QoS - Classification and Marking

9 important questions on QoS - Classification and Marking

Which 7 traffic descriptors are typically used for classification?

1. Internal (Locally significant to a router)
2. Layer 1 (Physical interface, subinterface, port)
3. Layer 2 (MAC address and 802.1Q/p Class of Service bits.
4. Layer 2.5 (MPLS Experimental (EXP) bits)
5. Layer 3 (Differentiated Service Code Points DSCP, IP Precedence IPP, and source/destination IP address
6. Layer 4 (TCP or UDP Ports)
7. Layer 7 (Next Generation Network-Based Application Recognition (NBAR2)

What is Next Generation Network-Bases Application Recognition, or NBAR2?

NBAR2 is a deep packet inspection engine that uses layer 3 to layer 7 information to classify application traffic.
It can recognize more than 1000 application and gets monthly protocol packs for new or changed applications.

What are the 2 modes of operation of NBAR2?

1. Protocol Discovery
Discovers and gets real-time statistics from application running on the network, which then can be used in defining QoS classes and policies using MQC configuration.

2. Modular QoS CLI (MQC)
Using MQC network traffic that matches specific network protocols can be assigned to a traffic class.
After which different QoS policies can be assigned per traffic class.
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Which Traffic Descriptors are used for Traffic Marking?

1. Internal (QoS Groups)
2. Layer 2 (802.1Q/p)
3 Layer 2.5 (MPLS EXP bits)
4 Layer 3 (DCSP and IPP)

Layer 2 and Layer 3 are the Traffic Descriptors used most often in Enterprise Networks.

What is the PCP or Priority Code Point field and what is it used for?

It is defined in the 802.1p standard and is a 3-bit field within the 802.1Q header.
It is used to mark packets as belonging to a specific CoS (Class of Service) and can use 8 different priority levels.
Level 0 is the lowest for background traffic and level 7 is the highest for Network Control

What is a drawback of using the PCP CoS markings?

Network frames lose their CoS marking when traversing a non-802.1Q link or Layer 3 network.
This can be overcome by mapping CoS markings to another marking.
For example CoS priority levels correspond directly to IPv4 IP Precedence ToS values so they can be mapped directly to each other.

What are the 2 options to mark Layer 3 packets?

1. ToS field, which is an 8-bit field where the first 3 bits are used for marking. Referred to as IP precedence or IPP. It has 6 usable classes of service 0 thru 5. Class 6 and 7 are used for internet network.

2. Differentiated Services Field, which is the same 8-bit field as the ToS field, however it has been redefined by newer standards.
Backwards compatible with ToS
It is composed of a 6 bit Differentiated Service Code Point (DSCP) fiels that allows classification of up to 64 classes. And a 2 bit Explicit Congestion Notification (ECN) field.

What is a DiffServ Behaviour Aggregate?

It is a collection of packets with the same DiffServ value crossing a link in a particular direction.
Routers can make Per-Hop Behaviour (PHB) decisions, which are based on DiffServ BAs

Which 4 PHBs have been defined and characterized for general use?

1. Class Selector PHB: First 3-bits of the DSCP fiels are used as CS bits. Makes DSCP backward compatible with IP Precedence.

2. Default Forwarding PHB: Used for best effort service

3. Assured Forwarding PHB: Used for guaranteed bandwidth service.

4. Expedited Forwarding PHB: Used for low-delay service

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