What is Differentiated Services Code Point (DSCP)?

Differentiated Service Code Points (DSCP) is an aspect of QoS (Quality of Service) used to classify and manage network traffic. The DSCP field is 6 bits in length and can take on values from 0 to 63.

DSCP is often described as “managed unfairness,” and with good reason. DSCP prioritizes packets by their attributes, prioritizing some information over others. For example, VoIP packets should be prioritized over email packets.

However, as the prioritization criteria become more in-depth, it quickly becomes unmanageable. After all, an IP packet has a 32-bit header, but the DSCP field, responsible for QoS differentiation, is limited to 6 bits. Despite this constraint, DSCP finds a way to differentiate various types of traffic within this compact 6-bit space. 

QoS and prioritizing network traffic are critical concepts for network engineers and are generally components of the Network+ Exam. Below, we’ll review how DSCP works, its core purpose, its benefits, and more.

What is a Differentiated Services Code Point?

DSCP is a field in the IP header used for QoS to prioritize and manage network traffic. It assigns a 6-bit value to packets, allowing for the classification of different types of traffic based on their importance or service requirements.

DSCP was outlined in RFC 2474 and created in 2000 to adapt to the growing number of traffic types. Before Y2K, IPv4 addresses only dedicated three bits to differentiate between traffic. So, in binary values, three bits would be eight different values. With DSCP, six bits are dedicated to traffic management, which allows for 64 combinations.

How Does DSCP Mark Headers?

DSCP accomplishes IP prioritization by marking packets with specific data. Marking is done at the IP’s source and acts as a mechanism for forwarding data based on priority. The “marks” themselves are bit combinations on the DSCP’s six-bit header. Let’s list out the three major DSCP markings found on IP headers:

Expedited Forwarding (EF)

EF, also known as DSCP 46, is used for data-intensive operations such as VoIP, media streaming, or gaming. EF is known as DSCP 46 because that’s how many bits it takes up in the six-bit header. EF looks like 101110 which is equal to 46 in decimal; take note it is six digits. EF receives the highest preferential treatment.

Assured Forwarding (AF) Classes

AF classes provided numerous levels of priority and drop precedence. A drop precedence indicates the possibility the packet will be dropped due to congestion issues. This is a QoS tactic that ensures higher priority packets will be forwarded.

There are twelve different AF classes, which are then subdivided into drop precedence: 

• Drop Precedence One

  • AF11 (DSCP 10)

  • AF12 (DSCP 12)

  • AF13 (DSCP 14)

• Drop Precedence Two

  • AF21 (DSCP 18)

  • AF22 (DSCP 20)

  • AF23 (DSCP 22)

• Drop Precedence Three

  • AF31 (DSCP 26)

  • AF32 (DSCP 28)

  • AF33 (DSCP 30)

• Drop Precedence Four

  • AF41 (DSCP 34)

  • AF42 (DSCP 36)

  • AF43 (DSCP 38)

Drop Precedence One are IP packets with the lowest possibility of being dropped, meaning they have the highest priority. Drop precedence four means they are the most likely to be dropped. The multitude levels of assured forwarding provided administrators with granular control over network precedence and priority.

Best Effort (BE)

DSCP 0 is generally used for default traffic without a specific prioritization requirement. These will always be at the back of the queue. 

How is DSCP Implemented?

DSCP is configured on the network router or any edge device, such as a firewall or gateway. While the administrator can set DSCP in multiple places, it is best practice to centralize DSCP management to ensure consistent QoS and packet forwarding prioritization.

While exact steps and terminology will depend on the router used, here are the general steps required to configure DSCP on an edge device.

Access Router Configuration

Log in to the router's management interface. This is typically done using a web browser and accessing the router's IP address. Generally, you will need a computer directly connected to the router via Ethernet. The username and password will be in the router’s manual.

Navigate to QoS or DSCP Configuration

Locate the section of the router's configuration related to Quality of Service (QoS) or DSCP. This might be under a QoS, Traffic Management, or Advanced Settings menu.

Define Traffic Classes

Identify the different types of traffic classes that you want to prioritize. For example, you may want to prioritize VoIP traffic, video conferencing, or critical business applications. Recall the three marks: AF, EF, and BE.

Assign DSCP Values

Assign an appropriate Differentiated Services Code Point (DSCP) value for each traffic class. DSCP values are 6-bit codes, typically represented in decimal form. For example, EF is DSCP 46. In the list above, each DSCP numerical code is annotated.

Apply DSCP Marking

Specify how the router should mark packets with the assigned DSCP values. This can involve configuring access control lists (ACLs) or policies that match specific criteria for marking packets.

Configure Traffic Policies

Create traffic policies that define the behavior for each DSCP value. Specify how the router should treat packets with different DSCP markings in terms of queuing, shaping, or dropping policies.

Apply QoS Policies to Interfaces

Associate the configured QoS policies with specific router interfaces. This ensures that the QoS policies are applied to traffic entering or leaving those interfaces.

Test and Monitor

Test the configured QoS policies by generating and monitoring traffic. Ensure that packets are being marked correctly and that the desired QoS treatment is applied as expected.

Adjust as Needed

Based on monitoring and testing, adjust the DSCP values or QoS policies as needed to achieve the desired Quality of Service.

Save Configuration

After double-checking the configurations, save the changes to the router's configuration to ensure that the settings persist across reboots.

What are the Benefits of DSCP?

There are myriad benefits to DSCP, and it clearly belongs in an admin's arsenal. The most obvious benefit is the granular control over data prioritization. This provides an enhanced user experience and ensures that mission-critical applications and data are transmitted with high fidelity.

DSCP also facilitates the optimization of network resources by allowing administrators to define appropriate drop precedences for unneeded packets. Lastly, DSCP empowers admins to tailor the Quality of Service experience to meet the network’s bandwidth and specific requirements.

What are the Limitations of DSCP?

While DSCP is a critical tool, it does have some drawbacks. For instance, DSCP adds network complexity, and its implementation can be challenging, particularly on large and dynamic networks. This complexity may result in hours of troubleshooting, testing, and configuring. 

Inconsistent application of QoS standards can lead to mixed signals and degraded quality. For example, suppose VoIP is treated as EF on the firewall but AF11 on one of the routers. In that case, this inconsistency will lead to unpredictable packet forwarding and potentially compromise the intended Quality of Service.

Final Thoughts

DSCP is an important topic for any network engineer to understand, especially for those preparing for the Network+ Exam. Remember that DSCP is a six-bit header attached to IP packets, and it includes three priority classes: Expedited Forwarding (EF), Assured Forwarding (AF), and Best Effort. 

Within each AF class, there are three drop precedences influencing the level of drop probability. The higher the drop precedence, the lower the priority. Lastly, when configuring DSCP on edge devices, ensuring consistency in priorities across different devices is critical to maintaining a cohesive Quality of Service policy.

Hopefully, this provides a better understanding of DSCP and will assist you and your organization in creating an efficient, robust network.

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