ZyXEL Communications 2802HWL-IX Manual Do Utilizador

 

All contents copyright (c) 2007 ZyXEL Communications Corporation.   

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Diffserv is a class of service (CoS) model that marks packets so that they receive specific per-hop treatment at 

DiffServ-compliant network device along the route based on the application types and traffic flow. Packets are 

marked with DSCP 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. 

 

Diffserv defines a new DS(Differentiated Services) field to replace the TOS(Type of Service) field in IP header. 

The DS field contains a 2-bit unused field and a 6-bit DSCP filed which can define up to 64 service levels. The 

following figure illustrates the DS field. 

 

    

 

 

DiffServ: 

Differentiated 

Service 

Field 

Diffserv Code Point 

6-bit 

Unused 

2-bit 

 

 

The DSCP value determines the forwarding behavior, the PHB(Per-Hop Behavior), that each packet gets across the 

Diffserv network. The PHB value is define in the RFC2597 and introduced where classes are developed such as 

Business, Telecommuter, Residential etc. that can be offered by an ISP as different levels of service. The following 

is the table illustrates the DSCP values: 

 

 

(PHB) 

  

  

(DSCP) 

  

Precedence 

Default 

  

  

  

  

0 

  

  

000000 

  

  

  

Assured Forwarding    

Probability 

Probability 

Probability 

  

  

Class 

AF11 

AF12 

AF13 

1