Re: [bitcoin-dev] Why not witnessless nodes?

From: Gregory Maxwell <greg@xiph•org>
To: Kalle Rosenbaum <kalle@rosenbaum•se>,
	 Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Why not witnessless nodes?
Date: Mon, 18 Dec 2017 20:42:34 +0000	[thread overview]
Message-ID: <CAAS2fgRuLWbQLw=2EQEODGHOp0=OrLkGguw=jJSCpQXEC_P+hQ@mail.gmail.com> (raw)
In-Reply-To: <CAPswA9ycPdTtm9PeD5a2R36cZ46HwnkwJu06FXuoE-F5Dx+eZQ@mail.gmail.com>

Because it would make no meaningful difference now, and if you are not
going to check the history there are much more efficient things to
do-- like not transfer it at all.

On Mon, Dec 18, 2017 at 8:32 AM, Kalle Rosenbaum via bitcoin-dev
<bitcoin-dev@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. 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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