ZyXEL Communications P-660HN-51 User Manual page 189

Chapter 17 Quality of Service (QoS)
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.
DSCP (6 bits) Unused (2 bits)
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.
P-660HN-51 User's Guide
189