IP QoS: As a Matter of Choice...

In recent years, data traffic over the Internet has increased exponentially.
The growth in network-reliant business services and applications in the past
decade have resulted in enterprise LANs getting choked with heterogeneous
traffic. VoIP, enterprise resource planning and e-business applications are
vying for the same limited bandwidth. These bandwidth-hungry applications have
driven the demand for excellent and intelligent networks, and due to this
demand, quality-of-service (QoS) became very important.

For example, a service provider may offer platinum, gold, and silver levels
of service. The platinum service could offer assurances on delay and jitters, as
well as bandwidth. Gold service may imply assured bandwidth with no tight bounds
on delay and jitters, while silver can be a best-effort service–a simple and
basic offering. So platinum and gold services are value-added offerings and
generate revenue.

The Internet Engineering Task Force has made efforts to develop
standards that provide QoS capabilities. These evolving standards are called:

DiffServ: Better Network Management
It eliminates bottlenecks through efficient management of current corporate network resources
	It allows customers to retain any existing Layer 3 ToS prioritization scheme that may be in use
	It allows customers to mix DiffServ-compliant devices with any existing ToS-enabled equipment in use
	It reduces the burden on network devices and is easily scalable as the network grows

l Integrated
Service or Intserv

l Differentiated
Service or DiffSer

The Intserv Model

The IS model proposes two types of services targeted at real-time traffic–guaranteed
and predictive services. This model is based on the assumption that resources
like bandwidth must be explicitly managed in order to meet application
requirements. This implies that resource reservation and admission control are
key building blocks of the service. RSVP, a signalling protocol that
applications may use to request resources from the network, is used to implement
this assumption. RSVP is an excellent protocol to provide QoS, but the following
factors have impeded large-scale deployment of RSVP on the Internet:

l The use of
per-flow state and per-flow processing raises scalability concerns for large
networks

l Only a small
number of hosts currently generate RSVP signalling

The DiffServ Model

In order to overcome the limitation of Intserv/RSVP, DiffServ enhancements
to the Internet protocol are intended to enable scalable service discrimination
on the Internet without the need for per-flow state and signaling at every hop.

Differentiated service is a multiple service model that can satisfy differing
QoS requirements. DiffServ networks classify packets into one of a small number
of aggregated flows of ‘classes’ based on the DiffServ code point (DSCP) in
the packet’s IP header.

n DS Field: The
DS field supersedes the existing definitions of the IPv4 type of service (ToS)
octet and the IPv6 traffic-class octet. Six bits of the DS field are used as the
DSCP to select the per-hop behavior (PHB) at each interface.

n Per-hop
Behaviors: PHBs are at the heart of the DiffServ architecture. RFC 2475 defines
PHB as the externally observable forwarding behavior applied at a DiffServ-compliant
node to a DiffServ behavior aggregate (BA). PHB refers to the packet scheduling,
queuing, policing, or shaping behavior of a node on any given packet belonging
to a BA, as configured by a service level agreement (SLA).

The four availbale standard PHBs are:

l Default PHB

l Class-selector
PHB

l Assured
forwarding PHB

l Expedited
forwarding PHB

Traffic Conditioning

Traffic conditioning performs metering, shaping, policing and/or re-marking
functions to ensure that the traffic entering the DiffServ domain conforms to
the rules specified in the TCA, in accordance with the domain’s service
provisioning policy.

A meter is used to measure the traffic stream against a traffic profile. The
state of the meter with respect to a particular packet, whether it is in- or
out-of-profile may be used to effect a marking, dropping, or shaping action.

Packet markers set the DiffServ filed of a packet to a particular code point,
adding the marked packet to a particular DiffServ behavior aggregate. Shapers
delay some or all of the packets in a traffic stream in order to bring the
stream in compliance with a traffic profile. A shaper usually has a finite-size
buffer, and packets may be discarded if there is insufficient buffer space to
hold the delayed packets. Droppers discard some or all of the packets in a
traffic stream, in order to bring the stream into compliance with a traffic
profile. This process is known as ‘policing’ the stream.

Clearly, DiffServe offers several advantages over IntServ.

Brijesh Kumar Shukla is with Network Programs India