TCP vs UDP - What Most People Don’t Know

You’ve probably heard things like:

• TCP is reliable, UDP isn’t
• TCP needs a connection, UDP doesn’t

UDP Lets You Broadcast, TCP Doesn’t

Broadcasting means sending one message to all devices on a network at the same time.
With UDP, a device can do exactly that.
TCP needs a separate connection for each device, so true broadcasting isn’t possible.

TCP Eats More Bandwidth, UDP Doesn’t

UDP has a fixed 8-byte header, keeping it light and fast.
TCP’s header starts at 20 bytes and can go up to 60 bytes with extra options.

Fun fact: For very tiny messages, TCP’s header can be bigger than the actual data, eating bandwidth.

TCP Recovers, UDP Moves On

TCP does this using retransmission — acknowledgments and sequence numbers.

• Retransmission resending lost data so nothing gets lost
• Sequence number each piece of data is numbered so the receiver knows the order
• Acknowledgment (ACK) receiver tells the sender, “I got this packet #3!”

Example

1. You send packets #1, #2, #3, #4
2. Packet #2 gets lost
3. Receiver sends ACKs: Got #1, #2 missing, Got #3, Got #4
4. Sender retransmits #2
5. Receiver now has all packets in order

In UDP, if packet #2 is lost,
there’s no retransmission (means no built-in) it’s gone.