Merge #3002

3002: consensus: draft the sketch implementation plan for Ouroboros Genesis r=nfrisby a=nfrisby

Recording the design sketch @edsko and I developed recently. This will guide the implementation effort. We're not sure when that will happen; we are choosing to assess the UTxO HD work impact/parallelizability/etc first.

Co-authored-by: Nicolas Frisby <nick.frisby@iohk.io>

ouroboros-consensus/docs/GenesisDecomposition.md

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+This file sketches the major points in the decomposition of the Genesis
+implementation. Edsko de Vries and Nicolas Frisby developed this plan of attack
+in 2021 March using chapter 21 of the Consensus Report.
+
+  * add _prefix selection_ in between ChainSync -> BlockFetch (Sections
+    21.1-21.4, 21.6.2 in the Chapter)
+
+      * implementation note: anticipate iteratively/incrementally advancing the
+        window within each function invocation
+
+        ```
+        -- PRECONDITION: all the fragments are anchored at the same point [some
+        -- upstream function in the pipeline should be able to ensure this
+        -- given the peers' fragments (each of which intersects with the
+        -- current chain) and the current chain]
+        --
+        -- Step 0: find the common prefix P of all fragments, anchor the lookahead window there
+        -- Step 1 & 2: as in The Report chapter
+        -- Step 3: prepend the common prefix P to the result of step 2
+        --
+        -- The chapter states "move the window", but this function doesn't do
+        -- the moving: "moving up past the select blocks" is achieved by
+        -- BlockFetch downloading them, ChainDB selecting, and ChainSync client
+        -- trimming them off the candidate fragments.
+        --
+        -- POSTCONDITION: the returned fragment is a prefix of one of the given
+        -- fragments. It's possible that a prefix of the returned fragment is
+        -- already on our selected chain.
+        selectPrefix ::
+             Ord tiebreaker
+          => (x -> tiebreaker)
+          -> NonEmpty (AnchoredFragment x, IsPeerTip)
+          ->           AnchoredFragment x   -- could be empty, indicating "wait for now"
+        ```
+
+      * prefix selection is _never_ compositional (wrt HFC combinator); it's
+        inherently motivated by arbitrary prefixes of chains, which is an
+        era-spanning concern.
+
+      * Genesis is fundamentally a change to how nodes catch-up, ie how they
+        sync the historical chain. Thus it must override key aspects of whatever
+        the contemporary protocol happened to be for those blocks. It is
+        all-or-nothing: you'd never apply Genesis to just part of a chain.
+
+        Genesis protects against attacks that would introduce adversarial forks
+        at some prefix of the current chain.
+
+        You'd never choose to enable Genesis for new eras but disable it for
+        some old eras. We want to use Genesis to compare prefixs, and a chain
+        that has reached the new era still has prefixes that have only reached
+        the old eras.
+
+        You'd never choose to enable Genesis for old eras but disable it for
+        some new eras. If you're using Genesis on a prefix of a chain, then
+        Genesis is affecting that entire chain -- it's nonsensical to "stop"
+        using Genesis during an execution. (Moreover, "opening the laptop lid"
+        may cause a node to need to "sync" again; see "alertness" below.)
+
+      * This method would only be invoked in NodeKernel.initInternalState, at
+        the ChainSync -> BlockFetch boundary. Abstractly: we apply it to all
+        candidates, then intersect each candidate with the result before giving
+        them to BF.
+
+      * CS now needs to include the "is peer's current tip" flag in its fragments
+
+      * We're planning to choose an interface that can express the current
+        behavior (ie longest prefix) as part of a staged implementation (ie us
+        developing multiple incremental PRs).
+
+  * managing the alert status, ie are we "caught up"? (Section 21.5)
+
+      * when alert, we can select among however many prefixes we have,
+        regardless of our valency (ie its no longer lower-bounded)
+
+      * corollary: when alert, Network Layer can reduce valency (The goal: _once
+        synced_, avoid redundant peers in order to lower resource usage. The
+        "syncing vs not syncing" dichotomy is one acceptable exclusion to the
+        average-case=worst-case guiding princple.)
+
+      * the general consensus from our meeting with the researchers was that the
+        most robust option is to always forge, even when behind. had considered
+        only forging when we're caught up, but that seems like a surface area
+        for an availability attack. (Section 21.5.5)
+
+      * specific case: OK for a non-alert node to forge during a ThreadNet test?
+        Old ThreadNet? New ThreadNet? (Frisby: it might even be _necessary_, due
+        to low-density chains arising there by design.)
+
+  * providing the alertness to the Network Layer
+
+      * _Become alert when_ both
+
+         1. We see every peer to its tip.
+
+         2. None of the peer's chain is binary-preferable to our current chain.
+
+      * _Become unalert when_ ... the wallclock is well ahead of the latest
+        meaningful peer interaction. Specifically: we become unalert as soon any
+        of our peer's tips is more than LIM behind our wallclock.
+
+      * With this division, the question becomes "having been caught-up, how
+        quickly could I fall dangerously behind?"
+
+      * We think the researchers will like this and will offer a suggestion for
+        the LIM value.
+
+  * making Network Layer sensitivity to alertness
+
+     * We anticipate adding an `:: STM IsAlert` field to the
+       `BlockFetchConsensusInterface` record.
+
+  * clock skew in relation to DoS attacks (Section 21.5.3)
+
+      * The `Left PastHorizonException{} -> retry` forecast case in CS client
+        should fail with "header invalid" instead of retry _when the header's
+        slot is ahead of the wallclock_ (cf `BlockchainTime.getCurrentSlot`).
+
+  * Frisby document to carefullyl consider support for/compatibility with
+    low-density chains
+
+      * current understanding: Genesis will not be blocked by low-density
+        chains, but any low-density chain will be a permanent opportunity for
+        an attacker _even despite_ Genesis (this is known; cf Distaster
+        Recovery Plan)
+
+  * key simplification: leave selection in ChainDB as-is (Section 21.6.1)
+
+      * This realization saved a lot of development time.
+
+  * SEPARATE WORK:
+
+      * TODO potential optimizations (Section 21.6.3)
+
+      * TODO testing etc - we'd _very much really_ like to use the ThreadNet
+        rewrite for this