Spark checkers retrieve CID content from Storage Providers and obtain the attestation with the payload.
In this step of the protocol, we zoom in on one Spark checker which is part of one committee in this round. This checker must now make the retrieval it has been assigned to perform in the previous step.
It sends a retrieval request to the Filecoin Storage Provider. The Storage Provider then sends back the response which is a CARv1 stream. However, the Storage Provider does not stop at the end of the CARv1 stream. As per IPIP-431, the Storage Provider also appends a 0x00 byte (to signal the end of the CAR stream) and then a metadata block.
The Metadata block includes the retrieval attestation, which includes a signature over the entire payload and some extra items that aid the retrieval checker with the next step: Proof of Data Possession.
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