ZyXEL Communications P-660R-F1 series, Руководство пользователя

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Chapter 11 Quality Of Service
P-660R-F1 Series User’s Guide
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11.5.3 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.
11.5.4 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 60
Internal Layer2 and Layer3 QoS Mapping
DSCP (6 bits) Unused (2 bits)
PRIORITY
QUEUE
LAYER 2 LAYER 3
IEEE 802.1P USER
PRIORITY
(ETHERNET
PRIORITY)
TOS (IP
PRECEDENCE)
DSCP
IP PACKET
LENGTH (BYTE)
0 1 0 000000
1 2
2 0 0 000000 >1100