An Argument for Increasing TCP's Initial Congestion Window
Venue
ACM SIGCOMM Computer Communications Review, vol. 40 (2010), pp. 27-33
Publication Year
2010
Authors
Nandita Dukkipati, Tiziana Refice, Yuchung Cheng, Jerry Chu, Tom Herbert, Amit Agarwal, Arvind Jain, Natalia Sutin
BibTeX
Abstract
TCP flows start with an initial congestion window of at most four segments or
approximately 4KB of data. Because most Web transactions are short-lived, the
initial congestion window is a critical TCP parameter in determining how quickly
flows can finish. While the global network access speeds increased dramatically on
average in the past decade, the standard value of TCP’s initial congestion window
has remained unchanged. In this paper, we propose to increase TCP’s initial
congestion window to at least ten segments (about 15KB). Through large-scale
Internet experiments, we quantify the latency benefits and costs of using a larger
window, as functions of network bandwidth, round-trip time (RTT), bandwidthdelay
product (BDP), and nature of applications. We show that the average latency of HTTP
responses improved by approximately 10% with the largest benefits being
demonstrated in high RTT and BDP networks. The latency of low bandwidth networks
also improved by a significant amount in our experiments. The average
retransmission rate increased by a modest 0.5%, with most of the increase coming
from applications that effectively circumvent TCP’s slow start algorithm by using
multiple concurrent connections. Based on the results from our experiments, we
believe the initial congestion window should be at least ten segments and the same
be investigated for standardization by the IETF.