This applies to _ALL_ IP packets, not just UDP. The lack of buffer credits/etc on IP mean that flow control is a function of the higher levels. Your TCP packets

This applies to _ALL_ IP packets, not just UDP. The lack of buffer credits/etc on IP mean that flow control is a function of the higher levels. Your TCP packets are also getting dropped for all the same reasons, its just that TCP backs off, and retransmits, so you don't see it as anything other than a slowdown.