Internet Engineering Task Force MMUSIC WG
Internet Draft Y. Nomura
Fujitsu Labs.
R. Walsh
Nokia
H. Asaeda
INRIA
H. Schulzrinne
Columbia University
draft-nomura-mmusic-img-framework-01.txt
June 30, 2003
Expires: December 2003
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
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Abstract
This document defines a framework for the delivery of Internet Media
Guides (IMGs). A media guide is a set of information (metadata)
describing the features of multimedia content in order to subscribe
to, manage and exchange multimedia content.
Several protocols and methods exist that are able to deliver metadata
over the Internet, giving information on Internet services, media and
content. This existing knowledge is used to provide a general
framework for Internet Media Guides and outlines existing scalability
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and flexibility issues, along with the desirable features that can
solve these problems.
Table of Contents
1 Introduction ........................................ 3
2 Terminology ......................................... 3
3 Problem Statement ................................... 4
3.1 Prior Methods for Server-Client Scenario ............ 4
3.1.1 Web-based Media Guide Browsing ...................... 5
3.1.2 Multicasting a Media Guide .......................... 5
3.1.3 Possible Solutions .................................. 8
3.2 Prior Methods for Server-Server Scenarios ........... 8
3.2.1 Content Distribution Internetworking (CDI) .......... 8
3.2.2 Web Intermediary (WEBI) ............................. 9
3.3 IETF Protocols ...................................... 9
3.3.1 HTTP ................................................ 9
3.3.2 FTP ................................................. 10
3.3.3 SAP ................................................. 10
3.3.4 SIP for Event Notification .......................... 11
4 IMG Common Framework Model .......................... 12
4.1 IMG Data-Type ....................................... 12
4.1.1 Complete Description ................................ 12
4.1.2 Delta Description ................................... 13
4.1.3 Pointer ............................................. 13
4.2 Operation Set for IMG Delivery ...................... 13
4.2.1 IMG ANNOUNCE ........................................ 13
4.2.2 IMG QUERY ........................................... 14
4.2.3 IMG RESOLVE ......................................... 14
4.2.4 IMG SUBSCRIBE ....................................... 14
4.2.5 IMG NOTIFY .......................................... 14
4.3 IMG Entities ........................................ 15
4.4 Overview of Protocol Operations ..................... 16
5 Deployment Scenarios for IMG Entities ............... 16
5.1 One-to-many Unidirectional Multicast ................ 17
5.2 One-to-one Bi-directional Unicast ................... 18
5.3 Combined Operations with Common Metadata ............ 19
6 Use case Requiring IMG Framework .................... 20
6.1 IP Datacast to a Wireless Receiver .................. 20
6.2 Regular fixed dial-up Internet Connection ........... 21
6.3 Broadband Always-on Fixed Internet Connection ....... 21
7 Security Considerations ............................. 21
8 Bibliography ........................................ 22
9 Acknowledgements .................................... 24
10 Author's Addresses .................................. 24
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1 Introduction
An Internet Media Guide (IMG) is a collection of multimedia session
descriptions expressed using SDP, SDPng or some similar session
description format. It is used to describe a collection of multimedia
sessions (e.g. television programmed schedules). IMGs 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.
This document is the result of investigating work on delivery
mechanisms for IMGs and generalizing work on session announcement and
initiation protocols, especially in the field of the IETF-MMUSIC
working group (SAP[1], SIP[2], SDP[3]).
Firstly, this document outlines the use of existing protocol to
create an IMG delivery infrastructure. It aims to organize existing
protocol into common model and show their capabilities and problems
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. These issues
and related problems with existing protocols are clarified in this
document.
Then, this document defines a generalized model for IMG delivery
mechanisms and their deployment in network entities with reference to
existing protocols. 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.
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 [4].
Internet Media Guide (IMG): An IMG is a set of meta-data
describing the features of multimedia content. For example,
meta-data may consist of the URI, title, air time,
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.
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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.
3 Problem Statement
The MMUSIC working group has long been investigating content, media
and service information delivery mechanisms and protocols, and has
itself produces Session Announcement Protocol (SAP), Session
Description Protocol (SDP) and Session' Initiation Protocol (SIP)
which, among other things, are protocols to distribute metadata about
multicast session information. Also in the IETF, HTTP is a well-known
information retrieval protocol using bi-directional transport and
widely used to deliver metadata to many hosts.
However, when the number of services, content and the receivers
becomes huge, we need a more scalable protocol for the metadata
delivery. In addition, comparing with other type of data, metadata
tends to change quite often, therefore an effective mechanism to
update the metadata is also required.
Although these protocols are widely used in the Internet they
mismatch our demands for scalable and flexible information
distribution in terms of the large amounts of metadata and a huge
number of receivers.
3.1 Prior Methods for Server-Client Scenario
There are three elementary scenarios for media delivery for server-
client models: Web-based Media Guide; Multicasting a Media Guide;
Fetching a Media Guide by Unicast. Each of these is described below.
The list is not intended to be exhaustive, but instead it is to give
a good foundation in the important types of media delivery mechanisms
in current usage.
For all these scenarios, it is assumed that there are a number of
suitable (e.g. multicast and broadcast) services available and
interested user groups having terminals capable of receiving the
services.
The IETF has previously standardized a number of protocols that are
relevant to the above scenarios. These are also briefly considered
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here.
3.1.1 Web-based Media Guide Browsing
A common Internet-world approach to discovering multicast - and
unicast - streaming media is to access web pages. These web pages
describe and categorize services in sufficient detail for the user,
and provide links to machine-readable files that provide enough
session and media parameters for client applications to access the
streaming service over a suitable IP network.
This scenario tends to be a human-oriented mechanism because the web
already provides appropriate infrastructure for human-oriented
information delivery and it is easy to deploy this technique.
However, this technique strongly depends on a web-based system,
namely client-driven delivery and a human readable data model for
browsing and selection.
Whenever a web client requires updated web pages, the client has to
reload the web page using the same URL. For mass media, the large
number of users polling a server causes scalability and congestion
concerns and so the technique is feasible only if the period between
reloadings is long and the amount of a media guide metadata or the
number of users is small. A well-behaved implementation significantly
limits the timeliness of receiver-side updates for mass audiences.
In any Internet access situation where there is a premium for
always-on connectivity, the need to keeping many point-to-point
sessions active merely for the purposes of browsing current and
future services may cause severe scalability concerns for the
distribution of resources. This is especially true of wireless
systems with finite, and potentially expensive, radio resources.
Additionally, this does not easily enable off-line reading of media
metadata. So finding the service descriptions for content that is
already delivered and locally stored becomes problematic. As does
browsing descriptions of available services at a future destination
and time, especially for multi-homing and mobile users. For these
kinds of use cases, an off-line storage of service descriptions is
necessary, and effective separation, filtering and maintenance of
this descriptive metadata is needed to keep the local storage at a
reasonable finite size.
3.1.2 Multicasting a Media Guide
A common broadcast-world (TV and radio) approach to discovering one-
to-many services is to push descriptions of streaming services to the
user terminals. Often this is done by means of data carousels that
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repeat the same set of service information at known or predictable
intervals. The repetition provides some robustness (i.e. tolerates
some errors in transmission) and enables receivers that are
switched-on later than the first service information transmission to
collect the full media guide - or at least the parts they are
interested in. In addition to carouselling the information, it is
also a common idea to notify of updates and delivery of changed
service information on-demand based on the changed event.
The primary advantage of this scheme is its massive scalability. No
uplink signaling is necessary and the user group size (for the media
guide) is unlimited. The downlink is more efficient than web-based
access on the broadcast radio as all receivers, which do not stay in
a power-saving mode, are able to receive the data simultaneously
using just one shared logical and physical link channel. However, to
prevent the downlink bandwidth usage from exploding, the service
information that is broadcast is fine-tuned according to the system
(for instance, this may be different for broadband and narrowband
users). Only the essential information is mandatory. Concise
information may be transmitted frequently and verbose information
infrequently. Therefore, the effect is a reasonable balance between
user and control data on the downlink radio channel.
Receivers may remain actively receiving for as long as desirable to
collect the service information, although it is common practice to
switch the communications interface to an idle, or sleep, mode once
the initial media guide is cached and only periodically reconnect to
a session for updates. Thus power-saving techniques are feasible, and
more effective, when the time of interesting service information
transmission is known in advance.
In the case of extremely small user groups, the repetition of service
information transmission in the downlink may cause more traffic than
separate individual connections to the sender. In this case, the
carousel method is not optimal and an on-demand multicast push or
unicast fetch may be more efficient.
There are a few disadvantages to these multicast schemes too. There
may be a significant delay before the interesting information is
transmitted. The information may be brief as verbose descriptions
would be unnecessary in most cases. The information is targeted at a
group and may not be well tuned to the wishes of an atypical user.
The IETF has developed two protocols for these kinds of applications.
SAP (version 2) and SDP are typically used to announce session
descriptions by multicast. SAP is the transport protocol over UDP/IP
and SDP is the description syntax for basic media and session
information. Both of these are independent such that, for example,
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SAP would be used to transport some other description syntax and SDP
could be delivered over some other transport protocol. The deployment
of SDP is increasing as it is also used with SIP. However, the
deployment of SAP is limited and it remains at the experimental
standard status.
SDP has a text-encoded syntax that is extensible but has well
recognized limitations. These are mostly overcome by the XML-based
SDPng[5] , which is intended for both two-way negotiation and also
unidirectional delivery.
SAP has a bit-encoded header and includes details of bandwidth
control by transmission frequency. SAP also has several know
limitations and, especially, robustness or reliability, payload size
and packing, bandwidth control, prioritization of
metadata/descriptions and authentication are either lacking or
performed in an unusual or unusable way for many applications.
Both SDP and SAP are products of the MMUSIC working group of the
IETF. There are no other current IETF standards that solve these
kinds of problems.
Tailoring multicast (and unicast) push-type delivery requires server
configuration facilitated by some kind of control messaging between
client and server. In the case of multicast, the crudest method is to
preconfigure multicast channels (or groups) with well-known subsets
of data at the sender and simply allow receivers to joining to the
relevant multicast groups (using MLD [6] or IGMP[7] for IP versions 6
and 4 respectively). The most significant limitation of this is
making the content well known. For instance, knowing in advance
exactly which metadata/descriptors will be updated in the future is
infeasible in most cases. This can be refined by making the type of
content well known so, for instance, a certain multicast channel
might contain metadata updates only relating to sports channel
services in the next hour.
The unicast equivalent of this is to maintain a unicast
connection/session between sender and receiver for the whole time a
receiver is interested in a service. This may be feasible in many
fixed-line systems for servers with only a few receivers, but both of
these become less attractive for both wireless links and large
numbers sender-receiver connections, especially as both of these
share a resource (the radio bandwidth or the server resources) and
thus limit the number of receivers which can be served, without
additional infrastructure investment.
More finely grained control would be to provide push content
categorized for multicast and individual, or categorized, for
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unicast, and use a control methods separated from the data delivery.
This can be facilitated by SIP or remote procedure call techniques
using SOAP[8] , CORBA[9] or ONC-RPC[10], among others.
3.1.3 Possible Solutions
Fetching media descriptions by unicast from the network is a
compromise, which enables personalized media guides that are
available off-line. Used alone, it is subject to similar scalability
issues as web-based media guides. However, it is well suited to being
used in combination with update notification by unicast or multicast
push so that the correct balance of scalability and personalization
of service guides can be made for the application and network
environment in question. This technique is feasible with current
protocols although not widely implemented, unlike web-base media
guides.
This also requires the use of consistently defined and identified
atomic metadata, which can be selected, delivered and maintained
regardless of the exact transport(s) used to get it to the receiver.
It would also benefit from a consistent metadata structure and syntax
to aid storage and rendering on the receiver.
Scalability may be increased further by the selective use of update
and notifications. An update would provide only the changed metadata
and a notification would notify a receiver that there has been a
change in metadata and would allow the receiver to decide on whether
it needs to receive or fetch that. Notifications are a common
technique in multiparty conferencing where users first subscribe to
the service and then are notified that they may participate. The use
of asynchronous notification allows users to subscribe in advance to
the service and be notified of the event as it occurs.
Fetching an object over IP is a well-standardized and deployed
activity. Primarily either HTTP[11] or FTP[12] is used over TCP/IP.
Metadata syntax describing media is mostly concentrated on
enhancements to SDP and SDPng (although other schemes such as XML-
based MPEG7[13] and TVAnytime [14] are deployable - also for
multicast metadata).
3.2 Prior Methods for Server-Server Scenarios
3.2.1 Content Distribution Internetworking (CDI)
The IETF CDI-WG has been investigating an architecture of network
elements, arranged for efficient delivery of digital content. CDI[15]
is a concept including request routing, the distribution and the
accounting techniques. Among those entities, the distribution
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techniques are related to IMG framework.
The unpublished distribution requirement document in CDI-WG tried to
specify interconnecting, or peering, the distribution systems of
Content Networks (CN). Distribution internetworking requires
advertising the capabilities of a CN offering distribution services,
moving content from one CN to another, and signaling requirements for
consistent storage and delivery of content.
According to CDI-WG definitions, content signaling is a mechanism
between CNs to propagate content metadata. The metadata includes
information such as the immediate invalidation of content or a change
in the sender or distribution method of content[16] .
This work considered parts of an IMG-like framework, although it
focused only in the server-to-server internetworking scope. However,
CDI-WG did not publish the requirement draft nor any protocols to
solve the problems.
3.2.2 Web Intermediary (WEBI)
The WEBI-WG in the IETF tried to define a new protocol named Resource
Update Protocol (RUP)[17] to achieve cache coherence in web
intermediary systems such as caching proxies and surrogates.
Therefore, the target system in WEBI-WG is a content cache network
consisting of caching entities. The target problem was to maintain
consistency in an environment where periodic revalidation is
unacceptable in terms of performance and/or cache consistency.
In addition, the scope of RUP included an updating mechanism for
metadata, which described content cache set.
However, WEBI-WG was closed without standardizing any protocol
document to solve the problems. The WEBI-WG problem definition is
also partly valid for an IMG framework and so may be reused,
especially in server-to-server scenarios.
3.3 IETF Protocols
3.3.1 HTTP
For the metadata distribution model, a HTTP client requests an object
(file containing metadata) by sending "GET Request", then the
metadata sender replies "GET Response" including the metadata in its
body
When the client wants to keep metadata up-to-date, the client must
reissue "GET Request" to the sender and obtain "GET Response" in
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order to avoid metadata becoming stale.
The mechanism has two major problems in terms of scalability and
consistency.
1. Since the client can not know when metadata changes, the
client must poll "GET Requests" to the metadata sender
periodically. This can overload the metadata sender with
unnecessary request messages, especially when the number of
clients high. Therefore, this approach has a scalability
limitations.
2. To decrease the load on the metadata sender, the client
must wait to send the request message for considerably long
period. It makes metadata inconsistent between the sender
and client when metadata changes during the client wait.
3.3.2 FTP
FTP is a client-server type protocol that is widely used. The
protocol aims to provide a control mechanism necessary for a file
transfer. This protocol simply consists of control messages, all of
which are initiated by the client. Thus, regarding the IMG delivery,
the protocol has the same drawbacks as HTTP. Additionally, since this
protocol depends on the file managing structure and file unit, there
is no means to a delta description and pointer operation, which we
discuss later.
3.3.3 SAP
The session information can be described using the Session
Description Protocol (SDP). SDP describes valuable metadata
information including session name, session time, sender and
multicast addresses, format of the media, etc., and the information
becomes the key to join the session.
SDP is text-based and extensible using a number of different options,
which can cause interoperability concerns for parser implementations
in ensuring all variations are supported. This is symptomatic of no
clear separation between application-specific descriptors and common
delivery-enabling descriptors. Furthermore, SDP does not give the
naturally hierarchical syntax nor many of the advanced features that
XML-based descriptors have to offer.
SDP is widely deployed although there is a consensus in the IETF
MMUSIC-WG (the home of SDP), that it should become obsoleted by a
transition to the XML-based SDPng, which is a work-in-progress.
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SDP does not specify how the information is transported. This is the
role of the Session Announcement Protocol (SAP). When the multicast
application starts sending session data, the sender announces its
media-specific information to prospective participants using SAP.
Therefore, although SAP is currently one of the necessary components
of a session announcement system based on the announcement model,
whenever a session is deleted or modified, SAP messages multicast
similarly to keep all the session directory instances synchronized.
Such periodic session announcements cause scalability problems when
the number of sessions increases. Additionally, since the SAP
announcements use the standard non reliable best-effort UDP/IP
semantics, improving SAP robustness in front of packet losses
requires transmitting the messages several times.
As another concern, SAP communication assumes that the whole network
establishes traditional many-to-many multicast communication, called
Any-Source Multicast (ASM), since it is required by the fact that
every data sender becomes a source of the SAP announcement group.
Nowadays Source-Specific Multicast (SSM) [18] realized by IGMPv3[7]
and MLDv2[19] is recognized as the a feasible multicast communication
model for a wide use throughout the Internet, as it eliminates many
complexities associated with the traditional ASM communication model.
If we can conceive that some Internet Service Providers would support
or permit only SSM because of its scalability advantages, avoiding a
multicast session directory that relies on the ASM model is more
reasonable.
3.3.4 SIP for Event Notification
SIP is a text-based request/response protocol. A SIP request consists
of a request-line, header field and message body like HTTP. A SIP
response also consists of a status-line, header field and body.
A SIP Event[20] has the same features as SIP but a SIP Event needs
only two messages, SUBSCRIBE and NOTIFY, to notify subscribers of an
event. Since the protocol intends to do this, it does not require SIP
messages to establish a session. The following diagram shows basic
protocol operations in a SIP Event.
Subscriber Notifier
|-----SUBSCRIBE---->| Request state subscription
|<-------200--------| Acknowledge subscription
|<------NOTIFY----- | Return current state information
|--------200------->|
|<------NOTIFY----- | Return current state information
|--------200------->|
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A SIP Event can notify update information, however there is no
standard protocol using SIP Event in order to carry metadata and
update information about metadata.
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
part of the delivery aspects, and in other cases descriptors 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 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 descriptors.
o Delivery Protocol -- 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 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-Type
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:
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.
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Note, this is not to be confused with a complete IMG, 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,
defining the difference from a previous version of the Compete
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 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 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
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 IMGs delivery. In this case, a IMG sender can initiate
unsolicited distribution for IMGs 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
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IMG ANNOUNCE operation.
4.2.2 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 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 whole IMG 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 notifications. Since the IMG
receiver doesn't know when metadata will be updated and the
notification will arrive, this operations does not synchronize with
the notification. The IMG receiver may wait for the notification 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 an updated IMG is available or part of
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the existing IMG 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 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 Resolver | IMG Notifier |
+------------------+------------------+------------------+
| ^ ^
message | | |
direction v v v
+------------------+------------------+------------------+
| IMG Listener | IMG Querier | IMG Subscriber |
+------------------+------------------+------------------+
| IMG Receiver |
+------------------+------------------+------------------+
Figure 1: Relationship with IMG Entities
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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
stack).
+--------------------------------------------------+
IMG | |
Data-type | 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
Some IMG may be distributed to a large number of receivers. If, for
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example, a particular IMG is public information and the sender
provides the same information for all receivers. This kind of IMG 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, possibly filter or modify its content, and then
forward it. The relayed network architecture is similar to content
distribution network architecture[15] (CDNs). Existing CDNs may carry
IMGs. Satellite and peer-to-peer networks may also carry IMGs.
+----------+ +----------+
| IMG | | IMG |
| Sender |---- ---->| Receiver |
+----------+ \ / +----------+
\ /
. \ +-----------+ / .
. -->|IMG |----- .
. -->|Transceiver| \ .
/ +-----------+ \
+----------+ / \ +----------+
| IMG | / ---->| IMG |
| Sender |---- | Receiver |
+----------+ +----------+
Figure 4: A Relay Network with an IMG Transceiver
IMG sender and receiver 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 or
proxy is used to combine or aggregate IMG data for some receivers. An
IMG receiver may be allowed to receive IMG metadata from any number
of senders.
The IMG 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 an IMG 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 IMGs from
different senders. For example, a rating service may add its own
content ratings or recommendations to existing meta-data.
5.1 One-to-many Unidirectional Multicast
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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
5.2 One-to-one Bi-directional Unicast
Both query/resolve (figure 6) and subscribe/notify (figure 7) message
exchange operations are feasible.
+----------+ +----------+
| IMG |<------(1)------| IMG |
| Resolver |-------(2)----->| Querier |
+----------+ +----------+
Figure 6: Query/Resolve Deployment Example
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Internet Draft IMG Framework June 30, 2003
+----------+ +------------+
| |<-------(1)--------| |
| IMG |--------(2)------->| IMG |
| Notifier | (time passes) | Subscriber |
| |--------(3)------->| |
+----------+ +------------+
Figure 7: Subscribe/Notify Deployment Example
5.3 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
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6 Use case Requiring IMG Framework
6.1 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 carouseled 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. Such that a receiver need only access the
channels of interest and thus 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
segmentation f 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.
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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).
6.2 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 an
IMG). 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 IMGs 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).
6.3 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 IMGs over multicast if the respective
operators require this to keep system-wide bandwidth usage feasible.
7 Security Considerations
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 IMGs may reveal information
about the habits and preferences of a user and may thus deserve
confidentiality protection, even though the information itself is
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public. Capturing and protecting this IMG information 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
the origin of an IMG 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 delivery 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 IMG using specific protocol instantiations will
naturally be subject to the security considerations of those
protocols.
8 Bibliography
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[1] M. Handley, C. E. Perkins, and E. Whelan, "Session announcement
protocol," RFC 2974, Internet Engineering Task Force, Oct. 2000.
[2] 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.
[3] M. Handley and V. Jacobson, "SDP: session description protocol,"
RFC 2327, Internet Engineering Task Force, Apr. 1998.
[4] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[5] D. Kutscher, J. Ott, and C. Bormann, "Session description and
capability negotiation," internet draft, Internet Engineering Task
Force, Mar. 2003. Work in progress.
[6] S. E. Deering, W. Fenner, and B. Haberman, "Multicast listener
discovery (MLD) for IPv6," RFC 2710, Internet Engineering Task Force,
Oct. 1999.
[7] B. Cain, S. E. Deering, I. Kouvelas, B. Fenner, and A.
Thyagarajan, "Internet group management protocol, version 3," RFC
3376, Internet Engineering Task Force, Oct. 2002.
[8] N. Mitra, "Soap version 1.2 part 0: Primer," W3C working draft,
World Wide Web Consortium (W3C), Dec. 2001.
[9] Object Management Group, "Common Object Request Broker
Architecture: Core Specification", Dec. 2002.
[10] R. Srinivasan, "RPC: remote procedure call protocol
specification version 2," RFC 1831, Internet Engineering Task Force,
Aug. 1995.
[11] 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.
[12] J. B. Postel and J. F. Reynolds, "File transfer protocol," RFC
959, Internet Engineering Task Force, Oct. 1985.
[13] 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.
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[14] T.-A. Forum, "Metadata specification S-3," TV-Anytime Forum
Specification SP003v1.2 Part A, TV, TV-Anytime Forum, June 2002.
[15] M. Day, B. Cain, G. Tomlinson, and P. Rzewski, "A model for
content internetworking (CDI)," RFC 3466, Internet Engineering Task
Force, Feb. 2003.
[16] L. Amini et al., "Distribution requirements for content
internetworking," internet draft, Internet Engineering Task Force,
Dec. 2002. Work in progress.
[17] I. Cooper, D. Li, M. Dahlin, and M. Hamilton, "Requirements for
a resource update protocol," internet draft, Internet Engineering
Task Force, Mar. 2002. Work in progress.
[18] H. Holbrook and B. Cain, "Source-specific multicast for IP,"
internet draft, Internet Engineering Task Force, May 2003. Work in
progress.
[19] R. Vida and L. Costa, "Multicast listener discovery version 2
(mldv2) for IPv6," internet draft, Internet Engineering Task Force,
June 2003. Work in progress.
[20] A. B. Roach, "Session initiation protocol (sip)-specific event
notification," RFC 3265, Internet Engineering Task Force, June 2002.
9 Acknowledgements
The authors would like to thank Joerg Ott, Juha-Pekka Luoma, Toni
Paila and Petri Koskelainen on for their ideas and input to this
document.
10 Author's 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
Hitoshi Asaeda
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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
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Y. Nomura et. al. [Page 25]