TCP Handshake

The TCP handshake, also known as the three-way handshake, is the process by which a TCP connection is established between a client and a server. This handshake ensures that both sides are ready for data transfer, and it also synchronizes the connection parameters (like sequence numbers) for reliable communication.

The three-way handshake consists of three steps:

1. SYN (Synchronize)
2. SYN-ACK (Synchronize-Acknowledge)
3. ACK (Acknowledge)

These three steps ensure that both the client and server are synchronized, and that both are ready to start exchanging data over the established TCP connection.

Step-by-Step Process of the TCP Handshake

Step 1: SYN (Synchronize)

The handshake starts with the client sending a SYN message to the server. This message is part of the initial request to establish a connection.

• Client → Server (SYN):
  • The client sends a SYN (Synchronize) packet to the server to initiate a connection.
  • The SYN packet contains an initial sequence number (let's say Seq = X), which is used to keep track of the data transmitted during the session.
  • The client also indicates that it is ready to begin communication.

Step 2: SYN-ACK (Synchronize-Acknowledge)

When the server receives the SYN message from the client, it acknowledges the request by sending a SYN-ACK message back to the client.

• Server → Client (SYN-ACK):
  • The server responds with a SYN-ACK packet.
  • The SYN-ACK serves two purposes:
    • SYN: It indicates that the server is willing to establish a connection.
    • ACK: The server acknowledges the client's SYN request by setting the ACK flag and returning an acknowledgment number of Seq = X + 1. This tells the client that the server has received the initial sequence number and is ready to proceed.
  • The server also generates its own initial sequence number (let's say Seq = Y), which will be used in subsequent messages.

Step 3: ACK (Acknowledge)

Finally, the client sends an ACK message back to the server to confirm that the connection is established.

• Client → Server (ACK):
  • The client sends an ACK packet back to the server, acknowledging the SYN-ACK message from the server.
  • The ACK packet has a sequence number of Seq = X + 1 (the acknowledgment of the server's SYN).
  • The acknowledgment number in the ACK message from the client is Seq = Y + 1, confirming that the server's sequence number has been received.

Once the server receives the ACK message from the client, the TCP connection is established, and data transmission can begin between the client and server.

Diagram of the TCP Three-Way Handshake

Here's a simple diagram illustrating the three-way handshake:

Client                               Server
   |                                    |
   | ------------ SYN (Seq=X) -------->  |
   |                                    |
   | <--------- SYN-ACK (Seq=Y, Ack=X+1)|
   |                                    |
   | ------------ ACK (Ack=Y+1) ------>  |
   |                                    |
   |          Connection Established    |
   |                                    |

• SYN: Client sends the synchronization request.
• SYN-ACK: Server acknowledges the client's request and sends its own synchronization request.
• ACK: Client acknowledges the server's synchronization, and the connection is established.

Key Concepts and Parameters in the TCP Handshake

1. Sequence Numbers:

   • Each side of the connection starts with an initial sequence number (ISN), which is a random number chosen at the beginning of the connection. This sequence number is used to keep track of the data that is being sent and received in the session.
   • During the handshake, the sequence numbers play a crucial role in ensuring that both sides are synchronized and that data can be properly acknowledged.

2. Acknowledgment Numbers:

   • In the ACK and SYN-ACK messages, the acknowledgment number indicates the next sequence number that the sender expects to receive from the other side. This ensures that both sides are aware of which data has been received.
   • For example, when the client sends an ACK, it is acknowledging that it has successfully received the SYN-ACK from the server and is now expecting the next sequence number.

3. Flags:

   • SYN (Synchronize): This flag is used to initiate a connection.
   • ACK (Acknowledge): This flag is used to acknowledge receipt of a message.
   • FIN (Finish): This flag is used to close a connection (used later in the connection termination phase).

4. Window Size:

   • The TCP handshake also negotiates the window size, which determines how much data the client or server is willing to buffer before receiving an acknowledgment. The window size is communicated in the Window Size field of the TCP header and is part of the SYN and SYN-ACK packets.

Why Is the Three-Way Handshake Important?

1. Reliable Connection Establishment:

   • The three-way handshake ensures that both the client and the server are ready for communication and that both sides are synchronized regarding the initial sequence numbers.

2. Prevents Half-Open Connections:

   • The handshake process prevents a situation where one side believes the connection is established, but the other side does not. Both sides must complete the three-way handshake before data is sent.

3. Prevents Data Loss:

   • By acknowledging each step in the handshake (i.e., the synchronization and sequence numbers), the handshake ensures that the receiver is ready and capable of receiving data, avoiding data loss.

4. Flow Control and Congestion Control:

   • The TCP handshake negotiates parameters like window size, which help to manage flow control and congestion control during data transmission, ensuring that data is transferred at an optimal rate.

5. Security Considerations:

   • The handshake helps prevent certain types of attacks, such as SYN flooding (a type of Denial of Service attack) by requiring proper acknowledgment of the SYN packets.
   • TCP sequence numbers also play a role in securing the connection, as they prevent attackers from hijacking the connection or injecting arbitrary data.

TCP Handshake in Real-World Use

In a typical scenario:

• Web Browsing: When you type a URL into your browser and press "Enter," your browser (the client) initiates a TCP connection with the web server (the server) through the three-way handshake.

• Email Communication: When sending or receiving email using a SMTP (Simple Mail Transfer Protocol) or IMAP server, the email client and server establish a TCP connection through the handshake before data (email messages) is exchanged.

• File Transfer: When using FTP or SFTP, a TCP connection is established with the server using the handshake process, ensuring that data can be reliably transferred.

TCP Handshake in Network Troubleshooting

Understanding the TCP three-way handshake is crucial when troubleshooting network connectivity issues. Here are some scenarios:

1. Connection Timeouts: If a TCP connection attempt times out before completing the handshake, it could indicate network issues such as a firewall blocking the connection, an incorrect IP address, or a server that is not responding to the SYN request.

2. Connection Refused: If the server refuses a connection, it may not be listening on the port specified, or there may be a firewall preventing the connection. The client won't even get to the point of completing the handshake.

3. SYN Flood Attacks: A SYN flood is a DoS (Denial of Service) attack in which an attacker sends a large number of SYN packets without completing the handshake. This exhausts the server’s resources, preventing legitimate connections. Monitoring the handshake process can help identify and mitigate such attacks.

The TCP three-way handshake is a fundamental process in TCP/IP networking that establishes a reliable connection between two devices. By using a SYN, SYN-ACK, and ACK sequence, it ensures that both the client and server are synchronized, ready for communication, and capable of exchanging data reliably. The handshake not only ensures proper data exchange but also plays a role in security, flow control, and congestion management.