OcNOS-SP : Quality of Service Guide : Quality of Service Configuration Guide : Quality of Service : Terminology
Terminology
Following is a brief description of terms and concepts used to describe QoS.
ACL
Access control lists (ACLs) classify traffic with the same characteristics.
CoS Value
Class of Service (CoS) is a 3-bit value used to classify the priority of Layer-2 frames upon entry into a network. QoS classifies frames by assigning priority-indexed CoS values to them, and gives preference to higher-priority traffic.CoS values range from zero to seven, seven being the highest priority.
DSCP Value
DSCP Value Differentiated Services Code Point (DSCP) is a 6-bit value used to classify the priority of Layer-3 packets upon entry into a network. DSCP values range from 0 to 63, 63 being the highest priority, 0 being best-effort traffic.
Classification
Classification distinguishes one kind of traffic from another by examining the fields in the packet. Classification is enabled only if QoS is globally enabled on the switch. By default, QoS is globally disabled, thus, no classification occurs. Classification occurs on an ingress physical port. Classification can be based on QoS ACLs, or class maps and policy maps.
Policing
Policing can occur on ingress interfaces. Policer limits the bandwidth consumed by a traffic flow with the results given to the marker. The two types of policers:
• Individual: QoS applies the bandwidth limits specified in the policer, separately, to each matched traffic class. An individual policer is configured within a policy map.
Marking
Marking determines how to handle a packet when it is out of profile. It assesses the policer and the configuration data to determine the action required for the packet, and then handles the packet using one of the following methods:
• Let the packet through without modification
• Drop the packet
Marking can occur on ingress and egress interfaces.
Queuing
Queuing maps packets to a queue. Each egress port can accommodate up 8 queues, prioritized as 0 lowest and 7 highest. The tagged packet incoming priority can be mapped to one of the 8 queues obtained from the filtering mechanism result. The untagged packet priority is also obtained from the filtering mechanism result. After the packets are mapped to a queue, they are scheduled.
Scheduling
Scheduling forwards or conditions packets using one of the following methods:
• Strict Priority-Based (SP), in which any high-priority packets are first transmitted. Lower-priority packets are transmitted only when the higher-priority queues are empty. A problem may occur when too many lower-priority packets are not transmitted. Strict Priority will be operating on the remaining bandwidth available for the port
• WFQ (Weighted Fair Queuing) weight-based scheduling – In this scheduling, some weight based bandwidth is allocated to all queues. In this scheduling, egress traffic will be served based on the configured weight distribution.
• Combination of WFQ and SP, the Remaining Bandwidth will be scheduled in the strict order for the SP Queues. The Remaining Bandwidth will be scheduled in the WFQ mode for WFQ Queues.
Class Map
A class map names and isolates specific traffic from other traffic. The class map defines the criteria used to match against a specific traffic flow to classify it further. The criteria can include:
• Matching the access group defined by the ACL
• Matching a specific list of CoS, DSCP, Exp and etc.
If there is more than one type of traffic to be classified, another class map can be created under a different name. After a packet is matched against the class-map criteria, it is further classified using a policy map.
Policy Map
A policy map specifies on which traffic class to act. This can be implemented as follows:
• Set a specific CoS or DSCP value in the traffic class.
• Specify the traffic bandwidth limitations for each matched traffic class (policer) and the action to take (marking) when the traffic is out of profile.
Policy maps have the following attributes:
• A policy map can contain Maximum 256 class-map per policy-map, each with different match criteria and policers.
• A separate policy-map class can exist for each type of traffic received through an interface.
• There can be only one policy map per interface per direction. The same policy map can be applied to multiple interfaces and directions.
• Before a policy map can be effective, it must be attached to an interface.
Last modified date: 10/20/2023