A SERVICE OF

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Chapter 14 Quality of Service (QoS)
P-870HN-51D User’s Guide
185
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.
14.8.1 Automatic Priority Queue Assignment
If you enable QoS on the P-870HN-51D, the P-870HN-51D 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-870HN-51D. On the P-870HN-51D, traffic assigned to higher priority queues
gets through faster while traffic in lower index queues is dropped if the network is
congested.
DSCP (6 bits) Unused (2 bits)
Table 69 Internal Layer2 and Layer3 QoS Mapping
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
12
2 0 0 000000 >1100