the need to exchange and agree on a baseline
metadata model and application specific metadata for the purposes of
interoperability testing between different implementations of IMG
related IETF protocols. However, we feel that the IETF standards
process is not required for this.
7 Security Considerations
The IMG framework is developed from the IMG Requirements document [2]
and so the selection of specific protocols and mechanism for use with
the IMG framework must also take into account the security
considerations of the IMG Requirements document. This framework
document does not mandate the use of specific protocols. However, an
IMG specification would inherit the security considerations of
specific protocols used, although this is outside the scope of this
document.
Protocol instantiations which are used to provide IMG operations will
have very different security considerations depending on their scope
and purpose. However, there are several general issues which are
valuable to consider and, in some cases, provide technical solutions
to deal with. These are described below.
Individual and Group Privacy: Customized IMG metadata may reveal
information about the habits and preferences of a user and may thus
deserve confidentiality protection, even though the information
itself is public. Capturing and protecting this IMG metadata requires
the same actions and measures as for other point-to-point and
multicast Internet communications. Naturally, the risk depends on the
amount of individual or group personalization the snooped sessions
contain. Further consideration is valuable at both transport and
metadata levels.
IMG Authenticity: In some cases, the receiver needs to be assured of
Y. Nomura et. al. [Page 23]
Internet Draft IMG Framework December 2, 2003
the origin of IMG metadata or its modification history. This can
prevent denial of service or hijacking attempts which give an IMG
receiver incorrect information in or about the metadata, thus
preventing successful access of the media or directing the IMG
receiver to the incorrect media possibly with tasteless material.
Action upon detection of unauthorized data insertion is out of scope
of this document.
Receiver Authorization: Some or all of any IMG sender's metadata may
be private or valuable enough to allow access to only certain
receivers and thus make it worth authenticating users. Encrypting the
data is also a reasonable step, especially where group communications
methods results in unavoidable snooping opportunities for
unauthorized nodes. Encryption and the required security parameters
exchange are outside the scope of this document.
Unidirectional Specifics: A difficulty that is faced by
unidirectional delivery operations is that many protocols providing
application-level security are based on bi-directional
communications. The application of these security protocols in case
of strictly unidirectional links is not considered in the present
document.
Malicious Code: Currently, IMGs are not envisaged to deliver
executable code at any stage. However, as some IMG transport
protocols may be capable of delivering arbitrary files, it is
RECOMMENDED that the FLUTE delivery service does not have write
access to the system or any other critical areas.
Protocol Specific Attacks: It is recommended that developers of any
IMG protocol take account of the above risks in addition to any
protocol design and deployment environment risks that may be
reasonably identified. Currently this framework document does not
recommend or mandate the use of any specific protocols, however the
deployment of IMGs using specific protocol instantiations will
naturally be subject to the security considerations of those
protocols.
Security Improvement Opportunity: The security properties of one
channel and protocol can be improved through the use of another
channel and protocol. For example, a secure unicast channel can be
used to deliver the keys and initialization vectors for an encryption
algorithm used on a multicast channel. The exploitation of this
opportunity is specific to the protocols used and is outside the
scope of this document.
8 Normative References
Y. Nomura et. al. [Page 24]
Internet Draft IMG Framework December 2, 2003
[1] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[2] Y. Nomura et al., "Protocol requirements for Internet media
guides," Internet Draft draft-ietf-mmusic-img-req-00, Internet
Engineering Task Force, Sept. 2003. Work in progress.
[3] M. Day, B. Cain, G. Tomlinson, and P. Rzewski, "A model for
content internetworking (CDI)," RFC 3466, Internet Engineering Task
Force, Feb. 2003.
[4] M. Handley and V. Jacobson, "SDP: session description protocol,"
RFC 2327, Internet Engineering Task Force, Apr. 1998.
[5] D. Kutscher, J. Ott, and C. Bormann, "Session description and
capability negotiation," Internet Draft draft-ietf-mmusic-sdpng-07,
Internet Engineering Task Force, Oct. 2003. Work in progress.
[6] ISO (International Organization for Standardization), "Overview
of the MPEG-7 standard," ISO Standard ISO/IEC JTC1/SC29/WG11 N4509,
International Organization for Standardization, Geneva, Switzerland,
Dec. 2001.
[7] T.-A. Forum, "Metadata specification S-3," TV-Anytime Forum
Specification SP003v1.2 Part A, TV, TV-Anytime Forum, June 2002.
[8] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, and T. Berners-
Lee, "Hypertext transfer protocol -- HTTP/1.1," RFC 2068, Internet
Engineering Task Force, Jan. 1997.
[9] M. Handley, C. E. Perkins, and E. Whelan, "Session announcement
protocol," RFC 2974, Internet Engineering Task Force, Oct. 2000.
[10] 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.
[11] A. B. Roach, "Session initiation protocol (sip)-specific event
notification," RFC 3265, Internet Engineering Task Force, June 2002.
[13] M. Luby, J. Gemmell, L. Vicisano, L. Rizzo, and J. Crowcroft,
"Asynchronous layered coding (ALC) protocol instantiation," RFC 3450,
Internet Engineering Task Force, Dec. 2002.
9 Informative References
Y. Nomura et. al. [Page 25]
Internet Draft IMG Framework December 2, 2003
[12] J. Rosenberg, "A presence event package for the session
initiation protocol (SIP)," internet draft, Internet Engineering Task
Force, Jan. 2003. Work in progress.
[14] T. Paila, "FLUTE - file delivery over unidirectional transport,"
Internet Draft draft-ietf-rmt-flute-06, Internet Engineering Task
Force, Nov. 2003. Work in progress.
10 Acknowledgements
The authors would like to thank Joerg Ott, Colin Perkins, Toni Paila
and Petri Koskelainen on for their ideas and input to this document.
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 Corporation
Nokia Research Center
P.O. Box 100, FIN-33721 Tampere
Finland
Email: rod,walsh@nokia.com
Juha-Pekka Luoma
Nokia Corporation
Nokia Research Center
P.O. Box 100, FIN-33721 Tampere
Finland
Email: juha-pekka.luoma@nokia.com
Hitoshi Asaeda
INRIA
Project PLANETE
2004, Route des Lucioles, BP93,
06902 Sophia Antipolis,
France
Email: Hitoshi.Asaeda@sophia.inria.fr
Henning Schulzrinne
Dept. of Computer Science
Columbia University
Y. Nomura et. al. [Page 26]
Internet Draft IMG Framework December 2, 2003
1214 Amsterdam Avenue
New York, NY 10027
USA
Email: schulzrinne@cs.columbia.edu
Y. Nomura et. al. [Page 27]
