Introductions to Class of Service(CoS) - Part 1

 What is Class of Service(CoS) and why do we need it ?

     Class of Service (CoS) refers to a mechanism used to prioritize and manage network traffic based on its importance or quality requirements. It allows network administrators to define different levels of service for various types of traffic, ensuring that critical applications receive the necessary bandwidth, latency, and reliability. By assigning appropriate priority and handling to different types of traffic, CoS helps maintain consistent and efficient network operation, especially in environments with diverse traffic demands and limited resources. IP routers normally will forward traffic as best effort(BE), which is to say independently without controlling the throughput or delay. normally, packets that arrive on a device are then placed in output queues in the order they arrive. CoS must be implemented consistently end to end throughout a network in order for it to function effectively.

 Cos features include traffic classifying, policing, queuing, scheduling, shaping, and marker rewriting.

• Classification: This step refers to examining incoming packets and associating them with a particular forwarding class. There are three ways JunOS devices classify traffic.
• Multi-Field Classifiers(MF): A MF Classifier is more complex and requires more configuration. The MF classifier uses a firewall filter(ACL) to match traffic criteria, such as port, protocol, and source addressing, then applies a forwarding class and loss priority(more on this later) to the packets.
  
• Behavioral Aggregate(BA) Classifiers: A BA Classifier is a little less complex and less flexible than MF classifiers. BA classifiers rely on markings placed in the headers of incoming packets, either IPv4, IPv6, or MPLS.
  
• Default IP Precedence Classifier: The Default IP Precedence Classifier is a classification mechanism used to assign IP precedence values to incoming packets based on their IP headers. IP precedence is a 3-bit field within the IP header that indicates the relative priority of the packet. The Default IP Precedence Classifier allows network administrators to apply a default classification to all unclassified IP traffic.
• Policing
• Traffic policing refers to the process of enforcing bandwidth limitations on network traffic. This involves monitoring the rate of incoming traffic and taking an action if it is exceeded.
  
• Queuing
• Traffic queuing refers to the process of organizing packets into different queues based on their assigned CoS classification and priority.
• Scheduling
• Scheduling determines how packets are prioritized and processed during periods of congestion, ensuring fair allocation of network resources.
• Shaping
• Shaping regulates the outgoing traffic rate, smoothing the flow and conforming it to predefined bandwidth limits.
• Marker Rewriting
• Marker rewriting involves modifying packet markings, such as Differentiated Services Code Point (DSCP) values, to ensure consistent and appropriate treatment throughout the network, allowing for effective CoS-based prioritization

 CoS processing on incoming packets

Classifiers and Policers perform the following operations on incoming packets:

1. The classifier examines incoming packets and assigns a forwarding class and loss priority to it.
2. Based on the forwarding class, the packets are assigned to an outbound transmission queue.
3. Input policers meter traffic to see if traffic flow exceeds its service level.
   

CoS processing on outgoing packets

The scheduler map and rewrite rules perofrm the following operations on outgoing packets:

1. The scheduler maps are applied to interfaces and associate the outgoing packets with a scheduler and a forwarding class(fowarding class = output queue). 
2. The scheduler defines how the packet is treated in the output transmission queue based on configured transmit rate, buffer size, priority, and drop profile. 
3. Output policers meter traffic and might change the forwarding class and loss priority of a packet if the traffic flow exceeds its service level. 
4. The rewrite rules writes information to the packet according to the forwarding class and loss priority of the packet.

 Re-call that packet classification involved examining the incoming packet and associating it with a forwarding class, which essentially is an output queue, and giving it a packet loss priority(PLP). The packet loss priority allows you to set a priority for dropping packets(low priority = low probability of being dropped and high priority = high probability of being dropped). 

Here is two ways to configure FC's:

set class-of-service forwarding-classes queue 0 best-effort
set class-of-service forwarding-classes queue 1 bronze-class
set class-of-service forwarding-classes queue 2 silver-class
set class-of-service forwarding-classes queue 3 network-class
set class-of-service forwarding-classes queue 4 gold-class
set class-of-service forwarding-classes queue 5 platinum-class
--or--
set class-of-service forwarding-classes class best-effort queue-num 0
set class-of-service forwarding-classes class bronze-class queue-num 1
set class-of-service forwarding-classes class silver-class queue-num 2
set class-of-service forwarding-classes class network-class queue-num 3
set class-of-service forwarding-classes class gold-class queue-num 4
set class-of-service forwarding-classes class platinum-class queue-num 5

 

part two coming soon..