Title:
On the Congestion Responsiveness of Aggregate Internet Traffic: Open-loop vs Closed-loop Session Arrivals
On the Congestion Responsiveness of Aggregate Internet Traffic: Open-loop vs Closed-loop Session Arrivals
Author(s)
Prasad, Ravi S.
Dovrolis, Constantine
Dovrolis, Constantine
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Abstract
A traffic aggregate is congestion responsive if
it reacts to network congestion by reducing its rate. The congestion
responsiveness of Internet traffic has been largely attributed
to TCP's congestion control. In this paper, we argue
that congestion control for individual transfers is not sufficient
to produce responsive aggregate traffic. The offered
load at a network link is generated from users/applications
that generate finite-length flows or groups of flows (sessions).
We examine two session generation models. First,
a closed-loop model where each user from a certain population
can generate a new session only after the completion
of her previous session. Second, an open-loop model where
sessions arrive independently of previous sessions. These
two models produce traffic with very different congestion responsiveness,
even if each flow is controlled by TCP. We introduce
two metrics to quantify the congestion responsiveness
of a traffic aggregate, the throughput responsiveness
and the flow rate responsiveness, and show that the closed-loop
model results in congestion responsive traffic, while the
open-loop model can lead to persistent overload and congestion
collapse. We then measure the congestion responsiveness
of the traffic at a university access link. These experiments
show that both responsiveness metrics are close
to zero, which explains why that link is often under persistent
overload. We also present an estimation methodology to
classify the traffic at a link as open-loop or closed-loop. Our
measurements at a dozen of access and core links show that
more than 70% of the traffic we analyzed follows the closed-loop
model. This implies that a major reason for the congestion
responsiveness of Internet traffic may be that most
traffic reacts to congestion at the session generation layer.
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Date Issued
2006
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Text
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Technical Report