Introduction to Differentiated Services (DiffServ) and HP-UX IPQoS

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port numbers. Packets are grouped into traffic flows based on BA or MF filters to be forwarded in a
pre-defined forwarding manner.
In our example, the XYZ Corporation will want to filter its VoIP traffic using one or more fields in the IP
header. These fields may include source/destination addresses, ports, or protocol id. The VoIP
application at XYZ uses the Real-Time Transport Protocol (RTP) to send data. RTP uses the UDP
protocol and UDP port number which will be used in the QoS configuration when filtering the VoIP
traffic.
Once the traffic has been classified (filtered) into like packet streams, the packets continue to the next
phase, Traffic Conditioning.
Conditioning
Traffic conditioning can include functions for metering, (re)marking, and shaping/dropping. These
functions bring packet streams into conformance (referred to as “in-profile”), if needed, to the agreed
upon service levels. It may be required that non-conforming (“out-of-profile”) packets be delayed or
dropped to satisfy the requirements for the particular traffic stream.
Metering
Metering measures the current state of a traffic stream against the expected state as defined by the
traffic profile. The state information is then used to trigger any necessary actions by marking,
shaping, and/or dropping.
Marking
Marking sets (or resets) the DS Codepoint in the leftmost 6-bits of the ToS field of the IPv4 header or
the Traffic Class field in IPv6. The packet is then assigned to a traffic aggregate or traffic flow
(referred to as a DS Behavior Aggregate) containing the same DSCP value. The DSCP value will map
to a forwarding behavior defined by a PHB and will be applied to all traffic within that Behavior
Aggregate. Several PHB standards exist today that define an “Assured Forwarding (AF) PHB Group”
and “Expedited Forwarding (EF) PHB” (RFC’s 2597 and 2598). To ensure backwards compatibility
with existing ToS values, refer to RFC 2474, “Definition of the Differentiated Services Field (DS Field)
in the IPv4 and IPv6 Headers”.
For the following IPv4 and IPv6 header diagrams, the DSCP value is shaded and the remaining 2-bits
(currently unused) have been shaded with lines.