Transmission Control Protocol (TCP) is considered one of the most important protocols
in the Internet. An important mechanism in TCP is the congestion control
mechanism which controls TCP sending rate and makes TCP react to congestion
signals. Nowadays in heterogeneous networks, TCP may work in networks with some
links that have lossy nature (wireless networks for example). TCP treats all packet
loss as if they were due to congestion. Consequently, when used in networks that
have lossy links, TCP reduces sending rate aggressively when there are transmission
(non-congestion) errors in an uncongested network.
One solution to the problem is to discriminate between errors; to deal with congestion
errors by reducing TCP sending rate and use other actions for transmission
errors. In this work we investigate the problem and propose a solution using an
end-to-end error discriminator. The error discriminator will improve the current
congestion window mechanism in TCP and decide when to cut and how much to
cut the congestion window.
We have identified three areas where TCP interacts with drops: congestion window
update mechanism, retransmission mechanism and timeout mechanism. All of
these mechanisms are part of the TCP congestion control mechanism. We propose
changes to each of these mechanisms in order to allow TCP to cope with transmission
errors. We propose a new TCP congestion window action (CWA) for transmission
errors by delaying the window cut decision until TCP receives all duplicate acknowledgments
for a given window of data (packets in flight). This will give TCP a clear
image about the number of drops from this window. The congestion window size is
then reduced only by number of dropped packets. Also, we propose a safety mechanism
to prevent this algorithm from causing congestion to the network by using
an extra congestion window threshold (tthresh) in order to save the safe area where
there are no drops of any kind. The second algorithm is a new retransmission action
to deal with multiple drops from the same window. This multiple drops action
(MDA) will prevent TCP from falling into consecutive timeout events by resending
all dropped packets from the same window. A third algorithm is used to calculate
a new back-off policy for TCP retransmission timeout based on the network¿s available
bandwidth. This new retransmission timeout action (RTA) helps relating the
length of the timeout event with current network conditions, especially with heavy
transmission error rates.
The three algorithms have been combined and incorporated into a delay based
error discriminator. The improvement of the new algorithm is measured along with
the impact on the network in terms of congestion drop rate, end-to-end delay, average
queue size and fairness of sharing the bottleneck bandwidth. The results show that
the proposed error discriminator along with the new actions toward transmission
errors has increased the performance of TCP. At the same time it has reduced the
load on the network compared to existing error discriminators. Also, the proposed
error discriminator has managed to deliver excellent fairness values for sharing the
bottleneck bandwidth.
Finally improvements to the basic error discriminator have been proposed by
using the multiple drops action (MDA) for both transmission and congestion errors.
The results showed improvements in the performance as well as decreases in the
congestion loss rates when compared to a similar error discriminator. / Ministry of Higher Education and
King Saud University in Saudi Arabia.
| Identifer | oai:union.ndltd.org:BRADFORD/oai:bradscholars.brad.ac.uk:10454/3347 |
| Date | January 2009 |
| Creators | Alnuem, M.A. |
| Contributors | Mellor, John E., Fretwell, Rod J. |
| Publisher | University of Bradford, Department of Computing |
| Source Sets | Bradford Scholars |
| Language | English |
| Detected Language | English |
| Type | Thesis, doctoral, PhD |
| Rights | <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/"><img alt="Creative Commons License" style="border-width:0" src="http://i.creativecommons.org/l/by-nc-nd/3.0/88x31.png" /></a><br />The University of Bradford theses are licenced under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/">Creative Commons Licence</a>. |
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