CoS Features in the Junos OS

For interfaces that carry IP version 4 (IPv4), IP version 6 (IPv6), or MPLS traffic, the Junos OS includes class-of-service (CoS) features that provide multiple classes of service for different applications. You can configure multiple forwarding classes for transmitting packets, defining which packets are placed into each output queue, scheduling the transmission service level for each queue, and managing congestion using a Random Early Detection (RED) algorithm.

The CoS features in the Junos OS include the following:

• Classifiers—Assign incoming packets to a forwarding class and loss priority, and direct packets to output queues based on the forwarding class. Two general types of classifiers are supported:

  • Behavior aggregate (BA) or code point traffic classifiers—Determine each packet’s forwarding class and loss priority. BA classifiers allow setting of the forwarding class and loss priority of a packet based on Differentiated Services (DiffServ) code point (DSCP) bits, IP precedence bits, MPLS EXP bits, and IEEE 802.1p bits. The default classifier is based on IP precedence bits.
  • Multifield traffic classifiers (also referred to as “MF traffic classifiers”)—Set a packet’s forwarding class and loss priority based on packet filter rules.
• DiffServ—Implemented as six bits of the type-of-service (ToS) byte in the IP header. The Junos OS uses DSCPs in the IP type of service (ToS) field to determine the forwarding class associated with each packet.
• Forwarding classes—Determine the forwarding, scheduling, and marking policies applied to packets as they transit the router. Four forwarding classes are supported: best effort, assured forwarding, expedited forwarding, and network control. Together with loss priority, the forwarding class defines the per-hop behavior.
• Forwarding policy options—Associate forwarding classes with next hops. Also enable creation of classification overrides, which assign forwarding classes to sets of prefixes.
• Layer 2 to Layer 3 CoS mapping—Set a Layer 3 packet’s forwarding class and loss priority value based on information in the Layer 2 packet header. Output involves mapping the forwarding class and loss priority value to a Layer 2-specific marking. You can configure the Junos OS to mark the Layer 2 and Layer 3 headers simultaneously.
• Loss priorities—Set a packet’s priority to be discarded. Typically, packets exceeding some service level are marked with a high loss priority. Loss priority affects the scheduling of a packet. Loss priority is set by configuring a classifier or a policer.
• MPLS EXP—Map MPLS EXP bit settings to forwarding classes and vice versa.
• Oversubscription of interface bandwidth (Gigabit Ethernet IQ and Channelized IQ PICs)—Configures shaping rates so that their sum exceeds the physical Ethernet bandwidth.
• Rewrite markers—Change the code-point value of outgoing packets. Rewriting, or marking, outbound packets is useful when the router is at the border of a network and must alter the code points to meet the policies of the targeted peer.
• Simple filters for metropolitan Ethernet applications (4-port and 8-port Gigabit Ethernet IQ2 PICs only)—Classify IPv4 traffic based on noncomplex filters.

• Transmission scheduling and rate control—Provides a variety of tools to manage traffic flows:

  • Fabric schedulers (T Series routers only)—Identify a packet as high or low priority based on its forwarding class.
  • Policers—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, to a different loss priority, or to both. Policers are defined with filters that can be associated with either input or output interfaces.
  • Schedulers—Define the priority, bandwidth, delay buffer size, rate control status, and RED drop profiles to be applied to a particular forwarding class for packet transmission.
• Two-rate tricolor marking—For T Series routers with Enhanced II Flexible PIC Concentrators (FPCs), configures traffic policing using two-rate tricolor marking (trTCM), which provides three levels of drop precedence (also called packet loss priority [PLP]). Two-rate TCM is a “color-aware” method of traffic policing—high, medium, and low loss priorities are mapped to the colors red, yellow, and green. The color of a packet, which is used or set by the TCM policer, corresponds to the packet’s loss priority. trTCM is defined in RFC 2698, A Two Rate Three Color Marker.
• Virtual channels and virtual channel groups (J Series Services Routers and SRX Series Services Gateways)—Direct traffic into a virtual channel and apply bandwidth limits to the channel.
• VPN outer label marking—Set outer label EXP bits based on MPLS EXP mapping.

The Junos OS supports CoS features on all interface types except the following:

• cau4—Channelized STM1 IQ interface (configured on the Channelized STM1 IQ PIC)
• ce1—Channelized E1 IQ interface (configured on the Channelized E1 IQ PIC or Channelized STM1 IQ PIC)
• coc1—Channelized OC1 IQ interface (configured on the Channelized OC12 IQ PIC)
• coc12—Channelized OC12 IQ interface (configured on the Channelized OC12 IQ PIC)
• cstm-1—Channelized STM1 IQ interface (configured on the Channelized STM1 IQ PIC)
• ct1—Channelized T1 IQ interface (configured on the Channelized DS3 IQ PIC or Channelized OC12 IQ PIC)
• ct3—Channelized T3 IQ interface (configured on the Channelized DS3 IQ PIC or Channelized OC12 IQ PIC)
• dsc—Discard interface
• fxp—Management and internal Ethernet interfaces
• lo—Loopback interface, which is internally generated
• pd—Interface that de-encapsulates packets on a Platform Independent Multicast (PIM) rendezvous point (RP) router
• pe—Interface on a first-hop RP that encapsulates packets destined for the RP router
• vt—Virtual loopback tunnel interface

Related Topics

• Supported CoS Standards