This document describes tools useful in relaying a data stream from a data device to a network tunnel, such as by injecting in-band control messages without impacting a data rate. For example, the tools may receive data packets on a first link and then relay the data packets to a data device on a second link. While doing so, the tools may periodically free an amount of link time by reducing the length of the relayed data packets by reducing the length of a header of each data packet. The tools may then consume the freed link time by sending a control message on the second link to the data device.