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QoS (Quality of Service)
A network with quality of service has the ability to deliver data traffic with a minimum amount of delay in an environment in which many users share the same network. QoS should not be confused with CoS (class of service). CoS classifies traffic into categories such as high, medium, and low (gold, silver, and bronze). Low-priority traffic is "drop eligible," while high-priority traffic gets the best service. However, if the network does not have enough bandwidth, even high-priority traffic may not get through. Traffic engineering, which enables QoS, is about making sure that the network can deliver the expected traffic loads.
A package-delivery service provides an analogy. You can request priority delivery for a package. The delivery service has different levels of priority (next day, two-day, and so on). However, prioritization does not guarantee the package will get there on time. It may only mean that the delivery service handles that package before handling others. To provide guaranteed delivery, various procedures, schedules, and delivery mechanisms must be in place. For example, Federal Express has its own fleet of planes and trucks, as well as a computerized package tracking system. Traffic engineers work out flight plans and schedule delivery trucks to make sure that packages are delivered as promised.
The highest quality of service is on a nonshared communication link such as a cable that directly connects two computers. No other users contend for access to the network. A switched Ethernet network in which one computer is attached to each switch port can deliver a high level of QoS. The only contention for the cable is between the computers that are exchanging data with one another. If the link is full duplex, there is no contention. Situations that cause QoS to degrade are listed here:
The starting point for providing QoS in any network is to control and avoid congestion. See "Congestion Control Mechanisms" for more information.
What can be done to improve QoS? The obvious solution is to overprovision network capacity and upgrade to the most efficient networking equipment. This is often a practical solution in the private network environment, but not for private WAN links. Another solution is to classify traffic into various priorities and place the highest priority traffic in queues that get better service. This is how bandwidth is divided up in packet-switched networks. Higher-level queues get to send more packets, and so get a higher percentage of the bandwidth. New optical networks in the Internet core provide QoS with excess bandwidth. A single fiber strand can support hundreds or even thousands of wavelength circuits (lambdas). Lambdas can provide single-hop optical pathways between two points with gigabit bandwidth. A single circuit can be dedicated to traffic that needs a specific service level. See "Optical Networks."
Service providers have been reluctant to implement QoS across their networks because of the management and logistics problems. If subscribers don't classify traffic in advance, then the provider will need edge devices that can classify traffic going into their networks. QoS features must also be set up from one end of a network to another, and that is often difficult to accomplish. QoS levels must be negotiated with every switch and router along a path. Still, QoS is getting easier to manage, and, in some cases, it is the only way to optimize network bandwidth.
Leading-edge service providers now offer a range of QoS service levels for Internet traffic. Subscribers specify QoS requirements in SLAs (service-level agreements). Some of the SLA specifications required for QoS are described here:
Of course, the range, location, and ownership of the network will make a big difference in how QoS is applied. An enterprise may wish to install QoS on its own intranet to support voice and video. QoS may also be applied to the LAN/WAN gateway to ensure that private WAN links or VPNs are appropriately loaded and provide quality service for intercompany voice calls, videoconferences, and so on. Most of the focus for QoS technologies is centered on the Internet because it lacks features that can provide QoS.
Service Levels: IP Versus ATM
The Internet is a connectionless packet-switching network, meaning that without any special QoS provisions, all services are best effort. In contrast, leased lines and ATM naturally support QoS because they deliver data in a predictable way. Leased lines such as T1 circuit use TDM (time division multiplexing), which provides fixed-size repeating slots for data. ATM uses fixed-size cells and has built-in traffic engineering parameters to ensure QoS.
Obtaining QoS in IP networks is not so easy, primarily for the following reasons:
Consider a typical LAN/WAN interface. It is an aggregation point where traffic from many sources inside the network comes together for transmission over the WAN link. If the WAN link has insufficient bandwidth, congestion will occur.
In the preceding scenario, all packets are equal. Packets for mission-critical applications may be dropped, while packets carrying the latest Dilbert cartoon get through. Classification is essential. Fortunately, packet classification is now easy with multilayer routing solutions from vendors such as Extreme Networks. See "Multilayer Switching." Still, the service these devices offer is more CoS oriented. Keep in mind that true QoS requires bandwidth management and traffic engineering across the networks that packets will travel.
ATM networks provide a number of native features to support QoS:
As a point of comparison, the Internet has no admittance controls, which is probably good, but it also means that long file transfers can consume bandwidth and prevent other packets from getting through. This is especially disruptive to real-time traffic.
The following sections describe the various techniques that may be used to provide QoS on the Internet and in enterprise networks. Note that some of these solutions provide only partial QoS, but are required to provide higher levels of service. The various solutions may be categorized as follows:
Congestion Management Techniques
Managing network congestion is a critical part of any QoS scheme. TCP has some rudimentary congestion controls. The technique relies on dropped packets. When a packet is dropped, the receiver fails to acknowledge receipt to the sender. The sender assumes that the receiver or the network must be congested and scales back its transmission rates. This reduces the congestion problem temporarily. The sender will eventually start to scale up its transmissions and the process may repeat.
Packets are dropped because a router queue is full or because a network device is using a congestion avoidance scheme, such as RED (random early detection). RED monitors queues to determine when they are getting full enough that they might overflow. It then drops packets in advance to signal senders that they should slow down. Fewer packets are dropped in this scheme.
The problem with RED is that it relies on dropping packets to signal congestion. ECN (explicit congestion control) is an end-to-end congestion avoidance mechanism in which a router that is experiencing congestion sets a notification bit in a packet and forwards the packet to the destination. The destination node then sends a "slow down" message back to the sender.
Traffic shaping is a technique that "smoothes out" the flow of packets coming from upstream sources so that downstream nodes are not overwhelmed by bursts of traffic. An upstream node may be a host, or it may be a network device that has a higher data rate than the downstream network. At the same time, some hosts with priority requirements may be allowed to burst traffic under certain conditions, such as when the network is not busy. A traffic shaper is basically a regulated queue that takes uneven and/or bursty flows of packets and outputs them in a steady predictable stream so that the network is not overwhelmed with traffic.
Classification, Admission, and Tagging
Any QoS scheme involves guaranteeing service levels to traffic flows. In a world of infinite bandwidth, all flows could be handled equally. But networks are still bandwidth limited and congestion problems occur due to improper network design. Therefore, traffic must be classified-and, in some cases, tagged-so that downstream devices know what to do with it. Basic classification techniques are outlined here:
Extreme Networks has a line of switches with built-in traffic classification features. Figure Q-1 shows an example of bandwidth allocation for various types of traffic.
Classification requires administrative decisions about how traffic should be classified and where it should be tagged. Administrators might classify traffic based on whether it is best effort and suitable for discard, real-time voice and video, network controls (e.g., OSPF messages), or mission critical.
The following classification schemes identify traffic near its source and mark packets before they enter the network. Network nodes only need to read the markings and forward packets appropriately.
The first scheme works over LANs, while Diff-Serv works over internetworks. The tag information in MAC-layer frames will be lost if the frame crosses a router. However, some method may be used to capture the information and use it to set Diff-Serv markings.
As mentioned, the IEEE defined a method for inserting a tag into an IEEE MAC-layer frame that contains bits to define class of service. During development, this was known as Project 802.1p, and you will see it referred to that way in much of the literature. It is now officially part of IEEE 802.1D-1998. The tag defines the following eight "user priority" levels that provide signals to network devices as to the class of service that the frame should receive:
A method for reordering and moving delay-sensitive real-time traffic to the front of a queue is also defined. A component of this scheme is GARP (Group Address Registration Protocol), which is used by LAN switches and network-attached devices to exchange information about current VLAN configurations. Note that 802.1D-1998 provides at the LAN level what Diff-Serv provides in layer 3 across internetworks. MAC-layer tags may be used to signal a class of service to Diff-Serv.
Two Web sites provide additional information:
The role of the IP ToS field has changed with the development of Diff-Serv. The original meaning of the ToS field was defined in RFC 791 (Internet Protocol, September 1981); however, it was never used in a consistent way. Most routers are aware of the field, but it has little meaning across public networks. Many enterprises have used it internally to designate various classes of service or to prioritize traffic across private WAN links.
The ToS field is divided into two sections: the Precedence field (three bits), and a field that is customarily called "Type-of-Service" or "TOS" (five bits). Interestingly, The Precedence field was intended for Department of Defense applications to signal a priority message in times of crisis or when a five-star general wanted to get a good tee time.
Diff-Serv redefined the field as the DS Field (Diff-Serv Field). RFC 2474 (Definition of the Differentiated Services Field in the IPv4 and IPv6 Headers, December 1998) describes this further. See "Differentiated Services (Diff-Serv)."
IETF QoS Solutions
The IETF has been working to define Internet QoS models for many years. The task has not been easy since packets must cross many networks, and providers must agree not only how QoS will be managed, but also how it is paid for. The primary QoS techniques developed by the IETF are Int-Serv (Integrated Services), Diff-Serv (Differentiated Services), and MPLS (Multiprotocol Label Switching), as described next. Each of these is discussed under its own heading elsewhere.
Policies and Policy Protocols
The final pieces of the QoS picture are policies, policy services, and policy signaling protocols. Most of the QoS systems just described use policy systems to keep track of how network users and network devices can access network resources. A defining feature of a policy system is that it works across a large network and provides policy information to appropriate devices with that network.
A policy architecture consists of the following components, which primarily manage the rules that govern how network resources may be used by specific users, applications, or systems. When rules are specified and programmed into policy systems, they are known as policies.
This architecture allows network administrators to specify policies for individuals, applications, and systems in a single place-the policy information system. The policy server then uses protocols such as LDAP (Lightweight Directory Access Protocol) or SQL to obtain this information and form policies that can be distributed to policy clients. Policy clients talk to policy servers via network protocols such as COPS (Common Open Policy Service) and SNMP (Simple Network Management Protocol). COPS is an intradomain mechanism for allocating bandwidth resources and it is being adapted for use in establishing policy associated with a Diff-Serv-capable networks.
This topic is covered in more detail under "Policy-Based Management." In addition, RFC 2768 (Network Policy and Services: A Report of a Workshop on Middleware, February 2000) provides useful information about policy.
Additional QoS Information
The following IETF working groups are developing QoS recommendations and standards. Refer to the working group pages for a list of related RFCs and other documents.
The following RFCs provide more information about QoS. More specific RFCs are listed under the headings just mentioned.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.