Minimum And Maximum Pipe Precedence - D-Link NetDefend DFL-210 User Manual

10.1.6. Precedences
Chapter 10. Traffic Management
These limits can be specified in kilobits per second and/or packets per second (if both are specified
then the first limit reached will be the limit used). In precedences are used then the total limit for the
pipe as a whole must be specified so the pipe knows when what its capacity is and therefore when
precedences are used.
The Best Effort Precedence
The precedence defined as the minimum pipe precedence has a special meaning: it acts as the Best
Effort Precedence. All packets arriving at this precedence will always be processed on a "first come,
first forwarded" basis and cannot be sent to another precendence.
Packets with a higher precedence and that exceed the limits of that precedence will automatically be
transferred down into this Best Effort precedence and they will no longer be treated differently from
packets with lower priorities. This approach is used since a precedence limit is also a guarantees for
that precendence.
Figure 10.3. Minimum and Maximum Pipe Precedence.
Precedences have no effect until the total bandwidth allocated for a pipe is reached. In other words
when the pipe is "full". At that point traffic is prioritized by NetDefendOS with higher precedence
packets being sent before lower precedence packets. The lower precedence packets are buffered. If
buffer space becomes exhausted then they are dropped.
Applying Precedences
Continuing from the previous example, we add the requirement that SSH and Telnet traffic is to
have a higher priority than all other traffic. To do this we add a Pipe Rule specifically for SSH and
Telnet and set the priority in the rule to be a higher priority, say 2. We specify the same pipes in this
new rule as are used for other traffic.
The effect of doing this is that the SSH and Telnet rule sets the higher priority on packets related to
these services and these packets are sent through the same pipe as other traffic. The pipe then makes
sure that these higher priority packets are sent first when the total bandwidth limit specified in the
pipe's configuration is exceeded. Lower priority packets will be buffered and sent when higher
priority traffic uses less than the maximum specified for the pipe. The buffering process is
sometimes referred to as "throttling back" since it reduces the flow rate.
The Need for Guarantees
A problem can occur however if the prioritized traffic is a continous stream such as real-time audio,
resulting in continuous use all available bandwidth and resulting in unacceptably long queuing times
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