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DSCP (Differentiated Services Code-Point)

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Introduction

Differentiated Services Code-Point (DSCP) is a mechanism that is used to classify and prioritize network traffic. It is an important part of Quality of Service (QoS) management and is used to ensure that network resources are used efficiently and effectively. DSCP is a way of marking packets so that routers and other network devices can identify the priority of the traffic and apply the appropriate QoS policies. In this essay, we will explain DSCP in detail, including its definition, purpose, history, and practical applications.

Definition of DSCP

DSCP is a field in the header of an IP packet that is used to indicate the priority of the packet. The DSCP field is 6 bits long and is located in the ToS (Type of Service) field of the IP header. The DSCP field is used to classify packets into different classes, each with its own priority level. DSCP is part of the larger set of QoS mechanisms that are used to ensure that network resources are used efficiently and effectively.

Purpose of DSCP

The purpose of DSCP is to provide a mechanism for classifying network traffic into different classes based on the priority of the traffic. This allows network administrators to define QoS policies that can be used to manage network resources more effectively. DSCP is particularly useful in networks that carry different types of traffic, such as voice, video, and data. Each type of traffic has different requirements in terms of bandwidth, latency, and jitter, and DSCP allows network administrators to prioritize traffic based on these requirements.

History of DSCP

The concept of DSCP was introduced in RFC 2474, which was published in 1998. The RFC defined a new field in the IP header, called the DS field, which was used to carry the DSCP value. The DS field was later renamed the ToS (Type of Service) field in RFC 3168, which was published in 2001. The DSCP mechanism was designed to be backward compatible with the original ToS field, which was only 3 bits long and had limited functionality. The DSCP mechanism was designed to be extensible, so that new classes of traffic could be added in the future if needed.

DSCP Classes

DSCP defines 64 different classes of traffic, which are identified by a 6-bit value in the DSCP field. The classes are divided into three categories: Default, Assured Forwarding, and Expedited Forwarding. The Default class is used for traffic that is not classified into any of the other classes. The Assured Forwarding (AF) classes are used for traffic that requires some level of assurance in terms of bandwidth and latency. The Expedited Forwarding (EF) class is used for traffic that requires the highest level of priority and is typically used for real-time traffic such as voice and video.

The 64 different DSCP values are divided into the following categories:

  • 8 values for the Default class (000000 to 001111)
  • 16 values for the Assured Forwarding (AF) classes (100000 to 101111 and 110000 to 111111)
  • 1 value for the Expedited Forwarding (EF) class (101110)

The AF classes are further divided into four sub-classes, each with a different priority level. The sub-classes are identified by the last 3 bits of the DSCP value. The four AF sub-classes are:

  • AF11 (DSCP value 001010)
  • AF12 (DSCP value 001100)
  • AF13 (DSCP value 001110)
  • AF21 (DSCP value 010010)
  • AF22 (DSCP value 010100)
  • AF23 (DSCP value 010110)
  • AF31 (DSCP value 011010)
  • AF32 (DSCP value 011100)
  • AF33 (DSCP value 011110)
  • AF41 (DSCP value 100010)
  • AF42 (DSCP value 100100)
  • AF43 (DSCP value 100110)

The AF sub-classes are designed to provide different levels of priority to different types of traffic. AF11, AF21, AF31, and AF41 provide the lowest level of priority, while AF13, AF23, AF33, and AF43 provide the highest level of priority within the AF classes.

The EF class provides the highest level of priority and is used for real-time traffic that requires minimal delay and jitter. The EF class is typically used for voice and video traffic, and is given priority over all other traffic in the network. The DSCP value for the EF class is 101110.

DSCP Mapping

DSCP values are mapped to different QoS policies in order to provide the appropriate level of service to each class of traffic. The QoS policies are typically defined by the network administrator and are based on the specific requirements of the network. The mapping of DSCP values to QoS policies is typically done using a QoS policy map.

A QoS policy map is a set of rules that are used to classify network traffic into different classes based on the DSCP value. The policy map then assigns the appropriate QoS policy to each class of traffic. The QoS policies can include bandwidth allocation, packet loss prioritization, and other parameters that affect the performance of the network.

Practical Applications of DSCP

DSCP is used in a wide range of applications, including enterprise networks, data centers, and service provider networks. In enterprise networks, DSCP is used to prioritize traffic based on the specific needs of the organization. Voice and video traffic may be given a higher priority than data traffic, for example, in order to ensure that real-time applications perform well.

In data center networks, DSCP is used to ensure that different applications receive the appropriate level of service. Critical applications, such as database servers, may be given a higher priority than less critical applications, such as web servers.

In service provider networks, DSCP is used to provide different levels of service to different customers. Service providers may offer different levels of service based on the DSCP value of the traffic, with higher levels of service provided to customers that require higher levels of priority.

Conclusion

Differentiated Services Code-Point (DSCP) is an important mechanism for managing network traffic in a variety of applications. It provides a way of classifying network traffic based on the priority of the traffic, and allows network administrators to define QoS policies that can be used to manage network resources more effectively. DSCP is a powerful tool for ensuring that network resources are used efficiently and effectively, and is an essential part of Quality of Service (QoS) management.