Precedences And Guarantees - D-Link DFL-1600 User Manual

250 Chapter 23. Traffic Shaping
simplistic, in that they do not know much about the types of traffic that
pass through them, and they know nothing about the direction either. A
pipe simply measures the amount of traffic that passes through it and
applies the configured limits in each precedence and/or user group. The
task of traffic filtering, categorizing, and prioritizing is done by Pipe Rules
covered in the next section.
D-Link firewalls are capable of handling hundreds of pipes simultaneously,
but in reality, only a handful of pipes are required for most setups. The
only occasion that uses dozens of pipes is the scenario where an individual
pipe is created for each service (protocol, or client in ISP cases).
23.2.1

Precedences and Guarantees

Depending on particular applications or manual configurations, traffics can
be treated as having different levels of importance.
In an IP version 4 packet, there is a 1-byte field called Type-of-Service(ToS)
in the header (shown in Table 23.1). This ToS field is used in Diff-Serv
approach to provide QoS by differentiating classes of service into different
priorities to support various network applications. The six left-most bits of
this field is called Differentiated Services Code Point(DSCP) and the last
two bits were not defined within the Diff-Serv model. The Diff-Serv
standard utilizes the higher 3 bits of DSCP for application priority setting,
which is organized into 8 precedence levels from 0 to 7; and the lower 3 bits
are used to offer finer granularity for precedence definitions. The priority of
an application increases with 0 the lowest and 7 the highest. The values 6
and 7 are reserved for network control packets, so the values through 0-5
can be set for priority based on IP networks or applications.
Corresponding to these 8 levels, a pipe in a D-Link firewall contains 4
precedences – Low, Medium, High, and Highest – for clarifying the
relative importance of the traffic. Each of these precedences maps to 2
levels in DSCP definition, for example, "Low" stands for level 0 and 1.
Traffic in precedence "Medium" will be passed on before traffic in
precedence "Low", traffic in precedence "High" before "Medium" and
"Low", and so on. The precedence assignment is controlled by the Pipe
Rules. In order to determine what precedence the traffic belongs to, each
packet buffer is assigned a precedence number before it is sent into a pipe.
The actual limiting of bandwidth is performed inside each precedence;
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