TCP header

The TCP segment header is at least 20 bytes: source and destination ports, a 32-bit sequence number, a 32-bit acknowledgement number, a data-offset/flags word (including SYN/ACK/FIN/RST/PSH/URG), a 16-bit window, checksum, and urgent pointer, followed by optional options padded to a 32-bit boundary.

wire format

spec: RFC 9293

Fixed 20-byte header (plus options). Reliable, ordered, connection-oriented transport. The 4-bit Data Offset gives the header length in 32-bit words so a parser knows where options end and payload begins.

field	size	meaning
Source Port	16 bits	Sending port.
Destination Port	16 bits	Receiving port.
Sequence Number	32 bits	Byte-stream sequence number of the first data octet (or the ISN+1 if SYN set).
Acknowledgement Number	32 bits	Next sequence number the sender expects, valid when the ACK flag is set.
Data Offset	4 bits	Header length in 32-bit words (5 = 20 bytes minimum, up to 15 = 60 bytes with options).
Reserved	4 bits	Must be zero (formerly part of a 6-bit reserved + flags layout).
Flags	8 bits	CWR, ECE, URG, ACK, PSH, RST, SYN, FIN control bits.
Window	16 bits	Receive window: bytes the sender of this segment is willing to accept (scaled by the Window Scale option if negotiated).
Checksum	16 bits	Ones-complement checksum over the header, payload, and a pseudo-header.
Urgent Pointer	16 bits	Offset of urgent data, valid when the URG flag is set.
Options	0-320 bits	Optional, e.g. MSS, Window Scale, SACK-Permitted, Timestamps; padded with zeros to a 32-bit boundary.

example:

01 BB C0 A8 ... (illustrative ports)

First two bytes 0x01BB = 443 as a destination port (HTTPS). Full segment parsing requires the IP header for length/pseudo-header checksum.

see: wire-format/ipv4-header

Minimum header is 20 bytes; Data Offset < 5 is invalid.
The checksum is computed over a pseudo-header (src/dst IP, protocol=6, TCP length) plus the TCP header and data.
SYN/SYN-ACK/ACK is the three-way handshake; FIN or RST tears the connection down.

wire format

provenance

see also