TCP Header vs UDP Header

TCP and UDP are both transport-layer protocols running over IP, but TCP's header is built for a reliable, connection-oriented byte stream while UDP's is a minimal 8-byte shim.

Aspect	TCP header	UDP header
Minimum size	20 bytes (up to 60 bytes with options)	8 bytes, always fixed
Fields	Source port, Dest port, Sequence no, Acknowledgment no, Data offset, Reserved, Flags (URG, ACK, PSH, RST, SYN, FIN), Window, Checksum, Urgent pointer, Options/padding	Source port, Dest port, Length, Checksum
Reliability	Seq/ack numbers, checksum, retransmission, duplicate detection	Checksum only (optional in IPv4), no retransmission
Flow control	Receiver's advertised window	None
Connection	SYN/FIN handshake and teardown	Connectionless
Multiplexing	Ports + connection state	Ports only
Overhead / use	Higher latency; file transfer, web, email	Lower latency; DNS, DHCP, RTP streaming, VoIP

TCP fields missing from the UDP header, and the reason for their absence

1. Sequence number - TCP numbers every byte of the stream so the receiver can reorder segments and detect gaps; UDP datagrams are independent and unordered, so there is nothing to sequence.
2. Acknowledgment number - needed for cumulative ACKs and reliable delivery; UDP has no ACK mechanism (an application such as DNS must build its own retry logic).
3. Data offset (header length) - TCP can grow to 60 bytes with options, so it must state how long it is; UDP's header is always exactly 8 bytes, so it carries an explicit Length field instead.
4. Window size - implements receiver-side flow control; UDP does no flow control and leaves pacing entirely to the application.
5. Control flags (SYN, ACK, FIN, RST, PSH, URG) - used for connection setup, orderly release, reset, push and urgent data; a connectionless protocol needs none of them.
6. Urgent pointer - points to urgent out-of-band data, which is meaningful only within an ordered byte stream.
7. Options (MSS, window scaling, SACK, timestamps) - parameters negotiated during connection setup; there is no connection to negotiate.

Key idea: every field that UDP lacks exists to provide connection, ordering, reliability or flow control - exactly the services UDP deliberately omits in exchange for low overhead and low delay.