When the NTP client sends an NTP packet, the source IP address is
normally set to the address of the interface through which the NTP
packet is sent.
Sets all the outgoing NTP packets' source ifIndex to one specific
ifIndex on the NTP client from which the source ifIndex is taken. This
ifIndex will be the index of the IP interface as NTP is IP/UDP based.
If the value is zero, it means no interface has configured.