iperf_udp.c issue of tos not set for the first test packet

[velamen2009]

Context

• Version of iperf3: 3.6+

• Operating system (and distribution, if any): tested on Ubuntu, MAC

Please fill out one of the "Bug Report" or "Enhancement Request"
sections, as appropriate.

Bug Report

At iperf_udp.c, line 431, client sends the first test packet, the tos is always 0x00.

• Expected Behavior
  The first test packet shall use the same tos as user's input.

Reason: Many vendors' scheduler only check the DSCP marking once when receive the first packet. If the user wants to send high QoS service such as 0xc0, since the first packet is the test packet with tos 0x00, scheduler will treat the service as 0x00, best effort, QoS will not work in this case.

• Possible Solution
  Send the first test packet with the same tos as user's setting

[bmah888]

Is this still a problem?

[davidBar-On]

Is this still a problem?

DSCP is still not set in the first UDP streams response from the server. Submitted PR #2047 with a suggested resolution.