About Transient Capture Buffer
Transient Capture Buffer (TCB) is a debug feature that monitors packet drop events. TCB provides more visibility into transactions around the vicinity of the packet drop. This feature is intended to debug rare unexpected packet drops.
TCB consists of the following:
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TCB buffer (Circular buffer) — Used to capture transactions on a set of memory management unit (MMU) resources around the vicinity of a specific drop event:
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Packet metadata (source/destination port, timestamp, Unicast queue number, Unicast queue depth, service pool depth, and so on)
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Raw packet data (80 bytes from start of the packet)
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Event buffer (FIFO buffer) — Used to:
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Record drop packets metadata
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Determine the reason for the drop
The following figure shows the workflow for TCB.
In the post-trigger phase, any drop happening in other queues of the capture scope is stored in event buffer. This buffer stores metadata of packets. Raw packet information is lost.
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Following are the configuration attributes for TCB:
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Capture Scope:
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Monitor Scope Type — Determines the scope type a TCB monitors. Supported scopes are:
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Unicast Queue (UCQ)
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Ingress port
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Egress port
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Monitor Scope Entity — Should be consistent with the Monitor Scope Type. Supported entities are:
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UCQ ID
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Port number
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Drop Event Trigger — Drop mechanisms that can cause a trigger. Supported triggers are:
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Ingress Admission Drop
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Egress Admission Drop
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Weighted Random Early Detection (WRED) Drop
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Pre-Trigger Phase Sample Probability — Packet sampling probability in the Pre-Trigger phase (1/16 to all)
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Post-Trigger Phase Sample Probability — Packet sampling probability in the Post-Trigger phase (1/16 to all)
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Freeze condition — The TCB state machine will enter the Frozen phase when either of the below freeze conditions are reached:
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Pre Freeze Capture Number — The number of packets captured between the Drop Event Trigger and Frozen phase
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Pre Freeze Capture Time — Time between the Drop Event Trigger to the Frozen phase (usec)
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Threshold Profiles — Eight threshold profiles available for each TCB instance. It has a start threshold and a stop threshold. The start threshold should be higher than the stop threshold.
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Threshold Profile Map — Each UCQ in the TCB scope can map to one threshold profile and different UCQs could map to one threshold profile. Supported maps are:
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Egress Admission Drop
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Weighted Random Drop
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