Suggest an editImprove this articleRefine the answer for “What is a TCP segment?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)**A TCP segment** is a **unit of data at the transport layer (L4)** used by the **TCP** protocol to transfer information between applications. **Key point:** a TCP segment is a piece of application data, wrapped by TCP with control information for reliable transmission.Shown above the full answer for quick recall.Answer (EN)Image**A TCP segment** is a **unit of data at the transport layer (L4)** used by the **TCP** protocol to transfer information between applications. In short: **a TCP segment is a piece of application data, wrapped by TCP with control information for reliable transmission**. --- ## Why a TCP segment is needed Applications transfer data in large blocks. TCP: - **splits the data stream into segments** - sends them over the network - **reassembles them in the correct order** on the receiver's side This lets TCP: - control delivery - detect loss - retransmit data --- ## What a TCP segment consists of A TCP segment consists of **two parts**: ### 1. **The TCP header** Contains the control information needed for reliability: - source port - destination port - sequence number - acknowledgment number - flags (SYN, ACK, FIN, and others) - window size (for flow control) - checksum It is the header that makes TCP a **reliable protocol**. --- ### 2. **The payload (data)** This is: - the application's data - a part of the overall data stream The payload size depends on: - the MTU - TCP settings - the state of the network --- ## What happens to a TCP segment next When transmitted over the network: 1. TCP forms a **segment** 2. It is passed to the network layer 3. The TCP segment is **encapsulated in an IP packet** 4. The IP packet is transmitted over the network Routers: - **do not see TCP segments** - work only with IP packets --- ## Important to remember - a TCP segment belongs to **L4** - it exists **only between the end nodes** - it provides: - reliability - order - no duplicates - UDP uses a similar unit, but without these guarantees --- ## In short, for the interview > **A TCP segment** is a unit of data at the transport layer containing the TCP header and application data, used for reliable transmission of information between applications over an end-to-end connection.For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.