Quality of Service (QoS) is a fundamental technology in networks that require reliable and predictable data delivery. It allows for traffic prioritization and management, ensuring that critical applications and services function at their best. At the heart of QoS lies the Differentiated Services Code Point (DSCP), which defines the level of service for IP packets. In this blog post, we’ll explore some commonly used DSCP values, their significance, and how they can be applied in real-world networking scenarios.
What is DSCP and Why Use It?
DSCP is part of the IP header that specifies the traffic class for each packet. It ensures that data is treated accordingly as it travels through routers and switches. By using DSCP, network devices can prioritize traffic, reduce latency, prevent jitter, and minimize packet loss, which is crucial for time-sensitive applications like VoIP or video streaming.
Common DSCP Values and Their Uses
Here are some of the commonly used DSCP values and their equivalent IP Precedence values, as well as guidance on when and where they should be used:
- Expedited Forwarding (EF) – DSCP 101 110 (46 Decimal)
Equivalent IP Precedence: Critical (101)
This value is reserved for voice RTP (Real-time Transport Protocol) streams. It guarantees timely delivery and is used in scenarios where voice traffic must not experience delays, such as in VoIP communications. - Best Effort (BE) – DSCP 000 000 (0 Decimal)
Equivalent IP Precedence: Routine (000)
The default service level for general traffic that does not need special treatment. It is applied to non-critical traffic such as bulk data transfers where delivery time is not a priority. - Assured Forwarding (AF) Group
This group includes several classes (AF11-AF43) with varying drop probabilities (Low, Medium, High). Each class ensures a certain level of delivery depending on network congestion:- AF11-AF13
They cater to applications that need assured delivery but can tolerate some delays, like business application data. - AF21-AF23
Typically used for video conferencing and streaming media which are less sensitive than voice but still require prioritization. - AF31-AF33
Ideal for mission-critical data, such as video surveillance feeds, where packet loss could be problematic. - AF41-AF43
Applied to applications that demand the highest quality, next to voice and video, like virtual desktop infrastructures.
- AF11-AF13
- Class Selector (CS) Group
These values are backward compatible with the IP Precedence field and range from CS1 to CS7 (DSCP 8 to 56 Decimal). They are utilized as follows:- CS1 (DSCP 8 Decimal)
Used for less critical traffic that should be de-prioritized during congestion scenarios, such as backup services. - CS2-CS4 (DSCP 16 to 32 Decimal)
Applied to traffic with increasing importance, often aligned with enterprise-specific policies. - CS5 (DSCP 40 Decimal)
Prioritizes traditional network control traffic like routing protocol packets. - CS6 (DSCP 48 Decimal)
Designated for network control traffic, ensuring that infrastructure communications maintain precedence. - CS7 (DSCP 56 Decimal)
The highest level reserved for network control and often used internally by network devices.
- CS1 (DSCP 8 Decimal)
Who Needs DSCP?
DSCP is used by network administrators to implement QoS policies. It’s particularly important for businesses and service providers that require:
- Stable and reliable voice and video communications.
- Prioritization of critical business applications.
- Efficient network traffic management to avoid congestion.
Where to Apply QoS?
QoS and DSCP markings should be applied at various points in the network:
- At the network edge, where traffic enters or exits an enterprise network.
- In the core network, especially in links between critical services and data centers.
- In wireless networks, where bandwidth is shared and can be limited.
- Across WAN links, where traffic prioritization is essential due to potentially limited bandwidth.
Conclusion
Understanding and correctly applying DSCP values is critical to managing a network that supports diverse applications with varying requirements. By using these DSCP markings wisely, network professionals can fine-tune their networks, delivering the right quality of service to each type of traffic. With the right QoS configurations in place, the network becomes a robust, dynamic, and highly responsive asset for any organization.
Cheat Sheet ( Table )
Unless noted as a restriction, you can mark both incoming and outgoing packets.
| DSCP Value | Decimal Value | Meaning | Drop Probability | Equivalent IP Precedence Value |
|---|---|---|---|---|
| 101 110 | 46 | Expedited Forwarding (EF) | N/A | 101 – Critical |
| 000 000 | 0 | Best Effort | N/A | 000 – Routine |
| 001 010 | 10 | AF11 | Low | 001 – Priority |
| 001 100 | 12 | AF12 | Medium | 001 – Priority |
| 001 110 | 14 | AF13 | High | 001 – Priority |
| 010 010 | 18 | AF21 | Low | 010 – Immediate |
| 010 100 | 20 | AF22 | Medium | 010 – Immediate |
| 010 110 | 22 | AF23 | High | 010 – Immediate |
| 011 010 | 26 | AF31 | Low | 011 – Flash |
| 011 100 | 28 | AF32 | Medium | 011 – Flash |
| 011 110 | 30 | AF33 | High | 011 – Flash |
| 100 010 | 34 | AF41 | Low | 100 – Flash Override |
| 100 100 | 36 | AF42 | Medium | 100 – Flash Override |
| 100 110 | 38 | AF43 | High | 100 – Flash Override |
| 001 000 | 8 | CS1 | 1 | |
| 010 000 | 16 | CS2 | 2 | |
| 011 000 | 24 | CS3 | 3 | |
| 100 000 | 32 | CS4 | 4 | |
| 101 000 | 40 | CS5 | 5 | |
| 110 000 | 48 | CS6 | 6 | |
| 111 000 | 56 | CS7 | 7 | |
| 000 000 | 0 | Default | ||
| 101 110 | 46 | EF |
IP Precedence Values
| Value | Description |
|---|---|
| 000 | Routine or Best Effort |
| 001 | Priority |
| 010 | Immediate |
| 011 | Flash (mainly used for voice signaling or for video) |
| 100 | Flash Override |
| 101 | Critical (mainly used for voice RTP) |
| 110 | Internet |
| 111 | Network |