Troubleshooting TCP performance tutorial

Greetings, This past week I have been trying to find the root cause of tcp performance problems of a few clients that are using a third party metro Ethernet for transport. RFC2544 tests (Layer 2) and iperf using UDP give good symmetric performance almost 100% the speed of the circuit. However all kind of TCP tests result in some kind of asymmetrical deficiency, either the upstream or downstream of the client is hugely different. The latency is not a huge factor since all the metro Ethernet connections have less than 2 ms. So the question basically if is there a good tutorial or white paper for troubleshooting tcp with emphasis of using tools like Wireshark to debug and track this kind of problems. Regards, Abel.