A generalized processor sharing approach to flow control in integrated services networks

The problem of how to assign network resources to the new users of an integrated services network is investigated in the context of rate based ow control. In packet networks that support varied tra c types such as voice, video, image and interactive data, many of the services may have stringent delay requirements on every packet submitted to the network. Thus, just providing average performance guarantees may not be adequate for these services. We propose a exible and e cient multiplexing scheme called Generalized Processor Sharing (GPS) that allows the network to make worst-case performance guarantees. The scheme is exible, in that one user may receive a \better" quality of service without adversely a ecting the performance guarantees of the others. In addition, GPS is shown to be fair to every packet that enters the system unlike other multiplexing schemes such as First-Come-First-Serve. A practical packet-by-packet transmission scheme that closely approximates Generalized Processor Sharing is also presented. This allows us to relate results for GPS to the packet-by-packet scheme in a precise manner. Both GPS and PGPS were suggested earlier in [6], in the context of managing congestion in gateways. The performance of a single server GPS system is analyzed from the standpoint of worst-case delay and burstiness when the sources are constrained by leaky buckets. The analysis yields a simple resource assignment scheme that allows the server to make worst-case delay and rate guarantees to every session in the system. Other multiplexing schemes such as FCFS and Strict Priority are also analyzed in the context of sources that are leaky bucket constrained. Next, we extend the results for the single server case to provide worst-case bounds on session delay and backlog for several broad classes of GPS networks. In Rate Proportional Processor Sharing Networks (RPPS), the server parameters are assigned in proportion to the average session rates. We bound the worst-case delay and backlog for every session in terms of its leaky bucket parameters. It is interesting that these bounds are independent of the behavior of the other sessions, and also of the network topology. Extensions of RPPS are discussed in which the same bounds on delay and backlog hold for a subset of the sessions. 2 We then show how to characterize the internal tra c (in terms of average rate and burstiness) for any acyclic network, and also for a broad class of non-acyclic networks called Consistent Relative Session Treatment networks. Next, we provide a method, based on an independent sessions assumption, to obtain bounds on session delay and backlog for any network for which internal tra c has been characterized. Our method analyzes the session i route as a whole, and yields bounds that are much better than those that result from adding the worst-case delays (backlogs) at each of the servers encountered by a session. We bound delay and backlog for each session, based on an e ciently computable universal service curve, and show that a \staggered" greedy regime achieves the worst-case bounds under the independent sessions assumption. Propagation delay is also incorporated into the model. Finally, we relate our analysis of arbitrary topology GPS networks to arbitrary topology PGPS networks. We nd that for small packet sizes, the behavior of the two schemes is virtually identical, and demonstrate the e ectiveness of PGPS in guaranteeing worst-case session delay under Rate Proportional Processor Sharing assignments. Thesis Supervisor: Robert G. Gallager Title: Fujitsu Professor of Electrical Engineering