  Decentralized Congestion Control (DCC) mechanisms have been a core part of
protocol stacks for vehicular networks since their inception and
standardization. The ETSI ITS-G5 protocol stack for vehicular communications
considers the usage of DCC not only in the network or access layers, but also
as a part of the cross-layer architecture that influences how often messages
are generated and transmitted. ETSI DCC mechanisms have evolved from a reactive
approach based on a finite state machine, to an adaptive approach that relies
on a linear control algorithm. This linear control algorithm, called LIMERIC,
is the basis of the mechanism used in the ETSI DCC Adaptive Approach. The
behavior of this algorithm depends on a set of parameters. Different values for
these parameters have been proposed in the literature, including those defined
in the ETSI specification. A recent proposal is Dual-$\alpha$, which chooses
parameters to improve convergence and fairness when the algorithm has to react
to fast changes in the use of the shared medium (transitory situations). This
article evaluates, by means of simulations, the performance of the ETSI DCC
Adaptive Approach and related algorithms, considering both steady state and
transitory situations. Results show that a bad selection of parameters can make
a DCC algorithm ineffective, that the ETSI DCC Adaptive algorithm performs well
in steady state conditions, and that Dual-$\alpha$ performs as well in steady
state conditions and outperforms the ETSI DCC Adaptive Approach in transitory
scenarios.
