END TO END ADAPTATION FOR THE WEB
Matching Content to Client Connections
Kristoffer Getchell, Martin Bateman, Colin Allison, Alan Miller
School of Computer Science, The University of St Andrews, St Andrews, FIFE, SCOTLAND
Keywords: Internet, monitoring, quality of service, adaptation, media, perception.
Abstract: The size and heterogeneity of the Internet means that the bandwidth available for a particular download ma
y
range from many megabits per second to a few kilobits. Yet Web Servers today provide a one size fits all
service and consequently the delay experienced by users accessing the same Web Page may range from a
few milliseconds to minutes. This paper presents a framework for making Web Servers aware of the Quality
of Service that is likely to be available for a user session, by utilising measurements of past traffic
conditions. The Web Server adapts the fidelity of content delivered to users in order to control the delay
experienced and thereby optimise the browsing experience. Where high bandwidth connectivity and low
congestion exist high fidelity content will be delivered, where the connectivity is low bandwidth, or the path
congested, lower fidelity content will be served, and delay controlled.
1 INTRODUCTION
Access bandwidths now range from a few kilobits
per second through a mobile phone up to gigabits
per second through Local Area Networks. Despite
the bandwidth at the core of the Internet increasing,
many bottlenecks continue to exist within the
system; even users with high bandwidth connectivity
may experience significant delay when engaging in
network communication. At present the Internet
relies upon the congestion control mechanisms
embedded in TCP (Jacobson, 1988) to prevent
congestion collapse (Jain and Ramakrishnan, 1988),
which would render it unusable, yet applications are
in a strong position to know how best to react to the
onset of congestion (Floyd et al., 1997). If an
application was made congestion aware it could
directly control the amount of data sent. If
congestion was high it could reduce the absolute
number of bytes transmitted as well as the rate at
which they were sent.
This paper addresses two questions: How is it
pos
sible for an application to become aware of
network conditions and, given this awareness, how
can a system be designed to allow application led
adaptation to occur. A framework, which consists of
three components, is proposed. A Network Monitor
provides a Server with measurements of the Quality
of Service (QoS) that the network is providing. A
Network Aware Web Server handles the dynamic
decisions required in order to determine the optimal
version of a site to send to a connecting client and a
Content Adaptation Tool allows content providers to
generate, from a single high quality site, versions
that are appropriate for different connection types.
From the user perspective a number of factors
cont
ribute to the perceived QoS offered, with both
the speed of presentation and the quality of
resources being important. A fast loading, simple
Web Site is not necessarily going to be considered to
be high quality by all users, and neither is a slow
loading, highly detailed one (Bouch and Sasse,
1999, Bouch et al., 2000, Bouch and Sasse, 2000).
From the content provider’s perspective, a trade-off
must be met whereby a balance between the speed at
which resources can be provided to the target
audience and the quality of the resources provided is
achieved. Currently this trade-off is managed
offline with content providers producing Web Sites
which will be viewable within reasonable periods of
time by the majority of Internet users. Those
Internet users with connections slower than the
speeds accounted for during the development of a
site will be left with a poor browsing experience,
whilst those with faster connections could have been
supplied with pages of a higher fidelity. As the
diversity of connection technologies continues to
expand, this disparity is set to grow yet further.
By considering the QoS offered to the browsing
clien
t by the network, an adaptation framework
131
Getchell K., Bateman M., Allison C. and Miller A. (2005).
END TO END ADAPTATION FOR THE WEB - Matching Content to Client Connections.
In Proceedings of the Second International Conference on e-Business and Telecommunication Networks, pages 131-136
DOI: 10.5220/0001416901310136
Copyright
c
 SciTePress

would afford content providers the ability to focus
on producing the highest quality resources, without
having to consider the limitations that slow network
links may cause. Lower fidelity resources can then
be generated automatically using adaptation tools.
This paper continues with a discussion of related
work in section 2 which leads to a discussion of the
QoS issues of relevance to this work. Section 4
describes the framework used to allow the Network
Aware Server to modify its behaviour as described
in section 5. An evaluation of the framework thus
far is provided in section 6, with section 7
concluding and drawing the paper to a close.
2 RELATED WORK
Numerous studies have advocated the adaptation of
content delivered in response to the load placed
upon a Web Server. A similar approach to those
outlined in (Pradhan and Claypool, 2002, Barnett,
2002) is adopted by our work. However we aim to
address the case where the total load on the Server is
manageable, but specific users are receiving poor
service because of congestion or connectivity issues.
With the expansion of e-commerce, the ways in
which people interact with online vendors is of great
importance to business sectors. (Bhatti et al., 2000)
discuss the issues surrounding user tolerance, with
regards to the levels of service offered, reiterating
the importance of user perception. Through
experimentation the level of user satisfaction is
tested, with the results showing that if a user is
provided with some feedback relatively quickly,
they are generally satisfied to wait for extended
periods while the remainder of their request is
fulfilled.
Taking the concept of adaptation to mobile
devices, (Abdelzaber and Bhatti, 1999, Shaha et al.,
2001) discuss the issues surrounding the need to
augment our traditional understanding of QoS in
order to make the best use of a new wave of access
device. Existing services designed for PC delivery
are not well suited to PDAs. Owing to the poor
quality of PDA delivered content it is hardly
surprising that the mobile device users are often left
frustrated and alienated. Media adaptation
techniques however, may help; allowing mobile
users to more readily gain access to the QoS adapted
versions of a particular resource.
3 QUALITY OF SERVICE ISSUES
The aim of this work is to provide mechanisms that
can maximise the QoS perceived by users for a
given set of network conditions. In order to do so it
is necessary to establish metrics for both the user
perceived and network observed QoS.
3.1 User Perceived QoS
There is no single mapping between network level
metrics and the way in which a user perceives QoS.
Here we identify three factors that contribute to
whether a user perceives a Web Browsing session as
satisfying or not.
1. Fidelity: The quality of data sent to clients is
vitally important – if the technical quality is too
low (i.e. videos are blurry, sound is skewed,
images are fuzzy or too small etc) then the user
will perceive poor resource quality. However if
the technical quality is too high (i.e. videos and
sounds are jumpy because there are delays in
getting data to the client, images are too large or
too slow to load etc) then the user will again
perceive poor quality.
2. Delay and Feedback: When a user is
interacting with a Web Site, they require
feedback for their actions. If it takes 2 minutes
to load each page on a site, then the length of
time required in order to supply feedback to the
user (i.e. the next page they asked for) would be
excessive and so user-perceived quality would
suffer. However if the pages load too quickly,
then it is possible that the user will not
recognise the true speeds – delays of anything
under 30ms are almost unnoticed by users and
as such resources should not be wasted trying to
reduce delays below 50ms or so (Abdelzaber
and Bhatti, 1999).
3. Consistency: If there is consistency of
presentation within a session the user will
perceive a higher QoS during their browsing
session (Bouch and Sasse, 2000).
3.2 Network QoS
QoS is usually quantified at the network level. At
this level there are two challenges; how to discover
the QoS that is being experienced by traffic and how
to communicate this information in a timely and
useful way to an application.
1. Round Trip Time (RTT): The length of time
to send a packet to a remote host and back to
originator. In general, the lower this value the
better – links with a high RTT appear sluggish
and unresponsive. However most users will not
notice delays around the 10s or possibly 100s of
ms range (Abdelzaber and Bhatti, 1999).
ICETE 2005 - GLOBAL COMMUNICATION INFORMATION SYSTEMS AND SERVICES
132

2. Jitter: The variation in packet delay
measurements recorded. Low jitter is better as
it indicates a more stable and therefore more
predictable link. Jitter is important for
interactive, real-time or streaming applications.
Figure 1: Network Monitor Positioning
3. Bandwidth: The amount of data that is
transferred per unit time. Together with RTT
this impacts on the delay experienced by a
client. To use an analogy with water
distribution; RTT is the length of the pipe and
bandwidth the width.
4. Packet Loss: The proportion of packets lost on
a link in a given time period. Lower loss values
are better, indicating a more efficient use of a
link: Lost packets are wasteful as the network
resource used to send the lost packet is wasted
and further wastage is introduced as a duplicate
packet has to be sent.
The approach adopted in this work is to alter the
fidelity of media in response to feedback on the level
of network congestion, thereby achieving the
appropriate trade off between technical quality and
delay. Consistency of presentation is achieved by
setting the fidelity at the start of a user session. In
the absence of sharp and prolonged changes in
network QoS, a consistent fidelity will be presented
throughout a session.
3.3 Adaptability of Media
When considering the types of media available
within Web Pages, they can broadly be categorised
into one of several categories, some of which may
be adjusted to account for the QoS that a connecting
client is receiving. Table 1 provides an overview of
the type of resources and the level of QoS adaptation
that can be applied to them:
Table 1: QoS Adaptability of Media
Media Type
QoS
Adjustable?
Text – HTML, TXT, PDF, etc. Limited
Graphics – GIF, JPG, TIFF etc. Highly
Video – AVI, MPEG, DIVX etc. Highly
Streaming Media Highly
Active Content – Flash, Java etc Limited
4 SYSTEM FRAMEWORK
There are three main components to the framework
developed: a Network Monitor, a Network Aware
Web Server and Content Adaptation Tools. Figure 1
illustrates the Network Monitor positioned so that it
is able to monitor all incoming and outgoing traffic,
with the resulting QoS data being distributed into a
QoS Multicast Zone, where all interested Servers
can then receive it; it is possible to have several
different Network Aware Servers providing a variety
of services to connecting clients, each receiving their
QoS information from a single Network Monitoring
Agent.
4.1 Network Monitor
The network monitor uses online passive monitoring
techniques (Mogul, 1990, Mogul, 1992) to discover
the QoS that flows are receiving. Passive monitoring
is used in preference to active probes for two
reasons. Firstly, no extra wide area traffic is
generated which could adversely impact upon the
client traffic. Secondly, by extracting measurements
from observed traffic more data will be available
from the locations where there is the most traffic;
predictions of expected network conditions will be
more accurate for those locations that use the service
most often.
The structure of the network monitor is shown in
figure 2 with its design, implementation and
evaluation described in more detail in (Ruddle,
2000, Ruddle et al., 2002). TCPDump is used to
capture packets, thus allowing the ability to filter
traffic and generate trace files for post-mortem
analysis. The network monitor extracts congestion
information from Transport Control Protocol (TCP)
data streams and feeds this information back to end
points. Information extraction occurs within a
connection layer where per-connection state is
maintained. Each reading is then passed to a
END TO END ADAPTATION FOR THE WEB: Matching Content to Client Connections
133

Figure 3: Network Aware Web Server Architecture
Fi
g
ure 2: Network Monitor Structure
location layer where statistics are maintained on a
per location basis.
At the connection layer state is maintained for all
open connections. As each packet arrives a hash
table look up is done. This locates the state for the
connection that the packet belongs to. Both TCP
Macro-state, which controls the set up and closure of
connections, and Micro-state, which controls the
flow of data, are monitored. Packet loss is detected
by the observation of duplicate acknowledgements,
or by the observation of a repeat packet. RTTs are
measured by observing the delay between the receipt
of an acknowledgement and the observation of the
packet that generated the acknowledgement. Only
one RTT measurement is attempted per window of
data. When a connection finishes all state associated
with that connection is reclaimed.
When events occur such as a packet being
observed, a loss being discovered or the RTT being
measured, these are communicated to the Location
Layer. The Location Layer maintains state for
aggregates of IP addresses where a Location is
defined as an aggregation of hosts to which traffic is
likely to experience similar network conditions.
State maintained for each location includes the
proportion of packets lost, the RTT and the
maximum bandwidth observed.
The observation of SYN packets allows new
connection requests to be detected and predictions of
likely network conditions for that connection to be
communicated to the local host in a timely fashion.
These predictions are contained within a Location
Information Packet. Extensions to the network
monitor which support the sharing of congestion
information between TCP and Real Time Protocol
(RTP) traffic, have been designed (Miller, 2002).
4.2 Network Aware Web Server
It is an important constraint of this architecture that
the Network Aware Web Server is able to make QoS
decisions automatically whilst working with existing
Web Browser software. As such the Network
Aware Web Server provides content in the same
way as a standard Web Server, fully conforming to
the HTTP/1.0 and HTTP/1.1 specifications (Berners-
Lee et al., 1996, Fielding et al., 1999).
In the current implementation the Network
Aware Web Server supplies different versions of
content depending on the QoS characteristics of a
link in a transparent and fully automatic manner.
Unlike other approaches (Abdelzaber and Bhatti,
1999), adapted resources are not generated on-the-
fly as this would take up valuable processing power
and introduce extra latency. Instead the Network
Aware Web Server chooses between different
versions of a resource that are held on backing store.
Assuming QoS data relating to a connecting client is
available, the Network Aware Web Server
implements only marginal changes to the model
implemented in a standard Web Server, with the
Connection Handler speaking to the Content
Provisioning Mechanism and then the Link
Evaluation Mechanism before returning the most
appropriate resource to the requesting client (figure
3). It is at this stage that indirection is used to send
to a client the most suitable resource, based on the
quality of the client link. Currently the framework
has been developed to support five distinct quality
levels – this is done in order to provide distinct sets
into which various links can be graded and thus
reduce the possible problems caused by a slightly
erroneous quality classification; there is less
likelihood that small errors will cause a change in
grading and so we protect the perceived user quality
by reducing unnecessary quality reclassifications,
and ensure a more consistent session presentation.
As a safeguard, should QoS data not be available the
Network Aware Web Server will revert to standard
operation, serving the non-QoS adapted version of
the requested resource as shown in figure 4.
Although not currently implemented, extensions
can be envisioned whereby content providers are
able to choose to allow their target audience to
ICETE 2005 - GLOBAL COMMUNICATION INFORMATION SYSTEMS AND SERVICES
134

Figure 5: Predicted vs. Actual Congestion
Figure 4: QoS Decision Mechanism
control the adaptation mechanisms thereby allowing
the user of a resource to determine whether QoS
Adaptation is performed or not.
5 RESOURCE MAPPING
Whilst mapping from measured network metrics to a
given QoS adapted version of a resource the priority
is to minimise the delay experienced by the user
whilst maximising the perceived QoS offered. This
work takes the view that by more closely matching
the amount of information to transfer with the
available bandwidth of a client’s connection it is
possible to maximise user perceived QoS by
minimising the delay experienced. In order to
achieve this trade-off a mapping function based on
the steady state behaviour of TCP is used. As
reported by (Mahdavi and Floyd, 1997), the steady
state behaviour of TCP provides a benchmark
against which the behaviour of other congestion
control schemes have been judged. The steady state
behaviour of TCP is given by equation 1 (Mathis et
al., 1997), where MSS is the maximum segment size
on a link, C is a known constant, RTT is the round
trip time and P is the probability of loss. Estimates
of RTT and P are provided by the LIS, MSS for a
connection is known and C is a known constant.
*
*
M
SS C
T
R
TT P
=
(1)
If the data flow is application limited low levels
of loss may suggest an available bandwidth in
excess of the actual physical bandwidth. This can be
accounted for by measuring the actual bandwidth
utilisation and setting the estimated available
bandwidth to the minimum of that estimated from
congestion feedback and actual utilisation.
The bandwidth category that a connection
request falls into (Q) is then given by equation 2:
*
min , max( )
*
MSS C
QT
RTT P
⎛
⎡⎤
==
⎜
⎢⎥
⎣⎦
⎝
B
⎞
⎟
⎠
(2)
In this way the expected available bandwidth for
a destination can be calculated based on past
measurements. The result may not correspond to the
actual access bandwidth, but may rather correspond
to the share of bandwidth available on the bottleneck
link. Having established the bandwidth available to a
destination the Server can then choose the
appropriate version of the site to transmit by
considering perceptual requirements.
In order to maximise consistency, once an
appropriate set of adapted resources have been
chosen, the system makes use of the concept of a
human session, the aim of which is to provide a
consistent fidelity through the lifetime of a session.
Here we define a human session as an amount of
time spanning a series of requests during which a
user might alternate between assimilating the
information within a page and downloading new
pages. For example a user receives Page 1 from a
site and spends 10 minutes reading it, he then
proceeds to request Page 2; ideally we want to
ensure that the quality of Page 2 is similar to the
quality of the Page 1 that has already been received
– this will ensure continuity throughout the browsing
session. In this situation the Human Session timeout
should not be set to anything below 10 minutes.
During a session all pages should aim to be served
from the same version of the site.
Regardless of any Human Session, changes in
the QoS readings are continually tracked during a
session. If an initial link quality evaluation is found
to be consistently lacking then it may be necessary
that remedial action is undertaken, resulting in the
link quality being re-graded, in order to provide the
most suitable browsing experience to the connecting
client. However such a decision should not be taken
lightly and cannot be based solely on one, or
possibly a handful of bad readings, instead historical
data should be considered, with the weight attached
to historical data diminishing over time as the data
becomes more and more out of date. In order to
achieve this data smoothing an Exponentially
Weighted Moving Average (EWMA) is used. It also
deals with the ageing of historical data, something
that the arithmetic mean is unable to do.
END TO END ADAPTATION FOR THE WEB: Matching Content to Client Connections
135

6 EVALUATION
The predictive powers of the network monitoring
agent has been evaluated using live Internet
experiments, between hosts in the US, the UK,
Spain, Germany and Eastern Europe. Figure 5 shows
the predicted verses the actual levels of congestion
experienced. The metric used is the proportion of
Implicit Congestion Notifications (ICNs). Packet
loss is taken as an implicit indication of congestion.
Yet in a single congestion event several packets may
be lost, consequently in these experiments only the
first packet loss in a window of packets is taken as
an ICN.
It is of interest that on links experiencing high
levels of congestion the accuracy of prediction
increases. Further more it was found that the
correlation between prediction and result remained
high for periods of time in excess of 20 minutes.
These results suggest that it is possible to use the
measurement of past traffic to predict the network
conditions that are likely to be experienced by future
traffic. Consequently the approach adopted here is
valid.
7 CONCLUSION
We have presented the design, implementation and
evaluation of a framework for making the Web QoS
Aware. The approach adopted is to use passive
monitoring of transport level headers to make
predictions about the QoS that is expected to be
experienced by a particular location. A mechanism
for translating a single high fidelity Web Site into a
set of Web Sites that are appropriate for different
bandwidths has been outlined. When a user session
starts the Web Server uses QoS information to
determine which bandwidth is appropriate. It then
serves up Web Pages from the appropriate Web Site.
This approach allows Web Designers to leverage the
increasing bandwidth many client have available
without producing Web Pages that are inaccessible
to others.
REFERENCES
Abdelzaber, T. & Bhatti, N., 1999. Web Server QoS
Management by Adaptive Content Delivery.
International Workshop on Quality of Service.
London.
Barnett, C. M., 2002. Adaptive Content Web Serving.
New Haven, CT, Yale University.
Berners-Lee, T., Fielding, R. & Frystyk, H., 1996. RFC
1945: Hypertext Transfer Protocol - HTTP/1.0. IETF.
Bhatti, N., Bouch, A. & Kuchinsky, A., 2000. Integrating
User-Perceived Quality into Web Server Design. 9th
International World Wide Web Conference.
Amsterdam.
Bouch, A., Kuchinsky, A. & Bhatti, N., 2000. Quality is in
the Eye of the Beholder: Meeting Users' Requirements
for Internet Quality of Service. CHI 2000.
Bouch, A. & Sasse, M. A., 1999. Network Quality of
Service: What do users need? 4th International
Distributed Conference. Madrid.
Bouch, A. & Sasse, M. A., 2000. The Case for Predictable
Media Quality in Networked Multimedia
Applications. MMCN 2000. San Jose, CA.
Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
L., Leach, P. & Berners-Lee, T., 1999. RFC 2616:
Hypertext Transfer Protocol - HTTP/1.1. IETF.
Floyd, S., Jacobson, V., Liu, C.-G., Mccanne, S. & Zhang,
L., 1997. A reliable multicast framework for light-
weight sessions and application level framing.
IEEE/ACM Transactions on Networking, 5, 784-803.
Jacobson, V., 1988. Congestion Control and Avoidance.
ACM Computer Communications Review, 4, 314-329.
Jain, R. & Ramakrishnan, K. K., 1988. Congestion
Avoidance in Computer Networks with a
Connectionless Network Layer: Concepts, Goals and
Methodology. IEEE Computer Networking
Symposium. Washington D.C.
Mahdavi, J. & Floyd, S., 1997. TCP-Friendly Unicast
Rate-Based Flow Control. Technical note sent to the
end2end-interest mailing list:
http://www.psc.edu/networking/papers/tcp_friendly.ht
ml
.
Mathis, M., Semke, J., Mahdavi, J. & Ott, T., 1997. The
Macroscopic Behavior of the Congestion Avoidance
Algorithm. Computer Communications Review, 27.
Miller, A., 2002. Best Effort Measurement Based
Congestion Control. Department of Computer Science.
Glasgow, University of Glasgow.
Mogul, J. C., 1990. Efficient use of Workstations for
Passive Monitoring of Local Area Networks. ACM
SIGCOMM, 20, 253-263.
Mogul, J. C., 1992. Observing TCP Dynamics in Real
Networks. ACM SIGCOMM, 22, 305-317.
Pradhan, R. & Claypool, M., 2002. Adaptive Multimedia
Content Delivery for Scalable Web Servers.
International Network Conference. Plymouth.
Ruddle, A., 2000. A Location Information Server for the
Internet. IEEE International Conference on Computer
Communications and Networks. Las Vegas, NV.
Ruddle, A., Allison, C. & Lindsay, P., 2002. A
Measurement Based Approach to TCP Congestion
Control. European Transactions on
Telecommunications, 13, 53-64.
Shaha, N., Desai, A. & Parashar, M., 2001. Multimedia
Content Adaptation for QoS Management over
Heterogeneous Networks. International Conference on
Internet Computing. Las Vegas, NV.
ICETE 2005 - GLOBAL COMMUNICATION INFORMATION SYSTEMS AND SERVICES
136