Differentiated Class of Service

Windows 2000 QoS supports layer 3 Differentiated Services (Diff-serv), also called Class of Service (CoS). Diff-serv extends QoS across networks that are not RSVP-enabled, such as large transit networks that make up the core of the Internet.

The QoS Packet Scheduler performs Diff-serv marking for any application that requests marking from the GQoS API or the Traffic Control API. The IP header of the packet is marked with a priority value in the Type of Service (ToS) fields, also referred to as the Diff-serv Code Point (DSCP). This marking determines the service level the packet receives when crossing a Diff-serv network segment.

Type of Service Field and DSCP   The DSCP value is established by setting the first six bits of the ToS field. Figure 9.6 illustrates the IP header with the ToS field enclosed.

Enlarge figure

Figure 9.6 IP Header with TOS Field

The default mappings for DSCP, in decimal, are listed in Table 9.5. These 6-bit DSCP values show up in the upper 6 bits of the ToS field as specified in RFC 2474.

Table 9.5 DSCP Default Priority Markings

The DSCP value has subsumed IP Precedence and is therefore backward compatible with IP Precedence. The IP Precedence contains the upper 3 bits of the DSCP field. These values are listed in Table 9.6:

Table 9.6 IP Precedence Markings

The QoS Packet Scheduler must be installed on any host that performs or interprets Diff-serv markings. If the layer 3 devices between the end nodes are not Diff-serv–capable or enabled, QoS cannot be guaranteed across that segment.

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