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.
• 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.

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