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Understanding How Class of Service Manages Congestion and Controls Service Levels in the Network

Usually, IP routers forward packets independently and without any control on throughput or delay. This is known as best-effort service. This service is as good as the network equipment and links, and the result is satisfactory for many traditional IP applications emphasizing data delivery, such as e-mail or Web browsing. However, IP applications such as real-time video and audio (or voice) require lower delay, jitter, and loss parameters than simple best-effort networks can provide during times of network congestion.

When a network experiences congestion and delay, some packets must be dropped. The Juniper Networks Junos operating system (Junos OS) class of service (CoS) enables you to assign traffic to classes and offer various levels of throughput and packet loss when congestion occurs.

CoS is the assignment of traffic flows to different service levels. Service providers can use router-based CoS features to define service levels that provide different delay, jitter (delay variation), and packet loss characteristics to particular applications served by specific traffic flows.

A router cannot compromise best-effort forwarding performance in order to deliver CoS features, because this merely trades one problem for another. When CoS features are enabled, they must allow routers to better process critical packets as well as best-effort traffic flows, even during times of congestion. Network throughput is determined by a combination of available bandwidth and delay. CoS guarantees a minimum bandwidth dedicated to a service class.

The main impact of CoS on network delay is in queuing delays, when packets are normally queued for output in the order of arrival, regardless of service class. Queuing delays increase with network congestion and often result in lost packets when queue buffers overflow. The other two elements of overall network delay, serial transmission delays determined by link speeds and propagation delays determined by media type, are not determined by CoS settings.

For interfaces that carry IPv4, IPv6, and MPLS traffic, you can configure the Junos OS CoS features to provide multiple classes of service for different applications. On the routing 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.

The Junos OS CoS features provide a set of mechanisms that you can use to provide differentiated services when best-effort traffic delivery is insufficient. In designing CoS applications, you must give careful consideration to your service needs, and you must thoroughly plan and design your CoS configuration to ensure consistency across all routing devices in a CoS domain. You must also consider all the routing devices and other networking equipment in the CoS domain to ensure interoperability among all equipment.

CoS Applications

You can configure CoS features to meet the needs of multiple applications. Because the components are generic, you can use a single CoS configuration syntax across multiple routing devices. CoS mechanisms are useful for two broad classes of applications. These applications can be referred to as in the box and across the network.

In-the-box applications use CoS mechanisms to provide special treatment for packets passing through a single node on the network. You can monitor the incoming traffic on each interface, using CoS to provide preferred service to some interfaces (that is, to some customers) while limiting the service provided to other interfaces. You can also filter outgoing traffic by the packet’s destination, thus providing preferred service to some destinations.

Across-the-network applications use CoS mechanisms to provide differentiated treatment to different classes of packets across a set of nodes in a network. In these types of applications, you typically control the ingress and egress routing devices to a routing domain and all the routing devices within the domain. You can use the Junos OS CoS features to modify packets traveling through the domain to indicate the packet’s priority across the domain.

Specifically, you modify the CoS code points in packet headers, remapping these bits to values that correspond to levels of service. When all routing devices in the domain are configured to associate the precedence bits with specific service levels, packets with the same code points traveling across the domain receive the same level of service from the ingress point to the egress point. For CoS to work in this case, the mapping between the code points and service levels must be identical across all routing devices in the domain.

The Junos OS CoS applications support the following range of mechanisms:

  • Differentiated Services (DiffServ)—The CoS application supports DiffServ, which uses a 6-bit differentiated services code point (DSCP) in the differentiated services field of the IPv4 and IPv6 packet header. For IPv6, DSCP is referred to as traffic class. The configuration uses DSCP values to determine the forwarding class associated with each packet. IPv4 traffic can also use the 3-bit IP precedence bits to classify traffic.

  • Layer 2 to Layer 3 CoS mapping—The CoS application supports mapping of Layer 2 (IEEE 802.1p) packet headers to routing device forwarding class and loss-priority values.

    Layer 2 to Layer 3 CoS mapping involves setting the forwarding class and loss priority based on information in the Layer 2 header. Output involves mapping the forwarding class and loss priority to a Layer 2-specific marking. You can mark the Layer 2 and Layer 3 headers simultaneously.

  • MPLS EXP—Supports configuration of mapping of MPLS experimental (EXP) bit settings to routing device forwarding classes and vice versa.

  • VPN outer-label marking—Supports setting of outer-label EXP bits, also known as CoS bits, based on MPLS EXP mapping.

CoS Standards

The standards for Junos OS class of service (CoS) capabilities are defined in the following RFCs:

  • RFC 2474, Definition of the Differentiated Services Field in the IPv4 and IPv6 Headers

  • RFC 2597, Assured Forwarding PHB Group

  • RFC 2598, An Expedited Forwarding PHB

  • RFC 2698, A Two Rate Three Color Marker