AFxx (Assured forwarding behavior group xx)
AFxx (Assured Forwarding Behavior Group xx) is a set of Quality of Service (QoS) classes that provide bandwidth assurance and low drop probability for network traffic. The AFxx classes are part of the Differentiated Services (DiffServ) architecture, which is used to manage network traffic by classifying it into different QoS classes and treating each class differently based on its importance or sensitivity to network delay and congestion.
The AFxx classes are defined in RFC 2597 and are numbered AF11 through AF43. Each AFxx class is defined by a set of parameters that determine its behavior in the network. These parameters include the Drop Probability, Minimum Bandwidth, Maximum Bandwidth, and Relative Weight. These parameters are used by network routers and switches to determine how to treat network traffic in each AFxx class.
The Drop Probability parameter is a measure of the likelihood that network traffic in a particular AFxx class will be dropped due to congestion in the network. The Drop Probability is defined using the following four levels: Low, Medium Low, Medium High, and High. The drop probability increases as the level of drop probability increases.
The Minimum Bandwidth parameter specifies the minimum amount of bandwidth that must be allocated to network traffic in a particular AFxx class. The Minimum Bandwidth parameter ensures that network traffic in a particular AFxx class will always have a minimum level of bandwidth, even when network congestion occurs.
The Maximum Bandwidth parameter specifies the maximum amount of bandwidth that can be allocated to network traffic in a particular AFxx class. The Maximum Bandwidth parameter ensures that network traffic in a particular AFxx class will never consume more than a certain amount of bandwidth, even when network capacity is available.
The Relative Weight parameter is used to prioritize network traffic in different AFxx classes. Network traffic in an AFxx class with a higher Relative Weight value will be given priority over network traffic in an AFxx class with a lower Relative Weight value. This ensures that network traffic in important AFxx classes, such as real-time voice and video, will be given priority over less important network traffic, such as file transfers.
The AFxx classes are organized into four groups, AF1x, AF2x, AF3x, and AF4x, with each group having a different level of Drop Probability and Relative Weight. The AF1x group has the lowest Drop Probability and Relative Weight, while the AF4x group has the highest Drop Probability and Relative Weight. The following table summarizes the different AFxx classes and their parameters:
| AFxx Class | Drop Probability | Minimum Bandwidth | Maximum Bandwidth | Relative Weight |
|---|---|---|---|---|
| AF11 | Low | 10% | 20% | 1 |
| AF12 | Medium Low | 10% | 20% | 2 |
| AF13 | Medium High | 10% | 20% | 3 |
| AF21 | Low | 20% | 30% | 1 |
| AF22 | Medium Low | 20% | 30% | 2 |
| AF23 | Medium High | 20% | 30% | 3 |
| AF31 | Low | 30% | 40% | 1 |
| AF32 | Medium Low | 30% | 40% | 2 |
| AF33 | Medium High | 30% | 40% | 3 |
| AF41 | Low | 40% | 50% | 1 |
The AF1x group is typically used for real-time voice and video traffic, which requires low delay and low drop probability. The AF2x group is used for interactive traffic, such as web browsing and email, which require low delay but can tolerate a higher drop probability. The AF3x group is used for bulk data transfer, such as file transfers, which require high bandwidth but can tolerate higher delay and drop probability. The AF4x group is used for best-effort traffic, which has no guaranteed bandwidth or drop probability.
When network traffic is classified into different AFxx classes, it is marked with a 6-bit Differentiated Services Code Point (DSCP) value in the IP header. The DSCP value is used by network routers and switches to determine how to treat network traffic in each AFxx class.
When network congestion occurs, network routers and switches use the Drop Probability parameter to determine which network traffic to drop. Network traffic in AFxx classes with a higher Drop Probability value will be dropped first, followed by network traffic in AFxx classes with a lower Drop Probability value. This ensures that network traffic in important AFxx classes, such as real-time voice and video, is given priority over less important network traffic, such as file transfers.
The Minimum Bandwidth and Maximum Bandwidth parameters are used to ensure that network traffic in each AFxx class is given a minimum and maximum amount of bandwidth, respectively. When network capacity is available, network routers and switches allocate bandwidth to network traffic in each AFxx class according to their Minimum Bandwidth and Maximum Bandwidth parameters. When network capacity is limited, network routers and switches prioritize network traffic in each AFxx class according to their Relative Weight parameter and Drop Probability parameter.
In summary, AFxx (Assured Forwarding Behavior Group xx) is a set of Quality of Service (QoS) classes that provide bandwidth assurance and low drop probability for network traffic. The AFxx classes are part of the Differentiated Services (DiffServ) architecture, which is used to manage network traffic by classifying it into different QoS classes and treating each class differently based on its importance or sensitivity to network delay and congestion. The AFxx classes are organized into four groups, with each group having a different level of Drop Probability and Relative Weight. The AF1x group is typically used for real-time voice and video traffic, while the AF4x group is used for best-effort traffic. The AFxx classes are marked with a DSCP value in the IP header, which is used by network routers and switches to determine how to treat network traffic in each AFxx class.