Hi, I am using iperf v 2.0.5 for testing 10G card. Its a dual port card. Two 16 core machines with 64GB RAM are connected back to back. In *TCP Bidirectional test* case sending throughput decrease as compare to *TCP Unidirectional test* case sending . All cases use default setting . No extra parameter is set. In case of 4 Client threads Output comes :-- Unidirectional Send Process --- *9.6* Gbps Bidirectional Send Process ---- *4.9* Gbps In case of 8 Client threads Output comes :-- Unidirectional Send Process --- *9.7* Gbps Bidirectional Send Process -- *6.4* Gbps In case of 16 Client threads Output comes :-- Unidirectional Send Process ---* 9.7* Gbps Bidirectional Send Process -- *8* Gbps Is this problem regarding TCP/IP stack or nic card ? Regards varun