{
  "id": "wire-format/tcp-header",
  "family": "wire-format",
  "slug": "tcp-header",
  "title": "TCP header",
  "summary": "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.",
  "kind": "wire-format",
  "aliases": [
    "TCP segment header",
    "Transmission Control Protocol header"
  ],
  "status": "standard",
  "verification": "verified",
  "tier": "B",
  "source_url": "https://www.rfc-editor.org/rfc/rfc9293#section-3.1",
  "source_version": "RFC 9293 (Transmission Control Protocol), Section 3.1 Header Format",
  "retrieved_date": "2026-05-29",
  "see_also": [
    "wire-format/ipv4-header",
    "wire-format/udp-header",
    "port/443"
  ],
  "ext_type": "wire-format@1",
  "ext": {
    "spec": "RFC 9293",
    "summary": "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.",
    "structure": [
      {
        "field": "Source Port",
        "size": "16 bits",
        "meaning": "Sending port."
      },
      {
        "field": "Destination Port",
        "size": "16 bits",
        "meaning": "Receiving port."
      },
      {
        "field": "Sequence Number",
        "size": "32 bits",
        "meaning": "Byte-stream sequence number of the first data octet (or the ISN+1 if SYN set)."
      },
      {
        "field": "Acknowledgement Number",
        "size": "32 bits",
        "meaning": "Next sequence number the sender expects, valid when the ACK flag is set."
      },
      {
        "field": "Data Offset",
        "size": "4 bits",
        "meaning": "Header length in 32-bit words (5 = 20 bytes minimum, up to 15 = 60 bytes with options)."
      },
      {
        "field": "Reserved",
        "size": "4 bits",
        "meaning": "Must be zero (formerly part of a 6-bit reserved + flags layout)."
      },
      {
        "field": "Flags",
        "size": "8 bits",
        "meaning": "CWR, ECE, URG, ACK, PSH, RST, SYN, FIN control bits."
      },
      {
        "field": "Window",
        "size": "16 bits",
        "meaning": "Receive window: bytes the sender of this segment is willing to accept (scaled by the Window Scale option if negotiated)."
      },
      {
        "field": "Checksum",
        "size": "16 bits",
        "meaning": "Ones-complement checksum over the header, payload, and a pseudo-header."
      },
      {
        "field": "Urgent Pointer",
        "size": "16 bits",
        "meaning": "Offset of urgent data, valid when the URG flag is set."
      },
      {
        "field": "Options",
        "size": "0-320 bits",
        "meaning": "Optional, e.g. MSS, Window Scale, SACK-Permitted, Timestamps; padded with zeros to a 32-bit boundary."
      }
    ],
    "example_hex": "01 BB C0 A8 ... (illustrative ports)",
    "example_decoded": "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"
    ],
    "notes": [
      "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."
    ]
  },
  "updated": "2026-05-29T00:00:00Z"
}