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IP QoS Architectures


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Int-Serv Architecture
Diff-Serv Architecture

IP QoS Architectures

In the new infrastructure, IP must provide business-quality solutions.

The key to providing QoS in the new service architectures is the ability to differentiate traffic and to provide differentiated service levels based on the types of traffic. For example, for real-time applications such as voice over IP (VoIP), the amount of available bandwidth and end-to-end delay is crucial compared to fax and e-mail transmissions, which are quite insensitive to bandwidth and delay issues.

To provide QoS from a network, the following must be satisfied:

  • User/application requirements should be known to the network; and
  • The network should have appropriate mechanisms for providing service levels that approximate these requirements.

The standard IP architecture was never designed to deliver on either of the two; it is based on a “best-effort” model where all network traffic is equally important and everyone receives service based on availability, without guarantees.

In the absence of QoS mechanisms, the industry traditionally has opted for over-provisioning bandwidth. While bandwidth over-provisioning continues, industry experts agree that QoS mechanisms are needed to address the needs of converging networks.

In general, end-to-end QoS in Internet is built from the concatenation of edge-to-edge QoS from each domain through which traffic passes, and ultimately depends on the QoS characteristics of the individual hops along any given route. The solution can be broken into three parts:

  • per-hop QoS,
  • traffic engineering, and
  • signaling/provisioning.

QoS mechanisms do not generate more bandwidth. They manipulate router/switch queues so that when congestion occurs, priority “VIP” traffic is serviced quickly, while less important traffic experiences delays and drops. The network applies packet-filtering criteria to identify and prioritize VIP traffic using either provisioned or signaled QoS: Provisioning assumes the network nodes are configured ahead of time, while signaling assumes that filtering criteria are communicated in real time, upon demand. Classification is based on class of service (CoS) or QoS criteria. QoS deals with individual flows; CoS, generally considered a subset of QoS, deals with aggregate classes of traffic. After being classified, packets are serviced by a predefined queuing discipline that determines their final service level.

Because not all QoS mechanisms are the same, selecting the right QoS mechanism for the network could affect results significantly.

Figure 3 - Per-hop classification, queuing, and scheduling (CQS) routing architecture

Standard QoS Approaches

A variety of standard QoS approaches are common. Some router/switches are capable of setting filters to classify traffic and map it to specific queues. Some of the more popular disciplines include:

  • Priority Queuing (PQ),
  • Class-Based Queuing (CBQ),
  • Weighted Fair Queuing (WFQ), and
  • Random Early Detection (RED).

In all these approaches, the entire QoS process (classification and queuing) is provisioned within a single node and requires no cooperation from others. However, the process of filtering packets based on multiple attributes causes

  • High overhead,
  • Does not scale well, and
  • Hard to create consistent multi-hop QoS by preconfiguring individual routers in a routing insensitive manner.

Figure 4 - A simplified Native IP Forwarding (NIF) engine

Advanced QoS Approaches

An important issue in the Internet, and consequently in every network connected to it, is support for multimedia applications (video, voice). These applications have specific requirements in terms of delay and bandwidth which challenge the original design goals of IP's best effort service model, and call for alternate service models and traffic management schemes that can offer the required quality of service (QoS). To this end, two QoS architectures have emerged in the IETF:

·         Integrated services architecture (IntServ), which provides end-to-end QoS on a per-flow basis; features soft states and end-to-end signaling.

·         Differentiated services architecture (DiffServ), which supports QoS for traffic aggregates; features class of flows and code points contained in the IP header’s differentiated services field.

Both proposals suggest solutions to overcome the QoS limitations in the current best-effort IP service architecture. Each system has, however, its own advantages and disadvantages, and its own role to perform in an appropriate segment of an IP network.

We now review these two proposals on how such QoS enabling schemes could be utilized to enhance the best effort service model of IP architecture







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Last modified: July 13, 2016


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