  The Laser Interferometer Space Antenna (LISA) is poised to revolutionize
astrophysics and cosmology in the late 2030's by unlocking unprecedented
insights into the most energetic and elusive astrophysical phenomena. The
mission envisages three spacecraft, each equipped with two lasers, on a
triangular constellation with 2.5 million-kilometer arm-lengths. Six
inter-spacecraft laser links are established on a laser-transponder
configuration, where five of the six lasers are offset-phase-locked to another.
The need to determine a suitable set of transponder offset frequencies
precisely, given the constraints imposed by the onboard metrology instrument
and the orbital dynamics, poses an interesting technical challenge. In this
paper we describe an algorithm that solves this problem via quadratic
programming. The algorithm can produce concrete frequency plans for a given
orbit and transponder configuration, ensuring that all of the critical
interferometric signals stay within the desired frequency range throughout the
mission lifetime, and enabling LISA to operate in science mode uninterruptedly.
