  Mobility management in a sliced 5G network introduces new and complex
challenges. In a network-sliced environment, user mobility has to be managed
among not only different base stations or access technologies but also
different slices. Managing user mobility among slices, or inter-slice mobility,
motivates the need for new solutions. This article, presented as a tutorial,
focuses on the problem of inter-slice mobility from the perspective of 3GPP
standards for 5G. It provides a detailed overview of the relevant 3GPP standard
principles. Accordingly, key technical gaps, challenges, and corresponding
research directions are identified towards achieving seamless inter-slice
mobility within the current 3GPP network slicing framework.
