Despite the persistent change and growth that characterizes the Internet,
the Transmission Control Protocol (TCP) still dominates at
the transport layer, carrying more than 90\% of the global traffic.
Despite its astonishing success, it has been observed that TCP
can cause poor end-to-end performance, especially for large transfers
and in network paths with high bandwidth-delay product.
In this thesis, we focus on mechanisms that can
address key problems in TCP performance, without
any modification in the protocol itself.
This evolutionary approach is important in practice, as the deployment
of clean-slate transport protocols in the Internet has been proved
to be extremely difficult.
Specifically, we identify a number of TCP-related problems
that can cause poor end-to-end performance.
These problems include poorly dimensioned socket buffer
sizes at the end-hosts, suboptimal buffer sizing at routers and switches,
and congestion unresponsive TCP traffic aggregates.
We propose solutions that can address these issues,
without any modification to TCP. <br> <br>
In network paths with significant available bandwidth, increasing
the TCP window till observing loss can result in
much lower throughput than the path's available bandwidth.
We show that changes in TCP are {em not required} to utilize all the
available bandwidth, and propose the application-layer
SOcket Buffer Auto-Sizing (SOBAS) mechanism to achieve this goal.
SOBAS relies on run-time estimation
of the round trip time (RTT) and receive rate, and limits its socket buffer
size when the receive rate approaches the path's available bandwidth.
In a congested network, SOBAS does not limit its socket buffer size.
Our experiment results show that SOBAS improves TCP throughput in uncongested
network without hurting TCP performance in congested networks.
<br> <br>
Improper router buffer sizing can also result in poor TCP throughput.
Previous research in router buffer sizing
focused on network performance metrics such as link utilization or loss rate.
Instead, we focus on the impact of buffer sizing on end-to-end TCP performance.
We find that the router buffer size that
optimizes TCP throughput is largely determined by
the link's output to input capacity ratio.
If that ratio is larger than one,
the loss rate drops exponentially with the buffer size
and the optimal buffer size is close to zero.
Otherwise, if the output to input capacity ratio is lower than one,
the loss rate follows a power-law reduction with the buffer size
and significant buffering is needed.
The amount of buffering required in this case depends on whether
most flows end in the slow-start phase or in the congestion avoidance phase.
<br> <br>
TCP throughput also depends on whether the cross-traffic reduces its
send rate upon congestion.
We define this cross-traffic property as {em congestion responsiveness}.
Since the majority of Internet traffic uses TCP, which reduces its send rate
upon congestion, an aggregate of many TCP flows is believed to be
congestion responsive. Here, we show that the congestion responsiveness of
aggregate traffic also depends on the flow arrival process. If the flow
arrival process follows an open-loop model, then even if the traffic consists
exclusively of TCP transfers, the aggregate traffic can still be unresponsive
to congestion. TCP flows that arrive in the network in a closed-loop manner
are always congestion responsive, on the other hand.
We also propose a scheme to estimate the fraction of traffic that
follows the closed-loop model in a given link, and give practical
guidelines to increase that fraction with simple application-layer
modifications.
| Identifer | oai:union.ndltd.org:GATECH/oai:smartech.gatech.edu:1853/22543 |
| Date | 08 January 2008 |
| Creators | Prasad, Ravi S. |
| Publisher | Georgia Institute of Technology |
| Source Sets | Georgia Tech Electronic Thesis and Dissertation Archive |
| Detected Language | English |
| Type | Dissertation |
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