Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring QoS (Rev. 03)
Overview
© 2008 Foundry Networks, Inc.
Page 9 of 62
Hybrid Queuing
This scheduling method combines Strict Priority and WRR scheduling. One or two queues are
serviced with strict priority whereas the rest of the queues are serviced in accordance with the
WRR algorithm.
Benefits of Hybrid Queuing
Hybrid Queuing guarantees immediate delivery of packets from high-ranking queues while
avoiding “starving” of lowest-ranking queues.
Differentiated Services
Differentiated Services (DiffServ) is a multiple service model that can satisfy differing QoS
requirements. However, unlike in the integrated service model, an application using DiffServ does
not explicitly signal the router before sending data.
For differentiated services, the network tries to deliver a particular kind of service based on the
QoS specified by each packet. This specification can occur in different ways, for example, using
the IP Precedence bit or the 6-bit Differentiated Services Code Point (DSCP) setting in IP packets,
or source and destination addresses. The network uses the QoS specification to classify, mark,
shape, and police traffic, and to perform intelligent queuing.
The differentiated services model is used for several mission-critical applications and for providing
end-to-end QoS. Typically, this service model is appropriate for aggregate flows because it
performs a relatively coarse level of traffic classification.
DiffServ Field Definition
A replacement header field, called the DS field, is defined by Differentiated Services. The DS field
supersedes the existing definitions of the IPv4 Type of Service (ToS) octet (RFC 791) and the IPv6
traffic class octet. Six bits of the DS field are used as the DSCP to select the Per Hop Behavior
(PHB) at each interface. A Currently Unused (CU) 2-bit field is reserved for explicit congestion
notification (ECN). The value of the CU bits is ignored by DS-compliant interfaces when
determining the PHB to apply to a received packet.
Figure 6
shows the location of the ToS octet within the IPv4 packet header.