Multiplex communications – Pathfinding or routing
Reexamination Certificate
2000-02-29
2004-10-26
Pham, Chi (Department: 2667)
Multiplex communications
Pathfinding or routing
Reexamination Certificate
active
06810031
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of high speed data packet processing for networking systems, and in particular to controlling bandwidth in networking systems which are characterized by high-speed switches that switch data packets having variable size and format requirements.
BACKGROUND OF THE INVENTION
In the field of networking systems, and in particular communication systems for the Internet, switch fabrics are used to direct data packets, for example, between different data packet processing modules. With the increasing speed in data transfer rates, improving efficiency and predictability of allocating and using bandwidth across switch fabrics of systems, such as routing devices, is increasingly crucial to maintaining the reliability of these devices at these high speeds. Such a need is particularly evident in data transfer over the Internet.
Historically, quality of service (QoS) on the Internet has been defined by a “best effort” approach. The “best effort” approach provides only one class of service to any connection, and all connections are handled with equal likelihood of experiencing congestion delays, with no priority assigned to any connection. With traditional Internet applications and transfer needs, this “best effort” approach was sufficient. However, new applications require significant bandwidth or reduced latencies. Bandwidth and latency are critical components of the QoS requirements specified for new applications. Bandwidth is the critical factor when large amounts of information must be transferred within a reasonable time period. Latency is the minimum time elapsed between requesting and receiving data and is important in real-time or interactive applications. In order to support these QoS guarantees through a network, it is essential that network nodes support such QoS.
Distribution of the available bandwidth across a switch fabric provides for trade-offs of bandwidth between different flows of data packets through a common switch fabric. This distribution permits the flexible allocation of QoS in accordance with the negotiated traffic contracts between users and service providers. Bandwidth distribution can affect the throughput performance of scheduling algorithms because such scheduling tries to match contracted throughput to the traffic arrival process. The ability to perform fast and reliable bandwidth distribution across the switch fabric permits the efficient utilization of the switch fabric bandwidth while maintaining rate guarantees to individual connections.
Known methods and schemes used to solve the problem of allocating bandwidth across a switch fabric were implemented through negotiation or through selective backpressure. In these known methods, bandwidth allocation is provided on a fixed length cell basis, and not on a more preferred variable length packet basis. For example, in these methods, each cell may be broadcast to output blocks which filter the cells and retain only those cells actually destined to the outputs comprising the block. The process is iterated down to the individual output port. This solution is similar to output buffering except that in this process, the “output” buffers are distributed throughout the switch fabric. As a result, the switch fabric can be made to be internally non-blocking with smaller speedup, and multicasting can be efficiently implemented. This implementation requires the replication of hardware in the form of switch fabric elements. The flow control needed to provide QoS is achieved by means of a Dynamic Bandwidth Allocation (DBA) protocol. In this protocol, at each input queue there is a virtual output queue associated with every input, with an explicit rate across the switching fabric which is negotiated between each input and output based on a set of thresholds which are maintained for each input queue. Each threshold is associated with a transmission rate from the input port into the switch fabric. In allocating these rates, the known method ensures that adequate bandwidth exists at the two points of contention: at the input link from the input buffer to the switch fabric, and at the output link, from the switch fabric to the output buffer. Real-time traffic bypasses the scheduling and is transported with priority across the switching fabric. The disadvantage in allocating bandwidth by this method is that the bandwidth is allocated in bursts which results in some loss of throughput.
In a known prior art device, the switch fabric consists of a non-blocking buffered Clos network. The middle stage module of the Clos network is not buffered in order to prevent sequencing problems of cells belonging to an individual connection. As a result, the modules need to schedule cells across the middle stage, with scheduling accomplished using a concurrent dispatching algorithm. Output buffering is emulated by utilizing selective backpressure across the switching fabric. However, the selective backpressure, combined with four levels of priority, in such a device provides a limited amount of flow control and cannot maintain guaranteed rates. The selective backpressure also complicates the multicasting function considerably.
In another known prior art system, high-bandwidth links implement a purely input buffered switch fabric with large throughput by using input scheduling based on the iSLIP-scheduling algorithm. The QoS provided by such a scheme is however limited.
Another known prior art system incorporates flow control by the use of statistical matching. In statistical matching, the matching process is initiated by the output ports, which generate a grant randomly to an input port based on the bandwidth reservation of that input port. Each input port receiving transfer grants selects one randomly by weighting the received grants by a probability distribution, which is computed by assuming that each output port distributes bandwidth independently based on the bandwidth reservation. However, matching is done on a cell-slot basis and the improvement in throughput achieved by statistical matching is limited.
Other prior art devices control data flow by means of the Weighted Probabilistic Iterative Matching(WPIM) algorithm. In WPIM, time is divided into cycles and credits are allocated to each input-output pair per cycle. The scheduling is then performed on a cell-slot basis by means of WPIM, with the additional feature that at each output port, when the credit of an input port is used up, its request is masked, making it more likely for the remaining input ports to be allocated in that particular slot. However, in WPIM, the computation of the credits does not take into account the outstanding credits, and is susceptible to large delays for traffic that is “bursty.”
Some prior art methods provide data flow control using a Real-Time Adaptive Bandwidth Allocation (RABA) algorithm which provides multi-phase negotiation for cells over a time frame, with a frame-balancing mechanism that uses randomization over a frame in order to reduce contention between cells destined to the same output port. Cells are transmitted only after being scheduled, which results in a latency overhead. In addition, there is control and latency overhead in the negotiation.
Performing bandwidth distribution at high speeds while maintaining rates for a large number of flows on a cell-time basis is demanding and particularly difficult to manage in a node where variable length packets are being switched across a common switch fabric. To perform the bandwidth distribution using a cell-time basis at these high speeds would require expensive and complex hardware.
Therefore, what is needed is a method and device for scheduling bandwidth in cycles across a switch fabric at a packet processing node that maintains allocated bandwidth to individual users, that maintains allocated bandwidth to groups of users who share bandwidth, and that provides high levels of throughput across the switch fabric with controlled buffer occupancy and low latency. Additionally, a method and device is needed that provides for meeting requi
Bordes Jean Pierre
Davis Curtis
Hegde Manju
Maher Monier
Schmid Otto Andreas
Celox Networks, Inc.
Jones Prenell
Pham Chi
Wilmer Cutler Pickering Hale and Dorr LLP
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