A standard IP-based network provides best effort data delivery service. Best effort service implies that the network delivers the data in a timely fashion, although there is no guarantee that it will. During times of congestion, packets can be delayed, sent sporadically, or dropped. For typical Internet applications, such as email and file transfers, a slight degradation in service is acceptable and in many cases unnoticeable. However, any degradation of service has undesirable effects on applications with strict timing requirements, such as voice and multimedia applications.
Quality of Service (QoS) is a technology that can provide consistent, predictable data delivery by distinguishing between packets that have strict timing requirements and those that are more tolerant of delay. Packets with strict timing requirements receive special treatment in a QoS-capable network. However, all components of the network must be QoS-capable. If one node is unable to meet the necessary timing requirements, a deficiency occurs in the network path, and the performance of the entire packet flow is compromised.
Two basic types of QoS exist:
- Integrated Services. Network resources are apportioned based on request and are reserved according to a network management policy, for example, RSVP.
- Differentiated Services. Network resources are apportioned based on traffic classification and priority, giving preferential treatment to data with strict timing requirements.
Smart switches support only Differentiated Services (DiffServ), which lets you create a differentiated service network with the following three QoS components:
- Class. Classify the incoming packets at layer 2, 3, and 4 by inspecting the following information for a packet:
- Source and destination MAC address
- EtherType
- VLAN ID
- Class of service (802.1p priority) value (first/only VLAN tag)
- IP service type octet (also known as: ToS bits, precedence value, DSCP value)
- IPv4 or IPv6 source and destination IP address
- Layer 4 protocol (such as TCP or UDP)
- Layer 4 source and destination ports
- Policy. Associate a collection of classes that you configure with one or more QoS policy statements. The result of this association is referred to as a policy. You must manually configure the various statements and rules to achieve the desired operation for the specific traffic. You can configure the following basic actions in the policy:
- Drop traffic
- Mark traffic for IP DSCP or IP preference
- Mark traffic for CoS(802.1p)
- Assign a queue
- Redirect traffic
- Apply a simple QoS policy
- Service. Assign a policy to a directional interface.
For information about configuring QoS on a smart switch, see the “How do I configure Quality of Service on a smart switch