Experimental research into TCP behaviours requires an in depth view of the networking stack’s internal state held for each TCP connection of interest. SIFTR (Statistical Information For TCP Research)  is a recently released, freely available FreeBSD 6.2 kernel module that intercepts TCP packets as they traverse the network stack within the kernel. The performance characteristics of SIFTR were obtained by stress testing the software under a range of conditions. This information can be used as a basis for estimating experimental error inherent in collected SIFTR data as well as broadly determining whether SIFTR might be suitable for a particular task. The experimental methodology is also completely described so that SIFTR’s operational limitations can be measured on different testbeds. With SIFTR running on 2004-era commodity PC hardware configured with maximum data logging granularity up to 100 TCP flows can achieve aggregate throughput of at least 204Mbps with a worst case skip rate of 16.2 skipped packets per Mbps throughput. SIFTR was also evaluated on a second testbed consisting of much newer 2007-era dual core commodity PC hardware. With SIFTR running on this hardware configured with maximum data logging granularity up to 100 TCP flows can achieve aggregate throughput of at least 565Mbps with a worst case skip rate of 31.1 skipped packets per Mbps throughput.