Fabric Access IC Bounding Cell Reorder Buffer Depth
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Solution Overview
Problem
In data communication networks with independent switch planes, buffering is required to reorder out-of-sequence cells, leading to potential buffer overflow when one switch plane fails or experiences heavy load, and increasing buffer size is costly and inefficient.
Innovation Solution
Implementing a technique to limit cell reorder buffer depth by using source-based backpressure, where backpressure information is sent to sources when the buffer reaches a threshold, using a reserved field in cells to manage congestion and prevent overflow, and dynamically allocating memory to optimize buffer size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the buffer size is increased to prevent buffer overflow when cells arrive out of sequence, then the reliability of data transfer is improved, but the cost of the switch fabric components increases significantly
Solution Approach 1:
The reorder buffer is divided into multiple independent buffer regions, each associated with a specific source port or source port class. This segmentation allows each buffer to be sized appropriately for its specific traffic pattern rather than providing a large universal buffer, reducing overall memory cost while maintaining reliability for each source's cell sequence
Solution Approach 2:
Different buffer allocation strategies are applied to different sources based on their traffic characteristics. Sources that frequently send out-of-sequence cells receive larger buffer allocations, while sources with orderly transmission receive smaller allocations. This local optimization ensures reliability where needed without uniformly increasing cost across all sources
2Productivity
If the reorder buffer depth is increased to accommodate more out-of-sequence cells, then the productivity of cell reordering is improved, but the device complexity increases
Solution Approach 1:
Buffer regions are pre-configured and associated with specific sources before cells arrive. When cells need reordering, the system simply places them in the pre-designated buffer region for that source, eliminating the need for complex dynamic buffer allocation decisions during operation and simplifying buffer management while maintaining high reordering capacity
Solution Approach 2:
A buffer depth bounding mechanism acts as an intermediary between the reorder buffer and the rest of the system. This mechanism monitors buffer depth and applies backpressure to sources when buffers approach capacity, preventing overflow without requiring extremely large buffers, thus maintaining productivity with controlled complexity
3Loss of information
If backpressure is applied to limit reorder buffer depth, then the loss of data is reduced, but the speed of cell transmission may be reduced
Solution Approach 1:
Backpressure is applied periodically or threshold-based rather than continuously. The system monitors buffer depth and only applies backpressure when thresholds are approached, allowing normal high-speed transmission to continue during typical operation while preventing data loss during congestion or failure conditions through intermittent backpressure application
Data Source
AI summary
Bounding cell reorder depth is disclosed. It is determined whether a reorder buffer that is associated with a switch fabric exceeds a depth limit. In the event that the reorder buffer exceeds the depth limit, backpressure with respect to a source associated with the reorder buffer is sent. In such cases source based back pressure information is included in a reserved field in a cell that is being sent into the switch fabric. In the event that the reorder buffer does not exceed the depth limit, the cell is sent to the switch fabric. In such cases the reserved field is present in the cell but is unused.