What indicates that this is a distributed denial$-$of$-$service $($DDoS$)$ attack?

There is a flood of SYN packets without matching SYN$-$ACK packets.

The destination address for all SYN packets is $128\mathrm{.}28\mathrm{.}1\mathrm{.}1.$

The hex value of the SYN flag is $0002.$

There are multiple source addresses for the $5$ Y packets with the destination address $128\mathrm{.}28\mathrm{.}1\mathrm{.}1.$