Class of Service Overview

When a network experiences congestion and delay, some packets must be dropped. Junos OS class of service (CoS) allows you to divide traffic into classes and offer various levels of throughput and packet loss when congestion occurs. This allows packet loss to happen according to the rules you configure.

For interfaces that carry IPv4, IPv6, or MPLS traffic, you can configure the Junos OS CoS features to provide multiple classes of service for different applications. On the device, you can configure multiple forwarding classes for transmitting packets, define which packets are placed into each output queue, schedule the transmission service level for each queue, and manage congestion using a random early detection (RED) algorithm.

Traffic shaping is the allocation of the appropriate amount of network bandwidth to every user and application on an interface. The appropriate amount of bandwidth is defined as cost-effective carrying capacity at a guaranteed CoS. You can use a J Series Services Router or an SRX Series Services Gateway to control traffic rate by applying classifiers and shapers.

The CoS features provide a set of mechanisms that you can use to provide differentiated services when best-effort delivery is insufficient.

Using Junos OS CoS features, you can assign service levels with different delay, jitter (delay variation), and packet loss characteristics to particular applications served by specific traffic flows. CoS is especially useful for networks supporting time-sensitive video and audio applications. To configure CoS features on a device, see Configuring Class of Service.

Note: Policing, scheduling, and shaping CoS services are not supported for pre-encryption and post-encryption packets going into and coming out of an IPsec VPN tunnel.

Junos OS supports the following RFCs for traffic classification and policing:

RFC 2474, Definition of the Differentiated Services Field in the IPv4 and IPv6
RFC 2475, An Architecture for Differentiated Services
RFC 2579, Assured Forwarding PHB Group
RFC 2598, An Expedited Forwarding PHB
RFC 2697, A Single Rate Three Color Marker
RFC 2698, A Two Rate Three Color Marker

For more information about CoS, see the Junos Class of Service Configuration Guide.

For information about which devices support the features documented in this chapter, see the Junos OS Feature Support Reference for SRX Series and J Series Devices.

This chapter contains the following topics: