Why is UDP called "unreliable"?

UDP is called "unreliable" because it does not guarantee data delivery and does not control the transmission process in any way.

More simply: UDP sends data and does not check whether it arrived.

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#What exactly makes UDP "unreliable"

#1. No delivery acknowledgments

UDP:

• does not wait for an ACK
• does not know whether the receiver got the message

If a packet is lost:

• the sender never finds out

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#2. No retransmission

UDP:

• does not resend lost packets
• does not try to recover the data

Once lost, it is lost forever.

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#3. No order guarantee

UDP:

• does not number messages
• does not assemble them in the correct sequence

Messages can arrive:

• in a different order
• or not arrive at all

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#4. No congestion control

UDP:

• does not reduce speed under network congestion
• may keep sending data

This can:

• worsen congestion
• lead to packet loss

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#5. No flow control

UDP:

• does not account for whether the receiver can keep up with processing the data
• may "flood" the application with messages

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#What UDP does still do

Important. UDP is not entirely "irresponsible":

• it preserves message boundaries
• it checks basic integrity (checksum)
• it delivers data on a best effort basis

But:

• if something goes wrong, UDP does not fix the situation

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#Why this is done on purpose

UDP is deliberately simple because that gives:

• minimal latency
• minimal overhead
• predictable delivery time

For many applications, this matters more than reliability.

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#Where "unreliability" is an advantage

UDP fits well when:

• reaction speed matters
• losses are acceptable

Examples:

• voice communication
• real-time video
• online games
• DNS