notes: QoS Mechanism

 

Quality of service (QoS) mechanisms are used to implement a coordinated QoS policy in
devices throughout the network. The moment an IP packet enters the network, it is classified and usually marked with its class identification. From that point on, the packet is treated by a variety of QoS mechanisms according to the packet classification. Depending upon the mechanisms it encounters, the packet could be expedited, delayed, compressed, fragmented, or even dropped. This lesson describes mechanisms for implementing QoS.

The main categories of tools used to implement QoS in a network are as follows:

1.  Classification and marking: The identifying and splitting of traffic into different classes
and the marking of traffic according to behavior and business policies.

2.  Congestion management: The prioritization, protection, and isolation of traffic based on
markings.

3.  Congestion avoidance: Discards specific packets based on markings to avoid network
congestion.

4.  Policing and shaping: Traffic conditioning mechanisms that police traffic by dropping
misbehaving traffic to maintain network integrity. These mechanisms also shape traffic to
control bursts by queuing traffic.

4.  Link efficiency: One type of link efficiency technology is packet header compression,
which improves the bandwidth efficiency of a link. Another technology is link fragmentation and interleaving (LFI), which can decrease the “jitter” of voice transmission
by reducing voice packet delay.

 

In a QoS-enabled network, classification is performed on every input interface.

Marking should be performed as close to the network edge as possible—in the originating
network device, if possible. Devices farther from the edge of the network, such as routers and switches, can be configured to trust or untrust the markings made by devices on the edge of the network. An IP Phone, for example, will not trust the markings of an attached PC, while a switch will generally be configured to trust the markings of an attached IP Phone.
It only makes sense to use congestion management, congestion avoidance, and traffic-shaping mechanisms on output interfaces, because these mechanisms help maintain smooth operation of links by controlling how much and which type of traffic is allowed on a link. On some router and switch platforms, congestion management mechanisms, such as weighted round robin (WRR) and modified deficit round robin (MDRR), can be applied on the input interface.
Congestion avoidance is typically employed on an output interface wherever there is a chance that a high-speed link or aggregation of links feeds into a slower link (such as a LAN feeding into a WAN).
Policing and shaping are typically employed on output interfaces to control the flow of traffic from a high-speed link to lower-speed links. Policing is also employed on input interfaces to control the flow into a network device from a high-speed link by dropping excess low-priority packets.
Both compression and LFI are typically used on slower-speed WAN links between sites to improve bandwidth efficiency.