What are the tools available to properly diagnose a network performance problem in Linux Server?

Below listed Linux tools are used to diagnose the network performance in Linux server.

netstat

    A command-line utility that prints network connections, routing tables, interface statistics, masquerade connections and multicast memberships. It retrieves information about the networking subsystem from the /proc/net/ file system. These files include:

        /proc/net/dev (device information)
        /proc/net/tcp (TCP socket information)
        /proc/net/unix (Unix domain socket information)

    For more information about netstat and its referenced files from /proc/net/, refer to the netstat man page: man netstat.
dropwatch
    A monitoring utility that monitors packets dropped by the kernel. For more information, refer to the dropwatch man page: man dropwatch.

ip
    A utility for managing and monitoring routes, devices, policy routing, and tunnels.

ethtool
    A utility for displaying and changing NIC settings.

/proc/net/snmp
    A file that displays ASCII data needed for the IP, ICMP, TCP, and UDP management information bases for an snmp agent. It also displays real-time UDP-lite statistics.