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technology/internet/TCP.md

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+---
+obj: concept
+wiki: https://en.wikipedia.org/wiki/Transmission_Control_Protocol
+rfc: https://datatracker.ietf.org/doc/html/rfc9293
+---
+
+# TCP
+The Transmission Control Protocol (TCP) is one of the core protocols of the Internet Protocol (IP) suite. It operates at the transport layer and provides reliable, connection-oriented communication between devices on a network. TCP ensures the ordered and error-checked delivery of data, making it suitable for applications where accuracy and reliability are paramount.
+
+## Key Characteristics
+### 1. **Connection-Oriented:**
+- TCP establishes a connection between two devices before data exchange. This connection is maintained throughout the communication session.
+### 2. **Reliability:**
+- TCP ensures the reliable delivery of data by using acknowledgments and retransmissions. If a segment is not acknowledged, it is retransmitted to ensure data integrity.
+### 3. **Ordered Data Delivery:**
+- Data sent over a TCP connection is delivered in the same order in which it was sent. This is achieved through sequence numbers assigned to each segment.
+### 4. **Flow Control:**
+- TCP incorporates flow control mechanisms to prevent a fast sender from overwhelming a slow receiver. It uses a sliding window mechanism to manage the flow of data.
+### 5. **Full-Duplex Communication:**
+- TCP supports full-duplex communication, allowing data to be sent and received simultaneously by both communicating devices.
+
+## TCP Operation
+### 1. **Three-Way Handshake:**
+- TCP begins with a three-way handshake to establish a connection. This involves SYN (synchronize), SYN-ACK (synchronize acknowledgment), and ACK (acknowledge) segments.
+### 2. **Data Transfer:**
+- Once the connection is established, data can be transferred between the devices. Each segment is acknowledged, and retransmissions occur in case of lost or corrupted data.
+### 3. **Connection Termination:**
+- TCP ensures a reliable connection termination through a four-way handshake involving FIN (finish) and ACK segments.
+
+## TCP Header
+The TCP header includes various fields, including:
+- **Source and Destination Port Numbers:** Identify the source and destination applications on the devices.
+- **Sequence Number and Acknowledgment Number:** Manage the ordering and acknowledgment of data segments.
+- **Window Size:** Specifies the size of the sender's receive window for flow control.
+- **Checksum:** Ensures the integrity of the TCP header and data.