Junos CoS Components

Junos CoS consists of many components that you can combine and tune to provide the level of services required by customers.

The Junos CoS components include:

• Code-point aliases—A code-point alias assigns a name to a pattern of code-point bits. You can use this name instead of the bit pattern when you configure other CoS components, such as classifiers, drop-profile maps, and rewrite rules.
• Classifiers—Packet classification refers to the examination of an incoming packet. This function associates the packet with a particular CoS servicing level. In the Junos OS, classifiers associate incoming packets with a forwarding class and loss priority and, based on the associated forwarding class, assign packets to output queues. Two general types of classifiers are supported:
  • Behavior aggregate or CoS value traffic classifiers—A behavior aggregate (BA) is a method of classification that operates on a packet as it enters the router. The CoS value in the packet header is examined, and this single field determines the CoS settings applied to the packet. BA classifiers allow you to set the forwarding class and loss priority of a packet based on the Differentiated Services code point (DSCP) value, DSCP IPv6 value, IP precedence value, MPLS EXP bits, and IEEE 802.1p value. The default classifier is based on the IP precedence value.
  • Multifield traffic classifiers—A multifield classifier is a second method for classifying traffic flows. Unlike a behavior aggregate, a multifield classifier can examine multiple fields in the packet. Examples of some fields that a multifield classifier can examine include the source and destination address of the packet as well as the source and destination port numbers of the packet. With multifield classifiers, you set the forwarding class and loss priority of a packet based on firewall filter rules.
• Forwarding classes—The forwarding classes affect the forwarding, scheduling, and marking policies applied to packets as they transit a router. The forwarding class plus the loss priority define the per-hop behavior. Four categories of forwarding classes are supported: best effort, assured forwarding, expedited forwarding, and network control. For Juniper Networks M Series Multiservice Edge Routers, four forwarding classes are supported; you can configure up to one each of the four types of forwarding classes. For M120 and M320 Multiservice Edge Routers, MX Series Ethernet Services Routers, and T Series Core Routers, 16 forwarding classes are supported, so you can classify packets more granularly. For example, you can configure multiple classes of expedited forwarding (EF) traffic: EF, EF1, and EF2.
• Loss priorities—Loss priorities allow you to set the priority of dropping a packet. Loss priority affects the scheduling of a packet without affecting the packet’s relative ordering. You can use the packet loss priority (PLP) bit as part of a congestion control strategy. You can use the loss priority setting to identify packets that have experienced congestion. Typically you mark packets exceeding some service level with a high loss priority. You set loss priority by configuring a classifier or a policer. The loss priority is used later in the work flow to select one of the drop profiles used by RED.
• Forwarding policy options—These options allow you to associate forwarding classes with next hops. Forwarding policy also allows you to create classification overrides, which assign forwarding classes to sets of prefixes.
• Transmission scheduling and rate control—These parameters provide you with a variety of tools to manage traffic flows:
  • Queuing—After a packet is sent to the outgoing interface on a router, it is queued for transmission on the physical media. The amount of time a packet is queued on the router is determined by the availability of the outgoing physical media as well as the amount of traffic using the interface.
  • Schedulers—An individual router interface has multiple queues assigned to store packets. The router determines which queue to service based on a particular method of scheduling. This process often involves a determination of which type of packet should be transmitted before another. Junos schedulers allow you to define the priority, bandwidth, delay buffer size, rate control status, and RED drop profiles to be applied to a particular queue for packet transmission.
  • Fabric schedulers—For M320 and T Series routers only, fabric schedulers allow you to identify a packet as high or low priority based on its forwarding class, and to associate schedulers with the fabric priorities.
  • Policers for traffic classes—Policers allow you to limit traffic of a certain class to a specified bandwidth and burst size. Packets exceeding the policer limits can be discarded, or can be assigned to a different forwarding class, a different loss priority, or both. You define policers with filters that can be associated with input or output interfaces.
• Rewrite rules—A rewrite rule sets the appropriate CoS bits in the outgoing packet. This allows the next downstream router to classify the packet into the appropriate service group. Rewriting, or marking, outbound packets is useful when the router is at the border of a network and must alter the CoS values to meet the policies of the targeted peer.