TCP vs UDP

Core concepts

TCP

TCP, or Transmission Control Protocol, is a connection-oriented and reliable transport-layer protocol. It provides ordered delivery and retransmission when packets are lost.

UDP

UDP, or User Datagram Protocol, is a lightweight transport-layer protocol that sends independent datagrams without building a connection first.

Connection and handshake

TCP

TCP requires a connection before data transfer.

Three-way handshake

1. The client sends SYN.
2. The server replies with SYN + ACK.
3. The client sends ACK.

Four-way termination

1. The client sends FIN.
2. The server replies with ACK.
3. The server sends its own FIN.
4. The client replies with ACK.

UDP

UDP is connectionless.

• No connection setup
• No teardown
• No handshake
• The sender can send datagrams immediately

Data transfer and reliability

TCP

• Splits data into segments
• Tracks sequence numbers and acknowledgements
• Reorders data if needed
• Retransmits lost packets
• Guarantees reliability and ordering

UDP

• Sends independent datagrams
• No built-in acknowledgement
• No retransmission
• No ordering guarantee
• Reliability, if needed, must be handled at the application layer

Latency and efficiency

TCP

• Higher latency because of connection setup, acknowledgements, and retransmission
• More overhead because of flow control, congestion control, and larger headers

UDP

• Lower latency
• Lower protocol overhead
• Better suited for real-time traffic where timeliness matters more than perfect delivery

Use cases

TCP

1. web browsing
2. file upload and download
3. email delivery
4. database communication
5. remote login and management

UDP

1. real-time audio and video
2. online games
3. DNS queries
4. IoT telemetry
5. SNMP

Header overhead

TCP

TCP headers are larger and contain fields such as:

• source port
• destination port
• sequence number
• acknowledgement number
• window size
• flags
• checksum
• optional extension fields

The minimum header size is 20 bytes, and it can grow larger if options are present.

UDP

UDP keeps a fixed 8-byte header:

• source port
• destination port
• length
• checksum

That simplicity is part of why UDP is fast.

Summary

TCP trades speed and simplicity for reliability, ordering, and delivery guarantees. UDP trades reliability for lower latency and smaller overhead.

When the workload cannot tolerate lost or out-of-order data, use TCP. When speed matters more than guaranteed delivery, UDP is often the better fit.