Quality-Of-Service in Packet Networks: Basic Mechanisms and Directions R
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Computer Networks 31Ž. 1999 169±189 Quality-of-service in packet networks: basic mechanisms and directions R. Guerin ), V. Peris IBM, T.J. Watson Research Center, P.O. Box 704, Yorktown Heights, NY 10598, USA Abstract In this paper, we review the basic mechanisms used in packet networks to support Quality-of-ServiceŽ. QoS guarantees. We outline the various approaches that have been proposed, and discuss some of the trade-offs they involve. Specifically, the paper starts by introducing the different scheduling and buffer management mechanisms that can be used to provide service differentiation in packet networks. The aim is not to provide an exhaustive review of existing mechanisms, but instead to give the reader a perspective on the range of options available and the associated trade-off between performance, functionality, and complexity. This is then followed by a discussion on the use of such mechanisms to provide specific end-to-end performance guarantees. The emphasis of this second part is on the need for adapting mechanisms to the different environments where they are to be deployed. In particular, fine grain buffer management and scheduling mechanisms may be neither necessary nor cost effective in high speed backbones, where ``aggregate'' solutions are more appropriate. The paper discusses issues and possible approaches to allow coexistence of different mechanisms in delivering end-to-end guarantees. q 1999 Elsevier Science B.V. All rights reserved. Keywords: QoS mechanisms; Real-time traffic; Scheduling; Buffer management; Aggregation 1. Introduction to a single, best-effort level of service which is the rule in today's Internet. The Internet is a continuously and rapidly evolv- Providing different levels of service in the net- ing entity, that is being used by an increasingly large work, however, introduces a number of new require- number of applications with diverse requirements. IP ments, that can be classified along two major axes: telephony is one of many new applications driving control path and data path. New data path mecha- the need for substantial changes in the current Inter- nisms are the means through which different services net infrastructure. Specifically, the emergence of ap- will be enforced. They are responsible for classifying plications with very different throughput, loss, or and mapping user packets to their intended service delay requirements is calling for a network capable class, and controlling the amount of network re- of supporting different levels of services, as opposed sources that each service class can consume. New control path mechanisms are needed to allow the users and the network to agree on service definitions, ) Corresponding author. Current address: Department of Elec- trical Engineering, Rm. 367 GRW, 200 South 33rd Street, identify which users are entitled to a given service, Philadelphia, PA 19104-6390, USA, E-mail: and let the network appropriately allocate resources [email protected]. to each service. 1389-1286r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0169-7552Ž. 98 00261-X 170 R. Guerin, V. PerisrComputer Networks 31() 1999 169±189 Data path mechanisms are the basic building gated service guarantees to the set of flows mapped blocks on which network QoS is built. They imple- into the same class. Here, the trade-off is again ment the actions that the network needs to be able to between fairness and efficiency, and complexity. take on user packets, in order to enforce different In Sections 2 and 3, we illustrate through a se- levels of service. As a result, the paper's first goal is lected set of scheduling and buffer management to provide a broad overview of the generic data path schemes, the many alternatives available to network approaches that have been developed to control ac- designers to control how user packets access network cess to resources in packet networks. resources. As mentioned above, the emphasis is just Resources that networks need to manage in order as much on reviewing the basic methods that have to support service differentiation primarily include been developed as on highlighting trade-offs. buffers and bandwidth. The corresponding mecha- Trade-offs are not the prerogative of data path nisms consist of buffer management schemes and mechanisms, and very similar issues exist on the scheduling algorithms, respectively. Buffer manage- control path. This applies to both the type of services ment schemes decide which packets can be stored as available and how they can be requested. For exam- they wait for transmission, while scheduling mecha- ple, services can vary greatly in the type of guaran- nisms control the actual transmissions of packets. tees they provide. Service guarantees can be quanti- The two are obviously related, e.g., scheduling fre- tative and range from simple throughput guarantees quent packet transmissions for a given flow, i.e., to hard bounds on end-to-end delay and lossless allocating it more bandwidth, can help reduce its transmissions. Alternatively, some proposed service need for buffers, and conversely limiting the amount guarantees are qualitative in nature, and only stipu- of buffer space a flow can use impacts the amount of late ``better'' treatment for some packets, e.g., they'll bandwidth it is able to consume. go through a higher priority queue and, therefore, This paper does not attempt an exhaustive review experience lower delay, or will be given a lower of all possible scheduling and buffer management discard threshold in case of congestion. Similarly, schemes. Instead, its aim is to identify the basic requests for service can be dynamic and extend all approaches that have been proposed and classify the way to applications, e.g., using the RSVP proto- them according to the design trade-off they repre- colwx 6 , or may be handled only through static config- sent. In particular, it is important to understand how uration information that defines bandwidth provi- different schemes fare in terms of fairnessŽ access to sioning between specific subnets. excess capacity.Ž , isolation protection from excess In general, control path trade-offs are along two traffic from other users.Ž , efficiency number of flows dimensions: the processing required to support the that can be accommodated for a given level of service, and the amount of information that needs to service.Ž , and complexity both in terms of implemen- be maintained. For example, the complexity of a call tation and control overhead. admission decision to determine if a new request can For example, certain buffer management schemes be accommodated, depends on both the service defi- maintain per flow buffer counts and use that infor- nition itself and how it maps onto the underlying mation to determine if a new packet should be scheduling and buffer management mechanisms. accommodated. Alternatively, other schemes base Similarly, a service such as the Guaranteed Service such decisions on more global information such as wx54 involves a number of parameters used to com- buffer content thresholds and service type indicators pute the end-to-end delay bound and buffer size that in packet headers. The finer granularity of per flow the service mandates. Examples illustrating some of information has benefits in terms of fairness and these issues and the associated trade-off are briefly efficiency, but comes at the cost of greater complex- reviewed in Section 4, that also describes and con- ity. Scheduling algorithms face similar trade-offs. trasts the two services currently being standardized For example, schedulers based on the weighted fair wx54,60 to provide QoS in IP networks. queueing algorithmwx 14,47 can be used to give rate In general, which trade-off is desirable or even and delay guarantees to individual flows, while class feasible is a function of the scalability requirements based mechanisms, e.g., CBQwx 19 , provide aggre- of each environment, i.e., the total amount of infor- R. Guerin, V. PerisrComputer Networks 31() 1999 169±189 171 mation and processing that can be accommodated. gram size Ž.M . The token bucket has a bucket depth, This is a decision that involves both the data path b, and a bucket rate, r. The token bucket, the peak and the control path. For example, per flow schedul- rate, and the maximum datagram size, together de- ing, buffer management, and state maintenance and fine the conformance test that identifies the user processing may be appropriate in a local campus, but packets eligible for service guarantees. The confor- can create scalability problems in the core of a mance test defines the maximum amount of traffic backbone where the number of individual flows will that the user can inject in the network, and for which be orders of magnitude larger. In such an environ- it can expect to receive the service guarantees it has ment, scalability is of prime concern and while per contracted. This maximum amount of traffic is ex- flow service mechanisms remain desirable, ap- pressed in terms of a traffic envelope AtŽ., that proaches that rely on service aggregation may be specifies an upper bound on the amount of traffic needed. We review such issues in Section 5, where generated in any time interval of duration t: we highlight some of the approaches that have been F q q proposed so far and point to some of the challenges AtŽ.min ŽM pt,b rt ..1 Ž. being faced. In particular, we discuss the need for Eq.Ž. 1 simply states that the user can send up to interoperability between fine grain and coarse grain b bytes of data at its full peak rate of p, but must solutions, and outline possible solutions. then lower its rate down to r. The presence of the factor M in the first term is because of the packet nature of transmissions, where up to one maximum 2.