What Is QoS?

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

What Is QoS?

In this section

  • QoS Concepts

  • Uses and Benefits of QoS

  • Network Technologies and Support for QoS

  • Windows Server 2003 QoS

  • Related Information

Quality of Service (QoS) for networks is an industry-wide set of standards and mechanisms for ensuring high-quality performance for critical applications. By using QoS mechanisms, network administrators can use existing resources efficiently and ensure the required level of service without reactively expanding or over-provisioning their networks.

Traditionally, the concept of quality in networks meant that all network traffic was treated equally. The result was that all network traffic received the network’s best effort, with no guarantees for reliability, delay, variation in delay, or other performance characteristics. With best-effort delivery service, however, a single bandwidth-intensive application can result in poor or unacceptable performance for all applications. The QoS concept of quality is one in which the requirements of some applications and users are more critical than others, which means that some traffic needs preferential treatment.

QoS Concepts

The goal of QoS is to provide preferential delivery service for the applications that need it by ensuring sufficient bandwidth, controlling latency and jitter, and reducing data loss. The following table describes these network characteristics.

Network Characteristics Managed by QoS

Network Characteristic Description


The rate at which traffic is carried by the network.


The delay in data transmission from source to destination.


The variation in latency.


The percentage of packets discarded by a router.

The Internet Engineering Task Force (IETF) defines two major models for QoS on IP-based networks: Integrated Services (Intserv) and Differentiated Services (Diffserv). These models encompass several categories of mechanisms that provide preferential treatment to specified traffic. The following table describes the three general categories of QoS mechanisms.


  • Different versions of Windows support different combinations of these mechanisms. Windows XP and Windows Server 2003 do not support resource reservation.
Category of QoS Mechanisms Description

Admission control

Determine which applications and users are entitled to network resources. These mechanisms specify how, when, and by whom network resources on a network segment (subnet) can be used.

Traffic control

Regulate data flows by classifying, scheduling, and marking packets based on priority and by shaping traffic (smoothing bursts of traffic by limiting the rate of flow). Traffic control mechanisms segregate traffic into service classes and control delivery to the network. The service class assigned to a traffic flow determines the QoS treatment the traffic receives.

The Intserv model integrates resource reservation and traffic control mechanisms to support special handling of individual traffic flows. The Diffserv model uses traffic control to support special handling of aggregated traffic flows.

Uses and Benefits of QoS

Network administrators can use QoS to guarantee throughput for mission-critical applications so that their transactions can be processed in an acceptable amount of time. Network administrators can also use QoS to manage User Data Protocol (UDP) traffic. Unlike Transmission Control Protocol (TCP), UDP is an inherently unreliable protocol that does not receive feedback from the network and, therefore, cannot detect network congestion. Network administrators can use QoS to manage the priority of applications that rely on UDP, such as multimedia applications, so that they have the required bandwidth even in times of network congestion, but do not overwhelm the network.

QoS provides the following benefits:

  • Gives administrators control over network resources and allows them to manage the network from a business, rather than a technical, perspective.

  • Ensures that time-sensitive and mission-critical applications have the resources they require, while allowing other applications access to the network.

  • Improves user experience.

  • Reduces costs by using existing resources efficiently, thereby delaying or reducing the need for expansion or upgrades.

Network Technologies and Support for QoS

QoS depends on support throughout the network. To achieve QoS from sender to receiver, all of the network elements through which a traffic flow passes — such as network interface cards, switches, routers, and bridges — must support QoS. If a network device along this path does not support QoS, the traffic flow receives the standard first-come, first-served treatment on that network segment.

Network technologies such as Frame Relay, asynchronous transfer mode (ATM), and the more traditional local area network (LAN) technologies (including Ethernet, Token Ring, and 802.11 wireless LAN) support QoS mechanisms. ATM, in particular, offers a high degree of support for QoS. Because ATM is a connection-oriented networking technology, it can establish a service contract that guarantees a specific quality of service and can allocate network resources. ATM enforces the service contract and allocates bandwidth at the hardware level. For more information about ATM QoS, see “How ATM Works.”

Service level guarantees and resource allocations such as those provided by ATM are not supported by the more traditional IEEE 802 LAN technologies. These LAN technologies do not support QoS mechanisms at the hardware level, but at higher levels of the Open Systems Interconnection (OSI) reference model only. For these types of technologies, QoS is primarily based on the concept of priority, in which one data transmission receives delivery preference over other transmissions.

Traditional IEEE 802 LAN technologies are connectionless, so a host that sends a transmission cannot detect the state of the network or the state of the destination host prior to the transmission. Traffic on these network technologies, therefore, is subject to delay at points along the way, making bandwidth availability and delivery time difficult to predict. Although high-priority traffic typically arrives at its destination before lower-priority traffic, high-priority traffic cannot be guaranteed to arrive within a specified time.

Although QoS is more difficult to implement on connectionless networks, there is growing interest in developing QoS for IP-based networks.

Windows Server 2003 QoS

QoS in Windows XP and Windows Server 2003 is different from QoS in Windows 2000. Windows 2000 supports both the Intserv and Diffserv models and includes resource reservation, admission control, and traffic control mechanisms. Windows XP and Windows Server 2003 support only the Diffserv model and include traffic control mechanisms.

The current industry trend is toward the use of the Diffserv model because those solutions that use the Intserv model have proven to be difficult to scale in large networks as well as difficult to design and develop. The Diffserv model is designed to be scalable; it offers a simple method of providing differentiated services for IP traffic.

QoS support in Windows Server 2003 has changed for the following reasons:

  • The Generic QoS (GQoS) application programming interface (API) is complex for developers to use.

  • Many applications cannot determine their traffic requirements in advance.

  • Resource allocation does not scale well in large networks.

Windows 2000 QoS versus Windows Server 2003 QoS

This section briefly describes the QoS mechanisms available in Windows 2000, Windows XP, and Windows Server 2003.

Resource Reservation

In Windows 2000, RSVP works with other components of the Intserv architecture to request network resources for traffic that has known characteristics. Windows XP and Windows Server 2003 do not support RSVP.

Admission Control

The Windows 2000 implementation of admission control is QoS Admission Control Service (ACS). ACS works with Subnet Bandwidth Manager (SBM) to manage congested subnets on IEEE 802-type networks. SBM is a signaling protocol used by ACS to indicate when a QoS request can be received. ACS then determines whether sufficient bandwidth is available and grants or rejects the request. Windows XP and Windows Server 2003 do not support ACS and SBM.

Traffic Control

In Windows 2000, as well as Windows XP and Windows Server 2003, the Generic Packet Classifier (GPC) classifies packets and the QoS Packet Scheduler schedules and marks packets and shapes traffic flows. Windows 2000, Windows XP, and Windows Server 2003 support prioritization at Layer 2 (Data Link layer) and at Layer 3 (Network layer) of the OSI reference model.


The two QoS application programming interfaces (APIs) included with Windows 2000, General Quality of Service (GQoS) and Traffic Control (TC), are also supported in Windows XP and Windows Server 2003. GQoS is an API used by developers to create QoS-aware applications. In Windows XP and Windows 2003, however, function calls to the GQoS API do not generate RSVP messages. The TC API can be used by traffic management applications running with administrative privileges to apply QoS to applications that are not QoS-aware.

The following table summarizes the QoS mechanisms and APIs available in Windows 2000 compared to Windows XP and Windows Server 2003.

QoS Mechanisms and APIs

QoS Mechanism Windows 2000 Windows XP and Windows Server 2003










Packet Classifier



Packet Scheduler





Yes, with limited functionality




The following resources contain additional information that is relevant to this section.