Resource Management Cells Research Articles

A performance prediction model for FECN rate-based flow control in ATM networks: a case study

Forward explicit congestion notification (FECN) plays an important role in rate-based flow control for asynchronous transfer mode (ATM) networks. This paper presents a case study in predicting and evaluating the performance of the FECN flow control. Two network models are under investigation; the first model assumes that a target ATM network has no traffic congestion, while the second model assumes that the network has exhibited heavy congestion. For the light-loaded network, we first estimate the number of cells issued by the source during each time interval and then calculate the required number of resource management (RM) cells in the same time interval. A general analytical form to predict allowable cell rate (ACR) at the source can be finally derived. For the heavy-loaded network, RM cells may be delayed and data cells may be discarded due to severe network congestion. Hence, to derive a general form of ACR, we need to estimate the round-trip time (RTT) of RM cells based on the queuing length of the traversed ATM switches. For the purpose of validation, we perform simulation through OPNET. The performance parameters of our interest include the ACR, the RTT of RM cells, the throughput and the cell loss ratio. We have shown that the differences between the analytical and the simulation results are quite small not only in the ACR adjusting rate, but also in the prediction of the system throughput, the CLR, and the RTT of RM cells.

Fast, Fair and Frugal Bandwidth Allocation in ATM Networks

Abstract. ATM networks are used to carry a variety of types of traffic. For some types of traffic, in particular Available Bit Rate (ABR) traffic, the bandwidth of a network is typically insufficient to satisfy the requests of all the sessions, and so some fair allocation scheme must be devised. The ATM Forum, the standards setting body for ATM networks, has specified that the fairness criterion for ABR traffic should be Max-Min Fairness , which intuitively means that raising the bandwidth of any session comes at the expense of some other session of no greater bandwidth. Protocols to allocate bandwidth to sessions in a max-min fair manner are an important part of a network design. For a protocol to be realistic, it must conform to the Resource Management (RM) cell mechanism specified by the ATM Forum. Such RM cells get sent as a constant fraction of all cells sent by the source; however, they have only a few fields. RM cells are the only means of communication allowed from link to link so any reasonable protocol is totally distributed and asynchronous, since the RM cell mechanism does not easily lend itself to synchronization. Finally, RM Cells must be handled very quickly at each link. We call a protocol frugal if at each link it performs O(1) computation per RM cell and uses O(1) space per session. Recently, several frugal RM cell protocols have been proposed for ABR traffic, but none have been shown to converge to the max-min fair state. Protocols that are known to converge in a linear number of maximum round-trip times require RM cell processing that is linear in the number of sessions that go through a link. In this paper we give a frugal RM cell protocol for ABR that matches the convergence time of the fastest known non-frugal protocol. A second type of ABR traffic is the Minimum Cell Rate (MCR) type, where every session can specify a minimum amount of bandwidth. The max-min fair allocation should then respect these MCR requests. We extend our results to give the first frugal RM cell protocol for MCR and achieve a quadratic convergence rate.

Correctness and performance of the ATM ABR rate control scheme

We study both correctness and performance of the source/destination protocols of the available bit rate (ABR) service in asynchronous transfer mode (ATM) networks. Although the basic protocol for rate-based congestion management is relatively simple, the protocol specification has to cope with several “real-world” cases such as failures and delayed/lost feedback which introduce complexity. Rigorous proof of the correct functioning of the protocol based on a formal specification is necessary. We use a formal model to show that the ABR source/destination protocol is free of livelocks, so that under all conditions both resource management (RM) and data cells will be transmitted. Furthermore, if there are data cells available, then the ABR protocol is deadlock free; otherwise, the system goes to a desirable sleep state waiting for data cells, as long as certain parameters are set appropriately at connection setup. We also show that the network options of explicit forward congestion indication (EFCI) and explicit rate (ER) interoperate correctly. In addition to ensuring the correct functioning of the protocol, it is essential that pathological situations do not result in very poor performance, which we view as another form of “incorrect operation”. We derive conditions that ensure that the source's allowed cell rate (ACR) is stable in the presence of delayed or lost feedback RM cells. We arrive at bounds on the number of consecutive RM cell losses tolerated while the ACR rate remains stable. We also provide an asymptotic estimate of ACR and the allowable RM cell loss probability to ensure that ACR is stable, statistically. The ABR protocol contributes to the feedback delay in two ways: the source delay of sending out the probe forward RM (FRM) cells and the destination delay of turning around the backward RM (BRM) cell. We provide a worst-case analysis of the delay in turning around RM cells at the destination station and the worst-case inter-departure time of FRM cells from the source.

Interoperability between EFCI and ER switch mechanisms for ABR traffic in an ATM network

The rate-based flow control mechanism in Asynchronous Transfer Mode (ATM) networks controls the transmission rate of Available Bit Rate (ABR) traffic sources, based on feedback information contained in the Resource Management (RM) cells coming from the destination and/or the network switching nodes. While the source and destination end systems behavior is fully described in the ATM Forum Traffic Management Specification, the network switch mechanism is implementation dependent. Since there is a variety of algorithms an ATM switch vendor can choose from, different performance characteristics are expected. This paper investigates the interoperability issues in terms of network performance and fairness of bandwidth allocation when an ATM network consists of switches using different rate control mechanisms, namely, the Explicit Forward Congestion Indication (EFCI) mode and the Explicit Rate (ER) mechanism. Simulation results show that these algorithms will interoperate as long as the switch implementation conforms to the end system behavior for congestion notification and usage of RM cells. Moreover, the location of an ER switch in a given topology might be critical to solving the “beat-down” phenomenon, where traffic passing through a larger number of switches might get less bandwidth than single hop traffic.

Evolution of controls for the available bit rate service

In the summer of 1993, the asynchronous transfer mode (ATM) community began a search for a mechanism to allocate bandwidth dynamically within an ATM network, while simultaneously preventing data loss. By the fall of 1994, the ATM Forum had introduced a new service category for this purpose, the available bit rate (ABR) service, and had selected a rate-based mechanism for its support. Simply put, the mechanism would consist of a stream of ABR resource management (RM) cells, generated by one end of the connection (the source) and looped back by the other (the destination), into which switches would encode the maximum rate at which the ABR source was to generate data. The ABR service is an addition to ATM that uses a feedback mechanism to control the transmission rates of traffic sources. Although the performance of the ABR service will depend critically on the quality of feedback sent by switches, the algorithms for determining this feedback are largely outside the scope of ATM standards and specifications. We describe how two of the properties implemented by the ABR source and destination, out-of-rate RM cells and a use-it-or-lose-it policy, were implicitly architectural decisions, and we explain how and why the ATM Forum ultimately revised these decisions. We then discuss how the ATM Forum addressed the issues of conformance testing, point-to-multipoint connections, and parameter negotiation for the ABR service. At the end, we reflect on the process by which the ABR specification was developed.