One document matched: draft-morita-tsvwg-pps-01.txt
Differences from draft-morita-tsvwg-pps-00.txt
TSV working group
Internet Draft Naotaka MORITA
Document: draft-morita-tsvwg-pps-01.txt NTT Corporation
Gunnar KARLSSON
KTH
Expires: April 2004 October 2003
Framework of Priority Promotion Scheme
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
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Abstract
The Priority Promotion Scheme (PPS) is a new scheme for traffic
control; more specifically, PPS involves applying a kind of admission
control to achieve end-to-end QoS for a series of packets on a
packet-based network. The main targets are interactive multimedia
services such as VoIP, video chat, and video conferencing. The
scheme is based on end-to-end measurement of network resources by end
systems. Before a session is established or even during a session,
the source end system senses, measures, or probes the availability of
network resources by sending out packets with priority one level
lower than that of normal packets. The result is modification of the
DiffServ Code Point (DSCP) value of the succeeding IP packets: the
priority is raised or promoted to firmly establish the session,
lowered to leave resources with existing sessions, or otherwise
adjusted so that the amount of packets does not exceed the available
capacity. The network, i.e., output links of the routers or L2
switches is only assumed to support the per-class form of priority
MORITA & KARLSSON Expires - April 2004 [Page 1]
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control that accompanies the DiffServ architecture. Having all end
systems follow the above behavior achieves end-to-end QoS without the
maintenance of per-flow state in each item of network equipment.
This document describes the reasons for the end-to-end measurement-
based approach and the general network architecture of PPS.
Conventions used in this document
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].
Table of Contents
1. Introduction...................................................3
2. The target service type - interactive multimedia...............3
3. Motivation for the focus on an end-system-oriented measurement-
based approach....................................................5
4. Basic procedure for the Priority Promotion Scheme..............6
4.1 Basic procedure for end systems............................6
4.2 Router behavior............................................7
4.3 Variation of measurement-based mechanisms..................7
4.4 Monitoring of terminal behavior............................8
4.5 Accommodation of variable-bit-rate sources.................9
5. Service models provided by the PPS.............................9
5.1 Admission control.........................................10
5.2 Quality improvement.......................................10
5.3 Available bit rate........................................10
5.4 Bit-rate increase.........................................10
6. The feasibility of probe-based admission control..............11
7. Functional architecture of the Priority Promotion Scheme......11
8. Requirements of the Priority Promotion Scheme.................11
8.1 Routers...................................................11
8.2 End systems...............................................12
8.3 SIP proxies...............................................13
8.4 Edge routers..............................................13
8.5 Media monitoring servers..................................13
9. Security Considerations.......................................14
10. IANA Considerations..........................................14
Acknowledgements.................................................14
Authors' Addresses...............................................14
References.......................................................14
Appendix: Probe-Based Admission Control (PBAC) - Current
experimental results and obervations.............................16
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1. Introduction
Emerging services such as VoIP, video chat, and video conferencing
require session-based QoS. A number of schemes for providing the
required QoS control have been put forward, but they either require
per-flow management of routers within the network or handle the
provision of QoS on a per-class basis, which requires the allocation
of large amounts of resources. In this document, a framework for a
new QoS scheme is proposed. The scheme is suitable for session-based
interactive multimedia and adds less complexity to the network than
previous approaches, while delivering per-flow QoS.
Karlsson [3] [4] originally proposed the basic concept. Here, we
clarify the requirements for routers, introduce enhancements to
session control using SIP, and show some alternative ways to
implement the required monitoring of end-system behavior. We refer
to this scheme as the "Priority Promotion Scheme".
One of the key functions of the Priority Promotion Scheme is the
behavior of routers. We introduce the MF-PHB (Measurable Forwarding
Per Hop Behavior) as a new per-hop behavior that provides the
required functionality. Whether or not MF-PHB is feasible on given
items of existing equipment will have to be verified. This framework
is intended as a guide for device manufacturers, network
administrators, and operators who need a way to provide QoS for
interactive multimedia services. It is not intended, in its current
state, for use by the majority of networks in the Internet. We make
this proposal now because we feel that the only way to achieve a
long-term solution for inter-domain QoS is to start putting intra-
domain solutions into practice and then incrementally expand the
scope of the work as more experience in deployment is gained.
In this document, we introduce a framework for Priority Promotion.
We describe the target service category, which we refer to as
"interactive multimedia services", in section 2. In section 3, we
explain our motivation in focusing on an end-system-oriented
measurement-based approach. The basic procedures of the Priority
Promotion Scheme are then explained in section 4. In section 5,
specific variant applications of the Priority Promotion Scheme are
presented to show the scheme's potential. The feasibility of a
measurement-based approach is presented in the appendix to this
document and section 6 states why the arguments in the appendix are
applicable to the PPS. The functional architecture of the scheme is
described in section 7. Finally, the requirements for individual
functional entities are summarized in section 8. MF-PHB (Measurable
Forwarding) that is necessary to realize PPS is defined in [5] and
the verification scenarios of MF-PHB is in [6].
2. The target service type - interactive multimedia
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The major targets of the Priority Promotion Scheme are multimedia and
interactive communications services provided through software tools
running on PCs and operated by human beings. We call such services
interactive multimedia (IMM) services. Typical examples of IMM are
VoIP, video chat, and video conferencing. Several characteristics
differentiate IMM services from existing data services. Web browsing
and, in many cases, file retrieval are based on client/server models
and the data transfers speeds required are not in general very high.
In contrast to this, IMM services are any-to-any and require
relatively high speeds in the range from less than 1 Mbps to several
Mbps. These IMM-inherent characteristics may cause large
fluctuations in traffic patterns and may not be predictable in
advance.
Other important characteristics of IMM services are the QoS
requirements: that is, the requirements for bandwidth guarantees and
short delays. The latter is because of the real-time nature of these
services. The former is because typical codecs are sensitive to
fluctuations in bandwidth, which lead to degradation of the QoS.
While several codecs adjust their information rates to suit the
available bandwidth, they impose higher processing loads on the end
systems; this approach also necessarily incurs noticeable and
possibly annoying fluctuation in the perceived quality. This implies
that once a session has been established, the bandwidth has to be
guaranteed until the end of the session. In other words, the session
should not be established unless the required bandwidth is available.
Note that one desirable extended interpretation of this concept is to
allow increases, but never decreases, in the bandwidth available to a
session. That is, improvement is acceptable but deterioration is not.
This is why we have included "promotion" in the name of the scheme.
Finally, a session of an IMM service is set up on-demand and may last
for time of the order of minutes to tens of minutes.
When we take the above-described characteristics and requirements of
IMM into account, we see that explicit admission control on a per-
flow basis is necessary. A common argument is that simple over-
provisioning is capable of meeting these requirements. As was stated
above, however, IMM combines the characteristics of relatively large
bandwidth requirements and strict QoS needs in general with
unpredictable traffic patterns. Therefore, we need a form of
session-based admission control to deliver QoS for IMM services.
It should be emphasized that admission control has a completely
different goal from the existing TCP core functionality. The goal of
admission control is to provide bandwidth guarantees with the
appropriate QoS for a certain maximum number of sessions. For
example, if the network is able to carry 100 Mbps and 100 users
request sessions with guarantees of 1 Mbps, nearly 100 sessions
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should be established. If 1000 users request the same 1-Mbps
guarantees, only around 100 sessions should be established. This is
quite different from existing data services provided through the TCP.
The idea of the TCP is to share network resources in a "fair" manner
among the sessions requested at any time. If the network is able to
carry 100 Mbps and 100 users request sessions, 100 sessions should be
established, each with roughly 1 Mbps throughput. If 1000 users
request sessions, all 1000 should be established, each with a
throughput around 0.1 Mbps. This is not suitable for IMM services.
The SIP provides one suitable way to control IMM services. Although
we focus on the SIP in this description, session-control protocols
for the PPS are not restricted in this way.
The application of a QoS policy which includes differentiation based
on the identity of the callers or callees in sessions has to be
studied as a separate issue. Issues include competition between VIP
calls and ordinary calls, or between preferential calls and ordinary
calls in times of disaster. If such a policy that caters for such
situations is to be applied along with simple admission control based
on resource availability, policy credential information from the SIP
or another signaling method may have to be incorporate into the PPS
framework.
3. Motivation for the focus on an end-system-oriented measurement-based
approach
As IP-based networks proliferate, overall network configurations
become increasingly complex. In terms of bandwidth available in the
access network, DSL alone includes many variants. 12-Mbps ADSL is
quite popular in Japan and higher-speed ADSL services will be
deployed in the near future, but the actual throughput is completely
dependent on conditions such as the distance from the central office
and interference among the lines.
Another point is the variations in the network configurations of
customers, including broadband routers. The broadband routers
initially offered for use with higher-speed access lines may not be
capable of providing the same maximum throughput as is stated in the
catalogue. A customer's PC may impose similar restrictions.
Furthermore, wireless access introduces further complications in
terms of the access environment. The network to which the customer
is connected adds a lot of variables.
In such a complicated situation, end-to-end guarantees of QoS are
difficult to achieve and the role of the end system becomes more
important, because only the end system is able to see the actual
conditions of communication. In the Priority Promotion Scheme, the
end systems measure, monitor, or probe levels of network resources so
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that they are able, if possible, to set up and maintain media streams
with required levels of QoS. We focus on an end-to-end approach
because only the end systems are able to judge the overall relevant
network situation.
We refer to the terminal points of the media stream, i.e. PCs or
residential gateways and routers, as end systems.
4. Basic procedure for the Priority Promotion Scheme
The Priority Promotion Scheme (PPS) is a new scheme for traffic
control; specifically, the PPS achieves end-to-end QoS for
interactive multimedia services by exercising admission control for
series of packets on a packet-based network. The scheme is based on
end-to-end measurement of network resources through coordination of
the end systems.
In this context, "priority" means priority or precedence at the
packet level as represented by the DiffServ Code Point (DSCP) in the
IP layer. If we apply the PPS in Layer 2, the priority is
represented by the user_priority field specified in 802.1D and Q. If
MPLS is used as an underlying transport, EXP field corresponds to the
code.
4.1 Basic procedure for end systems
PPS largely relies on end-system behavior for sending the probe
packets, which test the availability of network resources, and for
decisions on whether or not the succeeding (higher priority) packets
can in fact be sent.
Before a session is established and even, under certain conditions,
during sessions, the source-end system senses, measures, or probes to
detect the availability of network resources. This is done by sending
packets with priority one level lower than that of the non-probe
packets, i.e. those for established streams. Probe packets are given
lower priority so that existing flows of packets are maintained and
packet loss is confined to the probe packets; this gives a sharper
focus to the loss characteristics.
Criteria for successful receipt at the destination-end system can
include loss, delay, and delay jitter. The authors believe that loss
will usually be the crucial parameter, but are willing to enlarge the
scope of measurement to include the other two characteristics.
The conditions of receipt determine how the DSCP value for the
succeeding IP packets is adjusted: the priority is raised or promoted
to firmly establish the session, lowered to leave resources with
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existing sessions, or otherwise adjusted to control the amount of
packets such that the traffic fits into the available capacity.
The RTCP can be used to carry the report from the destination end
system. Whether or not the probing packets can carry real media data
depends on the required duration of measurement. If measurement will
take more than a couple of seconds, the probe packets should carry
real media so that the customer does not have to wait for completion
of the measurement period.
4.2 Router behavior
The PPS in principle requires that the network, i.e. each output link
of a router or Layer 2 switch, support per-class priority control.
Prioritization allows the end systems to measure remaining resources
without affecting existing streams. In addition to the simple
priority control required by the PPS in itself, existing classes
(Per-Hop Behaviors or PHBs) such as EF, AF, and BE should be
supported. That is, we have to implement an extension to the
DiffServ architecture. To clarify the requirements specific to the
PPS, we propose Measurable Forwarding as a new PHB (MF-PHB). A
detailed description of the MF-PHB has already been given [5].
Whether or not current DiffServ implementations are capable of
supporting this new PHB for the PPS without elaboration of the queue
configuration is not clear. However, having all end systems behave
in the way described above and all network elements implement the MF-
PHB ensures that the end-to-end QoS is achieved without having to
maintain per-flow states in individual items of network equipment.
A great advantage of the PPS is that it avoids persistent contention
among real-time streams. Note that we are talking about scheduling
priority in the DiffServ scheduler as opposed to a policy perspective
on call control preference or drop preference in a common queue.
4.3 Variation of measurement-based mechanisms
Measurement-based approaches have many basic variants. Any of the
end systems - the media proxy or home gateway, the edge router at the
ingress point of the network, or the border gateway - might be
assigned the role of measurement and decision entity.
The items for measurement from which we identify the remaining
bandwidth are packet loss and/or delay. Explicit congestion
notification initiated by the network may also provide supplementary
information.
For the sake of simplicity, we would like to focus on an approach
that is 1) end-system oriented, 2) loss-rate-based, 3) includes no
mechanism for explicit indication from the network.
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As we have previously noted, the above concept is not new. It was
originally proposed by Karlsson as probe-based admission control
(PBAC) [3][4]. Based on Karlsson's proposal, we would like to extend
the measurement-based approach to allow for various service models,
to clarify the behavior required of routers, and to take into account
monitoring of the correctness of end-terminal behavior.
4.4 Monitoring of terminal behavior
How we monitor, check, or audit the behavior of end systems is an
important issue for a commercial service. Since the Priority
Promotion Scheme is strongly reliant on the behavior of end systems,
incorrect behavior, whether accidental or intentional, will affect
the QoS for other customers.
Here, the items to be monitored include whether or not flows have
been given permission to enter or access the network, whether flows
are at the correct priority level, and whether flows are at the bit
rates indicated by probing or signaled by SIP. These are the
behaviors in the direction from source to destination. The behavior
in the direction from the destination to the source should also be
correct, and feedback reports on e.g. correctness of the conditions
of receipt might be included to monitor this. Furthermore, the
source behavior in response to such reports should be correct in
terms of not promoting priority when the report indicates bad
conditions. One of the benefits of the PPS is the allocation of
resource-management functions to the end systems, since this reduces
the burden on the network. If we implement functions of the kind
just described to monitor the correctness of the behavior of end-
systems, however, we place another burden on the network. There is a
tradeoff between the extent to which we should protect the network
and the costs of doing so.
The site of monitoring is another issue we face in designing the
network. One solution is to install checking mechanisms of the kind
described above in every edge router and have them monitor every
session. This is perfect in terms of protecting the network from all
kinds of incorrect behavior, but would cost too much.
Another practical solution is to introduce two-stage monitoring of
end-system behavior. The intention here is to classify items for
monitoring as either primary or secondary and having them checked at
the appropriate places. Primary monitoring may be implemented at the
edge routers and is triggered by session initiation. Secondary
monitoring might be done by a dedicated media-monitoring server. The
primary monitor checks every PPS-controlled media stream it handles.
Examples of items to check include whether the flow has been given
permission to enter the network, whether the flow rate is no greater
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than the probed bit rate, and the correctness of the usage of the
DSCPs. The secondary monitor checks the details of end-system
behavior. Whether or not the two monitoring stages are really used
will depend on the specific network environment, but both should be
available to allow flexibility in implementation.
4.5 Accommodation of variable-bit-rate sources
Any measurement-based form of admission control is more suitable with
constant bit rate (CBR) sources than with variable bit rate (VBR)
sources. CBR sources to which silence suppression is not applied are
often used in public voice communications in Japan. For interactive
multimedia, on the other hand, it is important that we take VBR into
account.
Another approach is possible, relying on declared traffic parameters
and deterministic capacity allocation rather than results of
measurement. The admission control system gets the declared
parameters, estimates the equivalent bandwidth, and then judges
whether or not admission is possible. The drawbacks here are the
difficulty of deriving truly representative parameters for each of
the many popular codecs and of estimating the total required
bandwidth when a new flow is offered.
VBR has quite different implications for a measurement-based approach
such as PPS. PPS requires no parameters, no estimation, and no
calculation. In addition, utilization of bandwidth is ideal because
measurement is of actual traffic. There is, however, a trade off.
The PPS depends on the usage of resources at the time of measurement.
Measurement for a particular session may occur when the flows already
present are at relatively low rates. The new session may then suffer
loss of QoS when the volume of flows returns to typical levels.
The tuning of the PPS to support VBR sources thus has to reflect
statistical variation, which can be done by probing over a longer
time or by sending the probing packets at a higher rate than the non-
probing packets. A new (elastic) mode of PHB provides a way of
avoiding such mechanisms and is introduced in the definition of the
MF-PHB[5].
Investigations with VBR sources including ON/OFF source have already
been done by Prof. Karlsson as is indicated by the Appendix of the
document.
5. Service models provided by the PPS
The Priority Promotion Scheme can be viewed as a kind of admission
control. However, it is not limited to the kind of
connection/session admission control we imagine if we think of the
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legacy telephone network. The probing can even be handled by the
media packets themselves. In this section, we examine the possible
service models provided by the PPS.
5.1 Admission control
Admission control alone is suitable for conventional service models
such as legacy switched services. The measurement is simply used for
admission control when the session is established. If the trial
fails, the session is not established. The user may retry, but the
terminal behavior does not specify the extent to which this is
possible. PPS is quite effective in this role as long as the
duration of probing is less than a couple of seconds.
5.2 Quality improvement
The case of PPS where the media packets are used for probing is
particularly applicable to quality improvement. The source starts by
sending media packets at probe level. If the conditions of receipt
are poor, the source stops sending the media packets at probe level,
and recommences sending them as packets of another class. After a
while, the source returns to probing; if this succeeds, the packets
are sent as packets of the higher (non-probing) MF-PHB class.
5.3 Available bit rate
In the available-bit-rate service model, the transmitter uses the
information on network conditions received in response to probing to
estimate the actual available bandwidth, selects the closest
bandwidth lower than the available bandwidth, and then sends the
media at the higher MF-PHB priority level. The transmission may be
made to fit the available bit rate by sending the video data with
less size or resolution than was originally desired or sending speech
data alone rather than a mix of video and speech. The quality of the
session is then maintained.
A further possible application of this approach is to send media data
at the full rate but only assign the higher MF-PHB priority to the
core part of the flow, which fits the available bit rate; the other
parts are sent but assigned to another class. This approach should
work well with hierarchical coding (in MPEG for example, I frames
would be sent with high priority and P or B frames with low priority).
5.4 Bit-rate increase
This is an extension to the available-bit-rate service model. If
initial probing indicated that the requested bit rate is not
available, the source sends at the lower rate than requested but
retries probing from time to time. When the requested rate becomes
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available, the source starts sending media packets at the requested
rate.
6. The feasibility of probe-based admission control
Karlsson has already investigated the characteristics of probe-based
admission control (PBAC). Although the overall system architecture
of PBAC is slightly different from the PPS, the basic dynamics are
the same and the analysis of PBAC is applicable to the PPS. A
summary of the analysis is thus given in the Appendix of this
document.
7. Functional architecture of the Priority Promotion Scheme
Figure 1 shows the functional architecture of the Priority Promotion
Scheme. The main functional elements are the two end systems, i.e.
the source and destination, the source-side edge router, the core
routers, the SIP proxy, and the media-monitoring server.
SIP proxy (Media-monitoring server)
|------| |------|
/---------| |------------| |
/ |------| |------|
/ | //
/ | //
|------| |------| |------| |------| |------|
| |=========| Edge |======| Core |======| Edge |======| |
|------| |------| |------| |------| |------|
End system End system
(Source) (Destination)
Figure 1. Functional architecture of the Priority Promotion Scheme
8. Requirements of the Priority Promotion Scheme
In this section, we describe the requirements for the various
functional entities.
8.1 Routers
Although the end systems play an important role in the Priority
Promotion Scheme, the scheme places a few other requirements on the
network. Specifically, the queuing mechanism or PHB (per-hop
behavior) for the PPS creates new requirements for network elements.
The Priority Promotion Scheme is intended to work with the existing
Diffserv PHBs, as was indicated in the introduction. However, to
clearly explain how the scheme would be implemented in this context,
we have to define a new PHB. We refer to this as measurable
forwarding (MF). The essential requirements for MF are as follows.
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- MF has two sub-classes, MF-High (MF-H) and MF-Middle (MF-M).
- MF-H and MF-M share the same capacity.
- MF-H takes priority over MF-M.
In other words, we have a total amount of MF-H and MF-M traffic as a
limit rather than separate limits for the two sub-classes. However,
since MF-M traffic will always defer to MF-H traffic, MF-M traffic
may experience markedly higher levels of jitter and loss than MF-H,
while one would expect MF-H traffic to experience very low levels of
jitter and loss.
Another view of MF is that, if a given amount of MF-M traffic for a
particular stream passes through a router, at least the same amount
of MF-H traffic for that stream must also be able to pass through.
In the absence of other DiffServ classes, configuring existing
commercially available routers to implement the MF-PHB should be
feasible. Further requirements are as follows.
1) The MF must co-exist with other PHBs, such as the EF, AF, and BE.
Existing implementations may not be capable of satisfying this
extended requirement.
2) MF should take priority over AF and BE. This is because the
target services are IMM services, where real-time variations in
traffic characteristics are crucially important.
The more detailed definition of MF-PHB and scenarios for its
verification are available in [5][6].
8.2 End systems
The transmitter should send trial packets before or at the beginning
of a session.
The receiver should record the results of trial-packet reception and
report this information to the transmitter.
The RTCP would be the best candidate to handle reporting of the
results of reception. Some improvements might be necessary to reduce
the measurement period and to make quick decisions. Actually, the
minimum measurement period is the key factor that determines the
usability of the Priority Promotion Scheme. This determines whether
or not the scheme is applicable to admission control, as was
described in section 5.
The transmitter then decides on the next action.
- If the conditions of reception are good, the transmitter sends the
remaining packets with the higher priority.
- If the conditions are not good, the transmitter gives up sending
monitor packets and either 1) sends the remaining packets with
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another class such as BE, 2) stops sending any media data and, after
a while, starts sending monitoring packets again, or 3) terminates
the session.
According to the service models described in section 5, further
actions are necessary.
Synchronization between the two directions of the media stream
remains a subject for further study.
8.3 SIP proxies
In principle, SIP is not directly related to the Priority Promotion
Scheme. However, for commercial applicability, the operator would
have to be able to monitor the service subscription of the customer
before establishing the call. Furthermore, if the edge router is
capable of monitoring user streams, an SIP proxy can send commands to
an edge router, requesting that it check on a particular end system's
behavior.
The specific signaling sequence may depend on the selected service
model.
If the policy is applied as was described in section 5, signaling is
where the policy credentials are exchanged.
8.4 Edge routers
As noted above, in some networks an SIP server might be available and
is able to instruct edge routers to monitor the behavior of end
systems. An edge router might monitor the following items.
- Packet-transmission rates: the transmitter should not send packets
at rates above the peak bit rate offered in the monitoring phase.
- Continuous sending of packets: if the transmitter pauses in the
sending of packets, the other end systems overestimate the remaining
network resources and incorrectly send higher-priority packets.
Transmitters should thus not pause during sending.
8.5 Media monitoring servers
In addition to primary monitoring by the edge routers, more detailed
monitoring may be required. The typical items to be monitored are as
follows:
- the accuracy of packet-reception information from receivers, and
the correctness of reactions of transmitters to this information; and
- if the received information indicates poor conditions, the
transmitter stops sending high-priority packets; if a next trial is
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allowed, a certain time interval should be maintained between the
initial trial and the next trial.
9. Security Considerations
To be described.
10. IANA Considerations
To be described.
Acknowledgements
The authors would like to thank Fred Baker, David Oran, Glenn Reitsma
and other technical experts at Cisco for some insightful suggestions.
Authors' Addresses
Naotaka Morita
Network Service Systems Laboratories
NTT Corporation
9-11, Midori-Cho 3-Chome,
Musashino-Shi, Tokyo
150-8585 Japan
E-mail: morita.naotaka@lab.ntt.co.jp
Gunnar KARLSSON
KTH, Royal Institute of Technology
Department of Microelectronics & Information Technology
Laboratory of Communication Networks
Isafjordsgatan 39
P.O.Box Electrum 229
SE-164 40 Kista, Sweden
E-mail: gk@imit.kth.se
References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9,
RFC 2026, October 1996.
2 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
3 Karlsson, K., "Providing Quality for Internet Video Services," in
Proc. of the CNIT/IEEE 10th International Tyrrhenian Workshop on
Digital Communications, Ischia, Italy, September 15-18, 1998.
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4 Fodor, (nee Elek), V., Karlsson, G., and Roenngren, R., "Admission
Control Based on End-to-End Measurements," in Proc. IEEE INFOCOM,
Tel-Aviv, Israel, March 26-30, 2000.
5 Morita, N., " Measurable Forwarding: A New per-Hop Behavior
(PHB) ," Internet draft, October 2003.
6 Morita, N., " Verification scenarios for Measurable Forwarding PHB
(Per-Hop Behavior)," Internet draft, October 2003.
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Appendix: Probe-Based Admission Control (PBAC) - Current experimental
results and obervations
1. System definitions
. Complete semantic definition of the probe-based admission control
[A1, A2].
. Multicast application of PBAC [A3]. The quality of service scheme
for multicast traffic is based on admission control for both
senders and receivers. The admission control is well suited to
multicast sessions with a single multimedia stream or with several
layered streams.
. Simple security model to verify the end host identities and secure
the probe phase and the admission decision [A4]. The scheme
verifies the end user's identities and secures the transmission
during the probing phase.
2. Analytical models
. Approximate mathematical model that relates probe and data packet
loss rate, queue buffer sizes and achieved link utilization for
the double queue system [A5]. The analysis is based on the
following steps: First, computation of the probability of a single
probe packet being successfully transmitted; second, computation
of the acceptance probability as a binomial distribution; third,
computation of the link utilization as a birth--death Markov
chain; and fourth, computation of the data packet loss for a
particular source type and the probe/data loss relationship.
. Numerical results with figures for probe packet loss probability,
acceptance probability as a function of the load on the system,
link utilization and data packet loss probabilities. The results
agree with the simulations and prove that the considered probe--
based admission control leads to a stable link utilization and has
a clear upper bound on the packet loss probability.
3. Performance evaluation
All the performance figures have been obtained with the NS-2
simulator. Different source types and source rates have been used:
sources with exponential and Pareto on--off holding times and traces
of real MPEG-2 encoded videos, with peak rates from 64 kb/s to 10
Mb/s. The sources are listed in Table 1. The following issues have
been investigated:
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. Performance and comparison of the proposed queuing schemes for the
controlled load service, a double queue system with two priorities
and a single queue system with a discard threshold for probe
packets [A2]. Both queue systems can be used with a proper buffer
and threshold dimensioning.
. The validity of the assumption of a normal distribution of the
probe packet loss for the admission decision [A2]. Histograms of
the probe packet loss prove the assumption valid.
. Stress test with short sessions and sessions that keep silent for
long periods of time [A2]. None of this special sessions have a
serious effect unless they represent a substantial percentage of
the link capacity (over 15 %). The performance of the system under
heavy stress (many simultaneous probes or sessions that keep
silent for periods of time longer than some probe lengths) is
stable. In general, as the situation worsens, the admission
control is conservative, allowing less ongoing sessions, but never
failing to keep the data packet loss under the threshold for
maximum session peak rates of less than 5% of the link capacity.
. Relationship between probe packet loss and session data loss for
different source types and peak rates [A1, A2]. Basically all
source types show between half to one order of magnitude
difference. All the figures show that there is a nearly linear
relationship between the probe and the data packet loss.
. Effect of multiple links scenarios with cross traffic [A1]. The
simulations prove that the bottleneck link dominates the behavior.
. Blocking and data packet loss probabilities and their relation to
the probe length and the location of a multicast receiver [A3].
The simulations prove that receivers in different branches of the
multicast tree have different blocking probabilities, depending on
the link loads on the different multicast branches.
. Performance evaluation of an implementation of the security model
proposed in [A4] with commodity hardware, focusing in the trade
off between security level and setup delay. The simple solution
does not require any change in the network nodes, just a
cryptographic interface in the access gateways and the end nodes.
Table 1: Parameters of the different test sources
Source On Time Off Time Peak Rate
Exponential 20 and 325ms 35.5 and 650ms 64kb/s to 10Mb/s
Pareto (fi=1.5) 20 and 325ms 35.5 and 650ms 64kb/s to 10Mb/s
Mixed 20 and 325ms 35.5 and 650ms 64kb/s to 10Mb/s
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PPS October 2003
Video Traces 360kb/s
(64kb/s average)
4. On-going work
. Software implementation of PBAC for Linux. A library to provide
the probing features is being developed, which will enable
software generators or end applications to perform the probing
before transmitting. The queuing system will be implemented using
the QoS capabilities of the Linux kernel (iproute2 (1)).
. A possible policing and metering tool for PBAC is under
investigation using Netramet (2).
References
[A1] Viktoria Elek, G. Karlsson, and R. Roenngren, "Admission control
based on end-to-end measurements," in Proc. of the 19th Infocom, (Tel
Aviv, Israel), pp. 623--630, IEEE, March 2000.
[A2] I. Mas Ivars and G. Karlsson, "PBAC: Probe--based admission
control," in Proc. of QofIS 2001, vol. 2156 of LNCS, (Coimbra,
Portugal), pp. 97--109, Springer, September 2001.
[A3] I. Mas Ivars, V. Fodor, and G. Karlsson, "Probe--based admission
control for multicast," in Proc. of the 10th IWQoS, (Miami Beach,
Florida), pp. 99--105, IEEE, May 2002.
[A4] M. Conte, I. Mas Ivars, V. Fodor, and G. Karlsson, "Policy
enforcing for probe--based admission control," in Proc. of NTS 16,
(Espoo, Finland), pp. 45--55, Helsinki University of Technology,
August 2002.
[A5] I. Mas Ivars, V. Fodor, and G. Karlsson, "The performance of
endpoint admission control based on packet loss," in Proc. of QofIS
2003, vol. 2856 of LNCS, (Stockholm, Sweden), Springer, October 2003.
(1) ftp://ftp.inr.ac.ru/ip-routing/
(2) http://www.auckland.ac.nz/net/NeTraMet/
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