User's Manual

Chapter 9 Quality of Service (QoS)
B222s User’s Guide
93
9.6.1 IEEE 802.1p
I EEE 802.1p specifies t he user priorit y field and defines up to eight separat e t raffic t ypes. The
following t able describes t he t raffic types defined in t he I EEE 802.1d st andard ( which incor porat es
the 802.1p) .
9.6.2 IP Precedence
Sim ilar t o I EEE 802.1p prioritizat ion at layer- 2, you can use I P precedence t o priorit ize packet s in a
layer- 3 net work. I P precedence uses t hree bits of t he eight- bit ToS ( Type of Service) field in t he I P
header. There are eight classes of services ( ranging from zero t o seven) in I P precedence. Zero is
the lowest priorit y level and seven is the highest .
9.6.3 DiffServ
QoS is used t o priorit ize source-t o- destination t raffic flows. All packet s in the flow are given the
sam e priorit y. You can use CoS ( class of service) t o give different priorit ies t o different packet
types.
DiffSer v ( Different iated Services) is a class of service (CoS) m odel t hat m arks packets so t hat they
receive specific per- hop t reat m ent at DiffServ- com pliant net work devices along t he rout e based on
the application t ypes and t raffic flow. Packets are m arked wit h DiffServ Code Points ( DSCPs)
indicating t he level of service desired. This allows t he int erm ediar y DiffServ- com pliant network
devices t o handle t he packets differently depending on t he code point s wit hout t he need to
negotiat e pat hs or rem em ber st ate inform at ion for every flow. I n addit ion, applicat ions do not have
to request a part icular ser vice or give advanced notice of where t he t raffic is going.
DSCP and Per-Hop Behavior
DiffServ defines a new DS ( Differentiat ed Services) field to replace the Type of Service (TOS) field
in the I P header. The DS field cont ains a 2- bit unused field and a 6- bit DSCP field w hich can define
up t o 64 service levels. The following figure illust rat es the DS field.
Table 33 I EEE 802.1p Prior it y Level and Traffic Type
PRIORITY
LEVEL
TRAFFIC TYPE
Level 7 Typically used for net work cont rol traffic such as rout er configurat ion m essages.
Level 6 Typically used for voice t raffic t hat is especially sensitiv e t o j it t er ( j it t er is t he
var iat ions in delay) .
Level 5 Typically used for video that consum es high bandwidt h and is sensitiv e t o j it ter.
Level 4 Typically used for controlled load, lat ency- sensitive t raffic such as SNA ( System s
Net work Archit ect ure) t ransact ions.
Level 3 Typically used for “ excellent effort ” or bet t er t han best effort and w ould include
im port ant business t raffic t hat can t olerat e som e delay.
Level 2 This is for “ spare bandwidth”.
Level 1 This is t ypically used for non- critical “ background” traffic such as bulk t ransfers t hat
are allow ed but that should not affect other applicat ions and users.
Level 0 Typically used for best- effort t raffic.