Netgear CSM4532 Software Administration Manual - Page 233
CoS Queuing Function and Behavior
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queues of the port. If a delay is necessary, packets are held in the queue until the scheduler authorizes the queue for transmission. As queues become full, packets have no place to be held for transmission and get dropped by the device. The drop precedence of a packet is an indication of whether the packet is more or less likely to be dropped during times of queue congestion. Often referred to as packet coloring, a low drop precedence (green) allows the packet to be transmitted under most circumstances, a higher drop precedence (yellow) subjects the packet to dropping when bursts become excessive, while the highest drop precedence (red) discards the packet whenever the queue is congested. In some hardware implementations, the queue depth can be managed using tail dropping or a weighted random early discard, or WRED, technique. These methods often use customizable threshold parameters that are specified on a per-drop-precedence basis. The switch supports Differentiated Services (DiffServ), which allows traffic to be classified into streams and given certain QoS treatment in accordance with defined per-hop behaviors. However, the DiffServ feature does not offer direct configuration of the hardware COS queue resources. The COS Queuing feature allows the switch administrator to directly configure certain aspects of device queuing to provide the desired QoS behavior for different types of network traffic when the complexities of DiffServ are not required. The priority of a packet arriving at an interface can be used to steer the packet to the appropriate outbound COS queue through a mapping table. With the CoS queuing feature, COS queue characteristics such as minimum guaranteed bandwidth, transmission rate shaping, etc. can be configured at the queue (or port) level. With support for the multistage scheduling architecture, the COS queue feature provides a method to configure Traffic Class Groups (TCGs) to extend the COS queue management. Multiple COS queues can be mapped to a single TCG. Each TCG can have a configured minimum guaranteed bandwidth allocation and a scheduling algorithm similar to the COS queue configuration. The TCG scheduling and bandwidth enforcement occurs after the COS queue scheduling and bandwidth enforcement is performed. Therefore all COS queues mapped to the same TCG share the scheduling and bandwidth properties of the TCG. 9.4.1. CoS Queuing Function and Behavior Like CoS mapping, CoS queuing uses the concept of trusted and untrusted ports. CoS queuing builds on includes user-configurable settings that affect hardware queue operation. 9.4.1.1. Trusted Port Queue Mappings A trusted port is one that takes at face value a certain priority designation within arriving packets. Specifically, a port may be configured to trust one of the following packet fields: • 802.1p User Priority • IP Precedence • IP DSCP Packets arriving at the port ingress are inspected and their trusted field value is used to designate the COS queue that the packet is placed when forwarded to the appropriate egress port. A mapping table associates the trusted field value with the desired COS queue. NETGEAR M4500 Series Switches Software Administration Manual 233