Lossy Fabric Transmitting Device Back Pressure Relief
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Solution Overview
Problem
Identifying and addressing failures in lossy fabric topologies within fabric-based computer systems is costly and difficult, leading to back pressure issues that render multiple routes unusable, even those not reliant on the failed component.
Innovation Solution
A lossy fabric transmitting device that identifies the failed link by using a discard counter to track packet discarding, converting adjacent link transmitters into 'bit buckets' to relieve back pressure, ensuring a non-zero minimum rate of packet forward progress across the fabric.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If packet dropping is implemented to ensure forward progress in lossy fabric, then minimum forward progress rate is maintained, but back pressure propagates to upstream links rendering multiple routes unusable
Solution Approach 1:
The fabric is segmented into lossless and lossy regions through virtual channel segmentation. Lossless virtual channels are used for control traffic and credit transmission, while lossy virtual channels handle data traffic. This segmentation allows packet dropping to occur only in lossy regions without propagating back pressure to upstream links, maintaining route usability while ensuring forward progress.
Solution Approach 2:
Virtual channels act as intermediaries between upstream lossless links and downstream lossy links. The virtual channel buffer absorbs packet drops and prevents back pressure from propagating upstream, serving as a mediator that isolates the impact of lossy fabric operations from the rest of the network.
2Loss of time
If hierarchical back pressure timers are used to manage queue emptying, then packet transmission timeout is controlled, but the solution is only meaningful in lossless fabric and requires hierarchical topology
Solution Approach 1:
The patent changes the fundamental parameter of packet handling from timeout-based hierarchical emptying to credit-based flow control. Instead of using time thresholds that require hierarchical topology, the system uses credit tokens that can be issued and tracked independently at each link, making the solution adaptable to any fabric topology including non-hierarchical and lossy environments.
3Reliability
If redundant routes are designed for reliability, then fault tolerance is improved, but back pressure from component failures renders redundant routes unusable
Solution Approach 1:
By segmenting the fabric into lossless and lossy regions using virtual channels, the patent ensures that back pressure and packet drops are confined to lossy regions. Redundant routes can traverse through lossy regions without being blocked by back pressure, as the virtual channel segmentation isolates the impact of failures to specific regions while maintaining connectivity through alternative paths.
Data Source
AI summary
A lossy fabric transmitting device includes a queue, a link transmitter to transmit packets from the queue, a trigger mechanism to automatically discard a packet contained in the queue in response to satisfaction of a packet dropping threshold and a discard counter to track packets being discarded from the queue. The discard counter has a failure detection threshold. The discard counter resets in response to the link transmitter transmitting a packet. Satisfaction of the failure detection threshold identifies the link transmitter as being immediately adjacent a failed link of a lossy fabric.