Adaptation Mechanisms for Network-Aware Applications
Diploma Thesis, Summer 1998
Supervisors: J. Bolliger, Prof. T. Gross
Institute for Computer Systems, ETH Zürich
Motivation and Objectives
Network-aware applications dynamically adjust their demand of network resources
(e.g., bandwidth) to the availability of the resources. They usually do so by reducing
the quality of the information transmitted to respect a user-specified time limit.
The adaptation mechanism is highly dependent on accurate information about the network
service quality.
The initial propagation delays inherent to so-called closed-loop control systems, which
ofent form the basis of network-aware applications, are problematic in the start-up phase
of such applications: On one hand, the application should start to send (useful) data right
away to get information about the properties of the underlying network as soon
as possible and not to waste resources. On the other hand, only after getting information
about service quality of the network, the application can decide how much data to send.
To solve this problem, a tradeoff between maximizing the utilization of the communication-channel
and minimizing the risk of violating the time limit has to be found.
In this diploma thesis various measures are studied, which defuse the starting problem
of network-aware applications. The mechanisms can be divided in two groups:
- Mechanisms to reduce the initial propagation delay by delivering
reliable information about the expected network service quality to the application earlier
(information provision).
- Mechanisms that make the data transmission process more flexible in such a way that an excessively
long transfer can be interrupted at any point in time without information loss by applying a
modified data encoding (risk minimization).
Results
Different versions of the concepts presented above were implemented and integrated in the
Chariot image search and retrieval system:
- Reordering of the sequence of the objects to be delivered according
to various properties of the request (1st group of mechanisms)
- A cache for network service quality values, used to extrapolate the conditions
at a given time (1st group)
- Progressive encoding of the data transmitted in combination with a modified
sending process, that can be interrupted at any time (2nd group)
Several experiments were conducted in order to evaluate the efficiency of these
mechanisms and how the mechanisms interact with each other.
Preliminary results indicate that these mechanisms in fact achieve that:
- application data can often be transmitted right away,
- the application receives feedback about the network service quality early on in a
connection,
- the utilization of the sending channel is improved, and
- the risk of violating the time limit due bad estimations of the expected
network service quality is reduced.
Since the behavior of the system is quite complex, the experiments took place
under various restrictions. Further experiments have to show the efficiency of the
integrated properties under real-world conditions.
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ETH Zürich: Department of Computer Science
Comments to Jürg Bolliger <
bolliger@inf.ethz.ch>
June 6, 1999