5-3 Differentiated Services (DiffServ) Explained

Differentiated Services (DiffServ) is a scalable and efficient method for classifying and managing network traffic to ensure quality of service (QoS). DiffServ operates by marking packets with Differentiated Services Code Points (DSCP) and applying per-hop behaviors (PHBs) to prioritize and manage traffic. Here, we will delve into the key concepts related to DiffServ.

1. Differentiated Services Code Points (DSCP)

DSCP is a field in the IP header that is used to classify and prioritize network traffic. Each DSCP value corresponds to a specific level of service, allowing routers to apply different treatment to packets based on their DSCP value. Common DSCP values include:

• EF (Expedited Forwarding): For real-time traffic like VoIP and video conferencing.
• AF (Assured Forwarding): For traffic that requires a guaranteed level of service, such as web browsing and email.
• BE (Best Effort): For traffic that does not require any specific priority, such as file transfers.

For example, you can configure a router to mark VoIP packets with the EF DSCP value, ensuring that they receive the highest priority in the network.

2. Per-Hop Behaviors (PHBs)

PHBs define how routers should handle packets based on their DSCP value. Each PHB specifies a set of actions that can be taken, such as prioritizing, dropping, or delaying packets. Common PHBs include:

• EF PHB: Ensures low delay, low jitter, and high reliability for real-time traffic.
• AF PHB: Provides a guaranteed level of service for traffic that requires assurance.
• BE PHB: Provides the default treatment for traffic that does not require any specific priority.

For instance, the EF PHB ensures that VoIP packets are processed quickly and with minimal delay, while the AF PHB guarantees a certain level of service for web browsing traffic.

3. Traffic Classification

Traffic Classification is the process of identifying and categorizing network traffic based on predefined criteria. This classification is essential for applying the appropriate DSCP values and PHBs. Common classification criteria include:

• Source and Destination IP Addresses: Classify traffic based on the IP addresses of the sender and receiver.
• Protocol Type: Classify traffic based on the protocol used, such as TCP, UDP, or ICMP.
• Port Numbers: Classify traffic based on the port numbers used, such as port 80 for HTTP or port 53 for DNS.

For example, you can classify traffic from a specific IP address as VoIP traffic and mark it with the EF DSCP value.

4. Traffic Policing and Shaping

Traffic Policing and Shaping are techniques used to control the rate of traffic passing through a network. Policing involves dropping or marking packets that exceed a predefined rate, while shaping involves delaying packets to ensure that the traffic stays within the specified rate.

For instance, you can configure a traffic policer to drop packets from a specific user if their upload rate exceeds 1 Mbps. Alternatively, you can configure a traffic shaper to delay packets from a specific application if their download rate exceeds 2 Mbps.

5. Examples and Analogies

Consider a highway with different lanes for different types of vehicles. The fast lane is reserved for emergency vehicles, the middle lane is for regular cars, and the slow lane is for trucks. Similarly, DiffServ ensures that critical traffic like VoIP and video conferencing is given the fast lane (EF PHB), while regular traffic like web browsing is given the middle lane (AF PHB), and background traffic like file transfers is given the slow lane (BE PHB).

Understanding DiffServ is crucial for network administrators who need to manage bandwidth efficiently and ensure that critical applications receive the necessary resources. By mastering DiffServ, you can ensure that your network operates efficiently, securely, and reliably.