Orphan Blocks

dcrd may download orphan blocks—blocks whose previous block header hash field refers to a block header this node hasn’t seen yet. In other words, orphan blocks have no known parent (unlike stale blocks, which have known parents but which aren’t part of the best block chain).

Difference Between Orphan And Stale Blocks

When dcrd downloads an orphan block, it will not validate it. Instead, it will send a getblocks message to the node which sent the orphan block, the broadcasting node will respond with an inv message containing inventories of any blocks the downloading node is missing (up to 500), the downloading node will request those blocks with a getdata message, and the broadcasting node will send those blocks with a block message. The downloading node will validate those blocks, and once the parent of the former orphan block has been validated, it will validate the former orphan block.