RFC 2212 (rfc2212) - Page 2 of 20
Specification of Guaranteed Quality of Service
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RFC 2212 Guaranteed Quality of Service September 1997 In brief, the concept behind this memo is that a flow is described using a token bucket and given this description of a flow, a service element (a router, a subnet, etc) computes various parameters describing how the service element will handle the flow's data. By combining the parameters from the various service elements in a path, it is possible to compute the maximum delay a piece of data will experience when transmitted via that path. It is important to note three characteristics of this memo and the service it specifies: 1. While the requirements a setup mechanism must follow to achieve a guaranteed reservation are carefully specified, neither the setup mechanism itself nor the method for identifying flows is specified. One can create a guaranteed reservation using a protocol like RSVP, manual configuration of relevant routers or a network management protocol like SNMP. This specification is intentionally independent of setup mechanism. 2. To achieve a bounded delay requires that every service element in the path supports guaranteed service or adequately mimics guaranteed service. However this requirement does not imply that guaranteed service must be deployed throughout the Internet to be useful. Guaranteed service can have clear benefits even when partially deployed. If fully deployed in an intranet, that intranet can support guaranteed service internally. And an ISP can put guaranteed service in its backbone and provide guaranteed service between customers (or between POPs). 3. Because service elements produce a delay bound as a result rather than take a delay bound as an input to be achieved, it is sometimes assumed that applications cannot control the delay. In reality, guaranteed service gives applications considerable control over their delay. In brief, delay has two parts: a fixed delay (transmission delays, etc) and a queueing delay. The fixed delay is a property of the chosen path, which is determined not by guaranteed service but by the setup mechanism. Only queueing delay is determined by guaranteed service. And (as the equations later in this memo show) the queueing delay is primarily a function of two parameters: the token bucket (in particular, the bucket size b) Shenker, et. al. Standards Track
