Internet DRAFT - draft-guenkova-mmusic-e2enp-sdpng
draft-guenkova-mmusic-e2enp-sdpng
MMUSIC Working Group T. Guenkova-Luy,
Internet-Draft A. Kassler
Document:draft-guenkova-mmusic-e2enp-sdpng-00.txt University of Ulm
Expires: September 25, 2002
J. Eisl
Siemens AG
D. Mandato
Sony International
(Europe) GmbH
March 25, 2002
Efficient End-to-End QoS Signaling - concepts and features
Status of this Memo
This document is an Internet-Draft and is subject to all
provisions of Section 10 of RFC2026.
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Abstract
This document analyzes issues for providing both sedentary and
mobile users of multiparty, multimedia communication services with
efficient mechanisms for coping with unstable network conditions
and limited resource availability, conforming to the users' QoS
requirements and expectations. To this extent, the concept of an
efficient end-to-end QoS signaling mechanism is needed, dealing
not only with capabilities/QoS negotiation, but also with
efficient re-negotiations and coordinated resource management.
Requirements of a protocol (thereinafter, the "End-to-End
Negotiation Protocol" - E2ENP) [1], as well as a possible
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implementation thereof, based on extensions of SDPng [2] and on
the use of SIP [3], are then discussed. Finally, differences
and/or synergies with existing solutions and proposals (especially
with [4] and [5]), as well as security considerations, are
provided.
Table of Contents
1. Introduction 2
1.1 Motivations 2
1.2 Problem Statement 2
1.3 Scope of this document 3
2. Definition of Terms 4
3. The concept 5
3.1 Coping with unstable environment conditions 6
3.2 Hierarchical QoS Specification 7
3.3 Inter-stream QoS constraints 7
3.4 Planning counteractions ahead 8
3.5 Independence from network aspects 8
3.6 Mapping of the quality settings on codec definition 9
3.7 Third-Party-Assisted Negotiation 9
4. The features 10
5. Security Considerations 11
6. Related Work 11
7. Conclusions 13
8. References 13
9. Author's addresses for comments and discussions 15
10. Acknowledgements 15
1. Introduction
1.1 Motivations
The Internet has proven to be a successful architecture for
delivering a broad set of electronic services (including - among
many others - telephony, electronic messaging, and audio/video
(A/V) services), not only to sedentary but also to nomadic users.
Micro/macro IP mobility and wireless IP technologies in fact pave
the way to the full integration of the Internet with the second
and third generation of mobile phone systems. In order to achieve
this goal, next generation IP networks and applications will have
to address the increasingly important challenges of wireless
access, mobility management, the provision of Quality of Service
(QoS), and multimedia services.
1.2 Problem Statement
A paramount problem that mobile users will face within this
context is how to cope with limited amounts of resources at the
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end-systems and within the network, under unstable environmental
conditions. It is expected that mobile users will no longer be
able to rely on their QoS contracts being supported over the whole
duration of a session by the network infrastructure, due to
various reasons like: wireless link quality degradations,
handovers, limited amount of resources. By assuming proper
resource overprovision in the backbone, one can expect that these
problems will most likely be concentrated in the access network,
especially in the radio part thereof, and even on the end-systems.
Since users typically have business relationships with specific
network providers (e.g. a subscription with an ISP or a prepaid
phone card), two possible types of handover may occur:
- horizontal handovers: occur within a given administrative
domain of a network provider, and within the same type of access
network;
- vertical handovers: occur across two different types of access
networks and/or across the administrative boundary between two
network providers.
When dealing with handovers, users must be prepared to face
changes in network resource availability, depending not only on
the type of access network, but also on the type of business
relationships the users may have with the various network
providers. In some extreme cases, the users might try to access
the network owned by a network provider, with which the user has
no business relationship at all, or which offers limited amount of
resources. Pricing aspects also play a key role.
Within this context, multiparty multimedia applications will need
an effective and efficient way of handling QoS fluctuations at the
network layer.
In particular, the sheer negotiation of capabilities (like codecs)
is hereby regarded as necessary but not sufficient for meeting the
users' QoS expectations. Intelligent, adaptive applications can in
fact provide predictive QoS on an end-to-end basis only if end-
peers are able to negotiate also QoS aspects directly affecting
the application performance, according to the users' QoS
expectations. This combined information enables applications
effectively choosing the best adaptation strategy in reaction to a
given QoS fluctuation [1].
1.3 Scope of this document
Coping with the aforementioned issues, this document presents a
concept for negotiation of capabilities and QoS at the application
level, triggered by QoS requirements of the end-user (the E2ENP
concept), which can be realized by either deriving a new specific
protocol, or enhancing existing ones. As an example of the latter
option, a possible E2ENP implementation based on extensions of
SDPng [2] and on the use of SIP [3] is also discussed.
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2. Definition of Terms
Adaptation Path (AP)
An ordered set of QoS contracts that end-systems use for employing
adaptation strategies in a preplanned way. The nominal QoS
contract indicates the one the end-system should preferably try to
enforce. Should this not (or no longer) be possible, the AP offers
alternative QoS contracts (and, optionally, specific rules for
choosing them) for degrading the QoS level in a controlled way.
Application Level QoS
An end-system internal representation of QoS specification (e.g.
XML description), derived from User Level QoS specification.
Capability
The ability to perform certain tasks and/or to handle certain
information types. A single capability is associated with a
certain amount of hardware and/or software resources (each
handling a given information type) and can be configured to
produce different QoS levels. On the other hand a given QoS level
can be associated with different capability sets (e.g. different
codecs can offer the same QoS level).
Intermediate Components (IM)
Any network element situated on the signaling and/or the data path
between two end-systems and which can understand the protocol the
end-systems use (at least on the network level). Examples are:
routers, proxies, independent services, etc.
Mediator
The role an end-peer can take for redirecting incoming calls to
another end-peer(s) according to some settings in the user
profile. A part of this role is to provide QoS satisfaction for
the user(s) in a way the Mediator itself (considering the end-
peer's multimedia capabilities) cannot handle.
Peer
A service or an end-device (associated with an end-user). This
term has equivalent meaning with the terms end-peer and party,
which are interchangeably used throughout this document.
QoS change
The change of the QoS contract initiated by the service user or by
an application within the context of a predefined AP.
QoS contract
An agreement between a service user and a service provider ,
specifying QoS requirements and constraints, as well as the
policies required to keep track of a single QoS level during the
given service execution.
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QoS level
A discrete region of the multidimensional QoS space characterizing
a service. Users perceive distinct QoS levels as the result of
applying certain QoS contracts to the given services.
QoS parameter
A functional representation of a single characteristic of a given
QoS aware service (e.g. network bitrate, network overall end-to-
end delay, but also parameters like video frame rate, video frame
size, audio sampling rate, number of audio channels, etc.), which
identifies a measurable QoS related quantity.
QoS profile
A collection of data specifying the end-peer preferences in terms
of QoS, concerning the usage of a given service. The QoS profile
is the end-system internal representation of the user's QoS
preferences within a QoS aware system. The QoS profile can contain
one or several Application Level QoS specifications.
QoS violation
The violation of one or several QoS contracts within an AP, caused
by the network and/or Service Provider and/or local system.
User Level QoS specification
The QoS as users expect to perceive, eventually expressed in non-
crisp terms by non-expert users. This QoS specification is an
application-specific issue, depending on user interface aspects.
3. The concept
This section defines requirements for an efficient end-to-end QoS
signaling mechanism for dealing with multiparty, multimedia
services. This concept also embraces the case of multi-streams
applications like online-games combined with A/V sessions.
The "handling of QoS" shall be achieved through the negotiation of
hardware/software configuration information and the negotiation
and enforcement of QoS contracts, which are derived from the user
preferences :
- a QoS contract shall capture user's QoS expectations, as well
as network provider policies for enabling a comprehensive QoS
adaptation process (this means for instance to control that the
QoS levels fit into the predefined policies, or to control the
amount of resources by up-/downgrading QoS upon detection of QoS
changes / QoS violations);
- a QoS contract shall also capture configuration information
concerning network resources, as well as the resources and
capabilities of the involved QoS aware end-systems (for instance,
some codecs like variable bitrate video-codecs can be differently
preset to produce different QoS directly perceivable by the user:
the sheer definition of codecs is thus not sufficient for
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determining the amount of local and network resources to reserve
when QoS should be supported).
Before starting a negotiation, each end-peer shall derive the
negotiable QoS contracts from existing information concerning both
users' QoS expectations and type/amount of available resources, by
possibly applying the following refinement/validation process:
1. users specify the User Level QoS;
2. applications derive Application Level QoS by mapping User Level
QoS into QoS contracts detailing specific aspects (e.g. video
frame rate, video frame size, image quality, etc);
3. the mapping of Application Level QoS on the end-system
capabilities (i.e. the end-system hardware and software
configuration - e.g. supported codecs), originates QoS contract
Sketches, which represent the Application Level QoS that the end-
system can theoretically support;
4. the validation of QoS contract Sketches against network
provider policies for supporting QoS, originates Validated
Application Level QoS contracts. These valid contracts constitute
a common vocabulary, which the local and the remote end-systems
can use for negotiating the establishment of QoS aware
communications, and for efficiently dealing with QoS re-
negotiations thereof. Those Application Level QoS which have
failed the validation, could still be used in special cases like
vertical handovers, dynamic end-system changes (e.g. due to end-
system up-/downgrading), etc.
For the sake of simplicity, this document refers thereinafter to
"Validated Application Level QoS contracts" as "Application Level
QoS". These are the negotiable QoS contracts end-peers are
expected to deal with during QoS negotiations and QoS re-
negotiations.
In order to allow end-peers reserving network resource in a
coordinated manner, during the negotiation process the sender side
can derive network-level QoS contracts (e.g. the TI(Traffic
Information) and SI(Sensitivity Information) information presented
in [4]) from the negotiable Application Level QoS.
3.1 Coping with unstable environment conditions
In order to allow users achieving QoS with limited amounts of
resources at the end-system and in the network, under unstable
environment conditions, the application developers and the users
shall be prepared to increase renegotiation speed, for instance by
proactively planning proper actions at negotiation time.
As aforementioned, QoS Contract information is complementary to
capability information. As such, the E2ENP may advantageously
negotiate QoS Contracts and capabilities independently. For
instance some codecs might be dynamically downloadable: having
negotiated QoS contracts independently of such codes, allows end-
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peers saving time (critical aspect of re-negotiations), by simply
referencing the pre-negotiated QoS contracts and binding the newly
available codecs with those contracts.
3.2 Hierarchical QoS Specification
Grouping streams is mainly useful for allowing QoS aware
applications to handle groups as a whole when balancing quality to
resource availability under certain environmental conditions.
For instance, in online gaming applications it can make sense to
augment the gaming experience by allowing each player to keep in
audio/visual contact with the other players. To this extent, it is
possible to distinguish (and therefore treat differently) various
groups of streams, each grouping all the streams between the given
player and another player. The description of these groups of
streams is subject to negotiation.
Furthermore, the developers and the users of multiparty multimedia
applications dealing with multiple streams may advantageously
arrange streams by recursively applying the grouping process,
based on high-level criteria aiming to improve resources
orchestration among multiple streams. For instance, a player can
arrange the A/V streams by identifying multiple concurrent
instances of audio- or videoconferences, one with the own team
members and one with each other team. This optional form of
streams grouping is managed locally by the end-peers (and thus is
not subject to negotiation).
The resulting hierarchical structure is topologically equivalent
to a tree, where each leaf represents a stream, and each internal
node represents a group of streams (or, recursively, of groups).
3.3 Inter-stream QoS constraints
Multi-media applications may deal with multiple streams of
different types (i.e. audio, video, and data). To this extent,
users of such applications may wish to specify their QoS
preferences for each single stream, but also any parameter that
might determine inter-stream behavior. Typically, videoconference
applications deal with voice and video streams, which must be
synchronized (time synchronization). If the media codec and stream
format itself does not provide means for synchronizing, it is a
requirement that inter-stream correlation information is available
and negotiated. This correlation can be generalized by introducing
the specification of QoS constraints applicable to a given bundle
of streams (QoS correlation). The decision of what level of
correlation should be enforced at the QoS level among a set of
streams, depends on the scenario and the requirements of the
application. The introduction of the time synchronization and QoS
correlation aspects augment the overall possibilities to specify
and manage QoS within a QoS aware system.
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3.4 Planning counteractions ahead
A possible way to cope with otherwise unpredictable events
resulting from QoS changes / QoS violations, is to enable the
mobile users' applications to efficiently and timely react to
those events, by planning proper counteractions ahead, in a
coordinated manner with the remote end-peers.
In this manner, whenever QoS changes / QoS violations occur,
agreements on how to most effectively adapt to the mutated
conditions can be timely reached among the peers.
These counteractions include:
- negotiating multiple alternative QoS levels for a stream;
- negotiating multiple alternative QoS levels for a group of
streams, to capture time synchronization, QoS correlation, and
other inter-stream QoS constraints;
- coordinating local-, remote-, and network-resource management.
3.5 Independence from network aspects
The end-peers are in general connected over one or a multiplicity
of interconnected networks, including also Intermediate Components
(IM). In terms of SIP the SIP-proxies are considered as IM.
Taking in account the functionality of the SIP-proxies [3], [6]
and their ability to interfere with the communication between two
end-peers, it is desirable that the E2ENP operates based on an
abstraction of the underlying network, in order to provide real
end-to-end negotiation and conformance of the so delivered QoS
information.
Since the IMs offer services that not only may influence the
information that end-peers eventually negotiate via E2ENP, but
also may enforce the results of the E2ENP process, IMs should be
informed about the decision made by the end-peers.
IMs may be informed by providing them with some standard-profile
information before the start of E2ENP, and/or by publishing the
agreed QoS contracts, e.g. via a directory service.
In order to best satisfy the user QoS expectations and the network
provider QoS policies, the following is suggested:
- the E2ENP should be able to be used in combination with (but
independently from) IM, which may result effective in preparing
and/or guaranteeing the QoS contracts agreed by the end-peers;
- the exchange of information (e.g. profiles, security,
authentication, provider policies, etc.) not directly affecting
the E2ENP-process, rather influencing the information that is
going to be negotiated, should be carried out before the E2ENP
starts.
In general the flows carrying E2ENP messaging (the signaling-path)
and the flows carrying the actual streams (the data-path) could be
routed differently, depending not only on network-related issues,
but also on application/service specific reasons. Thus the E2ENP
signaling-paths and the corresponding data-paths between any two
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given end-peers should be in general considered different.
Every time a signaling-path and/or data-path is built, there may
be some IMs (proxies, directory services, etc.) located along the
path, whose usage is application-specific, and which might
"understand" some of the protocols used by the end-peers. Some of
these entities might thus be able to "interfere" with the E2ENP,
thus disrupting the very "End-to-End" nature of the E2ENP. In
order to avoid this threat, it is suggested that:
- with respect to the E2ENP, Intermediate Components should
operate based only on information provided - directly or
indirectly - by the peers, in order to carry out application
specific tasks;
- exception cases when the involvement of the IMs is inevitable,
should also be considered. Thus the IMs should be allowed to
interfere with the E2ENP only by explicitly notifying the end-
peers about the occurred problems, and only when end-peers
misbehave, e.g. by disregarding system policies and/or in case of
system failure conditions.
3.6 Mapping of the quality settings on codec definition
When user-defined audio quality settings are associated with
codecs according to the standard static payload-type definitions
of the codecs [7], each QoS level can be mapped to one audio
payload type expressing such QoS level. On the other hand a single
variable bitrate video-codec can produce multiple QoS levels with
respect to the quality of the single video frame and - in some
cases - the quality of the color of a single frame. A user-defined
QoS level can be applied to a video by defining a compression
percentage for the performance of the video codec. However, some
video codecs have different number of compression levels. When an
application based on the users requirements selects a constant
video quality and accepts a variable bitrate coder, the
application would derive from the QoS level "visually perceivable
quality = X", an unique number expressing the compression level
for the video codec. Thus, in order to better apply QoS settings
to a codec, it is suggested that:
- applications should share a numbering range for the user
perceivable quality specification (overall visual quality and
color quality, if the user wants to specify a certain color
quality), in order to be able to uniquely map video quality to a
given codec;
- the numbering range for the user perceivable quality
specification should have enough resolution to uniquely map to the
compression levels of a given codec.
3.7 Third-Party-Assisted Negotiation
End-peers may leverage services like directory, allocation,
presence, etc. for redirecting connections over alternative end-
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systems, which can more appropriately meet users' QoS expectations
because of more appropriate capabilities.
In these cases, end-peers should be able to negotiate on behalf of
other end-peers, according to specific user profile information.
This type of negotiation is called third-party-assisted
negotiation. By third-party-assisted negotiation a pure Mediator
should only facilitate the delegation of a connection without
actively taking part in it.
4. The features
A possible E2ENP implementation can be realized by combining
special extensions of SDPng [2] with a particular use of SIP.
The following list indicates the proposed SDPng extensions.
- An important part of the implementation is the use of
modularity for the elements by applying SDPng and extensions of
it. This would enable to use SDPng with and without the new
extensions.
- The modularity of the SDPng extensions should enable the end-
to-end negotiating parties to quickly reach agreements on common
codecs, payload types and codec parametrizations thus possibly
optimizing the decisions on commonly supported QoS contracts.
- For describing the QoS contracts it is necessary to have SDPng
elements for mapping the Application Level QoS descriptions onto
the codecs and the payload types. A single QoS contract would be
in these terms the QoS settings of a single media stream.
- It is suggested to enrich at the sender side the negotiated
Application Level QoS contracts with the TI and SI information
presented in [4], for allowing the end-peers to perform resource
reservation in a coordinate manner.
- It is necessary to distinguish between the different streams
(audio, video) and data types (data, control) in the definition of
the QoS contracts for a communication session.
- Considering the video codecs, the parametrization indicated in
[7] is not sufficient to fully characterize the given codec from a
QoS perspective. That is why it is necessary to introduce codec
parametrization attributes like frame-rate, frame-size, color-
quality-range, overall-quality-range, etc. which should be
uniquely interpreted by the end-systems.
- It is necessary to introduce descriptions and parametrizations
for non-streaming data.
- It is necessary to provide means for managing user, system, and
provider policy information at least by defining which of the
proposed QoS contracts the QoS aware system may support during a
negotiation.
- For defining APs new SDPng elements are needed for formally
configuring the adaptation process correspondingly, in accordance
with the envisioned QoS changes and/or violations.
- For defining stream grouping it is necessary to introduce new
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SDPng elements with respect to (eventually hierarchical) group
definition, and QoS correlation / time synchronization aspects.
The following SIP features for implementing the E2ENP processes
are identified:
- Signaling for carrying out the negotiation and re-negotiation
- Means for coordinating resource management among multiple
parties (common example given in [8])
- Transporting and recognizing the E2ENP content expressed as
SDPng content
5. Security Considerations
This chapter discusses some aspects considering security with
respect to SDPng and SIP performance.
If the contents of the SIP message body (SDPng) are private and
should be treated according to security policies they might be
encrypted.
The use of Digital Certificates with and within SDPng message
bodies can serve the unique identification of the sent information
for both end-peers and IM. This would enable the end-peers and IM
verify and best satisfy the user, system and provider policies.
Additional techniques for confidentiality and integrity support,
with respect to the negotiated information, should also be
considered.
On the issue of firewalls, further thorough investigations
concerning IMs are required.
6. Related Work
This section discusses existing solutions and/or proposals in the
field of QoS aware end-to-end negotiations, and highlights the
shortcomings of the respective existing signaling mechanisms (in-
band, SIP) and description methods (SDPng) for QoS.
The authors of [7] and [9] describe possibilities of quick re-
negotiations via in-band signaling. However this kind of signaling
concerns only change of codecs and the redundant support of
differently coded media without considering the respective effects
when QoS should be supported.
The authors of [8] and [10] present a multi-phase call-setup
mechanism that makes network QoS and security establishment a
precondition to sessions initiated by the SIP and described by the
SDP. Thus the resource management is done only for the network
resources. However [8], [10] do not consider pre-negotiation of
QoS and the integration of local and peer resource management.
The authors of [11] describe a complete model for one-to-one
capabilities negotiation with SDP. However this model has problems
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by the definition of mutually referred information and information
on grouping streams because of the flat structure of the SDP.
Additionally this model concerns only capability negotiation but
no QoS support and QoS adaptation.
Ongoing work in [12] identifies the requirements of a framework
dealing with session description and endpoint capability
negotiation in multiparty multimedia conferencing scenarios.
Depending on user preferences, system capabilities, or other
constraints, different configurations can be chosen for the
conference. The need of a negotiation process among the end-peers
for determining a common set of potential system configurations
and for selection of one or many common configurations to be used
is identified, but not described. The authors of [12] also
identify the need for network resource reservation coupled with
session setup.
The proposal in [12] deals only with capability negotiations and
does neither consider a negotiation protocol for determining a
common set of QoS configuration nor integrate local, peer and
network resource reservation.
The most recent versions of SDPng proposal [2], [13] provide
detailed XML Schema specification and a prototype of the Audio
Codec and RTP Profiles. Additionally [13] describes a capability
negotiation process for establishing of a common capability
vocabulary between end-peers. However the proposals in [2], [13]
still do not consider the QoS specific issues.
In [5] an End-to-End User Perceived Quality of Service Negotiation
is described, with the assumption that some IMs and services may
strongly be involved in the end-decision about the negotiated QoS
information of the end-peers. The decision about QoS
configurations in [5] may be taken over some standard "contract
types". Although [5] mentions that signaling and data packets may
flow along different paths throughout the network, the authors of
[5] suggest that some IMs along the negotiation path may influence
the negotiation, though in general having nothing to do with the
data. According to such a protocol model the network is not
transparent.
The negotiation process presented in [5] does not allow
incremental negotiations, and furthermore [5] leverages static APs
with fixed transitions among capability configurations/network-
level QoS contracts only.
The model of [5] suggests negotiations of QoS only at stream level
without considering some stream dependencies like groups and
stream hierarchies.
The most recent work on the problem of User Perceived QoS [4]
introduces SDPng schema for defining traffic throughput and
sensitivity information. This information considers only the
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network reservation process without taking in account the
necessity for local (peer) QoS specific configurations and
reservations. The model in [4] is not taking in account the
Application Level QoS definitions, with respect to codec
configuration and a flexible adaptation process. However the E2ENP
can incorporate this proposal, by allowing senders to derive TI/SI
from Application Level QoS contracts, and to forward this
information as part of a proposal/counter-proposal to the
receiver(s). The model of [4] is static in terms of adaptation,
insofar as it reuses the fixed prioritization of alternative
options derived from SDP, thus losing the powerful flexibility
featured by XML. The description of the payload types in [4] is
not in accordance with the payload types as described in [7],
since the codec parametrization of the audio codecs is already
pre-defined for the static payload types. One interesting aspect
in [4] is the introduction of QoS Class format as a form of
predefined interoperable application level QoS definition. However
such a definition would also need high level QoS specification
considering the usage of different possible QoS configurations of
a single codec/payload type.
7. Conclusions
This document has introduced the concept and requirements of the
End-to-End QoS Negotiation Protocol (E2ENP). The E2ENP is a
signaling mechanism for effectively and efficiently performing
end-to-end QoS negotiations/re-negotiations and resource
management coordination, when dealing with multiparty multimedia
services under unstable network conditions. Special features are
the negotiation of common vocabulary and adaptation paths, that
describe alternative QoS contracts to be applied, when QoS
violations occur.
Furthermore, the E2ENP allows developers and/or users to capture
and enforce time synchronization and QoS correlation (and even
more complex) constraints among multiple streams, thus allowing
adaptation at different levels. This is envisioned to be a key
benefit in terms of QoS for current and future applications.
By modularly extending SDPng, and by properly defining SIP usages,
the authors expect that the induction of the E2ENP concept will
smoothly blend with legacy solutions, along the path towards the
next generation of QoS aware multiparty, multimedia services.
8. References
[1] IST-1999-10050 BRAIN Deliverable 1.2, Concepts for Service
adaptation, scalability and QoS handling on mobility enabled
networks, 31.03.2001 (http://www.ist-brain.org/)
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[3] IETF RFC 2543, SIP: Session Initiation Protocol, M. Handley et
al, ACIRI, March 1999.
[4] L. Bos, et al: SDPng extensions for Quality of service
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- SIP Extensions for Resource Management, IETF SIP working group,
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[9] IETF RFC 2198, RTP Payload for Redundant Audio Data, C.
Perkins et al., Network Working Group, September 1997.
[10] W.Marshall et al., SIP Extensions for Resource Management,
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9. Author's addresses for comments and discussions
Teodora Guenkova-Luy
Dept. Distributed Systems, University of Ulm,
Oberer Eselsberg, 89069 Ulm, Germany
Tel: +49 (0)731 502-4148
Fax: +49 (0)731 502-4142
e-Mail: guenkova@vs.informatik.uni-ulm.de
Andreas Kassler
Dept. Distributed Systems, University of Ulm,
Oberer Eselsberg, 89069 Ulm, Germany
Tel: +49 (0)731 502-4146
Fax: +49 (0)731 502-4142
e-Mail: kassler@informatik.uni-ulm.de
Jochen Eisl
Siemens Mobile Networks
Research & Concepts Dep.
Tel: +49 (0)89 722 62710
Fax: +49 (0)89 722 21882
e-Mail: jochen.eisl@icn.siemens.de
Davide Mandato
Broadband Wireless Technology Research (BWTR)
Sony International (Europe) GmbH
Advanced Technology Center Stuttgart
Heinrich-Hertz-Str. 1
70327 Stuttgart, Germany
Tel: +49 (0)711 5858-797
Fax: +49 (0)711 5858-468
e-mail: mandato@sony.de
10. Acknowledgements
This work has been performed in the framework of the IST project
IST-2000-28584 MIND, which is partly funded by the European Union.
The authors would like to acknowledge the contributions of their
colleagues, especially Markku Kojo.
The authors would also like to thank their colleagues from TZI,
University of Bremen, from Sony International (Europe), Stuttgart
and University of Ulm for their support.
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