HP-UX IPQoS A.01.00 Administrator's Guide (October 2005)

Introduction
Overview
Chapter 1 23
— The classifier selects a packet in a traffic stream based the content of a part of the packet
header. In HP-UX IPQoS, classification is based on filter attributes specified in the active
HP-UX IPQoS configuration.
— The meter checks compliance to a defined traffic profile and passes results to
shaper/policer to trigger appropriate actions for in- and out-of-profile packets. In HP-UX
IPQoS, metering is based on policy attributes specified in the active HP-UX IPQoS
configuration.
— The marker can write/rewrite the DSCP and VLAN priority values. In HP-UX IPQoS,
marking is based on policy attributes specified in the active HP-UX IPQoS configuration.
— The shaper delays (and the policer drops) some packets, so that traffic maintains
compliance with a defined traffic profile. In HP-UX IPQoS, shaping actions result from
maintaining accordance with metering specifications in the active HP-UX IPQoS
configuration. HP-UX IPQoS does not perform shaping based on marking characteristics.
Other nodes, such as DS-aware routers, may perform shaping as a result of marking
specifications.
DSCP Marking
Each IP packet has a DS field. For IPv4, this is the leftmost 6-bits of the TOS field; for IPv6,
this is the leftmost 6-bits of the Traffic Class field. The DS field can be used to mark a
packet so that it receives a particular forwarding behavior. The DiffServ model defines a set of
commonly used standard DSCP’s and their corresponding forwarding behaviors. The
forwarding behavior assigned to a DSCP is referred to as the per-hop behavior (PHB). The
PHB defines the forwarding precedence a packet receives in relation to other traffic on a
DiffServ-enabled network.
A common agreement about the use and interpretation of the possible values used in the DS
field is needed for interdomain use, multivendor interoperability, and overall consistency of
expected aggregate behaviors in a network. The IETF Differentiated Services Working Group
has standardized a common layout for the six-bit field of these octets. For more information,
refer to RFC 2474 Definition of the Differentiated Service Field (DS Field) in the IPv4 and
IPv6 Headers and RFC 2475 An Architecture for Differentiated Services, which define the
architecture and the general use of the bits within the DS field.
You can select a DSCP value from a group of well-known codepoints associated with the two
key forwarding types defined in the DiffServ model. The PHB types are Expedited
Forwarding (EF) for higher priority traffic, and Assisted Forwarding (AF) for other traffic.
The well-known codepoints, which would appear in the leftmost 6-bits of the TOS or Traffic
Class fields, are 46 (101110) for the EF PHB, and a range of codepoints for the AF PHB. For
more information, refer to RFC 2597 Assured Forwarding PHB Group and RFC 2598 An
Expedited Forwarding PHB.