ZyXEL Communications GS2220 Series - Differentiated Services; Diffe Re Ntia te D Se Rvic E S; Diffserv Overview; What You Can Do

Chapter 43 Differentiated Services
GS2220 Series User’s Guide
366
C HAPTER 43
Diffe re ntia te d Se rvic e s
43.1 DiffSe rv O ve rvie w
This chapter shows you how to configure Differentiated Services (DiffServ) on the Switch.
Quality of Service (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 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.
43.1.1 Wha t You C a n Do
• Use the DiffSe rv screen (
Section 43.2 on page 367) to activate DiffServ to apply marking rules or IEEE
802.1p priority mapping on the Switch.
• Use the DSC P Se tting screen (Section 43.3.1 on page 368) to change the DSCP-IEEE 802.1p mapping.
43.1.2 Wha t You Ne e d to Kno w
Read on for concepts on Differentiated Services that can help you configure the screens in this chapter.
DSCP a nd Pe r- Hop Be ha vio r
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 6-bit DSCP field which can define up to 64 service levels and the
remaining 2 bits are defined as currently unused (CU). The following figure illustrates the DS field.
Fig ure 277 DiffServ: Differentiated Service 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 PHB (Per-Hop Behavior), that each packet gets as it is forwarded across
the DiffServ network. Based on the marking rule different kinds of traffic can be marked for different
DSCP (6 bits) CU (2 bits)