ZyXEL P-2812HNU-51c User Manual download pdf (Page 325)

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Chapter 19 Quality of Service (QoS)

P-2812HNU-51c User’s Guide

325

DiffServ

QoS is used to prioritize source-to-destination traffic flows. All packets in the flow

are given the same priority. You can use CoS (class of service) to give different

priorities to different packet types.

DiffServ (Differentiated Services) 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.

DSCP and Per-Hop Behavior

DiffServ defines a new Differentiated Services (DS) 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.

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 kinds of forwarding. Resources

can then be allocated according to the DSCP values and the configured policies.

IP Precedence

Similar to IEEE 802.1p prioritization at layer-2, you can use IP precedence to

prioritize packets in a layer-3 network. IP precedence uses three bits of the eight-

bit ToS (Type of Service) field in the IP header. There are eight classes of services

(ranging from zero to seven) in IP precedence. Zero is the lowest priority level and

seven is the highest.

DSCP (6 bits) Unused (2 bits)

1 2 ... 320 321 322 323 324 325 326 327 328 329 330 ... 521 522

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ZyXEL P-2812HNU-51c User Manual Page 325

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