Diffserv; Automatic Priority Queue Assignment; Table 88 Internal Layer2 And Layer3 Qos Mapping - ZyXEL Communications P-660HN User Manual

Chapter 15 Quality of Service (QoS)

15.5.3 DiffServ

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.
Differentiated Services (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.
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.

15.5.4 Automatic Priority Queue Assignment

If you enable QoS on the ZyXEL Device, the ZyXEL 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 ZyXEL
Device. On the ZyXEL Device, traffic assigned to higher priority queues gets through faster
while traffic in lower index queues is dropped if the network is congested.

Table 88 Internal Layer2 and Layer3 QoS Mapping

PRIORITY
QUEUE
0
1
2
236
DSCP (6 bits)
LAYER 2 LAYER 3
IEEE 802.1P USER
PRIORITY TOS (IP
(ETHERNET PRECEDENCE)
PRIORITY)
1 0
2
0 0
Unused (2 bits)
DSCP
000000
000000
IP PACKET
LENGTH (BYTE)
>1100
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