Hoststack Throughput Testing over TCP/IP with iperf3
----------------------------------------------------

`iperf3 bandwidth measurement tool <https://github.com/esnet/iperf>`_
is used for measuring the maximum attainable bandwidth of the VPP Host
Stack connection across two instances of VPP running on separate DUT
nodes. iperf3 is a popular open source tool for active measurements
of the maximum achievable bandwidth on IP networks.

Because iperf3 utilizes the POSIX socket interface APIs, the current
test configuration utilizes the LD_PRELOAD mechanism in the linux
kernel to connect iperf3 to the VPP Host Stack using the VPP
Communications Library (VCL) LD_PRELOAD library (libvcl_ldpreload.so).

In the future, a forked version of iperf3 which has been modified to
directly use the VCL application APIs may be added to determine the
difference in performance of 'VCL Native' applications .vs. utilizing
LD_PRELOAD which inherently has more overhead and other limitations.

The test configuration is as follows:

       DUT1              Network               DUT2
[ iperf3-client -> VPP1 ]=======[ VPP2 -> iperf3-server]

where,

 1. iperf3 server attaches to VPP2 and LISTENs on VPP2:TCP port 5201.
 2. iperf3 client attaches to VPP1 and opens one or more stream
    connections to VPP2:TCP port 5201.
 3. iperf3 client transmits a uni-directional stream as fast as the
    VPP Host Stack allows to the iperf3 server for the test duration.
 4. At the end of the test the iperf3 client emits the goodput
    measurements for all streams and the sum of all streams.

 Test cases include 1 and 10 Streams with a 20 second test duration
 with the VPP Host Stack configured to utilize the Cubic TCP
 congestion algorithm.

 Note: iperf3 is single threaded, so it is expected that the 10 stream
 test does not show any performance improvement due to
 multi-thread/multi-core execution.