How to Set Message Priority Using TOS – Complete Step-by-Step Guide

To set message priority using the Type of Service (TOS) field in the IP header, you need to understand how TOS works and how to manipulate it within your application. The TOS field in the IPv4 header is an 8-bit field that was originally intended for quality of service (QoS) purposes.

Understanding the TOS Field

The TOS field is now commonly referred to as the Differentiated Services Code Point (DSCP) field. It allows you to specify the priority and handling of packets as they traverse a network.

Here is a breakdown of the TOS/DSCP field:

• Precedence (3 bits): Used to specify the priority of the packet.
• Type of Service (4 bits): Used to specify the desired service level (e.g., low delay, high throughput, high reliability).
• Unused (1 bit): Previously reserved for future use or unused.

Setting TOS in Code

You can set the TOS field using socket options in various programming languages. Below are examples in C and Python.

Example in C

In C, you can use the setsockopt function to set the TOS field for a socket.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>

int main() {
    int sockfd;
    struct sockaddr_in server_addr;
    int tos = 0x10; // Example TOS value for low delay

    // Create socket
    if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    // Set TOS
    if (setsockopt(sockfd, IPPROTO_IP, IP_TOS, &tos, sizeof(tos)) < 0) {
        perror("setsockopt");
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    // Set server address and port
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(8080);
    server_addr.sin_addr.s_addr = inet_addr("192.168.1.1");

    // Connect to server
    if (connect(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0) {
        perror("connect");
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    // Send data
    char *message = "Hello, server!";
    if (send(sockfd, message, strlen(message), 0) < 0) {
        perror("send");
    }

    // Close socket
    close(sockfd);

    return 0;
}

Example in Python

In Python, you can use the setsockopt method on a socket object to set the TOS field.

import socket

# Create socket
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

# Set TOS
tos = 0x10  # Example TOS value for low delay
sock.setsockopt(socket.IPPROTO_IP, socket.IP_TOS, tos)

# Connect to server
server_address = ('192.168.1.1', 8080)
sock.connect(server_address)

# Send data
message = 'Hello, server!'
sock.sendall(message.encode())

# Close socket
sock.close()

Common TOS Values

Here are some common TOS values you might use:

• 0x10: Low delay
• 0x08: High throughput
• 0x04: High reliability
• 0x02: Minimize monetary cost
• 0x00: Default (normal service)

Conclusion

Setting the TOS field allows you to specify the priority and handling of your network packets. This can be useful for ensuring that certain types of traffic receive the appropriate level of service in your network. The examples above demonstrate how to set this field in both C and Python.