Networking Quality of Service (QoS)

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Networking Quality of Service (QoS) refers to a variety of techniques that prioritize one type of traffic or program that operates across a network connection instead of relying only on "best effort" connectivity. QoS mechanisms are built into both Microsoft Windows 2000 and Windows XP.

We will look into the Networking Quality of Service for Windows XP.

QoS for Internet Connection Sharing

When one network is connected to another network through a slow link or connection, such as a dial-up line, a situation can exist that will increase the delay of traffic that is traversing the slow link. This delay occurs because of the speed mismatch between what the end stations in the communication know about, and the slow link. The slow link causes a bottleneck in the network path. This applies only to connection-oriented communication when you use TCP.

If the receiving client is running on a relatively fast network, such as a 100 megabytes per second Ethernet network, behind a computer that is running Windows XP with the Internet Connection Sharing service, and the server that this receiver is communicating with is behind a remote access on a fast network, the mismatch exists. In this scenario, the receiver's receive window is set to a large value that is based on the speed of the link the receiver is connected to. The sender starts out by sending at a slow rate, but if packets are not lost, the sender eventually sends almost a full window size of packets.

This scenario can affect the performance of other TCP connections that traverse the same network. Packets sit in a potentially large queue and wait to be transmitted over the slow network. If packet loss occurs, data has to be retransmitted, and this also congests the link.

The solution to this issue is to have the computer that is running Internet Connection Sharing on the edge of the network automatically set the receive window to a smaller size that is appropriate to the slow link. This setting overrides the receiver's specification. This setting will not adversely affect traffic, because the window size is being set as if the receiver were connected directly to the slow link. The QoS Packet Scheduler component that is running on the Internet Connection Sharing computer makes this window adjustment.

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Clarification about the use of QoS in end computers that are running Windows XP

As in Windows 2000, programs can take advantage of QoS through the QoS APIs in Windows XP. One hundred percent of the network bandwidth is available to be shared by all programs unless a program specifically requests priority bandwidth. This "reserved" bandwidth is still available to other programs unless the requesting program is sending data. By default, programs can reserve up to an aggregate bandwidth of 20 percent of the underlying link speed on each interface on an end computer. If the program that reserved the bandwidth is not sending sufficient data to use it, the unused part of the reserved bandwidth is available for other data flows on the same host.

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Incorrect claims about Windows XP QoS support

There have been claims in various published technical articles and newsgroup postings that Windows XP always reserves 20 percent of the available bandwidth for QoS. These claims are incorrect. The information in the "Clarification about QoS in end computers that are Running Windows XP" section correctly describes the behavior of Windows XP systems.