Analyze the QoS mechanism enhancements provided by Windows XP

Overview

Networked Quality of Service (QoS) refers to the various communication or program type prioritization techniques applied across the entire network connection. These technologies do not rely solely on "best effort" connections. QoS mechanisms are built into Microsoft Windows 2000 and Windows XP. This article describes the QoS enhancements provided by Windows XP. This article also provides a reference to the QoS features first introduced in Windows 2000.

QoS for Internet Connection Sharing (ICS)

When a network is connected to another network via a slow link or connection (such as a dial-up line), the communication delay through slow links may be increase. The reason for this delay is that the speed recognized by the communication terminal station does not match the speed of the slow link, resulting in a bottleneck of the network path. This only applies to connection-oriented communication (using TCP).

On a relatively fast network (such as 100 Mb/s Ethernet), if the receiving client is running behind a computer running Windows XP's ICS service, and the server that is communicating with this receiving client is located After remote access on a fast network, there is a mismatch. In this case, the receiving client's receiving window is set to a larger value based on the speed at which the receiving client connects to the link. The sender starts sending at a low rate, but if the packet is not lost, the speed will continue to increase and eventually packets of almost full window size will be sent.

This affects the performance of other TCP connections over the same network, so that their packets are waiting to be transmitted over a slow network in a queue that may be long. If packet loss occurs, the data must be retransmitted (up to full window size), making the link more clogged.

The corresponding solution is to have the computer running ICS at the edge of the network automatically set the receive window to a smaller size suitable for slow links, overwriting the specification of the receiving client. This setting does not adversely affect communication because the window size is set accordingly if the receiving client is directly connected to the slow link. This window adjustment is made by the QoS Packet Scheduler component running on the ICS computer.

QoS for Modem and Remote Access

As of January 2002, many users are still connected to the Internet via slow links, such as a connection speed of 56 kilobytes per second. Although the link speed is limited, many users still have to run multiple programs that access the network at the same time. This may include browsing, downloading, emailing, chatting, and even audio or video streaming. Most of these programs use TCP as the basic transport protocol, and each program uses its own connection.

The first program that uses a link initially has exclusive use to bring its connection to a stable state, thereby implementing a full TCP window of data in transit. When the next program starts transmitting data, the connection it uses is constrained by the slow start algorithm, which limits the amount of unacknowledged data that can be transferred. Since the established program is transferring a certain amount of data, the second program takes much longer to reach a steady state, and the same size of data transfer is much slower.

When Windows XP runs on a slow link, it implements a reasonable solution called "Insufficient Round Robin (DRR)". Windows 2000 also uses this scheme, but in Windows XP, this scheme is turned on by default when slow links are detected. This scenario allocates several data streams and assigns new application data streams to those flows. These flows are automatically serviced in a round-robin fashion, resulting in better network communication response and performance without requiring the user to do any manual configuration.

Clarification of QoS in Terminal Computers Running Windows XP

Like Windows 2000, in Windows XP, programs can also utilize QoS through the QoS Application Programming Interface (API). All programs can share 100% of the network bandwidth, with the exception of programs that require bandwidth priority. Other programs can also use this "reserved" bandwidth, except for the requestor that is sending data. By default, the program reserves a bandwidth that is up to 20 percent of the basic link speed of each interface on the terminal computer. If the amount of data sent by a program that reserves bandwidth does not completely run out of bandwidth, the unused portion of the reserved bandwidth can be used for other data streams on the same host.

For more information on the QoS Packet Scheduler, please refer to the Windows XP Help. The Windows 2000 Technical Library provides additional information about Windows 2000 QoS.

Some Mistakes in Correcting Windows XP QoS Support

Many published technical articles and newsgroup articles have repeatedly mentioned that Windows XP typically reserves 20% of the available bandwidth for QoS. From the above article we can conclude that "QoS reserves 20% of the available bandwidth" is wrong.