Build a Better Network with QoS

Today's networked applications—such as enterprise resource planning (ERP), data mining, e-commerce, and multimedia—are bandwidth hungry, time sensitive, and mission critical. These applications need networks to accommodate today's business priorities. Traditional networks can't recognize priority data because they handle network traffic in old-fashioned ways, such as best-effort or first-come first-served. For example, when application data enters a traditional network, the network allocates as much bandwidth as the application needs—until the network runs out of bandwidth. Mission-critical applications, such as SAP R/3, and time-sensitive applications, such as NetMeeting videoconferencing, can drown in a flood of less important network traffic—such as a PointCast Web newscast. Systems administrators end up facing network congestion, slow response, and packet-dropping problems.

Simply throwing more bandwidth at your network is not the ultimate solution to these problems, because you can't foresee what new bandwidth-hungry applications will be in use in several months. An ill-behaved application can easily bring your network down—and potentially shut down your business operation.

What Is QoS?
To gain more effective control of your network, you need to incorporate Quality of Service (QoS). In a QoS-enabled network, you can prioritize network traffic flow, allocate network bandwidth and resources to different applications and users, enforce security to applications and users entering your network, and link business needs with desired network behavior. For example, you can guarantee that an SAP R/3 application has the highest network priority and reserve a specific bandwidth for the finance department, and assign PointCast the lowest network priority and a limit on allowable bandwidth.

QoS has been in use for several years. IP supports QoS in the IP header. Asynchronous transfer mode (ATM) natively provides QoS in its virtual circuits and various bit-rate controls. Recently, the Internet Engineering Task Force (IETF) developed Resource Reservation Setup Protocol (RSVP) as a QoS standard for TCP/IP networks and the Internet. The Institute of Electrical and Electronics Engineers (IEEE) defined the 802.1p standard for QoS in all IEEE 802-type networks, such as Ethernet-standard networks.

Major network vendors, including 3Com, Bay Networks, and Cisco, have developed road maps to QoS implementation and started to deliver QoS-enabled network equipment and management tools. Microsoft is building QoS into Windows NT 5.0 and has released QoS APIs for developers to use in writing QoS-enabled applications. Designing and building QoS-enabled NT networks and applications might soon be at the top of your task list. In this article, I'll help you jump onto the QoS bandwagon by giving you an overview of QoS signaling techniques and queuing mechanisms. I'll describe QoS policy management and implementation methods. Finally, I'll explain Microsoft's implementation of QoS in NT 5.0.

QoS Signaling Techniques
QoS includes four reservation techniques—IP Precedence, 802.1p, RSVP, and Subnet Bandwidth Manager (SBM)—which are also called QoS signaling. QoS signaling lets routers and switches better control network traffic.

IP Precedence and 802.1p. When IETF designed IPv4, the current Internet network protocol, the task force defined a Type of Service (ToS) field in the IP header of IP packets. IP uses 3 bits of the 8-bit ToS field to designate IP Precedence. Using IP Precedence, systems administrators can partition their IP traffic into up to eight classes of service, numbered from 0 to 7. The higher the number, the higher the priority. For example, you might assign SAP R/3 priority number 7, SQL Server priority number 6, and NetMeeting priority number 5. Networks deliver higher-priority packets before they deliver lower-priority packets. QoS-enabled applications, computers, routers, and Layer 3 switches can set the IP Precedence designation for each IP packet when they generate or transmit the packet. By examining IP Precedence, routers and Layer 3 switches can easily prioritize arriving packets in different queue classes and deliver the packets based on their priority. IETF is researching how to use other bits in the IP header's ToS field for QoS. IETF has included the 8-bit Class and 20-bit Flow Label fields in the IP header of IPv6, the next generation IP, so that IPv6 will be ready to support more comprehensive QoS in the future. (To learn more about IPv6, see "The Next Generation IP in Action," June 1998.)

In contrast to IP Precedence, which lets administrators prioritize IP-only packets at Layer 3, IEEE defined the 802.1p standard for its 802-type (e.g., Ethernet and Token Ring) network at Layer 2. When IEEE developed the 802.1Q standard for virtual LANs (VLANs), it specified a VLAN tag that the standard appends to media access control (MAC) frames to carry VLAN information. This tag includes two types of information: 12 bits for the VLAN ID, and 3 bits for the priority designation. Although 802.1Q defines the tag format, the standard doesn't specify how to use the 3-bit priority field. The 802.1p standard, however, specifies how to use the 3-bit priority field. The 802.1p standard establishes eight levels of priority—0 to 7—similar to IP Precedence. NICs and switches supporting 802.1p can use the priority levels to route different types of network traffic. The 802.1p prioritization working at Layer 2 can support any upper-layer protocols, which means you can use 802.1p to define priorities for IPX, SNA, and AppleTalk applications, as well as IP applications. Layer 3 switches can also easily map 802.1p's eight priority levels to the corresponding eight priority levels in IP Precedence before the switches forward packets to routers.