It is nearly universally the case that TCP packets have the DF flag set. This is because it's most efficient to have TCP segments sized to fit into a single pa

It is nearly universally the case that TCP packets have the DF flag set. This is because it's most efficient to have TCP segments sized to fit into a single packet each - if part of a segment doesn't arrive, the entire segment will be resent anyway. So the DF flag is set, and the sending TCP reduces its estimate of the path-MTU whever it sees an ICMP "Fragmentation Needed" response.