Breaking an IP datagram (packet) into pieces in order to be sent across a transmission link with a frame size smaller than the datagram. Performed in a router, the header of the original IP packet is replicated with minor changes to each of the fragments. If one of the fragments is dropped, the original datagram must be fragmented again and retransmitted. IP Fragmentation Is Often Avoided IP fragments are problematic. Some firewalls and content switches may not allow them, and they add overhead to routers, which are designed for high-speed packet forwarding, not fragment creation and reassembly. To avoid IP fragmentation, Maximum Segment Size (MSS) messages are sent between two end points to find the maximum frame size that can be transmitted intact. In a network with multiple data links, the router uses Path Maximum Transmission Unit Discovery (PMTUD) to send error messages back to the sender whenever a link with a smaller frame size is encountered. The host then reduces its TCP packet size and retransmits. PMTUD does not work with UDP, only TCP.