ZyXEL Communications VMG892-B10A User Manual page 153

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.
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 55 Internal Layer2 and Layer3 QoS Mapping
PRIORITY
QUEUE
0
1
2
3
VMG8924-B10A and VMG8924-B30A Series User's Guide
DSCP (6 bits)
LAYER 2 LAYER 3
IEEE 802.1P USER
PRIORITY TOS (IP
(ETHERNET PRECEDENCE)
PRIORITY)
1 0
2
0 0
3 1
Unused (2 bits)
DSCP
000000
000000
001110
001100
001010
001000
Chapter 9 Quality of Service (QoS)
IP PACKET
LENGTH (BYTE)
>1100
250~1100
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