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-04.txt
April 13, 2004
Expires: October 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.
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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
model for IMG delivery mechanisms, and the use of existing protocol
to create an IMG delivery infrastructure.
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Table of Contents
1 Introduction ........................................ 2
2 Terminology ......................................... 3
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 Operation Set for IMG Delivery ...................... 6
3.2.1 IMG ANNOUNCE ........................................ 6
3.2.2 IMG QUERY ........................................... 7
3.2.3 IMG RESOLVE ......................................... 7
3.2.4 IMG SUBSCRIBE ....................................... 7
3.2.5 IMG NOTIFY .......................................... 8
3.2.6 Binding Between IMG Operations and Data Types ....... 8
3.3 IMG Entities ........................................ 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 ................... 13
4.4 Combined Operations with Common Metadata ............ 13
5 Applicability of Existing Protocols to the
Proposed Framework Model ....................................... 13
5.1 Existing Protocol Fit to the IMG Framework Model
5.2 Outstanding IMG Mechanism Needs ..................... 16
5.2.1 A Multicast Transport Protocol ...................... 16
5.2.2 Usage of Unicast Transport Protocols ................ 17
5.2.3 IMG Transfer Envelope ............................... 17
5.2.4 Baseline (Meta)Data Model Specification ............. 18
5.3 IMG Needs Fitting the IETF's Scope .................. 18
6 Security Considerations ............................. 19
7 IANA Considerations ................................. 21
8 References .......................................... 21
9 Acknowledgements .................................... 21
10 Authors' Addresses .................................. 22
11 Full Copyright Statement ............................ 22
1 Introduction
Internet Media Guides (IMGs) provide and deliver structured
collections of multimedia descriptions expressed using SDP, SDPng or
some similar description format. They are used to describe sets of
multimedia sessions (e.g. television program schedules, content
delivery schedules etc.) and refer to other networked resources
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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 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.
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
guideline for IMG delivery to efficiently deliver and describe IMG
metadata. The data types give common base descriptions on top of an
application-specific IMG metadata. IMG operations cover traditional
broadcast-style scenarios, multicast-based distributions, 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 perform one or more of
roles defined as the entities in IMG framework model. These are then
shown in a number of simplified deployment scenarios. The
requirements for IMG delivery mechanisms and descriptions can be
found in [1].
Then, 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 viewpoint 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.
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 [2].
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.
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IMG Element: The smallest atomic element of metadata that can be
transmitted separately by IMG operations and referenced
individually from other IMG elements.
IMG Metadata: A set of metadata consisting of one or more IMG
elements. IMG metadata describes the features of multimedia
content used to enable selection of and access to media
sessions containing content. For example, metadata may
consist of the URI, title, airtime, bandwidth needed, file
size, text summary, genre and access restrictions.
IMG Description: A collection of IMG metadata which has a
relationship to other IMG metadata. There are three data
types to describe the relationship: Complete IMG
Descriptions, Delta IMG Description and IMG pointer.
IMG Delivery: The process of exchanging IMG metadata both in
terms of large scale and atomic data transfers.
IMG Sender: An IMG sender is a logical entity that sends IMG
metadata to one or more IMG receivers.
IMG Receiver: An IMG receiver is a logical entity that receives
IMG metadata from an IMG sender.
IMG Transceiver: An IMG transceiver combines an IMG receiver and
sender. It may modify received IMG metadata or merge IMG
metadata received from a several different IMG senders.
IMG Operation: An atomic operation of an IMG transport protocol,
used between IMG sender(s) and IMG receiver(s) for the
delivery of IMG metadata and for the control of IMG
sender(s)/receiver(s).
IMG Transport Protocol: A protocol that transports IMG metadata
from an IMG sender to IMG receiver(s).
IMG Transport Session: An association between an IMG sender and
one or more IMG receivers within the scope of an IMG
transport protocol. An IMG transport session involves a
series of IMG transport protocol interactions that provide
delivery of IMG metadata from the IMG sender to the IMG
receiver(s).
IMG Transfer: A transfer of IMG metadata consisting of Complete
IMG Descriptions, Delta IMG Descriptions and/or IMG
Pointers.
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3 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
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 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, fundamental
session descriptions.
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 metadata 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 Description and IMG
Pointer.
3.1.1 Complete IMG Description
A Complete IMG Description provides a complete syntax and semantics
to describe a set of metadata, which does not need any additional
information from any other IMG element.
Note, this is not to be confused with "complete IMG metadata", which,
although vaguely defined here, represents the complete IMG metadata
database of an IMG sender (or related group of IMG senders --
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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 in question. Delta transfers may be used to
reduce network resource usage (it may be more bandwidth and
congestion friendly), for instance when data consistency is essential
and small and frequent changes occur to IMG elements. Thus, this
description itself cannot represent complete metadata set until it is
combined with existing, or future, description knowledge.
3.1.3 IMG Pointer
An IMG Pointer provides a simple identifier or locator, such as a
URI, that the IMG receiver is able to reference (or reference and
locate) specific metadata with. 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 metadata per se (although it may also appear as
a data field in Complete or Delta IMG descriptors).
3.2 Operation Set for IMG Delivery
A finite set of operations both meets the IMG requirements [1] and
fits the roles of existing protocols. These are crystallized in the
next few sections.
3.2.1 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 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 are considerably large number IMG receivers or
IMG receiver(s) do not have a guaranteed uplink connection to the IMG
sender(s). 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 is able to carry any of the IMG data
types.
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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) enables IMG receivers to subscribe/register to
the IMG ANNOUNCE operation.
3.2.2 IMG QUERY
If an IMG receiver needs to obtain IMG metadata, an IMG receiver can
send an IMG QUERY message 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 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 an IMG sender's complete IMG metadata may be
feasible, in others it may not.
3.2.3 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 is available to the IMG
sender, the IMG sender can resolve 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.
3.2.4 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 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). The 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
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the unsubscription may be provided implicitly by the expiry (timeout)
of a subscription before it is renewed. However, these details
belong to the messaging protocol and are beyond the scope of this
document.
Since the IMG receiver doesn't know when metadata will be updated 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 investigate whether an IMG SUBSCRIBE operation is from an
authorized IMG receiver.
3.2.5 IMG NOTIFY
An IMG NOTIFY is used asynchronously in response to an earlier IMG
SUBSCRIBE. An IMG NOTIFY generates a notify message indicating 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.
3.2.6 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
expire 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
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useful syntax should be excluded.
3.3 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, 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
3.4 Overview of Protocol Operations
Figure 2 gives an overview of the relationship between transport
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cases, IMG Operations and IMG data types (note, it is not a protocol
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
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. If, for example, some IMG metadata is public information
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and the IMG sender provides the same information for all IMG
receivers. This kind of IMG metadata may be distributed from one IMG
sender to multiple IMG 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.
+----------+ +----------+
| 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 content. IMG
metadata distributions 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 IMG 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 IMG senders is that the original authenticity is lost. Thus for
deployments requiring these kind of features, the original metadata
should be reasonably fragmented already (allowing the intermediary to
replace a small fragment without changing the authenticity of the
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remainder). It may be beneficial to use smaller fragments for more
volatile parts, and larger one for more stable parts.
4.2 One-to-many Unidirectional Multicast
This 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
+----------+ +----------+
| IMG | | IMG |
| Resolver | | Querier |
+----------+ +----------+
| |
|-----------IMG QUERY ---------->|
| |
|<---------IMG RESOLVE-----------|
| |
Figure 6: Query/Resolve Sequence Example
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4.3 One-to-one Bi-directional Unicast
Both query/resolve (figure 6) and subscribe/notify (figure 7) message
exchange operations are feasible. The "time passes" activities of
figure 7 are purely for 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.
5 Applicability of Existing Protocols to the Proposed Framework Model
5.1 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 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 unused overhead. Because it is expected that the
information conveyed by SDP is just a small subset of IMG metadata
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the use of SDP for other than session-level IMG parameters may not be
reasonable.
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
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. MPEG-7 is based on XML so
it is well suited for combining with other XML-based formats such as
SDPng.
TV-Anytime Forum: The TV-Anytime Forum [3] provides descriptions
based on XML schema for TV-specific program guides. TV-Anytime uses
the MPEG-7 User description profile to a limited extent (for user
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preferences and usage history) and also a TV-Anytime-specific data
model for other schema - which are optimised to describe broadcast
schedules, on-demand program guides and program events.
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
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 RFC 3265 [4]
provides a way to implement IMG SUBSCRIBE and IMG NOTIFY operations
via a bi-directional unicast connection. However, there are
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scalability problems with this approach, as RFC 3265 currently does
not consider multicast.
RTSP: The RTSP protocol defines a retrieval and update notification
mechanism, named DESCRIBE and ANNOUNCE, for the description of a
presentation or media object in order to initialize 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 IMG QUERY,
RESOLVE and SUBSCRIBE, and the ANNOUNCE could be a IMG NOTIFY for a
streaming session controlled by RTSP.
5.2 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.
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) [5] 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) [6]. 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.
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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 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 [7], SIP events [4], HTTP [8], 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.
5.2.3 IMG Transfer Envelope
Section 3.2.6 of this document discussed the need for 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 receiver(s).
Four options for IMG 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 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 RTCP [9]).
3. A metadata wrapper which points to and/or embeds the
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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.
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.
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 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 IMG transfer envelope to another.
5.2.4 Baseline (Meta)Data Model Specification
A minimal IMG data model may be useful to any implementer/deployment
of IMGs. The purpose would be to ensure that multiple metadata
syntaxes (SDP, MPEG-7, etc) can be related within the same body of
IMG knowledge, regardless of any specific metadata and data models
provided by the metadata syntaxes.
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.
5.3 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
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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
protocols. Thus, we recommend that the specification of this be taken
on as an official work item in the IETF.
The IMG transfer 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 IMG transfer 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.
6 Security Considerations
The IMG framework is developed from the IMG Requirements document [1]
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 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
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the amount of individual or group personalization the snooped IMG
metadata contains. Further consideration is valuable at both
transport and metadata levels.
IMG Authenticity: In some cases, the IMG receiver needs to be assured
of 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.
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. 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
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opportunity is specific to the protocols used and is outside the
scope of this document.
7 IANA Considerations
There are no IANA considerations within this document.
8 References
[1] Y. Nomura, R. Walsh, J-P. Luoma, J. Ott, and H. Schulzrinne,
"Protocol requirements for Internet media guides," Internet Draft
draft-ietf-mmusic-img-req-04, Internet Engineering Task Force, April
2004. Work in progress.
[2] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[3] 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.
[4] A. B. Roach, "Session initiation protocol (sip)-specific event
notification," RFC 3265, Internet Engineering Task Force, June 2002.
[5] 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.
[6] T. Paila, M. Luby, R. Lehtonen, V. Roca, R. Walsh, "FLUTE - file
delivery over unidirectional transport," Internet Draft draft-ietf-
rmt-flute-07, Internet Engineering Task Force, Dec. 2003. Work in
progress.
[7] 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.
[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] 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.
9 Acknowledgements
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The authors would like to thank Joerg Ott, Colin Perkins, Toni Paila
and Petri Koskelainen on for their ideas and input to this document.
10 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
1214 Amsterdam Avenue
New York, NY 10027
USA
Email: schulzrinne@cs.columbia.edu
11 Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
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except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
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Internet Draft IMG Framework April 13, 2004
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Y. Nomura et. al. [Page 23]
