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What are "message boundaries" in the following context?
One difference between TCP and UDP is that UDP preserves message
boundaries.
I understand the difference between TCP and UDP, but I’m unsure about the definition of "message boundaries".
Since UDP includes the destination and port information in each individual packet, could it be this that gives message a "boundary"?
No, message boundaries have nothing to do with destinations or ports. A “message boundary” is the separation between two messages being sent over a protocol. UDP preserves message boundaries. If you send “FOO” and then “BAR” over UDP, the other end will receive two datagrams, one containing “FOO” and the other containing “BAR”.
If you send “FOO” and then “BAR” over TCP, no message boundary is preserved. The other end might get “FOO” and then “BAR”. Or it might get “FOOBAR”. Or it might get “F” and then “OOB” and then “AR”. TCP does not make any attempt to preserve application message boundaries — it’s just a stream of bytes in each direction.