the incremental-fetch optimization - when a consumer already holds part of an evidence graph, ask only for what changed or what is missing, rather than re-shipping whole envelopes.
Two circles, one set-difference of names, and the 0.65x that makes repetition cheap.
Etymology and backstory
delta is engineering's universal word for difference (from the Greek letter, via calculus notation). Delta encoding is everywhere bandwidth matters: rsync's rolling hashes, git's packfile deltas, HTTP 304 conditional requests, CRDT deltas. Content addressing makes deltas UNUSUALLY clean: since every node has a stable cid, "what you are missing" is exactly the set-difference of cids - no heuristics, no rolling windows.
Ecosystem
rsync, git, OSTree, IPFS bitswap want-lists - all shipping missing-hash sets rather than whole objects.
In codexproof
the /delta re-fetch path measures 0.65x relative cost - re-fetching incrementally costs about two-thirds of a cold fetch in the evaluated scenario, the number appearing in the fallback-ratio family (1.78x, 297x, 11,903x, 148,737x for the pathological cold paths; 0.65x for the warm delta). Thirteen occurrences of delta in the paper mark it as a load-bearing eval concept: the amortization story that makes per- interrogation sealing economical under repeat consumption.
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