Suggest an editImprove this articleRefine the answer for “What fields make up a TCP header?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)**The TCP header** is the control part of a TCP segment, containing all the information needed for **reliable data transmission**. **Key point:** the TCP header is a set of fields TCP uses to manage the connection, order, and delivery of data.Shown above the full answer for quick recall.Answer (EN)Image**The TCP header** is the control part of a TCP segment, containing all the information needed for **reliable data transmission**. More simply: **the TCP header is a set of fields TCP uses to manage the connection, order, and delivery of data**. --- ## Main fields of the TCP header ### 1. **Source Port** - 16 bits - indicates **which application** the data was sent from --- ### 2. **Destination Port** - 16 bits - indicates **which application** the data is intended for --- ### 3. **Sequence Number** - 32 bits - the number of the first data byte in this segment - used for: - correct order - detecting loss --- ### 4. **Acknowledgment Number** - 32 bits - indicates **which next byte is expected** - used to acknowledge data delivery --- ### 5. **Data Offset** - shows the size of the TCP header - needed to determine **where the data begins** --- ### 6. **Reserved bits** - not used - kept "for the future" --- ### 7. **Control flags (Flags)** Small switch bits that control the connection: - **SYN** - establishing a connection - **ACK** - acknowledging receipt - **FIN** - correctly closing a connection - **RST** - forcibly resetting a connection - **PSH** - push data to the application immediately - **URG** - urgent data --- ### 8. **Window Size** - determines **how much data the receiver is ready to accept** - used for **flow control** --- ### 9. **Checksum** - checks the **integrity of the TCP segment** - if the checksum does not match, the segment is dropped --- ### 10. **Urgent Pointer** - used only if the **URG** flag is set - rarely used today --- ### 11. **TCP Options** - optional fields - used for: - MSS - Window Scaling - SACK, and others --- ## Minimum and maximum size - **Minimum TCP header size**: 20 bytes - **Maximum** (with options): up to 60 bytes --- ## What matters for the interview - the TCP header is **larger** than the UDP header - it is exactly these fields that make TCP: - reliable - order-managing - congestion-controlling - routers **do not analyze the TCP header** --- ## In short, for the interview > **The TCP header** consists of the source and destination ports, the sequence and acknowledgment numbers, control flags, the window size, the checksum, and additional options that provide reliable end-to-end data transmission.For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.