Given an ethernet payload, extract DS field and map it to User Priority if it’s an IP packet.
Otherwise, return None.
Note that this function does not check that the payload is a valid IP packet. It only requires
that the payload is long enough to contain the DS field.
Given the 6-bit DSCP from IPv4 or IPv6 header, convert it to User Priority
This follows RFC 8325 - https://tools.ietf.org/html/rfc8325#section-4.3
For list of DSCP, see https://www.iana.org/assignments/dscp-registry/dscp-registry.xhtml