Differentiated Services - ZyXEL Communications GS3700 Series User Manual

This chapt er shows you how t o configure Different iat ed Services ( DiffServ) on t he Swit ch.
42.1 DiffServ Overview
Qualit y of Service ( QoS) is used t o priorit ize source- t o- dest inat ion t raffic flows. All packet s in t he
flow are given t he sam e priorit y. You can use CoS ( class of service) t o give different priorit ies t o
different packet t ypes.
DiffServ is a class of service ( CoS) m odel t hat m arks packet s so t hat t hey receive specific per- hop
t reat m ent at DiffServ- com pliant net work devices along t he rout e based on t he applicat ion t ypes
and t raffic flow. Packet s are m arked wit h DiffServ Code Point s ( DSCPs) indicat ing t he level of
service desired. This allows t he int erm ediary DiffServ- com pliant net work devices t o handle t he
packet s different ly depending on t he code point s wit hout t he need t o negot iat e pat hs or rem em ber
st at e inform at ion for every flow. I n addit ion, applicat ions do not have t o request a part icular service
or give advanced not ice of where t he t raffic is going.
42.1.1 DSCP and Per-Hop Behavior
DiffServ defines a new DS ( Different iat ed Services) field t o replace t he Type of Service ( ToS) field in
t he I P header. The DS field cont ains a 6- bit DSCP field which can define up t o 64 service levels and
t he rem aining 2 bit s are defined as current ly unused ( CU) . The following figure illust rat es t he DS
field.
Figure 293 DiffServ: Different iat ed Service Field
DSCP is backward com pat ible wit h t he t hree precedence bit s in t he ToS oct et so t hat non- DiffServ
com pliant , ToS- enabled net work device will not conflict wit h t he DSCP m apping.
The DSCP value det erm ines t he PHB ( Per- Hop Behavior) , t hat each packet get s as it is forwarded
across t he DiffServ net work. Based on t he m arking rule different kinds of t raffic can be m arked for
different priorit ies of forwarding. Resources can t hen be allocat ed according t o t he DSCP values and
t he configured policies.
42.1.2 DiffServ Network Example
The following figure depict s a DiffServ net work consist ing of a group of direct ly connect ed DiffServ-
com pliant net work devices. The boundary node ( A in
( m arks wit h a DSCP value) t he incom ing packet s int o different t raffic flows ( Pla t inu m , Gold,
Silve r, Br onze ) based on t he configured m arking rules. A net work adm inist rat or can t hen apply
C
HAPTER

Differentiated Services

DSCP (6 bits)
GS3700/XGS3700 Series User's Guide
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CU (2 bits)
Figure 294) in a DiffServ net work classifies
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