TCP Connection Establishment (original) (raw)

Last Updated : 11 Jul, 2025

TCP (Transmission Control Protocol) is a core internet protocol that ensures reliable, ordered, and error-checked delivery of data between computers. It establishes a connection using a three-way handshake before data transfer begins, allowing both devices to synchronize and agree on communication parameters. TCP breaks data into packets, manages their delivery by resending lost packets, and reassembles them in the correct order. This makes TCP ideal for applications like web browsing, email, and file transfers where accuracy and reliability are important.

**TCP Connection Establishment

1. Sender starts the process with the following:

• **Sequence number (Seq=521): contains the random initial sequence number generated at the sender side.
• **Syn flag (Syn=1): request the receiver to synchronize its sequence number with the above-provided sequence number.
• **Maximum segment size (MSS=1460 B): sender tells its maximum segment size, so that receiver sends datagram which won't require any fragmentation. MSS field is present inside Option field in TCP header.
• **Window size (window=14600 B): sender tells about his buffer capacity in which he has to store messages from the receiver.

2. TCP is a full-duplex protocol so both sender and receiver require a window for receiving messages from one another.

• **Sequence number (Seq=2000): contains the random initial sequence number generated at the receiver side.
• **Syn flag (Syn=1): request the sender to synchronize its sequence number with the above-provided sequence number.
• **Maximum segment size (MSS=500 B): receiver tells its maximum segment size, so that sender sends datagram which won't require any fragmentation. MSS field is present inside Option field in TCP header.
  Since MSS receiver < MSS sender , both parties agree for minimum MSS i.e., 500 B to avoid fragmentation of packets at both ends.

Therefore, receiver can send maximum of 14600/500 = 29 packets.
This is the receiver's sending window size.

• **Window size (window=10000 B): receiver tells about his buffer capacity in which he has to store messages from the sender.

Therefore, sender can send a maximum of 10000/500 = 20 packets.
This is the sender's sending window size.

• **Acknowledgement Number (Ack no.=522): Since sequence number 521 is received by the receiver so, it makes a request for the next sequence number with Ack no.=522 which is the next packet expected by the receiver since Syn flag consumes 1 sequence no.
• **ACK flag (ACk=1): tells that the acknowledgement number field contains the next sequence expected by the receiver.

3. Sender makes the final reply for connection establishment in the following way:

• **Sequence number (Seq=522): since sequence number = 521 in 1 st step and SYN flag consumes one sequence number hence, the next sequence number will be 522.
• **Acknowledgement Number (Ack no.=2001): since the sender is acknowledging SYN=1 packet from the receiver with sequence number 2000 so, the next sequence number expected is 2001.
• **ACK flag (ACK=1): tells that the acknowledgement number field contains the next sequence expected by the sender.

Since the connection establishment phase of TCP makes use of 3 packets, it is also known as 3-way Handshaking (SYN, SYN + ACK, ACK).

**TCP Flags Used in Connection Establishment

TCP uses special flags to manage connections between devices such as:

• **SYN (Synchronize): This flag is used to start a new connection between two devices.
• **ACK (Acknowledge): It confirms that a message has been received successfully.
• **RST (Reset): This flag is used to abruptly close a connection due to errors or security reasons.
• **FIN (Finish): It signals that one device wants to close the connection properly.

**Common Issues in TCP Connection Establishment

Sometimes, problems can occur while setting up a connection. Here are some common issues:

• **SYN Flood Attacks: Hackers send many SYN requests without completing the connection, overloading the server.
• **Connection Timeout: If a device does not respond in time, the connection attempt fails.
• **Packet Loss: Some data packets might get lost due to network issues, causing delays or failures in connection.

**How to Optimize TCP Connection Establishment?

To make TCP connections faster and more reliable, here are some useful methods:

• **Reducing Latency with TCP Fast Open: This allows data to be sent earlier, reducing the delay when starting a connection.
• **Using Keep-Alive Mechanism: This keeps connections open longer, avoiding the need to set up new ones repeatedly.
• **Load Balancing for Faster Handshakes: Distributing traffic across multiple servers helps in handling many connections efficiently.

Practice GATE IT 2008 | Question 67 based on three-way handshake process.

Also, read about TCP Connection Termination.

**What does "established" mean in TCP?

"Established" in TCP means a connection is successfully set up between two devices, allowing data transfer after completing the three-way handshake.

**What is the TCP connection limit?

The limit depends on system resources and configurations, but most modern systems support thousands to millions of concurrent connections.

**What is the speed limit of a TCP connection?

TCP speed depends on factors like network bandwidth, latency, congestion, and window size, but it has no fixed limit.