Transmission Control Protocol (TCP)

The TCP (Transmission Control Protocol) three-way handshake is a process used to establish a connection between a client and a server. It ensures that both sides agree on the sequence numbers used to acknowledge data, sets the initial sequence number (ISN), and synchronizes the TCP buffers on both sides of the connection. Here's how the three-way handshake works:

1. Client to Server: SYN (synchronize)

   • The client sends a TCP segment with the SYN flag set to 1 and the sequence number set to a random value (ISN). This segment indicates the client's desire to establish a connection.

   • The client also specifies the maximum segment size (MSS) it can receive.

2. Server to Client: SYN-ACK (synchronize, acknowledge)

   • Upon receiving the SYN segment, the server responds with a TCP segment in which both the SYN and ACK flags are set to 1.

   • The server sends back its own ISN, which is incremented by one from the ISN it received from the client. This sequence number acknowledges the client's SYN segment.

   • The server also acknowledges the client's MSS, indicating the maximum segment size it can receive.

3. Client to Server: ACK (acknowledge)

   • After receiving the SYN-ACK segment, the client sends a final segment with the ACK flag set to 1. This segment acknowledges the server's SYN and ACK flags.

   • The sequence number in this segment is the server's ISN plus one, acknowledging the server's SYN segment.

At this point, the three-way handshake is complete, and the TCP connection is established. Data can now be transmitted between the client and server.

It's important to note that the three-way handshake is a fundamental part of the TCP connection establishment process and is used to ensure that both sides agree on sequence numbers and establish a reliable connection.

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The Transmission Control Protocol (TCP) header is a fundamental part of the TCP/IP protocol suite and is used for reliable, connection-oriented communication between devices on a network. The TCP header contains essential information required for the transmission, delivery, and acknowledgment of data packets. Here's an overview of the fields in a TCP header:

1. Source Port (16 bits):

   • Specifies the source port number, i.e., the port on the sender's machine from which the TCP segment originates.

   • It helps the receiving end identify the application or process to which the data should be delivered.

2. Destination Port (16 bits):

   • Specifies the destination port number, i.e., the port on the receiver's machine where the TCP segment should be delivered.

   • Similar to the source port, this field helps the receiving end identify the destination application or process.

3. Sequence Number (32 bits):

   • Indicates the sequence number of the first data byte in the current segment.

   • Sequence numbers are used for ordering and reassembly of TCP segments at the receiving end.

4. Acknowledgment Number (32 bits):

   • If the ACK flag is set, this field contains the next sequence number that the sender is expecting to receive from the other party.

   • It acknowledges receipt of all bytes up to and including the acknowledged sequence number minus 1.

5. Data Offset (4 bits):

   • Specifies the size of the TCP header in 32-bit words.

   • This field indicates where the data begins in the TCP segment, allowing for variable-length options to be included in the header.

6. Reserved (6 bits):

   • Reserved for future use and must be set to zero.

7. Flags (6 bits):

   • Various control flags used for signaling and controlling the behavior of the TCP connection. These flags include:

     • URG: Urgent pointer field significant

     • ACK: Acknowledgment field significant

     • PSH: Push Function

     • RST: Reset the connection

     • SYN: Synchronize sequence numbers

     • FIN: No more data from sender

8. Window Size (16 bits):

   • Specifies the size of the receiving window, indicating how much data the sender can send before needing an acknowledgment from the receiver.

   • It helps in flow control, allowing the sender to regulate the rate of data transmission based on the receiver's capacity.

9. Checksum (16 bits):

   • A checksum calculated over the TCP header, data, and a pseudo-header.

   • Used to detect errors in the TCP segment during transmission.

10. Urgent Pointer (16 bits):

    • If the URG flag is set, this field points to the last urgent data byte in the segment.

    • It's used for indicating the presence of urgent data within the TCP segment.

11. Options (Variable):

    • Optional fields used for additional functionality or features.

    • Examples of options include Maximum Segment Size (MSS), Timestamps, Window Scale, etc.

The TCP header provides crucial information for establishing, maintaining, and terminating TCP connections, ensuring reliable and orderly data transmission between network hosts.