Until recently, quality of service was not an issue in the enterprise campus because of the asynchronous nature of data traffic and the ability of network devices to tolerate buffer overflow and packet loss. However, with new applications such as voice and video, which are sensitive to packet loss and delay, buffers and not bandwidth are the key QoS issue in the enterprise campus. Figure 2 illustrates the typical oversubscription that occurs in LAN infrastructures.
This oversubscription, coupled with individual traffic volumes and the cumulative effects of multiple independent traffic sources, can result in the egress interface buffers becoming full instantaneously, thus causing additional packets to drop when they attempt to enter the egress buffer. The fact that campus switches use hardware-based buffers, which compared to the interface speed are much smaller than those found on WAN interfaces in routers, merely increases the potential for even short-lived traffic bursts to cause buffer overflow and dropped packets. Applications such as file sharing (both peer-to-peer and server-based), remote networked storage, network-based backup software, and emails with large attachments, can create conditions where network congestion occurs more frequently and/or for longer durations. Some of the negative effects of recent worm attacks have been an overwhelming volume of network traffic (both unicast and broadcast-storm based), increasing network congestion.
If no buffer management policy is in place, loss, delay, and jitter performance of the LAN may be affected for all traffic. Another situation to consider is the effect of failures of redundant network elements, which cause topology changes. For example, if a distribution switch fails, all traffic flows will be reestablished through the remaining distribution switch. Prior to the failure, the load balancing design shared the load between two switches, but after the failure all flows are concentrated in a single switch, potentially causing egress buffer conditions that normally would not be present. For applications such as voice, this packet loss and delay results in severe voice quality degradation. Therefore, QoS tools are required to manage these buffers and to minimize packet loss, delay, and delay variation (jitter).
The following types of QoS tools are needed from end to end on the network to manage traffic and ensure voice quality:
- Traffic classification
Classification involves the marking of packets with a specific priority denoting a requirement for class of service (CoS) from the network. The point at which these packet markings are trusted or not trusted is considered the trust boundary. Trust is typically extended to voice devices (phones) and not to data devices (PCs).
- Queuing or scheduling
Interface queuing or scheduling involves assigning packets to one of several queues based on classification for expedited treatment throughout the network.
- Bandwidth provisioning
Provisioning involves accurately calculating the required bandwidth for all applications plus element overhead.
The following sections discuss the use of these QoS mechanisms in a campus environment:
- Traffic Classification
- Interface Queuing
- Bandwidth Provisioning
- Impairments to IP Communications if QoS is Not Employed