MMUSIC Working Group Y. Nomura
Internet-Draft Fujitsu Labs.
Expires: June 23, 2006 R. Walsh
J-P. Luoma
Nokia
H. Asaeda
INRIA
H. Schulzrinne
Columbia University
December 19, 2005
A Framework for the Usage of Internet Media Guides
draft-ietf-mmusic-img-framework-09
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Copyright (C) The Internet Society (2005).
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 a generalized
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model for IMG delivery mechanisms, the use of existing protocols and
the need for additional work to create an IMG delivery
infrastructure.
Table of Contents
1 Introduction ........................................ 3
2 Terminology ......................................... 3
2.1 New Terms ........................................... 4
3 IMG Common Framework Model .......................... 5
3.1 IMG Data Types ...................................... 5
3.1.1 Complete IMG Description ............................ 5
3.1.2 Delta IMG Description ............................... 6
3.1.3 IMG Pointer ......................................... 6
3.2 IMG Entities ........................................ 6
3.3 Operation Set for IMG Delivery ...................... 7
3.3.1 IMG ANNOUNCE ........................................ 7
3.3.2 IMG QUERY ........................................... 8
3.3.3 IMG RESOLVE ......................................... 8
3.3.4 IMG SUBSCRIBE ....................................... 8
3.3.5 IMG NOTIFY .......................................... 9
3.3.6 Binding Between IMG Operations and Data Types ....... 9
3.4 Overview of Protocol Operations ..................... 9
4 Deployment Scenarios for IMG Entities ............... 10
4.1 Intermediary Cases .................................. 10
4.2 One-to-many Unidirectional Multicast ................ 12
4.3 One-to-one Bi-directional Unicast ................... 12
4.4 Combined Operations with Common Metadata ............ 14
5 Applicability of Existing Protocols to the
Proposed Framework Model ............................ 14
5.1 Existing Standard Fitting the IMG Framework Model ... 14
5.2 IMG Mechanism Needs Not Yet Met ..................... 16
5.2.1 A Multicast Transport Protocol ...................... 16
5.2.2 Usage of Unicast Transport Protocols ................ 17
5.2.3 IMG Envelope ........................................ 17
5.2.4 Metadata Data Model ................................. 18
6 Security Considerations ............................. 18
7 IANA Considerations ................................. 19
8 Normative References ................................ 20
9 Informative References .............................. 20
10 Acknowledgements .................................... 21
11 Authors' Addresses .................................. 21
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1 Introduction
Internet Media Guides (IMGs) provide and deliver structured
collections of multimedia descriptions expressed using SDP [2],
SDPng [3] or other description formats. They are used to describe
sets of multimedia services (e.g., television program schedules,
content delivery schedules) 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.
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 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.
This document defines a common framework model for IMG delivery
mechanisms and their deployment in network entities. There are
three fundamental components in IMG framework model: data types,
operation sets and entities. These components specify a set of
framework guidelines for efficient delivery and description of IMG
metadata. The data types give generalized means to deliver and
manage the consistency of application-specific IMG metadata. IMG
operations cover broadcast, multicast distribution, event
notification upon change, unicast-based push and interactive
retrievals similar to web pages.
Since we envision that any Internet host can be a sender and receiver
of IMG metadata, a host involved in IMG operations performs one or
more of the roles defined as the entities in IMG framework model.
The requirements for IMG delivery mechanisms and descriptions can be
found in the IMG requirements document [4].
This document outlines the use of existing protocols to create
an IMG delivery infrastructure. It aims to organize existing
protocols into a common model and show their capabilities and
limitations from the viewpoint of IMG delivery functions.
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].
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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 [4].
IMG Element: The smallest atomic element of metadata that can be
transmitted separately by IMG operations and referenced
individually from other IMG elements [4].
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 [4].
IMG Description: A collection of IMG metadata with a data
type indicating a self-contained set or a subset of IMG
metadata, or a reference to IMG metadata.
IMG Delivery: The process of exchanging IMG metadata both in
terms of large scale and atomic data transfers [4].
IMG Sender: An IMG sender is a logical entity that sends IMG
metadata to one or more IMG receivers [4].
IMG Receiver: An IMG receiver is a logical entity that receives
IMG metadata from an IMG sender [4].
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 [4].
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) [4].
IMG Transport Protocol: A protocol that transports IMG metadata
from an IMG sender to IMG receiver(s) [4].
IMG Transport Session: An association between an IMG sender and
one or more IMG receivers within the scope of an IMG
transport protocol. An IMG transport session involves a
time bound series of IMG transport protocol interactions
that provide delivery of IMG metadata from the IMG sender
to the IMG receiver(s) [4].
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IMG Transfer: IMG Transfer: A transfer of IMG metadata from an
IMG sender to IMG receiver(s).
3 IMG Common Framework Model
Two common elements are found in all of existing IMG candidate cases:
the need to describe the services and the need to deliver the
descriptions. In some cases, the descriptions provide multicast
addresses and thus are part of the transport configuration. In other
cases, descriptions are specific to the media application and may be
either meant for human or machine consumption. Thus, the technologies
can be roughly divided into three areas:
o Application-specific Metadata -- data describing the content
of services and media which are both specific to certain
applications and generally human readable.
o Delivery Descriptions -- the descriptions (metadata) that are
essential to enable (e.g., multicast) delivery. These include
framing (headers) for application-specific metadata, the
metadata element identification and structure, and fundamental
session data.
o Delivery Protocols -- the methods and protocols to exchange
descriptions between the senders and the receivers. An IMG
transport 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 to reduce the amount
of the messaging.
3.1 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 IMG Descriptions, Delta IMG Descriptions and IMG
Pointers.
3.1.1 Complete IMG Description
A complete IMG description provides a self-contained set of metadata
for one media object or service, i.e., it does not need additional
information from any other IMG element. This is not to be confused
with "complete IMG metadata", which, although vaguely defined here,
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represents the complete IMG metadata database of an IMG sender (or
related group of IMG senders -- potentially the complete Internet IMG
knowledge). An IMG sender will generally deliver only subsets of
metadata from its complete database in a particular IMG transport
session.
3.1.2 Delta IMG Description
A Delta IMG Description provides only part of a Complete IMG
Description, defining the difference from a previous version of the
Complete IMG Description. Delta IMG Descriptions may be used to
reduce network resource usage, for instance when data consistency
is essential and small and frequent changes occur to IMG elements.
Thus, this description does not represent a complete set of metadata
until it is combined with other metadata that may already exist or
arrive in the future.
3.1.3 IMG Pointer
An IMG pointer, typically a URI, identifies or locates metadata. This
may be used to separately obtain metadata (Complete or Delta IMG
Descriptions) or perform another IMG management function such as data
expiry (and erasure). The IMG Pointer may be used to reference IMG
metadata elements within the IMG transport session and across IMG
transport sessions. This pointer type does not include IMG metadata
per se (although it may also appear as a data field in Complete or
Delta IMG descriptors).
3.2 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, which are defined in
more detail in Section 3.3.
IMG Announcer: sends IMG ANNOUNCE
IMG Listener: receives IMG ANNOUNCE
IMG Querier: sends IMG QUERY to receive IMG RESOLVE
IMG Resolver: receives IMG QUERY then send IMG RESOLVE
IMG Subscriber: sends IMG SUBSCRIBE then receive IMG NOTIFY
IMG Notifier: receives IMG SUBSCRIBE then send IMG NOTIFY
Figure 1 shows the relationship between IMG entities and the IMG
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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 between IMG Entities, Senders and Receivers
3.3 Operation Set for IMG Delivery
A finite set of operations both meets the IMG requirements [4] and
fits the roles of existing protocols. These are crystallized in the
next few sections.
3.3.1 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 IMG 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 IMG senders).
This operation is useful when there is large numbers of IMG receivers
or the IMG receivers do not have a guaranteed uplink connection to
the IMG sender. The IMG sender may also include authentication data
in the announce operation so that IMG receivers may check the
authenticity of the metadata. This operation can carry any of the
IMG data types.
There is no restriction to prevent IMG ANNOUNCE from being used
in an asynchronous solicited manner, where a separate operation
(possibly out of band) enables IMG receivers to subscribe/register to
the IMG ANNOUNCE operation.
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3.3.2 IMG QUERY
If an IMG receiver needs to obtain IMG metadata, an IMG receiver can
use an IMG QUERY operation and initiate a receiver-driven IMG
transport 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 network connectivity 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. The extent indicates the number and grouping of metadata
descriptions. In some cases requesting an IMG sender's complete IMG
metadata collection, it may be feasible to request.
3.3.3 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 is available to
the IMG sender, the IMG sender can resolve the query; 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 IMG receivers
may check the authenticity of the metadata. This operation may
carry any of the IMG data types.
3.3.4 IMG SUBSCRIBE
If an IMG receiver wants to be notified when the IMG metadata it
holds is stale, the IMG receiver can use the IMG SUBSCRIBE operation
in advance in order to solicit IMG NOTIFY messages from an IMG
sender.
This operation may provide the IMG sender with specific details of
which metadata or notification services it is interested in in the
case where the IMG sender offers more than the simplest "all data"
service. This operation implicitly provides the functionality of
unsubscribing to inform an IMG sender that an IMG receiver wishes
to stop getting certain (or all) notifications. It should be noted
that unsubscription may be provided implicitly by the expiry
(timeout) of a subscription before it is renewed.
Since the IMG receiver does not know when metadata will be updated
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and the notify message will arrive, this operation does not
synchronize with the notify messages. The IMG receiver may wait for
notify messages for a long time. The IMG sender may authenticate the
IMG receiver to check whether an IMG SUBSCRIBE operation is from an
authorized IMG receiver.
3.3.5 IMG NOTIFY
An IMG NOTIFY is used asynchronously in response to an earlier IMG
SUBSCRIBE. An IMG NOTIFY operation 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 IMG sender may also include authentication data in the
IMG NOTIFY operation so that IMG receivers may check the authenticity
of the messages.
3.3.6 Binding Between IMG Operations and Data Types
There is a need to provide a binding between the various IMG
operations and IMG data types to allow management of each discrete
set of IMG metadata transferred using an IMG operation. This binding
must be independent of any particular metadata syntax used to
represent a set of IMG metadata, as well as be compatible with any
IMG transport protocol. The binding must uniquely identify the set of
IMG metadata delivered within an IMG transfer, regardless of the
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.
The binding must provide versioning to the transferred IMG metadata
so that changes can be easily handled and stale data identified, and
give temporal validity of the transferred IMG metadata. It must
invalidate the IMG metadata by indicating an expiry time, and may
optionally provide a time (presumably in the future) from when the
IMG metadata becomes valid. Temporal validity of IMG metadata may be
changeable for an IMG transfer, and even for specific versions of the
IMG transfer. Furthermore, the binding must be independent of the
metadata syntax(es) used for the IMG metadata, in the sense that no
useful syntax should be excluded.
3.4 Overview of Protocol Operations
Figure 2 gives an overview of the relationship between transport
cases, IMG Operations and IMG data types. It is not a protocol
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stack. Generalized multicast point-to-multipoint (P-to-M) and
unicast point-to-point (P-to-P) transports are shown.
+--------------------------------------------------+
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 Transport, IMG Operations and IMG Data types
4 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 an IMG sender to an IMG receiver, as shown in Figure 3.
+-------------+ +---------------+
| IMG Sender | | IMG Receiver |
| |--------------->| |
+-------------+ +---------------+
Figure 3: A Simple IMG Sender to IMG Receiver Relationship
4.1 Intermediary Cases
Some IMG metadata may be distributed to a large number of IMG
receivers, for example, when public metadata is distributed to all
IMG receivers of a certain group. This kind of IMG metadata may be
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distributed from one IMG sender to multiple IMG receivers (Figure 4)
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.
+----------+ +----------+
| 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 IMG receivers. An
IMG receiver may be allowed to receive IMG metadata from any number
of IMG senders.
IMG metadata is used to find, obtain, manage and play media content.
IMG metadata may be modified during IMG Transfer. 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 IMG
senders. For example, a rating service may add its own content
ratings or recommendations to existing metadata. An implication of
changing (or aggregating) IMG metadata from one or more IMG senders
is that the original authenticity is lost. Thus, IMG metadata
fragmented reasonably may be beneficial for the intermediary to
replace a small fragment without changing the authenticity of the
remainder. For example, smaller fragments may be appropriate for more
volatile parts, and larger ones may be appropriate for stable parts.
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4.2 One-to-many Unidirectional Multicast
One-to-many unidirectional multicast case implies many IMG receivers
and one or more IMG 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
Note, as defined in by the IMG requirements REL-4 [4], an IMG
transport protocol MUST support reliable message exchange.
This includes the one-to-many unidirectional multicast case,
however, the mechanism to provide this is beyond the scope of this
document.
4.3 One-to-one Bi-directional Unicast
In one-to-one bi-directional unicast case, both query/resolve
(Figure 6) and subscribe/notify (Figure 7) message exchange
operations are feasible.
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+----------+ +----------+
| IMG | | IMG |
| Resolver | | Querier |
+----------+ +----------+
| |
|<----------IMG QUERY -----------|
| |
|----------IMG RESOLVE---------->|
| |
Figure 6: Query/Resolve Sequence Example
+----------+ +------------+
| IMG | | IMG |
| Notifier | | Subscriber |
+----------+ +------------+
| |
|<---------IMG SUBSCRIBE---------|
: :
(time passes)
: :
|-----------IMG NOTIFY---------->|
: :
(time passes)
: :
|-----------IMG NOTIFY---------->|
| |
Figure 7: Subscribe/Notify Sequence Example
4.4 Combined Operations with Common Metadata
As shown in Figure 8, a common data model for multiple protocol
operations allows a diverse range of IMG senders and receivers to
provide consistent and interoperable sets of IMG metadata.
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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
5 Applicability of Existing Protocols to the Proposed Framework Model
5.1 Existing Standard Fitting the IMG Framework Model
SDP: The SDP format [2] could be used to describe session-level
parameters (e.g., scheduling, addressing and the use of media codecs)
to be included in Complete IMG Descriptions. Although there are
extension points in SDP allowing the format to be extended, there are
limitations in the flexibility of this extension mechanism. However,
SDP syntax cannot provide IMG Descriptions and IMG Pointers without
significant overhead. It is expected that the information conveyed by
SDP is just a small subset of IMG metadata, thus, the use of SDP for
other than session parameters may not be reasonable.
SDPng [3]: 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 should be much more flexible and
allow extensions and integration with other description formats.
MPEG-7: Descriptions based on the MPEG-7 standard [5] could provide
application-specific metadata describing the properties of multimedia
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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. MPEG-7 is based on XML so it is well
suited to be combined with other XML-based formats such as SDPng.
TV-Anytime Forum: The TV-Anytime Forum [6] provides descriptions
based on XML schema for TV-specific program guides. TV-Anytime uses
the MPEG-7 User description profile to a limited extent, only for
user preferences and usage history, and also a TV-Anytime-specific
data model for other schema. These are optimised to describe
broadcast schedules, on-demand program guides and program events.
HTTP: The HTTP protocol [7] 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. Thus, by itself, HTTP is not sufficient to
fulfill all of the IMG requirements [4] in a unicast deployment.
SAP: The announcement mechanism provided by SAP [8] 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, SAP lacks scalable and efficient reliability, extensibility
for payload size, congestion control, and only one description
allowed per SAP message due to lack of payload segmentation.
In principle, SAP could be extended to get around its limitations.
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 RFC 3265 [9]
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.
RTSP: The RTSP protocol [10] defines a retrieval and update
notification mechanism, named DESCRIBE and ANNOUNCE, for the
description of a presentation or media object in order to initialize
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a streaming session. These methods are subset of the entire streaming
control operations in RTSP, thus these could not be available for
individual mechanisms. However, the DESCRIBE method in RTSP could be
used to instantiate IMG QUERY, IMG RESOLVE and IMG SUBSCRIBE, and the
RTSP ANNOUNCE could be used to instantiate an IMG NOTIFY for a
streaming session controlled by RTSP.
5.2 IMG Mechanism Needs Not Yet Met
Several 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; (d) agreement on basic metadata
models to enable interoperability testing of the above. The following
sections describe each of these.
5.2.1 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) [11] protocol from the IETF
Reliable Multicast Transport (RMT) working group is very interesting
as it fulfills 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 working group had a fully
specified protocol using ALC called FLUTE (File Delivery over
Unidirectional Transport) [12]. FLUTE seems to be the only fully
specified transport and open specification on which a new IMG
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.
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In particular, any announcement protocol must feature sufficient
scalability, flexibility and reliability to meet IMG needs. Also, the
IMG ANNOUNCE operation must be supported and IMG NOTIFY capability
could be investigated for both hybrid unicast-multicast and
unidirectional unicast systems.
5.2.2 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 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 [13], SIP events [9], HTTP [7], etc.) In
particular, both IMG SUBSCRIBE-NOTIFY and IMG QUERY-RESOLVE operation
pairs must be enabled. We anticipate that the IMG QUERY-RESOLVE
operation can be achieved using HTTP, although other transport
protocol options may be beneficial to consider too.
5.2.3 IMG Envelope
An IMG envelope provides binding between IMG Operations and data
types. Such a binding can be realized by defining a common minimal
set of information needed to manage IMG metadata transfers, and by
including this information with any set of IMG metadata delivered to
IMG receivers.
Four options for IMG metadata transfer envelope delivery 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 would hinder
interoperability and transport protocol independence.
2. A separate envelope object, which points to the IMG
metadata 'object', delivered in-band with the metadata
transport protocol session. This might 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 RTCP [14]). This would also
enable schemes which deliver enveope and metadata
'objects' on by different media, also using more than a
single transport protocol.
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3. A metadata wrapper which points to and/or embeds the
service metadata into its 'super-syntax'. For example, an
XML would enable 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.
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.
When there are superset/subset relations between IMG descriptions,
it is assumed that the IMG descriptions of the subset inherit the
parameters of the superset. Thus, an IMG metadata transfer envelope
carrying the IMG descriptions of a superset may implicitly define
parameters of IMG descriptors belonging to its subset. The
relations between IMG descriptions may span from one envelope to
another according to a data model definition.
5.2.4 Metadata Data Model
A structured data model would allow reuse and extension of the set of
metadata and may enable use of multiple syntaxes (SDP, MPEG-7, etc.)
as part of the same body of IMG metadata.
Further work may be needed to meet application-specific requirements
at defining metadata and data models for the successful deployment of
IMGs in various environments. Existing (and future) work on these
would need to be mapped to the IMG data types and use of the IMG
transfer envelope concept as described above.
This document is a framework for the delivery of IMG metadata and
thus further discussion on the definition data models for IMGs is
beyond its scope.
6 Security Considerations
The IMG framework is developed from the IMG requirements document [4]
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
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IMG specification would inherit the security considerations of
specific protocols used.
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 if the original information
were public. Snooping and protecting this IMG metadata requires the
same actions and measures as for other point-to-point and multicast
Internet communications. Naturally, the risk of snooping depends on
the amount of individual or group personalization the snooped IMG
metadata contains.
IMG authenticity: In some cases, the IMG receiver needs to be assured
of the sender or 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.
IMG receiver authorization: Some or all of any IMG sender's metadata
may be private or valuable enough to allow access to only certain IMG
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.
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 IMG operations do not have write access to the
system or any other critical areas.
7 IANA Considerations
This document does not request any IANA action.
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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] 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.
[4] Y. Nomura, R. Walsh, J-P. Luoma, J. Ott, and H. Schulzrinne,
"Requirements for Internet Media Guides," Internet Draft
draft-ietf-mmusic-img-req-08, Internet Engineering Task Force,
December 2005. Work in progress.
[5] "Multimedia content description interface -- Part 1: Systems",
ISO/IEC 15938-1, July 2002.
[6] TV-Anytime Forum, "Broadcast and On-line Services: Search,
select, and rightful use of content on personal storage systems
("TV-Anytime Phase 1"); Part 2: System description," ETSI-TS 102
822-2: System Description, V1.1.1, October 2003.
[7] 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.
[8] M. Handley, C. E. Perkins, and E. Whelan, "Session announcement
protocol," RFC 2974, Internet Engineering Task Force, October 2000.
[9] A. B. Roach, "Session initiation protocol (sip)-specific event
notification," RFC 3265, Internet Engineering Task Force, June 2002.
[10] H. Schulzrinne, A. Rao, and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, Internet Engineering Task Force,
April 1998.
[11] M. Luby, J. Gemmell, L. Vicisano, L. Rizzo, and J. Crowcroft,
"Asynchronous layered coding (ALC) protocol instantiation," RFC 3450,
Internet Engineering Task Force, December 2002.
[12] T. Paila, M. Luby, R. Lehtonen, V. Roca, R. Walsh, "FLUTE -
file delivery over unidirectional transport," RFC 3926,
Internet Engineering Task Force, October 2004.
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[13] 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.
[14] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP:
a transport protocol for real-time applications," RFC 3550, Internet
Engineering Task Force, July 2003.
10 Acknowledgements
The authors would like to thank Spencer Dawkins, Jean-Pierre Evain,
Ted Hardie, Petri Koskelainen, Joerg Ott, Colin Perkins, Toni Paila,
Magnus Westerlund, and for their excellent 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 Research Center
P.O. Box 100, FIN-33721 Tampere
Finland
Email: rod.walsh@nokia.com
Juha-Pekka Luoma
Nokia Research Center
P.O. Box 100, FIN-33721 Tampere
Finland
Email: juha-pekka.luoma@nokia.com
Hitoshi Asaeda
INRIA
PLANETE Research Team
2004, Route des Lucioles, BP93,
06902 Sophia Antipolis,
France
Email: Hitoshi.Asaeda@sophia.inria.fr
Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue
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New York, NY 10027
USA
Email: schulzrinne@cs.columbia.edu
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