Cryptography Reference
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communication in wireless links. In the past, protocol designers have faced several chal-
lenges in incorporating the legacy layered architecture in the context of wireless com-
munication—for instance, the classic case of misconstruing a Transmission Control
Protocol (TCP) packet error on a wireless channel as an indicator of network congestion
in the communication channel by the sender of the packet.
TCP is considered to be a reliable, connection-oriented, end-to-end data-trans-
fer protocol. It achieves reliability by error or loss detection and retransmission and
is responsible for congestion control in an IP network. In current deployments of IP
networks, TCP assumes packet losses and unusual delays in the network as a direct
implication of congestion in the network. The receiver on the other end promptly sends
cumulative acknowledgments (ACKs) for successfully received segments, which then
assist the sender in determining the number of segments that have been received success-
fully. The sender determines the number of lost packets, either by the arrival of several
duplicate cumulative ACKs or by the absence of an ACK for a timeout interval equal
to the sum of the smoothed round-trip delay and four times its mean deviation. To
reduce congestion in the network, TCP reacts to packet losses, either by retransmitting
missing data and simultaneously invoking congestion control by reducing its transmis-
sion (congestion) window size or by backing off its repeated retransmissions.. However,
when packets are lost for reasons other than congestion, the transmission-rate adaptive
behavior of TCP would result in reduced throughput and eventually lead to suboptimal
performance of the network. In general, communication over wireless links is often
characterized by high bit-error rates due to channel fading, noise, or interference. In
such situations, TCP over wireless channels endure considerable throughput degrada-
tion and very high interactive delays because the sender misinterprets corruption for
congestion. Hence, to alleviate the effects of non-congestion-related losses on TCP over
wireless channels, several schemes have been proposed; one of these schemes is Explicit
Congestion Notification (ECN) (Floyd 1994).
Whenever the sender generates a TCP packet, it sets the ECN bit to zero. Upon
receiving this TCP packet, if the intermediate router detects congestion, it will set the
ECN bit to one. Such a TCP packet is said to be marked. This marked packet eventually
reaches the destination, and the receiver learns about the new state of the ECN bit in
the received TCP packet. Consequently, the receiver informs the sender about the new
state of this marked packet to the appropriate sender, which, in turn, adapts its transmis-
sion rate depending on the value of the ECN bit. In the current state of the art in TCP
deployment, the TCP sender interprets any packet loss to be the end result of conges-
tion. As a result, whenever packet losses occur over a wireless channel, the TCP sender
reacts immediately by reducing its packet transmission rate, which results in reduced
network throughput.
However, once ECN-enabled TCP is deployed, where the ECN bit can be used
to mark packets to indicate congestion, there is a means of differentiating between
congestion-related loss and wireless channel-related loss. Thus, the channel need not be
smoothed because the ECN mechanism provides a means of explicitly indicating con-
gestion. In Raisinghani and Iyer (2004), it has been analytically shown that in a single-
user environment, if packets are marked based solely on congestion information, there
is no significant degradation of TCP performance due to the time-varying nature of the
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