Differentiated Services Model
IPQoS includes the following modules, which are part of the Differentiated Services (Diffserv) architecture
that is defined in RFC 2475:
IPQoS adds the following enhancements to the Diffserv model:
802.1D datagram marker
This section introduces the Diffserv modules as they are used by IPQoS. You
need to know about these modules, their names, and their uses to
set up the QoS policy. For detailed information about each module, refer to
IPQoS Architecture and the Diffserv Model.
Classifier (ipgpc) Overview
In the Diffserv model, the classifier selects packets from a network traffic
flow. A traffic flow consists of a group of packets with identical information in
the following IP header fields:
In IPQoS, these fields are referred to as the 5-tuple.
The IPQoS classifier module is named ipgpc. The ipgpc classifier arranges traffic
flows into classes that are based on characteristics you configure in the IPQoS
For detailed information about ipgpc, refer to Classifier Module.
A class is a group of network flows that share similar characteristics. For
example, an ISP might define classes to represent the different service levels that
are offered to customers. An ASP might define SLAs that give different levels
of service to various applications. For an ASP's QoS policy, a class might
include outgoing FTP traffic that is bound for a particular destination IP address.
Outgoing traffic from a company's external web site might also be defined as
Grouping traffic into classes is a major part of planning your QoS
policy. When you create classes by using the ipqosconf utility, you are actually configuring
the ipgpc classifier.
For information on how to define classes, see How to Define the Classes for Your QoS Policy.
Filters are sets of rules that contain parameters called selectors. Each filter must
point to a class. IPQoS matches packets against the selectors of each filter
to determine if the packet belongs to the filter's class. You can filter
on a packet by using a variety of selectors, for example, the IPQoS
5-tuple and other common parameters:
Source address and destination addresses
Source port and destination port
Differentiated Services Codepoint (DSCP)
For example, a simple filter might include the destination port with the value
of 80. The ipgpc classifier then selects all packets that are bound for
destination port 80 (HTTP) and handles the packets as directed in the QoS
For information on creating filters, see How to Define Filters in the QoS Policy.
Meter (tokenmt and tswtclmt) Overview
In the Diffserv model, the meter tracks the transmission rate of traffic flows
on a per-class basis. The meter evaluates how much the actual rate
of the flow conforms to the configured rates to determine the appropriate outcome.
Based on the traffic flow's outcome, the meter selects a subsequent action. Subsequent
actions might include sending the packet to another action or returning the packet
to the network without further processing.
The IPQoS meters determine whether a network flow conforms to the transmission rate
that is defined for its class in the QoS policy. IPQoS includes two
Both metering modules recognize three outcomes: red, yellow, and green. You define the
actions to be taken for each outcome in the parameters red_action_name, yellow_action_name,
In addition, you can configure tokenmt to be color aware. A color-aware metering
instance uses the packet's size, DSCP, traffic rate, and configured parameters to determine
the outcome. The meter uses the DSCP to map the packet's outcome
to a green, yellow, or red.
For information on defining parameters for the IPQoS meters, refer to How to Plan Flow Control.
Marker (dscpmk and dlcosmk) Overview
In the Diffserv model, the marker marks a packet with a value that
reflects a forwarding behavior. Marking is the process of placing a value in
the packet's header to indicate how to forward the packet to the network.
IPQoS contains two marker modules:
dscpmk – Marks the DS field in an IP packet header with a numeric value that is called the Differentiated Services codepoint, or DSCP. A Diffserv-aware router can then use the DS codepoint to apply the appropriate forwarding behavior to the packet.
dlcosmk – Marks the virtual local area network (VLAN) tag of an Ethernet frame header with a numeric value that is called the user priority. The user priority indicates the class of service (CoS), which defines the appropriate forwarding behavior to be applied to the datagram.
dlcosmk is an IPQoS addition that is not part of the Diffserv model, as designed by the IETF.
For information on implementing a marker strategy for the QoS policy, see How to Plan Forwarding Behavior.
Flow Accounting (flowacct) Overview
IPQoS adds the flowacct accounting module to the Diffserv model. You can use
flowacct to gather statistics on traffic flows, and bill customers in agreement with
their SLAs. Flow accounting is also useful for capacity planning and system monitoring.
The flowacct module works with the acctadm command to create an accounting
log file. A basic log includes the IPQoS 5-tuple and two additional attributes,
as shown in the following list:
Number of packets
Number of bytes
You can also gather statistics on other attributes, as described in Recording Information About Traffic Flows, and
in the flowacct(7ipp) and acctadm(1M) man pages.
For information on planning a flow-accounting strategy, see How to Plan for Flow Accounting.
How Traffic Flows Through the IPQoS Modules
The next figure shows a path that incoming traffic might take through some
of the IPQoS modules.
Figure 32-1 Traffic Flow Through the IPQoS Implementation of the Diffserv Model
This figure illustrates a common traffic flow sequence on an IPQoS-enabled machine:
The classifier selects from the packet stream all packets that match the filtering criteria in the system's QoS policy.
The selected packets are then evaluated for the next action to be taken.
The classifier sends to the marker any traffic that does not require flow control.
Traffic to be flow-controlled is sent to the meter.
The meter enforces the configured rate. Then, the meter assigns a traffic conformance value to the flow-controlled packets.
The flow-controlled packets are then evaluated to determine if any packets require accounting.
The meter sends to the marker any traffic that does not require flow accounting.
The flow-accounting module gathers statistics on received packets. The module then sends the packets to the marker.
The marker assigns a DS codepoint to the packet header. This DSCP indicates the per-hop behavior that a Diffserv-aware system must apply to the packet.