Question

When an application asks the transport layer to use TCP to transmit a message, what additional messages will be sent by the transport layer in order to fulfill the application layer's request?

Short answer

TCP sends SYN, SYN-ACK, and ACK packets to establish a connection.

Step-by-step solution

Understanding TCP's Role

TCP (Transmission Control Protocol) is responsible for establishing a connection between two entities to ensure reliable data transmission. When the application layer requests TCP to send a message, the transport layer needs to initiate a connection-oriented session.

Initiating the Connection

To set up a TCP connection, the transport layer first sends a SYN (synchronize) packet to the destination host. The SYN packet is used to initiate and establish a connection by synchronizing sequence numbers.

Acknowledgment from Receiver

The destination host, upon receiving the SYN packet, responds with a SYN-ACK (synchronize-acknowledge) packet. This response indicates that the destination is ready to establish a connection and acknowledges the receipt of the initial SYN packet.

Completing the Handshake

The sender then sends an ACK (acknowledge) packet back to the destination host, confirming that it has received the SYN-ACK packet. This completes the three-way handshake, and a TCP connection is now established.

Data Transmission

Once the connection is established, the actual data from the application layer is sent. The transport layer segments the data into packets, and each packet is sent over the network with checksums for error-checking.

Closing the Connection

After the data transmission is complete, a similar exchange of packets is performed to terminate the connection. This typically involves sending a FIN (finish) packet, followed by an ACK to confirm the termination.

Key concepts

TCP Connection

A TCP connection forms the backbone of reliable communication over the internet. Imagine you're making a phone call; before you start talking, you need to establish a connection to ensure that you can hear and be heard. In the digital world, TCP (Transmission Control Protocol) serves this purpose. It links two devices, often referred to as the sender and receiver, creating a smooth pathway for data to flow back and forth.

When an application needs to send data, it relies on the transport layer to set up a TCP connection. A TCP connection is connection-oriented, meaning it establishes a clear route where data packets can reliably travel. This connection remains active as long as the two communicating parties need to exchange data, just like keeping a phone line open for a conversation.

Three-Way Handshake

The three-way handshake is the process TCP uses to establish a connection between two devices. Think of this as a formal introduction between two people who want to collaborate. It involves three key steps:

• SYN Packet: The process begins with the sender transmitting a SYN (synchronize) packet to the receiver. This packet is like extending a hand for a handshake, signaling the desire to start a communication session.
• SYN-ACK Packet: On receiving the SYN packet, the receiver sends back a SYN-ACK (synchronize-acknowledge) packet. This is akin to saying, "Hello! I'm ready to communicate." It acknowledges the initial contact.
• ACK Packet: To complete the handshake, the sender responds with an ACK (acknowledge) packet. This response confirms the initiation process, like receiving a nod that it’s time to start the conversation.

The three-way handshake ensures that both sender and receiver are synchronized and ready for reliable data transfer.

Data Transmission

Once the connection is successfully established, it's time for data transmission to begin. Think of this like a stream of water flowing smoothly from a tap; the water represents the chunks of data being sent from the sender to the receiver.

The transport layer takes the data from the application, breaks it into smaller, manageable pieces known as packets, and sends them across the network. Each packet is labeled with sequence numbers, which help in reassembling data on the receiving end in the correct order. This mechanism ensures the complete and correct delivery of data items, such as files or messages.

TCP implements error-checking procedures, where each packet includes a checksum. This checksum functions like a security tag, verifying the integrity of the data transmitted over potentially unreliable networks. Any corrupted packets are either retransmitted or corrected upon detection.

Reliable Data Transfer

TCP is renowned for its reliable data transfer capabilities, which involve mechanisms to ensure data integrity and delivery. This reliability is akin to a courier service that guarantees your package will arrive safely and in perfect condition.

Here’s how TCP achieves reliability:

• Sequencing: Each data packet has a unique sequence number. Sequence numbers allow the receiving device to arrange packets in the correct order, even if they arrive out of sequence.
• Acknowledgments: The receiver sends back acknowledgment packets for the data it correctly receives. If a sender doesn't receive an acknowledgment for a particular packet, it knows something went wrong, prompting a retransmission.
• Flow Control: TCP uses flow control techniques to prevent overwhelming the receiver with too much data at once. This is akin to ensuring each step in a queue is carefully managed.
• Error Correction: TCP performs thorough error checks. If errors are detected, TCP ensures corrupt packets are retransmitted, maintaining data accuracy.

These features collectively ensure that TCP provides a reliable channel for data transfer, much like guaranteeing a safe journey for each piece of data from sender to receiver.