High Priority Traffic Path


Problem description

When designing a network, traffic may belong to different classes requiring differentiated level of service by each network element.

As a concrete example, a network element managed by a service provider may be handling voice and data traffic. Voice traffic requires that the end-to-end latency and jitter is bounded to some msec accuracy in order to ensure quality-of-experience (QoE) for the participants in the voice call. Data traffic, typically, does not impose such performance requirements on the network. For example, when downloading a large file across the Internet, although the bandwidth requirements may be high there is usually no requirement that the file arrives within a bounded time interval.

Depending on the scheduling algorithms running on the network element, frames belonging to the data traffic may get transmitted before frames belonging to the voice traffic introducing unwanted latency or jitter. Therefore, in order to ensure deterministic latency and jitter characteristics end-to-end, each network element through which the voice traffic traverses must ensure that voice traffic is handled deterministically.

Hardware switches have typically been designed to ensure certain classes of traffic can be scheduled ahead of other classes and are also over-provisioned which further ensures deterministic behavior when handling high priority traffic. However, software switches (which includes virtual switches such as Open vSwitch) may require modification in order to achieve this deterministic behavior.

Use Cases

  1. Program classes of service

The End User specifies a number of classes of service. Each class of service will be represented by the value of a particular field in a frame. The class of service will determine the order which flows of this type will get handled with respect to other flows of differing classes of service. As such, each class of service will be associated with a priority. The End User will associate classes of service and priorities to ingress ports with the expectation that frames that arrive on these ingress ports will get scheduled following the specified priorities.

  1. Forward high priority network traffic

A remote network element sends traffic to Open vSwitch. The remote network element, indicates the class of service to which this flow of traffic belongs to by modifying a pre-determined but arbitrary field in the frame as specified in Use Case 1. Some examples include the Differentiated Services Code Point (DSCP) in an IP packet or the Priority Code Point (PCP) in an Ethernet frame. The order that frames get processed by Open vSwitch can be guaranteed by the values populated in these fields when the fields are different. If the fields are the same, ordering is not deterministic.

For example: Packet A is sent with a DSCP value of 0 and packet B is sent with a value of 46; 0 has a lower priority than 46. Packet A arrives before packet B. If Open vSwitch has been configured as such, Packet B will be transmitted before Packet A.

Proposed change




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Release Name Description
Colorado Introduced