CentOS

I am able to telnet on a port at remote machine. However, I am unable to send packets on a connected socket. I run the command (ethtool -k rx off tx off) and then I am able to send packets on same connected socket and then application working accurately.

My Query is, why I need to off rx and tx ?

I guess some device on the path (most probably a switch) doesn't support Frame control (IEEE 802.3x) - for example being disabled from the configuration.