Allied Telesis AT-S63 - Manual (Page 177)

AT-S63 Management Software Features Guide
Section II: Advanced Operations 177
DiffServ Domains
Differentiated Services (DiffServ) is a method of dividing IP traffic into
classes of service, without requiring that every router in a network
remember detailed information about traffic flows. DiffServ operates within
a DiffServ domain, a network or subnet that is managed as a single QoS
unit. Packets are classified according to user-specified criteria at the edge
of the network, divided into classes, and assigned the required class of
service. Then packets are marked with a Differentiated Services Code
Point (DSCP) tag to indicate the class of service to which they belong. The
DSCP value is written into the TOS field of the IP header. Routers within
the network then use this DSCP value to classify packets and assign QoS
appropriately. When a packet leaves the DiffServ domain, the DSCP value
can be replaced with a value appropriate for the next DiffServ domain.
A simple example of this process is shown in Figure 13, for limiting the
amount of bandwidth used by traffic from a particular IP address. In the
domain shown, this bandwidth limit is supplied by the class of service
represented by a DSCP value of 40. In the next DiffServ domain, this
traffic is assigned to the class of service represented by a DSCP value of
3.
Figure 13. DiffServ Domain Example
Non-DiffServ
traffic
Classify by source IP address
Mark with DSCP=40
Limit bandwidth
DiffServ Domain
Classify by DSCP=40
Limit bandwidth
Classify by DSCP=40
Limit bandwidth
Re-mark to DSCP=3
Next DiffServ
domain