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JUNOS CoS Components
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 software,
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 (MF) classifier is a second method for classifying traffic flows.
Unlike a behavior aggregate, an MF classifier can examine multiple
fields in the packet. Examples of some fields that an MF 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 MF 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 routing platform. 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 M-series routing platforms,
four forwarding classes are supported; you can configure up to one
each of the four types of forwarding classes. For the M120, M320,
MX-series, and T-series platforms, 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 platforms
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 routing platform is at the border of a network and must alter
the CoS values to meet the policies of the targeted peer.
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