[bitcoin-dev] Why not witnessless nodes?

[Eric Voskuil]

> On Dec 18, 2017, at 03:32, Kalle Rosenbaum via bitcoin-dev <bitcoin-dev at lists.linuxfoundation.org> wrote:
> 
> 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.

Why run a full node if you are not going to verify the chain?

> 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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