TCPUDPheaders

TCP and UDP headers are the two header types used by the transport layer in the Internet protocol suite. They accompany the payload of transport-layer segments or datagrams and determine how the data is delivered to the receiving application. TCP headers belong to a reliable, connection-oriented service, while UDP headers belong to a simpler, connectionless service. Both headers are described in RFCs (TCP in RFC 793, UDP in RFC 768) and are extended by later standards.

The TCP header is at least 20 bytes long and includes fields for source and destination ports, a 32-bit sequence number, a 32-bit acknowledgment number, data offset and reserved bits, a set of control flags (including URG, ACK, PSH, RST, SYN, FIN), a 16-bit window size, a 16-bit checksum, and a 16-bit urgent pointer. Optional TCP options may extend the header length beyond 20 bytes. The data offset field specifies the size of the header in 32-bit words. TCP provides reliability through acknowledgments, retransmission, order maintenance, and flow and congestion control.

The UDP header is fixed at 8 bytes and consists of source port, destination port, length, and checksum. UDP provides no sequencing, reliability guarantees, or in-band flow control. It is used for simple, time-sensitive exchanges such as DNS queries, voice, video, and applications that implement their own reliability mechanisms at the application layer.

Key differences include: TCP is connection-oriented and reliable with error checking and congestion control, while UDP is connectionless and best-effort with minimal overhead. The choice between them balances reliability, latency, and overhead requirements for the application.