December 2025 Update: I made a much better tool for the same purpose!
LIBPCAP Latency Meter is a small Java based tool that measures time difference between inbound and outbound TCP packets messages.
The tool intercepts inbound and outbound TCP packets containing your network messages. All captured packets carry time stamps. This tool matches outbound packets with corresponding inbound packets using some kind of correlation key (For example, value of specific FIX tags).
For example, this tool can be used measure latency of trading signals. FX market data feed usually contain a field QuoteEntryID(299), while FX orders may contain a field QuoteID(117). Tool can correlate inbound and outbound messages based on the value provided in these tags.
Simple statistics (min/max/avg) are printed out during run time and latency log of each signal is recorded into CSV file.
See github Project Wiki for more information.