Diffserv; Automatic Priority Queue Assignment - ZyXEL Communications P-793H User Manual

Chapter 15 Quality of Service (QoS)

15.6.3 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.

15.6.4 Automatic Priority Queue Assignment

If you enable QoS on the P-793H v2, the P-793H v2 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 P-793H v2. On the P-793H v2, traffic assigned to higher priority queues gets
232
DSCP (6 bits) Unused (2 bits)
P-793H v2 User's Guide