TCP/IP Basics for Windows Networking

This document provides a foundational understanding of the Transmission Control Protocol (TCP) and Internet Protocol (IP) as they relate to Windows networking programming. Understanding these core protocols is essential for developing robust and efficient network applications.

What is TCP/IP?

TCP/IP is a suite of communication protocols used to interconnect network devices on the internet and other computer networks. It is comprised of two primary protocols:

• Internet Protocol (IP): Responsible for addressing, routing, and fragmenting data packets so they can travel across networks and arrive at the correct destination.
• Transmission Control Protocol (TCP): Provides reliable, ordered, and error-checked delivery of a stream of bytes between applications running on hosts communicating via an IP network.

The Role of TCP

TCP is a connection-oriented protocol, meaning it establishes a connection between two endpoints before data transfer begins. This connection ensures:

• Reliability: TCP guarantees that data arrives at its destination. It uses acknowledgments and retransmissions to recover from lost packets.
• Ordered Delivery: Data segments are delivered to the application layer in the same order they were sent.
• Flow Control: TCP manages the rate at which data is sent to prevent a fast sender from overwhelming a slow receiver.
• Congestion Control: TCP helps to avoid network congestion by dynamically adjusting the sending rate based on network conditions.

TCP Handshake (Three-Way Handshake)

Before any data is exchanged, TCP establishes a connection using a process called the three-way handshake:

1. SYN (Synchronize): The client sends a SYN packet to the server to initiate a connection.
2. SYN-ACK (Synchronize-Acknowledge): The server responds with a SYN-ACK packet, acknowledging the client's SYN and sending its own SYN.
3. ACK (Acknowledge): The client sends an ACK packet back to the server, acknowledging the SYN-ACK.

Once this handshake is complete, the connection is established, and data transfer can begin.

TCP Data Transmission

Data is broken down into segments. Each segment includes:

• Source and Destination Ports: Identify the specific applications on the source and destination machines.
• Sequence Number: Used for ordering segments and detecting duplicates.
• Acknowledgment Number: Indicates the next expected sequence number from the other end.
• Flags: Control bits (e.g., SYN, ACK, FIN, RST) that manage the connection state.
• Checksum: Used for error detection.

// Conceptual representation of a TCP segment header
typedef struct {
    unsigned short source_port;
    unsigned short dest_port;
    unsigned int sequence_number;
    unsigned int acknowledgment_number;
    unsigned short data_offset : 4;
    unsigned short reserved : 6;
    unsigned short flags; // URG, ACK, PSH, RST, SYN, FIN
    unsigned short window;
    unsigned short checksum;
    unsigned short urgent_pointer;
    // Options (variable length)
    // Data
} TcpSegmentHeader;
        

The Role of IP

IP operates at a lower layer than TCP and is responsible for the logical addressing and routing of data packets across networks. It is a connectionless protocol, meaning each packet is treated independently.

• IP Addresses: Unique identifiers for devices on a network (e.g., IPv4: 192.168.1.1, IPv6: 2001:0db8:85a3:0000:0000:8a2e:0370:7334).
• Routing: Routers use IP addresses to determine the best path for packets to reach their destination.
• Packet Fragmentation: If a packet is too large for a particular network segment, IP can fragment it into smaller pieces, which are reassembled at the destination.

TCP/IP Stack in Windows

Windows implements the TCP/IP protocol suite as a stack of layers. Each layer provides services to the layer above it. When an application sends data:

1. The application data is passed down through the layers.
2. Each layer adds its own header information (e.g., TCP header, IP header).
3. The data is then sent over the network interface.

When data is received:

1. The data passes up through the layers.
2. Each layer processes its corresponding header, removing it.
3. Eventually, the original application data is delivered to the receiving application.

Common TCP Ports

Well-known ports are used by standard applications and services:

• Port 80: HTTP (Web browsing)
• Port 443: HTTPS (Secure web browsing)
• Port 25: SMTP (Email sending)
• Port 110: POP3 (Email retrieval)
• Port 21: FTP (File Transfer Protocol)