3Com 3C15500 - Network Director - PC, User Manual, Page: 629 / 924

Key Concepts 625
Classification is configured on a device as a set of rules. Each rule defines
a particular way of identifying the type of traffic. For example, a rule may
state that SNMP traffic can be identified as UDP packets that have either
the source or the destination UDP port set to the value 161.
Marking Due to the numerous possibilities for classifying packets, not every device
can classify the same packets in the same manner. Some devices are
capable of extremely complex classification (
complex classification
devices ), while other devices may only be able to match traffic based on
the content of one or two fields ( simple classification devices ). For
example, this can mean that while one device in your network is able to
identify traffic to and from your database server using the destination or
source IP address in the packets, another device may not be able to.
There are two standards that specify how a complex classification device
can modify packets in a straightforward manner so that a simple
classification device can still identify how important those packets are.
This modification of packets is known as marking and the two standards
are:
IEEE 8 02.1D — defines an extended MAC header (known as a tagged
header) which contains a user priority field (known as the 802.1p tag),
which takes one of eight values (0 - 7) to represent the priority of the
packet.
IETF RFC 2474 — defines the use of the Differentiated Services (DiffServ)
field in the IP header. This field can take one of sixty-four values (0 - 63)
which are known as DiffServ codepoints (or DSCPs) to represent the
quality of service requirements of the packet. There are no restrictions on
what a particular codepoint value may mean, however one use could be
to simply represent the priority of the packet.
More information on these standards is available in “User Priority Field”
on page 652 and “DiffServ Codepoint Field” on page 653 .