RED Drop Profiles Overview
You can configure two parameters to control congestion at the output stage. The first parameter defines the delay-buffer bandwidth, which provides packet buffer space to absorb burst traffic up to the specified duration of delay. Once the specified delay buffer becomes full, packets with 100 percent drop probability are dropped from the head of the buffer. For more information, see Configuring the Scheduler Buffer Size.
The second parameter defines the drop probabilities across the range of delay-buffer occupancy, supporting the random early detection (RED) process. When the number of packets queued is greater than the ability of the router or switch to empty a queue, the queue requires a method for determining which packets to drop from the network. To address this, the Junos OS provides the option of enabling RED on individual queues.
Depending on the drop probabilities, RED might drop many packets long before the buffer becomes full, or it might drop only a few packets even if the buffer is almost full.
A drop profile is a mechanism of RED that defines parameters that allow packets to be dropped from the network. Drop profiles define the meanings of the loss priorities.
When you configure drop profiles, there are two important values: the queue fullness and the drop probability. The queue fullness represents a percentage of the memory used to store packets in relation to the total amount that has been allocated for that specific queue. Similarly, the drop probability is a percentage value that correlates to the likelihood that an individual packet is dropped from the network. These two variables are combined in a graph-like format, as shown in Figure 1.
 | Note: You can only specify two fill levels for interpolated drop profiles on the Enhanced Queuing DPC for Juniper Network MX Series Ethernet Services Routers and EX Series switches. For more information about interpolated drop profiles on the Enhanced Queuing DPC for MX Series routers and EX Series switches, see Configuring WRED on Enhanced Queuing DPCs. |
Figure 1 shows both a segmented and an interpolated graph. Although the formation of these graph lines 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 by the router or switch. This random number is plotted against the drop profile using the current queue fullness of that particular queue. When the random number falls above the graph line, the packet is transmitted onto the physical media. When the number falls below graph the line, the packet is dropped from the network.
Figure 1: Segmented and Interpolated Drop Profiles
By defining multiple fill levels and drop probabilities, you create a segmented drop profile. The line segments are defined in terms of the following graphical model: in the first quadrant, the x axis represents the fill level, and the y axis represents the drop probability. The initial line segment spans from the origin (0,0) to the point (<l1>, <p1>); a second line runs from (<l1>, <p1>) to (<l2>, <p2>) and so forth, until a final line segment connects (100, 100). The software automatically constructs a drop profile containing 64 fill levels at drop probabilities that approximate the calculated line segments.
| Note: If you configure the interpolate statement, you can specify more than 64 pairs, but the system generates only 64 discrete entries. |
You specify drop probabilities in the drop profile section of the class-of-service (CoS) configuration hierarchy and reference them in each scheduler configuration. For each scheduler, you can configure multiple separate drop profiles, one for each combination of loss priority (low, medium-low, medium-high, or high) and protocol.
You can configure a maximum of 32 different drop profiles.
To configure RED drop profiles, include the following statements at the [edit class-of-service] hierarchy level:
