DiffServ
DiffServ groups network traffic into classes comprised of traffic needing the
same type of QoS treatment. For instance, voice traffic is separated from email
traffic. However, e-mail might be placed in the same class as web traffic. The
exact classes, traffic, and QoS policies used are a business decision.
These classes are distinguished from each other based on the value of certain
bits in the IP or ISL header or the 802.1Q tag. Each hop along the way must
be configured to treat the marked traffic the way you want—this is called
per-hop behavior (PHB).
■ In the Layer 3 IP header, you use the 8-bit Type of Service (ToS) field.
You can set either IP Precedence, using the top 3 bits, or DSCP using
the top 6 bits of the field. The bottom 2 bits are not used for setting
priority. The default DSCP value is zero, which corresponds to besteffort
delivery.
■ At Layer 2, with ISL, you can set 3 of the 4 bits in the ISL priority
field to reflect the class of service (CoS). With 802.1Q, you set the 3
802.1p bits to the CoS. The values of these 3 bits correspond to the IP
Precedence values.
Benefits of DiffServ include the many classes of service possible, and its
scalability. As a drawback, it can be complex to configure. It also does not
absolutely guarantee a level of service.