[bitcoin-dev] Why not witnessless nodes?

[Kalle Rosenbaum <kalle@rosenbaum.se>]

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Dear list,

I find it hard to understand why a full node that does initial block
download also must download witnesses if they are going to skip
verification anyway. If my full node skips signature verification for
blocks earlier than X, it seems the reasons for downloading the
witnesses for those blocks are:

* to be able to send witnesses to other nodes.

* to verify the witness root hash of the blocks

I suppose that it's important to verify the witness root hash because
a bad peer may send me invalid witnesses during initial block
download, and if I don't verify that the witness root hash actually
commits to them, I will get banned by peers requesting the blocks from
me because I send them garbage.

So both the reasons above (there may be more that I don't know about)
are actually the same reason: To be able to send witnesses to others
without getting banned.

What if a node could chose not to download witnesses and thus chose to
send only witnessless blocks to peers. Let's call these nodes
witnessless nodes. Note that witnessless nodes are only witnessless
for blocks up to X. Everything after X is fully verified.

Witnessless nodes would be able to sync faster because it needs to
download less data to calculate their UTXO set. They would therefore
more quickly be able to provide full service to SPV wallets and its
local wallets as well as serving blocks to other witnessless nodes
with same or higher assumevalid block. For witnessless nodes with
lower assumevalid they can serve at least some blocks. It could also
serve blocks to non-segwit nodes.

Do witnessless nodes risk dividing the network in two parts, one
witnessless and one with full nodes, with few connections between the
parts?

So basically, what are the reasons not to implement witnessless
nodes?

Thank you,
/Kalle

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