Introduction
Overview
—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.
