My firewall textbook says: "UDP breaks a message into numbered segments so that it can be transmitted."
My understanding was UDP had no sequence or other numbering scheme? That data was broken into packets and sent out with no ordered reconstruction on the other end, at least on this level. Am I missing something?
The book is just wrong here. The relevant section says:
User Datagram Protocol (UDP)—This protocol is similar to TCP in that it handles the addressing of a message. UDP breaks a message into numbered segments so that it can be transmitted. It then reassembles the message when it reaches the destination computer.
UDP does not include any mechanism to segment or reassemble messages; each message is sent as a single UDP datagram. If you look at the UDP "packet" (technically datagram) structure on page 108, there's no segment number or anything like that.
Mind you, segmentation can happen at other layers, either above or below UDP:
IP packets can be fragmented if they're too big for a network link's MTU (maximum transfer unit). This can happen to IP packets that contain UDP, TCP, or whatever. This is actually relevant for firewalls because creative fragmentation can sometimes be used to bypass packet filtering rules.
Some protocols that run on top of UDP also use something like numbered segments. For example, TFTP (trivial file transfer protocol) breaks files into "blocks", and transmits a block number in the header for each block. (And the receiver responds acknowledging the block number it's received -- it's like a drastically simplified version of TCP.) But this is part of the TFTP protocol, not part of UDP.
QUIC is another example of a protocol that runs over UDP and supports segmentation (and multiple connections, and...), and each packet contains a packet number. But again it's part of the QUIC protocol, not UDP.