TCP/IP Protocol Suite

The Transmission Control Protocol/Internet Protocol (TCP/IP) suite is a set of communication protocols used in the Internet and similar computer networks. It is the most widely used network protocol suite today.

Introduction to TCP

TCP (Transmission Control Protocol) is one of the main protocols of the Internet protocol suite. It operates at the transport layer and provides reliable, ordered, and error-checked delivery of a stream of bytes between applications running on hosts communicating via an IP network.

Key Features of TCP

Connection-Oriented: TCP establishes a connection before data transfer begins, ensuring both ends are ready.
Reliable Delivery: It guarantees that data sent will arrive at the destination without errors and in the correct order. This is achieved through mechanisms like sequence numbers, acknowledgments, and retransmissions.
Ordered Data Transfer: Data is delivered to the receiving application in the same order it was sent.
Flow Control: Prevents a fast sender from overwhelming a slow receiver.
Congestion Control: Manages network traffic to avoid overwhelming the network itself.

The TCP Three-Way Handshake

Before any data is transmitted, TCP uses a three-way handshake to establish a reliable connection between the client and the server. This process ensures that both parties are ready to communicate.

SYN (Synchronize): The client sends a SYN segment to the server, indicating its desire to establish a connection and proposing an initial sequence number.
SYN-ACK (Synchronize-Acknowledge): The server receives the SYN, acknowledges it, and sends back its own SYN segment with its own initial sequence number.
ACK (Acknowledge): The client receives the SYN-ACK, acknowledges it, and sends a final ACK segment back to the server. The connection is now established.

This handshake ensures both sides have agreed on synchronization numbers and are ready to send and receive data.

TCP Segments

Data is transmitted in TCP segments. Each segment contains a header and a payload (the actual data). The TCP header includes crucial information such as:

Source Port and Destination Port: Identifies the sending and receiving applications.
Sequence Number: Identifies the position of the segment in the byte stream.
Acknowledgment Number: Indicates the sequence number of the next byte the sender expects to receive.
Flags: Control bits like SYN, ACK, FIN, RST, PSH, URG, which control the state of the connection.
Window Size: Used for flow control.
Checksum: Used for error detection.

Common TCP Flags

SYN: Synchronize sequence numbers.
ACK: Acknowledgment number is valid.
FIN: Finish sending data.
RST: Reset the connection.
PSH: Push the data to the receiving application immediately.

TCP Connection Termination

TCP connections are terminated gracefully using a four-way handshake, similar to the establishment process:

The sender wishing to close the connection sends a FIN segment.
The receiver acknowledges the FIN with an ACK. The receiver may continue sending data until it is also ready to close.
When the receiver is ready to close, it sends its own FIN segment.
The original sender acknowledges the final FIN with an ACK.

TCP vs. UDP

While TCP provides reliability, UDP (User Datagram Protocol) is a simpler, connectionless protocol that offers faster but less reliable data transfer. The choice between TCP and UDP depends on the application's requirements:

TCP: Suitable for applications where data integrity and order are critical, such as web browsing (HTTP/HTTPS), email (SMTP), and file transfer (FTP).
UDP: Preferred for applications where speed is paramount and some data loss is acceptable, such as streaming media, online gaming, and DNS lookups.