With TCP still responsible for the bulk of data transfer over IP networks, increasing research effort is being made to optimise TCP’s behaviour for the increasingly diverse range of potential network conditions. TCP’s congestion control mechanism is one of the primary areas of focus for TCP research. In order to facilitate this type of TCP research using the FreeBSD operating system we have developed a light-weight modular congestion control framework for FreeBSD 7. This greatly lowers the time required by congestion control algorithm implementors and researchers to develop concrete implementations of new algorithms and evaluate them. It also helps to encourage congestion control research using an operating system known for its liberal licence stability and high performance networking stack. The current version of the patch which this report is based on is v0.9.1.