Understanding CoS Congestion Management
A congestion in a network occurs because of various parameters. Some packets must be dropped to avoid congestion and to facilitate easy flow of traffic in the network. On Juniper Networks EX Series Ethernet Switches, class of service (CoS) provides congestion management mechanisms to drop arriving packets on the basis of certain parameters when the queue is full. Based on the EX Series switch that you are using, packets are dropped depending on the priority of a packet or on both priority and drop probability of a packet.
You can specify parameters for dropping packets at the [edit class-of-service drop-profiles] hierarchy level and reference the parameters in a scheduler configuration.
This topic describes:
Weighted Tail Drop Congestion Management
A weighted tail drop (WTD) is a congestion management mechanism in which packets are dropped from the tail of the queue when the queue reaches a certain buffer capacity (that is, the fill level), and hence the name weighted tail drop. The packets that are dropped are those marked with a packet loss priority (PLP) of high. You can configure a WTD profile usually on edge devices in a network.
 | Note: A WTD profile is supported only on Juniper Networks EX2200, EX3200, EX3300, EX4200, EX4500, EX4550, and EX6200 Ethernet Switches. |
When you configure a WTD mechanism, you are essentially setting the value for queue fullness. The queue fullness represents a percentage of the memory, known as delay-buffer bandwidth, that is used to store packets in relation to the total amount of memory that has been allocated for that specific queue. The delay-buffer bandwidth provides packet buffer space to absorb burst traffic up to the specified duration of delay. When the specified delay buffer becomes full, packets are dropped from the tail of the buffer.
By default, if you do not configure any drop profile, WTD profile is in effect and functions as the primary mechanism for managing congestion.
| Note: The default WTD profile associated with the packets whose PLP is low cannot be modified. You can configure custom drop profile only for those packets whose PLP is high. |
Weighted Random Early Detection Congestion Management
In a weighted random early detection (WRED) congestion management mechanism, random packets with a PLP of low or high are gradually dropped (based on drop probability) when the queue reaches a certain buffer capacity (that is, fill level).
| Note: The WRED mechanism is supported only on standalone Juniper Networks EX8200 Ethernet Switches and Juniper Networks EX8200 Virtual Chassis. |
Following are the different implementations of WRED:
- Segmented drop profile
- Interpolated drop profile
From a high level, segmented drop profile is a stair-step-like drop profile, whereas interpolated drop profile is a smoother (curved) drop profile. Figure 1 and Figure 2 show graphical representations of a segmented drop profile and an interpolated drop profile, respectively. Regardless of the implementation, a drop profile is a graph where the x-axis represents the percentage of fill level (l) and the y-axis represents the percentage of drop probability (p). The origin (0,0) represents the drop profile in which the drop probability is 0 percent when the queue fullness is 0 percent, and the point (100,100) represents the drop profiles in which the drop probability is 100 percent when the queue fullness is 100 percent. Although the formation of graph lines in Figure 1 and Figure 2 is different, the application of the profile is the same. When a packet reaches the head of the queue, a random number between 0 and 100 is calculated. This random number is plotted against the drop profile graph using the current queue fullness of that particular queue. When the random number falls above the graph line, the packet is transmitted. When the number falls below the graph line, the packet is dropped from the network.
The following sections discuss the WRED drop profile implementations in detail:
Segmented Drop Profile
In a segmented drop profile configuration, you can define multiple data points for fill level and drop probability. Figure 1 shows a graphical representation of a segmented drop profile.
Figure 1: Graphical Representation of a Segmented Drop Profile
To create the profile's graph line, the software begins at the bottom-left corner of the graph, representing a 0 percent fill level and a 0 percent drop probability (that is, the point (0,0)). The configuration draws a line directly to the right until it reaches the first defined fill level (that is, 25 percent represented in the graph on the x-axis). The software then continues to draw the line vertically until the first drop probability is reached (that is, 25 percent represented in the graph on the y-axis). This process is repeated for all of the defined fill levels and drop probabilities until the top-right corner of the graph is reached (that is, point (100,100) in the graph).
Interpolated Drop Profile
An interpolated drop profile configuration forms a smoother graph line compared to the graph in a segmented drop profile configuration. An interpolated drop profile automatically generates 64 pairs of data points on the graph beginning at (0, 0) and ending at (100, 100). Along the way, the graph line intersects specific data points that you define for fullness and drop probability.
Figure 2 shows a graphical representation of an interpolated drop profile.
Figure 2: Graphical Representation of an Interpolated Drop Profile
