Dynalink RTA1025W 101, Quality of Service, First generation Precedence and type of service bits

Chapter 4: Web Configuration

Quality of Service

QoS (Quality of Service) is an industry-wide initiative to provide preferential treatment to certain subsets of data, enabling that data to traverse the Internet or intranet with higher quality transmission service.

There have been two generations of quality of service architectures in the Internet. The interpretation of the Type of Service Octet in the Internet Protocol header varies between these two generations.

The First generation: Precedence and type of service bits

The refined definition of the initial Type of Service Octet looks like this:

2^7	2^6	2^5	2^4	2^3	2^2	2^1	2^0
	Precedence			Type of Service Field

The Second generation: Differentiated services code point

The Differentiated Service Code Point is a selector for router's per-hop behaviors (PHB). As a selector, there is no implication that a numerically greater DSCP implies a better network service. RFC2474 redefined the Type of Service Octet to be:

2^7	2^6	2^5	2^4	2^3	2^2	2^1	2^0
	Differentiated Services Code Point					ECT	CE

The fields ECT and CE are nothing to do with quality of service. They are spare bits in the IP header used by Explicit Congestion Notification. As can be seen, the DSCP totally overlaps the old Precedence field. So if values of DSCP are carefully chosen then backward compatibility can be achieved. This leads to the notions of "class", each class being the group of DSCP with the same Precedence value. Values within a class would offer similar network services but with slight differences. Classes were initially defined as:

DSCP	Precedence	Purpose
0	0	Best effort
8	1	Class 1
16	2	Class 2
24	3	Class 3
32	4	Class 4
40	5	Express forwarding
48	6	Control
56	7	Control

Now, DSCP is what we are using for the QoS configuration on this device.

Among the classes you will see on the webpage, the BE (Best Effort) class possesses no guaranteed rates; the CS (Class Selector) values enable backward compatibility with the older IP-Precedence scheme ranges 0~7; the EF (Expedited Forwarding) class is a low-loss, low-latency, low-jitter, assured-bandwidth, end-to-end service; AF (Assured Forwarding) provides for the delivery of IP packets in four independently forwarded AF classes, AF1x through AF4x. Within each AF class, an IP packet can be assigned one of three different levels of drop precedence. This class is used when a service (application) requires a high probability of packets being forwarded, so long as the aggregate traffic from each site does not exceed the subscribed information rate (profile). Each of the four AF classes allocates a certain amount of forwarding resources, such as buffer space and bandwidth in each network node. When congestion occurs, the drop precedence of a packet determines the relative importance of the packet within the AF class.

You can start to configure the Bridge QoS/IP QoS rules on the Quality of Service webpage for your router.

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