Broadband electrostatic fluctuations are the most intense wave activity near the ramp of interplanetary shocks and Earth's bow shock. This wave activity consists mostly of wave packets, whose properties and sources have not been fully established. We present the first analysis of the phase speed, wavelength, and other related parameters of electrostatic wave packets associated with interplanetary shocks. We use Magnetospheric Multiscale measurements from one quasi-parallel and one quasi-perpendicular shock, both associated with coronal mass ejections. Based on the revealed parameters, we conclude that electrostatic wave packets observed in interplanetary shocks are ion-acoustic waves. The wave properties in both shocks are essentially identical and also similar to those previously reported at Earth's bow shock. This is an indication of potentially universal mechanisms producing broadband electrostatic fluctuations in different types of collisionless shocks. The present analysis contributes, therefore, not only to the physics of interplanetary shocks, but also advances the understanding of the sources of electrostatic waves in collisionless shocks in general.