The Junos OS CoS Components Used to Manage Congestion and Control Service Levels

DiffServ specifications establish a six-bit field in the IPv4 and IPv6 packet header to indicate the service class that should be applied to the packet. The bit values in the DiffServ field form DiffServ code points (DSCPs) that can be set by the application or a router on the edge of a DiffServ-enabled network.

Although CoS methods such as DiffServ specify the position and length of the DSCP in the packet header, the implementation of the router mechanisms to deliver DiffServ internally is vendor-specific. CoS functions in Junos OS are configured through a series of mechanisms that you can configure individually or in combination to define particular service offerings.

Figure 1 shows the components of the Junos CoS features, illustrating the sequence in which they interact.

Figure 1: Packet Flow Through CoS-Configurable Components

You can configure one or more of the following Junos CoS mechanisms:

• Classifiers—Packet classification refers to the examination of an incoming packet. This function associates the packet with a particular CoS servicing level. In Junos, 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 classifiers—A behavior aggregate (BA) is a method of classification that operates on a packet as it enters the routing device. 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.

    (You can also configure code-point aliases which assign 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.)

    See Understanding How Behavior Aggregate Classifiers Prioritize Trusted Traffic for more information on BA classifiers.

  • 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. Multifield classification is usually done at the edge of the network for packets that do not have valid or trusted behavior aggregate code points.

    See Overview of Assigning Service Levels to Packets Based on Multiple Packet Header Fields for more information on multifield classifiers.

• Forwarding classes—The forwarding classes affect the forwarding, scheduling, and marking policies applied to packets as they transit a routing device. Known as ordered aggregates in the DiffServ architecture, the forwarding class plus the loss priority determine the router’s per-hop behavior (PHB in DiffServ) for CoS. Four categories of forwarding classes are supported: best effort, assured forwarding, expedited forwarding, and network control. Up to 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.

  See Understanding How Forwarding Classes Assign Classes to Output Queues for more information on forwarding classes.

• 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 workflow to select one of the drop profiles used by RED.

  See Managing Congestion by Setting Packet Loss Priority for Different Traffic Flows for more information on packet loss priorities.

• Forwarding policy options—These options allow you to associate forwarding classes with next hops. Forwarding policy options also allow you to create classification overrides, which assign forwarding classes to sets of prefixes.

  See Forwarding Policy Options Overview for more information on forwarding policy options.

• 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 routing device, it is queued for transmission on the physical media. The amount of time a packet is queued on the routing device is determined by the availability of the outgoing physical media as well as the amount of traffic using the interface.

  • Schedulers—An individual routing device interface has multiple queues assigned to store packets. The routing device 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. The Junos OS 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.

    See How Schedulers Define Output Queue Properties for more information on schedulers.

  • 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 (hard policing), or can be assigned to a different forwarding class, a different loss priority, or both (soft policing). You define policers with filters that can be associated with input or output interfaces.

    See Controlling Network Access Using Traffic Policing Overview for more information on policers.

• Rewrite rules—A rewrite rule sets the appropriate CoS bits in the outgoing packet. This allows the next downstream routing device to classify the packet into the appropriate service group. Rewriting, or marking, outbound packets is useful when the routing device is at the border of a network and must alter the CoS values to meet the policies of the targeted peer.

  Typically, rewrites of the DSCPs on outgoing packets are done once, when packets enter the DiffServ portion of the network, either because the packets do not arrive from the customer with the proper DSCP bit set or because the service provider wants to verify that the customer has set the DSCP properly. CoS schemes that accept the DSCP and classify and schedule traffic solely on DSCP value perform behavior aggregate (BA) DiffServ functions and do not usually rewrite the DSCP. DSCP rewrites typically occur in multifield (MF) DiffServ scenarios.

  See Rewriting Packet Headers to Ensure Forwarding Behavior for more information on rewrite rules.