Two ways to send data.
TCP
Establishes a connection, confirms delivery, resends what is lost, and delivers in order.
Reliable, with overhead.
Used for web, mail, file transfer.
UDP
Sends without establishing a connection or confirming delivery.
Faster, with no guarantee.
Used for DNS, voice, video and gaming.
WHY THE CHOICE MATTERS
For a file, missing data is unacceptable.
For a voice call, a late packet is worthless anyway; better to continue.
WHAT THAT EXPLAINS
Why a call degrades gracefully while a download simply stalls.
WHAT AFFECTS EACH
TCP slows itself in response to loss UDP does not, so loss is heard or seen directly
WHAT TO KNOW WHEN CONFIGURING
Firewall rules specify which. A rule for one does not cover the other.
WHAT THAT CAUSES
A service configured with the wrong protocol in a rule, appearing blocked.
WHAT TO CHECK
Which protocol the service actually uses.
