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

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remaining types of traffic within a reasonable timeframe. Thus, there is no need to overprovision the
network bandwidth or build a parallel network for VoIP traffic.
Components of DiffServ
The following sections discuss the DiffServ environment from general concepts to more specific
information, including the role of HP-UX IPQoS in that environment.
In the DiffServ model, packets are marked with a DiffServ Code Point (DSCP) value which
corresponds to a specific traffic forwarding behavior referred to as a Per-Hop Behavior (PHB). The
DSCP is set by the source DS node and/or DS Boundary node located primarily at the edges of a DS
Domain. The Domain signifies a separate network or other administrative boundary of a DiffServ
compliant environment. The PHBs in use can be those defined by IETF RFC standards
1
or they can be
locally defined.
In the above example, DS domains in each building of XYZ Corporation operate under different QoS
policies and PHBs. A common set of policies will be needed at the DS boundary nodes to maintain
the service requirements for applications such as VoIP communications.
Traffic Classification and Conditioning
As the packet enters the Boundary node it undergoes the process of Traffic Classification and
Conditioning. The Traffic Classifier will direct the packets to Conditioners based on defined rules for
the particular traffic type. Conditioning will evaluate the packets and determine what action if any
needs to be taken on the packet to bring it into conformance within service level agreements.
Classification
The DiffServ model supports (2) types of Traffic Classifiers: Behavior Aggregate (BA) and the Multi-
Field (MF) Aggregate. The BA classifies traffic based on the value of the DSCP. The MF Aggregate
classifies packets based on multiple fields of the packet such as source or destination address(es) or
1
More information on standardized Per-Hop Behaviors can be found in RFC 2474 “Definition of the Differentiated Services Field (DS Field) in the
IPv4 and IPv6 Headers”, RFC 2597 “Assured Forwarding PHB Group”, and RFC 2598 “An Expedited Forward PHB”.
XYZ Corp Bldg 1
DS
Domain
DS
Boundary
Link
DS Boundary
Nodes
XYZ Corp Bldg 2
DS
Domain
Non-DS
Domain
Non-DS
Domain
DS Domain