QoS implementation for DIFFSERV
QOS
7-3
iMG/RG Software Reference Manual (Quality of Service)
Without the packet classifier, every packet arriving on any interface would be treated with the same priority.
The same profile can be added to more than one transport, so that the same set of rules are used by multiple
transports.
Each rule can test any of the following fields in the packet:
•
IP header DIFFSERV Codepoint (DSCP).
•
Source and/or destination IP address.
•
IP protocol (incorporating TCP/UDP/ICMP/GRE protocols).
•
For TCP and UDP packets, the source and/or destination port number.
This means that the Classifier device can be used in two ways (see
RFC 2475)
:
•
Multi-field (MF) Classifier
•
Behavior Aggregate (BA) Classifier
When used as a
Multi-Field (MF)
classifier, the Classifier device examines a combination of fields in the IP header
and payload, and, if configured to do so, may also act as a Marker to set the DS field of the IP header.
When used as a
Behaviour Aggregate (BA)
classifier, the Classifier device examines the DSCP value written into
the DS field of the IP header by the Ingress node of the DiffServ network.
7.1.3.2 Classifying packets
An incoming packet is tested against each rule in order of configuration. If all of the tests on the packet succeed,
the rule is ‘matched’ and the packet is assigned the traffic class, or Quality of Service Class (QoSC) associated
with that rule. The QoSC value in the packet comprises three fields and Classifier rules can be configured to set
these fields independently. The three fields that can be set for each packet are:
•
Scheduling Priority
. This value is used by the Scheduler, the Monitored Pools, the Priority Queue, to identify
different priority streams and to provide appropriate scheduling behavior. The Classifier device sets config-
ured scheduling priority for a traffic stream matching a classification rule. Multiple traffic streams may map to
the same scheduling priority. At present, 8 levels (values 0 to 7) of scheduling priority are defined. The value
7 is the highest priority and 0 is the lowest priority.
•
Meter Id.
A number of meter instances can be configured on a Meter device channel. Each meter instance
measures the temporal properties of a single traffic stream and is identified by a unique meter-id (number)
on a channel. The Classifier device sets the configured meter-id for a traffic stream matching a classification
rule. Note that there is no correlation between scheduling priority and meter-id. Different traffic streams
may be configured with different meter-ids but still have the same scheduling priority.
•
Drop Priority.
The drop priority can take three values: 0, 1 or 2. The values are described below:
•
0 lowest drop priority (green),
•
1 medium drop priority (yellow)
•
2 highest drop priority (red).
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