The Transmission Control Protocol (TCP) is one of the foundational protocols of the Internet, residing at the Transport Layer (Layer 4) of the TCP/IP model (and the OSI model). Its primary role is to provide reliable, ordered, and error-checked delivery of a stream of data between applications running on different hosts over an IP network.

While the Internet Protocol (IP) handles the delivery of packets from one host to another on a best-effort basis (meaning packets can be lost, duplicated, or arrive out of order), TCP builds upon IP to ensure that applications receive the data exactly as it was sent. This reliability is crucial for most internet applications.

Key Characteristics and Mechanisms of TCP:

1. Connection-Oriented:

   • Before any data is exchanged, TCP requires a connection to be established between the sender and receiver. This setup phase is known as the Three-Way Handshake.

   • Three-Way Handshake (SYN, SYN-ACK, ACK):

     1. SYN (Synchronize): The client sends a SYN packet to the server to initiate a connection and propose an initial sequence number.

     2. SYN-ACK (Synchronize-Acknowledge): The server responds with a SYN-ACK packet, acknowledging the client’s SYN, and proposing its own initial sequence number.

     3. ACK (Acknowledge): The client sends a final ACK packet, acknowledging the server’s SYN-ACK.

   • Once this handshake is complete, a full-duplex, reliable connection is established, and data transfer can begin.

2. Reliable Delivery:

   • Sequence Numbers: TCP assigns a sequence number to each byte of data it sends. This allows the receiver to correctly reassemble segments that might arrive out of order and to detect any missing segments.

   • Acknowledgements (ACKs): The receiver sends acknowledgements back to the sender for the data it has successfully received. These ACKs indicate the next sequence number the receiver expects.

   • Retransmission: If the sender does not receive an ACK for a segment within a certain timeout period, it assumes the segment was lost and retransmits it. This mechanism ensures that all data eventually reaches the destination.

   • Checksums: TCP calculates a checksum for each segment (header + data) to detect any corruption that might occur during transmission. If a receiver detects a corrupted segment, it discards it and does not send an ACK, prompting the sender to retransmit.

3. Ordered Data Transfer:

   • Even if IP delivers packets out of order, TCP uses sequence numbers to rearrange the incoming segments into the correct order before delivering them to the application.

4. Flow Control:

   • TCP uses a sliding window mechanism to prevent a fast sender from overwhelming a slow receiver.

   • The receiver advertises its available receive window size in its ACK packets. This window size tells the sender how much data it can send before needing to wait for further acknowledgements. This prevents buffer overflows at the receiver.

5. Congestion Control:

   • TCP employs sophisticated algorithms (like Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery) to adapt the data transmission rate based on perceived network congestion.

   • If congestion is detected (e.g., through packet loss or increased round-trip times), TCP reduces its sending rate to alleviate the congestion and prevent network collapse. This is a critical feature that helps the Internet operate smoothly.

6. Full-Duplex Communication:

   • Data can be transmitted in both directions simultaneously over a single TCP connection.

7. Port Numbers:

   • TCP uses port numbers (e.g., HTTP on port 80, HTTPS on port 443, SSH on port 22) to identify specific applications or services running on a host. This allows multiple applications to share the same network connection.

8. Connection Termination (Four-Way Handshake):

   • When an application finishes sending data, it initiates a graceful connection shutdown using a four-way handshake (FIN, ACK, FIN, ACK) to ensure all data has been sent and acknowledged by both sides.

TCP in the TCP/IP Suite:

TCP is part of the TCP/IP Protocol Suite, which is the set of communication protocols used for the Internet and other similar networks. The “TCP/IP” name often refers to the entire suite, but specifically highlights the crucial roles of TCP (for reliable transport) and IP (for addressing and routing).

Applications That Rely on TCP:

Because of its reliability, TCP is used by the vast majority of applications that require guaranteed data delivery:

• HTTP/HTTPS: Web Browse

• FTP: File transfer

• SMTP, POP3, IMAP: Email

• SSH: Secure remote access

• Telnet: (Older, insecure) remote access

• Database connections

• Most file sharing protocols