ZyXEL Communications P-660HN-F1A, User Manual

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Chapter 16 Quality of Service (QoS)
P-660HN-F1A User’s Guide
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16.10.2 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.
16.10.3 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.
Level 3 Typically used for “excellent effort” or better than best effort and would
include important business traffic that can tolerate some delay.
Level 2 This is for “spare bandwidth”.
Level 1 This is typically used for non-critical “background” traffic such as bulk
transfers that are allowed but that should not affect other applications and
users.
Level 0 Typically used for best-effort traffic.
Table 81 IEEE 802.1p Priority Level and Traffic Type
PRIORITY
LEVEL
TRAFFIC TYPE