ZyXEL SBG3300-NB00 User Guide - Page 219
DSCP and Per-Hop Behavior, IP Precedence, Automatic Priority Queue Assignment
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Chapter 10 Quality of Service (QoS) 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. Automatic Priority Queue Assignment If you enable QoS on the Device, the Device can automatically base on the IEEE 802.1p priority level, IP precedence and/or packet length to assign priority to traffic which does not match a class. The following table shows you the internal layer-2 and layer-3 QoS mapping on the Device. On the Device, traffic assigned to higher priority queues gets through faster while traffic in lower index queues is dropped if the network is congested. Table 52 Internal Layer2 and Layer3 QoS Mapping LAYER 2 LAYER 3 PRIORITY QUEUE 0 1 2 3 IEEE 802.1P USER PRIORITY (ETHERNET PRIORITY) 1 2 0 3 TOS (IP PRECEDENCE) 0 0 1 DSCP 000000 000000 001110 001100 001010 001000 IP PACKET LENGTH (BYTE) >1100 250~1100 SBG3300-N Series User's Guide 219