Software Defined Networking (SDN) is a new paradigm that facilitates network management and control. In our work, we explore the use of SDN for the control of network traffic on end-hosts. In particular, we use an OpenFlow software switch (OVS) to load balance application traffic across the multiple available network interfaces. A typical example is the simultaneous use of Wifi and 4G interfaces on a mobile device. In order to achieve optimal load balancing, it is critical to know the capacity of the last-hop links associated with the different interfaces. In this paper, we explore and adapt active packet probing mechanisms to the scenario of SDN-based end-host traffic control, in order to estimate the link capacity. In particular, we investigate the use of Variable Packet Size (VPS) probing, and demonstrate its viability via experiments.