ZyXEL Communications P-660HW-DX, User Manual

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Chapter 13 Bandwidth Management
P-660HW-Dx User’s Guide
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13.10 DiffServ
DiffServ is a class of service (CoS) model that marks packets so that they receive specific per-
hop treatment at DiffServ-compliant network devices along the route based on the application
types and traffic flow. Packets are marked with DiffServ Code Points (DSCPs) indicating the
level of service desired. This allows the intermediary DiffServ-compliant network devices to
handle the packets differently depending on the code points without the need to negotiate paths
or remember state information for every flow. In addition, applications do not have to request
a particular service or give advanced notice of where the traffic is going.
13.10.1 DSCP and Per-Hop Behavior
DiffServ defines a new DS (Differentiated Services) field to replace the Type of Service (TOS)
field in the IP header. The DS field contains a 2-bit unused field and a 6-bit DSCP field which
can define up to 64 service levels. The following figure illustrates the DS field.
DSCP is backward compatible with the three precedence bits in the ToS octet so that non-
DiffServ compliant, ToS-enabled network device will not conflict with the DSCP mapping.
Figure 107 DiffServ: Differentiated Service Field
The DSCP value determines the forwarding behavior, the PHB (Per-Hop Behavior), that each
packet gets across the DiffServ network. Based on the marking rule, different kinds of traffic
can be marked for different priorities of forwarding. Resources can then be allocated
according to the DSCP values and the configured policies.
PHB consists of two types of services: EF (Expedited Forwarding) and AF (Assured
Forwarding). EF has higher priority. EF guarantees services with minimal loss and delay. AF
has four sub-classes, each with three levels of importance (drop precedence). A high drop
precedence means low importance.
13.10.2 Rule Configuration
Click the Edit icon or select User Defined from the Service drop-down list in the Rule Setup
screen to configure a bandwidth management rule. Use bandwidth rules to allocate specific
amounts of bandwidth capacity (bandwidth budgets) to specific applications and/or subnets.
DSCP
(6-bit) Unused
(2-bit)
Table 77 Sub-Classes of AF Services
DIFFSERV PRIORITY
LOW DROP
PRECEDENCE
MEDIUM DROP
PRECEDENCE
HIGH DROP
PRECEDENCE
SUB-CLASS4 AF41 AF42 AF43
SUB-CLASS3 AF31 AF32 AF33
SUB-CLASS2 AF21 AF22 AF23
SUB-CLASS1 AF11 AF12 AF13