ZyXEL Communications P-660HW-DX 13.10 DiffServ

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.

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.

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