Laboratory
8
TCP: Transmission Control Protocol
A Reliable, Connection-Oriented, Byte-Stream Service
Objective
This lab is designed to demonstrate the congestion control algorithms implemented by the
Transmission Control Protocol (TCP). The lab provides a number of scenarios to simulate
these algorithms. You will compare the performance of the algorithms through the analysis
of the simulation results.
Overview
The Internet’s TCP guarantees the reliable, in-order delivery of a stream of bytes. It
includes a flow-control mechanism for the byte streams that allows the receiver to limit
how much data the sender can transmit at a given time. In addition, TCP implements a
highly tuned congestion-control mechanism. The idea of this mechanism is to throttle how
fast TCP sends data to keep the sender from overloading the network.
The idea of TCP congestion control is for each source to determine how much capacity is
available in the network, so that it knows how many packets it can safely have in transit. It
maintains a state variable for each connection, called the congestion window, which is
used by the source to limit how much data it is allowed to have in transit at a given time.
TCP uses a mechanism, called additive increase/multiplicative decrease, that decreases
the congestion window when the level of congestion goes up and increases the
congestion window when the level of congestion goes down. TCP interprets timeouts as a
sign of congestion. Each time a timeout occurs, the source sets the congestion window to
half of its previous value. This halving corresponds to the multiplicative decrease part of
the mechanism. The congestion window is not allowed to fall below the size of a single
packet (the TCP maximum segment size, or MSS). Every time the source successfully
sends a congestion window’s worth of packets, it adds the equivalent of one packet to the
congestion window; this is the additive increase part of the mechanism.
TCP uses a mechanism called slow start to increase the congestion window “rapidly” from
a cold start in TCP connections. It increases the congestion window exponentially, rather
than linearly. Finally, TCP utilizes a mechanism called fast retransmit and fast recovery.
Fast retransmit is a heuristic that sometimes triggers the retransmission of a dropped
packet sooner than the regular timeout mechanism
In this lab you will set up a network that utilizes TCP as its end-to-end transmission
protocol and analyze the size of the congestion window with different mechanisms.
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