QoS Implementation Methods
If you've decided to add QoS to your network, you can choose among three methods to implement your QoS-enabled network. The methods are networkcentric, network-based end-to-end, and application-based end-to-end.
Networkcentric. In the networkcentric method, you deploy QoS only in internetwork devices, such as switches and routers. You don't need to upgrade your legacy applications, servers, and workstations to support QoS. By using predefined QoS policies, the switches and routers can prioritize incoming traffic, reserve necessary bandwidth for the traffic, place packets into appropriate queues, and forward the packets. For example, a Layer 3 switch determines that SAP R/3 is originating incoming application data when the switch detects the data's TCP port in the packet. The switch can then apply a QoS policy to this traffic: for example, by setting IP Precedence to 7 and reserving 1Mbps bandwidth. The networkcentric method is the simplest QoS-enabling method, because it doesn't require changes in the user community. However, the networkcentric approach doesn't provide QoS on receiving computers. Bay Networks, Cisco, and Cabletron support the networkcentric method.
Network-based end-to-end. In the network-based end-to-end QoS-enabling method, you implement QoS not only in internetwork devices but also in computer NICs. A QoS-enabled NIC can classify application data passing through the NIC by setting its 802.1p and IP Precedence priorities based on predefined policies. You can thereby provide end-to-end QoS without modifying your existing applications. Currently, 3Com supports the network-based end-to-end method.
Application-based end-to-end. The application-based end-to-end QoS-enabling method requires implementation of QoS-enabled applications in workstations and servers. Using this method, you can fully exploit comprehensive QoS techniques, such as RSVP and SBM, from computer to computer in your network. Microsoft supports this approach in NT 5.0.
Microsoft QoS
Microsoft QoS consists of Generic QoS API, QoS Service Provider (SP), Traffic Control Interface, and Admission Control Service (ACS). Using Microsoft's Generic QoS API (part of Winsock 2), you can write a QoS-enabled application and specify the QoS requirement for the application. Then, QoS SP in the computer running the application can generate the RSVP message for the application. QoS SP then invokes Traffic Control Interface in the computer, which can classify the application traffic by defining its 802.1p and IP Precedence priorities, and schedule delivery time.
ACS provides QoS policy management, policy control, and SBM. ACS runs as a service on an NT 5.0 server and requires one ACS server per subnet in the network. Microsoft lets network vendors implement ACS in their routers and switches. ACS stores QoS policies in AD and uses LDAP to retrieve policy information from AD. You can link a QoS policy to a user, group, organizational unit, organization, or subnet in AD. When a client requests resource reservation, ACS checks the client's request against the QoS policies in the directory to verify the request.
Figure 4 shows how Microsoft ACS works with AD to implement QoS policy control. User John, who has logged on to the NT 5.0 domain from his computer, starts a QoS-enabled application, which requests a 56Kbps bandwidth reservation. QoS SP in the computer gets John a Kerberos ticket for ACS from the Kerberos server. The Kerberos ticket contains the user account information (e.g., domain\john) and session key. QoS SP forms a PATH message that includes the Kerberos ticket, and sends the message to the ACS server on the same subnet. The ACS server verifies the Kerberos ticket and checks domain\ john's permission to use 56Kbps through an LDAP lookup of John's user policy in AD. If the ACS server verifies the request, it sends the PATH message to the network. Otherwise, the ACS server sends John an error message.
QoS Becomes a Reality
If you're a systems administrator, you've probably dreamed of a tool that can give you better control of your network traffic. When you use QoS to prioritize network traffic and reserve network resources, you can ensure that low-importance network traffic won't bog down mission-critical and time-sensitive applications. By employing QoS policies, you can link network performance to your business priorities. And with Microsoft's support for QoS in NT 5.0 and network vendors' implementation of QoS in network equipment, you can build a QoS-enabled NT network. With QoS, your dream can become a reality.