D-Link 800 - DFL 800 - Security Appliance, User Manual

Page: 384 / 469

default precedence of 0.
Eight precedences exist, numbered from 0 to 7. Precedence 0 is the least important and 7 is the most
important. A precedence can be viewed as a separate traffic queue; traffic in precedence 2 will be
forwarded before traffic in precedence 0, precedence 4 forwarded before 2.
The meaning of a precedence comes from the fact that it is either higher or lower than another
precedence. If, for example, two precedences are used in a scenario, choosing 4 and 6 instead of 0
and 3 will makes no difference.
Figure 10.3. The Eight Pipe Precedences
Allocating Precedence
The way precedence is assigned to a packet is decided by the Pipe Rule that controls it and is done
in one of three ways:
• Use the precedence of the first pipe - Each pipe has a default precedence and packets take the
default precedence of the first pipe they pass through.
• Use the allocated precedence - The Pipe Rule explicitly allocates a precedence.
• Use the DSCP bits - Take the precedence from the DSCP bits in the packet. DSCP is a subset of
the Diffserv architecture where the Type of Service (ToS) bits are included in the IP packet
header.
Pipe Precedences
When a pipe is configured, a Default Precedence, a Minimum Precedence and a Maximum
Precedence can be specified. The Default Precedence is the precedence taken by a packet if it is not
explicitly assigned by a Pipe Rule as described in the preceding paragraph.
The minimum and maximum precedences define the precedence range that the pipe will handle. If a
packet arrives with an already allocated precedence below the minimum then its precedence is
changed to the minimum. Similarly, if a packet arrives with an already allocated precedence above
the maximum, its precedence is changed to the maximum.
For each pipe, separate bandwidth limits may be optionally specified for each precedence level.
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). If 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
10.1.6. Precedences Chapter 10. Traffic Management
384