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SYN Synchronized sequence Number

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In computer networking, SYN (Synchronized Sequence Number) is a control flag used in the Transmission Control Protocol (TCP) to establish a connection between two devices over a network. The SYN flag is part of the TCP three-way handshake process, which ensures reliable and orderly data transmission between the sender and receiver.

Here's a detailed explanation of SYN and its role in the TCP three-way handshake:

  1. TCP Three-Way Handshake: The TCP three-way handshake is a process that establishes a connection between a client and a server. It involves three steps: SYN, SYN-ACK, and ACK.
  2. SYN Flag: When a client wants to initiate a connection with a server, it sends a TCP segment with the SYN flag set to 1. This segment is known as a SYN segment. The SYN flag indicates that the client is requesting to synchronize sequence numbers with the server for reliable data transfer.
  3. Sequence Numbers: Sequence numbers are used in TCP to uniquely identify and order the segments of a TCP connection. Each segment is assigned a sequence number, allowing the receiver to reassemble the segments in the correct order.
  4. Initial Sequence Number (ISN): The SYN segment sent by the client includes an initial sequence number (ISN) that is randomly chosen. The ISN serves as the starting point for the sequence numbers used in subsequent segments of the connection.
  5. SYN-ACK: Upon receiving the SYN segment from the client, the server responds with a TCP segment known as the SYN-ACK segment. The SYN-ACK segment has both the SYN and ACK flags set. The SYN flag acknowledges the client's synchronization request, and the ACK flag acknowledges the client's ISN.
  6. ACK: After receiving the SYN-ACK segment, the client sends an acknowledgment segment (ACK segment) to the server. This segment has the ACK flag set and acknowledges the server's ISN.
  7. Establishing Connection: The three-way handshake is completed when the server receives the ACK segment from the client. At this point, the connection is established, and both the client and server have agreed upon initial sequence numbers for reliable data transfer.
  8. Connection Termination: The SYN flag is also used in the TCP connection termination process. When a device wants to terminate the connection, it sends a TCP segment with both the FIN (Finish) and ACK flags set. The FIN flag indicates the intention to close the connection, and the ACK flag acknowledges the receipt of the FIN segment.
  9. Reliable Data Transfer: The TCP three-way handshake, facilitated by the SYN flag, ensures reliable data transfer between the client and server. By synchronizing the sequence numbers and establishing a connection, both parties can reliably send and receive data in the correct order.

In summary, SYN (Synchronized Sequence Number) is a control flag used in the TCP three-way handshake process to establish a connection between a client and a server. The SYN flag is set in the initial segment sent by the client, indicating a request to synchronize sequence numbers. The server responds with a SYN-ACK segment, and the client acknowledges it with an ACK segment, completing the handshake and establishing the connection. The SYN flag plays a critical role in ensuring reliable and orderly data transmission over TCP connections.