HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 319

N o t e
The four ToS bits within the ToS field each request a different type of service
from forwarding nodes:
a one in the first bit requests low delay
a one in the second bit requests high throughput
a one in the third bit requests high reliability
a one in the fourth bit requests low cost
As you see above, a one in two of the ToS bits refers to a high setting, but in
the other two to a low setting. You will not be confused if you remember that
a one ToS bit always requests faster, better service.
In practice, networks rarely use the ToS bits. However, several protocols have
emerged to grant packets differentiated service according to the IP prece-
dence setting alone. These protocols include WFQ and LLQ.
The ProCurve Secure Router can both mark packets with an IP precedence
value and grant packets a differentiated service depending on a previously
marked value. It can:
mark packets with an IP precedence value so that the network to which
they are forwarded will grant them a specific type of service
read packets' IP precedence value for WFQ
read packets' IP precedence value and assign them to a class for CBWFQ
read packets' IP precedence and assign packets with a certain priority to
a low-latency queue
DiffServ. The DiffServ protocol redefines the ToS field as the Differentiated
Services (DS) field. It combines the three IP precedence bits and three of the
four under-used ToS bits into a six-bit Differentiated Service Code
Point (DSCP).
The DSCP supports 63 values to IP precedence's seven. (In both, a zero value
refers to routine traffic for which a priority has not been set.)
The last two bits of the DS field are reserved for flow control; these are the
congestion experienced bits.
Setting Up Quality of Service
Overview
7-7